Interactive Fly, Drosophila



Promoter Structure

The Dorsal protein (DL) regulates the transcriptional activity of several genes that determine cell fate along the dorsoventral axis of the embryo. Approximately 800 bp of 5'-flanking sequences upstream of the tld coding region were shown to drive an expression pattern indistinguishable from the wild-type pattern. A 423-bp fragment located within these sequences contains two DL binding sites and was shown to act as a silencer to mediate ventral repression. Point mutations in the sites abolish not only DNA binding but also ventral repression (Kirov, 1994).

A Dpp activity gradient specifies multiple thresholds of gene expression in the dorsal ectoderm of the early embryo. Some of these thresholds depend on a putative repressor, Brinker, which is expressed in the neurogenic ectoderm in response to the maternal Dorsal gradient and Dpp signaling. In this study it is shown that Brinker is a sequence-specific transcriptional repressor. It binds the consensus sequence, TGGCGc/tc/t, and interacts with the Groucho corepressor through a conserved sequence motif, FKPY. An optimal Brinker binding site is contained within an 800-bp enhancer from the tolloid gene, which has been identified as a genetic target of the Brinker repressor. A tolloid-lacZ transgene containing point mutations in this site exhibits an expanded pattern of expression, suggesting that Brinker directly represses tolloid transcription (Zhang, 2001).

The interplay between extracellular signaling molecules and localized transcriptional repressors is reminiscent of the segmentation pathway in the early Drosophila embryo. Pair-rule stripes of gene expression are established by broadly distributed transcriptional activators, such as Bicoid and Stat. The stripe borders are formed by localized gap repressors, including Hunchback, Kruppel, and Knirps. Similarly, the activation of tolloid and pannier might depend on broadly distributed Smad proteins, whereas the lateral limits of the expression patterns are established by the localized Brinker repressor. It is likely that vertebrates also employ one or more transcriptional repressors to restrict TGF-beta signaling interactions (Zhang, 2001).

Transcriptional Regulation

Embryonic target gene activation in the absence of brinker is independent of SMAD activity. Thus brk acts either parallel to or downstream of SMADs as a specific repressor of low and intermediate level Dpp target genes. brk is expressed like another dpp antagonist, short gastrulation (sog), in ventrolateral regions of the embryo abutting the dorsal dpp domain, and in brk mutants dpp expression expands to cover the entire ectoderm. In this situation sog is largely responsible for Dpp gradient formation, since brk;sog double mutant embryos have almost no polarity information in the ectoderm. The double mutants consist mainly of mesoderm and unstructured dorsal epidermis. Thus, brk and sog together specify the neuroectoderm of Drosophila embryos (Jazwinska, 1999).

The cuticle of brk mutant embryos has an enlarged region carrying dorsal hairs and a smaller region carrying ventral denticles. The number of sna-expressing neuroblasts in the ventral neurogenic region is reduced. This indicates that brk mutations lead to an expansion of dorsolateral fates and a reduction of ventrolateral fates. However, despite these lateral fate shifts, the number of Kr-expressing amnioserosa cells is not different from wild type. Thus, brk specifically affects cell fates depending on low or intermediate levels of Dpp signaling, while those that require peak levels are not altered. To identify the underlying causes of the visible changes in cell fate, the effect of brk was examined on the expression of two groups of dorsal/ventral (DV) patterning genes. The first group consists of dpp, zerknullt (zen) and tolloid (tld), whose expression is initiated very early in syncytial blastoderm stages. Since they are ventrally repressed by Dorsal (Dl) protein, their expression domains are confined to the dorsal 40% of the egg's circumference. In brk mutant embryos dpp, zen and tld expression is initiated normally. However, in contrast to wild type their expression domains expand ventrally during mid-cellularization. These data demonstrate that brk is not required for the early ventrolateral repression of these genes, but is essential to prevent their lateral expansion during cellularization. The second group of DV patterning genes includes rhomboid (rho), u-shaped (ush) and pannier (pnr), which are not direct targets of repression by maternal Dl. The initiation of their expression during cellularization requires prior formation of the Dpp activity gradient. Therefore, they are candidates for being direct targets of Dpp signaling in the embryo. They are expressed in domains straddling the dorsal midline that are 12 (rho), 14 (ush) and 32 (pnr) cells wide at cellular blastoderm (cell counts at approx. 50% egg length). The two narrowly expressed genes rho and ush are not changed in brk mutant embryos. This is also true for late zen expression, which in brk mutant embryos, as in wild type, refines to a narrow 5- to 6- cell-wide stripe along the dorsal midline despite the prior expansion. However, pnr expression expands in brk mutant embryos and low ectopic pnr levels can be seen in a broad lateral domain that stops about five cells short of mesodermal sna expression. Thus, brk does not affect the Dpp target genes that are expressed in dorsalmost regions and supposedly depend on highest Dpp levels. However, a target gene that is expressed in a wider domain, and is therefore presumably activated by intermediate levels of Dpp, is expanded. In summary, brk mutations affect the Dpp activity gradient in the embryo by expanding the domains of expression of dpp and one of its activators (tld) into ventrolateral regions. Despite the uniform expression of dpp in the entire ectoderm, Dpp activity levels appear to be only mildly increased in the ventrolateral region since only low-level (zen) or intermediate-level (pnr) target genes are ectopically expressed, causing a reduction in the size of the nervous system and ventral epidermis accompanied by an expansion of dorsal epidermis. Peak levels of Dpp in dorsalmost positions appear to be normal, judging from both target gene expression and cell type differentiation (Jazwinska, 1999).

The CBP histone acetyltransferase plays important roles in development and disease by acting as a transcriptional coregulator. A small reduction in the amount of Drosophila CBP (dCBP) leads to a specific loss of signaling by the TGF-ß molecules Dpp and Screw in the early embryo. The expression of Screw itself, and that of two regulators of Dpp/Screw activity, Twisted-gastrulation and the Tolloid protease, is compromised in dCBP mutant embryos. This prevents Dpp/Screw from initiating a signal transduction event in the receiving cell. Smad proteins, the intracellular transducers of the signal, fail to become activated by phosphorylation in dCBP mutants, leading to diminished Dpp/Screw-target gene expression. At a slightly later stage of development, Dpp/Screw-signaling recovers in dCBP mutants, but without a restoration of Dpp/Screw-target gene expression. In this situation, dCBP acts downstream of Smad protein phosphorylation, presumably via direct interactions with the Drosophila Smad protein Mad. It appears that a major function of dCBP in the embryo is to regulate upstream components of the Dpp/Screw pathway by Smad-independent mechanisms, as well as acting as a Smad coactivator on downstream target genes. These results highlight the exceptional sensitivity of components in the TGF-ß signaling pathway to a decline in CBP concentration (Lilja, 2003).

These results suggest that several transcription factors that regulate expression of Dpp/Screw signaling components require the dCBP coactivator for their function in Drosophila embryos, and implicate dCBP in regulation of the Dpp/Screw pathway independently of an interaction with Smad proteins. An additional role of dCBP is to regulate Dpp-target genes, acting at a step downstream of Smad protein phosphorylation. It is likely that direct interactions between dCBP and Mad/Medea contribute to regulation of Dpp target genes). Such interactions have been observed in vitro, both in mammalian systems and using Drosophila proteins. However, a major cause of impaired Dpp/Screw signaling in dCBP mutant embryos is due to reduced tolloid expression. This prevents Dpp/Screw from initiating a signaling event in cells that would normally receive the Dpp/Screw signal, presumably by a failure to cleave the Dpp-Sog and/or Screw-Sog complexes. In fact, a majority of embryos that do not express the Dpp/Screw-target gene rhomboid in dorsal cells, also do not contain phosphorylated Smad proteins. Furthermore, the pattern of phosphorylated Smad proteins correlates closely with that of tolloid expression. For example, in many early, cellularizing dCBP mutant embryos, an anterior patch of both tolloid expression and phosphorylated Smad staining remains. At later stages, tolloid expression recovers in dCBP mutant embryos, as does Dpp/Screw signaling as revealed by Smad protein phosphorylation. This recovery of tolloid expression at later stages of development might explain the recovery of phosphorylated Smad proteins in dCBP mutant embryos, by allowing Dpp/Screw to signal. For these reasons, regulation of tolloid expression appears to be a major means of controlling Dpp/Screw signaling by dCBP (Lilja, 2003).

It is likely that reduced screw expression also contributes to the reduction of phosphorylated Smad proteins observed in dCBP mutant embryos. In both screw and tolloid mutants, phosphorylation of Mad is eliminated. Furthermore, progressive reduction in Screw activity leads to a corresponding progressive deletion of dorsal-most cell fate, the amnioserosa. Tsg is required together with Sog to generate peak Dpp activity in dorsal midline cells. Reduced tsg expression in dCBP mutants may therefore contribute to the lack of Dpp/Screw-target gene expression. However, it is not believe that this lack can explain the defects in dCBP mutants, because in tsg mutants, low levels of Dpp signaling persist in a broad dorsal domain, leading to expanded rhomboid expression in dorsal cells. By contrast, in dCBP mutant embryos, expression of genes in response to a low threshold of Dpp activity, such as U-shaped and the dorsal rhomboid pattern, is eliminated (Lilja, 2003).

These experiments do not address whether dCBP regulation of tolloid, screw, and tsg expression is direct or indirect. However, since expression of these genes begins at about the time when zygotic transcription initiates in the embryo, and the effect of dCBP is evident from the onset of expression of tolloid and screw, the notion is favored that dCBP is acting directly on the enhancers of these genes. It is not yet understood whether HATs such as CBP primarily act to acetylate large chromosomal domains, or are directed to specific genes. In the case of the tolloid gene, the results indicate that dCBP is being recruited to the enhancer by a DNA-binding protein, since the isolated enhancer removed from its normal chromosomal location requires dCBP for its activity (Lilja, 2003).

Given its central position in gene regulation and the great number of mammalian transcription factors shown to interact with CBP, relatively few genes are affected by the dCBP mutation. For example, activation and repression mediated by the Dorsal protein are unaffected in the dCBP mutant embryos, as demonstrated by the expression patterns of Dorsal target genes. Also, no defects in early segmentation gene expression could be observed in germline clone mutants. However, the nej1 mutation used in this study to create dCBP mutant germline clone embryos is a weak mutation that results in a very modest reduction in dCBP levels. Since other means of reducing the dCBP amount by approximately two-fold results in similar gene expression defects, Smad proteins and the unidentified activators of tolloid, tsg, and screw expression are particularly sensitive to a decline in dCBP concentration. It may not be a coincidence that screw, tsg, tolloid, and Dpp-target gene expression are all specifically affected by a small dCBP reduction. Perhaps components of the Dpp/Screw signal transduction pathway have evolved to be coordinately regulated by a common coactivator. Given the phylogenetic conservation of the CBP protein and the TGF-alpha signal transduction pathway, as well as the ability of CBP and Smad proteins to interact in vitro, CBP is likely to play an equally important role in TGF-ß signaling in other metazoans (Lilja, 2003).

Graded maternal Short gastrulation protein contributes to embryonic dorsal-ventral patterning by delayed induction: Expression of tld and tok is ventrally restricted by the TGF-α ligand Gurken

Establishment of the dorsal–ventral (DV) axis of the Drosophila embryo depends on ventral activation of the maternal Toll pathway, which creates a gradient of the NFkappaB/c-rel-related transcription factor Dorsal. Signaling through the maternal BMP pathway also alters the dorsal gradient, probably by regulating degradation of the IkB homologue Cactus. The BMP4 homologue decapentaplegic (dpp) and the BMP antagonist short gastrulation (sog) are expressed by follicle cells during mid-oogenesis, but it is unknown how they affect embryonic patterning following fertilization. This study provides evidence that maternal Sog and Dpp proteins are secreted into the perivitelline space where they remain until early embryogenesis to modulate Cactus degradation, enabling their dual function in patterning the eggshell and embryo. Metalloproteases encoded by tolloid (tld) and tolkin (tok), which cleave Sog, are expressed by follicle cells and are required to generate DV asymmetry in the Dpp signal. Expression of tld and tok is ventrally restricted by the TGF-α ligand encoded by gurken, suggesting that signaling via the EGF receptor pathway may regulate embryonic patterning through two independent mechanisms: by restricting the expression of pipe and thereby activation of Toll signaling and by spatially regulating BMP activity (Carneiro, 2006).

This study has shown that sog, dpp, and tld act during oogenesis to promote the formation of dorsal anterior structures of the eggshell and to establish the embryonic DV axis. According to a proposed model, Sog is produced in follicle cells and is processed into different forms depending on DV location and stored in the perivitelline space. These forms of Sog then persist until early stages of embryogenesis at which time they act by a delayed induction mechanism to alter signaling mediated by maternally derived Dpp. It is proposed that an asymmetric distribution of Sog peptides is produced through the action of the ventrally localized Tld and Tok metalloproteases. Different forms of Sog act locally to inhibit Dpp signaling ventrally (e.g., N-Sog) or diffuse over considerable distances to concentrate Dpp dorsally (e.g., full-length Sog or C-Sog). According to this model, a dorsal-to-ventral gradient of Dpp activity is formed in the perivitelline space that counteracts and sharpens the inverse gradient of nuclear dorsal (Carneiro, 2006).

An important finding in this study is that Sog protein produced by follicle cells is secreted into the perivitelline space where it persists until the end of oogenesis and early embryogenesis, prior to initiation of zygotic sog expression. One way maternal Sog fragments might influence DV patterning in the embryo is to modify zygotic Dpp signaling. However, maternal Dpp signaling is involved in establishing the relative positions of the ventral mesoderm versus the lateral neuroectodermal territories, while zygotic Dpp activity determines the relative positions of dorsal and lateral domains. These distinct phenotypes suggest that maternal Sog acts by modulating the maternal rather than the zygotic component of Dpp signaling (Carneiro, 2006).

This analysis also suggests that the Dpp synthesized by follicle cells is secreted into the perivitelline space and stored there until advanced stages of oogenesis. These maternally synthesized Sog and Dpp proteins may act on the embryo following fertilization when signaling through the Toll pathway is initiated. Several lines of evidence support this hypothesis. (1) Through epistatic analysis, it was shown that maternal Dpp does not act upstream of the Toll receptor. Therefore, genes expressed in the follicle cell epithelium that regulate DV patterning exclusively via the Toll pathway should not be targets of maternal Dpp signaling. Alternatively, undescribed non-Toll mediators of DV patterning could potentially be targets of maternal Dpp in the follicular epithelium. (2) Blocking Tkv receptor function or reducing maternal Dpp activity (by 8xhssog, in follicle cells has no effect on the pattern of pip expression. It has been shown that maternal dpp does not alter grk expression. Thus, no evidence was found that the embryonic effects here described in this study are due to alterations in patterning of the follicular epithelium. (3) Maternal dpp signaling increases the levels of Cactus protein in the embryo by a mechanism that is independent of Toll. Finally, inhibition of Tkv with tkvDN expressed with an early maternal driver alters the embryonic expression domains of ventral and lateral genes such as vnd and snail, which are targets of dorsal activation but not of zygotic BMP signaling. tkvDN expression also alters expression of DV genes in lateralized embryos, which lack dorsal ectoderm and early zygotic dpp expression. In aggregate, these various data support the view that maternal dpp and sog act by delayed induction on the embryo itself. The possibility cannot be ruled out, however, that the embryonic DV phenotypes described in this study result from the combined effects of direct and indirect maternal dpp signaling with the predominant effect being direct (Carneiro, 2006).

Delayed inductive activities have been proposed for a variety of proteins synthesized during oogenesis. For example, activation of the terminal system relies on delayed inductive activity of the secreted product of the torsolike gene (tsl), which is expressed by follicle cells at the two poles of the oocyte and associates with the vitelline membrane. ndl has a dual action on chorion integrity and embryonic patterning. The embryonic patterning function of ndl is thought to be mediated by Nudel protein that is secreted into the perivitelline space where it associates with the embryonic plasma membrane and initiates a proteolytic cascade. It is proposed that Sog and Dpp secreted by follicle cells also serve two roles. First, they contribute to patterning the follicle cell epithelium and chorion, and secondly, they are transferred to and stored in the perivitelline space where it is proposed that they function after fertilization to modify Toll patterning in the embryo (Carneiro, 2006).

During embryogenesis, Sog protein diffuses dorsally from the neuroectoderm and may carry Dpp dorsally in a complex with Tld, Tsg, and Scw, resulting in the generation of peak Dpp activity in the dorsal midline. The spatial distribution of maternal Sog, Dpp, Tld and Tok during oogenesis could also create asymmetric BMP activity. Since tld and tok are expressed only in ventral follicle cells, a ventral-to-dorsal gradient of Sog fragments is likely to be produced. Because cleavage of Sog by Drosophila Tld and Tok is dependent on the amount of Dpp, cleavage of Sog by Tld and Tok should be increased near the source of Dpp, generating an oblique gradient of Sog fragments in the egg chamber. The existence of such a gradient is supported by the greater staining seen in anterior ventral cells with the anti-Sog 8A antiserum during stage 10B. However, greater asymmetry may exist as a result of differential distribution of an array of Sog fragments throughout the egg chamber. Unfortunately, visualization of such asymmetry would be hard to achieve due to limitations in the ability to recognize several fragments by existing Sog antisera (Carneiro, 2006).

The analysis of marked sog− and tld− follicle cell clones suggests that the mobility of Sog fragments in the extracellular compartment may contribute to creating a maternal Dpp activity gradient. Such clones resulted in different Sog staining patterns in the perivitelline space adjacent to the clones depending on where they were located along the DV axis. The staining pattern observed with the 8A antibody suggests that ventrally generated N-Sog cleavage products may be less diffusible than intact Sog or than C-Sog and remain restricted to their site of production. In contrast, full-length Sog and C-Sog fragments appear to diffuse more readily (Carneiro, 2006).

Diffusion of Dpp may also contribute to patterning the eggshell. The expression of dpp in anterior follicle cells is consistent with its role in the formation of dorsal anterior chorionic structures. An anterior-to-posterior gradient of Dpp activity in dorsal regions of the egg chamber is suggested by the Dpp-dependent activation of the A359 enhancer trap and graded repression of bunched along the AP axis. In addition, BR-C expression is lost in mad− clones away from the source of Dpp. sog is likely to contribute to establishing this BMP gradient since ventral sog−clones act non-cell-autonomously to decrease the size of the operculum. Since ventral tld− clones also alter the extent and angle of the operculum, Tld may process Sog to generate a fragment that diffuses and carries Dpp to a dorsal anterior location, concentrating and thus enhancing Dpp activity. Further evidence that a fragment with such activity exists derives from the observation that overexpression of a C-terminal Sog fragment generates chorionic phenotypes that strongly resemble dpp overexpression (Carneiro, 2006).

A dorsally produced form of Sog also appears to participate in patterning the eggshell since sog− clones located dorsally result in fusion of dorsal appendages along the dorsal midline. DV positioning of the dorsal appendages depends on several factors, most critically on EGFR signaling. In contrast, mild overexpression of dpp generates fusion of the dorsal appendages. Considering the well-established role of Sog in modulating Dpp activity, the fused appendage phenotype generated by dorsal sog− clones most likely reflects the loss of Dpp antagonism exerted by Sog (Carneiro, 2006).

In addition to the activities described above, N-Sog fragments which remain ventrally restricted could exert Supersog-like activity, antagonizing BMPs while acquiring resistance to further cleavage and degradation by Tld. This ventrally restricted activity most likely patterns the embryo but does not affect dorsal positioning of eggshell structures, which depends on the combined activity of Dpp/BMPR signaling and dorsally generated Grk/EGFR signals (Carneiro, 2006).

The assortment of Sog fragments in egg chambers is very similar to that in the embryo. Full-length and processed forms of Sog generated by Tld during oogenesis might remain asymmetrically distributed during embryogenesis and exert distinct activities. This hypothesis is in agreement with the effect of tld− and sog− follicle cell clones on the embryo. In the majority of cases, tld− follicle cell clones result in ventralized cuticles, indicating that Tld generates some activity that synergizes with Dpp. Reciprocally, the great majority of sog− follicle cell clones result in dorsalized cuticles and embryos, indicating that Sog primarily acts by antagonizing Dpp. Since only ventral sog− clones generate cuticle defects, ventrally produced Sog presumably generates a ventralizing activity that blocks Dpp locally. In contrast, since in a minority of cases ventral shifts are observed in embryonic gene expression domains resulting from sog− clones, as well as a minority of dorsalized cuticles from tld− clones, there may also be a form of Sog that can enhance Dpp signaling. This positive BMP promoting activity could be generated ventrally, as suggested above in the case of chorion patterning (Carneiro, 2006).

A model depicting the proposed effects of different Sog forms on formation of the chorion and embryonic patterning is presented. According to this model, ventrally restricted Tld cleaves Sog near the Dpp source in ventral anterior follicle cells generating N-Sog and C-Sog. It is suggested that N-Sog fragments remain restricted near ventral anterior cells to antagonize Dpp, while C-Sog fragments diffuse dorsally concentrating Dpp in dorsal anterior cells that direct formation of the operculum. This asymmetric production of Sog molecules would generate a dorsal-to-ventral gradient of Dpp, with the highest levels dorsally near the anterior Dpp source. Although direct visualization of the predicted resulting Dpp gradient in the embryo is hard to achieve with the tools available, it is proposed that such a similarly oriented gradient persists until early embryogenesis based on the asymmetric pattern of Dpp-GFP distribution during late oogenesis and the observed alterations in embryonic gene expression domains resulting from modifications in maternal Dpp signaling (Carneiro, 2006).

The slope of the Dl nuclear gradient ultimately defines the extent of the mesoderm (Mes), neuroectoderm (NE), and dorsal ectoderm (DE). A uniform increase or decrease in nuclear Dl along the DV axis can only alter the extent of the Mes and DE and positioning of the NE, while a change in the slope of the gradient will modify the extent of NE territories such as the vnd expression domain. Under all conditions that Dpp signaling was altered, modifications were observed in the width of the vnd domain. This suggests that graded maternal Dpp signaling helps determine the slope of the dorsal gradient. Earlier studies suggested that Dpp inhibits Cactus degradation and as a consequence decreases Dl translocation into the nucleus. Increased Dpp signaling should result in more Dl retained in the cytoplasm, with consequent narrowing of the mesoderm and ventral shift in lateral and dorsal expression domains. Conversely, inhibition of Dpp signaling would result in increased levels of Dl becoming available for nuclear translocation. Considering the proposal that maternal Dpp is highest dorsally, and that Cactus may also act to prevent Dl diffusion along the DV axis, decreasing Dpp should lower Cactus levels in dorsal–lateral regions of the embryo and result in the redistribution of free Dl from ventral to lateral regions. As a consequence of this redistribution of Dl, there would be a slight decrease in Dl levels ventrally and an increase laterally that would have the net effect of flattening the gradient. Such a mechanism would require a certain degree of mobility of dorsal dimers in the syncytial blastoderm. In future studies, it will be interesting to determine the relative mobilities of Dl/Cactus complexes in the cytoplasm (Carneiro, 2006).

Maternal BMP signaling may also increase the robustness of dorsal patterning. The prevailing view of DV patterning is that signaling through the Toll pathway is sufficient to generate threshold-dependent activation of several dorsal target genes along the entire DV axis. Activation of Toll triggered by the ON/OFF pip expression pattern must be transformed into a ventrally centered gradient of Toll signaling. Several mechanisms may contribute to generate this gradient, based on autoregulatory feedback mechanisms. Although the Toll system may be internally robust, regulatory inputs from other signaling pathways could also contribute further to its stability, such as suggested for the wntD pathway and for maternal Dpp. While a significant body of evidence supports the standard view that establishment of the dorsal gradient through the Toll pathway is central to DV axis specification, the maternal Dpp pathway may constitute an important secondary mechanism that sharpens and ensures robustness and stability of the dorsal gradient in response to a rapidly changing embryonic environment (Carneiro, 2006).

The initiating event in maternal DV patterning is localized activation of the Grk/EGFR pathway in dorsal cells. Grk functions by restricting the expression of both pip and tld/tok, providing two potentially independent means for spatially regulating the activity of Toll and Dpp. This dual action of the Grk/EGFR pathway is consistent with analysis in which it was found that embryonic cuticles from gd−; grk−; Tl[3] mothers displayed a phenotype distinct from those collected from gd−; Tl[3] mothers. While cuticles from both genotypes had denticle belts surrounding the entire circumference of the embryo, cuticles from gd−; grk−; Tl[3] mothers were more elongated than those from gd−; Tl[3] mothers and exhibited a more ventral character. This suggests that grk provides an additional signal for asymmetry downstream or in parallel to gd. It is suggested that the hypothetical system proposed acts downstream of grk/EGFR and in parallel to Toll may be the Dpp pathway (Carneiro, 2006).

Protein Interactions

DPP's activity is modulated by Tolloid, which also has a role in the determination of dorsal cell fate. The Tolloid protein functions by forming a complex containing DPP via protein-interacting EGF and C1r/s domains. The protease activity of Tolloid is necessary, either directly or indirectly, for the activation of the DPP complex (Finelli, 1994).

Noggin, a protein expressed in the Spemann organizer region of the Xenopus embryo, promotes dorsal cell fate within the mesoderm and neural development within overlying ectoderm. noggin, expressed in Drosophila, promotes ventral development, specifying ventral ectoderm and CNS in the absence of all endogenous ventral-specific zygotic gene expression. Noggin blocks DPP signaling upstream of DPP receptor activation. It is proposed that, whole most or all of the DPP produced in the dorsal-most region binds to its receptors, DPP produced more laterally has an increased probability of being bound by ventrally produced Short gastrulation, and that DPP can be released from this diffusible complex by the action of a third dorsal-specific gene, perhaps tolloid (Holley, 1996).

Local inhibition and long-range enhancement of Dpp signal transduction by Sog

Extracellular gradients of signaling molecules can specify different thresholds of gene activity in development. A gradient of Decapentaplegic (Dpp) activity subdivides the dorsal ectoderm of the Drosophila embryo into amnioserosa and dorsal epidermis. The proteins Short gastrulation (Sog) and Tolloid (Tld) are required to shape this gradient. Sog has been proposed to form an inhibitory complex with either Dpp or the related ligand Screw, and is subsequently processed by the protease Tld. Paradoxically, Sog appears to be required for amnioserosa formation, which is specified by peak Dpp signaling. Sog appears to be required for peak Dpp/Screw activity, since sog mutants lack amnioserosa. SOG transcripts are detected in two ventrolateral stripes within the presumptive neurogenic ectoderm. Several amnioserosa marker genes, including Kruppel, rhomboid and hindsight exhibit broadened patterns of expression that gradually diminish in older embryos. In contrast, the Race (Related to angiotensin converting enzyme) pattern is not transiently expanded in sog mutants; instead, by the onset of gastrulation, expression is nearly lost in central regions. Race may represent a more definitive marker for the presumptive amnioserosa than the genes used in previous studies (Ashe, 1999).

The misexpression of sog using the even-skipped stripe-2 enhancer redistributes Dpp signalling in a mutant background in which dpp is expressed throughout the embryo. Dpp activity is diminished near the Sog stripe and peak Dpp signaling is detected far from this stripe. However, a tethered form of Sog suppresses local Dpp activity without augmenting Dpp activity at a distance, indicating that diffusion of Sog may be required for enhanced Dpp activity and consequent amnioserosa formation. The long-distance stimulation of Dpp activity by Sog requires Tld, whereas Sog-mediated inhibition of Dpp does not. The heterologous Dpp inhibitor Noggin inhibits Dpp signaling but fails to augment Dpp activity. These results suggest an unusual strategy for generating a gradient threshold of growth-factor activity, whereby Sog and its protease specify peak Dpp signaling far from a localized source of Sog. Different models have been proposed to explain the requirement of Sog in generating peak Dpp activity. One invokes the diffusion of Sog-Dpp or Sog-Screw complexes away from the ventrolateral Sog stripes, thereby focusing Dpp and/or Screw at the dorsal midline. An alternative model suggests that a product resulting from the cleavage of Sog directly signals formation of the amnioserosa, possibly by augmenting the binding of Dpp or Screw to the receptors Thick veins and Saxophone (Ashe, 1999).

Processing of the Drosophila Sog protein creates a novel BMP inhibitory activity

Structurally unrelated neural inducers in vertebrate and invertebrate embryos have been proposed to function by binding to BMP4 or Dpp, respectively, and preventing these homologous signals from activating their receptor(s). The functions of various forms of the Drosophila Sog protein were examined using the discriminating assay of Drosophila wing development. Misexpression of Drosophila Sog, or its vertebrate counterpart Chordin, generates a very limited vein-loss phenotype. This sog misexpression phenotype is very similar to that of viable mutants of glass-bottom boat (gbb), which encodes a BMP family member. Consistent with Sog selectively interfering with Gbb signaling, Sog can block the effect of misexpressing Gbb, but not Dpp in the wing. In contrast to the limited BMP inhibitory activity of Sog, carboxy-truncated forms of Sog, referred to as Supersog, have been identified which when misexpressed cause a broad range of dpp minus mutant phenotypes. Evidence is provided that Twisted gastrulation (Tsg) functions in the embryo to generate a Supersog-like activity, perhaps by modifying the enzymic activity of Tolloid, the enzyme that processes Sog (Yu, 2000).

The predicted Sog protein is 1038 amino acids in length and contains four cysteine-rich (CR) domains in the extracellular domain. The metalloprotease Tld cleaves Sog at three major sites. Supersog1 is an N-terminal fragment of Sog including CR1 plus another 114 amino acids, and contains an additional 33 amino acids derived from vector sequences at its C terminus. Supersog2, which contains the same amino acids as Supersog1 but terminates abruptly at the end of Sog sequences, also generates Supersog phenotypes, albeit slightly weaker than those observed with Supersog1. Supersog4 is an N-terminal fragment of Sog ending 80 amino acids before CR2 and includes 130 sog 3' UTR derived amino acids (Yu, 2000).

In line with its phenotypic effects, Supersog can block the effects of both misexpressing Dpp and Gbb in the wing. Vertebrate Noggin, in contrast, acts as a general inhibitor of Dpp signaling, which can interfere with the effect of overexpressing Dpp, but not Gbb. Evidence suggests that Sog processing occurs in vivo and is biologically relevant. Overexpression of intact Sog in embryos and adult wing primordia leads to the developmentally regulated processing of Sog. This in vivo processing of Sog can be duplicated in vitro by treating Sog with a combination of the metalloprotease Tolloid (Tld) plus Twisted Gastrulation (Tsg), another extracellular factor involved in Dpp signaling. In accord with this result, coexpression of intact Sog and Tsg in developing wings generates a phenotype very similar to that of Supersog. Evidence is provided that tsg functions in the embryo to generate a Supersog-like activity, since Supersog can partially rescue tsg minus mutants. Consistent with this finding, sog minus and tsg minus mutants exhibit similar dorsal patterning defects during early gastrulation. These results indicate that differential processing of Sog generates a novel BMP inhibitory activity during development and, more generally, that BMP antagonists play distinct roles in regulating the quality as well as the magnitude of BMP signaling (Yu, 2000).

To determine whether Sog might be processed in vivo to generate Supersog-like molecules, an anti-Sog antibody directed against an amino fragment of Sog was used on immunoblots to analyze protein extracts from different stages and tissues of developing Drosophila. This antibody recognizes an epitope present in the stem portion of Supersog. An examination was made of the nature of Sog products produced both in wild-type individuals as well as in flies overexpressing Sog. This analysis reveals that Sog is processed in vivo, and that this processing is developmentally regulated. For example, in heat shocked early embryos carrying eight copies of an HS-sog construct, a 76 kDa band, a doublet of bands migrating at 42/40 kDa, and a 28 kDa band were observed, in addition to a 120 kDa band corresponding to full-length Sog. These bands are likely to represent various forms of Sog since they are strongly induced only in heat shocked HS-sog blastoderm stage embryos. Heat induction of HS-sog pupae results in the elevated production of prominent Sog fragments migrating at 76, 60, 50 and 42 kDa. In pupal wings, the same pattern of Sog fragments is present in overloaded extracts of wild-type pupal wings as observed in heat induced HS-sog wings, albeit at lower levels. This significant level of endogenous processing is not surprising given that wild-type pupal wings express high levels of Sog throughout intervein regions, which account for approximately 90% of cells in the wing (Yu, 2000).

Processing of exogenously provided Sog is developmentally regulated. During embryonic and pupal stages, when Sog is expressed in a significant fraction of cells and plays important developmental roles, distinct patterns of Sog fragments are produced. For example, during pupal development, 60, 50 and 42 kDa fragments are induced in heat shocked HS-sog wings, while in early embryos, a pair of induced bands migrating at 42/40 kDa is most prominent. In contrast, during late embryonic or third larval instar stages, only the full length Sog band is observed upon induction of HS-sog larvae. During these latter stages of development, sog is expressed in only a small percentage of cells and is not known to have any significant developmental function. Thus, Sog is processed in vivo at developmentally relevant times and in different patterns to generate fragments that are likely to have distinct activities from Sog in addition to being degraded into inert products (Yu, 2000).

The fact that pulses of Supersog1 expression delivered during the late blastoderm stage of development can partially rescue the tsg minus mutant embryos suggests that a Supersog-like activity might mediate part of tsg function in vivo. In addition, late blastoderm stage tsg minus mutant embryos display defects similar to those of sog mutants, suggesting that tsg is involved in a late function of Sog. Consistent with the view that tsg acts during early gastrulation, tsg minus mutants can not be rescued by driving expression of a tsg transgene under the control of the tld promoter, which is expressed only early during the blastoderm stage. In contrast, it is possible to rescue tsg minus mutants by driving tsg expression with promoters that continue to be expressed into early gastrulation. Several possible ways in which Supersog-like activities could contribute to this stage of development can be imagined, given that they have different ligand specificities from intact Sog and are stable to further proteolysis by Tld. Since Sog has been proposed to block the activity of Scw in embryos, it is likely that some other BMP is the preferred target of Supersog molecules. In addition, since Scw is only expressed transiently during the blastoderm stage of development, intact Sog would have no obvious target to inhibit beyond this stage. Perhaps a stable broad-spectrum BMP antagonist such as Supersog could inhibit the action of other BMPs expressed in the dorsal ectoderm during early stages of gastrulation (possibly Dpp itself) and thereby provide a form of molecular memory, which helps maintain the distinction between neural and non-neural ectoderm (Yu, 2000).

The observation that Supersog is less effective than Sog in blocking BMP signaling in the early embryo is consistent with the view that Supersog is not just a higher affinity version of Sog and suggests that Supersog is actually less effective than Sog at blocking the effect of Scw. The fact that Supersog does not inhibit Dpp itself during early blastoderm stages is likely to be the result of insufficient levels of Supersog being expressed by the heat shock vector. It is possible, however, that an endogenously produced Supersog activity (e.g. generated upon Tsg binding to Sog) has a higher affinity for Dpp than the artificially created Supersog1 construct. In any case, it is proposed that Supersog acts in the late blastoderm embryo or during early gastrulation stages rather than in the early blastoderm embryo, and that during this latter period, it is able to block the activity of a BMP (e.g. Dpp?) not recognized by Sog. It is tempting to consider a two step temporal model for the action of Sog and Supersog during embryonic dorsal-ventral patterning to account for the fact that sog mutants display a dorsal-ventral phenotype earlier than tsg minus mutants. According to one such scenario, the labile Tld-sensitive form of full-length Sog is produced from a localized source (i.e. the neuroectoderm) and diffuses dorsally to be degraded by Tld. Tld acts as a sink to create a transiently stable gradient of Sog, which creates a reciprocal gradient of Dpp activity. The Sog gradient created by this classic source/sink configuration would only be short-lived, however, since cells begin migrating when gastrulation begins. At this stage, the embryo elongates and the Dorsal gradient collapses, leading to loss of gene expression in early zygotic D/V domains. Following the establishment of the short-lived hypothetical Sog gradient, tsg expression is initiated in dorsal cells and leads to the production of stable Supersog-like molecules by switching the activity of Tld from degrading to activating Sog. Supersog-like molecules then could provide a stable record of high versus low BMP signaling domains during a subsequent step of development (Yu, 2000).

Creation of a Sog morphogen gradient in the Drosophila embryo

A variety of genetic evidence suggests that a gradient of Decapentaplegic (Dpp) activity determines distinct cell fates in the dorsal region of the Drosophila embryo, and that this gradient may be generated indirectly by an inverse gradient of the BMP antagonist Short gastrulation (Sog). It has been proposed that Sog diffuses dorsally from the lateral neuroectoderm where it is produced, and is cleaved and degraded dorsally by the metalloprotease Tolloid (Tld). This study shows directly that Sog is distributed in a graded fashion in dorsal cells and that Tld degradation limits the levels of Sog dorsally. In addition, Dynamin-dependent retrieval of Sog acts in parallel with degradation by Tld as a dorsal sink for active Sog (Srinivasan, 2002).

The finding that Tld collaborates with the Tolkin (Tok) protease to limit Sog diffusion indicates that these two closely related proteases are likely to share at least this one important substrate. Consistent with this possibility, in vitro studies indicate that Tok can cleave Sog in vitro, but with significantly reduced activity relative to Tld. Since Tld cleaves Sog in only a limited number of specific sites in vitro, it is likely that another class of extracellular protease degrades the products of Tld/Tok cleavage to peptide fragments, which may be too small to be recognized by either the 8A or 8B Sog antibodies. It is noteworthy that Tld degradation of Sog occurs on a much more rapid time scale in vivo (e.g., 30 min) than in vitro (e.g., several hours). This finding is consistent with the developmental timescale of Tld activity and suggests that additional factors present in vivo accelerate the action of Tld (Srinivasan, 2002).

The observation that Sog degradation fails to take place in dorsal cells of dpp- mutants is consistent with in vitro experiments in which Dpp is required as a cofactor for Tld-dependent cleavage of Sog. In contrast to in vitro studies in which either Dpp or Scw can act as cofactors, only Dpp serves as a critical cofactor function for in vivo degradation of Sog. An interesting difference between the ectopic Sog observed in dpp- versus Df(tld) embryos is that the staining is uniform in dpp- mutants but retains some degree of gradation in Df(tld) mutants. It is possible that another yet uncharacterized metalloprotease collaborates with Tld and Tok to degrade Sog in the early embryo. Alternatively, Sog might bind to a complex containing Dpp that is still present in Df(tld) mutant and limits Sog diffusion dorsally. The formation of this complex, or the ability of Sog to bind to it, may be strictly dependent on Dpp, so that in its absence, there is no restraint on Sog diffusion dorsally (Srinivasan, 2002).

An additional aspect of this study is the finding that Dynamin (shi) functions in parallel with Tld/Tok to limit active Sog levels in dorsal cells, which is required to generate a peak response to BMP signaling in dorsal-most cells. The fact that shi was not picked up previously as a D/V mutant in systematic screens for embryonic patterning mutants presumably reflects the pleiotropic requirement for Dynamin function, which is also required for Hh, Wg, Notch, and EGF-R signaling as well as various other cell biological processes involving membrane trafficking. While Dynamin function is not required for diffusion of Sog dorsally, it does appear to be required for the maintenance of the Sog gradient by removing Sog from the extracellular space. It is also possible that Dynamin plays other roles in promoting BMP signaling and that removing Sog from shits; sog RNAi embryos compensates for this reduced function. One argument against this latter possibility is that elimination of Dynamin function prior to the production and secretion of Sog does not compromise BMP signaling at that earlier stage. In any case, it is clear that an active form of Sog mediates the reduction of BMP signaling associated with loss of Dynamin function (Srinivasan, 2002).

In addition to inhibiting the activity of Scw and thereby reducing BMP signaling, there is evidence that Sog can exert other activities. For example, in the presence of the secreted protein Twisted gastrulation, Sog is cleaved in a different pattern by Tld in vitro to generate a truncated form of Sog consisting of CR1 and part of the stem. This truncated molecule, called Supersog, can inhibit Dpp as well as the auxiliary BMPs Scw and Glass bottom boat (Gbb). A major function of Tsg is to generate a Supersog-like activity in vivo, since expression of Supersog, but not intact Sog, can partially rescue tsg- mutant embryos. Supersog may play a persistent role in inhibiting BMP signaling following the transient expression of Scw, since it is refractory to degradation by Tld. There is also indirect genetic evidence that Sog acts at a long range to promote BMP signaling as judged by activation of the target gene RACE. Since Tld plays a dose-dependent role in generating this putative positive Sog activity, it too may be a processed form of Sog. It has also been proposed that some form of Sog might carry Dpp to the dorsal midline and thereby concentrate BMP along the dorsal midline. One line of evidence supporting this model is that the pattern of phosphorylation and activation of Mad observed in situ by staining with an anti-pMAD antibody reveals a narrow dorsal band of peak BMP activity with little evidence for a gradient diminishing ventrally. However, there is also a wealth of indirect genetic evidence that there are several intermediate levels of BMP activity that activate several dorsally expressed BMP target genes at different levels (Srinivasan, 2002).

Although the final BMP activity gradients generated in the early embryo and wing imaginal discs have strikingly similar shapes, the mechanisms for creating them are very different. In the wing disc, the spread of Dpp from its narrow localized source in the center of the disc is limited by sequestration by means of the BMP receptor Tkv. In contrast, in the embryo, a BMP activity gradient forms within a broad domain of uniform dpp expression in response to the creation of an inverse gradient of the BMP antagonist Sog, which diffuses into the dorsal domain from the adjacent neuroectoderm. This Sog gradient in dorsal cells is created by a combination of specific proteolytic degradation by Tld and Tok and Dynamin-mediated retrieval of Sog. It is noteworthy that Dynamin exerts opposite effects in the early embryo and wing disc. In the precellular embryo, where extracellular molecules may diffuse in a passive ink-in-water fashion in the surrounding perivitelline fluid, a Dynamin-dependent mechanism limits extracellular accumulation of Sog dorsally. In contrast, in the cellularized context of the wing imaginal disc, an active Dynamin-dependent transport process is required for Dpp movement between cells. The fact that completely different mechanisms can ultimately create similarly shaped BMP activity gradients highlights the flexibility of evolutionary processes, which can arrive at more than one type of solution to the same basic problem (Srinivasan, 2002).

Physical properties of Tld, Sog, Tsg and Dpp protein interactions are predicted to help create a sharp boundary in Bmp signals during dorsoventral patterning of the Drosophila embryo

Dorsal cell fate in Drosophila embryos is specified by an activity gradient of Decapentaplegic. Genetic and biochemical studies have revealed that the Sog, Tsg and Tld proteins modify Dpp activity at the post-transcriptional level. The predominant view is that Sog and Tsg form a strong ternary complex with Dpp that prevents it from binding to its cognate receptors in lateral regions of the embryo, while in the dorsalmost cells Tld is proposed to process Sog and thereby liberate Dpp for signaling. In this model, it is not readily apparent how Tld activity is restricted to the dorsal-most cells, since it is expressed throughout the entire dorsal domain. In this study, additional genetic and biochemical assays were developed to further probe the relationships between the Sog, Tsg, Tld and Dpp proteins. Using cell based assays, it has been found that the dynamic range over which Dpp functions for signaling is the same range in which Dpp stimulates the cleavage of Sog by Tld. In addition, the data support a role for Tsg in sensitizing the patterning mechanism to low levels of Dpp. It is proposed that the strong Dpp concentration dependence exhibited by the processing reaction, together with movement of Dpp by Sog and Tsg protein can help explain how Tld activity is confined to the dorsal-most region of the embryo through formation of a spatially dependent positive and negative reinforcement loop. Such a mechanism also explains how a sharp rather than smooth signaling boundary is formed (Shimmi, 2003).

According to the prevailing view, Sog, Tsg and Tld act to create a transport mechanism that helps promote Dpp diffusion from lateral regions of the embryos towards the dorsal side. According to this model, Sog would diffuse into the dorsal domain from its ventral lateral site of synthesis and capture Dpp, thereby preventing Dpp from binding to receptor. Net flux of Sog towards the dorsal side is envisioned to help transport Dpp and thereby increase its concentration in the dorsalmost tissue, which is destined to become the amnioserosa. Tld acts to liberate Dpp by cleaving Sog, and Dpp once released, will either be recaptured by another Sog molecule or bound to its receptors (Shimmi, 2003).

In order for the transport model to produce a Dpp concentration peak, the proper balance between binding affinities, diffusion rates and proteolytic processing is needed. Tsg has been suggested to have several activities that could influence this balance. In one model, Tsg would act to slow down the intrinsic rate of Sog cleavage by Tld. In this case, loss of Tsg is predicted to result in elevated processing of Sog. This should produce a sog loss-of-function phenotype, as is observed when molecular markers are examined. That data argues strongly against this possibility. First, it has been demonstrated that Tsg function is epistatic to Tld. If the tsg mutant phenotype is caused by excess Tld activity, then eliminating Tld should produce a tld loss-of-function phenotype. However, a tsg-like phenotype is observed where there is a general lowering and flattening of the Dpp activity gradient, as assayed by marker gene expression. In addition, biochemical studies reveal that Tsg actually enhances the ability of Tld to cleave Sog. Taken together, it is concluded that Tsg does not function during DV patterning to retard Tld proteolytic activity (Shimmi, 2003).

A second property has been attributed to Tsg: it alters the selection of Tld cleavage sites in Sog thereby producing novel Sog fragments with unique properties. In particular, a Sog fragment termed Supersog containing the first CR domain and a region of the spacer between CR1 and CR2 appears to be produced in vitro by the action of Tsg and Tld. Although the production of Supersog-like fragments are seen under the present reaction conditions described in this study, no enhancement in their production is seen upon Tsg addition. This may reflect loss of an unidentified component during purification or differences in the sensitivities of the CR1 antibodies used in the two studies. These issues are presently under examination. Whether Supersog-type molecules contribute to DV patterning in vivo is unclear. The fact that overexpression of Supersog can partially rescue tsg mutant embryos suggests that they could be important. A full resolution of the role of Supersog will need to await the results of in vivo rescue experiments employing mutants of the different Sog cleavage sites, especially those that lead to the production of Supersog-like fragments (Shimmi, 2003).

One of the primary findings in this report is that the rate of Sog cleavage is very sensitive to the level of the Dpp protein and varies substantially over a 10-fold range. Interestingly, this is the same Dpp concentration range within which low to maximal signaling occurs in S2 cell culture. Tsg sensitizes the system such that both the binding of Dpp to Sog as well as the rate of cleavage of Sog by Tld is stimulated by Tsg protein. Because in the invertebrate system, the binding of ligand to Sog is required for efficient processing of Sog, it is not surprising that the rate of Sog processing goes up in the presence of Tsg. This follows because, at a given concentration of Sog and Dpp, more complex will be formed in the presence of Tsg leading to a higher substrate concentration for the Tld protease. It is speculated that this system evolved in part to enable the embryo to produce a patterning mechanism that functions within the context of a very short developmental window. In Drosophila, the time between initial transcription of dpp during the early blastoderm stage and assignment of fate required for proper gastrulation is only about 40 minutes. In this short time-window, Dpp concentration must reach an effective signaling level. However, using a genomic Dpp-HA construct, it has been possible to visualize Dpp in the early embryo and it is present at much lower levels than in other tissues, such as the epidermis, at later stages of embryogenesis. It is proposed that under these conditions of low Dpp concentration, the presence of Tsg is required to enable Sog to bind to Dpp and to stimulate Sog cleavage in order to create a cyclic binding and release process that enables Dpp to be carried towards the dorsal midline. Furthermore, it is proposed that the intrinsic sensitivity of the cleavage reaction to the Dpp concentration is crucial for formation of a sharp signaling boundary. Thus, as the Dpp concentration drops in the lateral regions as a consequence of Dpp movement towards the dorsal side, the rate of Sog cleavage drops, allowing more Sog to enter this region and further reducing signaling in lateral regions. The movement of Dpp will simultaneously raise Dpp concentration in the dorsal region, further stimulating cleavage and clearance of Sog and thereby reinforcing Dpp signaling at the dorsal midline. This built-in positive and negative reinforcement mechanism should help establish sharp signaling boundaries by formation of steep ligand gradients, instead of the more gradual gradients that would form if Sog cleavage was not sensitive to the Dpp concentration (Shimmi, 2003).

In some vertebrate systems, DV patterning mechanisms have been conserved with respect to the molecules employed, but the polarity of axis over which they act has been inverted. Thus, in both amphibians and zebrafish, Bmp ligands specify ventral cell fates, whereas Bmp inhibitors, such as Chordin, are secreted from dorsal cells. In each of these systems, Tsg- and Tld-like proteins also contribute to axis formation, but the biochemical details of their associations appear different from those found in Drosophila. Two distinctions are most apparent and these probably have biological significance with respect to the patterning mechanism employed by these organisms. In Xenopus, the affinity of chordin for Bmps is significantly higher than Sog for Dpp; Bmps can be coimmunoprecipitated by chordin alone whereas this is not the case for the Drosophila components. In addition, once cleaved by Xolloid, at least some of the CR1 containing fragments of chordin continue to have significant affinity for the Bmp ligand preventing it from signaling (Shimmi, 2003 and references therein).

The second major difference between the Drosophila and Xenopus systems is that the Drosophila processing of Sog is dependant on prior binding of Sog to Dpp, while in Xenopus this is not the case. Rather, Chordin cleavage by Xolloid appears to be constitutive and is not enhanced by any tested ligand. Without ligand dependent cleavage, net movement of Bmps by Chordin diffusion may not readily occur nor would there be a mechanism to both positively and negatively reinforce the processing reaction. Indeed, recent studies have demonstrated that in the Drosophila embryo, Chordin does not have the ability to promote Dpp signaling at a distance, whereas Sog does. As a result, spatially enhanced Bmp concentrations and sharp signaling boundaries that result from net ligand movement by the activities of the Chordin, Xolloid and Tsg proteins may not occur in Xenopus. In fact there is no evidence in Xenopus that loss of Chordin activity actually results in a reduction in Bmp signaling in select regions of the embryo as occurs in Drosophila (Shimmi, 2003).

Despite these differences, Tsg may, nevertheless, play both positive and negative roles in modulating Bmp signaling; however, its mechanism is somewhat different. As processed fragments of Chordin still have reasonable affinity for ligand, they may need to be dislodged to allow for signaling. Tsg binding to Bmps appears to help promote this dislodgment and their ultimate degradation. In Drosophila, since Sog binds poorly to ligand in the absence of Tsg there is no need for Tsg to help promote dissociation of Sog fragments. Rather, it is its ability to help promote association of Sog with Dpp that is key to understanding its function. Tsg appears also to alter the rate of chordin proteolysis. Thus, at a high Tsg-to-chordin ratio, Chordin may be degraded and in this way Tsg might help promote signaling. It is possible that some combination of these properties is used in other vertebrates. For example, in zebrafish it has recently been shown that loss of chordin can enhance a phenotype that results from haplo-insufficiency for swirl, a gene that encodes Bmp2b. This paradoxical observation, that loss of an inhibitor exacerbates a phenotype resulting from loss of a ligand, is exactly analogous to the case of amnioserosa development in Drosophila where loss of Sog (an inhibitor) leads to less Dpp signaling in the dorsal domain. Detailed studies examining the ligand dependence of Chordin cleavage in zebrafish by minifin, the gene encoding a Tld homolog, have not been reported. It is possible therefore, that like Drosophila, this system may also employ a transport mechanism involving Tsg, Chordin and Tld that acts to boost Bmp signaling in specific tissues. It is interesting to note that the mouse homologs of Tsg, Chordin and Tld also exhibit their own distinct biochemical properties. Thus, a new Tld processing site in Chordin is induced by the presence of Tsg but this is not seen when the Xenopus components are used. Thus, it seems probable that the inherent complexity of this multi-component regulatory mechanism has provided numerous targets for evolutionary change. It is speculated that these changes account for the remarkable diversity that this mechanism exhibits with respect to the actual details by which it regulates Bmp signaling in different organisms (Shimmi, 2003).

Tlr and Tld appear to have evolved to achieve the proper balance between the inhibitory and positive activities of Sog in the PCV and early embryo, respectively

Tolloid (Tld) and Tolloid related (Tlr) belong to a family of developmentally important proteases that includes Bone Morphogenetic Protein 1 (Bmp1). Tld is required early in Drosophila development for proper patterning of dorsal embryonic structures, whereas Tlr is required later during larval and pupal stages of development. The major function of Tld is to augment the activity of Decapentaplegic (Dpp) and Screw (Scw), two members of the Bmp subgroup of the TgfbetaŸ superfamily, by cleaving the Bmp inhibitor Short gastrulation (Sog). Evidence is presented that Tlr also contributes to Sog processing. Tlr cleaves Sog in vitro in a Bmp-dependent manner at the same three major sites as does Tld. However, Tlr shows different site selection preferences and cleaves Sog with slower kinetics. To test whether these differences are important in vivo, the role of Tlr and Tld during development of the posterior crossvein (PCV) in the pupal wing was investigated. tlr mutants lack the PCV as a result of too little Bmp signaling. This is probably caused by excess Sog activity, since the phenotype can be suppressed by lowering Sog levels. However, Tld cannot substitute for Tlr in the PCV; in fact, misexpressed Tld can cause loss of the PCV. Reducing levels of Sog can also cause loss of the PCV, indicating that Sog has not only an inhibitory but also a positive effect on signaling in the PCV. It is proposed that the specific catalytic properties of Tlr and Tld have evolved to achieve the proper balance between the inhibitory and positive activities of Sog in the PCV and early embryo, respectively. It is further suggested that, as in the embryo, the positive effect of Sog upon Bmp signaling probably stems from its role in a ligand transport process (Serpe, 2005).

The major distinction between the two Drosophila proteases in terms of their Sog processing function is the time and tissue in which they act. Tld activity is primarily confined to the early embryo, while Tlr is required during pupal wing development. To some extent, the functional differences between them can be attributed simply to differences in expression pattern. In the pupal wing Tlr is far more abundantly expressed than Tld, and this alone might account for the lack of redundancy. In the early embryo, however, the situation is more complex. Both enzymes are expressed with similar profiles, but Tlr does not seem to be capable of providing sufficient Sog processing activity, even when several extra copies are provided as transgenes (Serpe, 2005).

It has been speculated that this difference in activity might reflect differences in activation of the proteases at the level of pro-peptide removal. Like all the members of the Bmp1 family, Tld and Tlr are secreted as pro-enzymes; the processing of the pro-peptide is necessary for the activation of proteolytic activity, since the N-terminal end of the astacin motif is buried inside the catalytic domain forming an internal salt bridge. Mutations at the processing site render the enzymes inactive, whereas removal of the pro-peptides produce activated forms of Tld and Tlr. Tlr has a much longer pro-peptide that could either aid or inhibit activation in a tissue-specific manner. However, the inability of Tlr to rescue Tld mutants does not appear to result from an inefficient activation step. Tld activation, both in the embryo and in S2 cells, is very inefficient with most of the protein found in the pro-enzyme state. By contrast, pro-peptide removal from Tlr is very efficient in S2 cells, and the same is true when Tlr is ectopically expressed in the embryo (Serpe, 2005).

Instead, it seems likely that the difference in kinetics of Sog processing by Tlr is the reason behind the inability of Tlr to rescue tld mutants. Tlr is much less efficient at cleaving Sog in vitro than Tld. Given the rapid developmental time of early embryogenesis, where patterning by Bmps during cellularization occurs within approximately a 30 minute time window, the slower kinetics of Sog processing by Tlr may not support proper patterning. Indeed, computational work has shown that a three to fourfold reduction in kinetic properties of Tld will completely disrupt the patterning process (Serpe, 2005).

Although the slow processing kinetics of Tlr towards Sog may prevent it from functioning effectively in early embryonic patterning, this property may be exactly what is required to achieve proper formation of the PCV. Unlike patterning in the early embryo, formation of the PCV, as assessed by profile of pMad accumulation, occurs over at least a 7 hour time frame. The slower processing rate of Tlr towards Sog may be required to achieve the appropriate balance of Sog destruction and diffusion that is necessary for proper patterning to occur. Consistent with this view, overexpression of various UAS-tld lines under the control of the A9-Gal4 driver in a tlr mutant background does not rescue PCV formation. In fact, in many cases overexpression of an activated Tld, or co-expression of wild type tld and tlr together produce loss of the crossvein tissue in a wild-type background. It is envisioned that, under these conditions, the increased level of enzymatic activity results in over-digestion of Sog, a situation that would phenocopy sog hypomorphs. Consistent with this view, hypomorphic sog allelic combinations also result in the loss of the PCV. In addition, large sog null clones can also result in loss of the PCV (Serpe, 2005).

In a Xenopus assay, it was found that Tlr is only slightly less efficient than Tld at reverting secondary axis induction caused by Sog. Although it is not known how well each enzyme is activated in this animal, it should be noted that the developmental time period over which the patterning process functions in Xenopus is long compared with early Drosophila development. The longer time frame may enable the less efficient protease to produce a similar biological response. Protease domain swap experiments suggest that the reduced processing rate does not involve evolution of intrinsic differences in the catalytic abilities of the protease domain itself, but rather changes occur in the way that the Sog substrate initially interacts with the enzyme. In summary, it is proposed that during evolution there was selection for particular biophysical properties of these two enzymes to properly match the developmental time frame over which the patterning mechanisms operate. It cannot be exclude however, that other differences besides kinetic activity might also play a role in providing functional specificity. For example, it is possible that variation in cleavage site selection might also contribute to the different biological activities of Tlr and Tld. It is worth noting in this regard that different fragments of Sog have been shown to have both positive and negative effects when overexpressed in the wing. However the in vivo roles of endogenous Sog fragments have not been defined (Serpe, 2005).

The results suggest that a proper balance of Sog and protease activity is necessary to pattern the PCV. Interestingly, the same situation holds true in the early embryo. In this case, Sog plays both positive and negative roles in patterning the dorsal domain. It is required in the dorsolateral regions to block Bmp signaling, but it also acts as an agonist to achieve peak levels of Bmp signals at the dorsal midline. Two types of models have been proposed to account for these dual activities. In one model, the different cleavage fragments of Sog are thought to provide either agonist or antagonist function, but the details of the mechanism are unclear. In the other model, both functions are proposed to come about as a result of Sog providing a transport mechanism that spatially redistributes Bmp ligands from the lateral region to the dorsal most cells. This transport mechanism also requires the activity of Tsg, a small cysteine-rich secreted protein which has been shown to form a tripartite complex with Sog and Dpp. The prevailing view is that as Sog diffuses into the dorsal domain it forms a high affinity complex with Tsg and Dpp. This complex is unable to bind to receptors and is responsible for the antagonistic activity of Sog. At the same time, the complex protects Dpp from degradation and receptor binding allowing it to diffuse and accumulate dorsally where it is released by Tld processing. The ability of Sog to redistribute the Bmp ligands accounts for the agonist function of Sog. Computational analyses have provided additional support for this model (Serpe, 2005).

It is proposed that the same type of mechanism may be responsible for patterning the PCV. Recent analysis has provided evidence that the longitudinal veins act as the source of Dpp for PCV specification. Dpp is thought to diffuse from these veins into a PCV competent zone. The exact mechanism by which the competent zone is specified is not clear, but low levels of Sog expression are required. tlr is expressed within the PCV competent zone during the initial stages of crossvein development, suggesting the Tlr:Sog ratio will be higher in this region. Furthermore, because processing of the Sog/Dpp complex by Tld-like enzymes is dependent on the Dpp concentration, the complex will be most efficiently processed in the center of the competent zone (Serpe, 2005).

According to this model, there is limited processing of Sog and therefore limited release of Dpp from its inhibitor in tlr mutants. Conversely, Sog also supplies a positive function for PCV formation, probably by providing a transport mechanism for Dpp, accounting for the partial loss of the PCV in hypomorphic sog mutants and complete loss of the PCV in large sog-null clones. The partial reversion of the tlr mutant phenotype by introduction of hypomorphic sog alleles is also consistent with the view that it is the balance between these two factors that is crucial for proper patterning. Interestingly, this is the way in which Sog was originally identified as an inhibitor of Dpp signaling in the embryo: weak sog alleles were isolated as partial suppressors of tld mutations. One difference is that, in the case of this partial reversion, lowering Sog levels is able to revert a null mutation instead of a hypomorphic condition, as was the case in the embryo. There are at least two possibilities that can explain this suppression effect. First, although these animals may be null for tlr, there could be some low level tld expression in the pupal wing. If so, then these wings would not be devoid of all Sog-processing activity and therefore lowering Sog levels might enable the limited amount of Tld to provide the proper production-destruction balance. Alternatively, neither Sog nor Tlr may be absolutely required for PCV formation. Instead, their functions may be simply to ensure that the patterning occurs reproducibly. Thus, in the absence of both Sog and Tlr, partial PCV formation may occur as a result of some Bmp ligand accumulating in the correct position. However, under these circumstances, the patterning mechanism would be unreliable and would produce different results on case-by-case basis. To prevent this from occurring, it is posited that evolution has selected for supplementary regulatory controls involving Sog and Tlr to ensure that the PCV always forms completely and reliably at the correct position (Serpe, 2005).

Shaping BMP Morphogen Gradients through Enzyme-Substrate Interactions

Bone morphogenetic proteins (BMPs) regulate dorsal/ventral (D/V) patterning across the animal kingdom; however, the biochemical properties of certain pathway components can vary according to species-specific developmental requirements. For example, Tolloid (Tld)-like metalloproteases cleave vertebrate BMP-binding proteins called Chordins constitutively, while the Drosophila Chordin ortholog, Short gastrulation (Sog), is only cleaved efficiently when bound to BMPs. This study identified Sog characteristics responsible for making its cleavage dependent on BMP binding. 'Chordin-like' variants that are processed independently of BMPs changed the steep BMP gradient found in Drosophila embryos to a shallower profile, analogous to that observed in some vertebrate embryos. This change ultimately affected cell fate allocation and tissue size and resulted in increased variability of patterning. Thus, the acquisition of BMP-dependent Sog processing during evolution appears to facilitate long-range ligand diffusion and formation of a robust morphogen gradient, enabling the bistable BMP signaling outputs required for early Drosophila patterning (Peluso, 2011).

To identify and characterize the Tld processing sites in Sog, the Sog cleavage fragments were purified and sequenced using tagged proteins generated in S2 insect cell cultures. The intermediate Sog cleavage fragments were captured using suboptimal amounts of enzyme and Dpp, the obligatory cosubstrate. The three major processing sites in Sog are in close proximity to the Cys-rich BMP binding modules. The positions of processing sites 1 and 3 correspond to the two major processing sites in Chordin. Sequencing of the N-termini revealed a conserved Asp residue at position P10, a hallmark of the astacin family of proteases that includes Tld, a conserved aliphatic residue (V) in position P3, and no significant homology with other Tld/BMP-1 substrates (Peluso, 2011).

Replacement of all four residues at processing site 1 (V183ALD) with Ala rendered Sog virtually uncleavable at this site in vitro. Full-length mutant Sog was still degraded over time, likely due to processing at the remaining unmodified sites, but the speed of its degradation was reduced. Additional Ala replacements at site 2 (V728PGD) further slowed down the Sog destruction. When only the conserved D (position P10) was replaced with E at processing site 1 were similar effects observed: undetectable cleavage at site 1 and a slower overall degradation of the mutant Sog. In this system, the precise cleavage kinetics at individual sites could not be observed, but the overall Sog destruction in various uncleavable Sog mutants was clearly slowed down, likely because of blocked/reduced cleavage at the modified site(s). Also any mutations at site 3 induced constitutive cleavage at this site, thus site 3 was kept intact for these studies (Peluso, 2011).

To test if these mutations (Sog-u) could render Sog uncleavable in vivo, Sog gain-of-function phenotypes were examined. Overexpression of Sog in the wing imaginal disc produces very mild phenotypes of venation defects. Sog together with Tsg produces a more potent BMP inhibitor; their combined overexpression inhibits BMP signaling and results in smaller wings with altered patterns of venation. Co-overexpression of Sog and Tsg with Tolloid-related (Tlr) is able to reverse the small wing phenotype and restore normal patterning in the case of wildtype Sog, but not in the case of Sog-u. In fact, overexpression of Sog-u by itself produced a significant loss of posterior crossvein, a structure that requires peak BMP signaling, suggesting that Sog-u is a better BMP inhibitor than the wild-type Sog. Moreover, the loss of posterior crossvein tissue was exacerbated when Tlr was coexpressed with Sog-u, indicating further reduction in the BMP activity. This is likely due to Tlr degrading endogenous Sog but not Sog-u. Thus, Sog-u appears resistant to cleavage and degradation in vivo and may act as a dominant negative by prolonged binding of the BMP ligands (Peluso, 2011).

Unlike in vertebrates, Drosophila Tld and Tlr process Sog only when bound to a BMP-type ligand. The binding of Sog to Tld requires several Tld protein interaction motifs besides the protease domain. Nevertheless, the requirement for the obligatory cosubstrate for Sog processing is thought to indicate a BMP-induced conformational modification that allows the Sog-BMP complex, but not Sog alone to fit into the catalytic pocket of the enzyme. In contrast, Chordin, which exhibits BMP-independent processing, should bind and fit into Tld's catalytic pocket without the need for a BMP-induced conformational change. Indeed, in spite of limited conservation between Sog and Chordin (40% similarity, 22% identity), it was found that Drosophila Tld can cleave the vertebrate Chordin in a BMP-independent manner (Peluso, 2011).

To focus on the enzyme-substrate interactions for Sog and Chordin, the Tld catalytic domain was modeled using the crystal structures available for related enzymes, the crayfish Astacin (the founder member of this zinc metalloprotease family), and the human Tld catalytic domain. As previously described, the catalytic pocket of Tld enzymes appears very tight in the proximity of the catalytic Zn, where scissile bonds align, and has a relatively wide cavity that accommodates residues P3 and P2 in the substrate. Bulky, hydrophobic residues in the substrate, such as in Chordin, might facilitate enzyme-substrate binding. Indeed, when the effect of changing processing site 1 was examined in Sog to either Chordin site 1 (Sog-1Ch1) or Chordin site 2 (Sog-1Ch2) indications were found of Tld cleavage at these sites in the absence of Dpp, although this cleavage was extremely weak. In addition, an aromatic residue in position P3 in the substrate could potentially stack against the aromatic ring, a key position near the active site, to further lower the substrate-enzyme binding energy and facilitate substrate binding (Peluso, 2011).

These predictions were tested and found that indeed changing several residues at the processing site could alter the cosubstrate requirements. For example, Sog processing at mutated site 1 (V183ALDV to FYGDP) occurred independently of the cosubstrate (Peluso, 2011).

This processing was enhanced when Dpp was added to the reaction partly because cleavage at unmodified sites 2 and 3 could not happen in the absence of the cosubstrate. Addition of Tsg similarly enhanced the processing of wild-type and mutant Sog. Nonetheless, when Dpp was in limiting amounts, the mutant Sog was processed more efficiently than the wild-type protein. Edman degradation confirmed that processing occurred at the expected G185-D covalent bond in the mutated site, and that mutagenesis did not create any promiscuous cleavage. Similar changes at processing site 2, separately or with site 1, further enhanced the speed of Sog degradation. The strongest effect was seen for a Sog variant in which both sites 1 and 2 were rendered BMP-independent for processing, designated Sog-i for 'independent of BMP for cleavage.' At the sequence level Sog-i is very different from Chordin, but it resembles Chordin in how it is processed by Tld: Sog-i exhibits significant BMP-independent processing by Tld, which is enhanced in the presence of BMP ligands. To emphasize these similarities Sog-i is referred as 'Chordinlike' Sog (Peluso, 2011).

Tests were performed to see if these changes impact Sog's ability to bind BMPs and/or inhibit their signaling. Purified Sog and Sog-i were found to be indistinguishable in their binding to Dpp homodimers and Dpp/Scw heterodimers in co-ip experiments. Also, in both cases, addition of Tsg equally increased Sog binding to the BMPs. The inhibitory activities of Sog and Sog-i on BMP signaling were compared in a cell-based assay; in the presence of Tsg, Sog inhibits Dpp-induced signaling in a concentration dependent manner. Equivalent amounts of Sog-i and Tsg produced a similar inhibitory response (Peluso, 2011).

While the BMP binding properties of Sog-i appeared to be largely unaffected, it was predicted that this 'Chordin-like' Sog would resemble Chordin when introduced into fly embryos, and be less efficient in promoting long-range BMP signaling. To model this process, a previously published spatiotemporal patterning model was modified by adding the BMP-independent processing of Sog. Briefly, the rate for Tld-mediated processing of Sog increases when the rate of BMP-dependent or BMP-independent cleavage increases. An increase in the rate of Tld processing will modify the Sog protein levels and the shapes of the Sog and Sog/Tsg distributions in the model. This results in a simultaneous reduction in the inhibition of Dpp signaling laterally and a reduction in the Dpp accumulation near the dorsal midline. To quantify the effect of BMP-independent cleavage of Sog-i on the net transport of BMP molecules toward the dorsal midline, the net diffusive flux of BMP ligand in the embryo was calculated by summing the contributions of free BMP and Sog-bound BMP. The flux provides the magnitude and direction of transport driven by the gradient of concentration. The Sog-i simulation clearly indicated a lower net transport toward the midline than the simulation with Sog-WT. Also investigated in this model was whether increased Sog-i expression could improve the transport toward the midline and whether the reduction in transport is solely the result of a reduction in Sog levels. Increasing the production of Sog-i increases the total amount of Sog-i in the system; however, transport of Dpp/Scw toward the midline is still reduced even with significantly increased levels of Sog-i greater than in Sog-WT embryos with normal patterning (Peluso, 2011).

To test the biological effect of these Sog variants on the BMP morphogen gradient profile, transgenic fly lines were constructed that allowed for normal spatial and temporal expression of Sog proteins at endogenous levels. The neural-ectoderm expression of tagged and nontagged Sog proteins, Sog-WT, Sog-WT-HA, and Sog-i-HA, in all of the transgenic lines obtained, overlapped the sog mRNA endogenous pattern. The relative Sog levels in these transgenic lines were quantified by immunofluorescence using anti-HA antibodies, anti-Sog antibodies, or both. It was found that indeed these transgenic lines have similar levels of Sog protein. In addition, all of the transgenic lines expressing Sog-WT (either HA-tagged or not-tagged) rescued the sogYL26 mutants and trans-heterozygous combinations (sog-/-) to viable, and fertile adults (Peluso, 2011).

The profile of the BMP morphogen gradient was examined in stage 5 embryos by following the accumulation of activated/ phosphorylated Mad (P-Mad), the effector of the BMP signaling pathway. In the absence of Sog, the facilitated diffusion of BMP ligands does not occur and Dpp remains uniformly distributed over the dorsal domain. No gradient of BMP activity is generated, thus the P-Mad levels are low and constant over the entire dorsal domain of sog-/- mutant embryos. In contrast, stage 5 wild-type embryos have a sharp, step gradient of BMP signaling, in which P-Mad levels are high in the dorsal most cells and rapidly drop off to undetectable levels in more lateral regions. The P-Mad positive domain is wider and slightly reduced in intensity in heterozygous (sog+/-) embryos. Among the sog alleles tested, the sogYL26/+ heterozygous embryos showed the widest P-Mad profile. The HA-tagged or untagged sog-WT transgenes were equally effective in restoring the sharp P-Mad profile in sog-/- embryos when in two copies, suggesting that the tag did not alter Sog activity. In contrast, addition of two sog-i copies to any sog-/- background produced a wide P-Mad positive domain with reduced signal intensities. In the latter embryos the boundaries of the P-Mad positive domains were more diffuse, with reduced slopes evident in the cross-section profile. Analogous studies of race expression, downstream of BMP signaling in the presumptive amnioserosa, indicated a similar effect. This suggests that Sog-i is indeed less efficient in supporting an adequate Dpp/Scw-Sog/Tsg flux toward the midline and consequently the formation of the steep BMP distribution profile (Peluso, 2011).

To quantify the differences in BMP signaling profiles between wild-type, sog+/-, and sog-/- embryos with 2x sog-i or 2x sog-WT transgenes, the P-Mad fluorescent staining of each embryo was decomposed into the product of an amplitude multiplied by the P-Mad distribution 'shape'. In brief, for each embryo, a region of interest was selected that encompasses a 4-cell-wide band centered at 33% embryo length. Each embryo was then processed through a Savitzky-Golay filter that removed noise while preserving the shape of the distribution and allowed for reliable calculation of the slope of the P-Mad profile (Peluso, 2011).

Shape was quantified by measuring the spatial-derivative of P-Mad in the cross-section. Starting on the left of a P-Mad cross-section plot, the derivative will be positive and change in magnitude at each position along the D/V axis directly proportional to the slope of P-Mad. As the slope decreases near the dorsal midline, the value of the derivative is approximately zero and then negative for the right side of the distribution where P-Mad is decreasing. The local average P-Mad slope for the population of 2x sog-WT embryos at each spatial location was virtually indistinguishable from the WT P-Mad slope. In contrast, the population average P-Mad slope for 2x sog-i embryos was noticeably shallower than WT and 2x sog-WT embryos with a lower magnitude of the spatial derivative near the midline and a higher magnitude in the lateral dorsal ectoderm. Moreover, differences in the overall intensity of P-Mad staining and/or in embryo-to-embryo variability do not account for this observation; even when the scaling for each population was chosen so the population means would have the same peak P-Mad levels, or when the absolute value of the local P-Mad slope was used for each individual in each population to calculate the population distributions of slopes, the differences remain clear: the replacement of sog-WT with sog-i leads to broader, shallower P-Mad profiles (Peluso, 2011).

The difference between the P-Mad profiles in WT and 2x sog-i was not equivalent to a decrease in the total amount of Sog protein in the system. Distributions of P-Mad slopes in both sog+/- and 2x sog-i differed from WT embryos, but the perturbations were not equivalent. For sog+/-, Sog protein levels were reduced approximately 50%, and the position of the peak slope shifted laterally away from the dorsal midline; however, the magnitude of the slope was still significantly greater than the magnitude of the slope for 2x sog-i embryos, though slightly less than WT. This means that the P-Mad profile in sog+/- is wider, but the steepness of the BMP activity gradient is similar to the steepness of the WT gradient. In contrast, the slope of the P-Mad profile in 2x sog-i embryos was significantly lower than WT embryos near where their peaks overlap and significantly greater than WT in the lateral dorsal ectoderm (Peluso, 2011).

Prior to gastrulation high levels of BMP signaling at the dorsal midline in early Drosophila embryos specify amnioserosa, an extraembryonic tissue required for gastrulation. The sharp and narrow BMP signaling domain in WT embryos induced formation of an amnioserosa field of approximately 200 cells in stage 13 embryos. The sog+/- heterozygous embryos have a wider BMP signaling domain that produced a larger amnioserosa field, about 50% bigger than that of the WT embryos. The spatial extent of the BMP signaling field above a certain threshold but below wild-type peak values appears to determine how many amnioserosa cells will be specified and consequently the size of the ensuing tissue (Peluso, 2011).

When sog-WT transgenes replaced the endogenous sog, the amnioserosa cell numbers were rescued to wild-type levels. However, addition of two sog-i copies to any sog-/- background tested produced statistically significant increases in the amnioserosa fields. The biological consequences of replacing Sog with Sog-i could not be explained simply by quantity differences between the sog-WT and sog-i transgenes. First, shallow BMP gradient profiles, broader target gene expression domains, along with increased cell allocation/ amnioserosa fields were observed using multiple independent sog-i transgenic lines with expression levels comparable with those of the sog-WT lines. Second, additional copies of sog-WT and sog-i transgenes did not significantly impact the BMP gradient profiles or amnioserosa fields. The P-Mad positive domains were more intense in either 4x groups, but in the sog-/-; 4x sog-i embryos the signaling domain remained wide, the boundaries diffuse and the slopes of the cross-section profiles reduced; also, the sog-/-; 4x sog-i embryos had significant embryo-to-embryo variability (Peluso, 2011).

To further search for alternative explanations for the Sog-i effects, Sog-WT and Sog-i versions of the 3D embryonic model were optimized against the 4x population data and it was asked whether the experimental observations could be captured by changes in Sog affinity to Dpp or changes in the processing rate of Sog by Tld independent of Dpp. It was found that the model with an enhanced processing rate achieved a greater fit: a modest increase in the processing of Sog by Tld without Dpp (increase BMP independent processing from about 8% to 19% of Tld processing rate in presence of Dpp), resulted in signaling profiles that matched the experimental data very well (Peluso, 2011).

In the 2x sog-WT simulations, the net reaction rate was negative in the lateral portions of the dorsal region and reached maximum near the dorsal midline. In the simulations for the 2x sog-i, the peak rate of Dpp release occurred laterally halfway between the neural-ectoderm and dorsal region. In contrast, no models obtained by decreasing the binding between Sog and Dpp (10x or more) could capture the experimentally observed loss of sharp boundaries. Thus, the shift in the net rate of cleavage, in conjunction with less effective net flux, produced less accumulation of Dpp near the dorsal midline in simulations of sog-i embryos (Peluso, 2011).

In conclusion, it was found that several residues at the Tld processing site make Sog dependent on a (BMP) cosubstrate for processing. Mutating these residues reduced the transport range of Sog-BMP complexes in vivo and altered the shape of the BMP signaling profiles and consequent cell fate allocation. Interestingly, BMP-dependent Chordin cleavage was also a requirement in mathematical modeling for scale invariance of Xenopus embryos. Here the cosubstrate requirement ensured transport of both ADMP and the BMP ligands and the reestablishment of a well-proportioned DV axis. How might shuttling of ligands persist in the absence of BMP-dependent cleavage of Chordin? An intriguing possibility is that Sizzled-mediated repression of Xolloid spatially restricts Chordin processing providing a nonuniform Chordin sink. In mathematical models of embryo patterning, lowering the processing rate of Tld results in signaling distributions that are sharper and result in a greater net transport of BMP ligands away from the Sog/Chordin source (Peluso, 2011).

An interesting and unexpected outcome of the comparison between sog-WT and sog-i embryos suggests that BMP-dependent Sog processing reduces embryo-to-embryo variability in P-Mad levels. Both sog-WT and sog-i embryos show sensitivity of P-Mad to gene dosage. However, when the coefficient of variation (standard deviation/width) within each genotype was calculated, it was found that embryos with one or two sog-WT copies showed less variability in signaling width than their 2x sog-i counterparts. The variability is greater at nearly all threshold positions, and the variability within this population increased dramatically at higher threshold levels. This suggests that BMP-dependent Sog destruction may reduce embryo-to-embryo variability between individuals in a population of the same genotype to provide robust patterning of the dorsal structures (Peluso, 2011).

Altogether, these results indicate that a 'Chordin-like' Sog is less able to reliably support patterning of the early Drosophila embryo. By modifying the Sog-Tld substrate-enzyme interaction with just a few residue changes, it appears that a new developmental function for Sog evolved that ensured reliable shuttling of BMPs and robust patterning. Further refinement of this shuttling mechanism, such as its speed or its directionality, expanded the repertoire of cell fate specification by BMP morphogen gradients and was likely exploited for diversified patterning during natural evolution (Peluso, 2011).

tolloid: Biological Overview | Evolutionary Homologs | Developmental Biology | Effects of Mutation | References

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