BIOLOGICAL OVERVIEW

During early embryogenesis in Drosophila, Caudal mRNA is distributed as a gradient with its highest level at the posterior of the embryo. This suggests that the Caudal homeodomain transcription factor might play a role in establishing the posterior domains of the embryo, which undergo gastrulation and give rise to the posterior gut. By generating embryos lacking both the maternal and zygotic mRNA contribution, caudal has been shown to be essential for invagination of the hindgut primordium and for further specification and development of the hindgut. Mature embryos lacking cad activity (maternal and/or zygotic contributions) were examined to assess the requirement for cad in establishing the structures that arise from the posterior ~15% of the blastoderm embryo, namely the posterior midgut, Malpighian tubules and hindgut. Embryos lacking both maternal and zygotic cad activity (referred to as cad m-z -, or cad-deficient), as well as embryos lacking maternal cad activity but carrying one zygotically active wild-type cad gene (cad m-z+) and embryos lacking only zygotic cad (cad m+z -) were generated. Staining with anti-Crumbs antibody, which labels the apical surface of the hindgut epithelium, reveals that cad activity, provided either maternally or zygotically, is essential for formation of the hindgut. When cad is provided only maternally (cad m+z-), the posterior gut appears essentially normal. When cad is provided only zygotically (cad m-z+), a hindgut tube is formed; this can range from almost normal to somewhat reduced in length and twisted. When cad activity is completely lacking (cad m-z-), the hindgut (as well as the anal pads and the eighth abdominal segment) is almost entirely absent. It is concluded that cad activity is essential for normal hindgut formation, and that while some of this activity must be provided maternally, most can be provided either maternally or zygotically. The Malpighian tubules and posterior midgut also arise from domains expressing cad, but form relatively normally in cad-deficient embryos. As revealed by their expression of forkhead (fkh), the tubule primordia form relatively normally in embryos lacking cad activity (Wu, 1998).

The stages of gastrulation can be observationally followed by using expression of brachyenteron byn as a marker for the hindgut primordium. In the wild-type embryo, byn is expressed in a ring at the circumference of the amnioproctodeal plate. The edges of this ring come together as the posterior midgut primordium invaginates during stages 6 and 7; the ring of the hindgut primordium then sinks inward during stage 8 and is completely internalized by the end of stage 9. The zygotically expressed cad stripe and the posterior wingless stripe are also expressed in the bordering ring (i.e., the hindgut primordium) of the invaginating amnioproctodeal plate. Strikingly, in cad-deficient embryos, the byn-expressing ring of hindgut primordium draws together, but fails to invaginate, remaining on the outside of the embryo. Thus, although internalization of the Malpighian tubule and posterior midgut primordia is normal in cad-deficient embryos, the gastrulation movements necessary for internalization of the hindgut primordium do not occur in embryos lacking cad activity (Wu, 1998).

The absence of the hindgut primordium from cad-deficient embryos suggests that Caudal regulates genes required for establishing and/or maintaining the hindgut primordium. tailless, forkhead, byn, bowl and wingless are likely targets for cad regulation, since all are required for some aspect of hindgut development: the hindgut is missing from both tll and fkh embryos, and severely reduced in wg, byn and bowl embryos. bowl, also called bowel, codes for a zinc finger transcription factor related to odd-skipped. Since maternally provided Caudal, which persists only through the blastoderm stage, is sufficient for essentially normal hindgut formation, the fact that all of these genes are expressed at the posterior of the embryo during the blastoderm stage means that they are potential targets for regulation by Caudal. The effect of absence of maternal and/or zygotic cad activity on the expression of these genes was assessed by in situ hybridization with appropriate probes. For tll, byn and bowl, absence of cad activity does not result in a detectable effect on expression. As described below, however, cad activity is essential for expression of fkh and wg. Both maternal and zygotic cad contributions are necessary for posterior wg expression. During early stage 5, just prior to its expression in 14 stripes that are required to establish the segmental pattern, wg is expressed in two domains at the anterior, and in a broad posterior stripe. This terminal wg stripe is located at approximately 8-12% EL, overlapping with the posterior of the zygotic cad stripe and with the position of the hindgut and Malpighian tubule primordia in the blastoderm fate map. Expression of the wg terminal stripe has been shown to be independent of other segmentation genes, but has not been otherwise characterized. All embryos from mutant cad germline mothers (even those expressing zygotic cad) fail to express the terminal stripe of wg. These results demonstrate that maternal cad activity is essential for the transcription of wg in the terminal stripe. Among embryos from wg heterozygous parents, approximately one-quarter (presumably those lacking only the zygotic component of cad expression) lack the terminal wg stripe. Thus both maternal and zygotic cad activities are required for expression of the terminal wg stripe (Wu, 1998).

The expression of the early cap of fkh also requires cad activity; approximately half of the embryos from mutant cad germline females mated to cad heterozygous males (i. e., cad m-z - embryos) show a dramatic reduction in both the size and intensity of the posterior cap of fkh expression. However, if cad is supplied either maternally or zygotically, fkh expression is normal. Thus expression of the posterior cap of fkh requires cad activity, which can be provided either maternally or zygotically. Later, by stage 10, fkh expression is as strong in cad-deficient as in wild-type embryos, indicating that this later expression is independent of cad activity. Since tll and hkb are also required to activate early fkh expression but are not themselves regulated by cad, cad must act combinatorially with these two genes to promote early fkh expression (Wu, 1998).

cad also regulates wg in combination with other genes. In addition to the demonstrated requirement for cad, expression of the posterior wg stripe requires positive input from fkh and tll, since the stripe is absent from the respective mutant embryos. Since embryos lacking either maternal or zygotic cad fail to express the posterior wg stripe, but still express fkh and tll, cad must act combinatorially with fkh and tll to promote formation of the posterior wg stripe. Expression of the terminal stripe thus requires the combinatorial action of cad, tll and fkh; the posterior limit of the stripe is known to be defined by repression by hkb (Wu, 1998).

The fact that tll, byn and bowl expression at the blastoderm stage are all apparently normal in cad-deficient embryos suggests that a hindgut primordium is established in the absence of cad activity. The lack of proper blastoderm stage expression of fkh and wg, however, indicates that this hindgut primordium is not properly specified. In byn mutant embryos one of the earliest phenotypic manifestations of an abnormally specified hindgut primordium is ectopic expression of the cell death gene reaper (rpr). To ask whether the extremely reduced hindgut in cad-deficient embryos might result from a similar course of programmed cell death, expression of rpr was examined in embryos lacking cad. A striking pattern of ectopic rpr expression is observed in cad minus embryos, beginning during stage 7 and continuing into stage 8 (gastrulation), in a ring at the circumference of the amnioproctodeal plate. However, the actual loss of cells that is presumably initiated by this ectopic rpr expression does not begin until after early stage 10, because the hindgut primordium is present at this stage in cad-deficient embryos, as indicated by its expression of byn and fkh. By stage 13, the cad-deficient embryo has a very short hindgut and no detectable anal pads; in sections of stage 13 embryos there are numerous apoptotic cells in the region of the hindgut remnant (Wu, 1998).

The amnioproctodeal plate of cad-deficient embryos, distinguished by the fact that it ectopically expresses rpr, becomes increasingly retarded in its anteriorward movements during stages 7 and 8. By early stage 10 (~4.5 hours AEL), when the germ band in the wild-type embryo is almost fully extended, the germ band in the cad minus embryo has extended only about 40% of the normal distance and the embryo appears twisted. This phenotype is particularly obvious when embryos are labeled for fkh expression (which is reestablished in cad-deficient embryos after the blastoderm stage): the hindgut primordium, which strongly expresses fkh, remains near the posterior of the embryo. Thus, in the absence of cad activity, there is a severe defect in germband extension (Wu, 1998).

The failure of the hindgut to become internalized in caudal-deficient embryos raises the question of whether cad might regulate a zygotically expressed gene required for the invagination of the amnioproctodeal plate. One gene known to be required for gastrulation is fog; fog mutant embryos lack not only the posterior midgut, but, as revealed by anti-Crb staining, the Malpighian tubules and hindgut as well. In the blastoderm stage embryo, fog expression is first activated in the region that will become the ventral furrow; shortly thereafter, expression is initiated in a posterior cap, in the region that will become the amnioproctodeal invagination. In cad-deficient embryos, fog expression in the prospective ventral furrow is normal, but is significantly reduced in the posterior cap. Thus, cad is required for the normal level of expression of fog in the prospective amnioproctodeal plate; decreased fog expression in cad-deficient embryos is likely responsible for the failure of the hindgut primordium to be internalized during gastrulation. Since fkh or wg mutant embryos do not display detectable defects in gastrulation, fog is the only gene presently known to mediate the effects of cad on gastrulation. In fog mutant embryos, none of the posterior gut primordia invaginate, while in cad-deficient embryos the posterior midgut and Malpighian tubule primordium do invaginate; thus, consistent with the in situ hybridization results, a low level of fog activity is present at the posterior of embryos lacking cad (Wu, 1998).

In addition to cad, three other genes (fkh, byn and wg,), which are required at the posterior of the Drosophila embryo for formation of the hindgut, are related to genes found throughout the metazoa, known as HNF-3 (alpha, beta, and gamma), Brachyury (also known as T) and Wnt, respectively. In many cases, these homologs are expressed in portions of the ‘blastopore equivalent’ at the posterior of the embryo, that overlap with domains of expression of cad (Cdx). In C. elegans, a Wnt homolog is expressed, and required for proper posterior development, in the same posterior blastomere where the cad homolog pal-1 functions. In sea urchin, HNF-3 and Brachyury homologs are expressed in the vegetal plate just prior to gastrulation. In fish and frog, Caudal, Brachyury and Wnt (Wnt8 and Wnt11) are initially expressed around most or all of the blastopore lip while HNF-3 expression is dorsally localized. As gastrulation proceeds, the expression of these genes becomes more restricted and non-overlapping, with HNF-3 and Brachyury expression becoming localized to the notochord and Wnt8 expression retreating from the dorsal position and becoming exclusively ventral. Patterns of expression of HNF-3 and Brachyury consistent with this general description have been found in ascidians (phylum Urochordata), amphioxus (phylum Chordata), chick and mouse. Required roles for some of these genes have been demonstrated by analysis of mutants: mouse HNF-3ß knockouts reveal requirements in the formation of the node, notochord and head process; fish no tail and mouse T mutants reveal a requirement for Brachyury in migration of mesoderm through the primitive streak and in formation of the notochord. There is thus a constellation of conserved genes -- cad (Cdx), fkh (HNF-3), wg (Wnt8 and Wnt11) and byn (Brachyury) -- whose overlapping expression patterns in the blastopore equivalent suggests function in a related process. The phenotypes of the available mutations in these genes suggest that the common function is to specify cell fate at the blastopore; in most cases, essential parts of this fate are internalization and forward migration, two of the cellular movements that occur during gastrulation (Wu, 1998 and references).

The striking conservation in expression (and likely in function) of cad suggests that the regulation of posterior terminal development in Drosophila by Caudal may represent a more ancient regulatory mechanism than the tor receptor and the two genes that it activates: tll and hkb. Of these three genes, a vertebrate homolog is known only for tll; the function of this vertebrate gene, Tlx, is related to that of Drosophila tll not in the posterior, but rather in the anterior, in the establishment of the brain. Thus the Torso receptor pathway and its activation of tll and hkb has probably been superimposed relatively recently (in evolutionary terms) upon a more ancient, Caudal-regulated network of gene activity controlling gastrulation and gut formation. The fact that the same four genes are expressed at both the blastopore equivalent of chordates and at the amnioproctodeal invagination of Drosophila suggests that these two highly dynamic domains are homologous. Given the regulatory hierarchy that is present in Drosophila, it is proposed that in embryos of the proximate ancestor to arthropods and chordates, the posterior was defined by a posterior-to-anterior gradient of Cad activity. Cad is thought to have then activated expression of downstream network of genes in control of invagination (gastrulation) and gut specification. Cad expression in the archenteron probably continued during evolution and played an essential developmental role, since this structure differentiated into the gut. Going beyond the bilaterian ancestor to chordates and arthropods, it is worth considering that this nexus of gene expression may have evolved even more basally in the metazoa. The foregoing, by homologizing the insect amnioproctodeal invagination with the echinoderm and vertebrate blastopore, does not fit with the classical definition of protostomes and deuterostomes. This view categorizes arthropods as protostomes, in which the mouth is derived from the primary invagination of gastrulation; chordates are categorized as deuterostomes, where the mouth arises from a secondary invagination. More recently, comparisons of gastrulation patterns in many different species, as well as construction of molecularly based cladograms, have called into question the utility of these classically defined groups. While there continues to be uncertainty in understanding of ‘protostome’ and ‘deuterostome’ phyla, the significant conclusion of the information presented here is that there may be a homology between the blastopore of vertebrates and the amnioproctodeal (posterior) invagination of insects (Wu, 1998 and references).

The actions of the homeotic gene caudal illustrate the significance of shifting maternal and zygotic gene expression, and the interaction of gene systems in the creation of evolutionary diversity among insects. caudal expression is driven from two promoters, each of which generates its own transcript. Maternally provided Caudal appears to activate fushi tarazu transcription in the embryo's posterior, and therefore is a strong factor in the essential segmentation [Images] of the posterior (Dearolf, 1989). Zygotic cad transcription under negative control of Hunchback appears to be important in refinement of the developmental process of the Drosophila tail. Zygotic cad may regulate Abd-B, a gene that acts to suppress some aspects of tail morphogenesis (Casanova, 1986).

The role of caudal in posterior segment formation is involved in the generation of diversity between insect species. Primitive insects develop segmentation by heterchronic gene activation. Each posterior segment forms independently in a time dependent manner. nanos activity eliminates Hunchback from the posterior precellular blastoderm region of primitive insects, and caudal, otherwise repressed by Hunchback, is able to function there at a later stage. Caudal proceeds to drive the addition of new segments over time. Thus, a gradient of Caudal would regulate the asynchronous segment development of primitive insects (Rivera-Pomar, 1995).

Drosophila pair-rule gene expression, in an array of seven evenly spaced stripes along the anterior-posterior axis of the blastoderm embryo, is controlled by distinct cis-acting stripe elements. In the anterior region, such elements mediate transcriptional activation in response to (1) the maternal concentration gradient of the anterior determinant Bicoid and (2) repression by spatially distinct activities of zygotic gap genes. In the posterior region, activation of hairy stripe 6 has been shown to depend on the activity of the gap gene knirps, suggesting that posterior stripe expression is exclusively controlled by zygotic regulators. The zygotic activation of hairy stripe 6 expression is preceded by activation in response to maternal caudal activity. Thus, transcriptional activation of posterior stripe expression is likely to be controlled by maternal and zygotic factors as has been observed for anterior stripes. To establish the potential of Cad and Kni to interact with the cis-acting DNA that mediates hairy stripe 6-like expression in the embryo, in vitro footprinting experiments were performed with the 532 bp hairy stripe 6-element DNA. Cad and Kni bind to thirty six in vitro binding sites, some of which overlap, throughout the element. The sequence of the Cad and Kni binding sites matches the consensus described for each of the two proteins. Most of the potential Cad and Kni binding sites are close to or overlapped by binding sites for Kruppel (eight sites), Hunchback (eight sites), and Tailless (five sites). Tests using fragments of the 532 bp enhancer and of another element, 284-HT, show that sequences mediating activation of reporter expression are not maintained within a minimal activation element but instead are dispersed throughout the enhancer (Hader, 1998).

The results suggest that activation and the expression level mediated by the hairy stripe 6-element depend on the number of activator binding sites; both activation and expression level are likely to involve additive rather than synergistic interactions. An identical transacting factor requirement is found for hairy stripe 6 and 7 expression. The arrangement of the corresponding binding sites for the common factors involved in the control of the two stripes share a high degree of similarity, but some of the factors exert opposite regulatory functions within the two enhancer elements (Hader, 1998).

The two opposing gradients of Bicoid and Cad provide a complementing transcriptional activator system along the entire axis of the preblastoderm embryo necessary for proper hairy stripe expression. The importance of Caudal as a posterior activator had been overlooked for some time. This is because Bicoid can partially compensate for the role of Cad as an activator of posterior gap genes. The activating role of Cad in the posterior region of the embryo, already suggested in the context of the fushi tarazu cis-acting control element, is substantiated by misexpression studies on hunchback, a regulator of zygotic caudal expression. The results of the misexpression studies imply that gene activation in response to Cad involves the combined action of maternal and zygotic caudal activities. These results indicate, however, that zygotic caudal activity cannot compensate for the lack of maternal caudal activity in the case of hairy. While Bcd is both necessary and sufficient for the activation of anterior genes and may require co-activations such as HB for establishing proper expression domains, maternal Cad seems to act by providing a basal level of activation on which other factors, such as Bcd, Kni or Kr, act to set the biologically relevant time and level of gene expression (Hader, 1998 and references).

GENE STRUCTURE

Length of Transcript - 2.6 (zygotic); 2.4 maternal

Bases in 5' UTR - 460; maternal transcript is shorter at 5' end

Exons - 2; Intron is 10.5k bases

Bases in 3' UTR - 619; maternal transcript is shorter at 3' end

PROTEIN STRUCTURE

Amino Acids - 472

The CAD homeodomain shares 58% homology to FTZ homeodomain and 53% homology to ANTP homeodomain (Mlodzik, 1985 and Macdonald, 1986).

date revised:  7 October 2021

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