Tests were made using Drosophila motoneurons of growth cone choices in response to muscle fiber mismatch. In numb mutants, multiple muscle fibers, including 7, 12, and 13, fail to develop. This allows testing for whether fibers distal to the target are involved in muscle fiber recognition, possibly by halting the growth cone advance. In mutant embryos, RP3 innervated muscle fiber 6 at the same frequency regardless of the absence of the distal muscle fiber 13. By contrast, RP1, which had lost its target entirely, frequently failed to innervate any muscle fiber during the period examined. These results indicate that each motorneuron growth cone has a primary target preference, a preference retained even when the numbers of the muscle fibers (and therefore their relative positions) are altered. Synaptic recognition by Drosophila motorneuron growth cones appears to rely on unique features of the individual muscle fibers (Chiba, 1993).
Terminal divisions of myogenic lineages in the Drosophila embryo generate sibling myoblasts that act as founders for larval muscles or form precursors of adult muscles. The formation of individual muscle fibers is seeded by a special class of founder myoblasts that fuse with neighboring mesodermal cells to form the syncytial precursors of particular muscle. Alternative fates adopted by sibling myoblasts are associated with distinct patterns of gene expression. During normal development (embryonic stage 11), two ventrally located progenitor cells divide once to produce three muscle founders and the precursor of an adult muscle (known as a persistent Twist cell because of its continued expression of twist). The more dorsal of the two progenitors divides, first giving rise to the founders of muscles VA1 and VA2, followed by the more ventral progenitors which produce the VA3 founder and the ventral adult persistent Twist precursor (VaP). As the progenitors divide, Numb is included in one of the two dorsal progenitors and in one of the two ventral progenitors. Thus the division of a muscle progenitor produces an unequal distribution of Numb between the founders: one contains Numb, the other does not. In numb mutants, some muscles are lost and others are transformed. For example VA1 and VaP are duplicated and VA2 and VA3 are lost. It is likely that all adult precursors are paired with larval founder cells as alternative fates. Each of the six persistent twist-expressing precursors in an abdominal hemisegment is duplicated in numb mutants. In the case of the lateral adult precursors, this duplication is associated with a loss of the segment border muscle; in the case of the three dorsal precursors, it is associated with the loss of dorsal muscles. Conversely, ectopic expression of numb leads to loss of adult muscle precursor and duplication of larval muscles. Lack of function of inscuteable produces phenotypes in the mesoderm similar to ectopic numb (Gomez, 1997).
Genes expressed in the progenitor cell are maintained in one sibling and repressed in the other. Kruppel, S59 and even skipped expression mark a subset of the developing muscles. In numb mutants the expression of Kruppel, S59 and even skipped is initiated normally but is lost from both founder cells after they are formed. Thus in numb mutants there are no muscles that express Kr, eve or S59. In contrast, when numb is ectopically expressed throughout the mesoderm, Kr, S59 and eve expression are maintained in both founders and in the muscle precursors to which they give rise. In these embryos, Kr, S59 and eve-expressing muscles are duplicated (Gomez, 1997).
In Drosophila, much has been learned about the specification of neuronal cell fates but little is known
about the lineage of mesodermal cells with different developmental fates. During development,
individual mesodermal precursor cells are initially singled out to become the founder cells for specific muscles.
The selection of muscle founder cells is thought to employ a Notch-mediated process of lateral
inhibition, similar to what is observed for the specification of neural precursors. These muscle founder
cells then seem to fuse with the surrounding, uncommitted myocytes, inducing the formation of muscle
fiber syncytia. In contrast, the differentiated progeny of neural precursor cells are usually the result of
a fixed pattern of asymmetric cell divisions that are directed, in part, by interactions among Numb (a
localized intracellular-receptor protein); Sanpodo (Spdo, a potential tropomodulin homolog), and Notch (a
transmembrane receptor protein). The roles of these neural lineage genes have been examined in
the cell fate specification of muscle and heart precursors. numb and spdo mutations have opposite effects in the specification of muscle founder cells. In all numb mutant embryos examined, the number of Kruppel and S59/NK1 expressing muscle cells is dramatically reduced or absent in stage 12/13 embryos. In spdo mutant embryos the number of S59 and Kr expressing muscle founders is increased (Park, 1998).
A progenitor cell that generates both a pericardial heart cell and a
somatic muscle founder cell was the focus of investigation. The two sibling cells studied were a single dorsal muscle, DA1, and a non-muscle pericardial cell (termed EPC), which is associated with the heart. Both cell types express Eve, however, they can be distinguished from one another morphologically. The precursor for both the EPCs and the putative founders of DA1 muscle emerges from a small cluster of mesodermal Eve-expressing cells in each hemisegment at mid-stage 11. At first, these mesodermal Eve cells are indistinguishable from one another, and they co-express Mef2, which is expressed in the entire early mesoderm and later in all (contractile) muscle types. Subsequently Mef2 expression ceases in the future EPCs as they begin to differentiate as non-muscle, pericardial cells. The putative DA1 founder seems to maintain Mef2 expression and begins fusing with surrounding myocytes. In Mef2 mutants, no fusion occurs but the putative muscle founders maintain expression of their muscle precursor markers, such as Eve. The asymmetric segregation of Numb into one of these
daughter cells antagonizes the function of Notch and Spdo by preventing the presumptive muscle
founder from assuming the same fate as its cardiac sibling. In numb mutants, most DA1 muscles are physically absent and the remaining ones lack Eve (and Kr) expression; in addition, the number of EPCs is doubled. These data suggest that in numb mutants, the putative DA1 founders are transformed into EPCs, because the Eve progenitor cells that normally give rise to DA1 founders and EPCs now only produce EPCs. Overexpression of numb or loss-of-spdo-function result in a failure to generate EPCs but allow for the formation of DA1 muscles. Similarly, expression of constitutively active Notch leads to a failure of DA1 muscle formation and an increase in the number of EPCs. Studies of double mutants indicate that spdo is epistatic to numb (in numb;spdo double mutant embryos, the spdo phenotype is apparent), suggesting that it acts downstream of numb. These results suggest that asymmetric cell
divisions, in addition to the previously-documented inductive mechanisms, play a major role in cardiac
and somatic muscle patterning. In addition, the cytoskeleton may have a role in the
asymmetrical localization of cell fate determinants (Park, 1998).
Each larval hemisegment comprises ~30 uniquely specified somatic muscles. These derive from muscle founders that arise as distinct sibling pairs from the division of muscle progenitor cells. The progenitor cell divisions of three mesodermal lineages (P2, P15, and P17) that generate muscle (and pericardial cell) founders have been analyzed. Each of these progenitors divides once at a specific stage in development to give rise to the founders for the Eve+ pericardial cells (EPCs), the Eve+ Kr+ dorsal acute 1 (DA1 = m1) muscle, and the Kr+ dorsal oblique 1 (DO1 = m9) muscle, respectively, and a sibling cell of unknown fate. These progenitors were examined because mutations in insc and nb have strong effects on the development of the EPC as well as DA1 and DO1, and suitable markers are available for the analyses of these cells and the division of their progenitors (Carmena, 1998).
Inscuteable and Numb proteins are localized as cortical crescents on opposite sides of dividing progenitor cells. Asymmetric segregation of Numb into one of the sibling myoblasts depends on inscuteable and is essential for the specification of distinct sibling cell fates. In contrast to the nervous system where Insc protein crescents are localized to the apical cortex of neuroblasts, there does not appear to be a fixed orientation for the Insc crescent in the different progenitors of the mesoderm. This appears to be due to the fact that unlike neuroblasts that align their mitotic spindles along the apical/basal axis, these progenitors do not divide with a fixed orientation. However, for any given type of progenitor, the Insc crescent accumulates at a similar position relative to the anterior/posterior or dorsal/ventral axis. Despite these differences in the orientation of the Insc crescent, there appears to be a tight correlation between the position of the Insc protein crescent and the orientation of the progenitor cell division as deduced from the staining of DNA; the location of the Insc crescent appears to center on one of the mitotic spindle poles (Carmena, 1998).
Loss of numb results in opposite cell fate transformations from loss of inscuteable - loss of either prevents sibling myoblasts from adopting distinct identities, resulting in duplicated or deleted mesodermal structures. Embryos homozygous for nb3, a putative amorphic allele of numb, were stained with anti-myosin heavy chain (MHC), which labels all somatic muscles, and anti-Eve, which stains DA1 and EPC. The loss of nb has a general effect causing many of the dorsal and ventral somatic muscles to be lost. However, the expressivity of the phenotype varies for each muscle, and not all somatic muscles are affected. Nevertheless, in nb3 homozygotes, DA1 is almost always absent, whereas DO1 is lost from >50% of the mutant hemisegments. In contrast, the number of EPC is increased. The overexpression of Nb can lead to the duplication of DA1 as well as the loss of EPC, effects that are opposite those caused by the loss of nb. However, because of the multiplicity of extra dorsal muscles associated with this overexpression paradigm, DO1 could not be scored. These observations are consistent with the notion that nb can act in a necessary and sufficient manner to specify mesodermal cell fate (Carmena, 1998).
A model is presented and tested for insc and nb loss- (and gain-) of-function phenotypes. In wild-type embryos, P15 (and P17) divide such that one progeny becomes the founder for DA1 (and DO1), henceforth referred to as FDA1 (and FDO1), as a consequence of inheriting all of the asymmetrically localized Nb protein. Its sibling cell does not inherit Nb and adopts an alternative (unknown) fate. In the absence of insc (or when nb is overexpressed), Nb is no longer asymmetrically distributed so both daughter cells derived from the P15 (and P17) division inherit Nb and both adopt an FDA1 (and FDO1) identity at the expense of its sibling. This leads to the duplication of DA1 (and DO1), which in fact is observed in insc mutants. In the case of EPC, for the wild-type P2 cell division, it is the progeny that fails to inherit Nb that becomes the FEPC, whereas its sibling (FEPCsib), which inherits Nb, adopts an alternative but unknown fate. Hence, in insc mutants (or when nb is overexpressed) both of the P2 progeny are Nb+ and adopt the fate of FEPCsib at the expense of the FEPC, leading to the loss of EPC. Conversely, in the absence of nb, both siblings derived from the progenitor cell division adopt the identity of the sibling which would normally not inherit Nb. As a result, the opposite cell fate transformations occur, leading ultimately to the loss of DA1 and DO1 and the gain of EPC (Carmena, 1998). Because insc and nb mutants show opposite mesodermal phenotypes, a double mutant was made and its phenotype examined to ascertain the hierarchical relationship between insc and nb. The insc, nb double homozygous embryos show loss of DA1 and DO1, as well as gain of EPC. Although qualitatively similar to those shown by nb mutant embryos, the double mutant embryos exhibit these phenotypes with higher levels of expression. These results suggest that nb acts downstream of insc, consistent with data showing that insc is required for wild-type Nb localization (Carmena, 1998).
A series of inductive signals are necessary to subdivide the
mesoderm in order to allow the formation of the progenitor
cells of the heart. Mesoderm-endogenous transcription
factors, such as those encoded by twist and tinman, seem to
cooperate with these signals to confer correct context and
competence for a cardiac cell fate. Additional factors are
likely to be required for the appropriate specification of
individual cell types within the forming heart. Similar to
tinman, the zinc finger- and homeobox-containing gene
zfh-1 is expressed in the early mesoderm and later in the
forming heart, suggesting a possible role in heart
development. zfh-1 is specifically
required for formation of the even-skipped (eve)-expressing
subset of pericardial cells (EPCs), without affecting the
formation of their siblings, the founders of a dorsal body
wall muscle (DA1). In addition to zfh-1, mesodermal eve
itself appears to be needed for correct EPC differentiation,
possibly as a direct target of zfh-1. Epistasis experiments
show that zfh-1 specifies EPC development independent
of numb, the lineage gene that controls DA1 founder versus
EPC cell fate. The combinatorial control
mechanisms that specify the EPC cell fate in a spatially
precise pattern within the embryo are discussed (Su, 1999).
zfh-1 and the components of the numb pathway are not the only
factors required for specifying EPC or DA1 founder fates (or for
eve expression characteristic of these fates). A transcription
factor encoded by the lethal-of-scute gene is expressed in a
cluster of mesodermal cells out of which the EPC and other
muscle progenitors emerge aided by a laterally inhibitory
mechanism. lethal-of-scute, however, as well as another
transcription factor encoded by the Krüppel gene, which is
expressed in the DA1 (and other muscle) founder cells, are only
weakly required for the corresponding muscles to form. In contrast, the
Drosophila EGF signal transduction pathway plays an essential
role in DA1 specification. For example, in the
absence of the secreted EGF-receptor ligand spitz, the
number of EPCs is normal but nearly all the DA1 muscles fail
to form.
Since DA1 founders and EPCs are likely to derive from
common precursors and the phenotype of spi mutants is the
opposite of zfh-1, it was decided to determine whether or not zfh-1 and spitz function as part of a
common genetic pathway. The phenotype of spitz;zfh-1
double mutants was examined. In these double mutants, neither EPC- nor
DA1-specific eve expression is present, suggesting
that the Egf-r pathway is required for DA1 differentiation
independently of zfh-1. This raises the question of whether or not
Egf-r pathway activation is required for providing the correct
DA1 differentiation context in a way that is reminiscent of zfh-1
function, which provides a context for EPC differentiation. If yes,
it would be expected that spitz, like zfh-1, functions independently
of the numb pathway. Indeed, when numb is mesodermally
overexpressed in spitz mutant embryos, a phenotype similar
to that of spitz;zfh-1 double mutants is observed: neither EPC- and
nor DA1-specific eve expression is observed. Taken together, these results suggest that correct cell
type-specific differentiation depends on both asymmetric
segregation of cell fate determinants during cell division as
well as on the appropriate regional context. In this case, the
context information (zfh-1 or Egf-r activity) does not need to
be originating from a spatially localized source, but may act in
concert with other mesodermal context determinants (e.g.,
tinman) (Su, 1999).
A model is provided of the genetic network regulating the specification and
differentiation of the EPC progenitors and their heart and muscle
associated progeny (EPC and DA1). Initially, the spatially coincident
activity of the transcription factor, Tinman, together with the
mesoderm-specific response induced by the patterning signals, Wg
and Dpp, are necessary to specify and position the most dorsal
portion of the mesoderm, which includes the EPC progenitors and
other cardiac precursors. The EPC progenitors then divide and
produce two types of progeny cells under the control of the lineage
gene numb. The daughter cell that inherits Numb protein will
differentiate as the DA1 muscle founder, because the Notch and spdo
encoded functions are inhibited, allowing Egf-r signaling (Spitz) to be
effective (perhaps in conjunction with Eve). In the daughter cell
without Numb, Notch signaling is operational and the transcription
factors Zfh-1 together with and/or mediated by Eve can effectively
contribute the correct differentiation of the EPC fate. Thus, three
levels of information appear to cooperate in the specification of a
particular cell fate: prepatterning or positional information,
asymmetric lineages and tissue context information (Su, 1999).
The Drosophila heart is a simple organ composed of two
major cell types: cardioblasts, which form the simple
contractile tube of the heart, and pericardial cells, which
flank the cardioblasts. A complete understanding of
Drosophila heart development requires the identification of
all cell types that comprise the heart and the elucidation
of the cellular and genetic mechanisms that regulate
the development of these cells. A new population of heart cells is reported here: the Odd
skipped-positive pericardial cells (Odd-pericardial cells).
Descriptive, lineage tracing and genetic
assays were used to clarify the cellular and genetic mechanisms that
control the development of Odd-pericardial cells. Odd
skipped marks a population of four pericardial cells per
hemisegment that are distinct from previously identified
heart cells. Within a hemisegment,
Odd-pericardial cells develop from three heart progenitors
and these heart progenitors arise in multiple
anteroposterior locations within the dorsal mesoderm. Two
of these progenitors divide asymmetrically such that each
produces a two-cell mixed-lineage clone of one Odd-pericardial
cell and one cardioblast. The third progenitor
divides symmetrically to produce two Odd-pericardial
cells. All remaining cardioblasts in a hemisegment arise
from two cardioblast progenitors, each of which produces
two cardioblasts. Furthermore, numb
and sanpodo mediate the asymmetric divisions of the two
mixed-lineage heart progenitors noted above (Ward, 2000).
Having established a wild-type profile of Odd-pericardial cell
development it was of interest to identify the genetic regulatory
mechanisms that govern Odd-pericardial cell development.
Genes that control
asymmetric divisions regulate Eve-pericardial cell
development. Thus, whether loss of
sanpodo or numb function affect Odd-pericardial cell and
cardioblast development was examined. Normally 4.2 Odd-pericardial cells and 6.0 cardioblasts develop within each
abdominal hemisegment of late-stage embryos. In
numb mutant embryos, 6.0 Odd-pericardial
cells and 4.2 cardioblasts were detected per
hemisegment. Conversely, 7.6 cardioblasts
and 2.7 Odd-pericardial cells per hemisegment were detected in
sanpodo mutant embryos. Thus, in numb mutant embryos roughly two extra Odd-pericardial cells and two fewer
cardioblasts were detected per hemisegment. Conversely, in sanpodo mutant
embryos roughly two fewer Odd-pericardial cells and two
additional cardioblasts form per hemisegment (Ward, 2000).
These results demonstrate that sanpodo promotes Odd-pericardial
cell development and opposes cardioblast
development. Conversely, numb opposes Odd-pericardial cell
development and promotes cardioblast development. In
addition, they suggest that two cardioblasts and two Odd-pericardial
cells arise via the asymmetric divisions of
numb/sanpodo dependent heart progenitors. These results are
consistent with the known requirement for Notch in pericardial cell development. Loss of numb function disrupts the precise alignment of
cardioblasts leading to 'broken rows' of cardioblasts in numb
mutant embryos (Ward, 2000).
Multiple models can explain the reciprocal effects of sanpodo
and numb on cardioblast and Odd-pericardial cell
development. For example, one model predicts that two mixed-lineage
heart progenitors each divide to yield one cardioblast
and one Odd-pericardial cell. A second model predicts the
existence of four progenitors: two would divide with each
producing one Odd-pericardial cell and one cell of unknown
fate; the other two progenitors would divide each producing
one cardioblast and one cell of unknown fate. In these and other
models, loss of numb or sanpodo function would equalize all
asymmetric divisions and could result in the observed Odd-pericardial
cell and cardioblast phenotypes (Ward, 2000).
An enhancer trap in the seven-up gene identifies the
two mixed-lineage heart progenitors.
Towards the end of the lineage analyses it was discovered
fortuitously that an enhancer trap in the gene seven-up labels
four heart cells in each abdominal hemisegment. This enhancer trap is referred to as svp-lacZ). Two of these cells reside
at the dorsal midline and are cardioblasts since they express Mef2. The other two cells reside just lateral and slightly
ventral and anterior to the svp-lacZ cardioblasts. These two
cells are Odd-pericardial cells because they express Odd. The relative
positioning of the svp-lacZ cardioblasts and Odd-pericardial
cells closely resembles that of the sibling cardioblasts and Odd-pericardial
cells marked by the mixed lineage heart clones. This suggests that the svp-lacZ heart cells may
identify the four progeny of the two mixed lineage heart
progenitors that arise in each hemisegment. If the four svp-lacZ
heart cells are the progeny of these two progenitors, then loss
of sanpodo function should convert all svp-lacZ heart cells to
cardioblasts and loss of numb function should convert all svp-lacZ
heart cells to Odd-pericardial cells. In sanpodo mutant
embryos, all four svp-lacZ cells acquire the cardioblast fate and in numb mutant embryos all four svp-lacZ cells
acquire the Odd-pericardial cell fate. The results
from these experiments demonstrate that svp-lacZ identifies the
progeny of the two mixed lineage heart progenitors and that
numb and sanpodo mediate the asymmetric divisions of these
mixed-lineage heart progenitors (Ward, 2000).
During the formation of the Drosophila heart, a combinatorial network that integrates signaling pathways and tissue-specific transcription factors specifies cardiac progenitors, which then undergo symmetric or asymmetric cell divisions to generate the final population of diversified cardiac cell types. Much has been learned concerning the combinatorial genetic network that initiates cardiogenesis, whereas little is known about how exactly these cardiac progenitors divide and generate the diverse population of cardiac cells. In this study, the cell lineages and cell fate determination in the heart have been examined by using various cell cycle modifications. By arresting the cardiac progenitor cell divisions at different developing stages, the exact cell lineages for most cardiac cell types have been determined. Once cardiac progenitors are specified, they can differentiate without further divisions. Interestingly, the progenitors of asymmetric cell lineages adopt a myocardial cell fate as opposed to a pericardial fate when they are unable to divide. These progenitors adopt a pericardial cell fate, however, when cell division is blocked in numb mutants or in embryos with constitutive Notch activity. These results suggest that a numb/Notch-dependent cell fate decision can take place even in undivided progenitors of asymmetric cell divisions. By contrast, in symmetric lineages, which give rise to a single type of myocardial-only or pericardial-only progeny, repression or constitutive activation of the Notch pathway has no apparent effect on progenitor or progeny fate. Thus, inhibition of Notch activity is crucial for specifying a myogenic cell fate only in asymmetric lineages. In addition, evidence is provided that whether or not Suppressor-of-Hairless can become a transcriptional activator is the key switch for the Numb/Notch activity in determining a myocardial versus pericardial cell fate (Han, 2003).
Previous studies have suggested that Notch activity controls two distinct processes during the specification of cardiac cell fates. First, it is required to single initial progenitors out of a field of competence by supporting the selection and inhibiting surrounding cells from adopting the same fate. Subsequent to the progenitor specification, Notch is required again for the specification of alternative cell fates of sibling cells produced during asymmetric cell divisions. In this
study, the cell autonomy of Notch was examined, by using eme-Gal4 (which confers expression in the mesodermal Eve lineage) to drive
activated forms of Notch and Su(H) exclusively in the mesodermal Eve lineages. Conditional ubiquitous expression of activated Notch was used to examine its lineage-specific function in other cardiac lineages. Notch was found to be required for specification of a non-myogenic fate in both the Eve and the Svp lineages of the cardiac mesoderm. By contrast, activation or inhibition of the Notch pathway does not affect cell fate decisions within the symmetric lineages. This suggests a mechanism by which cell type diversity may be increased during evolution by co-opting the Notch pathway during cell division
to distinguish between alternative fates of the daughter cells. The inability of activated Su(H) to autonomously influence cell fates in symmetric cardiac lineages further suggests that other factors or activities, not present in symmetric lineages, are crucial for the asymmetric lineage-specific functions of Notch and Su(H) (Han, 2003).
Interestingly, this influence of the Notch pathway on cell fate decision in asymmetric cardiac Eve and Svp lineages is not altered when cell division is arrested. Thus, cell division is not essential to distinguish between alternative cell fates. The data also suggest that the default cell fate of an asymmetrically dividing cardiac precursor in Drosophila is determined to assume a myogenic fate, owing to Numb-mediated inhibition of Notch, unless that fate is switched by the activation of target genes downstream of Su(H). Moreover, in a double mutant of Notch and numb one would expect to observe the same lineage phenotype as of Notch alone, i.e., a myogenic cell fate, since the primary role of Numb is to inhibit Notch signaling. Unfortunately, analysis of such double mutants is complicated by the earlier role of Notch in lateral inhibition (Han, 2003).
Another unresolved issue is the source of the Notch ligand that activates signal transduction within asymmetric cardiac lineages. If the myogenic cell were to produce the ligand for Notch activation in its pericardial sibling, then the undivided progenitor would have to secrete its own Notch ligand. This is unlikely, since production of the ligand is usually inhibited within the cell that experiences Notch signaling. In the asymmetric MP2 lineage of the Drosophila CNS, for example, ligand production appears to be required
in cells outside the MP2 lineage. A similar scenario may be operating in the asymmetric cardiac lineages (Han, 2003).
Within the Eve lineages, Notch activation is mimicked by Su(H) fused to the VP16, a potent transcriptional activation domain. Recent studies suggest that in the absence of Notch activity, DNA-bound Su(H) prevents activators from promoting transcription. When Notch ligands, such as Delta, bind to its receptor, Notch is cleaved to produce an intracellular domain fragment, N(icd), which is thought to enter the nucleus and interact directly with Su(H)
to recruit transcriptional co-activators and alleviate Hairless-mediated
repression, thus promoting transcription. In
support of this model, it has been found that Su(H) overexpression can mimic Notch activation only when linked directly to a transcriptional activator, but not in its wild-type form when it presumably associates with co-repressors, such as Hairless and Groucho, that prevent Su(H)-dependent transcriptional activation in
the absence of Notch signaling (Han, 2003).
The role of the PTB-containing, membrane-associated protein Numb in
preventing Notch activation in the nervous system is well established. To explore at which level Notch signaling is inhibited by
Numb in the cardiac lineages, numb was overexpressed simultaneously with N(icd) or Su(H)vp16 within the mesodermal Eve lineages.
Excess Numb was able to counteract activated Notch but not activated Su(H) function, suggesting that Numb can inhibit Notch activity after Notch has been cleaved, possibly by preventing its nuclear translocation, but is unlikely to
prevent the transcriptional activator function of Su(H) directly. Recent data suggest that Numb is involved in stimulating endocytosis of Notch, thus removing it from the cell surface and inhibiting its function. It is not clear, however, if this inhibition by endocytosis is at the level of the entire Notch receptor, or (also) at the level of N(icd) after it is cleaved off. Experiments described here provide strong evidence that Numb can indeed interfere with N(icd) function, but it remains to be determined if endocytosis is an obligatory intermediate in this inhibition of activated Notch (Han, 2003).
The Notch pathway may also have a role in vertebrates in specifying pericardial and other non-myogenic cell fates within the dorsolateral cardiogenic region of the anterolateral plate mesoderm. As in the Eve and Svp lineage of the Drosophila heart, activation of the Notch pathway decreases myocardial gene expression and increased expression of a pericardial marker, whereas inhibition of Notch signaling resulted in an increase of cardiac myogenesis. Similar results were
obtained with an activated form of RBP-J [a vertebrate homolog of
Drosophila Su(H) fused to vp16, as in this study]. These
data indicate that the Notch pathway may play a role in the specification of myocardial versus pericardial cell fates in both Drosophila and vertebrates. This raises the question of whether the mechanism of Notch mediated cell identity determination is also conserved between vertebrates and flies. Because it is not yet known if (Numb-controlled) asymmetric cell divisions are also involved in vertebrate heart development, the answer awaits future studies. However, recent studies on the role of Numb during cortical development suggest that it is likely to have a similar control function in cell fate specification in vertebrates as it does in flies (Han, 2003 and references therein).
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