miranda


DEVELOPMENTAL BIOLOGY

Embryonic

miranda transcripts can be detected in embryos at stage 3, probably due to the maternal contribution of Miranda mRNA. The transcripts start to accumulate in the procephalic and ventral neurogenic regions at stage 8. miranda is expressed in delaminating neuroblasts, cells in the posterior midgut primordia and sensory organ precursor cells in the developing peripheral nervous system. After germband retraction, the transcripts can not be detected from most of these regions; they remain only in the brain lobes and ventral region of the ventral nerve cord where cell divisions continue to take place. Thus, miranda is transiently expressed in neuroblasts, SOP cells and in cells of the procephalic neurogenic region at the time of asymmetric cell division and Prospero localization. Miranda protein shows the same expression pattern as the mRNA (Shen, 1997).

To generate different cell types, some cells can segregate protein determinants into one of their two daughter cells during mitosis. In Drosophila neuroblasts, the Par protein complex localizes apically and directs localization of the cell fate determinants Prospero and Numb and the adaptor proteins Miranda and Pon to the basal cell cortex, to ensure their segregation into the basal daughter cell. The Par protein complex has a conserved function in establishing cell polarity but how it directs proteins to the opposite side is unknown. A principal function of this complex is to phosphorylate the cytoskeletal protein Lethal (2) giant larvae [Lgl; also known as L(2)gl]. Phosphorylation by Drosophila atypical protein kinase C (aPKC), a member of the Par protein complex, releases Lgl from its association with membranes and the actin cytoskeleton. Genetic and biochemical experiments show that Lgl phosphorylation prevents the localization of cell fate determinants to the apical cell cortex. Lgl promotes cortical localization of Miranda, and it is proposed that phosphorylation of Lgl by aPKC at the apical neuroblast cortex restricts Lgl activity and Miranda localization to the opposite, basal side of the cell (Betschinger, 2003).

Lineage, cell polarity and inscuteable function in the peripheral nervous system of the Drosophila embryo: Dynamics of Miranda distribution

The stereotyped pattern of the Drosophila embryonic peripheral nervous system (PNS) makes it an ideal system to use to identify mutations affecting cell polarity during asymmetric cell division. However, the characterization of such mutations requires a detailed description of the polarity of the asymmetric divisions in the sensory organ lineages. The pattern of cell divisions generating the vp1-vp4a mono-innervated external sense (es) organs is described. Each sensory organ precursor (SOP) cell follows a series of four asymmetric cell divisions that generate the four es organs cells (the socket, shaft, sheath cells and the es neuron) together with one multidendritic (md) neuron. This lineage is distinct from any of the previously proposed es lineages. Strikingly, the stereotyped pattern of cell divisions in this lineage is identical to those described for the embryonic chordotonal organ lineage and for the adult thoracic bristle lineage. This analysis reveals that the vp2-vp4a SOP cells divide with a planar polarity to generate a dorsal pIIa cell and a ventral pIIb cell. The pIIb cell next divides with an apical-basal polarity to generate a basal daughter cell that differentiates as an md neuron. Inscuteable specifically accumulates at the apical pole of the dividing pIIb cell and regulates the polarity of the pIIb division. This study establishes for the first time the function of Inscuteable in the PNS, and provides the basis for studying the mechanisms controlling planar and apical-basal cell polarities in the embryonic sensory organ lineages (Orgogozo, 2001).

A detailed analysis of the vp1-vp4a lineages allowed for an investigation of the mechanisms regulating cell polarity in these lineages. Previous studies have indicated that insc is expressed in pI, suggesting a role for insc in regulating cell polarity in these lineages. The expression pattern of insc was examined in the vp1-vp4a lineages. Insc protein is not detectable in dividing pI, pIIa and pIIIb cells, but specifically accumulates in an apical crescent in dividing pIIb cells. The lack of insc expression in pI is further confirmed by the analysis of an insc-lacZ enhancer-trap marker. The expression of insc-lacZ is not detectable in pI and pIIa. However, it is first detected in the pIIb cell as it divides and specifically accumulates in both pIIb daughter cells. insc regulates the apical-basal polarity of the pIIb division The role of insc in regulating cell polarity was examined in the vp1-vp4a lineages. In insc mutant embryos, the vp1-vp4a pI divisions occur within the plane of the epithelium. The vp2, vp4 and vp4a pI cells divide with a d-v orientation with Numb localiz ing asymmetrically to the ventral pole of pI. Furthermore, the cell that divides next is always found at an antero-ventral position in both wild-type and insc mutant embryos, suggesting that the pIIa and pIIb cells are correctly specified. It is concluded that the loss of insc activity does not affect the polarity of the pI division. This is entirely consistent with the observation that the Insc protein is not present in the pI cell (Orgogozo, 2001).

To analyse the role of insc in the dividing pIIb cell, the asymmetric distribution of Miranda, an adaptor protein for Pros, was examined. In wild-type embryos, Miranda accumulates to the basal pole of pIIb at metaphase. In contrast, Miranda localizes asymmetrically to the basal pole in only 32% of insc mutant pIIb cells at metaphase. In the other pIIb cells, Miranda is either partly (52%) or largely (16%) delocalized around the cell cortex. This shows that insc is required for the basal localization of Miranda (Orgogozo, 2001).

In conclusion, the data show that the loss of insc activity results in cell polarity defects in the pIIb cell, as revealed by the mislocalization of Miranda at metaphase. This phenotype correlates with the abnormal accumulation of Pros into the apical pIIb daughter cell and with the mis-specification of the pIIIb cell (Orgogozo, 2001).

This study provides the first detailed description of each asymmetric cell division in an md-es lineage. The division of the vp2-vp4a pI cell is planar and takes place with a d-v polarity, revealing for the first time the existence of a planar polarity orienting asymmetric cell divisions in the embryo. Similarly, in the pupa, the pI cell divides in the plane of the epithelium and along the a-p axis. The polarity of this division is controlled by the Fz signaling pathway. In both pupae and embryos, the pIIb cell divides with an apical-basal polarity, with Numb, Pros and Miranda segregating to the basal cell. Moreover, Insc forms an apical crescent in the pIIb cell in the pupal lineage. This suggests that Insc regulates also the apical-basal polarity of the pIIb cell in the adult bristle lineage. It is clear, however, that a detailed analysis of the function of insc in regulating cell polarity in the adult PNS would have been much more difficult and time-consuming because insc mutations are embryonic lethal. In conclusion, this study clearly illustrates that the regulation of both planar and apical-basal polarities can now be studied in the embryonic PNS. This detailed analysis therefore provides the basis for future studies addressing the function of various candidate genes known to affect the development of the embryonic PNS (Orgogozo, 2001).

Monmuscle myosin II promotes the asymmetric segregation of cell fate determinants, such as Miranda, by cortical exclusion rather than active transport

Cell fate diversity can be achieved through the asymmetric segregation of cell fate determinants. In the Drosophila embryo, neuroblasts divide asymmetrically and in a stem cell fashion. The determinants Prospero and Numb localize in a basal crescent and are partitioned from neuroblasts to their daughters (GMCs). Nonmuscle myosin II regulates asymmetric cell division by an unexpected mechanism, excluding determinants from the apical cortex. Myosin II is activated by Rho kinase and restricted to the apical cortex by the tumor suppressor Lethal (2) giant larvae. During prophase and metaphase, myosin II prevents determinants from localizing apically. At anaphase and telophase, myosin II moves to the cleavage furrow and appears to “push” rather than carry determinants into the GMC. Therefore, the movement of myosin II to the contractile ring not only initiates cytokinesis but also completes the partitioning of cell fate determinants from the neuroblast to its daughter (Barros. 2003).

Class II myosins are barbed end-directed motors that form bipolar filaments. The filaments bind actin and initiate contraction when the two ends of the bipolar filament pull in opposite directions. Myosin's mode of action makes it unlikely that myosin II could transport cargo from one side of the cell to the other, except perhaps by progressive contraction along the cortex. The lack of colocalization of myosin II and Miranda in neuroblasts further implies that myosin II does not transport Miranda directly. The data suggest, first, that myosin II is required to maintain an intact cortical actin cytoskeleton and, second, that active myosin modifies the actin cytoskeleton at the apical cortex to exclude Miranda binding. The C. elegans myosin II may act in a similar fashion, as it appears to limit PAR-3 to the anterior of the zygote (Barros. 2003).

Several alternative approaches were taken to inactivate myosin II in neuroblasts. First, germline clones of Sqh were analyzed. In severe sqh1GLC embryos, levels of the regulatory light chain are greatly reduced from early development, and the heavy chain is found only in inactive aggregates. The actin cytoskeleton is disrupted, and neither Lgl nor Miranda localize to the cortex. Miranda concentrates instead at the spindle microtubules. Therefore, active myosin is necessary from early development to organize the actin cytoskeleton, which is in turn required for Lgl and Miranda to localize to the cortex (Barros. 2003).

Myosin II localizes to the apical cortex of metaphase neuroblasts. Why is myosin localization/activity asymmetric? Lgl binds myosin II heavy chain directly and inhibits myosin filament formation. This binding is regulated by phosphorylation of Lgl; this phosphorylation inhibits its interaction with myosin II in vitro. If Lgl negatively regulates myosin activity and localization, then myosin should be uniformly distributed in an lgl mutant. Indeed, in lgl1GLC mutants myosin II no longer concentrates apically but is found uniformly around the cortex. Most Miranda protein is released from the cortex and binds microtubules, again suggesting that myosin excludes Miranda from the cortex (Barros. 2003).

Myosin II localizes to the entire cortex in lgl mutants and thereby prevents Miranda binding basally. In Drosophila neuroblasts in which Lgl levels are reduced (zygotic lgl1 mutants), Miranda is released from the cortex. Miranda localization can be rescued by simultaneously reducing the level of myosin II (zip1 zygotic mutants). Reducing the level of active myosin may restore the balance between the levels of Lgl and myosin, enabling the remaining myosin to concentrate apically (Barros. 2003).

How does myosin II restrict neuroblast proteins to the basal side of the cell cortex? Myosin II and Miranda occupy primarily opposite sides at the neuroblast cortex: myosin II is concentrated at the apical cortex while Miranda localizes as a basal crescent. As myosin II shifts to the cleavage furrow, Miranda is segregated into the forming GMC. The apical F-actin compartment may be modified by myosin II to exclude binding of basal proteins like Miranda. Active myosin II requires Rho kinase activity and depends on inactivation of Lgl at the apical cortex by aPKC (Betschinger, 2003). Ectopic expression of a nonphosphorylatable form of Lgl, in which the conserved aPKC-dependent phosphorylation sites are mutated from Serines to Alanines (Lgl-3A), results in mislocalization of Miranda around the neuroblast cortex (Betschinger, 2003). The data support a spatially regulated interaction between myosin II and Lgl. Myosin is apically localized in wild-type neuroblasts, corresponding to the domain in which Lgl is inactivated by aPKC. In lgl mutants, myosin is no longer restricted apically but localizes around the entire cell cortex. Conversely, when nonphosphorylatable Lgl is expressed in neuroblasts, myosin is inhibited throughout the cell and drops off the cortex. It is proposed that myosin II is activated and can form filaments at the apical cortex, where phosphorylated Lgl is inactive and unable to bind myosin II. Myosin may then modify the actin cytoskeleton to prevent the binding of Miranda. At the basal cortex, in the absence of aPKC, Lgl is active and can bind and inhibit myosin. Myosin cannot form filaments, which are required for it to bind to the actin cortex. As a result, Miranda can bind to the basal cortex (Barros. 2003).

Larval

The division of postembryonic neuroblasts (Nbs) has been studied in the outer proliferation center (OPC) and central brain anlagen of Drosophila. Attention has been focussed on three aspects of these processes: the pattern of cellular division; the topological orientation of these divisions, and the expression of asymmetric cell fate determinants. Although larval Nbs are of embryonic origin, the results indicate that their properties appear to be modified during development. Several conclusions are summarized: (1) in early larvae, Nbs divide symmetrically to give rise to two Nbs while in the late larval brain most Nbs divide asymmetrically to bud off an intermediate ganglion mother cell (GMC) that very rapidly divides into two ganglion cells (GC); (2) symmetric and asymmetric divisions of OPC Nbs show tangential and radial orientations, respectively; (3) this change in the pattern of division correlates with the expression of Inscuteable, which is apically localized only in asymmetric divisions; (4) the spindle of an asymmetrically dividing Nb is always oriented on an apical-basal axis; (5) Prospero does not colocalize with Miranda in the cortical crescent of mitotic Nbs; (6) Prospero is transiently expressed in one of the two sibling GCs generated by the division of GMCs (Ceron, 2001).

In simple geometric terms, one may describe the OPC as a germ neuroepithelium forming a ring-like structure that covers the most lateral side of the lobe. Nbs occupy the external layer, close to the outside surface, and their progeny ganglion cells lay inside it, forming a thicker layer. This layered structure, which can be observed in frontal sections of optic lobes, allows an easy identification of the different cell types. If sectioning is similarly applied to BrdU-labeled optic lobes, one may observe that different time pulses give rise to different patterns of cell labeling in the OPC. Thus, short pulses result in preferential labeling of medium-size nuclei located just below Nbs that in turn are very often unlabeled. In contrast, longer pulses yield extensive labeling of large Nb nuclei and abundant small GC. Different pulse periods do not result in differential labeling of central brain (CB) Nbs and their progeny. Thus, short pulses yield pairs of labeled cells that consist of one Nb and a single daughter cell, while longer pulses produce labeling of one Nb together with a couple of daughter cells (Ceron, 2001).

The incorporation of BrdU in the progeny of Nbs during short pulses and the frequent observation of two labeled nuclei apparently undergoing cytokinesis very close to a Nb suggest the existence of GMCs that have a cell cycle shorter than their parent Nbs. Direct evidence for the existence of mitosis in those daughter cells was obtained by applying several immunochemical tools. Medium-size mitotic cells are detected just below the layer of OPC Nbs. Also, in the CB, where individual Nbs and their progeny can be observed, medium-size mitotic cells are detected immediately close to each Nb. In this case, all daughter cells are located at the same side of the Nb but no more than one is in mitosis. Interestingly, even in interphase Nbs, the centrosome is always located at the pole opposite that of the budding cells and the mitotic spindle of daughter cells is most often oriented at an oblique angle relative to that of the parent Nb. Altogether, labeling experiments with BrdU and mitotic markers demonstrate the presence of GMC-like cells in postembryonic proliferative anlagen (Ceron, 2001).

OPC Nbs stop producing more Nbs and begin to generate the final neuronal progeny around the third-instar larval period. This change in proliferative behavior could be explained by a change from an initial symmetric pattern of division to a late asymmetric one. Since asymmetric divisions of embryonic Nbs follow an apical/basal orientation, it would be also interesting to find out whether symmetric and asymmetric divisions of postembryonic Nbs have different orientations. This is indeed the case. The divisions of mitotic Nbs in the OPC of early third-instar larvae are preferentially oriented on an axis tangential to the surface, whereas those observed in late third-instar larvae show almost exclusively a radial orientation. Larval ventral ganglion Nbs, which divide asymmetrically, contain unequal centrosomes during mitosis. The larger centrosome is segregated into the resulting Nb and the smaller is inherited by the GMC. Radially oriented divisions of OPC Nbs have asymmetric centrosomes with the larger one close to the optic lobe surface, whereas tangentially oriented divisions have symmetric centrosomes. The metaphase plate of asymmetrically dividing Nbs is located close to the smaller (basal) centrosome. In contrast to the epithelial sheet-like organization of the OPC anlagen, CB Nbs are distributed in the most medial part of the optic lobe and each one shows a different direction of asymmetric division. Nevertheless, all the progeny of each Nb appear to be released by the same side and the interphase centrosome is maintained at the opposite side of the progeny (Ceron. 2001).

To determine whether the regulation of asymmetric divisions and the segregation of cell fate determinants of postembryonic Nbs follow a pattern similar to that described for embryos, the expression and localization of Insc, Mir, Numb, and Pros were examined in whole mounts of third-instar larval brain. Mir is widely expressed in all larval proliferative anlagen. During division, it shows a polarized distribution in the cell cortex of both CB and OPC Nbs, and it is segregated to the GMC during cytokinesis. Recently born GMCs show a very high expression of Mir both in the CB and in the OPC. Mir seems to be rapidly down-regulated in CB GMCs, whereas it seems to remain in OPC GMCs at high level for a rather long period of time, as judged by the relative higher proportion of labeled GMCs versus Nbs that can be detected in the OPC. Nevertheless, Mir seems to be completely down-regulated before GMCs begin mitosis (Ceron, 2001).

The tissue pattern of Pros expression in the larval brain is different from that of Mir. The expression of Pros protein in the CB and ventral (thoracic) anlagen is quite high, while in the OPC and inner proliferative center (IPC) it is rather low. Due to the higher level of expression, the localization of Pros can be studied in more detail in the CB. Pros protein is clearly observed only in the nucleus of daughter cells located away from the parent Nbs and, therefore, identified as GCs. Surprisingly, Pros protein is not consistently detected in dividing Nbs and GMCs either in the CB or in the OPC. This is especially clear by the lack of colocalization with Mir in the cortical crescent of dividing Nbs and newborn GMCs. The almost exclusive expression of Pros in GCs is also supported by the colocalization with Elav, a nuclear protein that is expressed in postmitotic cells and is absent in GMCs (Ceron, 2001).

It has been reported that Pros located at cortical sites of embryonic Nbs is highly phosphorylated compared to nuclear Pros. Nevertheless, this cannot be the reason for lack of detection of cortical Pros in Nbs of the larval optic lobe since the antiserum seems to recognize both phosphorylated and unphosphorylated forms of Pros (Ceron, 2001 and references therein).

The lack of Mir-Pros colocalization in postembryonic Nbs and GMCs opens the question of Mir's role in these cells. One possibility is that Mir might be involved in the localization of PROS mRNA, as has been shown for embryonic Nbs. To test this hypothesis, the expression of PROS mRNA was studied by in situ hybridization. PROS mRNA is detected in isolated cells of the optic lobe in both CB and OPC regions. In the CB, these cells correspond to single small daughter cells located closer to the Nb than those GCs that express Pros protein. In the OPC, it is rather obvious that Pros-expressing cells are located below the layer of GMCs. Also in the CB, PROS mRNA is detected neither in dividing GMCs nor in Mir-expressing cells. Therefore, it must be concluded that Pros is expressed at detectable levels only in GCs (Ceron, 2001).

In contrast to what it is known in the embryo and to previous data of the larval brain, these BrdU-labeling experiments clearly indicate that most postembryonic GMCs, especially those of the OPC, have a very transient life with cell cycle much shorter than that of parent Nbs. Another interesting difference is the large number of GMCs expressing high levels of Mir in the OPC. Taking into account the very short cell cycle of these intermediate cells, it is suggested that, in contrast to the rapid down-regulation observed in embryonic GMCs, Mir protein remains for a longer time in GMCs of the OPC. The rapid down-regulation of Mir in embryonic GMCs has been related to the requirement for a rapid release of the cell determinant Pros that has to translocate to the nucleus. The fact that Pros protein is not consistently detected in postembryonic GMCs makes it difficult to interpret the functional significance of this long lasting expression of Mir (Ceron, 2001).

Since Pros seems to be expressed neither in Nbs nor in GMCs, the expression of Numb, another asymmetric cell determinant of embryonic Nbs, was studied. Numb localizes in the cortical crescent of dividing Nbs of the OPC and CB; it is segregated to the membrane of GMCs where it seems to remain at low level, and it does not appear to be polarized during GMC division. Afterward, it seems to be down-regulated since it is hardly detected in GCs (Ceron, 2001).

A study of APC1 and APC2 examines asymmetric protein localization in larval neuroblasts

The tumor suppressor APC and its homologs, first identified for a role in colon cancer, negatively regulate Wnt signaling in both oncogenesis and normal development, and play Wnt-independent roles in cytoskeletal regulation. Both Drosophila and mammals have two APC family members. The functions of the Drosophila APCs is further explored using the larval brain as a model. Both proteins are expressed in the brain. APC2 has a highly dynamic, asymmetric localization through the larval neuroblast cell cycle relative to known mediators of embryonic neuroblast asymmetric divisions. Adherens junction proteins also are asymmetrically localized in neuroblasts. In addition they accumulate with APC2 and APC1 in nerves formed by axons of the progeny of each neuroblast-ganglion mother cell cluster. APC2 and APC1 localize to very different places when expressed in the larval brain: APC2 localizes to the cell cortex and APC1 to centrosomes and microtubules. Despite this, they play redundant roles in the brain; while each single mutant is normal, the zygotic double mutant has severely reduced numbers of larval neuroblasts. These experiments suggest that this does not result from misregulation of Wg signaling, and thus may involve the cytoskeletal or adhesive roles of APC proteins (Akong, 2002).

One striking feature of the asymmetric localization of APC2 is that it is present throughout the cell cycle and is particularly strong during interphase. During embryonic neuroblast divisions, most asymmetric markers are localized only during mitosis. However, less is known about their localization in larval neuroblasts. Several asymmetric markers in larval neuroblasts were examined, and their localization was compared with that of APC2. In embryonic neuroblasts, the transcription factor Prospero (Pros) and its mRNA are GMC determinants that are asymmetrically localized to the GMC daughter. Pros protein then becomes nuclear and helps direct cell fate. In larval neuroblasts, a similar localization is observed. Pros is not detectable in interphase neuroblasts, when the cortical APC2 crescent is strongest. A small amount of Pros transiently localizes to an asymmetric crescent during mitosis. Pros is present at low levels in GMC nuclei and at higher levels in the nuclei of ganglion cells (Akong, 2002).

Mira is basally localized in embryonic neuroblasts, and required there for localization of Pros protein and mRNA. In central brain neuroblasts, Mira is diffusely cytoplasmic during interphase, when the APC2 crescent is the strongest. As cells enter mitosis, Mira first becomes cortical and then begins to accumulate asymmetrically on the side of the neuroblast where the daughter will be born. By metaphase, Mira asymmetry is very pronounced. The center of the Mira crescent is always precisely aligned with one spindle pole. As a result, in cells with the spindle pointing toward the center of the APC2 crescent, the Mira and APC2 crescents substantially overlap, while in cells in which the spindle points to the edge of the APC2 crescent, the two crescents are offset. Mira is partitioned into the GMC during anaphase, while APC2 relocalizes to the cleavage furrow. Mira could still be detected in some GMCs, which are thought to be those that were recently born (Akong, 2002).

In contrast to Mira and Pros, Inscuteable (Insc) and Bazooka (Baz) localize to the apical sides of embryonic neuroblasts, where they play essential roles in asymmetric divisions. Insc is asymmetrically localized in larval neuroblasts. Insc localizes to the side of the neuroblast opposite that of APC2 through much, if not all, of the cell cycle. Interestingly, there is a weak Insc crescent during interphase, that becomes stronger through prophase and metaphase. During anaphase, Insc localizes to the neuroblast cortex but not the GMC daughter. Baz localization was similar to that of Insc, though no cortical localization during interphase was detected. During prophase and metaphase, Baz localizes to a crescent opposite APC2, and as the chromosomes begin to separate, Baz localizes to a tight cap opposite the future GMC. Together, these data confirm that larval and embryonic neuroblasts asymmetrically localize many of the same proteins, and that APC2 localizes on the GMC side (basal) of the neuroblast, overlapping Mira and opposite Baz and Insc, which localize apically (Akong, 2002).

Arm also localizes asymmetrically in neuroblasts. Extending this, an examination was made of the localization of Arm's adherens junction partners DE-cadherin and ß-catenin. When central brain neuroblasts undergo a sequential series of asymmetric divisions, the GMCs remain associated with their neuroblast mother, resulting in a cap of GMCs in association with each neuroblast. APC2 localizes strongly to the boundary between the neuroblast and each GMC, and more weakly to the borders between the GMCs. APC2 is present at lower levels in ganglion cells and differentiating neurons (Akong, 2002).

The adherens junction proteins DE-cadherin, Arm, and ß-catenin all show a striking and asymmetric localization pattern in central brain neuroblasts. All precisely colocalize both at the boundary between neuroblasts and GMCs and at the boundaries between GMCs. DE-cadherin, Arm, and ß-catenin are also all expressed in epithelial cells of the outer proliferation center. The localization of DE-cadherin and the catenins is consistent with the idea that cadherin-catenin-based adhesion could help ensure that GMCs remain associated with each other, via association with their neuroblast mother (Akong, 2002).

To further explore this, how successive GMCs are positioned relative to their older GMC sisters was examined using two different approaches. First Mira was used to mark the newborn GMCs and DE-cadherin was used to mark the neuroblast and all of her GMC daughters. Mira localizes to a crescent on the side of the neuroblast where the daughter will be born (basal side), and then is segregated into the daughter. Mira persists for some time in newborn GMCs, and it remains detectable in the other GMCs as well, thus allowing the position of newborn GMCs to be examined relative to their older sisters. In many cases, new GMCs are clearly born at the edge of the cluster of older GMCs. This is particularly striking in neuroblasts with many progeny. It is worth noting that the cluster of daughters is three-dimensional, comprising a 'cap' of daughters in three dimensions rather than a two-dimensional line of daughters. It is thus suspected that new daughters are born near the edge of this cap (Akong, 2002).

These data suggest that neuroblasts and their GMC progeny remain closely associated. The GMCs then divide to form ganglion cells and ultimately neurons. The data further suggest that these latter cells may also remain associated and send their axons together toward targets in the central brain. When sections were made more deeply into the brain, below each cluster of neuroblasts and GMCs, structures that appear to be axons were detected projecting from these groups of cells. These axons label with Arm, DE-cadherin, and APC1. Arm also localizes to the axons of the neuropil, while DE-cadherin and APC2 are present at low levels or are absent from this structure (Akong, 2002).

Effects of Mutation or Deletion

During development of the Drosophila central nervous system, neuroblast 6-4 in the thoracic segment (NB6-4T) divides asymmetrically into a medially located glial precursor cell and a laterally located neuronal precursor cell. To understand the molecular basis for this glia-neuron cell-fate decision, the effects of some known mutations on the NB6-4T lineage were examined. prospero mutations lead to a loss of expression of Glial cells missing; this is essential to trigger glial differentiation in the NB6-4T lineage. In wild-type embryos, Pros protein is localized at the medial cell cortex of dividing NB6-4T and segregates to the nucleus of the glial precursor cell. miranda and inscuteable mutations alter the behavior of Pros, resulting in failure to correctly switch the glial and neuronal fates. These results suggest that NB6-4T uses the same molecular machinery in the asymmetric cell division as other neuroblasts in cell divisions producing ganglion mother cells. Furthermore, outside the NB6-4T lineage most glial cells appear independently of Pros (Akiyama-Oda, 2000).

In a null allele of pros no cells express Gcm or Repo in the NB6-4T lineage. In contrast, in a null allele of miranda all cells of the lineage express the glial proteins. The double mutant pros;mira produces no glial cells in the NB6-4T lineage, the same result obtained with the pros mutation. These results indicated that both pros and mira are involved in a pathway leading to the glia-neuron cell-fate switch, and that pros is epistatic to mira in this pathway. The effects of the insc mutation on the glia-neuron cell-fate switch in the NB6-4T lineage are slightly different from those of the pros or mira mutations. In insc mutants, both glial and non-glial cells are generated from NB6-4T in many of the hemisegments examined, but glial fate arises randomly from either of the daughter cells. These involvements of pros, mira and insc suggest an analogy between the first cell division of NB6-4T and NB cell divisions that produce GMCs and no glia. Pros, Mira and Insc proteins behave similarly during the first division of NB6-4T to the usual NB divisions producing GMCs. In the analyses of wild-type and mutant embryos, the high levels of expression of the earliest glial protein Gcm, and of the later glial protein Repo, are correlated with the nuclear localization of Pros in NB6-4T daughter cells. Consistent with this, in a pros mutant in which the mutant Pros protein does not enter the nucleus even after cell division, no glial cells are observed in the NB6-4T lineage. These observations suggest an important role for Pros in the onset of glial differentiation in the NB6-4T lineage (Akiyama-Oda, 2000).

Asymmetric cell divisions and segregation of fate determinants are crucial events in the generation of cell diversity. Fly neuroblasts, the precursors that self-reproduce and generate neurons, represent a clear example of asymmetrically dividing cells. Less is known about how neurons and glial cells are generated by multipotent precursors. Flies provide the ideal model system to study this process. Indeed, neuroglioblasts (NGBs) can be specifically identified and have been shown to require the gcm fate determinant to produce glial cells, which otherwise would become neurons. The division of a specific NGB (NGB6-4T), which produces a neuroblast (NB) and a glioblast (GB), has been followed. To generate the glioblast, gcm RNA becomes progressively unequally distributed during NGB division and preferentially segregates. Subsequently, a GB-specific factor is required to maintain gcm expression. Both processes are necessary for gliogenesis, showing that the glial vs. neuronal fate choice is a two-step process. This feature, together with gcm subcellular RNA distribution and the behavior of the NGB mitotic apparatus identify a novel type of division generating cell diversity (Ragone, 2001).

Pros transcription factor is necessary to maintain gcm expression and thereby activate the glial program in the glioblast. Indeed, in the absence of Pros, gcm RNA progressively disappears from the GB. gcm displays several differences with respect to pros with regard to RNA localization. (1) asymmetric distribution is not evident before metaphase; (2) asymmetry occurs progressively during cell division rather than being sharply apical at interphase and basal at metaphase; (3) gcm transcripts are present at the cortex and in the cytoplasm. These differences suggest the existence of different RNA localization pathways in asymmetrically dividing cells. That Stau and Mira may participate to the process is suggested by the mislocalization of gcm RNA in stau and mira mutants. In addition, the gcm 3'UTR displays a stem-loop secondary structure, a conformation that is necessary for the interaction of Staufen with Bicoid 3'UTR. However, this mechanism is not sufficient to ensure a correct fate choice. Moreover, mira and stau are not fully penetrant with respect to gcm RNA distribution. Finally, and more importantly, the cytoplasmic localization of some gcm transcripts as well as the kinetics of asymmetry calls for a cortical microfilament independent mechanism. Thus, the same RNA may be the target of two localization pathways: this complements the observation that the same RNA binding protein may localize transcripts using pathways with different cytoskeletal requirements (Ragone, 2001).

In many organisms, single neural stem cells can generate both neurons and glia. How are these different cell types produced from a common precursor? In Drosophila, glial cells missing is necessary and sufficient to induce glial development in the CNS. GCM mRNA has been reported to be asymmetrically localized to daughter cells during precursor cell division, allowing the daughter cell to produce glia while the precursor cell generates neurons. In this study, it has been shown that (1) GCM mRNA is uniformly distributed during precursor cell divisions; (2) the Prospero transcription factor is asymmetrically localized into the glial-producing daughter cell; (3) Prospero is required to upregulate gcm expression and induce glial development, and (4) mislocalization of Prospero to the precursor cell leads to ectopic gcm expression and the production of extra glia. A model for the separation of glia and neuron fates in mixed lineages is proposed in which the asymmetric localization of Prospero results in upregulation of gcm expression and initiation of glial development in only precursor daughter cells (Freeman, 2001).

If GCM mRNA and protein are equally distributed into NGB 6-4T and its first-born G daughter cell, how are GCM mRNA and protein levels upregulated in G but not NGB 6-4T? To address this issue, mitotic NGB 6-4T were assayed for proteins known to be asymmetrically localized along the apical-basal axis of neuroblasts. The goal was to identify candidate genes that could differentially regulate gcm expression in the NGB 6-4T lineage. Insc protein marks the apical side of most or all mitotic neuroblasts and is necessary and sufficient for apical-basal spindle orientation. In NGB 6-4T, Insc is localized as an apical crescent at all stages of mitosis and is partitioned into the apically-positioned NGB 6-4T following cytokinesis. The mitotic GB 6-4A also shows apical Insc localization. Because Insc is sufficient to orient the mitotic spindle in all neuroblasts and epithelial cells assayed, the apical localization of Insc in NGB 6-4T and GB 6-4A provides strong confirmation that both cells divide along their apical-basal axis (Freeman, 2001).

Miranda, Prospero, Staufen, and Numb proteins mark the basal side of many or all mitotic neuroblasts and regulate the fate of daughter cells or their neuronal progeny. In NGB 6-4T, Miranda, Prospero, Staufen and Numb all form basal crescents from metaphase through telophase, and are partitioned into the basally positioned G daughter cell of NGB 6-4T after cytokinesis. The mitotic GB 6-4A also shows basal localization of Miranda, Prospero, Staufen and Numb. These results further confirm the apical-basal division axis of NGB 6-4T and GB 6-4A during glial producing divisions, and show that all of the above proteins are candidates for regulating gcm expression in the basal G daughter cell of NGB 6-4T (Freeman, 2001).

To determine if miranda, prospero, staufen or numb are involved in the development of glia in the NGB 6-4T lineage, embryos mutant for each gene were scored for the number and position of mature glia derived from NGB 6-4T. The three glia from NGB 6-4T express repo and have distinctive positions within the CNS: two near the midline and one between NGB 6-4T and the midline. These are the only Repo-positive glia adjacent to the midline at the ventral surface of the CNS, and thus are easy to identify unambiguously. Mutations in staufen and numb have no effect on glial development in the NGB 6-4T lineage. By contrast, prospero mutant embryos show striking loss of NGB 6-4T-derived glia, while miranda mutant embryos have a similar but weaker phenotype. It is concluded that prospero and miranda, but not staufen or numb, are required for normal glial development in the NGB 6-4T lineage (Freeman, 2001).

prospero is clearly necessary for upregulation of gcm and glial cell fate induction in the 6-4T and 7-4 lineages, but is it sufficient to induce gcm expression in these lineages? In miranda mutant embryos, prospero mRNA and protein are delocalized during neural precursor cell division, resulting in similar concentrations of Prospero segregating to both NGBs and their daughter cells. Interestingly, in miranda mutants ectopic gcm expression is found in NGB6-4T at stage 13, a time when this NGB is normally making neuronal progeny. miranda mutants also show ectopic expression of gcm in NGB 7-4 lineage during its window of glial production. Thus, mislocalization of Prospero to the NGB by removal of miranda function is sufficient to induce ectopic gcm expression in these NGBs (Freeman, 2001).

Does the upregulation of gcm in NGBs drive the production of extra glial progeny? To address this question, Repo expression was assayed in the NGB6-4T lineage in miranda mutants because the entire NGB 6-4T lineage can be identified. In miranda mutants only four Repo positive cells are typically found in the entire NGB 6-4T lineage, but neuronal progeny are completely absent. This phenotype is interpreted to indicate that the G cell produces three glia as usual, but that its sibling NGB differentiates directly into a Repo-positive glia cell, resulting in a termination of the lineage. Thus, it appears that Prospero mislocalized to the NGB can potently activate gcm expression in the NGB and transform it into a glial cell (Freeman, 2001).

Two additional phenotypic classes in miranda mutants are found: (1) a variable number of Repo-positive glia are produced (between two and four) and subsequent neuronal progeny are generated normally (12% of hemisegments); or (2) the wild-type pattern of three Repo-positive glia and neuronal progeny are produced. These phenotypes indicate that low level Prospero in the NGB is not always sufficient to induce a glial fate, and that reduced Prospero in the G cell may lead to fewer glial progeny (Freeman, 2001).

Mislocalization of Prospero to NGB 7-4 by removal of miranda function can also induce Repo expression in this NGB (25% of hemisegments), showing that NGB 7-4 can also be partially transformed towards a glial fate. It is not known if this NGB differentiates as a glial cell (like the Pros-positive NGB 6-4T), generates extra glial progeny, or if it can eventually produce neurons. miranda, prospero double mutants do not show upregulation of gcm in NGBs 6-4T or 7-4, demonstrating that the upregulation of gcm in these NGBs in miranda mutant embryos is due to Prospero protein that is delocalized into the NGB. These results indicate that Miranda, by asymmetrically localizing Prospero to NGB daughter cells, restricts gcm upregulation and induction of the glial developmental program to the progeny of NGBs 6-4T and 7-4 during their phases of glial production (Freeman, 2001).

In miranda mutant embryos, Prospero protein is delocalized at mitosis, allowing NGB/daughter cell siblings to inherit equal concentrations of Prospero. In these embryos, ectopically upregulated gcm is frequently seen in NGBs 6-4T and 7-4, and extra glia are derived from NGB 6-4T. These data indicate that prospero is a potent activator of gcm expression in the NGB 6-4T and 7-4 lineages. The extra glia observed could come from an extension of the glial portion of the NGB 6-4T lineage, or from a transformation of this NGB into a purely glial progenitor. The latter model is favored, because neurons are never observed in the NGB 6-4T lineage when extra glia are observed. Moreover, high levels of Gcm are correlated with pure glial lineages such as GB 6-4A and the GP, and gcm is known to positively autoregulate which may commit precursors with high Gcm to a glial-producing fate (Freeman, 2001).

In miranda mutant embryos, the ectopic expression of gcm in NGB 6-4T and 7-4 is in fact due to delocalization of Prospero and not simply the absence of Miranda, because miranda; prospero double mutants fail to upregulate gcm in NGBs. In both the NGB 6-4T and 7-4 lineage, the delocalization of Prospero has relatively little effect on glial production by the daughter cells, presumably because there is sufficient Prospero protein in these daughter cells to upregulate gcm expression. Thus, with respect to glial cell fate induction, the asymmetric localization of Prospero may be more important for removing Prospero from the NGB than for enriching Prospero in the daughter cell (Freeman, 2001).

Induction of tumor growth by altered stem-cell asymmetric division in Drosophila melanogaster

Loss of cell polarity and cancer are tightly correlated, but proof for a causative relationship has remained elusive. In stem cells, loss of polarity and impairment of asymmetric cell division could alter cell fates and thereby render daughter cells unable to respond to the mechanisms that control proliferation. To test this hypothesis, Drosophila larval neuroblasts were generated containing mutations in various genes that control asymmetric cell division and then their proliferative potential was assayed after transplantation into adult hosts. It was found that larval brain tissue carrying neuroblasts with mutations in raps (also called pins), mira, numb or pros grew to more than 100 times their initial size, invading other tissues and killing the hosts in 2 weeks. These tumors became immortal and can be retransplanted into new hosts for years. Six weeks after the first implantation, genome instability and centrosome alterations, two traits of malignant carcinomas, appeared in these tumors. Increasing evidence suggests that some tumors may be of stem cell origin. These results show that loss of function of any of several genes that control the fate of a stem cell's daughters may result in hyperproliferation, triggering a chain of events that subverts cell homeostasis in a general sense and leads to cancer (Caussinus, 2005).

Malignant transformation and loss of cell polarity are tightly correlated in human carcinomas. Likewise, Drosophila larval tissues with mutations in dlg1, l(2)gl or scrib have impaired apicobasal polarity and neoplastic growth in the imaginal epithelia and nervous system. There are several hypotheses to explain how loss of polarity contributes to neoplastic transformation. Most of them involve models in which changes in cellular architecture impinge directly on the cell cycle either by inhibiting signals that restrain cell proliferation or by enhancing mitogenic pathways. An alternative hypothesis is that loss of polarity in stem cells that divide asymmetrically impairs the mechanisms that specify the fate of the resulting daughter cells. If these daughter cells are unable to follow their normal developmental program, they may not respond to the mechanisms that control proliferation in the wild-type lineage (Caussinus, 2005).

Drosophila neuroblasts are stem cells whose asymmetric cell-division machinery is fairly well characterized and thus provide a good model to test this hypothesis. In the embryo, Insc integrates into the apical cortex of two neuroblast protein complexes, Baz-DmPar6-aPKC and Gialpha-Raps, by associating with Baz and Raps. These two complexes mediate the basal localization of Mira and Pon and their interacting proteins, Pros and Numb, whose segregation into the ganglion mother cell (GMC) is required for the unequal fate of the two neuroblast daughter cells. The basal localization of Mira and Pros, as well as the spindle orientation and asymmetry of daughter-cell sizes, require the functions provided by dlg1, l(2)gl and scrib. Larval neuroblasts originate from quiescent embryonic neuroblasts, and their asymmetric division seems to be controlled by the same molecular complexes, although minor differences have been reported (Caussinus, 2005).

To assess the effect of disrupted stem-cell asymmetric division on cell proliferation, larval neuroblasts were generated with mutations in aPKC, raps, mira, pros or numb and their proliferation potential was assayed after transplantation into adult hosts. No substantial growth of 101 pieces of wild-type larval brains were observed 2 weeks after transplantation. Similar results were observed for 109 implants that carried homozygous aPKCk06403 clones, none of which grew to any noticeable extent. In contrast, pieces of brains from rapsP89/raps P62 larvae or from larvae carrying homozygous numb03235, miraZZ176 or pros 17 clones grew to more than 100 times their original size, severely damaging and displacing the host's organs in the abdomen. Of the 103 flies studied in detail, 92% had one or more small tumor colonies derived from the implanted tissue but located at a long distance from the point of injection. The efficiency of tumor development ranged from 8% for numb03235 clones to 20% for rapsP89/rapsP62 tissue (Caussinus, 2005).

To assess further the growth potential of these tumors, they were cut into pieces and reimplanted into new hosts. More than 90% of these flies developed a tumor, even when they were implanted with numb 03235 tissue that had initially developed tumors in only 8% of implanted adults. This result suggests that the growing tumor mass adapts itself very rapidly to its new environment. Pieces of brain lobes from 9- to 12-d-old homozygous brat k06028 and l(3)mbt ts1 larvae, in which overgrowth was already apparent, developed tumors in 91% and 58%, respectively, of the implanted hosts (Caussinus, 2005).

All the tumors described here have been maintained in the laboratory, some for more than 2 years. This shows that the transformed cells became immortal and can proliferate without end, in contrast to cells of wild-type imaginal discs implanted into adult hosts, which remain alive after years of culture but very rarely proliferate. Among the established cell lines, substantial differences were observed in speed of growth, host lifespan or frequency or average number of additional tumor colonies, that could be attributed to the mutant background from which the tumors originated. Using the same criteria, these tumors were indistinguishable from dlg1, l(2)gl and scrib neuroblastomas (Caussinus, 2005).

Attempts were made to determine the kinds of cells that could be found in these tumors. Using green fluorescent protein as a clonal marker, it was observed that in tumors derived from tissue carrying numb 03235, miraZZ176 or pros17 clones induced by mitotic recombination, neither the wild-type twin nor the heterozygous background cells were able to proliferate upon implantation and were lost within 2 weeks. These cells accounted for most of the implanted mass, and so their inability to hyperproliferate provided a valuable internal control to substantiate the conclusion that tumor growth in this assay required the loss of the genes under study and was not just the result of dissection and transplantation into adult hosts. It also showed that the tumor growth induced by the loss of function of these genes was cell-autonomous (Caussinus, 2005).

Immunofluorescence staining for cell-specific markers identified the neuroblasts as relatively large cells, 8-12 microm in diameter, that expressed Mira. In miraZZ176 tumors, neuroblasts were identified by the expression of Wor. Ganglion cells were identified as small cells, 4-6 microm in diameter, that did not express Mira but did express Pros or, in pros 17-derived tumors, Numb. The intermediately sized cells that did not express Pros, some of which showed weak Mira staining, might be GMCs. Neuroblasts accounted for most of the mitotic activity observed in these tumors (86%). Daughter-cell size and Mira segregation during mitosis were symmetric in neuroblasts derived from rapsP89/rapsP62 tumors but asymmetric in those derived from numb03235 and pros 17 tumors. Daughter-cell size was also asymmetric in neuroblasts from miraZZ176 tumors (Caussinus, 2005).

Neither neuroblasts nor ganglion cells were markedly diluted or over-represented as the tumors aged from host to host. Therefore, like l(2)gl and dlg1 tumors, the tumors derived from numb03235, miraZZ176, pros17 and raps P89/rapsP62 were neuroblastomas that resulted from the uncontrolled division of neuroblast stem cells and were largely composed of the undifferentiated cell types that belong to this lineage. The mechanism by which these tumors grew is not understood, but it must account for the observed continuous expansion of both the neuroblast and the ganglion cell populations. One plausible mechanism could be a low frequency of neuroblast divisions that generate two neuroblast daughters. Real-time analysis of cell proliferation in these tumors may provide an answer to this issue (Caussinus, 2005).

In most solid human tumors, malignancy is very often correlated with genome instability, which is thought to contribute to multistage carcinogenesis. As in most animal cells, the frequency of natural cases of genome instability in wild-type Drosophila neuroblasts and GMCs is low (less than 10-3). This is also the case in numb03235, miraZZ176, pros 17 and rapsP89/raps P62 tumors shortly after transplantation. In 40-d-old tumors, however, 10%-15% of the cells presented different kinds of karyotype defects. Of the 340 karyotypes obtained from numb, mira, pros and raps tumors, 62% included segmental aneuploid; 9% were monosomic, trisomic or tetrasomic with respect to one or more chromosomes; 6% were triploid or tetraploid; and the remaining 23% included cells that could not be karyotyped owing to very high levels of ploidy, chromosome fragmentation or chromosome condensation (Caussinus, 2005).

The karyotypes obtained from cells in a single tumor were as different from one another as they were from the karyotypes of cells from other tumors, and none of the tumor lines that were established presented a distinct set of chromosome aberrations. Therefore, no substantial differences were observed attributable to the mutant condition that originated the tumor. In most tumor lines, the frequency of cells that contained abnormal karyotypes did not change noticeably over time, with one exception: 3 months after the first implantation, genome instability affected more than 95% of the cells in mirTF, a tumor line derived from miraZZ176. The absence or very low incidence of genome instability during the first round of implantation suggests that genome instability did not cause tumor formation in these tumor lines. But the onset of genome instability correlates well with a marked increase in the frequency of hosts that developed a tumor in later transplantations. Therefore, the possible contribution of genome instability to the evolution of these tumors remains to be assessed. Genome instability has also been reported in l(2)gl neuroblastomas (Caussinus, 2005).

In mammalian carcinomas, genome instability is tightly correlated with severe alterations of the centrosome cycle that affect the number of centrosomes per cell as well as centrosome size and shape. Supernumerary centrosomes can result in multipolar spindles and contribute to the generation of aneuploidy. Like the DNA cycle, the centrosome cycle is tightly controlled in wild-type neuroblasts, so that cells that have an abnormal number of centrosomes are exceptionally rare in wild-type tissue. This was not the case in numb03235, mira ZZ176, pros17 or raps P89/rapsP62 tumors: forty days after the first implantation, 15%-20% of those cells had more than two centrosomes. Some of these centrosomes were irregularly shaped, and their size range was much wider than that of control cells. A fraction of these could be centriole-less aggregates of pericentriolar material. The cells that had supernumerary centrosomes seemed to be hyperploid (Caussinus, 2005).

None of the mutant conditions from which these tumors originated has been reported to affect chromosome segregation or the centrosome cycle, which were both unaffected in early tumors. In addition, the cells of wild-type imaginal discs that have been kept for years in adult hosts maintain a stable genome and can differentiate into adult structures. Therefore, genome instability and impaired centrosome cycles in numb 03235, miraZZ176, pros17 and rapsP89/rapsP62 tumors cannot be considered a consequence of the mutant background or long-term exposure to the adult abdomen environment. Rather, the onset of genome instability and centrosome alterations suggests that once the mechanisms that control cell proliferation have been over-ridden, hyperproliferation triggers a chain of events that subverts cell homeostasis in a very general sense, including the DNA and centrosome cycles (Caussinus, 2005).

In summary, neoplastic transformation of Drosophila larval neuroblasts can be triggered by perturbing several of the functions that mediate asymmetric stem-cell division. In terms of growth rate, cell types, metastatic activity and extent of genome and centrosome instability, the resulting tumors are essentially indistinguishable from one another, regardless of the mutant from which they derive. The main conclusion that can be drawn from these data is that these tumors might have a common etiology: perturbation of neuroblast polarity and the resulting impairment of cell-fate determination. This argument is strengthened by the case of the homeobox-containing transcription factor Pros, which lies downstream of the other genes required for neuroblast asymmetric division (Caussinus, 2005).

The tumors in this study are practically indistinguishable from the neuroblastomas that arise in adults implanted with pieces of dlg1, l(2)gl or scrib mutant larval brains. Because these three neoplastic tumor suppressors are required for multiple aspects of neuroblast asymmetric cell division, including the basal localization of Mira, Numb and Pros, mislocalization of these proteins might explain, at least partially, the uncontrolled cell proliferation produced by loss of dlg1, l(2)gl or scrib function in larval neuroblasts (Caussinus, 2005).

The unequal segregation of cell-fate determinants resulting from asymmetric cell division is a fundamental mechanism for generating cellular diversity during development, organ homeostasis and repair. If impaired segregation of cell-fate determinants can cause the hyperproliferation of larval neuroblasts of Drosophila, it may similarly affect tissue stem cells in other species. At the moment, however, any parallel to stem-cell models of human cancer remains purely speculative. Consistent with this hypothesis, the inactivation of both Numb and Numb-like in the mouse dorsal forebrain leads to impaired neuronal differentiation, hyperproliferation of neural progenitors and delayed cell-cycle exit. In addition, loss of Lgl1 (also called Mlgl or Hugl), one of the two L(2)gl homologs in the mouse, results in a failure to asymmetrically localize Numb and leads to severe brain dysplasia (Caussinus, 2005).

In most human tumors, the identity of the first carcinogenic cell remains elusive. Indirect but growing evidence suggests that in some cases, the founders may be stem cells. Stem cells are self-renewing, have limitless replicative potential and produce differentiating cells, three features found in many cancers. Carcinomas occur in tissues that are maintained by a continuous supply of differentiating daughter cells originating from stem-cell division. Moreover, some of the signaling pathways that control stem-cell self-renewal, like the Notch, Wnt-ß-catenin and Hedgehog pathways, are known to have a role in carcinogenesis in these tissues. The results show that inactivation of any of several molecular mechanisms that control the asymmetry of the segregation of cell-fate determinants during stem-cell division may result in hyperproliferation of the stem-cell compartment and could contribute to cancer (Caussinus, 2005).


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miranda: Biological Overview | Regulation | Developmental Biology | Effects of Mutation

date revised: 10 February 2010

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