BIOLOGICAL OVERVIEW

Asymmetric cell divisions are critically important to generate diverse cell types in the development of multicellular organisms. Polarized distribution of cytoplasmic components and the proper alignment of the mitotic apparatus are prerequisite for asymmetric divisions. Genetic analysis of Drosophila and C. elegans has led the way in unraveling the origins of cell polarity during embryonic development. The recently cloned Drosophila gene bazooka codes for a protein implicated in the formation of the cell-cell junction called the zonula adherins. This protein is a homolog of C. elegans Par-3 protein, which contributes to cell polarity and spindle alignment in early C. elegans embryos. As these data suggest, there may be a common basis for the establishment of cell polarity throughout the metazoa. What is the origin of apical basal polarity in the epidermis of Drosophila and how does this get converted to a mechanism that faithfully orients the mitotic spindle thus influencing the asymmetric distribution of cytoplasmic determinants during asymmetric cell division? Answering this question provides a key cornerstone to understanding the origins of cellular diversity during development. This review will first describe the role of the Par genes in regulating spindle orientation in C. elegans. Then it will turn to the role of Bazooka in the establishment of the adherins junction. Finally, it will present the evidence that Bazooka plays a role in spindle orientation.

The role of the Par genes in regulating spindle orientation

In the C. elegans embryo, a series of early asymmetric cleavages produce six founder cells with different cleavage patterns and cell fates. Anterior-posterior (AP) polarity is established during the first cell cycle and correlates with a dynamic rearrangement of cytoplasm along the AP axis, which is defined by an extrinsic cue provided by sperm. Microfilaments accumulate at the anterior periphery; central cytoplasm flows toward the posterior pole, and cortical cytoplasm flows in the opposite direction. The first mitotic spindle is placed posteriorly and the first cleavage becomes asymmetric, producing a large cell, termed AB, to form the anterior and a small cell, P1, to form the posterior of the embryo. In concert with the cytoplasmic rearrangement, P granules (germline-specific ribonucleoprotein particles) become localized to the posterior pole. Several other cytoplasmic factors that play roles in cell fate specification are distributed asymmetrically after the first cleavage. For example, SKN-1, which is a putative transcription factor and is required to specify the fate of the EMS blastomere, is present at a higher level in the P1 cell, and MEX-3, a putative RNA-binding protein, is expressed at a higher level in the AB cell (Tabuse, 1998 and references).

The establishment of AP polarity in C. elegans embryos is known to require the activities of the maternally expressed par genes. Interference of normal functions of these genes causes extensive polarity defects in the 1-cell embryo and results in loss of many early asymmetries. Of the six par genes so far identified, par-1, par-2, par-3, par-5 and par-6 (Drosophila homolog: par-6) mutant embryos exhibit a symmetrically placed first cleavage spindle and the equal-sized AB and P1 blastomeres. P granules fail to be distributed exclusively to P blastomeres in par-1, par-3, par-4, par-5 and par-6 embryos. Both AB and P1 spindles at the second cleavage of par-1, par-2, par-4 and par-5 embryos are aligned transversely, like the wild-type AB spindle, while they are aligned longitudinally in par-3 and par-6 embryos. Three par genes (par-1, par-2 and par-3) have been molecularly characterized. PAR-1 contains an amino-terminal serine/threonine kinase domain and a carboxy-terminal domain with binding activity to non-muscle myosin. PAR-2 is a protein containing a zinc-binding domain of the ring finger class and a myosin-type ATP-binding site. PAR-3 is a novel protein (Etemad-Moghadam, 1995) with three PDZ domains. Consistent with their role in polarity, the PAR proteins are themselves distributed in a polar fashion in the P lineage blastomeres. PAR-1 and PAR-2 localize to the posterior periphery of the 1-cell embryo and the P1 cell and localize to the ventral periphery of P2 and P3 blastomeres, but are absent from the periphery of all other blastomeres. In contrast, PAR-3 is present in a distribution reciprocal to that of PAR-1 and PAR-2; it is localized to the anterior periphery of 1-cell embryos and P1, localizes to the dorsal side of P2 and P3, and is present uniformly at the periphery of all other blastomeres (Tabuse, 1998 and references).

PAR-3 appears to play a central role in polarity establishment. In par-3 1-cell embryos, PAR-1 and PAR-2 are no longer restricted to the posterior pole although they remain peripheral (Etemad-Moghadam, 1995), and the par-3 spindle orientation defect is epistatic to those of the other pars. Proper distribution of PAR-3 itself, however, is dependent on the activities of par-2, par-5 and par-6. Insight into how asymmetric cell divisions are controlled can be gained by identifying the proteins with which the PAR proteins interact. Screens for proteins that can bind to the carboxy-terminus of PAR-1 have identified a non-muscle myosin that was subsequently shown by RNA-mediated depletion of the protein to be required for successful asymmetric divisions and for proper distribution of PAR-1, PAR-2 and PAR-3. Mammalian atypical protein kinase Cs (aPKCs), PKCzeta and PKClambda, associate specifically with a mouse protein similar to C. elegans PAR-3 (Yasushi Izumi. et al., unpublished data, 1998 cited in Tabuse, 1998). Although aPKCs are expressed ubiquitously in animals and have been implicated in mitogenic signal transduction in mammalian cells and the maturation of the Xenopus oocyte, their physiological function is totally unknown. An aPKC of C. elegans, termed PKC-3, is implicated in regulation of asymmetic cell division. PKC-3 mutants die displaying Par-3-like phenotypes, and PKC-3 can bind to PAR-3 in vitro and is co-localized with PAR-3 at the anterior cortex of the 1-cell embryo. Furthermore, the two proteins are mutually dependent for their proper localization: PKC-3 localization is disrupted in par-3 embryos and PAR-3 localization is disrupted in PKC-3-depleted embryos. Other par genes that regulate PAR-3 distribution are also required for PKC-3 localization. Thus, C. elegans PKC-3 plays an indispensable role in establishing embryonic polarity through interaction with PAR-3 (Tabuse, 1998 and references).

The exact functional relationship between PKC-3 and PAR-3 is not clear. Three possibilities are suggested. (1) PAR-3 could be acting to recruit PKC-3 to the cell periphery where it acts as a signaling molecule. (2) PKC-3, like the par-6 product, acts to recruit PAR-3 to the cell periphery or maintain it there throughout the cell cycle or both. It could do this by phosphorylating PAR-3 directly or by modifying the cortical cytoskeleton. The two possibilities are not mutually exclusive; PAR-3 could recruit PKC-3 to the cell periphery where its kinase activity has the dual effect of providing a signal leading to anterior/posterior differences as well as maintaining PAR-3 at the periphery. (3) PKC-3 and PAR-3, perhaps along with the product of the par-6 gene, might act together to form a functional complex whose stability or localization requires the presence of both. In any case, it is likely that the PAR-3/PKC-3 connection is evolutionarily conserved and serves to establish or maintain cell polarity in other species (Tabuse, 1998).

The role of Bazooka in the establishment of the adherins junction

The discussion now turns to Bazooka, a Drosophila PAR-3 homolog, and its role in formation of the circumferential adherens junctions termed zonula adherens (ZA) and maintenance of polarized blastoderm epithelium in Drosophila. Antibodies to Armadillo, an adherins junction component, and to phosphotyrosine (PY) epitopes were used as markers to demonstrate the formation of the ZA during embryogenesis and to analyze defects caused by bazooka mutants and a second mutation termed stardust in ZA formation. The mutations baz and sdt belong to a group in which mutant embryos show severe abnormalities in the differentiation of the larval cuticles, including the genes crumbs (crb) and shotgun (shg). During cellularization, the formation of cell membranes begins with the generation of cleavage furrows extending from the periphery of the embryo in a radial direction, thus establishing the normal boundaries between cells in the ectoderm. New plasma membranes are assembled until a monolayer cell sheet has formed that shares many features with epithelial cell monolayers, including epithelial cell functions and polarized membrane transport. A major redistribution of Arm and PY occurs during cellularization and early gastrulation. At mid-cellularization, staining becomes stronger on the apical aspect than in the more basal part of the lateral domain. By the end of cellularization, Arm and PY staining are strongly reduced at the basal part of the cleavage furrow that separates nuclei of cells in the blastoderm. bazooka and stardust have been identified as X-linked genes required for the formation of the ZA in the embryo. Mutations in either of these genes lead to a zygotic embryonic lethal phenotype, which is characterized by either severe malformation or an absence of the embryonic epidermis (Wieschaus, 1984 and Tepass, 1993).

Hemizygous baz single mutants show normal Arm and NT staining up to stage 10 of embryogenesis. Even at this rather late stage, the ZA is disordered only locally while large regions of the epidermis are still normal. Similarly, sdt single mutant embryos exhibit abnormal ZA morphology only late in development. The effects of the baz;sdt double mutant are much more severe. In postgastrula embryos, adherens junction plaques are strongly reduced and often absent; these plaques are normally present at the apical-lateral junction, indicating the presence of the ZA. The relatively weak zygotic phenotype of baz can be explained by a strong maternal component for the expression of baz. baz null embryos were produced to analyze the ZA phenotype of embryos that lack both maternal and zygotic baz activity. baz null embryos show a early disruption of ZA formation like that seen in baz;sdt zygotic double mutants. In particular, the concentration of Arm in the apical region of cell contact at the beginning of gastrulation is absent in both baz null mutants and the double mutants. In addition to these early alterations in ZA formation, two morphological features of both mutant phenotypes are very similar: (1) when cellularization is complete, cell shapes in the mutant embryos are aberrant; instead of the highly columnar cell shape in the epithelium of wild-type embryos, the cells have irregular outlines and some appear bottle-shaped; (2) the regular hexagonal pattern of Arm staining seen on the surface of wild-type embryos is distorted in the double mutants; the apical cell surfaces appear to vary in size, indicating that the apical domains are expanded in some cells and constricted in others. No ZAs are found in baz null mutant embryos. Germ-line null armadillo mutants exhibit a phenotype similar to baz;sdb mutants, showing that Armadillo is also required for ZA formation. It is suggested that early stages in the assembly of the ZA are critical for the assembly and/or stability of the polarized blastoderm epithelium (Muller, 1996).

Lack of zygotic baz function results in a loss of the coherent epidermal tissue structure. During germ-band expansion, when three post-blastodermal cell divisions take place and neuroblasts delaminate from the neurogenic ectoderm, only slight irregularities in the epithelium can be detected. Widespread defects become obvious from the beginning of germ-band retraction onward: cell shape is modified; cells lose their contacts, and the epidermis adopts a highly irregular appearance (Kuchinke, 1998).

The role of Bazooka in spindle orientation

It is a defect of spindle orientation in zygotic bazooka mutants that links Bazooka to C. elegans PAR-3 and PAR-3's role in establishing the plane of spindle orientation in C. elegans cell division. One manifestation of a polarized phenotype is the orientation of the mitotic spindle. In the developing trunk epidermis of wild-type Drosophila embryos, the mitotic spindle is oriented parallel to the surface, resulting in two cells that remain integrated in the epithelium after cytokinesis. In baz mutant embryos, the mitotic spindle in epidermal cells occasionally adopts an aberrant orientation, leading to an inner and an outer cell after completion of division. This occurrence of the spindle phenotype with low penetrance is consistent with only a mild epithelial phenotype in the epidermis at a stage when all postblastodermal divisions are completed. This suggests that the strong defects observed in the epidermis at later stages of development must be attributed to an additional function of baz that is required late for the maintenance of the epithelial tissue structure (Kuchinke, 1998).

In delaminating neuroblasts of wild-type embryos, one of the centrosomes migrates basally, resulting in a spindle that is oriented perpendicular to the surface. Since the transcription factor Prospero is localized in a basal crescent in the cortical cytoplasm of the neuroblast during methaphase, only the basally located GMC will receive this protein. After cytokinesis, Pros is rapidly translocated into the nucleus of the GMC. In baz mutant embryos, the orientation of the mitotic spindle frequently deviates from the apico-basal axis. During division, Pros remains localized in a cortical crescent, which is, however, not always strictly basally positioned. Following division, Pros is correctly translocated into the nuclei of the smaller GMCs after cytokinesis but, in contrast to wild type, GMCs are often found localized in lateral positions relative to the neuroblasts instead of basal positions (Kuchinke, 1998). Defects in spindle orientation in baz mutants may be contrasted with those in inscuteable, another gene whose protein product is asymetrically localized. In insc mutants, spindle orientation and Pros localization are randomized and no longer coordinated; consequently, many GMCs do not receive Pros protein and, hence, fail to develop properly. This suggests a function of insc in the coordination of additional and/or different aspects of cellular polarity, such as spindle orientation and localization of cytoplasmic determinants (Kuchinke, 1998 and references).

Strikingly, spindle orientation and localization of GMCs are not completely randomized. In most cases, the GMCs can be found lying within a basal quadrant of the neuroblast, whereas hardly any GMCs can be found apical to the neuroblast. The phenotype clearly shows that neuroblasts of baz mutant embryos still develop an apico-basal polarity, which is manifested by the coordinated regulation of spindle orientation and Pros localization. Yet, many neuroblasts have lost the ability to orient their axis of polarity correctly with respect to the axis of the embryo. The fact that many neuroblasts with defective spindle orientation are lying below a phenotypically organized epithelium makes it unlikely that the misorientation of the apico-basal axis of neuroblasts is a consequence of defects in polarity of the overlying epithelium. Therefore, neuroblasts lacking baz still have an intrinsic apico-basal polarity (lost in insc mutants) but they fail to orient their axis of polarity with respect to the axis of the embryo, resulting in a misorientation of the spindle and, hence, a mispositioning of the GMC. It is proposed that Baz is a likely candidate for organizing a multiprotein complex by means of its recruitment of multiple cytoplasmic, cytoskeletal or integral membrane proteins to the apical pole of the cell (Kuchinke, 1998).

Cdc42 acts downstream of Bazooka to regulate neuroblast polarity through Par-6 aPKC

Cdc42 recruits Par-6-aPKC to establish cell polarity from worms to mammals. Although Cdc42 is reported to have no function in Drosophila neuroblasts, a model for cell polarity and asymmetric cell division, this study shows that Cdc42 colocalizes with Par-6-aPKC at the apical cortex in a Bazooka-dependent manner, and is required for Par-6-aPKC localization. Loss of Cdc42 disrupts neuroblast polarity: cdc42 mutant neuroblasts have cytoplasmic Par-6-aPKC, and this phenotype is mimicked by neuroblast-specific expression of a dominant-negative Cdc42 protein or a Par-6 protein that lacks Cdc42-binding ability. Conversely, expression of constitutively active Cdc42 leads to ectopic Par-6-aPKC localization and corresponding cell polarity defects. Bazooka remains apically enriched in cdc42 mutants. Robust Cdc42 localization requires Par-6, indicating the presence of feedback in this pathway. In addition to regulating Par-6-aPKC localization, Cdc42 increases aPKC activity by relieving Par-6 inhibition. It is concluded that Cdc42 regulates aPKC localization and activity downstream of Bazooka, thereby directing neuroblast cell polarity and asymmetric cell division (Atwood, 2007).

Little is currently known about how the Par complex is localized or regulated in Drosophila neuroblasts, despite the importance of this complex for neuroblast polarity, asymmetric cell division and progenitor self-renewal. This study shows that Cdc42 plays an essential role in regulating neuroblast cell polarity and asymmetric cell division. Baz localizes Cdc42 to the apical cortex where it recruits Par-6-aPKC, leading to polarization of cortical kinase activity that is essential for directing neuroblast cell polarity, asymmetric cell division, and sibling cell fate (Atwood, 2007).

Asymmetric aPKC kinase activity is essential for the restriction of components such as Mira and Numb to the basal cortex. The aPKC substrates Lgl and Numb are thought to establish basal polarity either by antagonizing activity of myosin II or by direct displacement from the cortex. This study found that Cdc42 recruits Par-6-aPKC to the apical cortex and that Cdc42 relieves Par-6 inhibition of aPKC kinase activity. In the absence of Cdc42, aPKC is delocalized and has reduced activity, resulting in uniform cortical Mira. Expression of Cdc42-DN leads to cortical overlap of inactive Par-6-aPKC and Mira indicating the importance of Cdc42-dependent activation of aPKC kinase activity. Expression of Cdc42-CA leads to cortical aPKC that displaces Mira from the cortex, presumably because Lgl is phosphorylated at the entire cell cortex. This is similar to what is seen when a membrane-targeted aPKC is expressed (Atwood, 2007).

Baz, Par-6 and aPKC have been considered to be part of a single complex (the Par complex). This study found that, when Cdc42 function is perturbed, Par-6 and aPKC localization is disrupted but Baz is unaffected. Why is Baz unable to recruit Par-6-aPKC in the absence of Cdc42? One explanation is that Cdc42 modulates the Par-6-Baz interaction, although Cdc42 has no direct effect on Par-6-Baz affinity. Alternatively, Baz might only be transiently associated with the Par-6-aPKC complex (e.g. as an enzyme-substrate complex); this is consistent with the observation that Baz does not colocalize with Par-6-aPKC in Drosophila embryonic epithelia and its localization is not dependent on either protein. How does Baz recruit Cdc42 to the apical cortex? Like other Rho GTPases, Cdc42 is lipid modified (prenylated), which is sufficient for cortical localization. Baz is known to bind GDP-exchange factors (GEFs), which may induce accumulation of activated Cdc42 at the apical cortex (Atwood, 2007).

The requirement of Par-6 for robust Cdc42 apical enrichment suggests that positive feedback exists in this pathway, a signaling pathway property that is also found in polarized neutrophils. More work is required to test the role of feedback in neuroblast polarity but one attractive model is that Baz establishes an initial polarity landmark at the apical cortex in response to external cues, which leads to localized Par-6-aPKC activity through Cdc42. Phosphorylation of Baz by aPKC might further increase asymmetric Cdc42 activation, perhaps by increased GEF association, thereby reinforcing cell polarity. Such a mechanism could generate the robust polarity observed in neuroblasts and might explain why expression of dominant Cdc42 mutants late in embryogenesis does not lead to significant defects in polarity (Atwood, 2007).

This study argues that Cdc42 functions downstream of Baz. Cdc42 is required for Baz-Par-6-aPKC localization in C. elegans embryos and mammalian neural progenitors. In C. elegans embryos, RNA interference of cdc42 disrupts Par-6 localization, whereas PAR-3 localization is slightly perturbed. In this case, Cdc42 is required for the maintenance but not establishment of PAR-3-Par-6 asymmetry; however, other proteins have been shown to localize Par complex members independently of Cdc42. Conditional deletion of cdc42 in the mouse brain causes significant Par-3 localization defects, although this may be caused by the loss of adherens junctions. More work will be required in these systems to determine if the pathway that has been proposed is conserved (Atwood, 2007).

This study has identified at least two functions of Cdc42 in neuroblasts: first, to recruit Par-6-aPKC to the apical cortex by direct interaction with its CRIB domain and, second, to promote aPKC activity by relieving Par-6 repression. aPKC activity is required to partition Mira and associated differentiation factors into the basal GMC; this ensures maintenance of the apical neuroblast fate as well as the generation of differentiated neurons. Polarized Cdc42 activity may also have a third independent function in promoting physically asymmetric cell division, because uniform cortical localization of active Cdc42 leads to same-size sibling cells. Loss of active Cdc42 at the cortex by overexpression of Cdc42-DN still results in asymmetric cell division, suggesting that other factors also regulate cell-size asymmetry, such as Lgl and Pins. In conclusion, these data show that Cdc42 is essential for the establishment of neuroblast cell polarity and asymmetric cell division, and defines its role in recruiting and regulating Par-6-aPKC function. These findings now allow Drosophila neuroblasts to be used as a model system for investigating the regulation and function of Cdc42 in cell polarity, asymmetric cell division and neural stem cell self-renewal (Atwood, 2007).

GENE STRUCTURE

PROTEIN STRUCTURE

Amino Acids - 1464

Structural Domains

Analysis of Baz sequence reveals no obvious hydrophobic regions, suggesting a localization of the protein in the cytoplasm. In the center of the protein, three repeated regions exhibit pronounced similarity to the PDZ motif. The PDZ motif is a globular domain of 80-110 amino-acid residues, which can be present once or several times within a protein and provides an interface for protein-protein interactions. This domain has been identified in a number of intracellular proteins, many of which are associated with the membrane or concentrated at sites of cell-cell contact, such as tight junctions, septate junctions (see Discs large and polychaetoid) or postsynaptic densities. Others, for example the Drosophila protein InaD, have been shown to control the assembly of a multiprotein signaling complex, mediated by specific interactions performed by individual PDZ domains (Kuchinke, 1998 and references).

The Drosophila Bazooka protein exhibits an overall similarity to the PDZ protein Par-3 of C. elegans. Similar to Baz, Par-3 also contains three PDZ domains. Strikingly, the third PDZ domain (PDZ3) of Bazooka is more similar to PDZ3 of Par-3 than to Bazooka PDZ2 or PDZ1. In both proteins, the PDZ1 domains are less similar to the consensus PDZ motif. They lack, for example, the characteristic Gly-Leu-Gly-Phe repeat, which occurs in PDZ2 of Baz and, slightly modified, in PDZ2 of Par-3 and PDZ3 of both proteins. The similarity between the two proteins extends beyond the three PDZ domains and includes two additional regions in the amino and carboxyl termini, which exhibit an amino acid identity of 41% and 39% in a region of 80 and 25 amino acids, respectively. In addition, one mouse and one human expressed sequence tag possess similarity to the amino-terminal region of the Baz protein. Since the remaining sequences of these proteins are not yet known, however, it is uncertain whether these represent homologous proteins (Kuchinke, 1998).

date revised: 16 January 2008

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