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Polarity in Cell Division

2000; Cell Press; Volume: 100; Issue: 6 Linguagem: Inglês

10.1016/s0092-8674(00)80695-9

1097-4172

Yuh Nung Jan, Lily Yeh Jan,

Developmental Biology and Gene Regulation

Asymmetric cell division is a basic means for generating cell fate diversity. In both unicellular and multicellular organisms, asymmetric cell division often results from preferential segregation of cell fate determinant(s) into one of the two daughter cells. Among eukaryotes, several biological processes have emerged as useful model systems for studying the mechanisms of asymmetric cell division, including the budding of the yeast Saccharomyces cerevisiae, the asymmetric division of the neural precursors of the fruit fly Drosophila melanogaster, and the asymmetric cleavages during early embryogenesis of the worm Caenorhabditis elegans. Studies of those model systems have revealed a number of general features of asymmetric cell division: a polarity cue is first set up in the mother cell, the cell fate determinants (either mRNAs or proteins) are then localized with respect to this cue, this localization requires actin filaments and certain myosin motors, and the axis of division is lined up with respect to the same cue so that the localized determinants are partitioned preferentially into one of the two progeny cells. Additional features exist in multicellular organisms. For example, in the Drosophila nervous system, the orientation of the neural precursor divisions is coordinated with the overall tissue polarity. Moreover, at least two types of polarity, apical-basal polarity and planar polarity, are used to orient the division of different neural precursors. In the last two years, significant progress has been made in unraveling the control of asymmetric cell divisions by polarity cues, and that is the focus of this minireview. In the Drosophila nervous system, the majority of neurons are derived from fixed lineage via multiple rounds of asymmetric cell division of the neural precursors. The central nervous system (CNS) neural precursors are named neuroblasts (NBs). Shortly after formation of the cellular blastoderm, the NBs of the embryonic ventral nerve cord delaminate basally from this epithelium into the interior of the embryo. Each NB divides asymmetrically in a stem cell fashion to produce an apical daughter that remains a NB and a smaller basal daughter called a ganglion mother cell (GMC) that divides again to give rise to two neurons. Two cell fate determinants, the Numb and Prospero proteins, become localized to the basal cortex of the NB as it enters mitosis. As the NB divides, its spindle is lined up along the apical-basal axis, so after division the determinants are segregated predominantly into the basal daughter, i.e., the GMC (Figure 1). Numb is a membrane-associated protein containing a protein–protein interaction domain known as the PTB domain. Numb functions in many binary cell fate decisions at least in part by antagonizing Notch activity to make two daughter cells different from each other, although its function in GMC is not clear. Prospero is a homeodomain-containing protein that controls cell fate by functioning as a transcriptional regulator (reviewed by 12Lu B Jan L Jan Y.-N Annu. Rev. Neurosci. 2000; 23: 531-556Crossref PubMed Scopus (116) Google Scholar). The peripheral nervous system (PNS) neural precursors are called sensory organ precursors (SOPs). The body of an adult fruit fly is covered with sensory bristles, each one derived from a single SOP. The back of the thorax, called the notum, is derived from the wing imaginal disc, a monolayer of epithelial cells. During pupal development, a subset of these epithelial cells are singled out to become SOPs. Each SOP divides asymmetrically into two secondary precursors, IIa and IIb, which then divide asymmetrically once and twice, respectively, to produce a total of five progeny. One migrates away as a glial cell (5Gho M Bellaiche Y Schweisguth F Development. 1999; 126: 3573-3584PubMed Google Scholar). The remaining four cells, a neuron, a sheath cell, a socket cell, and a hair cell, form the sensory bristle. The asymmetric division of the SOP shares many features with NB division. The SOP division also produces two daughters of different size. Some of the same cell fate determinants such as Numb and their adapter proteins, which serve to localize the cell fate determinants, are employed. Those molecules form a crescent at one pole of the SOP as it enters mitosis. As the SOP divides, the spindle lines up with the crescent so that the determinants are partitioned into the smaller daughter cell, the IIb cell (Figure 1). There are, however, important differences between SOP division and NB division. Whereas the NB delaminates from the epithelium, the SOP stays within the epithelium. Although the SOP is derived from epithelial cells that have apical-basal polarity, it uses a different type of polarity, called planar polarity or tissue polarity, which is perpendicular to apical-basal polarity, to orient its division (Figure 1). Consequently, different sets of molecules are required to control the orientation of the asymmetric divisions of the NB and the SOP. Studies over the last few years have revealed a genetic pathway for the control of asymmetric division of embryonic NBs (reviewed by 12Lu B Jan L Jan Y.-N Annu. Rev. Neurosci. 2000; 23: 531-556Crossref PubMed Scopus (116) Google Scholar). This genetic pathway is still quite incomplete and is surely an oversimplification (Figure 2A). In this pathway, Inscuteable is an important organizing molecule (8Kraut R Chia W Jan L.Y Jan Y.-N Knoblich J.A Nature. 1996; 383: 50-55Crossref PubMed Scopus (313) Google Scholar). In inscuteable loss-of-function mutant, Numb and Prospero become mislocalized. Inscuteable controls the localization of Numb, Prospero, and prospero mRNA indirectly by controlling the asymmetric localization of three adapter proteins: Partner of Numb (PON), Miranda, and Staufen. Inscuteable also controls the orientation of NB division. In the mitotic NB, the spindle is initially in the same orientation as those of the neighboring mitotic epithelial cells, i.e., parallel to the epithelial plane. At metaphase, however, the spindle undergoes a 90 degree rotation to line up along the apical-basal axis (7Kaltschmidt J.A Davidson C.M Brown N.H Brand A.H Nat. Cell Biol. 2000; 2: 7-12Crossref PubMed Scopus (243) Google Scholar). In inscuteable loss-of-function mutants, this rotation fails to occur. Conversely, misexpression of Inscuteable to the epithelial cells where it is normally not expressed causes a 90 degree rotation of the spindles (8Kraut R Chia W Jan L.Y Jan Y.-N Knoblich J.A Nature. 1996; 383: 50-55Crossref PubMed Scopus (313) Google Scholar). Inscuteable first becomes detectable at the apical stalk of the delaminating NB. After the NB rounds up and loses this apical stalk, Inscuteable forms a crescent on the apical cortex of the NB (Figure 3), which persists until anaphase of the NB division. The expression pattern of Inscuteable and the inscuteable mutant phenotype led to the suggestion that the NB uses Inscuteable as a mark to maintain the apical-basal polarity cue that it has inherited from the epithelium (8Kraut R Chia W Jan L.Y Jan Y.-N Knoblich J.A Nature. 1996; 383: 50-55Crossref PubMed Scopus (313) Google Scholar). What might link NB polarity, manifested as the apical Inscuteable crescent, with the apical-basal polarity of the epithelium? Studies of the gene bazooka have provided a missing link.Figure 3Expression Pattern of Bazooka, Inscuteable, and Pins in Polarized Epithelium, Delaminating NB, and Metaphase NBShow full captionThis figure is modified from Schober et al. 1999 by incorporating the Pins expression data from Yu et al. 2000. Apical is at the top.View Large Image Figure ViewerDownload Hi-res image Download (PPT) This figure is modified from Schober et al. 1999 by incorporating the Pins expression data from Yu et al. 2000. Apical is at the top. Loss-of-function bazooka mutants exhibit holes in the embryonic cuticle, a phenotype shared with mutants of several other genes: shotgun, crumbs, and stardust. Thus, this group of genes is essential for the integrity and/or polarity of the epithelium. When E. Knust's group cloned bazooka (9Kuchinke U Grawe F Knust E Curr. Biol. 1998; 8: 1357-1365Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar), they discovered that it is the fly homolog of the C. elegans par-3 gene, which is involved in the control of asymmetric cell division during early embryogenesis (3Etemad-Moghadam B Guo S Kemphues K.J Cell. 1995; 83: 743-752Abstract Full Text PDF PubMed Scopus (340) Google Scholar). Bazooka, like Par-3, contains three PDZ domains, which are likely to mediate protein–protein interactions. Bazooka localizes to the apical cortex of the epithelial cells and forms an apical crescent in mitotic NB. Those observations suggested that Bazooka links Inscuteable with the epithelial apical-basal polarity. Two recent papers demonstrated that this is indeed the case (14Schober M Schaefer M Knoblich J Nature. 1999; 402: 548-551Crossref PubMed Scopus (295) Google Scholar, 18Wodarz A Ramrath A Kuchinke U Knust E Nature. 1999; 402: 544-547Crossref PubMed Scopus (355) Google Scholar). Both groups removed zygotic and maternal Bazooka activities, either with germline clones or by using RNAi, and showed that embryos lacking both maternal and zygotic Bazooka exhibit much more severe phenotype than zygotic mutants. The NBs of those embryos lost much of their polarity. In the delaminating NBs, Inscuteable no longer forms an apical crescent and instead becomes delocalized in the cytoplasm, and the spindle orientation of the mitotic NBs is randomized. At metaphase, Numb, Prospero, and Miranda are localized all around the cortex of the NB instead of forming a basal crescent. How might Bazooka function? It is expressed in epithelial cells before NB delamination. Bazooka and Inscuteable colocalize in the apical stalk of the delaminating NB. After delamination, both proteins form a crescent on the apical cortex of the NB (Figure 3), and both disappear from the mitotic NB at anaphase. Bazooka and Inscuteable can be coimmunoprecipitated from embryonic extracts, suggesting that they form a complex in vivo. Moreover, Bazooka can bind directly to the Inscuteable asymmetric localization domain (14Schober M Schaefer M Knoblich J Nature. 1999; 402: 548-551Crossref PubMed Scopus (295) Google Scholar). When inscuteable or bazooka is transfected singly into cultured S2 cells, Inscuteable is diffusely localized to the cytoplasm and Bazooka is localized to the cell cortex. Cotransfection of inscuteable and bazooka into the same cells causes both proteins to colocalize at the cortex, suggesting that Bazooka can recruit Inscuteable to the cortex (18Wodarz A Ramrath A Kuchinke U Knust E Nature. 1999; 402: 544-547Crossref PubMed Scopus (355) Google Scholar). Together, these experiments strongly support the idea that Bazooka relays the initial apical-basal polarity cue from the epithelium to the NB by serving as an anchor for a multiprotein complex, containing Inscuteable, that forms at the apical cortex of the NB. In inscuteable mutants, Bazooka still forms a crescent but its protein level is reduced, suggesting that Inscuteable is not required for Bazooka localization but is required for Bazooka stability in the NB. Those two papers also revealed some previously unappreciated aspects of asymmetric divisions of the NBs. In embryos lacking Bazooka activity, Inscuteable becomes ubiquitously distributed in the NB, and Numb, Prospero, and Miranda are localized all around the cortex of the metaphase NB. By anaphase and telophase, however, Numb, Prospero, and Miranda are often asymmetrically segregated into the GMC. A similar phenotype is also observed in inscuteable mutants. These results suggest that there is a mechanism to asymmetrically localize Numb, Prospero, and Miranda during anaphase and telophase that is independent of Bazooka and Inscuteable. The nature of this mechanism is at present unknown. In a recent Cell paper, 19Yu F Morin X Cai Y Yang X Chia W Cell. 2000; 100: 399-409Abstract Full Text Full Text PDF PubMed Scopus (283) Google Scholar reported another important new component required for the asymmetric localization of Inscuteable. They used the central domain of Inscuteable, which is necessary and sufficient for its asymmetric localization as well as its ability to control spindle orientation, as bait in a yeast two-hybrid screen to identify a new gene they named partner of inscuteable (pins). The same gene was identified independently by coimmunoprecipitation of its gene product with Inscuteable (13Schaefer M Shevchenko A Shevchenko A Knoblich J Curr. Biol. 2000; in pressGoogle Scholar). As in the case of bazooka, the true loss-of-function pins mutant phenotype is revealed only when both maternal and zygotic pins activities are removed. In those mutant embryos, Inscuteable is initially localized correctly to the apical stalk of a delaminating NB but becomes ubiquitously distributed in the NB after delamination. Not surprisingly, pins mutants exhibit a phenotype similar to the inscuteable mutant phenotype, i.e., the spindle orientation becomes random, and Pon, Numb, Miranda, and Prospero are mislocalized. In inscuteable mutants, Pins becomes diffusely localized around the cortex of NBs. Thus, asymmetric localization of Pins and Inscuteable are mutually dependent. pins is predicted to encode a protein with 7 Tetratricopeptide (TRP) repeats, a protein–protein interaction motif, and three GoLoco domains that are thought to bind and regulate Gα, the α subunit of heterotrimeric G protein. There is a putative human homolog of unknown function, LGN. The expression of Pins in the NB lags behind that of Inscuteable (Figure 3) and is barely detectable in the delaminating NB. After NB delamination, Pins colocalizes with Inscuteable and forms a strong apical crescent. This expression pattern is consistent with the suggestion that Pins is required not for the initiation but for the maintenance of Inscuteable crescent. Pins and Inscuteable can be coimmunoprecipitated and thus are likely to form a complex in vivo. Moreover, Pins can bind Inscuteable directly; this binding occurs between the N terminus of Pins, which contains the TPR repeats, and the Inscuteable asymmetric localization domain (13Schaefer M Shevchenko A Shevchenko A Knoblich J Curr. Biol. 2000; in pressGoogle Scholar, 19Yu F Morin X Cai Y Yang X Chia W Cell. 2000; 100: 399-409Abstract Full Text Full Text PDF PubMed Scopus (283) Google Scholar). The emerging picture is that an apical complex forms in delaminating NBs to link NB polarity with epithelial apical polarity. This process involves at least two steps, an initiation step that requires Bazooka to recruit Inscuteable to the apical complex and a later maintenance step that requires Inscuteable to recruit Pins to the apical cortex. In this maintenance step, the localization of Bazooka, Inscuteable, and Pins are interdependent. In pins mutant embryos, the localization and/or stability of Bazooka and Inscuteable are affected. This apical complex is likely to contain additional components since the asymmetric localization of Par-3, the C. elegans homolog of Bazooka, is known to require the interaction of two other proteins: Par-6, another PDZ domain containing protein, and Pkc-3, a member of the protein kinase C family (6Hung T.-J Kemphues K.J Development. 1999; 126: 127-135PubMed Google Scholar, 16Tabuse Y Izumi Y Piano F Kemphues K.J Miwa J Ohno S Development. 1998; 125: 3607-3614Crossref PubMed Google Scholar). Homologs of both Par-6 and Pkc-3 exist in Drosophila (6Hung T.-J Kemphues K.J Development. 1999; 126: 127-135PubMed Google Scholar, 18Wodarz A Ramrath A Kuchinke U Knust E Nature. 1999; 402: 544-547Crossref PubMed Scopus (355) Google Scholar) and could be part of the apical complex. In addition, 13Schaefer M Shevchenko A Shevchenko A Knoblich J Curr. Biol. 2000; in pressGoogle Scholar found that Gα may also be part of the apical complex, raising the interesting possibility that a heterotrimeric G protein signaling cascade is involved in controlling asymmetric cell division. It will also be interesting to determine the exact relationship of this apical complex with the other genes that affect the epithelial integrity/polarity, such as crumb, stardust, discs lost, and scribble (1Bhat M.A Izaddoost S Lu Y Cho K.-O Choi K.-W Bellen H.J Cell. 1999; 96: 833-845Abstract Full Text Full Text PDF PubMed Scopus (231) Google Scholar, 2Bilder D Perrimon N Nature. 2000; 403: 676-680Crossref PubMed Scopus (532) Google Scholar). Developing SOPs express Inscuteable and presumably retain the epithelial apical-basal polarity cue. Surprisingly, the orientation of the SOP's division into IIa and IIb does not depend on Inscuteable but instead is controlled by planar polarity within the plane of the epithelium, orthogonal to the apical-basal axis. In the adult notum, planar polarity lies along the anterior-posterior axis and is manifested by all the sensory bristles' pointing posteriorly. 4Gho M Schweisguth F Nature. 1998; 393: 178-181Crossref PubMed Scopus (181) Google Scholar discovered that the SOP divides along the same anterior-posterior axis, with the Numb crescent always pointing toward the anterior, with the result that the anterior daughter IIb inherits the bulk of Numb protein. Moreover, they found that the orientation of SOP division is under the control of frizzled and dishevelled, members of the core group of planar polarity genes that control the polarity of various tissues such as the eye, the wing, and the notum (15Shulman J.M Perrimon N Axelrod J.D Trends Genet. 1998; 14: 452-458Abstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar). In frizzled or dishevelled loss-of-function mutants, the orientation of the Numb (and Pon) crescent and spindle in the SOP become randomized. Recently, an important new member of the core group of planar polarity gene flamingo, was cloned (17Usui T Shima Y Shimada Y Hirano S Burgees R.W Schwarz T.L Takeichi M Uemura T Cell. 1999; 98: 585-595Abstract Full Text Full Text PDF PubMed Scopus (551) Google Scholar). Flamingo is a seven-pass transmembrane cadherin that functions downstream from Frizzled and Dishevelled in controlling planar polarity in various tissues. Flamingo is also required to orient SOP divisions (11Lu B Usui T Uemura T Jan L Jan Y.-N Curr. Biol. 1999; 9 (b): 1247-1250Abstract Full Text Full Text PDF PubMed Google Scholar). In flamingo loss-of-function mutants, just as in frizzled and dishevelled mutants, the orientation of both the Numb crescent and the spindle become randomized. However, in all three mutants, the orientation of the Numb crescent and the spindle remain coupled. This suggests the existence of a yet unidentified molecule that carries out a function in the SOP analogous to that of Inscuteable in the NBs, i.e., to coordinate spindle orientation and the asymmetric localization of determinants (Figure 2B). Despite the considerable recent progress, many basic questions about the control of asymmetric division by polarity cues remain to be answered. For example: (1) What polarity cues are used to direct the successive asymmetric cell divisions within the NB lineage and the SOP lineage? In the case of SOPs in the notum, each division has a stereotyped orientation (5Gho M Bellaiche Y Schweisguth F Development. 1999; 126: 3573-3584PubMed Google Scholar). Of those divisions, we know the polarity cue only for the first division, i.e., that planar polarity controls the division of SOP into a posterior IIa and an anterior IIb. What polarity cues are used to control the subsequent divisions? Is apical-basal polarity used to control the IIb division that gives rise to an apical and a basal daughter? What about the division that is at an oblique angle with respect to both apical-basal axis and the epithelial plane? How do the secondary and tertiary precursors keep track of the polarity information they need to orient their division? In situations in which the precursor seems to face more than one polarity cue, e.g., the SOP expresses both Inscuteable and Flamingo, how does it choose to use one of them? Is the unused cue reserved to be used in later divisions? Similar questions apply to the NB lineage. The first division of the NB into another NB and a GMC is controlled by apical-basal polarity, but what about later rounds of NB division given that the Bazooka and Inscuteable crescent disappears after anaphase of the first NB division? (2) What links the polarity cue to spindle orientation? (3) How does the polarity cue at one pole of the cell control the movement of the determinants and adapter molecules so that they form a crescent at the opposite pole? At present, this is probably the largest gap in our knowledge (for a discussion, see 10Lu B Ackerman L Jan L.Y Jan Y.-N Mol. Cell. 1999; 4 (a): 883-891Abstract Full Text Full Text PDF PubMed Scopus (77) Google Scholar). (4) How do the mechanisms for asymmetric cell division used in different organisms compare? Most of the molecules involved in the asymmetric cell divisions in the Drosophila nervous system have homologs in vertebrates and C. elegans (Figure 2). Do they serve similar functions? It will be interesting to compare, for instance Drosophila embryonic neural precursor divisions with the first few cell divisions during C. elegans embryogenesis. Bazooka functions to link the apical-basal polarity from epithelium with NB polarity. In early C. elegans embryogenesis, there is no epithelial structure, so superficially the function of Par-3 appears to be different from Bazooka. However, once our knowledge is more complete, perhaps the similarity will become apparent. Perhaps the essential function of Par-3/Bazooka is to serve as an anchor to organize a special cortical domain and it is capable of responding to different polarity cues in different developmental systems. Several recent technical advances should accelerate the rate of progress. The various genome sequencing projects that are completed (yeast, worm, and fly) and soon to be completed (mouse and human) will greatly facilitate the identification of molecular components and the comparison of mechanisms used by different organisms. The increasing usage of GFP-tagged molecules allows monitoring of various aspects of asymmetric cell division events, such as spindle orientation (7Kaltschmidt J.A Davidson C.M Brown N.H Brand A.H Nat. Cell Biol. 2000; 2: 7-12Crossref PubMed Scopus (243) Google Scholar), movement of determinants or adapters (11Lu B Usui T Uemura T Jan L Jan Y.-N Curr. Biol. 1999; 9 (b): 1247-1250Abstract Full Text Full Text PDF PubMed Google Scholar), and cell lineaging (5Gho M Bellaiche Y Schweisguth F Development. 1999; 126: 3573-3584PubMed Google Scholar) in living organisms in real time. Such analysis will undoubtedly provide deeper insights into the process than experiments based on observations of fixed tissues.