InteractiveFly: GeneBrief

meru: Biological Overview | References

Polarity is a shared feature of most cells. In epithelia, apical-basal polarity often coexists, and sometimes intersects with planar cell polarity (PCP), which orients cells in the epithelial plane. From a limited set of core building blocks (e.g. the Par complexes for apical-basal polarity and the Frizzled/Dishevelled complex for PCP), a diverse array of polarized cells and tissues are generated. This suggests the existence of little-studied tissue-specific factors that rewire the core polarity modules to the appropriate conformation. In Drosophila sensory organ precursors (SOPs), the core PCP components initiate the planar polarization of apical-basal determinants, ensuring asymmetric division into daughter cells of different fates. This study shows that Meru, a RASSF9/RASSF10 homologue, is expressed specifically in SOPs, recruited to the posterior cortex by Frizzled/Dishevelled, and in turn polarizes the apical-basal polarity factor Bazooka (Par3). Thus, Meru belongs to a class of proteins that act cell/tissue-specifically to remodel the core polarity machinery (Banerjee, 2017).

Polarity is a fundamental feature of most cells and tissues. It is evident both at the level of individual cells and groups of cells (e.g. planar cell polarity (PCP) in epithelia. However, despite the fact that different cell types use a common set of molecules to establish and maintain polarity (Par complexes, Fz-PCP pathway), the organization of polarized cells and cell assemblies varies dramatically across different species and tissues. This implies the existence of factors that act in a cell or tissue-specific manner to modulate/rewire the core polarity machinery into the appropriate organization. Despite many advances in understanding of polarity in unicellular and multicellular contexts, little is known about the identity or function of such factors (Banerjee, 2017).

An example of polarity remodeling is the process of asymmetric cell division (ACD), where cells need to rearrange their polarity determinants into a machinery capable of asymmetrically segregating cell fate determinants, vesicles and organelles, as well as controlling the orientation of the mitotic spindle. ACDs result in two daughter cells of different fates and occur in numerous cell types and across species. Well-studied examples include budding in Saccharomyces cerevisiae, ACD in the early embryo of Caenorhabditis elegans, or ACD of progenitor cells in the mammalian stratified epidermis and neural stem cells in the mammalian neocortex. In Drosophila melanogaster, the study of germline stem cells, neuroblasts (neural stem cells) and sensory organ precursors (SOPs) has greatly contributed to understanding of the cell biology and molecular mechanisms of ACD (Banerjee, 2017).

SOPs (or pI cells) divide asymmetrically within the plane of the epithelium into pIIa and pIIb daughter cells. pIIa and pIIb themselves divide asymmetrically to give rise to the different cell types of the external sensory organs (bristles), which are part of the peripheral nervous system and allow the adult fly to sense mechanical or chemical stimuli. Individual SOPs are selected by Notch-dependent lateral inhibition from multicellular clusters of epithelial cells expressing proneural genes (proneural clusters) (Banerjee, 2017).

The unequal segregation of cell fate determinants (the Notch pathway modulators Numb and Neuralized), which specifies the different fates of the daughter cells, requires their asymmetric localization on one side of the cell cortex prior to mitosis. This is achieved by remodeling the PCP and apical-basal polarity systems in the SOP, and by orienting the spindle relative to the tissue axis. The epithelial sheet that forms the pupal notum (dorsal thorax), where the best-studied SOPs are located, is planar polarized along the anterior-posterior tissue axis, with the transmembrane receptor Frizzled (Fz) and its effector Dishevelled (Dsh) localizing to the posterior side of the cell cortex, while the transmembrane protein Van Gogh (Vang, also known as Strabismus) and its interactor Prickle (Pk) are found anteriorly. The apical-basal polarity determinants central to SOP polarity are the PDZ domain-containing scaffold protein Bazooka (Baz, or Par3), atypical Protein Kinase C (aPKC) and Partitioning defective 6 (Par6), which localize apically in epithelial cells and the basolaterally localized membrane-associated guanylate kinase homologues (MAGUK) protein Discs-large (Dlg). In most epithelial cells, these proteins localize uniformly around the cell cortex, whereas in SOPs they show a striking asymmetric localization during mitosis: the Baz-aPKC-Par6 complex is found at the posterior cell cortex, opposite an anterior complex consisting of Dlg, Partner of Inscuteable (Pins) and the G-protein subunit Gαi. The Fz-Dsh complex provides the spatial information for the Baz-aPKC-Par6 complex, while Vang-Pk positions the Dlg-Pins-Gαi complex (likely through direct interaction between Vang and Dlg). The asymmetric distribution of the polarity determinants then directs the positioning of cell fate determinants at the anterior cell cortex. Additionally, Fz-Dsh and Pins orient the spindle along the anterior-posterior axis by anchoring it on both sides of the cell via Mushroom body defective (Mud, mammalian NuMA) and Dynein (Banerjee, 2017).

The planar symmetry of the Baz-aPKC-Par6 complex in SOPs is initially broken in interphase via Fz-Dsh, and is independent of the Dlg-Pins-Gαi complex. Once this initial asymmetry is established, the core PCP components become dispensable for Par complex polarization at metaphase due to the mutual antagonism between the opposing polarity complexes, which then maintains asymmetry during cell division. Indeed, Baz is still polarized in fz mutants during mitosis, but losing both pins and fz results in Baz spreading uniformly around the cortex. Crucially, it is unclear how Fz-Dsh can transmit planar information to the Baz-aPKC-Par6 complex in SOPs but not in neighboring epithelial cells. The cell-type dependent coupling between PCP and apical-basal polarity suggests the involvement of unknown SOP-specific factors in this process (Banerjee, 2017).

The four N-terminal RASSFs (Ras association domain family) in humans (RASSF7-10) have been associated with various forms of cancer, but the exact processes in which these scaffolding proteins act remain mostly elusive. Drosophila RASSF8, the homologue of human RASSF7 and RASSF8, is required for junctional integrity via Baz (Zaessinger, 2015; Langton, 2009). Interestingly, human RASSF9 and RASSF10 were found in an interaction network with Par3 (the mammalian Baz homologue) and with several PCP proteins (Hauri, 2013). The Drosophila genes CG13875 and CG32150 are believed to be homologues of human RASSF9 and RASSF10, respectively and remarkably, CG32150 mRNA is highly enriched in SOPs (Banerjee, 2017).

This study shows that Meru, encoded by CG32150, is an SOP-specific factor, capable of linking PCP and apical-basal polarity. Meru localizes asymmetrically in SOPs based on the polarity information provided by Fz/Dsh, and is able to recruit Baz to the posterior cortex (Banerjee, 2017).

PCP provides the spatial information for the initial polarization of SOPs at interphase, resulting in the planar polarization of Baz, which is uniformly localized prior to SOP differentiation. How Fz/Dsh communicate with Baz and enable its asymmetric enrichment was unknown. Based on the current results and previous findings, the following model is proposed for the role of Meru in SOP polarization. Upon selection and specification of SOPs, Meru expression is transcriptionally activated by the AS-C transcription factors (Reeves and Posakony, 2005). At interphase, planar-polarized Fz/Dsh recruit Meru to the membrane and hence direct its polarization. Meru in turn positions and asymmetrically enriches Baz, promoting the asymmetry of aPKC-Par6. Upon entry into mitosis, Meru is also required to retain laterally localized Baz, thus supporting the antagonism between the opposing Dlg-Pins-Gα_i and Baz-aPKC-Par6 complexes, ultimately enabling the correct positioning of cell fate determinants (Banerjee, 2017).

The meru mutant cell fate phenotype (bristle duplication or loss) is weaker than the baz loss-of-function phenotype, which results in loss of entire SOPs. This is likely due to two factors: (1) unlike meru mutants, the full baz mutant phenotype is the result of a complete loss of Baz in all cells of the SOP lineage, which is known to cause multiple defects including apoptosis of many sensory organ cells as well as cell fate transformations; (2) since a small amount of Baz is retained at the cortex of some meru mutant cells, it is likely that this residual Baz can still be polarized through the antagonistic activity of Pins at metaphase and thus partially rescues SOP polarization. Indeed, it was observed that reduction of pins or baz levels by RNAi strongly enhanced the meru cell specification phenotype. Conversely, supplying excess levels of Baz in a meru mutant background presumably restores sufficient Baz at the cortex to rescue the meru specification defect, as long as Pins is present to drive asymmetry at mitosis. (Banerjee, 2017).

While a decrease in cortical Baz can account for the cell specification defects in meru mutants, it does not explain the spindle orientation phenotypee. This abnormal spindle alignment could either be due to a decrease in Fz/Dsh levels/activity, or a decrease in the ability of Dsh to recruit the spindle-tethering factor Mud. No gross abnormalities were detected in Fz levels in meru mutants, though the presence of Fz in all neighboring cells would make it difficult to detect subtle decreases in SOPs. Further work will be required to understand Meru's role in spindle orientation (Banerjee, 2017).

Analysis of Meru in Drosophila is in agreement with the association of human RASSF9 and RASSF10 with both Par3 and PCP proteins previously reported. However, while the interaction with Dsh is conserved between the fly and human proteins, the transmembrane protein Vangl1 (the mammalian homologue of Vang), rather than its antagonist Fz was recovered in the mammalian proteomic analysis. This could reflect species-specific differences or altered polarity in the transformed human embryonic kidney 293 cells used for the mammalian work. Although Meru (CG32150) was classified as a potential homologue of RASSF10, alignment of the protein sequences showed similar sequence identities for both human RASSF9 (31%) and RASSF10 (26%). Thus, further functional work on Meru, its Drosophila paralogue CG13875, as well as mammalian RASSF9 and RASSF10 is required to understand the evolutionary and functional relationships between these proteins (Banerjee, 2017).

Little is known about the in vivo functions of either RASSF9 or RASSF10 in other species. Xenopus RASSF10 is prominently expressed in the brain and other neural tissues of tadpoles, potentially indicating a function in neurogenesis, a process where ACDs are known to take place. Interestingly, mouse RASSF9 shows a cell-specific expression in keratinocytes of the skin and loss of RASSF9 results in differentiation defects of the stratified epidermis. Considering that Par3 is required for ACD of basal layer progenitors of the stratified epidermis this raises the exciting prospect that RASSF9 might regulate ACD in the mammalian skin (Banerjee, 2017).

The polarization of cells and tissues is essential for their architecture and ultimately allows them to fulfill their function. The polarity machinery can be considered as a series of modules that are combined in a cell or tissue-specific manner, and hence requires specific factors that can create a polarity network appropriate to each tissue and cell type. This study has identified Meru as an SOP-specific factor, which is able to link PCP (Fz-Dsh) with apical-basal polarity (Baz). The PCP proteins Vang and Pk promote the positioning of the opposing Dlg-Pins-Gα_i complex. Although Vang can directly bind to Dlg, the SOP and neuroblast-specific factor, Banderuola (aka Wide Awake) was recently shown to be required for Dlg localization and could thus constitute a link between the two polarity systems on the opposite side of the cortex (Banerjee, 2017).

There is increasing evidence that cell-type specific rewiring of the polarity modules may be a widespread phenomenon. For instance, in different parts of the embryonic epidermis, Baz is planar polarized by Rho-kinase or by the Fat-PCP pathway, while in the retina, Vang is responsible for Baz polarization. Apical-basal polarity can also operate upstream of PCP in some systems, as in Drosophila photoreceptor specification, where aPKC restricts Fz activity by inhibitory phosphorylation in a subset of photoreceptor precursors. Thus, tissue-specific factors are likely to operate in a number of different contexts (Banerjee, 2017).

The interplay between PCP and apical-basal polarity is also evident in other species, as Dishevelled has been reported to promote axon differentiation in rat hippocampal neurons by stabilizing aPKC, while Xenopus Dishevelled is required for Lethal giant larvae (Lgl) basal localization in the ectoderm. Interestingly, both mammalian Par3 and the Vang homologue Vangl2 are required for progenitor cell ACD in the developing mouse neocortex, raising the question as to whether PCP and apical-basal polarity are also connected in mammalian ACDs. It is therefore proposed that tissue-specific factors such as Meru might enable the diversity and plasticity observed across different polarized cells and tissues by rewiring the core polarity systems (Banerjee, 2017).

Search PubMed for articles about Drosophila Meru

Banerjee, J. J., Aerne, B. L., Holder, M. V., Hauri, S., Gstaiger, M. and Tapon, N. (2017). Meru couples planar cell polarity with apical-basal polarity during asymmetric cell division. Elife 6. PubMed ID: 28665270

Hauri, S., Wepf, A., van Drogen, A., Varjosalo, M., Tapon, N., Aebersold, R. and Gstaiger, M. (2013). Interaction proteome of human Hippo signaling: modular control of the co-activator YAP1. Mol Syst Biol 9: 713. PubMed ID: 24366813

Langton, P. F., Colombani, J., Chan, E. H., Wepf, A., Gstaiger, M. and Tapon, N. (2009). The dASPP-dRASSF8 complex regulates cell-cell adhesion during Drosophila retinal morphogenesis. Curr Biol 19(23): 1969-1978. PubMed ID: 19931458

Reeves, N. and Posakony, J. W. (2005). Genetic programs activated by proneural proteins in the developing Drosophila PNS. Dev Cell 8(3): 413-425. PubMed ID: 15737936

Zaessinger, S., Zhou, Y., Bray, S. J., Tapon, N. and Djiane, A. (2015). Drosophila MAGI interacts with RASSF8 to regulate E-Cadherin-based adherens junctions in the developing eye. Development 142(6): 1102-1112. PubMed ID: 25725070

date revised: 15 September 2017

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