The accuracy of cell division is fundamental for the maintenance of cell ploidy and genomic stability. During cell division, many events, like DNA replication, chromosome segregation, mitosis completion, and cytokinesis, must be tightly controlled. The deregulation of these events is closely associated with severe pathology. Among other factors, the accuracy of cell division relies on the correct placement of the division plane which is dependent on the polarity axis (Lu and Johnston 2013).
Both in unicellular organisms and in metazoan, the cell spindle position is regulated to be perpendicular or planar to the division plane, allowing this way to equally segregate the chromosomes between the two daughter cells. In either case, the correct position/orientation of the spindle is required to maintain cell architecture and tissues homeostasis, events that are on the origin of developmental differentiation and tissue regeneration. Incorrect spindle position/orientation has been associated to altered tissue architecture, abnormal differentiation, and altered proliferation (Lu and Johnston 2013) which have been linked to cancer, morphogenetic diseases, and aging.
Mob (Mps one binder) proteins are a family of kinase regulator proteins that has a major role in the control of cell division, proliferation, and cell polarity (Hergovich 2011; Tavares et al. 2012). The sequence and function of these proteins is highly conserved throughout eukaryotes (Hergovich 2011). A phylogenetic analysis showed that Mob proteins are usually encoded by more than one gene, ranging from two in yeast (mob1 and mob2) to three in Drosophila (Mob1, Mob2, and Mob3) and at least seven human Mob homologous genes (mob1A and mob1B – that encode two closely related proteins – mob2, mob3A, mob3B, mob3C, and mob4) (Chow et al. 2010; Hergovich 2011).
Although a complete picture of the role of the distinct MOB proteins in vivo is still missing, some of these proteins seem to have specialized functions (Chow et al. 2010; Hergovich 2011). Here we will review MOB1, a member of both the mitotic exit network (MEN) and Hippo pathway, two vias that regulate cell proliferation and cell division.
The Mob1 Functions
Mob1: A Pivotal Player in the Mitotic Exit Network
Like most of the members of MEN network, Mob1 localizes at mid-anaphase to the spindle pole bodies. At the end of yeast cell budding, Mob1 is re-localized to the bud neck where it stays until cytokinesis and abscission occurs (Luca et al. 2001). Interestingly, Mob1 also partially colocalizes with Cdc14 phosphatase at the kinetochores (Hergovich 2011). In S. pombe, Mob1 is also present at the spindle pole bodies during mitosis and accumulates in the medial ring where the septum is formed and posteriorly, where daughter cell separation follows (Salimova et al. 2000). The cellular localization of Mob1 is therefore dynamic through cell cycle, and finally it localizes in the region where abscission takes place.
The involvement of Mob1 in cytokinesis was demonstrated in other organisms since the depletion of Mob1 from the ciliates Tetrahymena thermophila (Tavares et al. 2012) and Stentor coeruleus (Slabodnick et al. 2014), the protozoan parasite Trypanosoma brucei (Hammarton et al. 2005), the plant Arabidopsis thaliana (Galla et al. 2011) and in human cells (Florindo et al. 2012) causes cytokinesis defects. Altogether these studies demonstrated that Mob1 regulates cytokinesis in the three major branches of the eukaryotic tree.
Mob1 Coactivator of LATS/NDR Protein Kinases of the Hippo Pathway
In metazoans, numerous studies support that Mob1 proteins’ role in cell division is conserved. In fact, studies in Drosophila melanogaster demonstrated that Mob1 works as a tumor suppressor protein since loss of function of this protein leads to cell proliferation and tumor development in many fly organs, which originated Mob1 name in the fly, Mats (Mob as a tumor suppressor) (Lai et al. 2005). In the fly, Mats is localized at the plasma membrane in developing tissues and when it is activated inhibits tissue growth (Hergovich 2011). Significantly, the function in cell proliferation control is also conserved in mammals. The human ortholog of Mats, MOB1A, was shown to rescue Mats mutant phenotypes. This study also showed that Mats is involved, not only in cell proliferation, but also in apoptosis. This protein facilitates cell death by negatively regulating DIAP1, a caspase inhibitor essential for cell survival (Lai et al. 2005).
MOB proteins activate NDR kinases that in humans are four, NDR1, NDR2, LATS1 (Large tumor suppressor 1), and LATS2, all involved in cell division control (Hergovich 2011). The activation of LATS1 kinase by Mob1 was reported to be required for proper cytokinesis as LATS1 or MOB1A depletion by siRNAs increased telophase duration (Hergovich 2011). In human cells, it was shown that MOB1A is distributed in the cytoplasm but also localizes at the centrosome in early mitosis and at the spindle midzone and midbody later in mitosis. The presence of this protein at the centrosome is dependent on PLK1, since PLK1 depleted cells do not exhibit MOB1-GFP at the spindle poles. This study also showed that MOB1 is present at the kinetochores (Wilmeth et al. 2010). MOB1A/B depletion disrupts the normal localization of Chromosomal Passenger Complex (CPC) at the spindle midzone during early anaphase. This seems to be related to a larger telophase duration (Wilmeth et al. 2010). Interestingly, it was also shown that the cytokinesis failure observed in MOB1 depleted cells is related to an overstabilization of microtubules at the end of telophase, in the midbody region. In fact, MOB1A depleted cells show higher levels of acetylated microtubules of midbody region with higher resistance to cold and nocodozole treatments. In addition to this, MOB1 depleted cells also showed increased motility, and after division daughter cells were kept together by ultrafine cytoplasmic bridges indicating abscission failure. However, the authors observed that the increased motility was also present after abscission (Florindo et al. 2012). These findings support the idea that MOB1A and/or the kinases regulated by it have biological roles beyond cell division, such as in microtubule dynamics.
In D.melanogaster, the Hippo pathway controls cell proliferation by the phosphorylation status of the transcription factor activator Yorkie (Yki) (Hergovich and Hemmings 2012). The main components of the Hippo pathway are the Ste20-like kinase Hippo, its scaffolding protein Salvador (Sav), the NDR family kinase Warts, and its activator Mats. During development, Yki is not phosphorylated by Wts, since the Hippo pathway kinase module is inactive and then can migrate to the nucleus to exert its transcriptional activation function (Fig. 3). When proliferation needs to be stopped, the Hippo pathway phosphorylation cascade is activated which culminates with the cytoplasmic retention of the effector Yki. Specifically, active Hpo forms a complex with Sav and the complex phosphorylates Wts kinase. Wts then forms a complex with Mats and phosphorylates Yki. Phosphorylated Yki is sequestered in the cytoplasm mainly via the binding to 14-3-3 proteins which avoids its transcription factor activator function (Hergovich and Hemmings 2012) (Fig. 3).
In mammals, the final effectors of the Hippo cascade are the proteins YAP or TAZ that are also transcription activator factors whose activity in the nucleus induces the expression of antiapoptotic and proliferation associated genes (Hergovich and Hemmings 2012) (Fig. 3).
Due to the importance of this pathway in tissue homeostasis, many authors have tried to understand which upstream signals control the Hippo pathway. Several studies have shown that Hippo can respond to cell and tissue organization. Indeed, Hippo pathway integrates information coming from cell pathways responsible for maintenance of the polar structure of epithelia, such as components of the apical-basal polarity protein complexes and also of the planar cell polarity machinery. It was also shown that Hippo pathway can also respond to mechanical alterations or tensions, transducing signals coming from cytoskeleton molecules, and the extracellular matrix (Yu and Guan 2013).
The fact that Hippo pathway controls the expression of antiapoptotic and proliferation genes together with the evidence that cell/tissue organization impinge in its activity strongly support that the Hippo cascade is critical for tumor and eventually cancer development.
However, recent findings have shown that Hippo pathway activity and regulation is far more complex. Apart from the canonical function of regulation of antiapoptotic genes, some components of the Hippo cascade also integrate signaling from well-established pro-apoptotic molecules (Fallahi et al. 2016). At the first glance, these data may be somehow contradictory, but possibly this noncanonical role could be related to specific features of distinct tissues and tumors and further investigation needs to be done.
Mob1 as Polarity Factor: Between Cytokinesis and Morphogenesis
Particularly in the case o Mob1 protein, new roles apart from the canonical control of cell proliferation arose from two independent studies developed in the protozoa ciliates Tetrahymena and Stentor (Tavares et al. 2012; Slabodnick et al. 2014). These works showed that Mob1 accounts for a mechanistic link between cytokinesis and morphogenesis (Tavares et al. 2012; Slabodnick et al. 2014). In both ciliates, Mob1 polarized cellular localization and its depletion showed that Mob1 is essential for the maintenance and regeneration of cell polarity, proper cell proportions, correct division plane placement, and cytokinesis conclusion. In yeast, MEN also responds to polarity factors since this pathway is only activated when a set of proteins, like Tem1 and Bub2/Bfa1 (spindle position checkpoint pathway, SPOC), are asymmetrically distributed in the two spindle pole bodies. Thus, some of these factors are more abundant at the spindle pole body that will migrate to the bud (Caydasi et al. 2010). In fact, in yeast whenever the mitotic spindle is mispositioned, SPOC pathway prevents the activation of MEN. Interestingly, and contrary to yeast an asymmetrical dividing organism, the two ciliate cells are permanently polarized and divide symmetrically, similarly to the case of epithelial cells in metazoans. The role of Mob1 as an intrinsic polarity factor is also supported by the observation that in human cells MOB1 centrosomal localization changes at the end of mitosis being Mob1 only detected in the centriole that moves closer to the midbody, which is probably the mother centriole. The asymmetry of mother and daughter centrioles in a centrosome, and consequently between duplicated centrosomes, is also a crucial feature of metazoan asymmetric cell divisions (Yamashita et al. 2007). Remarkably, it was demonstrated that proteins related to cell and tissue polarity associate asymmetrically to the mother centriole (Jakobsen et al. 2011). In fact, these studies have a significant importance concerning the study of cancer development because disruption of cell polarity is a feature of epithelial cancers. Loss of apical-basal polarity facilitates the deregulation of oncogene and tumor suppressor balances causing altered proliferation, apoptosis, and, finally, transformed cells invasion and metastasis (Halaoui and McCaffrey 2014).
The studies in ciliates have highlighted that in these unicellular organisms Mob1 is a global patterning protein that is required for proper development and regeneration (Chalker and Frankel 2014). However, again this seems to be replicated in a metazoan. In the plant Arabidopsis, Mob1A is required for proper plant development, the correct patterning of the root meristem, and also to the control of root growth under stress conditions (Pinosa et al. 2013). Thus, the mechanisms that regulate cytokinesis and morphogenesis seem to share common evolutionary origins by regulating the establishment of the most fundamental pattern common to all dividing cells – the establishment of the cellular division axis (Chalker and Frankel 2014). The work in Arabidopsis also shows that Mob1 is important to integrate signals coming from environment into the signaling pathways where it is a critical player. Probably, we still do not have the complete picture of all Mob1 cellular roles which is also supported by the observation that Mob1 depletion in Tetrahymena causes ciliogenesis delay.
Summary and Perspectives
The importance of Mob1 during cell division has now been well established, through the different pathways where it is present (MEN, SIN, and Hippo pathways). The most important challenge we face today is not only to determine the mechanisms that regulate the participation of Mob1 in these pathways but also to accommodate the new data coming from different aspects of the cell/tissue biology such as cell polarity. By doing so, we will start to understand the role of Mob1 as a sensor and distributor of signals throughout different signal pathways regulating cell proliferation, cell polarity, and apoptosis. To meet this challenge, it would be interesting to find the interacting partners of Mob1 protein throughout the cell cycle, especially in polarized cells. Having a broader vision of Mob1 functions in the cell will allow exploring this molecule as a potential target to better understand tumor development and cancer.
- Slabodnick MM, Ruby JG, Dunn JG, Feldman JL, DeRisi JL, Marshall WF. The kinase regulator Mob1 acts as a patterning protein for stentor morphogenesis. PLoS Biol 2014;12. doi: 10.1371/journal.pbio.1001861.