Mps1p regulates meiotic spindle pole body duplication in addition to having novel roles during sporulation

Exploring Plant Meiosis: Insights from the Kinetochore Perspective

The central player for chromosome segregation in both mitosis and meiosis is the macromolecular kinetochore structure, which is assembled by >100 structural and regulatory proteins on centromere DNA. Kinetochores play a crucial role in cell division by connecting chromosomal DNA and microtubule polymers. This connection helps in the proper segregation and alignment of chromosomes. Additionally, kinetochores can act as a signaling hub, regulating the start of anaphase through the spindle assembly checkpoint, and controlling the movement of chromosomes during anaphase. However, the role of various kinetochore proteins in plant meiosis has only been recently elucidated, and these proteins differ in their functionality from those found in animals. In this review, our current knowledge of the functioning of plant kinetochore proteins in meiosis will be summarized. In addition, the functional similarities and differences of core kinetochore proteins in meiosis between plants and other species are discussed, and the potential applications of manipulating certain kinetochore genes in meiosis for breeding purposes are explored.

Deprotection of centromeric cohesin at meiosis II requires APC/C activity but not kinetochore tension

Genome haploidization involves sequential loss of cohesin from chromosome arms and centromeres during two meiotic divisions. At centromeres, cohesin's Rec8 subunit is protected from separase cleavage at meiosis I and then deprotected to allow its cleavage at meiosis II. Protection of centromeric cohesin by shugoshin-PP2A seems evolutionarily conserved. However, deprotection has been proposed to rely on spindle forces separating the Rec8 protector from cohesin at metaphase II in mammalian oocytes and on APC/C-dependent destruction of the protector at anaphase II in yeast. Here, we have activated APC/C in the absence of sister kinetochore biorientation at meiosis II in yeast and mouse oocytes, and find that bipolar spindle forces are dispensable for sister centromere separation in both systems. Furthermore, we show that at least in yeast, protection of Rec8 by shugoshin and inhibition of separase by securin are both required for the stability of centromeric cohesin at metaphase II. Our data imply that related mechanisms preserve the integrity of dyad chromosomes during the short metaphase II of yeast and the prolonged metaphase II arrest of mammalian oocytes.

Mps1 promotes poleward chromosome movements in meiotic prometaphase

In prophase of meiosis I, homologous chromosomes pair and become connected by cross-overs. Chiasmata, the connections formed by cross-overs, enable the chromosome pair, called a bivalent, to attach as a single unit to the spindle. When the meiotic spindle forms in prometaphase, most bivalents are associated with one spindle pole and then go through a series of oscillations on the spindle, attaching to and detaching from microtubules until the partners of the bivalent become bioriented—attached to microtubules from opposite sides of the spindle. The conserved kinase, Mps1, is essential for the bivalents to be pulled by microtubules across the spindle in prometaphase. Here we show that MPS1 is needed for efficient triggering of the migration of microtubule-attached kinetochores toward the poles and promotes microtubule depolymerization. Our data support the model Mps1 acts at the kinetochore to coordinate the successful attachment of a microtubule and the triggering of microtubule depolymerization to then move the chromosome.

Role of E2Fs and mitotic regulators controlled by E2Fs in the epithelial to mesenchymal transition

The epithelial-to-mesenchymal transition (EMT) is a complex cellular process in which epithelial cells acquire mesenchymal properties. EMT occurs in three biological settings: development, wound healing and fibrosis, and tumor progression. Despite occurring in three independent biological settings, EMT signaling shares some molecular mechanisms that allow epithelial cells to de-differentiate and acquire mesenchymal characteristics that confer cells invasive and migratory capacity to distant sites. Here we summarize the molecular mechanism that delineates EMT and we will focus on the role of E2 promoter binding factors (E2Fs) in EMT during tumor progression. Since the E2Fs are presently undruggable due to their control in numerous pivotal cellular functions and due to the lack of selectivity against individual E2Fs, we will also discuss the role of three mitotic regulators and/or mitotic kinases controlled by the E2Fs (NEK2, Mps1/TTK, and SGO1) in EMT that can be useful as drug targets.

Impact statement

The study of the epithelial to mesenchymal transition (EMT) is an active area of research since it is one of the early intermediates to invasion and metastasis—a state of the cancer cells that ultimately kills many cancer patients. We will present in this review that besides their canonical roles as regulators of proliferation, unregulated expression of the E2F transcription factors may contribute to cancer initiation and progression to metastasis by signaling centrosome amplification, chromosome instability, and EMT. Since our discovery that the E2F activators control centrosome amplification and mitosis in cancer cells, we have identified centrosome and mitotic regulators that may represent actionable targets against EMT and metastasis in cancer cells. This is impactful to all of the cancer patients in which the Cdk/Rb/E2F pathway is deregulated, which has been estimated to be most cancer patients with solid tumors.

Mps1 promotes chromosome meiotic chromosome biorientation through Dam1

During meiosis, chromosomes attach to microtubules at their kinetochores and are moved by microtubule depolymerization. The Mps1 kinase is essential for this process. Phosphorylation of Dam1 by Mps1 allows kinetochores to move processively poleward along microtubules during the biorientation process.

In budding yeast meiosis, homologous chromosomes become linked by chiasmata and then move back and forth on the spindle until they are bioriented, with the kinetochores of the partners attached to microtubules from opposite spindle poles. Certain mutations in the conserved kinase, Mps1, result in catastrophic meiotic segregation errors but mild mitotic defects. We tested whether Dam1, a known substrate of Mps1, was necessary for its critical meiotic role. We found that kinetochore–microtubule attachments are established even when Dam1 is not phosphorylated by Mps1, but that Mps1 phosphorylation of Dam1 sustains those connections. But the meiotic defects when Dam1 is not phosphorylated are not nearly as catastrophic as when Mps1 is inactivated. The results demonstrate that one meiotic role of Mps1 is to stabilize connections that have been established between kinetochores and microtubles by phosphorylating Dam1.

Multiple Duties for Spindle Assembly Checkpoint Kinases in Meiosis

Cell division in mitosis and meiosis is governed by evolutionary highly conserved protein kinases and phosphatases, controlling the timely execution of key events such as nuclear envelope breakdown, spindle assembly, chromosome attachment to the spindle and chromosome segregation, and cell cycle exit. In mitosis, the spindle assembly checkpoint (SAC) controls the proper attachment to and alignment of chromosomes on the spindle. The SAC detects errors and induces a cell cycle arrest in metaphase, preventing chromatid separation. Once all chromosomes are properly attached, the SAC-dependent arrest is relieved and chromatids separate evenly into daughter cells. The signaling cascade leading to checkpoint arrest depends on several protein kinases that are conserved from yeast to man. In meiosis, haploid cells containing new genetic combinations are generated from a diploid cell through two specialized cell divisions. Though apparently less robust, SAC control also exists in meiosis. Recently, it has emerged that SAC kinases have additional roles in executing accurate chromosome segregation during the meiotic divisions. Here, we summarize the main differences between mitotic and meiotic cell divisions, and explain why meiotic divisions pose special challenges for correct chromosome segregation. The less-known meiotic roles of the SAC kinases are described, with a focus on two model systems: yeast and mouse oocytes. The meiotic roles of the canonical checkpoint kinases Bub1, Mps1, the pseudokinase BubR1 (Mad3), and Aurora B and C (Ipl1) will be discussed. Insights into the molecular signaling pathways that bring about the special chromosome segregation pattern during meiosis will help us understand why human oocytes are so frequently aneuploid.

Structured illumination with particle averaging reveals novel roles for yeast centrosome components during duplication

Duplication of the yeast centrosome (called the spindle pole body, SPB) is thought to occur through a series of discrete steps that culminate in insertion of the new SPB into the nuclear envelope (NE). To better understand this process, we developed a novel two-color structured illumination microscopy with single-particle averaging (SPA-SIM) approach to study the localization of all 18 SPB components during duplication using endogenously expressed fluorescent protein derivatives. The increased resolution and quantitative intensity information obtained using this method allowed us to demonstrate that SPB duplication begins by formation of an asymmetric Sfi1 filament at mitotic exit followed by Mps1-dependent assembly of a Spc29- and Spc42-dependent complex at its tip. Our observation that proteins involved in membrane insertion, such as Mps2, Bbp1, and Ndc1, also accumulate at the new SPB early in duplication suggests that SPB assembly and NE insertion are coupled events during SPB formation in wild-type cells.

DOI:http://dx.doi.org/10.7554/eLife.08586.001

Cells divide to produce two new daughter cells that each contain the same genetic material. First, the DNA of the parent cell is copied, then it must be physically separated into the daughter cells by a structure made of filaments called microtubules. To ensure that the DNA is separated into two equal parts, the microtubules must emerge from two points in the cell, known as spindle poles.

Each spindle pole is made of a group (or ‘complex’) of proteins and these have to be copied before the cell can divide. While we understand how DNA is copied, we do not know how cells copy proteins. The spindle pole in yeast—known as the spindle pole body—is an ideal model to study this problem because the proteins that form it have already been identified and it is easy to study yeast in the laboratory.

Burns et al. developed a new method to study the spindle pole body using fluorescent protein tags and a sophisticated microscopy technique. The experiments mapped the positions of 18 proteins within the spindle pole body during its duplication. Some of these proteins enable the spindle pole to insert into the membrane that surrounds the cell's nucleus. Unexpectedly, Burns et al. observed that this set of proteins interact with the new spindle pole as it forms, instead of afterwards as was previously believed.

Burns et al.'s findings suggest that the spindle pole body assembles into the membrane surrounding the nucleus at the same time as it is copied. The next challenges are to understand the details of how this works and to use the same method to study other large protein complexes in cells. Until now, highly detailed surveys of protein structures have been limited to a handful of proteins and conditions. The method developed by Burns et al. makes it possible to carry out studies that examine the movements of whole protein complexes during cell division.

DOI:http://dx.doi.org/10.7554/eLife.08586.002

SUMO localizes to the central element of synaptonemal complex and is required for the full synapsis of meiotic chromosomes in budding yeast

The synaptonemal complex (SC) is a widely conserved structure that mediates the intimate alignment of homologous chromosomes during meiotic prophase and is required for proper homolog segregation at meiosis I. However, fundamental details of SC architecture and assembly remain poorly understood. The coiled-coil protein, Zip1, is the only component whose arrangement within the mature SC of budding yeast has been extensively characterized. It has been proposed that the Small Ubiquitin-like MOdifier, SUMO, plays a role in SC assembly by linking chromosome axes with Zip1's C termini. The role of SUMO in SC structure has not been directly tested, however, because cells lacking SUMO are inviable. Here, we provide direct evidence for SUMO's function in SC assembly. A meiotic smt3 reduction-of-function strain displays reduced sporulation, abnormal levels of crossover recombination, and diminished SC assembly. SC structures are nearly absent when induced at later meiotic time points in the smt3 reduction-of-function background. Using Structured Illumination Microscopy we furthermore determine the position of SUMO within budding yeast SC structure. In contrast to previous models that positioned SUMO near Zip1's C termini, we demonstrate that SUMO lies at the midline of SC central region proximal to Zip1's N termini, within a subdomain called the “central element”. The recently identified SUMOylated SC component, Ecm11, also localizes to the SC central element. Finally, we show that SUMO, Ecm11, and even unSUMOylatable Ecm11 exhibit Zip1-like ongoing incorporation into previously established SCs during meiotic prophase and that the relative abundance of SUMO and Ecm11 correlates with Zip1's abundance within SCs of varying Zip1 content. We discuss a model in which central element proteins are core building blocks that stabilize the architecture of SC near Zip1's N termini, and where SUMOylation may occur subsequent to the incorporation of components like Ecm11 into an SC precursor structure.

The meiotic cell cycle enables sexually reproducing organisms to generate reproductive cells with half their chromosome complement. Chromosome ploidy is reduced during meiosis by virtue of prior associations established between homologous chromosomes (homologs). Such associations, which are ultimately secured by crossover recombination events, allow homologs to achieve an opposing orientation and segregate from one another at meiosis I. A multimeric protein structure, the synaptonemal complex (SC), mediates the intimate, lengthwise alignment of homologs during meiotic prophase and forms the context in which crossovers mature. The SC's tripartite structure is widely conserved but its composition and architecture remain incompletely understood in any organism. The Small Ubiquitin-like MOdifier (SUMO) localizes to SC in budding yeast. We show that SUMO is required for assembling mature SC and we furthermore demonstrate that SUMO and the recently identified SUMOylated protein, Ecm11, are components of the central element substructure of the budding yeast SC. Our findings suggest that SUMO and Ecm11 are core building blocks of SC, yet our data also suggest that SUMOylation may occur subsequent to Ecm11's incorporation into the SC structure. Finally, our study highlights Structured Illumination as a powerful tool for mapping the fine structure of budding yeast SC.

Mitotic Kinases and p53 Signaling

Mitosis is tightly regulated and any errors in this process often lead to aneuploidy, genomic instability, and tumorigenesis. Deregulation of mitotic kinases is significantly associated with improper cell division and aneuploidy. Because of their importance during mitosis and the relevance to cancer, mitotic kinase signaling has been extensively studied over the past few decades and, as a result, several mitotic kinase inhibitors have been developed. Despite promising preclinical results, targeting mitotic kinases for cancer therapy faces numerous challenges, including safety and patient selection issues. Therefore, there is an urgent need to better understand the molecular mechanisms underlying mitotic kinase signaling and its interactive network. Increasing evidence suggests that tumor suppressor p53 functions at the center of the mitotic kinase signaling network. In response to mitotic spindle damage, multiple mitotic kinases phosphorylate p53 to either activate or deactivate p53-mediated signaling. p53 can also regulate the expression and function of mitotic kinases, suggesting the existence of a network of mutual regulation, which can be positive or negative, between mitotic kinases and p53 signaling. Therefore, deciphering this regulatory network will provide knowledge to overcome current limitations of targeting mitotic kinases and further improve the results of targeted therapy.

The MPS1 family of protein kinases

MPS1 protein kinases are found widely, but not ubiquitously, in eukaryotes. This family of potentially dual-specific protein kinases is among several that regulate a number of steps of mitosis. The most widely conserved MPS1 kinase functions involve activities at the kinetochore in both the chromosome attachment and the spindle checkpoint. MPS1 kinases also function at centrosomes. Beyond mitosis, MPS1 kinases have been implicated in development, cytokinesis, and several different signaling pathways. Family members are identified by virtue of a conserved C-terminal kinase domain, though the N-terminal domain is quite divergent. The kinase domain of the human enzyme has been crystallized, revealing an unusual ATP-binding pocket. The activity, level, and subcellular localization of Mps1 family members are tightly regulated during cell-cycle progression. The mitotic functions of Mps1 kinases and their overexpression in some tumors have prompted the identification of Mps1 inhibitors and their active development as anticancer drugs.

Sporulation in the budding yeast Saccharomyces cerevisiae

In response to nitrogen starvation in the presence of a poor carbon source, diploid cells of the yeast Saccharomyces cerevisiae undergo meiosis and package the haploid nuclei produced in meiosis into spores. The formation of spores requires an unusual cell division event in which daughter cells are formed within the cytoplasm of the mother cell. This process involves the de novo generation of two different cellular structures: novel membrane compartments within the cell cytoplasm that give rise to the spore plasma membrane and an extensive spore wall that protects the spore from environmental insults. This article summarizes what is known about the molecular mechanisms controlling spore assembly with particular attention to how constitutive cellular functions are modified to create novel behaviors during this developmental process. Key regulatory points on the sporulation pathway are also discussed as well as the possible role of sporulation in the natural ecology of S. cerevisiae.

The Aurora kinase Ipl1 is necessary for spindle pole body cohesion during budding yeast meiosis

In budding yeast, the microtubule-organizing center is called the spindle pole body (SPB) and shares structural components with the centriole, the central core of the animal centrosome. During meiotic interphase I, the SPB is duplicated when DNA replication takes place. Duplicated SPBs are linked and then separate to form a bipolar spindle required for homolog separation in meiosis I. During interphase II, SPBs are duplicated again, in the absence of DNA replication, to form four SPBs that establish two spindles for sister-chromatid separation in meiosis II. Here, we report that the Aurora kinase Ipl1, which is necessary for sister-chromatid cohesion, is also required for maintenance of a tight association between duplicated SPBs during meiosis, which we term SPB cohesion. Premature loss of cohesion leads to SPB overduplication and the formation of multipolar spindles. By contrast, the Polo-like kinase Cdc5 is necessary for SPB duplication and interacts antagonistically with Ipl1 at the meiotic SPB to ensure proper SPB separation. Our data suggest that Ipl1 coordinates SPB dynamics with the two chromosome segregation cycles during yeast meiosis.

Mip1 associates with both the Mps1 kinase and actin, and is required for cell cortex stability and anaphase spindle positioning

The Mps1 family of protein kinases contributes to cell cycle control by regulating multiple microtubule cytoskeleton activities. We have uncovered a new Mps1 substrate that provides a novel link between Mps1 and the actin cytoskeleton. We have identified a conserved human Mps1 (hMps1) interacting protein we have termed Mps1 interacting protein-1 (Mip1). Mip1 defines an uncharacterized family of conserved proteins that contain coiled-coil and calponin homology domains. We demonstrate that Mip1 is a phosphoprotein that interacts with hMps1 in vitro and in vivo and is a hMps1 substrate. Mip1 exhibits dynamic localization during the cell cycle; Mip1 localizes to the actin cytoskeleton during interphase, the spindle in early mitosis, and the cleavage furrow during cytokinesis. Mip1 function is required to ensure proper spindle positioning at the onset of anaphase after cells begin furrow ingression. Cells depleted of Mip1 exhibit aberrant mitotic actin filament organization, excessive membrane blebbing, dramatic spindle rocking, and chromosome distribution errors during early cytokinesis producing high numbers of binucleate cells. Our data indicate that Mip1 is a newly recognized component of the actin cytoskeleton that interacts with hMps1 and that it is essential to ensure proper segregation of the genome during cell cleavage.

Membrane assembly modulates the stability of the meiotic spindle-pole body

Spore formation in Saccharomyces cerevisiae is driven by de novo assembly of new membranes termed prospore membranes. A vesicle-docking complex called the meiosis II outer plaque (MOP) forms on the cytoplasmic faces of the spindle-pole bodies at the onset of meiosis II and serves as the initiation site for membrane formation. In this study, a fluorescence-recovery assay was used to demonstrate that the dynamics of the MOP proteins change coincident with the coalescence of precursor vesicles into a membrane. Proteins within the MOP exchange freely with a soluble pool prior to membrane assembly, but after membranes are formed they remain stably within the MOP. By contrast, constitutive spindle-pole-body proteins display low exchange in both conditions. The MOP component Ady4p plays a role in maintaining the integrity of the MOP complex, but this role differs depending on whether the MOP is associated with docked vesicles or a fully formed membrane. These results suggest an architectural rearrangement of the MOP coincident with vesicle fusion.

Degradation of the human mitotic checkpoint kinase Mps1 is cell cycle-regulated by APC-cCdc20 and APC-cCdh1 ubiquitin ligases

Mps1 is a dual specificity protein kinase with key roles in regulating the spindle assembly checkpoint and chromosome-microtubule attachments. Consistent with these mitotic functions, Mps1 protein levels fluctuate during the cell cycle, peaking at early mitosis and abruptly declining during mitotic exit and progression into the G(1) phase. Although evidence in budding yeast indicates that Mps1 is targeted for degradation at anaphase by the anaphase-promoting complex (APC)-c(Cdc20) complex, little is known about the regulatory mechanisms that govern Mps1 protein levels in human cells. Here, we provide evidence for the ubiquitin ligase/proteosome pathway in regulating human Mps1 levels during late mitosis through G(1) phase. First, we showed that treatment of HEK 293T cells with the proteosome inhibitor MG132 resulted in an increase in both the polyubiquitination and the accumulation of Mps1 protein levels. Next, Mps1 was shown to co-precipitate with APC and its activators Cdc20 and Cdh1 in a cell cycle-dependent manner. Consistent with this, overexpression of Cdc20 or Cdh1 led to a marked reduction of endogenous Mps1 levels during anaphase or G(1) phase, respectively. In contrast, depletion of Cdc20 or Cdh1 by RNAi treatment both led to the stabilization of Mps1 protein during mitosis or G(1) phase, respectively. Finally, we identified a single D-box motif in human Mps1 that is required for its ubiquitination and degradation. Failure to appropriately degrade Mps1 is sufficient to trigger centrosome amplification and mitotic abnormalities in human cells. Thus, our results suggest that the sequential actions of the APC-c(Cdc20) and APC-c(Cdh1) ubiquitin ligases regulate the clearance of Mps1 levels and are critical for Mps1 functions during the cell cycle in human cells.

The cell cycle checkpoint kinase CHK2 mediates DNA damage-induced stabilization of TTK/hMps1

Cell cycle progression is monitored constantly to ensure faithful passage of genetic codes and genome stability. We have demonstrated previously that, upon DNA damage, TTK/hMps1 activates the checkpoint kinase CHK2 by phosphorylating CHK2 at Thr68. However, it remains to be determined whether and how TTK/hMps1 responds to DNA damage. In this report, we present evidence that TTK/hMps1 can be induced by DNA damage in normal human fibroblasts. Interestingly, the induction depends on CHK2 because CHK2-targeting small interfering RNA or a CHK2 inhibitor abolishes the increase. Such induction is mediated through phosphorylation of TTK/hMps1 at Thr288 by CHK2 and requires the CHK2 SQ/TQ cluster domain/forkhead-associated domain. In cells, TTK/hMps1 phosphorylation at Thr288 is induced by DNA damage and forms nuclear foci, which colocalize partially with gamma-H2AX. Reexpression of TTK/hMps1 T288A mutant in TTK/hMps1-knockdown cells causes a defect in G(2)/M arrest, suggesting that phosphorylation at this site participates in the proper checkpoint execution. Our study uncovered a regulatory loop between TTK/hMps1 and CHK2 whereby DNA damage-activated CHK2 may facilitate the stabilization of TTK/hMps1, therefore maintaining the checkpoint control.

TTK/Mps1 controls nuclear targeting of c-Abl by 14-3-3-coupled phosphorylation in response to oxidative stress

Upon exposure to genotoxic stress, the c-Abl tyrosine kinase is released from cytoplasmic 14-3-3 proteins and then is targeted to the nucleus. Phosphorylation of Thr735 in c-Abl is critical for binding to 14-3-3; however, kinases responsible for this phosphorylation are unknown. Here, we identify CLK1, CLK4, MST1, MST2 and TTK (also known as Mps1) as novel Thr735 kinases in vitro by expression cloning strategy using phosphospecific antibody. We also demonstrate that ectopic expression of these kinases is capable for phosphorylation of Thr735 in cells. Importantly, upon exposure to oxidative stress, phosphorylation of Thr735 is transiently upregulated, and the status of this phosphorylation remains unchanged in cells silenced for CLK1, CLK4, MST1 or MST2. By contrast, knockdown of TTK attenuates phosphorylation of Thr735, suggesting that TTK is a physiological kinase that phosphorylates Thr735. In concert with these results, we show that, in cells silenced for TTK, c-Abl is accumulated in the nucleus even in unstressed condition and no further targeting into the nucleus occurs after oxidative stress. Moreover, nuclear entrapment of c-Abl by knocking down TTK enhances oxidative stress-induced apoptosis. These findings provide evidence that TTK phosphorylates c-Abl at Thr735 and that this phosphorylation is of importance to the cytoplasmic sequestration of c-Abl.

Mps1 Activation Loop Autophosphorylation Enhances Kinase Activity

The Mps1 protein kinase is required for proper assembly of the mitotic spindle, checkpoint signaling, and several other aspects of cell growth and differentiation. Mps1 regulation is mediated by cell cycle-dependent changes in transcription and protein level. There is also a strong correlation between hyperphosphorylated mitotic forms of Mps1 and increased kinase activity. We investigated the role that autophosphorylation plays in regulating human Mps1 (hMps1) protein kinase activity. Here we report that hyperphosphorylated hMps1 forms are not the only active forms of the kinase. However, autophosphorylation of hMps1 within the activation loop is required for full activity in vitro. Mass spectrometry analysis of de novo synthesized enzyme in Escherichia coli identified autophosphorylation sites at residues Thr(675), Thr(676), and Thr(686), but phosphatase-treated and reactivated enzyme was only phosphorylated on Thr(676). Mutation of Thr(676) in hMps1 or the corresponding Thr(591) residue within yeast Mps1 reduces kinase activity in vitro. We find that overexpression of an hMps1-T676A mutation inhibits centrosome duplication in RPE1 cells. Likewise, yeast cells harboring mps1-T591A as the sole MPS1 allele are not viable. Our data strongly support the conclusion that site-specific Mps1 autophosphorylation within the activation loop is required for full activity in vitro and function in vivo.

Genetic evidence for a SPO1-dependent signaling pathway controlling meiotic progression in yeast

The yeast spindle pole body (SPB) plays a unique role in meiosis, initiating both spindle assembly and prospore membrane synthesis. SPO1, induced early in development, encodes a meiosis-specific phospholipase B (PLB) homolog required at three stages of SPB morphogenesis: MI, MII, and spore formation. Here we report in-depth analysis of the SPO1 gene including its transcriptional control by regulators of early gene expression, protein localization to the ER lumen and periplasmic space, and molecular genetic studies of its role in meiosis. Evidence is presented that multiple arrest points in spo1Delta occur independently, demonstrating that Spo1 acts at distinct steps. Loss of Spo1 is suppressed by high-copy glycosylphosphatidylinositol (GPI) proteins, dependent on sequence, timing, and strength of induction in meiosis. Since phosphatidylinositol (PI) serves as both an anchor component and a lipase substrate, we hypothesized that GPI-protein expression might substitute for Spo1 by decreasing levels of its potential substrates, PI and phosphatidylinositol phosphates (PIPs). Partial spo1Delta complementation by PLB3 (encoding a unique PLB capable of cleaving PI) and relatively strong Spo1 binding to PI(4)P derivatives (via a novel N-terminal lysine-rich fragment essential for Spo1 function) are consistent with this view. Epistasis of SPO1 mutations to those in SPO14 (encoding a PLD involved in signaling) and physical interaction of Spo1 with Spo23, a protein regulating PI synthesis required for wild-type sporulation, further support this notion. Taken together these findings implicate PI and/or PIPs in Spo1 function and suggest the existence of a novel Spo1-dependent meiosis-specific signaling pathway required for progression of MI, MII, and spore formation via regulation of the SPB.

SSP2 and OSW1, two sporulation-specific genes involved in spore morphogenesis in Saccharomyces cerevisiae

Spore formation in Saccharomyces cerevisiae requires the synthesis of prospore membranes (PSMs) followed by the assembly of spore walls (SWs). We have characterized extensively the phenotypes of mutants in the sporulation-specific genes, SSP2 and OSW1, which are required for spore formation. A striking feature of the osw1 phenotype is asynchrony of spore development, with some spores displaying defects in PSM formation and others spores in the same ascus blocked at various stages in SW development. The Osw1 protein localizes to spindle pole bodies (SPBs) during meiotic nuclear division and subsequently to PSMs/SWs. We propose that Osw1 performs a regulatory function required to coordinate the different stages of spore morphogenesis. In the ssp2 mutant, nuclei are surrounded by PSMs and SWs; however, PSMs and SWs often also encapsulate anucleate bodies both inside and outside of spores. In addition, the SW is not as thick as in wild type. The ssp2 mutant defect is partially suppressed by overproduction of either Spo14 or Sso1, both of which promote the fusion of vesicles at the outer plaque of the SPB early in PSM formation. We propose that Ssp2 plays a role in vesicle fusion during PSM formation.

Ascospore formation in the yeast Saccharomyces cerevisiae

Sporulation of the baker's yeast Saccharomyces cerevisiae is a response to nutrient depletion that allows a single diploid cell to give rise to four stress-resistant haploid spores. The formation of these spores requires a coordinated reorganization of cellular architecture. The construction of the spores can be broadly divided into two phases. The first is the generation of new membrane compartments within the cell cytoplasm that ultimately give rise to the spore plasma membranes. Proper assembly and growth of these membranes require modification of aspects of the constitutive secretory pathway and cytoskeleton by sporulation-specific functions. In the second phase, each immature spore becomes surrounded by a multilaminar spore wall that provides resistance to environmental stresses. This review focuses on our current understanding of the cellular rearrangements and the genes required in each of these phases to give rise to a wild-type spore.

Saccharomyces cerevisiae Sps1p regulates trafficking of enzymes required for spore wall synthesis

SPS1 encodes a sporulation-specific protein with homology to the Ste20/p21-activated kinase family. Deletion of SPS1 impinges on the formation of the spore wall, which surrounds each of the haploid nuclei generated by the meiotic divisions. Here, we demonstrate that the new internal membranes that surround the meiotic nuclei appear normal in the absence of Sps1p. Analyses of spore wall layers by immunohistochemistry suggest that the inner layers are not efficiently deposited. The defect in spore wall morphogenesis is most likely a consequence of mislocalization of enzymes required for the synthesis of the spore wall layers as both Chs3p, the major chitin synthase in yeast, and Gsc2/Fks2p, a glucan synthase transcriptionally upregulated during sporulation, fail to reach the prospore membrane in the sps1 mutant. Furthermore, localization of Chs3p to the prospore membrane is not dependent on Shc1p, a sporulation-specific homolog of Chs4p, which is required for recruitment of Chs3p to the bud neck in vegetative cells. Sps1p colocalized with Chs3p to peripheral and internal punctate structures and prospore membranes. We propose that Sps1p promotes sporulation, in part, by regulating the intracellular movement of proteins required for spore wall formation.

The meiotic defects of mutants in the Drosophila mps1 gene reveal a critical role of Mps1 in the segregation of achiasmate homologs

The conserved kinase Mps1 is necessary for the proper functioning of the mitotic and meiotic spindle checkpoints (MSCs), which monitor the integrity of the spindle apparatus and prevent cells from progressing into anaphase until chromosomes are properly aligned on the metaphase plate. In Drosophila melanogaster, a null allele of the gene encoding Mps1 was recently shown to be required for the proper functioning of the MSC, but it did not appear to exhibit a defect in female meiosis. We demonstrate here that the meiotic mutant ald1 is a hypomorphic allele of the mps1 gene. Both ald1 and a P-insertion allele of mps1 exhibit defects in female meiotic chromosome segregation. The observed segregational defects are substantially more severe for pairs of achiasmate homologs, which are normally segregated by the achiasmate (or distributive) segregation system, than they are for chiasmate bivalents. Furthermore, cytological analysis of ald1 mutant oocytes reveals both a failure in the coorientation of achiasmate homologs at metaphase I and a defect in the maintenance of the chiasmate homolog associations that are normally observed at metaphase I. We conclude that Mps1 plays an important role in Drosophila female meiosis by regulating processes that are especially critical for ensuring the proper segregation of nonexchange chromosomes.

Morphogenetic pathway of spore wall assembly in Saccharomyces cerevisiae

The Saccharomyces cerevisiae spore is protected from environmental damage by a multilaminar extracellular matrix, the spore wall, which is assembled de novo during spore formation. A set of mutants defective in spore wall assembly were identified in a screen for mutations causing sensitivity of spores to ether vapor. The spore wall defects in 10 of these mutants have been characterized in a variety of cytological and biochemical assays. Many of the individual mutants are defective in the assembly of specific layers within the spore wall, leading to arrests at discrete stages of assembly. The localization of several of these gene products has been determined and distinguishes between proteins that likely are involved directly in spore wall assembly and probable regulatory proteins. The results demonstrate that spore wall construction involves a series of dependent steps and provide the outline of a morphogenetic pathway for assembly of a complex extracellular structure.

Spindle regulation: Mps1 flies into new areas

Analysis of a mutation in the Drosophila Mps1 ortholog further demonstrates the universality of Mps1 function in the spindle checkpoint, but suggests Mps1 function in centrosome duplication might not be so conserved. The work also contributes new additions to a list of Mps1 functions that continues to grow with each new study.

Three-dimensional ultrastructure of Saccharomyces cerevisiae meiotic spindles

Meiotic chromosome segregation leads to the production of haploid germ cells. During meiosis I (MI), the paired homologous chromosomes are separated. Meiosis II (MII) segregation leads to the separation of paired sister chromatids. In the budding yeast Saccharomyces cerevisiae, both of these divisions take place in a single nucleus, giving rise to the four-spored ascus. We have modeled the microtubules in 20 MI and 15 MII spindles by using reconstruction from electron micrographs of serially sectioned meiotic cells. Meiotic spindles contain more microtubules than their mitotic counterparts, with the highest number in MI spindles. It is possible to differentiate between MI versus MII spindles based on microtubule numbers and organization. Similar to mitotic spindles, kinetochores in either MI or MII are attached by a single microtubule. The models indicate that the kinetochores of paired homologous chromosomes in MI or sister chromatids in MII are separated at metaphase, similar to mitotic cells. Examination of both MI and MII spindles reveals that anaphase A likely occurs in addition to anaphase B and that these movements are concurrent. This analysis offers a structural basis for considering meiotic segregation in yeast and for the analysis of mutants defective in this process.

Germ cell aneuploidy in zebrafish with mutations in the mitotic checkpoint gene mps1

Aneuploidy, resulting from chromosome missegregation during meiosis, is a major cause of human infertility and birth defects. However, its molecular basis remains incompletely understood. Here we have identified a spectrum of chromosome anomalies in embryos of zebrafish homozygous for a hypomorphic mutation in Mps1, a kinase required for the mitotic checkpoint. These aneuploidies are caused by meiotic error and result in severe developmental defects. Our results reveal Mps1 as a critical regulator of chromosome number in zebrafish, and demonstrate how slight genetic perturbation of a mitotic checkpoint factor can dramatically reduce the fidelity of chromosome segregation during vertebrate meiosis.

Cell signaling in yeast sporulation

Gametogenesis is essential for the propagation of all sexually reproducing organisms and consists of halving the chromosome number through meiosis, and the subsequent packaging of the haploid products into gametes. Meiosis and gamete formation must be tightly coupled to ensure the formation of viable progeny; perturbations result in infertility, inviability, and birth defects. In the yeast Saccharomyces cerevisiae, sexual reproduction occurs via sporulation and is similar in many respects to gametogenesis in mammals. An increasing number of conserved signaling molecules have been shown to be essential for yeast sporulation; recent studies reveal molecular insights into how these molecules regulate this intricate differentiation program.

Ady4p and Spo74p are components of the meiotic spindle pole body that promote growth of the prospore membrane in Saccharomyces cerevisiae

Spore formation in Saccharomyces cerevisiae occurs via the de novo synthesis of the prospore membrane during the second meiotic division. Prospore membrane formation is triggered by assembly of a membrane-organizing center, the meiotic outer plaque (MOP), on the cytoplasmic face of the spindle pole body (SPB) during meiosis. We report here the identification of two new components of the MOP, Ady4p and Spo74p. Ady4p and Spo74p interact with known proteins of the MOP and are localized to the outer plaque of the SPB during meiosis II. MOP assembly and prospore membrane formation are abolished in spo74Delta/spo74Delta cells and occur aberrantly in ady4Delta/ady4Delta cells. Spo74p and the MOP component Mpc70p are mutually dependent for recruitment to SPBs during meiosis. In contrast, both Ady4p and Spo74p are present at SPBs, albeit at reduced levels, in cells that lack the MOP component Mpc54p. Our findings suggest a model for the assembled MOP in which Mpc54p, Mpc70p, and Spo74p make up a core structural unit of the scaffold that initiates synthesis of the prospore membrane, and Ady4p is an auxiliary component that stabilizes the plaque.

Human Mps1 kinase is required for the spindle assembly checkpoint but not for centrosome duplication

Budding yeast Mps1p kinase has been implicated in both the duplication of microtubule-organizing centers and the spindle assembly checkpoint. Here we show that hMps1, the human homolog of yeast Mps1p, is a cell cycle-regulated kinase with maximal activity during M phase. hMps1 localizes to kinetochores and its activity and phosphorylation state increase upon activation of the mitotic checkpoint. By antibody microinjection and siRNA, we demonstrate that hMps1 is required for human cells to undergo checkpoint arrest in response to microtubule depolymerization. In contrast, centrosome (re-)duplication as well as cell division occur in the absence of hMps1. We conclude that hMps1 is required for the spindle assembly checkpoint but not for centrosome duplication.

Ady3p links spindle pole body function to spore wall synthesis in Saccharomyces cerevisiae

Spore formation in Saccharomyces cerevisiae requires the de novo synthesis of prospore membranes and spore walls. Ady3p has been identified as an interaction partner for Mpc70p/Spo21p, a meiosis-specific component of the outer plaque of the spindle pole body (SPB) that is required for prospore membrane formation, and for Don1p, which forms a ring-like structure at the leading edge of the prospore membrane during meiosis II. ADY3 expression has been shown to be induced in midsporulation. We report here that Ady3p interacts with additional components of the outer and central plaques of the SPB in the two-hybrid assay. Cells that lack ADY3 display a decrease in sporulation efficiency, and most ady3Delta/ady3Delta asci that do form contain fewer than four spores. The sporulation defect in ady3Delta/ady3Delta cells is due to a failure to synthesize spore wall polymers. Ady3p forms ring-like structures around meiosis II spindles that colocalize with those formed by Don1p, and Don1p rings are absent during meiosis II in ady3Delta/ady3Delta cells. In mpc70Delta/mpc70Delta cells, Ady3p remains associated with SPBs during meiosis II. Our results suggest that Ady3p mediates assembly of the Don1p-containing structure at the leading edge of the prospore membrane via interaction with components of the SPB and that this structure is involved in spore wall formation.

A Gip1p-Glc7p phosphatase complex regulates septin organization and spore wall formation

Sporulation of Saccharomyces cerevisiae is a developmental process in which a single cell is converted into four haploid spores. GIP1, encoding a developmentally regulated protein phosphatase 1 interacting protein, is required for spore formation. Here we show that GIP1 and the protein phosphatase 1 encoded by GLC7 play essential roles in spore development. The gip1Delta mutant undergoes meiosis and prospore membrane formation normally, but is specifically defective in spore wall synthesis. We demonstrate that in wild-type cells, distinct layers of the spore wall are deposited in a specific temporal order, and that gip1Delta cells display a discrete arrest at the onset of spore wall deposition. Localization studies revealed that Gip1p and Glc7p colocalize with the septins in structures underlying the growing prospore membranes. Interestingly, in the gip1Delta mutant, not only is Glc7p localization altered, but septins are also delocalized. Similar phenotypes were observed in a glc7-136 mutant, which expresses a Glc7p defective in interacting with Gip1p. These results indicate that a Gip1p-Glc7p phosphatase complex is required for proper septin organization and initiation of spore wall formation during sporulation.