The C. elegans MELK ortholog PIG-1 regulates cell size asymmetry and daughter cell fate in asymmetric neuroblast divisions

In the nematode Caenorhabditis elegans, neurons are generated from asymmetric divisions in which a mother cell divides to produce daughters that differ in fate. Here, we demonstrate that the gene pig-1 regulates the asymmetric divisions of neuroblasts that divide to produce an apoptotic cell and either a neural precursor or a neuron. In pig-1 mutants, these neuroblasts divide to produce daughters that are more equal in size, and their apoptotic daughters are transformed into their sisters, leading to the production of extra neurons. PIG-1 is orthologous to MELK, a conserved member of the polarity-regulating PAR-1/Kin1/SAD-1 family of serine/threonine kinases. Although MELK has been implicated in regulating the cell cycle, our data suggest that PIG-1, like other PAR-1 family members, regulates cell polarity.     

MOM-5 frizzled regulates the distribution of DSH-2 to control C. elegans asymmetric neuroblast divisions

Asymmetric cell divisions produce all 302 neurons of the C. elegans hermaphrodite. Here, we describe a role for a C. elegans Dishevelled homolog, DSH-2, in an asymmetric neuroblast division. In dsh-2 mutants, neurons normally descended from the anterior neuroblast daughter of the ABpl/rpppa blast cell were frequently duplicated, while non-neuronal cells produced by the posterior daughter cell were often missing. These observations indicate that in the absence of dsh-2 function, the posterior daughter cell was transformed into a second anterior-like cell. Loss of mom-5, a C. elegans frizzled homolog, produced a similar phenotype. We also show that the DSH-2 protein localized to the cell cortex in most cells of the embryo. In the absence of MOM-5/Fz, DSH-2 was localized to the cytoplasm, suggesting that MOM-5 regulates asymmetric cell division by controlling the localization of DSH-2. Although all neurons in C. elegans are produced by an invariant pattern of cell divisions, our results indicate that cell signaling may contribute to asymmetric neuroblast division during embryogenesis.     

C. elegans HAM-1 positions the cleavage plane and regulates apoptosis in asymmetric neuroblast divisions

Asymmetric cell division occurs when a mother cell divides to generate two distinct daughter cells, a process that promotes the generation of cellular diversity in metazoans. During Caenorhabditis elegans development, the asymmetric divisions of neural progenitors generate neurons, neural support cells and apoptotic cells. C. elegans HAM-1 is an asymmetrically distributed cortical protein that regulates several of these asymmetric neuroblast divisions. Here, we show that HAM-1 is a novel protein and define residues important for HAM-1 function and distribution to the cell cortex. Our phenotypic analysis of ham-1 mutant embryos suggests that HAM-1 controls only neuroblast divisions that produce apoptotic cells. Moreover, ham-1 mutant embryos contain many unusually large cell-death corpses. An investigation of this corpse phenotype revealed that it results from a reversal of neuroblast polarity. A misplacement of the neuroblast cleavage plane generates daughter cells of abnormal size, with the apoptotic daughters larger than normal. Thus, HAM-1 regulates the position of the cleavage plane, apoptosis and mitotic potential in C. elegans asymmetric cell divisions.     

Strabismus regulates asymmetric cell divisions and cell fate determination in the mouse brain

The planar cell polarity (PCP) pathway organizes the cytoskeleton and polarizes cells within embryonic tissue. We investigate the relationship between PCP signaling and cell fate determination during asymmetric division of neural progenitors (NPs) in mouse embryos. The cortex of Lp/Lp (Loop-tail) mice deficient in the essential PCP mediator Vangl2, homologue of Drosophila melanogaster Strabismus (Stbm), revealed precocious differentiation of neural progenitors into early-born neurons at the expense of late-born neurons and glia. Although Lp/Lp NPs were easily maintained in vitro, they showed premature differentiation and loss of asymmetric distribution of Leu-Gly-Asn–enriched protein (LGN)/partner of inscuteable (Pins), a regulator of mitotic spindle orientation. Furthermore, we observed a decreased frequency in asymmetric distribution of the LGN target nuclear mitotic apparatus protein (NuMa) in Lp/Lp cortical progenitors in vivo. This was accompanied by an increase in the number of vertical cleavage planes typically associated with equal daughter cell identities. These findings suggest that Stbm/Vangl2 functions to maintain cortical progenitors and regulates mitotic spindle orientation during asymmetric divisions in the vertebrate brain.     

DSH-2 regulates asymmetric cell division in the early C. elegans somatic gonad

Like other organs, the C. elegans gonad develops from a simple primordium that must undergo axial patterning to generate correct adult morphology. Proximal/distal (PD) polarity in the C. elegans gonad is established early during gonadogenesis by the somatic gonad precursor cells, Z1 and Z4. Z1 and Z4 each divide asymmetrically to generate one daughter with a proximal fate and one with a distal fate. PD polarity of the Z1/Z4 lineages requires the activity of a Wnt pathway that activates the TCF/LEF homolog pop-1. How the gonadal pathway controlling pop-1 is regulated by upstream factors has been unclear, as neither Wnt nor Dishevelled (Dsh) proteins have been shown to be required. Here we show that the C. elegansdsh homolog dsh-2 controls gonadal polarity. As in pop-1 mutants, dsh-2 hermaphrodites have Z1 and Z4 lineage defects indicative of defective PD polarity and are missing gonadal arms. Males have an elongated but disorganized gonad, also with lineage defects. DSH-2 protein is expressed in the Z1/Z4 gonadal precursor cells. Asymmetric distribution of nuclear GFP::POP-1 in Z1 and Z4 daughter cells is reversed in dsh-2 mutants, with higher levels in distal than proximal daughters. dsh-2 and the frizzled receptor homolog lin-17 have a strong genetic interaction, suggesting that they act in a common pathway. We suggest that DSH-2 functions as an upstream regulator of POP-1 in the somatic gonad to control asymmetric cell division, thereby establishing proximal-distal polarity of the developing organ.     

不对称分裂中命运决定子的调节机制

不对称细胞分裂是果蝇等无脊椎动物以及脊椎动物神经发生过程中神经干细胞分化的基本机制.命运决定子的极性定位及其选择性分配,作为不对称细胞分裂中的重要环节,在子细胞命运决定方面发挥至关重要的作用.本文综述了在中枢及外周神经系统发育期间,不对称分裂中调节Numb等命运决定子靶向定位的影响因素及命运决定子的效应机制,并简要探讨命运决定子调节机制的进化保守性.     

Human ACAP2 is a homolog of C. elegans CNT-1 that promotes apoptosis in cancer cells

Activation of caspases is an integral part of the apoptotic cell death program. Collectively, these proteases target hundreds of substrates, leading to the hypothesis that apoptosis is “death by a thousand cuts”. Recent work, however, has demonstrated that caspase cleavage of only a subset of these substrates directs apoptosis in the cell. One such example is C. elegans CNT-1, which is cleaved by CED-3 to generate a truncated form, tCNT-1, that acquires a potent phosphoinositide-binding activity and translocates to the plasma membrane where it inactivates AKT survival signaling. We report here that ACAP2, a homolog of C. elegans CNT-1, has a pro-apoptotic function and an identical phosphoinositide-binding pattern to that of tCNT-1, despite not being an apparent target of caspase cleavage. We show that knockdown of ACAP2 blocks apoptosis in cancer cells in response to the chemotherapeutic antimetabolite 5-fluorouracil and that ACAP2 expression is down-regulated in some esophageal cancers, leukemias and lymphomas. These results suggest that ACAP2 is a functional homolog of C. elegans CNT-1 and its inactivation or downregulation in human cells may contribute to cancer development.The caspases (cysteine aspartic acid proteases) are a class of proteases with diverse roles in cellular physiology including differentiation, inflammation and cell death.^1–3 Caspases play a critical role in apoptosis, where they collectively target hundreds of proteins. One prevailing view is that caspases drive apoptosis through a mass action effect due to hundreds of proteolytic cleavage events that lead to cellular disassembly and cell death.⁴ Recent studies, however, suggest that proteolysis of most substrates may simply be a bystander effect and that caspase cleavage of key proteins controlling a few specific cellular processes is what functionally drives apoptosis.⁵ Although much of the work to date has focused on factors acting upstream of caspase activation, it is becoming increasingly clear that events downstream of this commitment step are also tightly regulated and critically important for apoptosis. Presently, there is evidence of requirements for caspase-mediated control of the BCL2 family of anti-apoptotic proteins, mitochondrial elimination, chromosome fragmentation, phosphatidylserine externalization, and, as we have recently reported, inactivation of the AKT survival signaling pathway in programmed cell death (6-10 Therefore, a more thorough understanding of physiologically relevant caspase targets will increase our understanding of apoptosis in the context of animal development and disease.

Table 1

Human homologues of functional caspase targets in C. elegans. A summary of identified caspase substrates and caspase downstream events important for cell death execution in C. elegans and humans

The C. elegans Mi-2 chromatin-remodelling proteins function in vulval cell fate determination

The Mi-2 protein is the central component of the recently isolated NuRD nucleosome remodelling and histone deacetylase complex. Although the NuRD complex has been the subject of extensive biochemical analyses, little is known about its biological function. Here we show that the two C. elegans Mi-2 homologues, LET-418 and CHD-3, play essential roles during development. The two proteins possess both shared and unique functions during vulval cell fate determination, including antagonism of the Ras signalling pathway required for vulval cell fate induction and the proper execution of the 2 degrees cell fate of vulval precursor cells, a process under the control of LIN-12 Notch signalling.     

The HMX homeodomain protein MLS-2 regulates cleavage orientation, cell proliferation and cell fate specification in the C. elegans postembryonic mesoderm

The proper formation of a complex multicellular organism requires the precise coordination of many cellular events, including cell proliferation, cell fate specification and differentiation. The C. elegans postembryonic mesodermal lineage, the M lineage, allows us to study mechanisms coordinating these events at single cell resolution. We have identified an HMX homeodomain protein MLS-2 in a screen for factors required for M lineage patterning. The MLS-2 protein is present in nuclei of undifferentiated cells in the early M lineage and in a subset of head neurons. In the M lineage, MLS-2 activity appears to be tightly regulated at the fourth round of cell division, coincident with the transition from proliferation to differentiation. A predicted null allele of mls-2, cc615, causes reduced cell proliferation in the M lineage, whereas a semi-dominant, gain-of-function allele, tm252, results in increased cell proliferation. Loss or overexpression of mls-2 also affects cleavage orientation and cell fate specification in the M lineage. We show that the increased cell proliferation in mls-2(tm252) mutants requires CYE-1, a G1 cell cycle regulator. Furthermore, the C. elegans Myod homolog HLH-1 acts downstream of mls-2 to specify M-derived coelomocyte cell fates. Thus MLS-2 functions in a cell type-specific manner to regulate both cell proliferation and cell fate specification.     

Autophagy regulates ageing in C. elegans

The role of autophagy in ageing regulation has been suggested based on studies in C. elegans, in which knockdown of the expression of bec-1 (ortholog of the yeast and mammalian autophagy genes ATG6/VPS30 and beclin 1, respectively) shortens lifespan of the daf-2(e1370) mutant C. elegans. However, Beclin1/ATG6 is also known to be involved in other cellular functions in addition to autophagy. In the current study, we knocked down two other autophagy genes, atg-7 and atg-12, in C. elegans using RNAi. We showed that RNAi shortened the lifespan of both wild type and daf-2 mutant C. elegans, providing strong support for a role of autophagy in ageing regulation.     

The SCFSlimb E3 ligase complex regulates asymmetric division to inhibit neuroblast overgrowth

Drosophila larval brain neuroblasts divide asymmetrically to balance between self‐renewal and differentiation. Here, we demonstrate that the SCF^Slimb E3 ubiquitin ligase complex, which is composed of Cul1, SkpA, Roc1a and the F‐box protein Supernumerary limbs (Slimb), inhibits ectopic neuroblast formation and regulates asymmetric division of neuroblasts. Hyperactivation of Akt leads to similar neuroblast overgrowth and defects in asymmetric division. Slimb associates with Akt in a protein complex, and SCF^Slimb acts through SAK and Akt to inhibit neuroblast overgrowth. Moreover, Beta‐transducin repeat containing, the human ortholog of Slimb, is frequently deleted in highly aggressive gliomas, suggesting a conserved tumor suppressor‐like function.     

The phosphoinositide phosphatase MTM-1 regulates apoptotic cell corpse clearance through CED-5-CED-12 in C. elegans

Multicellular organisms use programmed cell death to eliminate unwanted or potentially harmful cells. Improper cell corpse removal can lead to autoimmune diseases. The development of interventional therapies that increase engulfment activity could represent an attractive approach to treat such diseases. Here, we describe mtm-1, the Caenorhabditis elegans homolog of human myotubularin 1, as a potential negative regulator of apoptotic cell corpse clearance. Loss of mtm-1 function leads to substantially reduced numbers of persistent cell corpses in engulfment mutants, which is a result of a restoration of engulfment function rather than of impaired or delayed programmed cell death. Epistatic analyses place mtm-1 upstream of the ternary GEF complex, which consists of ced-2, ced-5 and ced-12, and parallel to mig-2. Over-activation of engulfment results in the removal of viable cells that have been brought to the verge of death under limiting caspase activity. In addition, mtm-1 also promotes phagosome maturation in the hermaphrodite gonad, potentially through CED-1 receptor recycling. Finally, we show that the CED-12 PH domain can bind to PtdIns(3,5)P(2) (one target of MTM-1 phosphatase activity), suggesting that MTM-1 might regulate CED-12 recruitment to the plasma membrane.     

The MIG-15 NIK kinase acts cell-autonomously in neuroblast polarization and migration in C. elegans

Cell migration is a fundamental process in animal development, including development of the nervous system. In C. elegans, the bilateral QR and QL neuroblasts undergo initial anterior and posterior polarizations and migrations before they divide to produce neurons. A subsequent Wnt signal from the posterior instructs QL descendants to continue their posterior migration. Nck-interacting kinases (NIK kinases) have been implicated in cell and nuclear migration as well as lamellipodia formation. Studies here show that the C. elegans MIG-15 NIK kinase controls multiple aspects of initial Q cell polarization, including the ability of the cells to polarize, to maintain polarity, and to migrate. These data suggest that MIG-15 acts independently of the Wnt signal that controls QL descendant posterior migration. Furthermore, MIG-15 affects the later migrations of neurons generated from Q cell division. Finally, a mosaic analysis indicates that MIG-15 acts cell-autonomously in Q descendant migration.     

C. elegans cell cycles: invariance and stem cell divisions

The adult Caenorhabditis elegans nematode, a small roundworm, has a precisely defined number of somatic cells that create organs that are also found in larger animals, including intestine, muscles, skin, an excretory system and a primitive brain. Every cell has a defined role in this sophisticated, but tiny animal. Therefore, stringent control of the cell cycle is required to produce the almost invariant cell lineage that generates the C. elegans somatic body plan. The proliferation of germ cells is regulated differently, and occurs within a stem cell niche.     

C. elegans Rab GTPase 2 is required for the degradation of apoptotic cells

During apoptosis, the dying cell activates an intrinsic mechanism that quickly dismantles itself. The apoptotic cell corpses are then recognized and removed by neighboring cells or professional phagocytes. How dying cells are degraded after internalization is poorly understood. Here, we report the identification and characterization of unc-108, the Caenorhabditis elegans homolog of the human Rab GTPase 2, as a novel component involved in the degradation of apoptotic cells. unc-108 is expressed and functions in the engulfing cells and is likely to affect the degradation rather than the internalization of cell corpses. Similar to other Rab GTPases, unc-108 also affects endocytosis, acting in the endosomal trafficking from early to late endosome and late endosome to lysosome. UNC-108 co-localizes with RAB-5, RAB-7 and LMP-1 to the phagosome and promotes cell corpse degradation, possibly by mediating phagosome maturation.     

The DAF-2 insulin-like signaling pathway independently regulates aging and immunity in C. elegans

The Caenorhabditis elegans DAF-2 insulin-like signaling pathway, which regulates lifespan and stress resistance, has also been implicated in resistance to bacterial pathogens. Loss-of-function daf-2 and age-1 mutants have increased lifespans and are resistant to a variety of bacterial pathogens. This raises the possibility that the increased longevity and the pathogen resistance of insulin-like signaling pathway mutants are reflections of the same underlying mechanism. Here we report that regulation of lifespan and resistance to the bacterial pathogen Pseudomonas aeruginosa is mediated by both shared and genetically distinguishable mechanisms. We find that loss of germline proliferation enhances pathogen resistance and this effect requires daf-16, similar to the regulation of lifespan. In contrast, the regulation of pathogen resistance and lifespan is decoupled within the DAF-2 pathway. Long-lived mutants of genes downstream of daf-2, such as pdk-1 and sgk-1, show wildtype resistance to pathogens. However, mutants of akt-1 and akt-2, which we find to individually have modest effects on lifespan, show enhanced resistance to pathogens. We also demonstrate that pathogen resistance of daf-2, akt-1, and akt-2 mutants is associated with restricted bacterial colonization, and that daf-2 mutants are better able to clear an infection after challenge with P. aeruginosa. Moreover, we find that pathogen resistance among insulin-like signaling mutants is associated with increased expression of immunity genes during infection. Other processes that affect organismal longevity, including Jun kinase signaling and caloric restriction, do not affect resistance to bacterial pathogens, further establishing that aging and innate immunity are regulated by genetically distinct mechanisms.