Plk4/SAK/ZYG-1 in the regulation of centriole duplication

Disruption of Dhcr7 and Insig1/2 in cholesterol metabolism causes defects in bone formation and homeostasis through primary cilium formation

Human linkage studies suggest that craniofacial deformities result from either genetic mutations related to cholesterol metabolism or high-cholesterol maternal diets. However, little is known about the precise roles of intracellular cholesterol metabolism in the development of craniofacial bones, the majority of which are formed through intramembranous ossification. Here, we show that an altered cholesterol metabolic status results in abnormal osteogenesis through dysregulation of primary cilium formation during bone formation. We found that cholesterol metabolic aberrations, induced through disruption of either Dhcr7 (which encodes an enzyme involved in cholesterol synthesis) or Insig1 and Insig2 (which provide a negative feedback mechanism for cholesterol biosynthesis), result in osteoblast differentiation abnormalities. Notably, the primary cilia responsible for sensing extracellular cues were altered in number and length through dysregulated ciliary vesicle fusion in Dhcr7 and Insig1/2 mutant osteoblasts. As a consequence, WNT/β-catenin and hedgehog signaling activities were altered through dysregulated primary cilium formation. Strikingly, the normalization of defective cholesterol metabolism by simvastatin, a drug used in the treatment of cholesterol metabolic aberrations, rescued the abnormalities in both ciliogenesis and osteogenesis in vitro and in vivo. Thus, our results indicate that proper intracellular cholesterol status is crucial for primary cilium formation during skull formation and homeostasis.

Regulation of cilia abundance in multiciliated cells

Multiciliated cells (MCC) contain hundreds of motile cilia used to propel fluid over their surface. To template these cilia, each MCC produces between 100-600 centrioles by a process termed centriole amplification. Yet, how MCC regulate the precise number of centrioles and cilia remains unknown. Airway progenitor cells contain two parental centrioles (PC) and form structures called deuterosomes that nucleate centrioles during amplification. Using an ex vivo airway culture model, we show that ablation of PC does not perturb deuterosome formation and centriole amplification. In contrast, loss of PC caused an increase in deuterosome and centriole abundance, highlighting the presence of a compensatory mechanism. Quantification of centriole abundance in vitro and in vivo identified a linear relationship between surface area and centriole number. By manipulating cell size, we discovered that centriole number scales with surface area. Our results demonstrate that a cell-intrinsic surface area-dependent mechanism controls centriole and cilia abundance in multiciliated cells.

A Short CEP135 Splice Isoform Controls Centriole Duplication

Centriole duplication is coordinated such that a single round of duplication occurs during each cell cycle. Disruption of this synchrony causes defects including supernumerary centrosomes in cancer and perturbed ciliary signaling [1-5]. To preserve the normal number of centrioles, the level, localization, and post-translational modification of centriole proteins is regulated so that, when centriole protein expression and/or activity are increased, centrioles self-assemble. Assembly is initiated by the formation of the cartwheel structure that comprises the base of centrioles [6-11]. SAS-6 constitutes the cartwheel, and SAS-6 levels remain low until centriole assembly is initiated at S phase onset [3, 12, 13]. CEP135 physically links to SAS-6 near the site of microtubule nucleation and binds to CPAP for triplet microtubule formation [13, 14]. We identify two distinct protein isoforms of CEP135 that antagonize each other to modulate centriole duplication: full-length CEP135 (CEP135(full)) promotes new assembly, whereas a short isoform, CEP135(mini), represses it. CEP135(mini) represses centriole duplication by limiting the centriolar localization of CEP135(full) binding proteins (SAS-6 and CPAP) and the pericentriolar localization of γ-tubulin. The CEP135 isoforms exhibit distinct and complementary centrosomal localization during the cell cycle. CEP135(mini) protein decreases from centrosomes upon anaphase onset. We suggest that the decrease in CEP135(mini) from centrosomes promotes centriole assembly. The repression of centriole duplication by a splice isoform of a protein that normally promotes it serves as a novel mechanism to limit centriole duplication.

Deuterosome-mediated centriole biogenesis

The ability of cells to faithfully duplicate their two centrioles once per cell cycle is critical for proper mitotic progression and chromosome segregation. Multiciliated cells represent an interesting variation of centriole duplication in that these cells generate greater than 100 centrioles, which form the basal bodies of their motile cilia. This centriole amplification is proposed to require a structure termed the deuterosome, thought to be capable of promoting de novo centriole biogenesis. Here, we begin to molecularly characterize the deuterosome and identify it as a site for the localization of Cep152, Plk4, and SAS6. Additionally we identify CCDC78 as a centriole-associated and deuterosome protein that is essential for centriole amplification. Overexpression of Cep152, but not Plk4, SAS6, or CCDC78, drives overamplification of centrioles. However, in CCDC78 morphants, Cep152 fails to localize to the deuterosome and centriole biogenesis is impaired, indicating that CCDC78-mediated recruitment of Cep152 is required for deuterosome-mediated centriole biogenesis.

The importance of a single primary cilium

The centrosome is the main microtubule-organizing center in animal cells, and helps to influence the morphology of the microtubule cytoskeleton in interphase and mitosis. The centrosome also templates the assembly of the primary cilium, and together they serve as a nexus of cell signaling that provide cells with diverse organization, motility, and sensory functions. The majority of cells in the human body contain a solitary centrosome and cilium, and cells have evolved regulatory mechanisms to precisely control the numbers of these essential organelles. Defects in the structure and function of cilia lead to a variety of complex disease phenotypes termed ciliopathies, while dysregulation of centrosome number has long been proposed to induce genome instability and tumor formation. Here, we review recent findings that link centrosome amplification to changes in cilium number and signaling capacity, and discuss how supernumerary centrosomes may be an important aspect of a set of cilia-related disease phenotypes.

SmSak, the second Polo-like kinase of the helminth parasite Schistosoma mansoni: conserved and unexpected roles in meiosis

Polo-like kinases (Plks) are a family of conserved regulators of a variety of events throughout the cell cycle, expanded from one Plk in yeast to five Plks in mammals (Plk1-5). Plk1 is the best characterized member of the Plk family, homolog to the founding member Polo of Drosophila, and plays a major role in cell cycle progression by triggering G2/M transition. Plk4/Sak (for Snk (Serum-inducible kinase) akin kinase) is a unique member of the family, structurally distinct from other Plk members, with essential functions in centriole duplication. The genome of the trematode parasite Schistosoma mansoni contains only two Plk genes encoding SmPlk1 and SmSak. SmPlk1 has been shown already to be required for gametogenesis and parasite reproduction. In this work, in situ hybridization indicated that the structurally conserved Plk4 protein, SmSak, was largely expressed in schistosome female ovary and vitellarium. Expression of SmSak in Xenopus oocytes confirmed its Plk4 conserved function in centriole amplification. Moreover, analysis of the function of SmSak in meiosis progression of G2-blocked Xenopus oocytes indicated that, in contrast to SmPlk1, SmSak cannot induce G2/M transition in the absence of endogenous Plk1 (Plx1). Unexpectedly, meiosis progression was spontaneously observed in Plx1-depleted oocytes co-expressing SmSak and SmPlk1. Molecular interaction between SmSak and SmPlk1 was confirmed by co-immunoprecipitation of both proteins. These data indicate that Plk1 and Plk4 proteins have the potential to interact and cross-activate in cells, thus attributing for the first time a potential role of Plk4 proteins in meiosis/mitosis entry. This unexpected role of SmSak in meiosis could be relevant to further consider the function of this novel Plk in schistosome reproduction.