Primary cilia and the DNA damage response: linking a cellular antenna and nuclear signals

The maintenance of genome stability involves integrated biochemical activities that detect DNA damage or incomplete replication, delay the cell cycle, and direct DNA repair activities on the affected chromatin. These processes, collectively termed the DNA damage response (DDR), are crucial for cell survival and to avoid disease, particularly cancer. Recent work has highlighted links between the DDR and the primary cilium, an antenna-like, microtubule-based signalling structure that extends from a centriole docked at the cell surface. Ciliary dysfunction gives rise to a range of complex human developmental disorders termed the ciliopathies. Mutations in ciliopathy genes have been shown to impact on several functions that relate to centrosome integrity, DNA damage signalling, responses to problems in DNA replication and the control of gene expression. This review covers recent findings that link cilia and the DDR and explores the various roles played by key genes in these two contexts. It outlines how proteins encoded by ciliary genes impact checkpoint signalling, DNA replication and repair, gene expression and chromatin remodelling. It discusses how these diverse activities may integrate nuclear responses with those that affect a structure of the cell periphery. Additional directions for exploration of the interplay between these pathways are highlighted, with a focus on new ciliary gene candidates that alter genome stability.

Potential Therapeutic Targets for Olfactory Dysfunction in Ciliopathies Beyond Single-Gene Replacement

Olfactory dysfunction is a common disorder in the general population. There are multiple causes, one of which being ciliopathies, an emerging class of human hereditary genetic disorders characterized by multiple symptoms due to defects in ciliary biogenesis, maintenance, and/or function. Mutations/deletions in a wide spectrum of ciliary genes have been identified to cause ciliopathies. Currently, besides symptomatic therapy, there is no available therapeutic treatment option for olfactory dysfunction caused by ciliopathies. Multiple studies have demonstrated that targeted gene replacement can restore the morphology and function of olfactory cilia in olfactory sensory neurons and further re-establish the odor-guided behaviors in animals. Therefore, targeted gene replacement could be potentially used to treat olfactory dysfunction in ciliopathies. However, due to the potential limitations of single-gene therapy for polygenic mutation-induced diseases, alternative therapeutic targets for broader curative measures need to be developed for olfactory dysfunction, and also for other symptoms in ciliopathies. Here we review the current understanding of ciliogenesis and maintenance of olfactory cilia. Furthermore, we emphasize signaling mechanisms that may be involved in the regulation of olfactory ciliary length and highlight potential alternative therapeutic targets for the treatment of ciliopathy-induced dysfunction in the olfactory system and even in other ciliated organ systems.

Human Brain Organoids to Decode Mechanisms of Microcephaly

Brain organoids are stem cell-based self-assembling 3D structures that recapitulate early events of human brain development. Recent improvements with patient-specific 3D brain organoids have begun to elucidate unprecedented details of the defective mechanisms that cause neurodevelopmental disorders of congenital and acquired microcephaly. In particular, brain organoids derived from primary microcephaly patients have uncovered mechanisms that deregulate neural stem cell proliferation, maintenance, and differentiation. Not only did brain organoids reveal unknown aspects of neurogenesis but also have illuminated surprising roles of cellular structures of centrosomes and primary cilia in regulating neurogenesis during brain development. Here, we discuss how brain organoids have started contributing to decoding the complexities of microcephaly, which are unlikely to be identified in the existing non-human models. Finally, we discuss the yet unresolved questions and challenges that can be addressed with the use of brain organoids as in vitro models of neurodevelopmental disorders.

The Centrosome and the Primary Cilium: The Yin and Yang of a Hybrid Organelle

Centrosomes and primary cilia are usually considered as distinct organelles, although both are assembled with the same evolutionary conserved, microtubule-based templates, the centrioles. Centrosomes serve as major microtubule- and actin cytoskeleton-organizing centers and are involved in a variety of intracellular processes, whereas primary cilia receive and transduce environmental signals to elicit cellular and organismal responses. Understanding the functional relationship between centrosomes and primary cilia is important because defects in both structures have been implicated in various diseases, including cancer. Here, we discuss evidence that the animal centrosome evolved, with the transition to complex multicellularity, as a hybrid organelle comprised of the two distinct, but intertwined, structural-functional modules: the centriole/primary cilium module and the pericentriolar material/centrosome module. The evolution of the former module may have been caused by the expanding cellular diversification and intercommunication, whereas that of the latter module may have been driven by the increasing complexity of mitosis and the requirement for maintaining cell polarity, individuation, and adhesion. Through its unique ability to serve both as a plasma membrane-associated primary cilium organizer and a juxtanuclear microtubule-organizing center, the animal centrosome has become an ideal integrator of extracellular and intracellular signals with the cytoskeleton and a switch between the non-cell autonomous and the cell-autonomous signaling modes. In light of this hypothesis, we discuss centrosome dynamics during cell proliferation, migration, and differentiation and propose a model of centrosome-driven microtubule assembly in mitotic and interphase cells. In addition, we outline the evolutionary benefits of the animal centrosome and highlight the hierarchy and modularity of the centrosome biogenesis networks.

Primary cilia: biosensors of the male reproductive tract

BACKGROUND

The primary cilium is a microtubule-based organelle that extends transiently from the apical cell surface to act as a sensory antenna. Initially viewed as a cellular appendage of obscure significance, the primary cilium is now acknowledged as a key coordinator of signaling pathways during development and in tissue homeostasis.

OBJECTIVES

The aim of this review was to present the structure and function of this overlooked organelle,with an emphasis on its epididymal context and contribution to male infertility issues.

MATERIALS AND METHODS

A systematic review has been performed in order to include main references relevant to the aforementioned topic.

RESULTS

Increasing evidence demonstrates that primary cilia dysfunctions are associated with impaired male reproductive system development and male infertility issues.

DISCUSSION

While a large amount of data exists regarding the role of primary cilia in most organs and tissues, few studies investigated the contribution of these organelles to male reproductive tract development and homeostasis.

CONCLUSION

Functional studies of primary cilia constitute an emergent and exciting new area in reproductive biology research.

BBS4 is required for intraflagellar transport coordination and basal body number in mammalian olfactory cilia

Bardet-Beidl syndrome (BBS) manifests from genetic mutations encoding for one or more BBS proteins. BBS4 loss impacts olfactory ciliation and odor detection, yet the cellular mechanisms remain unclear. Here, we report that Bbs4-/- mice exhibit shorter and fewer olfactory sensory neuron (OSN) cilia despite retaining odorant receptor localization. Within Bbs4-/- OSN cilia, we observed asynchronous rates of IFT-A/B particle movements, indicating miscoordination in IFT complex trafficking. Within the OSN dendritic knob, the basal bodies are dynamic, with incorporation of ectopically expressed centrin-2 and γ-tubulin occurring after nascent ciliogenesis. Importantly, BBS4 loss results in the reduction of basal body numbers separate from cilia loss. Adenoviral expression of BBS4 restored OSN cilia lengths and was sufficient to re-establish odor detection, but failed to rescue ciliary and basal body numbers. Our results yield a model for the plurality of BBS4 functions in OSNs that includes intraciliary and periciliary roles that can explain the loss of cilia and penetrance of ciliopathy phenotypes in olfactory neurons.

TULP3 is required for localization of membrane-associated proteins ARL13B and INPP5E to primary cilia

The primary cilia are known as biosensors that transduce signals through the ciliary membrane proteins in vertebrate cells. The ciliary membrane contains transmembrane proteins and membrane-associated proteins. Tubby-like protein 3 (TULP3), a member of the tubby family, has been shown to interact with the intraflagellar transport-A complex (IFT-A) and to be involved in the ciliary localization of transmembrane proteins, although its role in the ciliary entry of membrane-associated proteins has remained unclear. Here, to determine whether TULP3 is required for the localization of ciliary membrane-associated proteins, we generated and analyzed TULP3-knockout (KO) hTERT RPE-1 (RPE1) cells. Immunofluorescence analysis demonstrated that ciliary formation was downregulated in TULP3-KO cells and that membrane-associated proteins, ADP-ribosylation factor-like 13B (ARL13B) and inositol polyphosphate-5-phosphatase E (INPP5E), failed to localize to primary cilia in TULP3-KO cells. These defects in the localization of ARL13B and INPP5E in TULP3-KO cells were rescued by the exogenous expression of wild-type TULP3, but not that of mutant TULP3 lacking the ability to bind IFT-A. In addition, the expression of TUB protein, another member of the tubby family whose endogenous expression is absent in RPE1 cells, also rescued the defective ciliary localization of ARL13B and INPP5E in TULP3-KO cells, suggesting that there is functional redundancy between TULP3 and TUB. Our findings indicate that TULP3 participates in ciliogenesis, and targets membrane-associated proteins to primary cilia via binding to IFT-A.

Insights into photoreceptor ciliogenesis revealed by animal models

Photoreceptors are polarized neurons, with very specific subcellular compartmentalization and unique requirements for protein expression and trafficking. Each photoreceptor contains an outer segment, the site of photon capture that initiates vision, an inner segment that houses the biosynthetic machinery and a synaptic terminal for signal transmission to downstream neurons. Outer segments and inner segments are connected by a connecting cilium (CC), the equivalent of a transition zone (TZ) of primary cilia. The connecting cilium is part of the basal body/axoneme backbone that stabilizes the outer segment. This report will update the reader on late developments in photoreceptor ciliogenesis and transition zone formation, specifically in mouse photoreceptors, focusing on early events in photoreceptor ciliogenesis. The connecting cilium, an elongated and narrow structure through which all outer segment proteins and membrane components must traffic, functions as a gate that controls access to the outer segment. Here we will review genes and their protein products essential for basal body maturation and for CC/TZ genesis, sorted by phenotype. Emphasis is given to naturally occurring mouse mutants and gene knockouts that interfere with CC/TZ formation and ciliogenesis.

Functional analyses of Pericentrin and Syne-2 interaction in ciliogenesis

Pericentrin (Pcnt) is a multifunctional scaffold protein and mutations in the human PCNT gene are associated with several diseases, including ciliopathies. Pcnt plays a crucial role in ciliary development in olfactory receptor neurons, but its function in the photoreceptor-connecting cilium is unknown. We downregulated Pcnt in the retina ex vivo and in vivo via a virus-based RNA interference approach to study Pcnt function in photoreceptors. ShRNA-mediated knockdown of Pcnt impaired the development of the connecting cilium and the outer segment of photoreceptors, and caused a nuclear migration defect. In protein interaction screens, we found that the outer nuclear membrane protein Syne-2 (also known as Nesprin-2) is an interaction partner of Pcnt in photoreceptors. Syne-2 is important for positioning murine photoreceptor cell nuclei and for centrosomal migration during early ciliogenesis. CRISPR/Cas9-mediated knockout of Syne-2 in cell culture led to an overexpression and mislocalization of Pcnt and to ciliogenesis defects. Our findings suggest that the Pcnt-Syne-2 complex is important for ciliogenesis and outer segment formation during retinal development and plays a role in nuclear migration.

Nuclear roles for cilia-associated proteins

Cilia appear to be derived, evolutionarily, from structures present in the ancestral (pre-ciliary) eukaryote, such as microtubule-based vesicle trafficking and chromosome segregation systems. Experimental observations suggest that the ciliary gate, the molecular complex that mediates the selective molecular movement between cytoplasmic and ciliary compartments, shares features with nuclear pores. Our hypothesis is that this shared transport machinery is at least partially responsible for the observation that a number of ciliary and ciliogenesis-associated proteins are found within nuclei where they play roles in the regulation of gene expression, DNA repair, and nuclear import and export. Recognizing the potential for such nuclear roles is critical when considering the phenotypic effects that arise from the mutational modification of ciliary proteins.

Consequences of Centrosome Dysfunction During Brain Development

Development requires cell proliferation, differentiation and spatial organization of daughter cells to occur in a highly controlled manner. The mode of cell division, the extent of proliferation and the spatial distribution of mitosis allow the formation of tissues of the right size and with the correct structural organization. All these aspects depend on cell cycle duration, correct chromosome segregation and spindle orientation. The centrosome, which is the main microtubule-organizing centre (MTOC) of animal cells, contributes to all these processes. As one of the most structurally complex organs in our body, the brain is particularly susceptible to centrosome dysfunction. Autosomal recessive primary microcephaly (MCPH), primordial dwarfism disease Seckel syndrome (SCKS) and microcephalic osteodysplastic primordial dwarfism type II (MOPD-II) are often connected to mutations in centrosomal genes. In this chapter, we discuss the consequences of centrosome dysfunction during development and how they can contribute to the etiology of human diseases.

Native Piezo2 Interactomics Identifies Pericentrin as a Novel Regulator of Piezo2 in Somatosensory Neurons

The ability of somatosensory neurons to perceive mechanical stimuli relies on specialized mechanotransducing proteins and their molecular environment. Only recently has the identity of a major transducer of mechanical forces in vertebrates been revealed by the discovery of Piezo2. Further work has established its pivotal role for innocuous touch in mice. Therefore, Piezo2 offers a unique platform for the molecular investigation of somatosensory mechanosensation. We performed a mass spectrometry-based interactomics screen on native Piezo2 in somatosensory neurons of mouse dorsal root ganglia (DRG). Stringent and quantitative data analysis yielded the identity of 36 novel binding partners of Piezo2. The biological significance of this data set is reflected by functional experiments demonstrating a role for Pericentrin in modulating Piezo2 activity and membrane expression in somatosensory neurons. Collectively, our findings provide a framework for understanding Piezo2 physiology and serve as a rich resource for the molecular dissection of mouse somatosensation.

ATR promotes cilia signalling: links to developmental impacts

Mutations in ATR(ataxia telangiectasia and RAD3-related) cause Seckel syndrome (ATR-SS), a microcephalic primordial dwarfism disorder. Hitherto, the clinical manifestation of ATR deficiency has been attributed to its canonical role in DNA damage response signalling following replication fork stalling/collapse. Here, we show that ATR regulates cilia-dependent signalling in a manner that can be uncoupled from its function during replication. ATR-depleted or patient-derived ATR-SS cells form cilia of slightly reduced length but are dramatically impaired in cilia-dependent signalling functions, including growth factor and Sonic hedgehog signalling. To better understand the developmental impact of ATR loss of function, we also used zebrafish as a model. Zebrafish embryos depleted of Atr resembled ATR-SS morphology, showed a modest but statistically significant reduction in cilia length and other morphological features indicative of cilia dysfunction. Additionally, they displayed defects in left-right asymmetry including ambiguous expression of southpaw, incorrectly looped hearts and randomized localization of internal organs including the pancreas, features typically conferred by cilia dysfunction. Our findings reveal a novel role for ATR in cilia signalling distinct from its canonical function during replication and strengthen emerging links between cilia function and development.

Specialized Cilia in Mammalian Sensory Systems

Cilia and flagella are highly conserved and important microtubule-based organelles that project from the surface of eukaryotic cells and act as antennae to sense extracellular signals. Moreover, cilia have emerged as key players in numerous physiological, developmental, and sensory processes such as hearing, olfaction, and photoreception. Genetic defects in ciliary proteins responsible for cilia formation, maintenance, or function underlie a wide array of human diseases like deafness, anosmia, and retinal degeneration in sensory systems. Impairment of more than one sensory organ results in numerous syndromic ciliary disorders like the autosomal recessive genetic diseases Bardet-Biedl and Usher syndrome. Here we describe the structure and distinct functional roles of cilia in sensory organs like the inner ear, the olfactory epithelium, and the retina of the mouse. The spectrum of ciliary function in fundamental cellular processes highlights the importance of elucidating ciliopathy-related proteins in order to find novel potential therapies.

Lack of centrioles and primary cilia in STIL(-/-) mouse embryos

Although most animal cells contain centrosomes, consisting of a pair of centrioles, their precise contribution to cell division and embryonic development is unclear. Genetic ablation of STIL, an essential component of the centriole replication machinery in mammalian cells, causes embryonic lethality in mice around mid gestation associated with defective Hedgehog signaling. Here, we describe, by focused ion beam scanning electron microscopy, that STIL(-/-) mouse embryos do not contain centrioles or primary cilia, suggesting that these organelles are not essential for mammalian development until mid gestation. We further show that the lack of primary cilia explains the absence of Hedgehog signaling in STIL(-/-) cells. Exogenous re-expression of STIL or STIL microcephaly mutants compatible with human survival, induced non-templated, de novo generation of centrioles in STIL(-/-) cells. Thus, while the abscence of centrioles is compatible with mammalian gastrulation, lack of centrioles and primary cilia impairs Hedgehog signaling and further embryonic development.

Kinesin family 17 (osmotic avoidance abnormal-3) is dispensable for photoreceptor morphology and function

In Caenorhabditis elegans, homodimeric [kinesin family (KIF) 17, osmotic avoidance abnormal-3 (OSM-3)] and heterotrimeric (KIF3) kinesin-2 motors are required to establish sensory cilia by intraflagellar transport (IFT) where KIF3 and KIF17 cooperate to build the axoneme core and KIF17 builds the distal segments. However, the function of KIF17 in vertebrates is unresolved. We expressed full-length and motorless KIF17 constructs in mouse rod photoreceptors using adeno-associated virus in Xenopus laevis rod photoreceptors using a transgene and in ciliated IMCD3 cells. We found that tagged KIF17 localized along the rod outer segment axoneme when expressed in mouse and X. laevis photoreceptors, whereas KIF3A was restricted to the proximal axoneme. Motorless KIF3A and KIF17 mutants caused photoreceptor degeneration, likely through dominant negative effects on IFT. KIF17 mutant lacking the motor domain translocated to nuclei after exposure of a C-terminal nuclear localization signal. Germ-line deletion of Kif17 in mouse did not affect photoreceptor function. A rod-specific Kif3/Kif17 double knockout mouse demonstrated that KIF17 and KIF3 do not act synergistically and did not prevent rhodopsin trafficking to rod outer segments. In summary, the nematode model of KIF3/KIF17 cooperation apparently does not apply to mouse photoreceptors in which the photosensory cilium is built exclusively by KIF3.

Pericentrin Is Related to Abnormal β-Cell Insulin Secretion through F-Actin Regulation in Mice

The aim of this study was to investigate the regulating effect of pericentrin (PCNT) on insulin secretion in the development of insulin resistance and to determine the underlying mechanism. PCNT expression was studied in different tissues of C57/B6 mice by reverse transcriptase-PCR and immunofluorescence. PCNT was highly expressed in organs involved in the regulation of metabolism, while cytoplasmic expression was only enriched in islet cells. PCNT expression was significantly lower in the central regions of insulin resistance (IR) mouse islets than in those of control mouse islets. PCNT expression was further studied in mouse MIN6 cells exposed to glucose stimulation, small interfering RNA (siRNA) against PCNT, and an ERK inhibitor (PD98095). The results revealed that PCNT expression in glucose-stimulated MIN6 cells reduced linearly with cytoplasmic insulin levels. MIN6 cells transfected with PCNT siRNA showed significantly decreased intracellular insulin and F-actin expression. The change in F-actin expression in MIN6 cells during PCNT siRNA interference showed a linear relationship with PCNT expression at different time points. The ERK inhibitor affected PCNT expression and F-actin expression linearly. The abnormal insulin secretion observed both in vivo and in vitro was associated with decreased PCNT expression, and F-actin was found to be the target of PCNT regulation.

Centrin 2 is required for mouse olfactory ciliary trafficking and development of ependymal cilia planar polarity

Centrins are ancient calmodulin-related Ca(2+)-binding proteins associated with basal bodies. In lower eukaryotes, Centrin2 (CETN2) is required for basal body replication and positioning, although its function in mammals is undefined. We generated a germline CETN2 knock-out (KO) mouse presenting with syndromic ciliopathy including dysosmia and hydrocephalus. Absence of CETN2 leads to olfactory cilia loss, impaired ciliary trafficking of olfactory signaling proteins, adenylate cyclase III (ACIII), and cyclic nucleotide-gated (CNG) channel, as well as disrupted basal body apical migration in postnatal olfactory sensory neurons (OSNs). In mutant OSNs, cilia base-anchoring of intraflagellar transport components IFT88, the kinesin-II subunit KIF3A, and cytoplasmic dynein 2 appeared compromised. Although the densities of mutant ependymal and respiratory cilia were largely normal, the planar polarity of mutant ependymal cilia was disrupted, resulting in uncoordinated flow of CSF. Transgenic expression of GFP-CETN2 rescued the Cetn2-deficiency phenotype. These results indicate that mammalian basal body replication and ciliogenesis occur independently of CETN2; however, mouse CETN2 regulates protein trafficking of olfactory cilia and participates in specifying planar polarity of ependymal cilia.

Congenital microcephaly

The underlying etiologies of genetic congenital microcephaly are complex and multifactorial. Recently, with the exponential growth in the identification and characterization of novel genetic causes of congenital microcephaly, there has been a consolidation and emergence of certain themes concerning underlying pathomechanisms. These include abnormal mitotic microtubule spindle structure, numerical and structural abnormalities of the centrosome, altered cilia function, impaired DNA repair, DNA Damage Response signaling and DNA replication, along with attenuated cell cycle checkpoint proficiency. Many of these processes are highly interconnected. Interestingly, a defect in a gene whose encoded protein has a canonical function in one of these processes can often have multiple impacts at the cellular level involving several of these pathways. Here, we overview the key pathomechanistic themes underlying profound congenital microcephaly, and emphasize their interconnected nature.

The ciliary cytoskeleton

Cilia and flagella are surface-exposed, finger-like organelles whose core consists of a microtubule (MT)-based axoneme that grows from a modified centriole, the basal body. Cilia are found on the surface of many eukaryotic cells and play important roles in cell motility and in coordinating a variety of signaling pathways during growth, development, and tissue homeostasis. Defective cilia have been linked to a number of developmental disorders and diseases, collectively called ciliopathies. Cilia are dynamic organelles that assemble and disassemble in tight coordination with the cell cycle. In most cells, cilia are assembled during growth arrest in a multistep process involving interaction of vesicles with appendages present on the distal end of mature centrioles, and addition of tubulin and other building blocks to the distal tip of the basal body and growing axoneme; these building blocks are sorted through a region at the cilium base known as the ciliary necklace, and then transported via intraflagellar transport (IFT) along the axoneme toward the tip for assembly. After assembly, the cilium frequently continues to turn over and incorporate tubulin at its distal end in an IFT-dependent manner. Prior to cell division, the cilia are usually resorbed to liberate centrosomes for mitotic spindle pole formation. Here, we present an overview of the main cytoskeletal structures associated with cilia and centrioles with emphasis on the MT-associated appendages, fibers, and filaments at the cilium base and tip. The composition and possible functions of these structures are discussed in relation to cilia assembly, disassembly, and length regulation.

Deficiency in origin licensing proteins impairs cilia formation: implications for the aetiology of Meier-Gorlin syndrome

Mutations in ORC1, ORC4, ORC6, CDT1, and CDC6, which encode proteins required for DNA replication origin licensing, cause Meier-Gorlin syndrome (MGS), a disorder conferring microcephaly, primordial dwarfism, underdeveloped ears, and skeletal abnormalities. Mutations in ATR, which also functions during replication, can cause Seckel syndrome, a clinically related disorder. These findings suggest that impaired DNA replication could underlie the developmental defects characteristic of these disorders. Here, we show that although origin licensing capacity is impaired in all patient cells with mutations in origin licensing component proteins, this does not correlate with the rate of progression through S phase. Thus, the replicative capacity in MGS patient cells does not correlate with clinical manifestation. However, ORC1-deficient cells from MGS patients and siRNA-mediated depletion of origin licensing proteins also have impaired centrosome and centriole copy number. As a novel and unexpected finding, we show that they also display a striking defect in the rate of formation of primary cilia. We demonstrate that this impacts sonic hedgehog signalling in ORC1-deficient primary fibroblasts. Additionally, reduced growth factor-dependent signaling via primary cilia affects the kinetics of cell cycle progression following cell cycle exit and re-entry, highlighting an unexpected mechanism whereby origin licensing components can influence cell cycle progression. Finally, using a cell-based model, we show that defects in cilia function impair chondroinduction. Our findings raise the possibility that a reduced efficiency in forming cilia could contribute to the clinical features of MGS, particularly the bone development abnormalities, and could provide a new dimension for considering developmental impacts of licensing deficiency.

Promoter hijack reveals pericentrin functions in mitosis and the DNA damage response

Centrosomes, the principal microtubule-organizing centers of animal somatic cells, consist of two centrioles embedded in the pericentriolar material (PCM). Pericentrin is a large PCM protein that is required for normal PCM assembly. Mutations in PCNT cause primordial dwarfism. Pericentrin has also been implicated in the control of DNA damage responses. To test how pericentrin is involved in cell cycle control after genotoxic stress, we disrupted the Pcnt locus in chicken DT40 cells. Pericentrin-deficient cells proceeded through mitosis more slowly, with a high level of monopolar spindles, and were more sensitive to spindle poisons than controls. Centriole structures appeared normal by light and electron microscopy, but the PCM did not recruit γ-tubulin efficiently. Cell cycle delays after ionizing radiation (IR) treatment were normal in pericentrin-deficient cells. However, pericentrin disruption in Mcph1-/- cells abrogated centrosome hyperamplification after IR. We conclude that pericentrin controls genomic stability by both ensuring appropriate mitotic spindle activity and centrosome regulation.

Arl13b in primary cilia regulates the migration and placement of interneurons in the developing cerebral cortex

Coordinated migration and placement of interneurons and projection neurons lead to functional connectivity in the cerebral cortex; defective neuronal migration and the resultant connectivity changes underlie the cognitive defects in a spectrum of neurological disorders. Here we show that primary cilia play a guiding role in the migration and placement of postmitotic interneurons in the developing cerebral cortex and that this process requires the ciliary protein, Arl13b. Through live imaging of interneuronal cilia, we show that migrating interneurons display highly dynamic primary cilia and we correlate cilia dynamics with the interneuron's migratory state. We demonstrate that the guidance cue receptors essential for interneuronal migration localize to interneuronal primary cilia, but their concentration and dynamics are altered in the absence of Arl13b. Expression of Arl13b variants known to cause Joubert syndrome induce defective interneuronal migration, suggesting that defects in cilia-dependent interneuron migration may in part underlie the neurological defects in Joubert syndrome patients.

Subdiffraction-resolution fluorescence microscopy reveals a domain of the centrosome critical for pericentriolar material organization

As the main microtubule-organizing centre in animal cells, the centrosome has a fundamental role in cell function. Surrounding the centrioles, the pericentriolar material (PCM) provides a dynamic platform for nucleating microtubules. Although the importance of the PCM is established, its amorphous electron-dense nature has made it refractory to structural investigation. By using SIM and STORM subdiffraction-resolution microscopies to visualize proteins critical for centrosome maturation, we demonstrate that the PCM is organized into two main structural domains: a layer juxtaposed to the centriole wall, and proteins extending farther away from the centriole organized in a matrix. Analysis of Pericentrin-like protein (PLP) reveals that its carboxy terminus is positioned at the centriole wall, it radiates outwards into the matrix and is organized in clusters having quasi-nine-fold symmetry. By RNA-mediated interference (RNAi), we show that PLP fibrils are required for interphase recruitment and proper mitotic assembly of the PCM matrix.

Pericentrin in health and disease: Exploring the patchwork of Pericentrin splice variants

Researchers around the world perform large-scale screens to identify disease-related gene defects in humans. One of the genes of interest is Pericentrin (PCNT), a gene which codes for a large coiled-coil protein with multiple functions in the cell. Recently, we showed that different Pericentrin (Pcnt) splice variants are differentially distributed among sensory tissues of the mouse, emphasizing the importance of a protein‘s spliceome for the function of a cell.

POC1A truncation mutation causes a ciliopathy in humans characterized by primordial dwarfism

Primordial dwarfism (PD) is a phenotype characterized by profound growth retardation that is prenatal in onset. Significant strides have been made in the last few years toward improved understanding of the molecular underpinning of the limited growth that characterizes the embryonic and postnatal development of PD individuals. These include impaired mitotic mechanics, abnormal IGF2 expression, perturbed DNA-damage response, defective spliceosomal machinery, and abnormal replication licensing. In three families affected by a distinct form of PD, we identified a founder truncating mutation in POC1A. This gene is one of two vertebrate paralogs of POC1, which encodes one of the most abundant proteins in the Chlamydomonas centriole proteome. Cells derived from the index individual have abnormal mitotic mechanics with multipolar spindles, in addition to clearly impaired ciliogenesis. siRNA knockdown of POC1A in fibroblast cells recapitulates this ciliogenesis defect. Our findings highlight a human ciliopathy syndrome caused by deficiency of a major centriolar protein.

Meier-Gorlin syndrome mutations disrupt an Orc1 CDK inhibitory domain and cause centrosome reduplication

Like DNA replication, centrosomes are licensed to duplicate once per cell division cycle to ensure genetic stability. In addition to regulating DNA replication, the Orc1 subunit of the human origin recognition complex controls centriole and centrosome copy number. Here we report that Orc1 harbors a PACT centrosome-targeting domain and a separate domain that differentially inhibits the protein kinase activities of Cyclin E-CDK2 and Cyclin A-CDK2. A cyclin-binding motif (Cy motif) is required for Orc1 to bind Cyclin A and inhibit Cyclin A-CDK2 kinase activity but has no effect on Cyclin E-CDK2 kinase activity. In contrast, Orc1 inhibition of Cyclin E-CDK2 kinase activity occurs by a different mechanism that is affected by Orc1 mutations identified in Meier-Gorlin syndrome patients. The cyclin/CDK2 kinase inhibitory domain of Orc1, when tethered to the PACT domain, localizes to centrosomes and blocks centrosome reduplication. Meier-Gorlin syndrome mutations that disrupt Cyclin E-CDK2 kinase inhibition also allow centrosome reduplication. Thus, Orc1 contains distinct domains that control centrosome copy number and DNA replication. We suggest that the Orc1 mutations present in some Meier-Gorlin syndrome patients contribute to the pronounced microcephaly and dwarfism observed in these individuals by altering centrosome duplication in addition to DNA replication defects.

Centrin depletion causes cyst formation and other ciliopathy-related phenotypes in zebrafish

Most bona fide centrosome proteins including centrins, small calcium-binding proteins, participate in spindle function during mitosis and play a role in cilia assembly in non-cycling cells. Although the basic cellular functions of centrins have been studied in lower eukaryotes and vertebrate cells in culture, phenotypes associated with centrin depletion in vertebrates in vivo has not been directly addressed. To test this, we depleted centrin2 in zebrafish and found that it leads to ciliopathy phenotypes including enlarged pronephric tubules and pronephric cysts. Consistent with the ciliopathy phenotypes, cilia defects were observed in differentiated epithelial cells of ciliated organs such as the olfactory bulb and pronephric duct. The organ phenotypes were also accompanied by cell cycle deregulation namely mitotic delay resulting from mitotic defects. Overall, this work demonstrates that centrin2 depletion causes cilia-related disorders in zebrafish. Moreover, given the presence of both cilia and mitotic defects in the affected organs, it suggests that cilia disorders may arise from a combination of these defects.

The centrosomal protein pericentrin identified at the basal body complex of the connecting cilium in mouse photoreceptors

Background

Pericentrin (Pcnt), a conserved protein of the pericentriolar material, serves as a multifunctional scaffold for numerous proteins and plays an important role in microtubule organization. Recent studies indicate that Pcnt mutations are associated with a range of diseases including primordial dwarfism and ciliopathies. To date, three Pcnt splice variants from orthologous genes in mice and humans are known.

Principal Findings

We generated a specific Pcnt antiserum detecting all known Pcnt splice variants and examined the cellular and subcellular distribution of Pcnt in ciliated tissues of the mouse, the olfactory epithelium and the retina. For the first time, we identified Pcnt and its centrosomal interaction partners at the basal body complex of mouse retinal photoreceptors. Photoreceptors are morphologically and functionally subdivided into the light sensitive outer segment and the inner segment comprising the metabolic function of the cell. The two compartments are linked via a modified, specialized, non-motile cilium, the connecting cilium. Here, Pcnt colocalized with the whole protein machinery responsible for transport processes between the two compartments. Surprisingly, photoreceptors expressed a small Pcnt splice transcript – most likely a modified variant of Pcnt S – which was not present in receptor neurons of the olfactory epithelium.

Conclusions

Our findings suggest distinct functional roles of several Pcnt variants in different ciliated tissues and sensory neurons, like the olfactory epithelium and the retina of the mouse. The individual patchwork of different Pcnt splice transcripts seems to reflect the complexity of Pcnt function, an assumption corroborated by the heterogeneous clinical manifestations associated with mutations in the Pcnt gene.

A systems-biology approach to understanding the ciliopathy disorders

'Ciliopathies' are an emerging class of genetic multisystemic human disorders that are caused by a multitude of largely unrelated genes that affect ciliary structure/function. They are unified by shared clinical features, such as mental retardation, cystic kidney, retinal defects and polydactyly, and by the common localization of the protein products of these genes at or near the primary cilium of cells. With the realization that many previously disparate conditions are a part of this spectrum of disorders, there has been tremendous interest in the function of cilia in developmental signaling and homeostasis. Ciliopathies are mostly inherited as simple recessive traits, but phenotypic expressivity is under the control of numerous genetic modifiers, putting these conditions at the interface of simple and complex genetics. In this review, we discuss the ever-expanding ciliopathy field, which has three interrelated goals: developing a comprehensive understanding of genes mutated in the ciliopathies and required for ciliogenesis; understanding how the encoded proteins work together in complexes and networks to modulate activity and structure-function relationships; and uncovering signaling pathways and modifier relationships.

Cdk5rap2 interacts with pericentrin to maintain the neural progenitor pool in the developing neocortex

Primary autosomal-recessive microcephaly (MCPH) and Majewski osteodysplastic primordial dwarfism type II (MOPDII) are both genetic diseases that result in decreased brain size at birth. MCPH is thought to arise from alterations in the size of the neural progenitor pool, but the cause of this defect has not been thoroughly explored. We find that one of the genes associated with MCPH, Cdk5rap2, is highly expressed in the neural progenitor pool and that its loss results in a depletion of apical progenitors and increased cell-cycle exit leading to premature neuronal differentiation. We link Cdk5rap2 function to the pericentriolar material protein pericentrin, loss of function of which is associated with MOPDII. Depletion of pericentrin in neural progenitors phenocopies effects of Cdk5rap2 knockdown and results in decreased recruitment of Cdk5rap2 to the centrosome. Our findings uncover a common mechanism, involving aberrations in the neurogenesis program, that may underlie the development of microcephaly in multiple diseases.

Role of primary cilia in brain development and cancer

The primary cilium, a hair-like extension from a cell's surface, acts as a sensory organelle to receive signals that regulate cellular behavior and physiology. Like most mammalian cells, neural progenitors and neurons have primary cilia. Recent studies show that this tiny projection plays important roles in brain development and diseases. Ciliary mutant mice show defects in brain patterning, progenitor proliferation, and specification of adult neural stem cells. Primary cilia also have dual opposing functions in the development of brain tumors. Ciliary defects are associated with genetic syndromes that frequently have neurological symptoms. Understanding the multifaceted roles that primary cilia have in brain development will provide important insights into the mechanism of brain development and diseases.

Pericentrin in cellular function and disease

Pericentrin is an integral component of the centrosome that serves as a multifunctional scaffold for anchoring numerous proteins and protein complexes. Through these interactions, pericentrin contributes to a diversity of fundamental cellular processes. Recent studies link pericentrin to a growing list of human disorders. Studies on pericentrin at the cellular, molecular, and, more recently, organismal level, provide a platform for generating models to elucidate the etiology of these disorders. Although the complexity of phenotypes associated with pericentrin-mediated disorders is somewhat daunting, insights into the cellular basis of disease are beginning to come into focus. In this review, we focus on human conditions associated with loss or elevation of pericentrin and propose cellular and molecular models that might explain them.