David K. Breslow

Primary cilia are micron-scale cellular compartments with key roles in cell signaling and disease. Cilium assembly has been delineated as a multi-step process that is disrupted in ciliopathies, yet many features of cilia disassembly remain incompletely understood. Here, we review recent insights into cilia disassembly. In addition to discussing the longstanding link between cilia disassembly and the cell cycle, we consider new findings that broaden our understanding of the molecular mechanisms and physiological contexts that govern cilia disassembly. We also synthesize recent evidence that mutual antagonism between assembly and disassembly pathways governs ciliation state. Together, these studies reveal that both excessive and impaired disassembly can perturb ciliary signaling and function, with consequences for development, tissue homeostasis and tumorigenesis. Motivated by these advances, we propose a new ciliopathy subclassification – disassembly-related ciliopathies – to encompass diseases in which aberrant cilia disassembly contributes to pathogenesis. Continued exploration of the mechanisms controlling cilia disassembly will provide both fundamental biological insights and open new avenues for targeted treatment of diseases caused by ciliary dysfunction.

Defective assembly of primary cilia causes ciliopathies, but cilia disassembly and its role in disease remain poorly understood. From a genome-wide CRISPR activation (CRISPRa) screen for negative regulators of ciliary function, we find here that the F2R G protein–coupled receptor, sterile alpha and TIR motif-containing 1 (SARM1) hydrolase, ryanodine receptors, peri-centrosomal calcium signaling, and RhoA form a functional pathway that is necessary and sufficient for cilia disassembly. Highlighting the significance of this pathway, several components are somatically mutated in focal cortical dysplasia (FCD), a neurological disorder characterized by intractable epilepsy. Supporting the functional impact of these variants, patient-derived SARM1 and RhoA mutations potentiate cilia loss, and a RhoA variant impairs cortical development. Conversely, SARM1 inhibition restores cilia in cells with FCD-associated alterations. Together, our work identifies a pathway for cilia disassembly, implicates aberrant pathway activation as a feature of FCD-associated mutations, and illustrates the potential of CRISPRa screening to provide insight into diseases caused by somatic mutations.

Summary: Centrosomes have critical roles in microtubule organization and in cell signaling. ^1–8 However, the mechanisms that regulate centrosome function are not fully defined, and thus how defects in centrosomal regulation contribute to disease is incompletely understood. From functional genomic analyses, we find here that PPP2R3C, a PP2A phosphatase subunit, is a distal centriole protein and functional partner of centriolar proteins CEP350 and FOP. We further show that a key function of PPP2R3C is to counteract the kinase activity of MAP3K1. In support of this model, MAP3K1 knockout suppresses growth defects caused by PPP2R3C inactivation, and MAP3K1 and PPP2R3C have opposing effects on basal and microtubule stress-induced JNK signaling. Illustrating the importance of balanced MAP3K1 and PPP2R3C activities, acute overexpression of MAP3K1 severely inhibits centrosome function and triggers rapid centriole disintegration. Additionally, inactivating PPP2R3C mutations and activating MAP3K1 mutations both cause congenital syndromes characterized by gonadal dysgenesis. ^9–15 As a syndromic PPP2R3C variant is defective in centriolar localization and binding to centriolar protein FOP, we propose that imbalanced activity of this centrosomal kinase-phosphatase pair is the shared cause of these disorders. Thus, our findings reveal a new centrosomal phospho-regulatory module, shed light on disorders of gonadal development, and illustrate the power of systems genetics to identify previously unrecognized gene functions.

Oral-facial-digital syndromes (OFDS) are a group of clinically and genetically heterogeneous disorders characterized by defects in the development of the face and oral cavity along with digit anomalies. Pathogenic variants in over twenty genes encoding ciliary proteins have been found to cause OFDS through deleterious structural or functional impacts on primary cilia. We identified by exome sequencing bi-allelic missense variants in a novel disease-causing ciliary gene RAB34 in four individuals from three unrelated families. Affected individuals presented a novel form of OFDS (OFDS-RAB34) accompanied by cardiac, cerebral, skeletal, and anorectal defects. RAB34 encodes a member of the Rab GTPase superfamily and was recently identified as a key mediator of ciliary membrane formation. Unlike many genes required for cilium assembly, RAB34 acts selectively in cell types that use the intracellular ciliogenesis pathway, in which nascent cilia begin to form in the cytoplasm. We find that the protein products of these pathogenic variants, which are clustered near the RAB34 C-terminus, exhibit a strong loss of function. Although some variants retain the ability to be recruited to the mother centriole, cells expressing mutant RAB34 exhibit a significant defect in cilium assembly. While many Rab proteins have been previously linked to ciliogenesis, our studies establish RAB34 as the first small GTPase involved in OFDS and reveal the distinct clinical manifestations caused by impairment of intracellular ciliogenesis.

Primary cilia form by two pathways: an extracellular pathway in which the cilium grows out from the cell surface and an intracellular pathway in which the nascent cilium forms inside the cell. Here we identify the GTPase Rab34 as a selective mediator of intracellular ciliogenesis. We find that Rab34 is required for formation of the ciliary vesicle at the mother centriole and that Rab34 marks the ciliary sheath, a unique sub-domain of assembling intracellular cilia. Rab34 activity is modulated by divergent residues within its GTPase domain, and ciliogenesis requires GTP binding and turnover by Rab34. Because Rab34 is found on assembly intermediates that are unique to intracellular ciliogenesis, we tested its role in the extracellular pathway used by MDCK cells. Consistent with Rab34 acting specifically in the intracellular pathway, MDCK cells ciliate independently of Rab34 and paralog Rab36. Together, these findings reveal a new context-specific molecular requirement for ciliary membrane biogenesis.

The centriole is an ancient microtubule-based organelle with a conserved nine-fold symmetry. Centrioles form the core of centrosomes, which organize the interphase microtubule cytoskeleton of most animal cells and form the poles of the mitotic spindle. Centrioles can also be modified to form basal bodies, which template the formation of cilia and play central roles in cellular signaling, fluid movement, and locomotion. In this review, we discuss developments in our understanding of the biogenesis of centrioles and cilia and the regulatory controls that govern their structure and number. We also discuss how defects in these processes contribute to a spectrum of human diseases and how new technologies have expanded our understanding of centriole and cilium biology, revealing exciting avenues for future exploration.