Dysfunction of Intraflagellar Transport Proteins beyond the Primary Cilium
Authors: Pamela V. Tran
Journal: Journal of the American Society of Nephrology (2014)

Intraflagellar transport (IFT) proteins are essential for bidirectional movement of structural proteins along the microtubular core of primary cilia, which are small, antenna-like sensory organelles that mediate signaling pathways. Disruption of IFT or of cilia perturbs ciliary-mediated signaling and causes human syndromes termed ciliopathies. These syndromes commonly affect renal tubules and result in renal cystic disease. TTC21B (also known as THM1) encodes a component of the IFT-A complex1 and is altered in 5% of patients with ciliopathies.2 In this issue of JASN, Cong et al. reveal that a mutation in TTC21B causes FSGS,3 a disease of progressive glomerular scarring that is not associated with primary cilia defects. Their findings are unprecedented and illuminate a novel extraciliary role for TTC21B. Although most FSGS cases are idiopathic, study of familial FSGS cases has shown that mutations cause defects in cell structure, actin cytoskeleton organization, calcium signaling, and mitochondrial and lysosomal function specifically in podocytes.4 Using whole-exome genome sequencing combined with homozygosity mapping, Cong et al. have now identified a homozygous p.P209L missense change in TTC21B in individuals with FSGS from two consanguineous families.3 Subsequent direct screening for this mutation revealed its occurrence in the homozygous form in affected individuals from five additional families. Together with Dr. N. Katsanis and others, this research group previously identified this same homozygous change in three families with nephronophthisis (NPHP), a cystic disease that causes thickening of the basement membrane of tubular epithelial cells.2 Re-examination of the kidneys of individuals with FSGS confirmed NPHP tubular features, and, conversely, kidneys of previously identified NPHP-affected individuals homozygous for the TTC21B p.P209L mutation showed FSGS lesions. These data suggest that TTC21B p.P209L homozygosity creates a novel class of inherited renal disorders with tubular and glomerular involvement. Analyses of rat renal tissue have demonstrated that immature podocytes possess a primary cilium that is lost during maturation.5 Cong et al. showed that TTC21B localized to the base of primary cilia of human undifferentiated podocytes, and relocalized to the cytoskeleton in differentiated podocytes lacking cilia.3 TTC21B deficiency causes shortened cilia with bulbous distal tips.1 Expression of TTC21B encoding the p.P209L mutation partially rescued the bulbous-tipped ciliary phenotype in TTC21B knockdown undifferentiated podocytes, indicating the mutation is hypomorphic with respect to ciliogenesis. Upon differentiation, TTC21B knockdown podocytes displayed abnormal organization of cytoplasmic actin filaments and microtubules, which remained altered after re-expression of TTC21B encoding the p.P209L mutation. Further, podocytes depleted of TTC21B or re-expressing mutant TTC21B exhibited dissociated centrioles and ectopic microtubule clusters in microtubule nucleation assays, and reduced cell migration, indicating defects in cytoskeletal dynamics. These data demonstrate for the first time widespread extraciliary effects of TTC21B deficiency on the cytoskeleton. Surprisingly, 14 years after the first studies revealing a link between cilia defects and renal cystogenesis, an IFT protein has only now been shown to be essential for podocyte integrity. Cong et al. noted that the individuals affected with FSGS and NPHP showed late-onset ESRD, occurring in the fourth decade of life.3 In the adolescent form of NPHP, patients typically develop ESRD around 15 years of age. Thus, if FSGS develops later than typical cases of NPHP, this may account for why glomerular lesions have not been previously recognized in patients with ciliopathies. Whether other IFT proteins play a role in podocyte integrity remains to be investigated. The IFT-A complex is a conglomerate of proteins; thus, if TTC21B functions in a complex in a similar manner in the cytoplasm, then other IFT proteins would likely be involved as well. Cong et al. showed that TTC21B-deficient podocytes differentiate normally in vitro, suggesting that the cytoskeletal alterations are independent of the primary cilia defect.3 Podocyte differentiation in a TTC21B-mutant model in vivo remains to be explored. Nevertheless, the findings of Cong et al. contribute to a growing body of evidence suggesting extraciliary roles for IFT proteins. IFT88 localizes to spindle poles during mitosis and is part of a complex that directs mitotic spindle orientation.6 IFT20 is present in the Golgi apparatus to sort and traffic proteins to the ciliary membrane.7 Further, ciliary proteins, NEK8, CEP164, and ZNF423, which are mutated in individuals with NPHP, localize to nuclear foci and mediate DNA damage response signaling.8,9 In summary, the findings of Cong et al. are the first to demonstrate that mutation of a ciliary gene causes glomerular disease.3 The molecular mechanism by which TTC21B affects the cytoskeleton remains to be investigated. Additionally, whether TTC21B extraciliary function extends to other underlying mechanisms of podocyte damage, such as calcium signaling and lysosomal function, are other areas for future exploration. The first reports linking defective primary cilia to renal cystogenesis caused a resurgence of ciliary research. Similarly, the novel findings of Cong et al. are likely to spur new investigations into the extraciliary role of IFT proteins in renal cell biology and pathobiology. Disclosures None. The author thanks Drs. D. Abrahamson and I. Saadi (University of Kansas Medical Center) for their very helpful comments. P.V.T. is supported by an American Society of Nephrology Gottschalk Research Scholar Award and a National Institutes of Health Center of Biomedical Research Excellence grant (P20-GM104936-06).