The tuberous sclerosis proteins regulate formation of the primary cilium via a rapamycin-insensitive and polycystin 1-independent pathway

Abstract

Tuberous sclerosis complex (TSC) is a tumor suppressor gene syndrome in which severe renal cystic disease can occur. Many renal cystic diseases, including autosomal dominant polycystic kidney disease (ADPKD), are associated with absence or dysfunction of the primary cilium. We report here that hamartin (TSC1) localizes to the basal body of the primary cilium, and that Tsc1−/− and Tsc2−/− mouse embryonic fibroblasts (MEFs) are significantly more likely to contain a primary cilium than wild-type controls. In addition, the cilia of Tsc1−/− and Tsc2−/− MEFs are 17–27% longer than cilia from wild-type MEFs. These data suggest a novel type of ciliary disruption in TSC, associated with enhanced cilia development. The TSC1 and TSC2 proteins function as a heterodimer to inhibit the activity of the mammalian target of rapamycin complex 1 (TORC1). The enhanced ciliary formation in the Tsc1−/− and Tsc2−/− MEFs was not abrogated by rapamycin, which indicates a TORC1-independent mechanism. Polycystin 1 (PC1), the product of the PKD1 gene, has been found to interact with TSC2, but Pkd1−/− MEFs did not have enhanced ciliary formation. Furthermore, while activation of mTOR has been observed in renal cysts from ADPKD patients, Pkd1−/− MEFs did not have evidence of constitutive mTOR activation, thereby underscoring the independent functions of the TSC proteins and PC1 in regulation of primary cilia and mTOR. Our data link the TSC proteins with the primary cilium and reveal a novel phenotype of enhanced ciliary formation in a cyst-associated disease.

TACC3-TSC2 maintains nuclear envelope structure and controls cell division

“…[41][42][43][44] Recently, a role for TSC proteins in the biology of centrosomes and basal body of cilia has been described through complementary approaches: TSC1 interacts with the mitotic regulator PLK1 in a phosphorylation-dependent manner and is necessary for proper maintenance of centrosome number; 45 and genetic and molecular interactions between the TSC1/TSC1-TSC2/TSC2 and PKD1/ PKD1 (polycystin 1) are implicated in cilia formation. [46][47][48] Here, TACC3 interactome mapping identified physical interactions with 16 different proteins, including TSC2 and additional regulators of cilia formation, CEP164, 49 and CP110. 50 In interphase, Tacc3 and Tsc2, but not Tsc1, co-localize and coimmunoprecipitate with components of the nuclear envelope and their deficiency causes alterations of this structure.…”

Lymphangioleiomyomatosis — a wolf in sheep’s clothing

“…63 and Figure 2B). The molecular mechanisms and clinical significance of these are generally less well understood than the canonical mechanisms, but include aggresome formation (72), regulation of the primary cilium (73,74), regulation of the cytoskeleton and RhoA via hamartin (75) or TOR complex 2 (TORC2) (71), and regulation of cellular differentiation and proliferation via B-Raf (76,77) and Notch (63,78).…”

Low-Dose Rapamycin (Sirolimus) Effects in Autosomal Dominant Polycystic Kidney Disease

“…Other factors that could diminish the effect of rapamycin on TKV include patients with limited rapamycin exposure (see Results), rapamycin not reaching target renal tubular epithelial cells (38), only low levels of mTOR activity being present in 65%-70% of ADPKD cysts (39), polycystic kidneys reaching a point of "irreversible damage" unresponsive to therapy (40), male sex accelerating the rate of renal function decline in ADPKD (41) and exhibiting a diminished response to mTOR inhibitors when the eGFR is ,60 ml/min per 1.73 m 2 (42).…”