Abstract

The assembly of primary cilia is dependant on intraflagellar transport (IFT), which mediates the bidirectional movement of proteins between the base and tip of the cilium. In mice, congenic mutations disrupting genes required for IFT (e.g., Tg737 or the IFT kinesin Kif3a) are embryonic lethal, whereas kidney-specific disruption of IFT results in severe, rapidly progressing cystic pathology [1Lin F. Hiesberger T. Cordes K. Sinclair A.M. Goldstein L.S. Somlo S. Igarashi P. Kidney-specific inactivation of the KIF3A subunit of kinesin-II inhibits renal ciliogenesis and produces polycystic kidney disease.Proc. Natl. Acad. Sci. USA. 2003; 100: 5286-5291Crossref PubMed Scopus (436) Google Scholar, 2Marszalek J.R. Ruiz-Lozano P. Roberts E. Chien K.R. Goldstein L.S. Situs inversus and embryonic ciliary morphogenesis defects in mouse mutants lacking the KIF3A subunit of kinesin-II.Proc. Natl. Acad. Sci. USA. 1999; 96: 5043-5048Crossref PubMed Scopus (423) Google Scholar, 3Murcia N.S. Richards W.G. Yoder B.K. Mucenski M.L. Dunlap J.R. Woychik R.P. The Oak Ridge Polycystic Kidney (orpk) disease gene is required for left-right axis determination.Development. 2000; 127: 2347-2355Crossref PubMed Google Scholar]. Although the function of primary cilia in most tissues is unknown, in the kidney they are mechanosenstive organelles that detect fluid flow through the tubule lumen [4Singla V. Reiter J.F. The primary cilium as the cell's antenna: Signaling at a sensory organelle.Science. 2006; 313: 629-633Crossref PubMed Scopus (761) Google Scholar]. The loss of this flow-induced signaling pathway is thought to be a major contributing factor to cyst formation [5Praetorius H.A. Spring K.R. The renal cell primary cilium functions as a flow sensor.Curr. Opin. Nephrol. Hypertens. 2003; 12: 517-520Crossref PubMed Scopus (200) Google Scholar, 6Nauli S.M. Alenghat F.J. Luo Y. Williams E. Vassilev P. Li X. Elia A.E. Lu W. Brown E.M. Quinn S.J. et al.Polycystins 1 and 2 mediate mechanosensation in the primary cilium of kidney cells.Nat. Genet. 2003; 33: 129-137Crossref PubMed Scopus (1514) Google Scholar, 7Al-Bhalal L. Akhtar M. Molecular basis of autosomal dominant polycystic kidney disease.Adv. Anat. Pathol. 2005; 12: 126-133Crossref PubMed Scopus (37) Google Scholar]. Recent data also suggest that there is a connection between ciliary dysfunction and obesity as evidenced by the discovery that proteins associated with human obesity syndromes such as Alström and Bardet-Biedl localize to this organelle [8Li G. Vega R. Nelms K. Gekakis N. Goodnow C. McNamara P. Wu H. Hong N.A. Glynne R. A Role for Alstrom Syndrome Protein, Alms1, in Kidney Ciliogenesis and Cellular Quiescence.PLoS Genet. 2007; 3: e8Crossref PubMed Scopus (113) Google Scholar]. To more directly assess the importance of cilia in postnatal life, we utilized conditional alleles of two ciliogenic genes (Tg737 and Kif3a) to systemically induce cilia loss in adults. Surprisingly, the cystic kidney pathology in these mutants is dependent on the time at which cilia loss was induced, suggesting that cyst formation is not simply caused by impaired mechanosensation. In addition to the cystic pathology, the conditional cilia mutant mice become obese, are hyperphagic, and have elevated levels of serum insulin, glucose, and leptin. We further defined where in the body cilia are required for normal energy homeostasis by disrupting cilia on neurons throughout the central nervous system and on pro-opiomelanocortin-expressing cells in the hypothalamus, both of which resulted in obesity. These data establish that neuronal cilia function in a pathway regulating satiety responses.