Ahi1, whose human ortholog is mutated in Joubert syndrome, is required for Rab8a localization, ciliogenesis and vesicle trafficking

Abstract

The primary non-motile cilium, a membrane-ensheathed, microtubule-bundled organelle, extends from virtually all cells and is important for development. Normal functioning of the cilium requires proper axoneme assembly, membrane biogenesis and ciliary protein localization, in tight coordination with the intraflagellar transport system and vesicular trafficking. Disruptions at any level can induce severe alterations in cell function, giving rise to a myriad of human genetic diseases known as ciliopathies. Here we show that the Abelson helper integration site 1 (Ahi1) gene, whose human ortholog is mutated in Joubert syndrome, regulates cilium formation via its interaction with Rab8a, a small GTPase critical for polarized membrane trafficking. We find that the Ahi1 protein localizes to a single centriole, the mother centriole, which becomes the basal body of the primary cilium. In order to determine whether Ahi1 functions in ciliogenesis, loss of function analysis of Ahi1 was performed in cell culture models of ciliogenesis. Knockdown of Ahi1 expression by shRNAi in cells or targeted deletion of Ahi1 (Ahi1 knockout mouse) leads to impairments in ciliogenesis. In Ahi1-knockdown cells, Rab8a is destabilized and does not properly localize to the basal body. Since Rab8a is implicated in vesicular trafficking, we next examined this process in Ahi1-knockdown cells. Defects in the trafficking of endocytic vesicles from the plasma membrane to the Golgi and back to the plasma membrane were observed in Ahi1-knockdown cells. Overall, our data indicate that the distribution and functioning of Rab8a is regulated by Ahi1, not only affecting cilium formation, but also vesicle transport.

BBS7 is required for BBSome formation and its absence in mice results in Bardet-Biedl syndrome phenotypes and selective abnormalities in membrane protein trafficking

“…These studies imply that BBS proteins function in specific vesicle trafficking. It has been shown that other cilia related proteins such as AHI1 and FUZZY are involved in general membrane protein trafficking rather than solely for cilia membrane protein trafficking (Gray et al, 2009;Hsiao et al, 2009). To test whether BBS7 is involved in general membrane protein trafficking, we examined clathrin-independent and clathrin-dependent endocytosis pathways in cultured wildtype and Bbs7 2/2 kidney primary cells using fluorophore labeled cholera toxin and transferrin receptor as markers, respectively.…”

“…Recently studies have shown that cilia-related genes, such as AHI1 and FUZZY, are involved in general membrane protein trafficking rather than solely for ciliary membrane protein trafficking (Gray et al, 2009;Hsiao et al, 2009). In contrast, absence of BBS7 does not affect either clathrin independent and or clathrin dependent endocytosis pathways.…”

“…Cholera toxin and transferrin receptor endocytosis assays were performed as described by Hsiao et al (Hsiao et al 2009). …”

Phosphoinositide 3-Kinase-C2α Regulates Polycystin-2 Ciliary Entry and Protects against Kidney Cyst Formation

“…39,40 Despite the reduced expression of PI3K-C2a leading to decreased Rab8 activation and a significant decline in ciliary polycystin-2, heterozygosity for a Pik3c2a-null allele did not directly induce kidney cyst development in mice. This suggests that the levels of polycystin-2 (or of the polycystin1/polycystin-2 complex) in Pik3c2a +/2 cilia remain above a threshold sufficient to protect from spontaneous cyst formation.…”

Hypothalamic Ahi1 Mediates Feeding Behavior through Interaction with 5-HT2C Receptor

“…Multiple lines of evidence have shown Ahi1 as a key regulator of neuronal development. Ahi1 mutation not only causes defects in intracellular trafficking (40) but also leads to abnormal development of neuronal interactions and networks. The dysfunction of Ahi1 causes developmental disorders.…”