Phosphoinositides Regulate Ciliary Protein Trafficking to Modulate Hedgehog Signaling

Abstract

SUMMARY Primary cilia interpret vertebrate Hedgehog (Hh) signals. Why cilia are essential for signaling is unclear. One possibility is that some forms of signaling require a distinct membrane lipid composition, found at cilia. We found that the ciliary membrane contains a particular phosphoinositide, PI(4)P, whereas a different phosphoinositide, PI(4,5)P2, is restricted to the membrane of the ciliary base. This distribution is created by Inpp5e, a ciliary phosphoinositide 5-phosphatase. Without Inpp5e, ciliary PI(4,5)P2 levels are elevated and Hh signaling is disrupted. Inpp5e limits the ciliary levels of inhibitors of Hh signaling, including Gpr161 and the PI(4,5)P2-binding protein Tulp3. Increasing ciliary PI(4,5)P2 levels or conferring the ability to bind PI(4)P on Tulp3 increases the ciliary localization of Tulp3. Lowering Tulp3 in cells lacking Inpp5e reduces ciliary Gpr161 levels and restores Hh signaling. Therefore, Inpp5e regulates ciliary membrane phosphoinositide composition, and Tulp3 reads out ciliary phosphoinositides to control ciliary protein localization, enabling Hh signaling.

Role of Ocrl1 and Inpp5E in primary cilia assembly and maintenance: a phosphatidylinositol phosphatase relay system?

“…Indeed, two recent studies 8,9 specifically addressed PIP composition within the PC. While one study used PI-specific …”

“…Recent studies have also unveiled the PIP content of the specialized signaling organelle known as the primary cilium, 8,9 which has a strikingly different PIP content from the PM, though in principle they are continuous with each other. The following section will highlight the importance of PIP regulation in the maintenance of the PC.…”

“…Further, loss of Inpp5E also led to increase in the levels of negative regulators of Shh signaling such as the IFT-A complex proteins Ift139 (also known as Thm1 or Ttc21b) and Ift140. 9 Taken together, these studies provide a mechanistic link between Inpp5E's phosphatase activity and modulation of Shh signal transduction at the PC for tissue patterning and development.…”

“…9 Both these studies concluded that the major PIP in the cilium is PI4P, while PI(4,5)P 2 is restricted to the ciliary base.…”

“…8,9 Importantly, Plotnikova et al 83 showed that Inpp5E directly interacts with AurkA, resulting in the phosphorylation of Inpp5E by AurkA and the enhancement of its catalytic activity toward PI(3,4,5)P 3 (particularly at the base of the PC where the kinase localizes 88 ). Interestingly, AurkA is one of the targets modulated by the Akt signaling pathway 89 and has been involved in the regulation of PC stability through complex formation with histone deacetylase 6 (HDAC6).…”