PIP2 determines length and stability of primary cilia by balancing membrane turnovers

Simon Stilling,Theodoros Kalliakoudas,Hannah Benninghoven-Frey,Björn H Falkenburger,Takanari Inoue

doi:10.1038/s42003-022-03028-1

Abstract

Primary cilia are sensory organelles on many postmitotic cells. The ciliary membrane is continuous with the plasma membrane but differs in its phospholipid composition with phosphatidylinositol 4,5-bisposphate (PIP2) being much reduced toward the ciliary tip. In order to determine the functional significance of this difference, we used chemically induced protein dimerization to rapidly synthesize or degrade PIP2 selectively in the ciliary membrane. We observed ciliary fission when PIP2 was synthesized and a growing ciliary length when PIP2 was degraded. Ciliary fission required local actin polymerisation in the cilium, the Rho kinase Rac, aurora kinase A (AurkA) and histone deacetylase 6 (HDAC6). This pathway was previously described for ciliary disassembly before cell cycle re-entry. Activating ciliary receptors in the presence of dominant negative dynamin also increased ciliary PIP2, and the associated vesicle budding required ciliary PIP2. Finally, ciliary shortening resulting from constitutively increased ciliary PIP2 was mediated by the same actin – AurkA – HDAC6 pathway. Taken together, changes in ciliary PIP2 are a unifying point for ciliary membrane stability and turnover. Different stimuli increase ciliary PIP2 to secrete vesicles and reduce ciliary length by a common pathway. The paucity of PIP2 in the distal cilium therefore ensures ciliary stability.

Highlights

Primary cilia are sensory organelles on many postmitotic cells
YFP-tagged FKBP-PIPK has been used in previous studies for acute synthesis of PIP2 at the plasma membrane[40,41]
It consists of the dimerization domain FKBP fused to an engineered PIP kinase that phosphorylates PIP to PIP2 upon addition of rapamycin (Fig. 1a)

Summary

Introduction

Primary cilia are sensory organelles on many postmitotic cells. The ciliary membrane is continuous with the plasma membrane but differs in its phospholipid composition with phosphatidylinositol 4,5-bisposphate (PIP2) being much reduced toward the ciliary tip. Ciliary fission required local actin polymerisation in the cilium, the Rho kinase Rac, aurora kinase A (AurkA) and histone deacetylase 6 (HDAC6) This pathway was previously described for ciliary disassembly before cell cycle re-entry. When ciliary G-protein coupled receptors (GPCRs) are activated with blocked retrograde transport, cilia secrete GPCRs in vesicles budding from the ciliary tip[12]. In spite of this continuous membrane turnover, ciliary length remains remarkably stable over time. The amount of PIP2 in the proximal ciliary membrane is as in the plasma membrane, but PIP2 is low in the distal cilium[33,34,35,36] This can be explained by the presence of specific PIP2 degrading enzymes localized in primary cilia. Loss of function mutations in some of these enzymes cause ciliopathies in humans[37,38,39], indicating that the paucity of PIP2 in the ciliary membrane is functionally relevant

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Conclusion