The Caenorhabditis elegans Tubby homolog dynamically modulates olfactory cilia membrane morphogenesis and phospholipid composition

Abstract

Plasticity in sensory signaling is partly mediated via regulated trafficking of signaling molecules to and from primary cilia. Tubby-related proteins regulate ciliary protein transport; however, their roles in remodeling cilia properties are not fully understood. We find that the C. elegans TUB-1 Tubby homolog regulates membrane morphogenesis and signaling protein transport in specialized sensory cilia. In particular, TUB-1 is essential for sensory signaling-dependent reshaping of olfactory cilia morphology. We show that compromised sensory signaling alters cilia membrane phosphoinositide composition via TUB-1-dependent trafficking of a PIP5 kinase. TUB-1 regulates localization of this lipid kinase at the cilia base in part via localization of the AP-2 adaptor complex subunit DPY-23. Our results describe new functions for Tubby proteins in the dynamic regulation of cilia membrane lipid composition, morphology, and signaling protein content, and suggest that this conserved family of proteins plays a critical role in mediating cilia structural and functional plasticity.

The Enigmatic Role of Lipids in Cilia Signaling

“…In the absence of Inpp5e , PI(4,5)P2 accumulates in the cilium, while ciliary PI(4)P levels drop ( Chávez et al, 2015 ; Garcia-Gonzalo et al, 2015 ; Figure 2C ). Similarly, in fly and worm sensory neurons, loss of INPP5E orthologs leads to increased PI(4,5)P2 levels in cilia ( Park et al, 2015 ; DiTirro et al, 2019 ). Together, these findings are consistent with the model that PI(4,5)P2 diffuses laterally from the plasma to the ciliary membrane, where it is converted to PI(4)P by Inpp5e.…”

“…In contrast, phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] is largely depleted from the ciliary membrane in mammals, Caenorhabditis elegans , Drosophila melanogaster , and Trypanosoma brucei . Instead, PI(4,5)P2 localizes to distinct membrane domains at the cilia base creating a sharp boundary in PPI composition ( Chávez et al, 2015 ; Garcia-Gonzalo et al, 2015 ; Jensen et al, 2015 ; Park et al, 2015 ; DiTirro et al, 2019 ; Dyson et al, 2017 ; Figure 2A ). In retinal pigmented epithelial cells and primary mouse embryonic fibroblasts, PI(4,5)P2 is concentrated at the TZ, which also contains phosphatidylinositol-3,4,5-trisphosphate [PI(3,4,5)P3] ( Dyson et al, 2017 ).…”

“…In retinal pigmented epithelial cells and primary mouse embryonic fibroblasts, PI(4,5)P2 is concentrated at the TZ, which also contains phosphatidylinositol-3,4,5-trisphosphate [PI(3,4,5)P3] ( Dyson et al, 2017 ). Conversely, in C. elegan s neurons and T. brucei , PI(4,5)P2 is enriched in an endocytic membrane domain (periciliary membrane compartment/ciliary pocket), which lies proximal to the TZ ( Demmel et al, 2014 ; Jensen et al, 2015 ; DiTirro et al, 2019 ). In photoreceptors, while PI(4,5)P2 is also largely excluded from the outer segment (OS), which is a modified cilium, PI(4)P localizes to the OS as well as inner segment and perinuclear regions ( Nasuhoglu et al, 2002 ; Finkelstein et al, 2020 ; Figure 2B ).…”

“…Recent studies in mammals and C. elegans demonstrated that, similar to ciliary protein composition, PPI content of the ciliary membrane is dynamic and can change in response to signaling. For example, C. elegans mutants in odr-1 , which encodes a receptor guanylyl cyclase, display elevated levels of intraciliary PI(4,5)P2 relative to wild type in a specialized sensory neuron type ( DiTirro et al, 2019 ). In mammals, activation of Hh signaling with SAG increases PI(3,4,5)P3 while decreasing PI(4,5)P2 levels at the TZ ( Dyson et al, 2017 ; Figure 2E ).…”

Local PI(4,5)P2pool dynamics detected by the coincidence biosensor tubbyCT maintain the integrity of ER-PM junctions during PLC signaling

“…So far, TULP proteins have a well-established role in the selective transport of G-proteincoupled receptors (GPCRs), channels, enzymes, and signaling proteins into cilia (Badgandi et al, 2017;DiTirro et al, 2019;Han et al, 2019;Legué and Liem, 2019;Sun et al, 2012). Intriguingly, a recent study showed that E-Syt3 is specifically expressed in hypothalamic nuclei involved in energy homeostasis and food intake (Zhang et al, 2020), and thus shows a striking overlap with neuronal tubby expression (including the arcuate nucleus) (Kleyn et al, 1996).…”