INPP5E interactome reveals novel connections to growth factor signaling

Primary cilia are sensory cell membrane protrusions whose malfunction causes diseases known as ciliopathies. Joubert syndrome (JBTS) is a rare recessive ciliopathy causing brain malformations and kidney cysts, among other manifestations. A key player in JBTS is INPP5E, a ciliary phosphoinositide lipid phosphatase. Although INPP5E regulates ciliary growth factor signaling, the molecular mechanisms remain poorly understood. Herein, we show that a constitutively active growth factor receptor (PDGFR-D842V) stimulates INPP5E tyrosine phosphorylation, as does the SRC tyrosine kinase. INPP5E tyrosine phosphorylation did not affect its enzyme activity but was associated with stronger binding to SH3GL1, an endocytic regulator, and SNX9, a phosphoinositide-binding protein involved, like INPP5E, in growth factor-induced ciliary ectovesicle release. Our INPP5E interactomic studies identified other growth factor signaling regulators, including among others: SIN1 (the phosphoinositide-binding subunit of the mTORC2 complex), STRAP (a TGF{beta} and PI3K/AKT signaling regulator), GRB2 (a growth factor receptor adaptor) and the JBTS-associated proteins AHI1 and NPHP1. Moreover, INPP5E strongly interacted with the phosphopeptide-binding 14-3-3 proteins, and did so only in presence of serine-85, a phosphorylated INPP5E residue. Lastly, we found that INPP5E differentially regulates two cilium-dependent growth factor signaling pathways in fibroblasts. Thus, while INPP5E downregulated PDGF-induced AKT phosphorylations, it upregulated TGF{beta}-induced SMAD2 and ERK phosphorylations. Altogether, our work provides important clues on the cilia-dependent actions of growth factors, with implications for ciliopathies like JBTS.