Modulation of inositol polyphosphate levels regulates neuronal differentiation

The modulation of inositol pentakisphosphate (IP5) and hexakisphosphate (IP6) intracellular levels controls the differentiation and survival of PC12 cells and primary neurons. These mechanisms are controlled by the levels of the protein kinase IP5-2K responsible for the conversion of IP5 into IP6.
The binding of neurotrophins to tropomyosin receptor kinase receptors initiates several signaling pathways, including the activation of phospholipase C-γ, which promotes the release of diacylglycerol and inositol 1,4,5-trisphosphate (IP3). In addition to recycling back to inositol, IP3 serves as a precursor for the synthesis of higher phosphorylated inositols, such as inositol 1,3,4,5,6-pentakisphosphate (IP5) and inositol hexakisphosphate (IP6). Previous studies on the effect of neurotrophins on inositol signaling were limited to the analysis of IP3 and its dephosphorylation products. Here we demonstrate that nerve growth factor (NGF) regulates the levels of IP5 and IP6 during PC12 differentiation. Furthermore, both NGF and brain-derived neurotrophic factor alter IP5 and IP6 intracellular ratio in differentiated PC12 cells and primary neurons. Neurotrophins specifically regulate the expression of IP5-2 kinase (IP5-2K), which phosphorylates IP5 into IP6. IP5-2K is rapidly induced after NGF treatment, but its transcriptional levels sharply decrease in fully differentiated PC12 cells. Reduction of IP5-2K protein levels by small interfering RNA has an effect on the early stages of PC12 cell differentiation, whereas fully differentiated cells are not affected. Conversely, perturbation of IP5-2K levels by overexpression suggests that both differentiated PC12 cells and sympathetic neurons require low levels of the enzyme for survival. Therefore maintaining appropriate intracellular levels of inositol polyphosphates is necessary for neuronal survival and differentiation.