Cellular consequences of inositol depletion



Inositol is an essential metabolite that plays a fundamental role in regulating cellular processes. The phosphorylation of inositol generates numerous inositol phosphates and phosphoinositides, many of which are signaling molecules that control membrane trafficking, calcium mobilization, chemotaxis, ion channel activity, cytoskeletal organization, and gene expression. Furthermore, phosphatidylinositol is a precursor for the synthesis of sphingolipids, which play an important role in signal transduction and trafficking.  Consistent with an essential role in cellular function, starvation for inositol leads to a rapid loss of viability. The crucial role of inositol-containing compounds in cell function is underscored by the link between perturbation of inositol metabolism and human disorders, including bipolar disorder, Alzheimer’s and Parkinson’s diseases, malignant hyperthermia, cancer, type 2 diabetes, Lowe syndrome, myotubular myopathy, and Charcot-Marie-Tooth disease. Therefore, elucidating the mechanisms underlying the control of inositol homeostasis is expected to have important implications for a broad range of illnesses.



The long-term goal of our research is to elucidate the molecular mechanisms underlying regulation of inositol homeostasis, to identify the cellular consequences of inositol depletion, and to determine the role of inositol depletion in the therapeutic mechanisms of drugs used to treat bipolar disorder.  To this end, we have identified three novel mechanisms of regulation of inositol synthesis – inhibition of synthesis by phosphorylation of the rate-limiting biosynthetic enzyme myo-inositol-3-P synthase (MIPS), perturbation of MIPS transcription by inositol pyrophosphates, and modulation of inositol synthesis by glycogen kinase 3 (GSK3).  In addition, we have shown that the drug valproate causes inositol depletion in yeast and human cells by indirectly inhibiting MIPS.  We have determined that a novel consequence of inositol depletion is perturbation of the vacuolar ATPase (V-ATPase) by altering homeostasis of the phosphoinositide PI3,5P2.  The V-ATPase is required for acidification and function of intracellular compartments, including lysosomes, secretory vesicles and synaptic vesicles. The proton gradient generated by the V-ATPase is utilized in the uploading and storage of neurotransmitters.  Therefore, our studies suggest that the therapeutic effects of inositol depletion by valproate may be mediated in part by perturbation of the V-ATPase.



Using the powerful yeast genetic model to elucidate the molecular mechanisms that control inositol homeostasis and mammalian cells to test the hypotheses generated from the yeast model, we are addressing the following questions: 



1. What are the molecular mechanisms underlying regulation of inositol synthesis by phosphorylation of MIPS, pyrophosphate regulation of MIPS transcription, and GSK3-activation?


2. What is the mechanism whereby inositol-depleting drugs inhibit synthesis of inositol in yeast and human cells?


3. How does inositol depletion perturb the V-ATPase?


4. Do inositol-depleting drugs affect sphingolipid metabolism?


5. How do inositol-depleting drugs perturb neurotransmission?

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