Inositol pyrophosphate inhibits synaptotagmin 1-dependent exocytosis
A research team led by Professor Seyun Kim of the Department of Biological Sciences at KAIST has identified one type of metabolite of inositol, the so-called inositol pyrophosphates, serving as the signaling molecule that potently controls neuronal signaling via synaptotagmin 1. Specific depletion of 5-IP7 in neuroendocrine PC12 cells or cultured hippocampal neurons markedly increased neurotransmitter release. Biophysical assays further revealed that 5-IP7 inhibits calcium-induced synaptic membrane fusion at a concentration of one order of magnitude lower than that required for other inositol phosphates. Mechanistic studies elucidated that 5-IP7 directly binds to synaptotagmin 1 (Syt1), a calcium sensor essential for cellular exocytosis, and suppresses its vesicle fusion activity. Thus, data that has been recently acquired allowed the researchers to propose 5-IP7 as a potent inhibitor of Syt1 actions on calcium-mediated synaptic vesicle exocytosis.
Inositol which is naturally enriched in species of corn and bean, is a key nutrient for the human diet. When the levels of inositol become greatly lowered, severe pathological symptoms including diabetes, anxiety, and hypercholesterolemia can present. Aside from its structural role for inositol as a component of the cellular membrane, phosphorylated inositol species called inositol phosphates have been considered as a secondary signaling messenger for controlling diverse physiologic events. Particularly, inositol trisphosphate (IP3) is well known for its classical signaling function in tightly regulating cytosolic calcium levels.
Inositol pentakisphosphate (5-IP7) containing a total of seven phosphates substituted onto an inositol ring was first discovered almost twenty years ago. 5-IP7 has recently been identified as playing a critical role in type 2 diabetes and obesity. 5-IP7 has been also speculated as a key molecule in the control of mood disorders and neurodegenerative diseases. However, the role and the mechanism of action of 5-IP7 in neurons and nerve transmission has remained largely unknown.
The Kim research group has worked on inositol pyrophosphates for several years and discovered that very small quantities of 5-IP7 can mediate various signal transduction pathways. In this study, they first observed that overexpression of inositol hexakisphosphate (IP6) kinase 1 (IP6K1), is able to suppress depolarization-induced neurotransmitter release from neuroendocrine PC12 cells. Conversely, IP6K1 depletion decreased intracellular 5-IP7 concentrations, leading to increased neurotransmitter release. Importantly, depletion of 5-IP7 via IP6K1 knockdown in cultured rat hippocampal neurons markedly increased action potential-driven synaptic vesicle exocytosis. By using a FRET-based, biophysical vesicle fusion assay, they observed that 5-IP7 exhibits significantly potent inhibitory activity toward synaptic vesicle exocytosis. Their mechanistic studies revealed that 5-IP7 can inactivate synaptotagmin 1 (Syt1) which is a calcium sensor essential for synaptic membrane fusion. Notably, 5-IP7 shows a 45-fold higher binding affinity for Syt1 compared to other inositol phosphates like IP6. The researchers further found that this 5-IP7-dependent inhibition of synaptic vesicle fusion can be abolished by increasing calcium levels. Thus, 5-IP7 seems to act through Syt1 binding to interfere with calcium ion action in triggering synaptic vesicle fusion. These findings thus establish an unexpected signaling function of 5-IP7 as a potent inhibitor of Syt1 in controlling the synaptic exocytotic pathway and helps toward the better understanding of the signaling mechanisms of inositol pyrophosphates. Future effort along these lines of research on synaptotagmin and 5-IP7 will bring new modes of 5-IP7-dependent neural regulation to light and can thus allow for the development of new approaches for the treatment of neurological disorders.
These research findings were made possible by close cooperation between various institutes: KAIST, Yonsei-IBS (Institute for Basic Science), Kyung Hee University, Sungkyunkwan University, KIST, University of Zurich in Switzerland, and Albert-Ludwigs-University Freiburg in Germany. This interdisciplinary research project was covered by a wide scope of academic fields, from inositol biochemistry to super resolution optical imaging.
The full article of this journal is available from:
Lee TS*, Lee JY*, Kyung JW, Yang Y, Park SJ, Lee S, Pavlovic I, Kong B, Jho YS, Jessen HJ, Kweon DH, Shin YK, Kim SH+, Yoon TY+, Kim S+. “Inositol pyrophosphates inhibit synaptotagmin-dependent exocytosis” Proc Natl Acad Sci U S A. 2016 Jul 19;113(29):8314-9. (*equally contributed to this work, +co-corresponding authors)
* lab webpage