ROLE OF INORGANIC POLYPHOSPHATE IN NEURONAL ACTIVITY AND SYNAPTIC PLASTICITY IN HIPPOCAMPAL CULTURES

Memory formation depends on the selective recruitment of hippocampal neurons into functional engrams, a process crucially determined by neuronal excitability and circuit dynamics. Identifying the endogenous factors that regulate these properties is essential for understanding how memory is encoded and consolidated. Inorganic polyphosphate (polyP) is a highly conserved polymer that influences neuronal excitability and astrocyte-neuron communication; however, its contribution to neuronal processes related to memory remains poorly understood. In this study, we investigated whether modulating polyP availability in hippocampal cultures affects synaptic plasticity.
Using complementary approaches, including calcium imaging, immunofluorescence, morphological analyses, and RT-qPCR, we assessed how modulating extracellular and intracellular polyP levels affects neuronal activity, intracellular signaling, gene expression, and excitatory synaptic organization in cultured hippocampal neurons. The application of elevated extracellular potassium (55 mM KCl) served as a reference depolarizing stimulus to reliably induce neuronal activation and explore the mechanisms of synaptic plasticity. We observed that increased polyP availability in hippocampal cultures enhanced neuronal activation signatures and promoted transcriptional responses associated with activity and plasticity. These changes were accompanied by structural and functional features consistent with increased synaptic interaction and elevated network-level calcium dynamics. Interestingly, we observed similar, though not identical, changes in neuronal activity and synaptic plasticity when applying KCl to hippocampal cultures.
Taken together, these findings identify polyP as a previously unknown modulator of hippocampal excitability and plasticity. Our results suggest that polyP contributes to the regulation of neuronal plasticity, highlighting polyP as a novel component in the molecular and cellular landscape that shapes cognitive processes.