HIPPOCALCIN SIGNALING WITHIN THE SPINES IN RESPONSE TO NMDA RECEPTORS ACTIVATION

Hippocalcin (HPCA), a neuronal Ca²⁺ sensor protein, is a potential key player in translating changes in intracellular Ca²⁺ concentration into plasticity of the excitatory synapses. This is largely due to its high Ca²⁺ affinity and distinctive ability for reversible, Ca²⁺-dependent incorporation into lipid membranes. Prior research suggested that HPCA was involved in both the endocytosis of synaptic glutamate receptors and interaction with the postsynaptic scaffold protein PSD-95 during the induction of the different types of long-term synaptic depression. Importantly, these suggestions were mainly based on results of coimmunoprecipitation experiments rather than on direct observations of protein-protein interaction in living neurons.

Our research mapped the spatiotemporal dynamics of HPCA translocation within individual dendritic spines following the selective activation of NMDA receptors (NMDARs) via iontophoretic NMDA application. Using live-cell imaging in combination with FRET approaches, we investigated the translocation of HPCA and potential interaction between HPCA and PSD-95 in response to NMDAR activation.

Activation of NMDARs causes HPCA to be preferentially recruited to perisynaptic regions, the possible localisations of the endocytic zones (EZs), the specialized sites for active synaptic receptor trafficking. However, FRET analysis showed no detectable interaction between HPCA and PSD-95, neither under basal conditions nor during NMDAR activation. These findings challenge previous reports of HPCA interaction with the postsynaptic density proteins. Instead, they provide strong evidence for a potential role for HPCA in the EZ signaling, where it may act as a mobile transducer regulating receptor trafficking at the level of individual spines.