Hippocalcin (HPCA) is a Ca2+-binding protein, and its Ca2+-dependent activation in hippocampal neurons is one of the necessary steps involved in many signal transduction mechanisms. In this work we have examined if synaptic glutamate receptor activation can result in HPCA signaling in spines of hippocampal neurons. Spontaneous and evoked synaptic activity induced excitatory postsynaptic potentials (EPSPs) in the hippocampal neurons leading to AP bursts and HPCA-YFP translocation to many sites in a neuronal dendritic tree including dendritic spines. In neurons clamped at −40 mV, episodes of presynaptic activity resulted in EPSCs associated with HPCA translocation mainly to dendritic spines while no translocation was observed at −70 mV . These results indicate that synaptic NMDAR activation is necessary for HPCA-YFP translocation. T- and R- rather than L-types of voltage activated Ca2+ channels also contribute to the observed translocation. FRET measurements between HPCA tagged by Yellow and Cyan Fluorescent Proteins have shown that the translocation was due to HPCA-FPs insertion in patches of spine membrane resulting in decrease of protein concentration in the cytosol of spines and diffusion of new HPCAFP molecules from the dendritic trunk. Thus, we have shown that hippocalcin may signal as a coincident detector in spines of hippocampal neurons by means of its robust insertion in spine plasma membrane.
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