DISSECTING THE IMPACT OF ENDOLYSOSOMAL CA²⁺ DYNAMICS ON HIPPOCAMPAL PRESYNAPTIC FUNCTION

Ca²⁺ signaling is well-known for its crucial role in neurotransmission. To keep the Ca²⁺ homeostasis for hippocampal neurons that constantly undergo synaptic plasticity, Ca²⁺ stores can always adjust their characteristics between the releaser and buffer to environmental changes. However, evidence is still lacking in whether and how the endolysosomal Ca²⁺ dynamics may influence presynaptic performance. In this study, we investigate endolysosomes containing the two major Ca²⁺-release channels, Transient Receptor Potential Mucolipin (TRPML) 1 and Two-pore Channel (TPC) 2 in mouse hippocampus at different postnatal stages. By immunofluorescence staining and immunohistochemistry, we identified a relatively stable subpopulation of endolysosomes at presynaptic compartments in the cell culture, and specify their distribution preference in hippocampal slices with an intact anatomical and more physiological context. We then discovered that the proportion of presynaptic endolysosomal subpopulation is neuronal activity dependent, suggesting its function as an active Ca²⁺ supplier for synaptic boutons. Pharmacological applications further indicates that endolysosomal Ca²⁺ dynamics can participate in regulating neurotransmitter release and presynaptic terminal maintenance. Taken together, our results shed new lights on the mechanisms underlying endolysosomal Ca²⁺ signaling in hippocampal presynaptic neurons. This will eventually help elucidate the contribution of Ca²⁺ dis-homeostasis involved in the pathophysiology of many neurological disorders where endolysosomal Ca²⁺ is disrupted and hold promise toward identifying potential therapeutic targets.