{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n Hippocalcin (HPCA) is a neuron‐specific calcium sensor protein that decodes intracellular Ca²⁺ signals via a Ca²⁺/myristoyl switch mechanism, which governs its reversible membrane translocation and subsequent engagement with downstream targets."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": " In so doing, HPCA modulates voltage‐dependent calcium channel activity and is implicated in shaping the slow afterhyperpolarization (sAHP) that helps regulate neuronal excitability and synaptic plasticity—processes critical for memory formation and higher brain functions."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "3"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n HPCA also plays a neuroprotective role. It assists in the extrusion of Ca²⁺ from neurons, thereby guarding against excitotoxicity and apoptosis induced by oxidative or ischemic stress. In experimental models, reduced HPCA levels are associated with heightened vulnerability to Ca²⁺ overload and cell death, while treatments with neuroprotective agents such as melatonin and quercetin help preserve its expression and function."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "4", "end_ref": "7"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n The importance of HPCA is further underscored by genetic studies showing that mutations in its coding gene are causative for autosomal‐recessive forms of primary isolated dystonia. In these cases, altered oligomerization of HPCA and subsequent dysregulation of Ca²⁺ influx (notably via N‐type channels) disrupt normal calcium signaling, which likely underlies the motor dysfunction observed in affected patients."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "8", "end_ref": "12"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Beyond its roles in calcium buffering and protective signaling, HPCA has been implicated in the regulation of neuronal differentiation and synaptogenesis—for example, through miRNA‐mediated control of its mRNA levels that affect neurite outgrowth and synaptic protein expression."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "13"}]}, {"type": "t", "text": " In addition, HPCA can interact with other neuronal regulators—such as NAIP, mixed lineage kinase 2 (MLK2) and phospholipase D2 (PLD2)—thereby influencing cell survival pathways and morphological differentiation."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "14", "end_ref": "16"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Collectively, these studies establish HPCA as a central modulator of calcium‐dependent signaling in neurons—with critical roles in regulating excitability, neural plasticity, differentiation, and survival. Disruption of HPCA function, whether by genetic mutation or pathological stress, can thus contribute to neurodegenerative and movement disorders.\n "}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Dermott W O'Callaghan, Alexei V Tepikin, Robert D Burgoyne "}, {"type": "b", "children": [{"type": "t", "text": "Dynamics and calcium sensitivity of the Ca2+/myristoyl switch protein hippocalcin in living cells."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Cell Biol (2003)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1083/jcb.200306042"}], "href": "https://doi.org/10.1083/jcb.200306042"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "14638856"}], "href": "https://pubmed.ncbi.nlm.nih.gov/14638856"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "M Kobayashi, T Masaki, K Hori, et al. 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