{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nExtracellular nucleotides transduce diverse signals via the P2Y receptor family, and the P2Y4 receptor is no exception. Quantitative mRNA profiling has revealed that P2Y4 is widely, yet selectively, expressed in a variety of human tissues—including brain, peripheral organs, and skin—even as part of a larger ensemble of P2Y receptors."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": " Structural studies comparing human and rat receptors have further shown that determinants in the amino‐terminal region and second extracellular loop critically influence whether ligands such as ATP and UTP act as agonists or antagonists, underscoring species‐specific pharmacological differences."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "2"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nIn cellular models of differentiation and proliferation, the P2Y4 receptor plays a significant role in mediating intracellular Ca²⁺ signals that control cell growth. In human keratinocytes, for instance, differential Ca²⁺ responses to purine and pyrimidine nucleotides point to a subpopulation in which P2Y4 activation contributes to enhanced proliferation."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": " Similarly, in human neuroblastoma cells, upregulation of P2Y4 during neuronal differentiation facilitates neuritogenesis and promotes the transcription of immediate-early genes, although prolonged receptor activation may ultimately lead to cell death."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nIn other contexts the functional role of P2Y4 is further diversified. In astrocytes, activation of the human P2Y4 receptor does not mediate protection against TNF‑α–induced apoptosis—a function attributed instead to its closely related P2Y6 counterpart."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "5"}]}, {"type": "t", "text": " Moreover, studies have shown that P2Y4 receptor trafficking is regulated by cargo proteins such as p24A and p23, which can retain the receptor within intracellular compartments, thereby modulating its signaling potential."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "6"}]}, {"type": "t", "text": " Beyond its role in neural cells, P2Y4 is localized at the sarcolemma of skeletal muscle fibers—suggesting a possible involvement in muscle metabolism that appears unaltered between diabetic and healthy subjects."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "7"}]}, {"type": "t", "text": " In the injured central nervous system, antagonism of P2Y4 function has been linked to an attenuation of trauma-induced thrombospondin‑1 expression, implicating this receptor in tissue repair processes"}, {"type": "fg", "children": [{"type": "fg_f", "ref": "8"}]}, {"type": "t", "text": ", while its overexpression in astrocytes of epileptic foci hints at a role in modulating glio-neuronal signaling pathways that may contribute to epileptogenesis."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "9"}]}, {"type": "t", "text": "\n"}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "D J Moore, J K Chambers, J P Wahlin, et al. 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