Non-Classical Glucocorticoid Receptor Action Regulates Gap Junction Intercellular Communication and Neural Progenitor Cell Proliferation

摘要

Glucocorticoids (GCs) are administered to neonates and to pregnant mothers for the treatment of complications arising from premature birth and for congenital adrenal hyperplasia; however, antenatal exposure to GCs may trigger adverse neurological side effects due in part to reduced neural progenitor cell (NPC) proliferation. While many established cell cycle regulators impact NPC proliferation, other molecules also influence proliferation. An example is the gap junction protein connexin 43 (Cx43), although its precise role and mechanism of regulation remain unresolved. Gap junction intercellular communication (GJIC) is influenced by GCs in some cells, but such hormone effects and resulting functional consequences have not been examined in coupled stem cells. We found that both continuous and transient exposure of embryonic (E14.5) mouse neurosphere cultures to the glucocorticoid dexamethasone (DEX) limits proliferation of coupled NPCs, which is manifested by both a reduction in S phase progression and enhanced cell cycle exit. A short (i.e. 1hr) DEX treatment also reduced GJIC as measured by live cell fluorescence recovery after photobleaching (FRAP). GC effects on GJIC in NPCs are transcription-independent and mediated through plasma membrane glucocorticoid receptors (GRs). This non-classical pathway appears to operate through lipid-raft associated GRs through a site-specific, MAPK-dependent phosphorylation of Cx43, which is linked to GR via caveolin-1 and c-src. These effects were absent in caveolin-1 knockout NPCs indicating that caveolin-1 is an essential component for this signaling pathway. As transient pharmacologic inhibition of GJIC triggers reduced S phase progression but not enhanced cell cycle exit, the non-classical GR signaling pathway may operate via distinct downstream effectors to alter the proliferative capacity of NPCs.