White matter injury is associated with neurological dysfunction in a variety of conditions, ranging from cerebral palsy to vascular dementia. Oligodendrocyte progenitor cells (OPCs) are able to regenerate themselves and initiate a repair response following injury; however, the release of hyaluronic acid from the ECM following white matter injury impairs OPC maturation and remyelination. In this episode, Stephen Back and colleagues identify a bioactive hyaluronan fragment (bHAf) that selectively blocks OPC differentiation via activation of a noncanonical TLR4/AKT/FoxO3 signaling pathway. These results elucidate a mechanism by which white matter injury prevents repair and suggest that strategies to overcome this block in OPC maturation have potential for promoting regeneration after injury.
Cerebral white matter injury (WMI) persistently disrupts myelin regeneration by oligodendrocyte progenitor cells (OPCs). We identified a specific bioactive hyaluronan fragment (bHAf) that downregulates myelin gene expression and chronically blocks OPC maturation and myelination via a tolerance-like mechanism that dysregulates pro-myelination signaling via AKT. Desensitization of AKT occurs via TLR4 but not TLR2 or CD44. OPC differentiation was selectively blocked by bHAf in a maturation-dependent fashion at the late OPC (preOL) stage by a noncanonical TLR4/TRIF pathway that induced persistent activation of the FoxO3 transcription factor downstream of AKT. Activated FoxO3 selectively localized to oligodendrocyte lineage cells in white matter lesions from human preterm neonates and adults with multiple sclerosis. FoxO3 constraint of OPC maturation was bHAf dependent, and involved interactions at the FoxO3 and MBP promoters with the chromatin remodeling factor Brg1 and the transcription factor Olig2, which regulate OPC differentiation. WMI has adapted an immune tolerance–like mechanism whereby persistent engagement of TLR4 by bHAf promotes an OPC niche at the expense of myelination by engaging a FoxO3 signaling pathway that chronically constrains OPC differentiation.
Taasin Srivastava, Parham Diba, Justin M. Dean, Fatima Banine, Daniel Shaver, Matthew Hagen, Xi Gong, Weiping Su, Ben Emery, Daniel L. Marks, Edward N. Harris, Bruce Baggenstoss, Paul H. Weigel, Larry S. Sherman, Stephen A. Back