Context-dependent effects of Ascl1 on glial cell lineage decisions in the injured adult mouse cortex
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Abstract
Brain injury typically results in neuronal or oligodendroglial loss accompanied by persistent functional impairment due to the limited regenerative capacity of the adult mammalian brain. In vivo conversion of non-neuronal cells into neurons or oligodendrocytes has emerged as a promising strategy for cell replacement and brain repair. The proneural transcription factor Ascl1 has been extensively investigated as a reprogramming factor, and its neurogenic capacity is strongly influenced by post-translational phosphorylation. During early postnatal development overexpression of a phospho-deficient mutant of Ascl1, Ascl1SA6, has been found to efficiently convert astrocytes into induced neurons (iNs), while wild-type Ascl1 enhances oligodendrocyte precursor cell (OPC) proliferation. These findings highlight potential strategies for regenerating neurons or expanding OPCs, respectively, but it is unclear whether these responses can be still elicited in the context of an adult brain injury.
In this study, I set out to examine the consequences of an inflammatory environment on the effects of Ascl1 on glial cell lineage using the adult cortical stab wound lesion model. Towards this, the reprogramming outcomes following retroviral-mediated expression of Ascl1 or Ascl1SA6 alone, or in combination with Bcl2, in reactive glia were assessed in the injured adult cortex. This milieu is characterized by inflammatory hallmarks including the presence of C3-expressing neurotoxic reactive astrocytes. In this context, Ascl1SA6 alone displayed only marginal neurogenic capacity, while co-expression of Bcl2 significantly enhanced neuronal conversion efficiency, with iNs surviving over time, despite remaining partially immature. Genetic fate-mapping revealed that iNs predominantly derived from OPCs rather than reactive astrocytes, which appeared refractory in this model. In contrast, ectopic expression of Ascl1 alone, or combined with Bcl2, promoted a robust and cell-autonomous proliferative response in OPCs following cortical injury. This proliferative effect expanded the oligodendroglial population which derived predominantly from pre-existing OPCs. Notably, Ascl1-mediated effect on OPCs was sustained in the long-term without altering their competence to undergo differentiation, as Ascl1 overexpressing OPCs gave rise to pre-myelinating and mature oligodendrocytes in the adult injured cortex.
Altogether, this study demonstrates that while the inflammatory environment of the injured adult cortex significantly constrains the efficacy of phospho-deficient Ascl1 to convert glia into neurons, wild-type Ascl1 is still capable of significantly enhancing OPC expansion. Thus, this study provides a basis for future work aimed at neuronal replacement and remyelination.