Cell-intrinsic and contextual determinants of aging by human glial progenitor cells

Abstract Glial progenitor cells (GPCs) pervade the adult human brain, and can give rise to new oligodendrocytes and astrocytes in response to myelin loss; yet they may fail to do so in chronic neuroinflammatory and age- related white matter diseases. Our goal is to identify the transcriptional and epigenetic basis for age-related glial progenitor failure, with the goal of identifying the repressive transcription factors and epigenetic states that restrict progenitor cell expansion and differentiation with age. By targeting these repressive networks, we hope to restore the functional viability of human GPCs, and by so doing prevent the myelin loss that characterizes both aging and those neurodegenerative and inflammatory disorders associated with white matter disease. By so doing, we hope to preserve not only the differentiation competence of the cells, but also their self-renewal, so that myelin- ogenesis may be induced from hGPCs without the progenitor depletion to be expected of strategies designed to trigger terminal oligodendrocytic differentiation. Achieving this in human GPCs, which differ substantially in their biology from mouse, and doing so in vivo, has proven a significant challenge to the field. To this end, we will ask: 1. To what extent is the aging of human ESC-derived GPCs cell-intrinsic and linked to prior cell division, both in vitro and in vivo? What are the transcriptional and epigenetic concomitants to hGPC aging in vivo, and which of these restrict hGPC expansion and differentiation? How do hGPCs, extracted back from neonatally- transplanted human chimeric mouse brains, change in their DNA methylation patterns, their ATAC-Seq-defined patterns of chromatin accessibility, and their consequent RNA expression, over the 2-year lifespan of a mouse? 2. To what extent are the effects of aging on hGPCs a function of the aged brain environment, rather than cell autonomous? In order to define the relationship of hGPC cell age to expansion and myelination competence - and the extent to which the age of the host influences hGPC fate – these experiments will include a set of reciprocal, heterochronic transplants, grafting aged cells into neonates, and new hGPCs into aged brains. 3. In aged GPCs, can genetic knock-down of those repressors implicated in the progression to adult hGPC phenotype restore the transcriptional signature, as well as the expansion and differentiation competence in vivo, of younger hGPCs? Is suppression of MAX, potentially together with a core set of other over-expressed repressors, sufficient to restore MYC-dependent mitotic expansion and host colonization by aged hGPCs? With this work, we expect to establish a granular understanding of the relative roles of cell-intrinsic, expansion- dependent senescence and host context in regulating the proliferation and remyelination competence of human GPCs. Furthermore, if the introduction of young hGPCs into an aged environment allows the selective colonization of the host white matter by those younger hGPCs, the implications may be profound, as disorders as varied as progressive multiple sclerosis and the neurodegenerative disorders might then become potential targets of cell replacement strategies based on the competitive advantages of young over aged glial progenitors.