TGF-β signaling mediates microglial resilience to age-dependent dorsal column myelin alterations

Microglia survey and regulate central nervous system myelination during development and homeostasis. However, whether microglia-myelin interactions are anatomically regulated remains unexplored. Herein, by exploring the spinal cord, which contains several white matter tracts, we demonstrate age-dependent myelin ultrastructural abnormalities occurring in the dorsal column (DC) but not the ventral column (VC). Transcriptomic analyses comparing aged and young spinal cords reveal major changes in genes regulating myelin, microglial function, and the TGF-β signaling pathway. The TGF-β signaling pathway is upregulated in the spinal cord with aging, and a TGFβ-enriched microenvironment is induced in the DC. Disrupting TGFβ1-TGFBR1/2 axis in microglia, by altering either ligand or receptor expression, leads to increased microglial activation, abnormal phagocytic behavior, and myelin loss in the DC, accompanied by neurological deficits more severe in older mice. snRNA-seq reveals a TGF-β signaling-sensitive microglial subset in the spinal cord, spatially restricted in the DC. These findings demonstrate that TGF-β signaling is indispensable for maintaining microglial resilience to myelin alterations in the DC during the aging process, highlighting a previously unresolved checkpoint mechanism of TGF-β signaling with regional specificity. To understand the transcriptomic changes of microglia in the spinal cord following Tgfbr2 deletion, we sorted 100 microglia from the spinal cords of Cx3cr1CreER:Tgfbr2fl/fl mice from the dorsal column (DC), ventral column (VC), and grey matter (GM) at 18 and 28 days post tamoxifen injection. Tgfbr2fl/fl mice were used as the wildtype control.