COL1A1 and early-onset autosomal dominant Alzheimer disease: A recent single-cell RNA-seq study showed that perivascular fibroblasts were significantly under-represented in the cortex of Alzheimer disease patients.47 The reduced fibroblast population may affect TIMP2 and MMP-2 levels and/or activities, which have been shown to regulate BBB integrity and Alzheimer disease pathology.48 In addition, Col1α1+ fibroblasts may repair BBB injury in multiple sclerosis and traumatic brain injury using a similar mechanism.