Alzheimer's disease through the lens of mesenchymal drift: the collapse of cellular identity.

Cellular identity is maintained by precise epigenetic regulation, but chronic stress and aging can disrupt this control, leading to mesenchymal drift (MD). Unlike the coordinated process of epithelial-mesenchymal transition (EMT), MD is a stochastic, lineage-independent phenomenon marked by increased epigenetic noise and the widespread activation of profibrotic gene programs. As originally defined, MD requires two components to co-occur: a compromised original cellular identity together with the acquisition or intensification of mesenchymal traits. Applying the MD framework to Alzheimer's disease (AD) provides a new perspective on its pathogenesis. In AD, brain cells do not merely survive or die due to protein toxicity; instead, they lose their specialized identities in an uncoordinated fashion. This transcriptomic drift has been reported across major cell types in the central nervous system: neural progenitor cells differentiate prematurely, mature neurons undergo transcriptomic dedifferentiation, endothelial cells develop features of vascular fibrosis, and glial cells lose their homeostatic functions. Direct enrichment of the MD signature in the AD brain, however, has so far been demonstrated only in astrocytes, oligodendrocytes, and pericytes, and, in our own reanalysis, in human-induced pluripotent stem cell (hiPSC)-derived neural progenitor cells carrying an APP mutation; we distinguish this evidence from observations documenting identity loss or a reactive state alone. We further caution that the novel cell populations identified in recent single-cell transcriptomic studies should not be assumed to represent genuinely new cell types: such clusters may equally reflect a blurring of existing cell lineages, or technical artifacts that produce the same appearance. A drifting transcriptomic profile also does not always signify a corresponding physiological change. These possibilities warrant careful attention, and we therefore emphasize the critical need to integrate transcriptomic data with in vivo functional and morphological validation. This integration is essential to determine whether these identity shifts contribute to AD progression or merely reflect underlying epigenetic instability.