Replicative senescence dictates the emergence of disease-associated microglia and contributes to Aß pathology.

The sustained proliferation of microglia is a key hallmark of Alzheimer's disease (AD), accelerating its progression. Here, we aim to understand the long-term impact of the early and prolonged microglial proliferation observed in AD, hypothesizing that extensive and repeated cycling would engender a distinct transcriptional and phenotypic trajectory. We show that the early and sustained microglial proliferation seen in an AD-like model promotes replicative senescence, characterized by increased ßgal activity, a senescence-associated transcriptional signature, and telomere shortening, correlating with the appearance of disease-associated microglia (DAM) and senescent microglial profiles in human post-mortem AD cases. The prevention of early microglial proliferation hinders the development of senescence and DAM, impairing the accumulation of Aß, as well as associated neuritic and synaptic damage. Overall, our results indicate that excessive microglial proliferation leads to the generation of senescent DAM, which contributes to early Aß pathology in AD.

The synaptic blocker botulinum toxin A decreases the density and complexity of oligodendrocyte precursor cells in the adult mouse hippocampus.

Oligodendrocyte progenitor cells (OPCs) are responsible for generating oligodendrocytes, the myelinating cells of the CNS. Life-long myelination is promoted by neuronal activity and is essential for neural network plasticity and learning. OPCs are known to contact synapses and it is proposed that neuronal synaptic activity in turn regulates their behavior. To examine this in the adult, we performed unilateral injection of the synaptic blocker botulinum neurotoxin A (BoNT/A) into the hippocampus of adult mice. We confirm BoNT/A cleaves SNAP-25 in the CA1 are of the hippocampus, which has been proven to block neurotransmission. Notably, BoNT/A significantly decreased OPC density and caused their shrinkage, as determined by immunolabeling for the OPC marker NG2. Furthermore, BoNT/A resulted in an overall decrease in the number of OPC processes, as well as a decrease in their lengths and branching frequency. These data indicate that synaptic activity is important for maintaining adult OPC numbers and cellular integrity, which is relevant to pathophysiological scenarios characterized by dysregulation of synaptic activity, such as age-related cognitive decline, Multiple Sclerosis and Alzheimer's disease.

Measuring Microglial Turnover in the Adult Brain.

Microglia are the main resident immunocompetent cells of the brain with key roles in brain development, homeostasis, and function. Recent reports have started to shed light on the homeostatic mechanisms regulating the composition and turnover of the microglial population under physiological conditions from development to ageing, but our knowledge of the dynamics of microglia is incomplete. Therefore, it appears relevant to provide a standardized approach to quantify the turnover of microglia, with direct application to create a greater understanding of the dynamics of this cell population, and how it may contribute to the pathogenesis and/or progression of neurological disorders. Here we describe a robust immunohistochemical method to analyze microglial proliferation in mouse brain, aiming at providing a shared and universal approach to analyze microglial dynamics across different laboratories.