Aging-induced alterations in microglial cells and their impact on neurodegenerative disorders

Senescence causes deterioration in the functioning and physiology of an organism. Microglia, the standing resident immune brain cells transform from neuroprotective to neurotoxic with age. Rapid process motility and cellular migration of microglia in the developing brain, and other characteristics are regarded to be crucial for immunological defense and tissue repair. As they mature, microglia not only differ in their morphology but also in their functioning. However, the exact mechanism related to the atrophies caused by aged microglia or their role in neurodegenerative diseases is still uncertain. The aim of this updated review is to provide insights of how aging microglial cells change and how this influences the development of neurodegenerative diseases. As life expectancy rises, there is an increase in the accumulation of iron, ROS/NOS, protein misfolding and insufficient clearing of debris. This is attributed to the age-dependent alterations in the genes linked to energy metabolism, mitochondrial and lysosome function, and neuroinflammation. Aging microglia often shifts towards a pro-inflammatory state with a reduction of anti-inflammatory cytokines. Aging microglia fail to clear amyloid-beta plaques, accelerates tau-pathology and enhances the chronic neuroinflammation, exacerbating the α-synuclein aggregation. These changes significantly impacted the onset of various neurogenerative disorders such as amyotrophic lateral sclerosis, Parkinson's disease, and Alzheimer's disease etc. However, it is important to note that these microglial aging effects might not be perceived as absolute, due to various limitations such as microglial heterogeneity, intercellular complexity across brain regions and variability in human aging owing to genetic and epigenetic variations. Regardless of this the future perspective of such insights are of immense relevance as novel therapeutic approaches can be formulated if the molecular and cellular mechanisms of aging microglial perturbations are understood. Future research should focus on restoring microglial homeostasis to mitigate the effects of aging on the brain and slowing the progression of neurodegenerative diseases.