The role of astroglia in Alzheimer's disease: pathophysiology and clinical implications

Astrocytes, also called astroglia, maintain homoeostasis of the brain by providing trophic and metabolic support to neurons. They recycle neurotransmitters, stimulate synaptogenesis and synaptic neurotransmission, form part of the blood–brain barrier, and regulate regional blood flow. Although astrocytes have been known to display morphological alterations in Alzheimer's disease for more than a century, research has remained neurocentric. Emerging evidence suggests that these morphological changes reflect functional alterations that affect disease. Genetic studies indicate that most of the risk of developing late onset Alzheimer's disease, the most common form of the disease, affecting patients aged 65 years and older, is associated with genes (ie, APOE, APOJ, and SORL) that are mainly expressed by glial cells (ie, astrocytes, microglia, and oligodendrocytes). This insight has moved the focus of research away from neurons and towards glial cells and neuroinflammation. Molecular studies in rodent models suggest a direct contribution of astrocytes to neuroinflammatory and neurodegenerative processes causing Alzheimer's disease; however, these models might insufficiently mimic the human disease, because rodent astrocytes differ considerably in morphology, functionality, and gene expression. In-vivo studies using stem-cell derived human astrocytes are allowing exploration of the human disease and providing insights into the neurotoxic or protective contributions of these cells to the pathogenesis of disease. The first attempts to develop astrocytic biomarkers and targeted therapies are emerging. Single-cell transcriptomics allows the fate of individual astrocytes to be followed in situ and provides the granularity needed to describe healthy and pathological cellular states at different stages of Alzheimer's disease. Given the differences between human and rodent astroglia, study of human cells in this way will be crucial. Although refined single-cell transcriptomic analyses of human post-mortem brains are important for documentation of pathology, they only provide snapshots of a dynamic reality. Thus, functional work studying human astrocytes generated from stem cells and exposed to pathological conditions in rodent brain or cell culture are needed to understand the role of these cells in the pathogenesis of Alzheimer's disease. These studies will lead to novel biomarkers and hopefully a series of new drug targets to tackle this disease.