Phagocytic Activities of Reactive Microglia and Astrocytes Associated with Prion Diseases Are Dysregulated in Opposite Directions.

Anshuman Sinha,Ilia V Baskakov,Kara Molesworth,Natallia Makarava,Olga Mychko,Rajesh Kushwaha

doi:10.3390/cells10071728

Abstract

Phagocytosis is one of the most important physiological functions of the glia directed at maintaining a healthy, homeostatic environment in the brain. Under a homeostatic environment, the phagocytic activities of astrocytes and microglia are tightly coordinated in time and space. In neurodegenerative diseases, both microglia and astrocytes contribute to neuroinflammation and disease pathogenesis, however, whether their phagocytic activities are up- or downregulated in reactive states is not known. To address this question, this current study isolated microglia and astrocytes from C57BL/6J mice infected with prions and tested their phagocytic activities in live-cell imaging assays that used synaptosomes and myelin debris as substrates. The phagocytic uptake by the reactive microglia was found to be significantly upregulated, whereas that of the reactive astrocytes was strongly downregulated. The up- and downregulation of phagocytosis by the two cell types were observed irrespective of whether disease-associated synaptosomes, normal synaptosomes, or myelin debris were used in the assays, indicating that dysregulations are dictated by cell reactive states, not substrates. Analysis of gene expression confirmed dysregulation of phagocytic functions in both cell types. Immunostaining of animal brains infected with prions revealed that at the terminal stage of disease, neuronal cell bodies were subject to engulfment by reactive microglia. This study suggests that imbalance in the phagocytic activities of the reactive microglia and astrocytes, which are dysregulated in opposite directions, is likely to lead to excessive microglia-mediated neuronal death on the one hand, and the inability of astrocytes to clear cell debris on the other hand, contributing to the neurotoxic effects of glia as a whole.

Highlights

Chronic neuroinflammation is considered as one of the major pathological manifestations of neurodegenerative diseases including Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis, and prion diseases [1]
We found that the reactive microglia upregulate, whereas the reactive astrocytes downregulate phagocytic uptakes
Astrocytes and microglia were isolated from adult animals according to the protocols that preserve glial reactive phenotypes [6]

Summary

Introduction

Chronic neuroinflammation is considered as one of the major pathological manifestations of neurodegenerative diseases including Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis, and prion diseases [1] In prion diseases, both astrocytes and microglia undergo profound transcriptional, morphological, and functional transformation resulting in reactive phenotypes [2,3,4,5]. Reactive phenotypes of astrocytes and microglia involve the global dysregulation of their homeostatic functions that manifest in the disturbances of multiple physiological pathways [2,3] Both microglia and astrocytes contribute to neuroinflammation. The degree of astrocyte activation along with the disturbance in their physiological pathways inversely correlated with the incubation time to prion disease suggesting that the reactive states contribute to the rate of disease progression [3]. Elimination of three microglia-derived factors TNF-α, IL-1α, and C1qa was sufficient to accelerate the progression of prion diseases [13]

Methods

Results

Conclusion