Single-Cell Transcriptomics and In Situ Morphological Analyses Reveal Microglia Heterogeneity Across the Nigrostriatal Pathway.

Oihane Uriarte Huarte,Alexander Skupin,Alessandro Michelucci,Rashi Halder,Manuel Buttini,Kamil Grzyb,Dimitrios Kyriakis,Michel Mittelbronn,Yolanda Pires-Afonso,Tony Heurtaux

doi:10.3389/fimmu.2021.639613

Abstract

Microglia are the resident immune effector cells of the central nervous system (CNS) rapidly reacting to various pathological stimuli to maintain CNS homeostasis. However, microglial reactions in the CNS may also worsen neurological disorders. Hence, the phenotypic analysis of microglia in healthy tissue may identify specific poised subsets ultimately supporting or harming the neuronal network. This is all the more important for the understanding of CNS disorders exhibiting regional-specific and cellular pathological hallmarks, such as many neurodegenerative disorders, including Parkinson’s disease (PD). In this context, we aimed to address the heterogeneity of microglial cells in susceptible brain regions for PD, such as the nigrostriatal pathway. Here, we combined single-cell RNA-sequencing with immunofluorescence analyses of the murine nigrostriatal pathway, the most affected brain region in PD. We uncovered a microglia subset, mainly present in the midbrain, displaying an intrinsic transcriptional immune alerted signature sharing features of inflammation-induced microglia. Further, an in situ morphological screening of inferred cellular diversity showed a decreased microglia complexity in the midbrain when compared to striatum. Our study provides a resource for the identification of specific microglia phenotypes within the nigrostriatal pathway, which may be relevant in PD.

Highlights

Microglia are the resident immune effectors of the brain that arise from the yolk sac and colonize the brain early during embryonic development [1]
Following the comprehensive characterization of the clusters (Figure 1D), we verified that identities, markers, and proportions of cell types matched previous single-cell dropletbased sequencing data from mouse brain tissue (Figure 1E) [33], indicating that our results were robust for analyses
We identified different patterns of microglial cell density, with CP, nucleus accumbens (NA) and substantia nigra pars reticulata (SNr) subregions composed by similar shapes than cortex, whereas microglia density in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) was closer to cerebellum

Summary

Introduction

Microglia are the resident immune effectors of the brain that arise from the yolk sac and colonize the brain early during embryonic development [1] Under homeostatic conditions, both during development and in the adult brain, microglia play key roles shaping the neuronal network through synaptic pruning and phagocytosis of apoptotic neurons [2,3,4,5]. Microglia scan the adult brain and can rapidly react to threatening conditions to mainly maintain the brain homeostasis In this context, improper immune responses, such as weakened or exaggerated microglia reactions, can play a critical role in the development and progression of neurological diseases with an immunological component [6,7,8]. It has not yet been elucidated if specific microglia subsets within the nigrostriatal pathway may play a causative or a protective role for the development and progression of PD [14,15,16]

Objectives

Methods

Results

Conclusion