Differential Roles of TREM2+ Microglia in Anterograde and Retrograde Axonal Injury Models

Gemma Manich,Nàdia Villacampa,Ariadna Regina Gómez-López,Mireia Recasens,Berta González,Bernardo Castellano,Beatriz Almolda,Kalpana Shrivastava

doi:10.3389/fncel.2020.567404

Abstract

Microglia are the main immune cells of the central nervous system (CNS), and they are devoted to the active surveillance of the CNS during homeostasis and disease. In the last years, the microglial receptor Triggering Receptor Expressed on Myeloid cells-2 (TREM2) has been defined to mediate several microglial functions, including phagocytosis, survival, proliferation, and migration, and to be a key regulator of a new common microglial signature induced under neurodegenerative conditions and aging, also known as disease-associated microglia (DAM). Although microglial TREM2 has been mainly studied in chronic neurodegenerative diseases, few studies address its regulation and functions in acute inflammatory injuries. In this context, the present work aims to study the regulation of TREM2 and its functions after reparative axonal injuries, using two-well established animal models of anterograde and retrograde neuronal degeneration: the perforant pathway transection (PPT) and the facial nerve axotomy (FNA). Our results indicate the appearance of a subpopulation of microglia expressing TREM2 after both anterograde and retrograde axonal injury. TREM2+ microglia were not directly related to proliferation, instead, they were associated with specific recognition and/or phagocytosis of myelin and degenerating neurons, as assessed by immunohistochemistry and flow cytometry. Characterization of TREM2+ microglia showed expression of CD16/32, CD68, and occasional Galectin-3. However, specific singularities within each model were observed in P2RY12 expression, which was only downregulated after PPT, and in ApoE, where de novo expression was detected only in TREM2+ microglia after FNA. Finally, we report that the pro-inflammatory or anti-inflammatory cytokine microenvironment, which may affect phagocytosis, did not directly modify the induction of TREM2+ subpopulation in any injury model, although it changed TREM2 levels due to modification of the microglial activation pattern. In conclusion, we describe a unique TREM2+ microglial subpopulation induced after axonal injury, which is directly associated with phagocytosis of specific cell remnants and show different phenotypes, depending on the microglial activation status and the degree of tissue injury.

Highlights

Microglial cells are the principal myeloid cells within the central nervous system (CNS), which play an immune surveillance function under homeostasis and after damage or pathological insult (Colonna and Butovsky, 2017; Wolf et al, 2017)
To determine if Triggering Receptor Expressed on Myeloid cells 2 (TREM2) microglial expression after perforant pathway transection (PPT) and facial nerve axotomy (FNA) was related to these functions, we studied the dynamics of microglial proliferation and phagocytosis (CD16/32, CD68) in both models, and we compared it to the time course of TREM2 expression
The results obtained in this work show that TREM2 expression is upregulated after both anterograde and retrograde axonal injury in a specific subpopulation of microglia

Summary

Introduction

Microglial cells are the principal myeloid cells within the central nervous system (CNS), which play an immune surveillance function under homeostasis and after damage or pathological insult (Colonna and Butovsky, 2017; Wolf et al, 2017). Single-cell RNA-sequencing analysis in the CNS tissue linked TREM2 with the differentiation of a newly identified specific microglial subtype appearing in mice in neurodegenerative conditions and aging, the so-called disease-associated microglia (DAM; Keren-Shaul et al, 2017; Deczkowska et al, 2018) or microglia associated to neurodegeneration (Krasemann et al, 2017) These microglia play a key role in chronic neurodegenerative conditions and show a unique transcriptional and functional signature highly differing from homeostatic microglia, characterized by the overexpression of other genes, such as Clec7a, Lgals, ApoE, and the downregulation of microglial homeostatic genes, such as P2ry or Tmem119

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