A Neurodegeneration-Specific Gene-Expression Signature of Acutely Isolated Microglia from an Amyotrophic Lateral Sclerosis Mouse Model

Abstract

Microglia are resident immune cells of the CNS that are activated by infection, neuronal injury and inflammation. Here we utilize flow cytometry and deep RNA sequencing of acutely isolated spinal cord microglia to define their activation in vivo. Analysis of resting microglia identified 29 genes that distinguish microglia from other CNS cells and peripheral macrophages/monocytes. We then analyzed molecular changes in microglia during neurodegenerative disease activation using the SOD1G93A mouse model of ALS. We find that SOD1G93A microglia are not derived from infiltrating monocytes, and that both potentially neuroprotective and toxic factors are concurrently up-regulated, including Alzheimer’s disease genes. Mutant microglia differed from SOD1WT, LPS activated microglia, and M1/M2 macrophages, that define an ALS-specific phenotype. Concurrent mRNA/FACS analysis revealed post-transcriptional regulation of microglia surface receptors, and T cell-associated changes in the transcriptome. These results provide insights into microglia biology and establish a resource for future studies of neuroinflammation.

Serotonin 2B receptor slows disease progression and prevents degeneration of spinal cord mononuclear phagocytes in amyotrophic lateral sclerosis

“…Our current work does not formally determine whether degenerating Iba1-positive cells are microglia and/or infiltrating monocytes. The extent of monocyte infiltration remains controversial in the field of ALS [9,12,42,1,46], contrary to Alzheimer's disease in which monocyte infiltration has been repeatedly observed [29,30]. Our gene expression results demonstrate consistent loss of El Oussini and collaborators 15 expression of multiple genes typical of homeostatic microglia but while the monocyte marker Ly6c1 remained unaffected (Fig 6).…”

“…5c), suggesting that the loss of Htr2b was associated with degeneration of mononuclear phagocytes in end stage SOD1(G86R) mice. To determine whether this degeneration of mononuclear phagocytes affected microglia and/or monocytes, we measured expression levels of genes known to be part of the homeostatic microglial signature [12,8,9,7], in particular Cx3cr1, Hexb, Tmem119, P2ry12, Olflm3 and SiglecH. All these genes displayed lower expression in SOD1(G86R) mice lacking the Htr2b gene than in SOD1(G86R) mice with the Htr2b gene (Fig.…”

“…Recently, genome-wide characterization of microglial and monocyte expression patterns led to the identification of a molecular signature of homeostatic microglia. Specifically, microglia express genes not expressed in circulating monocytes or other resident tissue macrophages such as P2ry12, Hexb, Olfml3, SiglecH, Tgfbr1 or Tmem119 [8,48,31,12,7]. Contrastingly, monocytes express surface markers not present on microglia, such as Ly6c1 [9].…”

“…An open question is whether the toxic effect of macrophages is mediated by resident microglia and/or by infiltrating blood monocytes. The extent of monocyte infiltration during ALS in mouse models remains controversial, with one study observing major monocytic infiltration [9], and others only very little if any [12,42,1,46]. Recently, genome-wide characterization of microglial and monocyte expression patterns led to the identification of a molecular signature of homeostatic microglia.…”

“…Contrastingly, monocytes express surface markers not present on microglia, such as Ly6c1 [9]. Importantly, microglia lose the expression of their homeostatic signature during disease progression [12,7], and develop unique expression patterns during experimental ALS distinct from inflammation-induced alteration [12,7]. This loss of typical microglial expression patterns could be directly correlated with degeneration of CNS macrophages, documented in different neurodegenerative diseases [58,57,69] including a transgenic model of ALS [24,58].…”