Microglia : health promoting pathways and therapeutic targets in ageing and neuroinflammation

Abstract

Microglia are the innate immune cells of the CNS with an embryonic origin and self-renew with a slow turnover throughout life. The health-promoting capacities of this cell are being acknowledged through updated and specific tools separating microglia from bone marrow-derived macrophages. Knowing that microglia depends on TGF-β signaling in acquiring a mature homeostatic phenotype, we also found this cytokine to mediate the integration of monocyte-derived cells into an empty myeloid CNS niche. Microglia or the integrated monocyte-derived macrophages, absent in TGF-β signaling, developed a damaging phenotype causing spontaneous de-myelination, clinical motor deficits, and death of experimental mice.

Multiple sclerosis (MS) is a de-myelinating autoimmune CNS disease, commonly with onset in young adults as a relapsing-remitting disease that over time converts to a progressive accumulation of clinical deficits. The etiology of MS is partly inherited but, in many aspects, unknown, although we have successful treatments targeting the adaptive immune system reducing relapses and likely delay progression. However, the progressive MS disease, believed to emanate from the cells residing in the CNS, is very limited in treatment options. Genetic association studies imply that the microglial cell harness a substantial part of the MS-pathogenicity. How this relates to disease phenotypes offering treatment targets is sparsely explored. As the human CNS is rather inaccessible, the use of the rodent animal model experimental autoimmune encephalomyelitis (EAE) has been instrumental in deciphering the MS pathology. In the recovery phase of this disease, the microglial clearance of myelin is of substantial importance. We found this process to depend on a lysosomal degradation process referred to as autophagy- or LC3- associated phagocytosis. During EAE, microglia lacking the autophagy gene Atg7 accumulated myelin debris and had reduced recirculation of scavenger receptors, causing a secondary impairment in tissue myelin-clearance. These cells also acquired an altered transcriptome associated with inflammatory microglia/macrophage phenotypes found in, e.g., MS, neurodegenerative disease, and stroke, while the genes of the homeostatic signature were downregulated. Autophagy is known to alter with age, and we targeted this by increasing autophagy-associated phagocytosis in aged microglia by treatment with the sugar molecule trehalose, which ameliorated EAE. Of note, trehalose metabolism and some autophagy-associated phagocytosis pathway components are associated with MS through risk allele analysis.

Microglial proliferation and survival rely on CSF-1 (M-CSF) or IL-34 mediated activation of the CSF-1 receptor (CSF1R). While microglial CSF-1 expression is elevated during inflammation, CSF1R associate with the homeostatic microglia. In the healthy CNS, neurons are the main source of IL-34, but a specific role of this cytokine in neuroinflammation remains to be evaluated. The ageing CNS is challenging for microglia in terms of adaptations to aid in health-promoting capacities in functions that, together with CSF1R signaling, engage canonical-autophagy. This degradation pathway controls the quality and quantity of, e.g., inflammatory mediators, receptors, and organelles. By deleting the key canonical-autophagy gene Ulk1, we found an age-associated subpopulation of microglia with highly activated ERK1/2 upon CSF1R engagement to be diminished, a loss not compensated by other microglia or myeloid cells. The loss of this population in aged mice caused neural and glial cell death and high mortality in EAE. In autophagy-competent aged mice, we could expand this CNS protective population specifically by IL-34 treatment and thereby ameliorate disease.

In this thesis, I present TGF-β as an essential factor in establishing a homeostatic CNS myeloid cell and a demand in aged microglia for canonical-autophagy to maintain a neuroprotective phenotype. The myelin processing by microglia through autophagyassociated phagocytosis is dissected in detail, and we can show how a decline in this pathway can be restored in aged microglia.