Microglia centered pathogenesis in ALS: insights in cell interconnectivity

Abstract

Amyotrophic lateral sclerosis (ALS) is the most common and most aggressive form of adult motor neuron (MN) degeneration. The cause of the disease is still unknown, but some protein mutations have been linked to the pathological process. Loss of upper and lower MNs results in progressive muscle paralysis and ultimately death due to respiratory failure. Although initially thought to derive from the selective loss of MNs, the pathogenic concept of non-cell-autonomous disease has come to the forefront for the contribution of glial cells in ALS, in particular microglia. Recent studies suggest that microglia may have a protective effect on MN in an early stage. Conversely, activated microglia contribute and enhance MN death by secreting neurotoxic factors, and impaired microglial function at the end-stage may instead accelerate disease progression. However, the nature of microglial-neuronal interactions that lead to MN degeneration remains elusive. We review the contribution of the neurodegenerative network in ALS pathology, with a special focus on each glial cell type from data obtained in the transgenic SOD1G93A rodents, the most widely used model. We further discuss the diverse roles of neuroinflammation and microglia phenotypes in the modulation of ALS pathology. We provide information on the processes associated with dysfunctional cell-cell communication and summarize findings on pathological cross-talk between neurons and astroglia, and neurons and microglia, as well as on the spread of pathogenic factors. We also highlight the relevance of neurovascular disruption and exosome trafficking to ALS pathology. The harmful and beneficial influences of NG2 cells, oligodendrocytes and Schwann cells will be discussed as well. Insights into the complex intercellular perturbations underlying ALS, including target identification, will enhance our efforts to develop effective therapeutic approaches for preventing or reversing symptomatic progression of this devastating disease.

Keywords: SOD1G93A transgenic mouse/rat; amyotrophic lateral sclerosis; microglia activation phenotypes; motor neuron; neurodegeneration; neuroinflammation; pathological cell–cell communication.

FIGURE 1

Neuron–microglia communication signaling pathways that modulate microglia cell phenotypes. Toll-like receptor (TLR) signaling contributes to classically activated microglia (M1) in response to damage-associated molecular patterns (DAMPs). Following recognition of DAMPs, TLRs activate downstream signaling cascades, activate nuclear factor-κB (NF-κB) inducing the transcription of inflammatory mediators associated with the M1-like microglial cell phenotype, such as the proinflammatory cytokines interleukin (IL)-1β and tumor necrosis factor (TNF)-α. The production of IL-1β can be achieved by the inflammasome activation by DAMPs, leading to caspase-1 activation that orchestrates the cleavage of pro-IL-1β to form active IL-1β, which leaves and binds to the IL-1 receptor, resulting in inflammation. Following activation, microglial cells augment the immune response by releasing metalloproteinases (MMPs) and increasing proinflammatory cytokines. High-mobility group box 1 (HMGB1) is an alarmin that signals cell damage in response to injury and are associated to all the described signaling events. HMGB1 released by activated microglia and damaged neurons concur for a vicious cycle mediating chronic, progressive neurodegeneration associated with neuroinflammation. HMGB1 can interact with receptors that include RAGE (receptor for advanced glycation end products), TLR-2, TLR-4, Mac-1 also known as CD11b, and possibly others. Alternatively activated microglia (M2) functions include phagocytosis. Milk fat globule factor-E8 (MFG-E8) produced by microglia recognizes phosphatidylserine (PS) as “eat me” signals expressed on the surface of apoptotic neurons, triggering a signaling cascade that stimulates phagocytosis to engulf the dying cell. In order to maintain a quiescent microglia phenotype (M0) under steady-state conditions, neurons suppress the activation of microglia through cell–cell contact (CD200–CD200R) and by the release of the chemokine ligand 1 (CX3CL1) mediated by the cathepsin S that binds to its receptor CX3CR1 on microglia.

FIGURE 2

Altered cross-talk between glial cells and motor neurons (MNs) in ALS disease. Many studies report the intervention of mutated superoxide dismutase-1 (SOD1)-expressing non-neuronal cells in the pathogenesis of the disease. In case of injury, astrocytes become activated in a process called astrogliosis, characterized by the up-regulation of intermediate filament glial fibrillary acidic protein (GFAP) and increased release of toxic products into the extracellular media, such as pro-inflammatory cytokines [tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β)] oxidative stressors [reactive oxygen species (ROS), nitric oxide (NO)], as well as glutamate and ATP mediated by the receptor P2X7. In addition, astrocytes also evidence a reduced expression of the glutamate transporter GLT-1, which additionally contribute to neuronal excitotoxicity. However, reactive astrocytes up-regulate nerve growth factor (NGF), which can modulate neuronal survival. Disorganization and destruction of the myelin sheath with a progressive loss of phospholipids and cholesterol is also observed. Moreover, oligodendrocytes also evidence a loss of the of the monocarboxylate transporter 1 (MCT1), thus compromising the energy supply to MNs. On the other hand, increased mutated SOD1 expression in Schwann cells may have intricate ways contributing to slow ALS progression. When mutated SOD1 accumulates within microglia, there is a pattern of activation. Microglia acquire different activation phenotypes (M0, M1, M2a, M2b, M2c, and dystrophic/senescent) and, consequently, produce a diversity of substances that may be either beneficial [insulin-like growth factor 1 (IGF-1), transforming growth factor-β (TGF-β), brain-derived neurotrophic factor (BDNF), and nerve growth factor (NGF)] or toxic (glutamate, ROS, NO) to other cells. Phagocytosis is mediated by the release of milk factor globule-8 (MFG-E8) from microglia M1 and M2c phenotypes that recognizes phosphatidylserine (PS) in apoptotic neurons. Reliable markers for M2a are high IL-1 receptor antagonist (IL-1Ra) and high arginase, for M2b are IL-10, TNF-α, IL-6, and IL-1β, for M2c are TGF-β and IL-10 and for M1 are TNF-α, IL-6, and IL-1β, increased inducible nitric oxide synthase (iNOS) and toll-like receptors (TLR)2 and 4.