Microglial Interactions With Other Cell Types in ALS
Astrocytes are the most abundant glial cell in the CNS, providing multiple mechanisms of support to neurons including metabolic homeostasis, synapse regulation and immune functions (Vasile et al., 2017). In an elegant study, it was demonstrated that microglia are able to influence astrocytic gene expression, resulting in both the loss of homeostatic functions and a transformation to a neurotoxic state (Liddelow et al., 2017). Like microglia, astrocyte dysfunction has been implicated in non-cell autonomous motor neuron death in ALS (Nagai et al., 2007; Yamanaka et al., 2008). Both loss of homeostatic functions and a toxic release of factors have been implicated in astrocytic non-cell autonomous mechanisms of motor neuron death in ALS (Serio and Patani, 2018). As significant crosstalk is known to occur between microglia and astrocytes (Jha et al., 2019), microglia-astrocyte crosstalk is likely to play a role in ALS (Figure 1).
Multiple cell types affect microglial responses in ALS. Both astrocytes and peripheral immune cells that infiltrate the CNS during ALS disease progression can affect microglial responses, resulting in an overall protective or toxic microglial phenotype. Changes in cellular composition, ageing and factors released from dying cells are also likely to affect the microglial phenotype in ALS. Components of this figure were created using Servier Medical Art templates, which are licensed under a Creative Commons Attribution 3.0 Unported License.
The interaction between microglia and astrocytes has been partially addressed in the mSOD1 mouse model. While no differences were observed in the morphology of astrocytes following removal of mSOD1 from microglia (Boillee et al., 2006), reducing microglial proliferation in the mSOD1 mouse decreased activation of astrocytes (Gowing et al., 2008). Concomitantly, removal of mSOD1 from astrocytes resulted in a decrease in microglial activation (Yamanaka et al., 2008). Further evidence for the influence of astrocytes on microglial activation comes from studies where transplantation of wild-type astrocyte precursors into the mSOD1 mouse spinal cord reduced microglial activation (Lepore et al., 2008) and transplantation of mSOD1 expressing astrocyte precursors into wild-type mice was sufficient to induce microglial activation (Papadeas et al., 2011).
The mechanisms that dictate the interplay between astrocytes and microglia in ALS are not well understood. While microglia release multiple factors that are able to influence astrocytic gene expression (Jha et al., 2019), none have been demonstrated to directly manipulate astrocytic gene expression in ALS models. Conversely, astrocytic release of TGF-β has been shown to affect microglia as an astrocyte-specific overexpressing TGF-β mSOD1 mouse reduced the expression of a subset of microglial factors that have been proposed to be neuroprotective (Endo et al., 2015). Somewhat surprisingly, owing to the vastly different embryological origin, microglia have been reported to undergo a fate transition to astrocytes when cultured from symptomatic mSOD1 rats (Trias et al., 2013). In aggregate, these studies argue for a deeper understanding of the cellular interplay and primacy of events between microglia and astrocytes.
Oligodendrocytes are responsible for providing metabolic support to neurons and maintaining the myelin sheath required for neuronal saltatory conduction of action potentials. Degeneration of oligodendrocytes has been found to occur in mSOD1 mice (Kang et al., 2013). This degeneration was accompanied by microglial activation and microglial localization to apoptotic oligodendrocytes. Furthermore, in this study removal of mSOD1 specifically from oligodendrocytes resulted in a delay in disease onset, which coincided with delayed microglial activation. Proliferation of oligodendrocyte progenitor cells (OPCs) was found to occur in the mSOD1 model to replace lost oligodendrocytes (Kang et al., 2013). Microglial reactive state can influence OPC differentiation as directing microglia to pro- or anti-inflammatory states respectively inhibited or promoted remyelination in the lysolecithin model of demyelination (Miron et al., 2013; Lombardi et al., 2019). However, the relationship between microglia and OPCs in the context of ALS is not well understood.
Peripheral Immune Cells
Breakdown of the blood–brain barrier during ALS disease progression enables the infiltration of peripheral immune cells into the CNS (Kawamata et al., 1992; Engelhardt et al., 1993; Garofalo et al., 2020). Invading immune cells have been implicated in both neurotoxic and neuroprotective responses in ALS (Beers et al., 2008, 2011a; Coque et al., 2019), which is more comprehensively reviewed elsewhere (Thonhoff et al., 2018; Beers and Appel, 2019). Interestingly, peripheral macrophages are reported to become 'microglia-like' and express several previously described microglial-specific genes following their entry into the CNS during chronic neuroinflammation in the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis, which may also be relevant in diseases such as ALS (Grassivaro et al., 2020).
Several studies have suggested that peripheral immune cells affect microglial morphology and gene expression during mSOD1 disease progression. Increased expression of microglial factors associated with neurotoxicity was observed at end stage in mSOD1/PU1knockout mice given bone marrow transplants from either Rag2 −/− or Cd4 −/− mice (Beers et al., 2008), which are unable to generate mature lymphocytes or CD4+ T cells, respectively. This suggests that these cells reduce harmful microglial activation in the mSOD1 mouse model. Interestingly, while mSOD1 mice lacking T cells increased factors associated with neurotoxicity there was a reduction in morphological activation, suggesting a disconnect between morphological change and activation at the level of gene expression in this context.
The regulation of microglia by T cells in the mSOD1 mouse may be temporally mediated, as mSOD1/RAG2 knockout mice had a delayed disease onset, which coincided with an increase in microglial activation associated with a neuroprotective state (Tada et al., 2011). The reason for this discrepancy in microglial activation state between early and late disease in mSOD1 mice may be a temporal shift in the composition of the infiltrating immune cell population during disease progression. It is important to note that these results are controversial as the finding of delayed disease onset in mSOD1/RAG2 knockout mice is in direct contradiction to findings of earlier disease onset in other studies using mSOD1/RAG2 knockout mice (Beers et al., 2008; Chiu et al., 2008), which was suggested to be due differences in environmental factors or the copy numbers of the SOD1 transgene between the strains of mice used (Tada et al., 2011).
It has been suggested that protective CD4+ cells enter the CNS at an earlier stage of disease progression than cytotoxic CD8+ cells, which results a shift in microglial activation from a neuroprotective to a neurotoxic state (Beers et al., 2008; Liao et al., 2012). A recent study has suggested that NK cells also shift microglial responses towards a neurotoxic state in mSOD1 mice, in addition to directly causing neurotoxicity through a perforin-dependent release of lysosomes (Garofalo et al., 2020). Depletion of the NK cell population with blocking antibodies reduced microglial proliferation, altered morphology and shifted microglial gene expression towards a phenotype associated with neuroprotection, coinciding with extended survival of mSOD1 mice. Together, these studies show that infiltrating immune cells affect microglial phenotypes, which correlate with modifying disease in mSOD1 models of ALS.
As previously stated, motor neurons are the primary cell type affected in ALS. Interestingly, while transcriptional changes in astrocytes and oligodendrocytes occurred after changes in motor neurons in mSOD1 mice (Sun et al., 2015), recent spatial transcriptomics has suggested that microglial dysfunction precedes changes in motor neurons (Maniatis et al., 2019). However, it is unlikely that microglia are the dominating cell type directly driving motor neuron death in ALS. As previously mentioned, expression of mSOD1 in microglia alone does not result in disease (Beers et al., 2006) and there is a lack of microglial-mediated motor neuron death in C9orf72−/− mice, even though widespread microglial activation occurs (O'Rourke et al., 2016). Furthermore, systemic neuroinflammation involving microglial activation is also observed in other diseases such as lupus and rheumatoid arthritis while no motor neuron death occurs (Schwartz et al., 2019).
Nevertheless, microglia may still play an important modulatory role in motor neuron death in ALS. The expression of mSOD1 specifically in motor neurons is not sufficient to cause disease and non-neuronal cell types including microglia have been shown to affect disease progression in mSOD1 mice (Pramatarova et al., 2001; Clement et al., 2003; Boillee et al., 2006). Thus, pathological mechanisms in both motor neurons and surrounding glia are required for disease to occur. This is supported by the high expression of ALS-associated genes such as C9orf72, OPTN and TBK1 in both motor neurons and glia. The molecular mechanisms dictating microglial-mediated protection or toxicity to motor neurons in ALS are discussed below.
Brain. 2020;143(12):3526-3539. © 2020 Oxford University Press