ASK1 signaling regulates phase-specific glial interactions during neuroinflammation

Neuroinflammation is well known to be associated with neurodegenerative diseases. Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein kinase kinase kinase that has been implicated in neuroinflammation, but its precise cellular and molecular mechanisms remain unknown. In this study, we generated conditional knockout (CKO) mice that lack ASK1 in T cells, dendritic cells, microglia/macrophages, microglia, or astrocytes, to assess the roles of ASK1 during experimental autoimmune encephalomyelitis (EAE). We found that neuroinflammation was reduced in both the early and later stages of EAE in microglia/macrophage-specific ASK1 knockout mice, whereas only the later-stage neuroinflammation was ameliorated in astrocyte-specific ASK1 knockout mice. ASK1 deficiency in T cells and dendritic cells had no significant effects on EAE severity. Further, we found that ASK1 in microglia/macrophages induces a proinflammatory environment, which subsequently activates astrocytes to exacerbate neuroinflammation. Microglia-specific ASK1 deletion was achieved using a CX3CR1^CreER system, and we found that ASK1 signaling in microglia played a major role in generating and maintaining disease. Activated astrocytes produce key inflammatory mediators, including CCL2, that further activated and recruited microglia/macrophages, in an astrocytic ASK1-dependent manner. Astrocyte-specific analysis revealed CCL2 expression was higher in the later stage compared with the early stage, suggesting a greater proinflammatory role of astrocytes in the later stage. Our findings demonstrate cell-type–specific roles of ASK1 and suggest phase-specific ASK1-dependent glial cell interactions in EAE pathophysiology. We propose glial ASK1 as a promising therapeutic target for reducing neuroinflammation.

Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein kinase (MAPK) kinase kinase that stimulates the c-Jun N-terminal kinase (JNK) and p38 MAPK pathways, and it mediates diverse biological signals leading to cell death, differentiation, and senescence (1, 2). Deletion of ASK1 in mice suppresses neuronal cell death from injury (3, 4), and recent studies indicate that ASK1 is involved in various neurodegenerative diseases, including amyotrophic lateral sclerosis, Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis (MS) (5–8). MS is an inflammatory disease of the central nervous system (CNS) characterized by localized areas of demyelination. Experimental autoimmune encephalomyelitis (EAE) is a classic model widely used to explore pathogenic mechanisms of MS, generated by administering a myelin basic protein peptide that induces an autoimmune response directed to myelin (9). During MS/EAE, activated microglia/macrophages are the first cells to respond to inflammatory insults within the CNS. Microglia/macrophages may be polarized into proinflammatory or antiinflammatory states, with each state having a distinct molecular phenotype and effector function, and targeting microglia/macrophages may have therapeutic benefits in MS/EAE treatment (10–12). Astrocytes, another subset of glia, are the most abundant cell population within the CNS. Astrocytes are involved in the regulation of synaptic function, plasticity, and maintaining brain homeostasis, and they are thought to contribute to the pathogenesis of MS/EAE by producing proinflammatory cytokine/chemokines such as CCL2 (13–16). In recent years, astrocytes have also been shown to polarize into different subtypes: A1 astrocytes are neurotoxic, and blocking the conversion of astrocytes into the A1 phenotype is neuroprotective (17, 18); although, nowadays, the activation state is described to be more diverse than the simple A1/A2 nomenclature (19, 20). Studies of intrinsic and external factors involved in astrocyte activation or polarization may provide information regarding how astrocytic function changes during disease, which may lead to the development of novel therapies.We previously reported that ASK1 deficiency ameliorated the severity of EAE, using conventional ASK1 knockout (ASK1 KO) mice (21). In this study, we selectively deleted ASK1 from five types of cells: T cells, dendritic cells, microglia/macrophages, microglia, and astrocytes, to dissect out the roles of ASK1 in different cell types during neuroinflammation. Our study revealed pathogenic roles of ASK1 signaling in innate immune cells and how they interact with each other in the progression of MS/EAE.