Mitochondrial damage-associated molecular patterns: Neuroimmunomodulators in central nervous system pathophysiology

Neuroinflammation contributes to a wide range of neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis. It is driven by non-neuronal glial cells, mainly microglia and astrocytes. Microglia are the resident immune cells of the central nervous system, while astrocytes are the main support cells for neuronal functions but can also participate in neuroimmune responses. Both these glial cell types can become reactive upon detection of certain endogenous intracellular molecules that appear in the extracellular space under specific circumstances; these can be pathology-associated abnormal structures, such as amyloid β proteins, or damage-associated molecular patterns released from injured cells, including their mitochondria. Once in the extracellular space, damage-associated molecular patterns act as ligands for specific pattern recognition receptors expressed by glia inducing their reactivity and neuroimmune responses. This review considers the following mitochondrial damage-associated molecular patterns: heme, cytochrome c, cardiolipin, adenosine triphosphate, mitochondrial DNA, mitochondrial transcription factor A, N-formyl peptides, and the tricarboxylic acid cycle metabolites: succinate, fumarate, and itaconate. We describe their well-established functions as damage-associated molecular patterns of the peripheral tissues before summarizing available evidence indicating these molecules may also play significant roles in the neuroimmune processes of the central nervous system. We highlight the pattern recognition receptors that mitochondrial damage-associated molecular patterns interact with and the cellular signaling mechanisms they modulate. Our review demonstrates that some mitochondrial damage-associated molecular patterns, such as cytochrome c, adenosine triphosphate, and mitochondrial transcription factor A, have already demonstrated significant effects on the central nervous system. In contrast, others including cardiolipin, mitochondrial DNA, N-formyl peptides, succinate, fumarate, and itaconate, will require additional studies corroborating their roles as damage-associated molecular patterns in the central nervous system. For all of the reviewed mitochondrial damage-associated molecular patterns, there is a shortage of studies using human cells and tissues, which is identified as a significant knowledge gap. We also assess the need for targeted research on the effects of mitochondrial damage-associated molecular patterns in the central nervous system pathologies where their roles are understudied. Such studies could identify novel treatment strategies for multiple neurodegenerative diseases, which are characterized by chronic neuroinflammation and currently lack effective therapies.