Novel molecular mechanisms and signaling pathways of microglial responses in amyotrophic lateral sclerosis (ALS)

Resumen

Amyotrophic Lateral Sclerosis (ALS) is an incurable and fatal neurodegenerative disease, with survival expectancy between 3-5 years from the clinical diagnose. This disease is characterized for the progressive death of motor neurons. A typical hallmark of ALS is neuroinflammation, with extensive microgliosis and astrogliosis within the affected areas of the central nervous system (CNS). It is believed that neuroinflammation, especially microglia activation, is involved in the onset and progression of the disease. This is known as the non-cell autonomous theory where motor neurons die not only due to intrinsic factors but also as a consequence of a chronic and exacerbated inflammation state. Another feature of ALS, which is considered as a protein misfolding disease together with other neurodegenerative diseases such as frontotemporal lobar dementia (FTLD) or Parkinson´s disease (PD), is the accumulation of intracellular protein inclusions. TAR DNA binding protein 43 (TDP-43) is the principal component of almost all intracellular aggregates found in ALS cases and in ca. 50% of those with Frontotemporal dementia (FTD). TDP-43 is a nuclear multifunctional protein known for its RNA/DNA binding activities, which is believed to play a central role in these disorders. TDP-43 aberrantly aggregates in the cytoplasm of motor neurons in pathological scenarios. Under these conditions, TDP-43 aggregates are formed in the cytoplasm, containing truncated, ubiquitylated and/or hyperphosphorylated forms of the protein, and, additionally, retro-translocation of the protein from the nucleus to the cytoplasm of degenerating motor neurons occurs. Until this date there is not much data on how TDP-43 proteinopathy affects protein homeostasis of microglial cells. More recently, increased levels of TDP-43 have been found in biological fluids, including cerebrospinal fluid (CSF) and plasma, of ALS and FTD patients. The extracellular content of TDP-43 may be a result of cell death caused by cytotoxic intracellular TDP-43 inclusions or, additionally, be released from cells to CSF via exosomes and/or other unknown pathways in ALS patients. Moreover, extracellular TDP-43 could play a critical role in sustained neuroinflammation in these disorders. It is known that microglial cells can be activated by extracellular aggregates of mutant superoxide dismutase 1 (mSOD1) via activation of Toll like receptor 2 (TLR2) and CD14 receptors but until now no data involving extracellular TDP-43 aggregates exist. In order to gain further insight into the physiopathological roles of TDP-43 aggregates in ALS, we studied the impact of exogenously added TDP-43 aggregates on the neuroinflammatory response of microglial cells in culture, and explored the molecular mechanisms operating upon exposure to such ALS-associated species. Furthermore, we investigated whether certain molecular chaperones that have been found at abnormal levels in extracellular fluids of ALS animal models and patients, are able to modify TDP-43-elicited microglial responses. Heat-shock proteins (HSPs) and other molecular chaperones are known for their traditional role linked to the maintenance of cellular proteostasis both under physiological and pathological scenarios such as in neurodegenerative proteinopathies. Importantly, several HSPs, in addition to binding and remodeling protein aggregates, have been found to display immunological activities, including the alteration of α-synuclein (α-syn) misfolding protein-elicited responses in vitro and in immunized mice. In attempt to elucidate new molecular elements involved in the signaling pathways of the neuroimmune response, we searched for TDP-43-interacting molecules in microglial cells. Remarkably, we uncovered MAPK/MAK/MRK overlapping kinase (MOK) as a new TDP-43 interactor and cell signalling mediator engaged not only in TDP-43-exposed microglia but also in a general inflammation model. Moreover, we were able to demonstrate that MOK protein kinase is indeed involved in the neuroinflammatory response and that it might be a central player in ALS disease and other neurodegenerative diseases.