Good riddance to bad rubbish: Waste disposal in motor neuron disease

Abstract

Motor neuron degeneration is accompanied by disruption to the blood brain and spinal cord barriers in motor neuron disease and has been attributed to the aggregation of a critical RNA processing molecule, TDP-43. Degradation of soluble monomeric TDP-43 is attributed to the ubiquitin proteasome system and aggregated TDP-43 degradation is attributed to autophagy, but it is unknown which or if either disposal system is at fault in motor neuron disease. Blood brain barrier disruption is evidenced by leakage of blood components across the barrier and loss of pericytes of the neurovascular unit at the blood brain barrier. It is unknown how pericytes degenerate or if blood brain barrier disruption precedes motor neuron degeneration. A tandem fluorescent timer was modified to report the relative age of TDP-43 for the purpose of testing TDP-43 turnover in post-mortem motor neuron disease pericytes compared to control pericytes. The ability of the timer to differentiate between new cytoplasmic TDP-43 and older nucleic TDP-43 was validated. Cytoplasmic TDP-43 was increased in age when treated with MG132, a ubiquitin proteasome system inhibitor and post-mortem motor neuron disease pericytes had older TDP-43 compared to control post-mortem pericytes. Despite these differences, proteasomal function and autophagy were intact in motor neuron disease pericytes. To further investigate TDP-43 turnover in pericytes, the effects of oxidative stress, which has been associated with MND development was tested. Acute oxidative stress, induced by Na-Arsenite, reduced the detection of TDP-43, most likely through loss of detection of RNA recognition motif-2. Detection of TDP-43 was restored with an antibody which did not rely on RNA recognition motif-2. In conclusion, a method to detect oxidatively stressed TDP-43 with the dual use of TDP-43 antibodies which are positive and negative for RRM2 detection was developed. This method of detection is a novel tool with powerful implications for studying pathological TDP-43 in post-mortem motor neuron disease brains with the potential to provide patient stratification, much needed for advancements in clinical trials.