The role of TDP-43 fragments in regular cellular functions and homeostatic failure

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive degeneration of motor neurons, leading to severe muscle weakness, loss of voluntary movement, and respiratory failure. A widely noted feature of the disease is the presence of TDP-43 proteinopathies. Under homeostatic conditions, the RNA/DNA-binding protein TDP-43 mainly resides in the nucleus, where it functions to regulate gene expression, controlling not only RNA transcription and splicing, but also stability and transport to the cytoplasm. Upon the arrival at ribosomes, TDP-43 may further moderate translation, acting as a global repressor of protein synthesis. However, in over 95% of ALS cases, TDP-43 mislocalises from the nucleus to the cytoplasm, where it enriches in cytoplasmic inclusions that are marked by the presence of misfolded, ubiquitinated, phosphorylated and fragmented protein species of TDP-43. Although recent studies have tried to untangle the relationship between TDP fragments on the one hand, and cytotoxicity as well as neurodegeneration on the other, the results are still a matter of debate. Here, we review our current understanding of the different TDP fragments derived from proteolytic cleavage as well as alternative splicing, addressing the different N-terminal and C-terminal species and evaluating differences in rodent and primate models. We focus our analysis on potential homeostatic functions of TDP fragments in the context of viral infections and myelination control, which could be pivotally interconnected. The findings illustrate several facets of fragmented TDP-43 protein species in scenarios of enhanced cellular stress. Gaining a detailed understanding could help to reveal new treatment options for ALS and other TDP-43 proteinopathies.