Aims: Smad ubiquitination regulatory factor-2 (Smurf2), an E3 ubiquitin ligase, can interact with Smad proteins and promote their ubiquitin-dependent degradation, thereby controlling the cellular levels of these signalling mediators. We previously reported that phosphorylated Smad2/3 (pSmad2/3) was sequestered in transactive response DNA-binding protein-43 (TDP-43) inclusions in the spinal cord of patients with amyotrophic lateral sclerosis (ALS). Recent biochemical and immunohistochemical studies on spinal cord and brain of ALS patients demonstrated that the composition of the TDP-43 inclusions is regionally distinct, suggesting different underlying pathogenic processes. We aimed to elucidate regional differences in pathomechanisms and composition of TDP-43 inclusions in relation to pSmad2/3 and Smurf2.

Methods: The spinal cord and brain tissues of 13 sporadic ALS (SALS) patients were investigated using immunohistochemical analysis.

Results: TDP-43-positive inclusions in lower motor neurones of SALS patients were immunopositive for Smurf2 and pSmad2/3. Multiple immunofluorescence staining for Smurf2, pSmad2/3, TDP-43 and ubiquitin revealed co-localization of these four proteins within the inclusions in lower motor neurones of SALS patients. Furthermore, the loss of nuclear pSmad2/3 immunoreactivity was observed in cells bearing TDP-43 inclusions. In contrast, TDP-43-positive inclusions in the extramotor neurones in the brain of SALS patients were noticeably negative for Smurf2 and pSmad2/3. In addition, pSmad2/3 immunoreactivity was preserved in the nuclei of inclusion-bearing cells.

Conclusions: This regional difference in the expression of Smurf2 and pSmad2/3 within TDP-43-positive inclusions might be one of the pathomechanisms underlying the loss of lower motor neurones and comparatively spared cortical neurones seen in ALS.