Detergent-resistant, ubiquitinated and hyperphosphorylated Tar DNA binding protein 43 (TDP-43, encoded by TARDBP) neuronal cytoplasmic inclusions are the pathological hallmark in ∼95% of amyotrophic lateral sclerosis and ∼60% of frontotemporal lobar degeneration cases. We sought to explore the role for the heat shock response in the clearance of insoluble TDP-43 in a cellular model of disease and to validate our findings in transgenic mice and human amyotrophic lateral sclerosis tissues. The heat shock response is a stress-responsive protective mechanism regulated by the transcription factor heat shock factor 1 (HSF1), which increases the expression of chaperones that refold damaged misfolded proteins or facilitate their degradation. Here we show that manipulation of the heat shock response by expression of dominant active HSF1 results in a dramatic reduction of insoluble and hyperphosphorylated TDP-43 that enhances cell survival, whereas expression of dominant negative HSF1 leads to enhanced TDP-43 aggregation and hyperphosphorylation. To determine which chaperones were mediating TDP-43 clearance we over-expressed a range of heat shock proteins (HSPs) and identified DNAJB2a (encoded by DNAJB2, and also known as HSJ1a) as a potent anti-aggregation chaperone for TDP-43. DNAJB2a has a J domain, allowing it to interact with HSP70, and ubiquitin interacting motifs, which enable it to engage the degradation of its client proteins. Using functionally deleted DNAJB2a constructs we demonstrated that TDP-43 clearance was J domain-dependent and was not affected by ubiquitin interacting motif deletion or proteasome inhibition. This indicates that TDP-43 is maintained in a soluble state by DNAJB2a, leaving the total levels of TDP-43 unchanged. Additionally, we have demonstrated that the levels of HSF1 and heat shock proteins are significantly reduced in affected neuronal tissues from a TDP-43 transgenic mouse model of amyotrophic lateral sclerosis and patients with sporadic amyotrophic lateral sclerosis. This implies that the HSF1-mediated DNAJB2a/HSP70 heat shock response pathway is compromised in amyotrophic lateral sclerosis. Defective refolding of TDP-43 is predicted to aggravate the TDP-43 proteinopathy. The finding that the pathological accumulation of insoluble TDP-43 can be reduced by the activation of HSF1/HSP pathways presents an exciting opportunity for the development of novel therapeutics.

Analysis of 226 exome-sequenced UK cases of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia identified 2 individuals who harbored a P497H and P506S UBQLN2 mutation, respectively (n = 0.9%). The P506S index case presented with behavioral variant frontotemporal dementia at the age of 54 years then progressed to ALS surviving 3 years. Three sons presented with (1) slowly progressive pure spastic paraplegia with an onset at 25 years and (2) ALS with disease onset of 25 years and survival of 2 years, and (3) ALS presenting symptoms at the age of 26 years, respectively. Analysis of postmortem tissue from the index case revealed frequent neuronal cytoplasmic UBQLN2-positive inclusions in the dentate gyrus and TDP-43-positive neuronal cytoplasmic inclusions in the frontal and temporal cortex and granular cell layer of the dentate gyrus of the hippocampus. Furthermore, a comprehensive analysis of published UBQLN2 mutations demonstrated that only proline-rich domain mutations contribute to a significantly earlier age of onset in male patients (p = 0.0026).

Mutations in CHCHD10 have recently been reported as a cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. To address the genetic contribution of CHCHD10 to ALS, we have screened a cohort of 425 UK ALS ± frontotemporal dementia patients and 576 local controls in all coding exons of CHCHD10 by Sanger sequencing. We identified a previously reported p.P34S variant that is also present in neurologically healthy controls (p = 0.58). Our results suggest that CHCHD10 is not a primary cause of ALS in UK cases.