Pathogenic TDP-43 accelerates the generation of toxic exon1 HTT in Huntington's disease knock-in mice.

Huntington's disease (HD) is caused by a CAG repeat expansion in exon1 of the HTT gene that encodes a polyglutamine tract in huntingtin protein. The formation of HTT exon1 fragments with an expanded polyglutamine repeat has been implicated as a key step in the pathogenesis of HD. It was reported that the CAG repeat length-dependent aberrant splicing of exon1 HTT results in a short polyadenylated mRNA that is translated into an exon1 HTT protein. Under normal conditions, TDP-43 is predominantly found in the nucleus, where it regulates gene expression. However, in various pathological conditions, TDP-43 is mislocalized in the cytoplasm. By investigating HD knock-in mice, we explore whether the pathogenic TDP-43 in the cytoplasm contributes to HD pathogenesis, through expressing the cytoplasmic TDP-43 without nuclear localization signal. We found that the cytoplasmic TDP-43 is increased in the HD mouse brain and that its mislocalization could deteriorate the motor and gait behavior. Importantly, the cytoplasmic TDP-43, via its binding to the intron1 sequence (GU/UG)n of the mouse Htt pre-mRNA, promotes the transport of exon1-intron1 Htt onto ribosome, resulting in the aberrant generation of exon1 Htt. Our findings suggest that cytoplasmic TDP-43 contributes to HD pathogenesis via its binding to and transport of nuclear un-spliced mRNA to the ribosome for the generation of a toxic protein product.

Suppressing UBE2N ameliorates Alzheimer's disease pathology through the clearance of amyloid beta.

INTRODUCTION: Aging is one of the risk factors for the early onset of Alzheimer's disease (AD). We previously discovered that the age-dependent increase in Ubiquitin Conjugating Enzyme E2 N (UBE2N) plays a role in the accumulation of misfolded proteins through K63 ubiquitination, which has been linked to AD pathogenesis. However, the impact of UBE2N on amyloid pathology and clearance has remained unknown. RESULTS: We observed the elevated UBE2N during the amyloid beta (Aß) generation in the brains of 5×FAD, APP/PS1 mice, and patients with AD, in comparison to healthy individuals. UBE2N overexpression exacerbated amyloid deposition in 5×FAD mice and senescent monkeys, whereas knocking down UBE2N via CRISPR/Cas9 reduced Aß generation and cognitive deficiency. Moreover, pharmacological inhibition of UBE2N ameliorated Aß pathology and subsequent transcript defects in 5×FAD mice. DISCUSSION: We have discovered that age-dependent expression of UBE2N is a critical regulator of AD pathology. Our findings suggest that UBE2N could serve as a potential pharmacological target for the advancement of AD therapeutics. HIGHLIGHTS: Ubiquitin Conjugating Enzyme E2 N (UBE2N) level was elevated during amyloid beta (Aß) deposition in AD mouse and patients' brains. UBE2N exacerbated Aß generation in the AD mouse and senescent monkey. Drug inhibition of UBE2N ameliorated Aß pathology and cognitive deficiency.

Loss of TDP-43 mediates severe neurotoxicity by suppressing PJA1 gene transcription in the monkey brain.

The nuclear loss and cytoplasmic accumulation of TDP-43 (TAR DNA/RNA binding protein 43) are pathological hallmarks of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Previously, we reported that the primate-specific cleavage of TDP-43 accounts for its cytoplasmic mislocalization in patients' brains. This prompted us to investigate further whether and how the loss of nuclear TDP-43 mediates neuropathology in primate brain. In this study, we report that TDP-43 knockdown at the similar effectiveness, induces more damage to neuronal cells in the monkey brain than rodent mouse. Importantly, the loss of TDP-43 suppresses the E3 ubiquitin ligase PJA1 expression in the monkey brain at transcriptional level, but yields an opposite upregulation of PJA1 in the mouse brain. This distinct effect is due to the species-dependent binding of nuclear TDP-43 to the unique promoter sequences of the PJA1 genes. Further analyses reveal that the reduction of PJA1 accelerates neurotoxicity, whereas overexpressing PJA1 diminishes neuronal cell death by the TDP-43 knockdown in vivo. Our findings not only uncover a novel primate-specific neurotoxic contribution to the loss of function theory of TDP-43 proteinopathy, but also underscore a potential therapeutic approach of PJA1 to the loss of nuclear TDP-43.

Loss of TDP-43 promotes somatic CAG repeat expansion in Huntington's disease knock-in mice.

TAR binding protein 43 (TDP-43) is normally present in the nucleus but mislocalized in the cytoplasm in a number of neurodegenerative diseases including Huntington's disease (HD). The nuclear loss of TDP-43 impairs gene transcription and regulation. However, it remains to be investigated whether loss of TDP-43 influences trinucleotide CAG repeat expansion in the HD gene, a genetic cause for HD. Here we report that CRISPR/Cas9 mediated-knock down of endogenous TDP-43 in the striatum of HD knock-in mice promoted CAG repeat expansion, accompanied by the increased expression of the DNA mismatch repair genes, Msh3 and Mlh1, which have been reported to increase trinucleotide repeat instability. Furthermore, suppressing Msh3 and Mlh1 by CRISPR/Cas9 targeting diminished the CAG repeat expansion. These findings suggest that nuclear TDP-43 deficiency may dysregulate the expression of DNA mismatch repair genes, leading to CAG repeat expansion and contributing to the pathogenesis of CAG repeat diseases.