ALS/FTD mutations in UBQLN2 are linked to mitochondrial dysfunction through loss-of-function in mitochondrial protein import

Abstract

UBQLN2 mutations cause amyotrophic lateral sclerosis (ALS) with frontotemporal dementia (FTD), but the pathogenic mechanisms by which they cause disease remain unclear. Proteomic profiling identified ‘mitochondrial proteins’ as comprising the largest category of protein changes in the spinal cord (SC) of the P497S UBQLN2 mouse model of ALS/FTD. Immunoblots confirmed P497S animals have global changes in proteins predictive of a severe decline in mitochondrial health, including oxidative phosphorylation (OXPHOS), mitochondrial protein import and network dynamics. Functional studies confirmed mitochondria purified from the SC of P497S animals have age-dependent decline in nearly all steps of OXPHOS. Mitochondria cristae deformities were evident in spinal motor neurons of aged P497S animals. Knockout (KO) of UBQLN2 in HeLa cells resulted in changes in mitochondrial proteins and OXPHOS activity similar to those seen in the SC. KO of UBQLN2 also compromised targeting and processing of the mitochondrial import factor, TIMM44, resulting in accumulation in abnormal foci. The functional OXPHOS deficits and TIMM44-targeting defects were rescued by reexpression of WT UBQLN2 but not by ALS/FTD mutant UBQLN2 proteins. In vitro binding assays revealed ALS/FTD mutant UBQLN2 proteins bind weaker with TIMM44 than WT UBQLN2 protein, suggesting that the loss of UBQLN2 binding may underlie the import and/or delivery defect of TIMM44 to mitochondria. Our studies indicate a potential key pathogenic disturbance in mitochondrial health caused by UBQLN2 mutations.

Multi-omics profiling identifies a deregulated FUS-MAP1B axis in ALS/FTD–associated UBQLN2 mutants

“…In agreement with the latter, proteins and genes associated with RNA processing terms such as “regulation of mRNA stability,” “poly(A) RNA binding,” or “translation” were prominently represented in our proteomic and transcriptomic data sets besides additional terms such as “mitochondrion,” “extracellular exosome,” and Golgi-associated terms. Although morphological changes of mitochondria and the Golgi are established consequences of UBQLN2 mutations ( Lin et al, 2021 ), the connection between UBQLN2 and extracellular exosomes has so far not been explored in great detail. However, emerging evidence links exosomes to ALS pathology.…”

Stress Granule Homeostasis, Aberrant Phase Transition, and Amyotrophic Lateral Sclerosis

“…92,93 UBQLN2 also acts as a molecular chaperone, maintaining the conformation of nuclear geneencoded mitochondrial transmembrane proteins. 94,95 UBQLN2 participates in the cellular stress response. When cells were exposed to oxidative (SA) or heat stress, endogenous UBQLN2 partly colocalized with SGs.…”

“…As a shuttle factor of the ubiquitin-proteasome system, UBQLN2 noncovalently interacts with the polyubiquitin chain of ubiquitinated substrates with its UBA domain and the 26S proteasome with its UBL domain and plays an important role in the protein quality control system. UBQLN2 mutations can disrupt the cellular protein quality control system, especially the protein degeneration system, so that the cell will undergo “protein stress”. , UBQLN2 also acts as a molecular chaperone, maintaining the conformation of nuclear gene-encoded mitochondrial transmembrane proteins. , …”

UBQLN proteins in health and disease with a focus on UBQLN2 in ALS/FTD

“…Besides their roles in facilitating protein degradation, there is growing evidence that UBQLN proteins enhance biosynthesis of membrane proteins, possibly by shielding hydrophobic segments in transmembrane proteins prior to membrane insertion [9,13,[32][33][34][35]. This function has been attributed to its STI1 domains that are known to bind heat shock-related proteins, but the exact mechanism by which they do so remains to be clarified [36][37][38].…”