A new Drosophila model of Ubiquilin knockdown shows the effect of impaired proteostasis on locomotive and learning abilities

Ubiquilin (UBQLN) plays a crucial role in cellular proteostasis through its involvement in the ubiquitin proteasome system and autophagy. Mutations in the UBQLN2 gene have been implicated in amyotrophic lateral sclerosis (ALS) and ALS with frontotemporal lobar dementia (ALS/FTLD). Previous studies reported a key role for UBQLN in Alzheimer's disease (AD); however, the mechanistic involvement of UBQLN in other neurodegenerative diseases remains unclear. The genome of Drosophila contains a single UBQLN homolog (dUbqn) that shows high similarity to UBQLN1 and UBQLN2; therefore, the fly is a useful model for characterizing the role of UBQLN in vivo in neurological disorders affecting locomotion and learning abilities. We herein performed a phenotypic and molecular characterization of diverse dUbqn RNAi lines. We found that the depletion of dUbqn induced the accumulation of polyubiquitinated proteins and caused morphological defects in various tissues. Our results showed that structural defects in larval neuromuscular junctions, abdominal neuromeres, and mushroom bodies correlated with limited abilities in locomotion, learning, and memory. These results contribute to our understanding of the impact of impaired proteostasis in neurodegenerative diseases and provide a useful Drosophila model for the development of promising therapies for ALS and FTLD.     

Dystrobrevin is required postsynaptically for homeostatic potentiation at the Drosophila NMJ

Evolutionarily conserved homeostatic systems have been shown to modulate synaptic efficiency at the neuromuscular junctions of organisms. While advances have been made in identifying molecules that function presynaptically during homeostasis, limited information is currently available on how postsynaptic alterations affect presynaptic function. We previously identified a role for postsynaptic Dystrophin in the maintenance of evoked neurotransmitter release. We herein demonstrated that Dystrobrevin, a member of the Dystrophin Glycoprotein Complex, was delocalized from the postsynaptic region in the absence of Dystrophin. A newly-generated Dystrobrevin mutant showed elevated evoked neurotransmitter release, increased bouton numbers, and a readily releasable pool of synaptic vesicles without changes in the function or numbers of postsynaptic glutamate receptors. In addition, we provide evidence to show that the highly conserved Cdc42 Rho GTPase plays a key role in the postsynaptic Dystrophin/Dystrobrevin pathway for synaptic homeostasis. The present results give novel insights into the synaptic deficits underlying Duchenne Muscular Dystrophy affected by a dysfunctional Dystrophin Glycoprotein complex.     

Depletion of Ubiquilin induces an augmentation in soluble ubiquitinated Drosophila TDP-43 to drive neurotoxicity in the fly

The proteostasis machinery has critical functions in metabolically active cells such as neurons. Ubiquilins (UBQLNs) may decide the fate of proteins, with its ability to bind and deliver ubiquitinated misfolded or no longer functionally required proteins to the ubiquitin-proteasome system (UPS) and/or autophagy. Missense mutations in UBQLN2 have been linked to X-linked dominant amyotrophic lateral sclerosis with frontotemporal dementia (ALS-FTD). Although aggregation-prone TAR DNA-binding protein 43 (TDP-43) has been recognized as a major component of the ubiquitin pathology, the mechanisms by which UBQLN involves in TDP-43 proteinopathy have not yet been elucidated in detail. We previously characterized a new Drosophila Ubiquilin (dUbqn) knockdown model that produces learning/memory and locomotive deficits during the proteostasis impairment. In the present study, we demonstrated that the depletion of dUbqn markedly affected the expression and sub-cellular localization of Drosophila TDP-43 (TBPH), resulting in a cytoplasmic ubiquitin-positive (Ub⁺) TBPH pathology. Although we found that the knockdown of dUbqn widely altered and affected the turnover of a large number of proteins, we herein showed that an augmented soluble cytoplasmic Ub⁺-TBPH is as a crucial source of neurotoxicity following the depletion of dUbqn. We demonstrated that dUbqn knockdown-related neurotoxicity may be rescued by either restoring the proteostasis machinery or reducing the expression of TBPH. These novel results extend our knowledge on the UBQLN loss-of-function pathomechanism and may contribute to the identification of new therapeutics for ALS-FTD and aging-related diseases.