Reduced body weight gain in ubiquilin-1 transgenic mice is associated with increased expression of energy-sensing proteins

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

Ubiquilin (UBQLN) proteins are a dynamic and versatile family of proteins found in all eukaryotes that function in the regulation of proteostasis. Besides their canonical function as shuttle factors in delivering misfolded proteins to the proteasome and autophagy systems for degradation, there is emerging evidence that UBQLN proteins play broader roles in proteostasis. New information suggests the proteins function as chaperones in protein folding, protecting proteins prior to membrane insertion, and as guardians for mitochondrial protein import. In this review, we describe the evidence for these different roles, highlighting how different domains of the proteins impart these functions. We also describe how changes in the structure and phase separation properties of UBQLNs may regulate their activity and function. Finally, we discuss the pathogenic mechanisms by which mutations in UBQLN2 cause amyotrophic lateral sclerosis and frontotemporal dementia. We describe the animal model systems made for different UBQLN2 mutations and how lessons learnt from these systems provide fundamental insight into the molecular mechanisms by which UBQLN2 mutations drive disease pathogenesis through disturbances in proteostasis.

Modeling Sporadic Alzheimer's Disease by Efficient Direct Reprogramming of the Elderly Derived Disease Dermal Fibroblasts into Neural Stem Cells

Alzheimer's disease (AD) is a devastating neurodegenerative disorder, neuropathologically characterized by hyperphosphorylation of tau and formation of amyloid plaques. Most AD cases are sporadic with no clear cause. Cell models play an important role in understanding the pathogenesis of sporadic AD, and the cell reprogramming and epigenetic techniques have provided new avenue to model the disorder. However, since most sporadic AD patients are late-onset, it poses a challenge to reprogram elderly somatic cells into stem cells. Here, we report that combination of overexpressing a single transcription factor, hSOX, with nine small molecules, was able to directly reprogram elderly (55-75 years of age) sporadic AD and the age-matched healthy individual dermal fibroblasts into the induced neural stem cells (iNSCs). These cells possessed the typical neural stem cell properties and were able to be further differentiated into neurons and glia in vitro and in vivo. More importantly, AD iNSC-derived neurons showed hyperphosphorylation at several sites of tau and increased release of Aβ into culture medium, indicating the replication of the major neuropathological hallmarks. Thus, we described a new technique to directly convert elderly AD dermal fibroblasts into iNSCs that may serve as a useful tool for studying the pathogenesis of sporadic AD and for drug discovery to treat the disorder.

Overexpression of UBQLN1 reduces neuropathology in the P497S UBQLN2 mouse model of ALS/FTD

Missense mutations in UBQLN2 cause X-linked dominant inheritance of amyotrophic lateral sclerosis with frontotemporal dementia (ALS/FTD). UBQLN2 belongs to a family of four highly homologous proteins expressed in humans that play diverse roles in maintaining proteostasis, but whether one isoform can substitute for another is not known. Here, we tested whether overexpression of UBQLN1 can alleviate disease in the P497S UBQLN2 mouse model of ALS/FTD by crossing transgenic (Tg) mouse lines expressing the two proteins and characterizing the resulting genotypes using a battery of pathologic and behavioral tests. The pathologic findings revealed UBQLN1 overexpression dramatically reduced the burden of UBQLN2 inclusions, neuronal loss and disturbances in proteostasis in double Tg mice compared to single P497S Tg mice. The beneficial effects of UBQLN1 overexpression were primarily confirmed by behavioral improvements seen in rotarod performance and grip strength in male, but not female mice. Paradoxically, although UBQLN1 overexpression reduced pathologic signatures of disease in P497S Tg mice, female mice had larger percentage of body weight loss than males, and this correlated with a corresponding lack of behavioral improvements in the females. These findings lead us to speculate that methods to upregulate UBQLN1 expression may reduce pathogenicity caused by UBQLN2 mutations, but may also lead to gender-specific outcomes that will have to be carefully weighed with the therapeutic benefits of UBQLN1 upregulation.

Attenuation of Ischemic Stroke-Caused Brain Injury by a Monoamine Oxidase Inhibitor Involves Improved Proteostasis and Reduced Neuroinflammation

Mitochondrial dysfunction and oxidative stress play a key role in ischemia/reperfusion (I/R) induced brain injury. We previously showed that ubiquilin-1 (Ubqln1), a ubiquitin-like protein, improves proteostasis and protects brains against oxidative stress and I/R induced brain injury. We demonstrate here that nialamide (NM), a non-selective monoamine oxidase (MAO) inhibitor, upregulated Ublqn1 and protected neurons from oxygen-glucose deprivation- and I/R-caused cell death in in vitro and in vivo, respectively. Post-ischemic administration of the NM in a stroke mouse model even at 3 h following I/R still reduced neuronal injury and improved functional recovery and survival. Treating stroke animals with NM also increased the association of Ubqln1 with mitochondria and decreased the total oxidized and polyubiquitinated protein levels. Intriguingly, NM-enhanced proteostasis was also associated with reduced I/R-caused neuroinflammation, as reflected by attenuated activation of microglia and astrocytes as well as reduced TNF-α level. Thus, our results suggest that MAO inhibition-induced neuroprotection following I/R involves improved proteostasis and reduced neuroinflammation.

Reduced body weight gain in ubiquilin-1 transgenic mice is associated with increased expression of energy-sensing proteins

Ubiquilin-1 (Ubqln1), a ubiquitin-like protein, is implicated in a variety of pathophysiological processes, but its role in mediating body weight gain or metabolism has not been determined. Here, we demonstrate that global overexpression of Ubqln1 in a transgenic (Tg) mouse reduces the animal's body weight gain. The decreased body weight gain in Tg mice is associated with lower visceral fat content and higher metabolic rate. The Ubqln1 Tg mice exhibited reduced leptin and insulin levels as well as increased insulin sensitivity manifested by homeostatic model assessment of insulin resistance. Additionally, the reduced body weight in Tg mice was associated with the upregulation of two energy-sensing proteins, sirtuin1 (SIRT1) in the hypothalamus and AMP-activated protein kinase (AMPK) in the skeletal muscle. Consistent with the in vivo results, overexpression of Ubqln1 significantly increased SIRT1 and AMPK levels in the mouse embryonic fibroblast cell culture. Thus, our results not only establish the link between Ubqln1 and body weight regulation but also indicate that the metabolic function of Ubqln1 on body weight may be through regulating energy-sensing proteins.