Role of Ubiquilin-2 in Proteostasis and Tau Aggregation in Tauopathies
Introduction
Tauopathies are a group of neurologic disorders that include frontotemporal dementia (FTD) and Alzheimer's disease (AD), the most common forms of dementia (Wang and Mandelkow, 2016). A hallmark of tauopathies is the accumulation of toxic aggregates composed of hyperphosphorylated tau protein throughout the CNS. These aggregates are toxic and lead to neuronal death (Wang and Mandelkow, 2016). Indeed, the density of tau aggregates, also known as neurofibrillary tangles, directly correlate with the clinical progression of cognitive decline and signs of dementia in AD (Nelson et al., 2012).
Tau, a microtubule-associated protein encoded by the MAPT gene, promotes microtubule assembly and regulates microtubule stability in neurons (Weingarten et al., 1975). In pathologic states such as tauopathies, an imbalance in the activity of kinases and phosphatases can lead to tau being hyperphosphorylated at multiple sites. This makes tau more likely to dissociate from microtubules and become susceptible to aggregation (Hanger et al., 2009). An additional contributor to the formation of tau pathology is the disruption of protein regulation and homeostasis by the ubiquitin–proteasome system and autophagy–lysosome system (Lee et al., 2013; Weng and He, 2021). For example, the tau mutation P301S can inhibit autophagy-mediated tau degradation, which accelerates tau aggregation (Caballero et al., 2018).
Ubiquilin-2 (UBQLN2), a ubiquitin-like protein, contributes to both the ubiquitin–proteasome system and the autophagy–lysosome system. Specifically, UBQLN2 helps shuttle ubiquitin-tagged, misfolded, or nonfunctional proteins to the proteasome for degradation (Rothenberg et al., 2010). In addition, UBQLN2 can interact with LAMP2A to promote chaperone-mediated autophagy and the formation of autophagosomes that target larger aggregates for degradation (Hjerpe et al., 2016). Specific missense mutations in UBQLN2 have been shown to cause familial forms of amyotrophic lateral sclerosis and FTD (Deng et al., 2011). Although UBQLN2 levels are decreased in various tauopathies (Nölle et al., 2013), the effects of UBQLN2 on the progression of tau pathology in tauopathies are unclear and warrant further investigation.
A recent study in the Journal of Neuroscience investigated the interaction between UBQLN2 and tau protein using cell lines, transgenic mice, and human tissue samples (Gerson et al., 2022). In HEK293 cells, coexpression of wild-type tau and UBQLN2 led to increased tau degradation and decreased tau levels. As expected, siRNA-mediated knockdown of UBQLN2 increased overall tau levels. This interactive effect was specific to UBQLN2 since other ubiquilin proteins expressed in the brain, including UBQLN1 and UBQLN4, did not significantly alter tau levels. This suggests that UBQLN2 is the major ubiquilin involved in the regulation of tau protein levels in the brain.
The effects of UBQLN2 on tau levels were further confirmed in mouse models of tauopathy. In the brains of the htau mouse model that expressed the wild-type human tau protein (Andorfer et al., 2003), higher-molecular-weight UBQLN2 gradually accumulated with age. Similarly, in PS19 mice that express the missense tau mutation P301S, brain lysates contained more soluble UBQLN2 species compared with controls. The authors also confirmed that UBQLN2 was present in insoluble fractions of brain samples from humans with progressive supranuclear palsy, a subtype of FTD and another tauopathy. These results show that UBQLN2 accumulates with age and that the development of insoluble UBQLN2 aggregates impairs its regulation of protein degradation that further contribute to the progression of tau pathology.
To better understand the effects of UBQLN2 and its interaction with tau in vivo, Gerson et al. (2022) generated several transgenic mouse lines. The PS19 transgenic mouse line that expresses P301S tau mutation was crossed with different UBQLN2 lines to generate P301S x UBQLN2-Hi mice, P301S x UBQLN2-Low, and P301S x UBQLN2 KO mice that have high (∼14×), low (∼1×), or no UBQLN2 expression, respectively. The P301S x UBQLN2-Hi mice showed increased tau phosphorylation within hippocampal areas and the prefrontal cortex at multiple sites including pSer202, Thr205, pThr231, and pS396, as detected by various phosphorylation-specific antibodies.& Many of these sites are classically hyperphosphorylated in tauopathies including Alzheimer's disease (Hanger et al., 2009). Coimmunofluorescence with tau and UBQLN-2 antibodies revealed that the increased tau phosphorylation was primarily found in close proximity of neurons expressing high levels of UBQLN2. The increase in tau phosphorylation is related to UBQLN2 expression, since both UQLN2-Hi and UBQLN2-Low mice have the similar effect of enhancing tau phosphorylation. However, UBQLN2-Low mice express much lower levels of UBQLN2 and are composed of soluble UBQLN2 without aggregates (Sharkey et al., 2018). This suggests that UBQLN2 aggregates are not necessary to promote increased phosphorylation of tau at multiple sites.
In addition to phosphorylation, UBQLN2 exacerbated tau aggregation and toxicity. in P301S x UBQLN2-Hi mice, which had more Thioflavin-positive tau aggregates compared with P301S mice of the same age. The PS19 tau transgenic mice that express the P301S tau mutation have a median survival of ∼9 months because of motor paralysis from tau expression in the spinal cord (Yoshiyama et al., 2007). The expression of both high and low UBQLN2 levels in the P301S x UBLQN2-Hi and P301S x UBQLN2-Low mice led to earlier paralysis and death. This suggests that UBQLN2 contributes to enhancing tau toxicity in the PS19 model by increasing both phosphorylation and tau aggregation.
Previous studies have shown that phosphorylated tau is primarily degraded by autophagic pathways rather than proteasome pathways (Rodríguez-Martín et al., 2013). Thus, Gerson et al. (2022) also assessed expression levels of proteins involved in autophagic degradation pathways. UBQLN2 overexpression in P301S mice increased LAMP2A levels and decreased LC3A levels, both of which are involved in chaperone-mediated autophagy and macroautophagy pathways. This was expected because UBQLN2 has been shown to directly bind to LAMP2A (Hjerpe et al., 2016), and LAMP2A acts as a receptor for initiation of chaperone-mediated autophagy. Similarly, LC3A is important for forming autophagosome complexes involved in protein processing. UBQLN2 expression also increased overall ubiquitin levels, but it did not significantly alter heat shock proteins Hsp70 and Hsp90B, both of which are chaperone proteins involved in protein folding and quality control. This suggests that UBQLN2 may regulate tau through other autophagic pathways that do not depend on heat shock proteins. UBQLN2 is likely involved in the degradation of different tau species such as soluble tau by activating autophagy pathways; however, this process is not effective at the removal of tau aggregates and may indirectly lead to increased tau phosphorylation and accumulation. This is evident in UBQLN2 overexpression that may have led to UBQLN2 accumulation and functional impairment, similar to UBQLN2 knockdown.
Overall, increased UBQLN2 expression enhanced tau toxicity by increasing phosphorylation and tau aggregation, and altering degradation pathways. The UBQLN2-dependent increase in tau phosphorylation may play a major role in tau pathogenesis. Tau phosphorylation is a major driver of tau pathology, and different phosphorylated tau species such as pThr181, pThr217, and pThr231 have been identified as early CSF and blood biomarkers in AD and other tauopathies (Suárez-Calvet et al., 2020). Although UBQLN2 did not significantly alter GSK-3β (glycogen synthase kinase-3β) and MARK2 (microtubule affinity regulating kinase 2), kinases that phosphorylate tau (Gerson et al., 2022), there are many other tau-targeting kinases such as cyclin-dependent kinase 5 and tau-tubulin kinase that might be affected by UBQLN2 (Hanger et al., 2009). It is also possible that UBQLN2 decreases the activity of tau-specific protein phosphatases, which then contribute to increased net phosphorylation levels without significantly affecting kinases (Liu et al., 2005). Overall, increased tau phosphorylation likely contributes to tau aggregation and decreased survival found in PS19 x UBQLN2-Hi mice.
UBQLN2 may further enhance tau toxicity by causing dysregulation of protein degradation pathways such as autophagy. Gerson et al. (2022) showed that UBQLN can alter expression levels of certain proteins such as LAMP2A and LC3A, and it is possible that UBQLN2 regulation simultaneously affects other degradation pathways. For example, UBQLN2 has been shown to interact with ubiquitin regulatory X domain-containing protein 8 and is associated with endoplasmic reticulum-associated protein degradation (Xia et al., 2014), another degradation pathway that might be involved in tauopathies. Follow-up studies could elucidate other proteins or pathways that might be used as potential therapeutic targets. Currently, several lead compounds and drugs, including lithium, cilostazol, and methylene blue, have been identified as potential autophagy inducers that promote tau degradation and have ongoing clinical trials involving human patients (Jiang and Bhaskar, 2020).
In summary, Gerson et al. (2022) have established the role of UBQLN2 in the regulation of tau protein degradation. In transgenic mouse models of tauopathy, UBQLN2 significantly exacerbated tau hyperphosphorylation and promoted the formation of toxic tau aggregates because of the dysregulation of protein homeostasis. Since multiple pathways are involved in protein degradation and can have complex interactions, further studies should identify specific proteins that can be used as therapeutic targets to promote removal of tau aggregates in different tauopathies.
Footnotes
- Received May 1, 2022.
- Revision received June 14, 2022.
- Accepted June 17, 2022.
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This work is supported by funding from the National Institute on Aging F30 NRSA Fellowship (Grant F30-AG-067673). I thank my mom for support.
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The authors declare no competing financial interests.
- Correspondence should be addressed to Yuxing Xia at yuxingxia{at}ufl.edu
- Copyright © 2022 the authors
