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Ushering in the Cardiac Role of Ubiquilin1 The Journal of Clinical Investigation COMMENTARY Ushering in the cardiac role of Ubiquilin1 Xi Fang, Christa Trexler, and Ju Chen Department of Medicine, UCSD, La Jolla, California, USA. teins in the 19S proteasome, while the UBA domain binds polyubiquitinated proteins. Protein quality control (PQC) mechanisms are essential for maintaining In a proposed “shuttle-factor” model, cardiac function, and alterations in this pathway influence multiple forms of UBL/UBA proteins bind to ubiquitinated heart disease. Since heart disease is the leading cause of death worldwide, proteins via the UBA domain and subse- understanding how the delicate balance between protein synthesis and quently interact with the proteasome via degradation is regulated in the heart demands attention. The study by Hu the UBL domain to ensure that substrates et al. reveals that the extraproteasomal ubiquitin receptor Ubiquilin1 arrive at the proteasome without protein (Ubqln1) plays an important role in cardiac ubiquitination-proteasome aggregation (5, 6). coupling, particularly in response to myocardial ischemia/reperfusion injury, Ubiquilin (Ubqln)/protein linking inte- thereby suggesting that this may be a new avenue for therapeutics. grin-associated protein to cytoskeleton (PLIC) proteins, which are mammalian orthologs of yeast Dsk2, belong to the UBL/UBA family (7, 8). The Ubiquilin fam- The recognition of ubiquitinated UPS-mediated protein degradation ily consists of five paralogous, structur- proteins by the proteasome involves two major steps: ubiquitination, ally conserved members that are named Cardiomyocytes are long-lived, terminal- whereby a polyubiquitin chain is attached UBQLN1 through 4, and UBQLNL (Ubiqui- ly differentiated cells that do not prolif- to the target protein via isopeptide bonds, lin-like). Each family member harbors an erate; thus, they are extremely sensitive and degradation, whereby ubiquitinated N-terminal UBL domain and a C-terminal to increasing concentrations of misfold- proteins are degraded by the 26S protea- UBA domain as well as a stress-inducible ed or malfunctioning proteins and rely some, a multicatalytic protease consisting heat shock chaperone-binding motif (STI) on an efficient protein quality control of a 19S regulatory particle (RP) and a 20S domain(s) in the middle (7). (PQC) system to maintain normal car- core particle (CP) that degrades proteins diac structure and function (1). The first into small oligopeptides (4). The collabo- The role of Ubiquilin1 in the heart line of defense of the PQC system is the rative action of these two steps is crucial Thus far, most studies investigating chaperone-cochaperone system, ensur- for effective PQC. Initial recognition of the function of Ubiquilin proteins have ing that proteins are appropriately folded. substrates by the proteasome is mediat- focused on the nervous system, as mul- However, some proteins slip through this ed by a ubiquitin “tag” on the substrate tiple reports have linked Ubiquilins to checkpoint, leading to accumulation of (4). Two classes of ubiquitin receptors, several neurodegenerative diseases (5). misfolded proteins (2, 3). Furthermore, intraproteasomal and extraproteasomal, In vivo animal models further demon- damaged, oxidized, mutant, and normal recruit “tagged” ubiquitinated proteins to strate important functions of Ubiquilin proteins that are no longer needed must the proteasome (refs. 5, 6, and Figure 1). proteins in the brain (9). For example, also be removed in a timely fashion to Intraproteasomal receptors include two Ubqln1-overexpressing transgenic mice maintain heart function (1). Thus, the sec- 19S proteasomal subunits, Rpn10/S5a and show increased tolerance to ischemia/ ond line of defense of the PQC is the ubiq- Rpn13/ADRM1, which bind to polyubiqui- reperfusion (I/R) stress, while Ubqln1 uitin-proteasome system (UPS), which tin via ubiquitin interacting motifs (UIM) deletion in neurons exacerbates neuronal rapidly and effectively recognizes specific (5, 6). Extraproteasomal receptors include damage after stroke (9). A potential role substrates to remove misfolded, oxidized, ubiquitin-like (UBL)/ubiquitin- associated of Ubqln1 in diseased heart, however, mutant, damaged, or superfluous proteins (UBA) proteins that deliver ubiquitinat- remains to be addressed. In the current (1, 4). Defects in the UPS occur in multiple ed proteins to the 26S proteasome. The issue of the JCI, Hu and colleagues tack- cardiac disease settings, underlining the UBL/UBA family of proteins contains an le this important gap in scientific under- importance of this system in the heart and N-terminal UBL domain, one or more standing through a variety of novel in vivo suggesting that optimizing or augmenting C-terminal UBA domain or domains, and and in vitro methods (10) (Figure 1). The this pathway may have therapeutic poten- a variable central region. The UBL domain authors showed that Ubqln1, which binds tial for cardiac patients (1). binds to the UIM motif of Rpn10/S5a pro- a broad range of ubiquitinated substrates, colocalizes with the proteasome in cardio- myocytes and significantly increases in the Related Article: p. 5294 soluble fraction following I/R stress. Con- sistent with previous reports in HEK293 Authorship note: XF and CT contributed equally to this work. Conflict of interest: The authors have declared that no conflict of interest exists. cells (11), Ubqln1 was recruited to the ER- Reference information: J Clin Invest. 2018;128(12):5195–5197. https://doi.org/10.1172/JCI124567. associated degradation (ERAD) machin- jci.org Volume 128 Number 12 December 2018 5195 COMMENTARY The Journal of Clinical Investigation Figure 1. Pathways to transfer ubiquitinated proteins to the proteasome. (A) A ubiquitin tag is added to proteins that need to be degraded, including unfolded, misfolded, nonnative, or oxidized proteins. Two main pathways recruit ubiquitinated proteins to the proteasome: the first relies on intraproteasomal polyubiquitin receptors Rpn10/Rpn13 in the 19S RP, which directly binds polyubiquitin; the second depends upon extraproteasomal ubiquitin receptors, such as Ubqln1 (yellow). Ubqln1 functions as a “shuttle-factor” by binding to ubiquitinated proteins via its UBA domain and subsequently interacts with Rpn10 in the proteasome via its UBL domain. (B) During I/R injury, unfolded, misfolded, and oxidized proteins are increased, putting greater demand on the UPS. Hu et al. reveal that Ubqln1 expression increases with I/R and that loss of Ubqln1 exacerbates, while ubiquitous overexpression of an Ubqln1 transgene ameliorates, myocardial I/R injury. In cultured cardiomyocytes, Ubqln1 overexpression facilitated proteasome-mediated degradation of oxidized proteins. ery in response to ER stress in cardiomyo- increased the degradation of the UPS surro- It is clear that both K48- and K63-linked cytes. Cardiomyocyte-specific deletion of gate substrate GFPdgn (9) as well as oxidized ubiquitinated proteins are increased in the Ubqln1 in mice (referred to as CKO mice) proteins (Figure 1) without enhancing auto- Ubqln1-CKO heart. Although K48-linked resulted in late-onset cardiomyopathy as phagic flux, suggesting that the protective polyubiquitin chains are mostly involved in well as increased accumulation of pro- role of Ubqln1 in response to myocardial I/R proteasomal degradation, K63-linked ubiq- teins with lys residues K63- and K48- injury may be through enhanced clearing of uitination generally serves as a docking site linked polyubiquitin chains (Figure 1). The both ubiquitinated and oxidized proteins. for mediating protein-protein interactions authors also utilized a transgenic mouse and has diverse proteasome-independent line expressing GFPdgn, a well-charac- Implications, challenges, roles (4). Thus, whether accumulated terized surrogate substrate of the UPS (9), and future directions K63-linked polyubiquitin proteins have and observed an accumulation of GFPdgn This study reveals a novel cardiac role of functional proteasome- independent con- in the myocardium of Ubqln1-CKO mice. Ubqln1 and its therapeutic potential in the sequences that may contribute to the However, activity of proteasomal pepti- setting of I/R injury of the heart. Findings observed CKO phenotypes remains to dases was not diminished, demonstrating in this study raise a number of important be addressed. In addition to transferring that coupling between ubiquitination and questions for future areas of investigation ubiquitinated substrates to the 26S prote- proteasomal degradation was altered in in the field. Characterization of the pref- asome, findings in other cell types suggest Ubqln-CKO mice. Together, these obser- erence or specificity of substrates targeted that Ubqln1 is also present within the inner vations demonstrate that Ubqln1 plays an by Ubqln1 would help us to fully under- core of aggresomes and functions to deliv- essential role in maintaining cardiomyo- stand the role of Ubqln1 in cardiomyo- er ubiquitinated proteins to aggregates cyte proteostasis. cytes. The authors showed that Ubqln1 (12–14) as well as to regulate aggresome Importantly, Hu et al. also demonstrat- facilitates proteasomal degradation of formation via its UBL domain (13). Aggre- ed that Ubqln1 deletion in cardiomyocytes oxidized proteins and interacts with the some formation was not examined by Hu et exacerbates myocardial I/R injury. Com- ERAD machinery in cultured cardiomyo- al.; thus the potential role of Ubqln1 protein pared with control mice, Ubqln1-CKO mice cytes. It would be interesting
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