The Emerging Role of UCHL1 in Neurological and Musculoskeletal Diseases

2.1. Canonical Deubiquitination Function

The canonical deubiquitinating activity of UCHL1 regulates diverse cellular processes by directly interacting with and stabilizing target proteins. For instance, the endogenous association of UCHL1 with β‐catenin prevents its degradation, thereby enhancing β‐catenin/TCF‐dependent transcription [4]. In models of cardiac hypertrophy and cardiovascular remodeling, UCHL1 deubiquitinates and stabilizes the epidermal growth factor receptor (EGFR), and this phenotype can be reversed via UCHL1 knockdown or pharmacological inhibition with LDN‐57444 [13]. In oncogenic contexts, UCHL1 interacts with and stabilizes the nuclear pore component POM121, preserving neuroendocrine identity and facilitating the POM121‐dependent nuclear translocation of E2F1 and c‐MYC to accelerate cancer progression [14]. In clear cell renal cell carcinoma, wild‐type UCHL1, but not its catalytically inactive C90A mutant, deubiquitinates and stabilizes lysine demethylase 4B (KDM4B), which upregulates VEGF signaling and confers therapeutic resistance to bevacizumab [15]. UCHL1 also plays a complex role in tumor suppression pathways by deubiquitinating p53 and p14(ARF) resulting in p53 stabilization and the activation of downstream p53 signaling pathway in nasopharyngeal carcinoma pathogenesis [16]. Conversely, in other malignancies, UCHL1 enhances metastatic potential by preventing the von Hippel–Lindau‐driven ubiquitination and degradation of hypoxia‐inducible factor‐1α (HIF‐α) thereby stabilizing this crucial regulatory subunit [17].

This regulatory control over HIF‐1α is also vital in nonmalignant tissues. Within the cartilage microenvironment, UCHL1‐mediated stabilization of HIF‐1α activates the HIF‐1α‐SOX9 axis and downregulates MMP13, which enhances the chondrogenic differentiation and matrix synthesis of mesenchymal stem cells [18]. Concurrently, UCHL1 mitigates chondrocyte apoptosis by promoting HIF‐1α‐dependent mitophagy to maintain mitochondrial homeostasis, underscoring its protective role in osteoarthritis (OA) [19]. Recent investigations have also uncovered a sophisticated negative feedback loop composed of the RANKL‐MAPK‐UCHL1‐sCD13 axis; here, UCHL1 stabilizes CD13 and promotes the shedding of soluble CD13, which suppresses MAPK signaling and osteoclastogenesis to delay OA progression [20]. UCHL1 also acts as a critical checkpoint in bone homeostasis and cellular differentiation. Although UCHL1 is upregulated during RANKL‐induced osteoclastogenesis, it deubiquitinates and stabilizes TAZ, which suppresses NFATc1 and its downstream effector genes to ultimately inhibit osteoclast differentiation [21].

In reproductive biology, UCHL1 deubiquitinates and stabilizes voltage‐dependent anion channel 2 (VDAC2), thereby enhancing the synthesis of pregnenolone—the precursor of estradiol [22]. Finally, UCHL1 maintains the stability of AKT1, as its inhibition or ablation accelerates the K48‐linked ubiquitination and proteasomal degradation of AKT1, thereby suppressing endothelial proliferation and pulmonary vascular remodeling [23]. Similarly, UCHL1 stabilizes 6‐phosphofructo‐2‐kinase/fructose‐2,6‐bisphosphatase 3 (PFKFB3) via deubiquitination, which stimulates glycolysis in astrocytes, sustains metabolic reprogramming, and inhibits neuronal ferroptosis to exert robust neuroprotective effects [24].

2.2. Noncanonical Regulatory Functions

Not all reported functions of UCHL1 can be categorized as classical, substrate‐specific deubiquitination. Crucially, a given UCHL1 function should be classified as truly DUB‐independent only when it is preserved following complete catalytic inactivation validated by catalytic‐dead mutants, activity‐deficient rescue experiments, or physical binding assays that demonstrate a scaffold‐like activity independent of ubiquitin hydrolysis. While numerous published studies demonstrate that UCHL1 interacts with macromolecular signaling complexes, alters protein subcellular localization, or modulates downstream pathway activation, the absolute dependence of these phenotypes on its enzymatic activity has not always been directly evaluated. Consequently, until strict DUB independence is experimentally substantiated, these phenomena are more accurately described as noncanonical activities that operate beyond classical deubiquitination (Figure 1).

Notably, UCHL1 interacts with the NACHT domain of the NLRP3 inflammasome, regulating its macromolecular assembly and subsequent IL‐1β production in human macrophages and microglia, and this noncanonical interaction provides a unified molecular basis connecting joint synovitis and neuroinflammation [12]. Beyond scaffolding functions, it has been proposed that UCHL1 can exhibit a distinct ligase‐like activity upon homodimerization, which promotes the accumulation of α‐synuclein through a mechanism distinct from its baseline hydrolytic activity [25]. However, this putative ligase function remains mechanistically debated, and its operational relationship to the catalytic cysteine residue and classical hydrolase activity requires careful interpretation. Furthermore, posttranslational modifications heavily dictate these noncanonical roles; for example, C‐terminal farnesylation actively promotes the membrane association of UCHL1 and alters its intracellular interaction network, thereby accelerating α‐synuclein accumulation and subsequent neurotoxicity [26].

3.1. Transcriptional Regulation

Several upstream transcription factors and epigenetic modifiers tightly control UCHL1 expression across distinct tissue types (Figure 2). For instance, RE1‐silencing transcription factor (REST) plays a critical regulatory role in cardiac pathology. Genetic deletion of REST exacerbates myocardial fibrosis and inflammation following pressure overload and this phenotype driven in part by the secondary upregulation of UCHL1, which actively promotes pathological hypertrophy [27]. Significantly, pharmacological inhibition of UCHL1 via LDN‐57444 in REST conditional knockout mice successfully mitigates hypertrophic fibrosis and inflammatory signaling by dampening the NF‐κB and STAT3 pathways [27].

In parallel, the NF‐κB pathway itself serves as a prominent upstream regulator of UCHL1 transcription, though its effects appear to be highly context‐dependent. In models of renal podocyte injury, immune complex‐mediated inflammation activates NF‐κB, which directly drives UCHL1 gene expression [28]. Similarly, investigations into cardiac remodeling identify NF‐κB as a direct transcriptional activator of UCHL1 [29]. Conversely, early foundational studies indicated that under certain experimental conditions, NF‐κB activation can suppress UCH‐L1 promoter transcription, underscoring a complex, context‐specific regulatory mechanism [30].

Beyond these pathways, specificity protein 1 (Sp1) has also been implicated in UCHL1 modulation. In an H₂O₂‐induced model of age‐related hearing loss, the Sp1‐mediated suppression of UCHL1 transcription markedly attenuates cellular senescence and oxidative stress within cochlear hair cells [31]. Finally, epigenetic mechanisms significantly influence UCHL1 expression in malignant contexts. In triple‐negative breast cancer (TNBC), ten‐eleven translocation 1 (TET1) and TET3 actively enhance UCHL1 transcription by demethylating its promoter; this locus activation subsequently sustains Krüppel‐like factor 5 (KLF5) protein levels via UCHL1‐mediated deubiquitination to drive cancer progression [14].

3.2. Epigenetic Regulation

DNA methylation represents a primary epigenetic mechanism governing UCHL1 expression, particularly within malignant contexts. Aberrant epigenetic silencing of UCHL1 via promoter CpG island hypermethylation serves as a strong prognostic indicator, predicting advanced disease development and poor clinical outcomes in renal cell carcinoma [32]. Similarly, in prostate cancer (PCa), UCHL1 silencing has been definitively linked to promoter hypermethylation, which correlates with a profound reduction in UCHL1 transcript and protein levels in malignant tissues compared to adjacent, benign prostate epithelium [33]. Furthermore, the loss of UCHL1 expression driven by CpG promoter hypermethylation is significantly associated with metastatic progression in gastroenteropancreatic neuroendocrine tumors (GEP‐NETs) [34]. This epigenetic silencing pattern extends to nasopharyngeal carcinoma as well, where promoter hypermethylation has been identified as the predominant molecular mechanism responsible for downregulating UCHL1 expression and disrupting its downstream tumor‐suppressive pathways [35].

3.3. Noncoding RNA‐Mediated Regulation

Posttranscriptional networks driven by noncoding RNAs add an additional layer of precision to UCHL1 expression control. For instance, miR‐922 directly inhibits the translation of UCHL1 mRNA by binding to its 3′ prime untranslated region, thereby significantly reducing endogenous UCHL1 protein expression levels. This microRNA‐mediated depletion of UCHL1 subsequently disrupts the delicate homeostatic balance of tau protein phosphorylation, ultimately accelerating the tau pathology characteristic of Alzheimer's disease [36]. Conversely, long noncoding RNAs (LncRNAs) can actively enhance UCHL1 expression. In dopaminergic neurons, long noncoding antisense RNA (AS‐Uchl1) acts as a translational upregulator by pairing with Uchl1 mRNA to promote its translation, thereby increasing UCHL1 protein abundance and supporting neuronal homeostasis [37].

3.4. Posttranslational Modifications

Beyond transcriptional and epigenetic control, UCHL1 function is dynamically modulated at the protein level by an array of posttranslational modifications that alter its enzymatic activity, stability, and cellular localization. For example, S‐nitrosylation of UCHL1 directly disrupts its core hydrolase activity. This modification impairs UPS function, linking nitric oxide‐mediated oxidative stress to the dysregulation of neuronal proteostasis [38]. Similarly, high‐throughput proteomic analyses of Alzheimer's disease brain tissue have identified UCHL1 as a primary target of pathological oxidation. Extensive oxidative modification of its critical cysteine residues severely compromises both its protein solubility and its catalytic activity [39, 40].

UCHL1 abundance is also tightly regulated by the ubiquitination machinery itself. The E3 ubiquitin ligase Parkin‐mediated K63‐linked chains can modify UCHL1, targeting the enzyme for degradation via the autophagy‐lysosome pathway rather than the classic proteasome, and this cross‐system targeting acts as a major checkpoint to downregulate steady‐state UCHL1 levels and functional activity [41]. Furthermore, UCHL1 partitions into distinct biophysical compartments existing as both a soluble cytosolic protein and a distinct membrane‐bound species termed UCHL1^M [26]. This membrane association is driven by lipid modification, specifically C‐terminal farnesylation, which anchors the protein to intracellular lipid bilayers, such as the endoplasmic reticulum. Pathologically, elevated UCHL1^M expression in transfected models correlates closely with the stabilization and accumulation of α‐synuclein, driving subsequent neurotoxicity and Parkinson's disease progression [26].

Taken together, these upstream regulatory mechanisms demonstrate that UCHL1 is subject to an intricate network of transcriptional, epigenetic, and posttranslational controls. Collectively, these inputs dictate its highly tissue‐specific expression patterns, baseline stability, and ultimate functional outcomes in both homeostatic and diseased states.

4.1. Neurological Disorders

The relevance of UCHL1 to neurological disorders must be interpreted within the broader structural and functional framework of ubiquitin‐dependent proteostasis. Because neurons are long‐lived, postmitotic cells with virtually no regenerative capacity, they are exquisitely dependent on flawless protein quality‐control networks. In neurodegenerative diseases, the accumulation of misfolded or aggregation‐prone proteins can rapidly overwhelm these cellular degradation pathways, culminating in the formation of cytotoxic, ubiquitin‐positive inclusions, the impairment of baseline proteasomal activity, and the disruption of critical crosstalk between the UPS and the autophagy–lysosome system. Consequently, UCHL1 is mechanistically relevant to neurodegeneration not merely because its expression levels fluctuate in diseased tissues, but because it actively governs the core machinery of ubiquitin recycling, protein turnover, and stress‐related signaling pathways that dictate neuronal homeostasis and survival [11, 42, 43] (Figure 3).

4.2. Cancer

The divergent roles of UCHL1 across various malignancies can be reconciled through two primary, opposing molecular mechanisms: widespread epigenetic silencing, which abrogates its intrinsic tumor‐suppressive properties in specific tissue types, and targeted substrate stabilization, which hyperactivates oncogenic signaling cascades in others. Consequently, UCHL1 cannot be simplistically classified as a universal oncogene or a classic tumor suppressor. Instead, it functions as a context‐dependent proteostatic regulator whose final phenotypic output is dictated by promoter methylation status, cellular lineage, and the specific repertoire of downstream substrates available within the tumor microenvironment.

Accumulating studies demonstrate that distinct members of the UCH enzyme family modulate diverse signaling pathways critical to malignant transformation and metastatic dissemination [60]. This operational divergence is clearly illustrated by the opposing roles UCHL1 plays in breast versus prostate malignancies. In breast cancer, UCHL1 is frequently overexpressed and functions as a key oncogenic driver. Mechanistic investigations have revealed that elevated UCHL1 directly stabilizes estrogen receptor α (ERα) and hyperactivates the PI3K/AKT signaling axis, thereby accelerating tumor cell proliferation, driving local tissue invasion, and conferring therapeutic resistance to standard endocrine therapies [61]. Conversely, in prostate cancer (PCa), the UCHL1 locus undergoes profound transcriptional silencing driven by extensive promoter hypermethylation. Experimental re‐expression of UCHL1 in PCa models successfully suppresses cell proliferation and restricts in vivo tumor growth. This tumor‐suppressive phenotype is mechanistically executed through the robust activation of the p53/p14(ARF) signaling axis alongside the concomitant downregulation of oncogenic PI3K/AKT signaling [33]. Together, these findings highlight how the microenvironmental context and lineage background completely invert UCHL1's functional consequences.

Similarly, colorectal and ovarian carcinomas frequently exhibit profound epigenetic silencing of UCHL1, and this loss of expression removes key homeostatic checkpoints, thereby supporting accelerated tumor expansion and enhanced cell survival [62]. In contrast, UCHL1 shifts to an active promoter of malignancy in other anatomical contexts. For example, in gastric cancer cell lines, the functional ablation or decreased expression of UCHL1 significantly impedes cell proliferation, migration, and invasive capacity [63]. Furthermore, in high‐grade neuroendocrine lung cancers, UCHL1 is strongly upregulated and tightly correlated with highly aggressive clinical behaviors. In these neuroendocrine models, UCHL1 knockdown robustly restricts cell proliferation and compromises metastatic dissemination, highlighting its potential utility as both an adverse prognostic indicator and a viable therapeutic target [14, 35]. Ultimately, the dual role of UCHL1 across the oncogenic spectrum reflects a complex equilibrium between cell‐heritable epigenetic regulation and lineage‐specific substrate signaling, emphasizing that any translational application requires a highly tumor‐context‐specific interpretation.

4.3. Cardiovascular Disorders

Beyond its role in cell proliferation, UCHL1 serves as a critical modulator of fibroblast activation and vascular signaling. In both cardiac and vascular injury models, UCHL1 fosters transforming growth factor‐β (TGF‐β)/NF‐κB‐linked fibrotic pathways. Consequently, its targeted inhibition or genetic ablation significantly restricts pathological fibroblast activation and mitigates adverse postmyocardial infarction remodeling [29, 64]. Furthermore, UCHL1 contributes to vascular remodeling under chronic pressure states. In models of pulmonary arterial hypertension (PAH), the pharmacological inhibition or ablation of UCHL1 markedly ameliorates clinical phenotypes. Mechanistically, this therapeutic effect is driven by accelerating the K48‐linked polyubiquitination and subsequent proteasomal degradation of AKT1 within endothelial cells, thereby suppressing aberrant endothelial proliferation and vascular thickening [23]. Because adverse tissue remodeling, fibroblast hyperactivation, persistent inflammatory signaling, and altered matrix repair responses represent shared pathogenic hallmarks of several chronic musculoskeletal disorders, these cardiovascular insights carry broader translational implications.

4.4. Musculoskeletal Disorders