E3 ligase Smurf1 protects against misfolded SOD1 in neuronal cells by promoting its K63 ubiquitylation and aggresome formation

TRIB3 mediates vascular calcification by facilitating self-ubiquitination and dissociation of Smurf1 in chronic kidney disease

The osteogenic environment promotes vascular calcium phosphate deposition and aggregation of unfolded and misfolded proteins, resulting in ER stress in chronic kidney disease (CKD). Controlling ER stress through genetic intervention is a promising approach for treating vascular calcification. In this study, we demonstrated a positive correlation between ER stress-induced tribble homolog 3 (TRIB3) expression and progression of vascular calcification in human and rodent CKD. Increased TRIB3 expression promoted vascular smooth muscle cell (VSMC) calcification by interacting with the C2 domain of the E3 ubiquitin-protein ligase Smurf1, facilitating its K48-related self-ubiquitination at Lys381 and Lys383 and subsequent dissociation from the plasma membrane and nuclei. This degeneration of Smurf1 accelerated the stabilization of the osteogenic transcription factors RUNX family transcription factor 2 (Runx2) and SMAD family member 1 (Smad1). C/EBP homologous protein and activating transcription factor 4 are upstream transcription factors of TRIB3 in an osteogenic environment. Genetic KO of TRIB3 or rescue of Smurf1 ameliorated VSMC and vascular calcification by stabilizing Smurf1 and enhancing the degradation of Runx2 and Smad1. Our findings shed light on the vital role of TRIB3 as a scaffold in ER stress and vascular calcification and offer a potential therapeutic option for CKD.

SMURF1 mediates damaged lysosomal homeostasis by ubiquitinating PPP3CB to promote the activation of TFEB

The calcium-activated phosphatase PPP3/calcineurin dephosphorylates TFEB (transcription factor EB) to trigger its nuclear translocation and the activation of macroautophagic/autophagic targets. However, the detailed molecular mechanism regulating TFEB activation remains poorly understood. Here, we highlighted the importance of SMURF1 (SMAD specific E3 ubiquitin protein ligase 1) in the activation of TFEB for lysosomal homeostasis. SMURF1 deficiency prevents the calcium-triggered ubiquitination of the catalytic subunit of PPP3/calcineurin in a manner consistent with defective autophagic degradation of damaged lysosomes. Mechanically, PPP3CB/CNA2 plays a bridging role in the recruitment of SMURF1 by LGALS3 (galectin 3) upon lysosome damage. Importantly, PPP3CB increases the dissociation of the N-terminal tail (NT) and C-terminal carbohydrate-recognition domain (CRD) of LGALS3, which may promote the formation of open conformers in a PPP3CB dephosphorylation activity-dependent manner. In addition, PPP3CB is ubiquitinated at lysine 146 by the recruited SMURF1 in response to intracellular calcium stimulation. The K63-linked ubiquitination of PPP3CB enhances the recruitment of TFEB. Moreover, TFEB directly interacts with both PPP3CB and the regulatory subunit PPP3R1 which facilitate the conformational correction of TFEB for its activation for the transcription of TFEB-targeted genes. Altogether, our results highlighted a critical mechanism for the regulation of PPP3/calcineurin activity via its ubiquitin ligase SMURF1 in response to lysosomal membrane damage, which may account for a potential target for the treatment of stress-related diseases.Abbreviation AID: autoinhibitory domain; ATG: autophagy related; CD: catalytic domain; CRD: carbohydrate-recognition domain; CsA: cyclosporin A; DMSO: dimethyl sulfoxide; ESCRT: endosomal sorting complexes required for transport; GSK3B: glycogen synthase kinase 3 beta; LAMP1: lysosomal associated membrane protein 1; LGALS3: galectin 3; LLOMe: L-leucyl-L-leucine methyl ester; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; ML-SA1: mucolipin synthetic agonist 1; MTORC1: mechanistic target of rapamycin kinase complex 1; NT: N-terminal tail; PPP3CB: protein phosphatase 3 catalytic subunit beta; PPP3R1: protein phosphatase 3 regulatory subunit B, alpha; SMURF1: SMAD specific E3 ubiquitin protein ligase 1; SQSTM1/p62: sequestosome 1; TFEB: transcription factor EB; VCP/p97: valosin containing protein; YWHA/14-3-3: tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein.

Ubiquitin-Proteasome-Mediated Protein Degradation and Disorders of the Central Nervous System

Ubiquitin-proteasome-mediated proteolysis post-translationally regulates the amounts of many proteins that are critical for the normal physiology of the central nervous system. Research carried out over the last several years has revealed a role for components of the ubiquitin-proteasome pathway (UPP) in many neurodegenerative diseases such as Parkinson's disease and Huntington's disease. Studies have also shown a role for the UPP in mental disorders such as schizophrenia and autism. Even though dysregulation of protein degradation by the UPP is a contributory factor to the pathology underlying many nervous system disorders, the association between the components of the UPP and these diseases is far from simple. In this review, we discuss the connections between the UPP and some of the major mental disorders and neurodegenerative diseases.

UPS-dependent strategies of protein quality control degradation

The degradation of damaged proteins is critical for tissue integrity and organismal health because damaged proteins have a high propensity to form aggregates. E3 ubiquitin ligases are key regulators of protein quality control (PQC) and mediate the selective degradation of damaged proteins, a process termed 'PQC degradation' (PQCD). The degradation signals (degrons) that trigger PQCD are based on hydrophobic sites that are normally buried within the native protein structure. However, an open question is how PQCD-specialized E3 ligases distinguish between transiently misfolded proteins, which can be efficiently refolded, and permanently damaged proteins, which must be degraded. While significant progress has been made in characterizing degradation determinants, understanding the key regulatory signals of cellular and organismal PQCD pathways remains a challenge.

Ubiquitin-Proteasome System in the Different Stages of Dominantly Inherited Alzheimer's Disease

This study explored the role of the ubiquitin-proteasome system (UPS) in dominantly inherited Alzheimer's disease (DIAD) by examining changes in cerebrospinal fluid (CSF) levels of UPS proteins along with disease progression, AD imaging biomarkers (PiB PET, tau PET), neurodegeneration imaging measures (MRI, FDG PET), and Clinical Dementia Rating® (CDR®). Using the SOMAscan assay, we detected subtle increases in specific ubiquitin enzymes associated with proteostasis in mutation carriers (MCs) up to two decades before the estimated symptom onset. This was followed by more pronounced elevations of UPS-activating enzymes, including E2 and E3 proteins, and ubiquitin-related modifiers. Our findings also demonstrated consistent correlations between UPS proteins and CSF biomarkers such as Aβ42/40 ratio, total tau, various phosphorylated tau species to total tau ratios (ptau181/T181, ptauT205/T205, ptauS202/S202, ptauT217/T217), and MTBR-tau243, alongside Neurofilament light chain (NfL) and the CDR®. Notably, a positive association was observed with imaging markers (PiB PET, tau PET) and a negative correlation with markers of neurodegeneration (FDG PET, MRI), highlighting a significant link between UPS dysregulation and neurodegenerative processes. The correlations suggest that the increase in multiple UPS proteins with rising tau levels and tau-tangle associated markers, indicating a potential role for the UPS in relation to misfolded tau/neurofibrillary tangles (NFTs) and symptom onset. These findings indicate that elevated CSF UPS proteins in DIAD MCs could serve as early indicators of disease progression and suggest a link between UPS dysregulation and amyloid plaque, tau tangles formation, implicating the UPS as a potential therapeutic target in AD pathogenesis.

Proteostasis in neurodegenerative diseases

Proteostasis is essential for normal function of proteins and vital for cellular health and survival. Proteostasis encompasses all stages in the "life" of a protein, that is, from translation to functional performance and, ultimately, to degradation. Proteins need native conformations for function and in the presence of multiple types of stress, their misfolding and aggregation can occur. A coordinated network of proteins is at the core of proteostasis in cells. Among these, chaperones are required for maintaining the integrity of protein conformations by preventing misfolding and aggregation and guide those with abnormal conformation to degradation. The ubiquitin-proteasome system (UPS) and autophagy are major cellular pathways for degrading proteins. Although failure or decreased functioning of components of this network can lead to proteotoxicity and disease, like neuron degenerative diseases, underlying factors are not completely understood. Accumulating misfolded and aggregated proteins are considered major pathomechanisms of neurodegeneration. In this chapter, we have described the components of three major branches required for proteostasis-chaperones, UPS and autophagy, the mechanistic basis of their function, and their potential for protection against various neurodegenerative conditions, like Alzheimer's, Parkinson's, and Huntington's disease. The modulation of various proteostasis network proteins, like chaperones, E3 ubiquitin ligases, proteasome, and autophagy-associated proteins as therapeutic targets by small molecules as well as new and unconventional approaches, shows promise.

SMURF1 controls the PPP3/calcineurin complex and TFEB at a regulatory node for lysosomal biogenesis

Macroautophagy/autophagy is a homeostatic process in response to multiple signaling, such as the lysosome-dependent recycling process of cellular components. Starvation-induced MTOR inactivation and PPP3/calcineurin activation were shown to promote the nuclear translocation of TFEB. However, the mechanisms via which signals from endomembrane damage are transmitted to activate PPP3/calcineurin and orchestrate autophagic responses remain unknown. This study aimed to show that autophagy regulator SMURF1 controlled TFEB nuclear import for transcriptional activation of the lysosomal biogenesis. We showed that blocking SMURF1 affected lysosomal biogenesis in response to lysosomal damage by preventing TFEB nuclear translocation. It revealed galectins recognized endolysosomal damage, and led to recruitment of SMURF1 and the PPP3/calcineurin apparatus on lysosomes. SMURF1 interacts with both LGALS3 and PPP3CB to form the LGALS3-SMURF1-PPP3/calcineurin complex. Importantly, this complex further stabilizes TFEB, thereby activating TFEB for lysosomal biogenesis. We determined that LLOMe-mediated TFEB nuclear import is dependent on SMURF1 under the condition of MTORC1 inhibition. In addition, SMURF1 is required for PPP3/calcineurin activity as a positive regulator of TFEB. SMURF1 controlled the phosphatase activity of the PPP3CB by promoting the dissociation of its autoinhibitory domain (AID) from its catalytic domain (CD). Overexpression of SMURF1 showed similar effects as the constitutive activation of PPP3CB. Thus, SMURF1, which bridges environmental stress with the core autophagosomal and autolysosomal machinery, interacted with endomembrane sensor LGALS3 and phosphatase PPP3CB to control TFEB activation.Abbreviations: ATG: autophagy-related; LLOMe: L-Leucyl-L-Leucine methyl ester; ML-SA1: mucolipin synthetic agonist 1; MTOR: mechanistic target of rapamycin kinase; PPP3CB: protein phosphatase 3 catalytic subunit beta; RPS6KB1/p70S6K: ribosomal protein S6 kinase B1; SMURF1: SMAD specific E3 ubiquitin protein ligase 1; TFEB: transcription factor EB.

SMURF1 attenuates endoplasmic reticulum stress by promoting the degradation of KEAP1 to activate NRF2 antioxidant pathway

Cancer cells consistently utilize the unfolded protein response (UPR) to encounter the abnormal endoplasmic reticulum (ER) stress induced by the accumulation of misfolded proteins. Extreme activation of the UPR could also provoke maladaptive cell death. Previous reports have shown that NRF2 antioxidant signaling is activated by UPR and serves as noncanonical pathway to defense and reduce excessive ROS levels during ER stress. However, the mechanisms of regulating NRF2 signaling upon ER stress in glioblastoma have not been fully elucidated. Here we identify that SMURF1 protects against ER stress and facilitates glioblastoma cell survival by rewiring KEAP1-NRF2 pathway. We show that ER stress induces SMURF1 degradation. Knockdown of SMURF1 upregulates IRE1 and PERK signaling in the UPR pathway and prevents ER-associated protein degradation (ERAD) activity, leading to cell apoptosis. Importantly, SMURF1 overexpression activates NRF2 signaling to reduce ROS levels and alleviate UPR-mediated cell death. Mechanistically, SMURF1 interacts with and ubiquitinates KEAP1 for its degradation (NRF2 negative regulator), resulting in NRF2 nuclear import. Moreover, SMURF1 loss reduces glioblastoma cell proliferation and growth in subcutaneously implanted nude mice xenografts. Taken together, SMURF1 rewires KEAP1-NRF2 pathway to confer resistance to ER stress inducers and protect glioblastoma cell survival. ER stress and SMURF1 modulation may provide promising therapeutic targets for the treatment of glioblastoma.