Specificity for latent C termini links the E3 ubiquitin ligase CHIP to caspases

AAV mediated carboxyl terminus of Hsp70 interacting protein overexpression mitigates the cognitive and pathological phenotypes of APP/PS1 mice

JOURNAL/nrgr/04.03/01300535-202501000-00033/figure1/v/2024-05-14T021156Z/r/image-tiff The E3 ubiquitin ligase, carboxyl terminus of heat shock protein 70 (Hsp70) interacting protein (CHIP), also functions as a co-chaperone and plays a crucial role in the protein quality control system. In this study, we aimed to investigate the neuroprotective effect of overexpressed CHIP on Alzheimer's disease. We used an adeno-associated virus vector that can cross the blood-brain barrier to mediate CHIP overexpression in APP/PS1 mouse brain. CHIP overexpression significantly ameliorated the performance of APP/PS1 mice in the Morris water maze and nest building tests, reduced amyloid-β plaques, and decreased the expression of both amyloid-β and phosphorylated tau. CHIP also alleviated the concentration of microglia and astrocytes around plaques. In APP/PS1 mice of a younger age, CHIP overexpression promoted an increase in ADAM10 expression and inhibited β-site APP cleaving enzyme 1, insulin degrading enzyme, and neprilysin expression. Levels of HSP70 and HSP40, which have functional relevance to CHIP, were also increased. Single nuclei transcriptome sequencing in the hippocampus of CHIP overexpressed mice showed that the lysosomal pathway and oligodendrocyte-related biological processes were up-regulated, which may also reflect a potential mechanism for the neuroprotective effect of CHIP. Our research shows that CHIP effectively reduces the behavior and pathological manifestations of APP/PS1 mice. Indeed, overexpression of CHIP could be a beneficial approach for the treatment of Alzheimer's disease.

DSFworld: A flexible and precise tool to analyze differential scanning fluorimetry data

Differential scanning fluorimetry (DSF) is a method to determine the apparent melting temperature (Tma) of a purified protein. In DSF, the raw unfolding curves from which Tma is calculated vary widely in shape and complexity. However, the tools available for calculating Tma are only compatible with the simplest of DSF curves, hindering many otherwise straightforward applications of the technology. To overcome this limitation, we designed new mathematical models for Tma calculation that accommodate common forms of variation in DSF curves, including the number of transitions, the presence of high initial signal, and temperature-dependent signal decay. When tested these models against DSFbase, an open-source database of 6235 raw, real-life DSF curves, these models outperformed the existing standard approaches of sigmoid fitting and maximum of the first derivative. To make these models accessible, we created an open-source software and website, DSFworld (https://gestwickilab.shinyapps.io/dsfworld/). In addition to these improved fitting capabilities, DSFworld also includes features that overcome the practical limitations of many analysis workflows, including automatic reformatting of raw data exported from common qPCR instruments, labeling of data based on experimental variables, and flexible interactive plotting. We hope that DSFworld will enable more streamlined and accurate calculation of Tma values for DSF experiments.

Tau truncation in the pathogenesis of Alzheimer's disease: a narrative review

ABSTRACT

Alzheimer's disease is characterized by two major neuropathological hallmarks-the extracellular β-amyloid plaques and intracellular neurofibrillary tangles consisting of aggregated and hyperphosphorylated Tau protein. Recent studies suggest that dysregulation of the microtubule-associated protein Tau, especially specific proteolysis, could be a driving force for Alzheimer's disease neurodegeneration. Tau physiologically promotes the assembly and stabilization of microtubules, whereas specific truncated fragments are sufficient to induce abnormal hyperphosphorylation and aggregate into toxic oligomers, resulting in them gaining prion-like characteristics. In addition, Tau truncations cause extensive impairments to neural and glial cell functions and animal cognition and behavior in a fragment-dependent manner. This review summarizes over 60 proteolytic cleavage sites and their corresponding truncated fragments, investigates the role of specific truncations in physiological and pathological states of Alzheimer's disease, and summarizes the latest applications of strategies targeting Tau fragments in the diagnosis and treatment of Alzheimer's disease.

Exploration of the Binding Determinants of Protein Phosphatase 5 (PP5) Reveals a Chaperone-Independent Activation Mechanism

The protein phosphatase 5 (PP5) is normally recruited to its substrates by the molecular chaperones, heat shock protein 70 (Hsp70) and heat shock protein 90 (Hsp90). This interaction requires the tetratricopeptide repeat (TPR) domain of PP5, which binds to an EEVD motif at the extreme C-termini of cytosolic Hsp70 and Hsp90 isoforms. In addition to bringing PP5 into proximity with chaperone-bound substrates, this interaction also relieves auto-inhibition in PP5's catalytic domain, promoting its phosphatase activity. To better understand the molecular determinants of this process, we screened a large, pentapeptide library for binding to PP5. This screen identified the amino acid preferences at each position, which we validated by showing that the optimal sequences bind 4- to 7-fold tighter than the natural EEVD motifs and stimulate PP5's enzymatic activity. The enhanced affinity for PP5's TPR domain was confirmed using a protein-adaptive differential scanning fluorimetry (paDSF) assay. Using this increased knowledge of structure-activity relationships, we re-examined affinity proteomics results to look for potential EEVD-like motifs in the C-termini of known PP5-binding partners. This search identified elongator acetyltransferase complex subunit 1 (ELP1/IKBKAP) as a putative partner and, indeed, we found that its C-terminal sequence, LSLLD, binds directly to PP5's TPR domain in vitro. Consistent with this idea, mutation of ELP1's terminal aspartate was sufficient to interrupt the interaction with PP5 in vitro and in cells. Together, these findings reveal the sequence preferences of PP5's TPR domain and expand the scope of PP5's functions to include chaperone-independent complexes.

N/C-degron pathways and inhibitor development for PROTAC applications

Ubiquitination is a fascinating post-translational modification that has received continuous attention since its discovery. In this review, we first provide a concise overview of the E3 ubiquitin ligases, delving into classification, characteristics and mechanisms of ubiquitination. We then specifically examine the ubiquitination pathways mediated by the N/C-degrons, discussing their unique features and substrate recognition mechanisms. Finally, we offer insights into the current state of development pertaining to inhibitors that target the N/C-degron pathways, as well as the promising advances in the field of PROTAC (PROteolysis TArgeting Chimeras). Overall, this review offers a comprehensive understanding of the rapidly-evolving field of ubiquitin biology.

Exploring the significance of caspase-cleaved tau in tauopathies and as a complementary pathology to phospho-tau in Alzheimer's disease: implications for biomarker development and therapeutic targeting

Tauopathies are neurodegenerative diseases that typically require postmortem examination for a definitive diagnosis. Detecting neurotoxic tau fragments in cerebrospinal fluid (CSF) and serum provides an opportunity for in vivo diagnosis and disease monitoring. Current assays primarily focus on total tau or phospho-tau, overlooking other post-translational modifications (PTMs). Caspase-cleaved tau is a significant component of AD neuropathological lesions, and experimental studies confirm the high neurotoxicity of these tau species. Recent evidence indicates that certain caspase-cleaved tau species, such as D13 and D402, are abundant in AD brain neurons and only show a modest degree of co-occurrence with phospho-tau, meaning caspase-truncated tau pathology is partially distinct and complementary to phospho-tau pathology. Furthermore, these caspase-cleaved tau species are nearly absent in 4-repeat tauopathies. In this review, we will discuss the significance of caspase-cleaved tau in the development of tauopathies, specifically emphasizing its role in AD. In addition, we will explore the potential of caspase-cleaved tau as a biomarker and the advantages for drug development targeting caspase-6. Developing specific and sensitive assays for caspase-cleaved tau in biofluids holds promise for improving the diagnosis and monitoring of tauopathies, providing valuable insights into disease progression and treatment efficacy.

Mirror-image ligand discovery enabled by single-shot fast-flow synthesis of D-proteins

Widespread adoption of mirror-image biological systems presents difficulties in accessing the requisite D-protein substrates. In particular, mirror-image phage display has the potential for high-throughput generation of biologically stable macrocyclic D-peptide binders with potentially unique recognition modes but is hindered by the individualized optimization required for D-protein chemical synthesis. We demonstrate a general mirror-image phage display pipeline that utilizes automated flow peptide synthesis to prepare D-proteins in a single run. With this approach, we prepare and characterize 12 D-proteins - almost one third of all reported D-proteins to date. With access to mirror-image protein targets, we describe the successful discovery of six macrocyclic D-peptide binders: three to the oncoprotein MDM2, and three to the E3 ubiquitin ligase CHIP. Reliable production of mirror-image proteins can unlock the full potential of D-peptide drug discovery and streamline the study of mirror-image biology more broadly.

Alzheimer proteopathic tau seeds are biochemically a forme fruste of mature paired helical filaments

Aggregation prone molecules, such as tau, form both historically well characterized fibrillar deposits (neurofibrillary tangles) and recently identified phosphate-buffered saline (PBS) extract species called proteopathic seeds. Both can cause normal endogenous tau to undergo templated misfolding. The relationship of these seeds to the fibrils that define tau-related diseases is unknown. We characterized the aqueous extractable and sarkosyl insoluble fibrillar tau species derived from human Alzheimer brain using mass spectrometry and in vitro bioassays. Post-translational modifications (PTMs) including phosphorylation, acetylation and ubiquitination are identified in both preparations. PBS extract seed competent tau can be distinguished from sarkosyl insoluble tau by the presence of overlapping, but less abundant, PTMs and an absence of some PTMs unique to the latter. The presence of ubiquitin and other PTMs on the PBS-extracted tau species correlates with the amount of tau in the seed competent size exclusion fractions, with the bioactivity and with the aggressiveness of clinical disease. These results demonstrate that the PTMs present on bioactive, seed competent PBS extract tau species are closely related to, but distinct from, the PTMs of mature paired helical filaments, consistent with the idea that they are a forme fruste of tau species that ultimately form fibrils.

Recognition and reprogramming of E3 ubiquitin ligase surfaces by α-helical peptides

Molecules that induce novel interactions between proteins hold great promise for the study of biological systems and the development of therapeutics, but their discovery has been limited by the complexities of rationally designing interactions between three components, and because known binders to each protein are typically required to inform initial designs. Here, we report a general and rapid method for discovering α-helically constrained (Helicon) polypeptides that cooperatively induce the interaction between two target proteins without relying on previously known binders or an intrinsic affinity between the proteins. We show that Helicons are capable of binding every major class of E3 ubiquitin ligases, which are of great biological and therapeutic interest but remain largely intractable to targeting by small molecules. We then describe a phage-based screening method for discovering "trimerizer" Helicons, and apply it to reprogram E3s to cooperatively bind an enzyme (PPIA), a transcription factor (TEAD4), and a transcriptional coactivator (β-catenin).

Rep protein accommodates together dsDNA and ssDNA which enables a loop-back mechanism to plasmid DNA replication initiation

For DNA replication initiation in Bacteria, replication initiation proteins bind to double-stranded DNA (dsDNA) and interact with single-stranded DNA (ssDNA) at the replication origin. The structural-functional relationship of the nucleoprotein complex involving initiator proteins is still elusive and different models are proposed. In this work, based on crosslinking combined with mass spectrometry (MS), the analysis of mutant proteins and crystal structures, we defined amino acid residues essential for the interaction between plasmid Rep proteins, TrfA and RepE, and ssDNA. This interaction and Rep binding to dsDNA could not be provided in trans, and both are important for dsDNA melting at DNA unwinding element (DUE). We solved two crystal structures of RepE: one in a complex with ssDNA DUE, and another with both ssDNA DUE and dsDNA containing RepE-specific binding sites (iterons). The amino acid residues involved in interaction with ssDNA are located in the WH1 domain in stand β1, helices α1 and α2 and in the WH2 domain in loops preceding strands β1' and β2' and in these strands. It is on the opposite side compared to RepE dsDNA-recognition interface. Our data provide evidence for a loop-back mechanism through which the plasmid replication initiator molecule accommodates together dsDNA and ssDNA.

Rare antibody phage isolation and discrimination (RAPID) biopanning enables identification of high-affinity antibodies against challenging targets

In vitro biopanning platforms using synthetic phage display antibody libraries have enabled the identification of antibodies against antigens that were once thought to be beyond the scope of immunization. Applying these methods against challenging targets remains a critical challenge. Here, we present a new biopanning pipeline, RAPID (Rare Antibody Phage Isolation and Discrimination), for the identification of rare high-affinity antibodies against challenging targets. RAPID biopanning uses fluorescent labeled phage displayed fragment antigen-binding (Fab) antibody libraries for the isolation of high-affinity binders with fluorescent activated sorting. Subsequently, discriminatory hit screening is performed with a biolayer interferometry (BLI) method, BIAS (Biolayer Interferometry Antibody Screen), where candidate binders are ranked and prioritized according to their estimated kinetic off rates. Previously reported antibodies were used to develop the methodology, and the RAPID biopanning pipeline was applied to three challenging targets (CHIP, Gαq, and CS3D), enabling the identification of high-affinity antibodies.

Mechanism and evolutionary origins of alanine-tail C-degron recognition by E3 ligases Pirh2 and CRL2-KLHDC10

In ribosome-associated quality control (RQC), nascent polypeptides produced by interrupted translation are modified with C-terminal polyalanine tails ("Ala-tails") that function outside ribosomes to induce ubiquitylation by E3 ligases Pirh2 (p53-induced RING-H2 domain-containing) or CRL2 (Cullin-2 RING ligase2)-KLHDC10. Here, we investigate the molecular basis of Ala-tail function using biochemical and in silico approaches. We show that Pirh2 and KLHDC10 directly bind to Ala-tails and that structural predictions identify candidate Ala-tail-binding sites, which we experimentally validate. The degron-binding pockets and specific pocket residues implicated in Ala-tail recognition are conserved among Pirh2 and KLHDC10 homologs, suggesting that an important function of these ligases across eukaryotes is in targeting Ala-tailed substrates. Moreover, we establish that the two Ala-tail-binding pockets have convergently evolved, either from an ancient module of bacterial provenance (Pirh2) or via tinkering of a widespread C-degron-recognition element (KLHDC10). These results shed light on the recognition of a simple degron sequence and the evolution of Ala-tail proteolytic signaling.

Phosphorylation of a Cleaved Tau Proteoform at a Single Residue Inhibits Binding to the E3 Ubiquitin Ligase, CHIP

Microtubule-associated protein tau (MAPT/tau) accumulates in a family of neurodegenerative diseases, including Alzheimer's disease (AD). In disease, tau is aberrantly modified by post-translational modifications (PTMs), including hyper-phosphorylation. However, it is often unclear which of these PTMs contribute to tau's accumulation or what mechanisms might be involved. To explore these questions, we focused on a cleaved proteoform of tau (tauC3), which selectively accumulates in AD and was recently shown to be degraded by its direct binding to the E3 ubiquitin ligase, CHIP. Here, we find that phosphorylation of tauC3 at a single residue, pS416, is sufficient to block its interaction with CHIP. A co-crystal structure of CHIP bound to the C-terminus of tauC3 revealed the mechanism of this clash and allowed design of a mutation (CHIPD134A) that partially restores binding and turnover of pS416 tauC3. We find that pS416 is produced by the known AD-associated kinase, MARK2/Par-1b, providing a potential link to disease. In further support of this idea, an antibody against pS416 co-localizes with tauC3 in degenerative neurons within the hippocampus of AD patients. Together, these studies suggest a discrete molecular mechanism for how phosphorylation at a specific site contributes to accumulation of an important tau proteoform.

The E3 Ubiquitin Ligase, CHIP/STUB1, Inhibits Aggregation of Phosphorylated Proteoforms of Microtubule-associated Protein Tau (MAPT)

Hyper-phosphorylated tau accumulates as insoluble fibrils in Alzheimer's disease (AD) and related dementias. The strong correlation between phosphorylated tau and disease has led to an interest in understanding how cellular factors discriminate it from normal tau. Here, we screen a panel of chaperones containing tetratricopeptide repeat (TPR) domains to identify those that might selectively interact with phosphorylated tau. We find that the E3 ubiquitin ligase, CHIP/STUB1, binds 10-fold more strongly to phosphorylated tau than unmodified tau. The presence of even sub-stoichiometric concentrations of CHIP strongly suppresses aggregation and seeding of phosphorylated tau. We also find that CHIP promotes rapid ubiquitination of phosphorylated tau, but not unmodified tau, in vitro. Binding to phosphorylated tau requires CHIP's TPR domain, but the binding mode is partially distinct from the canonical one. In cells, CHIP restricts seeding by phosphorylated tau, suggesting that it could be an important barrier in cell-to-cell spreading. Together, these findings show that CHIP recognizes a phosphorylation-dependent degron on tau, establishing a pathway for regulating the solubility and turnover of this pathological proteoform.

The N-degron pathway: From basic science to therapeutic applications

The N-degron pathway is a degradative system in which single N-terminal (Nt) amino acids regulate the half-lives of proteins and other biological materials. These determinants, called N-degrons, are recognized by N-recognins that link them to the ubiquitin (Ub)-proteasome system (UPS) or autophagy-lysosome system (ALS). In the UPS, the Arg/N-degron pathway targets the Nt-arginine (Nt-Arg) and other N-degrons to assemble Lys48 (K48)-linked Ub chains by UBR box N-recognins for proteasomal proteolysis. In the ALS, Arg/N-degrons are recognized by the N-recognin p62/SQSTSM-1/Sequestosome-1 to induce cis-degradation of substrates and trans-degradation of various cargoes such as protein aggregates and subcellular organelles. This crosstalk between the UPS and ALP involves reprogramming of the Ub code. Eukaryotic cells developed diverse ways to target all 20 principal amino acids for degradation. Here we discuss the components, regulation, and functions of the N-degron pathways, with an emphasis on the basic mechanisms and therapeutic applications of Arg/N-degrons and N-recognins.

Mechanism and evolutionary origins of Alanine-tail C-degron recognition by E3 ligases Pirh2 and CRL2-KLHDC10

In Ribosome-associated Quality Control (RQC), nascent-polypeptides produced by interrupted translation are modified with C-terminal polyalanine tails ('Ala-tails') that function outside ribosomes to induce ubiquitylation by Pirh2 or CRL2-KLHDC10 E3 ligases. Here we investigate the molecular basis of Ala-tail function using biochemical and in silico approaches. We show that Pirh2 and KLHDC10 directly bind to Ala-tails, and structural predictions identify candidate Ala-tail binding sites, which we experimentally validate. The degron-binding pockets and specific pocket residues implicated in Ala-tail recognition are conserved among Pirh2 and KLHDC10 homologs, suggesting that an important function of these ligases across eukaryotes is in targeting Ala-tailed substrates. Moreover, we establish that the two Ala-tail binding pockets have convergently evolved, either from an ancient module of bacterial provenance (Pirh2) or via tinkering of a widespread C-degron recognition element (KLHDC10). These results shed light on the recognition of a simple degron sequence and the evolution of Ala-tail proteolytic signaling.

Dual truncation of tau by caspase-2 accelerates its CHIP-mediated degradation

Intraneuronal aggregates of the microtubule binding protein Tau are a hallmark of different neurodegenerative diseases including Alzheimer's disease (AD). In these aggregates, Tau is modified by posttranslational modifications such as phosphorylation as well as by proteolytic cleavage. Here we identify a novel Tau cleavage site at aspartate 65 (D65) that is specific for caspase-2. In addition, we show that the previously described cleavage site at D421 is also efficiently processed by caspase-2, and both sites are cleaved in human brain samples. Caspase-2-generated Tau fragments show increased aggregation potential in vitro, but do not accumulate in vivo after AAV-mediated overexpression in mouse hippocampus. Interestingly, we observe that steady-state protein levels of caspase-2 generated Tau fragments are low in our in vivo model despite strong RNA expression, suggesting efficient clearance. Consistent with this hypothesis, we find that caspase-2 cleavage significantly improves the recognition of Tau by the ubiquitin E3 ligase CHIP, leading to increased ubiquitination and faster degradation of Tau fragments. Taken together our data thus suggest that CHIP-induced ubiquitination is of particular importance for the clearance of caspase-2 generated Tau fragments in vitro and in vivo.

Characterization of degradation signals at protein C-termini

Protein termini are critical for protein functions. They are often more accessible than internal regions and thus are frequently subjected to various modifications that affect protein function. Protein termini also contribute to regulating protein lifespan. Recent studies have revealed a series of degradation signals located at protein C-termini, termed C-degrons or C-end degrons. C-degrons have been implicated as underlying a protein quality surveillance system that eliminates truncated, cleaved and mislocalized proteins. Despite the importance of C-degrons, our knowledge of them remains sparse. Here, we describe an established framework for the characterization of C-degrons by Global Protein Stability (GPS) profiling assay, a fluorescence-based reporter system for measuring protein stability in cellulo. Furthermore, we apply an approach that couples GPS with random peptide libraries for unbiased and context-independent characterization of C-degron motifs. Our methodology provides a robust and efficient platform for analyzing the degron potencies of C-terminal peptides, which can significantly accelerate our understanding of C-degrons.

E3 ligase autoinhibition by C-degron mimicry maintains C-degron substrate fidelity

E3 ligase recruitment of proteins containing terminal destabilizing motifs (degrons) is emerging as a major form of regulation. How those E3s discriminate bona fide substrates from other proteins with terminal degron-like sequences remains unclear. Here, we report that human KLHDC2, a CRL2 substrate receptor targeting C-terminal Gly-Gly degrons, is regulated through interconversion between two assemblies. In the self-inactivated homotetramer, KLHDC2's C-terminal Gly-Ser motif mimics a degron and engages the substrate-binding domain of another protomer. True substrates capture the monomeric CRL2KLHDC2, driving E3 activation by neddylation and subsequent substrate ubiquitylation. Non-substrates such as NEDD8 bind KLHDC2 with high affinity, but its slow on rate prevents productive association with CRL2KLHDC2. Without substrate, neddylated CRL2KLHDC2 assemblies are deactivated via distinct mechanisms: the monomer by deneddylation and the tetramer by auto-ubiquitylation. Thus, substrate specificity is amplified by KLHDC2 self-assembly acting like a molecular timer, where only bona fide substrates may bind before E3 ligase inactivation.

The Cys/N-degron pathway in the ubiquitin-proteasome system and autophagy

The N-degron pathway is a degradative system in which the N-terminal residues of proteins modulate the half-lives of proteins and other cellular materials. The majority of amino acids in the genetic code have the potential to induce cis or trans degradation in diverse processes, which requires selective recognition between N-degrons and cognate N-recognins. Of particular interest is the Cys/N-degron branch, in which the N-terminal cysteine (Nt-Cys) induces proteolysis via either the ubiquitin (Ub)-proteasome system (UPS) or the autophagy-lysosome pathway (ALP), depending on physiological conditions. Recent studies provided new insights into the central role of Nt-Cys in sensing the fluctuating levels of oxygen and reactive oxygen species (ROS). Here, we discuss the components, regulations, and functions of the Cys/N-degron pathway.

Conformationally responsive dyes enable protein-adaptive differential scanning fluorimetry

Flexible in vitro methods alter the course of biological discoveries. Differential Scanning Fluorimetry (DSF) is a particularly versatile technique which reports protein thermal unfolding via fluorogenic dye. However, applications of DSF are limited by widespread protein incompatibilities with the available DSF dyes. Here, we enable DSF applications for 66 of 70 tested proteins (94%) including 10 from the SARS-CoV2 virus using a chemically diverse dye library, Aurora, to identify compatible dye-protein pairs in high throughput. We find that this protein-adaptive DSF platform (paDSF) not only triples the previous protein compatibility, but also fundamentally extends the processes observable by DSF, including interdomain allostery in O-GlcNAc Transferase (OGT). paDSF enables routine measurement of protein stability, dynamics, and ligand binding.

CHIP: A Co-chaperone for Degradation by the Proteasome and Lysosome

Protein homeostasis relies on a balance between protein folding and protein degradation. Molecular chaperones like Hsp70 and Hsp90 fulfill well-defined roles in protein folding and conformational stability via ATP-dependent reaction cycles. These folding cycles are controlled by associations with a cohort of non-client protein co-chaperones, such as Hop, p23, and Aha1. Pro-folding co-chaperones facilitate the transit of the client protein through the chaperone-mediated folding process. However, chaperones are also involved in proteasomal and lysosomal degradation of client proteins. Like folding complexes, the ability of chaperones to mediate protein degradation is regulated by co-chaperones, such as the C-terminal Hsp70-binding protein (CHIP/STUB1). CHIP binds to Hsp70 and Hsp90 chaperones through its tetratricopeptide repeat (TPR) domain and functions as an E3 ubiquitin ligase using a modified RING finger domain (U-box). This unique combination of domains effectively allows CHIP to network chaperone complexes to the ubiquitin-proteasome and autophagosome-lysosome systems. This chapter reviews the current understanding of CHIP as a co-chaperone that switches Hsp70/Hsp90 chaperone complexes from protein folding to protein degradation.

A STUB1 ubiquitin ligase/CHIC2 protein complex negatively regulates the IL-3, IL-5, and GM-CSF cytokine receptor common β chain (CSF2RB) protein stability

The IL-3, IL-5, and GM-CSF family of cytokines play an essential role in the growth, differentiation, and effector functions of multiple hematopoietic cell types. Receptors in this family are composed of cytokine-specific α chains and a common β chain (CSF2RB), responsible for the majority of downstream signaling. CSF2RB abundance and stability influence the magnitude of the cellular response to cytokine stimulation, but the exact mechanisms of regulation are not well understood. Here, we use genetic screens in multiple cellular contexts and cytokine conditions to identify STUB1, an E3 ubiquitin ligase, and CHIC2 as regulators of CSF2RB ubiquitination and protein stability. We demonstrate that Stub1 and Chic2 form a complex that binds Csf2rb and that genetic inactivation of either Stub1 or Chic2 leads to reduced ubiquitination of Csf2rb. The effects of Stub1 and Chic2 on Csf2rb were greatest at reduced cytokine concentrations, suggesting that Stub1/Chic2-mediated regulation of Csf2rb is a mechanism of reducing cell surface accumulation when cytokine levels are low. Our study uncovers a mechanism of CSF2RB regulation through ubiquitination and lysosomal degradation and describes a role for CHIC2 in the regulation of a cytokine receptor.

STUB1 is an intracellular checkpoint for interferon gamma sensing

Immune checkpoint blockade (ICB) leads to durable and complete tumour regression in some patients but in others gives temporary, partial or no response. Accordingly, significant efforts are underway to identify tumour-intrinsic mechanisms underlying ICB resistance. Results from a published CRISPR screen in a mouse model suggested that targeting STUB1, an E3 ligase involved in protein homeostasis, may overcome ICB resistance but the molecular basis of this effect remains unclear. Herein, we report an under-appreciated role of STUB1 to dampen the interferon gamma (IFNγ) response. Genetic deletion of STUB1 increased IFNGR1 abundance on the cell surface and thus enhanced the downstream IFNγ response as showed by multiple approaches including Western blotting, flow cytometry, qPCR, phospho-STAT1 assay, immunopeptidomics, proteomics, and gene expression profiling. Human prostate and breast cancer cells with STUB1 deletion were also susceptible to cytokine-induced growth inhibition. Furthermore, blockade of STUB1 protein function recapitulated the STUB1-null phenotypes. Despite these encouraging in vitro data and positive implications from clinical datasets, we did not observe in vivo benefits of inactivating Stub1 in mouse syngeneic tumour models-with or without combination with anti-PD-1 therapy. However, our findings elucidate STUB1 as a barrier to IFNγ sensing, prompting further investigations to assess if broader inactivation of human STUB1 in both tumors and immune cells could overcome ICB resistance.

Discovery and Structure-Based Design of Macrocyclic Peptides Targeting STUB1

Recent evidence suggests that deletion of STUB1─a pivotal negative regulator of interferon-γ sensing─may potentially clear malignant cells. However, current studies rely primarily on genetic approaches, as pharmacological inhibitors of STUB1 are lacking. Identifying a tool compound will be a step toward validating the target in a broader therapeutic sense. Herein, screening more than a billion macrocyclic peptides resulted in STUB1 binders, which were further optimized by a structure-enabled in silico design. The strategy to replace the macrocyclic peptides' hydrophilic and solvent-exposed region with a hydrophobic scaffold improved cellular permeability while maintaining the binding conformation. Further substitution of the permeability-limiting terminal aspartic acid with a tetrazole bioisostere retained the binding to a certain extent while improving permeability, suggesting a path forward. Although not optimal for cellular study, the current lead provides a valuable template for further development into selective tool compounds for STUB1 to enable target validation.

Multivalent protein-protein interactions are pivotal regulators of eukaryotic Hsp70 complexes

Heat shock protein 70 (Hsp70) is a molecular chaperone and central regulator of protein homeostasis (proteostasis). Paramount to this role is Hsp70's binding to client proteins and co-chaperones to produce distinct complexes, such that understanding the protein-protein interactions (PPIs) of Hsp70 is foundational to describing its function and dysfunction in disease. Mounting evidence suggests that these PPIs include both "canonical" interactions, which are universally conserved, and "non-canonical" (or "secondary") contacts that seem to have emerged in eukaryotes. These two categories of interactions involve discrete binding surfaces, such that some clients and co-chaperones engage Hsp70 with at least two points of contact. While the contributions of canonical interactions to chaperone function are becoming increasingly clear, it can be challenging to deconvolute the roles of secondary interactions. Here, we review what is known about non-canonical contacts and highlight examples where their contributions have been parsed, giving rise to a model in which Hsp70's secondary contacts are not simply sites of additional avidity but are necessary and sufficient to impart unique functions. From this perspective, we propose that further exploration of non-canonical contacts will generate important insights into the evolution of Hsp70 systems and inspire new approaches for developing small molecules that tune Hsp70-mediated proteostasis.

Structural Basis for Chaperone‐Independent Ubiquitination of Tau Protein by Its E3 Ligase CHIP

The multi‐site ubiquitination of Tau protein found in Alzheimer's disease filaments hints at the failed attempt of neurons to remove early toxic species. The ubiquitin‐dependent degradation of Tau is regulated in vivo by the E3 ligase CHIP, a quality controller of the cell proteome dedicated to target misfolded proteins for degradation. In our study, by using site‐resolved NMR, biochemical and computational methods, we elucidate the structural determinants underlying the molecular recognition between the ligase and its intrinsically disordered substrate. We reveal a multi‐domain dynamic interaction that explains how CHIP can direct ubiquitination of Tau at multiple sites even in the absence of chaperones, including its typical partner Hsp70/Hsc70. Our findings thus provide mechanistic insight into the chaperone‐independent engagement of a disordered protein by its E3 ligase.

Site-specific ubiquitination: Deconstructing the degradation tag

Ubiquitin is a small eukaryotic protein so named for its cellular abundance and originally recognized for its role as the posttranslational modification (PTM) "tag" condemning substrates to degradation by the 26S proteasome. Since its discovery in the 1970s, protein ubiquitination has also been identified as a key regulatory feature in dozens of non-degradative cellular processes. This myriad of roles illustrates the versatility of ubiquitin as a PTM; however, understanding the cellular and molecular factors that enable discrimination between degradative versus non-degradative ubiquitination events has been a persistent challenge. Here, we discuss recent advances in uncovering how site-specificity - the exact residue that gets modified - modulates distinct protein fates and cellular outcomes with an emphasis on how ubiquitination site specificity regulates proteasomal degradation. We explore recent advances in structural biology, biophysics, and cell biology that have enabled a broader understanding of the role of ubiquitination in altering the dynamics of the target protein, including implications for the design of targeted protein degradation therapeutics.

Two distinct classes of cochaperones compete for the EEVD motif in heat shock protein 70 to tune its chaperone activities

Chaperones of the heat shock protein 70 (Hsp70) family engage in protein-protein interactions with many cochaperones. One "hotspot" for cochaperone binding is the EEVD motif, found at the extreme C terminus of cytoplasmic Hsp70s. This motif is known to bind tetratricopeptide repeat domain cochaperones, such as the E3 ubiquitin ligase CHIP. In addition, the EEVD motif also interacts with a structurally distinct domain that is present in class B J-domain proteins, such as DnaJB4. These observations suggest that CHIP and DnaJB4 might compete for binding to Hsp70's EEVD motif; however, the molecular determinants of such competition are not clear. Using a collection of EEVD-derived peptides, including mutations and truncations, we explored which residues are critical for binding to both CHIP and DnaJB4. These results revealed that some features, such as the C-terminal carboxylate, are important for both interactions. However, CHIP and DnaJB4 also had unique preferences, especially at the isoleucine position immediately adjacent to the EEVD. Finally, we show that competition between these cochaperones is important in vitro, as DnaJB4 limits the ubiquitination activity of the Hsp70-CHIP complex, whereas CHIP suppresses the client refolding activity of the Hsp70-DnaJB4 complex. Together, these data suggest that the EEVD motif has evolved to support diverse protein-protein interactions, such that competition between cochaperones may help guide whether Hsp70-bound proteins are folded or degraded.

How the ends signal the end: Regulation by E3 ubiquitin ligases recognizing protein termini

Specificity of eukaryotic protein degradation is determined by E3 ubiquitin ligases and their selective binding to protein motifs, termed "degrons," in substrates for ubiquitin-mediated proteolysis. From the discovery of the first substrate degron and the corresponding E3 to a flurry of recent studies enabled by modern systems and structural methods, it is clear that many regulatory pathways depend on E3s recognizing protein termini. Here, we review the structural basis for recognition of protein termini by E3s and how this recognition underlies biological regulation. Diverse E3s evolved to harness a substrate's N and/or C terminus (and often adjacent residues as well) in a sequence-specific manner. Regulation is achieved through selective activation of E3s and also through generation of degrons at ribosomes or by posttranslational means. Collectively, many E3 interactions with protein N and C termini enable intricate control of protein quality and responses to cellular signals.

Destabilization of macrophage migration inhibitory factor by 4-IPP reduces NF-κB/P-TEFb complex-mediated c-Myb transcription to suppress osteosarcoma tumourigenesis

BACKGROUND

As an inflammatory factor and oncogenic driver protein, the pleiotropic cytokine macrophage migration inhibitory factor (MIF) plays a crucial role in the osteosarcoma microenvironment. Although 4-iodo-6-phenylpyrimidine (4-IPP) can inactivate MIF biological functions, its anti-osteosarcoma effect and molecular mechanisms have not been investigated. In this study, we identified the MIF inhibitor 4-IPP as a specific double-effector drug for osteosarcoma with both anti-tumour and anti-osteoclastogenic functions.

METHODS

The anti-cancer effects of 4-IPP were evaluated by wound healing assay, cell cycle analysis, colony formation assay, CCK-8 assay, apoptosis analysis, and Transwell migration/invasion assays. Through the application of a luciferase reporter, chromatin immunoprecipitation assays, and immunofluorescence and coimmunoprecipitation analyses, the transcriptional regulation of the NF-κB/P-TEFb complex on c-Myb- and STUB1-mediated proteasome-dependent MIF protein degradation was confirmed. The effect of 4-IPP on tumour growth and metastasis was assessed using an HOS-derived tail vein metastasis model and subcutaneous and orthotopic xenograft tumour models.

RESULTS

In vitro, 4-IPP significantly reduced the proliferation and metastasis of osteosarcoma cells by suppressing the NF-κB pathway. 4-IPP hindered the binding between MIF and CD74 as well as p65. Moreover, 4-IPP inhibited MIF to interrupt the formation of downstream NF-κB/P-TEFb complexes, leading to the down-regulation of c-Myb transcription. Interestingly, the implementation of 4-IPP can mediate small molecule-induced MIF protein proteasomal degradation via the STUB1 E3 ligand. However, 4-IPP still interrupted MIF-mediated communication between osteosarcoma cells and osteoclasts, thus promoting osteoclastogenesis. Remarkably, 4-IPP strongly reduced HOS-derived xenograft osteosarcoma tumourigenesis and metastasis in an in vivo mouse model.

CONCLUSIONS

Our findings demonstrate that the small molecule 4-IPP targeting the MIF protein exerts an anti-osteosarcoma effect by simultaneously inactivating the biological functions of MIF and promoting its proteasomal degradation. Direct destabilization of the MIF protein with 4-IPP may be a promising therapeutic strategy for treating osteosarcoma.

Colloidal Aggregators in Biochemical SARS-CoV-2 Repurposing Screens

To fight COVID-19, much effort has been directed toward in vitro drug repurposing. Here, we investigate the impact of colloidal aggregation, a common screening artifact, in these repurposing campaigns. We tested 56 drugs reported as active in biochemical assays for aggregation by dynamic light scattering and by detergent-based enzyme counter screening; 19 formed colloids at concentrations similar to their literature IC₅₀'s, and another 14 were problematic. From a common repurposing library, we further selected another 15 drugs that had physical properties resembling known aggregators, finding that six aggregated at micromolar concentrations. This study suggests not only that many of the drugs repurposed for SARS-CoV-2 in biochemical assays are artifacts but that, more generally, at screening-relevant concentrations, even drugs can act artifactually via colloidal aggregation. Rapid detection of these artifacts will allow the community to focus on those molecules that genuinely have potential for treating COVID-19.

End-Binding E3 Ubiquitin Ligases Enable Protease Signaling

Post-translational modifications (PTMs) direct the assembly of protein complexes. In this context, proteolysis is a unique PTM because it is irreversible; the hydrolysis of the peptide backbone generates separate fragments bearing a new N and C terminus. Proteolysis can "re-wire" protein-protein interactions (PPIs) via the recruitment of end-binding proteins to new termini. In this review, we focus on the role of proteolysis in specifically creating complexes by recruiting E3 ubiquitin ligases to new N and C termini. These complexes potentiate proteolytic signaling by "erasing" proteolytic modifications. This activity tunes the duration and magnitude of protease signaling events. Recent work has shown that the stepwise process of proteolysis, end-binding by E3 ubiquitin ligases, and fragment turnover is associated with both the nascent N terminus (i.e., N-degron pathways) and the nascent C terminus (i.e., the C-degron pathways). Here, we discuss how these pathways might harmonize protease signaling with protein homeostasis (i.e., proteostasis).

Exploration of Aberrant E3 Ligases Implicated in Alzheimer's Disease and Development of Chemical Tools to Modulate Their Function

The Ubiquitin Proteasome System (UPS) is responsible for the degradation of misfolded or aggregated proteins via a multistep ATP-dependent proteolytic mechanism. This process involves a cascade of ubiquitin (Ub) transfer steps from E1 to E2 to E3 ligase. The E3 ligase transfers Ub to a targeted protein that is brought to the proteasome for degradation. The inability of the UPS to remove misfolded or aggregated proteins due to UPS dysfunction is commonly observed in neurodegenerative diseases, such as Alzheimer's disease (AD). UPS dysfunction in AD drives disease pathology and is associated with the common hallmarks such as amyloid-β (Aβ) accumulation and tau hyperphosphorylation, among others. E3 ligases are key members of the UPS machinery and dysfunction or changes in their expression can propagate other aberrant processes that accelerate AD pathology. The upregulation or downregulation of expression or activity of E3 ligases responsible for these processes results in changes in protein levels of E3 ligase substrates, many of which represent key proteins that propagate AD. A powerful way to better characterize UPS dysfunction in AD and the role of individual E3 ligases is via the use of high-quality chemical tools that bind and modulate specific E3 ligases. Furthermore, through combining gene editing with recent advances in 3D cell culture, in vitro modeling of AD in a dish has become more relevant and possible. These cell-based models of AD allow for study of specific pathways and mechanisms as well as characterization of the role E3 ligases play in driving AD. In this review, we outline the key mechanisms of UPS dysregulation linked to E3 ligases in AD and highlight the currently available chemical modulators. We present several key approaches for E3 ligase ligand discovery being employed with respect to distinct classes of E3 ligases. Where possible, specific examples of the use of cultured neurons to delineate E3 ligase biology have been captured. Finally, utilizing the available ligands for E3 ligases in the design of proteolysis targeting chimeras (PROTACs) to degrade aberrant proteins is a novel strategy for AD, and we explore the prospects of PROTACs as AD therapeutics.

Chaperone-assisted E3 ligase CHIP: A double agent in cancer

The carboxy-terminus of Hsp70-interacting protein (CHIP) is a ubiquitin ligase and co-chaperone belonging to Ubox family that plays a crucial role in the maintenance of cellular homeostasis by switching the equilibrium of the folding-refolding mechanism towards the proteasomal or lysosomal degradation pathway. It links molecular chaperones viz. HSC70, HSP70 and HSP90 with ubiquitin proteasome system (UPS), acting as a quality control system. CHIP contains charged domain in between N-terminal tetratricopeptide repeat (TPR) and C-terminal Ubox domain. TPR domain interacts with the aberrant client proteins via chaperones while Ubox domain facilitates the ubiquitin transfer to the client proteins for ubiquitination. Thus, CHIP is a classic molecule that executes ubiquitination for degradation of client proteins. Further, CHIP has been found to be indulged in cellular differentiation, proliferation, metastasis and tumorigenesis. Additionally, CHIP can play its dual role as a tumor suppressor as well as an oncogene in numerous malignancies, thus acting as a double agent. Here, in this review, we have reported almost all substrates of CHIP established till date and classified them according to the hallmarks of cancer. In addition, we discussed about its architectural alignment, tissue specific expression, sub-cellular localization, folding-refolding mechanisms of client proteins, E4 ligase activity, normal physiological roles, as well as involvement in various diseases and tumor biology. Further, we aim to discuss its importance in HSP90 inhibitors mediated cancer therapy. Thus, this report concludes that CHIP may be a promising and worthy drug target towards pharmaceutical industry for drug development.

Tau Protein Interaction Partners and Their Roles in Alzheimer's Disease and Other Tauopathies

Tau protein plays a critical role in the assembly, stabilization, and modulation of microtubules, which are important for the normal function of neurons and the brain. In diseased conditions, several pathological modifications of tau protein manifest. These changes lead to tau protein aggregation and the formation of paired helical filaments (PHF) and neurofibrillary tangles (NFT), which are common hallmarks of Alzheimer's disease and other tauopathies. The accumulation of PHFs and NFTs results in impairment of physiological functions, apoptosis, and neuronal loss, which is reflected as cognitive impairment, and in the late stages of the disease, leads to death. The causes of this pathological transformation of tau protein haven't been fully understood yet. In both physiological and pathological conditions, tau interacts with several proteins which maintain their proper function or can participate in their pathological modifications. Interaction partners of tau protein and associated molecular pathways can either initiate and drive the tau pathology or can act neuroprotective, by reducing pathological tau proteins or inflammation. In this review, we focus on the tau as a multifunctional protein and its known interacting partners active in regulations of different processes and the roles of these proteins in Alzheimer's disease and tauopathies.

The structure of an Hsp90-immunophilin complex reveals cochaperone recognition of the client maturation state

The Hsp90 chaperone promotes folding and activation of hundreds of client proteins in the cell through an ATP-dependent conformational cycle guided by distinct cochaperone regulators. The FKBP51 immunophilin binds Hsp90 with its tetratricopeptide repeat (TPR) domain and catalyzes peptidyl-prolyl isomerase (PPIase) activity during folding of kinases, nuclear receptors, and tau. Here we determined the cryoelectron microscopy (cryo-EM) structure of the human Hsp90:FKBP51:p23 complex to 3.3 Å, which, together with mutagenesis and crosslinking analyses, reveals the basis for cochaperone binding to Hsp90 during client maturation. A helix extension in the TPR functions as a key recognition element, interacting across the Hsp90 C-terminal dimer interface presented in the closed, ATP conformation. The PPIase domain is positioned along the middle domain, adjacent to Hsp90 client binding sites, whereas a single p23 makes stabilizing interactions with the N-terminal dimer. With this architecture, FKBP51 is positioned to act on specific client residues presented during Hsp90-catalyzed remodeling.

Tying up loose ends: the N-degron and C-degron pathways of protein degradation

Selective protein degradation by the ubiquitin-proteasome system (UPS) is thought to be governed primarily by the recognition of specific motifs — degrons — present in substrate proteins. The ends of proteins — the N- and C-termini – have unique properties, and an important subset of protein–protein interactions involve the recognition of free termini. The first degrons to be discovered were located at the extreme N-terminus of proteins, a finding which initiated the study of the N-degron (formerly N-end rule) pathways, but only in the last few years has it emerged that a diverse set of C-degron pathways target analogous degron motifs located at the extreme C-terminus of proteins. In this minireview we summarise the N-degron and C-degron pathways currently known to operate in human cells, focussing primarily on those that have been discovered in recent years. In each case we describe the cellular machinery responsible for terminal degron recognition, and then consider some of the functional roles of terminal degron pathways. Altogether, a broad spectrum of E3 ubiquitin ligases mediate the recognition of a diverse array of terminal degron motifs; these degradative pathways have the potential to influence a wide variety of cellular functions.

An E3 ligase guide to the galaxy of small-molecule-induced protein degradation

Induced protein degradation accomplishes elimination, rather than inhibition, of pathological proteins. Key to the success of this novel therapeutic modality is the modification of proteins with ubiquitin chains, which is brought about by molecular glues or bivalent compounds that induce proximity between the target protein and an E3 ligase. The human genome encodes ∼600 E3 ligases that differ widely in their structures, catalytic mechanisms, modes of regulation, and physiological roles. While many of these enzymes hold great promise for drug discovery, few have been successfully engaged by small-molecule degraders. Here, we review E3 ligases that are being used for induced protein degradation. Based on these prior successes and our growing understanding of the biology and biochemistry of E3 ligases, we propose new ubiquitylation enzymes that can be harnessed for drug discovery to firmly establish induced protein degradation as a specific and efficient therapeutic approach.

CHIP-mediated hyperubiquitylation of tau promotes its self-assembly into the insoluble tau filaments

The tau protein is a highly soluble and natively unfolded protein. Under pathological conditions, tau undergoes multiple post-translational modifications (PTMs) and conformational changes to form insoluble filaments, which are the proteinaceous signatures of tauopathies. To dissect the crosstalk among tau PTMs during the aggregation process, we phosphorylated and ubiquitylated recombinant tau in vitro using GSK3β and CHIP, respectively. The resulting phospho-ub-tau contained conventional polyubiquitin chains with lysine 48 linkages, sufficient for proteasomal degradation, whereas unphosphorylated ub-tau species retained only one-three ubiquitin moieties. Mass-spectrometric analysis of in vitro reconstituted phospho-ub-tau revealed seven additional ubiquitylation sites, some of which are known to stabilize tau protofilament stacking in the human brain with tauopathy. When the ubiquitylation reaction was prolonged, phospho-ub-tau transformed into insoluble hyperubiquitylated tau species featuring fibrillar morphology and in vitro seeding activity. We developed a small-molecule inhibitor of CHIP through biophysical screening; this effectively suppressed tau ubiquitylation in vitro and delayed its aggregation in cultured cells including primary cultured neurons. Our biochemical findings point to a "multiple-hit model," where sequential events of tau phosphorylation and hyperubiquitylation function as a key driver of the fibrillization process, thus indicating that targeting tau ubiquitylation may be an effective strategy to alleviate the course of tauopathies.

Molecular basis for arginine C-terminal degron recognition by Cul2FEM1 E3 ligase

Degrons are elements within protein substrates that mediate the interaction with specific degradation machineries to control proteolysis. Recently, a few classes of C-terminal degrons (C-degrons) that are recognized by dedicated cullin-RING ligases (CRLs) have been identified. Specifically, CRL2 using the related substrate adapters FEM1A/B/C was found to recognize C degrons ending with arginine (Arg/C-degron). Here, we uncover the molecular mechanism of Arg/C-degron recognition by solving a subset of structures of FEM1 proteins in complex with Arg/C-degron-bearing substrates. Our structural research, complemented by binding assays and global protein stability (GPS) analyses, demonstrates that FEM1A/C and FEM1B selectively target distinct classes of Arg/C-degrons. Overall, our study not only sheds light on the molecular mechanism underlying Arg/C-degron recognition for precise control of substrate turnover, but also provides valuable information for development of chemical probes for selectively regulating proteostasis.

Praja2 suppresses the growth of gastric cancer by ubiquitylation of KSR1 and inhibiting MEK-ERK signal pathways

Gastric cancer (GC) is a common malignant tumor, which has a high incidence and fatality. Therefore, it is important to clarify the molecular mechanism of the occurrence and development for GC and to find more effective treatments and targeted drugs. In this study, we found that the kinase suppressor of Ras1 (KSR1) was increased in GC tissues and cell lines. Silencing of KSR1 inhibited the proliferation, migration and invasion of MKN-45 cells. E3 ligase Praja2 was downregulated in GC tissues and cell lines. In addition, praja2 promoted ubiquitylation of KSR1, but inhibited MEK-ERK signal pathways. Functional analysis indicated overexpression of praja2 inhibited the proliferation, migration and invasion of MKN-45 cells, while MG132 or FGF2 treatment removed the inhibitory effects of praja2 on GC progression. In vivo tumorigenesis experiments indicated praja2 inhibited tumor growth via KSR1-MEK-ERK axis. In conclusion, praja2 promoted the ubiquitylation and degradation of KSR1, which disturbed MEK- ERK signaling and inhibited GC progression. Our study might provide a novel target for GC clinical treatment.

Emerging Evidences for an Implication of the Neurodegeneration-Associated Protein TAU in Cancer

Neurodegenerative disorders and cancer may appear unrelated illnesses. Yet, epidemiologic studies indicate an inverse correlation between their respective incidences for specific cancers. Possibly explaining these findings, increasing evidence indicates that common molecular pathways are involved, often in opposite manner, in the pathogenesis of both disease families. Genetic mutations in the MAPT gene encoding for TAU protein cause an inherited form of frontotemporal dementia, a neurodegenerative disorder, but also increase the risk of developing cancer. Assigning TAU at the interface between cancer and neurodegenerative disorders, two major aging-linked disease families, offers a possible clue for the epidemiological observation inversely correlating these human illnesses. In addition, the expression level of TAU is recognized as a prognostic marker for cancer, as well as a modifier of cancer resistance to chemotherapy. Because of its microtubule-binding properties, TAU may interfere with the mechanism of action of taxanes, a class of chemotherapeutic drugs designed to stabilize the microtubule network and impair cell division. Indeed, a low TAU expression is associated to a better response to taxanes. Although TAU main binding partners are microtubules, TAU is able to relocate to subcellular sites devoid of microtubules and is also able to bind to cancer-linked proteins, suggesting a role of TAU in modulating microtubule-independent cellular pathways associated to oncogenesis. This concept is strengthened by experimental evidence linking TAU to P53 signaling, DNA stability and protection, processes that protect against cancer. This review aims at collecting literature data supporting the association between TAU and cancer. We will first summarize the evidence linking neurodegenerative disorders and cancer, then published data supporting a role of TAU as a modifier of the efficacy of chemotherapies and of the oncogenic process. We will finish by addressing from a mechanistic point of view the role of TAU in de-regulating critical cancer pathways, including the interaction of TAU with cancer-associated proteins.

Prion-Like Propagation Mechanisms in Tauopathies and Traumatic Brain Injury: Challenges and Prospects

The accumulation of tau protein in the form of filamentous aggregates is a hallmark of many neurodegenerative diseases such as Alzheimer's disease (AD) and chronic traumatic encephalopathy (CTE). These dementias share traumatic brain injury (TBI) as a prominent risk factor. Tau aggregates can transfer between cells and tissues in a "prion-like" manner, where they initiate the templated misfolding of normal tau molecules. This enables the spread of tau pathology to distinct parts of the brain. The evidence that tauopathies spread via prion-like mechanisms is considerable, but work detailing the mechanisms of spread has mostly used in vitro platforms that cannot fully reveal the tissue-level vectors or etiology of progression. We review these issues and then briefly use TBI and CTE as a case study to illustrate aspects of tauopathy that warrant further attention in vivo. These include seizures and sleep/wake disturbances, emphasizing the urgent need for improved animal models. Dissecting these mechanisms of tauopathy progression continues to provide fresh inspiration for the design of diagnostic and therapeutic approaches.

Deep profiling of protease substrate specificity enabled by dual random and scanned human proteome substrate phage libraries

Proteolysis is a major posttranslational regulator of biology inside and outside of cells. Broad identification of optimal cleavage sites and natural substrates of proteases is critical for drug discovery and to understand protease biology. Here, we present a method that employs two genetically encoded substrate phage display libraries coupled with next generation sequencing (SPD-NGS) that allows up to 10,000-fold deeper sequence coverage of the typical six- to eight-residue protease cleavage sites compared to state-of-the-art synthetic peptide libraries or proteomics. We applied SPD-NGS to two classes of proteases, the intracellular caspases, and the ectodomains of the sheddases, ADAMs 10 and 17. The first library (Lib 10AA) allowed us to identify 10⁴ to 10⁵ unique cleavage sites over a 1,000-fold dynamic range of NGS counts and produced consensus and optimal cleavage motifs based position-specific scoring matrices. A second SPD-NGS library (Lib hP), which displayed virtually the entire human proteome tiled in contiguous 49 amino acid sequences with 25 amino acid overlaps, enabled us to identify candidate human proteome sequences. We identified up to 10⁴ natural linear cut sites, depending on the protease, and captured most of the examples previously identified by proteomics and predicted 10- to 100-fold more. Structural bioinformatics was used to facilitate the identification of candidate natural protein substrates. SPD-NGS is rapid, reproducible, simple to perform and analyze, inexpensive, and renewable, with unprecedented depth of coverage for substrate sequences, and is an important tool for protease biologists interested in protease specificity for specific assays and inhibitors and to facilitate identification of natural protein substrates.

Luminescence complementation assay for measurement of binding to protein C-termini in live cells

Protein-protein interactions (PPIs) involving the extreme C-terminus serve important scaffolding and regulatory functions. Here, we leveraged NanoBiT technology to build a luminescent complementation assay for use in studying this subcategory of PPI. As a model system, we fused one component of NanoBiT to the disordered C-terminus of heat shock protein (Hsp70) and the other to its binding partner, the tetratricopeptide repeat (TPR) domain of CHIP/STUB1. We found that HEK293 cells that stably express these chimeras under a doxycycline promoter produced a robust luminescence signal. This signal was sensitive to mutations and it was further tuned by the expression of competitive C-termini. Using this system, we identified a promising, membrane permeable inhibitor of the Hsp70-CHIP interaction. More broadly, we anticipate that NanoBiT is well-suited for studying PPIs that involve C-termini.

Hsp70:CHIP Ubiquitinates Dysfunctional but Not Native Neuronal NO Synthase

Heat shock protein (Hsp) 70 modulators are being developed to enhance the removal of toxic proteins in a variety of protein misfolding diseases. In the course of our studies on neuronal nitric oxide synthase (nNOS), a client of the Hsp90 and Hsp70 chaperone system, we have established that inactivation of nNOS by heme or tetrahydrobiopterin (BH₄) alteration and loss triggers ubiquitination by the Hsp70-associated E3 ligase c-terminus of Hsp70-interacting protein (CHIP) and subsequent degradation in cells. Although in cells Hsp90 and Hsp70 work together to maintain protein quality control, in this study, we specifically developed an assay to assess the selectivity of the Hsp70:CHIP complex for inactivated nNOS. We developed a highly sensitive ELISA to measure Hsp70:CHIP-dependent nNOS ubiquitination without interference from direct ubiquitination by CHIP, as evidenced by Bcl-2 associated athanogene 1-M completely abolishing ubiquitination. To further validate the assay we demonstrated, JG-98, a rhodocyanin compound that acts on Hsp70 but not its inactive structural analog JG-258, enhances the ubiquitination of nNOS 3-fold. Utilizing this assay, we have shown that the Hsp70:CHIP complex preferentially ubiquitinates heme-deficient nNOS (apo-nNOS) over heme-containing nNOS (holo-nNOS). Moreover, depletion of nNOS-bound BH₄ triggers ubiquitination of holo-nNOS by the Hsp70:CHIP complex. Most importantly, JG-98 was shown to enhance the ubiquitination of only dysfunctional nNOS while leaving the native functional nNOS untouched. Thus, the finding that enhancing Hsp70:CHIP-mediated ubiquitination does not affect native proteins has important pharmacological implications. Moreover, development of a facile in vitro method for Hsp70:CHIP-mediated ubiquitination will be beneficial for testing other Hsp70 modulators. SIGNIFICANCE STATEMENT: The heat shock protein 70 (Hsp70):c-terminus of Hsp70-interacting protein (CHIP) complex facilitates the ubiquitination and subsequent degradation of several hundred-client proteins, and activation of Hsp70 has been suggested as a therapeutic strategy to enhance the degradation of disease-causing proteins. The current study shows that the pharmacological activation of Hsp70 enhances the ubiquitination of dysfunctional but not native nNOS, and it suggests that this therapeutic strategy will likely be highly selective.

HSP70 Multi-Functionality in Cancer

The 70-kDa heat shock proteins (HSP70s) are abundantly present in cancer, providing malignant cells selective advantage by suppressing multiple apoptotic pathways, regulating necrosis, bypassing cellular senescence program, interfering with tumor immunity, promoting angiogenesis and supporting metastasis. This direct involvement of HSP70 in most of the cancer hallmarks explains the phenomenon of cancer "addiction" to HSP70, tightly linking tumor survival and growth to the HSP70 expression. HSP70 operates in different states through its catalytic cycle, suggesting that it can multi-function in malignant cells in any of these states. Clinically, tumor cells intensively release HSP70 in extracellular microenvironment, resulting in diverse outcomes for patient survival. Given its clinical significance, small molecule inhibitors were developed to target different sites of the HSP70 machinery. Furthermore, several HSP70-based immunotherapy approaches were assessed in clinical trials. This review will explore different roles of HSP70 on cancer progression and emphasize the importance of understanding the flexibility of HSP70 nature for future development of anti-cancer therapies.