Examining Targeted Protein Degradation from Physiological and Analytical Perspectives: Enabling Translation between Cells and Subjects

Metabolomic and proteomic applications to exercise biomedicine

Objectives

'OMICs encapsulates study of scaled data acquisition, at the levels of DNA, RNA, protein, and metabolite species. The broad objectives of OMICs in biomedical exercise research are multifarious, but commonly relate to biomarker development and understanding features of exercise adaptation in health, ageing and metabolic diseases.

Methods

This field is one of exponential technical (i.e., depth of feature coverage) and scientific (i.e., in health, metabolic conditions and ageing, multi-OMICs) progress adopting targeted and untargeted approaches.

Results

Key findings in exercise biomedicine have led to the identification of OMIC features linking to heritability or adaptive responses to exercise e.g., the forging of GWAS/proteome/metabolome links to cardiovascular fitness and metabolic health adaptations. The recent addition of stable isotope tracing to proteomics ('dynamic proteomics') and metabolomics ('fluxomics') represents the next phase of state-of-the-art in 'OMICS.

Conclusions

These methods overcome limitations associated with point-in-time 'OMICs and can be achieved using substrate-specific tracers or deuterium oxide (D2O), depending on the question; these methods could help identify how individual protein turnover and metabolite flux may explain exercise responses. We contend application of these methods will shed new light in translational exercise biomedicine.

Targeted Protein Degradation: Advances, Challenges, and Prospects for Computational Methods

The therapeutic approach of targeted protein degradation (TPD) is gaining momentum due to its potentially superior effects compared with protein inhibition. Recent advancements in the biotech and pharmaceutical sectors have led to the development of compounds that are currently in human trials, with some showing promising clinical results. However, the use of computational tools in TPD is still limited, as it has distinct characteristics compared with traditional computational drug design methods. TPD involves creating a ternary structure (protein-degrader-ligase) responsible for the biological function, such as ubiquitination and subsequent proteasomal degradation, which depends on the spatial orientation of the protein of interest (POI) relative to E2-loaded ubiquitin. Modeling this structure necessitates a unique blend of tools initially developed for small molecules (e.g., docking) and biologics (e.g., protein-protein interaction modeling). Additionally, degrader molecules, particularly heterobifunctional degraders, are generally larger than conventional small molecule drugs, leading to challenges in determining drug-like properties like solubility and permeability. Furthermore, the catalytic nature of TPD makes occupancy-based modeling insufficient. TPD consists of multiple interconnected yet distinct steps, such as POI binding, E3 ligase binding, ternary structure interactions, ubiquitination, and degradation, along with traditional small molecule properties. A comprehensive set of tools is needed to address the dynamic nature of the induced proximity ternary complex and its implications for ubiquitination. In this Perspective, we discuss the current state of computational tools for TPD. We start by describing the series of steps involved in the degradation process and the experimental methods used to characterize them. Then, we delve into a detailed analysis of the computational tools employed in TPD. We also present an integrative approach that has proven successful for degrader design and its impact on project decisions. Finally, we examine the future prospects of computational methods in TPD and the areas with the greatest potential for impact.

Measurement of protein in vivo turnover rate with metabolic labeling using LC-MS

Understanding the protein dynamics of a drug target is important for pharmaceutical research because it provides insight into drug design, target engagement, pharmacodynamics and drug efficacy. Nonradioactive isotope labeling has been the method of choice for protein turnover measurement thanks to the advancement of high-resolution mass spectrometry. While the changes in proteome in cell cultures can be monitored precisely, as the culture media can be completely replaced with ² H-, ¹⁵ N- or ¹³ C-labeled essential amino acids, quantifying rates of protein synthesis in vivo is more challenging. The amount of isotope tracer that can be administered into the body is relatively small compared with the existing protein, thus requiring more sensitive detection, and the precursor-product labeling relationship is more complicated to interpret. The purpose of this review is to provide an overview of the principles of in vivo protein turnover studies using deuterium water (² H₂ O) with an emphasis on targeted protein analysis by hybrid LC-MS assay platforms. The pursuit of these opportunities will facilitate drug discovery and research in preclinical and clinical stages.

Translational PK-PD for targeted protein degradation

Targeted protein degradation has emerged from the chemical biology toolbox as one of the most exciting areas for novel therapeutic development across the pharmaceutical industry. The ability to induce the degradation, and not just inhibition, of target proteins of interest (POIs) with high potency and selectivity is a particularly attractive property for a protein degrader therapeutic. However, the physicochemical properties and mechanism of action for protein degraders can lead to unique pharmacokinetic (PK) and pharmacodynamic (PD) properties relative to traditional small molecule drugs, requiring a shift in perspective for translational pharmacology. In this review, we provide practical insights for building the PK-PD understanding of protein degraders in the context of translational drug development through the use of quantitative mathematical frameworks and standard experimental assays. Published datasets describing protein degrader pharmacology are used to illustrate the applicability of these insights. The learnings are consolidated into a translational PK-PD roadmap for targeted protein degradation that can enable a systematic, rational design workflow for protein degrader therapeutics.

Protein turnover models for LC-MS data of heavy water metabolic labeling

Protein turnover is vital for cellular functioning and is often associated with the pathophysiology of a variety of diseases. Metabolic labeling with heavy water followed by liquid chromatography coupled to mass spectrometry is a powerful tool to study in vivo protein turnover in high throughput and large scale. Heavy water is a cost-effective and easy to use labeling agent. It labels all nonessential amino acids. Due to its toxicity in high concentrations (20% or higher), small enrichments (8% or smaller) of heavy water are used with most organisms. The low concentration results in incomplete labeling of peptides/proteins. Therefore, the data processing is more challenging and requires accurate quantification of labeled and unlabeled forms of a peptide from overlapping mass isotopomer distributions. The work describes the bioinformatics aspects of the analysis of heavy water labeled mass spectral data, available software tools and current challenges and opportunities.

Recent Developments in PROTAC-Mediated Protein Degradation: From Bench to Clinic

Proteolysis-targeting chimeras (PROTACs), an emerging paradigm-shifting technology, hijacks the ubiquitin-proteasome system for targeted protein degradation. PROTACs induce ternary complexes between an E3 ligase and POI, and this induced proximity leads to polyUb chain formation on substrates and eventual proteasomal-mediated POI degradation. PROTACs have shown great therapeutic potential by degrading many disease-causing proteins, such as the androgen receptor and BRD4. The PROTAC technology has advanced significantly in the last two decades, with the repertoire of PROTAC targets increased tremendously. Herein, we describe recent developments of PROTAC technology, focusing on mechanistic and kinetic studies, pharmacokinetic study, spatiotemporal control of PROTACs, covalent PROTACs, resistance to PROTACs, and new E3 ligands.

Proteome dynasmics and bioinformatics reveal major alterations in the turnover rate of functionally related cardiac and plasma proteins in a dog model of congestive heart failure

Protein pool turnover is a critically important cellular homeostatic component, yet it has been little explored in the context of heart failure (HF) pathophysiology. We employed in vivo ²H labeling/ proteome dynamics for non-biased discovery of turnover alterations involving functionally linked cardiac and plasma proteins in canine tachypacing-induced HF, an established preclinical model of dilated cardiomyopathy. Compared to control, dogs with congestive HF displayed bidirectional turnover changes of 28 cardiac proteins, i.e. reduced half-life of several key enzymes involved in glycolysis, homocysteine metabolism and glycogenesis, and increased half-life of proteins involved in proteolysis. Changes in plasma proteins were more modest: only 5 proteins, involved in various functions including proteolysis inhibition, hemoglobin, calcium and ferric-iron binding, displayed increased or decreased turnover rates. In other dogs undergoing cardiac tachypacing, we infused for 2 weeks the myokine Follistatin-like protein 1 (FSTL1), known for its ameliorative effects on HF-induced alterations. Proteome dynamics proved very sensitive in detecting the partial or complete prevention, by FSTL1, of cardiac and plasma protein turnover alterations. In conclusion, our study unveiled, for the first time in a large mammal, numerous HF-related alterations that may serve as the basis for future mechanistic research and/or as conceptually new molecular markers.

PROTACs, molecular glues and bifunctionals from bench to bedside: Unlocking the clinical potential of catalytic drugs

The vast majority of currently marketed drugs rely on small molecules with an 'occupancy-driven' mechanism of action (MOA). Therefore, the efficacy of these therapeutics depends on a high degree of target engagement, which often requires high dosages and enhanced drug exposure at the target site, thus increasing the risk of off-target toxicities (Churcher, 2018 [1]). Although small molecule drugs have been successfully used as treatments for decades, tackling a variety of disease-relevant targets with a defined binding site, many relevant therapeutic targets remain challenging to drug due, for example, to lack of well-defined binding pockets or large protein-protein interaction (PPI) interfaces which resist interference (Dang et al., 2017 [2]). In the quest for alternative therapeutic approaches to address different pathologies and achieve enhanced efficacy with reduced side effects, ligand-induced targeted protein degradation (TPD) has gained the attention of many research groups both in academia and in industry in the last two decades. This therapeutic modality represents a novel paradigm compared to conventional small-molecule inhibitors. To pursue this strategy, heterobifunctional small molecule degraders, termed PROteolysis TArgeting Chimeras (PROTACs) have been devised to artificially redirect a protein of interest (POI) to the cellular protein homeostasis machinery for proteasomal degradation (Chamberlain et al., 2019 [3]). In this chapter, the development of PROTACs will first be discussed providing a historical perspective in parallel to the experimental progress made to understand this novel therapeutic modality. Furthermore, common strategies for PROTAC design, including assays and troubleshooting tips will be provided for the reader, before presenting a compendium of all PROTAC targets reported in the literature to date. Due to the recent advancement of these molecules into clinical trials, consideration of pharmacokinetics and pharmacodynamic properties will be introduced, together with the biotech landscape that has developed from the success of PROTACs. Finally, an overview of subsequent strategies for targeted protein degradation will be presented, concluding with further scientific quests triggered by the invention of PROTACs.