Cation-independent mannose 6-phosphate receptor: From roles and functions to targeted therapies

The Alzheimer's Disease Gene SORL1 Regulates Lysosome Function in Human Microglia

The SORL1 gene encodes the sortilin-related receptor protein SORLA, a sorting receptor that regulates endo-lysosomal trafficking of various substrates. Loss of function variants in SORL1 are causative for Alzheimer's disease (AD) and decreased expression of SORLA has been repeatedly observed in human AD brains. SORL1 is highly expressed in the central nervous system, including in microglia, the tissue-resident immune cells of the brain. Loss of SORLA leads to enlarged lysosomes in hiPSC-derived microglia-like cells (hMGLs). However, how SORLA deficiency contributes to lysosomal dysfunction in microglia and how this contributes to AD pathogenesis is not known. In this study, we show that loss of SORLA results in decreased lysosomal degradation and lysosomal enzyme activity due to altered trafficking of lysosomal enzymes in hMGLs. Phagocytic uptake of fibrillar amyloid beta 1-42 and synaptosomes is increased in SORLA-deficient hMGLs, but due to reduced lysosomal degradation, these substrates aberrantly accumulate in lysosomes. An alternative mechanism of lysosome clearance, lysosomal exocytosis, is also impaired in SORL1-deficient microglia, which may contribute to an altered immune response. Overall, these data suggest that SORLA has an important role in the proper trafficking of lysosomal hydrolases in hMGLs, which is critical for microglial function. This further substantiates the microglial endo-lysosomal network as a potential novel pathway through which SORL1 may increase AD risk and contribute to the development of AD. Additionally, our findings may inform the development of novel lysosome and microglia-associated drug targets for AD.

Mannose-6-phosphate-PEG-lipid conjugates improve liposomal uptake

Targeted liposomes are a keystone of nanomedicine, offering a precise and efficient means to deliver therapeutic agents directly to diseased tissues or cells. By incorporating targeting ligands on their surface, liposomes enhance the specificity of drug delivery, improving efficacy and reducing toxicity. Mannose-6-phosphate (M6P) is a crucial molecular tag for internalization and intracellular sorting of macromolecular structures to the lysosome. Taking advantage of this mechanism, we designed and developed liposomal systems to enhance therapeutic delivery to the lysosomes. The synthesized M6P-based targeting molecules were covalently coupled to a phospholipid using a polyethylene glycol (PEG) linker. The prepared ligands were successfully incorporated into the liposomes, yielding a size of roughly 100 nm and a zeta potential of around -40 mV. Incorporating the M6P-based ligand enhances the internalization of liposomes in a concentration-dependent manner, increasing uptake by up to 14-fold in several tested cell lines. In contrast, structurally similar monosaccharides and equally charged ligands failed to replicate this effect, highlighting the specificity of M6P-mediated internalization. Our studies demonstrate that M6P-mediated uptake predominantly occurs via a clathrin-mediated pathway, and once internalized, 72 % of the M6P-coated liposomes are associated with the lysosomal compartment. This study highlights the potential of M6P-based liposomal carriers as a modular platform for targeted lysosomal delivery, offering a promising therapeutic approach for lysosomal storage diseases.

Protein-Based Degraders: From Chemical Biology Tools to Neo-Therapeutics

The nascent field of targeted protein degradation (TPD) could revolutionize biomedicine due to the ability of degrader molecules to selectively modulate disease-relevant proteins. A key limitation to the broad application of TPD is its dependence on small-molecule ligands to target proteins of interest. This leaves unstructured proteins or those lacking defined cavities for small-molecule binding out of the scope of many TPD technologies. The use of proteins, peptides, and nucleic acids (otherwise known as "biologics") as the protein-targeting moieties in degraders addresses this limitation. In the following sections, we provide a comprehensive and critical review of studies that have used proteins and peptides to mediate the degradation and hence the functional control of otherwise challenging disease-relevant protein targets. We describe existing platforms for protein/peptide-based ligand identification and the drug delivery systems that might be exploited for the delivery of biologic-based degraders. Throughout the Review, we underscore the successes, challenges, and opportunities of using protein-based degraders as chemical biology tools to spur discoveries, elucidate mechanisms, and act as a new therapeutic modality.

Journey of PROTAC: From Bench to Clinical Trial and Beyond

Proteolysis-targeting chimeras (PROTACs) represent a transformative advancement in drug discovery, offering a method to degrade specific intracellular proteins. Unlike traditional inhibitors, PROTACs are bifunctional molecules that target proteins for elimination, enabling the potential treatment of previously "undruggable" proteins. This concept, pioneered by Crews and his team, introduced the use of small molecules to link a target protein to an E3 ubiquitin ligase, inducing ubiquitination and subsequent degradation of the target protein. By promoting protein degradation rather than merely inhibiting function, PROTACs present a novel therapeutic strategy with enhanced specificity and effectiveness, especially in areas such as cancer and neurodegenerative diseases. Since their initial discovery, the field of PROTAC research has rapidly expanded with numerous PROTACs now designed to target a wide range of disease-relevant proteins. The substantial research, investment, and collaboration across academia and the pharmaceutical industry reflect the growing interest in PROTACs. This Review discusses the journey of PROTACs from initial discovery to clinical trials, highlighting advancements and challenges. Additionally, recent developments in fluorescent and photogenic PROTACs, used for real-time tracking of protein degradation, are presented, showcasing the evolving potential of PROTACs in targeted therapy.

Lysosome-targeting chimeras containing an endocytic signaling motif trigger endocytosis and lysosomal degradation of cell-surface proteins

Lysosome-targeting degradation technologies have emerged as a promising therapeutic strategy for the selective depletion of target extracellular and cell-surface proteins by harnessing a cell-surface effector protein such as lysosome-targeting receptors (LTRs) or transmembrane E3 ligases that direct lysosomal degradation. We recently developed a lysosome-targeting degradation platform termed signal-mediated lysosome-targeting chimeras (SignalTACs) that functions independently of an LTR or E3 ligase; these are engineered fusion proteins comprising a target binder, a cell-penetrating peptide (CPP), and a lysosomal sorting signal motif (P1). Herein, we present the next-generation SignalTACs containing a single endocytic signal that bypasses the need for a CPP. We demonstrate that the fusion with a 10-amino acid endocytic signaling peptide (P3) derived from the cation-independent mannose-6-phosphate receptor (CI-M6PR) induces robust internalization and lysosomal degradation of the target protein. The P3-based SignalTAC exhibited enhanced antitumor efficacy compared to the parent antibody. We envision that the fusion of the endocytic signaling peptide P3 to a target binder may allow the construction of an effective degrader for membrane-associated targets. Furthermore, mechanistic studies identified different drivers for the activities of the P3- and P1-based SignalTACs, which is expected to provide crucial insights toward the harnessing of the intrinsic signaling pathways to direct protein trafficking and degradation.

Programming DNA Nanoassemblies into Polyvalent Lysosomal Degraders for Potent Degradation of Pathogenic Membrane Proteins

Lysosome-targeting chimera (LYTAC) shows great promise for protein-based therapeutics by targeted degradation of disease-associated membrane or extracellular proteins, yet its efficiency is constrained by the limited binding affinity between LYTAC reagents and designated proteins. Here, we established a programmable and multivalent LYTAC system by tandem assembly of DNA into a high-affinity protein degrader, a heterodimer aptamer nanostructure targeting both pathogenic membrane protein and lysosome-targeting receptor (insulin-like growth factor 2 receptor, IGF2R) with adjustable spatial distribution or organization pattern. The DNA-based multivalent LYTACs showed enhanced efficacy in removing immune-checkpoint protein programmable death-ligand 1 (PD-L1) and vascular endothelial growth factor receptor 2 (VEGFR2) in tumor cell membrane that respectively motivated a significant increase in T cell activity and a potent effect on cancer cell growth inhibition. With high programmability and versatility, this multivalent LYTAC system holds considerable promise for realizing protein therapeutics with enhanced activity.

Targeting Neurological Aspects of Mucopolysaccharidosis Type II: Enzyme Replacement Therapy and Beyond

Mucopolysaccharidosis type II (MPS II) is a rare, pediatric, neurometabolic disorder due to the lack of activity of the lysosomal hydrolase iduronate 2-sulfatase (IDS), normally degrading heparan sulfate and dermatan sulfate within cell lysosomes. The deficit of activity is caused by mutations affecting the IDS gene, leading to the pathological accumulation of both glycosaminoglycans in the lysosomal compartment and in the extracellular matrix of most body districts. Although a continuum of clinical phenotypes is described, two main forms are commonly recognized-attenuated and severe-the latter being characterized by an earlier and faster clinical progression and by a progressive impairment of central nervous system (CNS) functions. However, attenuated forms have also been recently described as presenting some neurological involvement, although less deep, such as deficits of attention and hearing loss. The main treatment for the disease is represented by enzyme replacement therapy (ERT), applied in several countries since 2006, which, albeit showing partial efficacy on some peripheral organs, exhibited a very poor efficacy on bones and heart, and a total inefficacy on CNS impairment, due to the inability of the recombinant enzyme to cross the blood-brain barrier (BBB). Together with ERT, whose design enhancements, performed in the last few years, allowed a possible brain penetration of the drug through the BBB, other therapeutic approaches aimed at targeting CNS involvement in MPS II were proposed and evaluated in the last decades, such as intrathecal ERT, intracerebroventricular ERT, ex vivo gene therapy, or adeno-associated viral vector (AAV) gene therapy. The aim of this review is to summarize the main clinical aspects of MPS II in addition to current therapeutic options, with particular emphasis on the neurological ones and on the main CNS-targeted therapeutic approaches explored through the years.

Subcellular targeting strategies for protein and peptide delivery

Cytosolic delivery of proteins and peptides provides opportunities for effective disease treatment, as they can specifically modulate intracellular processes. However, most of protein-based therapeutics only have extracellular targets and are cell-membrane impermeable due to relatively large size and hydrophilicity. The use of organelle-targeting strategy offers great potential to overcome extracellular and cell membrane barriers, and enables localization of protein and peptide therapeutics in the organelles. Although progresses have been made in the recent years, organelle-targeted protein and peptide delivery is still challenging and under exploration. We reviewed recent advances in subcellular targeted delivery of proteins/peptides with a focus on targeting mechanisms and strategies, and highlight recent examples of active and passive organelle-specific protein and peptide delivery systems. This emerging platform could open a new avenue to develop more effective protein and peptide therapeutics.

Therapeutic antibody engineering for efficient targeted degradation of membrane proteins in lysosomes

Targeted degradation of pathological proteins is a promising approach to enhance the effectiveness of therapeutic monoclonal antibodies (mAbs) in cancer therapy. In this study, we demonstrate that this objective can be efficiently achieved by the grafting of mannose 6-phosphate analogues called AMFAs2 onto the therapeutic antibodies trastuzumab and cetuximab, both directed against membrane antigens. The grafting of AMFAs confers to these antibodies the novel property of being internalized via the mannose 6-phosphate receptor (M6PR) pathway. AMFA conjugation to these mAbs significantly increases their cellular uptake and leads to enhanced degradation of the target antigens in cancer cells. This results in a drastic inhibition of cancer cell proliferation compared to unconjugated mAbs, as demonstrated in various cancer cell lines, and an increased therapeutic efficacy in mouse and zebrafish xenografted models. These findings highlight the potential of this technology to improve therapeutic outcomes in cancer treatment.