Recycling compartments and the internal vesicles of multivesicular bodies harbor most of the cholesterol found in the endocytic pathway

Resolving the two-body problem: A postulated role for the V0 sector of the V0V1-ATPase in exosome biogenesis and multivesicular body fate

Because the discovery of the multivesicular body (MVB) as the origin of secreted vesicles or exosomes, the question arose and still looms-what distinguishes an MVB destined for fusion with the plasma membrane (EXO-MVB) facilitating exosome release from an MVB involved in transport of content to the lysosome (LYSO-MVB). Do they have independent origins? Hence, the two-body problem. We hypothesize that a key to this conundrum is the membrane spanning V0 sector of the proton pump, V0V1-ATPase. The V0V1-ATPase participates in the acidification of intracellular compartments, although V0 can function separately from V1 and different V0 isoforms are endowed with membrane binding capabilities that allow the V0V1-ATPase to selectively localize to different endocytic compartments including early and late endosomes and lysosomes. We propose that V0, in collaboration with cholesterol and phosphoinositides, plays a central role in the early endosome as a nucleation center to direct the de novo assembly of an EXO-MVB scaffold. The EXO-MVB scaffold may play multiple roles-operating as an assembly platform, participating in membrane fission as well as providing downstream navigational queues necessary for exosome secretion. Thus, V0 may represent an influential nexus, a starting point, in exosome biogenesis.

Maistero-2, a Novel Probe for Sterols: Application for Visualizing Cellular Cholesterol

Maistero-2 is a novel, non-toxic cholesterol-binding protein derived from an edible mushroom Grifola frondosa mRNA. Maistero-2 specifically binds to lipid membranes containing 3-hydroxy sterols with a lower cholesterol concentration threshold than cholesterol-binding domain 4 (D4) of perfringolysin O (PFO) and anthrolysin O (ALO). Maistero-2 binding is particularly sensitive to the size and conformation of the A-, B-, and D-ring of sterols but not very sensitive to modifications of the isooctyl side chain commonly found in phytosterols. EGFP-maistero-2 has been demonstrated to be a suitable tool to visualize the increase of cell surface cholesterol during the differentiation of Neuro 2a cells and heterogeneous cholesterol distribution between CD63-positive and LAMP1-positive late endosomes/lysosomes in HeLa cells. This chapter describes the detailed protocols for examining the binding of maistero-2 to lipids and model membranes and the labeling of cell surface and intracellular cholesterol by EGFP-maistero-2.

Two NPC1 homologous proteins are involved in asexual reproduction, pathogenicity, and lipid trafficking in Phytophthora sojae

Niemann-Pick type C (NPC) disease is characterized by lysosomal lipid storage disorders and defects in lipid trafficking, primarily due to mutations in the NPC1 protein. Two NPC1 homologous proteins are present in the genome of Phytophthora sojae, named as PsNPC1-1 and PsNPC1-2. Both proteins exhibit high sequence identity, consistent conserved functional domains, similar gene expression patterns, and comparable subcellular localization. Deletion of a single PsNPC1 gene did not result in significant phenotypic changes. However, simultaneous deletion of both PsNPC1 genes led to reduced mycelial growth, decreased sporangial production, impaired pathogenicity, and an inability to release normal zoospores in P. sojae. Furthermore, dysfunction of PsNPC1s did not completely block the absorption and utilization of exogenous sterols by P. sojae. While lipidome analysis revealed that the relative contents of fatty acyls, sphingolipids and saccharolipids were significantly elevated in the double-gene deletion mutant, alongside obvious alterations in glycerophospholipid and glycerolipid metabolism. Additionally, we observed a significant down-regulation of PsCDP-AP protein along with its interactions with both PsNPC1s. Deletion of PsCDP-AP also impaired asexual reproduction and virulence of P. sojae. These findings demonstrate that both PsNPC1 proteins may collaborate with other key regulators to modulate asexual reproduction, pathogenicity and lipid trafficking in P. sojae.

Tackling exosome and nuclear receptor interaction: an emerging paradigm in the treatment of chronic diseases

Nuclear receptors (NRs) function as crucial transcription factors in orchestrating essential functions within the realms of development, host defense, and homeostasis of body. NRs have garnered increased attention due to their potential as therapeutic targets, with drugs directed at NRs demonstrating significant efficacy in impeding chronic disease progression. Consequently, these pharmacological agents hold promise for the treatment and management of various diseases. Accumulating evidence emphasizes the regulatory role of exosome-derived microRNAs (miRNAs) in chronic inflammation, disease progression, and therapy resistance, primarily by modulating transcription factors, particularly NRs. By exploiting inflammatory pathways such as protein kinase B (Akt)/mammalian target of rapamycin (mTOR), nuclear factor kappa-B (NF-κB), signal transducer and activator of transcription 3 (STAT3), and Wnt/β-catenin signaling, exosomes and NRs play a pivotal role in the panorama of development, physiology, and pathology. The internalization of exosomes modulates NRs and initiates diverse autocrine or paracrine signaling cascades, influencing various processes in recipient cells such as survival, proliferation, differentiation, metabolism, and cellular defense mechanisms. This comprehensive review meticulously examines the involvement of exosome-mediated NR regulation in the pathogenesis of chronic ailments, including atherosclerosis, cancer, diabetes, liver diseases, and respiratory conditions. Additionally, it elucidates the molecular intricacies of exosome-mediated communication between host and recipient cells via NRs, leading to immunomodulation. Furthermore, it outlines the implications of exosome-modulated NR pathways in the prophylaxis of chronic inflammation, delineates current limitations, and provides insights into future perspectives. This review also presents existing evidence on the role of exosomes and their components in the emergence of therapeutic resistance.

Infertility: Focus on the therapeutic potential of extracellular vesicles

Infertility is a well-known problem that arises from a variety of reproductive diseases. Until now, researchers have tried various methods to restore fertility, including medication specific to the cause, hormone treatments, surgical removals, and assisted reproductive technologies. While these methods do produce results, they do not consistently lead to fertility restoration in every instance. The use of exosome therapy has significant potential in treating infertility in patients. This is because exosomes, microvesicles, and apoptotic bodies, which are different types of vesicles, play a crucial role in transferring bioactive molecules that aid in cell-to-cell communication. Reproductive fluids can transport a variety of molecular cargos, such as miRNAs, mRNAs, proteins, lipids, and DNA molecules. The percentage of these cargos in the fluids can be linked to their physiological and pathological status. EVs are involved in several physiological and pathological processes and offer interesting non-cellular therapeutic possibilities to treat infertility. EVs (extracellular vesicles) transplantation has been shown in many studies to be a key part of regenerating different parts of the reproductive system, including the production of oocytes and the start of sperm production. Nevertheless, the existing evidence necessitates testifying to the effectiveness of injecting EVs in resolving reproductive problems among humans. This review focuses on the current literature about infertility issues in both females and males, specifically examining the potential treatments involving extracellular vesicles (EVs).

Prostate cancer-derived extracellular vesicles metabolic biomarkers: Emerging roles for diagnosis and prognosis

Prostate cancer (PCa) is a global health concern, ranking as the most common cancer among men in Western countries. Traditional diagnostic methods are invasive with adverse effects on patients. Due to the heterogeneous nature of PCa and their multifocality, tissue biopsies often yield false-negative results. To address these challenges, researchers are exploring innovative approaches, particularly in the realms of proteomics and metabolomics, to identify more reliable biomarkers and improve PCa diagnosis. Liquid biopsy (LB) has emerged as a promising non-invasive strategy for PCa early detection, biopsy selection, active surveillance for low-risk cases, and post-treatment and progression monitoring. Extracellular vesicles (EVs) are lipid-bilayer nanovesicles released by all cell types and play an important role in intercellular communication. EVs have garnered attention as a valuable biomarker resource in LB for PCa-specific biomarkers, enhancing diagnosis, prognostication, and treatment guidance. Metabolomics provides insight into the body's metabolic response to both internal and external stimuli, offering quantitative measurements of biochemical alterations. It excels at detecting non-genetic influences, aiding in the discovery of more accurate cancer biomarkers for early detection and disease progression monitoring. This review delves into the potential of EVs as a resource for LB in PCa across various clinical applications. It also explores cancer-related metabolic biomarkers, both within and outside EVs in PCa, and summarises previous metabolomic findings in PCa diagnosis and risk assessment. Finally, the article addresses the challenges and future directions in the evolving field of EV-based metabolomic analysis, offering a comprehensive overview of its potential in advancing PCa management.

CD63 sorts cholesterol into endosomes for storage and distribution via exosomes

Extracellular vesicles such as exosomes are now recognized as key players in intercellular communication. Their role is influenced by the specific repertoires of proteins and lipids, which are enriched when they are generated as intraluminal vesicles (ILVs) in multivesicular endosomes. Here we report that a key component of small extracellular vesicles, the tetraspanin CD63, sorts cholesterol to ILVs, generating a pool that can be mobilized by the NPC1/2 complex, and exported via exosomes to recipient cells. In the absence of CD63, cholesterol is retrieved from the endosomes by actin-dependent vesicular transport, placing CD63 and cholesterol at the centre of a balance between inward and outward budding of endomembranes. These results establish CD63 as a lipid-sorting mechanism within endosomes, and show that ILVs and exosomes are alternative providers of cholesterol.

The role of exosomal molecular cargo in exosome biogenesis and disease diagnosis

Exosomes represent a type of extracellular vesicles derived from the endosomal pathway that transport diverse molecular cargoes such as proteins, lipids, and nucleic acids. These cargoes have emerged as crucial elements impacting disease diagnosis, treatment, and prognosis, and are integral to the process of exosome formation. This review delves into the essential molecular cargoes implicated in the phases of exosome production and release. Emphasis is placed on their significance as cancer biomarkers and potential therapeutic targets, accompanied by an exploration of the obstacles and feasible applications linked to these developments.

Emerging Roles of Neuronal Extracellular Vesicles at the Synapse

Extracellular vesicles (EVs) are secreted from most, if not all, cell types and are implicated in short- and long-distance signaling throughout the body. EVs are also secreted from neurons and represent an emergent neuronal communication platform. Understanding the functional implications of EV signaling to recipient neurons and glia requires understanding the cell biology involved in EV biogenesis, cargo loading, secretion, uptake, and signal transduction in the recipient cell. Here we review these major questions of EV biology while highlighting recent new insights and examples within the nervous system, such as modulating synaptic function or morphogenesis in recipient neurons.

Dysregulation of cellular membrane homeostasis as a crucial modulator of cancer risk

Cellular membranes serve as an epicentre combining extracellular and cytosolic components with membranous effectors, which together support numerous fundamental cellular signalling pathways that mediate biological responses. To execute their functions, membrane proteins, lipids and carbohydrates arrange, in a highly coordinated manner, into well-defined assemblies displaying diverse biological and biophysical characteristics that modulate several signalling events. The loss of membrane homeostasis can trigger oncogenic signalling. More recently, it has been documented that select membrane active dietaries (MADs) can reshape biological membranes and subsequently decrease cancer risk. In this review, we emphasize the significance of membrane domain structure, organization and their signalling functionalities as well as how loss of membrane homeostasis can steer aberrant signalling. Moreover, we describe in detail the complexities associated with the examination of these membrane domains and their association with cancer. Finally, we summarize the current literature on MADs and their effects on cellular membranes, including various mechanisms of dietary chemoprevention/interception and the functional links between nutritional bioactives, membrane homeostasis and cancer biology.

Influenza A virus propagation requires the activation of the unfolded protein response and the accumulation of insoluble protein aggregates

Influenza A virus (IAV) employs multiple strategies to manipulate cellular mechanisms and support proper virion formation and propagation. In this study, we performed a detailed analysis of the interplay between IAV and the host cells' proteostasis throughout the entire infectious cycle. We reveal that IAV infection activates the inositol requiring enzyme 1 (IRE1) branch of the unfolded protein response, and that this activation is important for an efficient infection. We further observed the accumulation of virus-induced insoluble protein aggregates, containing both viral and host proteins, associated with a dysregulation of the host cell RNA metabolism. Our data indicate that this accumulation is important for IAV propagation and favors the final steps of the infection cycle, more specifically the virion assembly. These findings reveal additional mechanisms by which IAV disrupts host proteostasis and uncovers new cellular targets that can be explored for the development of host-directed antiviral strategies.

Annexin A7 mediates lysosome repair independently of ESCRT-III

Lysosomes are crucial organelles essential for various cellular processes, and any damage to them can severely compromise cell viability. This study uncovers a previously unrecognized function of the calcium- and phospholipid-binding protein Annexin A7 in lysosome repair, which operates independently of the Endosomal Sorting Complex Required for Transport (ESCRT) machinery. Our research reveals that Annexin A7 plays a role in repairing damaged lysosomes, different from its role in repairing the plasma membrane, where it facilitates repair through the recruitment of ESCRT-III components. Notably, our findings strongly suggest that Annexin A7, like the ESCRT machinery, is dispensable for membrane contact site formation within the newly discovered phosphoinositide-initiated membrane tethering and lipid transport (PITT) pathway. Instead, we speculate that Annexin A7 is recruited to damaged lysosomes and promotes repair through its membrane curvature and cross-linking capabilities. Our findings provide new insights into the diverse mechanisms underlying lysosomal membrane repair and highlight the multifunctional role of Annexin A7 in membrane repair.

Insights into the function of ESCRT complex and LBPA in ASFV infection

The African swine fever virus (ASFV) is strongly dependent on an intact endocytic pathway and a certain cellular membrane remodeling for infection, possibly regulated by the endosomal sorting complexes required for transport (ESCRT). The ESCRT machinery is mainly involved in the coordination of membrane dynamics; hence, several viruses exploit this complex and its accessory proteins VPS4 and ALIX for their own benefit. In this work, we found that shRNA-mediated knockdown of VPS4A decreased ASFV replication and viral titers, and this silencing resulted in an enhanced expression of ESCRT-0 component HRS. ASFV infection slightly increased HRS expression but not under VPS4A depletion conditions. Interestingly, VPS4A silencing did not have an impact on ALIX expression, which was significantly overexpressed upon ASFV infection. Further analysis revealed that ALIX silencing impaired ASFV infection at late stages of the viral cycle, including replication and viral production. In addition to ESCRT, the accessory protein ALIX is involved in endosomal membrane dynamics in a lysobisphosphatydic acid (LBPA) and Ca2+-dependent manner, which is relevant for intraluminal vesicle (ILV) biogenesis and endosomal homeostasis. Moreover, LBPA interacts with NPC2 and/or ALIX to regulate cellular cholesterol traffic, and would affect ASFV infection. Thus, we show that LBPA blocking impacted ASFV infection at both early and late infection, suggesting a function for this unconventional phospholipid in the ASFV viral cycle. Here, we found for the first time that silencing of VPS4A and ALIX affects the infection later on, and blocking LBPA function reduces ASFV infectivity at early and later stages of the viral cycle, while ALIX was overexpressed upon infection. These data suggested the relevance of ESCRT-related proteins in ASFV infection.

HIV-1 Gag targeting to the plasma membrane reorganizes sphingomyelin-rich and cholesterol-rich lipid domains

Although the human immunodeficiency virus type 1 lipid envelope has been reported to be enriched with host cell sphingomyelin and cholesterol, the molecular mechanism of the enrichment is not well understood. Viral Gag protein plays a central role in virus budding. Here, we report the interaction between Gag and host cell lipids using different quantitative and super-resolution microscopy techniques in combination with specific probes that bind endogenous sphingomyelin and cholesterol. Our results indicate that Gag in the inner leaflet of the plasma membrane colocalizes with the outer leaflet sphingomyelin-rich domains and cholesterol-rich domains, enlarges sphingomyelin-rich domains, and strongly restricts the mobility of sphingomyelin-rich domains. Moreover, Gag multimerization induces sphingomyelin-rich and cholesterol-rich lipid domains to be in close proximity in a curvature-dependent manner. Our study suggests that Gag binds, coalesces, and reorganizes pre-existing lipid domains during assembly.

Kinetic modelling of sterol transport between plasma membrane and endo-lysosomes based on quantitative fluorescence and X-ray imaging data

Niemann Pick type C1 and C2 (NPC1 and NPC2) are two sterol-binding proteins which, together, orchestrate cholesterol transport through late endosomes and lysosomes (LE/LYSs). NPC2 can facilitate sterol exchange between model membranes severalfold, but how this is connected to its function in cells is poorly understood. Using fluorescent analogs of cholesterol and quantitative fluorescence microscopy, we have recently measured the transport kinetics of sterol between plasma membrane (PM), recycling endosomes (REs) and LE/LYSs in control and NPC2 deficient fibroblasts. Here, we use kinetic modeling of this data to determine rate constants for sterol transport between intracellular compartments. Our model predicts that sterol is trapped in intraluminal vesicles (ILVs) of LE/LYSs in the absence of NPC2, causing delayed sterol export from LE/LYSs in NPC2 deficient fibroblasts. Using soft X-ray tomography, we confirm, that LE/LYSs of NPC2 deficient cells but not of control cells contain enlarged, carbon-rich intraluminal vesicular structures, supporting our model prediction of lipid accumulation in ILVs. By including sterol export via exocytosis of ILVs as exosomes and by release of vesicles-ectosomes-from the PM, we can reconcile measured sterol efflux kinetics and show that both pathways can be reciprocally regulated by the intraluminal sterol transfer activity of NPC2 inside LE/LYSs. Our results thereby connect the in vitro function of NPC2 as sterol transfer protein between membranes with its in vivo function.

Exosomes Interactions with the Blood-Brain Barrier: Implications for Cerebral Disorders and Therapeutics

The Blood-Brain Barrier (BBB) is a selective structural and functional barrier between the circulatory system and the cerebral environment, playing an essential role in maintaining cerebral homeostasis by limiting the passage of harmful molecules. Exosomes, nanovesicles secreted by virtually all cell types into body fluids, have emerged as a major mediator of intercellular communication. Notably, these vesicles can cross the BBB and regulate its physiological functions. However, the precise molecular mechanisms by which exosomes regulate the BBB remain unclear. Recent research studies focused on the effect of exosomes on the BBB, particularly in the context of their involvement in the onset and progression of various cerebral disorders, including solid and metastatic brain tumors, stroke, neurodegenerative, and neuroinflammatory diseases. This review focuses on discussing and summarizing the current knowledge about the role of exosomes in the physiological and pathological modulation of the BBB. A better understanding of this regulation will improve our understanding of the pathogenesis of cerebral diseases and will enable the design of effective treatment strategies.

The expanding organelle lipidomes: current knowledge and challenges

Lipids in cell membranes and subcellular compartments play essential roles in numerous cellular processes, such as energy production, cell signaling and inflammation. A specific organelle lipidome is characterized by lipid synthesis and metabolism, intracellular trafficking, and lipid homeostasis in the organelle. Over the years, considerable effort has been directed to the identification of the lipid fingerprints of cellular organelles. However, these fingerprints are not fully characterized due to the large variety and structural complexity of lipids and the great variability in the abundance of different lipid species. The process becomes even more challenging when considering that the lipidome differs in health and disease contexts. This review summarizes the information available on the lipid composition of mammalian cell organelles, particularly the lipidome of the nucleus, mitochondrion, endoplasmic reticulum, Golgi apparatus, plasma membrane and organelles in the endocytic pathway. The lipid compositions of extracellular vesicles and lamellar bodies are also described. In addition, several examples of subcellular lipidome dynamics under physiological and pathological conditions are presented. Finally, challenges in mapping organelle lipidomes are discussed.

A Review of Extracellular Vesicles in COVID-19 Diagnosis, Treatment, and Prevention

The 2019 novel coronavirus disease (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is ongoing, and has necessitated scientific efforts in disease diagnosis, treatment, and prevention. Interestingly, extracellular vesicles (EVs) have been crucial in these developments. EVs are a collection of various nanovesicles which are delimited by a lipid bilayer. They are enriched in proteins, nucleic acids, lipids, and metabolites, and naturally released from different cells. Their natural material transport properties, inherent long-term recycling ability, excellent biocompatibility, editable targeting, and inheritance of parental cell properties make EVs one of the most promising next-generation drug delivery nanocarriers and active biologics. During the COVID-19 pandemic, many efforts have been made to exploit the payload of natural EVs for the treatment of COVID-19. Furthermore, strategies that use engineered EVs to manufacture vaccines and neutralization traps have produced excellent efficacy in animal experiments and clinical trials. Here, the recent literature on the application of EVs in COVID-19 diagnosis, treatment, damage repair, and prevention is reviewed. And the therapeutic value, application strategies, safety, and biotoxicity in the production and clinical applications of EV agents for COVID-19 treatment, as well as inspiration for using EVs to block and eliminate novel viruses are discussed.

Extracellular vesicle-matrix interactions

The extracellular matrix in microenvironments harbors a variety of signals to control cellular functions and the materiality of tissues. Most efforts to synthetically reconstitute the matrix by biomaterial design have focused on decoupling cell-secreted and polymer-based cues. Cells package molecules into nanoscale lipid membrane-bound extracellular vesicles and secrete them. Thus, extracellular vesicles inherently interact with the meshwork of the extracellular matrix. In this Review, we discuss various aspects of extracellular vesicle-matrix interactions. Cells receive feedback from the extracellular matrix and leverage intracellular processes to control the biogenesis of extracellular vesicles. Once secreted, various biomolecular and biophysical factors determine whether extracellular vesicles are locally incorporated into the matrix or transported out of the matrix to be taken up by other cells or deposited into tissues at a distal location. These insights can be utilized to develop engineered biomaterials where EV release and retention can be precisely controlled in host tissue to elicit various biological and therapeutic outcomes.

How statin drugs affect exosomes?

Statins reduce serum cholesterol and isoprenoids by the inhibition of cholesterol synthesis in the mevalonate pathway. Exosomes are extracellular vesicles (30-200 nm) released by all cells that regulate cell-to-cell communication in health and disease by transferring functional proteins, metabolites and nucleic acids to recipient cells. There are many reports that show an effect of statins on exosomes, from their production and release to their content and performance. In this review, we have summarized existing data on the impact of statins on the biosynthesis, secretion, content, uptake and function of exosomes.

Ca2+ and Annexins - Emerging Players for Sensing and Transferring Cholesterol and Phosphoinositides via Membrane Contact Sites

Maintaining lipid composition diversity in membranes from different organelles is critical for numerous cellular processes. However, many lipids are synthesized in the endoplasmic reticulum (ER) and require delivery to other organelles. In this scenario, formation of membrane contact sites (MCS) between neighbouring organelles has emerged as a novel non-vesicular lipid transport mechanism. Dissecting the molecular composition of MCS identified phosphoinositides (PIs), cholesterol, scaffolding/tethering proteins as well as Ca²⁺ and Ca²⁺-binding proteins contributing to MCS functioning. Compelling evidence now exists for the shuttling of PIs and cholesterol across MCS, affecting their concentrations in distinct membrane domains and diverse roles in membrane trafficking. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at the plasma membrane (PM) not only controls endo-/exocytic membrane dynamics but is also critical in autophagy. Cholesterol is highly concentrated at the PM and enriched in recycling endosomes and Golgi membranes. MCS-mediated cholesterol transfer is intensely researched, identifying MCS dysfunction or altered MCS partnerships to correlate with de-regulated cellular cholesterol homeostasis and pathologies. Annexins, a conserved family of Ca²⁺-dependent phospholipid binding proteins, contribute to tethering and untethering events at MCS. In this chapter, we will discuss how Ca²⁺ homeostasis and annexins in the endocytic compartment affect the sensing and transfer of cholesterol and PIs across MCS.

Neuronal Ganglioside and Glycosphingolipid (GSL) Metabolism and Disease : Cascades of Secondary Metabolic Errors Can Generate Complex Pathologies (in LSDs)

Glycosphingolipids (GSLs) are a diverse group of membrane components occurring mainly on the surfaces of mammalian cells. They and their metabolites have a role in intercellular communication, serving as versatile biochemical signals (Kaltner et al, Biochem J 476(18):2623-2655, 2019) and in many cellular pathways. Anionic GSLs, the sialic acid containing gangliosides (GGs), are essential constituents of neuronal cell surfaces, whereas anionic sulfatides are key components of myelin and myelin forming oligodendrocytes. The stepwise biosynthetic pathways of GSLs occur at and lead along the membranes of organellar surfaces of the secretory pathway. After formation of the hydrophobic ceramide membrane anchor of GSLs at the ER, membrane-spanning glycosyltransferases (GTs) of the Golgi and Trans-Golgi network generate cell type-specific GSL patterns for cellular surfaces. GSLs of the cellular plasma membrane can reach intra-lysosomal, i.e. luminal, vesicles (ILVs) by endocytic pathways for degradation. Soluble glycoproteins, the glycosidases, lipid binding and transfer proteins and acid ceramidase are needed for the lysosomal catabolism of GSLs at ILV-membrane surfaces. Inherited mutations triggering a functional loss of glycosylated lysosomal hydrolases and lipid binding proteins involved in GSL degradation cause a primary lysosomal accumulation of their non-degradable GSL substrates in lysosomal storage diseases (LSDs). Lipid binding proteins, the SAPs, and the various lipids of the ILV-membranes regulate GSL catabolism, but also primary storage compounds such as sphingomyelin (SM), cholesterol (Chol.), or chondroitin sulfate can effectively inhibit catabolic lysosomal pathways of GSLs. This causes cascades of metabolic errors, accumulating secondary lysosomal GSL- and GG- storage that can trigger a complex pathology (Breiden and Sandhoff, Int J Mol Sci 21(7):2566, 2020).

Exosomal MicroRNAs as Novel Cell-Free Therapeutics in Tissue Engineering and Regenerative Medicine

Extracellular vesicles (EVs) are membrane-bound vesicles (50-1000 nm) that can be secreted by all cell types. Microvesicles and exosomes are the major subsets of EVs that exhibit the cell-cell communications and pathological functions of human tissues, and their therapeutic potentials. To further understand and engineer EVs for cell-free therapy, current developments in EV biogenesis and secretion pathways are discussed to illustrate the remaining gaps in EV biology. Specifically, microRNAs (miRs), as a major EV cargo that exert promising therapeutic results, are discussed in the context of biological origins, sorting and packing, and preclinical applications in disease progression and treatments. Moreover, advanced detection and engineering strategies for exosomal miRs are also reviewed. This article provides sufficient information and knowledge for the future design of EVs with specific miRs or protein cargos in tissue repair and regeneration.

ER as master regulator of membrane trafficking and organelle function

The endoplasmic reticulum (ER), which occupies a large portion of the cytoplasm, is the cell's main site for the biosynthesis of lipids and carbohydrate conjugates, and it is essential for folding, assembly, and biosynthetic transport of secreted proteins and integral membrane proteins. The discovery of abundant membrane contact sites (MCSs) between the ER and other membrane compartments has revealed that, in addition to its biosynthetic and secretory functions, the ER plays key roles in the regulation of organelle dynamics and functions. In this review, we will discuss how the ER regulates endosomes, lysosomes, autophagosomes, mitochondria, peroxisomes, and the Golgi apparatus via MCSs. Such regulation occurs via lipid and Ca2+ transfer and also via control of in trans dephosphorylation reactions and organelle motility, positioning, fusion, and fission. The diverse controls of other organelles via MCSs manifest the ER as master regulator of organelle biology.

Endolysosomal cholesterol export: More than just NPC1

NPC1 plays a central role in cholesterol egress from endolysosomes, a critical step for maintaining intracellular cholesterol homeostasis. Despite recent advances in the field, the full repertoire of molecules and pathways involved in this process remains unknown. Emerging evidence suggests the existence of NPC1-independent, alternative routes. These may involve vesicular and non-vesicular mechanisms, as well as release of extracellular vesicles. Understanding the underlying molecular mechanisms that bypass NPC1 function could have important implications for the development of therapies for lysosomal storage disorders. Here we discuss how cholesterol may be exported from lysosomes in which NPC1 function is impaired.

Neurological aspects of SARS-CoV-2 infection: lipoproteins and exosomes as Trojan horses

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) primarily targets lipid-producing cells for viral tropism. In this review, we connect systemic lipid couriers, particularly high-density lipoproteins (HDLs) and exosomes, with the neurological facets of SARS-CoV-2 infection. We discuss how SARS-CoV-2 preferentially targets lipid-secreting cells and usurps host cell lipid metabolism for efficient replication and systemic spreading. Besides providing natural veils for viral materials against host immunity, the inherent properties of some of these endogenous lipid particles to traverse the blood-brain barrier (BBB) also offer alternative routes for SARS-CoV-2 neurotropism. Importantly, virus-driven neurological aberrations mediated by HDLs and exosomes are fueled by lipid rafts, which are implicated in the production and transmigration of these lipid particles across the BBB. Finally, we discuss how repurposing existing drugs targeting lipid rafts and cholesterol homeostasis may be beneficial toward alleviating the global coronavirus disease 2019 (COVID-19) disease burden.

Effect of Exosomes from Patients with Grade One Cervical Intraepithelial Neoplasia on Cell Cultures: A Preliminary Study

Purpose

Exosomes are extracellular membrane vesicles. Their content directly reflects the metabolic state of the cells from which they originate and play an important role in cellular functions and pathological states, for example, cancer. The aim was to establish the effect of exosomes from patients diagnosed with CIN1 (grade one cervical intraepithelial neoplasia) on the viability of HeLa cells in culture. It had not been documented, nor had the vesicles obtained by cervicovaginal samples taken by the patients themselves (self-taken vaginal).

Patients and Methods

Exosomes were obtained from self-taken vaginal by patients diagnosed with CIN1 and healthy. The exosomes were characterized by determining the AChE (acetylcholinesterase) activity, obtaining a protein profile, and obtaining images of these by STEM. The effect on cell viability was made in HeLa and HaCaT cells in culture.

Results

Vesicles between 185 nm and 415 nm were observed by STEM. Exosomes show a "protective" effect when those patients without injury are confronted with HeLa cells. On the other hand, exosomes promote viability when they come from injured patients in the presence of the same cells.

Conclusion

Exosomes can be used to identify ideal biomarkers for the early diagnosis of CC (cervical cancer), follow-up of patients, and even treatment given the effects observed on cell cultures.

The sodium proton exchanger NHE9 regulates phagosome maturation and bactericidal activity in macrophages

Acidification of phagosomes is essential for the bactericidal activity of macrophages. Targeting machinery that regulates pH within the phagosomes is a prominent strategy employed by various pathogens that have emerged as major threats to public health. Nascent phagosomes acquire the machinery for pH regulation through a graded maturation process involving fusion with endolysosomes. In addition, meticulous coordination between proton pumping and leakage mechanisms is crucial for maintaining optimal pH within the phagosome. However, relative to mechanisms involved in acidifying the phagosome lumen, little is known about proton leakage pathways in this organelle. Sodium proton transporter NHE9 is a known proton leakage pathway located on the endosomes. As phagosomes acquire proteins through fusions with endosomes during maturation, NHE9 seemed a promising candidate for regulating proton fluxes on the phagosome. Here, using genetic and biophysical approaches, we show NHE9 is an important proton leakage pathway associated with the maturing phagosome. NHE9 is highly expressed in immune cells, specifically macrophages; however, NHE9 expression is strongly downregulated upon bacterial infection. We show that compensatory ectopic NHE9 expression hinders the directed motion of phagosomes along microtubules and promotes early detachment from the microtubule tracks. As a result, these phagosomes have shorter run lengths and are not successful in reaching the lysosome. In accordance with this observation, we demonstrate that NHE9 expression levels negatively correlate with bacterial survival. Together, our findings show that NHE9 regulates lumenal pH to affect phagosome maturation, and consequently, microbicidal activity in macrophages.

A novel sterol-binding protein reveals heterogeneous cholesterol distribution in neurite outgrowth and in late endosomes/lysosomes

We identified a mushroom-derived protein, maistero-2 that specifically binds 3-hydroxy sterol including cholesterol (Chol). Maistero-2 bound lipid mixture in Chol-dependent manner with a binding threshold of around 30%. Changing lipid composition did not significantly affect the threshold concentration. EGFP-maistero-2 labeled cell surface and intracellular organelle Chol with higher sensitivity than that of well-established Chol probe, D4 fragment of perfringolysin O. EGFP-maistero-2 revealed increase of cell surface Chol during neurite outgrowth and heterogeneous Chol distribution between CD63-positive and LAMP1-positive late endosomes/lysosomes. The absence of strictly conserved Thr-Leu pair present in Chol-dependent cytolysins suggests a distinct Chol-binding mechanism for maistero-2.

Phosphatidylinositol phosphates modulate interactions between the StarD4 sterol trafficking protein and lipid membranes

There is substantial evidence for extensive nonvesicular sterol transport in cells. For example, lipid transfer by the steroidogenic acute regulator-related proteins (StarD) containing a StarT domain has been shown to involve several pathways of nonvesicular trafficking. Among the soluble StarT domain–containing proteins, StarD4 is expressed in most tissues and has been shown to be an effective sterol transfer protein. However, it was unclear whether the lipid composition of donor or acceptor membranes played a role in modulating StarD4-mediated transport. Here, we used fluorescence-based assays to demonstrate a phosphatidylinositol phosphate (PIP)-selective mechanism by which StarD4 can preferentially extract sterol from liposome membranes containing certain PIPs (especially, PI(4,5)P₂ and to a lesser degree PI(3,5)P2). Monophosphorylated PIPs and other anionic lipids had a smaller effect on sterol transport. This enhancement of transport was less effective when the same PIPs were present in the acceptor membranes. Furthermore, using molecular dynamics (MD) simulations, we mapped the key interaction sites of StarD4 with PIP-containing membranes and identified residues that are important for this interaction and for accelerated sterol transport activity. We show that StarD4 recognizes membrane-specific PIPs through specific interaction with the geometry of the PIP headgroup as well as the surrounding membrane environment. Finally, we also observed that StarD4 can deform membranes upon longer incubations. Taken together, these results suggest a mechanism by which PIPs modulate cholesterol transfer activity via StarD4.

Sterolight as imaging tool to study sterol uptake, trafficking and efflux in living cells

Information about cholesterol subcellular localization and transport pathways inside cells is essential for understanding and treatment of cholesterol-related diseases. However, there is a lack of reliable tools to monitor it. This work follows the fate of Sterolight, a BODIPY-labelled sterol, within the cell and demonstrates it as a suitable probe for visualization of sterol/lipid trafficking. Sterolight enters cells through an energy-independent process and knockdown experiments suggest caveolin-1 as its potential cellular carrier. Intracellular transport of Sterolight is a rapid process, and transfer from ER and mitochondria to lysosomes and later to lipid droplets requires the participation of active microtubules, as it can be inhibited by the microtubule disruptor nocodazole. Excess of the probe is actively exported from cells, in addition to being stored in lipid droplets, to re-establish the sterol balance. Efflux occurs through a mechanism requiring energy and may be selectively poisoned with verapamil or blocked in cells with mutated cholesterol transporter NPC1. Sterolight is efficiently transferred within and between different cell populations, making it suitable for monitoring numerous aspects of sterol biology, including the live tracking and visualization of intracellular and intercellular transport.

Targeting the liver X receptor with dendrogenin A differentiates tumour cells to secrete immunogenic exosome-enriched vesicles

Tumour cells are characterized by having lost their differentiation state. They constitutively secrete small extracellular vesicles (sEV) called exosomes when they come from late endosomes. Dendrogenin A (DDA) is an endogenous tumour suppressor cholesterol‐derived metabolite. It is a new class of ligand of the nuclear Liver X receptors (LXR) which regulate cholesterol homeostasis and immunity. We hypothesized that DDA, which induces tumour cell differentiation, inhibition of tumour growth and immune cell infiltration into tumours, could functionally modify sEV secreted by tumour cells. Here, we have shown that DDA differentiates tumour cells by acting on the LXRβ. This results in an increased production of sEV (DDA‐sEV) which includes exosomes. The DDA‐sEV secreted from DDA‐treated cells were characterized for their content and activity in comparison to sEV secreted from control cells (C‐sEV). DDA‐sEV were enriched, relatively to C‐sEV, in several proteins and lipids such as differentiation antigens, “eat‐me” signals, lipidated LC3 and the endosomal phospholipid bis(monoacylglycero)phosphate, which stimulates dendritic cell maturation and a Th1 T lymphocyte polarization. Moreover, DDA‐sEV inhibited the growth of tumours implanted into immunocompetent mice compared to control conditions. This study reveals a pharmacological control through a nuclear receptor of exosome‐enriched tumour sEV secretion, composition and immune function. Targeting the LXR may be a novel way to reprogram tumour cells and sEV to stimulate immunity against cancer.

Exosomes as Promising Nanostructures in Diabetes Mellitus: From Insulin Sensitivity to Ameliorating Diabetic Complications

Diabetes mellitus (DM) is among the chronic metabolic disorders that its incidence rate has shown an increase in developed and wealthy countries due to lifestyle and obesity. The treatment of DM has always been of interest, and significant effort has been made in this field. Exosomes belong to extracellular vesicles with nanosized features (30-150 nm) that are involved in cell-to-cell communication and preserving homeostasis. The function of exosomes is different based on their cargo, and they may contain lipids, proteins, and nucleic acids. The present review focuses on the application of exosomes in the treatment of DM; both glucose and lipid levels are significantly affected by exosomes, and these nanostructures enhance lipid metabolism and decrease its deposition. Furthermore, exosomes promote glucose metabolism and affect the level of glycolytic enzymes and glucose transporters in DM. Type I DM results from the destruction of β cells in the pancreas, and exosomes can be employed to ameliorate apoptosis and endoplasmic reticulum (ER) stress in these cells. The exosomes have dual functions in mediating insulin resistance/sensitivity, and M1 macrophage-derived exosomes inhibit insulin secretion. The exosomes may contain miRNAs, and by transferring among cells, they can regulate various molecular pathways such as AMPK, PI3K/Akt, and β-catenin to affect DM progression. Noteworthy, exosomes are present in different body fluids such as blood circulation, and they can be employed as biomarkers for the diagnosis of diabetic patients. Future studies should focus on engineering exosomes derived from sources such as mesenchymal stem cells to treat DM as a novel strategy.

Lysosomal Changes in Mitosis

The recent discovery demonstrating that the leakage of cathepsin B from mitotic lysosomes assists mitotic chromosome segregation indicates that lysosomal membrane integrity can be spatiotemporally regulated. Unlike many other organelles, structural and functional alterations of lysosomes during mitosis remain, however, largely uncharted. Here, we demonstrate substantial differences in lysosomal proteome, lipidome, size, and pH between lysosomes that were isolated from human U2OS osteosarcoma cells either in mitosis or in interphase. The combination of pharmacological synchronization and mitotic shake-off yielded ~68% of cells in mitosis allowing us to investigate mitosis-specific lysosomal changes by comparing cell populations that were highly enriched in mitotic cells to those mainly in the G1 or G2 phases of the cell cycle. Mitotic cells had significantly reduced levels of lysosomal-associated membrane protein (LAMP) 1 and the active forms of lysosomal cathepsin B protease. Similar trends were observed in levels of acid sphingomyelinase and most other lysosomal proteins that were studied. The altered protein content was accompanied by increases in the size and pH of LAMP2-positive vesicles. Moreover, mass spectrometry-based shotgun lipidomics of purified lysosomes revealed elevated levels of sphingolipids, especially sphingomyelin and hexocylceramide, and lysoglyserophospholipids in mitotic lysosomes. Interestingly, LAMPs and acid sphingomyelinase have been reported to stabilize lysosomal membranes, whereas sphingomyelin and lysoglyserophospholipids have an opposite effect. Thus, the observed lysosomal changes during the cell cycle may partially explain the reduced lysosomal membrane integrity in mitotic cells.

Pathways and Mechanisms of Cellular Cholesterol Efflux-Insight From Imaging

Cholesterol is an essential molecule in cellular membranes, but too much cholesterol can be toxic. Therefore, mammalian cells have developed complex mechanisms to remove excess cholesterol. In this review article, we discuss what is known about such efflux pathways including a discussion of reverse cholesterol transport and formation of high-density lipoprotein, the function of ABC transporters and other sterol efflux proteins, and we highlight their role in human diseases. Attention is paid to the biophysical principles governing efflux of sterols from cells. We also discuss recent evidence for cholesterol efflux by the release of exosomes, microvesicles, and migrasomes. The role of the endo-lysosomal network, lipophagy, and selected lysosomal transporters, such as Niemann Pick type C proteins in cholesterol export from cells is elucidated. Since oxysterols are important regulators of cellular cholesterol efflux, their formation, trafficking, and secretion are described briefly. In addition to discussing results obtained with traditional biochemical methods, focus is on studies that use established and novel bioimaging approaches to obtain insight into cholesterol efflux pathways, including fluorescence and electron microscopy, atomic force microscopy, X-ray tomography as well as mass spectrometry imaging.

The microenvironment‐ a general hypothesis on the homeostatic function of extracellular vesicles

Extracellular vesicles (EVs), exosomes and microvesicles, is a burgeoning field of biological and biomedical research that may change our understanding of cell communication in plants and animals while holding great promise for the diagnosis of disease and the development of therapeutics. However, the challenge remains to develop a general hypothesis about the role of EVs in physiological homeostasis and pathobiology across kingdoms. While they can act systemically, EVs are often seen to operate locally within a microenvironment. This microenvironment is built as a collection of microunits comprised of cells that interact with each other via EV exchange, EV signaling, EV seeding, and EV disposal. We propose that microunits are part of a larger matrix at the tissue level that collectively communicates with the surrounding environment, including other end‐organ systems. Herein, we offer a working model that encompasses the various facets of EV function in the context of the cell biology and physiology of multicellular organisms.

Neutral Sphingomyelinase 2 controls exosomes secretion via counteracting V-ATPase-mediated endosome acidification

During endosome maturation, neutral sphingomyelinase 2 (nSMase2, encoded by SMPD3) is involved in budding of intraluminal vesicles (ILVs) into late endosomes or multivesicular bodies (MVBs). Fusion of these with the plasma membrane results in secretion of exosomes or small extracellular vesicles (sEVs). Here, we report that nSMase2 activity controls sEV secretion through modulation of vacuolar H⁺-ATPase (V-ATPase) activity. Specifically, we show that nSMase2 inhibition induces V-ATPase complex assembly that drives MVB lumen acidification and consequently reduces sEV secretion. Conversely, we further demonstrate that stimulating nSMase2 activity with the inflammatory cytokine TNFα (also known as TNF) decreases acidification and increases sEV secretion. Thus, we find that nSMase2 activity affects MVB membrane lipid composition to counteract V-ATPase-mediated endosome acidification, thereby shifting MVB fate towards sEV secretion.

This article has an associated First Person interview with the first author of the paper.

Summary: Changing neutral sphingomyelinase 2 activity regulates small extracellular vesicle secretion through modulation of V-ATPase activity.

The Biology and Function of Extracellular Vesicles in Cancer Development

Extracellular vesicles (EVs) exert their biological functions by delivering proteins, metabolites, and nucleic acids to recipient cells. EVs play important roles in cancer development. The anti-tumor effect of EVs is by their cargos carrying proteins, metabolites, and nucleic acids to affect cell-to-cell communication. The characteristics of cell-to-cell communication can potentially be applied for the therapy of cancers, such as gastric cancer. In addition, EVs can be used as an effective cargos to deliver ncRNAs, peptides, and drugs, to target tumor tissues. In addition, EVs have the ability to regulate cell apoptosis, autophagy, proliferation, and migration of cancer cells. The ncRNA and peptides that were engaged with EVs were associated with cell signaling pathways in cancer development. This review focuses on the composition, cargo, function, mechanism, and application of EVs in cancers.

Extracellular Vesicles Physiological Role and the Particular Case of Disease-Spreading Mechanisms in Polyglutamine Diseases

Recent research demonstrated pathological spreading of the disease-causing proteins from one focal point across other brain regions for some neurodegenerative diseases, such as Parkinson's and Alzheimer's disease. Spreading mediated by extracellular vesicles is one of the proposed disease-spreading mechanisms. Extracellular vesicles are cell membrane-derived vesicles, used by cells for cell-to-cell communication and excretion of toxic components. Importantly, extracellular vesicles carrying pathological molecules, when internalized by "healthy" cells, may trigger pathological pathways and, consequently, promote disease spreading to neighboring cells. Polyglutamine diseases are a group of genetic neurodegenerative disorders characterized by the accumulation of mutant misfolded proteins carrying an expanded tract of glutamines, including Huntington's and Machado-Joseph disease. The pathological spread of the misfolded proteins or the corresponding mutant mRNA has been explored. The understanding of the disease-spreading mechanism that plays a key role in the pathology progression of these diseases can result in the development of effective therapeutic approaches to stop disease progression, arresting the spread of the toxic components and disease aggravation. Therefore, the present review's main focus is the disease-spreading mechanisms with emphasis on polyglutamine diseases and the putative role played by extracellular vesicles in this process.

Endosomal Cholesterol in Viral Infections - A Common Denominator?

Cholesterol has gained tremendous attention as an essential lipid in the life cycle of virtually all viruses. These seem to have developed manifold strategies to modulate the cholesterol metabolism to the side of lipid uptake and de novo synthesis. In turn, affecting the cholesterol homeostasis has emerged as novel broad-spectrum antiviral strategy. On the other hand, the innate immune system is similarly regulated by the lipid and stimulated by its derivatives. This certainly requires attention in the design of antiviral strategies aiming to decrease cellular cholesterol, as evidence accumulates that withdrawal of cholesterol hampers innate immunity. Secondly, there are exceptions to the rule of the abovementioned virus-induced metabolic shift toward cholesterol anabolism. It therefore is of interest to dissect underlying regulatory mechanisms, which we aimed for in this minireview. We further collected evidence for intracellular cholesterol concentrations being less important in viral life cycles as compared to the spatial distribution of the lipid. Various routes of cholesterol trafficking were found to be hijacked in viral infections with respect to organelle-endosome contact sites mediating cholesterol shuttling. Thus, re-distribution of cellular cholesterol in the context of viral infections requires more attention in ongoing research. As a final aim, a pan-antiviral treatment could be found just within the transport and re-adjustment of local cholesterol concentrations. Thus, we aimed to emphasize the importance of the regulatory roles the endosomal system fulfils herein and hope to stimulate research in this field.

Crosstalk between autophagy inhibitors and endosome-related secretory pathways: a challenge for autophagy-based treatment of solid cancers

Autophagy is best known for its role in organelle and protein turnover, cell quality control, and metabolism. The autophagic machinery has, however, also adapted to enable protein trafficking and unconventional secretory pathways so that organelles (such as autophagosomes and multivesicular bodies) delivering cargo to lysosomes for degradation can change their mission from fusion with lysosomes to fusion with the plasma membrane, followed by secretion of the cargo from the cell. Some factors with key signalling functions do not enter the conventional secretory pathway but can be secreted in an autophagy-mediated manner.Positive clinical results of some autophagy inhibitors are encouraging. Nevertheless, it is becoming clear that autophagy inhibition, even within the same cancer type, can affect cancer progression differently. Even next-generation inhibitors of autophagy can have significant non-specific effects, such as impacts on endosome-related secretory pathways and secretion of extracellular vesicles (EVs). Many studies suggest that cancer cells release higher amounts of EVs compared to non-malignant cells, which makes the effect of autophagy inhibitors on EVs secretion highly important and attractive for anticancer therapy. In this review article, we discuss how different inhibitors of autophagy may influence the secretion of EVs and summarize the non-specific effects of autophagy inhibitors with a focus on endosome-related secretory pathways. Modulation of autophagy significantly impacts not only the quantity of EVs but also their content, which can have a deep impact on the resulting pro-tumourigenic or anticancer effect of autophagy inhibitors used in the antineoplastic treatment of solid cancers.

Extracellular vesicles in cardiovascular disease: Biological functions and therapeutic implications

Extracellular vesicles (EVs), including exosomes and microvesicles, are lipid bilayer particles naturally released from the cell. While exosomes are formed as intraluminal vesicles (ILVs) of the multivesicular endosomes (MVEs) and released to extracellular space upon MVE-plasma membrane fusion, microvesicles are generated through direct outward budding of the plasma membrane. Exosomes and microvesicles have same membrane orientation, different yet overlapping sizes; their cargo contents are selectively packed and dependent on the source cell type and functional state. Both exosomes and microvesicles can transfer bioactive RNAs, proteins, lipids, and metabolites from donor to recipient cells and influence the biological properties of the latter. Over the last decade, their potential roles as effective inter-tissue communicators in cardiovascular physiology and pathology have been increasingly appreciated. In addition, EVs are attractive sources of biomarkers for the diagnosis and prognosis of diseases, because they acquire their complex cargoes through cellular processes intimately linked to disease pathogenesis. Furthermore, EVs obtained from various stem/progenitor cell populations have been tested as cell-free therapy in various preclinical models of cardiovascular diseases and demonstrate unequivocally encouraging benefits. Here we summarize the findings from recent research on the biological functions of EVs in the ischemic heart disease and heart failure, and their potential as novel diagnostic biomarkers and therapeutic opportunities.

Delivery of Oligonucleotide Therapeutics: Chemical Modifications, Lipid Nanoparticles, and Extracellular Vesicles

Oligonucleotides (ONs) comprise a rapidly growing class of therapeutics. In recent years, the list of FDA-approved ON therapies has rapidly expanded. ONs are small (15-30 bp) nucleotide-based therapeutics which are capable of targeting DNA and RNA as well as other biomolecules. ONs can be subdivided into several classes based on their chemical modifications and on the mechanisms of their target interactions. Historically, the largest hindrance to the widespread usage of ON therapeutics has been their inability to effectively internalize into cells and escape from endosomes to reach their molecular targets in the cytosol or nucleus. While cell uptake has been improved, "endosomal escape" remains a significant problem. There are a range of approaches to overcome this, and in this review, we focus on three: altering the chemical structure of the ONs, formulating synthetic, lipid-based nanoparticles to encapsulate the ONs, or biologically loading the ONs into extracellular vesicles. This review provides a background to the design and mode of action of existing FDA-approved ONs. It presents the most common ON classifications and chemical modifications from a fundamental scientific perspective and provides a roadmap of the cellular uptake pathways by which ONs are trafficked. Finally, this review delves into each of the above-mentioned approaches to ON delivery, highlighting the scientific principles behind each and covering recent advances.

Membrane trafficking: Retrofusion as an escape route out of the endosome

Intraluminal vesicles accumulate within the endosomal lumen before lysosomal delivery or extracellular release. A new study reports the development of an elegant assay showing that these vesicles can escape from the endosomal lumen by 'back-fusion' or 'retrofusion' with the endosomal limiting membrane.

The late endosome-resident lipid bis(monoacylglycero)phosphate is a cofactor for Lassa virus fusion

Arenavirus entry into host cells occurs through a low pH-dependent fusion with late endosomes that is mediated by the viral glycoprotein complex (GPC). The mechanisms of GPC-mediated membrane fusion and of virus targeting to late endosomes are not well understood. To gain insights into arenavirus fusion, we examined cell-cell fusion induced by the Old World Lassa virus (LASV) GPC complex. LASV GPC-mediated cell fusion is more efficient and occurs at higher pH with target cells expressing human LAMP1 compared to cells lacking this cognate receptor. However, human LAMP1 is not absolutely required for cell-cell fusion or LASV entry. We found that GPC-induced fusion progresses through the same lipid intermediates as fusion mediated by other viral glycoproteins–a lipid curvature-sensitive intermediate upstream of hemifusion and a hemifusion intermediate downstream of acid-dependent steps that can be arrested in the cold. Importantly, GPC-mediated fusion and LASV pseudovirus entry are specifically augmented by an anionic lipid, bis(monoacylglycero)phosphate (BMP), which is highly enriched in late endosomes. This lipid also specifically promotes cell fusion mediated by Junin virus GPC, an unrelated New World arenavirus. We show that BMP promotes late steps of LASV fusion downstream of hemifusion–the formation and enlargement of fusion pores. The BMP-dependence of post-hemifusion stages of arenavirus fusion suggests that these viruses evolved to use this lipid as a cofactor to selectively fuse with late endosomes.

Pathogenic arenaviruses pose a serious health threat. The viral envelope glycoprotein GPC mediates attachment to host cells and drives virus entry via endocytosis and low pH-dependent fusion within late endosomes. Understanding the host factors and processes that are essential for arenavirus fusion may identify novel therapeutic targets. To delineate the mechanism of arenavirus entry, we examined cell-cell fusion induced by the Old World Lassa virus GPC proteins at low pH. Lassa GPC-mediated fusion was augmented by the human LAMP1 receptor and progressed through lipid curvature-sensitive intermediates, such as hemifusion (merger of contacting leaflets of viral and cell membrane without the formation of a fusion pore). We found that most GPC-mediated fusion events were off-path hemifusion structures and that the transition from hemifusion to full fusion and fusion pore enlargement were specifically promoted by an anionic lipid, bis(monoacylglycero)phosphate, which is highly enriched in late endosomes. This lipid also specifically promotes fusion of unrelated New World Junin arenavirus. Our results imply that arenaviruses evolved to use bis(monoacylglycero)phosphate to enter cells from late endosomes.

Cellular cholesterol and how to find it

Cholesterol is an essential component of eukaryotic cellular membranes. Information about its subcellular localization and transport pathways inside cells are key for the understanding and treatment of cholesterol-related diseases. In this review we give an overview over the most commonly used methods that contributed to our current understanding of subcellular cholesterol localization and transport routes. First, we discuss methods that provide insights into cholesterol metabolism based on readouts of downstream effects such as esterification. Subsequently, we focus on the use of cholesterol-binding molecules as probes that facilitate visualization and quantification of sterols inside of cells. Finally, we explore different analogues of cholesterol which, when taken up by living cells, are integrated and transported in a similar fashion as endogenous sterols. Taken together, we highlight the challenges and advantages of each method such that researchers studying aspects of cholesterol transport may choose the most pertinent approach for their problem.

Acid Sphingomyelinase, a Lysosomal and Secretory Phospholipase C, Is Key for Cellular Phospholipid Catabolism

Here, we present the main features of human acid sphingomyelinase (ASM), its biosynthesis, processing and intracellular trafficking, its structure, its broad substrate specificity, and the proposed mode of action at the surface of the phospholipid substrate carrying intraendolysosomal luminal vesicles. In addition, we discuss the complex regulation of its phospholipid cleaving activity by membrane lipids and lipid-binding proteins. The majority of the literature implies that ASM hydrolyses solely sphingomyelin to generate ceramide and ignores its ability to degrade further substrates. Indeed, more than twenty different phospholipids are cleaved by ASM in vitro, including some minor but functionally important phospholipids such as the growth factor ceramide-1-phosphate and the unique lysosomal lysolipid bis(monoacylglycero)phosphate. The inherited ASM deficiency, Niemann-Pick disease type A and B, impairs mainly, but not only, cellular sphingomyelin catabolism, causing a progressive sphingomyelin accumulation, which furthermore triggers a secondary accumulation of lipids (cholesterol, glucosylceramide, GM2) by inhibiting their turnover in late endosomes and lysosomes. However, ASM appears to be involved in a variety of major cellular functions with a regulatory significance for an increasing number of metabolic disorders. The biochemical characteristics of ASM, their potential effect on cellular lipid turnover, as well as a potential impact on physiological processes will be discussed.

In situ Tissue Regeneration in the Cornea from Bench to Bedside

Corneal blindness accounts for 5.1% of visual deficiency and is the fourth leading cause of blindness globally. An additional 1.5-2 million people develop corneal blindness each year, including many children born with or who later develop corneal infections. Over 90% of corneal blind people globally live in low- and middle-income regions (LMIRs), where corneal ulcers are approximately 10-fold higher compared to high-income countries. While corneal transplantation is an effective option for patients in high-income countries, there is a considerable global shortage of corneal graft tissue and limited corneal transplant programs in many LMIRs. In situ tissue regeneration aims to restore diseases or damaged tissues by inducing organ regeneration. This can be achieved in the cornea using biomaterials based on extracellular matrix (ECM) components like collagen, hyaluronic acid, and silk. Solid corneal implants based on recombinant human collagen type III were successfully implanted into patients resulting in regeneration of the corneal epithelium, stroma, and sub-basal nerve plexus. As ECM crosslinking and manufacturing methods improve, the focus of biomaterial development has shifted to injectable, in situ gelling formulations. Collagen, collagen-mimetic, and gelatin-based in situ gelling formulas have shown the ability to repair corneal wounds, surgical incisions, and perforations in in-vivo models. Biomaterial approaches may not be sufficient to treat inflammatory conditions, so other cell-free therapies such as treatment with tolerogenic exosomes and extracellular vesicles may improve treatment outcomes. Overall, many of the technologies described here show promise as future medical devices or combination products with cell or drug-based therapies. In situ tissue regeneration, particularly with liquid formulas, offers the ability to triage and treat corneal injuries and disease with a single regenerative solution, providing alternatives to organ transplantation and improving patient outcomes.