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Endocytosis is a critical step in the process by which many therapeutic nanomedicines reach their intracellular targets. Our understanding of cellular uptake mechanisms has developed substantially in the past five years. However, these advances in cell biology have not fully translated to the nanoscience and therapeutics literature. Misconceptions surrounding the role of different endocytic pathways and how to study these pathways are hindering progress in developing improved nanoparticle therapies. Here, we summarize the latest insights into cellular uptake mechanisms and pathways. We highlight limitations of current systems to study endocytosis, particularly problems with non-specific inhibitors. We also summarize alternative genetic approaches to robustly probe these pathways and discuss the need to understand how cells endocytose particles in vivo. We hope that this critical assessment of the current methods used in studying nanoparticle uptake will guide future studies at the interface of cell biology and nanomedicine.Successful nanomedicine approaches rely on the efficient cellular uptake of nanoparticles, yet endocytic mechanisms remain challenging to probe. In this Review the authors describe the different cellular endocytic pathways and provide a critical discussion of the available tools and systems for their study.

IntroductionIschaemic heart disease is the leading cause of morbidity and mortality worldwide. During myocardial infarction, occlusion of the coronaries reduces blood flow to the heart causing ischaemic injury and eventual cell death. Restoration of blood flow by percutaneous coronary intervention and/or thrombolytics, can paradoxically lead to further myocardial damage and cell death. The extent of ischaemic- reperfusion (IR) injury determines the size of the infarct and the subsequent recovery of the patient, therefore understanding the molecular mechanisms responsible for IR injury is vital in developing novel therapeutic approaches. Initial events in IR injury include metabolic acidosis, followed by Na+ and Ca2+ overload inside the myocyte. Elevated Ca2+ prompts opening of the mitochondrial transition pore (mPTP), triggering release of coenzyme A (CoA) into the cytoplasm where it is made into palmitoyl CoA. This can be utilised by zDHHC-PATs, zDHHC5 in particular, to palmitoylate membrane proteins at the cell surface. This hyper- palmitoylation event drives a process called massive endocytosis (MEND), where ca. 70% of the plasma membrane becomes engulfed, resulting in cell damage and death.AimThe aim of this study was to determine the changes that occur in the cardiac palmitome following IR injury, in the presence and absence of cyclosporine A (an inhibitor of the mPTP) by quantitative proteomics.MethodsThe hearts of male rats were perfused respective to three different conditions to mimic ischaemic reperfusion injury with and without treatment with CsA. The ventricles were frozen and analysed using Acyl-RAC. The triptic peptides were isolated and were sent for analysis by mass spectrometry.Abstract BS50 Table 1Rat Heart Perfusion ConditionsResults and ConclusionThe data revealed that 87 proteins had increased palmitoylation under IR conditions. Of these, the increase in palmitoylation on IR could be blocked for 27 proteins with CsA treatment, suggesting that their palmitoylation may be linked to opening of the mPTP. When the biological function of the CsA sensitive proteins was considered, three proteins were identified that may play an important role in IR injury, namely PACSIN3 (plays a role in endocytosis and possibly MEND), as well as Tescalcin and NHE1 (which may be responsible for the Na+ and Ca2+ overload).Abstract BS50 Figure 1

Macrophages are dynamic cells that mature under the influence of signals from the local microenvironment into either classically (M1) or alternatively (M2) activated macrophages with specific functional and phenotypic properties. Although the phenotypic identification of M1 and M2 macrophages is well established in mice, this is less clear for human macrophages. In addition, the persistence and reversibility of polarized human phenotypes is not well established. Human peripheral blood monocytes were differentiated into uncommitted macrophages (M0) and then polarized to M1 and M2 phenotypes using LPS/IFN-γ and IL-4/IL-13, respectively. M1 and M2 were identified as CD64(+)CD80(+) and CD11b(+)CD209(+), respectively, by flow cytometry. Polarized M1 cells secreted IP-10, IFN-γ, IL-8, TNF-α, IL-1β, and RANTES, whereas M2 cells secreted IL-13, CCL17, and CCL18. Functionally, M2 cells were highly endocytic. In cytokine-deficient medium, the polarized macrophages reverted back to the M0 state within 12 days. If previously polarized macrophages were given the alternative polarizing stimulus after 6 days of resting in cytokine-deficient medium, a switch in polarization was seen (i.e., M1 macrophages switched to M2 and expressed CD11b(+)CD209(+) and vice versa). In summary, we report phenotypic identification of human M1 and M2 macrophages, their functional characteristics, and their ability to be reprogrammed given the appropriate stimuli.

We previously found that ophthalmic formulations containing nanoparticles prepared by a bead mill method lead to an increase in bioavailability in comparison with traditional formulations (solution type). However, the transcorneal penetration pathway for ophthalmic formulations has not been explained yet. In this study, we investigated the mechanism of transcorneal penetration in the application of ophthalmic formulations containing indomethacin nanoparticles (IMC-NPs). IMC-NPs was prepared by the bead mill method. For the analysis of energy-dependent endocytosis, corneal epithelial (HCE-T) cell monolayers and removed rabbit cornea were thermoregulated at 4°C, where energy-dependent endocytosis is inhibited. In addition, for the analysis of different endocytosis pathways using pharmacological inhibitors, inhibitors of caveolae-mediated endocytosis (54 µM nystatin), clathrin-mediated endocytosis (40 µM dynasore), macropinocytosis (2 µM rottlerin) or phagocytosis (10 µM cytochalasin D) were used. The ophthalmic formulations containing 35-200 nm sized indomethacin nanoparticles were prepared by treatment with a bead mill, and no aggregation or degradation of indomethacin was observed in IMC-NPs. The transcorneal penetration of indomethacin was significantly decreased by the combination of nystatin, dynasore and rottlerin, and the decreased penetration levels were similar to those at 4°C in HCE-T cell monolayers and rabbit cornea. In the in vivo experiments using rabbits, dynasore and rottlerin tended to decrease the transcorneal penetration of indomethacin (area under the drug concentration - time curve in the aqueous humor [AUC ]), and the AUC in the nystatin-treated rabbit was significantly lower than that in non-treatment group. In addition, the AUC in rabbit corneas undergoing multi-treatment was obviously lower than that in rabbit corneas treated with each individual endocytosis inhibitor. We found that three energy-dependent endocytosis pathways (clathrin-dependent endocytosis, caveolae-dependent endocytosis and macropinocytosis) are related to the trans-corneal penetration of indomethacin nanoparticles. In particular, the caveolae-dependent endocytosis is strongly involved.

Segmented filamentous bacteria (SFB) are anaerobic, spore-forming, clostridia-like organisms that are important immune modulators in the mammalian gut. For some reason, SFB do not provoke inflammatory responses. Ladinsky et al. probed the mechanistic basis of this soothing effect in mice. SFB attach tightly to intestinal epithelial cells via a hook-like structure. Bacterial material is extruded from the hook and enters the host cell by endocytosis. An extruded SFB protein called P3340 is shuttled by the host protein cell division control protein 42 homolog (CDC42) through the endosomelysosome vesicular pathway to the basolateral side of the intestinal epithelial cell. Here, it prompts an immunomodulatory SFB-specific CD4 T helper 17 cell response, possibly via intestinal macrophages. Science , this issue p. eaat4042 Gut bacteria transfer immunogenic proteins into intestinal epithelial cells via adhesion-directed endocytosis, which affects host T cells. Commensal bacteria influence host physiology, without invading host tissues. We show that proteins from segmented filamentous bacteria (SFB) are transferred into intestinal epithelial cells (IECs) through adhesion-directed endocytosis that is distinct from the clathrin-dependent endocytosis of invasive pathogens. This process transfers microbial cell wall–associated proteins, including an antigen that stimulates mucosal T helper 17 (T H 17) cell differentiation, into the cytosol of IECs in a cell division control protein 42 homolog (CDC42)–dependent manner. Removal of CDC42 activity in vivo led to disruption of endocytosis induced by SFB and decreased epithelial antigen acquisition, with consequent loss of mucosal T H 17 cells. Our findings demonstrate direct communication between a resident gut microbe and the host and show that under physiological conditions, IECs acquire antigens from commensal bacteria for generation of T cell responses to the resident microbiota.

Clathrin-mediated endocytosis is a key process in vesicular trafficking that transports a wide range of cargo molecules from the cell surface to the interior. Clathrin-mediated endocytosis was first described over 5 decades ago. Since its discovery, over 50 proteins have been shown to be part of the molecular machinery that generates the clathrin-coated endocytic vesicles. These proteins and the different steps of the endocytic process that they mediate have been studied in detail. However, we still lack a good understanding of how all these different components work together in a highly coordinated manner to drive vesicle formation. Nevertheless, studies in recent years have provided several important insights into how endocytic vesicles are built, starting from initiation, cargo loading and the mechanisms governing membrane bending to membrane scission and the release of the vesicle into the cytoplasm.

Recently identified Parkinson’s disease (PD) genes involved in synaptic vesicle endocytosis, such as DNAJC6 (auxilin), have further implicated synaptic dysfunction in PD pathogenesis. However, how synaptic dysfunction contributes to the vulnerability of human dopaminergic neurons has not been previously explored. Here, we demonstrate that commonly mutated, PD-linked leucine-rich repeat kinase 2 (LRRK2) mediates the phosphorylation of auxilin in its clathrin-binding domain at Ser627. Kinase activity-dependent LRRK2 phosphorylation of auxilin led to differential clathrin binding, resulting in disrupted synaptic vesicle endocytosis and decreased synaptic vesicle density in LRRK2 patient-derived dopaminergic neurons. Moreover, impaired synaptic vesicle endocytosis contributed to the accumulation of oxidized dopamine that in turn mediated pathogenic effects such as decreased glucocerebrosidase activity and increased α-synuclein in mutant LRRK2 neurons. Importantly, these pathogenic phenotypes were partially attenuated by restoring auxilin function in mutant LRRK2 dopaminergic neurons. Together, this work suggests that mutant LRRK2 disrupts synaptic vesicle endocytosis, leading to altered dopamine metabolism and dopamine-mediated toxic effects in patient-derived dopaminergic neurons.

ABSTRACT Antimicrobial peptides have gained much attention in clinical research due to their extensive possibilities of application beyond antimicrobial use. The modification of antimicrobial peptides enables the peptides to target particular cancer cells, improving the specificity and efficiency of the treatment. In this study, TP2‐D‐Tox, a derivative of TP‐D‐Tox, demonstrated a superior anti‐tumor activity towards renal carcinoma, Caki‐2, and breast carcinoma, SK‐BR‐3. TP‐Tox was previously reported to inhibit tumor growth in a mouse model, increasing the overall survival. TP‐ and TP2‐D‐Tox were shown to penetrate the cells via clathrin‐mediated endocytosis, triggered by binding to the subunits of non‐muscle myosin IIa and S100A9. HSPB1 was observed to have a protective effect towards TP2‐D‐Tox against the immediate proteolytic inactivation. The intracellular presence of the peptides evoked mitochondrial permeability transition, generation of reactive oxygen species, and formation of MLKL oligomers in the plasma membrane. Our investigation revealed that TP‐ and TP2‐D‐Tox induced a similar but distinctly regulated cell death in Caki‐2 and SK‐BR‐3 cells. Both peptides established toxicity without activating any caspases, suggesting the possibility of TP‐ and TP2‐D‐Tox as a promising approach to bypass the caspase‐dependent apoptosis‐resistance issue impairing therapeutic responses of many cancer treatments. ()‐D‐Tox peptide binding to the ligands triggers their internalization via clathrin‐mediated endocytosis. Intracellular presence of ()‐D‐Tox peptide induces a transient loss of mitochondrial membrane integrity and oligomerization of MLKL. Translocation of the oligomers to the plasma membrane causes a leakage of the cytosolic content and ultimately leads to cell death.

Endocytosis is one of the major ways cells communicate with their environment. This process is frequently hijacked by pathogens. Endocytosis also participates in the oncogenic transformation. Here, we review the approaches to inhibit endocytosis, discuss chemical inhibitors of this process, and discuss potential clinical applications of the endocytosis inhibitors.

Endocytosis in mammalian cells is a fundamental cellular machinery that regulates vital physiological processes, such as the absorption of metabolites, release of neurotransmitters, uptake of hormone cellular defense, and delivery of biomolecules across the plasma membrane. A remarkable characteristic of the endocytic machinery is the sequential assembly of the complex proteins at the plasma membrane, followed by internalization and fusion of various biomolecules to different cellular compartments. In all eukaryotic cells, functional characterization of endocytic pathways is based on dynamics of the protein complex and signal transduction modules. To coordinate the assembly and functions of the numerous parts of the endocytic machinery, the endocytic proteins interact significantly within and between the modules. Clathrin-dependent and -independent endocytosis, caveolar pathway, and receptor mediated endocytosis have been attributed to a greater variety of physiological and pathophysiological roles such as, autophagy, metabolism, cell division, apoptosis, cellular defense, and intestinal permeabilization. Notably, any defect or alteration in the endocytic machinery results in the development of pathological consequences associated with human diseases such as cancer, cardiovascular diseases, neurological diseases, and inflammatory diseases. In this review, an in-depth endeavor has been made to illustrate the process of endocytosis, and associated mechanisms describing pathological manifestation associated with dysregulated endocytosis machinery.