Time course and temperature dependence of synaptic vesicle endocytosis

Metabolic regulation of single synaptic vesicle exo- and endocytosis in hippocampal synapses

Glucose has long been considered a primary energy source for synaptic function. However, it remains unclear to what extent alternative fuels, such as lactate/pyruvate, contribute to powering synaptic transmission. By detecting individual release events in hippocampal synapses, we find that mitochondrial ATP production regulates basal vesicle release probability and release location within the active zone (AZ), evoked by single action potentials. Mitochondrial inhibition shifts vesicle release closer to the AZ center and alters the efficiency of vesicle retrieval by increasing the occurrence of ultrafast endocytosis. Furthermore, we uncover that terminals can use oxidative fuels to maintain the vesicle cycle during trains of activity. Mitochondria are sparsely distributed along hippocampal axons, and we find that terminals containing mitochondria display enhanced vesicle release and reuptake during high-frequency trains. Our findings suggest that mitochondria not only regulate several fundamental features of synaptic transmission but may also contribute to modulation of short-term synaptic plasticity.

Intelligent berberine-loaded erythrocytes attenuated inflammatory cytokine productions in macrophages

Erythrocytes are impressive tools for drug delivery, especially to macrophages. Therefore, berberine was loaded into erythrocytes using both hypotonic pre-swelling and endocytosis methods to target macrophages. Physicochemical and kinetic parameters of the resulting carrier cells, such as drug loading/release kinetics, osmotic fragility, and hematological indices, were determined. Drug loading was optimized for the study using Taguchi experimental design and lab experiments. Loaded erythrocytes were targeted to macrophages using ZnCl2 and bis-sulfosuccinimidyl-suberate, and targeting was evaluated using flow cytometry and Wright-Giemsa staining. Differentiated macrophages were stimulated with lipopolysaccharide, and the inflammatory profiles of macrophages were evaluated using ELISA, western blotting, and real-time PCR. Findings indicated that the endocytosis method is preferred due to its low impact on the erythrocyte's structural integrity. Maximum loading achieved (1386.68 ± 22.43 μg/ml) at 1500 μg/ml berberine treatment at 37 °C for 2 h. Berberine successfully inhibited NF-κB translation in macrophages, and inflammatory response markers such as IL-1β, IL-8, IL-23, and TNF-α were decreased by approximately ninefold, sixfold, twofold, eightfold, and twofold, respectively, compared to the LPS-treated macrophages. It was concluded that berberine-loaded erythrocytes can effectively target macrophages and modulate the inflammatory response.

Length-Dependent Cellular Internalization of Nanobody-Functionalized Poly(2-oxazoline) Nanorods

The ability to target specific tissues and to be internalized by cells is critical for successful nanoparticle-based targeted drug delivery. Here, we combined "stealthy" rod-shaped poly(2-oxazoline) (POx) nanoparticles of different lengths with a cancer marker targeting nanobody and a fluorescent cell internalization sensor via a heat-induced living crystallization-driven self-assembly (CDSA) strategy. A significant increase in association and uptake driven by nanobody-receptor interactions was observed alongside nanorod-length-dependent kinetics. Importantly, the incorporation of the internalization sensor allowed for quantitative differentiation between cell surface association and internalization of the targeted nanorods, revealing unprecedented length-dependent cellular interactions of CDSA nanorods. This study highlights the modularity and versatility of the heat-induced CDSA process and further demonstrates the potential of POx nanorods as a modular nanomedicine platform.

Gadolinium Effects on Liposome Fluidity and Size Depend on the Headgroup and Side Chain Structure of Key Mammalian Brain Lipids

The lanthanide metal gadolinium has been used in the healthcare industry as a paramagnetic contrast agent for years. Gadolinium deposition in brain tissue and kidneys has been reported following gadolinium-based contrast agent administration to patients undergoing MRI. This study demonstrates the detrimental effects of gadolinium exposure at the level of the cell membrane. Biophysical analysis using fluorescence spectroscopy and dynamic light scattering illustrates differential interactions of gadolinium ions with key classes of brain membrane lipids, including phosphatidylcholines and sphingomyelins, as well as brain polar extracts and biomimetic brain model membranes. Electrostatic attraction to negatively charged lipids like phosphatidylserine facilitates metal complexation but zwitterionic phosphatidylcholine and sphingomyelin interaction was also significant, leading to membrane rigidification and increases in liposome size. Effects were stronger for fully saturated over monounsaturated acyl chains. The metal targets key lipid classes of brain membranes and these biophysical changes could be very detrimental in biological membranes, suggesting that the potential negative impact of gadolinium contrast agents will require more scientific attention.

Deciphering variations in the endocytic uptake of a cell-penetrating peptide: the crucial role of cell culture protocols

Delivery tools, including cell-penetrating peptides (CPPs), are often inefficient due to a combination of poor endocytosis and endosomal escape. Aspects that impact the delivery of CPPs are typically characterized using tissue culture models. One problem of using cell culture is that cell culture protocols have the potential to contribute to endosomal uptake and endosomal release of CPPs. Hence, a systematic study to identify which aspects of cell culturing techniques impact the endocytic uptake of a typical CPP, the TMR-TAT peptide (peptide sequence derived from HIV1-TAT with the N-terminus labeled with tetramethylrhodamine), was conducted. Aspects of cell culturing protocols previously found to generally modulate endocytosis, such as cell density, washing steps, and cell aging, did not affect TMR-TAT endocytosis. In contrast, cell dissociation methods, media, temperature, serum starvation, and media composition all contributed to changes in uptake. To establish a range of endocytosis achievable by different cell culture protocols, TMR-TAT uptake was compared among protocols. These protocols led to changes in uptake of more than 13-fold, indicating that differences in cell culturing techniques have a cumulative effect on CPP uptake. Taken together this study highlights how different protocols can influence the amount of endocytic uptake of TMR-TAT. Additionally, parameters that can be exploited to improve CPP accumulation in endosomes were identified. The protocols identified herein have the potential to be paired with other delivery enhancing strategies to improve overall delivery efficiency of CPPs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10616-023-00591-1.

Spatially non-overlapping Ca2+ signals drive distinct forms of neurotransmission

Calcium (Ca2+) signaling is tightly regulated within a presynaptic bouton. Here, we visualize Ca2+ signals within hippocampal presynaptic boutons using GCaMP8s tagged to synaptobrevin, a synaptic vesicle protein. We identify evoked presynaptic Ca2+ transients (ePreCTs) that derive from synchronized voltage-gated Ca2+ channel openings, spontaneous presynaptic Ca2+ transients (sPreCTs) that originate from ryanodine sensitive Ca2+ stores, and a baseline Ca2+ signal that arises from stochastic voltage-gated Ca2+ channel openings. We find that baseline Ca2+, but not sPreCTs, contributes to spontaneous glutamate release. We employ photobleaching as a use-dependent tool to probe nano-organization of Ca2+ signals and observe that all three occur in non-overlapping domains within the synapse at near-resting conditions. However, increased depolarization induces intermixing of these Ca2+ domains via both local and non-local synaptic vesicle turnover. Our findings reveal nanosegregation of Ca2+ signals within a presynaptic terminal that derive from multiple sources and in turn drive specific modes of neurotransmission.

Vesicle formation induced by thermal fluctuations

The process of fission and vesicle formation depends on the geometry of the membrane that will split. For instance, a flat surface finds it difficult to form vesicles because of the lack of curved regions where to start the process. Here we show that vesicle formation can be promoted by temperature, by using a membrane phase field model with Gaussian curvature. We find a phase transition between fluctuating and vesiculation phases that depends on temperature, spontaneous curvature, and the ratio between bending and Gaussian moduli. We analysed the energy dynamical behaviour of these processes and found that the main driving ingredient is the Gaussian energy term, although the curvature energy term usually helps with the process as well. We also found that the chemical potential can be used to investigate the temperature of the system. Finally we address how temperature changes the condition for spontaneous vesiculation for all geometries, making it happen in a wider range of values of the Gaussian modulus.

PROTAC Linkerology Leads to an Optimized Bivalent Chemical Degrader of Polycomb Repressive Complex 2 (PRC2) Components

Bivalent chemical degraders, otherwise known as proteolysis-targeting chimeras (PROTACs), have proven to be an efficient strategy for targeting overexpressed or mutated proteins in cancer. PROTACs provide an alternative approach to small-molecule inhibitors, which are restricted by occupancy-driven pharmacology, often resulting in acquired inhibitor resistance via compensatory increases in protein expression. Despite the advantages of bivalent chemical degraders, they often have suboptimal physicochemical properties and optimization for efficient degradation remains highly unpredictable. Herein, we report the development of a potent EED-targeted PRC2 degrader, UNC7700. UNC7700 contains a unique cis-cyclobutane linker and potently degrades PRC2 components EED (DC50 = 111 nM; Dmax = 84%), EZH2WT/EZH2Y641N (DC50 = 275 nM; Dmax = 86%), and to a lesser extent SUZ12 (Dmax = 44%) after 24 h in a diffuse large B-cell lymphoma DB cell line. Characterization of UNC7700 and related compounds for ternary complex formation and cellular permeability to provide a rationale for the observed improvement in degradation efficiency remained challenging. Importantly, UNC7700 dramatically reduces H3K27me3 levels and is anti-proliferative in DB cells (EC50 = 0.79 ± 0.53 μM).

Bile Acid Conjugation on Solid Nanoparticles Enhances ASBT-Mediated Endocytosis and Chylomicron Pathway but Weakens the Transcytosis by Inducing Transport Flow in a Cellular Negative Feedback Loop

Bile acid-modified nanoparticles provide a convenient strategy to improve oral bioavailability of poorly permeable drugs by exploiting specific interactions with bile acid transporters. However, the underlying mechanisms are unknown, especially considering the different absorption sites of free bile acids (ileum) and digested fat molecules from bile acid-emulsified fat droplets (duodenum). Here, glycocholic acid (GCA)-conjugated polystyrene nanoparticles (GCPNs) are synthesized and their transport in Caco-2 cell models is studied. GCA conjugation enhances the uptake by interactions with apical sodium-dependent bile acid transporter (ASBT). A new pathway correlated with both ASBT and chylomicron pathways is identified. Meanwhile, the higher uptake of GCPNs does not lead to higher transcytosis to the same degree compared with unmodified nanoparticles (CPNs). The pharmacological and genomics study confirm that GCA conjugation changes the endocytosis mechanisms and downregulates the cellular response to the transport at gene levels, which works as a negative feedback loop and explains the higher cellular retention of GCPNs. These findings offer a solid foundation in the bile acid-based nanomedicine design, with utilizing advantages of the ASBT-mediated uptake, as well as inspiration to take comprehensive consideration of the cellular response with more developed technologies.

Alix is required for activity-dependent bulk endocytosis at brain synapses

In chemical synapses undergoing high frequency stimulation, vesicle components can be retrieved from the plasma membrane via a clathrin-independent process called activity-dependent bulk endocytosis (ADBE). Alix (ALG-2-interacting protein X/PDCD6IP) is an adaptor protein binding to ESCRT and endophilin-A proteins which is required for clathrin-independent endocytosis in fibroblasts. Alix is expressed in neurons and concentrates at synapses during epileptic seizures. Here, we used cultured neurons to show that Alix is recruited to presynapses where it interacts with and concentrates endophilin-A during conditions triggering ADBE. Using Alix knockout (ko) neurons, we showed that this recruitment, which requires interaction with the calcium-binding protein ALG-2, is necessary for ADBE. We also found that presynaptic compartments of Alix ko hippocampi display subtle morphological defects compatible with flawed synaptic activity and plasticity detected electrophysiologically. Furthermore, mice lacking Alix in the forebrain undergo less seizures during kainate-induced status epilepticus and reduced propagation of the epileptiform activity. These results thus show that impairment of ADBE due to the lack of neuronal Alix leads to abnormal synaptic recovery during physiological or pathological repeated stimulations.

The adaptor protein Alix (PDCD6IP) is necessary for membrane shaping underlying various biological processes including endocytosis. This study shows that Alix mediates activity-dependent bulk endocytosis and is required for correct synaptic physiology under normal and pathological conditions.

Interaction between nanometer calcium oxalate and renal epithelial cells repaired with carboxymethylated polysaccharides

OBJECTIVE

Injury of renal tubular epithelial cells (HK-2) is an important cause of kidney stone formation. In this article, the repairing effect of polysaccharide (PCP0) extracted from the traditional Chinese medicine Poria cocos and its carboxymethylated derivatives on damaged HK-2 cells was studied, and the differences in adhesion and endocytosis of the cells to nanometer calcium oxalate monohydrate (COM) before and after repair were explored.

METHODS

Sodium oxalate (2.8 mmol/L) was used to damage HK-2 cells to establish a damage model, and then Poria cocos polysaccharides (PCPs) with different carboxyl (COOH) contents were used to repair the damaged cells. The changes in the biochemical indicators of the cells before and after the repair and the changes in the ability to adhere to and internalize nano-COM were detected.

RESULTS

The natural PCPs (PCP0, COOH content = 2.56%) were carboxymethylated, and three carboxylated modified Poria cocos with 7.48% (PCP1), 12.07% (PCP2), and 17.18% (PCP3) COOH contents were obtained. PCPs could repair the damaged HK-2 cells, and the cell viability was enhanced after repair. The cell morphology was gradually repaired, the proliferation and healing rate were increased. The ROS production was reduced, and the polarity of the mitochondrial membrane potential was restored. The level of intracellular Ca²⁺ ions decreased, and the autophagy response was weakened.

CONCLUSION

The cells repaired by PCPs inhibited the adhesion to nano-COM and simultaneously promoted the endocytosis of nano-COM. The endocytic crystals mainly accumulated in the lysosome. Inhibiting adhesion and increasing endocytosis could reduce the nucleation, growth, and aggregation of cell surface crystals, thereby inhibiting the formation of kidney stones. With the increase of COOH content in PCPs, its ability to repair damaged cells, inhibit crystal adhesion, and promote crystal endocytosis all increased, that is, PCP3 with the highest COOH content showed the best ability to inhibit stone formation.

A Peptide-Based Fluorescent Sensor for Anionic Phospholipids

Anionic phospholipids are key cell signal mediators. The distribution of these lipids on the cell membrane and intracellular organelle membranes guides the recruitment of signaling proteins leading to the regulation of cellular processes. Hence, fluorescent sensors that can detect anionic phospholipids within living cells can provide a handle into revealing molecular mechanisms underlying lipid-mediated signal regulation. A major challenge in the detection of anionic phospholipids is related to the presence of these phospholipids mostly in the inner leaflet of the plasma membrane and in the membranes of intracellular organelles. Hence, cell-permeable sensors would provide an advantage by enabling the rapid detection and tracking of intracellular pools of anionic phospholipids. We have developed a peptide-based, cell-permeable, water-soluble, and ratiometric fluorescent sensor that entered cells within 15 min of incubation via the endosomal machinery and showed punctate labeling in the cytoplasm. The probe could also be introduced into living cells via lipofection, which allows bypassing of endosomal uptake, to image anionic phospholipids in the cell membrane. We validated the ability of the sensor toward detection of intracellular anionic phospholipids by colocalization studies with a fluorescently tagged lipid and a protein-based anionic phospholipid sensor. Further, the sensor could image the externalization of anionic phospholipids during programmed cell death, indicating the ability of the probe toward detection of both intra- and extracellular anionic phospholipids based on the biological context.

Lipid raft-mediated and upregulated coordination pathways assist transport of glycocholic acid-modified nanoparticle in a human breast cancer cell line of SK-BR-3

Bile acid transporter-targeting has been proven to be an effective strategy to improve drug delivery to hepatocytes and enterocytes. With increasing discoveries of bile acid transporter expression on tumor cells, bile acid-modified anticancer drugs are gradually attaining interests. In our previous study, we confirmed the efficacy of glycocholic acid-conjugated polystyrene nanoparticles (GCPN) on apical sodium bile acid transporter (ASBT)-expressed SK-BR-3 cells. However, the transport mechanisms remain unknown, due to the nanosized carriers are unlikely to be pumped through the narrow cavities of ASBT. To clarify their transport pathways, in this article, pharmacological inhibition and gene knocking-down studies were performed, which revealed that GCPN were primarily internalized via non-caveolar lipid raft-mediated endocytosis. Proteomics was analyzed to explore the in-depth mechanisms. In total 561 proteins were identified and statistical overrepresentation test was used to analyze the gene ontology (GO) upregulated pathways based on the highly expressed proteins. It was found that multiple pathways were upregulated and might coordinate to assist the location of the GCPN-ASBT complex and the recycling of ASBT. Among the highly expressed proteins, myelin and lymphocyte protein 2 (MAL2) was selected and confirmed to colocalize with GCPN, which further supported the lipid raft-mediated process. These findings will help set up a platform for design the bile acid-modified nanomedicines and regulate their transport to improve their anticancer efficacy.

Transsynaptic N-Cadherin Adhesion Complexes Control Presynaptic Vesicle and Bulk Endocytosis at Physiological Temperature

At mammalian glutamatergic synapses, most basic elements of synaptic transmission have been shown to be modulated by specific transsynaptic adhesion complexes. However, although crucial for synapse homeostasis, a physiological regulation of synaptic vesicle endocytosis by adhesion molecules has not been firmly established. The homophilic adhesion protein N-cadherin is localized at the peri-active zone, where the highly temperature-dependent endocytosis of vesicles occurs. Here, we demonstrate an important modulatory role of N-cadherin in endocytosis at near physiological temperature by synaptophysin-pHluorin imaging. Different modes of endocytosis including bulk endocytosis were dependent on N-cadherin expression and function. N-cadherin modulation might be mediated by actin filaments because actin polymerization ameliorated the knockout-induced endocytosis defect. Using super-resolution imaging, we found strong recruitment of N-cadherin to glutamatergic synapses upon massive vesicle release, which might in turn enhance vesicle endocytosis. This provides a novel, adhesion protein-mediated mechanism for efficient coupling of exo- and endocytosis.

The relationship between fasting-induced torpor, sleep, and wakefulness in laboratory mice

STUDY OBJECTIVES

Torpor is a regulated and reversible state of metabolic suppression used by many mammalian species to conserve energy. Whereas the relationship between torpor and sleep has been well-studied in seasonal hibernators, less is known about the effects of fasting-induced torpor on states of vigilance and brain activity in laboratory mice.

METHODS

Continuous monitoring of electroencephalogram (EEG), electromyogram (EMG), and surface body temperature was undertaken in adult, male C57BL/6 mice over consecutive days of scheduled restricted feeding.

RESULTS

All animals showed bouts of hypothermia that became progressively deeper and longer as fasting progressed. EEG and EMG were markedly affected by hypothermia, although the typical electrophysiological signatures of non-rapid eye movement (NREM) sleep, rapid eye movement (REM) sleep, and wakefulness enabled us to perform vigilance-state classification in all cases. Consistent with previous studies, hypothermic bouts were initiated from a state indistinguishable from NREM sleep, with EEG power decreasing gradually in parallel with decreasing surface body temperature. During deep hypothermia, REM sleep was largely abolished, and we observed shivering-associated intense bursts of muscle activity.

CONCLUSIONS

Our study highlights important similarities between EEG signatures of fasting-induced torpor in mice, daily torpor in Djungarian hamsters and hibernation in seasonally hibernating species. Future studies are necessary to clarify the effects on fasting-induced torpor on subsequent sleep.

Cytosolic protein delivery via metabolic glycoengineering and bioorthogonal click reactions

Cytosolic protein delivery holds great potential for the development of protein-based biotechnologies and therapeutics. Currently, cytosolic protein delivery is mainly achieved with the assistance of various carriers. Herein, we present a universal and effective strategy for carrier-free cytosolic protein delivery via metabolic glycoengineering and bioorthogonal click reactions. Ac₄ManNAz (AAM), an azido-modified N-acetylmannosamine analogue, was first employed to label tumor cell surfaces with abundant azido groups via glycometabolism. Then, proteins including RNase A, cytochrome C (Cyt C), and bovine serum albumin (BSA) were covalently modified with dibenzocyclooctyne (DBCO). Based on the highly efficient bioorthogonal click reactions between DBCO and azido, DBCO-modified proteins could be efficiently internalized by azido-labeled cancer cells. RNase A-DBCO could largely maintain its enzymatic activity and, thus, led to notable anti-tumor efficacy in HeLa and B16F10 cells in vitro and in B16F10 xenograft tumors in vivo. This study therefore provides a simple and powerful approach for carrier-free protein delivery and would have broad applicability in anti-tumor protein therapy.

Cytosolic delivery of membrane-penetrating QDs into T cell lymphocytes: implications in immunotherapy and drug delivery

We report single-particle characterization of membrane-penetrating semiconductor quantum dots (QDs) in T cell lymphocytes. We functionalized water-soluble CdSe/CdZnS QDs with a cell-penetrating peptide composed of an Asp-Ser-Ser (DSS) repeat sequence. DSS and peptide-free control QDs displayed concentration-dependent internalization. Intensity profiles from single-particle imaging revealed a propensity of DSS-QDs to maintain a monomeric state in the T cell cytosol, whereas control QDs formed pronounced clusters. Single-particle tracking showed a direct correlation between individual QD clusters' mobility and aggregation state. A significant portion of control QDs colocalized with an endosome marker inside the T cells, while the percentage of DSS-QDs colocalized dropped to 9%. Endocytosis inhibition abrogated the internalization of control QDs, while DSS-QD internalization only mildly decreased, suggesting an alternative cell-entry mechanism. Using 3D single-particle tracking, we captured the rapid membrane-penetrating activity of a DSS-QD. The ability to characterize membrane penetrating activities in live T cells creates inroads for the optimization of gene therapy and drug delivery through the use of novel nanomaterials.

Calcium-Independent Exo-endocytosis Coupling at Small Central Synapses

At presynaptic terminals, neurotransmitters are released by synaptic vesicle exocytosis at the active zone. In order to maintain efficient neurotransmission and proper synaptic structure, sites of vesicle fusion must be cleared rapidly by endocytosis. Therefore, the coupling of exo- and endocytosis is crucial. Despite many years of research, the exact molecular and biophysical requirements for the coupling of exo- and endocytosis remain unclear. We investigate whether endocytosis can be triggered in a calcium-independent fashion by evoking calcium-independent exocytosis using a hypertonic sucrose solution. We demonstrate that endocytosis can be triggered, in the absence of calcium influx, in a clathrin-independent manner that relies on actin polymerization. Our findings point to a central role of membrane tension dependent on actin for efficient coupling of exo- and endocytosis.

Temperature effects on synaptic transmission and neuronal function in the visual thalamus

Numerous neuronal properties including the synaptic vesicle release process, neurotransmitter receptor complement, and postsynaptic ion channels are involved in transforming synaptic inputs into postsynaptic spiking. Temperature is a significant influencer of neuronal function and synaptic integration. Changing temperature can affect neuronal physiology in a diversity of ways depending on how it affects different members of the cell’s ion channel complement. Temperature’s effects on neuronal function are critical for pathological states such as fever, which can trigger seizure activity, but are also important in interpreting and comparing results of experiments conducted at room vs physiological temperature. The goal of this study was to examine the influence of temperature on synaptic properties and ion channel function in thalamocortical (TC) relay neurons in acute brain slices of the dorsal lateral geniculate nucleus, a key synaptic target of retinal ganglion cells in the thalamus. Warming the superfusate in patch clamp experiments with acutely-prepared brain slices led to an overall inhibition of synaptically-driven spiking behavior in TC neurons in response to a retinal ganglion cell spike train. Further study revealed that this was associated with an increase in presynaptic synaptic vesicle release probability and synaptic depression and altered passive and active membrane properties. Additionally, warming the superfusate triggered activation of an inwardly rectifying potassium current and altered the voltage-dependence of voltage-gated Na⁺ currents and T-type calcium currents. This study highlights the importance of careful temperature control in ex vivo physiological experiments and illustrates how numerous properties such as synaptic inputs, active conductances, and passive membrane properties converge to determine spike output.

Therapeutic Delivery of Polymeric Tadpole Nanostructures with High Selectivity to Triple Negative Breast Cancer Cells

Targeted delivery of therapeutic drugs using nanoparticles to the highly aggressive triple negative breast cancer cells has the potential to reduce side effects and drug resistance. Cell entry into triple negative cells can be enhanced by incorporating cell binding receptor molecules on the surface of the nanoparticles to enhance receptor-mediated entry pathways, including clatherin or caveolae endocytosis. However, for highly aggressive cancer cells, these pathways may not be effective, with the more rapid and high volume uptake from macropinocytosis or phagocytosis being significantly more advantageous. Here we show, in the absence of attached cell binding receptor molecules, that asymmetric polymer tadpole nanostructure coated with a thermoresponsive poly(N-isopropylacrylamide) polymer with approximately 50% of this polymer in a globular conformation resulted in both high selectivity and rapid uptake into the triple breast cancer cell line MDA-MB-231. We found that the poly(N-isopropylacrylamide) surface coating in combination with the tadpole's unique shape had an almost 15-fold increase in cell uptake compared to spherical particles with the same polymer coating, and that the mode of entry was most likely through phagocytosis. Delivery of the tadpole attached with doxorubicin (a prodrug, which can be released at pHs < 6) showed a remarkable 10-fold decrease in the IC₅₀ compared to free doxorubicin. It was further observed that cell death was primarily through late apoptosis, which may allow further protection from the body's own immune system. Our results demonstrate that by tuning the chemical composition, polymer conformation and using an asymmetric-shaped nanoparticle, both selectivity and effective delivery and release of therapeutics can be achieved, and such insights will allow the design of nanoparticles for optimal cancer outcomes.

Protein Kinase C and Calmodulin Serve As Calcium Sensors for Calcium-Stimulated Endocytosis at Synapses

Calcium influx triggers and facilitates endocytosis, which recycles vesicles and thus sustains synaptic transmission. Despite decades of studies, the underlying calcium sensor remained not well understood. Here, we examined two calcium binding proteins, protein kinase C (PKC) and calmodulin. Whether PKC is involved in endocytosis was unclear; whether calmodulin acts as a calcium sensor for endocytosis was neither clear, although calmodulin involvement in endocytosis had been suggested. We generated PKC (α or β-isoform) and calmodulin (calmodulin 2 gene) knock-out mice of either sex and measured endocytosis with capacitance measurements, pHluorin imaging and electron microscopy. We found that these knock-outs inhibited slow (∼10-30 s) and rapid (<∼3 s) endocytosis at large calyx-type calyces, and inhibited slow endocytosis and bulk endocytosis (forming large endosome-like structures) at small conventional hippocampal synapses, suggesting the involvement of PKC and calmodulin in three most common forms of endocytosis-the slow, rapid and bulk endocytosis. Inhibition of slow endocytosis in PKC or calmodulin 2 knock-out hippocampal synapses was rescued by overexpressing wild-type PKC or calmodulin, but not calcium-binding-deficient PKC or calmodulin mutant, respectively, suggesting that calcium stimulates endocytosis by binding with its calcium sensor PKC and calmodulin. PKC and calmodulin 2 knock-out inhibited calcium-dependent vesicle mobilization to the readily releasable pool, suggesting that PKC and calmodulin may mediate calcium-dependent facilitation of vesicle mobilization. These findings shed light on the molecular signaling link among calcium, endocytosis and vesicle mobilization that are crucial in maintaining synaptic transmission and neuronal network activity.SIGNIFICANCE STATEMENT Vesicle fusion releases neurotransmitters to mediate synaptic transmission. To sustain synaptic transmission, fused vesicles must be retrieved via endocytosis. Accumulating evidence suggests that calcium influx triggers synaptic vesicle endocytosis. However, how calcium triggers endocytosis is not well understood. Using genetic tools together with capacitance measurements, optical imaging and electron microscopy, we identified two calcium sensors, including protein kinase C (α and β isoforms) and calmodulin, for the most commonly observed forms of endocytosis: slow, rapid, and bulk. We also found that these two proteins are involved in calcium-dependent vesicle mobilization to the readily releasable pool. These results provide the molecular signaling link among calcium, endocytosis, and vesicle mobilization that are essential in sustaining synaptic transmission and neuronal network activity.