Exocytosis and endocytosis: modes, functions, and coupling mechanisms

Is clathrin a membrane fission protein?

Membrane fission is thought to involve helix-forming proteins to constrict the Ω-shaped profile's neck. Recent studies suggest that membrane pit-coating proteins, especially clathrin, may also mediate fission via polymerization on the Ω-profile's base or head to generate neck constriction, which underlies various endocytic modes previously attributed as clathrin (Ω-profile head) independent.

SNAP-25: A biomarker of synaptic loss in neurodegeneration

Synaptic dysfunction is one of the most important markers of neurodegenerative diseases, which contribute to cognitive decline and the loss of neurons. Synaptosomal-associated protein 25 (SNAP-25) is a member of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, which plays a significant role in the exocytosis of synaptic vesicles and the release of neurotransmitters. Recent studies have shown that expression levels of SNAP-25 are altered in various neurodegenerative disorders, including Alzheimer's disease (AD), Huntington's disease (HD), and Creutzfeldt-Jakob disease (CJD). These investigations led to the consideration of SNAP-25 as a potential biomarker of synaptic degeneration. Understanding the role of SNAP-25 in neurodegeneration will aid in early diagnosis, disease monitoring, and therapeutic development, and will also provide new insights into synaptic dysfunction as a main feature of neurodegenerative diseases. Therefore, this paper explores the physiological role of SNAP-25, its involvement in synaptic pathology, and the implications of its dysregulation in neurodegenerative conditions, such as AD, HD, and CJD. Literature regarding cerebrospinal fluid (CSF) SNAP-25 levels as a diagnostic marker were reviewed and its applications in detecting the progression of the disease have been discussed. Additionally, the limitations of SNAP-25 as a biomarker, including variability across studies and the need for further validation have been addressed.

Actin maintains synaptic transmission by restraining vesicle release probability

Despite decades of pharmacological studies, how the ubiquitous cytoskeletal actin regulates synaptic transmission remains poorly understood. We addressed this issue with a tissue-specific knockout of actin β-isoform or γ-isoform, combined with recordings of postsynaptic EPSCs, presynaptic capacitance jumps or fluorescent synaptophysin-pHluorin changes, and electron microscopy in large calyx-type and small conventional hippocampal synapses. We found that actin restrains basal synaptic transmission during single action potential firings by lowering the readily releasable vesicle's release probability. Such an inhibition of basal synaptic transmission is turned into facilitation during repetitive firings by slowing down depletion of the readily releasable vesicle pool and, thus, short-term synaptic depression, leading to more effective synaptic transmission for a longer time. These mechanisms, together with the previous finding that actin promotes vesicle replenishment to the readily releasable pool, may control synaptic transmission and short-term synaptic plasticity at many synapses, contributing to neurological disorders caused by actin cytoskeleton impairment.

Characterization of the encystment-inducing activity of extracellular fluid from encysting vegetative cells in the terrestrial ciliated protozoa Colpoda cucullus

The terrestrial ciliated protozoan Colpoda cucullus transforms into a dehydration-resistant cyst upon sensing adverse signals. Recently, we identified a pheromone secreted by encysting vegetative cells of C. cucullus, termed "encystment-inducing pheromone" (EnIP), and characterized its properties. Overpopulated vegetative cells spontaneously encysted in ion-free ultrapure water without external stimuli. The external solution of encysting cells markedly induced encystment in vegetative cells under low cell density conditions, suggesting that EnIP, secreted by encysting cells into the external solution, induced encystment in vegetative cells. Further investigations revealed the following properties of EnIP: 1) EnIP retains encystment-inducing activity even in the presence of EGTA. 2) EnIP exhibits a concentration-dependent encystment effect. 3) EnIP is released within hours at high cell density. 4) EnIP is released by exocytosis. 5) EnIP loses its activity after 1-2 days. 6) EnIP is inactivated by heating and treatment with proteolytic enzymes. 7) The nominal molecular weight of EnIP was estimated to range between 10 and 100 kDa. These findings lead to the conclusion that encystment-induced C. cucullus vegetative cells secrete EnIP, a protein pheromone, with encystment-inducing activity for other cells. This study contributes to understanding microbial communication and reveals a novel mechanism for protist survival in harsh environments.

Coding principles and mechanisms of serotonergic transmission modes

Serotonin-mediated intercellular communication has been implicated in myriad human behaviors and diseases, yet how serotonin communicates and how the communication is regulated remain unclear due to limitations of available monitoring tools. Here, we report a method multiplexing genetically encoded sensor-based imaging and fast-scan cyclic voltammetry, enabling simultaneous recordings of synaptic, perisynaptic, proximate and distal extrasynaptic serotonergic transmission. Employing this method alongside a genetically encoded sensor-based image analysis program (GESIAP), we discovered that heterogeneous firing patterns of serotonergic neurons create various transmission modes in the mouse raphe nucleus and amygdala, encoding information of firing pulse frequency, number, and synchrony using neurotransmitter quantity, releasing synapse count, and synaptic and/or volume transmission. During tonic and low-frequency phasic activities, serotonin is confined within synaptic clefts due to efficient retrieval by perisynaptic transporters, mediating synaptic transmission modes. Conversely, during high-frequency, especially synchronized phasic activities, or when transporter inhibition, serotonin may surpass transporter capacity, and escape synaptic clefts through 1‒3 outlet channels, leading to volume transmission modes. Our results elucidate a mechanism of how channeled synaptic enclosures, synaptic properties, and transporters collaborate to define the coding principles of activity pattern-dependent serotonergic transmission modes.

Loss of Fanconi anemia proteins causes a reliance on lysosomal exocytosis

Mutations in the FA pathway lead to a rare genetic disease that increases risk of bone marrow failure, acute myeloid leukemia, and solid tumors. FA patients have a 500 to 800-fold increase in head and neck squamous cell carcinoma compared to the general population and the treatment for these malignancies are ineffective and limited due to the deficiency in DNA damage repair. Using unbiased CRISPR-interference screening, we found the loss of FA function renders cells dependent on key exocytosis genes such as SNAP23. Further investigation revealed that loss of FA pathway function induced deficiencies in lysosomal health, dysregulation of autophagy and increased lysosomal exocytosis. The compromised cellular state caused by the loss of FA genes is accompanied with decreased lysosome abundance and increased lysosomal membrane permeabilization in cells. We found these signatures in vitro across multiple cell types and cell lines and in clinically relevant FA patient cancers. Our findings are the first to connect the FA pathway to lysosomal exocytosis and thus expands our understanding of FA as a disease and of induced dependencies in FA mutant cancers.

Dense-core vesicles contain exosomes in secretory cells

Dense-core vesicles (DCVs) are found in various types of cells, such as neurons, pancreatic β- cells, and chromaffin cells. These vesicles release transmitters, peptides, and hormones to regulate diverse functions, such as the stress response, immune response, behavior, and blood glucose levels. In traditional electron microscopy after chemical fixation, it is often reported that the dense cores occupy a portion of the vesicle toward the center and are surrounded by a clear halo. With electron microscopy after cryofixation in adrenal chromaffin cells, we report here that we did not observe halos, but dense cores filling up the entire vesicles suggesting that halos are likely the product of chemical fixation. More importantly, we observed that a fraction of DCVs contained 36-168 nm clear-core vesicles. A similar fraction of DCVs labeled with fluorescent false neurotransmitter FFN 511 or the dense-core matrix protein chromogranin A (CGA) were colocalized with fluorescently labeled or endogenous CD63 or ALIX, the membrane or lumen marker of ∼40-160 nm exosomes. These results suggest that DCVs contain exosomes. Since exosomes are generally thought to reside within multivesicular bodies in the cytosol and are released to the extracellular space to mediate diverse cell-to-cell communications, our findings suggest that DCV fusion from many cell types is a new source for releasing exosomes to mediate intercellular communications. Given that DCV fusion mediates many physiological functions, such as stress responses, immune responses, behavior regulation, and blood glucose regulation, exosome release from DCV fusion might contribute to mediating these important functions.

Elucidating the uptake and trafficking of nanostructured lipid carriers as delivery systems for miRNA

Cationic nanostructured lipid carriers (cNLCs) represent promising non-viral carriers for nucleic acids, such as miRNAs, forming stable self-assembled miRNA complexes due to electrostatic interactions. Prepared by high-pressure homogenization, cNLC formulations, both with and without Nile Red dye demonstrated stable particle sizes in the range of 100-120 nm and positive surface charges (>30 mV), which are necessary for effective cellular uptake. The miRNA complexes formed at mass ratios of 1:2.5 and 1:5 showed similar stability and size, with positive zeta potentials, as well as high cell viability (> 80 %) in 3T3-L1 and MCF-7 cell lines. The cellular uptake studies of miRNA:cNLC complexes in both cell lines revealed that uptake was time- and concentration-dependent, with rapid initial uptake in 30 min and a zig-zag pattern over 24 h. To elucidate the endocytosis mechanism of miRNA:cNLC complexes, 3T3-L1 and MCF-7 cells were incubated with different inhibitors (chlorpromazine, 5-[N-ethyl-N-isopropyl] amiloride, dynasore, nystatin, or sodium azide with 2-deoxy-d-glucose). Results showed significant inhibition of uptake at low temperatures and with ATP depletion, suggesting endocytosis, particularly macropinocytosis, as the main uptake mechanism in 3T3-L1 cells. In MCF-7 cells, the uptake was less inhibited by the substances, indicating the need for more specific methods to fully decipher the endocytic mechanisms involved. Confocal laser scanning microscopy images revealed that the complexes are internalized in vesicles, and are primarily localized in the juxtanuclear region, suggesting trafficking through the endolysosomal system. Colocalization study with LysoTracker™ Green DND-26 showed significant colocalization of miRNA:cNLC complexes with lysosomes in 3T3-L1 cells, indicating trafficking through the endolysosomal system. In MCF-7 cells, colocalization was lower, suggesting macropinocytosis as the primary uptake mechanism. Additional studies showed partial colocalization between labeled NLCs and miRNA, indicating that about 50 % of miRNA is released from NLCs within 30 min post-transfection.

MpPUB9, a U-box E3 ubiquitin ligase, acts as a positive regulator by promoting the turnover of MpEXO70.1 under high salinity in Marchantia polymorpha

Marchantia polymorpha, occupying a basal position in the monophyletic assemblage of land plants, displays a notable expansion of plant U-box (PUB) proteins compared with those in animals. We elucidated the roles of MpPUB9 in regulating salt stress tolerance in M. polymorpha. MpPUB9 expression was rapidly induced by high salinity and dehydration. MpPUB9 possessed an intact U-box domain in the N-terminus. MpPUB9-Citrine localized to punctate structures and was peripherally associated with microsomal membranes. Phenotypic analyses demonstrate that the hyponastic and epinastic thallus growth phenotypes, which were induced by the overexpression and suppression of MpPUB9, may provoke salt stress-resistant and -susceptible phenotypes, respectively. MpPUB9 was also found to directly interact with the exocyst protein MpEXO70.1, leading to its ubiquitination. Under high-salinity conditions, though the stability of MpPUB9 was dramatically increased, MpEXO70.1 showed slightly faster turnover rates. Transcriptome analyses showed that salt treatment and the overexpression of MpPUB9 co-upregulated the genes related to the modulation of H2O2 and cell wall organization. Overall, our results suggest that MpPUB9 plays a crucial role in the positive regulation of salt stress tolerance, resulting from its interaction with MpEXO70.1 and modulating turnover of the protein under high-salt conditions via the coordination of UPS with autophagy.

Fission of double-membrane tubes under tension

The division of a cellular compartment culminates with the scission of a highly constricted membrane neck. Scission requires lipid rearrangements, topology changes, and transient formation of nonbilayer intermediate structures driven by curvature stress. Often, a side effect of this stress is pore-formation, which may lead to content leakage and thus breaching of the membrane barrier function. In single-membrane systems, leakage is avoided through the formation of a hemifusion (HF) intermediate, whose structure is still a subject of debate. The consequences of curvature stress have not been explored in double-membrane systems, such as the mitochondrion. Here, we combine experimental and theoretical approaches to study neck constriction and scission driven by tension in biomimetic lipid systems, namely single- and double-membrane nanotubes (sNTs and dNTs), respectively. In sNTs, constriction by high tension gives rise to a metastable HF intermediate (seen as stalk or worm-like micelle), whereas poration is universally slower in a simple neck. In dNTs, high membrane tension causes sequential rupture of each membrane. In contrast, low tension leads to the HF of both membranes, which may lead to a leaky fusion pathway, or may progress to further fusion of the two membranes along a number of transformation pathways. These findings provide a new mechanistic basis for fundamental cellular processes.

Algorithm for semi-automatic detection of insulin granule exocytosis in human pancreatic β-cells

Image processing and analysis are two significant areas that are highly important for interpreting enormous amounts of data obtained from microscopy-based experiments. Several image analysis tools exist for the general detection of fundamental cellular processes, but tools to detect highly distinct cellular functions are few. One such process is exocytosis, which involves the release of vesicular content out of the cell. The size of the vesicles and the inherent differences in the imaging parameters demand specific analysis platforms for detecting exocytosis. In this direction, we have developed an image-processing algorithm based on Lagrangian particle tracking. The tool was developed to ensure that there is efficient detection of punctate structures initially developed by mathematical equations, fluorescent beads and cellular images with fluorescently labelled vesicles that can exocytose. The detection of these punctate structures using the tool was compared with other existing tools, such as find maxima in ImageJ and manual detection. The tool not only met the precision of existing solutions but also expedited the process, resulting in a more time-efficient solution. During exocytosis, there is a sudden dip in the intensity of the fluorescently labelled vesicles that look like punctate structures. The algorithm precisely locates the vesicles' coordinates and quantifies the variations in their respective intensities. Subsequently, the algorithm processes and retrieves pertinent information from large datasets surpassing that of conventional methods under our evaluation, affirming its efficacy. Furthermore, the tool exhibits adaptability for the image analysis of diverse cellular processes, requiring only minimal modifications to ensure accurate detection of exocytosis.

ExoJ - a Fiji/ImageJ2 plugin for automated spatiotemporal detection and analysis of exocytosis

Exocytosis is a dynamic physiological process that enables the release of biomolecules to the surrounding environment via the fusion of membrane compartments to the plasma membrane. Understanding its mechanisms is crucial, as defects can compromise essential biological functions. The development of pH-sensitive optical reporters alongside fluorescence microscopy enables the assessment of individual vesicle exocytosis events at the cellular level. Manual annotation represents, however, a time-consuming task that is prone to selection biases and human operational errors. Here, we introduce ExoJ, an automated plugin based on Fiji/ImageJ2 software. ExoJ identifies user-defined genuine populations of exocytosis events, recording quantitative features including intensity, apparent size and duration. We designed ExoJ to be fully user-configurable, making it suitable for studying distinct forms of vesicle exocytosis regardless of the imaging quality. Our plugin demonstrates its capabilities by showcasing distinct exocytic dynamics among tetraspanins and vesicular SNARE protein reporters. Assessment of performance on synthetic data shows that ExoJ is a robust tool that is capable of correctly identifying exocytosis events independently of signal-to-noise ratio conditions. We propose ExoJ as a standard solution for future comparative and quantitative studies of exocytosis.

Synergistic regulation of fusion pore opening and dilation by SNARE and synaptotagmin-1

Fusion pore opening is a transient intermediate state of synaptic vesicle exocytosis, which is highly dynamic and precisely regulated by the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex and synaptotagmin-1 (Syt1). Yet, the regulatory mechanism is not fully understood. In this work, using single-channel membrane fusion electrophysiology, we determined that SNAREpins are important for driving fusion pore opening and dilation but incapable of regulating the dynamics. When Syt1 was added, the closing frequency of fusion pores significantly increased, while the radius of fusion pores mildly decreased. In response to Ca2+, SNARE/Syt1 greatly increased the radius of fusion pores and reduced their closing frequency. Moreover, the residue F349 in the C2B domain of Syt1, which mediates Syt1 oligomerization, was required for clamping fusion pore opening in the absence of Ca2+, probably by extending the distance between the two membranes. Finally, in Ca2+-triggered fusion, the primary interface between SNARE and Syt1 plays a critical role in stabilizing and dilating the fusion pore, while the polybasic region of Syt1 C2B domain has a mild effect on increasing the radius of the fusion pore. In summary, our results suggest that Syt1, SNARE, and the anionic membrane synergically orchestrate the dynamics of fusion pore opening in synaptic vesicle exocytosis.

Arp2/3-dependent endocytosis ensures Cdc42 oscillations by removing Pak1-mediated negative feedback

The GTPase Cdc42 regulates polarized growth in most eukaryotes. In the bipolar yeast Schizosaccharomyces pombe, Cdc42 activation cycles periodically at sites of polarized growth. These periodic cycles are caused by alternating positive feedback and time-delayed negative feedback loops. At each polarized end, negative feedback is established when active Cdc42 recruits the Pak1 kinase to prevent further Cdc42 activation. It is unclear how Cdc42 activation returns to each end after Pak1-dependent negative feedback. We find that disrupting branched actin-mediated endocytosis disables Cdc42 reactivation at the cell ends. Using experimental and mathematical approaches, we show that endocytosis-dependent Pak1 removal from the cell ends allows the Cdc42 activator Scd1 to return to that end to enable reactivation of Cdc42. Moreover, we show that Pak1 elicits its own removal via activation of endocytosis. These findings provide a deeper insight into the self-organization of Cdc42 regulation and reveal previously unknown feedback with endocytosis in the establishment of cell polarity.

Transient pores in hemifusion diaphragms

Exchange of material across two membranes, as in the case of synaptic neurotransmitter release from a vesicle, involves the formation and poration of a hemifusion diaphragm (HD). The nontrivial geometry of the HD leads to environment-dependent control, regarding the stability and dynamics of the pores required for this kind of exocytosis. This work combines particle simulations, field-based calculations, and phenomenological modeling to explore the factors influencing the stability, dynamics, and possible control mechanisms of pores in HDs. We find that pores preferentially form at the HD rim, and that their stability is sensitive to a number of factors, including the three line tensions, membrane tension, HD size, and the ability of lipids to "flip-flop" across leaflets. Along with a detailed analysis of these factors, we discuss ways that vesicles or cells may use them to open and close pores and thereby quickly and efficiently transport material.

Roles of membrane mechanics-mediated feedback in membrane traffic

Synthesizing the recent progresses, we present our perspectives on how local modulations of membrane curvature, tension, and bending energy define the feedback controls over membrane traffic processes. We speculate the potential mechanisms of, and the control logic behind, the different membrane mechanics-mediated feedback in endocytosis and exo-endocytosis coupling. We elaborate the path forward with the open questions for theoretical considerations and the grand challenges for experimental validations.

Biology of Exfoliation of Plasma Membrane-Derived Vesicles and the Radiation Response: Historical Background, Applications in Biodosimetry and Cell-Free Therapeutics, and Quantal Mechanisms for Their Release and Function with Implications for Space Travel

This historical review of extracellular vesicles in the setting of exposure to ionizing radiation (IR) traces our understanding of how vesicles were initially examined and reported in the literature in the late 1970s (for secreted exosomes) and early 1980s (for plasma membrane-derived, exfoliated vesicles) to where we are now and where we may be headed in the next decade. An emphasis is placed on biophysical properties of extracellular vesicles, energy consumption and the role of vesiculation as an essential component of membrane turnover. The impact of intercellular signal trafficking by vesicle surface and intra-vesicular lipids, proteins, nucleic acids and metabolites is reviewed in the context of biomarkers for estimating individual radiation dose after exposure to radiation, pathogenesis of disease and development of cell-free therapeutics. Since vesicles express both growth stimulatory and inhibitory molecules, a hypothesis is proposed to consider superposition in a shared space and entanglement of molecules by energy sources that are external to human cells. Implications of this approach for travel in deep space are briefly discussed in the context of clinical disorders that have been observed after space travel.

Phospholipid Scramblase 1 Controls Efficient Neurotransmission and Synaptic Vesicle Retrieval at Cerebellar Synapses

Phospholipids (PLs) are asymmetrically distributed at the plasma membrane. This asymmetric lipid distribution is transiently altered during calcium-regulated exocytosis, but the impact of this transient remodeling on presynaptic function is currently unknown. As phospholipid scramblase 1 (PLSCR1) randomizes PL distribution between the two leaflets of the plasma membrane in response to calcium activation, we set out to determine its role in neurotransmission. We report here that PLSCR1 is expressed in cerebellar granule cells (GrCs) and that PLSCR1-dependent phosphatidylserine egress occurred at synapses in response to neuron stimulation. Synaptic transmission is impaired at GrC Plscr1 -/- synapses, and both PS egress and synaptic vesicle (SV) endocytosis are inhibited in Plscr1 -/- cultured neurons from male and female mice, demonstrating that PLSCR1 controls PL asymmetry remodeling and SV retrieval following neurotransmitter release. Altogether, our data reveal a novel key role for PLSCR1 in SV recycling and provide the first evidence that PL scrambling at the plasma membrane is a prerequisite for optimal presynaptic performance.

L-Type Calcium Channel Modulates Low-Intensity Pulsed Ultrasound-Induced Excitation in Cultured Hippocampal Neurons

As a noninvasive technique, ultrasound stimulation is known to modulate neuronal activity both in vitro and in vivo. The latest explanation of this phenomenon is that the acoustic wave can activate the ion channels and further impact the electrophysiological properties of targeted neurons. However, the underlying mechanism of low-intensity pulsed ultrasound (LIPUS)-induced neuro-modulation effects is still unclear. Here, we characterize the excitatory effects of LIPUS on spontaneous activity and the intracellular Ca2+ homeostasis in cultured hippocampal neurons. By whole-cell patch clamp recording, we found that 15 min of 1-MHz LIPUS boosts the frequency of both spontaneous action potentials and spontaneous excitatory synaptic currents (sEPSCs) and also increases the amplitude of sEPSCs in hippocampal neurons. This phenomenon lasts for > 10 min after LIPUS exposure. Together with Ca2+ imaging, we clarified that LIPUS increases the [Ca2+]cyto level by facilitating L-type Ca2+ channels (LTCCs). In addition, due to the [Ca2+]cyto elevation by LIPUS exposure, the Ca2+-dependent CaMKII-CREB pathway can be activated within 30 min to further regulate the gene transcription and protein expression. Our work suggests that LIPUS regulates neuronal activity in a Ca2+-dependent manner via LTCCs. This may also explain the multi-activation effects of LIPUS beyond neurons. LIPUS stimulation potentiates spontaneous neuronal activity by increasing Ca2+ influx.

c-Src-induced vascular malformations require localised matrix degradation at focal adhesions

Endothelial cells lining the blood vessel wall communicate intricately with the surrounding extracellular matrix, translating mechanical cues into biochemical signals. Moreover, vessels require the capability to enzymatically degrade the matrix surrounding them, to facilitate vascular expansion. c-Src plays a key role in blood vessel growth, with its loss in the endothelium reducing vessel sprouting and focal adhesion signalling. Here, we show that constitutive activation of c-Src in endothelial cells results in rapid vascular expansion, operating independently of growth factor stimulation or fluid shear stress forces. This is driven by an increase in focal adhesion signalling and size, with enhancement of localised secretion of matrix metalloproteinases responsible for extracellular matrix remodelling. Inhibition of matrix metalloproteinase activity results in a robust rescue of the vascular expansion elicited by heightened c-Src activity. This supports the premise that moderating focal adhesion-related events and matrix degradation can counteract abnormal vascular expansion, with implications for pathologies driven by unusual vascular morphologies.

Adhesion between EVs and tumor cells facilitated EV-encapsulated doxorubicin delivery via ICAM1

Doxorubicin (Dox) is an anti-tumor drug with a broad spectrum, whereas the cardiotoxicity limits its further application. In clinical settings, liposome delivery vehicles are used to reduce Dox cardiotoxicity. Here, we substitute extracellular vesicles (EVs) for liposomes and deeply investigate the mechanism for EV-encapsulated Dox delivery. The results demonstrate that EVs dramatically increase import efficiency and anti-tumor effects of Dox in vitro and in vivo, and the efficiency increase benefits from its unique entry pattern. Dox-loading EVs repeat a "kiss-and-run" motion before EVs internalization. Once EVs touch the cell membrane, Dox disassociates from EVs and directly enters the cytoplasm, leading to higher and faster Dox import than single Dox. This unique entry pattern makes the adhesion between EVs and cell membrane rather than the total amount of EV internalization the key factor for regulating the Dox import. Furthermore, we recognize ICAM1 as the molecule mediating the adhesion between EVs and cell membranes. Interestingly, EV-encapsulated Dox can induce ICAM1 expression by irritating IFN-γ and TNF-α secretion in TME, thereby increasing tumor targeting of Dox-loading EVs. Altogether, EVs and EV-encapsulated Dox synergize via ICAM1, which collectively enhances the curative effects for tumor treatment.

Unexpected regulatory functions of cyprinid Viperin on inflammation and metabolism

BACKGROUND

Viperin, also known as radical S-adenosyl-methionine domain containing protein 2 (RSAD2), is an interferon-inducible protein that is involved in the innate immune response against a wide array of viruses. In mammals, Viperin exerts its antiviral function through enzymatic conversion of cytidine triphosphate (CTP) into its antiviral analog ddhCTP as well as through interactions with host proteins involved in innate immune signaling and in metabolic pathways exploited by viruses during their life cycle. However, how Viperin modulates the antiviral response in fish remains largely unknown.

RESULTS

For this purpose, we developed a fathead minnow (Pimephales promelas) clonal cell line in which the unique viperin gene has been knocked out by CRISPR/Cas9 genome-editing. In order to decipher the contribution of fish Viperin to the antiviral response and its regulatory role beyond the scope of the innate immune response, we performed a comparative RNA-seq analysis of viperin-/- and wildtype cell lines upon stimulation with recombinant fathead minnow type I interferon.

CONCLUSIONS

Our results revealed that Viperin does not exert positive feedback on the canonical type I IFN but acts as a negative regulator of the inflammatory response by downregulating specific pro-inflammatory genes and upregulating repressors of the NF-κB pathway. It also appeared to play a role in regulating metabolic processes, including one carbon metabolism, bone formation, extracellular matrix organization and cell adhesion.

Deconstructing synthetic biology across scales: a conceptual approach for training synthetic biologists

Synthetic biology allows us to reuse, repurpose, and reconfigure biological systems to address society's most pressing challenges. Developing biotechnologies in this way requires integrating concepts across disciplines, posing challenges to educating students with diverse expertise. We created a framework for synthetic biology training that deconstructs biotechnologies across scales-molecular, circuit/network, cell/cell-free systems, biological communities, and societal-giving students a holistic toolkit to integrate cross-disciplinary concepts towards responsible innovation of successful biotechnologies. We present this framework, lessons learned, and inclusive teaching materials to allow its adaption to train the next generation of synthetic biologists.

Microtubule-associated protein MAP7 promotes tubulin posttranslational modifications and cargo transport to enable osmotic adaptation

Cells remodel their cytoskeletal networks to adapt to their environment. Here, we analyze the mechanisms utilized by the cell to tailor its microtubule landscape in response to changes in osmolarity that alter macromolecular crowding. By integrating live-cell imaging, ex vivo enzymatic assays, and in vitro reconstitution, we probe the impact of cytoplasmic density on microtubule-associated proteins (MAPs) and tubulin posttranslational modifications (PTMs). We find that human epithelial cells respond to fluctuations in cytoplasmic density by modulating microtubule acetylation, detyrosination, or MAP7 association without differentially affecting polyglutamylation, tyrosination, or MAP4 association. These MAP-PTM combinations alter intracellular cargo transport, enabling the cell to respond to osmotic challenges. We further dissect the molecular mechanisms governing tubulin PTM specification and find that MAP7 promotes acetylation and inhibits detyrosination. Our data identify MAP7 in modulating the tubulin code, resulting in microtubule cytoskeleton remodeling and alteration of intracellular transport as an integrated mechanism of cellular adaptation.

Clathrin mediates membrane fission and budding by constricting membrane pores

Membrane budding, which underlies fundamental processes like endocytosis, intracellular trafficking, and viral infection, is thought to involve membrane coat-forming proteins, including the most observed clathrin, to form Ω-shape profiles and helix-forming proteins like dynamin to constrict Ω-profiles' pores and thus mediate fission. Challenging this fundamental concept, we report that polymerized clathrin is required for Ω-profiles' pore closure and that clathrin around Ω-profiles' base/pore region mediates pore constriction/closure in neuroendocrine chromaffin cells. Mathematical modeling suggests that clathrin polymerization at Ω-profiles' base/pore region generates forces from its intrinsically curved shape to constrict/close the pore. This new fission function may exert broader impacts than clathrin's well-known coat-forming function during clathrin (coat)-dependent endocytosis, because it underlies not only clathrin (coat)-dependent endocytosis, but also diverse endocytic modes, including ultrafast, fast, slow, bulk, and overshoot endocytosis previously considered clathrin (coat)-independent in chromaffin cells. It mediates kiss-and-run fusion (fusion pore closure) previously considered bona fide clathrin-independent, and limits the vesicular content release rate. Furthermore, analogous to results in chromaffin cells, we found that clathrin is essential for fast and slow endocytosis at hippocampal synapses where clathrin was previously considered dispensable, suggesting clathrin in mediating synaptic vesicle endocytosis and fission. These results suggest that clathrin and likely other intrinsically curved coat proteins are a new class of fission proteins underlying vesicle budding and fusion. The half-a-century concept and studies that attribute vesicle-coat contents' function to Ω-profile formation and classify budding as coat-protein (e.g., clathrin)-dependent or -independent may need to be re-defined and re-examined by considering clathrin's pivotal role in pore constriction/closure.

Human neurons lacking amyloid precursor protein exhibit cholesterol-associated developmental and presynaptic deficits

Amyloid precursor protein (APP) produces aggregable β-amyloid peptides and its mutations are associated with familial Alzheimer's disease (AD), which makes it one of the most studied proteins. However, APP's role in the human brain remains unclear despite years of investigation. One problem is that most studies on APP have been carried out in cell lines or model organisms, which are physiologically different from human neurons in the brain. Recently, human-induced neurons (hiNs) derived from induced pluripotent stem cells (iPSCs) provide a practical platform for studying the human brain in vitro. Here, we generated APP-null iPSCs using CRISPR/Cas9 genome editing technology and differentiate them into matured human neurons with functional synapses using a two-step procedure. During hiN differentiation and maturation, APP-null cells exhibited less neurite growth and reduced synaptogenesis in serum-free but not serum-containing media. We have found that cholesterol (Chol) remedies those developmental defects in APP-null cells, consistent with Chol's role in neurodevelopment and synaptogenesis. The phenotypic rescue was also achieved by coculturing those cells with wild-type mouse astrocytes, suggesting that APP's developmental role is likely astrocytic. Next, we examined matured hiNs using patch-clamp recording and detected reduced synaptic transmission in APP-null cells. This change was largely due to decreased synaptic vesicle (SV) release and retrieval, which was confirmed by live-cell imaging using two SV-specific fluorescent reporters. Adding Chol shortly before stimulation mitigated the SV deficits in APP-null iNs, indicating that APP facilitates presynaptic membrane Chol turnover during the SV exo-/endocytosis cycle. Taken together, our study in hiNs supports the notion that APP contributes to neurodevelopment, synaptogenesis, and neurotransmission via maintaining brain Chol homeostasis. Given the vital role of Chol in the central nervous system, the functional connection between APP and Chol bears important implications in the pathogenesis of AD.

Triggers, cascades, and endpoints: connecting the dots of coral bleaching mechanisms

The intracellular coral-dinoflagellate symbiosis is the engine that underpins the success of coral reefs, one of the most diverse ecosystems on the planet. However, the breakdown of the symbiosis and the loss of the microalgal symbiont (i.e. coral bleaching) due to environmental changes are resulting in the rapid degradation of coral reefs globally. There is an urgent need to understand the cellular physiology of coral bleaching at the mechanistic level to help develop solutions to mitigate the coral reef crisis. Here, at an unprecedented scope, we present novel models that integrate putative mechanisms of coral bleaching within a common framework according to the triggers (initiators of bleaching, e.g. heat, cold, light stress, hypoxia, hyposalinity), cascades (cellular pathways, e.g. photoinhibition, unfolded protein response, nitric oxide), and endpoints (mechanisms of symbiont loss, e.g. apoptosis, necrosis, exocytosis/vomocytosis). The models are supported by direct evidence from cnidarian systems, and indirectly through comparative evolutionary analyses from non-cnidarian systems. With this approach, new putative mechanisms have been established within and between cascades initiated by different bleaching triggers. In particular, the models provide new insights into the poorly understood connections between bleaching cascades and endpoints and highlight the role of a new mechanism of symbiont loss, i.e. 'symbiolysosomal digestion', which is different from symbiophagy. This review also increases the approachability of bleaching physiology for specialists and non-specialists by mapping the vast landscape of bleaching mechanisms in an atlas of comprehensible and detailed mechanistic models. We then discuss major knowledge gaps and how future research may improve the understanding of the connections between the diverse cascade of cellular pathways and the mechanisms of symbiont loss (endpoints).

Vesicle fusion and release in neurons under dynamic mechanical equilibrium

Vesicular fusion plays a pivotal role in cellular processes, involving stages like vesicle trafficking, fusion pore formation, content release, and membrane integration or separation. This dynamic process is regulated by a complex interplay of protein assemblies, osmotic forces, and membrane tension, which together maintain a mechanical equilibrium within the cell. Changes in cellular mechanics or external pressures prompt adjustments in this equilibrium, highlighting the system's adaptability. This review delves into the synergy between intracellular proteins, structural components, and external forces in facilitating vesicular fusion and release. It also explores how cells respond to mechanical stress, maintaining equilibrium and offering insights into vesicle fusion mechanisms and the development of neurological disorders.

Reversible structural change of [Co2Fe2] complexes between diamagnetic hydrogen-bonded 1D chains and paramagnetic complexes within a layered structure of amphiphilic anions

The combination of amphiphilic ions and metal complexes may enable the construction of assemblies in which the assembly structure and electronic state of the metal complexes change concertedly. In this work, an alternating layered structure of [Co2Fe2] complexes and amphiphilic anions was constructed. In the crystal structure, [Co2Fe2] complexes and water molecules formed a hydrogen-bonded supramolecular one-dimensional (1D) chain in the hydrophilic layer. A reversible structural change between the 1D chain and discrete [Co2Fe2] complexes was found to occur concertedly with an electron transfer-coupled spin transition (ETCST) of the [Co2Fe2] complex and desorption/adsorption of water molecules.

Generating Concentration Gradients across Membranes for Molecular Dynamics Simulations of Periodic Systems

The plasma membrane forms the boundary between a living entity and its environment and acts as a barrier to permeation and flow of substances. Several computational means of calculating permeability have been implemented for molecular dynamics (MD) simulations-based approaches. Except for double bilayer systems, most permeability studies have been performed under equilibrium conditions, in large part due to the challenges associated with creating concentration gradients in simulations utilizing periodic boundary conditions. To enhance the scientific understanding of permeation and complement the existing computational means of characterizing membrane permeability, we developed a non-equilibrium method that enables the generation and maintenance of steady-state gradients in MD simulations. We utilize PBCs advantageously by imposing a directional bias to the motion of permeants so that their crossing of the boundary replenishes the gradient, like a previous study on ions. Under these conditions, a net flow of permeants across membranes may be observed to determine bulk permeability by a direct application of J=PΔc. In the present study, we explore the results of its application to an exemplary O2 and POPC bilayer system, demonstrating accurate and precise permeability measurements. In addition, we illustrate the impact of permeant concentration and the choice of thermostat on the permeability. Moreover, we demonstrate that energetics of permeation can be closely examined by the dissipation of the gradient across the membrane to gain nuanced insights into the thermodynamics of permeability.

Synaptic endocytosis in adult adipose stromal cell-derived neurons

Synapses are essential for facilitating the transmission of information between neurons and for executing neurophysiological processes. Following the exocytosis of neurotransmitters, the synaptic vesicle may quickly undergo endocytosis to preserve the structural integrity of the synapse. When converting adipose-derived stromal cells (ADSCs) into neurons, the ADSCs have already demonstrated comparable morphology, structure, and electrophysiological characteristics to neurons. Nevertheless, there is currently no published study on the endocytotic function of neurons that are produced from ADSCs. This study aimed to examine synaptic endocytosis in neurons derived from ADSCs by qualitatively and quantitatively analyzing the presence of Ap-2, Clathrin, Endophilin, Dynamin, and Hsc70, which are the key proteins involved in clathrin-mediated endocytosis (CME), as well as by using FM1-43 and cadmium selenide quantum dots (CdSe QDs). Additionally, single-cell RNA sequencing (scRNA-seq) was used to look at the levels of both neuronal markers and markers related to CME at the same time. The results of this study provide evidence that synapses in neurons produced from ADSCs have a role in endocytosis, mainly through the CME route.

Deep Learning-Assisted Automated Multidimensional Single Particle Tracking in Living Cells

The translational and rotational dynamics of anisotropic optical nanoprobes revealed in single particle tracking (SPT) experiments offer molecular-level information about cellular activities. Here, we report an automated high-speed multidimensional SPT system integrated with a deep learning algorithm for tracking the 3D orientation of anisotropic gold nanoparticle probes in living cells with high localization precision (<10 nm) and temporal resolution (0.9 ms), overcoming the limitations of rotational tracking under low signal-to-noise ratio (S/N) conditions. This method can resolve the azimuth (0°-360°) and polar angles (0°-90°) with errors of less than 2° on the experimental and simulated data under S/N of ∼4. Even when the S/N approaches the limit of 1, this method still maintains better robustness and noise resistance than the conventional pattern matching methods. The usefulness of this multidimensional SPT system has been demonstrated with a study of the motions of cargos transported along the microtubules within living cells.

Recent advancements of fluorescent biosensors using semisynthetic probes

Fluorescent biosensors are crucial experimental tools for live-cell imaging and the quantification of different biological analytes. Fluorescent protein (FP)-based biosensors are widely used for imaging applications in living systems. However, the use of FP-based biosensors is hindered by their large size, poor photostability, and laborious genetic manipulations required to improve their properties. Recently, semisynthetic fluorescent biosensors have been developed to address the limitations of FP-based biosensors using chemically modified fluorescent probes and self-labeling protein tag/peptide tags or DNA/RNA-based hybrid systems. Semisynthetic biosensors have unique advantages, as they can be easily modified using different probes. Moreover, the self-labeling protein tag, which labels synthetically developed ligands via covalent bonds, has immense potential for biosensor development. This review discusses the recent progress in different types of fluorescent biosensors for metabolites, protein aggregation and degradation, DNA methylation, endocytosis and exocytosis, membrane tension, and cellular viscosity. Here, we explain in detail the design strategy and working principle of these biosensors. The information presented will help the reader to create new biosensors using self-labeling protein tags for various applications.

Insights into the role of derailed endocytic trafficking pathway in cancer: From the perspective of cancer hallmarks

The endocytic trafficking pathway is a highly organized cellular program responsible for the regulation of membrane components and uptake of extracellular substances. Molecules internalized into the cell through endocytosis will be sorted for degradation or recycled back to membrane, which is determined by a series of sorting events. Many receptors, enzymes, and transporters on the membrane are strictly regulated by endocytic trafficking process, and thus the endocytic pathway has a profound effect on cellular homeostasis. However, the endocytic trafficking process is typically dysregulated in cancers, which leads to the aberrant retention of receptor tyrosine kinases and immunosuppressive molecules on cell membrane, the loss of adhesion protein, as well as excessive uptake of nutrients. Therefore, hijacking endocytic trafficking pathway is an important approach for tumor cells to obtain advantages of proliferation and invasion, and to evade immune attack. Here, we summarize how dysregulated endocytic trafficking process triggers tumorigenesis and progression from the perspective of several typical cancer hallmarks. The impact of endocytic trafficking pathway to cancer therapy efficacy is also discussed.

Wound Repair of the Cell Membrane: Lessons from Dictyostelium Cells

The cell membrane is frequently subjected to damage, either through physical or chemical means. The swift restoration of the cell membrane's integrity is crucial to prevent the leakage of intracellular materials and the uncontrolled influx of extracellular ions. Consequently, wound repair plays a vital role in cell survival, akin to the importance of DNA repair. The mechanisms involved in wound repair encompass a series of events, including ion influx, membrane patch formation, endocytosis, exocytosis, recruitment of the actin cytoskeleton, and the elimination of damaged membrane sections. Despite the absence of a universally accepted general model, diverse molecular models have been proposed for wound repair in different organisms. Traditional wound methods not only damage the cell membrane but also impact intracellular structures, including the underlying cortical actin networks, microtubules, and organelles. In contrast, the more recent improved laserporation selectively targets the cell membrane. Studies on Dictyostelium cells utilizing this method have introduced a novel perspective on the wound repair mechanism. This review commences by detailing methods for inducing wounds and subsequently reviews recent developments in the field.

Platelet C3G: a key player in vesicle exocytosis, spreading and clot retraction

C3G is a Rap1 GEF that plays a pivotal role in platelet-mediated processes such as angiogenesis, tumor growth, and metastasis by modulating the platelet secretome. Here, we explore the mechanisms through which C3G governs platelet secretion. For this, we utilized animal models featuring either overexpression or deletion of C3G in platelets, as well as PC12 cell clones expressing C3G mutants. We found that C3G specifically regulates α-granule secretion via PKCδ, but it does not affect δ-granules or lysosomes. C3G activated RalA through a GEF-dependent mechanism, facilitating vesicle docking, while interfering with the formation of the trans-SNARE complex, thereby restricting vesicle fusion. Furthermore, C3G promotes the formation of lamellipodia during platelet spreading on specific substrates by enhancing actin polymerization via Src and Rac1-Arp2/3 pathways, but not Rap1. Consequently, C3G deletion in platelets favored kiss-and-run exocytosis. C3G also controlled granule secretion in PC12 cells, including pore formation. Additionally, C3G-deficient platelets exhibited reduced phosphatidylserine exposure, resulting in decreased thrombin generation, which along with defective actin polymerization and spreading, led to impaired clot retraction. In summary, platelet C3G plays a dual role by facilitating platelet spreading and clot retraction through the promotion of outside-in signaling while concurrently downregulating α-granule secretion by restricting granule fusion.

Endocytosis and Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and is the most common cause of dementia. The pathogenesis of AD still remains unclear, including two main hypotheses: amyloid cascade and tau hyperphosphorylation. The hallmark neuropathological changes of AD are extracellular deposits of amyloid-β (Aβ) plaques and intracellular neurofibrillary tangles (NFTs). Endocytosis plays an important role in a number of cellular processes including communication with the extracellular environment, nutrient uptake, and signaling by the cell surface receptors. Based on the results of genetic and biochemical studies, there is a link between neuronal endosomal function and AD pathology. Taking this into account, we can state that in the results of previous research, endolysosomal abnormality is an important cause of neuronal lesions in the brain. Endocytosis is a central pathway involved in the regulation of the degradation of amyloidogenic components. The results of the studies suggest that a correlation between alteration in the endocytosis process and associated protein expression progresses AD. In this article, we discuss the current knowledge about endosomal abnormalities in AD.

Membrane transformations of fusion and budding

Membrane fusion and budding mediate fundamental processes like intracellular trafficking, exocytosis, and endocytosis. Fusion is thought to open a nanometer-range pore that may subsequently close or dilate irreversibly, whereas budding transforms flat membranes into vesicles. Reviewing recent breakthroughs in real-time visualization of membrane transformations well exceeding this classical view, we synthesize a new model and describe its underlying mechanistic principles and functions. Fusion involves hemi-to-full fusion, pore expansion, constriction and/or closure while fusing vesicles may shrink, enlarge, or receive another vesicle fusion; endocytosis follows exocytosis primarily by closing Ω-shaped profiles pre-formed through the flat-to-Λ-to-Ω-shape transition or formed via fusion. Calcium/SNARE-dependent fusion machinery, cytoskeleton-dependent membrane tension, osmotic pressure, calcium/dynamin-dependent fission machinery, and actin/dynamin-dependent force machinery work together to generate fusion and budding modes differing in pore status, vesicle size, speed and quantity, controls release probability, synchronization and content release rates/amounts, and underlies exo-endocytosis coupling to maintain membrane homeostasis. These transformations, underlying mechanisms, and functions may be conserved for fusion and budding in general.

Direct Quantification of Nanoplastics Neurotoxicity by Single-Vesicle Electrochemistry

Nanoplastics are recently recognized as neurotoxic factors for the nervous systems. However, whether and how they affect vesicle chemistry (i.e., vesicular catecholamine content and exocytosis) remains unclear. This study offers the first direct evidence for the nanoplastics-induced neurotoxicity by single-vesicle electrochemistry. We observe the cellular uptake of polystyrene (PS) nanoplastics into model neuronal cells and mouse primary neurons, leading to cell viability loss depending on nanoplastics exposure time and concentration. By using single-vesicle electrochemistry, we find the reductions in the vesicular catecholamine content, the frequency of stimulated exocytotic spikes, the neurotransmitter release amount of single exocytotic event, and the membrane-vesicle fusion pore opening-closing speed. Mechanistic investigations suggest that PS nanoplastics can cause disruption of filamentous actin (F-actin) assemblies at cytomembrane zones and change the kinetic patterns of vesicle exocytosis. Our finding shapes the first quantitative picture of neurotoxicity induced by high-concentration nanoplastics exposure at a single-cell level.

KTt-45, a T-type calcium channel blocker, acts as an anticancer agent by inducing apoptosis on HeLa cervical cancer cell line

The abnormal expression in the T-type calcium channels is involved in various cancer types, thus inhibiting T-type calcium channels is one of approaches in cancer treatment. The fact that KTt-45 acted as a T-type calcium channel inhibitor as well as a pain-relief agent prompts us to address if KTt-45 plays any role against cancer cells. The results showed that KTt-45 caused cytotoxic effects towards HeLa cervical, Raji lymphoma, MCF-7 breast cancer, and A549 lung cancer cell lines with IC50 values less than 100 μM, in which highly selective toxicity was against HeLa cells (IC50 = 37.4 μM, SI > 3.2). Strikingly, the KTt-45 induced an accumulation of cytoplasmic vacuoles after 48 h treatment and mitochondrial-dependent apoptosis activation as evidenced by morphological features, chromatin condensation, nuclear fragmentation, and significant activation of caspase-9 as well as caspase-3. In conclusion, KTt-45 could inhibit cell growth and trigger mitochondrial-dependent apoptosis in HeLa cervical cancer cells. The results, taken together, strongly demonstrated that KTt-45 is a potential agent for further study on anticancer drug development which not only targets cancer cells but also helps to relieve neuropathic pain in cancer patients.

Synaptotagmin 1-triggered lipid signaling facilitates coupling of exo- and endocytosis

Exocytosis and endocytosis are essential physiological processes and are of prime importance for brain function. Neurotransmission depends on the Ca2+-triggered exocytosis of synaptic vesicles (SVs). In neurons, exocytosis is spatiotemporally coupled to the retrieval of an equal amount of membrane and SV proteins by compensatory endocytosis. How exocytosis and endocytosis are balanced to maintain presynaptic membrane homeostasis and, thereby, sustain brain function is essentially unknown. We combine mouse genetics with optical imaging to show that the SV calcium sensor Synaptotagmin 1 couples exocytic SV fusion to the endocytic retrieval of SV membranes by promoting the local activity-dependent formation of the signaling lipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at presynaptic sites. Interference with these mechanisms impairs PI(4,5)P2-triggered SV membrane retrieval but not exocytic SV fusion. Our findings demonstrate that the coupling of SV exocytosis and endocytosis involves local Synaptotagmin 1-induced lipid signaling to maintain presynaptic membrane homeostasis in central nervous system neurons.

Interleukin‑22 alleviates arginine‑induced pancreatic acinar cell injury via the regulation of intracellular vesicle transport system: Evidence from proteomic analysis

Acute pancreatitis (AP) is a severe inflammatory condition characterized by the activation of pancreatic enzymes within acinar cells, leading to tissue damage and inflammation. Interleukin (IL)-22 is a potential therapeutic agent for AP owing to its anti-inflammatory properties and ability to promote tissue repair. The present study evaluated the differentially expressed proteins in arginine-induced pancreatic acinar cell injury following treatment with IL-22, and the possible mechanisms involved in IL-22-mediated alleviation of AP. AR42J cells were stimulated using L-arginine to establish an acinar cell injury model in vitro and the damaged cells were subsequently treated with IL-22. The characteristics of the model and the potential therapeutic effects of IL-22 were examined by CCK-8 assay, flow cytometry, TUNEL assay, transmission electron microscopy and ELISA. Differentially expressed proteins in cells induced by arginine and treated with IL-22 were assessed using liquid chromatography-mass spectrometry. The identified proteins were further subjected to Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analysis to elucidate their functional roles. The present study demonstrated that arginine-stimulated cells showed significant pathological changes resembling those in AP, which were alleviated after IL-22 treatment. Proteomic analysis then demonstrated that in IL-22-treated cells, proteins related to the formation and fusion of autophagosomes with lysosomes were significantly downregulated, whereas endocytosis related proteins were enriched in the upregulated proteins. After IL-22 treatment, western blotting demonstrated reduced expression of autophagy-associated proteins. In conclusion, by inhibiting the formation and fusion of autophagosomes with lysosomes, IL-22 may have mitigated premature trypsinogen activation, subsequently minimizing acinar cell injury induced by L-arginine. This was accompanied by concurrent upregulation of endocytosis, which serves a pivotal role in sustaining regular cellular material transport and signal propagation. This research underscored the potential of IL-22 in mitigating arginine-induced AR42J injury, which could be valuable in refining treatment strategies for AP.

How Merkel cells transduce mechanical stimuli: A biophysical model of Merkel cells

Merkel cells combine with Aβ afferents, producing slowly adapting type 1(SA1) responses to mechanical stimuli. However, how Merkel cells transduce mechanical stimuli into neural signals to Aβ afferents is still unclear. Here we develop a biophysical model of Merkel cells for mechanical transduction by incorporating main ingredients such as Ca2+ and K+ voltage-gated channels, Piezo2 channels, internal Ca2+ stores, neurotransmitters release, and cell deformation. We first validate our model with several experiments. Then we reveal that Ca2+ and K+ channels on the plasma membrane shape the depolarization of membrane potentials, further regulating the Ca2+ transients in the cells. We also show that Ca2+ channels on the plasma membrane mainly inspire the Ca2+ transients, while internal Ca2+ stores mainly maintain the Ca2+ transients. Moreover, we show that though Piezo2 channels are rapidly adapting mechanical-sensitive channels, they are sufficient to inspire sustained Ca2+ transients in Merkel cells, which further induce the release of neurotransmitters for tens of seconds. Thus our work provides a model that captures the membrane potentials and Ca2+ transients features of Merkel cells and partly explains how Merkel cells transduce the mechanical stimuli by Piezo2 channels.

Membrane microdomains: Structural and signaling platforms for establishing membrane polarity

Cell polarity results from the asymmetric distribution of cellular structures, molecules, and functions. Polarity is a fundamental cellular trait that can determine the orientation of cell division, the formation of particular cell shapes, and ultimately the development of a multicellular body. To maintain the distinct asymmetric distribution of proteins and lipids in cellular membranes, plant cells have developed complex trafficking and regulatory mechanisms. Major advances have been made in our understanding of how membrane microdomains influence the asymmetric distribution of proteins and lipids. In this review, we first give an overview of cell polarity. Next, we discuss current knowledge concerning membrane microdomains and their roles as structural and signaling platforms to establish and maintain membrane polarity, with a special focus on the asymmetric distribution of proteins and lipids, and advanced microscopy techniques to observe and characterize membrane microdomains. Finally, we review recent advances regarding membrane trafficking in cell polarity establishment and how the balance between exocytosis and endocytosis affects membrane polarity.

The Arp2/3 complex promotes periodic removal of Pak1-mediated negative feedback to facilitate anticorrelated Cdc42 oscillations

The conserved GTPase Cdc42 is a major regulator of polarized growth in most eukaryotes. Cdc42 periodically cycles between active and inactive states at sites of polarized growth. These periodic cycles are caused by positive feedback and time-delayed negative feedback loops. In the bipolar yeast S. pombe, both growing ends must regulate Cdc42 activity. At each cell end, Cdc42 activity recruits the Pak1 kinase which prevents further Cdc42 activation thus establishing negative feedback. It is unclear how Cdc42 activation returns to the end after Pak1-dependent negative feedback. Using genetic and chemical perturbations, we find that disrupting branched actin-mediated endocytosis disables Cdc42 reactivation at the cell ends. With our experimental data and mathematical models, we show that endocytosis-dependent Pak1 removal from the cell ends allows the Cdc42 activator Scd1 to return to that end to enable reactivation of Cdc42. Moreover, we show that Pak1 elicits its own removal via activation of endocytosis. In agreement with these observations, our model and experimental data show that in each oscillatory cycle, Cdc42 activation increases followed by an increase in Pak1 recruitment at that end. These findings provide a deeper insight into the self-organization of Cdc42 regulation and reveal previously unknown feedback with endocytosis in the establishment of cell polarity.

How micron-sized exocrine vesicles release content: A comparison with sub-micron endocrine vesicles

Exocytosis releases vesicular contents to mediate physiological functions. In this issue, Biton et al. (https://doi.org/10.1083/jcb.202302112) found four modes of releasing micron-sized exocrine vesicles and the underlying mechanisms involving actomyosin and BAR domain proteins. We highlight their discovery, compare it with much smaller/faster neuroendocrine vesicle fusion, and draw distinct and conserved principles regarding their membrane transformations, pore dynamics, and underlying mechanisms.

Live-cell imaging of endocytosed synaptophysin around individual hippocampal presynaptic active zones

In presynaptic terminals 4 types of endocytosis, kiss-and-run, clathrin-mediated, bulk and ultrafast endocytosis have been reported to maintain repetitive exocytosis of neurotransmitter. However, detailed characteristics and relative contribution of each type of endocytosis still need to be determined. Our previous live-cell imaging study demonstrated individual exocytosis events of synaptic vesicle within an active-zone-like membrane (AZLM) formed on glass using synaptophysin tagged with a pH-sensitive fluorescent protein. On the other hand, individual endocytosis events of postsynaptic receptors were recorded with a rapid extracellular pH exchange method. Combining these methods, here we live-cell imaged endocytosed synaptophysin with total internal reflection fluorescence microscopy in rat hippocampal culture preparations. Clathrin-dependent and -independent endocytosis, which was seemingly bulk endocytosis, occurred within several seconds after electrical stimulation at multiple locations around AZLM at room temperature, with the locations varying trial to trial. The contribution of clathrin-independent endocytosis was more prominent when the number of stimulation pulses was large. The skewness of synaptophysin distribution in intracellular vesicles became smaller after addition of a clathrin inhibitor, which suggests that clathrin-dependent endocytosis concentrates synaptophysin. Ultrafast endocytosis was evident immediately after stimulation only at near physiological temperature and was the predominant endocytosis when the number of stimulation pulses was small.

Gonadal Feedback Alters the Relationship between Action Potentials and Hormone Release in Gonadotropin-Releasing Hormone Neurons in Male Mice

In vertebrates, the pulsatile release of gonadotropin-releasing hormone (GnRH) from neurons in the hypothalamus triggers secretion of anterior pituitary gonadotropins, which activate steroidogenesis, and steroids in turn exert typically homeostatic negative feedback on GnRH release. Although long-term episodic firing patterns of GnRH neurons in brain slices resemble the pulsatile release of GnRH and LH in vivo, neither the relationship between GnRH neuron firing and release nor whether this relationship is influenced by gonadal feedback are known. We combined fast-scan cyclic voltammetry and patch-clamp to perform simultaneous measurements of neuropeptide release with either spontaneous action potential firing or in response to neuromodulator or action-potential-spike templates in brain slice preparations from male mice. GnRH release increased with higher frequency spontaneous firing to a point; release reached a plateau after which further increases in firing rate did not elicit further increased release. Kisspeptin, a potent GnRH neuron activator via a Gq-coupled signaling pathway, triggered GnRH release before increasing firing rate, whether globally perfused or locally applied. Increasing the number of spikes in an applied burst template increased release; orchidectomized mice had higher sensitivity to the increased action potential number than sham-operated mice. Similarly, Ca2+ currents triggered by these burst templates were increased in GnRH neurons of orchidectomized mice. These results suggest removal of gonadal feedback increases the efficacy of the stimulus-secretion coupling mechanisms, a phenomenon that may extend to other steroid-sensitive regions of the brain.SIGNIFICANCE STATEMENT Pulsatile secretion of GnRH plays a critical role in fertility. The temporal relationship between GnRH neuron action potential firing and GnRH release remains unknown as does whether this relationship is influenced by gonadal feedback. By combining techniques of fast-scan cyclic voltammetry and patch-clamp we, for the first time, monitored GnRH concentration changes during spontaneous and neuromodulator-induced GnRH neuron firing. We also made the novel observation that gonadal factors exert negative feedback on excitation-secretion coupling to reduce release in response to the same stimulus. This has implications for the control of normal fertility, central causes of infertility, and more broadly for the effects of sex steroids in the brain.

Three membrane fusion pore families determine the pathway to pore dilation

During exocytosis secretory vesicles fuse with a target membrane and release neurotransmitters, hormones, or other bioactive molecules through a membrane fusion pore. The initially small pore may subsequently dilate for full contents release, as commonly observed in amperometric traces. The size, shape, and evolution of the pore is critical to the course of contents release, but exact fusion pore solutions accounting for membrane tension and bending energy constraints have not been available. Here, we obtained exact solutions for fusion pores between two membranes. We find three families: a narrow pore, a wide pore, and an intermediate tether-like pore. For high tensions these are close to the catenoidal and tether solutions recently reported for freely hinged membrane boundaries. We suggest membrane fusion initially generates a stable narrow pore, and the dilation pathway is a transition to the stable wide pore family. The unstable intermediate pore is the transition state that sets the energy barrier for this dilation pathway. Pore dilation is mechanosensitive, as the energy barrier is lowered by increased membrane tension. Finally, we study fusion pores in nanodiscs, powerful systems for the study of individual pores. We show that nanodiscs stabilize fusion pores by locking them into the narrow pore family.

Functional genomics identify causal variant underlying the protective CTSH locus for Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent age-related neurodegenerative disease, which has a high heritability of up to 79%. Exploring the genetic basis is essential for understanding the pathogenic mechanisms underlying AD development. Recent genome-wide association studies (GWASs) reported an AD-associated signal in the Cathepsin H (CTSH) gene in European populations. However, the exact functional/causal variant(s), and the genetic regulating mechanism of CTSH in AD remain to be determined. In this study, we carried out a comprehensive study to characterize the role of CTSH variants in the pathogenesis of AD. We identified rs2289702 in CTSH as the most significant functional variant that is associated with a protective effect against AD. The genetic association between rs2289702 and AD was validated in independent cohorts of the Han Chinese population. The CTSH mRNA expression level was significantly increased in AD patients and AD animal models, and the protective allele T of rs2289702 was associated with a decreased expression level of CTSH through the disruption of the binding affinity of transcription factors. Human microglia cells with CTSH knockout showed a significantly increased phagocytosis of Aβ peptides. Our study identified CTSH as being involved in AD genetic susceptibility and uncovered the genetic regulating mechanism of CTSH in pathogenesis of AD.