Connexin43 promotes exocytosis of damaged lysosomes through actin remodelling.

A robust and efficient cellular response to lysosomal membrane damage prevents leakage from the lysosome lumen into the cytoplasm. This response is understood to happen through either lysosomal membrane repair or lysophagy. Here we report exocytosis as a third response mechanism to lysosomal damage, which is further potentiated when membrane repair or lysosomal degradation mechanisms are impaired. We show that Connexin43 (Cx43), a protein canonically associated with gap junctions, is recruited from the plasma membrane to damaged lysosomes, promoting their secretion and accelerating cell recovery. The effects of Cx43 on lysosome exocytosis are mediated by a reorganization of the actin cytoskeleton that increases plasma membrane fluidity and decreases cell stiffness. Furthermore, we demonstrate that Cx43 interacts with the actin nucleator Arp2, the activity of which was shown to be necessary for Cx43-mediated actin rearrangement and lysosomal exocytosis following damage. These results define a novel mechanism of lysosomal quality control whereby Cx43-mediated actin remodelling potentiates the secretion of damaged lysosomes.

Role of lipids in interorganelle communication.

Cell homeostasis and function rely on well-orchestrated communication between different organelles. This communication is ensured by signaling pathways and membrane contact sites between organelles. Many players involved in organelle crosstalk have been identified, predominantly proteins and ions. The role of lipids in interorganelle communication remains poorly understood. With the development and broader availability of methods to quantify lipids, as well as improved spatiotemporal resolution in detecting different lipid species, the contribution of lipids to organelle interactions starts to be evident. However, the specific roles of various lipid molecules in intracellular communication remain to be studied systematically. We summarize new insights in the interorganelle communication field from the perspective of organelles and discuss the roles played by lipids in these complex processes.

Loss of the lysosomal protein CLN3 modifies the lipid content of the nuclear envelope leading to DNA damage and activation of YAP1 pro-apoptotic signaling.

Batten disease is characterized by early-onset blindness, juvenile dementia and death during the second decade of life. The most common genetic causes are mutations in the CLN3 gene encoding a lysosomal protein. There are currently no therapies targeting the progression of the disease, mostly due to the lack of knowledge about the disease mechanisms. To gain insight into the impact of CLN3 loss on cellular signaling and organelle function, we generated CLN3 knock-out cells in a human cell line (CLN3-KO), and performed RNA sequencing to obtain the cellular transcriptome. Following a multi-dimensional transcriptome analysis, we identified the transcriptional regulator YAP1 as a major driver of the transcriptional changes observed in CLN3-KO cells. We further observed that YAP1 pro-apoptotic signaling is hyperactive as a consequence of CLN3 functional loss in retinal pigment epithelia cells, and in the hippocampus and thalamus of CLN3exΔ7/8 mice, an established model of Batten disease. Loss of CLN3 activates YAP1 by a cascade of events that starts with the inability of releasing glycerophosphodiesthers from CLN3-KO lysosomes, which leads to perturbations in the lipid content of the nuclear envelope and nuclear dysmorphism. This results in increased number of DNA lesions, activating the kinase c-Abl, which phosphorylates YAP1, stimulating its pro-apoptotic signaling. Altogether, our results highlight a novel organelle crosstalk paradigm in which lysosomal metabolites regulate nuclear envelope content, nuclear shape and DNA homeostasis. This novel molecular mechanism underlying the loss of CLN3 in mammalian cells and tissues may open new c-Abl-centric therapeutic strategies to target Batten disease.

Upregulation of cholesterol synthesis by lysosomal defects requires a functional mitochondrial respiratory chain.

Mitochondria and lysosomes are two organelles that carry out both signaling and metabolic roles in cells. Recent evidence has shown that mitochondria and lysosomes are dependent on one another, as primary defects in one cause secondary defects in the other. Although there are functional impairments in both cases, the signaling consequences of primary mitochondrial dysfunction and lysosomal defects are dissimilar. Here, we used RNA sequencing to obtain transcriptomes from cells with primary mitochondrial or lysosomal defects to identify the global cellular consequences associated with mitochondrial or lysosomal dysfunction. We used these data to determine the pathways affected by defects in both organelles, which revealed a prominent role for the cholesterol synthesis pathway. We observed a transcriptional upregulation of this pathway in cellular and murine models of lysosomal defects, while it is transcriptionally downregulated in cellular and murine models of mitochondrial defects. We identified a role for the posttranscriptional regulation of transcription factor SREBF1, a master regulator of cholesterol and lipid biosynthesis, in models of mitochondrial respiratory chain deficiency. Furthermore, we found that retention of Ca2+ in lysosomes of cells with mitochondrial respiratory chain defects contributes to the differential regulation of the cholesterol synthesis pathway in the mitochondrial and lysosomal defects tested. Finally, we verified in vivo, using a model of mitochondria-associated disease in Caenorhabditis elegans that normalization of lysosomal Ca2+ levels results in partial rescue of the developmental delay induced by the respiratory chain deficiency.

Up-regulation of cholesterol synthesis by lysosomal defects requires a functional mitochondrial respiratory chain.

Mitochondria and lysosomes are two organelles that carry out both signaling and metabolic roles in the cells. Recent evidence has shown that mitochondria and lysosomes are dependent on one another, as primary defects in one cause secondary defects in the other. Nevertheless, the signaling consequences of primary mitochondrial malfunction and of primary lysosomal defects are not similar, despite in both cases there are impairments of mitochondria and of lysosomes. Here, we used RNA sequencing to obtain transcriptomes from cells with primary mitochondrial or lysosomal defects, to identify what are the global cellular consequences that are associated with malfunction of mitochondria or lysosomes. We used these data to determine what are the pathways that are affected by defects in both organelles, which revealed a prominent role for the cholesterol synthesis pathway. This pathway is transcriptionally up-regulated in cellular and mouse models of lysosomal defects and is transcriptionally down-regulated in cellular and mouse models of mitochondrial defects. We identified a role for post-transcriptional regulation of the transcription factor SREBF1, a master regulator of cholesterol and lipid biosynthesis, in models of mitochondrial respiratory chain deficiency. Furthermore, the retention of Ca 2+ in the lysosomes of cells with mitochondrial respiratory chain defects contributes to the differential regulation of the cholesterol synthesis pathway in the mitochondrial and lysosomal defects tested. Finally, we verified in vivo , using models of mitochondria-associated diseases in C. elegans , that normalization of lysosomal Ca 2+ levels results in partial rescue of the developmental arrest induced by the respiratory chain deficiency.

Intersectin and Endophilin condensates prime synaptic vesicles for release site replenishment.

Neurotransmitter is released from dedicated sites of synaptic vesicle fusion within a synapse. Following fusion, the vacated sites are replenished immediately by new vesicles for subsequent neurotransmission. These replacement vesicles are assumed to be located near release sites and used by chance. Here, we find that replacement vesicles are clustered around this region by Intersectin-1. Specifically, Intersectin-1 forms dynamic molecular condensates with Endophilin A1 near release sites and sequesters vesicles around this region. In the absence of Intersectin-1, vesicles within 20 nm of the plasma membrane are reduced, and consequently, vacated sites cannot be replenished rapidly, leading to depression of synaptic transmission. Similarly, mutations in Intersectin-1 that disrupt Endophilin A1 binding result in similar phenotypes. However, in the absence of Endophilin, this replacement pool of vesicles is available but cannot be accessed, suggesting that Endophilin A1 is needed to mobilize these vesicles. Thus, our work describes a distinct physical region within a synapse where replacement vesicles are harbored for release site replenishment.

Lack of peroxisomal catalase affects heat shock response in Caenorhabditis elegans.

Exact mechanisms of heat shock-induced lifespan extension, although documented across species, are still not well understood. Here, we show that fully functional peroxisomes, specifically peroxisomal catalase, are needed for the activation of canonical heat shock response and heat-induced hormesis in Caenorhabditis elegans Although during heat shock, the HSP-70 chaperone is strongly up-regulated in the WT and in the absence of peroxisomal catalase (ctl-2(ua90)II), the small heat shock proteins display modestly increased expression in the mutant. Nuclear foci formation of HSF-1 is reduced in the ctl-2(ua90)II mutant. In addition, heat-induced lifespan extension, observed in the WT, is absent in the ctl-2(ua90)II strain. Activation of the antioxidant response and pentose phosphate pathway are the most prominent changes observed during heat shock in the WT worm but not in the ctl-2(ua90)II mutant. Involvement of peroxisomes in the cell-wide cellular response to transient heat shock reported here gives new insight into the role of organelle communication in the organism's stress response.

Colocalization of different neurotransmitter transporters on synaptic vesicles is sparse except for VGLUT1 and ZnT3.

Vesicular transporters (VTs) define the type of neurotransmitter that synaptic vesicles (SVs) store and release. While certain mammalian neurons release multiple transmitters, it is not clear whether the release occurs from the same or distinct vesicle pools at the synapse. Using quantitative single-vesicle imaging, we show that a vast majority of SVs in the rodent brain contain only one type of VT, indicating specificity for a single neurotransmitter. Interestingly, SVs containing dual transporters are highly diverse (27 types) but small in proportion (2% of all SVs), excluding the largest pool that carries VGLUT1 and ZnT3 (34%). Using VGLUT1-ZnT3 SVs, we demonstrate that the transporter colocalization influences the SV content and synaptic quantal size. Thus, the presence of diverse transporters on the same vesicle is bona fide, and depending on the VT types, this may act to regulate neurotransmitter type, content, and release in space and time.

Fine-tuning activity-dependent bulk endocytosis via kinases and phosphatases.

The regulation of activity-dependent bulk endocytosis, the dominant mode of membrane retrieval in response to intense neuronal activity, is poorly understood. In this JCB issue, Peng et al. (2021. J. Cell. Biol.https://doi.org/10.1083/jcb.202011028) propose a novel molecular mechanism for the coordination of activity-dependent bulk endocytosis that builds on Minibrain kinase and its presynaptic substrate synaptojanin-1.

Targeting synaptic plasticity in schizophrenia: insights from genomic studies.

Patients with schizophrenia experience cognitive dysfunction and negative symptoms that do not respond to current drug treatments. Historical evidence is consistent with the hypothesis that these deficits are due, at least in part, to altered cortical synaptic plasticity (the ability of synapses to strengthen or weaken their activity), making this an attractive pathway for therapeutic intervention. However, while synaptic transmission and plasticity is well understood in model systems, it has been challenging to identify specific therapeutic targets for schizophrenia. New information is emerging from genomic findings, which converge on synaptic plasticity and provide a new window on the neurobiology of schizophrenia. Translating this information into therapeutic advances will require a multidisciplinary and collaborative approach.

Spatial and Temporal Aspects of Exocytosis Studied on the Isolated Plasma Membranes.

Exocytosis of large-dense core vesicles in neuroendocrine cells is a highly regulated, calcium-dependent process, mediated by networks of interrelated proteins and lipids. Here, I describe experimental procedures for studies of selective spatial and temporal aspects of exocytosis at the plasma membrane, or in its proximity, using adrenal chromaffin cells. The assay utilizes primary cells subjected to a brief ultrasonic pulse, resulting in the formation of thin, flat inside-out plasma membranes with attached secretory vesicles and elements of cell cytoskeleton. In this model, secretion of plasma membrane-attached secretory vesicles was found to be dependent on calcium and sensitive to clostridial neurotoxins. Depending on the probe selected for secretory vesicle cargo, protein, and/or lipid detection, this simple assay is versatile, fast and inexpensive, and offers excellent spatial resolution.

Cooperativity of membrane-protein and protein-protein interactions control membrane remodeling by epsin 1 and affects clathrin-mediated endocytosis.

Membrane remodeling is a critical process for many membrane trafficking events, including clathrin-mediated endocytosis. Several molecular mechanisms for protein-induced membrane curvature have been described in some detail. Contrary, the effect that the physico-chemical properties of the membrane have on these processes is far less well understood. Here, we show that the membrane binding and curvature-inducing ENTH domain of epsin1 is regulated by phosphatidylserine (PS). ENTH binds to membranes in a PI(4,5)P2-dependent manner but only induces curvature in the presence of PS. On PS-containing membranes, the ENTH domain forms rigid homo-oligomers and assembles into clusters. Membrane binding and membrane remodeling can be separated by structure-to-function mutants. Such oligomerization mutants bind to membranes but do not show membrane remodeling activity. In vivo, they are not able to rescue defects in epidermal growth factor receptor (EGFR) endocytosis in epsin knock-down cells. Together, these data show that the membrane lipid composition is important for the regulation of protein-dependent membrane deformation during clathrin-mediated endocytosis.

ATP hydrolysis by yeast Hsp104 determines protein aggregate dissolution and size in vivo.

Signs of proteostasis failure often entwine with those of metabolic stress at the cellular level. Here, we study protein sequestration during glucose deprivation-induced ATP decline in Saccharomyces cerevisiae. Using live-cell imaging, we find that sequestration of misfolded proteins and nascent polypeptides into two distinct compartments, stress granules, and Q-bodies, is triggered by the exhaustion of ATP. Both compartments readily dissolve in a PKA-dependent manner within minutes of glucose reintroduction and ATP level restoration. We identify the ATP hydrolase activity of Hsp104 disaggregase as the critical ATP-consuming process determining compartments abundance and size, even in optimal conditions. Sequestration of proteins into distinct compartments during acute metabolic stress and their retrieval during the recovery phase provide a competitive fitness advantage, likely promoting cell survival during stress.

PI(4,5)P2-dependent regulation of exocytosis by amisyn, the vertebrate-specific competitor of synaptobrevin 2.

The functions of nervous and neuroendocrine systems rely on fast and tightly regulated release of neurotransmitters stored in secretory vesicles through SNARE-mediated exocytosis. Few proteins, including tomosyn (STXBP5) and amisyn (STXBP6), were proposed to negatively regulate exocytosis. Little is known about amisyn, a 24-kDa brain-enriched protein with a SNARE motif. We report here that full-length amisyn forms a stable SNARE complex with syntaxin-1 and SNAP-25 through its C-terminal SNARE motif and competes with synaptobrevin-2/VAMP2 for the SNARE-complex assembly. Furthermore, amisyn contains an N-terminal pleckstrin homology domain that mediates its transient association with the plasma membrane of neurosecretory cells by binding to phospholipid PI(4,5)P2 However, unlike synaptrobrevin-2, the SNARE motif of amisyn is not sufficient to account for the role of amisyn in exocytosis: Both the pleckstrin homology domain and the SNARE motif are needed for its inhibitory function. Mechanistically, amisyn interferes with the priming of secretory vesicles and the sizes of releasable vesicle pools, but not vesicle fusion properties. Our biochemical and functional analyses of this vertebrate-specific protein unveil key aspects of negative regulation of exocytosis.

Endophilin-A coordinates priming and fusion of neurosecretory vesicles via intersectin.

Endophilins-A are conserved endocytic adaptors with membrane curvature-sensing and -inducing properties. We show here that, independently of their role in endocytosis, endophilin-A1 and endophilin-A2 regulate exocytosis of neurosecretory vesicles. The number and distribution of neurosecretory vesicles were not changed in chromaffin cells lacking endophilin-A, yet fast capacitance and amperometry measurements revealed reduced exocytosis, smaller vesicle pools and altered fusion kinetics. The levels and distributions of the main exocytic and endocytic factors were unchanged, and slow compensatory endocytosis was not robustly affected. Endophilin-A's role in exocytosis is mediated through its SH3-domain, specifically via a direct interaction with intersectin-1, a coordinator of exocytic and endocytic traffic. Endophilin-A not able to bind intersectin-1, and intersectin-1 not able to bind endophilin-A, resulted in similar exocytic defects in chromaffin cells. Altogether, we report that two endocytic proteins, endophilin-A and intersectin-1, are enriched on neurosecretory vesicles and regulate exocytosis by coordinating neurosecretory vesicle priming and fusion.

Vps34 derived phosphatidylinositol 3-monophosphate modulates megakaryocyte maturation and proplatelet production through late endosomes/lysosomes.

BACKGROUND: Development of platelet precursor cells, megakaryocytes (MKs), implies an increase in their size; formation of the elaborate demarcation membrane system (DMS); and extension of branched cytoplasmic structures, proplatelets, that will release platelets. The membrane source(s) for MK expansion and proplatelet formation have remained elusive. OBJECTIVE: We hypothesized that traffic of membranes regulated by phosphatidylinositol 3-monophosphate (PI3P) contributes to MK maturation and proplatelet formation. RESULTS: In immature MKs, PI3P produced by the lipid kinase Vps34 is confined to perinuclear early endosomes (EE), while in mature MKs PI3P shifts to late endosomes and lysosomes (LE/Lys). PI3P partially colocalized with the plasma membrane marker phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2 ) and with LE/Lys in mature MKs, suggests that PI3P-containing LE/Lys membranes contribute to MK expansion and proplatelet formation. Consistently, we found that sequestration of PI3P, specific pharmacological inhibition of Vps34-mediated PI3P production, or depletion of PI3P by PI3-phosphatase (MTM1)-mediated hydrolysis potently blocked proplatelet formation. Moreover, Vps34 inhibition led to the intracellular accumulation of enlarged LE/Lys, and decreased expression of surface LE/Lys markers. Inhibiting Vps34 at earlier MK stages caused aberrant DMS development. Finally, inhibition of LE/Lys membrane fusion by a dominant negative mutant of the small GTPase Rab7 or pharmacological inhibition of PI3P conversion into PI(3,5)P2 led to enlarged LE/Lys, reduced surface levels of LE/Lys markers, and decreased proplatelet formation. CONCLUSION: Our results suggest that PI3P-positive LE/Lys contribute to the membrane growth and proplatelet formation in MKs by their translocation to the cell periphery and fusion with the plasma membrane.

Regulation of synaptic vesicle acidification at the neuronal synapse.

The vacuolar H+ -adenosine triphosphatases (vATPases) acidify multiple intracellular organelles, including synaptic vesicles (SVs) and secretory granules. Acidification of SVs represents a critical point during the SV cycle: without acidification, neurotransmitters cannot be loaded into SVs. Despite the obvious importance of the vesicle acidification process for neurotrasmission and the life of complex organisms, little is known about the regulation of vATPase at the neuronal synapse. In addition, the composition of the vATPase complex on the SVs is unclear. Here, we summarize the key features of vATPase found on SVs, and propose a model of how vATPase activity is regulated during the SV cycle. It is anticipated that the information from the SV lumen is communicated to SV surface in order to signal successful acidification and neurotransmitter loading: we postulate here that the regulators of the vATPase activity exist (e.g., Rabconnectin-3) that promote the recruitment of SV peripheral proteins and, consequently, SV fusion.

AP180 promotes release site clearance and clathrin-dependent vesicle reformation in mouse cochlear inner hair cells.

High-throughput neurotransmission at ribbon synapses of cochlear inner hair cells (IHCs) requires tight coupling of neurotransmitter release and balanced recycling of synaptic vesicles (SVs) as well as rapid restoration of release sites. Here, we examined the role of the adaptor protein AP180 (also known as SNAP91) for IHC synaptic transmission by comparing AP180-knockout (KO) and wild-type mice using high-pressure freezing and electron tomography, confocal microscopy, patch-clamp membrane capacitance measurements and systems physiology. AP180 was found predominantly at the synaptic pole of IHCs. AP180-deficient IHCs had severely reduced SV numbers, slowed endocytic membrane retrieval and accumulated endocytic intermediates near ribbon synapses, indicating that AP180 is required for clathrin-dependent endocytosis and SV reformation in IHCs. Moreover, AP180 deletion led to a high prevalence of SVs in a multi-tethered or docked state after stimulation, a reduced rate of SV replenishment and a hearing impairment. We conclude that, in addition to its role in clathrin recruitment, AP180 contributes to release site clearance in IHCs.This article has an associated First Person interview with the first author of the paper.

Cells Control BIN1-Mediated Membrane Tubulation by Altering the Membrane Charge.

The Bridging integrator 1 (BIN1)/Amphiphysin/Rvs (BAR) protein family is an essential part of the cell's machinery to bend membranes. BIN1 is a muscle-enriched BAR protein with an established role in muscle development and skeletal myopathies. Here, we demonstrate that BIN1, on its own, is able to form complex interconnected tubular systems in vitro, reminiscent of t-tubule system in muscle cells. We further describe how BIN1's electrostatic interactions regulate membrane bending: the ratio of negatively charged lipids in the bilayer altered membrane bending and binding properties of BIN1 and so did the manipulation of BIN1's surface charge. We show that the electrostatically mediated BIN1 membrane binding depended on the membrane curvature-it was less affected in liposomes with high curvature. Curiously, BIN1 membrane binding and bending was diminished in cells where the membrane's charge was experimentally reduced. Membrane bending was also reduced in BIN1 mutants where negative or positive charges in the BAR domain have been eliminated. This phenotype, characteristic of BIN1 mutants linked to myopathies, was rescued when the membrane charge was made more negative. The latter findings also show that cells can control tubulation at their membranes by simply altering the membrane charge and through it, the recruitment of BAR proteins and their interaction partners (e.g. dynamin).