Hyperosmotic stress stimulates autophagy via polycystin-2

New hemisynthetic derivatives of sphaeropsidin phytotoxins triggering severe endoplasmic reticulum swelling in cancer cells

Sphaeropsidins are iso-pimarane diterpenes produced by phytopathogenic fungi that display promising anticancer activities. Sphaeropsidin A, in particular, has been shown to counteract regulatory volume increase, a process used by cancer cells to avoid apoptosis. This study reports the hemi-synthesis of new lipophilic derivatives obtained by modifications of the C15,C16-alkene moiety. Several of these compounds triggered severe ER swelling associated with strong proteasomal inhibition and consequently cell death, a feature that was not observed with respect to mode of action of the natural product. Significantly, an analysis from the National Cancer Institute sixty cell line testing did not reveal any correlations between the most potent derivative and any other compound in the database, except at high concentrations (LC50). This study led to the discovery of a new set of sphaeropsidin derivatives that may be exploited as potential anti-cancer agents, notably due to their maintained activity towards multidrug resistant models.

Cardiovascular perspectives of the TRP channel polycystin 2

Calcium release from the endoplasmic reticulum (ER) is predominantly driven by two key ion channel receptors, inositol 1, 4, 5-triphosphate receptor (InsP3R) in non-excitable cells and ryanodine receptor (RyR) in excitable and muscle-based cells. These calcium transients can be modified by other less-studied ion channels, including polycystin 2 (PC2), a member of the transient receptor potential (TRP) family. PC2 is found in various cell types and is evolutionarily conserved with paralogues ranging from single-cell organisms to yeasts and mammals. Interest in the mammalian form of PC2 stems from its disease relevance, as mutations in the PKD2 gene, which encodes PC2, result in autosomal dominant polycystic kidney disease (ADPKD). This disease is characterized by renal and liver cysts, and cardiovascular extrarenal manifestations. However, in contrast to the well-defined roles of many TRP channels, the role of PC2 remains unknown, as it has different subcellular locations, and the functional understanding of the channel in each location is still unclear. Recent structural and functional studies have shed light on this channel. Moreover, studies on cardiovascular tissues have demonstrated a diverse role of PC2 in these tissues compared to that in the kidney. We highlight recent advances in understanding the role of this channel in the cardiovascular system and discuss the functional relevance of PC2 in non-renal cells.

Polycystin-2 Mediated Calcium Signalling in the Dictyostelium Model for Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) occurs when the proteins Polycystin-1 (PC1, PKD1) and Polycystin-2 (PC2, PKD2) contain mutations. PC1 is a large membrane receptor that can interact and form a complex with the calcium-permeable cation channel PC2. This complex localizes to the plasma membrane, primary cilia and ER. Dysregulated calcium signalling and consequential alterations in downstream signalling pathways in ADPKD are linked to cyst formation and expansion; however, it is not completely understood how PC1 and PC2 regulate calcium signalling. We have studied Polycystin-2 mediated calcium signalling in the model organism Dictyostelium discoideum by overexpressing and knocking down the expression of the endogenous Polycystin-2 homologue, Polycystin-2. Chemoattractant-stimulated cytosolic calcium response magnitudes increased and decreased in overexpression and knockdown strains, respectively, and analysis of the response kinetics indicates that Polycystin-2 is a significant contributor to the control of Ca2+ responses. Furthermore, basal cytosolic calcium levels were reduced in Polycystin-2 knockdown transformants. These alterations in Ca2+ signalling also impacted other downstream Ca2+-sensitive processes including growth rates, endocytosis, stalk cell differentiation and spore viability, indicating that Dictyostelium is a useful model to study Polycystin-2 mediated calcium signalling.

Hyperosmotic stress induces 2-cell-like cells through ROS and ATR signaling

Mouse embryonic stem cells (ESCs) display pluripotency features characteristic of the inner cell mass of the blastocyst. Mouse embryonic stem cell cultures are highly heterogeneous and include a rare population of cells, which recapitulate characteristics of the 2-cell embryo, referred to as 2-cell-like cells (2CLCs). Whether and how ESC and 2CLC respond to environmental cues has not been fully elucidated. Here, we investigate the impact of mechanical stress on the reprogramming of ESC to 2CLC. We show that hyperosmotic stress induces 2CLC and that this induction can occur even after a recovery time from hyperosmotic stress, suggesting a memory response. Hyperosmotic stress in ESCs leads to accumulation of reactive-oxygen species (ROS) and ATR checkpoint activation. Importantly, preventing either elevated ROS levels or ATR activation impairs hyperosmotic-mediated 2CLC induction. We further show that ROS generation and the ATR checkpoint act within the same molecular pathway in response to hyperosmotic stress to induce 2CLCs. Altogether, these results shed light on the response of ESC to mechanical stress and on our understanding of 2CLC reprogramming.

PKD2/polycystin-2 inhibits LPS-induced acute lung injury in vitro and in vivo by activating autophagy

Polycystin-2 (PC2), which is a transmembrane protein encoded by the PKD2 gene, plays an important role in kidney disease, but its role in lipopolysaccharide (LPS)-induced acute lung injury (ALI) is unclear. We overexpressed PKD2 in lung epithelial cells in vitro and in vivo and examined the role of PKD2 in the inflammatory response induced by LPS in vitro and in vivo. Overexpression of PKD2 significantly decreased production of the inflammatory factors TNF-α, IL-1β, and IL-6 in LPS-treated lung epithelial cells. Moreover, pretreatment with 3-methyladenine (3-MA), an autophagy inhibitor, reversed the inhibitory effect of PKD2 overexpression on the secretion of inflammatory factors in LPS-treated lung epithelial cells. We further demonstrated that overexpression of PKD2 could inhibit LPS-induced downregulation of the LC3BII protein levels and upregulation of SQSTM1/P62 protein levels in lung epithelial cells. Moreover, we found that LPS-induced changes in the lung wet/dry (W/D) weight ratio and levels of the inflammatory cytokines TNF-α, IL-6 and IL-1β in the lung tissue were significantly decreased in mice whose alveolar epithelial cells overexpressed PKD2. However, the protective effects of PKD2 overexpression against LPS-induced ALI were reversed by 3-MA pretreatment. Our study suggests that overexpression of PKD2 in the epithelium may alleviate LPS-induced ALI by activating autophagy.

Impact of context-dependent autophagy states on tumor progression

Macroautophagy is a cellular quality-control process that degrades proteins, protein aggregates and damaged organelles. Autophagy plays a fundamental role in cancer where, in the presence of stressors (for example, nutrient starvation, hypoxia, mechanical pressure), tumor cells activate it to degrade intracellular substrates and provide energy. Cell-autonomous autophagy in tumor cells and cell-nonautonomous autophagy in the tumor microenvironment and in the host converge on mechanisms that modulate metabolic fitness, DNA integrity and immune escape and, consequently, support tumor growth. In this Review, we will discuss insights into the tumor-modulating roles of autophagy in different contexts and reflect on how future studies using physiological culture systems may help to understand the complexity and open new therapeutic avenues.

Actin cytoskeletal reorganization is involved in hyperosmotic stress-induced autophagy in tubular epithelial cells

Tubular epithelial cells are routinely exposed to severe changes in osmolarity. Although the autophagic activity of cells is an indispensable process to maintain cellular homeostasis and respond to stressors, the effect of hyperosmotic stress on autophagic activity in tubular epithelial cells remains unknown. The aim of this study was to determine the effect of hyperosmotic stress on autophagy in rat kidney tubular epithelial cells focusing on the role of actin and microtubule cytoskeletons. Normal rat kidney (NRK)-52E cells exposed to mannitol-induced hyperosmotic stress. As a result, NRK-52E cells showed elevated protein levels of the autophagosome marker LC3-II, indicating enhancement of the autophagic flux. Hyperosmotic stress also transiently decreased cell volume and caused the reorganization of actin and microtubule cytoskeletal structures in NRK-52E cells. The inhibition of the actin cytoskeleton reorganization by cytochalasin D impaired the increase in the levels of LC3-II; however, disassembly of the microtubules following treatment with nocodazole did not affect the increase. These results indicate that hyperosmotic stress can induce autophagy mediated by the reorganization of the actin cytoskeleton in tubular epithelial cells.

High glucose induces an early and transient cytoprotective autophagy in retinal Müller cells

PURPOSE

We investigated the autophagic response of rat Müller rMC-1 cells during a short-term high glucose challenge.

METHODS

rMC-1 cells were maintained in 5 mM glucose (LG) or exposed to 25 mM glucose (HG). Western blot analysis was used to evaluate the expression levels of markers of autophagy (LC3-II, p62) and glial activation (AQP4), as well as the activation of TRAF2/JNK, ERK and AKT pathways. Autophagic flux assessment was performed using the autophagy inhibitor chloroquine. ROS levels were measured by flow cytometry using dichlorofluorescein diacetate. ERK involvement in autophagy induction was addressed using the ERK inhibitor FR180204. The effect of autophagy inhibition on cell viability was evaluated by SRB assay.

RESULTS

Activation of autophagy was observed in the first 2-6 h of HG exposure. This early autophagic response was transient, not accompanied by an increase in AQP4 or in the phospho-activation of JNK, a key mediator of cellular response to oxidative stress, and required ERK activity. Cells exposed to HG had a lower viability upon autophagy inhibition by chloroquine, as compared to those maintained in LG.

CONCLUSION

A short-term HG challenge triggers in rMC-1 cells a process improving the ability to cope with stressful conditions, which involves ERK and an early and transient autophagy activation.

Molecular Mechanisms of Epigenetic Regulation, Inflammation, and Cell Death in ADPKD

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disorder, which is caused by mutations in the PKD1 and PKD2 genes, characterizing by progressive growth of multiple cysts in the kidneys, eventually leading to end-stage kidney disease (ESKD) and requiring renal replacement therapy. In addition, studies indicate that disease progression is as a result of a combination of factors. Understanding the molecular mechanisms, therefore, should facilitate the development of precise therapeutic strategies for ADPKD treatment. The roles of epigenetic modulation, interstitial inflammation, and regulated cell death have recently become the focuses in ADPKD. Different epigenetic regulators, and the presence of inflammatory markers detectable even before cyst growth, have been linked to cyst progression. Moreover, the infiltration of inflammatory cells, such as macrophages and T cells, have been associated with cyst growth and deteriorating renal function in humans and PKD animal models. There is evidence supporting a direct role of the PKD gene mutations to the regulation of epigenetic mechanisms and inflammatory response in ADPKD. In addition, the role of regulated cell death, including apoptosis, autophagy and ferroptosis, have been investigated in ADPKD. However, there is no consensus whether cell death promotes or delays cyst growth in ADPKD. It is therefore necessary to develop an interactive picture between PKD gene mutations, the epigenome, inflammation, and cell death to understand why inherited PKD gene mutations in patients may result in the dysregulation of these processes that increase the progression of renal cyst formation.

Severe Hyperosmotic Stress Issues an ER Stress-Mediated “Death Sentence” in H9c2 Cells, with p38-MAPK and Autophagy “Coming to the Rescue”

With several cardiovascular pathologies associated with osmotic perturbations, researchers are in pursuit of identifying the signaling sensors, mediators and effectors involved, aiming at formulating novel diagnostic and therapeutic strategies. In the present study, H9c2 cells were treated with 0.5 M sorbitol to elicit hyperosmotic stress. Immunoblotting as well as cell viability analyses revealed the simultaneous but independent triggering of multiple signaling pathways. In particular, our findings demonstrated the phosphorylation of eukaryotic translation initiation factor 2 (eIF2α) and upregulation of the immunoglobulin heavy-chain-binding protein (BiP) expression, indicating the onset of the Integrated Stress Response (IRS) and endoplasmic reticulum stress (ERS), respectively. In addition, autophagy was also induced, evidenced by the enhancement of Beclin-1 protein expression and of AMP-dependent kinase (AMPK) and Raptor phosphorylation levels. The involvement of a Na+/H+ exchanger-1 (NHE-1) as well as NADPH oxidase (Nox) in 0.5 M sorbitol-induced eIF2α phosphorylation was also indicated. Of note, while inhibition of ERS partially alleviated the detrimental effect of 0.5 M sorbitol on H9c2 cellular viability, attenuation of p38-MAPK activity and late phase autophagy further mitigated it. Deciphering the mode of these pathways’ potential interactions and of their complications may contribute to the quest for effective clinical interventions against associated cardiovascular diseases.

Effect of extracellular hyperosmolality during normothermia and hyperthermia on the autophagic response in peripheral blood mononuclear cells from young men

Heat-stress induced dehydration is associated with extracellular hyperosmolality. To counteract the associated stress, cells employ cytoprotective mechanisms, including autophagy, however, the autophagic response to hyperosmotic stress has yet to be evaluated in humans. Thus, we investigated autophagy and associated cellular stress pathways (the heat shock response [HSR], apoptosis, and the acute inflammatory response) to isosmotic and hyperosmotic conditions with and without hyperthermia in twelve young men (mean [SD]; 25 [5] years). Participants received a 90-min intravenous infusion of either isosmotic (ISO; 0.9% NaCl; serum osmolality of 293 [4] mOsm/kg) or hyperosmotic (HYP; 3.0% NaCl; 300 [6] mOsm/kg) saline, followed by passive whole-body heating using a water perfused suit to increase esophageal temperature by ~0.8⁰C. Peripheral blood mononuclear cells were harvested at baseline (pre-infusion), post-infusion, and after heating, and changes in protein content were analyzed via Western blotting. Post-infusion, the LC3-II/I ratio was higher in HYP compared to ISO infusion (p<0.001), although no other protein changes were observed (all p>0.050). Following passive heating, autophagy increased in HYP, as demonstrated by an increase in LC3-II from baseline (p=0.004) and an elevated LC3-II/I ratio compared to ISO (p=0.035), and a decrease in p62 when compared to the ISO condition (p=0.019). This was accompanied by an elevation in cleaved caspase-3 following heating in the HYP condition (p<0.010), however, the HSR and acute inflammatory response did not change under any condition (all p>0.050). Taken together, our findings indicate that serum hyperosmolality induces autophagy and apoptotic signaling during mild hyperthermia with minimal autophagic activation during normothermia.

Data-driven identification of inherent features of eukaryotic stress-responsive genes

Living organisms are continuously challenged by changes in their environment that can propagate to stresses at the cellular level, such as rapid changes in osmolarity or oxygen tension. To survive these sudden changes, cells have developed stress-responsive mechanisms that tune cellular processes. The response of Saccharomyces cerevisiae to osmostress includes a massive reprogramming of gene expression. Identifying the inherent features of stress-responsive genes is of significant interest for understanding the basic principles underlying the rewiring of gene expression upon stress. Here, we generated a comprehensive catalog of osmostress-responsive genes from 5 independent RNA-seq experiments. We explored 30 features of yeast genes and found that 25 (83%) were distinct in osmostress-responsive genes. We then identified 13 non-redundant minimal osmostress gene traits and used statistical modeling to rank the most stress-predictive features. Intriguingly, the most relevant features of osmostress-responsive genes are the number of transcription factors targeting them and gene conservation. Using data on HeLa samples, we showed that the same features that define yeast osmostress-responsive genes can predict osmostress-responsive genes in humans, but with changes in the rank-ordering of feature-importance. Our study provides a holistic understanding of the basic principles of the regulation of stress-responsive gene expression across eukaryotes.

Interdependent Regulation of Polycystin Expression Influences Starvation-Induced Autophagy and Cell Death

Autosomal dominant polycystic kidney disease (ADPKD) is mainly caused by deficiency of polycystin-1 (PC1) or polycystin-2 (PC2). Altered autophagy has recently been implicated in ADPKD progression, but its exact regulation by PC1 and PC2 remains unclear. We therefore investigated cell death and survival during nutritional stress in mouse inner medullary collecting duct cells (mIMCDs), either wild-type (WT) or lacking PC1 (PC1KO) or PC2 (PC2KO), and human urine-derived proximal tubular epithelial cells (PTEC) from early-stage ADPKD patients with PC1 mutations versus healthy individuals. Basal autophagy was enhanced in PC1-deficient cells. Similarly, following starvation, autophagy was enhanced and cell death reduced when PC1 was reduced. Autophagy inhibition reduced cell death resistance in PC1KO mIMCDs to the WT level, implying that PC1 promotes autophagic cell survival. Although PC2 expression was increased in PC1KO mIMCDs, PC2 knockdown did not result in reduced autophagy. PC2KO mIMCDs displayed lower basal autophagy, but more autophagy and less cell death following chronic starvation. This could be reversed by overexpression of PC1 in PC2KO. Together, these findings indicate that PC1 levels are partially coupled to PC2 expression, and determine the transition from renal cell survival to death, leading to enhanced survival of ADPKD cells during nutritional stress.

m6A reader YTHDC1 modulates autophagy by targeting SQSTM1 in diabetic skin

Dysregulation of macroautophagy/autophagy contributes to the delay of wound healing in diabetic skin. N⁶-methyladenosine (m⁶A) RNA modification is known to play a critical role in regulating autophagy. In this study, it was found that SQSTM1/p62 (sequestosome 1), an autophagy receptor, was significantly downregulated in two human keratinocyte cells lines with short-term high-glucose treatment, as well as in the epidermis of diabetic patients and a db/db mouse model with long-term hyperglycemia. Knockdown of SQSTM1 led to the impairment of autophagic flux, which was consistent with the results of high-glucose treatment in keratinocytes. Moreover, the m⁶A reader protein YTHDC1 (YTH domain containing 1), which interacted with SQSTM1 mRNA, was downregulated in keratinocytes under both the acute and chronic effects of hyperglycemia. Knockdown of YTHDC1 affected biological functions of keratinocytes, which included increased apoptosis rates and impaired wound-healing capacity. In addition, knockdown of endogenous YTHDC1 resulted in a blockade of autophagic flux in keratinocytes, while overexpression of YTHDC1 rescued the blockade of autophagic flux induced by high glucose. In vivo, knockdown of endogenous Ythdc1 or Sqstm1 inhibited autophagy in the epidermis and delayed wound healing. Interestingly, we found that a decrease of YTHDC1 drove SQSTM1 mRNA degradation in the nucleus. Furthermore, the results revealed that YTHDC1 interacted and cooperated with ELAVL1/HuR (ELAV like RNA binding protein 1) in modulating the expression of SQSTM1. Collectively, this study uncovered a previously unrecognized function for YTHDC1 in modulating autophagy via regulating the stability of SQSTM1 nuclear mRNA in diabetic keratinocytes.

Abbreviations: ACTB: actin beta; AGEs: glycation end products; AL: autolysosome; AP: autophagosome; ATG: autophagy related; AKT: AKT serine/threonine kinase; ANOVA: analysis of variance; BECN1: beclin 1; Co-IP: co-immunoprecipitation; DEGs: differentially expressed genes; DM: diabetes mellitus; ELAVL1: ELAV like RNA binding protein 1; FTO: FTO alpha-ketoglutarate dependent dioxygenase; G: glucose; HaCaT: human keratinocyte; GO: Gene Ontology; GSEA: Gene Set Enrichment Analysis; HE: hematoxylin-eosin; IHC: immunohistochemical; IRS: immunoreactive score; KEAP1: kelch like ECH associated protein 1; KEGG: Kyoto Encyclopedia of Genes and Genomes; m⁶A: N⁶-methyladenosine; M: mannitol; MANOVA: multivariate analysis of variance; MAP1LC3: microtubule associated protein 1 light chain 3; MAP1LC3B: microtubule associated protein 1 light chain 3 beta; MeRIP: methylated RNA immunoprecipitation; METTL3: methyltransferase 3, N6-adenosine-methytransferase complex catalytic subunit; MTOR: mechanistic target of rapamycin kinase; MTORC1: mechanistic target of rapamycin complex 1; NBR1: NBR1 autophagy cargo receptor; NFE2L2: nuclear factor, erythroid 2 like 2; NG: normal glucose; NHEK: normal human epithelial keratinocyte; OE: overexpressing; p-: phospho-; PI: propidium iodide; PPIN: Protein-Protein Interaction Network; RBPs: RNA binding proteins; RIP: RNA immunoprecipitation; RNA-seq: RNA-sequence; RNU6–1: RNA, U6 small nuclear 1; ROS: reactive oxygen species; siRNAs: small interfering RNAs; SQSTM1: sequestosome 1; SRSF: serine and arginine rich splicing factor; T2DM: type 2 diabetes mellitus; TEM: transmission electron microscopy; TUBB: tubulin beta class I; WT: wild-type; YTHDC1: YTH domain containing 1.

PKD2/polycystin-2 induces autophagy by forming a complex with BECN1

Macroautophagy/autophagy is an intracellular process involved in the breakdown of macromolecules and organelles. Recent studies have shown that PKD2/PC2/TRPP2 (polycystin 2, transient receptor potential cation channel), a nonselective cation channel permeable to Ca2+ that belongs to the family of transient receptor potential channels, is required for autophagy in multiple cell types by a mechanism that remains unclear. Here, we report that PKD2 forms a protein complex with BECN1 (beclin 1), a key protein required for the formation of autophagic vacuoles, by acting as a scaffold that interacts with several co-modulators via its coiled-coil domain (CCD). Our data identified a physical and functional interaction between PKD2 and BECN1, which depends on one out of two CCD domains (CC1), located in the carboxy-terminal tail of PKD2. In addition, depletion of intracellular Ca2+ with BAPTA-AM not only blunted starvation-induced autophagy but also disrupted the PKD2-BECN1 complex. Consistently, PKD2 overexpression triggered autophagy by increasing its interaction with BECN1, while overexpression of PKD2D509V, a Ca2+ channel activity-deficient mutant, did not induce autophagy and manifested diminished interaction with BECN1. Our findings show that the PKD2-BECN1 complex is required for the induction of autophagy, and its formation depends on the presence of the CC1 domain of PKD2 and on intracellular Ca2+ mobilization by PKD2. These results provide new insights regarding the molecular mechanisms by which PKD2 controls autophagy.Abbreviations: ADPKD: autosomal dominant polycystic kidney disease; ATG: autophagy-related; ATG14/ATG14L: autophagy related 14; Baf A1: bafilomycin A1; BCL2/Bcl-2: BCL2 apoptosis regulator; BCL2L1/BCL-XL: BCL2 like 1; BECN1: beclin 1; CCD: coiled-coil domain; EBSS: Earle's balanced salt solution; ER: endoplasmic reticulum; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GFP: green fluorescent protein; GOLGA2/GM130: golgin A2; GST: glutathione s-transferase; LAMP1: lysosomal associated membrane protein 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTORC1: mechanistic target of rapamycin kinase complex 1; NBR1: NBR1 autophagy cargo receptor; PIK3C3/VPS34: phosphatidylinositol 3-kinase catalytic subunit type 3; PKD2/PC2: polycystin 2, transient receptor potential cation channel; RTN4/NOGO: reticulon 4; RUBCN/RUBICON: rubicon autophagy regulator; SQSTM1/p62: sequestosome 1; UVRAG: UV radiation resistance associated; WIPI2: WD repeat domain, phosphoinositide interacting 2.

Kinetics of osmotic stress regulate a cell fate switch of cell survival

Exposure of cells to diverse types of stressful environments differentially regulates cell fate. Although many types of stresses causing this differential regulation are known, it is unknown how changes over time of the same stressor regulate cell fate. Changes in extracellular osmolarity are critically involved in physiological and pathophysiological processes in several tissues. We observe that human cells survive gradual but not acute hyperosmotic stress. We find that stress, caspase, and apoptosis signaling do not activate during gradual stress in contrast to acute treatments. Contrary to the current paradigm, we see a substantial accumulation of proline in cells treated with gradual but not acute stresses. We show that proline can protect cells from hyperosmotic stress similar to the osmoprotection in plants and bacteria. Our studies found a cell fate switch that enables cells to survive gradually changing stress environments by preventing caspase activation and protect cells through proline accumulation.

Calcitriol inhibits apoptosis via activation of autophagy in hyperosmotic stress stimulated corneal epithelial cells in vivo and in vitro

BACKGROUND

Previously, calcitriol has been demonstrated as a potential therapeutic agent for dry eye, whilst its role on corneal epithelium death remains unclear. This study aims to investigate the relationship between apoptosis and autophagy on dry eye related scenario, as well as the effect of calcitriol and its potential mechanism.

METHODS

In vitro, immortalized human corneal epithelial cells (iHCEC) were cultured in hyperosmotic medium with or without various concentrations of calcitriol and other reagents. In vivo, Wistar rats were applied with benzalkonium chloride to induce dry eye. Then rats were topically treated with calcitriol (10^-6 M) for 14 days. Autophagy flux (LC3B-II and SQSTM1/P62) was examined by western blotting or immunostaining. To test cell apoptosis, western blotting for cleaved caspase-3, Annexin V/PI double staining and TUNEL assay were used. CCK-8 assay was performed to detect the cell viability. Small interfering RNA was used to knock down the expression of vitamin D receptor in iHCECs.

RESULTS

Autophagy activation could protect iHCECs against HS induced apoptosis in vitro, and calcitriol was able to augment autophagy flux via VDR signaling, shown as the remarkably elevated expression of LC3B-II, as well as the declined p62 expression. In vivo results further supported the protective role of calcitriol on corneal epithelium apoptosis through promoting autophagy in dry eye rats.

CONCLUSION

The current study indicated that autophagy was an adaptive change of corneal epithelial cells in response to hyperosmotic stress and calcitriol could prevent cells from apoptosis via further activation of autophagy through VDR pathway.

Safety assessment of the innovative functional food ingredient from Cannabis sativa L. wastes

Xylooligosaccharides (XOS) are the oligomers of β-1,4 linked xylose monomers and they have health promoting effect by modulating the beneficial microorganisms in intestine. In this study, hydrolysate obtained from hemp (Cannabis sativa) shives was investigated in terms of its in vitro toxicological impacts at cellular and genetic levels and antioxidant activity. The hydrolysate was found to contain 0.264 mg mL-1 of xylose, 0.789 mg mL-1 of xylobiose and 0.171 mg mL-1 of xylotriose in addition to hydroxymethlyfurfural (HMF) and furfural (F) at concentrations of 0.545 mg mL-1 and 0.107 mg mL-1, respectively. The cells, colon epithelial cells (CoN) and colon cancer cells (Caco-2), exposed to 5.00 mg mL-1 or lower XOS hydrolysate showed very similar growth profiles to the untreated control cells. At the genetic level, the oxidative responses of the cell types to XOS hydrolysate were different as measured by NFE2L2 (Nuclear factor, erythroid-derived 2-like 2) gene expression. Regarding antioxidant activity, the amount of XOS hydrolysate (IC50) that cleared 50 % of the 2,2-diphenyl-l-picrylhydrazyl (DPPH) in the medium was calculated as 0.12 mg mL-1. To conclude, based on in vitro studies, XOS hydrolysate obtained from lignocellulosic hemp shives emerges as an innovative, alternative and safe functional food candidate.

New emerging roles of Polycystin-2 in the regulation of autophagy

Polycystin-2 (PC2) is a calcium channel that can be found in the endoplasmic reticulum, the plasmatic membrane, and the primary cilium. The structure of PC2 is characterized by a highly ordered C-terminal tail with an EF-motif (calcium-binding domain) and a canonical coiled-coil domain (CCD; interaction domain), and its activity is regulated by interacting partners and post-translational modifications. Calcium mobilization into the cytosol by PC2 has been mainly associated with cell growth and differentiation, and therefore mutations or dysfunction of PC2 lead to renal and cardiac consequences. Interestingly, PC2-related pathologies are usually treated with rapamycin, an autophagy stimulator. Autophagy is an intracellular degradation process where recycling material is sequestered into autophagosomes and then hydrolyzed by fusion with a lysosome. Interestingly, several studies have provided evidence that PC2 may be required for autophagy, suggesting that PC2 maintains a physiologic catabolic state.

Multiple distinct pathways lead to hyperubiquitylated insoluble TDP-43 protein independent of its translocation into stress granules

Insoluble, hyperubiquitylated TAR DNA-binding protein of 43 kDa (TDP-43) in the central nervous system characterizes frontotemporal dementia and ALS in many individuals with these neurodegenerative diseases. The causes for neuropathological TDP-43 aggregation are unknown, but it has been suggested that stress granule (SG) formation is important in this process. Indeed, in human embryonic kidney HEK293E cells, various SG-forming conditions induced very strong TDP-43 ubiquitylation, insolubility, and reduced splicing activity. Osmotic stress-induced SG formation and TDP-43 ubiquitylation occurred rapidly and coincided with colocalization of TDP-43 and SG markers. Washout experiments confirmed the rapid dissolution of SGs, accompanied by normalization of TDP-43 ubiquitylation and solubility. Surprisingly, interference with the SG process using a protein kinase R-like endoplasmic reticulum kinase inhibitor (GSK2606414) or the translation blocker emetine did not prevent TDP-43 ubiquitylation and insolubility. Thus, parallel pathways may lead to pathological TDP-43 modifications independent of SG formation. Using a panel of kinase inhibitors targeting signaling pathways of the osmotic shock inducer sorbitol, we could largely rule out the stress-activated and extracellular signal-regulated protein kinase modules and glycogen synthase kinase 3β. For arsenite, but not for sorbitol, quenching oxidative stress with N-acetylcysteine did suppress both SG formation and TDP-43 ubiquitylation and insolubility. Thus, sodium arsenite appears to promote SG formation and TDP-43 modifications via oxidative stress, but sorbitol stimulates TDP-43 ubiquitylation and insolubility via a novel pathway(s) independent of SG formation. In conclusion, pathological TDP-43 modifications can be mediated via multiple distinct pathways for which SGs are not essential.

Trehalose supplementation during porcine oocytes in vitro maturation improves the developmental capacity of parthenotes

Autophagy is a critical process in early mammalian embryogenesis. Mammalian target of rapamycin (mTOR) inhibitors are major regulators of autophagy. However, mTOR plays a vital role in major signaling pathways controlling cell growth and metabolism; thus, more secure autophagy activation methods should be considered. The present study investigated the effects of supplementary trehalose, a novel mTOR-independent autophagy enhancer, on oocyte maturation and embryonic development after parthenogenetic activation (PA). Trehalose treatment during in vitro maturation (IVM) did not affect the nuclear maturation rates of oocytes. Oocytes treated with 25 mM trehalose during IVM had a significantly higher (P < 0.05) blastocyst formation rate (64.2%) after PA compared to that in control oocytes (52.0%). Blastocyst quality was also improved in the trehalose-treated group. The total cell numbers for blastocyst formation and expanded blastocyst formation were significantly increased in the trehalose-treated group (52.2% and 27.7%, respectively) compared to those in the control group (36.9% and 11.0%, respectively). Trehalose treatment led to the increased expression of LC3, an autophagy marker, in metaphase II oocytes and 4-cell stage embryos. Gene expression analysis revealed that the expression of several autophagy related genes (LAMP2, pATG5, and LC3) increased, while the Bax/Bcl2 ratio and pro-apoptotic Bak transcript levels were decreased in the trehalose-treated group. In conclusion, these results indicate that treatment with trehalose during IVM improved the developmental potential of porcine embryos by down-regulation of pro-apoptotic genes and up-regulation of autophagy-related genes and marker. Trehalose may be useful for the large-scale production of high-quality porcine blastocysts in vitro.

The Role of Water Homeostasis in Muscle Function and Frailty: A Review

Water, the main component of the body, is distributed in the extracellular and intracellular compartments. Water exchange between these compartments is mainly governed by osmotic pressure. Extracellular water osmolarity must remain within very narrow limits to be compatible with life. Older adults lose the thirst sensation and the ability to concentrate urine, and this favours increased extracellular osmolarity (hyperosmotic stress). This situation, in turn, leads to cell dehydration, which has severe consequences for the intracellular protein structure and function and, ultimately, results in cell damage. Moreover, the fact that water determines cell volume may act as a metabolic signal, with cell swelling acting as an anabolic signal and cell shrinkage acting as a catabolic signal. Ageing also leads to a progressive loss in muscle mass and strength. Muscle strength is the main determinant of functional capacity, and, in elderly people, depends more on muscle quality than on muscle quantity (or muscle mass). Intracellular water content in lean mass has been related to muscle strength, functional capacity, and frailty risk, and has been proposed as an indicator of muscle quality and cell hydration. This review aims to assess the role of hyperosmotic stress and cell dehydration on muscle function and frailty.

Hyperosmotic Stress Induces Unconventional Autophagy Independent of the Ulk1 Complex

Autophagy is considered an adaptive mechanism against hyperosmotic stress. Although the process has been reported to be triggered by the inhibition of mTORC1, the precise downstream mechanisms remain elusive. Here, we demonstrate that hyperosmotic-stress-induced autophagy is different from conventional macroautophagy in mouse embryonic fibroblasts (MEFs) and human T24 cells. Our results indicated that cytoplasmic puncta for the isolation membrane markers WIPI2 and Atg16L increased after hyperosmotic stress. They were found to partially colocalize with puncta for a selective autophagy substrate, SQSTM1/p62, and were shown to be diminished by inhibitors of phosphatidylinositol 3-kinase (PI3K) or by knockdown of human Vps34 (hVps34), a component of PI3K. In addition, flux assays showed that SQSTM1/p62 and NcoA4 were degraded by the lysosomal pathway. Surprisingly, Ulk1, which is essential for starvation-induced macroautophagy, remained inactivated under hyperosmotic stress, which was partially caused by mTOR activity. Accordingly, the Ulk1 complex was not nucleated under hyperosmotic stress. Finally, autophagy proceeded even in MEFs deficient in RB1CC1/FIP200 or Atg13, which encode components of the Ulk1 complex. These data suggest that hyperosmotic-stress-induced autophagy represents an unconventional type of autophagy that bypasses Ulk1 signaling.

TMEM33 regulates intracellular calcium homeostasis in renal tubular epithelial cells

Mutations in the polycystins cause autosomal dominant polycystic kidney disease (ADPKD). Here we show that transmembrane protein 33 (TMEM33) interacts with the ion channel polycystin-2 (PC2) at the endoplasmic reticulum (ER) membrane, enhancing its opening over the whole physiological calcium range in ER liposomes fused to planar bilayers. Consequently, TMEM33 reduces intracellular calcium content in a PC2-dependent manner, impairs lysosomal calcium refilling, causes cathepsins translocation, inhibition of autophagic flux upon ER stress, as well as sensitization to apoptosis. Invalidation of TMEM33 in the mouse exerts a potent protection against renal ER stress. By contrast, TMEM33 does not influence pkd2-dependent renal cystogenesis in the zebrafish. Together, our results identify a key role for TMEM33 in the regulation of intracellular calcium homeostasis of renal proximal convoluted tubule cells and establish a causal link between TMEM33 and acute kidney injury.

Hyperosmotic Stress Initiates AMPK-Independent Autophagy and AMPK- and Autophagy-Independent Depletion of Thioredoxin 1 and Glyoxalase 2 in HT22 Nerve Cells

Background

Hyperosmotic stress is an important pathophysiologic condition in diabetes, severe trauma, dehydration, infection, and ischemia. Furthermore, brain neuronal cells face hyperosmotic stress in ageing and Alzheimer's disease. Despite the enormous importance of knowing the homeostatic mechanisms underlying the responses of nerve cells to hyperosmotic stress, this topic has been underrepresented in the literature. Recent evidence points to autophagy induction as a hallmark of hyperosmotic stress, which has been proposed to be controlled by mTOR inhibition as a consequence of AMPK activation. We previously showed that methylglyoxal induced a decrease in the antioxidant proteins thioredoxin 1 (Trx1) and glyoxalase 2 (Glo2), which was mediated by AMPK-dependent autophagy. Thus, we hypothesized that hyperosmotic stress would have the same effect.

Methods

HT22 hippocampal nerve cells were treated with NaCl (37, 75, or 150 mM), and the activation of the AMPK/mTOR pathway was investigated, as well as the levels of Trx1 and Glo2. To determine if autophagy was involved, the inhibitors bafilomycin (Baf) and chloroquine (CQ), as well as ATG5 siRNA, were used. To test for AMPK involvement, AMPK-deficient mouse embryonic fibroblasts (MEFs) were used.

Results

Hyperosmotic stress induced a clear increase in autophagy, which was demonstrated by a decrease in p62 and an increase in LC3 lipidation. AMPK phosphorylation, linked to a decrease in mTOR and S6 ribosomal protein phosphorylation, was also observed. Deletion of AMPK in MEFs did not prevent autophagy induction by hyperosmotic stress, as detected by decreased p62 and increased LC3 II, or mTOR inhibition, inferred by decreased phosphorylation of P70 S6 kinase and S6 ribosomal protein. These data indicating that AMPK was not involved in autophagy activation by hyperosmotic stress were supported by a decrease in p^S555-ULK1, an AMPK phosphorylation site. Trx1 and Glo2 levels were decreased at 6 and 18 h after treatment with 150 mM NaCl. However, this decrease in Trx1 and Glo2 in HT22 cells was not prevented by autophagy inhibition by Baf, CQ, or ATG5 siRNA. AMPK-deficient MEFs under hyperosmotic stress presented the same Trx1 and Glo2 decrease as wild-type cells.

Conclusion

Hyperosmotic stress induced AMPK activation, but this was not responsible for its effects on mTOR activity or autophagy induction. Moreover, the decrease in Trx1 and Glo2 induced by hyperosmotic stress was independent of both autophagy and AMPK activation.

Polycystin-2 Is Required for Starvation- and Rapamycin-Induced Atrophy in Myotubes

Muscle atrophy involves a massive catabolism of intracellular components leading to a significant reduction in cellular and tissue volume. In this regard, autophagy, an intracellular mechanism that degrades proteins and organelles, has been implicated with muscle breakdown. Recently, it has shown that polycystin-2 (PC2), a membrane protein that belongs to the transient receptor potential (TRP) family, is required for the maintenance of cellular proteostasis, by regulating autophagy in several cell types. The role of PC2 in the control of atrophy and autophagy in skeletal muscle remains unknown. Here, we show that PC2 is required for the induction of atrophy in C2C12 myotubes caused by nutrient deprivation or rapamycin exposure. Consistently, overexpression of PC2 induces atrophy in C2C12 myotubes as indicated by decreasing of the myogenic proteins myogenin and caveolin-3. In addition, we show that inhibition of mTORC1, by starvation or rapamycin is inhibited in cells when PC2 is silenced. Importantly, even if PC2 regulates mTORC1, our results show that the regulation of atrophy by PC2 is independent of autophagy. This study provides novel evidence regarding the role of PC2 in skeletal muscle cell atrophy.

The Precision Control of Autophagic Flux and Vesicle Dynamics-A Micropattern Approach

Autophagy failure is implicated in age-related human disease. A decrease in the rate of protein degradation through the entire autophagy pathway, i.e., autophagic flux, has been associated with the onset of cellular proteotoxity and cell death. Although the precision control of autophagy as a pharmacological intervention has received major attention, mammalian model systems that enable a dissection of the relationship between autophagic flux and pathway intermediate pool sizes remain largely underexplored. Here, we make use of a micropattern-based fluorescence life cell imaging approach, allowing a high degree of experimental control and cellular geometry constraints. By assessing two autophagy modulators in a system that achieves a similarly raised autophagic flux, we measure their impact on the pathway intermediate pool size, autophagosome velocity, and motion. Our results reveal a differential effect of autophagic flux enhancement on pathway intermediate pool sizes, velocities, and directionality of autophagosome motion, suggesting distinct control over autophagy function. These findings may be of importance for better understanding the fine-tuning autophagic activity and protein degradation proficiency in different cell and tissue types of age-associated pathologies.