Discovery of endoplasmic reticulum calcium stabilizers to rescue ER-stressed podocytes in nephrotic syndrome

Calcium signaling crosstalk between the endoplasmic reticulum and mitochondria, a new drug development strategies of kidney diseases

Calcium (Ca2+) acts as a second messenger and constitutes a complex and large information exchange system between the endoplasmic reticulum (ER) and mitochondria; this process is involved in various life activities, such as energy metabolism, cell proliferation and apoptosis. Increasing evidence has suggested that alterations in Ca2+ crosstalk between the ER and mitochondria, including alterations in ER and mitochondrial Ca2+ channels and related Ca2+ regulatory proteins, such as sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), inositol 1,4,5-trisphosphate receptor (IP3R), and calnexin (CNX), are closely associated with the development of kidney disease. Therapies targeting intracellular Ca2+ signaling have emerged as an emerging field in the treatment of renal diseases. In this review, we focused on recent advances in Ca2+ signaling, ER and mitochondrial Ca2+ monitoring methods and Ca2+ homeostasis in the development of renal diseases and sought to identify new targets and insights for the treatment of renal diseases by targeting Ca2+ channels or related Ca2+ regulatory proteins.

A novel ryanodine receptor 2 inhibitor, M201-A, enhances natriuresis, renal function and lusi-inotropic actions: Preclinical and phase I study

BACKGROUND AND PURPOSE

The ryanodine receptor 2 (RyR2) is present in both the heart and kidneys, and plays a crucial role in maintaining intracellular Ca2+ homeostasis in cells in these organs. This study aimed to investigate the impact of M201-A on RyR2, as well as studying its effects on cardiac and renal functions in preclinical and clinical studies.

EXPERIMENTAL APPROACH

Following the administration of M201-A (1,4-benzothiazepine-1-oxide derivative), we monitored diastolic Ca2+ leak via RyR2 and intracellular Ca2+ concentration in isolated rat cardiomyocytes and in cardiac and renal function in animals. In a clinical study, M201-A was administered intravenously at doses of 0.2 and 0.4 mg·kg-1 once daily for 20 min for four consecutive days in healthy males, with the assessment of haemodynamic responses.

KEY RESULTS

In rat heart cells, M201-A effectively inhibited spontaneous diastolic Ca2+ leakage through RyR2 and exhibited positive lusi-inotropic effects on the rat heart. Additionally, it enhanced natriuresis and improved renal function in dogs. In human clinical studies, when administered intravenously, M201-A demonstrated an increase in natriuresis, glomerular filtration rate and creatinine clearance, while maintaining acceptable levels of drug safety and tolerability.

CONCLUSIONS AND IMPLICATIONS

The novel drug M201-A inhibited diastolic Ca2+ leak via RyR2, improved cardiac lusi-inotropic effects in rats, and enhanced natriuresis and renal function in humans. These findings suggest that this drug may offer a potential new treatment option for chronic kidney disease and heart failure.

The 14th International Podocyte Conference 2023: from podocyte biology to glomerular medicine

The 14th International Podocyte Conference took place in Philadelphia, Pennsylvania, USA from May 23 to 26, 2023. It commenced with an early-career researchers' meeting on May 23, providing young scientists with a platform to present and discuss their research findings. Throughout the main conference, 29 speakers across 9 sessions shared their insights on podocyte biology, glomerular medicine, novel technologic advancements, and translational approaches. Additionally, the event featured 3 keynote lectures addressing engineered chimeric antigen receptor T cell- and mRNA-based therapies and the use of biobanks for enhanced disease comprehension. Furthermore, 4 brief oral abstract sessions allowed scientists to present their findings to a broad audience. The program also included a panel discussion addressing the challenges of conducting human research within the American Black community. Remarkably, after a 5-year hiatus from in-person conferences, the 14th International Podocyte Conference successfully convened scientists from around the globe, fostering the presentation and discussion of crucial research findings, as summarized in this review. Furthermore, to ensure continuous and sustainable education, research, translation, and trial medicine related to podocyte and glomerular diseases for the benefit of patients, the International Society of Glomerular Disease was officially launched during the conference.

Calcium signalling and transport in the kidney

The kidney plays a pivotal role in regulating calcium levels within the body. Approximately 98% of the filtered calcium is reabsorbed in the nephron, and this process is tightly controlled to maintain calcium homeostasis, which is required to facilitate optimal bone mineralization, preserve serum calcium levels within a narrow range, and support intracellular signalling mechanisms. The maintenance of these functions is attributed to a delicate balance achieved by various calcium channels, transporters, and calcium-binding proteins in renal cells. Perturbation of this balance due to deficiency or dysfunction of calcium channels and calcium-binding proteins can lead to severe complications. For example, polycystic kidney disease is linked to aberrant calcium transport and signalling. Furthermore, dysregulation of calcium levels can promote the formation of kidney stones. This Review provides an updated description of the key aspects of calcium handling in the kidney, focusing on the function of various calcium channels and the physiological stimuli that control these channels or are communicated through them. A discussion of the role of calcium as an intracellular second messenger and the pathophysiology of renal calcium dysregulation, as well as a summary of gaps in knowledge and future prospects, are also included.

Talin mechanotransduction in disease

Talin protein (Talin 1/2) is a mechanosensitive cytoskeleton protein. The unique structure of the Talin plays a vital role in transmitting mechanical forces. Talin proteins connect the extracellular matrix to the cytoskeleton by linking to integrins and actin, thereby mediating the conversion of mechanical signals into biochemical signals and influencing disease progression as potential diagnostic indicators, therapeutic targets, and prognostic indicators of various diseases. Most studies in recent years have confirmed that mechanical forces also have a crucial role in the development of disease, and Talin has been found to play a role in several diseases. Still, more studies need to be done on how Talin is involved in mechanical signaling in disease. This review focuses on the mechanical signaling of Talin in disease, aiming to summarize the mechanisms by which Talin plays a role in disease and to provide references for further studies.

ER Stress-Perturbed Intracellular Protein O-GlcNAcylation Aggravates Podocyte Injury in Diabetes Nephropathy

Diabetes nephropathy (DN) is the leading cause of end-stage renal disease (ESRD) worldwide, and podocyte injury is the central contributor to the progression of DN. Despite the emerging evidence that has established the importance of podocyte endoplasmic reticulum (ER) stress in the pathogenesis of DN, abnormal protein O-GlcNAcylation is also augmented. Currently, the mechanism associating these two hyperglycemia-induced disorders remains poorly understood. This study intended to elucidate whether ER stress drives hyper-protein O-GlcNAcylation to cause podocyte injury in DN. We used both type 1 and type 2 DN models to confirm the occurrence of ER stress and excessive protein O-GlcNAcylation, and then podocyte purification was also conducted for further investigation. Nephroseq V5 data were mined and in vitro studies were applied to reveal the involvement of ER stress and hyper-O-GlcNAcylation in podocyte injury. Our results indicated that ER stress was induced in both type 1 and type 2 DN, and the human RNA-seq data from Nephroseq V5 showed that O-GlcNAcylation-related genes were significantly upregulated in the DN patients. We further demonstrated that ER stress occurred prior to hyper-O-GlcNAc modification and that pharmacologically inhibited protein O-GlcNAcylation can help decrease the podocyte apoptosis induced by hyperglycemia. Together, these discoveries will aid in uncovering the activation of the ER stress-O-GlcNAcylation axis in podocyte injury under DN, which will help open up new therapeutic approaches for preventing DN progression.

Endoplasmic reticulum stress mediates environmental particle-induced inflammatory response in bronchial epithelium

While the detailed mechanisms for how particulate matter (PM) causes adverse health effects in the lungs remain largely unknown, endoplasmic reticulum (ER) stress has been implicated in PM-induced lung injury. The present study was undertaken to examine how/if ER stress might regulate PM-induced inflammation, and to begin to define potential underlying molecular mechanisms. Here, ER stress hallmarks were examined in human bronchial epithelial (HBE) cells exposed to PM. To confirm roles of certain pathways, siRNA targeting ER stress genes and an ER stress inhibitor were employed. Expression of select inflammatory cytokines and related signaling pathway components by the cells were assessed as well. The results showed that PM exposure induced elevations in two ER stress hallmarks, i.e. GRP78 and IRE1α, in time-and/or dose-related manners in the HBE cells. Inhibition of ER stress by siRNA for GRP78 or IRE1α significantly alleviated the PM-induced effects. Further, ER stress appeared to regulate PM-induced inflammation - likely through downstream autophagy and NF-κB pathways - as implied by studies showing that inhibition of ER stress by siRNA of GRP78 or IRE1α caused significant amelioration of PM-induced autophagy and subsequent activation of NF-κB pathways. Moreover, the ER stress inhibitor 4-PBA were used to confirm the protective effects against PM-induced outcomes. Together, the results suggest ER stress plays a deleterious role in PM-induced airway inflammation, possibly through activation of autophagy and NF-κB signaling. Accordingly, protocols/treatments that could lead to inhibited ER stress could potentially be effective for treatment of PM-related airway disorders.

Podocyte injury of diabetic nephropathy: Novel mechanism discovery and therapeutic prospects

Diabetic nephropathy (DN) is a severe complication of diabetes mellitus, posing significant challenges in terms of early prevention, clinical diagnosis, and treatment. Consequently, it has emerged as a major contributor to end-stage renal disease. The glomerular filtration barrier, composed of podocytes, endothelial cells, and the glomerular basement membrane, plays a vital role in maintaining renal function. Disruptions in podocyte function, including hypertrophy, shedding, reduced density, and apoptosis, can impair the integrity of the glomerular filtration barrier, resulting in elevated proteinuria, abnormal glomerular filtration rate, and increased creatinine levels. Hence, recent research has increasingly focused on the role of podocyte injury in DN, with a growing emphasis on exploring therapeutic interventions targeting podocyte injury. Studies have revealed that factors such as lipotoxicity, hemodynamic abnormalities, oxidative stress, mitochondrial dysfunction, and impaired autophagy can contribute to podocyte injury. This review aims to summarize the underlying mechanisms of podocyte injury in DN and provide an overview of the current research status regarding experimental drugs targeting podocyte injury in DN. The findings presented herein may offer potential therapeutic targets and strategies for the management of DN associated with podocyte injury.

MANF stimulates autophagy and restores mitochondrial homeostasis to treat autosomal dominant tubulointerstitial kidney disease in mice

Misfolded protein aggregates may cause toxic proteinopathy, including autosomal dominant tubulointerstitial kidney disease due to uromodulin mutations (ADTKD-UMOD), a leading hereditary kidney disease. There are no targeted therapies. In our generated mouse model recapitulating human ADTKD-UMOD carrying a leading UMOD mutation, we show that autophagy/mitophagy and mitochondrial biogenesis are impaired, leading to cGAS-STING activation and tubular injury. Moreover, we demonstrate that inducible tubular overexpression of mesencephalic astrocyte-derived neurotrophic factor (MANF), a secreted endoplasmic reticulum protein, after the onset of disease stimulates autophagy/mitophagy, clears mutant UMOD, and promotes mitochondrial biogenesis through p-AMPK enhancement, thus protecting kidney function in our ADTKD mouse model. Conversely, genetic ablation of MANF in the mutant thick ascending limb tubular cells worsens autophagy suppression and kidney fibrosis. Together, we have discovered MANF as a biotherapeutic protein and elucidated previously unknown mechanisms of MANF in the regulation of organelle homeostasis, which may have broad therapeutic applications to treat various proteinopathies.

Pharmacological Regulation of Endoplasmic Reticulum Structure and Calcium Dynamics: Importance for Neurodegenerative Diseases

The endoplasmic reticulum (ER) is the largest organelle of the cell, composed of a continuous network of sheets and tubules, and is involved in protein, calcium (Ca2+), and lipid homeostasis. In neurons, the ER extends throughout the cell, both somal and axodendritic compartments, and is highly important for neuronal functions. A third of the proteome of a cell, secreted and membrane-bound proteins, are processed within the ER lumen and most of these proteins are vital for neuronal activity. The brain itself is high in lipid content, and many structural lipids are produced, in part, by the ER. Cholesterol and steroid synthesis are strictly regulated in the ER of the blood-brain barrier protected brain cells. The high Ca2+ level in the ER lumen and low cytosolic concentration is needed for Ca2+-based intracellular signaling, for synaptic signaling and Ca2+ waves, and for preparing proteins for correct folding in the presence of high Ca2+ concentrations to cope with the high concentrations of extracellular milieu. Particularly, ER Ca2+ is controlled in axodendritic areas for proper neurito- and synaptogenesis and synaptic plasticity and remodeling. In this review, we cover the physiologic functions of the neuronal ER and discuss it in context of common neurodegenerative diseases, focusing on pharmacological regulation of ER Ca2+ Furthermore, we postulate that heterogeneity of the ER, its protein folding capacity, and ensuring Ca2+ regulation are crucial factors for the aging and selective vulnerability of neurons in various neurodegenerative diseases. SIGNIFICANCE STATEMENT: Endoplasmic reticulum (ER) Ca2+ regulators are promising therapeutic targets for degenerative diseases for which efficacious drug therapies do not exist. The use of pharmacological probes targeting maintenance and restoration of ER Ca2+ can provide restoration of protein homeostasis (e.g., folding of complex plasma membrane signaling receptors) and slow down the degeneration process of neurons.

Podocyte-specific deletion of ubiquitin carboxyl-terminal hydrolase L1 causes podocyte injury by inducing endoplasmic reticulum stress

Ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) is a unique component of the ubiquitin-proteasome system (UPS), which has multiple activities in maintaining intracellular ubiquitin levels. We previously reported the aberrant low expression of UCHL1 in podocytes of non-immune complex-mediated glomerulonephritis, and recent studies indicate that anti-UCHL1 antibody was responsible for the refractory minimal change disease (MCD), but the specific effect of UCHL1 to the podocytopathy has not been determined. Therefore, we generated podocyte-specific UCHL1 gene knockout (UCHL1cre/cre) rats model. Podocyte-specific UCHL1 knockout rats exhibited severe kidney damage, including segmental/global glomerulosclerosis, kidney function damage and severe proteinuria, compared with littermate control. Subsequently, by carrying out mass spectrometry analysis of isolated glomeruli of rats, abnormal protein accumulation of ECM-receptor Interaction was found in UCHL1cre/cre rats. Mechanistic studies in vivo and in vitro revealed that aberrant protein accumulation after UCHL1 deficiency induced endoplasmic reticulum (ER) stress, unfolded protein reaction (UPR) to reduce the protein level of podocyte skeleton proteins, and CHOP mediated apoptosis as well, which related to the dysfunction of the ubiquitin-proteasome system with decreased free monomeric ubiquitin level, thereby affecting protein ubiquitination and degradation. In addition, inhibition of ER stress by 4-PBA could attenuate the degree of ER stress and podocyte dysfunction. Our study indicates that UCHL1 is a potential target for preventing podocytes injury in some non-immune complex-mediated glomerulopathy.

Analysis of gene expression and use of connectivity mapping to identify drugs for treatment of human glomerulopathies

Background

Human glomerulonephritis (GN)—membranous nephropathy (MN), focal segmental glomerulosclerosis (FSGS) and IgA nephropathy (IgAN), as well as diabetic nephropathy (DN) are leading causes of chronic kidney disease. In these glomerulopathies, distinct stimuli disrupt metabolic pathways in glomerular cells. Other pathways, including the endoplasmic reticulum (ER) unfolded protein response (UPR) and autophagy, are activated in parallel to attenuate cell injury or promote repair.

Methods

We used publicly available datasets to examine gene transcriptional pathways in glomeruli of human GN and DN and to identify drugs.

Results

We demonstrate that there are many common genes upregulated in MN, FSGS, IgAN, and DN. Furthermore, these glomerulopathies were associated with increased expression of ER/UPR and autophagy genes, a significant number of which were shared. Several candidate drugs for treatment of glomerulopathies were identified by relating gene expression signatures of distinct drugs in cell culture with the ER/UPR and autophagy genes upregulated in the glomerulopathies (“connectivity mapping”). Using a glomerular cell culture assay that correlates with glomerular damage in vivo, we showed that one candidate drug – neratinib (an epidermal growth factor receptor inhibitor) is cytoprotective.

Conclusion

The UPR and autophagy are activated in multiple types of glomerular injury. Connectivity mapping identified candidate drugs that shared common signatures with ER/UPR and autophagy genes upregulated in glomerulopathies, and one of these drugs attenuated injury of glomerular cells. The present study opens the possibility for modulating the UPR or autophagy pharmacologically as therapy for GN.

MANF stimulates autophagy and restores mitochondrial homeostasis to treat toxic proteinopathy

Misfolded protein aggregates may cause toxic proteinopathy, including autosomal dominant tubulointerstitial kidney disease due to uromodulin mutations (ADTKD- UMOD ), one of the leading hereditary kidney diseases, and Alzheimer’s disease etc. There are no targeted therapies. ADTKD is also a genetic form of renal fibrosis and chronic kidney disease, which affects 500 million people worldwide. For the first time, in our newly generated mouse model recapitulating human ADTKD- UMOD carrying a leading UMOD deletion mutation, we show that autophagy/mitophagy and mitochondrial biogenesis are severely impaired, leading to cGAS- STING activation and tubular injury. Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a novel endoplasmic reticulum stress-regulated secreted protein. We provide the first study that inducible tubular overexpression of MANF after the onset of disease stimulates autophagy/mitophagy and clearance of the misfolded UMOD, and promotes mitochondrial biogenesis through p-AMPK enhancement, resulting in protection of kidney function. Conversely, genetic ablation of endogenous MANF upregulated in the mutant mouse and human tubular cells worsens autophagy suppression and kidney fibrosis. Together, we discover MANF as a novel biotherapeutic protein and elucidate previously unknown mechanisms of MANF in regulating organelle homeostasis to treat ADTKD, which may have broad therapeutic application to treat various proteinopathies.

An ex vivo culture model of kidney podocyte injury reveals mechanosensitive, synaptopodin-templating, sarcomere-like structures

Chronic kidney diseases are widespread and incurable. The biophysical mechanisms underlying them are unclear, in part because material systems for reconstituting the microenvironment of relevant kidney cells are limited. A critical question is how kidney podocytes (glomerular epithelial cells) regenerate foot processes of the filtration apparatus following injury. Recently identified sarcomere-like structures (SLSs) with periodically spaced myosin IIA and synaptopodin appear in injured podocytes in vivo. We hypothesized that SLSs template synaptopodin in the initial stages of recovery in response to microenvironmental stimuli and tested this hypothesis by developing an ex vivo culture system that allows control of the podocyte microenvironment. Results supported our hypothesis. SLSs in podocytes that migrated from isolated kidney glomeruli presented periodic synaptopodin-positive clusters that nucleated peripheral, foot process-like extensions. SLSs were mechanoresponsive to actomyosin inhibitors and substrate stiffness. Results suggest SLSs as mechanobiological mediators of podocyte recovery and as potential targets for therapeutic intervention.

Blocking CHOP-dependent TXNIP shuttling to mitochondria attenuates albuminuria and mitigates kidney injury in nephrotic syndrome

Albuminuria is a hallmark of glomerular disease of various etiologies. It is not only a symptom of glomerular disease but also a cause leading to glomerulosclerosis, interstitial fibrosis, and eventually, a decline in kidney function. The molecular mechanism underlying albuminuria-induced kidney injury remains poorly defined. In our genetic model of nephrotic syndrome (NS), we have identified CHOP (C/EBP homologous protein)-TXNIP (thioredoxin-interacting protein) as critical molecular linkers between albuminuria-induced ER dysfunction and mitochondria dyshomeostasis. TXNIP is a ubiquitously expressed redox protein that binds to and inhibits antioxidant enzyme, cytosolic thioredoxin 1 (Trx1), and mitochondrial Trx2. However, very little is known about the regulation and function of TXNIP in NS. By utilizing Chop-/- and Txnip-/- mice as well as 68Ga-Galuminox, our molecular imaging probe for detection of mitochondrial reactive oxygen species (ROS) in vivo, we demonstrate that CHOP up-regulation induced by albuminuria drives TXNIP shuttling from nucleus to mitochondria, where it is required for the induction of mitochondrial ROS. The increased ROS accumulation in mitochondria oxidizes Trx2, thus liberating TXNIP to associate with mitochondrial nod-like receptor protein 3 (NLRP3) to activate inflammasome, as well as releasing mitochondrial apoptosis signal-regulating kinase 1 (ASK1) to induce mitochondria-dependent apoptosis. Importantly, inhibition of TXNIP translocation and mitochondrial ROS overproduction by CHOP deletion suppresses NLRP3 inflammasome activation and p-ASK1-dependent mitochondria apoptosis in NS. Thus, targeting TXNIP represents a promising therapeutic strategy for the treatment of NS.

Activation of TRPC6 by AngⅡ Induces Podocyte Injury and Participates in Proteinuria of Nephrotic Syndrome

Background: Nephrotic syndrome (NS) is a common glomerular disease, and podocyte injury is the character of primary NS, usually caused by minimal change disease and membranous nephropathy. Podocytopathy is primarily associated with glomerular proteinuria. Losartan, an angiotensin receptor blocker (ARB), is commonly used in the treatment of NS, and the AngiotensinⅡ (AngⅡ)–transient receptor potential ion channel 6 (TRPC6) axis has been reported to act on podocytes to regulate proteinuria in NS. Therefore, the purpose of this study was to explore the relationship in between AngⅡ–TRPC6, podocyte injury, and proteinuria based on the adriamycin (ADR) NS rat model.

Method: All male rats were divided into three groups: control group, model group, and ARB group. The rats in the model group were induced by ADR, and the rats in the ARB group received losartan after induction of renal injury for 4 weeks. The changes in parameters related to renal dysfunction, and glomerular and podocyte structural damage, such as AngⅡ, AngⅡ type I receptor (AT1R), TRPC6, CaN, Caspase-3, Nephrin, and Podocin, were analyzed. Furthermore, the kidneys were isolated for study via transmission electron microscopy (TEM), immunohistochemistry, and western blot (WB) after the rats were sacrificed. In vitro, immortalized mouse MPC5 podocytes were used to investigate the regulatory effect of flufenamic acid (Flu) and SAR7334 (SAR) on the AngⅡ-TRPC6 signaling axis. Flow cytometry and WB were conducted to determine the relationship between podocyte injury and AngⅡ-TRPC6.

Results:In vivo results showed that NS rats developed massive albuminuria and abnormal renal function, accompanied by abnormally increased levels of AngⅡ, TRPC6, AT1R, and CaN and a decreased expression of actin molecules in podocytes, extensive fusion of foot processes (FP), loss of glomerular structural integrity, collapse of podocyte structure, and skeletal reorganization. In vitro experiments indicated that both AngⅡ and Flu (the specific agonist of TRPC6) stimulated the expressions of TRPC6, AT1R, and Caspase-3 in podocytes. The AngⅡ receptor–blocker losartan and TRPC6-specific inhibitor SAR blocked the overexpression of the aforementioned proteins. In addition, SAR also attenuated the degradation of podocyte structural proteins and inhibited the fluorescence intensity of intracellular calcium (Ca2+) and cell apoptosis.

Conclusion: The involvement of AngⅡ in the occurrence of NS proteinuria may be related to podocyte injury induced by activated TRPC6.

UPR Responsive Genes Manf and Xbp1 in Stroke

Stroke is a devastating medical condition with no treatment to hasten recovery. Its abrupt nature results in cataclysmic changes in the affected tissues. Resident cells fail to cope with the cellular stress resulting in massive cell death, which cannot be endogenously repaired. A potential strategy to improve stroke outcomes is to boost endogenous pro-survival pathways. The unfolded protein response (UPR), an evolutionarily conserved stress response, provides a promising opportunity to ameliorate the survival of stressed cells. Recent studies from us and others have pointed toward mesencephalic astrocyte-derived neurotrophic factor (MANF) being a UPR responsive gene with an active role in maintaining proteostasis. Its pro-survival effects have been demonstrated in several disease models such as diabetes, neurodegeneration, and stroke. MANF has an ER-signal peptide and an ER-retention signal; it is secreted by ER calcium depletion and exits cells upon cell death. Although its functions remain elusive, conducted experiments suggest that the endogenous MANF in the ER lumen and exogenously administered MANF protein have different mechanisms of action. Here, we will revisit recent and older bodies of literature aiming to delineate the expression profile of MANF. We will focus on its neuroprotective roles in regulating neurogenesis and inflammation upon post-stroke administration. At the same time, we will investigate commonalities and differences with another UPR responsive gene, X-box binding protein 1 (XBP1), which has recently been associated with MANF’s function. This will be the first systematic comparison of these two UPR responsive genes aiming at revealing previously uncovered associations between them. Overall, understanding the mode of action of these UPR responsive genes could provide novel approaches to promote cell survival.

The role of the PI3K/AKT signalling pathway in the corneal epithelium: recent updates

Phosphatidylinositol 3 kinase (PI3K)/AKT (also called protein kinase B, PKB) signalling regulates various cellular processes, such as apoptosis, cell proliferation, the cell cycle, protein synthesis, glucose metabolism, and telomere activity. Corneal epithelial cells (CECs) are the outermost cells of the cornea; they maintain good optical performance and act as a physical and immune barrier. Various growth factors, including epidermal growth factor receptor (EGFR) ligands, insulin-like growth factor 1 (IGF1), neurokinin 1 (NK-1), and insulin activate the PI3K/AKT signalling pathway by binding their receptors and promote antiapoptotic, anti-inflammatory, proliferative, and migratory functions and wound healing in the corneal epithelium (CE). Reactive oxygen species (ROS) regulate apoptosis and inflammation in CECs in a concentration-dependent manner. Extreme environments induce excess ROS accumulation, inhibit PI3K/AKT, and cause apoptosis and inflammation in CECs. However, at low or moderate levels, ROS activate PI3K/AKT signalling, inhibiting apoptosis and stimulating proliferation of healthy CECs. Diabetes-associated hyperglycaemia directly inhibit PI3K/AKT signalling by increasing ROS and endoplasmic reticulum (ER) stress levels or suppressing the expression of growth factors receptors and cause diabetic keratopathy (DK) in CECs. Similarly, hyperosmolarity and ROS accumulation suppress PI3K/AKT signalling in dry eye disease (DED). However, significant overactivation of the PI3K/AKT signalling pathway, which mediates inflammation in CECs, is observed in both infectious and noninfectious keratitis. Overall, upon activation by growth factors and NK-1, PI3K/AKT signalling promotes the proliferation, migration, and anti-apoptosis of CECs, and these processes can be regulated by ROS in a concentration-dependent manner. Moreover, PI3K/AKT signalling pathway is inhibited in CECs from individuals with DK and DED, but is overactivated by keratitis.

MANF: an emerging therapeutic target for metabolic diseases

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is an endoplasmic reticulum-resident protein and a secretory factor and has beneficial effects in multiple diseases. Recent evidence shows that its circulating levels in humans are dynamically regulated under various metabolic diseases, including diabetes, obesity, fatty liver, and cardiovascular diseases, suggesting that MANF may play a role in these pathological states. Also, its downregulation in mice impairs glucose homeostasis, promotes lipid accumulation in the liver, reduces energy expenditure, and induces inflammation. Conversely, MANF overexpression prevents or mitigates some of these metabolic disturbances. In particular, systemic MANF administration alleviates dietary obesity and related metabolic disorders in obese mice. We therefore propose that MANF might be a promising target for treating chronic metabolic diseases.

The Transient Receptor Potential Channel Yvc1 Deletion Recovers the Growth Defect of Calcineurin Mutant Under Endoplasmic Reticulum Stress in Candida albicans

Transient receptor potential (TRP) channel Yvc1 was related with hyphal growth, oxidative stress response, and pathogenicity. Calcineurin subunit Cnb1 was activated immediately in yeasts when exposed to severe stimulation. However, the relationship between Yvc1 and Cnb1-governed calcium ions and endoplasmic reticulum (ER) stress response remains unrevealed. In this study, we found that the mutant cnb1Δ/Δ was sensitive to TN, which was related with the overexpression of membrane calcium ion channels that could increase the cytosol calcium concentration. However, the growth of the cnb1Δ/Δyvc1Δ/Δ mutant was recovered and its cell vitality was better than the cnb1Δ/Δ strain. Meanwhile, the cellular calcium concentration was decreased and its fluctuation was weakened under ER stress in the cnb1Δ/Δyvc1Δ/Δ strain. To verify the regulation role of Yvc1 in the calcium concentration, we found that the addition of CaCl₂ led to the worse viability, while the growth state was relieved under the treatment of EGTA in the cnb1Δ/Δ strain. In conclusion, the deletion of YVC1 could reduce the cellular calcium and relieve the ER stress sensitivity of the cnb1Δ/Δ strain. Thereby, our findings shed a novel light on the relationship between the Yvc1-governed cellular calcium concentration and ER stress response in C. albicans.

Calcium Signaling Mediates Cell Death and Crosstalk with Autophagy in Kidney Disease

The kidney is an important organ for the maintenance of Ca²⁺ homeostasis in the body. However, disruption of Ca²⁺ homeostasis will cause a series of kidney diseases, such as acute kidney injury (AKI), chronic kidney disease (CKD), renal ischemia/reperfusion (I/R) injury, autosomal dominant polycystic kidney disease (ADPKD), podocytopathy, and diabetic nephropathy. During the progression of kidney disease, Ca²⁺ signaling plays key roles in various cell activities such as necrosis, apoptosis, eryptosis and autophagy. Importantly, there are complex Ca²⁺ flux networks between the endoplasmic reticulum (ER), mitochondria and lysosomes which regulate intracellular Ca²⁺ signaling in renal cells and contribute to kidney disease. In addition, Ca²⁺ signaling also links the crosstalk between various cell deaths and autophagy under the stress of heavy metals or high glucose. In this regard, we present a review of Ca²⁺ signaling in cell death and crosstalk with autophagy and its potential as a therapeutic target for the development of new and efficient drugs against kidney diseases.

MANF: A Novel Endoplasmic Reticulum Stress Response Protein-The Role in Neurological and Metabolic Disorders

The mesencephalic astrocyte-derived neurotrophic factor (MANF), also named as arginine-rich protein (ARP) or arginine-rich mutated in early-stage tumors (ARMET), is a novel evolutionary conserved protein related to unfolded protein response. Growing evidence suggests that MANF critically involves in many ER stress-related diseases with a protective effect. Here, we review the function of MANF based on its structure in neurological and metabolic disorders and summarize its potential applications in disease diagnosis and therapies.

Autonomous Calcium Signaling in Human and Zebrafish Podocytes Controls Kidney Filtration Barrier Morphogenesis

BACKGROUND

Podocytes are critical to maintaining the glomerular filtration barrier, and mutations in nephrotic syndrome genes are known to affect podocyte calcium signaling. However, the role of calcium signaling during podocyte development remains unknown.

METHODS

We undertook live imaging of calcium signaling in developing podocytes, using zebrafish larvae and human kidney organoids. To evaluate calcium signaling during development and in response to channel blockers and genetic defects, the calcium biosensor GCaMP6s was expressed in zebrafish podocytes. We used electron microscopy to evaluate filtration barrier formation in zebrafish, and Fluo-4 to detect calcium signals in differentiating podocytes in human kidney organoids.

RESULTS

Immature zebrafish podocytes (2.5 days postfertilization) generated calcium transients that correlated with interactions with forming glomerular capillaries. Calcium transients persisted until 4 days postfertilization, and were absent after glomerular barrier formation was complete. We detected similar calcium transients in maturing human organoid glomeruli, suggesting a conserved mechanism. In both models, inhibitors of SERCA or IP3 receptor calcium-release channels blocked calcium transients in podocytes, whereas lanthanum was ineffective, indicating the calcium source is from intracellular podocyte endoplasmic-reticulum stores. Calcium transients were not affected by blocking heartbeat or by blocking development of endothelium or endoderm, and they persisted in isolated glomeruli, suggesting podocyte-autonomous calcium release. Inhibition of expression of phospholipase C-γ1, but not nephrin or phospholipase C-ε1, led to significantly decreased calcium activity. Finally, blocking calcium release affected glomerular shape and podocyte foot process formation, supporting the critical role of calcium signaling in glomerular morphogenesis.

CONCLUSIONS

These findings establish podocyte cell-autonomous calcium signaling as a prominent and evolutionarily conserved feature of podocyte differentiation and demonstrate its requirement for podocyte foot process formation.

Intracellular Calcium Homeostasis and Kidney Disease

Kidney disease is a serious health problem that burdens our healthcare system. It is crucial to find the accurate pathogenesis of various types of kidney disease to provide guidance for precise therapies for patients suffering from these diseases. However, the exact molecular mechanisms underlying these diseases have not been fully understood. Disturbance of calcium homeostasis in renal cells plays a fundamental role in the development of various types of kidney disease, such as primary glomerular disease, diabetic nephropathy, acute kidney injury and polycystic kidney disease, through promoting cell proliferation, stimulating extracellular matrix accumulation, aggravating podocyte injury, disrupting cellular energetics as well as dysregulating cell survival and death dynamics. As a result, preventing the disturbance of calcium homeostasis in specific renal cells (such as tubular cells, podocytes and mesangial cells) is becoming one of the most promising therapeutic strategies in the treatment of kidney disease. The endoplasmic reticulum and mitochondria are two vital organelles in this process. Calcium ions cycle between the endoplasmic reticulum and mitochondria at the conjugation of these two organelles known as the mitochondria-associated endoplasmic reticulum membrane, maintaining calcium homeostasis. The pharmacologic modulation of cellular calcium homeostasis can be viewed as a novel therapeutic method for renal diseases. Here, we will introduce calcium homeostasis under physiological conditions and the disturbance of calcium homeostasis in kidney diseases. We will focus on the calcium homeostasis regulation in renal cells (including tubular cells, podocytes and mesangial cells), especially in the mitochondria- associated endoplasmic reticulum membranes of these renal cells.

Endoplasmic reticulum-associated degradation is required for nephrin maturation and kidney glomerular filtration function

Podocytes are key to the glomerular filtration barrier by forming a slit diaphragm between interdigitating foot processes; however, the molecular details and functional importance of protein folding and degradation in the ER remain unknown. Here, we show that the SEL1L-HRD1 protein complex of ER-associated degradation (ERAD) is required for slit diaphragm formation and glomerular filtration function. SEL1L-HRD1 ERAD is highly expressed in podocytes of both mouse and human kidneys. Mice with podocyte-specific Sel1L deficiency develop podocytopathy and severe congenital nephrotic syndrome with an impaired slit diaphragm shortly after weaning and die prematurely, with a median lifespan of approximately 3 months. We show mechanistically that nephrin, a type 1 membrane protein causally linked to congenital nephrotic syndrome, is an endogenous ERAD substrate. ERAD deficiency attenuated the maturation of nascent nephrin, leading to its retention in the ER. We also show that various autosomal-recessive nephrin disease mutants were highly unstable and broken down by SEL1L-HRD1 ERAD, which attenuated the pathogenicity of the mutants toward the WT allele. This study uncovers a critical role of SEL1L-HRD1 ERAD in glomerular filtration barrier function and provides insights into the pathogenesis associated with autosomal-recessive disease mutants.

Hepatocyte-derived MANF alleviates hepatic ischaemia-reperfusion injury via regulating endoplasmic reticulum stress-induced apoptosis in mice

BACKGROUND

Endoplasmic reticulum (ER) perturbations are novel subcellular effectors involved in the ischaemia-reperfusion injury. As an ER stress-inducible protein, mesencephalic astrocyte-derived neurotrophic factor (MANF) has been proven to be increased during ischaemic brain injury. However, the role of MANF in liver ischaemia reperfusion (I/R) injury has not yet been studied.

METHODS

To investigate the role of MANF in the process of liver ischaemia-reperfusion, Hepatocyte-specific MANF knockout (MANF^hep-/- ) mice and their wild-type (WT) littermates were used in our research. Mice partial (70%) warm hepatic I/R model was established by vascular occlusion. We detected the serum levels of MANF in both liver transplant patients and WT mice before and after liver I/R injury. Recombinant human MANF (rhMANF) was injected into the tail vein before 1 hour occlusion. AST, ALT and Suzuki score were used to evaluate the extent of I/R injury. OGD/R test was performed on primary hepatocytes to simulate IRI in vitro. RNA sequence and RT-PCR were used to detect the cellular signal pathway activation while MANF knockout.

RESULTS

We found that MANF expression and secretion are dramatically up-regulated during hepatic I/R. Hepatocyte-specific MANF knockout aggravates the I/R injury through the over-activated ER stress. The systemic administration of rhMANF before ischaemia has the potential to ameliorate I/R-triggered UPR and liver injury. Further study showed that MANF deficiency activated ATF4/CHOP and JNK/c-JUN/CHOP pathways, and rhMANF inhibited the activation of the two proapoptotic pathways caused by MANF deletion.

CONCLUSION

Collectively, our study unravels a previously unknown relationship among MANF, UPR and hepatic I/R injury.

Endoplasmic Reticulum Calcium Homeostasis in Kidney Disease: Pathogenesis and Therapeutic Targets

Calcium (Ca²⁺) homeostasis is a crucial determinant of cellular function and survival. Endoplasmic reticulum (ER) acts as the largest intracellular Ca²⁺ store that maintains Ca²⁺ homeostasis through the ER Ca²⁺ uptake pump, sarco/ER Ca²⁺ ATPase, ER Ca²⁺ release channels, inositol 1,4,5-trisphosphate receptor channel, ryanodine receptor, and Ca²⁺-binding proteins inside of the ER lumen. Alterations in ER homeostasis trigger ER Ca²⁺ depletion and ER stress, which have been associated with the development of a variety of diseases. In addition, recent studies have highlighted the role of ER Ca²⁺ imbalance caused by dysfunction of sarco/ER Ca²⁺ ATPase, ryanodine receptor, and inositol 1,4,5-trisphosphate receptor channel in various kidney diseases. Despite progress in the understanding of the importance of these ER Ca²⁺ channels, pumps, and binding proteins in the pathogenesis of kidney disease, treatment is still lacking. This mini-review is focused on: i) Ca²⁺ homeostasis in the ER, ii) ER Ca²⁺ dyshomeostasis and apoptosis, and iii) altered ER Ca²⁺ homeostasis in kidney disease, including podocytopathy, diabetic nephropathy, albuminuria, autosomal dominant polycystic kidney disease, and ischemia/reperfusion-induced acute kidney injury.

Inhibiting calpain 1 and 2 in cyclin G associated kinase-knockout mice mitigates podocyte injury

Evidence for reduced expression of cyclin G associated kinase (GAK) in glomeruli of patients with chronic kidney disease was observed in the Nephroseq human database, and GAK was found to be associated with the decline in kidney function. To examine the role of GAK, a protein that functions to uncoat clathrin during endocytosis, we generated podocyte-specific Gak-knockout mice (Gak-KO), which developed progressive proteinuria and kidney failure with global glomerulosclerosis. We isolated glomeruli from the mice carrying the mutation to perform messenger RNA profiling and unearthed evidence for dysregulated podocyte calpain protease activity as an important contributor to progressive podocyte damage. Treatment with calpain inhibitor III specifically inhibited calpain-1/-2 activities, mitigated the degree of proteinuria and glomerulosclerosis, and led to a striking increase in survival in the Gak-KO mice. Podocyte-specific deletion of Capns1, essential for calpain-1 and calpain-2 activities, also improved proteinuria and glomerulosclerosis in Gak-KO mice. Increased podocyte calpain activity-mediated proteolysis of IκBα resulted in increased NF-κB p65-induced expression of growth arrest and DNA-damage-inducible 45 beta in the Gak-KO mice. Our results suggest that loss of podocyte-associated Gak induces glomerular injury secondary to calcium dysregulation and aberrant calpain activation, which when inhibited, can provide a protective role.

Neuroplastin Modulates Anti-inflammatory Effects of MANF

Endoplasmic reticulum (ER) stress is known to induce pro-inflammatory response and ultimately leads to cell death. Mesencephalic astrocyte-derived neurotrophic factor (MANF) is an ER-localized protein whose expression and secretion is induced by ER stress and a crucial survival factor. However, the underlying mechanism of how MANF exerts its cytoprotective activity remains unclear due to the lack of knowledge of its receptor. Here we show that Neuroplastin (NPTN) is such a receptor for MANF. Biochemical analysis shows the physiological interaction between MANF and NPTN on the cell surface. Binding of MANF to NPTN mitigates the inflammatory response and apoptosis via suppression of NF-kβ signaling. Our results demonstrate that NPTN is a cell surface receptor for MANF, which modulates inflammatory responses and cell death, and that the MANF-NPTN survival signaling described here provides potential therapeutic targets for the treatment of ER stress-related disorders, including diabetes mellitus, neurodegeneration, retinal degeneration, and Wolfram syndrome.

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Neuroplastin (NPTN) is a plasma membrane receptor for MANF

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NPTN regulates MANF-mediated suppression of inflammation

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NPTN regulates cell survival mediated by MANF under ER stress

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MANF-NPTN survival pathway provides potential therapeutic targets for ER stress-related disorders

Mesencephalic Astrocyte-Derived Neurotrophic Factor, a Prognostic Factor of Cholangiocarcinoma, Affects Sorafenib Sensitivity of Cholangiocarcinoma Cells by Deteriorating ER Stress

Purpose

Intrahepatic cholangiocarcinoma (ICC) is an aggressive malignant tumor characterized by high malignancy and poor prognosis. Although the efficacy of sorafenib against cholangiocarcinoma cell lines has been demonstrated in vivo and in vitro, limited clinical data are available on the efficacy of sorafenib in patients with cholangiocarcinoma. Sorafenib can enhance endoplasmic reticulum (ER) stress-mediated apoptosis, and ER stress and unfolded protein response are also the mechanisms by which cancer cells resist drug therapy. Mesencephalic astrocyte-derived neurotrophic factor (MANF), initially identified as a neurotrophic factor, can be regulated by ER stress activation. There are no available studies on the diagnostic value and therapeutic significance of MANF in ICC. Hence, the purpose of this study was to evaluate the role of MANF in cholangiocarcinoma, investigating the possibility of whether sorafenib could become a reliable strategy for cholangiocarcinoma therapy.

Methods

In this study, the expression level of MANF in ICC patients was investigated by bioinformatic analysis and the results were verified by tissue microarray assay. Cholangiocarcinoma cell lines were also used to determine how MANF regulates the therapeutic effect of sorafenib and to identify the underlying mechanisms.

Results

The results showed that MANF was correlated with poor prognosis and MANF knockdown could facilitate sorafenib-mediated apoptosis and increase the sensitivity of sorafenib treatment by activating excessive ER stress.

Conclusion

MANF is a prognostic marker of cholangiocarcinoma. MANF knockdown increases sorafenib-mediated ER stress and apoptosis in the cholangiocarcinoma cell lines. This mechanism may lead to a new therapeutic strategy in cholangiocarcinoma.

Trophic activities of endoplasmic reticulum proteins CDNF and MANF

Mesencephalic astrocyte-derived neurotrophic factor (MANF) and cerebral dopamine neurotrophic factor (CDNF) are endoplasmic reticulum (ER) luminal proteins that confer trophic activities in a wide range of tissues under diverse pathological conditions. Despite initially being classified as neurotrophic factors, neither protein structurally nor functionally resembles bona fide neurotrophic factors. Their highly homologous structures comprise a unique globular, saposin-like domain within the N-terminus joined by a flexible linker to a C-terminus containing a SAP-like domain, CXXC motif and an ER retention sequence. Neurotrophic factors exert effects by binding to cognate receptors in the plasma membrane; however, no cell surface receptors have been identified for MANF and CDNF. Both can act as unfolded protein response (UPR) genes that modulate the UPR and inflammatory processes. The trophic activity of MANF and CDNF extends beyond the central nervous system with MANF being crucial for the development of pancreatic β cells and both have trophic effects in a variety of diseases related to the liver, heart, skeletal tissue, kidney and peripheral nervous system. In this article, the unique features of MANF and CDNF, such as their structure and mechanisms of action related to ER stress and inflammation, will be reviewed. Recently identified interactions with lipids and membrane trafficking will also be described. Lastly, their function and therapeutic potential in different diseases including a recent clinical trial using CDNF to treat Parkinson's disease will be discussed. Collectively, this review will highlight MANF and CDNF as broad-acting trophic factors that regulate functions of the endoplasmic reticulum.

Organelle Stress and Crosstalk in Kidney Disease

Organelles play important roles in maintaining cellular homeostasis. Organelle stress responses, especially in mitochondria, endoplasmic reticula (ER), and primary cilia, are deeply involved in kidney disease pathophysiology. Mitochondria are the center of energy production in most eukaryotic cells. Renal proximal tubular cells are highly energy demanding and abundant in mitochondria. Mitochondrial dysfunctions in association with energy metabolism alterations produce reactive oxygen species and promote inflammation in proximal tubular cells, resulting in progression of kidney disease. The ER play critical roles in controlling protein quality. Unfolded protein response (UPR) pathways are the adaptive response to ER stress for maintaining protein homeostasis. UPR pathway dysregulation under pathogenic ER stress often occurs in glomerular and tubulointerstitial cells and promotes progression of kidney disease. The primary cilia sense extracellular signals and maintain calcium homeostasis in cells. Dysfunction of the primary cilia in autosomal dominant polycystic kidney disease reduces the calcium concentration in proximal tubular cells, leading to increased cell proliferation and retention of cyst fluid. In recent years, the direct interaction at membrane contact sites has received increased attention in association with the development of imaging technologies. The part of the ER that is directly connected to mitochondria is termed the mitochondria-associated ER membrane (MAM), which regulates calcium homeostasis and phospholipid metabolism in cells. Disruption of MAM integrity collapses cellular homeostasis and leads to diseases such as diabetes and Alzheimer disease. This review summarizes recent research on organelle stress and crosstalk, and their involvement in kidney disease pathophysiology. In addition, potential treatment options that target organelle stress responses are discussed.

Tripterygium glycoside fraction n2 ameliorates adriamycin-induced nephrotic syndrome in rats by suppressing apoptosis

ETHNOPHARMACOLOGICAL RELEVANCE

Tripterygium wilfordii Hook F. (TwHF), a traditional Chinese herb medicine, has been widely used for clinical treatment of various rheumatic immune diseases. Tripterygium glycosides (TG) extracted from TwHF has been verified to process multiple bioactivities, including immunosuppressive, anti-inflammatory and anti-cancer effects. However, the clinical application of TG is limited due to its severe toxicity and narrow therapeutic window. For the clinical safety of TG usage, attenuation of toxicity is the key issue to be solved.

PURPOSE

Tripterygium glycoside fraction n2 (TG-n2) is a detoxified mixture obtained from TG using a new preparation method. In our previous study, we have demonstrated that TG-n2 has a lower toxicity than TG. The aim of the present study was to screen the renal protective effect of TG-n2 in nephrotic syndrome (NS) induced by adriamycin (ADR) in rats and its effect on apoptosis, as well as the effective difference between TG-n2 and TG.

MATERIALS AND METHODS

The ADR-induced NS rat model was established. Rats were intravenously injected with ADR (6 mg/kg), then treated with either TG-n2 (10 mg/kg/day) or TG (10 mg/kg/day) by oral gavage for 4 weeks. Clinical indexes in each group were determined. HE staining and electron microscopic analysis were used to evaluate renal histopathological damage. Caspase-3 activity reagent and TUNEL staining were used to estimate renal apoptosis. Protein levels of caspase-3, caspase-9, caspase-8, caspase-12, Bax, Bcl-2, p53, TNF-R1, FLIP and podocin were measured by Western Blot.

RESULTS

TG-n2 and TG intervention ameliorated renal function as assessed by the levels of 24-h proteinuria, Cr, BUN, TC, TG, ALB and LDL-c. TG-n2 and TG alleviated the decrease of podocin protein expression and morphological injury of podocyte as screened by Western Blot and electron microscopic analysis. Besides, renal tubular injury was reduced as inspected by light microscopic analysis. TG-n2 and TG could significantly inhibit the apoptosis and activity of caspase-3 in kidney tissues as examined by fluorescence microscopic analysis and reagent. After intervention of TG-n2 and TG, protein levels of cleaved caspase-3, cleaved caspase-8, cleaved caspase-9, Bax, p53 and TNF-R1 in renal issues were significantly decreased compared with ADR group. In contrast, protein level of Bcl-2 was elevated remarkedly.

CONCLUSIONS

Our data suggested that attenuated TG-n2 may have a similar protective effect with TG in ADR-induced NS in rats by inhibiting activation of apoptosis.

Urinary ERdj3 and mesencephalic astrocyte-derived neutrophic factor identify endoplasmic reticulum stress in glomerular disease

Podocyte injury and endoplasmic reticulum (ER) stress have been implicated in the pathogenesis of various glomerular diseases. ERdj3 (DNAJB11) and mesencephalic astrocyte-derived neurotrophic factor (MANF) are ER chaperones lacking the KDEL motif, and may be secreted extracellularly. Since podocytes reside in the urinary space, we examined if podocyte injury is associated with secretion of KDEL-free ER chaperones from these cells into the urine, and if chaperones in the urine reflect ER stress in glomerulonephritis. In cultured podocytes, ER stress increased ERdj3 and MANF intracellularly and in culture medium, whereas GRP94 (KDEL chaperone) increased only intracellularly. ERdj3 and MANF secretion was blocked by the secretory trafficking inhibitor, brefeldin A. Urinary ERdj3 and MANF increased in rats injected with tunicamycin (in the absence of proteinuria). After induction of passive Heymann nephritis (PHN) and puromycin aminonucleoside nephrosis (PAN), there was an increase in glomerular ER stress, and appearance of ERdj3 and MANF in the urine, coinciding with the onset of proteinuria. Rats with PHN were treated with the chemical chaperone, 4-phenyl butyrate (PBA), starting at the time of disease induction, or after disease was established. In both protocols, 4-PBA reduced proteinuria and urinary ER chaperone secretion, compared with PHN rats treated with saline (control). In conclusion, urinary ERdj3 and MANF reflect glomerular ER stress. 4-PBA protected against complement-mediated podocyte injury and the therapeutic response could be monitored by urinary ERdj3 and MANF.

Ryanodine receptor 1 mediated dexamethasone-induced chondrodysplasia in fetal rats

BACKGROUND

Osteoarthritis is caused by cartilage dysplasia and has fetal origin. Prenatal dexamethasone exposure (PDE) induced chondrodysplasia in fetal rats by inhibiting transforming growth factor β (TGFβ) signaling. This study aimed to determine the effect of dexamethasone on fetal cartilage development and illustrate the underlying molecular mechanism.

METHODS

Dexamethasone (0.2 mg/kg.d) was injected subcutaneously every morning in pregnant rats from gestational day (GD) 9 to GD21. Harvested fetal femurs and tibias at GD21 for immunofluorescence and gene expression analysis. Fetal chondrocytes were treated with dexamethasone (100, 250 and 500 nM), endoplasmic reticulum stress (ERS) inhibitor, and ryanodine receptor 1 (RYR1) antagonist for subsequent analyses.

RESULTS

In vivo, prenatal dexamethasone exposure (PDE) decreased the total length of the fetal cartilage, the proportion of the proliferation area and the cell density and matrix content in fetal articular cartilage. Moreover, PDE increased RYR1 expression and intracellular calcium levels and elevated the expression of ERS-related genes, while downregulated the TGFβ signaling pathway and extracellular matrix (ECM) synthesis in fetal chondrocytes. In vitro, we verified dexamethasone significantly decreased ECM synthesis through activating RYR 1 mediated-ERS.

CONCLUSIONS

PDE inhibited TGFβ signaling pathway and matrix synthesis through RYR1 / intracellular calcium mediated ERS, which ultimately led to fetal dysplasia. This study confirmed the molecular mechanism of ERS involved in the developmental toxicity of dexamethasone and suggested that RYR1 may be an early intervention target for fetal-derived adult osteoarthritis.

MANF Promotes Diabetic Corneal Epithelial Wound Healing and Nerve Regeneration by Attenuating Hyperglycemia-Induced Endoplasmic Reticulum Stress

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a neurotrophic factor widely expressed in mammalian tissues, and it exerts critical protective effects on neurons and other cell types in various disease models, such as those for diabetes. However, to date, the expression and roles of MANF in the cornea, with or without diabetic keratopathy (DK), remain unclear. Here, we demonstrate that MANF is abundantly expressed in normal corneal epithelial cells; however, MANF expression was significantly reduced in both unwounded and wounded corneal epithelium in streptozotocin-induced type 1 diabetic C57BL/6 mice. Recombinant human MANF significantly promoted normal and diabetic corneal epithelial wound healing and nerve regeneration. Furthermore, MANF inhibited hyperglycemia-induced endoplasmic reticulum (ER) stress and ER stress-mediated apoptosis. Attenuation of ER stress with 4-phenylbutyric acid (4-PBA) also ameliorated corneal epithelial closure and nerve regeneration. However, the beneficial effects of MANF and 4-PBA were abolished by an Akt inhibitor and Akt-specific small interfering RNA (siRNA). Finally, we reveal that the subconjunctival injection of MANF-specific siRNA prevents corneal epithelial wound healing and nerve regeneration. Our results provide important evidence that hyperglycemia-suppressed MANF expression may contribute to delayed corneal epithelial wound healing and impaired nerve regeneration by increasing ER stress, and MANF may be a useful therapeutic modality for treating DK.

Solution structure of human myeloid-derived growth factor suggests a conserved function in the endoplasmic reticulum

Human myeloid-derived growth factor (hMYDGF) is a 142-residue protein with a C-terminal endoplasmic reticulum (ER) retention sequence (ERS). Extracellular MYDGF mediates cardiac repair in mice after anoxic injury. Although homologs of hMYDGF are found in eukaryotes as distant as protozoans, its structure and function are unknown. Here we present the NMR solution structure of hMYDGF, which consists of a short α-helix and ten β-strands distributed in three β-sheets. Conserved residues map to the unstructured ERS, loops on the face opposite the ERS, and the surface of a cavity underneath the conserved loops. The only protein or portion of a protein known to have a similar fold is the base domain of VNN1. We suggest, in analogy to the tethering of the VNN1 nitrilase domain to the plasma membrane via its base domain, that MYDGF complexed to the KDEL receptor binds cargo via its conserved residues for transport to the ER.