Mitochondria-associated endoplasmic reticulum membranes promote mitochondrial fission through AKAP1-Drp1 pathway in podocytes under high glucose conditions

Inhibition of SGLT2 protects podocytes in diabetic kidney disease by rebalancing mitochondria-associated endoplasmic reticulum membranes

BACKGROUND

Sodium-glucose cotransporter 2 (SGLT2) inhibitors have changed the therapeutic landscape for diabetic kidney disease (DKD) patients, but their underlying mechanisms are complicated and not fully understood. Mitochondria-associated endoplasmic reticulum membranes (MAMs), the dynamic contact sites between mitochondria and the endoplasmic reticulum (ER), serve as intracellular platforms important for regulating cellular fate and function. This study explored the roles and mechanisms of SGLT2 inhibitors in regulating MAMs formation in diabetic podocytes.

METHODS

We assessed MAMs formation in podocytes from DKD patients' renal biopsy samples and induced an increase in MAMs formation in cultured human podocytes by transfecting OMM-ER linker plasmid to investigate the effects of MAMs imbalance on podocyte injury. Empagliflozin-treated diabetic mice and podocyte-specific SGLT2 knockout diabetic mice (diabetic states were induced by streptozotocin and a high-fat diet), empagliflozin-treated podocytes, SGLT2-downregulated podocytes, and SGLT2-overexpressing podocytes were used to investigate the effects and mechanisms of SGLT2 inhibitors on MAMs formation in diabetic podocytes.

RESULTS

MAMs were increased in podocytes and were associated with renal dysfunction in DKD patients. Increased MAMs aggravated HG-induced podocyte injury. The expression of SGLT2 was increased in diabetic podocytes. In addition, empagliflozin-treatment and podocyte-specific SGLT2 knockout attenuated MAMs formation and podocyte injury in diabetic mice. Empagliflozin treatment and SGLT2 knockdown decreased podocyte MAMs formation by activating the AMP-activated protein kinase (AMPK) pathway, while SGLT2 overexpression had the opposite effect.

CONCLUSIONS

Inhibition of SGLT2 attenuates MAMs imbalance in diabetic podocytes by activating the AMPK pathway. This study expands our knowledge of the roles of SGLT2 inhibitors in improving DKD podocyte injury and provides new insights into DKD treatment.

Novel insight into the role of A-kinase anchoring proteins (AKAPs) in ischemic stroke and therapeutic potentials

Ischemic stroke, a devastating disease associated with high mortality and disability worldwide, has emerged as an urgent public health issue. A-kinase anchoring proteins (AKAPs) are a group of signal-organizing molecules that compartmentalize and anchor a wide range of receptors and effector proteins and have a major role in stabilizing mitochondrial function and promoting neurodevelopmental development in the central nervous system (CNS). Growing evidence suggests that dysregulation of AKAPs expression and activity is closely associated with oxidative stress, ion disorder, mitochondrial dysfunction, and blood-brain barrier (BBB) impairment in ischemic stroke. However, the underlying mechanisms remain inadequately understood. This review provides a comprehensive overview of the composition and structure of A-kinase anchoring protein (AKAP) family members, emphasizing their physiological functions in the CNS. We explored in depth the molecular and cellular mechanisms of AKAP complexes in the pathological progression and risk factors of ischemic stroke, including hypertension, hyperglycemia, lipid metabolism disorders, and atrial fibrillation. Herein, we highlight the potential of AKAP complexes as a pharmacological target against ischemic stroke in the hope of inspiring translational research and innovative clinical approaches.

Mitochondria-associated endoplasmic reticulum membrane (MAM): a dark horse for diabetic cardiomyopathy treatment

Diabetic cardiomyopathy (DCM), an important complication of diabetes mellitus (DM), is one of the most serious chronic heart diseases and has become a major cause of heart failure worldwide. At present, the pathogenesis of DCM is unclear, and there is still a lack of effective therapeutics. Previous studies have shown that the homeostasis of mitochondria and the endoplasmic reticulum (ER) play a core role in maintaining cardiovascular function, and structural and functional abnormalities in these organelles seriously impact the occurrence and development of various cardiovascular diseases, including DCM. The interplay between mitochondria and the ER is mediated by the mitochondria-associated ER membrane (MAM), which participates in regulating energy metabolism, calcium homeostasis, mitochondrial dynamics, autophagy, ER stress, inflammation, and other cellular processes. Recent studies have proven that MAM is closely related to the initiation and progression of DCM. In this study, we aim to summarize the recent research progress on MAM, elaborate on the key role of MAM in DCM, and discuss the potential of MAM as an important therapeutic target for DCM, thereby providing a theoretical reference for basic and clinical studies of DCM treatment.

The role of endoplasmic reticulum-mitochondria-associated membranes in diabetic kidney disease

Diabetic kidney disease (DKD) is the leading cause of end-stage renal disease worldwide. The pathomechanisms of DKD are multifactorial, yet haemodynamic and metabolic changes in the early stages of the disease appear to predispose towards irreversible functional loss and histopathological changes. Recent studies highlight the importance of endoplasmic reticulum-mitochondria-associated membranes (ER-MAMs), structures conveying important cellular homeostatic and metabolic effects, in the pathology of DKD. Disruption of ER-MAM integrity in diabetic kidneys is associated with DKD progression, but the regulation of ER-MAMs and their pathogenic contribution remain largely unknown. Exploring the cell-specific components and dynamic changes of ER-MAMs in diabetic kidneys may lead to the identification of new approaches to detect and stratify diabetic patients with DKD. In addition, these insights may lead to novel therapeutic approaches to target and/or reverse disease progression. In this review, we discuss the association of ER-MAMs with key pathomechanisms driving DKD such as insulin resistance, dyslipidaemia, ER stress, and inflammasome activation and the importance of further exploration of ER-MAMs as diagnostic and therapeutic targets in DKD.

The endoplasmic reticulum: Homeostasis and crosstalk in retinal health and disease

The endoplasmic reticulum (ER) is the largest intracellular organelle carrying out a broad range of important cellular functions including protein biosynthesis, folding, and trafficking, lipid and sterol biosynthesis, carbohydrate metabolism, and calcium storage and gated release. In addition, the ER makes close contact with multiple intracellular organelles such as mitochondria and the plasma membrane to actively regulate the biogenesis, remodeling, and function of these organelles. Therefore, maintaining a homeostatic and functional ER is critical for the survival and function of cells. This vital process is implemented through well-orchestrated signaling pathways of the unfolded protein response (UPR). The UPR is activated when misfolded or unfolded proteins accumulate in the ER, a condition known as ER stress, and functions to restore ER homeostasis thus promoting cell survival. However, prolonged activation or dysregulation of the UPR can lead to cell death and other detrimental events such as inflammation and oxidative stress; these processes are implicated in the pathogenesis of many human diseases including retinal disorders. In this review manuscript, we discuss the unique features of the ER and ER stress signaling in the retina and retinal neurons and describe recent advances in the research to uncover the role of ER stress signaling in neurodegenerative retinal diseases including age-related macular degeneration, inherited retinal degeneration, achromatopsia and cone diseases, and diabetic retinopathy. In some chapters, we highlight the complex interactions between the ER and other intracellular organelles focusing on mitochondria and illustrate how ER stress signaling regulates common cellular stress pathways such as autophagy. We also touch upon the integrated stress response in retinal degeneration and diabetic retinopathy. Finally, we provide an update on the current development of pharmacological agents targeting the UPR response and discuss some unresolved questions and knowledge gaps to be addressed by future research.

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.

Biofactors regulating mitochondrial function and dynamics in podocytes and podocytopathies

Podocytes are terminally differentiated kidney cells acting as the main gatekeepers of the glomerular filtration barrier; hence, inhibiting proteinuria. Podocytopathies are classified as kidney diseases caused by podocyte damage. Different genetic and environmental risk factors can cause podocyte damage and death. Recent evidence shows that mitochondrial dysfunction also contributes to podocyte damage. Understanding alterations in mitochondrial metabolism and function in podocytopathies and whether altered mitochondrial homeostasis/dynamics is a cause or effect of podocyte damage are issues that need in-depth studies. This review highlights the roles of mitochondria and their bioenergetics in podocytes. Then, factors/signalings that regulate mitochondria in podocytes are discussed. After that, the role of mitochondrial dysfunction is reviewed in podocyte injury and the development of different podocytopathies. Finally, the mitochondrial therapeutic targets are considered.