Glycolaldehyde induces endoplasmic reticulum stress and apoptosis in Schwann cells

Immortalized Schwann cell lines as useful tools for pathogenesis-based therapeutic approaches to diabetic peripheral neuropathy

Growing evidence suggests that hyperglycemia-related abnormalities in Schwann cells play a pivotal role in the development and progression of diabetic peripheral neuropathy (DPN). Several immortalized Schwann cell lines have been established in our laboratory and utilized for the study of DPN; IMS32 from normal ICR mice, 1970C3 from normal C57BL/6 mice, IWARS1 and IKARS1 from wild-type and aldose reductase-deficient C57BL/6 mice, and IFRS1 from normal Fischer 344 rats. These cell lines retain biological features of Schwann cells and display high proliferative activities that enable us to perform molecular and biochemical analyses. In addition, these cells have exhibited metabolic alterations under exposure to diabetes-associated conditions, such as hyperglycemia, dyslipidemia, glycative and oxidative stress load. Herein, recent studies with these cell lines regarding the pathogenic factors of DPN (augmentation of the polyol and other collateral glycolysis pathways, glycative and oxidative stress-induced cell injury, autophagic and proteostatic disturbances, etc.) and therapeutic strategies targeting these factors are introduced.

Unveiling the Role of Schwann Cell Plasticity in the Pathogenesis of Diabetic Peripheral Neuropathy

Diabetic peripheral neuropathy (DPN) is a prevalent complication of diabetes that affects a significant proportion of diabetic patients worldwide. Although the pathogenesis of DPN involves axonal atrophy and demyelination, the exact mechanisms remain elusive. Current research has predominantly focused on neuronal damage, overlooking the potential contributions of Schwann cells, which are the predominant glial cells in the peripheral nervous system. Schwann cells play a critical role in neurodevelopment, neurophysiology, and nerve regeneration. This review highlights the emerging understanding of the involvement of Schwann cells in DPN pathogenesis. This review explores the potential role of Schwann cell plasticity as an underlying cellular and molecular mechanism in the development of DPN. Understanding the interplay between Schwann cell plasticity and diabetes could reveal novel strategies for the treatment and management of DPN.

Cilostazol Ameliorates Motor Dysfunction and Schwann Cell Impairment in Streptozotocin-Induced Diabetic Rats

This study investigated the effects of cilostazol on motor dysfunction, spinal motor neuron abnormalities, and schwannopathy in rats with diabetes. Diabetes mellitus (DM) was induced in rats via femoral intravenous streptozotocin (STZ) injection (60 mg/kg). After successful DM induction, cilostazol was administered on day 15 via oral gavage (100 mg/kg/day) for 6 weeks until sacrifice. Behavioral assays, including motor function, were performed weekly. The sciatic nerve, L5 spinal cord, and spinal ventral root were collected to evaluate the expression of the glial fibrillary acidic protein (GFAP), myelin protein zero (P0), and choline acetyltransferase (ChAT) by immunofluorescence and Western blotting. DM rats displayed decreased running speeds, running distances, and toe spread but increased foot pressure. In addition, loss of non-myelinating Schwann cells and myelin sheaths was observed in the sciatic nerve and L5 spinal ventral root. Reduced numbers of motor neurons were also found in the L5 spinal ventral horn. Cilostazol administration significantly potentiated running speed and distance; increased hind paw toe spread; and decreased foot pressure. In the sciatic nerve and L5 spinal ventral root, cilostazol treatment significantly improved non-myelinated Schwann cells and increased myelin mass. ChAT expression in motor neurons in the spinal ventral horn was improved, but not significantly. Cilostazol administration may protect sensorimotor function in diabetic rats.

Mechanism of Schwann cells in diabetic peripheral neuropathy: A review

Diabetic peripheral neuropathy (DPN) is the most common neuropathy in the world, mainly manifested as bilateral symmetry numbness, pain or paresthesia, with a high rate of disability and mortality. Schwann cells (SCs), derived from neural ridge cells, are the largest number of glial cells in the peripheral nervous system, and play an important role in DPN. Studies have found that SCs are closely related to the pathogenesis of DPN, such as oxidative stress, endoplasmic reticulum stress, inflammation, impaired neurotrophic support and dyslipidemia. This article reviews the mechanism of SCs in DPN.

Oxidative Stress and Mitochondrial Dysfunction Associated with Peripheral Neuropathy in Type 1 Diabetes

Significance: This review highlights the many intracellular processes generating reactive oxygen species (ROS) in the peripheral nervous system in the context of type 1 diabetes. The major sources of superoxide and hydrogen peroxide (H₂O₂) are described, and scavenging systems are explained. Important roles of ROS in regulating normal redox signaling and in a disease setting, such as diabetes, contributing to oxidative stress and cellular damage are outlined. The primary focus is the role of hyperglycemia in driving elevated ROS production and oxidative stress contributing to neurodegeneration in diabetic neuropathy (within the dorsal root ganglia [DRG] and peripheral nerve). Recent Advances: Contributors to ROS production under high intracellular glucose concentration such as mitochondria and the polyol pathway are discussed. The primarily damaging impact of ROS on multiple pathways including mitochondrial function, endoplasmic reticulum (ER) stress, autophagy, and epigenetic signaling is covered. Critical Issues: There is a strong focus on mechanisms of diabetes-induced mitochondrial dysfunction and how this may drive ROS production (in particular superoxide). The mitochondrial sites of superoxide/H₂O₂ production via mitochondrial metabolism and aerobic respiration are reviewed. Future Directions: Areas for future development are highlighted, including the need to clarify diabetes-induced changes in autophagy and ER function in neurons and Schwann cells. In addition, more clarity is needed regarding the sources of ROS production at mitochondrial sites under high glucose concentration (and lack of insulin signaling). New areas of study should be introduced to investigate the role of ROS, nuclear lamina function, and epigenetic signaling under diabetic conditions in peripheral nerve.

Glycolaldehyde, an Advanced Glycation End Products Precursor, Induces Apoptosis via ROS-Mediated Mitochondrial Dysfunction in Renal Mesangial Cells

Glycolaldehyde (GA) is a reducing sugar and a precursor of advanced glycation end products (AGEs). The role of precursor and precursor-derived AGEs in diabetes and its complications have been actively discussed in the literature. This study aimed to elucidate the mechanism of GA-induced apoptosis in renal cells. Immunoblotting results showed that GA (100 μM) caused cytotoxicity in murine renal glomerular mesangial cells (SV40 MES 13) and induced apoptosis via major modulators, decreasing Bcl-2 and increasing Bax, cytochrome c, and cleaved caspase-3/-9 expression. GA-derived AGE accumulation and receptor for AGE (RAGE) expression increased in mesangial cells; however, cells that were cotreated with aminoguanidine (AG) showed no increase in GA-derived AGE concentration. Furthermore, reactive oxygen species (ROS) production was increased by GA, while AG inhibited AGE formation, leading to a decrease in ROS levels in mesangial cells. We evaluated apoptosis through fluorescence-activated cell sorting, and used TUNEL staining to study DNA fragmentation. Additionally, we measured ATP generation and used MitoTracker staining to access changes in mitochondrial membrane potential. This study showed that GA increased AGE concentration, RAGE expression, and excessive ROS generation, leading to renal mesangial cell damage via GA-induced apoptosis pathway caused by mitochondrial dysfunction.

Misfolded proteins bind and activate death receptor 5 to trigger apoptosis during unresolved endoplasmic reticulum stress

Disruption of protein folding in the endoplasmic reticulum (ER) activates the unfolded protein response (UPR)—a signaling network that ultimately determines cell fate. Initially, UPR signaling aims at cytoprotection and restoration of ER homeostasis; that failing, it drives apoptotic cell death. ER stress initiates apoptosis through intracellular activation of death receptor 5 (DR5) independent of its canonical extracellular ligand Apo2L/TRAIL; however, the mechanism underlying DR5 activation is unknown. In cultured human cells, we find that misfolded proteins can directly engage with DR5 in the ER-Golgi intermediate compartment, where DR5 assembles pro-apoptotic caspase 8-activating complexes. Moreover, peptides used as a proxy for exposed misfolded protein chains selectively bind to the purified DR5 ectodomain and induce its oligomerization. These findings indicate that misfolded proteins can act as ligands to activate DR5 intracellularly and promote apoptosis. We propose that cells can use DR5 as a late protein-folding checkpoint before committing to a terminal apoptotic fate.

Proteins are chains of building blocks called amino acids, folded into a flexible 3D shape that is critical for its biological activity. This shape depends on many factors, but one is the chemistry of the amino acids. Because the internal and external environments of cells are mostly water-filled, correctly folded proteins often display so-called hydrophilic (or ‘water-loving’) amino acids on their surface, while tucking hydrophobic (or ‘water-hating’) amino acids on the inside.

A compartment within the cell called the endoplasmic reticulum folds the proteins that are destined for the outside of the cell. It can handle a steady stream of protein chains, but a sudden increase in demand for production, or issues with the underlying machinery, can stress the endoplasmic reticulum and hinder protein folding. This is problematic because incorrectly folded proteins cannot work as they should and can be toxic to the cell that made them or even to other cells. Many cells handle this kind of stress by activating a failsafe alarm system called the unfolded protein response. It detects the presence of incorrectly shaped proteins and sends signals that try to protect the cell and restore protein folding to normal. If that fails within a certain period of time, it switches to signals that tell the cell to safely self-destruct. That switch, from protection to self-destruction, involves a protein called death receptor 5, or DR5 for short. DR5 typically triggers the cell’s self-destruct program by forming molecular clusters at the cell’s surface, in response to a signal it receives from the exterior. During a failed unfolded protein response, DR5 seems instead to act in response to signals from inside the cell, but it was not clear how this works.

To find out, Lam et al. stressed the endoplasmic reticulum in human cells by forcing it to fold a lot of proteins. This revealed that DR5 sticks to misfolded proteins when they leave the endoplasmic reticulum. In response, DR5 molecules form clusters that trigger the cell's self-destruct program. DR5 directly recognized hydrophobic amino acids on the misfolded protein’s surface that would normally be hidden inside. When Lam et al. edited these hydrophobic regions to become hydrophilic, the DR5 molecules could no longer detect them as well. This stopped the cells from dying so easily when they were under stress. It seems that DR5 decides the fate of the cell by detecting proteins that were incorrectly folded in the endoplasmic reticulum.

Problems with protein folding occur in many human diseases, including metabolic conditions, cancer and degenerative brain disorders. Future work could reveal whether controlling the activation of DR5 could help to influence if and when cells die. The next step is to understand how DR5 interacts with incorrectly folded proteins at the atomic level. This could aid the design of drugs that specifically target such receptors.

ER stress response mediates diabetic microvascular complications

Endoplasmic reticulum (ER) homeostasis orchestrates the folding, modification, and trafficking of secretory and membrane proteins to the Golgi compartment, thus governing cellular functions. Alterations in ER homeostasis result in the activation of signaling pathways, such as the unfolded protein response (UPR), to regain ER homeostasis. Nevertheless, failure of UPR leads to activation of autophagy-mediated cell death. Several recent studies emphasized the association of the ER stress (ERS) response with the initiation and progression of diabetes. In this review, we highlight the contribution of the ERS response, such as UPR and autophagy, in the initiation and progression of diabetes and associated microvascular complications, including diabetic nephropathy (DN), retinopathy, and neuropathy, in various experimental models, as well as in humans. We highlight the ERS as a putative therapeutic target for the treatment of diabetic microvascular complications and, thus, the urgent need for the development of improved synthetic and natural inhibitors of ERS.