Proanthocyanidins protect against early diabetic peripheral neuropathy by modulating endoplasmic reticulum stress

Abstract

Diabetic peripheral neuropathy (DPN) is the most common and troublesome complication of type 2 diabetes mellitus (T2DM). Recent findings reveal an important role of endoplasmic reticulum (ER) stress in the development of DPN and identify a potential new therapeutic target. Schwann cells (SC), the myelinating cells in peripheral nervous system, are highly susceptible to ER homeostasis. Grape seed proanthocyanidins (GSPs) have been reported to improve DPN of type 1 diabetic rats and relieve ER stress in skeletal muscles and pancreas of T2DM. We investigated the potential role of ER stress in SC in regulating DPN of T2DM and assessed whether early intervention of GSPs would prevent DPN by modulating ER stress. The present study was performed in Sprague–Dawley rats made T2DM with low-dose streptozotocin and a high-carbohydrate/high-fat diet and in rat SC cultured in serum from type 2 diabetic rats. Diabetic rats showed a typical characteristic of T2DM and slowing of nerve conduction velocity (NCV) in sciatic/tibial nerves. The lesions of SC, Ca²⁺ overload and ER stress were present in sciatic nerves of diabetic rats, as well as in cell culture models. GSPs administration significantly decreased the low-density lipoprotein level and increased NCV in diabetic rats. GSPs or their metabolites also partially prevented cell injury, Ca²⁺ overload and ER stress in animal and cell culture models. Therefore, ER stress is implicated in peripheral neuropathy in animal and cell culture models of T2DM. Prophylactic GSPs treatment might have auxiliary preventive potential for DPN partially by alleviating ER stress.

Introduction

Diabetic peripheral neuropathy (DPN) is the most common and troublesome complication of diabetes, leading to the greatest disability and mortality [1]. As changing lifestyles lead to increased caloric consumption and reduced physical activity, DPN continues to increase in numbers and significance in countries where the prevalence of type 2 diabetes mellitus (T2DM) is rising [2]. Incidence and prevalence of DPN increase with age and increasing duration of diabetes [3]. Once nerve damage is done, it is almost totally irreversible [4]. Moreover, up to 50% of DPN may be asymptomatic, and patients are at risk of insensate injury to their feet [5]. Therefore, the early recognition and prevention of DPN is the best strategy and may result in a reduced incidence of ulceration and consequently amputation.

The observation that the lesions of Schwann cells (SC) existed earlier than ongoing demyelination and axonal degeneration highlights the significance of SC in the pathogenesis of DPN [6]. As an extension of their plasma membrane, SC generate unique, lipid-rich multilamellar myelin sheaths to ensure successful conduction of the action potential along axons and maintain the axonal integrity [7]. During the active phase of myelination, SC must synthesize tremendous amounts of myelin membrane proteins, cholesterol and membrane lipids through the secretory pathway [8]. Therefore, SC are highly sensitive to perturbations of the secretory pathway, particularly endoplasmic reticulum (ER) homeostasis [9]. Recent studies have demonstrated that ER stress is emerging as an important mechanism of metabolic diseases [10], [11]. Based on diabetic models, the contribution of ER stress in target organs (pancreatic β-cells, liver, adipose tissue and skeletal muscle) to the pathogenesis of diabetes has been well established [12], [13], [14], [15], [16]. Within the context of diabetic complications, ER stress has also been demonstrated to be linked to retinopathy, cardiomyopathy, nephropathy, endothelial dysfunction and neuropathy in central nervous system [17], [18], [19], [20], [21]. Furthermore, recent in vivo studies have reported that ER stress plays a key role in the pathogenesis of peripheral neuropathy in prediabetic and type 1 diabetic animals [22], [23]. However, much less attention has been focused on the effect of ER stress on lesions of SC in models of DPN. Indeed, it has become increasingly clear that ER stress in myelinating cells is an important feature of chemically induced and inherited neuropathies [9], [24], [25]. Given an in vitro study of palmitic acid and high-glucose-induced SC that supports the hypothesis linking ER stress in SC and DPN [26], the relevant mechanisms of ER stress on DPN need further elucidation.

Natural plant components with minimal or no side effects are being pursued as alternatives to pharmaceutical interventions. Grape seed proanthocyanidins (GSPs), which are derived from grape seeds, refer to a group of proanthocyanidins mostly containing dimers, trimers and other oligomers of catechin and epicatechin and their gallic acid esters. A previous study confirmed that 6 months of 250 or 500 mg/kg body weight (BW) GSPs treatment was safe and did not cause any detrimental effects in vivo [27]. Previous studies based on chemically induced type 1 diabetic animals reported that GSPs exerted anti-diabetic property [28], [29]. Our recent study also found that GSPs partially ameliorated hyperglycemia and insulin resistance in type 2 diabetic rats in part by alleviation of ER stress in skeletal muscles and pancreas [15], [16]. Moreover, GSPs showed activities that improved type 1 diabetic complications, including cardiomyopathy, nephropathy, retinopathy and neuropathies in central and peripheral nervous systems [30], [31], [32], [33], [34]. However, the effect of GSPs on the peripheral nerves of T2DM has not been elaborated.

In the present study, using Sprague–Dawley rats made T2DM with low-dose streptozotocin and a high-carbohydrate/high-fat diet, we aimed to investigate whether early intervention of GSPs would result in the prevention of early structural and functional abnormalities in sciatic nerves and to assess whether ER stress in sciatic nerves was the target of GSPs. Meanwhile, given the previous finding that only (+)-catechin (C) and (−)-epicatechin (EC) were identified in both rat plasma and nervous system after oral administration of GSPs [35], we used rat SC cultured in serum from type 2 diabetic rats to establish the contribution of ER stress in lesions of SC and to clarify whether the pretreatment of C and EC could protect SC via alleviating ER stress.