Ganglioside GM1 effects on the expression of nerve growth factor (NGF), Trk-A receptor, proinflammatory cytokines and on autoimmune diabetes onset in non-obese diabetic (NOD) mice

Abstract

NOD (non-obese diabetic) mice develop type 1 diabetes mellitus spontaneously and with a strong similarity to the human disease. Differentiation and function of pancreas β cells are regulated by a variety of hormones and growth factors, including the nerve growth factor (NGF). Gangliosides have multiple immunomodulatory activities with immunosuppressive properties, decreasing lymphoproliferative responses and modulating cytokine production. In the present study, serum, pancreas islets and spleen mononuclear cells from NOD mice treated with monosialic ganglioside GM1 (100 mg/kg/day) and the group control which received saline solution were isolated to investigate the proinflammatory cytokines (IL-1β, IFN-γ, IL-12, TNF-α), NGF and its high-affinity receptor TrkA, peri-islet Schwann cells components (GFAP, S100-β) expression and the relationship with diabetes onset and morphological aspects. Our results suggest that GM1 administration to female NOD mice beginning at the 4th week of life is able to reduce the index of inflammatory infiltrate and consequently the expression of diabetes, modulating the expression of proinflammatory cytokines (IL-12, IFN-γ, TNF-α and IL-1β). Furthermore, GM1 increases GFAP, S-100β and NGF in pancreas islets, factors involved in β cell survival.

Introduction

NOD (non-obese diabetic) mice are a very useful and important tool for autoimmune type 1 diabetes studies [1], [2]. Until approximately 3–4 weeks of life, histological studies have shown a progressive migration of immune cells to the pancreas islets. These cells surround the islet (peri-insulitis) without significant β cell destruction. In the subsequent weeks, the infiltrating immune cells gradually invade the islet (insulitis) leading, in most of the female NOD mice, to severe insulitis and insulin-producing cell damage and overt diabetes [3]. The majority of immune cells are CD4 T cells and CD8 T cells, although NK cells, B cells, dendritic cells and macrophages can also be identified in the islet injury NOD [4].

A variety of cytokines are expressed in genes or proteins, or in both, related to the insulitis present in the autoimmune diabetes-prone NOD mice [5]. In this context, insulitis is characterized by the increase of proinflammatory cytokine expression (IL-1β and TNF-α) as well as type I cytokines (IFN-γ), which are released by infiltrating mononuclear cells, such as activated macrophages and T-cells [5], [6].

The pancreas islets are surrounded by Schwann cells, recognized by the expression of glial fibrillary acidic protein (GFAP) and S100-β, which encircle the endocrine pancreas tissue. Recent data has suggested that the reorganization and/or partial destruction of Schwann cells begins as an early manifestation of autoimmunity in type 1 diabetes during the development of insulitis [7], [8].

Nerve growth factor (NGF), one of the most studied neurotrophic factors, plays a crucial role in the survival and development of cerebral and peripheral neurons [9]. However, numerous studies reveal that NGF does not only act in the cells of the peripheral and central nervous systems, but also outside the nervous system, especially in the endocrine and immune systems [10], [11], [12]. The NGF is produced and released by a variety of cells and its biological effect on immunohematopoietic cells, mediated by the low affinity receptor (p75^NTR) and by the high-affinity receptor (TrkA), is similar to the nervous cells [11], [13].

NGF secreted by β-cells is also an autocrine regulator of survival and preserves insulin biosynthesis and secretion. These observations may contribute to a better understanding of the physiopathology of diabetes, where serum insulin and NGF levels are diminished [14]. Pancreas islets express NGF functional receptors that, activated by the neurotrophin, exert an effect on these cells inducing morphological and physiological modifications in pancreas β-cells, including an increase in insulin secretion and the extension of neurite-like process [15]. The low and high-affinity NGF receptor expression was demonstrated in islets of rats during the fetal period and adult life [16], [17]. The cellular localization of NGF and TrkA is regulated by growth requirements, suggesting that this system plays an important role in the development of β-cells [15].

Gangliosides constitute a heterogeneous family of sialic acid-containing glycosphingolipids found as normal cell membrane components. These compounds can be found in the majority of cellular types, but with a relative abundance in the nervous system where they influence the development and/or differentiation of neurons [18], [19]. They have multiple immunomodulatory properties, decreasing the lymphoproliferative responses and modulating cytokine production [20], [21]. Particularly, monosialoganglioside GM1 seems to be effective in the reduction of CD4 surface expression on T cells from different species [22]. Other authors have demonstrated that GM1 mask epitopes of CD4 by induction of molecular internalization into endocytic milieu, where they are eventually degraded [23]. In neuronal cell culture, GM1 induces a variety of responses similar to the NGF feature. In vivo, there are evidences that GM1, as well as NGF prevents apoptosis of cholinergic neurons after cortical damage [24]. Together, these data suggest that GM1 and other gangliosides exert neurotrophic activity similar to NGF and, for this reason, can be used to restore neurological functions [25]. Apparently, GM1 mimics the effect of NGF through an interaction with the high-affinity NGF receptor, TrkA, and subsequent specific activation of signaling proteins [26]. This type of interaction and induction of tyrosine phosphorylation by GM1 could also explain the ability of this ganglioside to enhance in vitro and in vivo NGF effects [18].

The aim of the present study is to investigate the effects of exogenous GM1 administration in female NOD mice regarding the incidence of diabetes, cytokine, peri-islets Schwann cells components (GFAP, S-100β), NGF and TrkA expression.