Glial cell line-derived neurotrophic factor in the vitreous of patients with proliferative diabetic retinopathy

THE ASSOCIATION OF DECREASED SERUM GDNF LEVEL WITH HYPERGLYCEMIA AND DEPRESSION IN TYPE 2 DIABETES MELLITUS

Objective: Comorbidity of diabetes and depression is a critical problem. Decreased glial-derived neurotrophic factor (GDNF) has been demonstrated in depression, but no evidence of a relationship between GDNF and diabetes has been shown. The present studies were designed to investigate the relationship between GDNF and metabolism. Methods: In Study 1, we performed a case-control study in which subjects with type 2 diabetes mellitus (T2DM), prediabetes (p-DM), and normal glucose tolerance (NGT) were included. In Study 2, we performed a cross-sectional study in 296 patients having pre-existing diabetes in whom the levels of serum GDNF, blood glucose, blood lipids, blood pressure, body mass index, scores from the Patient Health Questionnaire (PHQ-9), the EuroQol-5 scale, and the diabetes distress scale were measured, as well as single-nucleotide polymorphisms of GDNF including rs884344, rs3812047, and rs2075680. Results: In Study 1, serum GDNF concentration was significantly lower in the T2DM group than in the NGT group (NGT: 11.706 ± 3.918 pg/mL; p-DM: 10.736 ± 3.722 pg/mL; type 2 diabetes mellitus [T2DM group]: 9.884 ± 2.804 pg/mL, P = .008). In Study 2, significantly decreased serum GDNF levels were observed in subjects with poor glycemic control or depression (glycated hemoglobin [HbA1c] <7.0% without depression: 11.524 ± 2.903 pg/mL; HbA1c ≥7.0% without depression: 10.625 ± 2.577 pg/mL; HbA1c <7.0% with depression: 10.355 ± 2.432 pg/mL; HbA1c ≥7.0% with depression: 8.824 ± 2.102 pg/mL, P = .008). Double-factor variance analysis showed that glycemic control and depression were independent factors for the GDNF level. Moreover, the serum GDNF level was significantly inversely associated with the fasting plasma glucose, 2 hours postprandial plasma glucose, HbA1c, and PHQ-9 score. Conclusion: Glycemic dysregulation was an independent factor for the GDNF level. These findings suggest that GDNF level might be involved in the pathophysiology of T2DM and depression through various pathways. Abbreviations: BP = blood pressure; CHO = cholesterol; DDS = diabetes distress scale; DM = diabetes mellitus; EQ-5D = the health-related dimensions of the EuroQol-5 scale; FPG = fasting plasma glucose; GDNF = glial-derived neurotrophic factor; HbA1c = glycated hemoglobin; HDL = high-density lipoprotein; LDL = low-density lipoprotein; NGT = normal glucose tolerance; PHQ-9 = Patient Health Questionnaire; p-DM = prediabetes; PPG = postprandial plasma glucose; SNP = single-nucleotide polymorphism; T2DM = type 2 diabetes mellitus; TG = triglyceride.

Human vitreous in proliferative diabetic retinopathy: Characterization and translational implications

Diabetic retinopathy (DR) is one of the leading causes of visual impairment in the working-age population. DR is a progressive eye disease caused by long-term accumulation of hyperglycaemia-mediated pathological alterations in the retina of diabetic patients. DR begins with asymptomatic retinal abnormalities and may progress to advanced-stage proliferative diabetic retinopathy (PDR), characterized by neovascularization or preretinal/vitreous haemorrhages. The vitreous, a transparent gel that fills the posterior cavity of the eye, plays a vital role in maintaining ocular function. Structural and molecular alterations of the vitreous, observed during DR progression, are consequences of metabolic and functional modifications of the retinal tissue. Thus, vitreal alterations reflect the pathological events occurring at the vitreoretinal interface. These events are caused by hypoxic, oxidative, inflammatory, neurodegenerative, and leukostatic conditions that occur during diabetes. Conversely, PDR vitreous can exert pathological effects on the diabetic retina, resulting in activation of a vicious cycle that contributes to disease progression. In this review, we recapitulate the major pathological features of DR/PDR, and focus on the structural and molecular changes that characterize the vitreal structure and composition during DR and progression to PDR. In PDR, vitreous represents a reservoir of pathological signalling molecules. Therefore, in this review we discuss how studying the biological activity of the vitreous in different in vitro, ex vivo, and in vivo experimental models can provide insights into the pathogenesis of PDR. In addition, the vitreous from PDR patients can represent a novel tool to obtain preclinical experimental evidences for the development and characterization of new therapeutic drug candidates for PDR therapy.

Assessment of Neurotrophins and Inflammatory Mediators in Vitreous of Patients With Diabetic Retinopathy

Purpose

To assess vitreous levels of inflammatory cytokines and neurotrophins (NTs) in diabetic retinopathy (DR) and elucidate their potential roles.

Methods

A prospective study was performed on 50 vitreous samples obtained from patients with DR (n = 22) and the nondiabetic controls (n = 28). All patients were candidates for vitrectomy. Inflammatory cytokine and NT levels were determined with ELISA. Potential source and role of NTs was determined by using human retinal Müller glia and mouse photoreceptor cells and challenging them with TNF-α or IL-1β, followed by detection of NTs and cell death.

Results

Vitreous NT levels of all DR patients were significantly higher than those of nondiabetic controls (nerve growth factor [NGF, P = 0.0001], brain-derived neurotrophic factor [BDNF, P = 0.009], neurotrophin-3 [NT-3, P < 0.0001], neurotrophin-4 [NT-4, P = 0.0001], ciliary neurotrophic factor [CNTF, P = 0.0001], and glial cell–derived neurotrophic factor [GDNF, P = 0.008]). Similarly, the levels of inflammatory mediators IL-1β (P < 0.0001), IL-6 (P = 0.0005), IL-8 (P < 0.0001), and TNF-α (P < 0.0001) were also higher in eyes with DR. Interestingly, inflammatory cytokine and NT levels, particularly TNF-α (P < 0.05), IL-8 (P < 0.004), NT-3 (P = 0.012), NGF (P = 0.04), GDNF (P = 0.005), and CNTF (P = 0.002), were higher in eyes with nonproliferative diabetic retinopathy (NPDR) than in eyes with active proliferative diabetic retinopathy (PDR). Cytokine stimulation of Müller glia resulted in production of NTs, and GDNF treatment reduced photoreceptor cell death in response to inflammation and oxidative stress.

Conclusions

Together, our study demonstrated that patients with DR have higher levels of both inflammatory cytokines and NTs in their vitreous. Müller glia could be the potential source of NTs under inflammatory conditions to exert neuroprotection.

Identification of proteins associated with clinical and pathological features of proliferative diabetic retinopathy in vitreous and fibrovascular membranes

Purpose

To identify the protein profiles in vitreous associated with retinal fibrosis, angiogenesis, and neurite formation in epiretinal fibrovascular membranes (FVMs) in patients with proliferative diabetic retinopathy (PDR).

Methods

Vitreous samples of 5 non-diabetic control patients with vitreous debris and 7 patients with PDR membranes were screened for 507 preselected proteins using the semi-quantitative RayBio® L-series 507 antibody array. From this array, 60 proteins were selected for a custom quantitative antibody array (Raybiotech, Human Quantibody® array), analyzing 7 control patients, 8 PDR patients with FVMs, and 5 PDR patients without FVMs. Additionally, mRNA levels of proteins of interest were measured in 10 PDR membranes and 11 idiopathic membranes and in retinal tissues and cells to identify possible sources of protein production.

Results

Of the 507 proteins screened, 21 were found to be significantly elevated in PDR patients, including neurogenic and angiogenic factors such as neuregulin 1 (NRG1), nerve growth factor receptor (NGFR), placental growth factor (PlGF) and platelet derived growth factor (PDGF). Angiopoietin-2 (Ang2) concentrations were strongly correlated to the degree of fibrosis and the presence of FVMs in patients with PDR. Protein correlation analysis showed PDGF to be extensively co-regulated with other proteins, including thrombospondin-1 and Ang2. mRNA levels of glial-derived and brain/derived neurotrophic factor (GDNF and BDNF) were elevated in PDR membranes. These results were validated in a second study of 52 vitreous samples of 32 PDR patients and 20 control patients.

Conclusions

This exploratory study reveals protein networks that potentially contribute to neurite outgrowth, angiogenesis and fibrosis in the formation of fibrovascular membranes in PDR. We identified a possible role of Ang2 in fibrosis and the formation of FVMs, and of the neurotrophic factors NRG1, PDGF and GDNF in neurite growth that occurs in all FVMs in PDR.

Inflammatory mechanisms of idiopathic epiretinal membrane formation

The pathogenesis of idiopathic epiretinal membranes (iERMs), a common pathology found in retina clinics, still eludes researchers to date. Ultrastructural studies of iERMs in the past have failed to identify the cells of origin due to the striking morphologic changes of cells involved via transdifferentiation. Thus, immunohistochemical techniques that stain for the cytostructural components of cells have confirmed the importance of glial cells and hyalocytes in iERM formation. The cellular constituents of iERMs are thought to consist of glial cells, fibroblasts, hyalocytes, etc. that, in concert with cytokines and growth factors present in the vitreous, lead to iERM formation. Recently, research has focused on the role of the posterior hyaloid in iERM formation and contraction, particularly the process of anomalous PVD as it relates to iERM formation. Recent advances in proteomics techniques have also elucidated the growth factors and cytokines involved in iERM formation, most notably nerve growth factor, glial cell line-derived growth factor, and transforming growth factor β1.

Experimental effect of retinoic acids on apoptosis during the development of diabetic retinopathy

Purpose

This study was conducted to investigate whether retinoic acids (RAs) had any effect on apoptosis during the development of diabetic retinopathy.

Methods

To investigate whether RAs had any effect on apoptosis during the development of diabetic retinopathy, we housed 32 C57BL/6 male mice and induced diabetes in 24 by intra peritoneal injections of streptozotocin (STZ; Sigma, St Louis, MO) and treated 16 of the diabetic mice with the RAs, all-trans-retinoic acid (ATRA) (seven mice) and 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)carboxamido] benzoic acid (Am580) (nine mice). The other eight mice were used as diabetic controls. We then measured apoptosis in the retina by TdT-dUTP terminal nick-end labeling assay.

Results

RAs inhibited the apoptosis of retinal cells in diabetic retinopathy. Many apoptotic cells were observed in retinas of the eight diabetic control mice (mean value and SD: 37.8 ± 6.9), whereas when diabetic mice were treated with RAs, the number of apoptotic cells significantly decreased (mean value and SD: 9.9 ± 6.4 for the seven ATRA-treated diabetic mice and 9.8 ± 5.9 for the nine Am580-treated diabetic mice) (p < 0.05).

Conclusion

Treatment with RAs decreases apoptosis during the development of diabetic retinopathy.

Glial cell-derived cytokines attenuate the breakdown of vascular integrity in diabetic retinopathy

The blood-retinal barrier (BRB) is a biological unit comprised of specialized capillary endothelial cells firmly connected by intercellular tight junctions and endothelium-surrounding glial cells. The BRB is essential for maintaining the retinal microenvironment and low permeability and is compromised in an early phase during the progression of diabetic retinopathy. Here, we demonstrate that retinoic acid receptor (RAR)alpha stimulants preferentially act on glial cells rather than endothelial cells, resulting in the enhanced expression of glial cell line-derived neurotrophic factor (GDNF) through recruitment of the RARalpha-driven trans-acting coactivator to the 5'-flanking region of the gene promoter. Conversely, RARalpha decreases expression of vascular endothelial growth factor (VEGF)/vascular permeability factor. These gene expression alterations causally limit vascular permeability by modulating the tight junction function of capillary endothelium in a paracrine manner in vitro. The phenotypic transformation of glial cells mediated by RARalpha is sufficient for significant reductions of vascular leakage in the diabetic retina, suggesting that RARalpha antagonizes the loss of tight junction integrity induced by diabetes. These findings reveal that glial cell-derived cytokines such as GDNF and VEGF regulate BRB function, implying that the glial cell can be a possible therapeutic target in diabetic retinopathy.

The role of cytokines and trophic factors in epiretinal membranes: Involvement of signal transduction in glial cells

Idiopathic epiretinal membranes (ERMs) in the macular region can cause a reduction in vision and sometimes recurs after surgical removal, but its pathogenic mechanisms are still unknown. On the other hand, the presence of secondary ERMs has been associated with various clinical conditions including proliferative diabetic retinopathy (PDR) and proliferative vitreoretinopathy (PVR). Recent studies have shown a significant association between clinical grades of PDR or PVR, and the expression levels of specific cytokines and/or growth factors in the vitreous fluid. Expression of these factors and their receptors are also observed in secondary ERMs. ERMs are composed of many cell types such as retinal pigment epithelial cells and vascular endothelial cells, however the role of glial cells is yet unclear. Interestingly, glial cells in ERMs express some trophic factor receptors and transcription factors, such as NF-kappaB, suggesting an involvement of glial signal transduction in the pathogenesis of ERMs. In this review, we summarize recent progress regarding the clinical and laboratory findings of ERMs.