Insulin-like growth factor binding protein-1 levels in diabetic proliferative retinopathy

Mousetap, a Novel Technique to Collect Uncontaminated Vitreous or Aqueous and Expand Usefulness of Mouse Models

Vitreous or aqueous humour taps are widely used in patients or large animals with retinal diseases to monitor disease biomarkers, search for novel biomarkers, assess the integrity of the blood-retinal barrier, or perform pharmacokinetic or pharmacodynamics studies. Although there are many useful mouse models of retinal diseases, the small size of mouse eyes has precluded vitreous or aqueous taps. Herein we describe a novel technique, mousetap, which allows collection of vitreous or aqueous humour uncontaminated by blood or tissue surrounding the vitreous cavity. Mousetap was used to obtain vitreous samples from several mouse models of retinal vascular diseases and vitreous albumin measured by ELISA was highly reproducible among mice of the same model. The mean vitreous albumin concentration differed widely among control mice and mice of different models and correlated with fluorescein angiographic assessment of vascular leakage severity. Protein arrays showed increases in levels of several vasoactive proteins in the vitreous from mice with oxygen-induced ischemic retinopathy compared with age-matched controls; almost all of these proteins are increased in the vitreous of patients with the most common human ischemic retinopathy, proliferative diabetic retinopathy. Thus, mousetap facilitates the use of mice for studies previously reserved for large animal models and patients.

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.

The relationship between the circulating IGF system and the presence of retinopathy in Type 1 diabetic patients

BACKGROUND

Patients with proliferative diabetic retinopathy (PDR) have increased vitreous levels of insulin-like growth factor (IGF)-I, IGF-II and IGF binding proteins (IGFBPs). This accumulation is probably caused by increased leakiness of the blood-retina barrier and influx of circulating IGFs and IGFBPs. To date, interest has focused on the role of circulating total IGF-I in the development of PDR, and there are only sparse data on circulating levels of free IGF-I and IGFBPs.

METHODS

We compared fasting serum samples from matched groups of Type 1 diabetic patients with no retinopathy (n = 29), non-PDR (n = 13) and PDR (n = 16). We also included matched controls (n = 26). Serum was analysed for free and total IGF-I and -II, free plus dissociable IGF-I, IGFBP-1, -2 and -3, IGFBP-1-bound IGF-I as well as IGFBP-3 proteolysis.

RESULTS

When compared with controls, diabetic patients (n = 58) showed reduced (P < 0.0005) levels of free and total IGFs, free plus dissociable IGF-I and IGFBP-3, whereas levels of IGFBP-1, IGFBP-1-bound IGF-I and IGFBP-2 were elevated. IGFBP-3 proteolysis remained unaltered. When comparing diabetic patients with different degrees of retinopathy, IGFBP-2 and IGFBP-1-bound IGF-I were the only parameters that differed significantly. Patients with retinopathy (non-PDR as well as PDR) had elevated IGFBP-2 (P < 0.03) and reduced IGFBP-1-bound IGF-I (P < 0.03), when compared with patients without retinopathy. Noteworthy, both parameters correlated (0.11 < r2 < 0.14, P < 0.02) with the severity of retinopathy.

CONCLUSION

Our study gives no evidence of a direct role of circulating free IGF-I in the development of PDR, whereas IGFBP-2 and IGFBP-1-bound IGF-I showed a relationship with the degree of retinopathy. However, further investigations are needed in order to clarify the basis and clinical relevance of this finding.

Diabetic retinopathy is associated with decreased serum levels of free IGF-I and changes of IGF-binding proteins

In patients with diabetic retinopathy, elevated serum levels of total circulating insulin-like growth factor-I (IGF-I) have been implicated as an important mediator of the disease. There is no study, however, measuring free IGF-I levels in patients with diabetic retinopathy which mediate the biological effects of IGF-I and are modulated by a complex system of six specific IGF binding proteins (IGFBPs) and several IGFBP proteases.

Insulin-like growth factor-1 retinal microangiopathy in the pig eye

OBJECTIVE

Human recombinant insulin-like growth factor-1 (hrIGF-1), a ubiquitous angiogenic growth factor, was injected into the vitreous cavity of pigs to investigate the clinical and histopathologic consequences of supraphysiologic levels of this angiogenic growth factor on the retinal vasculature.

DESIGN

Young male pigs were injected with 600 microg hrIGF-1 into the vitreous cavity and were observed with serial examinations by ophthalmoscopy, fundus photography, and fluorescein angiography for varying periods up to 6 months. In a separate set of experiments, a dose-response relation was explored in animals injected with varying doses of IGF-1.

MAIN OUTCOME MEASURES

Histopathologic analysis included light and transmission electron microscopy and modified elastase digestion. Quantitative morphometric measurements were made of capillary basement membrane thickness and endothelial cell and pericyte densities of the retinal capillaries.

RESULTS

Early clinical features of IGF-1-injected eyes included marked arteriolar tortuosity, vitreitis, and retinal vessel and optic nerve head vascular fluorescein leakage. By 4 weeks, hyperfluorescent dots consistent with microaneurysms appeared and increased in number until 8 weeks postinjection. Clinical findings did not change appreciably after 8 weeks. Elastase digestion showed microaneurysms of the retinal capillaries and no ischemia or pericyte ghosts. Quantitative analysis of the digested specimens showed increased endothelial density by 1 month after injection (P < 0.05). Transmission electron microscopic cross-sections of capillaries showed significant basement membrane thickening by 3 months (P < 0.05). Lower doses of IGF-1 showed fewer clinical and histopathologic changes, and no significant changes were noted with a single 6 microg injection. Suspending hrIGF-1 in acidic buffer produced less intraocular inflammation than use of bovine serum albumin at neutral pH.

CONCLUSIONS

A single intravitreous injection of a large dose of hrIGF-1 produces a retinal microangiopathy that has a prolonged time of onset and remains stable from 2 to 6 months after injection. Some aspects of this angiopathy resemble diabetic retinopathy, suggesting growth factor effects in the morphologic vascular changes of diabetes.