It has long been known that uncontrolled glucose in the blood through diabetes leads to a variety of complications. One devastating to patients is diabetic retinopathy; a progressive loss of sight leading to clinical blindness with remarkable speed. Just how the increase in systemic glucose accomplishes this pathology has been an area of intense research over the past decade.
Diabetic retinopathy occurs through progressive loss of retinal function and is accompanied by characteristic edematous patterns indicative of leaky vasculature in the choroid. This edema is thought to be a cause of vision loss through either retinal ganglionic starvation and retinal detachment through edematous pressure from the lower retinal layers. It has long been known that VEGF, as well as IGF-1, controls vascular permeability and, indeed, the eponymous factors are upregulated in the diabetic retina. But the how of it? How does inappropriate VEGF expression lead to edema?
Spoerri and coworkers set out to answer this question and have succeeded admirably. They developed a set of hammerhead ribozymes designed to cleave VEGF receptors 1/2 and IGF-1R mRNAs. In isolated human retinal endothelial cells (HRECs), the ribozymes successfully knocked down their respective targets as measured by qPCR and FACS. Surprisingly, the knockdown of VEGF-R2 led to a concomitant reduction in VEGF-R1 suggesting a coregulation hitherto unseen.
The researchers then added excess glucose to the growth medium simulating a diabetic pathology. In response the HRECs overexpressed VEGF, IGF and upregulated phosphorylation of their receptors – the main receptor modification activating their signaling function.
Since vascular permeability is mediated through the function of tight junctions, Spoerri et al., performed histochemistry against the pain player in tight junction function: Occludin. Occludin, in conjunction with claudins and the zonula occludens proteins ZO-1, 2 and 3 form the tight junctions between endothelial cells.
Upon the addition of glucose the localization, and to some extent the expression of occludin was lost highlighting the induction of leakage by glucose. Upon transfection with AAVs encoding the ribozyme receptor cleavers for VEGF-R1, VEGF-R2 and IGF-1R, this modulation of occludin was blocked; definitive evidence of VEGF signaling in tight junction breakdown and consequent leakage.
The authors postulate that the disappearance and reorganisation of the occluding protein may be linked with phosphorylation since previous work has shown that activation of VEGF signaling results in phosphorylation of occluding and ZO-1, leading directly to leakage. Current research trends are perfectly placed to take advantage of this new data and develop appropriate kinase inhibitors as well as receptor antagonists and/or tight junction protectors as therapies for diseases with an edematous pathology.
Message posted by: Simon Chandler