During the course of angiogenesis the

During the course of angiogenesis, the overactivated angiogenic stimulators are responsible for carrying out abnormal proliferation in the retinal microvasculature, resulting in the formation of various immature blood vessels. These newly formed blood vessels become the prime spots for leakage of the vascular content due to their petite nature. The above event also results in increased permeability of the vascular apparatus, ultimately leading to vitreous hemorrhages. This leads to increased osmotic stress in the retinal cells, accounting for their apoptosis. However, a more prominent effect of hyperosmolarity observed in retina is the induction of various pathways leading to the initiation of retinal angiogenesis. Amongst these pathways, hyperosmolarity-induced inflammation serves as one of the most detrimental stimuli for the activation of several angiogenic factors. COX-2 (cyclooxygenase-2) and MMPs (matrix metalloproteinases)-mediated inflammatory processes are highly accountable for causing such pathogenic alterations. These two inflammatory mediators are activated upon hyperglycemia-induced osmotic stress in retina and their pro-angiogenic roles are described as follows [15]. COX-2 increases the production of eicosanoids, such as prostaglandin E2, prostaglandin I2 and thromboxane A2. Further downstream signaling of these eicosanoids induces increased production of vascular endothelial growth factor (which upregulates pro-angiogenic events such as vascular sprouting, endothelial cell migration and tube formation), upregulation of epidermal growth factor receptor-mediated angiogenesis and enhanced survival of retinal endothelial glucose transport proteins via Akt (protein kinase B)-signaling pathway [16]. Amongst the family of MMPs, MMP-1, MMP-2 and MMP-9 produce pro-angiogenic effects by degrading extracellular matrix and thus releasing the angiogenic factors bound to them as well as aiding the generation of their promigratory fragments [17]. Hence, osmotic stress-induced inflammatory mediators augment the commencement of retinal angiogenesis. Moreover, several studies have concluded additional molecular targets of hyperglycemia-induced osmotic stress. It is known that osmotic pressure critically regulates the functioning of certain key fluid and ion exchangers, namely sodium-hydrogen exchangers (NHEs) and aquaporins (AQPs). Apart from their prime function of fluid and ion exchange, these exchangers are also involved in the regulation of vascular tone and endothelial cell migration. Osmotic stress (during hyperglycemia) induces several dysfunctions in the physiology of these exchangers, ultimately leading to their upregulation. Due to their presence on retinal endothelial as well as neuronal cells, overactivated NHEs and AQPs lead to several pathological alterations, mentioned as follows. Overactivation of NHEs leads to overexpression of hypertonicity-responsive genes. It also markedly decreases the levels of inducible nitric oxide synthase (iNOS) mRNA levels, subsequently reducing the production of nitric oxide, thereby leading to endothelial dysfunction and vascular injury. According to several studies, AQPs are critical regulators of angiogenesis. Osmotic stress-induced overexpression of AQPs results in increased endothelial cell migration and proliferation. Apart from endothelium-related dysfunctions, AQP4 has also been demonstrated to induce neuro-excitatory responses, leading to speculations that they might be involved in excitotoxicity-mediated retinal neuronal apoptosis, accounting for further inflammatory responses. Thus, the above discussion concludes a detrimental role of osmotic stress in the inducing pathological inflammation and angiogenesis [18,19]. Besides, the stress produced on the posterior wall of the vitreous chamber induces several anatomical changes in the eye. Out of these numerous changes, constriction of vitreous chamber is the one responsible for the terminal blindness associated with proliferative diabetic retinopathy. The osmotic stress-induced constriction of the vitreous chamber leads to detachment of retina from its underlying tissue. Hence, the prime devastating events occurring in diabetic retinopathy such as macular edema and eventual blindness are said to be the major consequences of angiogenesis [20].