br Effective therapeutic window of AR actions on ROP Retinal

Effective therapeutic window of AR actions on ROP Retinal vasculature undergoes critical developmental changes postnatally: from P7 onward the superficial Zanamivir mg start sprouting vertically in retina to form first the deep then the intermediated vascular plexus in the retina of C57BL/6 mice (Smith et al., 1994, Stahl et al., ). Pathologic neovascularization formation might be particular sensitive to pharmacological manipulation at this stage. Furthermore, ROP is defined as a two-stage disease: the first stage is characterized by the vaso-obliteration while the second stage is characterized by hypoxia of the avascularized retina and resultant increase in neovascularization (Lutty and McLeod, 2003). Thus, it is critical to define the specific postnatal developmental stages and the specific disease course (hyperoxic vs hypoxic phases) that is sensitive to adenosine receptor modulation. Adenosine-based actions are also presumably to be most evident at the hypoxic phase with the surge of adenosine level. Indeed, our preliminary study indicates the protection against pathological angiogenesis at the hypoxic phase (P17) by A2AR KO and by A2AR antagonists (KW6002) and by caffeine (non-selective adenosine receptor antagonists). However, detailed analysis indicated that A1R KO reduced avascular areas at during the hyperoxic phase (P12) (Zhang et al., 2015). Similarly, A2AR KO and caffeine was effective in protecting OIR not only at P17, but also at P12 (unpublished data). Our findings highlight the important function of adenosine signaling in modulating retinal vascular function even under hyperoxic environments. This may indicate that retinal vasculature development at P7-12 might be particular sensitive to interference since angiogenic sprouting from retinal superficial capillaries into the vitreous take places at P7. However, this finding is somewhat surprising since despite clearly vaso-obliteration at the retina center at the hyperoxic phase, there is no “hypoxia” in retina as shown by in vivo detection with nitroimidazole EF5 (Scott and Fruttiger, 2010) and the adenosine concentration and the expression of ecto-5′ nucleotidase (CD73) are low during the hyperoxic phase. The exact reason for this is not clear. If the hypothesis that the hyperoxia phase is the critical to AR-mediated protection against OIR is validated by future investigations, this finding suggests that the hyperoxic damage to developing retinal vasculatures is the primary and critical effect during ROP pathogenesis despite the fact that pathological angiogenesis is most evident at the hypoxic phase of ROP (Zhang et al., 2015).
Cellular (type) mechanism of adenosine receptor actions on ROP ROP pathology is characterized by abnormal/pathological angiogenesis and the endothelial cell is a final common pathway of abnormal endothelial proliferation (Xu et al., 2012). In retina, endothelial tip cells are mainly located at the leading edge of vascular plexus and the fusion sites of the remodeling area. Numerous studies support the role of adenosine signaling in endothelial cell proliferation and migration in vitro and vascular growth in vivo (Adair, 2005). In retinal endothelial cells, A2AR activation increases production of VEGF and GLUT-1 (Takagi et al., 1996, Takagi et al., 1998), indicating a pro-angiogenic effect of A2AR. Consistent with this, genetic inactivation of the A2AR reduces endothelial cell proliferation in OIR model (Liu et al., 2010). Thus, increased endothelial sprouting and proliferation likely play a major role in control of pathological angiogenesis in ROP (Xu et al., 2012, Horowitz and Simons, 2008, Carmeliet and Tessier-Lavigne, 2005). However, the exact role of the endothelial A2ARs remains to be determined by study using endothelial A2AR KO mice. Future studies of caffeine and KW6002 control of gene expression in the tip cell fraction of sprouting vessels in OIR would shed light on the transcriptional mechanism underlying AR control of tip cells in promoting retinal vascularization.