GTMB 7 - Gene Therapy & Molecular Biology

Zhang et al: Angiogenic Gene Therapy for Improving Islet Graft Vascularizationcontrol, in comparison to control islets that aretransplanted in the absence of VEGF protein. These resultssuggest that local VEGF delivery to islet grafts improvesthe outcome of islet transplantation by enhancing isletrevascularization (Sigrist et al, 2002).To improve islet graft vascularization, we havedelivered the human vascular endothelial growth factor(hVEGF) cDNA by adenoviral-gene transfer to mouseislets, followed by transplantation under the renal capsulein streptozotocin-induced diabetic mice (Zhang et al,2003). We showed that all the renal capsules containingthe hVEGF vector-transduced islets (250 islets) displayedsignificantly higher functional islet mass, as measured byinsulin immunostaining, and greater vascular density, asdetermined by immunostaining of CD31, the plateletendothelial cell adhesion molecule-1 (PECAM-1)(Watanabe et al, 2000). As a result, diabetic micereceiving the hVEGF vector-treated islets exhibitednormoglycemia with improved glucose tolerance. Incontrast, diabetic mice receiving an equivalent islet massthat were pre-transduced with a control vector maintainedmoderate hyperglycemia with impaired glucose tolerance.These results provide the proof-of-principle thatangiogenic gene transfer to islets prior to islettransplantation allows local production of VEGF in isletgrafts, which in turn stimulates graft angiogenesis andaugments islet revascularization (Zhang et al, 2003).While therapeutic angiogenesis, so called biobypass,has been considered an alternative modality for treatingcoronary and peripheral artery diseases, based on theefficacy and safety of plasmid- or adenoviral vectormediatedVEGF delivery in angiogenesis in a number ofpreclinical studies and clinical trials (Isner, 2002;Koransky et al, 2002; Mercadier and Logeart, 2002;Rasmussen et al, 2002; Sylven, 2002, Khan et al, 2003;Kusumanto et al, 2003), our view is that a similarangiogenic strategy should be explored to accelerate isletgraft angiogenesis, allowing rapid and adequate isletrevascularization post transplantation. Such an approach,when used in conjunction with islet transplantation, hasthe potential for improving the success rate and clinicaloutcome of islet transplantation with long-term glycemiccontrol at a reduced cost of islets.2. Ex vivo gene delivery to isletsThe rationale for enhancing islet graftvascularization by angiogenic gene transfer is as follows:islets are transduced in culture with a vector expressingangiogenic molecules, such as VEGF, followed bytransplantation into a diabetic subject, as illustratedschematically in Figure 2. Using an adenoviral-mediatedgene delivery system, we have validated this concept byshowing that VEGF production in newly transplantedislets significantly improves islet revascularization andfunctional islet mass (Zhang et al, 2003). It is noteworthythat adenoviral vectors are associated withimmunogenecity. In addition, islets are terminallydifferentiated post-mitotic cells, which poses a greatchallenge for ex vivo gene delivery to islets by vectorswhose transduction depends on cell division (Ito andKedes, 1997; Robbins and Ghivizzani, 1998). However,recent advances in both viral and nonviral vectordevelopment have made it feasible to transfer genes tointact islets ex vivo at reasonable efficiencies withoutadversely affecting the architecture and function of islets.Below is a focused review of a number of vector systemsthat are currently in use for ex vivo gene transfer toisolated islets.Figure 2. Schematic representation of angiogenic gene transfer in conjunction with islet transplantation. Islets are isolated and incubatedin culture media in the presence of a gene vector that expresses angiogenic molecules. After transduction, islets are transplantedintraportally into the liver of a diabetic subject.158