Activin Receptor-Like Kinase 5 Inhibition Reverses Impairment of Endothelial Cell Viability by Endogenous Islet Mesenchymal Stromal Cells

Clarkin, Claire E., King, Aileen J., Dhadda, Paramjeet, Chagastelles, Pedro, Nardi, Nance, Wheeler-Jones, Caroline P. and Jones, Peter M. (2013) Activin Receptor-Like Kinase 5 Inhibition Reverses Impairment of Endothelial Cell Viability by Endogenous Islet Mesenchymal Stromal Cells. Stem Cells, 31 (3), 547-559. (doi:10.1002/stem.1305).

Abstract

Following islet transplantation, islet graft revascularization is compromised due to loss of endothelial cells (ECs) during islet culture. TGF-? signaling pathways are essential for vascular homeostasis but their importance for islet EC function is unclear. We have identified a population of multipotent mesenchymal stromal cells (MSCs) within islets and investigated how modulation of TGF-? signaling by these cells influences islet EC viability. Cultured islets exhibited reduced expression of EC markers (VEGFR2; VE-cadherin; CD31) which was associated with diminished but sustained expression of endoglin a marker of both ECs and MSCs. Double fluorescent labelling of islets in situ with the EC marker CD31 disclosed a population of CD31-negative cells which were positive for endoglin. In vitro co-culture of microvascular ECs with endoglin-positive, CD31-negative islet MSCs reduced VEGFR2 protein expression, disrupted EC angiogenic behaviour and increased EC detachment. Medium conditioned by islet MSCs significantly decreased EC viability and increased EC caspase 3/7 activity. EC:MSC co-cultures showed enhanced Smad2 phosphorylation consistent with altered ALK5 signalling. Pharmacological inhibition of ALK5 activity with SB431542 (SB) improved EC survival upon contact with MSCs, and SB-treated cultured islets retained EC marker expression and sensitivity to exogenous VEGF164. Thus, endoglin-expressing islet MSCs influence EC ALK5 signalling in vitro, which decreases EC viability, and changes in ALK5 activity in whole cultured islets contribute to islet EC loss. Modifying TGF-? signalling may enable maintenance of islet ECs during islet isolation and thus improve islet graft revascularization post-transplantation

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