Transmembrane stem cell factor protein therapeutics enhance revascularization in ischemia without mast cell activation

Diabetes mellitus affects approximately 350 million people worldwide, leading to the death of about 4.6 million people per year. As a complication of diabetes, 30-40 percent of patients age 50 and older develop peripheral artery disease (PAD). The current standard of care treatments for PAD includes surgical revascularization with bypass grafting or percutaneous interventions. However, these interventions cannot be performed in a significant portion of patients, and many do not respond to these therapies. An alternative approach for treating PAD is to use proteins to stimulate the body to create new vasculature, thus restoring blood flow through its own regenerative processes. Stem cell factor (SCF) is a cytokine that acts through the receptor tyrosine kinase c-Kit to regulate hematopoiesis and has been a candidate protein for treating PAD. Clinical use of soluble SCF would be highly beneficial but has been limited due to toxicity related to mast cell activation. SCF also exists in a transmembrane form that has differential activity from soluble SCF and has not been explored as a therapeutic agent. To explore the transmembrane SCF (tmSCF) as a therapeutic we created formulations of tmSCF embedded in proteoliposomes or in lipid nanodiscs. Mouse models of anaphylaxis and ischemia revealed the tmSCF-based therapies did not activate mast cells and were effective in improving the recovery from ischemia in both wild type and diabetic mice. We also found that the formulation of the lipid nanocarrier to deliver tmSCF altered the biological response and trophism of the tmSCF-based treatments. Proteoliposomal tmSCF preferentially acted on mature endothelial cells to induce angiogenesis while tmSCF nanodiscs had greater activity in inducing stem cell mobilization from the bone marrow and recruitment to ischemic sites. A mechanistic analysis of the effects of the treatments on mast cells, mature endothelial cells and endothelial progenitor cells, revealed that the nanocarriers altered the relative utilization of clathrin- versus caveolin-mediated uptake pathways of c-Kit in response to the treatments. Overall, our studies support tmSCF-based therapies can provide therapeutic benefits without off-target effects on mast cells and that lipid nanocarriers can be used to tailor the properties of membrane protein-based therapeutics.