Christopher Kontos

Skeletal muscle injury in peripheral artery disease (PAD) has been attributed to vascular insufficiency, however evidence has demonstrated that muscle cell responses play a role in determining outcomes in limb ischemia. Here, we demonstrate that genetic ablation of Pax7+ muscle progenitor cells (MPCs) in a model of hindlimb ischemia (HLI) inhibited muscle regeneration following ischemic injury, despite a lack of morphological or physiological changes in resting muscle. Compared to control mice (Pax7WT), the ischemic limb of Pax7-deficient mice (Pax7Δ) was unable to generate significant force 7 or 28 days after HLI. A significant increase in adipose was observed in the ischemic limb 28 days after HLI in Pax7Δ mice, which replaced functional muscle. Adipogenesis in Pax7Δ mice corresponded with a significant increase in PDGFRα+ fibro/adipogenic progenitors (FAPs). Inhibition of FAPs with batimastat decreased muscle adipose but increased fibrosis. In vitro, Pax7Δ MPCs failed to form myotubes but displayed increased adipogenesis. Skeletal muscle from patients with critical limb threatening ischemia displayed increased adipose in more ischemic regions of muscle, which corresponded with fewer satellite cells. Collectively, these data demonstrate that Pax7+ MPCs are required for muscle regeneration after ischemia and suggest that muscle regeneration may be an important therapeutic target in PAD.

Heart regeneration requires multiple cell types to enable cardiomyocyte (CM) proliferation. How these cells interact to create growth niches is unclear. Here, we profile proliferation kinetics of cardiac endothelial cells (CECs) and CMs in the neonatal mouse heart and find that they are spatiotemporally coupled. We show that coupled myovascular expansion during cardiac growth or regeneration is dependent upon VEGF-VEGFR2 signaling, as genetic deletion of Vegfr2 from CECs or inhibition of VEGFA abrogates both CEC and CM proliferation. Repair of cryoinjury displays poor spatial coupling of CEC and CM proliferation. Boosting CEC density after cryoinjury with virus encoding Vegfa enhances regeneration. Using Mendelian randomization, we demonstrate that circulating VEGFA levels are positively linked with human myocardial mass, suggesting that Vegfa can stimulate human cardiac growth. Our work demonstrates the importance of coupled CEC and CM expansion and reveals a myovascular niche that may be therapeutically targeted for heart regeneration.

Angiogenesis is defined as the growth and proliferation of blood vessels from existing vascular structures (1, 2). Since blood vessels sub-serve the critical biological function of delivering oxygen and removing toxins from target organs, this part of the circulatory system plays a critical role in normal body homeostasis. Therefore, evolutionary pressures have put a great deal of emphasis on the development of a complex circulatory system in larger animals, and the growth of new blood vessels in the adult is tightly regulated to prevent disruption of the delicate homeostatic balance. In a number of physiologic and pathologic states, however, metabolic changes in a target organ may result in the need to modulate the delivery of oxygen and the removal of waste products, which may best be achieved by an increased vascular supply to that organ. This chapter will: (i) review some basic concepts of blood vessel growth and development (i.e., angiogenesis); (ii) describe some of the growth factors and receptors that mediate angiogenesis; (iii) discuss examples of the signal transduction pathways that appear to be critical for this process; (iv) highlight potential targets for therapeutic angiogenesis agents that have been studied in different human disease states with basic fibroblast growth factor.

The Angiopoietin-Tie (Ang-Tie) pathway is a key signaling pathway regulating vascular stability and permeability, and it significantly intersects and crosstalk with the vascular endothelial growth factor (VEGF) signaling pathway, a major signaling pathway regulating angiogenesis and vascular permeability. Disrupted Ang-Tie and VEGF signaling is linked to vascular dysfunction related to cancer, systemic inflammation, cardiovascular disease, and diabetic macular edema. Ang-Tie pathway is regulated by various molecular mechanisms, including ligand-receptor interactions, receptor multimerization, binding of co-receptor Tie1, inhibition by vascular endothelial protein tyrosine phosphatase (VE-PTP), receptor extracellular domain shedding, junctional localization, trafficking, and turnover. VEGF pathway is regulated by ligand binding, homo- and heterodimerization of VEGF receptors VEGFR1 and VEGFR2, interaction with neuropilin, thrombospondin-1/CD47, receptor internalization, recycling, and degradation. Ang-Tie and VEGF signaling pathways also share crosstalk mechanisms and integrative downstream signaling through phosphoinositide 3-kinases (PI3K)/Akt, sphingosine-dependent Erk activation, Src sequestration via RhoA/mDia, and calcium cycling, forming a complex reaction network that requires an integrative model to study the signaling outcomes on a systems level. The present study uses a mechanistic computational model of the Ang-Tie and VEGF signaling pathways, their regulatory mechanisms, crosstalk, and integrative downstream signaling to quantitatively characterize the molecular control of vascular growth, permeability, and stability by endothelial cells. The model captures and reproduces key experimental observations of the Ang-Tie/VEGF signaling network with detailed molecular mechanisms. The model significantly expands our previous computational models of the Ang-Tie signaling pathway [1,2] and integrates our systems biology understanding of the VEGF pathway and their crosstalk. The model serves as a platform for quantitatively predicting the signaling outcome of varying ligand concentrations and receptor expressions in physiological and pathological conditions, identifying molecular mechanisms and targets for therapeutic interventions targeting the signaling network, and simulating the effects of drugs and their combinations on the signaling outcome on a network level.

Postgraduate physician-scientist training programs (PSTPs) enhance the experiences of physician-scientist trainees following medical school graduation. PSTPs usually span residency and fellowship training, but this varies widely by institution. Applicant competitiveness for these programs would be enhanced, and unnecessary trainee anxiety relieved, by a clear understanding of what factors define a successful PSTP matriculant. Such information would also be invaluable to PSTP directors and would allow benchmarking of their admissions processes with peer programs. We conducted a survey of PSTP directors across the US to understand the importance they placed on components of PSTP applications. Of 41 survey respondents, most were from internal medicine and pediatrics residency programs. Of all components in the application, two elements were considered very important by a majority of PSTP directors: (a) having one or more first-author publications and (b) the thesis advisor's letter. Less weight was consistently placed on factors often considered more relevant for non-physician-scientist postgraduate applicants - such as US Medical Licensing Examination scores, awards, and leadership activities. The data presented here highlight important metrics for PSTP applicants and directors and suggest that indicators of scientific productivity and commitment to research outweigh traditional quantitative measures of medical school performance.