UNC-CH Department of Cell and Molecular Physiology

Education:

BS, University of Missouri 1974
PhD, University of Missouri 1981

I. Development of the Collateral Circulation in Normal Tissues and in Ischemic Disease— Genetic and Environmental Determinants, and Cell Signaling Mechanisms.

Occlusive vascular disease of the heart, brain and peripheral limbs is the primary cause of morbidity and mortality in the US. Angiogenesis and growth of collateral vessels are major adaptations that limit end-organ damage. Collaterals (COLs) are tiny, ~25 µm diameter arteriole-arteriole anastomoses connecting adjacent arterial trees that are rare in number but present in most tissues. These vessels are unique -- they defy the canonical artery-capillary-venous anatomic patterning (eprhin etc), normally have little pressure drop or net flow across them, yet persist in a quiescent state despite their low/disturbed shear stress environment. When the artery supplying an adjacent tree critically narrows, COLs enlarge to become “endogenous bypass vessels” able to limit ischemic injury. The degree of protection depends on their pre-existing density and capacity to outwardly remodel into large 100-500 µm diameter conduit arteries that is initiated by increased shear stress --a process termed arteriogenesis. In fact, the ability of angiogenesis to increase O₂ delivery following occlusion is largely dependent on COL remodeling. Thus, COLs constitute a unique “third circulation” besides the arterial-venous and lymphatic circulations. Yet, compared to angiogenesis, much less is known about the mechanisms directing collateral growth. And remarkably, no studies have determined how or when COLs develop. COL density and arteriogenesis vary widely among species and humans, suggesting a genetic basis. Yet nothing is known about the source of this variation. We have found that COLs develop during the late embryonic-to-early postnatal period in mice. Further, we have found dramatic differences in COL density in several in-bred mouse strains that appear to be genetically linked, with one strain virtually lacking a collateral circulation and thus have severe myocardial infarctions, stroke and peripheral vascular disease. These findings create an excellent opportunity to identify factors specifying COL formation. Our findings, using eQTL and QTL analysis, and recombinant inbred and chromosome substitution mouse strain sets suggest that multiple genes are involved. We have identified the first two genes that regulate COL formation in normal tissues. One of these is also a key determinant of COL growth/remodeling in ischemia. Identification of the factors directing COL formation in normal tissues and potential genetic polymorphisms underlying this variation are intriguing fundamental questions. Furthermore, their answers may allow us to identify individuals at risk from too-few COLs, as well as lead to therapies to induce formation of new COLs – a goal that has thus far eluded investigators.

Some Specific Areas of Current Investigation (primarily in vivo studies in mice):

Identify the molecular determinants of formation of the COL circulation in the embryo and stabilization/maturation of them in the neonate.
Identify the basis for the large variability in collateral circulatory formation among and within species, including human: genetic and environmental.
Characterize the unique phenotype and signaling mechanisms that exist in endothelial and mural cells that permit formation and persistence of COLs.
Identify the basis for decline in COL capacity with aging

Non-invasive scanning infrared laser Doppler perfusion images of the hind limb ventral surface from the same mouse obtained before and at 3 and 21 days after femoral artery ligation (center three figures, left-to-right, respectively). Note the appearance of collaterals between the genu artery and branches of the distal saphenous artery and between the lateral caudal femoral artery and the saphenous artery. Relative flow velocity indicated by 16-hue pseudocolor, where gray and white represent zero and maximal flux [range of perfusion units (PU) = 0-5000]. Panels in upper right and lower left are the post-mortem angiogram and color photomicrograph of the medial adductor region from a mouse (different from the mouse in the Doppler-images) 21 days after ligation and following maximal dilation and filling of the arterial circulation with barium sulfate. The tortuous arcading deep perforating collateral artery connecting the profundus and popliteal arteries in the adductor region can be seen in the angiogram, whereas the superficial collaterals connecting the lateral caudal and saphenous arteries can be seen in both angiogram and color micrograph. From Chalothorn D, Zhang H, Clayton JA, Thomas SA, Faber JE. "Catecholaminesaugment collateral vessel growth and angiogenesis in hind limb ischemia."
Am J Physiol Heart Circ Physiol 2005 (in press).

In the past decade, we have found that norepinephrine (NE) induces growth of smooth muscle cells (SMCs) and adventitial fibroblasts in vitro and in vivo. This growth factor-like activity, confirmed by others, is accentuated in arteries undergoing hypertrophic changes in pathologic (eg, balloon injury) and adaptive physiologic settings (eg, flow remodeling), even at baseline levels of sympathetic tone. The responsible α₁-adrenoceptor (AR) type differs from the one that generally mediates constriction. Adrenergic-induced growth is generalized to other settings besides injury, eg, intimal hyperplasia in pulmonary hypertension, collateral wall thickening, and low-Flow Induced Negative (inward) hypertrophic Remodeling (FINR). The key signaling elements have been identified: α1-AR → NAD(P)H-oxidase → ROS/H₂O₂ → pro-HB-EGF-cleavage → HB-EGF → EGFR → Raf1 → MEK → ERK1/2 → cell hypertrophy, proliferation and migration, collagen accumulation, thickening of intima, media and adventitia. Not only do these findings have potential relevance to disease, but they also attach a function to particular α₁-AR subtypes present on SMCs and adventitial fibroblasts that until now had not been ascribed a function. Possible support for this mechanism in humans has recently appeared; chronic treatment of prostatic hypertrophy with an α_1A-AR antagonist was accompanied by a 72% reduction in the development of symptoms of athe-rosclerosis-induced ischemic heart disease. Current studies are aimed at determining if this trophic pathway exists in human vessels and if it contributes to or worsens atherogenesis.