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 developed countries. Angiogenesis (increase in capillary density) and growth of collateral vessels are major adaptations that limit tissue damage. My laboratory’s interests concentrate on mechanisms of adaptive physiological and pathological blood vessel formation, growth and remodeling, with primary focus on the collateral circulation (“endogenous bypass vessels”). We study these processes at all levels in the mouse embryo, neonate and adult—from the influence of genetic polymorphisms to the responsible cell signaling pathways.
Collaterals are tiny ~25 µm diameter arteriole-arteriole anastomoses connecting adjacent arterial trees that are low in number but present in most tissues. These vessels are unique – during formation they defy the guidance molecules (eprhin-eph, etc) that insure the artery-capillary-vein anatomic patterning that characterizes the general circulation. They also normally have little pressure drop or net flow across them, yet escape pruning and persist despite their low/disturbed shear stress environment. When the artery supplying an adjacent tree critically narrows, these “native” pre-existing collaterals enlarge (remodel) their diameter up to 10-fold (a process initiated by increase shear stress) to become “endogenous bypass vessels” able to limit ischemic injury. The degree of protection they afford to tissues depends on their pre-existing density and diameter, plus their capacity to outwardly remodel. Thus, collaterals 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 collaterals develop in healthy tissues. Native collateral density and diameter vary widely among species and humans, suggesting a genetic basis. Yet nothing is known about the source of this variation.
We have found that collaterals develop during the late embryonic-to-early postnatal period in mice. We have also: 1) introduced several new methods, eg, resolution-boosting modifications to laser Doppler perfusion imaging and x-ray/light micro-angiography; 2) developed new model tissues for study of the collateral circulation in the mouse (eg, cerebral, coronary and abdominal skeletal muscle); 3) discovered that the extent (ie, density, diameter, length—thus conductance) of the native collateral circulation in healthy inbred mouse strains varies dramatically from natural genetic polymorphisms. These differences have a major impact on variation in severity of tissue injury and collateral-dependent flow recovery in models of ischemic disease (stroke, myocardial infarction, peripheral arterial disease). These findings create an excellent opportunity to identify the pathways specifying collateral formation. 4) identified the first three genes (Vegfa, Flt1 and Clic4) whose expression significantly impacts formation of the native collateral circulation; 5) used quantitative trait loci and haplotype association mapping to identify novel candidate genes responsible for variation in extent of the collateral circulation; 6) begun to investigate the applicability of these findings to human.
Besides genetic factors, we are also studying whether environmental factors (eg, risk factors for cardiovascular disease) and gene-environment interactions affect the extent of the native collateral circulation and alter collateral remodeling. Our recent work has demonstrated that eNOS-derived nitric oxide (NO) is required for maintenance of collateral density during natural growth to adulthood. Other work has found that collateral rarefaction occurs with advanced aging in association with impaired NO signaling, resulting in greatly enhance ischemic injury in brain and hindlimb after arterial obstruction. Identification of the factors directing collateral formation in normal tissues and genetic polymorphisms underlying this variation are intriguing fundamental questions. Furthermore, their answers may allow us to identify individuals at risk from too-few collaterals, as well as lead to therapies to induce formation of new collaterals – a goal that has thus far eluded investigators. We also study collateral vascular biology with other collaborators at UNC and elsewhere. Our research secondarily studies ischemic and tumor angiogenesis (increased growth of capillaries).
Some Specific Areas of Current Investigation:
- Identify the determinants of formation of the collateral circulation in the embryo, stabilization / maturation of collaterals in the neonate, and maintenance of them in the adult.
- Identify genetic polymorphisms responsible for the large variability in collateral circulatory formation among inbred mouse strains; extension of these findings to “outbred” humans.
- Characterize the unique cell phenotype and expression patterns that we have found exist in collateral endothelial and mural cells that may function in the formation and persistence of collaterals.
- Identify the basis for decline in collateral extent caused by aging and endothelial dysfunction. Determine if other cardiovascular risk factors and diseases (environmental factors) cause similar collateral rarefaction. Investigate approaches to prevent this.
- Determine how new collaterals form in adult ischemic tissue and how to augment this.