High vascular endothelial growth factor levels in NZW mice do not correlate with collagen deposition in allergic asthma

Genetic architecture underlying variation in extent and remodeling of the collateral circulation

RATIONALE

Collaterals are arteriole-to-arteriole anastomoses that connect adjacent arterial trees. They lessen ischemic tissue injury by serving as endogenous bypass vessels when the trunk of 1 tree becomes narrowed by vascular disease. The number and diameter ("extent") of native (preexisting) collaterals, plus their amount of lumen enlargement (growth/remodeling) in occlusive disease, show remarkably wide variation among inbred mouse strains (eg, C57BL/6 and BALB/c), resulting in large differences in tissue injury in models of occlusive disease. Evidence suggests similar large differences exist among healthy humans.

OBJECTIVE

To identify candidate loci responsible for genetic-dependent collateral variation.

METHODS AND RESULTS

Cerebral collateral number and diameter were determined in 221 C57BL/6xBALB/c F2 progeny, followed by linkage analysis to identify quantitative trait loci (QTL) for collateral number and diameter. Four QTL were obtained for collateral number, including epistasis between 2 loci. A QTL that was identical to the strongest QTL for collateral number on chromosome 7 (logarithm of the odds [LOD]=29, effect size=37%) was also mapped for collateral diameter (LOD=17, effect size=30%). Chromosome substitution strain analysis confirmed this locus. We also obtained a unique QTL on chromosome 11 for collateral remodeling after middle cerebral artery occlusion. Association mapping within the chromosome 7 QTL interval using collateral traits measured for 15 inbred strains delineated 172-kbp (P=0.00002) and 290-kbp (P=0.0004) regions on chromosome 7 containing 2 and 7 candidate genes, respectively.

CONCLUSIONS

We conclude that collateral extent and remodeling are unique, highly heritable complex traits, with 1 QTL predominantly affecting native collateral number and diameter.

Formation and maturation of the native cerebral collateral circulation

The native (pre-existing) collateral circulation minimizes tissue injury if obstructive vascular disease develops. Evidence suggests that large differences in collateral extent exist among healthy individuals, presumably from as-yet unknown genetic and/or environmental factors. Little is known regarding when or how native collaterals form-information needed to identify these factors. We examined collateral development between the middle and anterior cerebral artery trees in BALB/c and C57BL/6 mouse embryos-strains with marked differences in adult collateral density and diameter (85% fewer, 50% smaller in BALB/c). The circulation was dilated, fixed and stained. By E15.5, a "primary collateral plexus" was beginning to form in both strains. By E18.5, plexus vessel number peaked but was 60% less and diameter smaller in BALB/c (P<0.001). Earlier time points were examined to determine if these differences correlated with differences in patterning of the general circulation. At approximately E9.0, the primary capillary plexus was similar between strains, but by E12.5 branching was less and diameter larger in BALB/c (P<0.05). Between E12.5-E18.5-during pial artery tree development-small differences in tree size, branch number and distance between branches did not correlate with the large difference in collaterogenesis. Pruning of nascent collaterals between P1-P21 was comparable in both strains, yielding the adult density, but diameter and tortuosity increased less in BALB/c. Pericyte recruitment to nascent collaterals was comparable, despite lower VEGF-A and PDGF-B expression in BALB/c mice. These findings demonstrate that collaterals form late during vascular development and undergo postnatal maturation and that differences in genetic background have dramatic effects on these processes.