Understanding angiogenesis during aging: opportunities for discoveries and new models

Abstract

Microvascular network growth and remodeling are common denominators for most age-related pathologies. For multiple pathologies (myocardial infarction, stroke, hypertension), promoting microvascular growth, termed angiogenesis, would be beneficial. For others (cancer, retinopathies, rheumatoid arthritis), blocking angiogenesis would be desirable. Most therapeutic strategies, however, are motivated based on studies using adult animal models. This approach is problematic and does not account for the impaired angiogenesis or the inherent network structure changes that might result from age. Considering the common conception that angiogenesis is impaired with age, a need exists to identify the causes and mechanisms of angiogenesis in aged scenarios and for new tools to enable comparison of aged versus adult responses to therapy. The objective of this article is to introduce opportunities for advancing our understanding of angiogenesis in aging through the discovery of novel cell changes along aged microvascular networks and the development of novel ex vivo models.

Keywords: aging; angiogenesis; endothelial cell; microcirculation; pericyte.

Fig. 1.

Opportunities for understanding angiogenesis during aging. Angiogenesis, characterized by capillary sprouting off existing vessels, involves multiple cell-cell and cell-protein interactions. Main players are endothelial cells, vascular pericytes, growth factors (GF), and the extracellular matrix (ECM). Evidence for impaired angiogenesis during aging motivates the need to identify how these interactions are altered in aged tissues and during age-related pathologies and highlight opportunities for basic science discovery and the development of new biomimetic models for aging research. GF, growth factor.

Fig. 2.

Aged microvascular networks display increased pericyte coverage along capillaries. A–D: representative images from 9-mo-old and aged 24-mo-old Fisher 344 rats labeled for platelet endothelial cell adhesion molecule (PECAM) and α-smooth muscle actin (α-SMA) (A and B) or PECAM and neural/glial antigen 2 (NG2) (C and D). PECAM identifies endothelial cells. α-SMA and NG2 identify perivascular cells, including pericytes along capillaries. Arrow indicates a capillary segment void of pericyte wrapping. E and F: quantification of capillary coverage by α-SMA-positive (E) or NG2-positive (F) pericyte labeling. The number of PECAM-positive capillaries with observable pericyte labeling was quantified across entire tissues (α-SMA: n = 13 tissues from rats for each group; NG2: n = 15 tissues from 7 rats for each group). *P < 0.05, ***P < 0.001 (Student’s t-test). Values are shown as means ± SE. Scale bars, 50 µm.

Fig. 3.

Application of the rat mesentery culture model for aging research. A: in the rat mesentery culture model, mesenteric tissues can be harvested from adult and aged rats and cultured for comparison of microvascular growth dynamics. Advantages of this tissue culture model include time lapse observation of angiogenesis across intact microvascular networks and the ability to probe cell-cell interactions (2, 54). B: representative images of an aged mesenteric network from an aged 24-mo-old Fisher 344 rat immediately after harvesting and after culturing for 3 days in minimum essential media supplemented with 10% fetal bovine serum (FBS). Platelet endothelial cell adhesion molecule (PECAM) labeling identifies the hierarchy of intact networks, including arterioles (A), venules (V), and capillaries. Angiogenesis in the cultured tissues is supported by the observation of regions with high vascular density (*) and capillary sprouting (arrows). C: data demonstrating the feasibility of comparing angiogenic responses in mesenteric tissues from adult (9 mo) and aged (24 mo) Fisher 344 rats in unstimulated (UN) and stimulated (ST) conditions (n = 8 tissues from 5 rats/group). NS, nos. of vascular segments per area and capillary sprouts per vascular area were not significantly different between adult and aged networks (P > 0.05, 2-way ANOVA, followed by a Bonferroni pairwise comparison test). *Significant difference between unstimulated and stimulated conditions (P < 0.05, 2-way ANOVA followed by a Bonferroni pairwise comparison test). These results suggest that angiogenesis in aged scenarios can be rescued by exogenous delivery of growth cues. Values are shown as means ± SE. Scale bars, 100 µm.