Lack of Pericytes Leads to Endothelial Hyperplasia and Abnormal Vascular Morphogenesis

TLDR

Pericytes are believed to regulate endothelial cell proliferation, survival, migration, differentiation, and vascular branching during vessel formation. The study aimed to determine how pericyte deficiency affects brain angiogenesis. The authors employed PDGF‑B and PDGFR‑β knockout mice as in vivo models of pericyte‑deficient brain angiogenesis. Pericyte loss induced endothelial hyperplasia, enlarged capillaries, abnormal endothelial morphology, altered junctional protein distribution, increased permeability, and upregulation of VEGF‑A and metabolic stress genes, resulting in systemic vascular abnormalities and an edematous phenotype.

Abstract

The association of pericytes (PCs) to newly formed blood vessels has been suggested to regulate endothelial cell (EC) proliferation, survival, migration, differentiation, and vascular branching. Here, we addressed these issues using PDGF-B– and PDGF receptor-β (PDGFR-β)–deficient mice as in vivo models of brain angiogenesis in the absence of PCs. Quantitative morphological analysis showed that these mutants have normal microvessel density, length, and number of branch points. However, absence of PCs correlates with endothelial hyperplasia, increased capillary diameter, abnormal EC shape and ultrastructure, changed cellular distribution of certain junctional proteins, and morphological signs of increased transendothelial permeability. Brain endothelial hyperplasia was observed already at embryonic day (E) 11.5 and persisted throughout development. From E 13.5, vascular endothelial growth factor-A (VEGF-A) and other genes responsive to metabolic stress became upregulated, suggesting that the abnormal microvessel architecture has systemic metabolic consequences. VEGF-A upregulation correlated temporally with the occurrence of vascular abnormalities in the placenta and dilation of the heart. Thus, although PC deficiency appears to have direct effects on EC number before E 13.5, the subsequent increased VEGF-A levels may further abrogate microvessel architecture, promote vascular permeability, and contribute to formation of the edematous phenotype observed in late gestation PDGF-B and PDGFR-β knock out embryos.