Mechanotransduction in embryonic vascular development

Abstract

A plethora of biochemical signals provides spatial and temporal cues that carefully orchestrate the complex process of vertebrate embryonic development. The embryonic vasculature develops not only in the context of these biochemical cues, but also in the context of the biomechanical forces imparted by blood flow. In the mature vasculature, different blood flow regimes induce distinct genetic programs, and significant progress has been made toward understanding how these forces are perceived by endothelial cells and transduced into biochemical signals. However, it cannot be assumed that paradigms that govern the mature vasculature are pertinent to the developing embryonic vasculature. The embryonic vasculature can respond to the mechanical forces of blood flow, and these responses are critical in vascular remodeling, certain aspects of sprouting angiogenesis, and maintenance of arterial-venous identity. Here, we review data regarding mechanistic aspects of endothelial cell mechanotransduction, with a focus on the response to shear stress, and elaborate upon the multifarious effects of shear stress on the embryonic vasculature. In addition, we discuss emerging predictive vascular growth models and highlight the prospect of combining signaling pathway information with computational modeling. We assert that correlation of precise measurements of hemodynamic parameters with effects on endothelial cell gene expression and cell behavior is required for fully understanding how blood flow-induced loading governs normal vascular development and shapes congenital cardiovascular abnormalities.

Fig. 1

Integration of molecular responses to shear stress in laminar and disturbed flow regimes. Transcription factors and pathways affected by laminar flow are shown in blue, whereas transcription factors and pathways affected by disturbed flow are shown in purple. The relationship between disturbed flow and KLF2 (marked by question mark) has been demonstrated only in embryonic development. Please see text for details.

Fig. 2

Loss of alk1 function in zebrafish embryos results in enlarged arteries and arteriovenous shunts. Alk1 expression is dependent on blood flow and plays an important role in limiting arterial caliber; in its absence, alk1-dependent arteries exhibit increased endothelial cell number and decreased endothelial cell density (compare D, alk1 mutant to C, wild type). Arterial enlargement results in flow-dependent development of arteriovenous malformations (asterisks in B). A,B: endothelial cell cytoplasm is green, red blood cells are magenta. C,D: endothelial cell nuclei are green, endothelial cell membranes are grayscale. The boxed region in A is similar to the region shown at higher magnification in independent embryos in C and D. 48 hours post-fertilization, dorsal views, anterior left.

Fig. 3

Correlation of 3D high resolution wall shear stress mapping to gene expression data in chick aortic arches. Subject-specific 3D high resolution wall shear stress mapping at HH18 (left panel) is consistent with published data on expression of the laminar shear stress-responsive genes, NOS3 and KLF2, at HH18 [right panels; reprinted with permission from (Groenendijk et al. 2004)].

Fig. 4

Demonstration of micro-PIV analysis of the chick embryonic aortic arches. Aortic arch-specific flow profile data can be used for numerical model validation and wall shear stress estimate. Using 0.5 μm fluorescent particles injected into the single ventricle, right aortic arch 2 and 3 velocity vectors (green) are computed at HH21. Embryo is oriented right side up. R-II labels the right second arch and R-III labels the right third arch. Data were collected with a standard 15 Hz PIV system. Arch vessels ~100 μm in diameter.