Mechanotransduction in Development: A Focus on Angiogenesis

Abstract

Cells respond to external mechanical cues and transduce these forces into biological signals. This process is known as mechanotransduction and requires a group of proteins called mechanosensors. This peculiar class of receptors include extracellular matrix proteins, plasma membrane proteins, the cytoskeleton and the nuclear envelope. These cell components are responsive to a wide spectrum of physical cues including stiffness, tensile force, hydrostatic pressure and shear stress. Among mechanotransducers, the Transient Receptor Potential (TRP) and the PIEZO family members are mechanosensitive ion channels, coupling force transduction with intracellular cation transport. Their activity contributes to embryo development, tissue remodeling and repair, and cell homeostasis. In particular, vessel development is driven by hemodynamic cues such as flow direction and shear stress. Perturbed mechanotransduction is involved in several pathological vascular phenotypes including hereditary hemorrhagic telangiectasia. This review is conceived to summarize the most recent findings of mechanotransduction in development. We first collected main features of mechanosensitive proteins. However, we focused on the role of mechanical cues during development. Mechanosensitive ion channels and their function in vascular development are also discussed, with a focus on brain vessel morphogenesis.

Keywords: angiogenesis; blood–brain barrier; development; mechanotransduction.

Figure 1

Mechanical signals from the ECM to the nucleus. Extracellular matrix mechanical cues are captured on cell surface by integrins and transmitted to cytoskeletal actin by the focal adhesion proteins. Likewise, tension from adjacent cell is transmitted by adherens junctions. Actin directly continues with nesprin 1/2, on nuclear envelope. By the SUN proteins, actin mechanical remodelling is trasmitted by the nesprins to the nuclear lamins, triggering chromatin remodelling and modification of the epigenetic pattern. ECM: extracellular matrix; FAK: focal adhesion kinase. Image created by the BioRender tool (https://www.biorender.com/).

Figure 2

Mammalian TRP channels. The six mammal TRP channel classes include TRPC, TRPM, TRPA, TRPML, TRPP, TRPV. (a) TRPC channels show 4 ankyrin domains at the N-terminus that are recognized by the activated phospholipase C, upon stimulation. Downstream, intracellular calcium is mobilized. (b) TRPM channels exhibit 4 melastatin homology domains at the N-terminus; cation gating upon activation results in regulation of gene expression. (c) The TRPA1 channel has 16 ankyrin repeats at the N-terminus and contributes to calcium release from the endoplasmic reticulum. (d) The TRPML channels are expressed on endosomal and lysosomal membranes, forming either heterodimers (TRPML1/2) or homodimers (TRPML2/2). (e) The TRPP, also known as polycystins, are the most important mechanosensors of the primary cilium. (f) The TRPV channels are divided into two subclasses, the calcium-selective TRPV5/6 activate the MAPK-p38 cascade, regulating ECM remodeling and cell adhesion; the cation-aspecific TRPV1/2/3/4 activate the phospholipase cascades. ANK: ankyrin; CC: coiled-coil; CTD: carboxyl-terminal domain; DAG: diacyl-glycerol; ER: endoplasmic reticulum; IP3: inositol trisphosphate; MAPK: MAP-kinase; MHR: melastatin homology domains; eNOS: nitric oxide synthase; PC: polycystin; PL: phospholipase; RNS: reactive nitrogenous species; ROS: reactive oxygen species. Image created by the BioRender tool.

Figure 3

PIEZO channels. (a) PIEZO1 activation results in intracellular calcium influx; within the cell, protein kinase C can be activated. Downstream responses depend on stimulus properties and cell type. (b) PIEZO2 activation results in cation gating and cell depolarization. PKC: protein-kinase C. Image created by the BioRender tool.

Figure 4

Mechanical regulation during embryo development. Extracellular matrix stiffness and cell density lead migration of neural crest cells; in particular, GPCRs and PIEZO1 drive mechanotransduction. Neural tube closure, instead, is driven by the β-actin gradient, increasing from dorsal to ventral cells. Likewise, differentiation of the cells of the ventral notochord region is driven by higher rigidity. ECM: extracellular matrix; GPCR: G-protein coupled receptor; PLC: phospholipase C. Image created by the BioRender tool.

Figure 5

Mechanical regulation of vessel development. Vascular development can be divided into three moments: vasculogenesis, angiogenesis and specification. During vasculogenesis (a), mechanical stimuli generated by early blood flow act on progenitor endothelial cells and are mainly transduced by the TRPV4/PIEZO1 mechanosensitive ion channels, driving capillary plexus formation. During sprouting angiogenesis (b) both external and intracellular cues control endothelial tip cell protrusion. During late angiogenesis, mechanical properties of ECM surrounding mural cells control the early stages of arteriovenous differentiation. Finally, during specification (c), EC acquire their final identity as lymphatic ECs by expressing Prox1; likewise, blood flow velocity and direction drive vessel caliber during pruning. Finally, vascular regression contributes to the formation of the final vascular network. EC: endothelial cell; ECM: extracellular matrix; ETC: endothelial tip cell. Image created by the BioRender tool.

Figure 6

Mechanical signals in tip/stalk phenotype acquisition. At the angiogenetic front, during the angiogenic phase, EC can differentiate into either the stalk or tip phenotype. Tip cells exhibit by a protrusive behavior, driven by the BMPR2, activated by blood flow. Likewise, fibrillin binding to the integrin-syndecan complex directs differentiation toward the tip phenotype. In stalk cells, instead, the BMP9/Alk9 interaction promotes the β-IV-spectrin expression, responsible for VEGFR2 internalization. BMP: bone morphogenetic protein; BMPR2: bone morphogenetic protein receptor 2; FBN: fibrillin; VEGFR2: vascular endothelial growth factor receptor 2. Image created by the BioRender tool.

Figure 7

Mechanics of the neurovascular development. During early embryo development, blood flow drives endothelial cell (EC) adhesion by enhancing adherens junction-mediated claudin 5 expression. Likewise, PIEZO1 responds to flow by stabilizing adherens junctions between endothelial cells. The Notch1-Jag1-Notch3 signals encourages EC/pericyte adhesion. During neurovascular coupling, not only chemical but also hemodynamic stimuli activate both neurons and astrocytes. Signals are transmitted to ECs by the pericytes. In ECs, a vasomotor response in generated and transmitted to pericyte, regulating vascular tone and blood pressure. Image created by the BioRender tool.