Factors that regulate in vitro brain angiogenesis
The formation of brain capillaries during development occurs through the convergence of multiple signaling pathways [2, 34]. Vascular endothelial growth factor (VEGF) released by the developing neural tube initiates formation of the perineural vascular plexus (PNVP) via vasculogenesis. From the PNVP, BMECs invade the brain parenchyma via angiogenesis driven by chemical cues released by developing neurons (e.g. wnt7a/b) and mechanical interactions with the brain parenchyma [2, 34]. The culmination of brain angiogenesis during development results in a hierarchical BBB with profound heterogeneity in structure and phenotype [35–37]. However, after development, angiogenesis is generally restricted to pathological conditions which alter BBB structure and phenotype [37–39]. Here we develop an in vitro model of brain angiogenesis using iPSC-derived BMECs (dhBMECs) to study developmental and pathological angiogenesis. We explore multiple factors that alter angiogenic phenotype of brain microvascular endothelial cells, including growth factors, ECM composition, and oxidative stress.
Critical chemical cues implicated in developmental brain angiogenesis include vascular endothelial growth factor (VEGF)  and wnt7a/b (WNT) . WNT signaling is specifically required for brain angiogenesis and is harnessed during differentiation of hiPSC-derived BMECs . However, other growth factors, including basic fibroblast growth factor (bFGF), are also implicated in promoting brain angiogenesis [40, 41]. Here we find that all three growth factors are likely pro-angiogenic for dhBMECs. In previous studies of primary brain microvascular endothelial cells using a tube formation assay , hypoxia was found to increase VEGF expression but was insufficient to promote formation of new vessels. In contrast, we find that angiogenic factors are sufficient to promote angiogenesis of dhBMECs within a 3D microenvironment, supporting the development of more physiological angiogenesis models.
Additionally, ECM composition and stiffness are key regulators of angiogenesis [43–48]. Numerous studies have shown that increased ECM stiffness reduces angiogenesis, likely by limiting cell proliferation and migration [46–48]. Pro-angiogenic ECM proteins include collagen I, fibronectin, and laminin . In studies specific to BMECs, fibronectin and laminin were shown to promote angiogenic and maturation phenotype, respectively . The extracellular space in the brain is comprised of hyaluronic acid, lecticans, proteoglycan link proteins, and tenascins [50, 51]. However, as the human brain is highly cellular by volume, non-brain-specific ECM components are commonly used to mimic the physical properties of the brain [3, 52, 53]. For example, 3D BBB models commonly utilize non-brain ECM components including collagen I [27, 54–58] and fibrin [20–22, 59]. We previously characterized and compared the stiffness of collagen I hydrogels to native mouse brain, and showed that 6 mg mL− 1 collagen is a reasonable proxy for brain stiffness . Additionally, materials with stiffnesses much lower than native brain were not conducive to the formation of stable BBB microvessels . Thus, we chose to only explore ECM materials with sufficient stiffness to form perfusable microvessel models, despite their absence within the brain parenchyma. We find that addition of growth factor-reduced Matrigel (primarily composed of laminin) to a collagen I matrix increased angiogenic phenotype.
Under homeostatic conditions angiogenesis is not prevalent in the adult brain; however, brain angiogenesis is associated with pathological conditions, including neurodegenerative disease, brain cancer, and stroke . Production of reactive oxygen species (ROS) is associated with these conditions and may contribute to BBB disruption and pathological angiogenesis . Reactive oxygen species promote angiogenesis via both VEGF-dependent and independent mechanisms . Previous work utilizing primary rat BMECs found that H2O2 displays a concentration-dependent influence on angiogenic behavior: concentrations below 10 µM increased tube length using a Matrigel tube formation assay, while concentrations above 10 µM decreased tube length . Here, the use of BBB beads provides spatial and temporal resolution to study the time-dependent effects of oxidative stress, which have previously been ignored. We find that H2O2 exerts a bimodal and concentration-dependent effect on brain angiogenesis.
Model advantages and limitations
2D models of brain angiogenesis (i.e. transwell assay or Matrigel tube forming assay) are unable to recapitulate the spatial dynamics of BMEC sprouting. Recently, 3D models of the brain microvasculature have been engineered via mimicry of vasculogenesis or angiogenesis using co-cultured primary ECs or BMECs, pericytes, and astrocytes [20–22]. Additionally, a microfluidic model of neurogenesis and angiogenesis was formed using co-cultured mesenchymal stem cells, primary BMECs, and neural stem cells, but was not tested for functional BBB properties . The use of hiPSC-derived BMECs in models of brain angiogenesis has not yet been demonstrated. Here, we build the first iPSC-derived in vitro model of brain angiogenesis. This model supports controlled studies of microenvironmental cues and genetic mutations on angiogenesis, without confounding factors present in vivo. Additionally, previous models have utilized fibrin for creating brain microvascular networks via angiogenesis and vasculogenesis-like processes [20–22]. However, fibrin is not conducive with dhBMEC microvessel formation and is rapidly degraded (Supp. Figure 3). Although neither collagen I nor fibrin is found in native brain ECM, collagen I densities used in this study are similar to the mechanical stiffness of native brain.
There are two main limitations to our model. (1) Brain angiogenesis in vivo occurs in the presence of complex cell-cell interactions, which are neglected in our model: BMECs interact with neurons, neural progenitor cells, pericytes, and glial progenitors during brain angiogenesis. As previously discussed, neurons and neural progenitor cells release critical chemical stimuli including wnt ligands and VEGF, which we introduce to promote sprouting in our model. Pericytes are an important cellular component of the neurovascular unit as they physically support new capillaries and are required for the formation of the BBB during development [62, 63]. Astrocytes are not critically involved in angiogenesis, as they are not present during initial brain vascularization; however, postnatally, they release ligands that maintain BBB integrity . Lastly, radial glia cells guide spatial patterning of angiogenesis as a physical scaffold for endothelial cell migration [34, 65]. Recent reports of an isogenic multicellular iPSC-based BBB transwell assay provide the foundations for building more complex angiogenesis models . Additionally, we previously incorporated iPSC-derived pericytes into a 3D microvessel BBB model, showing that they do not significantly alter barrier properties . Future work is required to examine how iPSC-derived pericytes and other cells of the BBB may alter angiogenesis in vitro. (2) The stability of angiogenic vessels is not addressed: the adult cerebrovasculature is highly stable, with limited angiogenesis . For example, over 30 days changes in capillary length, diameter or branching were not observed in the adult mouse somatosensory and motor cortex . Thus, models of the cerebrovasculature should aim to mimic physiological structural and phenotypic stability. We previously explored the stability of BBB microvessels, finding that microvessels reach quiescence over several days (when rates of cell division match cell apoptosis) . However, stability of microvessels formed via angiogenesis has not been addressed. As growth factor expression can display unique temporal and spatial expression patterns , transient administration or removal of growth factors may aid in generating stable microvessels. Future work will explore how removal of growth factors after angiogenesis occurs alters the structural and phenotypic stability of tissue-engineered cerebrovascular models. Additionally, many other stimuli influence the morphology of microvasculature in vitro, including flow and shear stress [45, 69], which could be harnessed to promote stability.
Engineering BBB hierarchy
To promote sprouting and anastomosis of capillaries between adjacent tissue-engineered microvessels we applied angiogenic factors which maximized growth rates. In previous work, capillary growth rates of ~ 40 µm day− 1 were sufficient to anastomose adjacent HUVEC microvessels . Here we observed more modest growth rates for dhBMECs (~ 20 µm day− 1). Previously it has been found that iPSC-derived endothelial cells exhibit reduced angiogenic potential compared to primary ECs (HUVECs), likely due to differences in MMP production . Due to limitations with primary and immortalized BMEC sources we did not explore cell source-dependent angiogenic differences. Importantly, our hierarchical model allows probing of how BBB phenotype changes across the vascular tree. Recently, we demonstrated use of BBB microvessels for studying hyperosmotic BBB disruption , but do not know if capillaries are more susceptibility to opening.