Stromal Cells Promote Neovascular Invasion Across Tissue Interfaces

SARS-CoV-2 Rapidly Infects Peripheral Sensory and Autonomic Neurons, Contributing to Central Nervous System Neuroinvasion before Viremia

Neurological symptoms associated with COVID-19, acute and long term, suggest SARS-CoV-2 affects both the peripheral and central nervous systems (PNS/CNS). Although studies have shown olfactory and hematogenous invasion into the CNS, coinciding with neuroinflammation, little attention has been paid to susceptibility of the PNS to infection or to its contribution to CNS invasion. Here we show that sensory and autonomic neurons in the PNS are susceptible to productive infection with SARS-CoV-2 and outline physiological and molecular mechanisms mediating neuroinvasion. Our infection of K18-hACE2 mice, wild-type mice, and golden Syrian hamsters, as well as primary peripheral sensory and autonomic neuronal cultures, show viral RNA, proteins, and infectious virus in PNS neurons, satellite glial cells, and functionally connected CNS tissues. Additionally, we demonstrate, in vitro, that neuropilin-1 facilitates SARS-CoV-2 neuronal entry. SARS-CoV-2 rapidly invades the PNS prior to viremia, establishes a productive infection in peripheral neurons, and results in sensory symptoms often reported by COVID-19 patients.

Isolated Fragments of Intact Microvessels: Tissue Vascularization, Modeling, and Therapeutics

The microvasculature is integral to nearly every tissue in the body, providing not only perfusion to and from the tissue, but also homing sites for immune cells, cellular niches for tissue dynamics, and cooperative interactions with other tissue elements. As a microtissue itself, the microvasculature is a composite of multiple cell types exquisitely organized into structures (individual vessel segments and extensive vessel networks) capable of considerable dynamics and plasticity. Consequently, it has been challenging to include a functional microvasculature in assembled or fabricated tissues. Isolated fragments of intact microvessels, which retain the cellular composition and structures of native microvessels, are proving effective in a variety of vascularization applications including tissue in vitro disease modeling, vascular biology, mechanistic discovery, and tissue prevascularization in regenerative therapeutics and grafting. In this review, we will discuss the importance of recapitulating native tissue biology and the successful vascularization applications of isolated microvessels.

Dynamic Biophysical Cues Near the Tip Cell Microenvironment Provide Distinct Guidance Signals to Angiogenic Neovessels

The formation of new vascular networks via angiogenesis is a crucial biological mechanism to balance tissue metabolic needs, yet the coordination of factors that influence the guidance of growing neovessels remain unclear. This study investigated the influence of extracellular cues within the immediate environment of sprouting tips over multiple hours and obtained quantitative relationships describing their effects on the growth trajectories of angiogenic neovessels. Three distinct microenvironmental cues-fibril tracks, ECM density, and the presence of nearby cell bodies-were extracted from 3D time series image data. The prominence of each cue was quantified along potential sprout trajectories to predict the response to multiple microenvironmental factors simultaneously. Sprout trajectories significantly correlated with the identified microenvironmental cues. Specifically, ECM density and nearby cellular bodies were the strongest predictors of the trajectories taken by neovessels (p < 0.001 and p = 0.016). Notwithstanding, direction changing trajectories, deviating from the initial neovessel orientation, were significantly correlated with fibril tracks (p = 0.003). Direction changes also occurred more frequently with strong microenvironmental cues. This provides evidence for the first time that local matrix fibril alignment influences changes in sprout trajectories but does not materially contribute to persistent sprouting. Together, our results suggest the microenvironmental cues significantly contribute to guidance of sprouting trajectories. Further, the presented methods quantitatively distinguish the influence of individual microenvironmental stimuli during guidance.

Vascularized Tissue Organoids

Tissue organoids hold enormous potential as tools for a variety of applications, including disease modeling and drug screening. To effectively mimic the native tissue environment, it is critical to integrate a microvasculature with the parenchyma and stroma. In addition to providing a means to physiologically perfuse the organoids, the microvasculature also contributes to the cellular dynamics of the tissue model via the cells of the perivascular niche, thereby further modulating tissue function. In this review, we discuss current and developing strategies for vascularizing organoids, consider tissue-specific vascularization approaches, discuss the importance of perfusion, and provide perspectives on the state of the field.

Stromal Vascular Fraction Reverses the Age-Related Impairment in Revascularization following Injury

Adipose-derived stromal vascular fraction (SVF) has emerged as a potential regenerative therapy, but few studies utilize SVF in a setting of advanced age. Additionally, the specific cell population in SVF providing therapeutic benefit is unknown. We hypothesized that aging would alter the composition of cell populations present in SVF and its ability to promote angiogenesis following injury, a mechanism that is T cell-mediated. SVF isolated from young and old Fischer 344 rats was examined with flow cytometry for cell composition. Mesenteric windows from old rats were isolated following exteriorization-induced (EI) hypoxic injury and intravenous injection of one of four cell therapies: (1) SVF from young or (2) old donors, (3) SVF from old donors depleted of or (4) enriched for T cells. Advancing age increased the SVF T-cell population but reduced revascularization following injury. Both young and aged SVF incorporated throughout the host mesenteric microvessels, but only young SVF significantly increased vascular area following EI. This study highlights the effect of donor age on SVF angiogenic efficacy and demonstrates how the ex vivo mesenteric-window model can be used in conjunction with SVF therapy to investigate its contribution to angiogenesis.

Matrix anisotropy promotes angiogenesis in a density-dependent manner

Angiogenesis is necessary for wound healing, tumorigenesis, implant inosculation, and homeostasis. In each situation, matrix structure and mechanics play a role in determining whether new vasculatures can establish transport to new or hypoxic tissues. Neovessel growth and directional guidance are sensitive to three-dimensional (3-D) matrix anisotropy and density, although the individual and integrated roles of these matrix features have not been fully recapitulated in vitro. We developed a tension-based method to align 3-D collagen constructs seeded with microvessel fragments in matrices of three levels of collagen fibril anisotropy and two levels of collagen density. The extent and direction of neovessel growth from the parent microvessel fragments increased with matrix anisotropy and decreased with density. The proangiogenic effects of anisotropy were attenuated at higher matrix densities. We also examined the impact of matrix anisotropy in an experimental model of neovessel invasion across a tissue interface. Matrix density was found to dictate the success of interface crossing, whereas interface curvature and fibril alignment were found to control directional guidance. Our findings indicate that complex configurations of matrix density and alignment can facilitate or complicate the establishment or maintenance of vascular networks in pathological and homeostatic angiogenesis. Furthermore, we extend preexisting methods for tuning collagen anisotropy in thick constructs. This approach addresses gaps in tissue engineering and cell culture by supporting the inclusion of large multicellular structures in prealigned constructs.NEW & NOTEWORTHY Matrix anisotropy and density have a considerable effect on angiogenic vessel growth and directional guidance. However, the current literature relies on 2-D and simplified models of angiogenesis (e.g., tubulogenesis and vasculogenesis). We present a method to align 3-D collagen scaffolds embedded with microvessel fragments to different levels of anisotropy. Neovessel growth increases with anisotropy and decreases with density, which may guide angiogenic neovessels across tissue interfaces such as during implant inosculation and tumorigenesis.

Methods for vascularization and perfusion of tissue organoids

Tissue organoids or "mini organs" can be invaluable tools for understanding health and disease biology, modeling tissue dynamics, or screening potential drug candidates. Effective vascularization of these models is critical for truly representing the in vivo tissue environment. Not only is the formation of a vascular network, and ultimately a microcirculation, essential for proper distribution and exchange of oxygen and nutrients throughout larger organoids, but vascular cells dynamically communicate with other cells to modulate overall tissue behavior. Additionally, interstitial fluid flow, mediated by a perfused microvasculature, can have profound influences on tissue biology. Thus, a truly functionally and biologically relevant organoid requires a vasculature. Here, we review existing strategies for fabricating and incorporating vascular elements and perfusion within tissue organoids.

The Evaluation of Neovessel Angiogenesis Behavior at Tissue Interfaces

Angiogenesis, the formation of new vessel elements from existing vessels, is important in homeostasis and tissue repair. Dysfunctional angiogenesis can contribute to numerous pathologies, including cancer, ischemia, and chronic wounds. In many instances, growing vessels must navigate along or across tissue-associated boundaries and interfaces tissue interfaces. To understand this dynamic, we developed a new model for studying angiogenesis at tissue interfaces utilizing intact microvessel fragments isolated from adipose tissue. Isolated microvessels retain their native structural and cellular complexity. When embedded in a 3D matrix, microvessels, sprout, grow, and connect to form a neovasculature. Here, we discuss and describe methodology for one application of our microvessel-based angiogenesis model, studying neovessel behavior at tissue interfaces.

Microvascular fragments in microcirculation research and regenerative medicine

Adipose tissue-derived microvascular fragments (MVF) are functional vessel segments, which rapidly reassemble into new microvasculatures under experimental in vitro and in vivo conditions. Accordingly, they have been used for many years in microcirculation research to study basic mechanisms of endothelial cell function, angiogenesis and microvascular network formation in two- and three-dimensional environments. Moreover, they serve as vascularization units for musculoskeletal regeneration and implanted biomaterials as well as for the treatment of myocardial infarction and the generation of prevascularized tissue organoids. Besides, multiple factors determining the vascularization capacity of MVF have been identified, including their tissue origin and cellular composition, the conditions for their short- and long-term storage as well as their implantation site and the general health status and medication of the recipient. The next challenging step is now the successful translation of all these promising experimental findings into clinical practice. If this succeeds, a multitude of future therapeutic applications may significantly benefit from the remarkable properties of MVF.

Quantifying Vascular Density in Tissue Engineered Constructs Using Machine Learning

Given the considerable research efforts in understanding and manipulating the vasculature in tissue health and function, making effective measurements of vascular density is critical for a variety of biomedical applications. However, because the vasculature is a heterogeneous collection of vessel segments, arranged in a complex three-dimensional architecture, which is dynamic in form and function, it is difficult to effectively measure. Here, we developed a semi-automated method that leverages machine learning to identify and quantify vascular metrics in an angiogenesis model imaged with different modalities. This software, BioSegment, is designed to make high throughput vascular density measurements of fluorescent or phase contrast images. Furthermore, the rapidity of assessments makes it an ideal tool for incorporation in tissue manufacturing workflows, where engineered tissue constructs may require frequent monitoring, to ensure that vascular growth benchmarks are met.

Vascularized adipocyte organoid model using isolated human microvessel fragments

Tissue organoids are proving valuable for modeling tissue health and disease in a variety of applications. This is due, in part, to the dynamic cell-cell interactions fostered within the 3D tissue-like space. To this end, the more that organoids recapitulate the different cell-cell interactions found in native tissue, such as that between parenchyma and the microvasculature, the better the fidelity of the model. The microvasculature, which is comprised of a spectrum of cell types, provides not only perfusion in its support of tissue health, but also important cellular interactions and biochemical dynamics important in tissue phenotype and function. Here, we incorporate whole, intact human microvessel fragments isolated from adipose tissue into organoids to form both MSC and adipocyte vascularized organoids. Isolated microvessels retain their native structure and cell composition, providing a more complete representation of the microvasculature within the organoids. Microvessels expanded via sprouting angiogenesis within organoids comprised of either MSCs or MSC-derived adipocytes and grew out of the organoids when placed in a 3D collagen matrix. In MSC organoids, a ratio of 50 MSCs to 1 microvessel fragment created the optimal vascularization response. We developed a new differentiation protocol that enabled the differentiation of MSCs into adipocytes while simultaneously promoting microvessel angiogenesis. The adipocyte organoids contained vascular networks, were responsive in a lipolysis assay, and expressed the functional adipocyte markers adiponectin and PPARγ. The presence of microvessels promoted insulin receptor expression by adipocytes and modified IL-6 secretion following a TNF-alpha challenge. Overall, we demonstrate a robust method for vascularizing high cell-density organoids with potential implications for other tissues as well.

Scaffold Architecture and Matrix Strain Modulate Mesenchymal Cell and Microvascular Growth and Development in a Time Dependent Manner

BACKGROUND

Volumetric tissue-engineered constructs are limited in development due to the dependence on well-formed vascular networks. Scaffold pore size and the mechanical properties of the matrix dictates cell attachment, proliferation and successive tissue morphogenesis. We hypothesize scaffold pore architecture also controls stromal-vessel interactions during morphogenesis.

METHODS

The interaction between mesenchymal stem cells (MSCs) seeded on hydroxyapatite scaffolds of 450, 340, and 250 μm pores and microvascular fragments (MVFs) seeded within 20 mg/mL fibrin hydrogels that were cast into the cell-seeded scaffolds, was assessed in vitro over 21 days and compared to the fibrin hydrogels without scaffold but containing both MSCs and MVFs. mRNA sequencing was performed across all groups and a computational mechanics model was developed to validate architecture effects on predicting vascularization driven by stiffer matrix behavior at scaffold surfaces compared to the pore interior.

RESULTS

Lectin staining of decalcified scaffolds showed continued vessel growth, branching and network formation at 14 days. The fibrin gel provides no resistance to spread-out capillary networks formation, with greater vessel loops within the 450 μm pores and vessels bridging across 250 μm pores. Vessel growth in the scaffolds was observed to be stimulated by hypoxia and successive angiogenic signaling. Fibrin gels showed linear fold increase in VEGF expression and no change in BMP2. Within scaffolds, there was multiple fold increase in VEGF between days 7 and 14 and early multiple fold increases in BMP2 between days 3 and 7, relative to fibrin. There was evidence of yap/taz based hippo signaling and mechanotransduction in the scaffold groups. The vessel growth models determined by computational modeling matched the trends observed experimentally.

CONCLUSION

The differing nature of hypoxia signaling between scaffold systems and mechano-transduction sensing matrix mechanics were primarily responsible for differences in osteogenic cell and microvessel growth. The computational model implicated scaffold architecture in dictating branching morphology and strain in the hydrogel within pores in dictating vessel lengths.