Hereditary Hemorrhagic Telangiectasia (HHT) is a rare congenital disease in which fragile vascular malformations focally develop in multiple organs. These can be small (telangiectasias) or large (arteriovenous malformations, AVMs) and may rupture leading to frequent, uncontrolled bleeding. There are few treatment options and no cure for HHT. Most HHT patients are heterozygous for loss-of-function mutations for Endoglin (ENG) or Alk1 (ACVRL1), however, why loss of these genes manifests as vascular malformations remains poorly understood. To complement ongoing work in animal models, we have developed a microphysiological system model of HHT. Based on our existing vessel-on-a-chip (VMO) platform, our fully human cell-based HHT-VMO recapitulates HHT patient vascular lesions. Using inducible ACVRL1 (Alk1)-knockdown, we control timing and extent of endogenous Alk1 expression in primary human endothelial cells (EC) in the HHT-VMO. HHT-VMO vascular lesions develop over several days, and are dependent upon timing of Alk1 knockdown. Interestingly, in chimera experiments AVM-like lesions can be comprised of both Alk1-intact and Alk1-deficient EC, suggesting possible cell non-autonomous effects. Single cell RNA sequencing data are consistent with microvessel pruning/regression as contributing to AVM formation, while loss of PDGFB expression implicates mural cell recruitment. Finally, lesion formation is blocked by the VEGFR inhibitor pazopanib, mirroring the positive effects of this drug in patients. In summary, we have developed a novel HHT-on-a-chip model that faithfully reproduces HHT patient lesions and that is sensitive to a treatment effective in patients. The VMO-HHT can be used to better understand HHT disease biology and identify potential new HHT drugs.Significance:This manuscript describes development of an organ-on-a-chip model of Hereditary Hemorrhagic Telangiectasia (HHT), a rare genetic disease involving development of vascular malformations. Our VMO-HHT model produces vascular malformations similar to those seen in human HHT patients, including small (telangiectasias) and large (arteriovenous malformations) lesions. We show that VMO-HHT lesions are sensitive to a drug, pazopanib, that appears to be effective in HHT human patients. We further use the VMO-HHT platform to demonstrate that there is a critical window during vessel formation in which the HHT gene, Alk1, is required to prevent vascular malformation. Lastly, we show that lesions in the VMO-HHT model are comprised of both Alk1-deficient and Alk1-intact endothelial cells.

Hereditary hemorrhagic telangiectasia is an autosomal dominant trait affecting approximately 1 in 5000 people. A pathogenic DNA sequence variant in the ENG, ACVRL1 or SMAD4 genes, can be found in the majority of patients. The 12th International Scientific HHT Conference was held on June 8–11, 2017 in Dubrovnik, Croatia to present and discuss the latest scientific achievements, and was attended by over 200 scientific and clinical researchers. In total 174 abstracts were accepted of which 58 were selected for oral presentations. This article covers the basic science and clinical talks, and discussions from three theme-based workshops. We focus on significant emergent themes and unanswered questions. Understanding these topics and answering these questions will help to define the future of HHT research and therapeutics, and ultimately bring us closer to a cure.

The microvasculature is an essential component of nearly all tissues, with most cells residing within 200 μm of a vessel. Endothelial cells form the inner wall of microvessels and control which nutrients, cells and drugs cross into the underlying tissue. Given these critical roles, it makes sense to include functioning microvessels when creating tissue models, and these can be generated by pre-patterning of channels, 3D printing, or by “biology-directed” vasculogenesis and angiogenesis. These methods have been used to create tissue-specific vascularized micro-organs, vascularized microtumors, and vascular beds for studying tumor cell and immune cell extravasation. Recent innovations in 3D printing and high-throughput technologies have enabled more complex geometries and facilitated more comprehensive studies. Future developments hold promise for increasingly relevant models of healthy and diseased tissues, with implications for studying biological mechanisms and screening for new drugs.

The blood–brain barrier is a dynamic and highly organized structure that strictly regulates the molecules allowed to cross the brain vasculature into the central nervous system. The blood–brain barrier pathology has been associated with a number of central nervous system diseases, including vascular malformations, stroke/vascular dementia, Alzheimer’s disease, multiple sclerosis, and various neurological tumors including glioblastoma multiforme. There is a compelling need for representative models of this critical interface. Current research relies heavily on animal models (mostly mice) or on two-dimensional (2D) in vitro models, neither of which fully capture the complexities of the human blood–brain barrier. Physiological differences between humans and mice make translation to the clinic problematic, while monolayer cultures cannot capture the inherently three-dimensional (3D) nature of the blood–brain barrier, which includes close association of the abluminal side of the endothelium with astrocyte foot-processes and pericytes. Here we discuss the central nervous system diseases associated with blood–brain barrier pathology, recent advances in the development of novel 3D blood–brain barrier -on-a-chip systems that better mimic the physiological complexity and structure of human blood–brain barrier, and provide an outlook on how these blood–brain barrier-on-a-chip systems can be used for central nervous system disease modeling. Impact statement The field of microphysiological systems is rapidly evolving as new technologies are introduced and our understanding of organ physiology develops. In this review, we focus on Blood–Brain Barrier (BBB) models, with a particular emphasis on how they relate to neurological disorders such as Alzheimer’s disease, multiple sclerosis, stroke, cancer, and vascular malformations. We emphasize the importance of capturing the three-dimensional nature of the brain and the unique architecture of the BBB – something that until recently had not been well modeled by in vitro systems. Our hope is that this review will provide a launch pad for new ideas and methodologies that can provide us with truly physiological BBB models capable of yielding new insights into the function of this critical interface.

Null mutations in for pigment epithelium‐derived factor (PEDF), the protein product of the SERPINF1 gene, are the cause of osteogenesis imperfecta (OI) type VI. The PEDF‐knockout (KO) mouse captures crucial elements of the human disease, including diminished bone mineralization and propensity to fracture. Our group and others have demonstrated that PEDF directs human mesenchymal stem cell (hMSC) commitment to the osteoblast lineage and modulates Wnt/β‐catenin signaling, a major regulator of bone development; however, the ability of PEDF to restore bone mass in a mouse model of OI type VI has not been determined. In this study, PEDF delivery increased trabecular bone volume/total volume by 52% in 6‐mo‐old PEDF‐KO mice but not in wild‐type mice. In young (19‐d‐old) PEDF‐KO mice, PEDF restoration increased bone volume fraction by 35% and enhanced biomechanical parameters of bone plasticity. A Wnt‐green fluorescent protein reporter demonstrated dynamic changes in Wnt/β‐catenin signaling characterized by early activation and marked suppression during terminal differentiation of hMSCs. Continuous Wnt3a exposure impeded mineralization of hMSCs, whereas the combination of Wnt3a and PEDF potentiated mineralization. Interrogation of the PEDF sequence identified a conserved motif found in other Wnt modulators, such as the dickkopf proteins. Mutation of a single amino acid on a 34‐mer PEDF peptide increased mineralization of hMSC cultures compared with the native peptide sequence. These results indicate that PEDF counters Wnt signaling to allow for osteoblast differentiation and provides a mechanistic insight into how the PEDF null state results in OI type VI.—Belinsky, G. S., Sreekumar, B., Andrejecsk, J. W., Saltzman, W. M., Gong, J., Herzog, R. I., Lin, S., Horsley, V., Carpenter, T. O., Chung, C. Pigment epithelium‐derived factor restoration increases bone mass and improves bone plasticity in a model of osteogenesis imperfecta type VI via Wnt3a blockade. FASEB J. 30, 2837‐2848 (2016). www.fasebj.org

A key attribute for nanoparticles (NPs) that are used in medicine is the ability to avoid rapid uptake by phagocytic cells in the liver and other tissues. Poly(ethylene glycol) (PEG) coatings has been the gold standard in this regard for several decades. Here, we examined hyperbranched polyglycerols (HPG) as an alternate coating on NPs. In earlier work, HPG was modified with amines and subsequently conjugated to poly(lactic acid) (PLA), but that approach compromised the ability of HPG to resist non-specific adsorption of biomolecules. Instead, we synthesized a copolymer of PLA-HPG by a one-step esterification. NPs were produced from a single emulsion using PLA-HPG: fluorescent dye or the anti-tumor agent camptothecin (CPT) were encapsulated at high efficiency in the NPs. PLA-HPG NPs were quantitatively compared to PLA-PEG NPs, produced using approaches that have been extensively optimized for drug delivery in humans. Despite being similar in size, drug release profile and in vitro cytotoxicity, the PLA-HPG NPs showed significantly longer blood circulation and significantly less liver accumulation than PLA-PEG. CPT-loaded PLA-HPG NPs showed higher stability in suspension and better therapeutic effectiveness against tumors in vivo than CPT-loaded PLA-PEG NPs. Our results suggest that HPG is superior to PEG as a surface coating for NPs in drug delivery.

Transplantation of endothelial cells (ECs) for therapeutic vascularization or tissue engineering is a promising method for increasing tissue perfusion. Here, we report on a new approach for enhanced EC transplantation using targeted nanoparticle transfection to deliver proangiogenic microRNA‐132 (miR‐132) to cultured ECs before their transplantation, thereby sensitizing cells to the effects of endogenous growth factors. We synthesized biodegradable PLGA polymer nanoparticles (NPs) that were loaded with miR‐132 and coated with cyclic RGD (cRGD) peptides that target integrin αvβ3 expressed on cultured human umbilical vein ECs (HUVECs), increasing NP uptake through clathrin‐coated pits. Unlike previously reported NPs for miR delivery, these NPs slowly release RNA for several weeks. The endocytosed NPs remain in clathrin‐coated vesicles from which they mediate intracellular delivery of siRNA or miRNA. Transfection of HUVECs with miR‐132 enhances growth factor‐induced proliferation and migration in 2D culture, producing a 1.8‐ and 5‐fold increase, respectively. However, while the effects of conventional transfection were short‐lived, NP transfection produced protein knockdown and biological effects that were significantly longer in duration (≥6 d). Transfection of HUVECs with miR‐132 NP resulted in a 2‐fold increase in the number of microvessels per square millimeter compared to lipid after transplantation into immunodeficient mice and led to a higher number of mural cell‐invested vessels than control transfection. These data suggest that sustained delivery of miR‐132 encapsulated in a targeted biodegradable polymer NP is a safe and efficient strategy to improve EC transplantation and vascularization.—Devalliere, J., Chang, W. G., Andrejecsk, J. W., Abrahimi, P., Cheng, C. J., Jane‐wit, D., Saltzman, W. M., Pober, J. S. Sustained delivery of proangiogenic microRNA‐132 by nanoparticle transfection improves endothelial cell transplantation. FASEB J. 28, 908–922 (2014). www.fasebj.org

AIM Angiogenic sprouts arise from microvessels formed by endothelial cells (ECs) invested by pericytes (PCs). The aim of this study was to examine the role of PCs in angiogenic sprouting, an understudied phenomenon. METHODS AND RESULTS We adapted a human EC spheroid model to examine PC effects on vascular endothelial growth factor-A-induced EC sprouting in vitro by using Bcl-2-transduced human umbilical vein ECs to reduce apoptosis in collagen gels. Human placental PCs, separated from endothelial spheroids by a transwell, or addition of PC-conditioned media increased EC sprouting primarily through hepatocyte growth factor (HGF). Mixed endothelial-PC spheroids formed similar numbers of endothelial sprouts as endothelial spheroids but the sprouts from mixed spheroids were invested by PCs within 24 h. PCs were recruited to the sprouts by platelet-derived growth factor (PDGF)-BB; inhibition of PDGF signalling reduced PC coverage and increased EC sprouting. Transplanted endothelial spheroids give rise to sprouts in vivo that evolve into perfused microvessels. Mixed endothelial-PC spheroids form similar numbers of microvessels as endothelial-only spheroids, but acquire human PC investment and have reduced average lumen diameter. CONCLUSIONS PCs promote endothelial sprouting by elaborating HGF, but when recruited to invest endothelial sprouts by PDGF-BB, limit the extent of sprouting in vitro and lumen diameter in vivo.

Paracrine signals, essential for the proper survival and functioning of tissues, may be mimicked by delivery of therapeutic proteins within engineered tissue constructs. Conventional delivery methods are of limited duration and are unresponsive to the local environment. We developed a system for sustained and regulated delivery of paracrine signals by encapsulating living cells of one type in alginate beads and co-suspending these cell-loaded particles along with unencapsulated cells of a second type within a 3D protein gel. This system was applied to vascular tissue engineering by placing human placental microvascular pericytes (PCs) in the particulate alginate phase and human umbilical vein endothelial cells (HUVECs) in the protein gel phase. Particle characteristics were optimized to keep the encapsulated PCs viable for at least two weeks. Encapsulated PCs were bioactive in vitro, secreting hepatocyte growth factor, an angiogenic protein, and responding to externally applied HUVEC-derived signals. Encapsulated PCs influenced HUVEC behavior in the surrounding gel by enhancing the formation of vessel-like structures when compared to empty alginate bead controls. In vivo, encapsulated PCs modulated the process of vascular self-assembly by HUVECs in 3D gels following implantation into immunodeficient mice. We conclude that alginate encapsulated cells can provide functional paracrine signals within engineered tissues.

Rapid induction and stabilization of new microvascular networks is essential for the proper functioning of engineered tissues. Many efforts to achieve this goal have used proangiogenic proteins—such as vascular endothelial growth factors—to induce the formation of new microvessels. These proteins have demonstrated promise in improving vascularization, but it is also clear that the spatial and temporal presentation of these signals is important for achieving proper vascular function. Delivery systems that present proteins in a localized and sustained manner can promote the formation and stabilization of microvascular networks by precisely presenting proangiogenic proteins at desired locations, and for specified durations. Further, these systems allow for some control over the sequence of release of multiple proteins, and it has become clear that such coordination is critical for the development of fully functional and mature vascular structures. This review focuses on the actions of proangiogenic proteins and the innovations in controlled release technologies that precisely deliver these to stimulate microvascular network formation and stabilization.

Early perfusion is essential for survival and function of engineered tissues. A promising solution to this problem is the development of stable microvessels within an engineered construct. Transplantation of human endothelial cells within protein gels has been shown to form functional vascular networks both in vitro and after implantation into immunodeficient mice. Co-transplantation of human pericytes, a supporting cell in microvessels, can significantly improve the maturation and stability of vessels formed compared to endothelial cells alone. We hypothesize that the optimization of a system for simultaneous delivery of endothelial cells and pericytes will improve vascularization and vascular stability in engineered tissues. Pericytes are contractile, which can interfere with the formation of vascular structures in protein gels. Here we evaluate strategies to prevent gel contraction.

Transplantation of endothelial cells (EC) for therapeutic vascularization is a promising approach in tissue engineering but has yet to be proven effective in clinical trials. This cell-based therapy is hindered by significant apoptosis of EC upon transplantation as well as poor recruitment of host mural cells to stabilize nascent vessels. Here, we address these deficiencies by augmenting endothelial cell transplantation with dual delivery of vascular endothelial growth factor (VEGF) – to improve survival of transplanted EC – and monocyte chemotactic protein-1 (MCP-1) – to induce mural cell recruitment. We produced alginate microparticles that deliver VEGF and MCP-1 with distinct release kinetics and that can be integrated into a collagen/fibronectin (protein) gel construct for delivery of EC. Combined delivery of VEGF and MCP-1 increased functional vessel formation from transplanted EC and also led to a higher number of smooth muscle cell-invested vessels than did EC therapy alone. Despite the well-known role of MCP-1 in inflammation, these beneficial effects were accomplished without a long-term increase in monocyte/macrophage recruitment or a shift to a pro-inflammatory (M1) macrophage phenotype. Overall, these data suggest a potential benefit of combined delivery of MCP-1 and VEGF from EC-containing hydrogels as a strategy for therapeutic vascularization.

A successful engineered tissue construct must acquire a functional microvascular network that rapidly connects to the host circulation. Suspended endothelial cells (EC) within a construct will spontaneously organize into a microvascular system, but maturation depends on recruitment of host mural cells and is slow and limited. We have integrated protein delivery with transplantation of EC to create multifunctional hydrogels for use in tissue engineering. These gels consist of a protein matrix that facilitates formation of nascent endothelium and alginate microparticles capable of sustained release of multiple protein therapeutics. We have shown that delivery of vascular endothelial growth factor (VEGF) accelerates the development of graft perfusion. In this study, we examine the effects of concurrent delivery of VEGF and monocyte chemoattractant protein-1 (MCP-1). We show MCP-1 release can be achieved at physiologically relevant levels for 48-72 hours, enhancing recruitment of host monocytes, a cell population that may foster angiogenesis and vascular remodeling (arteriogeneisis). Furthermore, our results indicate combining VEGF with MCP-1 delivery facilitates improved engraftment of transplanted EC in a mouse model. These data herald the utility of combined delivery of arteriogenic (MCP-1) and angiogenic (VEGF) growth factors from EC-containing hydrogels as a promising biomaterial strategy for advanced tissue engineering.