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 adsorptions 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.

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.

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.

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.

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.