New generations of synthetic biomaterials are being developed at a rapid pace for use as three-dimensional extracellular microenvironments to mimic the regulatory characteristics of natural extracellular matrices (ECMs) and ECM-bound growth factors, both for therapeutic applications and basic biological studies. Recent advances include nanofibrillar networks formed by self-assembly of small building blocks, artificial ECM networks from protein polymers or peptide-conjugated synthetic polymers that present bioactive ligands and respond to cell-secreted signals to enable proteolytic remodeling. These materials have already found application in differentiating stem cells into neurons, repairing bone and inducing angiogenesis. Although modern synthetic biomaterials represent oversimplified mimics of natural ECMs lacking the essential natural temporal and spatial complexity, a growing symbiosis of materials engineering and cell biology may ultimately result in synthetic materials that contain the necessary signals to recapitulate developmental processes in tissue- and organ-specific differentiation and morphogenesis.

https://sisu.ut.ee/sites/default/files/quantum/files/synthetic_biomaterials_as_instructive_extracellular_microenvironments_for_morphogenesis_in_tissue_engineering_lutolf_m.p_hubbell_j.a_2005_0.pdf

Synthetic biomaterials as instructive extracellular microenvironments for morphogenesis in tissue engineering

Blood vessels constitute the first organ in the embryo and form the largest network in our body but, sadly, are also often deadly. When dysregulated, the formation of new blood vessels contributes to numerous malignant, ischemic, inflammatory, infectious and immune disorders. Molecular insights into these processes are being generated at a rapidly increasing pace, offering new therapeutic opportunities that are currently being evaluated.

Angiogenesis in health and disease.

Biodegradable nanoparticles for drug and gene delivery to cells and tissue

We have used the pH-induced self-assembly of a peptide-amphiphile to make a nanostructured fibrous scaffold reminiscent of extracellular matrix. The design of this peptide-amphiphile allows the nanofibers to be reversibly cross-linked to enhance or decrease their structural integrity. After cross-linking, the fibers are able to direct mineralization of hydroxyapatite to form a composite material in which the crystallographic c axes of hydroxyapatite are aligned with the long axes of the fibers. This alignment is the same as that observed between collagen fibrils and hydroxyapatite crystals in bone.

http://science.sciencemag.org/content/sci/294/5547/1684.full.pdf

Self-assembly and mineralization of peptide-amphiphile nanofibers

The maturation of nascent vasculature, formed by vasculogenesis or angiogenesis, requires recruitment of mural cells, generation of an extracellular matrix and specialization of the vessel wall for structural support and regulation of vessel function. In addition, the vascular network must be organized so that all the parenchymal cells receive adequate nutrients. All of these processes are orchestrated by physical forces as well as by a constellation of ligands and receptors whose spatio-temporal patterns of expression and concentration are tightly regulated. Inappropriate levels of these physical forces or molecules produce an abnormal vasculature--a hallmark of various pathologies. Normalization of the abnormal vasculature can facilitate drug delivery to tumors and formation of a mature vasculature can help realize the promise of therapeutic angiogenesis and tissue engineering.

Molecular regulation of vessel maturation.

The development of tissues and organs is typically driven by the action of a number of growth factors. However, efforts to regenerate tissues (e.g., bone, blood vessels) typically rely on the delivery of single factors, and this may partially explain the limited clinical utility of many current approaches. One constraint on delivering appropriate combinations of factors is a lack of delivery vehicles that allow for a localized and controlled delivery of more than a single factor. We report a new polymeric system that allows for the tissue-specific delivery of two or more growth factors, with controlled dose and rate of delivery. The utility of this system was investigated in the context of therapeutic angiogenesis. We now demonstrate that dual delivery of vascular endothelial growth factor (VEGF)-165 and platelet-derived growth factor (PDGF)-BB, each with distinct kinetics, from a single, structural polymer scaffold results in the rapid formation of a mature vascular network. This is the first report of a vehicle capable of delivery of multiple angiogenic factors with distinct kinetics, and these results clearly indicate the importance of multiple growth factor action in tissue regeneration and engineering.

Polymeric system for dual growth factor delivery

The exposure duration and tissue distribution will likely dictate the success of vascular endothelial growth factor (VEGF) in therapeutic angiogenesis. We hypothesized that these variables can be regulated via the manner in which the VEGF is incorporated into polymer constructs (formed with a gas foaming technique) used for its delivery. VEGF was incorporated directly into poly(lactide-co-glycol- ide) (PLG) scaffolds or pre-encapsulated in PLG micro- spheres used to fabricate scaffolds. Protein release kinetics and tissue distribution were determined using iodinated VEGF. VEGF was positioned predominantly adjacent to scaffold pores when incorporated directly and was released rapidly (40 - 60% in 5 days). Pre-encapsulation led to the VEGF being more deeply embedded and resulted in a de- layed release. Alterations in polymer composition, scaffold size, and matrix composition generated minor variations in release kinetics. In vivo, the released VEGF generated local protein concentrations above 10 ng/mL at distances up to 2 cm from the implant site for the 21 days of the experiment, with negligible release into the systemic circulation, and significantly enhanced local angiogenesis. These data indi- cate that VEGF can be administered in a sustained and localized fashion in vivo, and the timing of VEGF delivery can be altered with the mechanism of incorporation into polymer scaffolds used for its delivery. © 2006 Wiley Peri- odicals, Inc. J Biomed Mater Res 79A: 176 -184, 2006

https://deepblue.lib.umich.edu/bitstream/handle/2027.42/55823/30771_ftp.pdf;sequence=1

Temporally regulated delivery of VEGF in vitro and in vivo.

Neovascularisation is a promising alternative therapeutic approach to re-establish blood flow in ischaemic tissues of patients suffering from coronary artery or peripheral artery disease. Often, these patients are not suitable candidates for current revascularisation procedures such as coronary angioplasty or bypass surgery. Several strategies are presently under investigation to induce vascularisation by stimulating the body's natural processes of vasculogenesis, angiogenesis and arteriogenesis. These strategies involve transplantation of various cell types into the ischaemic site and the delivery of recombinant angiogenic agents through direct protein administration or gene transfer. We will examine the basic approaches for these neovacularisation strategies and their therapeutic potential as demonstrated in animal models and human trials to date.

Alessandra B. Ennett

Papers

Polymeric system for dual growth factor delivery

Temporally regulated delivery of VEGF in vitro and in vivo.

Tissue engineering strategies for in vivo neovascularisation

Materials for Angiogenesis on Demand