Engineering dynamics of growth factors and other therapeutic ligands

Growth factors and early mesoderm morphogenesis: insights from the sea urchin embryo

The early morphogenesis of the mesoderm is critically important in establishing the body plan of the embryo. Recent research has led to a better understanding of the mechanisms that underlie this process, and growth factor signaling pathways have emerged as key regulators of the directional movements of mesoderm cells during gastrulation. In this review, we undertake a comparative analysis of the various essential functions of growth factor signaling pathways in regulating early mesoderm morphogenesis, with an emphasis on recent advances in the sea urchin embryo. We focus on the roles of the vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) pathways in the migration of primary mesenchyme cells and the formation of the embryonic endoskeleton. We compare the functions of VEGF and FGF in sea urchins with the roles that these and other growth factors play in regulating mesoderm migration during gastrulation in Drosophila and vertebrates.

Combination antibody treatment down-regulates epidermal growth factor receptor by inhibiting endosomal recycling

Due to its common dysregulation in epithelial-based cancers and extensive characterization of its role in tumor growth, epidermal growth factor receptor (EGFR) is a highly validated target for anticancer therapies. There has been particular interest in the development of monoclonal antibodies (mAbs) targeting EGFR, resulting in two approved mAb-based drugs and several others in clinical trials. It has recently been reported that treatment with combinations of noncompetitive mAbs can induce receptor clustering, leading to synergistic receptor down-regulation. We elucidate three key aspects of this phenomenon. First, we show that highly potent combinations consisting of two noncompetitive mAbs that target EGFR domain 3 reduce surface receptor levels by up to 80% with a halftime of 0.5-5 h in both normal and transformed human cell lines to an extent inversely proportional to receptor density. Second, we find the mechanism underlying down-regulation to be consistent with recycling inhibition. Third, in contrast to the agonism associated with ligand-induced down-regulation, we demonstrate that mAb-induced down-regulation does not activate EGFR or its downstream effectors and it leads to synergistic reduction in migration and proliferation of cells that secrete autocrine ligand. These new insights will aid in ongoing rational design of EGFR-targeted antibody therapeutics.

Spatiotemporal control over growth factor signaling for therapeutic neovascularization

Many of the qualitative roles of growth factors involved in neovascularization have been delineated, but it is unclear yet from an engineering perspective how to use these factors as therapies. We propose that an approach that integrates quantitative spatiotemporal measurements of growth factor signaling using 3-D in vitro and in vivo models, mathematic modeling of factor tissue distribution, and new delivery technologies may provide an opportunity to engineer neovascularization on demand.

Long-range signal transmission in autocrine relays

Intracellular signaling induced by peptide growth factors can stimulate secretion of these molecules into the extracellular medium. In autocrine and paracrine networks, this can establish a positive feedback loop between ligand binding and ligand release. When coupled to intercellular communication by autocrine ligands, this positive feedback can generate constant-speed traveling waves. To demonstrate that, we propose a mechanistic model of autocrine relay systems. The model is relevant to the physiology of epithelial layers and to a number of in vitro experimental formats. Using asymptotic and numerical tools, we find that traveling waves in autocrine relays exist and have a number of unusual properties, such as an optimal ligand binding strength necessary for the maximal speed of propagation. We compare our results to recent observations of autocrine and paracrine systems and discuss the steps toward experimental tests of our predictions.

Disposition and receptor-site binding of (125)I-EGF after topical administration to skin wounds

The rhEGF topical delivery systems have been hindered by a number of shortcomings which have led to the search of new development strategies. In this study we report the evaluation of cumulative profiles of 10, 5 and 1 microg/ml solutions of (125)I-rhEGF, in a rat full-thickness skin wound model, as well as the drug-induced modulation in the expression of the EGF receptor after lesion. The tissue-associated radioactivity, expressed as the percentage of the dose administered per grams of tissue (%D/g), peaks at 2 h after administration of all doses. (125)I-rhEGF degraded species were detected chromatographically, but no diffusion of the peptide to the surrounding skin was documented. Despite the dose, the EGF receptor expression was increased within 2 h after wounding, followed by a slow decline up to 12 h below baseline. Twelve hours after punch, differences were evident between all treated groups and control. These results demonstrate that (125)I-rhEGF saline solutions are rapidly cleared from application sites, probably by protease-driven cleavage and receptor-mediated endocytosis. Finally, we must be aware that the results herein discussed should be taken into account during the drug delivery system design in order to guarantee the necessary steady-state rhEGF levels upon wound healing process.

Comparative mitogenic potencies of EGF and TGF alpha and their dependence on receptor-limitation versus ligand-limitation

Transforming growth factor alpha (TGF alpha) has been reported to be a more potent agonist when compared to epidermal growth factor (EGF) in several systems while acting via their common receptor, the epidermal growth factor receptor (EGFR). It has been postulated that this increased potency is mediated by the increased recycling of EGFR upon activation by TGF alpha as against receptor activation by EGF. The authors test this hypothesis by simultaneously measuring mitogenesis and the dynamics of surface receptor number in response to these ligands in NR6 mouse fibroblasts expressing the EGFR. The data demonstrates that increased receptor recycling due to endosomal dissociation of TGF alpha can indeed realise an increased mitogenic potency relative to EGF under appropriate cellular and experimental conditions (i.e. situations in which the increase in the number of occupied receptors due to receptor sparing by TGF alpha represents additional mitogenic signalling capacity). However, this difference in receptor trafficking does not uniquely determine the relative potencies of these ligands since TGF alpha is a less potent mitogen compared to EGF when experimental conditions are dominated by the effects of ligand trafficking on growth factor availability. Thus, the relative potencies of these growth factors are determined in a given context by the relative importance of ligand and receptor trafficking effects which determine the availability of these signalling components. These results are consistent with a suggested model of hormone responsiveness which favours dissociative ligands (such as TGF alpha) in receptor-limited situations and non-dissociative ligands (such as EGF) in the case of ligand limitation.

Advances in hematopoietic stem cell culture

Recent advances in our understanding of the earliest stages of hematopoietic cell differentiation, and how these may be manipulated under defined conditions in vitro, have set the stage for the development of robust bioprocess technology applicable to hematopoietic cells. Sensitive and specific assays now exist for measuring the frequency of hematopoietic stem cells with long-term in vivo repopulating activity from human as well as murine sources. The production of natural or engineered ligands through recombinant DNA and/or combinatorial chemistry strategies is providing new reagents for enhancing the productivity of hematopoietic cell cultures. Multifactorial and dose-response analyses have yielded new insight into the different types and concentrations of factors required to optimize the rate and the extent of amplification of specific subpopulations of primitive hematopoietic cells. In addition, the rate of cytokine depletion from the medium has also been found to be dependent on the types of cell present. The discovery of these cell-type-specific parameters affecting cytokine concentrations and responses has introduced a new level of complexity into the design of optimized hematopoietic bioprocess systems.

Localized delivery of proteins in the brain: can transport be customized?

Certain central nervous system (CNS) diseases are characterized by the degeneration of specific cell populations. One strategy for treating neurodegenerative diseases is long-term, controlled delivery of proteins such as epidermal growth factor (EGF) and nerve growth factor (NGF). Since proteins permeate through brain capillaries very slowly, local administration using polymeric implants, continuous infusion pumps, or transplanted, protein-secreting cells may be required to achieve therapeutic concentrations in the tissue. The efficiency of local distribution, and hence effectiveness of local therapy, depends on the rate of protein migration through tissue. The rate of dispersion of molecules in a quiescent, isotropic medium can be characterized by the molecular diffusion coefficient, D, which can be measured by techniques such as quantitative autoradiography, iontophoresis, and fluorescence photobleaching recovery (FPR). These methods are reviewed, with an emphasis on their application to measurement of D for proteins in the brain. Biophysical techniques yield quantitative descriptions of local protein distribution and may enable discrimination of mechanisms of protein transport in the brain. This capability suggests a new paradigm for design of protein therapies, in which proteins and delivery systems are collectively customized to provide sustained protein availability over predetermined volumes of tissue.