Endocytic Relay as a Potential Means for Enhancing Ligand Transport through Cellular Tissue Matrices: Analysis and Possible Implications for Drug Delivery

Engineering dynamics of growth factors and other therapeutic ligands

Peptide growth factors and other receptor-binding cytokine ligands are of interest in contemporary molecular health care approaches in applications such as wound healing, tissue regeneration, and gene therapy. Development of effective technologies based on operation of these regulatory molecules requires an ability to deliver the ligands to target cells in a reliable and well-characterizable manner. Quantitative information concerning the fate of peptide ligands within tissues is necessary for adequate interpretation of experimental observations at the tissue level and for truly rational engineering design of ligand-based therapies. To address this need, we are undertaking efforts to elucidate effects of key molecular and cellular parameters on temporal and spatial distribution of cytokines in cell population and cell/matrix systems. In this article we summarize some of our recent findings on dynamics of growth factor depletion by cellular endocytic trafficking, growth factor transport through cellular matrices, and growth factor production and release by autocrine cell systems. (c) 1996 John Wiley & Sons, Inc.

Controlled growth factor delivery for tissue engineering

Growth factors play a crucial role in information transfer between cells and their microenvironment in tissue engineering and regeneration. They initiate their action by binding to specific receptors on the surface of target cells and the chemical identity, concentration, duration, and context of these growth factors contain information that dictates cell fate. Hence, the importance of exogenous delivery of these molecules in tissue engineering is unsurprising, considering their importance for tissue regeneration. However, the short half-lives of growth factors, their relatively large size, slow tissue penetration, and their potential toxicity at high systemic levels, suggest that conventional routes of administration are unlikely to be effective. In this review, we provide an overview of the design criteria for growth factor delivery vehicles with respect to the growth factor itself and the microenvironment for delivery. We discuss various methodologies that could be adopted to achieve this localized delivery, and strategies using polymers as delivery vehicles in particular.

Dpp gradient formation by dynamin-dependent endocytosis: receptor trafficking and the diffusion model

Developing cells acquire positional information by reading the graded distribution of morphogens. In Drosophila, the Dpp morphogen forms a long-range concentration gradient by spreading from a restricted source in the developing wing. It has been assumed that Dpp spreads by extracellular diffusion. Under this assumption, the main role of endocytosis in gradient formation is to downregulate receptors at the cell surface. These surface receptors bind to the ligand and thereby interfere with its long-range movement. Recent experiments indicate that Dpp spreading is mediated by Dynamin-dependent endocytosis in the target tissue, suggesting that extracellular diffusion alone cannot account for Dpp dispersal. Here, we perform a theoretical study of a model for morphogen spreading based on extracellular diffusion, which takes into account receptor binding and trafficking. We compare profiles of ligand and surface receptors obtained in this model with experimental data. To this end, we monitored directly the pool of surface receptors and extracellular Dpp with specific antibodies. We conclude that current models considering pure extracellular diffusion cannot explain the observed role of endocytosis during Dpp long-range movement.

Resonance energy transfer for assessing the molecular integrity of proteins for local delivery

It remains unclear whether the limitations to the therapeutic potential of angiogenic growth factors stem from pharmacokinetic concerns related to inadequate delivery or from a reduced sensitivity of target tissues. Here, we report a novel method using resonance energy transfer to assess the molecular integrity of proteins after local delivery. As an example, we labeled fibroblast growth factor-2 with a fluorescent donor and nonfluorescent acceptor pair, tetramethylrhodamine and QSY-7, and demonstrate in an ex vivo bovine carotid artery model that this growth factor is not limited by proteolytic constraints imposed by the tissue. Our data indicate that FGF-2 is unlikely to be degraded within the arterial wall and suggest that pharmacokinetic limitations alone cannot fully explain the muted response seen thus far in therapeutic angiogenesis. In general, resonance energy transfer may serve as a novel approach to assess the molecular integrity of protein-based therapies in local delivery.

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.

Effective intercellular communication distances are determined by the relative time constants for cyto/chemokine secretion and diffusion

A cell's ability to effectively communicate with a neighboring cell is essential for tissue function and ultimately for the organism to which it belongs. One important mode of intercellular communication is the release of soluble cyto- and chemokines. Once secreted, these signaling molecules diffuse through the surrounding medium and eventually bind to neighboring cell's receptors whereby the signal is received. This mode of communication is governed both by physicochemical transport processes and cellular secretion rates, which in turn are determined by genetic and biochemical processes. The characteristics of transport processes have been known for some time, and information on the genetic and biochemical determinants of cellular function is rapidly growing. Simultaneous quantitative analysis of the two is required to systematically evaluate the nature and limitations of intercellular signaling. The present study uses a solitary cell model to estimate effective communication distances over which a single cell can meaningfully propagate a soluble signal. The analysis reveals that: (i) this process is governed by a single, key, dimensionless group that is a ratio of biological parameters and physicochemical determinants; (ii) this ratio has a maximal value; (iii) for realistic values of the parameters contained in this dimensionless group, it is estimated that the domain that a single cell can effectively communicate in is approximately 250 micron in size; and (iv) the communication within this domain takes place in 10-30 minutes. These results have fundamental implications for interpretation of organ physiology and for engineering tissue function ex vivo.

Controlling receptor/ligand trafficking: effects of cellular and molecular properties on endosomal sorting

Receptor-mediated endocytosis is the process by which cells internalize ligands that have specifically interacted with cell surface receptors. Within intracellular endosomal compartments, receptor/ligand complexes can be targeted to lysosomes for degradation, recycled back to the plasma membrane, or sorted separately to these destinations. We have developed a mechanistic mathematical model that can account for the spectrum of experimentally observed endosomal sorting outcomes. The central hypothesis of this model is that receptors may be selectively retained by putative endosomal retention components and that this process may be modulated by receptor occupancy. This hypothesis is supported by the recent discovery of an endosomal retention component involved in targeting epidermal growth factor receptors to lysosomes. In this paper, we use the model to predict how changes in key cellular and molecular parameters alter sorting outcomes. This analysis provides guidance for rationally modulating the sorting process in a variety of biomedical applications, either by the manipulation of cellular parameters or the design of ligand properties.