My work is focused on the development of biofunctional nanoparticles, specifically small micelles decorated with biologically relevant peptides. I aim to understand these structures from a variety of perspectives. The first topic in my research is a fundamental investigation of the thermodynamics of micelles with a peptide headgroup, allowing predictions about micellar shape and size to be made. The second topic examines how to make and control micelles decorated with many different peptides. Can we arrange peptides in nano-scale patterns to mimic complex protein function? The third topic is the culmination of lessons learned from the first two: How do our nanoparticles interact with living organisms, like cells in a petri dish, or with tissue in a live mouse?
Below, I go into more depth on each of these topics, in reverse order.
I. Application: Stabilization of atherosclerotic plaques
Atherosclerotic plaques phenotypically differ in their susceptibility to rupture. While there are multiple mechanisms leading to rupture, some general conclusions can be made. Vulnerable plaques have a thin fibrous cap with adherent inflammatory cells. They plaques most commonly lose their stability due to rupture of this fibrous cap, whereas stable plaques have a thicker proteinaceous cap and lack inflammatory cells.
The cap of vulnerable plaques is composed primarily of fibrilar Types I and III collagen on the surface of the plaque. In contrast, Types IV and V are most common in stable plaques and their networked structure penetrates into the core of the plaque. Furthermore, Types I and III support adhesion of inflammatory cells through binding to the alpha 2 , beta 1 and alpha 3 , beta 1 integrin adhesion receptors on inflammatory cells.
My work seeks to stabilize vulnerable plaques through the use of self-assembled biofunctional structures. Stabilization in this context refers to three concepts: mechanical reinforcement, inflammatory reduction, and blocking of destructive enzyme activity. Mechanical reinforcement can be accomplished either by establishing covalent bonds between collagen strands, or with molecules that can reversibly associate (i.e. through triple helix formation) with collagen. Inflammatory reduction and enzyme blocking can be accomplished with molecules that bind and inactivate sites to which destructive enzymes or cells can bind.
To achieve stabilization, multifunctional molecules can be designed to present collagen with an arsenal of biological molecules in the concentrations and spatial arrangements appropriate to their function. My goal is to construct a general approach to self-assembly of biologically active molecules that allows control over the composition, structure and function of the end product. For this, protein analogous micelles are a good platform, mixing different peptide / alkyl tail combination and observing the micro-scale phases that can be achieved under various solution conditions.
II. Nano-scale patterns
While many platforms are available for the creation of biologically functionalized nanoparticles (quantum dots, polymeric particles, emulsified fluorocarbon particles), the protein-analogous micelle (see below) is unique in several ways. Self-assembly properties of surfactants allow a range of sizes and shapes to be created with the same class of molecules. As a fluid system, these particles can easily be created with mixed surfactants to achieve properties that may be different than that of the individual components. The goal of this project is to create a methodology for analyzing the nanoheterogeneity in mixed micelles. With such a framework in place, it will be possible to explore ways to create complex nano-distributions on a micelle surface that will improve functionality.
III. Fundamental thermodynamics
Our approach to biofunctional nanoparticles is to create protein-analogous micelles (PAMs). A PAM is the structure that results when a peptide is covalently linked to a hydrophobic moiety and self-assembled in aqueous solution above its critical micelle concentration (CMC). Such a structure is analogous to a protein in that its hydrophobic core provides structure for hydrophilic, functional peptides available to the external environment. A variety of shapes are seen in PAMs, ranging from spheres to cylinders to bilayers capable of forming vesicles. Cylindrical structures have been used by others to support cell growth while small spheres and liposomes have applications to targeting and disease therapy as discussed above. In particular, the size difference between micelles (~5 nm) and liposomes (~100 nm) can result in vastly different mechanisms of internalization by their cellular targets. Thus, size and shape are highly related to the function of the PAM.
PAMs are unique nanoparticles that present new challenges in their design. A PAM is a highly fluid structure in which a vast number of components can be mixed through a random self-assembly process. Several targeting and therapeutic peptides can be incorporated into a PAM with the same ease of assembly that a one-component PAM undergoes. Such fluidity may be exploited to allow components to rearrange spontaneously to create a specific, functional pattern. Furthermore, the high valency of peptides presented on the surface of a PAM can increase the efficacy of the particle. Interestingly, peptides incorporated into a PAM often exhibit more secondary structure than just the peptide alone, possibly resulting in increased biological function. In turn, changing the structure of the headgroup can affect the resulting aggregate geometry. Interaction between secondary structure and PAM shape is the major theme of this work.