2007 Progress Report – Executive Summary
The National Center for Design of Biomimetic Nanoconductors (NCDBN) is devoted to developing technology associated with self-assembled membranes on nanoporous supports, and applying that technology to scientific exploration and fighting disease. We also do molecular design of protein channels and nanoscale design of smart nanoparticles, and may apply those activities outside of the supported membranes framework.
A concept at the heart of the work of NCDBN is the “functional protocell”, which was only indirectly part of our original vision, but has emerged as central within the past year, due to technical advances within our Center.
The functional protocell is defined as a nanoporous solid surrounded by a membrane. The cavities in the solid can be filled with any desired electrolyte up to the limit of solubility. The surrounding membrane can contain any combination of membrane proteins. Thus the functional protocell can be imbued with any array of intracellular and membrane processes that are desired. It can be considered analogous to either a cell or an organelle such as mitochondrion or a chloroplast (which started life as bacteria the order of 1018 nanoyears ago).
The phrase “functional protocell” should be distinguished from the term “protocell” that is commonly applied to a (so far hypothetical) minimal assembly of molecules that would have all the essential properties of cellular life, including self-replication. The “functional protocell” does not have all the essential properties of life, but would have specific designed properties that would make it technologically or biomedically useful. In some ways, the functional protocells would have the basic components of cells – a membrane that could do molecular recognition and transport, and an intracellular network of reactions that could sustain cell-like functioning. But it would have no properties that are not specifically built into it; especially the “functional protocell” would not selfreplicate.
The rationale for creating the functional protocell stems from the observation that cells have enormous flexibility and specificity of behavior compared to human-made entities on the same size scale. The key is the combination of a complex surface coating for specific recognition and transport tasks, and complex contents comprised of networks of molecules organized to do specific tasks. Cells and viruses are often questionable therapeutic agents because of unpredictable side effects. Living cells and viruses may be modified by engineering to give some desirable properties, but because their functioning and their effects on human function are not completely understood, undesirable side effects can not be fully predicted and eliminated. Completely synthetic functional protocells, on the other hand, would only have components that are put into them, and could be guaranteed not to replicate.
At this writing we have made the first prototype of the functional protocell and have made a preliminary assessment of possible therapeutic and scientific uses. We envisage that the functional protocell could be useful for:
“Smart” dialysis. In this application blood would be passed through a filter containing functional protocells. The protocell membranes would be equipped with proteins that would:
- use active transport to accelerate the passage of materials that pass through existing dialysis membranes, such as urea and potassium ions.
- take up toxic materials that may be present in the blood in low but damaging concentrations. A notable possibility here is to deploy in the smart membrane heavy metal transport proteins from bacteria or from plants to cleanse the blood of toxic metals. Essentially this would extend the technique of phytoremediation (using biological transport to clean up pollution) to dialysis. Here the application is not limited to renal disease, but could be used to remediate other poisoning situations such as lead. It should also be possible to absorb cytotoxins in the membranes
After a dialysis session the protocells would be either “recharged” (if feasible), or replaced.
Combating Infection. In this mode of functioning the protocell would have a surface coating that would recognize and bind to viruses or other pathogens. Once bound, a reaction would be triggered that would neutralize or kill the pathogen.
One can reasonably envisage several modes of functioning, as follows:
- The protocells would be deployed in the airway or the GI tract, for pathogens that reside in those spaces.
- The protocells could be injected and exert their action in extracellular fluid.
- The protocells could be injected and engineered to
- The protocells could be used in a hemodialysis machine. In this instance the blood would be passed through a filter containing a suspension of honeypot protocells and blood borne pathogens would be removed by association with the surface membranes of the protocells. An obvious disadvantage of the dialysis mode would be the need to connect the patient to the machine for the treatment. On the other hand, a great advantage to the dialysis mode would be that the protocells themselves would not enter the general circulation, so that there would not be concerns about the side effects of residual protocell contents in the body.
- The protocells could be injected.
One can also envisage a “benign invader” functional protocell that could target pathogens inside cells and could deliver drugs to cell interiors.
Building a Biocompatible Battery that would be recharged by biological metabolism. This was actually our founding mission, and we have made progress in the last year on both experimental and theoretical fronts. On the experimental front we have been able to create a network of synthetic cells that produces a voltage from one side of the network to the other. On the theoretical side we have made a dynamical model of the electric organ of the electric eel (the biological proof of concept for the biobattery) that can serve as an engineering design tool for the synthetic biobattery. On the other hand we have come to the realization that the functional protocells of the biobattery, and the scaffold to house them, constitute exceptionally difficult problems relative to other functional protocell applications, by virtue of the fact that the cells must be polar and contiguous in a constrained geometric relationship.
Another initiative that has emerged in NCDBN in the past year is the design of a direct protein therapy for cystic fibrosis. The common genetic defect in cystic fibrosis is in a gene that codes for the chloride channel CFTR. While the gene is expressed in several epithelia, the lethal symptoms are in the airway. The airway mucus becomes drier and more viscous, and thus cannot be moved by the cilia towards the mouth to clear inhaled foreign materials. The mucus becomes susceptible to opportunistic infection. Researchers in NCDBN have acquired the ability to convert a bacterial toxin, alpha-hemolysin, into an anion selective ion channel. The cystic fibrosis initiative within NCDBN is to explore the question of whether this finding by NCDBN can be the basis of direct protein therapy, in which a designed synthetic protein channel would play the role that the defective CFTR can not.
In another medical-directed initiative, NCDBN is utilizing an experimental technique developed by us (individually addressable arrays of membrane patches on a single nanoporous surface) and a computational technique newly applied by us to membranes (Mean Field Langevin Dynamics) to study in more detail than has previously been possible the changes in biological membrane organization that accompany oxidative stress. Changes in membrane properties under oxidative stress are implicated in many diseases, so we believe that the basic knowledge that we generate will ultimately be medically applied.
An important feature of our Center is the synergy between experiment and theory. Our cystic fibrosis initiative will depend equally on experiment and theory to build a useful model for the lung epithelium, our molecular design of channels will be guided by theoretical simulation of the properties of putative channel structures, as well as the above mentioned oxidative stress in membranes.
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