Nanomedicine Development Centers
Cell Propulsion LabCenter for Cell ControlCenter for Protein Folding MachineryNanomedicine Center for Nucleoprotein MachinesNanotechnology Center for Mechanics in Regenerative MedicineNational Center for Design of Biomimetic NanoconductorsOptical Control of Biological FunctionPhi29 DNA-Packaging Motor for Nanomedicine

Cell Propulsion Lab

Executive Summary

Living cells are extremely sophisticated sensor-actuator devices that detect specific environmental cues, process this information, and generate specific mechanical responses such as growth, shape change, or directed movement. These processes are controlled by networks of signal transduction and cytoskeletal proteins that form a dynamic, self-organizing system. Our long-term goal is to understand the fundamental engineering principles underlying these nanoscale molecular control systems. If we understand these principles, we may be able to alter these systems to "reprogram" cells such that they carry out new therapeutic functions. Alternatively, we may be able to build artificial, biologically-inspired cell-like, nanoscale assemblies that can perform targeted therapeutic functions. In the long-term, the ability to precisely program cells or synthesize molecular assemblies with cell-like behaviors would have broad, revolutionary, therapeutic potential. As our primary testbed, our center is focusing on understanding how cells achieve signal-guided movement (chemotaxis), with the goal of learning how to reprogram or build cells that can function as smart search and delivery vehicles.

Introduction

Fantastic Voyage

One of the most popular and captivating visions of nanomedicine is that epitomized by the 1966 film "Fantastic Voyage," in which nanorobots could be injected into our bodies to hunt down pathogens and tumors, or restore damaged organs and nerves. Of course, this was science fiction, but is this really the future? Maybe, but probably not as envisioned in this film. For example, it is unrealistic to build a minature robot operating at the nanoscale that exactly resembles the macroscopic mechanical machines that we encounter everyday (Figure 1a). This is because the behavior of materials at these dimensions differs significantly from behavior at larger scales -- molecular structures operate in unique and interesting ways. Nonetheless, something akin to therapeutic microscale robots does already exist — our own cells are complex mechanical and sensing devices that can carry out highly complex tasks (Figure 1b). For example, our immune cells can detect infection, move to these sites, and respond by destroying the infectious agent either by phagocytosis or by secreting countermeasures such as antibodies. Cells can form new repair structures, such as clots and bone. These are only a few of the many tasks that our cells, which are about 10-40 microns in diameter, are genetically programmed to carry out.

Figure 1
Figure. 1. Visions of nanomedicine. (a) artist rendition of an imaginary nanorobot injecting a payload into a red blood cell. (b) Natural cells, such as this neutrophil sending out protrusions to attack invading bacteria, are capable of functioning as therapeutic agents, except that we do not know how to reprogram them to carry out new or modified functions.

Employing cells for targeted therapeutic purposes, however, faces several fundamental barriers. Real robots are useful because we can program them to carry out specific tasks that we desire. We can also program them with highly specific sets of instructions, so that this task is carried out only when, where, and how we specify. What we lack in the case of cellular "robots" is this ability to reprogram and precisely control the behavior of cells. Thus, we cannot modify cells to carry out novel tasks that they normally do not perform. Natural cells cannot always function as we wish — our immune cells can be tricked by evasive tumors or pathogens; certain nerve cells fail to regenerate — and it is these missing functions that are therapeutically most critical. Our inability to program cells also prevents us from specifying that a cell only carryout its function under a very precise time and set of conditions. The execution of a task at the wrong time and place might do no good and could be harmful. An essential element of a robot's utility lies in our ability to precisely control the robot. Thus, the overall goal of our center is to learn how cellular sensor-actuator systems are designed, and how they could be reprogrammed.

Directed cell movement as a testbed

As a testbed for understanding how to reprogram cells we are focusing on directed cell motility. Many cells can detect specific extracellular signals, process this information, and use the information to activate complex mechanical programs, such as directional cell movement or morphing into a new shape. Such programs are critical in immune cell movement and phagocytosis; they are also used in developing cells like neurons as they send out axons over great distances towards specific targets.

Pathway to Medicine: Programmed search and delivery cells

If we could reprogram cells to detect and move to novel signals, we could generate powerful search and delivery "robots". It would be extremely useful to have cells that could search the body for an elusive target, such as a microscopic tumor. Engineered smart cells could in principle be designed to detect and integrate a host of different information to pinpoint the tumor, including detection of generic environmental conditions (such as low oxygen, which is typical in tumors) and detection of tumor specific molecules. These could be integrated and linked to the cytoskeletal movement machinery to allow for efficient search of the tumor. Once at site, the cells could be used to deliver a number of payloads, ranging from dyes for imaging of the tumor, to chemotherapeutics that would kill the tumor.

How can we program cellular robots?

Macroscopic robots are controlled by electronic and mechanical subdevices that are linked by wires (Fig. 2a). What controls cellular behavior is, at the surface, very different. Cells contain a complex, self-organizing network of regulatory and mechanical molecules. Thus, to reprogram cells, we need to understand how this intracellular nanoscale machinery works, how it is wired, and how it can be manipulated. The overriding goal of our center is to learn how to program cellular systems, or more appropriately, cell-like systems, so that we can build novel and precisely controlled therapeutic agents. Ultimately, to fulfill this vision, we need to learn how to build and program cells or cell-like assemblies in which we can flexibly and precisely tune what they sense and how they respond.

While understanding how to program cells is a daunting task, genomic research over the last decade has revealed that biological control circuits are surprisingly modular — the same molecular toolkits are used repeatedly to generate diverse biological functions. This suggests that there is a fundamental logic to how complex biological tasks are programmed, and that we have the potential to understand and exploit this programming logic. For a behavior like directed cell motility, environmental cues are sensed by receptor molecules, this information is processed by signaling networks — in this case primarily by kinase/phosphatase and GTPase networks — which, in turn, engage and activate the actin cytoskeleton, which functions as a mechanical actuation system that ultimately moves the cell. Thus these cellular systems have functional modules that are analogous to those found in macroscopic robots (Fig. 2b), although these cellular control modules are composed of nanoscale molecules. Our goal is to understand how these molecular modules work and how they are linked together. Then, perhaps we can link them in novel ways to build cells that show novel, targeted behavior. Interestingly, this kind of relinking of components resembles a major mechanism in the natural evolution of new cellular responses.

Figure 2
Figure 2. Both robots (a) and cells (b) are built from components and subsystems with distinct modular functions (e.g sensing, processing, mechanical output), although the composition nature of these components is very different. Different robots can be built by linking these components in different ways. Similarly it may be possible to build cells with novel behaviors by linking cellular subsystems in novel ways (as appears to happen in evolution).

NDC Goals

We are pursuing the following specific aims:

Understand the design principles underlying the natural molecular sensor-actuator systems used to generate directed cell motility

We are studying fundamental mechanisms of the signaling and actin cytoskeletal systems that are used to build circuits for cell movement and shape changes. We are interested in understanding how subtle gradients of signals are interpreted by the cell to yield directed, asymmetric activation of the actin cytoskeleton.

Determine ways to relink and reprogram cellular signaling subsystem components

Cell signaling systems are built from receptors, kinases, phosphatases, GTPases, and other regulatory components. How are these components wired in specific ways, and how can we rewire them? A growing body of evidence suggests that many of these components are functionally linked via specific protein interaction modules. These modules can organize multiple proteins into physical complexes and can also gate the activity of signaling enzymes. Thus, we are trying to use a toolkit of protein interaction domains to link signaling components in new ways to generate new cellular behaviors. We are also developing engineered protein-protein interaction modules that are optimized for cellular engineering (i.e. have minimal cross-talk with endogenous molecules).

Determine ways to regulate force generation systems

Ultimately to bring about movement, the signaling systems must activate the actin cytoskeleton in a spatially precise manner, and in a way that yields force and protrusion of the membrane. We are investigating how regulated polymerization systems can be used to perform spatially directed work in a cell-like environment. We are investigating the use of non-actin polymers to perform work in a cell. We are also investigating whether hybrid materials (synthetic nanomaterials linked to biological regulatory components) could be used to generate nanostructures that assemble in a regulated manner to perform mechanical work at the nanometer to micron scale.

Apply this understanding to build cells or cell-like particles with novel motility control that can be used for targeting within an animal

We are taking both a top-down and bottom up approach towards building minimal motile particles that can be used as search and delivery vehicles. In the top-down approach, we are trying to reprogram motile cells (neutrophils) so that they move towards novel signals. We are also trying to generate cytoplasts (cell fragments lacking nuclei) that can move towards specified targets. In the bottom-up approach, we are trying to reconstitute minimal cell-like molecular assemblies that can show regulated movement or shape change, from either biological molecules or from nanomaterial components that are regulated in a biologically-inspired manner. These cells, cell-fragments, or assemblies, could potentially be used as search-and-delivery vehicles in the body. We will test the particles engineered using both approaches for targeting functions within a mouse.

Accomplishments

To date we have made significant progress in characterizing the design principles of natural motility control systems, and have made significant first steps in learning how to manipulate the relevant signaling and force generating subsystems.

Characterization of natural cell motility systems

Reprogramming cell signaling networks controlling actin motility

Reconstitution/Engineering of regulated force generation systems in vitro

References

Background

Current NDC papers

Contact Information

Center Website

Key Investigators