Cellular Biophysics

Axon in collagen Axon navigating in a 3D collagen gel. The actin from the axon is stained in green, the collagen fibrils in red.

Axon Motility and Guidance
How do axons sense molecular gradients and use these signals to find their correct targets? How does the extracellular matrix modulate axon motility? In collaboration with Herb Geller an the NIH, we are studying the dynamics of axons navigating in collagen matrices. We have developed a novel technology for generating precisely controlled molecular gradients in collagen gels, and are using it to study the behavior of developing and regenerating axons. The growing neurons are led by a complex, dynamic structure called a growth cone, and we are trying to understand how it works as a sensing element. Recently we have begun investing the biomechanics of the axon outgrowth through live cell imaging of neurons transfected with fluorescently labeled cytoskeletal proteins. Funded by NIH. Click on the link for recent publications.

Giardia lamblia is one of the most prevalent intestinal protozoan pathogens worldwide, causing widespread infection in areas without access to clean water. In much of the world, the lack of clean water results in nearly universal infection of the population by the age of three. After ingestion of the infective cyst stage by the host, the parasite differentiates in the lumen of the small intestine into the trophozoite form, attaches to intestinal epithelial cells (the lower intestine) and replicates. Its presence often results in severe gastrointestinal symptoms, including diarrhea, vomiting and weight loss.
In collaboration with Heidi Elmendorf’s lab, we are helping to answer questions about Giardia’s cytoskeleton and how attaches itself to the intestinal wall. Ongoing experiments are helping to determine the mechanism of attachment employed by Giardia which is believed to be, for the most part, due either to a suction mechanism or to a gripping mechanism. Funded by NIH.

Living P. falciparum parasite inside a human red blood cell

In collaboration with Professor Paul Roepe and his group, we are imaging the structure and dynamics of the malarial parasite after invasion of a red blood cell. Using a high speed, high resolution confocal microscope, we have investigated the volume regulation of intracellular compartments in order to understand the mechanism of drug resistance. Funding from NIH.

structural Dynamics

Rat tail collagen network adhered to a polyacrylamide gel substrate

Soft biopolymer networks undergo substantial bulk stiffening and contraction when subject to shear strain.  These nonlinear rheological signatures have been well-documented for a wide range of semiflexible and stiff biopolymers, but the underlying geometric fiber rearrangements have not been measured and the resulting propagation of stress through the microscopic network has not been experimentally assessed.  We apply precisely controlled shear strain to collagen gels adhered to thin elastic polyacrylamide gel substrates embedded with fluorescent microspheres while simultaneously imaging the three-dimensional networks with a coupled confocal-rheometer. We observe dramatic network realignment in the direction of shear driven by the buckling, rotation, and stretching of undulated constituent fibers and simultaneously measure stress inhomogeneities at the collagen-polyacrylamide interface.  With the addition of fluorescent tracer particles in the collagen network, we observe nonaffine network deformations strongly correlated with the concurrently measured bulk stiffening.  These observations elucidate the physical mechanisms governing contraction, strain-stiffening, and our recent observation of the system-size dependence of the stiffening effect. Watch Movie


Image of a composite gel made of f-actin (magenta) and microtubules (green)

Hydrogel Mechanics
Hydrogels are three dimensional hydrated crosslinked or entangled polymer networks, in which most of the volume is occupied by fluid. We study the mechanics of composite hydrogels, formed by a network of polymerized actin and microtubules. We also study the interaction of the polymer network with the fluid in polyacrylamide. The tools we use for this include: optical traps, confocal microscopy and rheology.