The classic method for characterizing gait in mice is paw inking, where the mouse's paws are dipped into ink and their pawprints are observed as they walk on a piece of paper. The investigator can then measure the distance between pawprints. This is relevant in characterizing murine models of human disorders, such as Lou Gehrig's disease, with which patients have an abnormal gait. Other indices of locomotor function in mice are the rotor-rod test and the wire-grip test, neither of which are particularly clinically relevant.

Mouse Specifics Inc., Boston, a developer of phenotyping tools for analyzing the effects of genes and drugs in mouse models of human disease, is developing the Murine Gait Imaging System, a noninvasive system for characterizing gait in mice.

The Amyotrophic Lateral Sclerosis (ALS) Association, Calabasas Hills, Calif., is providing partial funding for this video-based system which is being developed in collaboration with The Jackson Laboratory, Bar Harbor, Maine, for its National Institutes of Health (NIH)-funded Neuroscience Mutagenesis Facility.

"Since the paw-inking method is not very robust and provides only a 2D static indication of gait, Jackson Labs wanted something more comprehensive," says Thomas Hampton, PhD, founder, and CEO of Mouse Specifics.

The imaging technology consists of a small treadmill with a transparent belt. "We built the treadmill to accommodate a camera between the belt and the underside of the mouse so we can videotape the mouse running," says Hampton.

The imaging system allows one to visually watch four paws appearing and disappearing as the mouse takes sequential steps through its gait cycle. Part of the technology is based on video subtraction methodologies dependent on the color of the paw.

"The software we developed looks at the color of the paws, subtracts out everything that doesn't match paw color, and converts the whole movie to black and white. So you end up with a black and white movie of the paw prints. As the paw comes down on the belt, you see progressively more of a white pawprint and as the paw leaves the belt, just prior to the swing phase, you see progressively less of a white pawprint," says Hampton.

There are several phases in the gait cycle. The portion of the paw coming down during stance is called the breaking phase; as the mouse runs forward, the limb is in the propulsion phase; the duration that the paw is in the air is termed the swing phase; and the amount of time a paw is on the ground is called stance time. "What's neat is that each one of these phases correlates to physiological mechanisms," says Hampton.

Initial pilot data that Mouse Specifics has collected demonstrate abnormal gait dynamics in SOD1 (superoxide dismutase 1) mice, a mouse model of ALS. The study showed significant differences in gait in mice at 9.5 weeks of age compared to controls. "That's significant, because the other methods, such as rotor-rod and wire-grip, aren't able to show differences between normal and diseased mice until 13 to 15 weeks of age. Our system provides early detection, so one can start giving treatments earlier and investigate efficacy of treatments by preventing these differences in gait indices," says Hampton.

Hampton has also received funding from the National Down Syndrome Society, New York. Children with Down syndrome develop a waddling gait and a flat-footed posture. Hampton was a recent recipient of the Charles J. Epstein Down Research Award for the application of the Gait Imaging System to investigate gait defects in trisomic mice, with an eye to better understanding and treatment of the gait defects seen in children with Down syndrome.

The imaging system is capable of a very high throughput, enabling an unlimited number of mice to be tested with one treadmill. A normal mouse will take about four steps every second to achieve a typical walking speed of 24 cm/s. Sufficient images for analyses are collected within seconds. Because they are not compromised with anesthesia in the process, the mice can be used immediately for other experiments.

"We think the potential for this system is huge. We are looking at mouse models for arthritis, muscular dystrophy, diabetic neuropathies, pain, and orthopedic applications, among others," says Hampton.

Company: Mouse Specifics Inc., Boston
Web site: www.mousespecifics.com