Spinal-cord neurons from embryos of the African clawed frog and a small peptide found in the venom of the Chilean Rose tarantula were used in the research.
The findings are published in the January 2, 2013 issue of the Journal of Neuroscience.
The School of Medicine and Public Health research team, headed by neuroscience professor Timothy Gomez, is examining broadly how the nervous system, and in particular the spinal cord, develops. Scientists are interested in this area because a number of neurological disorders – such as various forms of autism – are linked to defects in how nerve cells grow and connect with each other. Understanding how nerve cells grow and connect in particular ways may ultimately lead to interventions to ensure the cells grow properly.
Nerve cells, or neurons, send and receive electrical signals from other neurons, muscles and a number of other tissues throughout the body. These signals travel along axons – long, slender projections that connect with other cells through synapses, which are junctions where chemical messages are transmitted. In the developing nervous system, axons elongate and are guided by “growth cones” at their tips. Growth cones are essential to direct axons toward their proper target cells.
Chemical cues in the environment of developing neurons help determine how and where growth cones move, and form synapses that will carry nerve impulses. But the Gomez team, working in the budding area of mechanobiology, decided to test whether the physical properties of the cellular environment may also activate and direct axon growth.
They focused their attention on mechanosensitive (MS) channels – particular proteins within the cell membrane that can respond to external stimuli by opening and closing to permit certain ions (calcium and sodium) to move into the cell.
Patrick Kerstein, a Neuroscience Training Program graduate student, used spinal-cord neurons from embryos of the African clawed frog, a frequent model for such studies. He also used a small peptide found in the venom of the Chilean Rose tarantula, as this peptide is known to specifically block MS channels. Kerstein blocked MS channels in living neurons and then recorded images to show the effects on various cellular signals and how nerve cells grow.
They found that blocking the MS calcium channels in these cells stimulates axon extension and promotes attractive turning (moving toward a stimulus) when presented as a gradient. Moreover, they showed that calcium influx through MS channels activates calpain, which is a protease that breaks apart a second protein, called talin. This chain reaction ultimately inhibits the growth of the neurons.
By adjusting the rigidity of the underlying material on which the cell was growing, they demonstrated that mechanical forces within the growth cone were activating these channels. The spinal-cord nerve cells grew faster on flexible material because there was less calcium coming through the channels. As the mechanical environment of developing embryos can vary widely between tissues, this result suggests that substratum rigidity – the stiffness of the underlying material the cell adheres to – may contribute to the normal assembly of nerve circuits.
“The results are significant because we have identified a component in the cell – a calcium channel – that is involved in sensing how rigid the underlying material is in the environment of developing axons,” said Gomez. “This suggests that both mechanical and chemical cues work together to assemble the developing nervous system.”
These findings are relevant to the understanding of the basic mechanisms that govern brain development, and have far-reaching implications for axon regeneration. Spinal-cord regeneration is known to be limited at the site of the scar by chemical factors deposited during tissue repair, while these new findings indicate the mechanical environment may also be limiting.
“Our findings suggest that the mechanical environment of the glial scar, which is known to be more rigid than uninjured tissue, may also limit regeneration, said Gomez. “We believe that treatments that target the MS channels or downstream inhibitory pathways may be good candidates to promote spinal-axon regeneration.”
In fact, Gomez notes that this tarantula peptide is currently being tested for treatment of other human diseases such as muscular dystrophy and certain cardiac arrhythmias.
The research was supported by National Institutes of Health grant NS41564, NIH F31NS053076 to Dr. Bridget Jacques-Fricke at the University of Minnesota, NIH T32GM007507 to the Neuroscience Training Program and a Dana Foundation grant to Dr. Gomez.
University of Wisconsin School of Medicine and Public Health