Birth of a neuron: Imaging technique tracks nervous systemgrowth and repair, Cornell-Harvard group reports

A biomedical-imaging technique that would highlight the cytoskeletal infrastructure of nerve cells and map the nervous system as it develops and struggles to repair itself has been proposed by biophysics researchers at Cornell and Harvard universities.

Reporting in Proceedings of the National Academy of Sciences (PNAS June 10, 2003) , the researchers say that besides the new imaging technique’s obvious applications in studying the dynamics of nervous system development, it could answer the puzzle about which errant pathways initiate damage to brain cells, a key question about the onset of Alzheimer’s disease.

The PNAS report, “Uniform polarity microtubule assemblies imaged in native brain tissue by second harmonic generation microscopy,” is the work of Watt W. Webb, professor of applied physics at Cornell and leader of the research program. His laboratory collaborators in the School of Applied and Engineering Physics are graduate students Daniel A. Dombeck and Harshad D. Vishwasrao and research associate Karl A. Kasischke, M.D. Martin Ingelsson and Bradley T. Hyman of Massachusetts General Hospital, the largest teaching hospital of Harvard Medical School, also are collaborators.

In developing nerve cells, microtubules are the pioneering extensions from the cell body that grow to form two kinds of processes: the dendrites (branches that collect and conduct impulses inward to the cell body) and the axon (the single, longer process that conducts impulses away from the neuron cell body). Microtubules, made of tiny polymers, are a major part of the cellular cytoskeleton and are responsible for mechanical support. The proposed imaging procedure capitalizes on a structural polarity that exists in the polymers, making the characteristics at one end different from the other.

The researchers predict that their system to image microtubule polarity deep within living brain tissue could expedite the study of neuronal development and repair, the dynamics of migrating cells and neurodegenerative disease.

“Never before has there been a satisfactory way of detecting polarity in microtubule assemblies in living brain tissue,” says Webb. “Now we can follow the development of microtubules in vivo to see how architectural changes are occurring in nerve cells or in any other living cells where microtubules are found.”

Dombeck says that changes in microtubule polarity are the key to how neurons grow and find their orientation in the developing brain. The cellular processes are depicted in brilliant detail by the new imaging technique, he notes, because of a quantum physical optics phenomenon called second harmonic generation.

“In sound waves, we can hear the second harmonic of a vibrating guitar string when the guitar body resonates and produces a tone twice as high in pitch as the original tone. The same thing happens with light waves — although no one knew it until lasers were invented — when a laser beam hits certain kinds of materials in our bodies,” says Dombeck. “Sometimes a second harmonic is generated at exactly twice the energy, or half the wavelength, of the original light. Microtubules with uniform polarity generate a second harmonic, but microtubules with mixed polarity don’t. We get destructive interference instead, so that axons light up, and dendrites and everything else with nonuniform polarity in the microtubules stay dark.”

To demonstrate the imaging system, the biophysicists depicted axon bundles and individual axons in rat hippocampal brain tissue as well as axons growing from cell bodies in culture dishes. Other demonstrations in non-neuronal structures showed microtubules in the mitotic spindles of dividing cells, and microtubule-based cilia that line the inner walls of the aquaductus cerebri and waggle to propel fluid through the brainstem. When individual, successive images are assembled into a video, cell division can be followed and the fluid-propelling motion of brainstem duct cilia can be studied in detail.

At the most fundamental level, imaging studies might explain the role of microtubule-polarity in developing brain tissue, helping to decipher how the brain becomes “wired.” The technique may even reveal changes in microtubule polarity, showing where, when and why neurofilamentary tangles of axons form with precipitates of tau protein in the brains of Alzheimer’s patients, giving the new technique clinical significance, Webb says.

Funding support for the microtubule polarity studies came from the National Science Foundation, National Institutes of Health, Hellmuth Hertz Foundation, Wenner-Gran Foundation and the Alzheimer’s Association.