In my laboratory we use the tools of modern molecular and cell biology to study axonal development. Neurofilaments are one of the principal components of the axonal cytoskeleton, and their molecular composition changes during axonal development. We hypothesize that these changes influence the structural properties of growing axons, and help them to accomodate the varying requirements for plasticity and stability that arise during development. To explore this hypothesis, we study the neurofilaments of axons in the frog, Xenopus laevis.

Xenopus laevis

Electron micrograph of X. laevis optic axon. nf, neurofilaments; mt, microtubules; mf, microfilaments

In mammals, the neurofilaments of injured peripheral axons, which can regenerate, resemble those of newly developing axons; whereas the neurofilaments of mammalian central nervous system axons, which cannot regenerate, remain adult-like after injury. The ability of axons to regenerate in mammals is controlled, at least in part, by substances produced by glial cells found along axonal pathways.

Frog optic axons, unlike those of mammals, successfully regenerate fully functional connections following nerve injury. We have shown that the neurofilament composition of these injured axons resembles that of newly developing ones. Moreover, we discovered that these regenerating axons modulate their neurofilament compositions in response to cues emanating from other cells along the visual pathway. We believe that studying what regulates neurofilament protein expression may provide clues to the riddle of why axons vary in their ability to regenerate.

We also study how changes in axonal neurofilament composition influence axonal growth and development. We alter the neurofilaments of developing axons by injecting antibodies and mRNA molecules into frog embryos. These mRNAs encode either normal or mutated frog neurofilament proteins. The axons altered by these procedures are then studied in the intact frog embryo and in tissue culture.

Tadpole that was injected at the 2-cell stage with Beta-galactosidase mRNA

Cultured neuron and muscle cell expressing Green Fluorescent Protein

Students in my laboratory learn modern techniques of molecular and cell biology as they study the factors that influence neurofilament protein gene expression and explore how neurofilaments influence axonal growth.