Dec 1 (Thursday) Joseph Fetcho (Cornell University)

Optical and genetic approaches toward understanding motor system function and dysfunction

Abstract:

Modern optical and genetic methods are providing powerful tools for the
study of motor systems. These approaches facilitate work designed to
explore the contributions of neurons to behavior, to examine the
functional disruptions following injury, and to test strategies designed
to restore normal function.  Our work focuses on understanding spinal
cord function and dysfunction in zebrafish, which are especially amenable
to optical and genetic approaches.  Two applications of these techniques
will be presented - one directed toward studies of normal function and
the other toward restoration of function after injury.

Normal function: The differentiation of spinal cord is directed by
transcription factors that are thought to be expressed by subsets of
spinal interneurons.  We have been exploring the links between
transcription factor expression and the functional organization of spinal
interneurons.  Interneurons marked by the transcription factor Engrailed 1
were labeled by GFP in living fish by using BAC recombination to place GFP
under control of the zebrafish engrailed promoter.  The structure and
function of the cells was explored by confocal microscopy, in vivo calcium
imaging, and electrophysiology. Our data indicate that engrailed marks
most and probably all of the glycinergic inhibitory neurons with
ipsilateral ascending axons in larval spinal cord, and only them.  These
cells are the only known glycinergic spinal interneurons with ipsilateral
axons in larval zebrafish.   The cells are rhythmically active in
swimming. They inhibit interneurons in sensory reflex circuits to gate the
flow of sensory information and have widely distributed axonal arbors,
with individual cells having apparent contacts both with ventral
motoneurons and more dorsal sensory interneurons. This “primitive”
multifunction engrailed population may have diverged during the evolution
of chicks and mammals to give rise to several different ipsilateral
glycinergic inhibitory cell types with more restricted functional roles.

Restoration of function after injury: A major obstacle to the recovery of
function after spinal cord injury is the problem of inducing regeneration
of severed spinal axons long after a lesion. Regeneration of spinal
neurons in mammals is limited by both a poor environment for growing axons
and by the inability of some neurons to re-grow axons even when placed in
an environment permissive for growth. A potential role for cyclic AMP in
promoting regeneration was suggested by evidence that it is upregulated in
dorsal root ganglion cells after a peripheral lesion and that this
increase enhances their regenerative response to a subsequent central
lesion. We used in vivo imaging in zebrafish to show that cyclic AMP
induces robust regeneration of severed central Mauthner axons well after
an injury and over multiple body segments past the lesion site.  This
regeneration is associated with both a recovery of function at the level
of post-synaptic interneurons as revealed by in vivo calcium imaging, and
a restoration of the movements initiated by the regenerating axon.
actin-myosin interactions.