Project Summary
The long term objective of this work is to uncover guidance mechanisms that are critical to axon
targeting during development, plasticity, and regeneration following injury. The discrete nature
of taste papillae and the taste buds within their epithelium make it an excellent model for
studying axon targeting mechanisms. Taste axons innervate taste buds but not adjacent
epithelium, and somatosensory axons that penetrate non-taste epithelium surround the taste
bud but do not enter it. It is also likely that both types of axons recognize non-target and specific
target cells within their target zones, further limiting exploration. Surprisingly little is known about
the molecules and mechanisms that underlie targeting in this system. Diffusible molecules such
as Sema3A and neurotrophins have roles in repelling axons from and attracting axons toward
epithelial targets, respectively, but are unlikely to fully account for the ability of taste and
somatosensory axons to distinguish between adjacent populations of target and non-target
epithelial cells. Cell-attached cues could provide the signaling resolution needed for
discrimination between adjacent target and non-target cell populations. EphA receptors
(EphA's) and ephrinA's are cell-attached cues that can mediate growth promotion, repulsion
(non-target recognition), or stabilization (target recognition). The first aim of the proposed work
is to determine which ephrinA's and EphA's are expressed in lingual gustatory papillae and in
the sensory nerves that innervate them during embryonic development. Multiple approaches will
be used to localize these proteins: in situ hybridization, immunohistochemistry, detection using
synthetic proteins containing the extracellular domain of their binding partners, and detection of
marker proteins substituted for EphA's or ephrinA's in mutant mice. Second, to determine if
EphA/ephrinA signaling promotes or repels taste and somatosensory neurites, ganglion
explants will be grown on substrates that have been micro-patterned with alternating stripes of
ephrinA's or EphA's and control proteins. To determine if EphA's or ephrinA's are necessary for
normal targeting, nerve trajectories in the tongues of mutant mice lacking multiple ephrinA's
and/or EphA's will be evaluated. Identifying guidance molecules and understanding their roles in
targeting axons to papillae and taste buds may uncover guidance mechanisms that apply to
other parts of the nervous system in which the fidelity of initial innervation is not as obvious as in
the tongue. These mechanisms may also be employed throughout life as peripheral epithelial
targets routinely undergo turnover in the absence of injury and are also re-innervated after
injury.
