Hedgehog signaling in taste cell maintenance and regeneration

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PROJECT SUMMARY/ABSTRACT Taste sensation is a critical component of basic human survival, as it contributes centrally to the discrimination of substances whose ingestion sustains us and others that are detrimental to our well-being. Even in modern times, with safe and abundant sources of nutrients all around us, taste sensation becomes a significant clinical issue when its loss, often associated with the cytotoxic treatments employed in cancer therapy, causes unwanted weight reduction and a significant decrement in quality of life. An intriguing manifestation of this problem occurs in cancer patients treated with a Hedgehog (Hh) pathway antagonist, who experience cumulative loss of taste sensation over time, suggesting that Hh pathway activity may play a critical in maintaining the taste system. An additional clue to the basis of taste maintenance is the 140 year-old observation that surgical denervation causes taste bud degeneration. Finally, we observe that gustatory neurons and their projections display the lipid- modified Hh protein ligand encoded by Sonic hedgehog (Shh) on their surface. These observations together lead to the central hypothesis addressed in this proposal: that a neuronal Hh signal induces and specifies the position of de novo taste receptor cell (TRC) formation from stem or progenitor cells of the lingual epithelium. Our proposal addresses TRC replacement to offset TRC turnover during ordinary homeostasis, as well as the wholesale regeneration required when near-complete loss is inflicted by chemotherapy or Hh pathway antagonism. This hypothesis also represents a new biological principle, namely, that precisely localized delivery of a potent inductive signal (Hh) by processes from distant neurons can sustain postnatal tissue pattern (maintenance) or impose correct pattern during de novo organ formation after severe injury (regeneration). To determine how neuronal Shh signaling supports TRC maintenance (Aim 1), we propose to identify the critical features that allow Shh-expressing neurons to induce TRCs de novo, determine the temporal characteristics of the requirement for neuronal provision of the Shh signal, and determine the potential contribution of epithelial Shh signal to TRC maintenance. To understand regeneration (Aim 2) we will treat mice with a Hh pathway antagonist that is particularly efficient in ablating TRCs, then track TRC regeneration during a recovery period to determine whether Hedgehog pathway activity is limiting, determine the cellular presentation of the Shh signal, and identify other cues that may contribute to regeneration. Finally, we will determine the mechanism of Shh delivery via neuronal processes (Aim 3) by using specially marked Shh protein and testing the contributions of other factors known to function in release of Hh protein in other settings. The results of our study will expand our biological understanding of taste organ homeostasis and of sensory organ regeneration generally, but also should help improve our ability to prevent loss of taste or facilitate its recovery in patients undergoing cytotoxic cancer therapy.