Not so sweet: Investigating the role of c-Kit in sweet cell homeostasis

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Project Summary The sense of taste is mediated by multicellular taste buds that each house 50-100 rapidly renewing taste receptor cells (TRCs). TRCs are categorized into three main types: type I glial-like cells, type II cells that detect sweet, bitter or umami, and type III cells that detect sour. As TRCs renew, the three types are maintained in relatively stable proportions, allowing the sense of taste to remain stable over time. However, rapid turnover of taste cells makes the taste system prone to disruption by certain drugs and diseases. Cancer patients with metastatic renal cell carcinoma (mRCC) being treated with tyrosine kinase inhibitors (TKIs) often experience taste dysfunction, or dysgeusia. The primary targets of TKIs used to treat mRCC are the receptor tyrosine kinases (RTKs) VEGFR and PDGFRβ, which are not expressed in taste tissue. However, these TKIs also inhibit many off-target RTKs like Met, Ret, PDGFRα, and c-Kit. According to our single-cell RNA sequencing (scRNAseq) data, these RTKs are expressed in taste tissue in subsets of progenitors and differentiated taste cells. This suggests inhibition of these off-target RTKs may be the cause of dysgeusia and that some or all of these RTKs are necessary for proper TRC renewal and taste homeostasis. To test the role of off-target RTKs in taste homeostasis, I treated lingual organoids with the TKIs Axitinib, Cabozantinib and Sunitinib, which inhibit different combinations of off-target RTKs and frequently cause dysgeusia in patients. I found these drugs did not affect progenitor cell proliferation but instead decreased the expression of certain differentiated taste cell markers. Specifically, all three TKIs decreased expression of the sweet cell marker Tas1r2, and Tas1r2 was the only marker affected by all three drugs. Importantly, the only off-target RTK inhibited by all three drugs is c-Kit, which we find in our scRNAseq data to be most highly expressed in sweet-sensing type II TRCs. These data strongly implicate c-Kit in sweet cell homeostasis. Further analysis of our scRNAseq data reveals that c-Kit's ligand - stem cell factor (SCF), is expressed by type I and type III TRCs, raising the possibility of c-Kit mediated crosstalk within taste buds. These data in sum lead me to my hypothesis that c-Kit signaling, stimulated by crosstalk with other TRC types, is necessary for the differentiation and/or survival of sweet-sensing type II TRCs. To test this hypothesis, my first aim is to determine if c-Kit inhibition by TKI treatment leads to deficits in sweet taste in vivo by treating mice with Axitinib and performing behavioral, electrophysiological and cellular assays. My second aim is to determine if c-Kit is necessary for sweet cell differentiation or survival. I will use two genetic c-Kit knockout models under different Cre drivers to knock out c-Kit expression at different points in TRC differentiation. Lastly, I will investigate c-Kit mediated crosstalk by genetically knocking out SCF from within taste buds. Completion of this project will illuminate the role of the previously unstudied RTK c-Kit in taste homeostasis.