Genome-wide analysis of gene expression in primate taste buds reveals links to diverse processes

Abstract

Efforts to unravel the mechanisms underlying taste sensation (gustation) have largely focused on rodents. Here we present the first comprehensive characterization of gene expression in primate taste buds. Our findings reveal unique new insights into the biology of taste buds. We generated a taste bud gene expression database using laser capture microdissection (LCM) procured fungiform (FG) and circumvallate (CV) taste buds from primates. We also used LCM to collect the top and bottom portions of CV taste buds. Affymetrix genome wide arrays were used to analyze gene expression in all samples. Known taste receptors are preferentially expressed in the top portion of taste buds. Genes associated with the cell cycle and stem cells are preferentially expressed in the bottom portion of taste buds, suggesting that precursor cells are located there. Several chemokines including CXCL14 and CXCL8 are among the highest expressed genes in taste buds, indicating that immune system related processes are active in taste buds. Several genes expressed specifically in endocrine glands including growth hormone releasing hormone and its receptor are also strongly expressed in taste buds, suggesting a link between metabolism and taste. Cell type-specific expression of transcription factors and signaling molecules involved in cell fate, including KIT, reveals the taste bud as an active site of cell regeneration, differentiation, and development. IKBKAP, a gene mutated in familial dysautonomia, a disease that results in loss of taste buds, is expressed in taste cells that communicate with afferent nerve fibers via synaptic transmission. This database highlights the power of LCM coupled with transcriptional profiling to dissect the molecular composition of normal tissues, represents the most comprehensive molecular analysis of primate taste buds to date, and provides a foundation for further studies in diverse aspects of taste biology.

Figure 1. LCM of macaque taste tissue.

Intact FG papilla section (A), residual tissue after LCM (B), and isolated FG taste bud area (C). Intact LE section (adjacent to FG papilla) (D), residual tissue after LCM (E), and isolated LE areas (F). (G–K) Collection of top and bottom TB fractions by LCM. Intact CV papilla section (G), section with bottom fraction removed (H), isolated bottom fraction (I), section with top fraction removed (J), and isolated top fraction (K). Scale bar is 20 µm in A and represents panels A–C, 40 µm in D and represents panels D–F, and 40 µm in G and represents panels G–K.

Figure 2. Expression of CXCL14 mRNA in macaque CV taste tissue.

(A) Mean microarray expression values±SEM for CXCL14. (B) in situ hybridization showing CXCL14 expression in CV taste buds. Scale bar is 30 µm. (C) Zoom of CV taste buds expressing CXCL14. Scale bar is 10 µm.

Figure 3. Expression of IKBKAP mRNA in macaque taste tissue.

(A) Mean microarray expression values±SEM for IKBKAP. (B–G) in situ hybridization showing IKBKAP expression in PKD1L3 cells in CV taste buds. IKBKAP expression was visualized using colorimetric detection (purple color, left panels). Taste genes (TRPM5 and PKD1L3) were visualized using fluorescent detection (red color; center panels). Merged images (right panels) show signals from IKBKAP and taste genes. (B) IKBKAP, (C), TRPM5 (marker of sweet, bitter, and umami cells), and (D) merge showing expression of IKBKAP and TRPM5 in different cells. (E) IKBKAP, (F), PKD1L3 (sour cell marker), and (G) merge showing expression of IKBKAP in PKD1L3 cells. Scale bar is 15 µm in B and represents panel B–G.

Figure 4. Expression of KIT mRNA in macaque taste tissue.

(A) Mean microarray expression values±SEM for KIT. (B–P) in situ hybridization showing KIT expression in TAS1R1 cells in CV taste buds. KIT expression was visualized using colorimetric detection (purple color, left panels). Taste genes (TRPM5) and taste receptors (TAS1R1, TAS1R2, TAS1R3, and TAS2Rs) were visualized using fluorescent detection (red color; center panels). Merged images (right panels) show signals from KIT and taste genes. (B) KIT, (C), TRPM5 (marker of sweet, bitter, and umami cells), and (D) merge showing coexpression of KIT in a subset of TRPM5 cells. (E) KIT, (F), TAS1R1 (umami receptor), and (G) merge showing expression of KIT in a subset of TAS1R1 cells. KIT was expressed in approximately half of TAS1R1 cells. (H) KIT, (I) TAS1R2 (sweet receptor), and (J) merge showing expression of KIT and TAS1R2 in different cells. (K) KIT, (L) TAS1R3 (sweet and umami co-receptor), and (M) merge showing expression of KIT in a subset of TAS1R3 cells (these cells would also express TAS1R1). (N) KIT, (O) TAS2Rs (bitter receptors), and (P) merge showing expression of KIT and TAS2Rs in different cells. Scale bar is 15 µm in B and represents panels B–P.