Vasoactive intestinal peptide-null mice demonstrate enhanced sweet taste preference, dysglycemia, and reduced taste bud leptin receptor expression

Abstract

Objective: It is becoming apparent that there is a strong link between taste perception and energy homeostasis. Recent evidence implicates gut-related hormones in taste perception, including glucagon-like peptide 1 and vasoactive intestinal peptide (VIP). We used VIP knockout mice to investigate VIP's specific role in taste perception and connection to energy regulation.

Research design and methods: Body weight, food intake, and plasma levels of multiple energy-regulating hormones were measured and pancreatic morphology was determined. In addition, the immunocytochemical profile of taste cells and gustatory behavior were examined in wild-type and VIP knockout mice.

Results: VIP knockout mice demonstrate elevated plasma glucose, insulin, and leptin levels, with no islet beta-cell number/topography alteration. VIP and its receptors (VPAC1, VPAC2) were identified in type II taste cells of the taste bud, and VIP knockout mice exhibit enhanced taste preference to sweet tastants. VIP knockout mouse taste cells show a significant decrease in leptin receptor expression and elevated expression of glucagon-like peptide 1, which may explain sweet taste preference of VIP knockout mice.

Conclusions: This study suggests that the tongue can play a direct role in modulating energy intake to correct peripheral glycemic imbalances. In this way, we could view the tongue as a sensory mechanism that is bidirectionally regulated and thus forms a bridge between available foodstuffs and the intricate hormonal balance in the animal itself.

FIG. 1.

Expression of taste cell markers and VIP in circumvallate papillae of wild-type (WT) and VIP knockout (KO) mice. Type II cell marker (PLCβ2) (A and B), type III cell marker (NCAM) (C and D), and type IV cell marker (Shh) (E and F) are expressed in taste cells of both wild-type and VIP knockout mice. Scale bars, 20 μm. Blue is TO-PRO-3 nuclear stain. G and H: VIP (red) is expressed in a subset of taste cells of wild-type mice. I and J: VIP is not expressed within the VIP knockout mouse taste bud. Scale bars, 20 μm. Sections are representative of three mice. (A high-quality digital representation of this figure is available in the online issue.)

FIG. 2.

Coexpression of VIP receptors (VPAC1/VPAC2) and PLCβ2 in circumvallate papillae of wild-type (WT) and VIP knockout (KO) mice. A–C: VPAC1 (red) and PLCβ2 (green) are colocalized in a subset of PLCβ2-positive cells (yellow cells) in wild-type mice. The arrows denote cells expressing both. D–F: VPAC1 (red) and PLCβ2 (green) are colocalized in a subset of PLCβ2-positive cells (yellow cells) in VIP knockout mice. G–I: VPAC2 (red) and PLCβ2 (green) are colocalized in a subset of PLCβ2-positive cells (yellow cells) in wild-type mice. The arrows denote cells expressing both. J–L: VPAC2 (red) and PLCβ2 (green) are colocalized in a subset of PLCβ2-positive cells (yellow cells) in VIP knockout mice. The arrows denote cells expressing both. Scale bars, 20 μm. Blue is TO-PRO-3 nuclear stain. (A high-quality digital representation of this figure is available in the online issue.)

FIG. 3.

Expression of taste-related proteins in circumvallate papillae of wild-type (WT) and VIP knockout (KO) mice. A and B: Sweet taste receptor (T1R3, green) is expressed in both wild-type and VIP knockout mice. C and D: GLP-1 (green) is expressed in wild-type and VIP knockout mice. E and F: Leptin receptor (green) is expressed in wild-type and VIP knockout mice. Scale bars, 20 μm. Blue is TO-PRO-3 nuclear stain. G: The percentage of immunopositive cells quantified in both wild-type and VIP knockout mice for the following markers: PLCβ2, NCAM, Shh, T1R3, GLP-1, and leptin receptor. Cells were scored as immunoreactive only if a nuclear profile was present within the cell. Values are the means ± SEM. **P < 0.01, ***P < 0.001. TCs, taste cells. (A high-quality digital representation of this figure is available in the online issue.)

FIG. 4.

VIP knockout (KO) mice possess alterations in multiple hormones controlling appetite and energy balance. Multiplex hormone analysis from plasma samples of wild-type (WT) and VIP knockout mice reveal multiple distinctions between the two animal groups. A and B: Significantly higher nonfasting and fasting glucose occurs in the VIP knockout mice. The VIP knockout mice also possessed significantly higher plasma insulin (C) and leptin (D) levels. No significant differences between wild-type and VIP knockout for amylin were noted (E). VIP knockout mice demonstrated higher levels of plasma gastrointestinal polypeptide (F), GLP-1 (G), and peptide tyrosine tyrosine (PYY) (H). No significant difference in the plasma levels of PP were noted between wild-type and VIP knockout mice (I). Each panel depicts means ± SEM from eight animals in each group. *P < 0.05, **P < 0.01.

FIG. 5.

Pancreatic islet sizing in VIP knockout (KO) and wild-type (WT) mice. Representative pancreatic islets from wild-type (A) or VIP knockout (B) mice demonstrate similar insulin (red fluorescence) and glucagon (green fluorescence) expression profiles. C: Mean islet area (mm²) measured in wild-type or VIP knockout pancreata. D: Similar mean α-cell (α) and β-cell (β) percentage in the islets of VIP knockout and wild-type mice. E: Relative distribution of the different islet sizes (1 = small; 2 = middle; 3 = large) for VIP knockout and wild-type mice. Values are expressed as the means ± SEM. (A high-quality digital representation of this figure is available in the online issue.)

FIG. 6.

Altered sweet, bitter, and sour taste responses of wild-type (WT) and VIP knockout (KO) mice in brief-access taste tests (A–D). Taste responses, expressed as tastant/water lick ratios and as a function of stimulus concentration, of VIP knockout (closed triangle, dotted line) and wild-type (open square, solid line) to sucrose (A), citric acid (CA; B), denatonium benzoate (DB; C), and sodium chloride (NaCl; D). Values are expressed as means ± SEM. Curves were fit as described in research design and methods. *P < 0.05, ***P < 0.001.

FIG. 7.

Normal leptin receptor and IRS2 expression in the hypothalami of wild-type (WT) and VIP knockout (KO) mice. A: Representative Western blots of three wild-type (1–3) or VIP knockout (1–3) mice hypothalami. The tissue extracts were probed with specific antisera for the leptin receptor (Ob-Rb), IRS2, and β-actin as a loading control. For the Ob-Rb blot, a plasma membrane extract was used, whereas cytoplasmic tissue extracts were used for the IRS2 and actin blots. B–E: Mean ± SEM of the relative band intensities for the specific Western blots measured as relative absorbance units minus background absorbance per square pixel (AU-B/px²). F: Mean food intake (g) in wild-type and VIP knockout mice. G: Mean body weight of wild-type and VIP knockout mice. Values are expressed as means ± SEM.

FIG. 8.

A diagrammatic summary of the complex physiological parameters observed in the VIP knockout (KO) mice compared with the wild-type (WT) controls. The VIP knockout mice present with a diabetic-like state, and they actively attempt to ameliorate this condition with a series of physiological alterations (high circulating leptin, low tongue Ob-Rb expression, high tongue GLP-1 expression, normal hypothalamic Ob-Rb expression) that—in concert—could attenuate the animals' euglycemic disruption by enhancing their sweet taste and potentially reducing their desire to consume sweet foods.