Involvement of T1R3 in calcium-magnesium taste

Abstract

Calcium and magnesium are essential for survival but it is unknown how animals detect and consume enough of these minerals to meet their needs. To investigate this, we exploited the PWK/PhJ (PWK) strain of mice, which, in contrast to the C57BL/6J (B6) and other inbred strains, displays strong preferences for calcium solutions. We found that the PWK strain also has strong preferences for MgCl2 and saccharin solutions but not representative salty, sour, bitter, or umami taste compounds. A genome scan of B6 x PWK F2 mice linked a component of the strain difference in calcium and magnesium preference to distal chromosome 4. The taste receptor gene, Tas1r3, was implicated by studies with 129.B6ByJ-Tas1r3 congenic and Tas1r3 knockout mice. Most notably, calcium and magnesium solutions that were avoided by wild-type B6 mice were preferred (relative to water) by B6 mice null for the Tas1r3 gene. Oral calcium elicited less electrophysiological activity in the chorda tympani nerve of Tas1r3 knockout than wild-type mice. Comparison of the sequence of Tas1r3 with calcium and saccharin preferences in inbred mouse strains found 1) an inverse correlation between calcium and saccharin preference scores across primarily domesticus strains, which was associated with an I60T substitution in T1R3, and 2) a V689A substitution in T1R3 that was unique to the PWK strain and thus may be responsible for its strong calcium and magnesium preference. Our results imply that, in addition to its established roles in the detection of sweet and umami compounds, T1R3 functions as a gustatory calcium-magnesium receptor.

Fig. 1.

Preferences of male C57BL/6J and PWK/PhJ mice for various taste solutions. CaCl₂, calcium chloride; CaLa, calcium lactate; QHCl, quinine hydrochloride; NaLa, sodium lactate; IMP, inosine monophosphate (umami). Dotted horizontal lines show indifference (50% preference).

Fig. 2.

Interval maps of distal chromosome 4 based on genome screens of CaCl₂, CaLa, MgCl₂, and saccharin preference scores of 484 B6 × PWK F₂ mice. Marker symbols are shown at top. Faint horizontal lines are genome-wide significance levels for P = 0.05 (lower) and P = 0.01 (upper). LOD, logarithm of the odds ratio.

Fig. 3.

Preferences of 129.B6ByJ-Tas1r3 congenic mice possessing a B6ByJ allele of Tas1r3 (B6ByJ/129 heterozygotes; n = 11) and their littermate controls (129/129 homozygotes; n = 19) for various concentrations of saccharin, CaCl₂, CaLa, and MgCl₂. The 129.B6ByJ-Tas1r3 congenic mice carry a 194-kb chromosomal segment including Tas1r3 from the B6ByJ strain that is introgressed onto the 129 strain background. Dotted horizontal lines show indifference (50% preference).

Fig. 4.

Preferences of Tas1r3 knockout (−/−; n = 14), heterozygous (+/−; n = 36), and wild type (+/+; n = 19) mice for various concentrations of 5 taste compounds. Dotted horizontal lines show indifference (50% preference).

Fig. 5.

Tas1r3 single nucleotide polymorphism (SNP) analysis of 40 inbred strains of mice. Left columns give each marker's name, position (MGI, Build 37), and effect on amino acid (AA) sequence, if any. Column labeled “nt of B6” shows nucleotide present in C57BL/6J strain. In body of table, B = same nucleotide as B6; A = alternative to B6 (i.e., other nucleotide); blank = no genotype available. Strains are arranged from left to right so as to display similarities. PWK/PhJ and PWD/Ph (not shown) have the same haplotype; 129S1/SvImJ and 129P3/J (not shown) have the same haplotype. The I60T substitution (at 155,237,265 bp; shaded) is believed to be primarily responsible for determining responsiveness of Tas1r3 to saccharin. The critical SNP at “Sequenced” (shaded) is unique to the PWK/PhJ and PWD/PhJ strains (highlighted with circle) and may thus be responsible for these strains high calcium preferences. Preference data for 75 mM CaLa in bottom row are extracted from Ref. .

Fig. 6.

Scatter plots showing relationship between preferences for calcium and sweeteners in: (A) 480 B6 × PWK F₂ hybrid mice (r = −0.05, not significant); (B) males of 28 inbred strains with 30 mM CaCl₂ preference from Ref. and 1.6 mM saccharin preference from Ref. (r = −0.77, P < 0.00001); (C) males of 11 strains with 25 mM CaCl₂ preference from Ref. and 2.5% sucrose preference from Fig. 2 of Ref. (r = −0.87, P = 0.0005). •, B6 genotype at rs13478082, which is believed to confer high sweet preference; ○, alternate genotype at rs13478082.