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Comparative taste biology of the pig: a potential human model

Eugeni Roura
Centre for Nutrition and Food Sciences, Queensland Alliance for Agriculture and Food Innovation, University of Queensland, Australia

Abstract / Resumen

El gusto es uno de los principales factores relacionados con el consumo de alimentos y tiene un impacto directo e inmediato sobre la iniciación y la cantidad de comida ingerida. La investigación acerca del sabor ha aumentado su interés en los últimos años con el descubrimiento de los receptores del gusto. Todos los mamíferos estudiados, excepto los gatos y los osos panda, tienen la capacidad de detectar los cinco sabores básicos.

Si comparamos la percepción del gusto entre las diferentes especies de mamíferos podemos encontrar diferencias importantes. Los seres humanos tienen aproximadamente de tres a cuatro veces menos papilas gustativas que los cerdos o las vacas. Sin embargo, la detección de azúcares simples es muy similar en los mamíferos, con la excepción del gato. Por ejemplo, los cerdos y los humanos tienen respuestas muy similares a los azúcares naturales. En cambio, la percepción de los edulcorantes artificiales difiere significativamente entre los humanos y otros mamíferos. En el caso del sabor umami , solo L-glutámico y L-glutamina han sido identificados como umami en seres humanos.

Los cerdos presentan características anatómicas, regímenes dietéticos y estrategias digestivas mucho más cercanas a los seres humanos que otro tipo de animal. En estos animales, las preferencias de comportamiento y los mecanismos fisiológicos del gusto de los azúcares, el glutamato monosódico (MSG), cloruro de quinina, la sal y el ácido cítrico se sabe que son similares a los de las personas.

Según estudios del grupo de investigación del Centro de Nutrición y Ciencias de la Alimentación de la Universidad de Queensland (Australia), en general los receptores umami en los cerdos muestran mayor grado de semejanza con los seres humanos que con los roedores.

Aunque se puede seguir confiando en el modelo roedor de laboratorio, el desarrollo de un modelo de cerdo añade conocimientos complementarios relevantes.

Biodiversity is a result of the adaptation of living species to available ecological niches featured in part by the competition for food. The search of animals for food is driven by their peripheral chemosensory system mainly taste, smell and sight. Peripheral senses interpret those stimuli relevant to the nutritional value of foods. The peripheral sense most directly related to nutritional evaluation is taste. Consequently, taste is one of the main determinants of food consumption and has a direct and immediate impact on meal initiation and presumably meal size. Taste research has flourished with the discovery of taste receptors (TRs) around a decade ago, and since then the understanding regarding the involvement of taste in food intake mechanisms has advanced significantly. Five different tastes have been defined and characterized down to their molecular mechanisms: sweet, umami, salty, sour and bitter. Sweet taste is related to carbohydrates such as sugars. Umami taste (known as the monosodium glutamate taste in humans) responds to protein-derived nutrients such as L-amino acids (L-AA) and peptides. Both sweet and umami tastes are sensed through two different heterodimer transmembrane receptors the T1R2/T1R3 and the T1R1/T1R3 respectively. Salty and sour tastes are perceived through candidate transmembrane channels ENaC (for salt) and PKD family (for sour). Finally, bitter taste is sensed through a big family of receptors (the T2Rs) that identify dietary anti-nutritional factors and potential toxicants in food to prevent excessive ingestion. All mammals studied, except cats and panda bears, have the capability for sensing at least the five primary tastes. Cats lack a functional sweet T1R2 and the panda bear lacks a functional umami receptor gene T1R1. Similar to the rest of the studied mammals, pigs have the three active T1R related to sweet and umami tastes.

The comparative biology of taste sensing shows important differences among mammalian species. Pigs and cows, with roughly 20000 taste buds, have the highest number of buds in the oral cavity of all studied animals. Humans have roughly 3 to 4 times less taste buds than pigs or cows which may advocate for a lower sensitivity in primates compared to ungulates. However, sensing of simple sugars is very similar across mammals with the exception of the cat. For example, pigs and humans have very similar responses to natural sugars. In contrast, the perception of artificial and non-carbohydrate high intensity sweeteners (HIS) differ significantly between humans and other well-studied mammals and the binding of HIS to the sweet receptor dimer T1R2/T1R3 appears to be species dependent. On the other hand, the umami taste receptor genes seem to be highly conserved and to have a very wide distribution across vertebrates from fish to avian species and primates. Nevertheless, only L-Glutamic and L-Glutamine have been identified as being umami in humans. In turn, the umami T1R1/T1R3 receptor of the mouse is more responsive to cysteine, methionine, arginine and threonine than to glutamate. Laboratory rodents have been widely used to gain basic knowledge on the characterization and metabolic implications of TRs in taste and non-taste tissues. Noticeably gaining specific understanding of the involvement of the taste machinery in human digestive physiology and the hunger-satiety cycle has been hampered by the physiological distance between rodents and humans particularly when it comes to nutritional studies. On the contrary, pigs have anatomical features, dietary regimes and digestive strategies that are far closer to humans than the rodent model.

In pigs, behavioural preferences for and physiological mechanisms to sense sugars, monosodium glutamate (MSG), quinine chloride, salt and citric acid are known to be similar to those in humans. Other umami compounds known to elicit a hedonic response in pigs include an array of L-AA (Glutamic, Alanine, Asparagine, Hyrdorxyproline, Serine and Threonine) smaller than in rodents. Recently the porcine umami taste receptor T1R1/T1R3 gene sequences have become available. Our group has been involved in a comparative study of the T1R1 sequences, Venus flytrap (VFT) ligand binding domains and L-Amino Acid agonists of several mammalian species including human, pig and laboratory rodents. Compared to the human gene ortholog, sequence identities where highest for the dog and the pig and lowest for the mouse and the rat. Human T1R1 VFT domain contains 10 different amino acid residues critically involved in ligand binding. All 10 amino acid residues are conserved, both type and location, within the pig VFT domain. In turn, the Arginine and Histidine (both charged polar) residues at key positions 307 and 308 appear as threonine and tyrosine (both neutral polar) residues in mouse and rat sequences. As a consequence the molecular modelling of the T1R1 VFT domain offers a good explanation regarding the differences of in vivo preference data for L-AA in rats, pigs and humans. Overall t he umami sensing in pigs show higher degree of resemblance to humans than that of the laboratory rodents. Whilst we can keep relying on the laboratory rodent model, the development of a pig model will add complementary knowledge relevant for and closer to humans.

The bitter receptor (T2R) is a large mammalian gene family ranging from 21 (dog) to 41 (mouse) members. The mammalian bitter gene family has been found to show a strong adaptive response through gene inactivation or pseudogenization. Unfortunately very little information is known about the bitter taste system in pigs and how it might relate to human bitter sensing. However, the pig genome is available since October 2009 and «in silico» search from our group show a significant resemblance of the pig and human repertoires.

The recent discovery of TRs in non-taste tissues such as the gastrointestinal tract (GIT) mucosa or smooth muscle of the respiratory tract opens new venues on the relevance of such receptors in human health. In addition, the presence of TRs in non-taste tissues increases the need for novel invasive studies and the potential interest of the pig as a model for scientific research should gain momentum. The taste machinery in non-taste tissues is believed to be involved in nutrient sensing and absorption and the on/off setting of the hunger-satiety cycle. In addition, these taste mechanisms respond to the nutritional status of an individual. In the hunger-satiety cycle, the response to taste stimuli decreases as food is eaten until satiety arrives and increases back once in a hunger state. Both oral and non-oral taste sensing cells are responsive to anorectic (e.g., leptin, CCK) and orexigenic (i.e., ghrelin, orexins) hormones . For example, during a nutritional sodium deficiency, high levels of the renal hormone, aldosterone, induce the expression of the sodium channel ENaC. This up regulation of sodium receptors increases the taste sensitivity for sodium and aids in the specific appetite for this nutrient. Thus, the nutritional state tunes specific taste sensitivities to help maintain homeostasis. These findings have significant potential implications related to human food intake disorders and epidemics such as anorexia, overweight, obesity or type II diabetes that will likely drive future research on taste biology.




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