Naim Akhtar Khan

The gustatory pathway is involved in CD36-mediated orosensory perception of long-chain fatty acids in the mouse

The sense of taste informs the body about the quality of ingested foods. Tastant‐mediated signals are generated by a rise in free intracellular calcium levels ([Ca2+]i) in the taste bud cells and then are transferred to the gustatory area of brain via connections between the gustatory nerves (chorda tym‐ pani and glossopharyngeal nerves) and the nucleus of solitary tract in the brain stem. We have recently shown that lingual CD36 contributes to fat preference and early digestive secretions in the mouse. We show here that 1) the induction of an increase in [Ca2+]i by linoleic acid is CD36‐dependent in taste receptor cells, 2) the spontaneous preference for or conversely con ditioned aversion to linoleic acid requires intact gusta tory nerves, and 3) the activation of gustatory neurons in the nucleus of the solitary tract elicited by a linoleic acid deposition on the tongue in wild‐type mice cannot be reproduced in CD36‐null animals. We conclude that the CD36‐mediated perception of long‐chain fatty acids involves the gustatory pathway, suggesting that the mouse may have a “taste“ for fatty foods. This system would constitute a potential physiological advantage under conditions of food scarcity by leading the mouse to select and absorb fatty foods. However, it might also lead to a risk of obesity and associated diseases in a context of constantly abundant food.—Gaillard, D., Laugerette, F., Darcel, N., El‐Yassimi, A., Passilly‐De grace, P., Hichami, A., Khan, N. A., Montmayeur, J.‐P., Besnard, P. The gustatory pathway is involved in CD36‐ mediated orosensory perception of long‐chain fatty acids in the mouse. FASEB J. 22, 1458–1468 (2008)

Linoleic Acid Induces Calcium Signaling, Src Kinase Phosphorylation, and Neurotransmitter Release in Mouse CD36-positive Gustatory Cells

We have recently demonstrated that the cells expressing CD36, localized apically on the taste buds of mouse lingual circumvallate papillae, act as gustatory cells. In the present study we isolated these CD36-positive cells from mouse circumvallate papillae and investigated intracellular signaling events, triggered by a long-chain polyunsaturated fatty acid, i.e. linoleic acid (LA). LA induced increases in free intracellular calcium concentrations, [Ca2+]i, by recruiting calcium from endoplasmic reticulum pool via inositol 1,4,5-triphosphate production followed by calcium influx via opening of store-operated calcium (SOC) channels. LA also induced phosphorylation of Src-protein-tyrosine kinases (Src-PTKs), particularly of Fyn59 and Yes62. LA-evoked phosphorylation of Fyn59 and Yes62 was implicated in the activation of SOC channels. Reverse transcription-quantitative PCR revealed that the CD36-positive gustatory cells possessed mRNA of enzymes like tryptophan hydroxylase-1, l-aromatic amino acid decarboxylase, tyrosine hydroxylase, and dopamine β-hydroxylase, involved in the synthesis of monoamine neurotransmitters. Interestingly, the addition of LA to these cells induced the release of 5-hydroxytryptamine and noradrenalin to the extracellular environment. The LA-induced release of these neurotransmitters was curtailed by SOC channel blockers and Src-PTK inhibitors. These results altogether demonstrate that LA binds to mouse CD36-positive gustatory cells, induces Src-PTKs phosphorylation, triggers calcium signaling, and evokes the release of 5-hydroxytryptamine and noradrenalin, which in turn may be implicated in the downstream signaling to the afferent nerve fibers, thus transmitting the output signal from taste buds to the central nervous system.

Implication of acyl chain of diacylglycerols in activation of different isoforms of protein kinase C

We synthesized diacylglycerols (DAGs) containing ω‐6 or ω‐3 polyunsaturated fatty acids [i.e., 1‐stearoyl‐2‐arachidonoyl‐sn‐glycerol (SAG), 1‐stearoyl‐2‐docosahexaenoyL‐sn‐glycerol (SDG), and 1‐stearoyl‐2‐eicosapentaenoyl‐sn‐glycerol (SEG)] and assessed their efficiency on activation of conventional (α, βI, γ) and novel (ε, δ) protein kinase C (PKC). SAG exerted significantly higher stimulatory effects than SDG and SEG on activation of PKCα and PKCδ. Activation of PKCβI by SEG and SDG was higher than that by SAG. Activation of PKCγ did not differ significantly among DAG molecular species. Addition of SAG to assays containing SEG and SDG exerted additive effects on activation of α and ε, but not on βI and γ, isoforms of PKC. SDG‐ and SEG‐induced activation of PKCδ was significantly curtailed by the addition of SAG. Three DAG species significantly curtailed the PMA‐induced activation of βl, γ, and δ, but not of α and ε, isoforms of PKC. Our study demonstrates for the first time that in vitro activation of different PKC isoenzymes vary in response to different DAG species, and one can envisage that this differential regulation may be responsible for their in vivo effects on target organs.—Madani, S., Hichami, A., Legrand, A., Belleville, J., Khan, N. A. Implication of acyl chain of diacylglycerols in activation of different isoforms of protein kinase C. FASEB J. 15, 2595–2601 (2001)

Regulatory activity of polyunsaturated fatty acids in T-cell signaling.

n-3 Polyunsaturated fatty acids (PUFA) are considered to be authentic immunosuppressors and appear to exert beneficial effects with respect to certain immune-mediated diseases. In addition to promoting T-helper 1 (Th1) cell to T-helper 2 (Th2) cell effector T-cell differentiation, n-3 PUFA may also exert anti-inflammatory actions by inducing apoptosis in Th1 cells. With respect to mechanisms of action, effects range from the modulation of membrane receptors to gene transcription via perturbation of a number of second messenger cascades. In this review, the putative targets of anti-inflammatory n-3 PUFA, activated during early and late events of T-cell activation will be discussed. Studies have demonstrated that these fatty acids alter plasma membrane micro-organization (lipid rafts) at the immunological synapse, the site where T-cells and antigen-presenting cells (APC) form a physical contact for antigen initiated T-cell signaling. In addition, the production of diacylglycerol and the activation of different isoforms of protein kinase C (PKC), mitogen-activated protein kinase (MAPK), calcium signaling, and nuclear translocation/activation of transcriptional factors, can be modulated by n-3 PUFA. Advantages and limitations of diverse methodologies to study the membrane lipid raft hypothesis, as well as apparent contradictions regarding the effect of n-3 PUFA on lipid rafts will be critically presented.

Oro-sensory perception of dietary lipids: new insights into the fat taste transduction.

The sense of taste informs the organism about the quality of ingested food. Five basic taste modalities, e.g., sweet, sour, bitter, salty and umami have so far been identified. Recent compelling evidence from rodent and human studies raise the possibility for an additional sixth taste modality devoted to the perception of lipids. Recent studies strongly suggest that lingual CD36, being implicated in the perception of dietary fat, may act as a gustatory lipid sensor. Knocking down of CD36 gene decreases the spontaneous preference for long chain fatty acids (LCFA) in mice subjected to a free choice situation. Lingual CD36, after activation by LCFA, is able to trigger specific signalling mechanisms, e.g., increase in free intracellular calcium concentrations, ([Ca(2)(+)]i), phosphorylation of protein-tyrosine kinase (PTK) and release of the neurotransmitters like serotonin and nor-adrenaline into synaptic clefts. This signalling cascade is likely responsible for physiologic responses, induced by the detection of lipids in the oral cavity (i.e., lingual fat preference and cephalic phase of digestion). This review provides recent insights into the molecular mechanisms involved in the oro-sensory perception of lipids.

CD36- and GPR120-Mediated Ca2+ Signaling in Human Taste Bud Cells Mediates Differential Responses to Fatty Acids and Is Altered in Obese Mice

BACKGROUND & AIMS It is important to increase our understanding of gustatory detection of dietary fat and its contribution to fat preference. We studied the roles of the fat taste receptors CD36 and GPR120 and their interactions via Ca(2+) signaling in fungiform taste bud cells (TBC). METHODS We measured Ca(2+) signaling in human TBC, transfected with small interfering RNAs against messenger RNAs encoding CD36 and GPR120 (or control small interfering RNAs). We also studied Ca(2+) signaling in TBC from CD36(-/-) mice and from wild-type lean and obese mice. Additional studies were conducted with mouse enteroendocrine cell line STC-1 that express GPR120 and stably transfected with human CD36. We measured release of serotonin and glucagon-like peptide-1 from human and mice TBC in response to CD36 and GPR120 activation. RESULTS High concentrations of linoleic acid induced Ca(2+) signaling via CD36 and GPR120 in human and mice TBC, as well as in STC-1 cells, and low concentrations induced Ca(2+) signaling via only CD36. Incubation of human and mice fungiform TBC with lineoleic acid down-regulated CD36 and up-regulated GPR120 in membrane lipid rafts. Obese mice had decreased spontaneous preference for fat. Fungiform TBC from obese mice had reduced Ca(2+) and serotonin responses, but increased release of glucagon-like peptide-1, along with reduced levels of CD36 and increased levels of GPR120 in lipid rafts. CONCLUSIONS CD36 and GPR120 have nonoverlapping roles in TBC signaling during orogustatory perception of dietary lipids; these are differentially regulated by obesity.

Cell signaling mechanisms of oro-gustatory detection of dietary fat: advances and challenges.

CD36 and two G-protein coupled receptors (GPCR), i.e., GPR120 and GPR40, have been implicated in the gustatory perception of dietary fats in rodents. These glycoproteins are coupled to increases in free intracellular Ca²⁺ concentrations, [Ca²⁺](i), during their activation by dietary long-chain fatty acids (LCFA). The transient receptor potential type M5 (TRPM5) channel, activated by [Ca²⁺](i), participates in downstream signaling in taste bud cells (TBC). The mice, knocked-out for expression of CD36, GPR120, GPR40 or TRPM5 have a reduced spontaneous preference for fat. The delayed rectifying K⁺ (DRK) channels believed to lie downstream of these receptors are also important players in fat taste transduction. The trigeminal neurons by triggering increases in [Ca²⁺](i) may influence the taste signal to afferent nerve fibers. Why are there so many taste receptor candidates for one taste modality? We discuss the recent advances on the role of CD36, GPR120, GPR40, TRPM5 and DRK channels, in signal transduction in TBC. We shed light on their cross-talk and delineate their roles in obesity as a better understanding of the molecular mechanisms behind their regulation could eventually lead to new strategies to fight against this condition.

Docosahexaenoic acid reduces suppressive and migratory functions of CD4 + CD25 + regulatory T-cells

Immunological tolerance is one of the fundamental aspects of the immune system. The CD4+CD25+ regulatory T (Treg) cells have emerged as key players in the development of tolerance to self and foreign antigens. However, little is known about the endogenous factors and mechanisms controlling their suppressive capacity on immune response. In this study, we observed that docosahexaenoic acid (DHA), an n-3 polyunsaturated fatty acid, diminished, in a dose-dependent manner, the capacity of Treg cells to inhibit the CD4+CD25− effector T-cell proliferation. DHA not only reduced the migration of Treg cells toward chemokines but also downregulated the mRNA expression of CCR-4 and CXCR-4 in Treg cells. DHA also curtailed ERK1/2 and Akt phosphorylation and downregulated the Smad7 levels in these cells. Contradictorily, DHA upregulated the mRNA expression of Foxp3, CTLA-4, TGF-β, and IL-10; nonetheless, this fatty acid increased the expression of p27KIP1 mRNA, known to be involved in Treg cell unresponsiveness. In Foxp3-immunoprepitated nuclear proteins, DHA upregulated histone desacetylase 7 levels that would again participate in the unresposnsiveness of these cells. Finally, a DHA-enriched diet also diminished, ex vivo, the suppressive capacity of Treg cells. Altogether, these results suggest that DHA, by diminishing Treg cell functions, may play a key role in health and disease.

N-3 fatty acids modulate antioxidant status in diabetic rats and their macrosomic offspring.

Objective:We investigated the role of dietary n-3 polyunsaturated fatty acids (n-3 PUFA) in the modulation of total antioxidant status in streptozotocin (STZ)-induced diabetic rats and their macrosomic offspring.Design:Female wistar rats, fed on control diet or n-3 PUFA diet, were rendered diabetic by administration of five mild doses of STZ on day 5 and were killed on days 12 and 21 of gestation. The macrosomic (MAC) pups were killed at the age of 60 and 90 days.Measurements:Lipid peroxidation was measured as the concentrations of plasma thiobarbituric acid reactive substances (TBARS), and the total antioxidant status was determined by measuring (i) plasma oxygen radical absorbance capacity (ORAC), (ii) plasma vitamin A, E and C concentrations, and (iii) antioxidant enzymes activities in erythrocytes. The plasma lipid concentrations and fatty acid composition were also determined.Results:Diabetes increased plasma triglyceride and cholesterol concentrations, whereas macrosomia was associated with enhanced plasma cholesterol and triglyceride levels, which diminished by feeding n-3 PUFA diet. N-3 PUFA diet also reduced increased plasma TBARS and corrected the decreased ORAC values in diabetic rats and their macrosomic offspring. EPAX diet increased the diminished vitamin A levels in diabetic mothers and vitamin C concentrations in macrosomic pups. Also, this diet improved the decreased erythrocyte superoxide dismutase and glutathione peroxidase activities in diabetic and macrosomic animals.Conclusion:Diabetes and macrosomia were associated with altered lipid metabolism, antioxidant enzyme activities and vitamin concentrations. N-3 PUFA diet improved hyperlipidemia and restored antioxidant status in diabetic dams and MAC offspring.

STIM1 regulates calcium signaling in taste bud cells and preference for fat in mice.

Understanding the mechanisms underlying oro-gustatory detection of dietary fat is critical for the prevention and treatment of obesity. The lipid-binding glycoprotein CD36, which is expressed by circumvallate papillae (CVP) of the mouse tongue, has been implicated in oro-gustatory perception of dietary lipids. Here, we demonstrate that stromal interaction molecule 1 (STIM1), a sensor of Ca(2+) depletion in the endoplasmic reticulum, mediates fatty acid-induced Ca(2+) signaling in the mouse tongue and fat preference. We showed that linoleic acid (LA) induced the production of arachidonic acid (AA) and lysophosphatidylcholine (Lyso-PC) by activating multiple phospholipase A2 isoforms via CD36. This activation triggered Ca(2+) influx in CD36-positive taste bud cells (TBCs) purified from mouse CVP. LA also induced the production of Ca(2+) influx factor (CIF). STIM1 was found to regulate LA-induced CIF production and the opening of multiple store-operated Ca(2+) (SOC) channels. Furthermore, CD36-positive TBCs from Stim1-/- mice failed to release serotonin, and Stim1-/- mice lost the spontaneous preference for fat that was observed in wild-type animals. Our results suggest that fatty acid-induced Ca(2+) signaling, regulated by STIM1 via CD36, might be implicated in oro-gustatory perception of dietary lipids and the spontaneous preference for fat.