In the present study, we aimed to evaluate the association between gene expression levels of TAS1R2, TAS1R3, TAS2R38, OR51E1, LEPR, and GHRL genes which are linked to gustatory, olfactory, and, appetite receptors in COVID-19 patients and their correlation among SARS-CoV-19 Delta and Omicron BA.1 variants of concern (VoCs).
Different scenarios have been drawn to explain COVID-19 relation to loss of taste, one of them explains taste disfunction in the manifestation of the SARS-CoV-2 infection could be due to the damage to the taste nerves after the virus infects the central nervous system. While this appears improbable, evidence from the human study found that ageusia is associated with low severity in COVID-19 individuals once who do not have encephalitis (Meunier et al., 2021); although, a recent study observed that SARS-CoV-2 can directly infect the central nervous system but still it is unclear and the low incidence of SARS-CoV-2-related to central nervous system injury (Fabbri et al., 2022).
Another possible explanation is assumed that the regeneration of taste buds that express the taste receptors is limited after infection of the epithelial cells. Evidently, taste buds have large levels of Toll-like receptors (TLR) and interferon (IFN) receptors, and their activation may inhibit taste cell renewal (Wang et al., 2009). As a result of the “cytokine storm” (excessive or uncontrolled production of immune cells and cytokines) caused by SARS-CoV-2 in distant cells, ageusia could be the result of decreased taste bud renewal. The cytokine storm may also allow SARS-CoV-2 to infect taste buds cells. The angiotensin-converting enzyme 2 (ACE2) is identified as the main receptor for SARS-CoV-2 entrance into the host cell. Moreover, in the presence of IFN, the ACE2 has been demonstrated to be overexpressed. As a result, distant IFN production from infected keratinocytes might lead to the ACE2 expression in taste bud cells, which could then be infected with SARS-CoV-2 (Meunier et al., 2021).
Taste receptors are expressed in the upper airways. The non-ciliated solitary chemosensory cells (SCCs) epithelial cells, express the sweet taste receptors formed by two members of the taste 1 receptor family TAS1R2 and TAS1R3, as well as the bitter taste receptors TAS2s (Maina et al., 2018). These cells have an important role in immunity response by activation of the taste receptors. (Imada et al., 2010).
Our results reveal that expression levels of the TAS1R2 (two-fold) and TAS1R3 (~ two-fold) genes were significantly decreased in COVID-19 patients who were infected with Delta variant compared to the non-infected individual. Additionally, the expression levels of TAS1R2 (two-fold) and TAS1R3 (~ two-fold) genes were increased in COVID-19 patients who were infected with Omicron BA.1 variant compared to COVID-19 patients who were infected with Delta variant. This study demonstrated the possible scenario of down-regulation of sweet taste receptors involving SARS-CoV-2 Delta infection. These findings pointed to ACE2 being expressed in taste buds and high replication of SARS-CoV-2 in infected gustatory cells in the taste bud causing inflammation and eventually destroying the cells, especially with the SARS-CoV-2 Delta infections, which is the most disruptive and symptomatic variant causing hospitalizations and deaths in comparison to other variants. The injury to the gustatory cells could cause the gustatory system to malfunction.
Previous studies are revealed that the sweet taste receptors regulate energy balance, glucose hemostasis, and food intake, (Lee & Owyang, 2019). As sweet sensation encourages people to eat more, hence a study in the duodenum and parts of the mouse brain involved in energy regulation, observed that the TAS1R3 gene is downregulated in obese patients’ stomachs (Widmayer et al., 2011). The TAS1R2 gene expression in the brain is reduced in obese mice whereas, the TAS1R2 is downregulated by high glucose, while TAS1R3 is downregulated by elevated leptin levels, both of which are outcomes of obesity (Chao et al., 2016). In our study, the TAS1R2 gene expression was positively correlated with GHRL, as well as TAS1R3 gene expression was positively correlated with GHRL gene expression among the COVID-19 patients. This might be associated with the individual’s health condition which was, unfortunately lacking in our data.
TAS2Rs are associated with the respiratory system (Shah et al., 2009). The TAS2Rs have been discovered to be capable of recognizing bacterial pathogens and triggering downstream reactions within minutes. One of the most studied isoforms of the TAS2R family is the TAS2R38 gene, which induces a response to different bitter compounds such as bacterial bitter compound acyl-homoserine lactones (AHLs) (Maina et al., 2018). The TAS2R38 gene is activated by the microbial bitter products, inducing canonical taste-signaling pathway immune response, releasing calcium Ca+ 2 from the endoplasmic reticulum. The Ca+ 2 increasing via nitric oxide synthase activates nitric oxide (NO) production. NO tends to increase ciliary beat frequency by activating protein kinase G and causing damage to infectious microbial compounds (Carey et al., 2017). TAS2Rs, ensure defense responses primarily for ciliated sinonasal epithelial cells occurring in the first layer of upper airway immunity (Maina et al., 2018) which associates with SARS-CoV-2, causing immune responses to destabilization (Aoe, 2020).
Hence, we analyze the expression of the TAS2R38 gene, whereas activation of bitter taste receptors shows to have anti-inflammatory effects and its upregulation might have a protective role in some conditions (Orsmark-Pietras et al., 2013), controversially the findings showed that the expression levels of the TAS2R38 gene were four-fold lower in COVID-19 patients who were infected with Delta variant. The comparison of this group to the control group was not as well as no significant differences found between the two COVID-19 variant groups. Supporting Risso et al., (2022), the TAS2R38 genotypes do not alter infection-driven dysgeusia and are not responsible for variances in the presence or severity of COVID-19 (Risso et al., 2022).
Furthermore, TAS2s activate the enteroendocrine cells, resulting in ghrelin level elevation, and an increase in food intake, causing gastric fulfillment, which in long term reduces food intake, in mice. Our study confirmed a positive correlation of the TAS2R38 expression with the LEPR gene and among COVID-19 patients. In another study the food avoidance behavior was observed in mice over-expressing TAS2R38 receptors, suggesting a possible therapeutic target for appetite suppression medicines (Dalesio et al., 2018).
Olfactory receptors (ORs) are expressed in the olfactory epithelium in the nasal cavity and influence the sense of smell. Apart from nasal tissues, the ORs are observed in non-nasal tissues as well, however, the function of these ectopic ORs in cell signaling, survival, and proliferation is still not known well (Pronin & Slepak, 2021). The ORs were primarily identified from nasal epithelium tissue (Buck & Axel, 1991), and a few years ago the ORs genes expression in human and rodents cell lines and tissues was detected in the airways, brain, blood vessels and guts, and some other organs (Dalesio et al., 2018).
Loss of smell was observed in several COVID-19 patients who did not have any coryzal symptoms or substantial nasal infection (Levinson et al., 2020). This discovery is most likely the cause of virus-induced damage to olfactory receptor neurons in the olfactory epithelium. In some individuals, a cytokine storm occurs, affecting the neurological system and sensory organs of smell (Kanjanaumporn et al., 2020). Moreover, the presence of both receptors, ACE2 and transmembrane serine protease 2 (TMPRSS2), are essential for effective SARS-CoV-2 infection in humans in non-neural cells of the olfactory epithelium (Butowt & Bilinska, 2020).
Here, we report the expression level of the OR51E1 gene in COVID-19 patients in comparison with non-infected individuals, the expression levels of the OR51E1 gene (two-and-half-fold) were significantly higher in COVID-19 patients who were infected with SARS-CoV-2 Delta variants compared to the control group, this may be explained that the virus induces damage to olfactory receptor neurons as well as might be correlated with olfactory dysfunction symptoms in COVID-19 positive patients. Although, more clinical studies are urged in this area to elucidate the role of ORs in the context of COVID-19 diseases. A recent in-silico study that was performed to explain the messenger RNA expression profiles for various genes among COVID-19 patients showed that the OR51E1 gene was upregulated within COVID-19 patients’ groups (Jha et al., 2022). On the other hand, there was no statistical difference in the expression of this gene between COVID-19 variants in our study.
The other gene that was targeted in this study was the GHRL, which regulates various physiological processes such as nutritional intake, metabolism, sleep, inflammation, and memory. As the GHRL gene is expressed in different tissues (Groschl et al., 2005), in many inflammatory diseases, it was reported that the level of ghrelin in plasma was increased, including sepsis, ulcerative colitis, rheumatoid arthritis, ankylosing spondylitis, pancreatitis, Crohn’s disease (Baatar et al., 2011). The changes related to sensory perception, hunger, and food-associated pleasure, has been reported in COVID-19 patient (Høier et al., 2021), (Kanjanaumporn et al., 2020). Therefore, it is important to understand the expression level of the GHRL gene and the possible correlation with SARS-CoV-2 infection. Thus, our results emphasized that the expression levels of the GHRL (three-fold) gene were significantly higher in COVID-19 patients who were infected with Delta variant compared to the control group; this might be associated with the high expression level of this gene in various inflammatory diseases and its role in the inflammatory response, as well and can be explained as upregulation in ghrelin receptors in response to inflammation may serve as a mechanism of protective feedback to initiate as an inflammatory response.
Comparing the expression profile of the GHRL gene among the SARS-CoV-2 variant, in this study, however, there was no significant difference between the studied groups, although this gene is observed to be down-regulated in different conditions, further studies might be needed to understand the accurate association with COVID-19 patients. Nevertheless, the current findings indicated that the GHRL gene expression was positively correlated with the OR51E1 gene expression in the positive cases. Overall, the OR51E1 protein plays an important role in physiological function (Dalesio et al., 2018), more studies are needed for further understanding of this correlation in expression.
The LEPR gene encodes the leptin hormone (LEP) that is mostly produced in white adipose tissue and is linked to the amount of body fat. The LEPR gene is located in many tissues other than the hypothalamus, which is its classical target organ such as the placenta, lung, stomach mucosa, endometrium, immune cells, liver, and (Al-Shibli et al., 2019). It works by activating LEPR in the hypothalamus and peripheral organs like the liver to regulate food intake. LEP affects the central nervous system, causing a decrease in food intake and an increase in energy consumption (Viesti et al., 2014).
On the other hand, leptin protein and the LEPR gene have shown to have an important role in inflammation, whereas the LEPR gene is expressed all over the immune system (Guglielmi et al., 2021), and a study of COVID-19 patients shows increased levels of leptin, especially in severe cases in comparison with non-infected individuals (Wang et al., 2021), this might offer a reliable correlation with our study, as it was reported in COVID-19 patients with appetite destruction symptoms, hence our findings interestingly indicated that the expression profile of the LEPR (four-fold) genes was significantly higher in COVID-19 patients who were infected with Delta (p = 0.001).
Also, COVID-19 patients who were infected with Omicron BA.1 variants had two-fold higher LEPR gene expression levels compared to the control group (p = 0.021). Moreover, the comparison of two SARS-CoV-2 variants showed that the expression level of the LEPR gene was found to be slightly significantly down-regulated within COVID-19 patients who were infected with Omicron BA.1 (two-folds) (p = 0.001). Given the potential arguments of the COVID-19 infection might induce neurological damage and it is conceivable that the virus could have a significant impact on food intake, driven by taste and smell dysfunctions, and perhaps the virus spreading to brain areas involved in the hedonic regulation of food intake (Meunier et al., 2021).
There are notable limitations in our study, the limited sample size, and the unknown gene expression level of the detected genes in those individuals before infection. The other limitation is unavailable data on the health condition of those individuals and their clinical history. Despite that, a follow-up study for the same patient after COVID-19 recovery may give insight into the disease progression and relation with gene expression.