Anthony Huang, PhD

NIH/NIDCD
SIUSOM Research Seed Grants (RSG).
AAA (the American Association of Anatomists) Fellows Grant Award Program (FGAP).

Immune gustatory processing: immune responses to drugs shape peripheral taste signals

Taste sensations are considered to be an important factor for detecting and digesting food to maintain good nutrition and leading a healthy lifestyle. Many medications produce altered taste perceptions, which then leads to a direct effect on food-related behaviors. Considering that taste disturbances of patients with have long been observed but never understood, ongoing taste research has identified cellular mechanisms of a variety of drugs that cause taste impairments in cancer patients. These findings facilitate a better understanding of the paradigm that immune responses initiated by drugs can alter taste signal transduction pathways during sensory information processing in taste buds. Inevitably, these findings also may facilitate the development of taste modifiers, which ameliorate taste disturbances associated with the chemotherapy on the compensation of taste sensations for losses in medications.

Significance statement
Taste signals transmitted to the brain are substantially determined by tastants activating taste receptors. But they are also substantially modified by the action of drugs during gustatory information processing in taste buds. The findings that immune responses alter the mechanisms of cellular signaling in taste cells explain the basis for taste dysfunctions of cancer patients that has long been observed but never understood.

Introduction
There is an increasing interest in causes of taste disorders by researchers striving to improve the health-related quality of life, especially for individuals who are suffering from diseases. Taste plays an important role in food detection and any taste impairments potentially impact food-related behaviors in ways that lead to problems with weight management and poor life quality. Taste deficits often result from medical treatments. For instance, a common side effect of chemotherapy is abnormal taste, which is seen in about 15–55% of cancer sufferers [1,2,3••,4]. With the increase in global consumption of drugs to treat diseases, the incidences of adverse effects caused by drugs, such as xerostomia (dry mouth), hyposalivation, mucositis and taste disorders (particularly the inability to detect basic tastes among patients evaluated by psychophysical methods), escalate [5,6••,7,8, 9, 10,11•]. Inevitably, taste disturbances can have a substantial negative impact on general health by affecting appetite and influencing food choices, and, as a result, can lead to a significant reduction in quality of life.

Taste buds are peripheral taste organs responsible for sensing taste compounds in food and drink and contain taste receptor cells for recognizing sweet, bitter, umami, sour and salty compounds (and possibly, fat). Distinctive turnover mechanisms promote homeostasis in taste buds, with new cells generated from progenitor cells to replace dying cells throughout life [12,13,14•]. Initiation of taste signaling is coordinated through the activation of G protein-coupled receptors (GPCR) or ion channels expressed on corresponding taste receptor cells. These cells then release the main transmitter, ATP, to activate primary afferent sensory fibers that carry the gustatory information to the brain [15, 16, 17]. Intriguingly, despite the important function of taste sensations that guides dietary selection while avoiding toxins and indigestible materials, taste perceptions may be altered by medications [1,2,4,6••,19,11•,18].

In the taste bud, serotonin (5-HT) is identified to be secreted from taste bud cells and exerts negative paracrine feedback, decreasing the secretion of the afferent transmitter, ATP [20]. These results supported the psychophysical demonstration that human taste thresholds were altered by selective 5-HT reuptake inhibitors such as Prozac [18,21]. Conceivably, altered taste perceptions seen in patients with drug-induced taste disturbances may reflect changes in the peripheral taste organs of the gustatory system [3••,6••,22,23••]. The notion that the functions of taste buds are responsible for the process of altering taste thresholds in disease is beginning to be understood.

Fundamental taste research is essential both to understand the source of dysfunctions and to suggest possible therapies. While the causes of taste disorders are multifactorial, defective peripheral taste signaling resulting in the decreased ability to taste certain types of foods is a common pathophysiological contributor to taste disturbances. Numerous pioneering studies have examined loss of taste due to diseases and treatments over the past few years. The modification of taste signals as well as the aberration in homeostasis of taste buds affected by cancer drugs are likely contributors to taste disturbances associated with medications.

Many drugs cause taste disturbances
Pharmaceutical drugs play a key role in protecting and maintaining human health, and restore health of those who have acute and chronic diseases. Common complaints from patients receiving pharmacological treatments are taste impairments [6••,10,11•,24, 25, 26]. The incidences of adverse chemosensory effects from drugs depend on specific medications [6••,11•]. Despite the fragmented literature regarding loss of taste, hypogeusia (decreased taste sensation) and dysgeusia (distorted taste sensation) are most often described in clinical reports and in the scientific literature to classify the severity of taste disturbances (dysfunctions) of patients [6••,10,11•,24,25]. Nevertheless, the incidence of adverse chemosensory effects from drugs is still quite variable, with several factors involved: including individual differences associated with dosage of drugs, drug interactions initiated by using multiple drugs simultaneously, and the adverse sensory properties of the drug itself, such as bitter or metallic tastes [6••]. Of particular note, taste complaints related to drugs with dysgeusia or hypogeusia as a side effect were predominantly reported in patients taking drugs categorized as either immunomodulating or antineoplastic [11•]. Although the recent literature has summarized the potential incidence, dosage correlation, and the potential mechanisms of drug-induced taste disorders [6••,10,11•,25], the exact mechanisms by which drugs disrupt normal taste signaling cascades remain vague. Therefore, to test direct effects of the taste signaling pathways involving systematic drug administration, recent research has focused on aberrant cellular mechanisms in taste bud cells [3••,23••]. These efforts offered insights into whether pharmaceutical drugs alter the authentic cascades of taste signal transduction pathways such as biochemical targets, cell lineages, and transmission of signals.

Immunomodulatory drugs affect taste bud cells
Evidence accumulated over the past decade has highlighted the presence in taste buds, of various molecules involved in innate immune responses. This suggests that defense responses may regulate the structure and functions of taste buds [27, 28, 29, 30]. Inflammatory stimuli derived from pathogens act on taste tissues via Toll-like receptors (TLRs) [28,31]. Activating TLRs upregulates the expression of cytokines such as interferons (IFNs) and tumor necrosis factors (TNFs), which induce apoptosis and cause abnormal cell turnover in taste buds [27,28,31,32]. Since that early work, the list of pro-inflammatory cytokines has been broadened to include lipopolysaccharide (LPS) that mimics bacterial or viral infection [31]. These findings suggest that taste cells directly respond to pathogenic challenges in the oral cavity, and that immune interactions appear to play a role in taste disorders associated with many conditions and diseases. Despite several potential mechanisms of taste disturbances that have been proposed for drug-induced taste disorders, specific mechanisms have yet to be determined. Immune interactions have not been shown directly to influence peripheral taste signals.

It is certainly possible that a medication sometimes causes changes in the body’s ability to recognize taste sensations [1,2,3••,6••,11•,19]. Notably, numerous studies have identified the cellular mechanisms of imiquimod, an immune response modifier approved by the US Food and Drug Administration (FDA) for treating basal cell carcinoma, a common malignant neoplasm worldwide. Imiquimod initiates proinflammatory cytokine pathways during treatments on cancer patients. Specifically, imiquimod acts on target cells via TLR7, a pattern recognition receptor recognizing pathogen-associated molecular patterns (PAMPs) [33, 34, 35, 36], and thus induces the production of proinflammatory cytokines, including IFN-α and TNF-α [33,35,37, 38, 39, 40]. Importantly, patients on imiquimod therapy complain of taste loss [1]. Clinical reports indicate that Imiquimod enters the circulation and generates peak serum concentrations ranging from 0.5–15 μM [41, 42, 43]. This suggests that imiquimod may penetrate into gustatory epithelia containing taste buds. On isolated taste buds, imiquimod acts on taste cells via TLR7 receptors [23••], found mostly on Type II and Type III cells [28]. Imiquimod appears to activate phospholipase C (PLC) or other intracellular molecular signaling pathways either directly or indirectly by binding to a second messenger [44,45], to trigger transmitter secretion, and, as a result, to shape taste signals transmitted to the brain [23••]. Parenthetically, initiation of taste sensations consists of a highly complex set of cell-to-cell interactions within taste buds, via transmitters, before propagating signals to the brain. Reviews by Roper and Chaudhari [46] and Huang [47] offer additional details regarding the roles of cell-to-cell interactions on taste.

Alternatively, in spite of the modulation of taste signal transmission attributable to the effect of immune responses, imiquimod may also trigger PLC activation in the Golgi apparatus and nucleus [44,45], in which mRNA export, DNA repair, and gene transcription occur [48,49]. Several reports using in vitro cell culture [50] and rodent models [51] have identified nuclear PLCβ, an abundantly represented family of nuclear isozymes which affect G1 progression and thus participate directly in controlling the cell cycle [49,52]. Imiquimod, acting as a potent cell autonomous inhibitor of oncogenic hedgehog signaling, a signaling pathway that transmits information to embryonic cells required for proper cell differentiation [53], may cause taste changes due to the alteration of intracellular molecular signaling pathways in taste buds. Hence, it is feasible that the interruption of taste cell turnover by cancer drugs results in taste disturbances. This notion has been tested in recent studies, which state that impacted physiology of taste by cancer drugs results in taste changes due to the local effects in taste buds [3••,22]. Figure 1 summarizes the postulated scenarios in the schematic taste bud.

Figure 1. (a) Schematic drawing of the tongue epithelium containing a taste bud. Taste cells are divided into certain morphotypes, which are termed types I (I), II (II), III (III), and taste progenitor cells (also named basal cell, B), originally on the basis of their histologic and ultrastructural characteristics. Red boxes represent approximate positions of the taste bud in B and C. n, nerve fibers. (b) Schematic diagram of a gustatory processing unit shows the pathways of imiquimod within taste buds. Imiquimod freely passed across the membrane of Type III cells and triggered certain PLC isoforms in the nucleus (Nu). Activation of PLC initiated Ca2+ release from internal stores, suggesting IP3 acts on the endoplasmic reticulum (ER) via IP3 receptors (IP3Rs). Imiquimod evoked secretion of 5-HT, which then provided negative feedback onto Type II cells to reduce taste-evoked ATP secretion [23]. Parenthetically, imiquimod may cause taste changes due to the alteration of intracellular oncogenic signaling pathways in taste bud cells. En, endosome. (c) In basal cells, the activated smoothened homolog (SMO) and glioma-associated oncogene homologs (GLI) could be suppressed by vismodegib, these lead to the inhibition of target genes to inhibit the growth and proliferation of progenitor taste cells. In summary, new insights into the immune response explain contributions to taste dysfunctions by adversely affected taste transmission as well as by local effects in taste buds.

Disruption of taste cell renewal alters taste sensations
Nearly all types of basal cell carcinoma are associated with genetic alterations in the hedgehog signaling pathway [1,3••,19,53]. Vismodegib, an antineoplastic agent indicated for the treatment of patients with basal cell carcinoma, has an acceptable and manageable tolerability profile characterized by a number of class-related treatment-emergent adverse events, including muscle spasms, alopecia (baldness), weight loss and asthenia (fatigue) [2,19]. Notably, vismodegib also has the adverse effect of taste disturbances [19].

With the recognition of taste cell turnover, which was originally believed to maintain the homeostasis of taste papillae via the hedgehog signaling pathway [12,14•], the renewal of taste cells was thought to be the major mechanism underlying vismodegib-induced taste disturbances [3••,22]. Yang et al. [3••] simulated the effect of vismodegib-treated patients using a mouse model and thus identified mechanisms responsible for taste disturbances during medical treatments. Their findings, that blocking the hedgehog signaling pathway causes profound alternations of taste cells expressing biochemical machinery and taste bud structures for taste sensing (Figure 1c), facilitate the understanding of alterations in vismodegib-induced taste disturbances in cancer patients. In addition, the results of a similar type of approach using LDE225, a hedgehog inhibitor, were consistent with the earlier research, which stated that antineoplastic agents result in the disruption of taste progenitor cells that leads to the alteration of taste perceptions [54]. Hence, the notion that the alterations of human taste perceptions related to vismodegib treatments are most likely due to the aberrant taste cell turnover and the integrity of taste bud functions would be congruous. Collectively, these data provide an explanation for the changes of human taste perceptions due to medications, because vismodegib most likely results in similar alterations in human taste buds.

Conclusions
Considering that new drugs are regularly introduced as treatments for diseases, the prevalence of drug-induced taste disorders in medications will likely only escalate. While our understanding of the impact of gustatory mechanisms initiated by immunomodulating or antineoplastic drugs that cause taste disturbances is still in its infancy, recent studies in human and rodents have begun to establish the gustatory correlates, which pave the way for an exciting field of inquiry. In the coming decade, comprehensive investigations on the cellular mechanisms (intracellular signaling, cell lineage, signal transduction pathways, etc.) of taste bud cells affected by pharmaceutical drugs will be important. Such investigations will increase understanding of altered taste perceptions in terms of approaches to investigate taste disturbances from medications.

Immune gustatory processing