Progress and renewal in gustation: new insights into taste bud development

Tongue and Taste Organ Biology and Function: Homeostasis Maintained by Hedgehog Signaling

The tongue is an elaborate complex of heterogeneous tissues with taste organs of diverse embryonic origins. The lingual taste organs are papillae, composed of an epithelium that includes specialized taste buds, the basal lamina, and a lamina propria core with matrix molecules, fibroblasts, nerves, and vessels. Because taste organs are dynamic in cell biology and sensory function, homeostasis requires tight regulation in specific compartments or niches. Recently, the Hedgehog (Hh) pathway has emerged as an essential regulator that maintains lingual taste papillae, taste bud and progenitor cell proliferation and differentiation, and neurophysiological function. Activating or suppressing Hh signaling, with genetic models or pharmacological agents used in cancer treatments, disrupts taste papilla and taste bud integrity and can eliminate responses from taste nerves to chemical stimuli but not to touch or temperature. Understanding Hh regulation of taste organ homeostasis contributes knowledge about the basic biology underlying taste disruptions in patients treated with Hh pathway inhibitors.

Anorexia of aging and its role for frailty

PURPOSE OF REVIEW

The purpose of this review is to examine the concept of anorexia of aging, including its complex pathophysiology and the multifaceted interventions required to prevent adverse health consequences from this geriatric syndrome.

RECENT FINDINGS

Anorexia of aging is extremely common, occurring in up to 30% of elderly individuals; however, this diagnosis is frequently missed or erroneously attributed to a normal part of the aging process. With aging, impairments in smell and taste can limit the desire to eat. Alterations in stress hormones and inflammatory mediators can lead to excess catabolism, cachexia, and reduced appetite. In addition, mood disorders, such as anxiety and depression, are powerful inhibitors of appetite. Anorexia of aging, with its negative consequences on weight and muscle mass, is a risk factor for the development of frailty and is important to screen for, as early intervention is key to reversing this debilitating condition.

SUMMARY

Anorexia of aging is a complex geriatric syndrome and a direct risk factor for frailty and thus should not be accepted as normal consequence of aging. Early diagnosis and formulating a plan for targeted interventions is critical to prevent disability and preserve function in elderly patients.

AP1 transcription factors are required to maintain the peripheral taste system

The sense of taste is used by organisms to achieve the optimal nutritional requirement and avoid potentially toxic compounds. In the oral cavity, taste receptor cells are grouped together in taste buds that are present in specialized taste papillae in the tongue. Taste receptor cells are the cells that detect chemicals in potential food items and transmit that information to gustatory nerves that convey the taste information to the brain. As taste cells are in contact with the external environment, they can be damaged and are routinely replaced throughout an organism's lifetime to maintain functionality. However, this taste cell turnover loses efficiency over time resulting in a reduction in taste ability. Currently, very little is known about the mechanisms that regulate the renewal and maintenance of taste cells. We therefore performed RNA-sequencing analysis on isolated taste cells from 2 and 6-month-old mice to determine how alterations in the taste cell-transcriptome regulate taste cell maintenance and function in adults. We found that the activator protein-1 (AP1) transcription factors (c-Fos, Fosb and c-Jun) and genes associated with this pathway were significantly downregulated in taste cells by 6 months and further declined at 12 months. We generated conditional c-Fos-knockout mice to target K14-expressing cells, including differentiating taste cells. c-Fos deletion caused a severe perturbation in taste bud structure and resulted in a significant reduction in the taste bud size. c-Fos deletion also affected taste cell turnover as evident by a decrease in proliferative marker, and upregulation of the apoptotic marker cleaved-PARP. Thus, AP1 factors are important regulators of adult taste cell renewal and their downregulation negatively impacts taste maintenance.

Adeno-Associated Virus-Mediated Gene Transfer into Taste Cells In Vivo

The sense of taste is achieved by cooperation of many signaling molecules expressed in taste cells, which code and transmit information on quality and intensity of taste to the nervous system. Viral vector-mediated gene transfer techniques have been proven to be useful to study and control function of a gene product in vivo However, there is no transduction method for taste cells in live animals. Here, we have established a method for inducing foreign gene expression in mouse taste cells in vivo by recombinant adeno-associated virus (AAV) vector. First, using enhanced green fluorescent protein (EGFP) as a reporter, we screened 6 AAV serotypes along with a recombinant lentivirus vector for their ability to transduce taste cells. One week after viral injection into the submucosa of the tongue, EGFP expression in fungiform taste cells was observed only in animals injected with AAV-DJ, a synthetic serotype. Next, time course of AAV-DJ-mediated EGFP expression in fungiform taste cells was evaluated. Intragemmal EGFP signals appeared after a delay, rapidly increased until 7 days postinjection, and gradually decreased over the next few weeks probably because of the cell turnover. Finally, the taste cell types susceptible to AAV-DJ transduction were characterized. EGFP expression was observed in PLCβ2-immunoreactive type II and aromatic l-amino acid decarboxylase (AADC)-immunoreactive type III taste cells as well as in cells immunonegative for both PLCβ2 and AADC, demonstrating that AAV-DJ does not discriminate functional taste cell types. In conclusion, the method established in this study will be a promising tool to study the mechanism of taste.

Characterization and Management of Hedgehog Pathway Inhibitor-Related Adverse Events in Patients With Advanced Basal Cell Carcinoma

Abnormal activation of hedgehog pathway signaling is a key driver in the pathogenesis of basal cell carcinoma (BCC). Vismodegib, a first-in-class small-molecule inhibitor of hedgehog pathway signaling, is approved by regulatory authorities for the treatment of adults who have metastatic BCC or locally advanced BCC that has recurred after surgery, or who are not candidates for surgery and who are not candidates for radiation. A second inhibitor, sonidegib, was also recently approved for the same patient group with locally advanced BCC. Adverse events (AEs) commonly observed in hedgehog pathway inhibitor (HPI)-treated patients include muscle spasms, ageusia/dysgeusia, alopecia, weight loss, and asthenia (fatigue). These AEs are thought to be mechanistically related to inhibition of the hedgehog pathway in normal tissue. Although the severity of the majority of AEs associated with HPIs is grade 1-2, the long-term nature of these AEs can lead to decreased quality of life, treatment interruption, and in some cases discontinuation, all of which might affect clinical outcome. The incidence, clinical presentation, putative mechanisms, and management strategies for AEs related to HPIs in advanced BCC are described. These observations represent the first step toward the development of mechanism-based preventive and management strategies. Knowledge of these AEs will allow health care professionals to provide appropriate counseling and supportive care interventions, all of which will contribute to improved quality of life and optimal benefit from therapy.

IMPLICATIONS FOR PRACTICE

The hedgehog pathway inhibitors (HPIs) vismodegib and sonidegib represent a therapeutic breakthrough for patients with advanced basal cell carcinoma. However, the nature of the low-grade adverse events (AEs) commonly observed in HPI-treated patients, including muscle spasms, ageusia/dysgeusia, alopecia, weight loss, and fatigue, can impact clinical outcomes as a result of decreased quality of life and treatment discontinuation. The incidence, clinical presentation, putative mechanisms, and management strategies for AEs related to administration of HPIs are described, with the goal of enabling health care professionals to provide appropriate counseling and supportive care interventions to their patients.

LGN plays distinct roles in oral epithelial stratification, filiform papilla morphogenesis and hair follicle development

Oral epithelia protect against constant challenges by bacteria, viruses, toxins and injury while also contributing to the formation of ectodermal appendages such as teeth, salivary glands and lingual papillae. Despite increasing evidence that differentiation pathway genes are frequently mutated in oral cancers, comparatively little is known about the mechanisms that regulate normal oral epithelial development. Here, we characterize oral epithelial stratification and describe multiple distinct functions for the mitotic spindle orientation gene LGN (Gpsm2) in promoting differentiation and tissue patterning in the mouse oral cavity. Similar to its function in epidermis, apically localized LGN directs perpendicular divisions that promote stratification of the palatal, buccogingival and ventral tongue epithelia. Surprisingly, however, in dorsal tongue LGN is predominantly localized basally, circumferentially or bilaterally and promotes planar divisions. Loss of LGN disrupts the organization and morphogenesis of filiform papillae but appears to be dispensable for embryonic hair follicle development. Thus, LGN has crucial tissue-specific functions in patterning surface ectoderm and its appendages by controlling division orientation.

Feedback, Lineages and Self-Organizing Morphogenesis

Feedback regulation of cell lineage progression plays an important role in tissue size homeostasis, but whether such feedback also plays an important role in tissue morphogenesis has yet to be explored. Here we use mathematical modeling to show that a particular feedback architecture in which both positive and negative diffusible signals act on stem and/or progenitor cells leads to the appearance of bistable or bi-modal growth behaviors, ultrasensitivity to external growth cues, local growth-driven budding, self-sustaining elongation, and the triggering of self-organization in the form of lamellar fingers. Such behaviors arise not through regulation of cell cycle speeds, but through the control of stem or progenitor self-renewal. Even though the spatial patterns that arise in this setting are the result of interactions between diffusible factors with antagonistic effects, morphogenesis is not the consequence of Turing-type instabilities.