Figure 6. Schematic representation of organs-on-a-chip platform. (a) Organs-on-a-chip platforms. Source: 99. Reproduced with permission from Trends Cell Biol. (b) A microfluidic device with liver, tumor, and marrow. Source: 100. Reproduced with permission from Royal Society of Chemistry.
4.3. Gene regulation
The development of sequencing technology has also provided great convenience to the research of organoids. To understand the genes and signaling pathways involved in the development of taste buds, cDNAs were generated from organoids at different days, and performed RNA-Seq (RNA sequencing). The results showed that there are multiple signaling pathways and related genes during taste bud organoids development and different transcriptome landscape at the different growth stages (Figure 7a). Moreover, known transcripts of taste receptors were further clarified that appearing at late-stage organoids, and Tas2r126 was found as an emerging gene in taste bud development (Figure 7b)[71]. Then, the author obtained taste bud organoids derived from Lgr5+ cells of neonatal and adult mice. The confocal images showed neonatal mice generated more taste receptor cells than adult mice[70].
The taste system is regulated by many factors. Take Gli3, TRPV4, Sirt1, TNF, and IL-6 as examples. The members of the Glis family include Gli1,2,3 which are the key transcription factors of the Shh signaling pathway. They exist in two forms: activator and repressor. Hh blocked the inhibition of Smo activity by the twelve-times transmembrane protein Ptc, which reduce the production of the C-terminal truncated Gli repressor GliR. This promote the production of Gli activator GliA, and regulate the expression of target genes[101]. Because the regeneration of adult taste cells is affected by many factors such as aging, drug treatment, and various diseases. Gli3 is the main effector of Shh signaling pathway in adults. So, the researchers wanted to figure out what role Gli3 plays in the renewal of adult taste cells. A study has shown that Gli3 acted as a negative regulator to inhibit the proliferation of taste stem cells and the maturation of taste cells in taste bud organoids[97].
The Trp gene family encodes transient receptor potential (TRP) proteins which have different structures[102] and participates in a variety of physiological functions, affecting cell signal transduction[103]. In mammals, 28 Trp genes have been identified. Among them, TRP-melastatin 5 (TRPM5), polycystic kidney disease-1-like 3 (PKD1L3), polycystic kidney disease-2-like 1 (PKD2L1), and TRPV1t are expressed in taste cells[28,104]. The role of TRPM5 in taste perception has been well studied. In type II taste cells lacking synaptic connections, TRPM5 is activated by corresponding stimuli to generate action potentials, deliver ATP, and promote cell signal transduction[105-107]. However, TRPM5 is not the only ion channel that transmits bitter, sweet and umami tastes. TRPM4 has attracted considerable attention, both scholarly because its mRNA is present in taste cells[108] and its role in taste transduction has not been described[26]. Therefore, both TRPM4 and TRPM5 contribute to taste perception. Their absence will impair the sense of taste to a certain extent. PKD1L3 and PKD2L1 were co-expressed in taste receptor cells and form ion channels through interaction to realize taste perception[109,110]. And, TRPV1t regulates the perception of salty taste in the taste system[111]. A recent study related to TRPV4, another member of the TRP family, has suggested that TRPV4 realized the perception of sour taste by regulating the differentiation of type III taste cells[25].
The loss of taste is affected by many factors, such as aging and medication. In the aging process, the energy storage in the cell will be reduced, which will activate the sirtuins family and participate in the regulation of physiological processes. The sirtuins family includes seven members, which regulate energy metabolism through the protein lysine deacetylations. An updated study about sirt1 showed that SIRT1 inhibitors promoted Lgr5+ taste bud stem cell survival and mitigated radiation-induced oral mucositis in mice[12]. An initial study revealed that taste bud organoids had fast induction of TNF and IL-6 which was similar to native mouse taste epithelia[69]. In other words, taste buds have a unique mechanism to cope with inflammation. The epidermal growth factor (EGF) is a cell secretion factor that regulate the growth and development of hair and teeth. The first proof of the potential regulatory role of EGF in the fungal papilla model was reported in 2008. The author added human recombinant EGF for stimulation and found that EGF increased the number of fungal papillae and promoted the interpapillary tongue through PI3K/Akt, MEK/ERK, and p38 MAPK signals. The proliferation of epithelial cells prevents the rapid increase in the number of papillae induced by SHH destruction[112].
Genes are the basic genetic units that control biological traits. They produce proteins through translation to transmit information and control growth and development. Therefore, changes in the structure and function of genes will lead to many diseases. Gene therapy refers to the use of genetic engineering techniques to transfer normal genes into the cells of diseased patients to replace diseased genes, thereby expressing the lacking products, or by shutting down or reducing abnormally expressed genes, etc., to achieve certain treatment purpose of these genetic diseases. Using taste organoids to study the expression changes of important genes in the process of taste transmission undoubtedly provides a strong support for gene therapy. At the same time, it can speed up the research of taste transduction mechanism.