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.