Imagine a tooth that fails to develop properly not because of bacteria or trauma, but because a single gene stops functioning. This is the question that prompted drg. Anrizandy Narwidina, MDSc, Sp.KGA, Ph.D., and a team of researchers from several institutions in Japan to explore one of the most fundamental layers of tooth development: the molecular mechanisms that control dentin-forming cells.
Their findings, published in Biochemical and Biophysical Research Communications in February 2023, revealed something that had not previously been reported. A gene called Iroquois homeobox 3, or Irx3, previously known primarily for its roles in the nervous and cardiovascular systems, appears to be a critical determinant of the proliferation and differentiation of odontoblasts—the mesenchymal cells responsible for producing dentin, the major hard component of teeth.
One Gene, Two Different Cellular Worlds
The study began with a simple question: of the six members of the Irx gene family (Irx1 to Irx6), which ones are active in dental tissue? The team extracted dental tissue from postnatal day 1 (P1) mice and analysed mRNA expression using RT-PCR. The results showed that five of the six genes—Irx1, Irx2, Irx3, Irx5, and Irx6—were detected in dental tissue, while Irx4 was not.
What was interesting was not simply the presence of these genes but their distribution patterns. When the analysis was narrowed to two odontogenic cell lines—the dental epithelial M3H1 cells and dental mesenchymal mDP cells—only Irx3 was expressed in both. M3H1 cells contained Irx1, Irx2, and Irx3, whereas mDP cells contained only Irx3 and Irx5. The only gene present in both epithelial and mesenchymal compartments was Irx3.
Visual confirmation through immunostaining of E13.5-stage tooth embryos reinforced the finding: IRX3 protein was detected in both dental epithelial cells and the dental mesenchymal condensation area. This strategic position strongly suggests that Irx3 plays a role at the intersection of two cellular worlds that communicate with each other during odontogenesis.
When the Gene Is Silenced, Cells Stop Developing
To establish the function of Irx3, the researchers used siRNA-mediated knockdown, a technique that selectively suppresses the expression of a target gene. When Irx3 was silenced in mDP cells, the effects were dramatic. Cell numbers decreased significantly during the first 24–48 hours, indicating impaired proliferation. Furthermore, the cells' ability to differentiate into odontoblasts was also compromised: the formation of mineralised nodules, assessed using Alizarin Red S staining, declined sharply compared with control cells.
“These results indicate that Irx3 plays an important role, at least in part, through regulation of Wnt5a expression during odontoblast proliferation and differentiation.” — Anrizandy Narwidina and colleagues, Biochemical and Biophysical Research Communications, 2023
Experiments using three different siRNA sequences, designed to minimise off-target effects, produced similar results: without Irx3, mineralisation of mDP cells was disrupted.
Irx3 Works Through Wnt5a
The next question was how Irx3 performs its function. The researchers evaluated the expression of various growth factors known to be involved in odontoblast differentiation, including several members of the Wnt and Bmp families. Quantitative PCR analysis identified three molecules that were strongly expressed in mDP cells: Wnt3a, Wnt5a, and Bmp4.
However, when Irx3 was silenced, only Wnt5a was significantly suppressed. The expression of Wnt3a and Bmp4 was unaffected. This finding suggests that the role of Irx3 in odontogenesis is specific: it regulates the differentiation of dental mesenchymal cells, at least in part, by controlling Wnt5a expression—a molecule previously known to contribute to the formation of mineralised nodules and the induction of mineralisation-related genes such as dentine matrix protein-1.
The relationship between Irx3 and Wnt5a did not emerge unexpectedly. In the kidneys, Irx3 interacts with Wnt5a during tubular maturation. In Wilms tumour models, cells deficient in Irx3 show reduced WNT5A expression. The same pattern has now been confirmed in dental tissue, strengthening the hypothesis that the Irx3–Wnt5a relationship represents a regulatory axis conserved across different tissues.
From the Laboratory to Understanding Dental Anomalies
This discovery is more than a molecular biology achievement. Understanding how Irx3 regulates odontoblast proliferation and differentiation opens a new window into developmental dental anomalies whose underlying mechanisms have remained unclear. Mutations in genes belonging to the Irx family have previously been associated with Hamamy syndrome, a rare genetic disorder involving hypodontia and craniofacial dysmorphism.
The study by drg. Anrizandy Narwidina and colleagues fills an important knowledge gap: how dental papilla cells receive the “instructions” to proliferate, differentiate, and ultimately form dentin that serves as the structural backbone of the tooth. The answer, at least in part, lies in a single gene working quietly at the intersection of two cellular worlds—and it is no longer hidden.
Source DOI: https://doi.org/10.1016/j.bbrc.2023.02.004
Authors: Nanda Ayu, drg. Achmad Zam Zam Aghasy, M.Kes.
Photo: Pexels