Gum Nerve Cells Become Tooth-Specific Mesenchymal Stem Cells


Stem cells self-renew and also produce progeny that differentiate into more mature cell types. The neurons and glia that compose nervous systems are examples of mature cells and these cells can be produced from embryonic stem cells, induced pluripotent stem cells, or neural stem cells. However, the reverse does not occur during development; more mature cells do not de-differentiate into less mature cells types. Development tends to be a one-direction event.

However, researchers have now discovered that inside teeth, nervous system cells can transform back into stem cells. This unexpected source of stem cells potentially offers stem cell scientists a new starting point from which to grow human tissues for therapeutic or research purposes without using embryos.

“More than just applications within dentistry, this finding can have very broad implications,” says developmental biologist Igor Adameyko of the Karolinska Institute in Stockholm, who led this new work. “These stem cells could be used for regenerating cartilage and bone as well.”

The soft “tooth pulp” in the center of teeth has been known to contain a small population of tooth-specific mesenchymal stem cells, which can typically differentiate into tooth-specific structures, bones, and cartilage. However, no one has conclusively determined where these stem cells came from. Adameyko hypothesized that if he could trace their developmental lineage, he should be able to recapitulate their development in the laboratory. This might offer new ways of growing stem cells for tissue regeneration.

Adameyko and his and colleagues had already studied glial cells, which are nervous system cells that surround neurons and support them. Several of the nerves that wind through the mouth and gums help transmit pain signals from the teeth to the brain are associated with glial cells.

Adameyko and others used fluorescent labels to mark the glial cells in the gum. When the gum-specific glial cells were observed over time, some of these cells migrated away from neurons in the gums into teeth, where they differentiated into mesenchymal stem cells. These same cells then matured into tooth cells. This work was reported in the journal Nature.

a–c, Incisor traced for 3 days from adult PLP-CreERT2/R26YFP mouse. Note protein gene product 9.5 (PGP9.5)+ nerve fibres (a). b, c, Magnified areas from a. d, e, Incisor traced for 30 days from adult PLP-CreERT2/R26YFP mouse. Note collagen IV+ blood vessels (d). e, YFP+ odontoblasts and adjacent pulp cells. f, Incisor traced for 30 days from Sox10-CreERT2/R26YFP mouse. g–k, Incisor traced for 40 days from PLP-CreERT2/R26Confetti incisor. h–j, Magnified areas from g. Arrow in h indicates a cluster of odontoblasts; arrow in j points at CFP+ and RFP+ cells in proximity to a cervical loop at the base of CFP+ and RFP+ streams shown in g and i. k, Streams of CFP+ and RFP+ pulp cells next to i and j. l, m, Incisor traced for 40 days from PLP-CreERT2/R26Confetti mouse with YFP+ and RFP+ pulp cells adjacent to clusters of odontoblasts with corresponding colours. m, Magnified region from l. n, Stream of pulp cells (arrows) in proximity to the cervical loop; yellow and red isosurfaces mark YFP+ and RFP+ cells. o, p, Progenies of individual MSCs intermingle with neighbouring clones in pulp (o) and odontoblast layer (p), projections of confocal stacks. q, r, Clonal organization of mesenchymal compartment in adult incisor. a–n, Dotted line, enamel organ and mineralized matrix. Scale bars, 100 µm (a, d, f, g, k, l); 50 µm (b, c, e, m–p). CL1 and CL2 indicate labial and lingual aspects of cervical loop. d.p.i., days post-injection. s, Incidence of mesenchymal clones depending on fraction of odontoblasts within the clone. t–v, Proximity of dental MSCs (dMSCs) to cervical loop (CL) correlates with clonal size and proportion of odontoblasts in clone.
a–c, Incisor traced for 3 days from adult PLP-CreERT2/R26YFP mouse. Note protein gene product 9.5 (PGP9.5)+ nerve fibres (a). b, c, Magnified areas from a. d, e, Incisor traced for 30 days from adult PLP-CreERT2/R26YFP mouse. Note collagen IV+ blood vessels (d). e, YFP+ odontoblasts and adjacent pulp cells. f, Incisor traced for 30 days from Sox10-CreERT2/R26YFP mouse. g–k, Incisor traced for 40 days from PLP-CreERT2/R26Confetti incisor. h–j, Magnified areas from g. Arrow in h indicates a cluster of odontoblasts; arrow in j points at CFP+ and RFP+ cells in proximity to a cervical loop at the base of CFP+ and RFP+ streams shown in g and i. k, Streams of CFP+ and RFP+ pulp cells next to i and j. l, m, Incisor traced for 40 days from PLP-CreERT2/R26Confetti mouse with YFP+ and RFP+ pulp cells adjacent to clusters of odontoblasts with corresponding colours. m, Magnified region from l. n, Stream of pulp cells (arrows) in proximity to the cervical loop; yellow and red isosurfaces mark YFP+ and RFP+ cells. o, p, Progenies of individual MSCs intermingle with neighbouring clones in pulp (o) and odontoblast layer (p), projections of confocal stacks. q, r, Clonal organization of mesenchymal compartment in adult incisor. a–n, Dotted line, enamel organ and mineralized matrix. Scale bars, 100 µm (a, d, f, g, k, l); 50 µm (b, c, e, m–p). CL1 and CL2 indicate labial and lingual aspects of cervical loop. d.p.i., days post-injection. s, Incidence of mesenchymal clones depending on fraction of odontoblasts within the clone. t–v, Proximity of dental MSCs (dMSCs) to cervical loop (CL) correlates with clonal size and proportion of odontoblasts in clone.

Before this experiment, it was generally believed that nervous system cells were unable to de-differentiate or revert back to a flexible stem cell state. Therefore, Adameyko said that it was very surprising to see such a process in action. He continued: “Many people in the community were convinced … that one cell type couldn’t switch to the other. But what we found is that the glial cells still very much maintain the capacity” to become stem cells. If stem cell researchers and physicians could master those chemical cues in the teeth pulp that signals glial cells to transform into mesenchymal stem cells, they could generate a new way to grow and make stem cells in the lab.

“This is really exciting because it contradicts what the field had thought in terms of the origin of mesenchymal stem cells,” says developmental biologist Ophir Klein of the University of California, San Francisco, who was not involved in the new work. But it’s also just the first step in understanding the interplay between the different cell populations in the body, he adds. “Before we really put the nail in the coffin in terms of where mesenchymal stem cells are from, it’s important to confirm these findings with other techniques.” If that confirmation comes, a new source of stem cells for researchers will be invaluable, he says.

Identifying Barriers to Cell Reprogramming


A new study from the laboratory of Miguel Ramalho-Santos, associate professor of obstetrics, gynecology and reproductive sciences at the University of California, San Francisco (UCSF), might lead to a faster way to derive stem cells that can be used for regenerative therapies.

Induced pluripotent stem cells or iPSCs, which are made from adult cells by means of genetic engineering and cell culture techniques, behave much like embryonic stem cells. These adult cell-derived stem cells are pluripotent and can be differentiated into heart, liver, nerve and muscle cells. This present work by Ramalho-Santos and his colleagues builds upon the reprogramming protocols that have been developed to de-differentiate mature adults cells into iPSCs.

Ramalho-Santos and his co-workers have been interested in understanding the reprogramming process more completely in order to increase the efficiency and safety of this process. In particular, the Ramalho-Santos laboratory has been examining the cellular barriers that prevent adult cells from being reprogrammed in order to circumvent them and increase the efficiency of stem-cell production. In this present work, Ramalho-Santos’ group identified many of these cellular barriers to reprogramming.

“Our new work has important implications for both regenerative medicine and cancer research,” said Ramalho-Santos, who is also a member of the Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research at UCSF.

In 2012, Shinya Yamanaka from Kyoto University won the Nobel Prize in Physiology or Medicine for his discovery of iPSCs. Yamanaka discovered ways to turn back the clock on adult cells, but the protocol that he developed and others have used for years is inefficient, slow, and tedious. The percentage of adult cells successfully converted to iPS cells is usually rather low, and the resultant cells often retain traces of their earlier lives as mature, fully-differentiated cells.

To make iPSCs, researchers force the expression of pluripotency-inducing genes in adult cells. These four genes (Oct4, Klf4, Sox2, cMyc) have become known as the so-called “Yamanaka factors” and they work to turn back the clock on cellular maturation. However, as Ramalho-Santos explained: “From the time of the discovery of iPS cells, it was appreciated that the specialized cells from which they are derived are not a blank slate. They express their own genes that may resist or counter reprogramming.”

So what are those barriers? Ramalho-Santos continued: “Now, by genetically removing multiple barriers to reprogramming, we have found that the efficiency of generation of iPS cells can be greatly increased.” This discovery will contribute to accelerating the production of safe and efficient iPSCs and other types of other reprogrammed cells, according to Ramalho-Santos.

Instead of identifying individual genes that act as barriers to reprogramming, Ramalho-Santos and others discovered that sets of genes acted in combination to establish barriers to reprogramming. “At practically every level of a cell’s functions there are genes that act in an intricately coordinated fashion to antagonize reprogramming,” Ramalho-Santos explained. These existing mechanisms probably help mature, adult cells maintain their identities and functional roles. Ramalho-Santos explained it this way: “Much like the Red Queen running constantly to remain in the same place in Lewis Carroll’s ‘Through the Looking-Glass,’ adult cells appear to put a lot of effort into remaining in the same state.” Ramalho-Santos also added that apart from maintaining the integrity of our adult tissues, the barrier genes probably serve important roles in other diseases, including in the prevention of certain cancers

To identify these barriers, Ramalho-Santos and his team had to employ cutting-edge genetic, cellular and bioinformatics technologies. They collaborated with other UCSF labs headed by Jun Song, assistant professor of epidemiology and biostatistics, and Michael McManus, associate professor of microbiology and immunology.

They conducted genome-wide RNAi screens that revealed known and novel barriers to human cell reprogramming. Of these, a protein called ADAM29 antagonizes reprogramming as does clathrin-mediated endocytosis, which antagonizes reprogramming by enhancing TGF-β signaling. Also it became apparent that different barrier pathways have a combined effect on reprogramming efficiency. Additionally, genes involved in transcription, chromatin regulation, ubiquitination, dephosphorylation, vesicular transport, and cell adhesion also act as barriers to reprogramming.

Barriers to reprogramming

The hopes are that this knowledge will produce iPSCs faster that are safer to use and differentiate more completely.