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.

Teaching Old Neural Stem Cells New Tricks


In our brains, cells called neurons produce nerve impulses and are responsible for thinking, learning memory, reasoning, and so on. Neurons do not exist in isolation, but in combination with cells called glial cells that support the neurons, nourish them, and protects them from stress damage. Neurons and glial cells are replenished by brain-specific neural stem cell populations in the brain.

Unfortunately, the neural stem cell population in our brains tends to produce far fewer neurons as they age. This deficit of new neurons can play a role in the onset of neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease. Also, our own “senior moments” when we forget where we placed our iPod or car keys comes from a loss of neurons as we age.

Fortunately, some recent research might change this trend. A team from Japan’s Keio University, and the Riken national research institute, has reported the discovery of a small RNA molecule (micro-RNA) that controls neuron production in young mice. When this micro-RNA was manipulated in older mice, their neural stem cells started to make neurons again. The Japanese team also has reasons to believe that the same mechanism is at work in human brains as well. This research was reported in the journal Proceedings of the National Academies of Science. The mechanism is believed to exist in humans as well.

Senior Author Hideyuki Okano said, “We observed the neurogenic-to-gliogenic switching in developing NSCs.” Translation: Okano and his team examined embryonic mouse brains and their neural stem cell (NSC) populations. They found what many other groups have previously observed: that the developing embryonic brain NSCs create neurons first, then switch over to making glial cells later. Okano’s team also discovered the microRNA-17/106-p38 axis that is responsible for this initial neuron-to-glial cell switch during embryonic development.

When they manipulated this embryonic microRNA-17/106-p38 pathway in older, post-natal NSCs in culture, these older post-natal NSCs switched from making glial cells to producing neurons.

In culture, NSCs are difficult to control, since getting large supplies of neurons from cell cultures that various research groups call NSCs is very difficult.

Nevertheless, “there is general agreement that neurogenesis (make neurons) largely precedes gliogenesis (making glial cells) during CNS development in vertebrates,” Okano explained. And adult NSCs, according to Okano, clearly can produce neurons in the body, “whereas they exhibit strong gliogenic characteristics under culture conditions in vitro (that is, in the laboratory).”

Adult NSCs in two regions of the brain—the subventricular zone and hippocampus—also “make neurons, even though transplant studies have shown us that the adult CNS is a gliogenic environment.”

Subventricular Zone

So it seems clear that old NSCs can make neurons, at least under certain conditions. However, it is very difficult to determine the age at which NSCs begin making substantially more glial cells than neurons. According to Okano, “It is difficult to clearly explain the association between total glial cell number and changes in NSC abilities. Moreover, there is less evidence about gliogenic ability of aged NSCs because most of studies about NSCs have mainly focused on the neurogenic ability. “

Still, Okano says: “There are some reports about decline of neurogenesis ability of NSCs with age. These reports indicate that reduction in paracrine Wnt3 factors, and increase of (chemokine) CCL11 concentration in blood, impaired adult neurogenesis in the hippocampus, for example.”

Could the group’s microRNA approach improve memory in humans? Okano believes so, but says more work needs to be done.

“We observed the neurogenic effect by overexpression of miR-17 in primary cultured neurospheres” – spheres of a variety of cells, including NSCs—“derived from the SVZ at postnatal day 30. Similar phenomenon by overexpression of miR-106b-25 cluster has been reported by another group.”

Okano also warns that his approach has only been attempted in cultured cells. He cautioned, “There is no evidence using knock-out mice. Therefore, the functions of them in adult neurogenesis and learning/memory functions are still unclear.”

Next, Okano’s group will develop “a useful method for precise manipulation of cytogenesis from NSCs. “

However, he says, “we think that further understanding of basic molecular mechanisms underlying the neural development is also an important issue.” He will study the ways in which his microRNA system interacts with other glia-producing genes. He wants to fully understand the mechanisms underlying “the end of neurogenic competence and acquisition of gliogenic competence.”

Finally, the group will “examine the significance of miR-17/p38 pathway in various somatic stem cells other than NSCs,” he says.

Finding the Optimal Spot for Stem Cell Injections In Spinal Cord Injured Patients


A gaggle of laboratory animal experiments and clinical studies in human patients have established that stem cell injections into the spinal cord after spinal cord injury promote functional recovery (see Beattie, M. S., et al., Exp. Neurol. 148(2):453‐463; 1997; Bennett, D. L., et al., J. Neurosci. 20(1):427‐437; 2000; Kim HK, et al., PLos One 4(3): e4987 2009; Lu, P.; Tuszynski, M. H. Exp. Neurol. 209(2):313‐320; 2008; McTigue, D. M., et al., J. Neurosci. 18(14):5354-5365; 1998; Widenfalk, J.; Lundströmer, K. J. Neurosci. 21(10):3457‐3475; 2001; also see Salazar DL, et al., PLoS ONE, August 2010; Hooshmand M, et al., PLoS ONE, June 2009; Cummings BJ, et al., Neurological Research, July 2006; and Cummings BJ, et al., PNAS, September 19, 2005).  Stem Cell, Inc., for example, has conducted several tests with human patients using their HuCNS-SC human neural stem cell line, and transplantation of these stem cells promotes functional recovery in human patients who have suffered spinal cord injury.

However, one factor that has yet to be properly determined is the best site for stem cell injection. Previous work by scientists at the Keio University School of Medicine in Japan has shown that injection of neural stem cells and neural progenitor cells (NS/PCs) into non-injured sites by either intravenous or intrathecal (introduced directly into the space under the arachnoid membrane of the brain or spinal cord) administration failed to produce sufficient engraftment of stem cells at the site of injury.

Arachnoid space

Instead cells were trapped in the lungs and kidneys, and many mice even developed fatal lung conditions as a result of intravenous administration (see Takahashi Y., et al., Cell Transplant. 2011;20(5):727-39). These data convinced them that intralesional application of the stem cells (injections directly into the damaged site of the spinal cord) might be the most effective and reliable method for NS/PC tranplantations.

A new study by the Keio group has attempted to ascertain the efficacy of the intralesional injections. Mice with spinal cord injuries were injected with NS/PCs that had been derived from mice that expression glowing proteins. This allowed the injected cells to be tracked with bio-luminescence imaging (BLI).

The principal investigator of this research is Masaya Nakamura from the Department of Orthopedic Surgery at the Keio University School of Medicine. Dr. Nakamura and his team gave mice spinal contusions at the level of the tenth thoracic vertebra. Then some mice were given low doses and others high doses of NS/PCs that were derived from fetal mice (for those who are interested, low dose – 250,000 cells per mouse; high dose – 1 million cells per mouse) nine days after spinal cord injury. These mice were further divided into two groups: those injected at the lesion epicenter (E), those injected at sites at the front and back of the lesion (RC for “rostral/caudal”). Thus there were four groups total: High dose E, High dose RC, Low dose E, and Low dose RC.

All four groups showed better functional recovery than the control group, which was injected with phosphate buffered saline. BLI showed that the number of cells that survived in each of the four cell-transplanted groups was about the same across these groups.  Thus injecting more cells does not lead to greater numbers of surviving neural stem cells.  This makes sense, since the damaged spinal cord in  very inhospitable place for transplanted cells.

However, when the mice were examined for the expression of particular brain-derived neurotropic factors, the expression of such genes was higher in the RC-injected mice than in the E-injected mice. These results seems to explain why the transplanted NS/PCs differentiated more readily into neurons in the RC-injected mice rather than a type of glial cell known as an astrocyte, as was the case in the E-injected mice.

Human Astrocytes
Human Astrocytes

Nakamura and his team interpreted these results to mean that the environments of the E and RC sites can both support the survival of transplanted NS/PCs during the sub-acute phase of spinal cord injury. The authors conclude with a practical note: “Therefore, we conclude that it is optimal to graft a certain threshold number of NS/PCs into the epicenter lesion during the sub-acute phase of SCI, and thereby avoid causing further iatrogenic injury to the intact RC regions of the spinal cord.”

Hopefully Nakamura’s work will be translated into further human clinical trials. One feature of this study is that a particular threshold of stem cells survive when injected into the spinal cord and injecting larger numbers of cells does not increase the number of surviving cells. Injecting more cells might only contribute to the cell debris in the spinal cord. This is certainly a good thing to know when conducting clinical trials with neural stem cells in spinal cord-injured patients.

How Neural Stem Cells Become Neurons and Glia


How do neural stem cells differentiate into neurons or glia? A new paper from researchers at the University of California, Los Angeles (UCLA) seeks to explain this very phenomenon.

Neurons serve as the conductive cells of the nervous system. They transmit electrochemical signals from one neuron to another and provide signals to muscles, glands, and so on. They are responsible for consciousness, thought, learning and memory, and personality.

Despite their immense utility, neurons are not the only cells in the nervous system. Glial cells or just glia support neurons, hold them in place, and supply neurons with oxygen and nutrients and protect them from pathogens.

Glial Cells

When mouse neural stem cells were grown in culture, Wange Lu, associate professor of biochemistry and molecular biology at the Keck School of Medicine, and his colleagues came upon a protein called SMEK1 that promotes the differentiation of neural stem and progenitor cells. SMEK1 also keeps neural stem cells in check by preventing them from dividing uncontrollably.

When Lu and others took a more detailed look at the role of SMEK1, they discovered that it does not work alone, but in concert with a protein called Protein Phosphatase 4 (PP4) to suppress the function of a third protein called PAR3. PAR3 discourages the birth of new neurons (neurogenesis), and PAR3 inhibition leads to the differentiation of neural stem progenitor cells into neurons and glia.

“These studies reveal the mechanisms of how the brain keeps the balance of stem cells and neurons when the brain is formed,” said Wange. “If this process goes wrong, it leads to cancer, or mental retardation or other neurological diseases.”

Neural stem and progenitor cells offer tremendous promise as a future treatment for neurodegenerative disorders, and understanding their differentiation is the first step towards co-opting the therapeutic potential of these cells. This could offer new treatments for patients who suffer from Alzheimer’s, Parkinson’s and many other currently incurable diseases.

This work is interesting. It was published in Cell Reports 5, 593–600, November 14, 2013. My only criticism of some of the thinking in this paper is that neural stem cell lines are usually made from aborted fetuses. I realize that some of these neural stem cell lines come from medical abortions in which the baby had already died, but many of them come from aborted babies. If we are going to use neural stem cells for therapeutic purposes, then we should make them from induced pluripotent stem cells and take them from aborted babies.

Broad Center Scientists Discover Pathway that Controls Neural Stem Cells


A neuroscientist and his coworkers at the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA have published a major findings in the control of nerve cell regeneration. Nerve cells arise from neural stem cells (NSCs), and NSCs can divide and form neurons, the conductive cells in the nervous system, or glial cells, which act as support cells. What are the triggers that cause NSCs to form one or the other?

A research team at UCLA led by Bennett Novitch, assistant professor of neurobiology, has found some answers to this question. According to Novitch, ““One of the greatest mysteries in developmental biology is what constitutes the switch between stem cell proliferation and differentiation. In our studies of the formation of motor neurons — the cells that are essential for movement — we were able to uncover what controls the early expansion of neural stem and progenitor cells, and more importantly, what stops their proliferation when there are enough precursors built up. If the neurons don’t form at the proper time, it could lead to deficits in their numbers and to catastrophic, potentially fatal neurological defects.”

Novitvh’s laboratory has shown that a finely-tuned network of gene expression drives the NSCs to initially divide and then cease dividing and differentiate into neurons and other types of nerve cells.

During the 12 weeks of development (the first trimester), NSCs and progenitor cells that are programmed to make specific nerve cells for a specific zone where the cells stick tightly to each other. This adhesion is medicated by a molecule called “N-Cadherin.” By sticking to each other, the cells expand without differentiating into nerve cells. Once the time comes for cells to motor neurons (those neurons that direct skeletal muscles to contract and move parts of the skeleton), two proteins, Foxp2 and Foxp4, increase in concentration and shut off N-Cadherin expression. The cessation of N-Cadherin expression causes the NSCs to break apart and initiate differentiation.

Novitch explained further, ““We have these cells in a dividing state, making more of themselves. And to make neurons, that process has to be stopped and those contacts between the cells disassembled. Until now, it has not been clear how the cells are pulled apart.”

Elimination of Foxp protein function prevents the normal formation of motor neurons and other mature cells in the nervous system. NSCs that lack Foxp, do not show downregulation of N-cadherin gene. This illustrates the fine tuning of gene expression that is necessary for the normal development of the nervous system.

Novitch discussed the significance of his group’s findings further, ““It’s a fundamental discovery. Most studies have focused on defining what promotes the adhesiveness and self-renewal of neural stem cells, rather than what breaks these contacts. We were also surprised to see how small changes in the degree of cell adhesion can markedly alter the development and structure of the nervous system. It’s all about balance — if you have too many or too few stem and precursor cells, the result could be disastrous.”

Another possible role for the Fox proteins is in cancer, since the inability of cells to exit the cell cycle and differentiate could cause them to divide uncontrollably and accumulate mutations that cause them to grow faster and faster. Alternatively, the Fox proteins might also play a central part in establishing neural networks that development abnormally in patients with cognitive or speech-acquisition disabilities.