Key Protein Reveals Secret Of Stem Cell Pluripotency

A protein that helps maintain mouse stem cell pluripotency has been identified by researchers at the RIKEN Omics Science Center. The finding, published in the August issue of Stem Cells (first published online July 26, 2011), points the way to advances in regenerative medicine and more effective culturing techniques for human pluripotent stem cells.

Through their capacity to differentiate into any other type of cell, embryonic stem cells (ES cells) and induced-pluripotent stem cells (iPS cells) promise a new era of cell-based treatments for a wide range of conditions and diseases. Cultivating such cells, however, commonly relies on the use of so-called “feeder” cells to maintain pluripotency in cell culture conditions. Feeder cells keep stem cells in their undifferentiated state by releasing nutrients into the culture medium, but they have the potential to introduce contamination which, in humans, can lead to serious health risks.

Previous research has shown that mouse pluripotent stem cells can be cultured without feeder cells through the addition of a cytokine called Leukemia Inhibitory Factor (LIF) to the culture media (“feeder-free” culture). LIF is secreted by mouse feeder cells and activates signal pathways reinforcing a stem cell regulatory network. The researchers discovered early in their investigation, however, that the amount of LIF secreted from feeder cells is much less than the amount needed to maintain pluripotency in feeder-free conditions. This points to other, as-of-yet unknown contributing factors.

To clarify these factors, the research group analyzed differences in gene expression between mouse iPS cells cultured on feeder cells and those cultured in feeder-free (LIF treated) conditions. Their results revealed 17 genes whose expression level is higher in feeder conditions. To test for possible effects on pluripotency, they then selected 7 chemokines (small proteins secreted by cells) from among these candidates and overexpressed them in iPS cells grown in feeder-free conditions. They found that one chemokine in particular, CC chemokine ligand 2 (CCL2), enhances the expression of key pluripotent genes via activation of a well-known signal pathway known as Jak/Stat3.

While CCL2 is known for its role in recruiting certain cells to sites of infection or inflammation, the current research is the first to demonstrate that it also helps maintain iPS cell pluripotency. The findings also offer broader insights applicable to the cultivation of human iPS/ES cells, setting the groundwork for advances in regenerative medicine.

Bone Marrow Stem Cells and the Damaged Spinal Cord

Alan Trounson is an Australian stem cell scientist. He did quite a bit of original research in animal reproduction and human in vitro fertilization, but his more recent research interests have been embryonic stem cell research. Recently, he moved to the United States, lured by California’s Stem Cell Initiative and the promise of large amounts of research funds for embryonic stem cell research. He has written a summary of the clinical trials that involve stem cells, but he has used this article to advertise for the California Institute for Regenerative Medicine (CIRM).

In this BMC Medicine article, Trounson and his colleagues write:  “Clinical trials involving use of MSCs for the treatment of neurological disorders is also relatively common (Figure 1), despite little evidence for their conversion to neural cells in vivo.”  

It is often the strategy of embryonic stem cell advocates to cast adult stem cells in as negative light as possible.  This is an unfortunate strategy, since such advocates can tend to oversell or undersell particular therapies and clinical trials.  With respect to mesenchymal stem cells, the ability of these cells to become neurons (the cells that conduct nerve impulses) within the damaged spinal cord is controversial.  For that matter, the ability of most stem cells, even embryonic stem cells and their derivatives, to form neurons within the damaged spinal cord is controversial.  Implantation of undifferentiated embryonic stem cells into the damaged spinal cord can result in tumor formation (Michael J. Howard, et al, “Transplantation of apoptosis-resistant embryonic stem cells into the injured rat spinal cord,” Somatosensory  & Motor Research 22, no. 1-2 (2005): 37-44).   Also, the introduction of undifferentiated cells into an environment as toxic and limiting as a damaged spinal cord often causes those cells to differentiate into a nervous system-specific cell called an “astrocyte.”  While not all astrocytes are created equally, some astrocytes cause chronic pain (also known as allodynia; see Jeannette E Davies, et al, “Transplanted astrocytes derived from BMP- or CNTF-treated glial-restricted precursors have opposite effects on recovery and allodynia after spinal cord injury,” Journal of Biology 7 (2008): 24).  Therefore, it is clear from a variety of studies that the best way to help the spinal cord injury is to differentiate stem cells into the desired cell type and then implant those differentiated cells into the damaged spinal cord.  Therefore, Trounson seems to be dissing mesenchymal stem cells for being able to do something that not even his precious embryonic stem cells can do.

More to the point, bone marrow mesenchymal stem cells can differentiate into neurons and other nervous system-specific cells, but Trounson seems to ignore this evidence:

1.  Mesenchymal stem cells from bone marrow can be induced to form neurons in culture.  In the journal Stem Cells, volume 23, 2005, pages 383-391, Kyung Jin Cho and colleagues used retinoic acid to efficiently convert human mesenchymal stem cells extracted from bone marrow into cells that expressed neurons-specific genes, like MAP2 and nestin, assumed a neuron-like shape that included projections of the cells that looked like axons, decorated the membranes around the axons with proteins that are used by neurons to form connections (synapses) with other neurons, and also secreted a particular neurotransmitter called substance P.  These cells also showed the electrophysiology of true neurons.  These cells were clearly converted into neurons.

Other papers have shown similar results with different protocols.  Zhaohui Cheng and colleagues at the Tongji Medical College in Wuhan, China converted bone marrow mesenchymal stem cells into “neuron-like cells.”  However,  even though these cells expressed nestin and neurofilament, which are specific to neurons, there were no electrophysiological studies that were performed.  Therefore the evidence that these cells were wholly committed to the neuronal fate is unsatisfactory in this case (Journal of Huazhong University of Science and Technology 2009, 29(3): 296-9).

2.  Injection of MSCs into the brains of rodents that had chemical-induced Parkinson’s disease caused substantial improvement in the ability of the animals to perform standardized movement and coordination tests.  Staining the brains of the animals that had received MSC implantations revealed that the injected cells expressed enzymes that certain neurons use to synthesize the neurotransmitter dopamine.  Dopamine is used by the neurons of the midbrain and these are the neurons that die off during active Parkinson’s disease (Li Y, et al, “Intracerebral transplantation of bone marrow stromal cells in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mouse model of Parkinson’s disease,” Neuroscience Letters 316, no. 2 (2001): 67-70).  This shows that MSCs can form dopaminergic neurons in the brains of rodents (in vivo).

3.  In another paper, administration of the pesticide ouabain to the ear of guinea pigs killed off their spiral ganglion neurons and decreased their hearing.  Implantation of human MSCs from bone marrow increased the number of spiral ganglion neurons and restored some hearing ability to the guinea pigs.  Some of these spiral ganglion neurons expressed human genes, which means they could have only come from the transplanted MSCs.  This demonstrates the ability of MSCs to form neurons in vivo (Yong-Bum Cho, et al., “Transplantation of Neural Differentiated Human Mesenchymal Stem Cells into the Cochlea of an Auditory-neuropathy Guinea Pig Model,” J Korean Med Sci. 26, no. 4 (2011): 492-498).

4.  MSCs can be successfully differentiated into nervous system-specific cells (Schwann cells).  Furthermore, implantation of these differentiated cells into damaged spinal cords improves axon myelination and animal motor function (Gerburg Keilhoff, et al, “Transdifferentiation of mesenchymal stem cells into Schwann cell-like myelinating cells,” European Journal of Cell Biology 85, no. 1 (2006): 11-24).  This shows that MSCs can act form neural cells in culture and keep that identity when transplanted into the injured spinal cord.

With respect to the damaged spinal cord, several studies strongly suggest that MSCs improve the damaged spinal cord by mechanisms that do not include the formation of neurons.  Instead, MSCs seem to help the damaged spinal cord by improving the environment within it so that stem cell populations already present in the spinal cord can divide, differentiate and repair the spinal cord (P. Lu, L.L. Jones, M.H. Tuszynski,BDNF-expressing marrow stromal cells support extensive axonal growth at sites of spinal cord injury,” Experimental Neurology 191, no. 2 (2005): 344-360).  Therefore, Trounson’s statement is misleading, since it assumes that the only way stem cells can aid the damaged spinal cord is by transdifferentiating into neurons or glial cells, which is not the case.

Trounson diminishes MSCs as a potential treatment for spinal cord injuries even though clinical trials are being conducted to determine the safety of efficacy of these stem cells to improve the condition of spinal cord injury patients.  These trials are underway because animal studies show that MSCs do provide relief to spinal injured animals, and to not acknowledge this is to not accurately represent the state of stem cell research for the sake of an agenda.