Since the first isolation of human embryonic stem cells in 1998(1), scientists and doctors have been excited by the potential of using these pluripotent cells (having the ability to develop into any cell type) to grow tissues in the lab for research and implantation to replace damaged or diseased body parts. Recent years have seen major breakthroughs in the nascent field of regenerative medicine, including investigations into the efficacy of stem cell treatments in response to heart attack(2) and stroke(3), and the improvement of motor neuron function following spinal cord injury(4). In vitro maturation of stem cells into pancreatic β-cells (insulin secreting cells) for implantation into patients with type-1 diabetes (T1D) presents an opportunity for regenerative therapies to expand their usefulness and break into mainstream medicine by treating large patient cohorts.
In addition to a large (and expanding) need, T1D also is a strong candidate for a regenerative medicine breakthrough because the pathology is driven by the death of a single cell type, avoiding the complications that would arise from having to integrate multiple tissue types into a functional implant. T1D arises from an autoimmune attack on the insulin secreting islet β cells of the pancreases. Loss of insulin regulation results in hyperglycemia, which drives pathologies in a number of systems throughout the body. Currently there are only two therapeutic interventions. The overwhelmingly common one is exogenous insulin injection multiple times daily in order to regulate blood glucose levels. This involves constant monitoring and can still result in dangerous swings in blood glucose if the treatment protocol is not followed closely enough. The second option is β-cell transplant from a cadaver. In addition to requiring major surgery, this treatment is undesirable in that it requires lifelong immune suppression and the grafts seldom last longer than 5 years(5).
A longer lasting transplant that doesn’t require immunosuppression would represent a huge step forward in the treatment of T1D. Recently, two groups(6,7) have published protocols for differentiating stem cells into pancreatic β-cells. These groups analyzed what is known about the chemical signaling that takes place in the maturation of fetal pancreatic cells into mature, insulin secreting β-cells, and recapitulated this sequence of signaling events in vitro. Each group was able to generate β-cells from both embryonic stem cells, and induced pluripotent stem cells(8). As more and more parents save the umbilical cords of their children, there will be an increased pool of embryonic stem cells available for use in regenerative medicine, and as the protocol is also effective in induced pluripotent stem cells, β-cells that do not require immune modulation for implantation can be created for any patient.
The cells produced in this manner mimic the transcriptional profile(9), and a number of functionally relevant aspects of mature β-cells. They are mono-hormonal, meaning they only produce insulin and not one or both of the other two major pancreatic hormones, which is important because poly-hormonal cells do not secrete insulin in response to glucose. When examined under an electron microscope they display insulin packaged into granules identical to mature β-cells, ready to be secreted. They also show calcium ion flux upon exposure to glucose, which is the first step in a signaling cascade that results in insulin secretion. Most importantly, the in vitro differentiated cells show a repeated secretion of insulin upon multiple glucose stimulations.
Promising finds in the laboratory often don’t translate into effective treatments when taken out of controlled situations and put into the chaos of real biological systems. What’s so exciting about both of these studies is that the β-cells produced by either method effectively reverse T1D in mouse models of the disease. In both cases, fasting blood glucose levels dropped back near normal within about 2 weeks of implantation into diabetic mice. This was shown to be a direct result of the insulin secretion of the implanted cells, as hyperglycemia returned to the mice within 48 hours of removal of the grafts, which coincided with clearance of human insulin from the animals’ bloodstreams. Additionally, one of the studies tracked morbidity in diabetic mice following implantation. 5 of 6 control animals given an implant of non-functional poly-hormonal islet cells had died by the end of the ~4 month trial period, compared to 1 of 6 animals that received functional β-cell transplants.
While these studies advance the state of the science toward clinical relevance, certain hurdles still remain. While the cells developed in these studies functionally resemble β-cells, they are not transcriptionally identical. Slight differences in the transcriptional identity of the cell might prove relevant in certain genetic backgrounds, which would necessitate a fine tuning of the differentiation protocol to get it as close as possible to normal β-cells. In some of the animals receiving β-cell transplants fasting glucose levels actually dropped below what is considered normal in animals with functioning glucose clearance. It’s possible that other types of cells within the pancreas help fine-tune to insulin response of the β-cells. Another concern is that using model organisms with such short life spans will make it impossible to test the longevity of the transplants on time scales relevant to human life, especially considering most T1D cases are diagnosed in childhood. However, none of these complications are insurmountable, and it looks like regenerative medicine is on its way go claiming its first widespread clinical success.
References:
2) Clifford DM, Fisher SA, Brunskill SJ, Doree C, Mathur A, Watt S, Martin-Rendon E. Stem cell treatment for acute myocardial infarction. Cochrane Database of Systematic Reviews 2012, Issue 2. Art. No.: CD006536. DOI: 10.1002/14651858.CD006536.pub3.
3) Bang, O. Y., Lee, J. S., Lee, P. H. and Lee, G. (2005), Autologous mesenchymal stem cell transplantation in stroke patients. Ann Neurol., 57: 874–882. doi: 10.1002/ana.20501
4) MacDonald, J., Liu, X., Qu, Y., et al. (1999), Transplanted Embryonic Stem Cells Survive, differentiate and promote recovery in injured rat spinal cord. Nat. Med., Volume 5, Issue 12: 1410-1412.
5) Atkinson, M.A., Eisenbarth, G.S., and Michels, A.W. (2014) Type 1 Diabetes-progress and prospects. The Lancet, Volume 383, Issue 9911: 4-10.
6) Rezania, A., Bruin, J.E., Arora, P., et al. (2014) Reversal of diabetes with insulin producing cells derived in vitro from human pluripotent stem cells. Nat. Biotechnology, 32: 1121-1133.
7) Pagliuca, F.W., Millman, J.R., Gurtler, M., et al. (2014) Generation of Functional Human Pancreatic β Cells in vitro. Cell Stem Cell, Volume 15, Issue 5: 535-536.
8) Induced pluripotent stem cells are mature cells that are dedifferentiated back into a pluripotent state, resembling embryonic stem cells. This can be accomplished using a number of cell types as a starting point, and is essentially the reverse process of differentiating a stem cell. You expose the cells to a sequential cocktail of signaling molecules to revert them back into stem cells.
9) The transcription profile of a cell is a snap shot of the complete collection of mRNAs within that cell at a given time. This shows the entire collection of genes that are being expressed at that moment, and which splice variants (different versions of the same gene). This is the most detailed way to identify a cell type.
