There are a few papers and commentaries that you might find interesting in the latest issue of Regenerative Medicine. The one I'll point out here offers a retrospective and a forecast for the use of pluripotent stem cells in medicine. It is authored by one of the more outspoken figures from the last decade of research and development, but is worth reading regardless of that point. All industries tend to follow what has come to be known as a hype cycle as they reach critical mass and transition into broad adoption and large scale development. Stem cell medicine as a whole had its initial peak of attention and overhyped expectations, followed by a consequent period of disillusionment as people realized that it wasn't a silver bullet for everything, and that, yes, there was actually going to be quite a lot of work involved in turning the new knowledge of stem cell biology and promising early results in transplant therapies into the bigger, better next generation of medicine. All of that largely took place almost decade ago. As is always the case, it is after the initial hype and crash is out of the way that the real work begins in earnest, and at a far greater scale. The ongoing development of cell therapies is now well into this quieter, more productive period of growth; the engineering of reliable treatments that improve upon the prior state of the art.
Not all cell therapies are relevant to aging and rejuvenation, but since it is the case that comparatively simple forms of stem cell transplant can produce a number of benefits in age-damaged tissues, and are an incremental improvement over existing therapies for a number of conditions, a large fraction of the development initiatives in this industry are focused on age-related disease. So far the most reliable benefits are produced by classes of therapy in which stem cells provide signaling that reduces inflammation and spurs greater regenerative activity in native cell populations. In most cases the transplanted cells don't integrate or stick around for the long term, however. Stem cell treatments for the sort of inflammatory joint issues prevalent in the old are an example of the type, as are most treatments involving mesenchymal stem cells. Regeneration of internal organ damage and effective treatment of age-related disease has proven to be a more elusive goal, however. Benefits are observed, but reliability is a real challenge. A great deal of the data is hidden from view, given that most patients are treated via medical tourism and formal trials are an expensive and slow business.
Given all of this, it is fair to say that a large fraction of the effort and funding in stem cell medicine has little to do with addressing aging directly. The therapies are compensatory in nature, and do not target the causes of degenerative aging relevant to the space of cell therapies. Insofar as aging is in part a problem of cell loss on the one hand and a mix of damage and declining activity in stem cell populations on the other, we would want to see cell therapies that can replace lost cells (such as muscle cells in order to reduce frailty) and deliver fresh new stem cell populations that will integrate into tissues and take up the work of their predecessors. The latter is the preferable approach, as new stem cells that work as they did when youthful should solve the problem of lost cells and weakened tissues as a matter of course. The challenge here is that it appears that much of the problem of stem cell decline in aging is driven by changes in signaling in tissue, which in turns results from the varied forms of cell and tissue damage that cause aging. Stem cell decline is a reaction to damage, possibly an evolved response that serves to balance death by cancer on the one hand and death by failing organs on the other. Replacing stem cell populations with new, pristine cells is certainly needed, but will probably be of only limited benefit without inroads into other forms of rejuvenation therapy that can lift the burden of damage and thus revert the signaling environment to a more youthful state.
Pluripotent stem cells (PSCs) can differentiate into virtually any cell type in the body, making them attractive for both regenerative medicine and drug discovery. Over the past 10 years, technological advances and innovative platforms have yielded first-in-man PSC-based clinical trials and opened up new approaches for disease modeling and drug development. Induced PSCs have become the foremost alternative to embryonic stem cells and accelerated the development of disease-in-a-dish models. Over the years and with each new discovery, PSCs have proven to be extremely versatile.
In 2006, it had been 8 years since the initial isolation of human embryonic stem cells (hESCs) and incremental scientific progress was being made. However, ethical dilemmas regarding the use and/or destruction of human embryos as well as legislative barriers in several countries hindered hESC research endeavors. Moreover, the need to source several hundred embryos for the creation of hESC lines to cover the diversity of human leukocyte antigen (HLA) phenotypes made clinical translation of embryonic stem cell (ESC) based therapies seem difficult. This situation precipitated major initiatives to find alternatives. Single blastomere technology is one such alternative; it was developed in 2006 as a nondestructive ESC derivation method and was first demonstrated for mouse ESCs, then adapted for human ESCs in the same year. With this technique, a single cell or 'blastomere' is isolated from a morula (8-cell) stage embryo and, after culture and expansion, can give rise to an ESC line.
Somatic cell nuclear transfer (SCNT) is another alternative for generating hESCs without the destruction of naturally made embryos. This technique has been used successfully in other species such as calves, pigs and mice since the late 1990s and early 2000s, yet for various reasons including the availability of federal funding, institutional review board (IRB) requirements and public sentiment, it took until 2013 for it to be successfully applied to humans. In SCNT, the nucleus of an unfertilized egg is removed and replaced with the nucleus from a somatic cell. Precise culture conditions coupled with maternal factors within the egg promote the reprogramming of the somatic cell nucleus back to a pluripotent state and can give rise to an ESC line. Despite these successes, SCNT has not been widely used for ESC derivation due to the need for high-quality eggs and precise microsurgical techniques. Moreover, the requirement for egg donation is a significant barrier to its widespread use.
Arguably the most important alternative to conventional methods for hESC generation was the invention of induced PSC (iPSC) technology in 2006 and its application to human cells in 2007. iPSC technology avoids the use and destruction of human eggs and/or embryos altogether, thereby largely circumventing ethical controversy. iPSCs are generated through the reprogramming of somatic cells back to an embryonic-like state; the addition of exogenous reprogramming factors triggers this reprogramming process. iPSC technology revolutionized the field of PSC research. Today, generating iPSCs takes many shapes and forms, with different reprogramming factors, different methods for introducing factors to cells, different starting cell types, among others. The technology has undergone a fascinating evolution from its first report in 2006 to the present day and it will continue to evolve in years to come. In 2009-2011, right around the same time that various second-generation reprogramming methods were being developed, reports were starting to emerge that iPSCs were not equivalent to ESCs and that differentiation potential of iPSCs was either impaired or skewed based on the starting somatic cell type. Differences in the somatic cell type used for reprogramming, the specific reprogramming method employed, as well as the extent of culturing are thought to influence the degree of disparity between various iPSC lines and/or ESCs. Yet, in some instances, epigenetic memory can be reduced or even eliminated through subsequent passaging of iPSC clones, or alternatively by differentiation and secondary reprogramming, whereas errors that arise during reprogramming may be corrected through the use of chromatin modifying drugs. Improvements and modifications made to reprogramming methods over the past decade have helped improve the safety and quality of iPSCs such that the development of iPSC-based therapies is moving forward rapidly. In years to come, the development of iPSC-based therapies may overtake conventional hESC-based ones since their generation does not involve the destruction of embryos or even the use of any unfertilized eggs. This is particularly appealing for the long-discussed generation of banks of HLA-matched PSCs to cover patient diversity on a larger scale and reduce or avoid the need for concomitant immunosuppression.
PSCs may be useful for treating a wide variety of diseases given their ability to differentiate, theoretically, into every cell type in the body. The last 5-6 years have seen the PSC field begin to deliver on this promise, with a handful of clinical trials being approved in spinal cord injury, macular degeneration (AMD), diabetes and heart disease. Starting it off in 2009, Geron received investigational new drug (IND) approval to begin testing its hESC-derived oligodendrocyte precursors, GRNOPC1 in a Phase I trial for spinal cord injury. In 2010, a few months before Geron transplanted GRNOPC1 into its first patient, Advanced Cell Technology received IND approval to begin testing hESC-derived retinal pigment epithelium (RPE) for age-related macular degeneration. Around the same time that ACT's 2014 safety data were being published, Japan's RIKEN Institute successfully transplanted the world's first iPSC-derived therapy into humans. They too chose the eye and (wet) AMD as a first indication but decided to transplant autologous iPSC-derived RPE into patients instead of using an off-the-shelf allogeneic cellular product. Given the risks of first-in-human PSC-based therapies, the eye is considered a logical place to begin developing therapies. First, the eye is a locally contained environment, providing a natural barrier to any potentially deleterious cells spreading systemically. Second, its immune-privileged nature may make it more accepting of transplanted allogeneic cells in the long-term. Third, the lens provides a way to noninvasively image the transplantation site over time and functional readouts such as visual acuity are easy to obtain. Indeed, numerous groups have active trials listed. More than a decade of PSC research and development has also led to clinical trials for PSC-derived therapies in other disease areas. In 2014, Viacyte received IND approval to begin a Phase I/II trial to treat Type 1 diabetes. In addition to the above trials, a PSC-derived therapy was approved for an ischemic heart disease Phase I clinical trial in 2013.
The last decade has also seen incredible progress on the development of other PSC-based therapies, some very close to beginning clinical trials. Several groups have made great progress in generating PSC-derived dopaminergic (DA) neurons for the treatment of Parkinson's disease (PD). A long-standing goal for PSC research has been the in vitro generation of glucose-responsive, insulin-producing mature pancreatic β cells to treat diabetes. In 2014, a new protocol was finally able to overcome this challenge and resulted in the in vitro generation of β cells expressing mature pancreatic β cell markers. PSCs are being developed for therapeutic use in various other diseases as well. For example, autologous iPSCs are being generated for patients with the blistering skin disorder, epidermolysis bullosa as part of a cell replacement strategy. In the eye, retinal progenitors are being developed from both ESCs and iPSCs to use as a cell replacement therapy for retinal degenerative diseases, such as retinitis pigmentosa (RP), whereby transplantation of the progenitors would lead to in vivo differentiation and functional engraftment by mature photoreceptors. PSCs are also being developed to maintain the health of endogenous cells at risk for degeneration in various diseases. For example, iPSC-derived macrophages are being manipulated for therapeutic use in Alzheimer's disease (AD) patients. These macrophages have been engineered to express high levels of the β-amyloid-degrading enzyme, neprilysin 2, in an effort to reduce the burden of disease-associated plaques and spare the health of existing neurons in AD. Similarly, in amyotrophic lateral sclerosis (ALS), iPSC-derived neural stem cells may provide therapeutically useful support to endogenous neurons.
Gene editing technologies have been developed to correct disease-causing genetic mutations, functionally replace and/or knock-out expression of dysfunctional genes. Regardless of the editing system employed, the objectives of PSC-based gene editing endeavors fall into two major categories: improving disease models and drug screening systems through the creation of isogenic controls, and gene editing for cell-based therapies. Proof of principle studies includes a report where Crispr/Cas gene editing was used to correct the mutation of the β-globin gene in iPSCs from a β-thalassemia patient. These corrected iPSCs displayed improved differentiation capacity into various types of hematopoietic progenitors and may be one day used as a source of autologous hematopoietic stem cells for transplantation and repopulation of the hematopoietic system. Similarly, Crispr/Cas9 was used to correct a mutation in the gene encoding the RP GTPase regulator in iPSCs derived from a patient with X-linked RP. These corrected cells could in principle be differentiated into photoreceptors or their progenitors and used in cell replacement strategies for RP patients.
The dramatic progress made over the past decade will almost certainly translate into exciting new advancements in decades to come. First-in-man PSC-based clinical trials have thus far shown that PSC-derivatives are safe to use in humans, and provide the impetus for continued clinical trial testing. To date, trials have almost exclusively employed hESCs, yet that is likely to change in the future. Improvements in iPSC quality should enable these ethically sound alternatives to hESCs to catch up or even pass hESC usage in clinical trials. As differentiation procedures and 3D technologies improve, PSCs will become ever more integral to drug screening efforts and disease modeling, although it is unlikely they will ever fully replace the use of in vivo disease models. Another major advancement that will likely drive PSC research in years to come involves the marriage of gene editing technology with PSCs. The ability to precisely correct disease-causing mutations, create isogenic controls and potentially eliminate immunogenicity of PSC derivatives make gene editing in PSCs an incredibly important endeavor. The PSC field will likely produce additional exciting breakthroughs in the coming decade - advancements that could one day make incurable diseases curable.