Because so many of you commented on the ALS entry, I decided to write more about stem cell treatments for this disease.
Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that results from death of so-called “upper” and “lower” motor neurons. Motor neurons enervate skeletal muscles, and the activity of motor neurons allows those muscles to contract. Without contraction of skeletal muscles, the skeleton cannot move and ambulatory movement as we know it, becomes impossible.
There is only one treatment for ALS and that is a drug called riluzole (Rilutek). When neurons start to die, they dump enormous quantities of neurotransmitters into the spaces surrounding the cells, and this neurotransmitter dump causes nearby neurons to die from neurotransmitter overdose. Blocks the glutamate receptor and prevents large quantities of glutamate from binding to the surfaces of neurons en mass and killing them. Riluzole, however, only buys ALS patients time and increases survival by a matter of months (3-5 months). ALS patients die approximately within three-to-five years after receiving their diagnosis. Death typically results from the weakness of those skeletal muscles that are responsible for airway and respiratory control (See Borasio, G. & Miller, R. Clinical characteristics and management of ALS. Semin. Neurosci. 2002;21:155–166).
As you can see, better treatment options are required, and cell transplantation has recently been proposed as a treatment for various neurological disorders (see Miller, R. H. The promise of stem cells for neural repair. Brain Res. 2006;1091:258–264). In 2009, the FDA approved the first phase I trial of intraspinal stem cells as a treatment of ALS. This trial is ongoing, with completion anticipated sometime in 2012.
What causes motor neurons to all of a sudden start dying off? In a small subset of cases, genetic mutations in genes such as super¬oxide dismutase 1 are the reason for motor neuron die-off (see Rothstein, J. D. Current hypotheses for the underlying biology of amyotrophic lateral sclerosis. Ann. Neurol. 2009;65(Suppl. 1):S3–S9 & Ilieva, H., Polymenidou, M. & Cleveland, D. W. Non-cell autonomous toxicity in neurodegenerative disorders: ALS and beyond. J. Cell Biol. 2009;187:761–772). In the vast majority of cases, the exact mechanisms of motor neuron degeneration in ALS are poorly understood. ALS pathogenesis involves multiple cell types and many mechanisms. The events that cause neuronal death in ALS patients include inflammation, oxidative stress, overdose of the neurotransmitter glutamate, and loss of neurotrophic support. Therefore, the spinal is converted into a toxic waste dump that is completely inhospitable for the survival of neurons. The best way to treat this disease is to maintain or restore motor neuron function and roll back the toxic environment in the spinal cord. Also replacing dead neurons is the goal of cell transplantation therapies.
Cell transplantation strategies come in two forms: A) transplantation of neurons (the conductive cells in the nervous system; and B) transplantation of glial cells (the support cells in the nervous system). Neuron transplantation is possible, since neurons can be derived from embryonic stem cells or from existing neural stem cell lines. Neuron transplantation has been studied in an ALS model in rodents. See the following papers: a) Bonner, J. F., Blesch, A., Neuhuber, B. & Fischer, I. Promoting directional axon growth from neural progenitors grafted into the injured spinal cord. J. Neurosci. Res. 88, 1182–1192 (2010). b) Silani, V., Calzarossa, C., Cova, L. & Ticozzi, N. Stem cells in amyotrophic lateral sclerosis: motor neuron protection or replacement? CNS Neurol. Disord. Drug Targets 9, 314–324 (2010). c) Xu, L., Ryugo, D. K., Pongstaporn, T., Johe, K. & Koliatsos, V. E. Human neural stem cell grafts in the spinal cord of SOD1 transgenic rats: differentiation and structural integration into the segmental motor circuitry. J. Comp. Neurol. 514, 297–309 (2009). d) Yan, J. et al. Extensive neuronal differentiation of human neural stem cell grafts in adult rat spinal cord. PLoS Med. 4, e39 (2007). Unfortunately, when it comes to transplanting motor neurons, there are some daunting practical issues: grafted neurons must receive functional synapses, send axons through inhibitory white matter, and direct axons over long distances to the target muscles in order to retain neuromuscular function. Given these limitations, direct replacement of motor neuron populations is unlikely to provide a viable treatment option for ALS.
Transplantation of glial cells, such as astrocytes and microglia is a much more practical possibility for ALS treatment. Astrocytes and microglia contribute to ALS pathology by impaired metabolic support, compromised neuron–glia crosstalk, or release of toxic metabolites. By replacing diseased glia cells, the pathology of ALS can be effectively short-circuited and the environment of the spinal cord is ameliorated. Experiments, once again in rodents, have shown that transplantation of astrocytes that express the wild-type SOD1 allele can reduce the degeneration and death of motor neurons expressing mutant SOD1 (see Boucherie, C., Schafer, S., Lavand’homme, P., Maloteaux, J. M. & Hermans, E. Chimerization of astroglial population in the lumbar spinal cord after mesenchymal stem cell transplantation prolongs survival in a rat model of amyotrophic lateral sclerosis. J. Neurosci. Res. 2009;87:2034–2046; & Clement, A. M. et al. Wild-type nonneuronal cells extend survival of SOD1 mutant motor neurons in ALS mice. Science 2003;302:113–117). Other experiments that transplanted glial-restricted progenitor (GRP) cells into the spinal cords of mutant SOD1 rats showed that such GRPs differentiates into astrocytes that restored the levels of astrocyte physiology, decreased glutamate levels in the spinal cord and extended the survival of the transplanted rats (Lepore, A. C. et al. Focal transplantation-based astrocyte replacement is neuroprotective in a model of motor neuron disease. Nat. Neurosci. 2008;11:1294–1301). These experiments indicate that cellular replacement therapies might support motor neurons in ALS by maintaining a more hospitable microenvironment in the spinal cord.
Another therapeutic strategy for ALS patients is to use growth factors to protect the axons that extend from the motor neurons in the spine to the skeletal muscles in the limbs and body wall. Axonal defects that include degeneration of the neuromuscular junction and distal axon are some of the earliest hallmarks of ALS. Degeneration of axons occurs before the onset of symptoms and the death of the motor neurons (Fischer LR & Glass JD. Axonal degeneration in motor neuron disease. Neurodegener. Dis. 4, 431–442 (2007). This suggests that axonal dysfunction is a consequence of a loss of trophic support. To understand trophic support, consider that you have to take a very long trip to bring something to a client. Without gas stations along the way, you would never make it to your destination, since your care would run out of gas. Axons that extend from neurons are the same way. In order to make it all the way to their target muscle, they need gas stations along the way. Once they attach to their target muscle, the muscle secreted molecules that allows the axon to survive. However, glial cells along the way provided molecules that helped the axon survive as well. Without this constant input of pro-survival molecules (so-called “trophic factors”), the axons retract and lose their neuromuscular junction with the muscle. This is what is meant by trophic support.
Transplanted stem cells that secrete neurotrophic factors might provide a strategy to protect the diseased neurons. Trophic support of axons by growth factors and insulin-like growth factor I (IGF‑I) have been shown to provide neuroprotection in both in vitro and in vivo models of ALS and reduce motor neuron degeneration (see Sakowski, S. A. et al. Neuroprotection using gene therapy to induce vascular endothelial growth factor‑A expression. Gene Ther. 16, 1292–1299 (2009) & Sakowski, S. A., Schuyler, A. D. & Feldman, E. L. Insulin-like growth factor‑I for the treatment of amyotrophic lateral sclerosis. Amyotroph. Lateral Scler. 10, 63–73 (2009). Unfortunately, recent clinical trials showed that subcutaneous delivery of IGF‑I had no therapeutic benefit in ALS, since the growth factor could not reach the motor neurons in the spinal cord (Sorenson, E. J. et al. Subcutaneous IGF‑1 is not beneficial in 2‑year ALS trial. Neurology 71, 1770–1775 (2008). Intraspinal transplantation of stem cells that are capable of secreting IGF‑I could potentially overcome this limitation and provide therapeutic levels of IGF‑I directly to motor neurons.
More and more data point to the efficacy of treatments that improvement the spinal cord environment. Transplantation of stem cells that secrete growth factors might do exactly this task. Cortical human neural progenitor cells engineered to secrete glial cell-derived neurotrophic factor (GDNF) provide protection for motor neurons after transplantation into the spinal cords of SOD1G93A transgenic rats (which develop a rodent form of ALS; see Suzuki, M. et al. GDNF secreting human neural progenitor cells protect dying motor neurons, but not their projection to muscle, in a rat model of familial ALS. PLoS ONE 2, e689 (2007). These cells also rapidly differentiated into astrocytes and replaces diseased glial cells. Therefore, transplanted stem cells provided multiple therapeutic benefits. Similarly, transplantation of neural progenitor cells producing either GDNF or IGF‑I into SOD1G93A mice decreases loss of motor neuron loss (Park, S. et al. Growth factor-expressing human neural progenitor cell grafts protect motor neurons but do not ameliorate motor performance and survival in ALS mice. Exp. Mol. Med. 41, 487–500 (2009). Intramuscular delivery of GDNF-producing mesenchymal stem cells in a rat model of ALS also increases neuromuscular contacts, motor neuron survival and lifespan (Suzuki, M. et al. Direct muscle delivery of GDNF with human mesenchymal stem cells improves motor neuron survival and function in a rat model of familial ALS. Mol. Ther. 16, 2002–2010 (2008). Cellular therapies might, therefore, represent a source of neurotrophic support for diseased motor neurons in ALS.
Human spinal stem cells are derived from spinal cord progenitors and differentiate into both neurons and glia. Several rodent studies have confirmed the therapeutic potential of intraspinal HSSC transplantation. Following transplantation, these cells express excitatory amino acid transporters that can restore functional glutamate reuptake around vulnerable motor neurons. Additionally, HSSCs also release neurotrophic factors (Yan, J. et al. Extensive neuronal differentiation of human neural stem cell grafts in adult rat spinal cord. PLoS Med. 4, e39 (2007). Grafted HSSCs express several growth factors, including GDNF and brain-derived neurotrophic factor, and also form synaptic contacts with host motor neurons (Xu, L., Ryugo, D. K., Pongstaporn, T., Johe, K. & Koliatsos, V. E. Human neural stem cell grafts in the spinal cord of SOD1 transgenic rats: differentiation and structural integration into the segmental motor circuitry. J. Comp. Neurol. 514, 297–309 (2009). Several publications show that transplantation of Intraspinal HSSCs delays symptom onset and extends the lifespan in rodent ALS models (see following papers: a) Xu, L. et al. Human neural stem cell grafts ameliorate motor neuron disease in SOD‑1 transgenic rats. Transplantation 82, 865–875 (2006); b) Yan, J. et al. Combined immunosuppressive agents or CD4 antibodies prolong survival of human neural stem cell grafts and improve disease outcomes in amyotrophic lateral sclerosis transgenic mice. Stem Cells 24, 1976–1985 (2006); c) Xu, L., Shen, P., Hazel, T., Johe, K. & Koliatsos, V. E. Dual transplantation of human neural stem cells into cervical and lumbar cord ameliorates motor neuron disease in SOD1 transgenic rats. Neurosci. Lett. 494, 222–226 (2011). Thus, transplanted HSSCs differentiate into multiple cell types, improve the spinal cord milieu, provide neurotrophic support, and form functional synaptic contacts with motor neurons in the spinal cord, forming a multifaceted attack on ALS progression. HSSCs are the ideal stem cell for ALS treatment.
Since cellular therapies have the ability to improve survival and motor function of ALS rodents, the next stop is human trials. In 2009, the FDA approved a human trial that involved intraspinal injection of HSSCs. Choosing the right stem cell for ALS patients will require several clinical trials, but with this trial, HSSCs were chosen. They were injected at two levels in the spinal cord; lumbar and cervical levels. Some patients were injected at both levels and others were injected at one but not the other level. Injections were performed with a special device designed specifically for stabilized injection into the spinal cord at specific locations.
This figure comes from Nicholas M. Boulis, Thais Federici, Jonathan D. Glass, J. Simon Lunn, Stacey A. Sakowski & Eva L. Feldman. Translational stem cell therapy for amyotrophic lateral sclerosis. Nature Reviews Neurology, advance online publication, Published online 13 December 2011 | doi:10.1038/nrneurol.2011.191.
The clinical trial will be completed in 2012 and the data should be published soon thereafter. A new era in ALS treatment is dawning and stem cells are leading the way.