New Cell Type Derived from Embryonic Stem Cells for Possible Treatment of Brain Diseases

This story comes from my alma mater, UC Irvine. Go anteaters!! No really, UC Irvine’s mascot is the anteater, not to be confused with the aardvark.

Edwin Monuki at the Sue and Bill Gross Stem Cell Research Center with his graduate student Momoko Watanabe and other colleagues devised culture conditions to differentiate embryonic stem cells in “choroid plexus epithelial cells.” Monuki and he team were able to make choroid plexus epithelial cells (CPECs) from mouse and human embryonic stem cell (ESC) lines.

Now you are probably reading this and screaming, “what the heck are CPECs?” Calm down, we will explain:

The central nervous system is surrounded by a clear fluid known as cerebrospinal fluid or CSF. CSF flows all around the brain and spinal cord and also flows inside it. The CSF has several functions. These functions include buoyancy, protection, stability, and prevention of stroke (ischemia).

The CSF provides buoyancy to the brain, since the brain is large a potentially heavy. However, by filling the brain from within and around it with CSF decreases the density of the brain. CSF allows the brain to maintain a density that prevent it from collapsing under its own weight. If the brain were denser, then it would compact and cut off the blood supply of the cells in the lower part of the brain. This would kill off neurons.

Protection provided by the CSF comes during those times the head is struck. The CSF prevents the brain from coming into contact with the skull. The stability provided by the CSF

CSF flows throughout the inside of the brain through cavities known as “ventricles.” These ventricles provide reservoirs through which the CSF flows and is absorbed back into the bloodstream. This constant movement of the CSF through the CNS rinses it and removes metabolic wastes from the central nervous system. It also guarantees an even distribution of neural materials through the central nervous system.

CSF also helps prevent strokes, since the low pressure of the CSF in the skull translates into low intracranial pressure, which facilitates blood perfusion.

Now that have seen that CSF is very important in the life of the brain, where does it come from?  The answer is the CPECs make CSF as a fine filtrate from blood plasma. A minority of the CSF is also made by the walls of the brain ventricles. Nevertheless, CSF circulates from the lateral ventricles to the interventricular foramen, to the third ventricle, through the cerebral aqueduct, to the fourth ventricle, through the median aperture and lateral apertures to the subarachnoid space over the brain and spinal cord where arachnoid granulations return the CSF to the bloodstream.

Thus, CPECs make CSF, which helps remove metabolic wastes and other toxic compounds from the brain. In various neurodegenerative diseases, the choroid plexus and CPECs degenerate too and fail to efficiently remove debris and other rubbish from the central nervous system. Transplantation experiments in rodents have demonstrated that implanting a healthy set of CPECs can restore CSF function and slow down the damage done to the brain by neurodegenerative diseases (see Matsumoto et al., Neurosci Lett. 2010 Jan 29;469(3):283-8).  The problem is the lack of good cultures of CPECs.

Monuki and Watanabe and company seem to have fixed this problem. Monuki commented on his publication, “Our method is promising, because for the first time we can use stem cells to create large amounts of these epithelial cells (CPECs), which could be utilized in different ways to treat neurodegenerative diseases.” Monuki is an associate professor of pathology and laboratory medicine and cell and developmental biology at UC Irvine.

To make CPECs from ESCs, Monuki and his team differentiated cultured ESCs into neural stem cells. The neural stem cells were then differentiated into CPECs. This strategy makes a great deal of sense, since a neural stem cell population seems to exist in the CPECs (see Itokazu Y et al., Glia. 2006 Jan 1;53(1):32-42.

According to Monuki, there are three ways that these cells could be used to treat neurodegenerative diseases. First, the CPECs could be used to make more CSF and that would help flush out the proteins and other toxic compounds that kill off neurons. The down side of this is that it would also increase intracranial pressure, which is not optimal. Secondly,. the CPECs could be engineered into superpumps that transport high concentrations of therapeutic agents into the brain. Third, cultured CPECs could be used as a model system to screen drugs and other agents that shore up the endogenous CPECs in the patient’s brain.

Monuki’s next step is to develop an effective drug screening system in order conduct proof-of-concept studies to determine how CPECs afect the brain in mouse models of Huntington’s disease and other pediatric neurological diseases.

Cell Transplantation Treatments for Amyotrophic Lateral Sclerosis (ALS)

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.

a | Platform anchored to patient’s spine consists of two bridge rails (blue), one of which is scored at 2-mm intervals to aid regular positioning of injections. Gondola (green) compensates for slight movements in the platform application. Mechanical Z drive (orange) allows precise raising and lowering of a floating cannula. b | Cannula tip is positioned 1 mm medial to dorsal root entry zone. c | Needle penetrates into spinal cord ~4 mm from pial surface. d | Once needle tip is positioned at the target, metal outer sleeve is pulled up, leaving flexible tubing exposed.

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.