Transplantation of Neurons Made from Bioreactor-Grown Human Neural Precursor Cells Restores Brains of Rats With Huntington’s Disease


Huntington’s disease (HD) is an inherited disorder of the central nervous system characterized by progressive dementia, involuntary movements, and emotional deterioration.  The brain is affected in patients with HD and the part of the brain that takes the biggest beating is the “neostriatum.”

The term “neostriatum” is almost certainly not a word that you hear terribly often in conversation.  Therefore I will try to explain what it is.  The outer layers of the brain are known as the cerebral cortex and they are composed of so-called “grey matter.”  The cerebral cortex consists of grey matter because it is loaded with cells known as neurons.  Beneath the cortical layer of the brain, lies a whole host of extensions of these neurons that reside in the cerebral cortex.  These extensions are called “axons,” and beneath the cerebral cortex lies white matter, which consists, largely, of bundles of axons.  Think of the neurons are plugs and the axons as extension cords.  The neurons are plugged into each other by means of extension cords that extend from the cerebral cortex.

Now the cerebral cortex is not the only game in town.  There are also clusters of neurons that lie beneath the cerebral cortex called “nuclei.”  One of these nuclei beneath the cerebral cortex plays an extremely important role in voluntary motion, and this structure is called the “basal ganglia.”  Here’s a picture to make things a little clearer:

Basal ganglia

As you can see in the figure, the striatum consists of two structures: the putamen and the caudate nucleus.  The striatum or striate nucleus receives neural inputs from the cerebral cortex and inputs this neural information to the basal ganglia.

From a functional perspective, the striatum helps coordinate motivation with body movement.  It facilitates and balances motivation with both higher-level and lower-level functions – for example, inhibiting one’s behavior in a complex social interaction and fine-motor functions involved in inhibiting small voluntary movement.

Some of the neural outputs from the striatum are excitatory – they stimulate other neurons.  Other signals are inhibitory – they prevent the neurons to which they are connected from becoming stimulated.  Inhibitory neurons release a chemical called “GABA.”  These GABA-using neurons are very important for the work of the striatum, and it is exactly these neurons that die off at the greatest rate in patients with HD.  Therefore, treatments for patients with HD have focused on replacing or protecting these GABA-using neurons.

Experimentally, you can induce an HD-like disease in rodents if you inject a chemical into their brains called quinolinic acid.  Quinolinic acid causes many of the GABA-using neurons in the striatum to pack up and die, and for this reason, this chemical is heavily used in the laboratories of scientists who study HD and HD treatments.

In a paper by Marcus McLeod and others who did their work in the laboratory of Ivar Mendez, who was at Dalhousie University in Nova Scotia, Canada, but has since moved to the University of Saskatchewan, GABA-using neurons were made from cultured human neural precursor cells (hNPCs) and then implanted into the brains of rats that had been injected with quinolinic acid.  The results were spectacularly successful.  This work was published in the journal Cell Transplantation.

A definite twist with this particular paper is the way the GABA-using neurons were grown in culture; they were grown in bioreactors.  Bioreactors are devices that support biological cells, processes, or organisms.  They keep the environment of the cells constant, and provide a far superior way to grow cells or tissues in the laboratory.  McLeod and his colleagues used human neural progenitor cells and grew them to large numbers in bioreactors.  These expanded hNPCs were then differentiated them into GABA-using neurons and then injected into the brains of rats who has been treated with quinolinic acid.

The rat model allows the scientist to inject only one side of the brain with quinolinic acid.  This leaves the intact side of the brain as a control tissue that can be compared with the injected one.  The injected rats showed the characteristic death of the GABA-using neurons and the behavioral features that result from the death of these neurons.  Such animals do not walk normally when they are led through a cylinder, and they have trouble finding their way through a maze.  The animals that received the transplantations of the GABA-using neurons, however, performed almost as well in these tests as normal rats; not quite as well, but almost as well.  The rats treated with quinolinic acid did quite poorly, as expected.

Upon post-mortem examination, the rats transplanted with GABA-using neurons shows a host of new GABA-using neurons in their striatums.  These cells also underwent further maturation after transplantation, and they also made connections with other neurons.

Now this paper shows that the injected cells not only survived the transplantations, but they also matured, made connections and promoted recovery of many of the behavioral symptoms of HD.  This procedure certainly has promise.

Having said all that, there are two caveats to these experiments.  The rodent model is a good model as far as it goes, but it seems clear that the actual human disease turns the environment of the brain into a very inhospitable place.  Transplanted cells in the case of human HD patients do not usually survive terribly well.  It seems to me that treatments like this must be coupled with other treatments that seek to improve the actual cerebral environment.  The second caveat to this experiment is that the neural progenitor cells were taken 10-week-old from aborted fetuses.  While these scientists did not perform the abortions that ended the lives of these babies, it is more than little troubling that this research was done using the corpses of those babies whose lives were prematurely ended.

Nevertheless, despite these caveats, this paper represents a definite advance in the regenerative strategies available to treat HD patients.

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Mouse Model of Huntington’s Disease Shows Replacement of Lost Neurons by Endogenous Stem Cell Populations


Huntington’s disease (HD) is a debilitating and invariably fatal disease that results from mutations in the IT15 gene. IT151 stands for “interesting transcript 15,” but it is more commonly referred to as the “huntingtin” gene. Mutations in the front of the gene (exon 1 for those who are interested) expand a run of CAG codons, and these mutations are probably the result of DNA polymerase slippage. Because CAG codons encode the amino acid glutamine, the mutant proteins contain long polyglutamine repeats and these repeats tend to clump inside neurons.

These protein aggregates form in neurons of the “striatum.” The striatum is a region of the brain that is also called the striate nucleus of the striate body. The striatum receives its name from the fact that it is organized in striped layers of gray and white matter. The striate nucleus is part of the cerebrum or forebrain.

Striatum

Mutant Huntington (Htt) protein has a toxic that causes cell death by means of unknown mechanisms. Clinically, the most obvious symptoms of HD involve involuntary movements of the arms, legs, and face. But the severe cognitive and personality changes are the most devastating to HD patients and most troubling for their caregivers.

Researchers are using animal models of HD to study the disease pathogenesis, to elucidate areas of the brain involved in structural and functional decline, and to evaluate potential therapeutic interventions. These animal models include injecting toxins into the brain to kill off those populations of neurons that typically die in HD patients, and transgenic models in which animals are bred with either extra mutant copies of the Htt gene or a pair of copies of the mutant Htt gene that have replaced the original, normal copies. All of these model systems have limitations, but they are all useful in some way for assessing the pathology of HD.

This long introduction leads us to new data from the laboratory of Steve Goldman, the co-director of the University of Rochester Medical Center’s Center for Translational Medicine. Goldman and his colleagues triggered the production of new neurons in mice that had a rodent form of HD. These new neurons successfully integrated into the brain’s existing neural networks and dramatically extended the survival of the mice.

“This study demonstrates the feasibility of a completely new concept to treat Huntington’s disease, by recruiting the brain’s endogenous neural stem cells to regenerate cell lost the disease,” said Goldman.

One of the types of neurons most commonly affected in HD patients is the medium spinal neuron, which is critical to motor control. Goldman banked on findings from previous studies in his laboratory on canaries. Songbirds such as canaries have the ability to lay down new neurons in the adult brain when mating season comes. The male birds, in response to a flush of male sex hormones,, grow a gaggle of new neurons in the vocal control centers of the brain, and this provides the bird the means to sing specific songs in order to attract mates. This event is known as adult neurogenesis, and Goldman and Fernando Nottebohm of the Rockefeller University discovered this phenomenon in the early 1980s.

“Our work with canaries essentially provided us with the information we needed to understand how to add new neurons to adult brain tissue,” said Goldman. Once we mastered how this happened in birds, we set about how to replicate the process in the adult mammalian brain.”

Humans possess the ability to make new neurons, but Goldman’s lab demonstrated in the 1990s that a font of neuronal precursor cells exist in the lining of the ventricles (these are structures at the very center of the brain and spinal cord that are filled with cerebrospinal fluid). In early development, these cells are actively producing neurons.

Shortly after birth, the neural stem cells stop generating neurons and produce support cells called glia. Some parts of the human brain continue to produce neurons into adulthood, the most prominent example is the hippocampus, where memories are formed and stored. However, the striatum, new neuron production is switched off in adulthood.

Goldman sought to switch neuron production back on in the striatum. He tested a cadre of growth factors that would switch the neural stem cells of the striatum (a region that is ravaged by HD) from producing new glia to producing new neurons. Goldman, however, had some help from his recent work in canaries. Namely that once mating season was upon the birds, targeted expression of brain-derived neurotrophic factor (BDNF) flared up in the vocal centers of the brain, where many new neurons were being produced.

Goldman used genetically engineered viruses to express BDNF and another protein called “Noggin” in the striatum. Goldman and others found that a single intraventricular injection of the adenoviruses expressing BDNF and Noggin triggered the sustained recruitment of new neurons in both normal of R6/2 (HD) mice. These treated mice also showed that the newly formed neurons were recruited to form new medium spiny neurons; the ones destroyed in HD. These new neurons also matured and achieved circuit integration.

Medium Spinal Neuron
Medium Spinal Neuron

Also the treated mice showed delayed deterioration of motor function and substantially increased survival.

When the same experiments were conducted in squirrel monkeys, there was a similar addition of new striatal neurons.

Thus, induced neuronal addition may therefore represent a promising avenue for decreasing the ravages of HD and increasing cognitive ability.

Huntington Disease therapy fails


Huntington Disease in a fatal, inherited disease that causes degeneration of the central nervous system. It clinically manifests itself as severe movement and cognitive problems, and the patient gradually loses control of their body in a slow, painful slide to death that is difficult to watch.

A treatment for Huntington Disease that generated a far amount of hope in the 1990s was to transplant healthy neural tissue from fetuses. In particular, the striatum — the brain region most severely affected in Huntington disease was replaced by fetal neural tissue. Unfortunately, this tissue came from babies who were killed by selective abortion. Unfortunately, clinical follow-up of this approach has shown that technique does not work (Cicchetti, F. et al. Proc. Natl Acad. Sci. USA advance online publication doi:10.1073/pnas.0904239106 (2009).

Source: http://www.glaucoma.org/uploads/eye-anatomy-2012_650.gif
Source: http://www.glaucoma.org/uploads/eye-anatomy-2012_650.gif

University of South Florida neurosurgeon Thomas Freeman and his colleagues have conducted a post-mortem analysis of the brains of three people with Huntington’s disease who received fetal striatal-tissue transplants a decade before they died. The results were rather clear – instead of slowing or stopping the progression of the disease, the grafts degenerated even more severely than the patients’ own tissue.

Early results for this procedure generated some hope.  Animal experiment in rats (Kendall, A. L. et al. Nature Med. 4, 727-729 (1998) and non-human primates (Isacson, O. et al. Nature Med. 1, 1189-1194 (1995) showed that transplanted tissue could replace lost striatal neurons and improve behavioural symptoms.  Also, early clinical results tended to support the efficacy of this technique.  Patients who had received these striatial grafts showed modest improvements and autopsies showed that the grafts of fetal neural tissue had survived and integrated into the brain (see Hauser, R. A. et al. Neurology 58, 687-695 (2002), and Bachoud-Lévi, A.-C. et al. Lancet Neurol. 5, 303-309).

Unfortunately, this more recent examination shows that the animal models were deceptive.  The animals were treated with chemicals that destroyed the striatum but left the rest of the brain intact.  In the case of human patients, the entire brain is diseased, and dying neurons release extensive amounts of neurotransmitters that kill neurons by overdosing them on these neurotransmitters.  The transplanted tissue is killed by neurotransmitter overdose.

This has implications for stem cell treatments of Huntington Disease.  Transplanted stem cells or neural progenitor cells will be subjected to this same cocktail of death.  Therefore another strategy is needed.

Fortunately, some cells can surround transplanted cells and protect them from death by neurotransmitter overdose.  For example, co-transplantation of testicular Sertoli cells with neural grafts not only produce an area of localized immuno-suppression (due to local secretion of GDNF by the Sertoli cells) but they can push stem cells into dopaminergic neurons, which are killed in Parkinson Disease (see Halberstadt C, Emerich DF, Gores P. Expert Opin Biol Ther. 4, (2004): 813-25).  Therefore, some new thinking on this front might provide a new treatment scheme.