Drug Corrects Brain Abnormalities in Mice With Down Syndrome


Down syndrome (DS) results when human babies have three copies of chromosome 21 rather than the normal two copies. However, three copies of pieces of chromosome 21 can also cause DS, and the region of chromosome 21 called the “Down Syndrome Critical Region” can also cause the symptoms of DS. The Down Syndrome Critical Region is located 21q21–21q22.3. Within this region are several genes, that, when present in three copies, seem to be responsible for the symptoms of DS. These genes are APP or amyloid beta4 precursor protein, SOD1 or Superoxide dismutase, DYRK or Tyrosine Phosphorylation-Regulated Kinase 1A, IFNAR or Interferon, Alpha, Beta, and Omega, Receptor, DSCR1 or the Down Syndrome Critical Region Gene 1 (some sort of signaling protein), COL6A1 or Collagen, type I, alpha 1, ETS2 or Avian Erythroblastosis Virus E26 Oncogene Homolog 2, and CRYAz or alpha crystalline (a protein that makes the lens of the eye).

All of these genes have been studied in laboratory animals, and the overproduction of each one of them can produce some of the symptoms of DS. For example, APP overproduction in mice leads to the death of neurons in the brain and inadequate transport of growth factors in the brain (see A.Salehi et al., Neuron, July 6, 2006; and S.G. Dorsey et al., Neuron, July 6, 2006). Also, the overexpression of CRYA1 seems to cause the increased propensity of DS patients to suffer from cataracts. Likewise, overexpression of ETS2 leads to the head and facial abnormalities in mice that are normally seen in human DS patients (Sumarsono SH, et al. (1996). Nature 379 (6565): 534–537).

People can also have only portions of the DS Critical Region triplicated and this leads to graded types of DS that only have some but not all of the symptoms of DS.

Why all this introduction to DS? It is among the most frequent genetic causes of intellectual disability. Therefore, finding a way to improve the cognitive abilities of DS patients is a major goal. T

There is a mouse strain called Ts65Dn mice that recapitulates some major brain structural and behavioral symptoms of DS and these include reduced size and cellularity of the cerebellum and learning deficits associated with the hippocampus.

Roger Reeves at Johns Hopkins University has used a drug that activates the hedgehog signaling pathway to reverse the brain deficits of Ts65Dn mice. Yes you read that right.

A single treatment given the newborn mice of the Sonic hedgehog pathway agonist SAG 1.1 (SAG) results in normal cerebellar morphology in adults.

cerebellum

But wait, there’s more. SAG treatment at birth also improved the hippocampal structure and function. The hippocampus is involved in learning and memory.

hippocampus

SAG treatment resulted in behavioral improvements and normalized performance in a test called the “Morris water maze task for learning and memory. The Morris water maze test essentially takes a mouse from a platform in shallow water and then moves the mouse through the maze and then leave it there. The mouse has to remember how they got there and retrace their steps to get back to the platform before they get too tired from all that swimming. Normally Ts65Dn mice do very poorly at this test. However, after treating newborn Ts65Dn mice with SAG, they improved their ability to find their way back.

SAG treatment also produced other effects in the brain. For example, the ratios of different types of receptors in the brain associated with memory are skewed in Ts65Dn mice, but after treatment with SAG, these ratios became far more normal. Also, the physiology of learning and memory was also more normal in the brains of SAG-treated Ts65Dn mice.

These results are extremely exciting. They confirm an important role for the hedgehog pathway in cerebellar development. Also, they suggest that the development of the cerebellum (a small lobe at the back of the brain involved in coordination and fine motor skills, direct influences the development of the hippocampus. These results also suggest that it might be possible to provide a viable therapeutic intervention to improve cognitive function for DS patients.

This excitement must be tempered. This is an animal model and not a perfect animal model. Also, it is unclear if such a compound will work in humans. Much more work must be done, but this is a fascinating start.

Recovery of the Brain After a Stroke


A stroke results when the brain suffers from “ischemia” or a lack of blood flow for an extended period of time. Blockage in the small vessels that feed blood to the brain can cause a trans-ischemic attack (TIA) or stroke. The lack of oxygen causes localized death of brain cells. The dying cells dump a whole gaggle of molecules into the spaces surrounding nearby brain cells, and these cell-derived molecules can actually poison surrounding cells, thus increasing the area that dies as a result of a stroke.

Stroke pathology

New work from by Henry Ford Hospital researchers in Detroit, Michigan suggests that some of the molecules released by brain cells during a stroke might actually help the brain heal after a stroke. Small RNA molecules or microRNAs that are packaged into lipid-bound vesicles in cells known as exosomes are released by stem cells after a stroke and seem to contribute to neurological recovery.

Exosomes are secreted vesicles that were first discovered nearly 30 years ago. They were, at first, considered little more than garbage cans whose job was to discard unwanted cellular components. However, once cell biologists began to study these little structures, evidence began to accumulate that these dumpsters also act as messengers that convey information to distant tissues. Exosomes contain cell-specific payloads of proteins, lipids, and genetic material that are transported to other cells, where they alter function and physiology.

Exosome_Basics

Therefore, it is little wonder that exosomes can also transport microRNAs. In this present study from the laboratory of Michael Chopp, rats were given experimentally induced strokes, and then the neurological recovery of the rats was examined at the molecular level.

Chopp and his colleagues first isolated mesenchymal stem cells (MSCs) from the bone marrow of their laboratory rats. Then they genetically engineered these MSCs to release exosomes laden with specific microRNAs; in particular miR-133b.

MicroRNAs are a class of post-transcriptional regulators. Since they are usually only about 22 base pairs in length, they are far too short to encode anything. microRNAs usually bind to complementary sequences in the 3’ untranslated region of messenger RNAs, and this binding silences the RNA, which simply means that the RNA cannot be recognized by ribosomes and will not be translated into protein, or that the RNA is degraded by special enzymes that target RNAs bound by microRNAs. Single microRNAs target hundreds genes at a time, and some 60% of all genes are regulated by microRNAs. MicroRNAs are abundantly present in all human cells. They are also highly conserved in organisms ranging from the unicellular algae Chlamydomonas reinhardtii to mitochondria in vertebrates, which suggest that they are a vital part of genetic regulation throughout the plant and animal kingdoms.

The Actions of Small Silencing RNAs (A) Messenger RNA cleavage specified by a miRNA or siRNA. Black arrowhead indicates site of cleavage. (B) Translational repression specified by miRNAs or siRNAs. (C) Transcriptional silencing, thought to be specified by heterochromatic siRNAs.
The Actions of Small Silencing RNAs
(A) Messenger RNA cleavage specified by a miRNA or siRNA. Black arrowhead indicates site of cleavage.
(B) Translational repression specified by miRNAs or siRNAs.
(C) Transcriptional silencing, thought to be specified by heterochromatic siRNAs.

The microRNA known as miR-133b has been shown to enhance the death of prostate cancer cells when they are delivered to them (see Patron JP, Fendler A, Bild M, Jung U, Müller H, et al. (2012) MiR-133b Targets Antiapoptotic Genes and Enhances Death Receptor-Induced Apoptosis. PLoS ONE 7(4): e35345. doi:10.1371/journal.pone.0035345). However, because different cell types show different responses to the same reagents, exposing brain cells to this microRNA after a stroke might elicit a very different response.

By raising or lowering the amount of miR-133b in MSCs, Chopp and his colleagues were able to determine the effects of miR-133b on brains cells after a stroke. Chopp and others injected their genetically engineered MSCs into the bloodstream of rats 24 hours after inducing a stroke in these animals. When the exosomes of the MSCs were enriched in miR-133b, the neurological recovery in the rats was amplified, but when injected MSCs were deprived of miR-133b, the neurological recovery was substantially less.

To measure neurological recovery, researchers separated the disabled rats into several groups and injected each groups with saline, nongenetically-engineered MSCs, MSCs with low levels of miR-133b, and MSCs with high levels of miR-133b. The rats were given behavioral tests 3, 7, and 14 days after treatment. These tests measured the gait of the animals on a grid to determine if the rats could walk on an unevenly spaced grid (foot-fault test). The second test determined how long it took the rats to remove a piece of adhesive tape that was stuck to their front paws.

in every test, the rats injected with miR-133b-enriched MSCs showed superior levels of neurological recovery. Autopsies of these same animals revealed that the rats treated with miR-133b-enriched MSCs had enhanced rewiring of the brain and axonal outgrowth. In the areas of the brain adversely affected by the stroke, the rats showed increased axonal plasticity and neurite remodeling.

Most stroke victims recover some ability to use their hands and other body parts on a voluntary basis, but almost half of all stroke victims are left with some weakness on one side of their body and many are permanently disabled by the stroke.

No treatment presently exists for improving or restoring this lost motor function in stroke patients, mainly because of mysteries about how the brain and nerves repair themselves.

Chopp said, “This study may have solved one of these mysteries by showing how certain stem cells play a role in the brain’s ability to heal itself to differing degrees after stroke or other trauma. Chopp also serves as the scientific director of the Henry Ford Neuroscience Institute.