Signals * promising developments for neurology

September 1, 2005

A team of neuroscientists from Purdue University and Nagoya University in Nagoya, Japan, has identified acrolein as a key neurotoxin in the neurodegenerative process that follows traumatic spinal cord injury. Acrolein, a toxic industrial by-product, naturally occurs in the body in negligible quantities as free radical molecules. However, it can quickly accrue to toxic levels in response to trauma, stress, or exposure to environmental carcinogens. Carriage

Is Acrolein Interception Key to Thwarting Neurodegeneration?

A team of neuroscientists from Purdue University and Nagoya University in Nagoya, Japan, has identified acrolein as a key neurotoxin in the neurodegenerative process that follows traumatic spinal cord injury. Acrolein, a toxic industrial by-product, naturally occurs in the body in negligible quantities as free radical molecules. However, it can quickly accrue to toxic levels in response to trauma, stress, or exposure to environmental carcinogens. Carriage of high levels of acrolein has been associated with such slow-developing, so-far untreatable neurologic diseases as Parkinson, Huntington, and Alzheimer disease.

The team, led by Riyi Shi, MD, PhD, associate professor of neuroscience and biomedical engineering at Purdue, decided to study acrolein activity in the setting of another untreatable disorder associated with progressive decline--traumatic spinal cord injury--to better define the role of acrolein in neurodegeneration. Using immunoblotting techniques, they studied the accumulation of the toxin in 25 guinea pigs after compression injury to the spinal cord. The research appears in the March issue of Neurochemical Research.

"We found that levels of acrolein peak 24 hours after [injury], and they remain high for at least a week. Because acrolein has such a long life span and is so toxic, we theorize that it is primarily responsible for the secondary damage that keeps injured spines from healing," explained Shi.

If this is the case, antioxidating agents may prove useful in the treatment of neurodegenerative disease and CNS injury, according to Shi. "Hypertension drugs, which bind to acrolein and detoxify it, are already under study for their added potential to promote liver health. We would like to see whether they also could be modified to treat the conditions we are interested in," said Shi.

The team has begun studying the use of antioxidants in the prevention of acrolein-meditated cell death. Preliminary data on the value of these therapies are in hand, Shi told Applied Neurology, but the investigators are holding off on the release of the data until their study on therapeutics is complete. "But I can tell you that the data look very promising," Shi said.

Shi's collaborators in this research are his student Jian Luo, PhD, who is currently a postdoctoral fellow at Stanford University, and Koji Uchida, PhD, associate professor at the Graduate School of Bioagricultural Sciences at Nagoya University. The citation for their research article is Luo J, Uchida K, Shi R. Accumulation of acrolein-protein adducts after traumatic spinal cord injury. Neurochem Res. 2005;30:291-295. *

Polymers and Nerve Repair

Shi also has been involved in long-term research with coinvestigator Richard B. Borgens, PhD, studying the use of polyethylene glycol (PEG) in the repair of spinal cord injury. Borgens is the Mari Hulman George Professor of Applied Neuroscience and director of the Center for Paralysis Research at Purdue's School of Veterinary Medicine. He made headlines late last year when he reported that "nearly 75%" of a study population of 19 paraplegic dogs that had received an injection of PEG within 72 hours of injury, along with standard veterinary care (corticosteroid therapy, physical rehabilitation, and surgical intervention), rapidly regained function such that they "were able to resume a normal life."

PEG, which can be injected or taken orally, apparently binds only to injured (typically crushed) nerves as a kind of protective adhesive that prevents them from fully rupturing and allows them to heal. Borgens, with Shi, began this course of study in the late 1990s, using experimental rodent models. The dogs in the current study, which was conducted by Borgens and a team from his institution and from Texas A&M University, were veterinary patients receiving PEG as experimental therapy for traumatic spinal cord injury. They were matched with a control group consisting of historical cases of injured dogs that had only received standard care.

The team is currently looking into the mechanism of action of PEG. Borgens cautioned that use in humans is a very long way off, in part because the mechanisms of spinal cord­associated mobility and paralysis in humans are more complex than those of animals. He added that veterinary use is still in the experimental stage.

The citation for this research is Laverty PH, Leskovar H, Breur GJ, et al. A preliminary study of intravenous surfactants in paraplegic dogs: polymer therapy in canine clinical SCI. J Neurotrauma. 2004;21:1767-1777. *

A Novel Approach to Gene Therapy for Diabetic Neuropathy

Researchers from Britain's University of Manchester and from a Richmond, CA­based biotechnology company, Sangamo BioSciences, have hit on a way to cause up-regulation of endogenous vascular endothelial growth factor-A (VEGF-A) to protect against nerve conduction velocity deficits responsible for diabetic neuropathy. "This approach to gene therapy is quite different from previous attempts at treatment, as we do not inject a gene that produces a 'foreign' copy of a therapeutic protein. This is the normal approach and causes problems from immunological side effects," said principal investigator David R. Tomlinson, DSc, PhD, BSc, a professor in the Faculty of Life Sciences at Manchester University.

The novel technique involves administration of a specially engineered zinc finger DNA-binding protein transcription factor (ZFP TF) that stimulates the expression of the endogenous VEGF-A gene and causes comprehensive up-regulation of multiple VEGF-A isoforms in cells. The value of the technique in inhibiting neurodegeneration was demonstrated in rat models of diabetic neuropathy. Four groups of study animals were administered an intramuscular injection of placebo to the right gastrocnemius and an intramuscular injection of ZFP TF­encoded plasmid DNA (SB-509) to the left gastrocnemius at a dose of either 62.5 mg, 125 mg, 250 mg, or 500 mg. The treated rats were compared with a group of nontreated, nondiabetic controls.

Although neurologic deficits in motor and sensory nerve conduction velocities were detected in the placebo-treated site and in the site treated with the lowest dose of SB-509, significant dose-related protection of motor and sensory nerve conduction velocities was observed in the sites treated with SB-509 at doses of 125 mg, 250 mg, and 500 mg, according to Tomlinson, who presented the findings at the recent 65th Scientific Session of the American Diabetes Association, which convened from June 10 to 14 in San Diego. The research, which has moved into clinical trials, marks a potential turning point for the treatment of diabetic neuropathy: it represents a potential cure for patients otherwise at risk for losing limbs.

A phase 1 clinical trial, sponsored by Sangamo BioSciences, Inc., to evaluate the safety of the experimental treatment and its effect on lower limb diabetic neuropathy has just begun. For more information about the trial, including enrollment information, visit NCT00110500?order=18. *

Insights on the Role of Tau in AD

Amyloid-b, tau protein, and neurofibrillary tangles all have been implicated in the development and progression of Alzheimer disease (AD) and other neurodegenerative disorders. Much has been written about amyloid-b. Indeed, another clinical trial--a phase 1 study to test the safety of an agent shown to inhibit amyloid precursor protein and amyloid-b in experimental models--is about to begin. The agent, a positive phenserine D-isomer dubbed Posiphen by its manufacturer, Avonyx, was approved by the FDA for Investigational New Drug status on August 1.

Meanwhile, the recent research news has focused on the role of tau protein. A team from the Neuroscience Research Institute at the University of California in Santa Barbara asserts that tau protein causes pathology when it becomes phosphorylated, and that the enzyme implicated in tau phosphorylation is Cdk5.

The team researched 58,000 small molecules in search of at least one that would inhibit Cdk5 phosphorylating action on tau. They whittled down the search to 400 molecules that met certain criteria and tested the ability of these molecules to inhibit Cdk5. Three molecules emerged from the bunch as candidates for drug development. The first binds to the Cdk5 adenosine triphosphate (ATP) pocket; the second binds to the edge of the pocket, displacing adjacent amino acid residues. The third molecule does not target the locale of the Cdk5 ATP pocket but competes with tau at low concentrations.

Kenneth Kosik, MD, Harriman Professor of Neuroscience Research at the University and codirector of the Institute, called the research an "important step forward toward developing treatments" for AD, and added that much needs to be done in terms of further laboratory testing, leading to experimental trials in animal models of AD.

The research article appears in the July issue of Chemistry and Biology. The reference citation is: Ahn JS, Radhakrishnan ML, Mapelli M, et al. Defining Cdk5 ligand chemical space with small molecule inhibitors of tau phosphorylation. Chem Biol. 2005;12:811-823.

Another group of researchers demonstrated that manipulation of tau protein expression had a striking effect on memory capacity in mice genetically engineered to develop a tau protein­mediated dementia. A team led by Karen H. Ashe, MD, PhD, a professor in the Department of Neurology and the Department of Neuroscience at the University of Minnesota in Minneapolis, said that their findings suggest that abnormal tau--not neurofibrillary tangles--are responsible for AD pathology and that memory potentially can be restored in AD patients by addressing nuances related to amyloid-b and tau. Ashe theorizes, based on her team's past and current research, that amyloid plaques and neurofibrillary tangles may be by-products rather than causes of AD.

The team engineered mice that carry an abnormal form of tau associated with frontotemporal dementia and parkinsonism linked to chromosome 17 (FDTP-17) and related tau abnormalities in AD. When the mouse pups were aged 1 month, they were trained to find a hidden platform on which to rest after a swim in a water maze. The researchers removed the platform after the pups demonstrated the ability to remember its location. They then examined the amount of time the mice spent searching for the platform in the correct locale. As the mice aged and the abnormal tau­mediated time clock for AD set in, the memory capacity of the mice dwindled. However, when the tau transgene was pharmacologically turned off, not only was the disease progression arrested, but the mice also regained their memory function.

"We were quite surprised with the results of the experiment because we had expected that we would halt the progression of memory loss, but we actually restored memory in these mice," said Ashe. She pointed out that nerve cell death occurred and consequent cognitive deficits developed in the mice; however, memory capacity could be salvaged nevertheless. The young animals that lacked neuronal loss showed full recovery of memory function when the tau transgene was turned off. The older animals that had some neuronal loss also demonstrated the ability to recover memory function, but not to the same degree as was seen previously. The poorer outcome was attributed to the higher degree of nerve cell loss associated with disease progression.

Examination of the brains of the experimental mice showed that the extent of neurofibrillary tangles was unrelated to the impact of tau and memory function. Indeed, the team reported that the tangles continued to grow as memory function improved. "There's such a stark contrast between accumulation of neurofibrillary tangles and the improvement in memory that it becomes obvious that the neurofibrillary tangles could not be the cause of memory loss," Ashe said.

She added that the findings hold promise for patients with AD but that researchers still need to identify the precise molecules to inhibit in order to prevent memory loss. She and her colleagues are currently searching for these molecules.

For more information on this study of mice and memory, see Santacruz K, Lewis J, Spires T, et al. Tau suppression in a neurodegenerative mouse model improves memory function. Science. 2005;309:476-481. Also see the review article: Ashe KH. Mechanisms of memory loss in Abeta and tau mouse models. Biochem Soc Trans. 2005;33(pt 4):591-594. *

Glutamate Receptor May be Key to Ameliorating Absence Seizures

Animal research led to the discovery by a team from Wake Forest University School of Medicine in Winston-Salem, NC, of how to interrupt disordered corticothalamic communication to prevent absence seizures. "We know that the cortex communicates with the thalamus continuously, and current theories suggest that when the 'conversation' gets too loud, seizures can occur. We wanted to see if there was a way to calm the dialogue," said study coauthor Georgia Alexander, who is completing graduate work in the Neuroscience Program at the university.

Working with ferret models of petit mal epilepsy, she and Dwayne Godwin, PhD, an associate professor in the Department of Neurobiology and Anatomy, found that inhibition of corticothalamic synaptic transmission was mediated by a group 2 metabotropic glutmate receptor (mGluR). The researchers were able to reduce the amplitude of corticogeniculate excitatory postsynaptic currents by applying group 2 mGluR agonists (LY39268 and DCG-IV) to neurons in the thalamus.

The Wake Forest team pointed out that although mGluR agonists do not prevent dysrhythmic synaptic excitation, they can mute it, and because this type of receptor appears to be silent during normal synaptic processing, it may be more specific to the high-frequency activity associated with the initiation of dysrhythmic thalamic discharges. The study by Alexander and Godwin is the first to show the role of mGluR in the regulation of corticothalamic synaptic transmission. Their hope is that additional in vivo research will lead to the development of drug therapies that target mGluR in an effort to advance treatment of absence and other types of epileptic seizures.

For more information, see Alexander GM, Godwin DW. Presynaptic inhibition of corticothalamic feedback by metabotropic glutamate receptors. J Neurophysiol. 2005;94: 163-175. *