Editor's note: Science Scan is a roundup of recently published biotechnology-relevant research.
Studying mice, neuroscientists have successfully prevented a molecular event in brain cells that is required for storing spatial memories. Unlike regular mice, the engineered rodents quickly forgot where to find a familiar resting place in a pool of water, the researchers report in the March 7, 2003, issue of Cell.
Their paper is titled "Phosphorylation of the AMPA receptor GluR1 subunit is required for synaptic plasticity and retention of spatial memory." Its authors are at The Johns Hopkins University School of Medicine in Baltimore.
Their experiments are believed to be the first to prove that subtly altering the chemistry of a certain protein can profoundly affect a brain cell's ability to respond to external stimulation. This process, called neuronal plasticity, has long been thought to underlie learning and memory. By genetically changing part of a receptor that binds glutamate - the most important excitatory chemical in the brain - the co-authors created a version of the protein that could not be modified by adding phosphate groups. In their laboratory experiments, preventing the receptor's activity kept it from responding normally to external stimulation, and limited how long an animal could store newly registered recollections.
"Since 1986," the paper's senior author, Richard Huganir, observed, "phosphorylation has been recognized as a key to modulating receptor responses to neurotransmitters like glutamate. Ours is the first demonstration that phosphorylation of a particular target protein mediates the processes we believe are behind learning and memory. This new work," he added, "shows that phosphorylation of this target protein does indeed affect an animal's ability to remember."
Mice with the "phosphate-free" version of the protein, known as GluR1, learned as readily as normal mice to find a hidden platform in a pool of water, but they couldn't recall its position eight hours later. In contrast, normal animals remembered what they'd learned even after 24 hours.
"The spatial learning and memory of rodents is highly developed," noted research psychologist Michela Gallagher, a co-author. "The neuronal processes behind this form of learning," she said, "are a convenient and measurable test of learning and memory. The neuronal plasticity involved in spatial learning may also play a large role," Gallagher went on, "in the wiring' of the brain during development, and in conditions such as epilepsy, addiction, chronic pain and other disorders in which repeated experience creates new memories."
Huganir and his research associates explored the role of receptor phosphorylation in two neuronal processes: long-term depression (LTD) and short-term depression (STD). Those affect a neuron's ability to communicate with neighboring nerve cells at points called synapses. By improving communication with a specific neuron and inhibiting communication with others, new neuronal pathways are formed - each thought to represent a specific memory. Transmission at a given synapse depends on how the local receptors change in response to stimulation - either artificial (applying an electrical stimulation) or natural (looking for the underwater platform in a pool).
Glutamate not only elicits many normal neuronal responses but excessive amounts can actually cause the nerve cells to die. So-called glutamate toxicity is thought to contribute to certain neurological diseases, including epilepsy, stroke and amyotrophic lateral sclerosis. Understanding how glutamate receptors are regulated, the researchers suggest, "could one day affect the treatment of these disorders."
Ritonavir, Oral HIV Drug Forced From Market By Rival Compound, Analyzed By Crystallography
The ability of a compound to exist in multiple solid-state structures has significant impact on the physical properties, performance and safety of an active pharmaceutical ingredient and its formulated products. Hence, control of drug-substance polymorphism is of major importance to drug discovery and development, and is monitored carefully by the regulatory agencies.
One high-profile case of drug polymorphism was ritonavir, a peptidomimetic compound introduced in 1996 to treat HIV-1 infection. Then, in 1998, a lower-energy, more-stable polymorph appeared. It caused slow dissolution of ritonavir's marketed dosage form and compromised the drug's oral bioavailability. That event forced removal of ritonavir's oral capsule formulation from the market.
Scientists at the Massachusetts Institute of Technology in Cambridge, Mass., and TransForm Pharmaceuticals Inc., of Lexington, Mass., carried out high-throughput crystallization experiments to comprehensively explore ritonavir form diversity. Their research is reported in the Proceedings of the National Academy of Sciences dated March 4, 2003. It's titled: "Elucidation of crystal form diversity of the HIV protease inhibitor ritonavir by high-throughput crystallization."
Neuroglobin, An O2-Binding Cerebral Protein, Proves Protective Against Ischemic Stroke In Rat Brain
A recently discovered protein, neuroglobin by name, aims to protect the brain after a stroke. An oxygen-binding molecule with similarities to hemoglobin and myoglobin, it was discovered just two years ago. However, researchers have yet to fully understand its functions within the body. Knowing that neuroglobin is present primarily in the brain, scientists at the Buck Institute for Age Research in Novato, Calif., investigated whether the protein prevents damage during the oxygen deprivation of an ischemic stroke.
To test the protein's effects, researchers injected rats with either antisense nucleotide sequences that block neuroglobin expression, or viral vectors that increase its expression. They simulated a stroke by the conventional ploy of clamping a critical artery in the animal's brain. After the stroke, rats that expressed high levels of neuroglobin performed significantly better on tests of motor, sensory and reflex function, compared to animals that expressed low levels of the experimental protein. Moreover, overexpression of neuroglobin reduced the area of the brain damaged by the oxygen-deprivation episode.
Their work is reported in the Proceedings of the National Academy of Sciences, published the week of March 3, 2003. Its title: "Neuroglobin protects the brain from experimental stroke in vivo."