Cells are often described as factories, a metaphor that adequately describes the swarm of specialized tasks constantly underway in each of the human body’s 100 trillion cells. The factory floor of the cell is so busy and complex that scientists are still discovering new machinery responsible for important jobs, with no clear end in sight. The neurons of the brain have been especially difficult to analyze given their role as communicators, ceaselessly sending and receiving chemical messages called neurotransmitters. Many different proteins are needed to release these signals, and when just one is missing, it can cause disaster.
The CLC family is a group of ion channel proteins known by such disasters. When these channels are missing or not working properly, motor disorders such as myotonia can result, suggesting how important their normal function is to the nervous system. Through the use of genetically-modified mice, where the gene for a single protein can be switched off, scientists can determine what a protein’s job is in the cell’s factory. But the process requires working methodically backwards, analyzing the big problems caused by a defective factory and retracing the steps back to where the target protein should have been working.
Yesterday in Nature Neuroscience, the laboratory of Deborah Nelson, professor of neurobiology, pharmacology and cell physiology, reported on one such investigation of a CLC family member. CLC-3 has not been tied as of yet to any human disease, but when it is deleted in mice, there’s no missing the consequences. Without CLC-3, the hippocampus, a region of the brain involved in learning and memory, slowly degenerates over the first months of a mouse’s life until it has completely vanished by the end of their first year. The retinas of the eye also degenerate in CLC-3 knockout mice, causing blindness during their first month of life. What could CLC-3, a humble ion channel that allows chloride ions to pass through its gate when activated, be doing in normal circumstances to avert such neurological catastrophe?
Vladimir Riazanski and Ludmila Deriy, research associates in Nelson’s laboratory, started with a clue about where CLC-3 lives in the cells of the hippocampus. Before they are released, neurotransmitters must be concentrated into packages called synaptic vesicles, sort of like a car being filled up at a gas station. A 2001 study of CLC-3 found that the protein is located on these synaptic vesicles in hippocampal neurons, suggesting a role for the ion channel in this packaging process. Experiments recording electrical activity from hippocampal regions of CLC-3 knockout and normal mice indicated that something was wrong with the transmission of GABA, the inhibitory neurotransmitter, when CLC-3 went missing.
So Riazanski and his collaborators zeroed in on the process of filling vesicles with GABA in the neurons of the hippocampus. By isolating those extremely small vesicles (on the scale of nanometers), the researchers could look very closely at what CLC-3 is doing to package GABA. The vesicles lacking the ion channel acidified more slowly, researchers discovered – a logical result of losing a channel that allows for the influx of acidifying chloride ions. But without acidification, the GABA vesicles can not be filled as efficiently, leaving vesicles with lower amounts of GABA or no GABA at all. It’s as though the gas station inside GABA neurons is missing its attendants – there’s plenty of fuel, but nobody around to properly fill up the vesicles.
Without sufficient inhibitory GABA being released, surrounding neurons can become over-excited to the point of death, Nelson said, which may explain the hippocampal and retinal damage seen in knockout mice.
“This is the first study to show any effect of CLC-3 on inhibitory transmission,” Nelson said. “It’s this loss of GABA transmission that probably contributes to the imbalance between excitatory and inhibitory signals within the mouse hippocampus, and eventually gives rise to excitotoxicity and cellular loss.”
When neurons become over-excited, the clinical result is usually epilepsy – disordered brain activity that induces seizures. Mice lacking CLC-3 did not appear to suffer from epilepsy, but defects in the channel could contribute to seizure disorders in other species, such as humans. However, in these early days of genetic screnning, CLC channels have yet to be linked to any human cases of epilepsy.
The effect of dysfunctional CLC-3 upon GABA transmission could reach beyond the brain as well. In the pancreas, the role of GABA reverses from an inhibitor to a stimulator, in this case triggering the release of the hormone insulin. Without normal GABA stimulation of insulin release, diabetic symptoms could result, Nelson said. Thus, someone suffering from this double whammy of disease may be the unknowing victim of a CLC-3 mutation, a missing gas station attendant that causes major problems.
“What we would love right now is to be able to screen epileptic populations to look for mutations in the CLC genes,” Nelson said. “We don’t have any candidate mutations at the moment, but these experiments suggest that ClC-3 might be a suitable place to begin our search.”
Riazanski, V., Deriy, L., Shevchenko, P., Le, B., Gomez, E., & Nelson, D. (2011). Presynaptic CLC-3 determines quantal size of inhibitory transmission in the hippocampus Nature Neuroscience DOI: 10.1038/nn.2775