10 October. In tomorrow's Science, researchers describe how developing a new method dubbed "pharmacological knock-in" has enabled them to fill in the missing pieces of an unusual signal transduction pathway that links changes in a neuron's membrane potential to the expression of genes important for learning, memory, and the neuron's survival. Other studies have implicated several of the players in this pathway in Alzheimer's disease or neurodegeneration. The paper also helps explain how calcium influx can have different functional consequences depending on its route of entry into the cell.
Ricardo Dolmetsch, Michael Greenberg, and colleagues at Harvard Medical School focused on L-type calcium channels, which reside on neuronal dendrites and cell bodies. Researchers already know that these are not classic channel that sense changes in the membrane potential and then merely open or close to allow or choke off the flow of ions across the membrane. "The channel is acting almost as a receptor in the way that growth factor receptors act," says Greenberg. That is, this calcium channel can lead to gene activation, and it does so through CREB, a transcription factor known to turn on genes involved in memory formation (Greenberg et al., Sheng et al.).
Earlier research had also found that the ion's point of entry determines its effect. For example, calcium coming through the L-type channel activates CREB but calcium passing through NMDA receptors does not. In this paper, the scientists developed a method that allowed them to study the L-type channel independent of the neuron's other calcium channel types. Dolmetsch et al. transfected primary neurons with mutant channels that are resistant to pharmacological channel inhibitors, then blocked the endogenous channels and so were able to study in detail signaling by the inserted channels. This method could also be used to sort out whether, and how, the NMDA channel signals to the nucleus, Greenberg said.
The researchers found that the calcium-binding protein calmodulin, which is tethered to the inside mouth of the channel, senses and binds to incoming calcium and then associates with a particular two-amino-acid motif on the channel's intracellular side. Then it probably forms a complex with other signaling proteins nearby, possibly including ras, and activates the map kinase signal transduction pathway, which culminates in the phosphorylation of CREB and gene expression.
This research is important because it helps cut through a body of sometimes confusing research on calcium's roles in neurons. This ion can, within milliseconds, effect synaptic vesicle release at presynaptic axon terminals when it has entered through N-type channels following membrane depolarization by an action potential. When flowing in through NMDA receptors located on dendritic spines, calcium has local effects facilitating synaptic plasticity. The present study now shows how its entry through L-type receptors can transmit signals from dendrites to the nucleus and effect gene expression within minutes to hours.
The relevance of this research to disease is indirect. Generally, calcium influx through the L-type receptor is protective, while excessive calcium entering through NMDA receptors can become toxic to neurons. One of the genes expressed through the calcium-CREB pathway studied here is that for brain-derived-neurotrophic factor (BDNF), a growth factor known to regulate neuronal survival (Ghosh A, et al). BDNF has been implicated in Alzheimer's (Marvanova M et al., Kunugi H et al., Siegel GJ et al.), and A-beta42 appears to inhibit CREB-mediated expression of BDNF (Tong L et al.)—Gabrielle Strobel
See also Perspective by Stephen Ikeda in the same issue.
No Available References
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