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
et al., Sheng
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
L et al.)-Gabrielle Strobel.
Reference:Dolmetsch RE, Pajvani U, Fife K, Spotts JM, Greenberg ME. Signaling to the nucleus by an L-type calcium channel-calmodulin complex through the MAP kinase pathway. Science. 2001 Oct 12;294(5541):333-9. Abstract
See also Perspective by Stephen Ikeda in the same issue.