Scientific Understanding of Consciousness
Dendritic Spines Dispersed Signals Integrated in Nuclear ERK
Science 29 November 2013: Vol. 342 no. 6162 pp. 1107-1111
Long-Distance Integration of Nuclear ERK Signaling Triggered by Activation of a Few Dendritic Spines
Shenyu Zhai, Eugene D. Ark, Paula Parra-Bueno, Ryohei Yasuda
Department of Neurobiology, Duke University Medical Center, Durham, NC 27710, USA.
Max Planck Florida Institute for Neuroscience, Jupiter, FL 33458, USA.
Howard Hughes Medical Institute, Duke University Medical Center, Durham, NC 27710, USA.
The late phase of long-term potentiation (LTP) at glutamatergic synapses, which is thought to underlie long-lasting memory, requires gene transcription in the nucleus. However, the mechanism by which signaling initiated at synapses is transmitted into the nucleus to induce transcription has remained elusive. Here, we found that induction of LTP in only three to seven dendritic spines in rat CA1 pyramidal neurons was sufficient to activate extracellular signal–regulated kinase (ERK) in the nucleus and regulate downstream transcription factors. Signaling from individual spines was integrated over a wide range of time (>30 minutes) and space (>80 micrometers). Spatially dispersed inputs over multiple branches activated nuclear ERK much more efficiently than clustered inputs over one branch. Thus, biochemical signals from individual dendritic spines exert profound effects on nuclear signaling.
Activity-dependent gene transcription is essential for the maintenance of long-term potentiation (LTP) and memory consolidation. Induction of LTP in single dendritic spines activates signaling that can either be restricted to the stimulated spine or spread into the parent dendrite over 5 to 10 μm. However, it is not known whether signaling initiated at single dendritic spines can be transmitted into the nucleus to regulate gene transcription. Extracellular signal–regulated kinase (ERK) is important, both for signaling within the stimulated spine and adjacent dendrites and also for activating transcription factors in the nucleus during LTP. Thus, ERK signaling may play an important role in relaying signals from the stimulated spines to the nucleus.
To monitor the activity of ERK in the nucleus, we ballistically transfected cultured organotypic hippocampal slices of rats with nuclear-targeted ERK activity reporter (EKARnuc) and imaged CA1 pyramidal neurons with two-photon fluorescence lifetime imaging microscopy (2pFLIM). The expression of EKARnuc was highly localized to the nucleus. Using the weak EKAR expression in the cytosol, we employed fluorescence intensity measurements to monitor structural plasticity of dendritic spines on secondary and tertiary apical dendritic branches.
In most of the experiments, we stimulated proximal branches within 200 μm from the soma. However, when we stimulated distal branches at more than 200 μm away from the soma, ERK activation showed a long delay (~40 min) before it started to increase to the level similar to that caused by proximal stimulation. This slow process suggests that fast biochemical processes such as Ca2+ waves and electrochemical signaling are unlikely to underlie the nuclear ERK activation induced by activating a few spines.
Next, we varied the number of dendritic branches on which the stimulated spines reside to find out which input pattern—clustered or dispersed—produces nuclear ERK activation more efficiently. Clustered stimulation of all seven spines on a single branch failed to induce any nuclear ERK activity increase. In contrast, stimulating three or seven spines distributed over two to seven branches resulted in marked activation of nuclear ERK. Thus, signal integration over multiple dendritic branches is required to induce nuclear activation of ERK.
What is the range of the spatiotemporal integration of the nuclear ERK activation? Stimulation applied to branches separated by more than 30 μm, significantly increased nuclear ERK activity.. When a second set of stimulation was applied to the same branch or a nearby branch (within 30 μm), we did not observe a significant increase in nuclear ERK activity. Thus, nuclear ERK is activated more efficiently by a spatially distributed pattern of stimulation.
Is nuclear ERK activation induced by stimulation of a few dendritic spines sufficient to regulate gene transcription? Results show that stimulation of a few spines regulates activities of transcription factors CREB and Elk-1 through ERK.
Induction of structural LTP in three to seven spines led to nuclear ERK activation and subsequent regulation of downstream transcription factors CREB and Elk-1. Because each CA1 pyramidal neuron has roughly 10,000 synapses, activation of only a tiny fraction (<0.1%) of its synapses can activate nuclear signaling that regulates gene transcription.
The long-distance spatiotemporal integration in inducing nuclear ERK activation may have important implications for the functional organization of dendritic inputs. Many studies have shown that synaptic potentiation tends to occur in a spatially clustered fashion due to electrical integration and biochemical cross talk within a short stretch of dendrite. However, because potentiated synapses would recruit stronger local membrane depolarization and biochemical signaling in the surrounding region in a positive-feedback manner, this mechanism potentially leads to accumulation of potentiated synapses in one dendritic branch. The nuclear signaling efficiently induced by spatially dispersed inputs may be important for counterbalancing the tendency to accumulate potentiated spines in one branch and developing balanced spatial distribution of synaptic weights.
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