How does a memory come to be
With so many synapses in a dendritic tree?
With umpteen connections betwixt cells,
How does a nucleus come to tell
When and which genes transcribed will be
Sufficient to mediate plasticity?
The Martin Lab at UCLA
Believes that they have found a way.
When transmitter binding allows calcium in
A protein is released on a dendritic swim
CRTC is the instigator:
CREB-regulated transcriptional coactivator.
In pancreatic β islet cells it was first described
How CRTC ensures CREB’s targets are transcribed:
Phosphorylated and bound to protein 14-3-3
CRTC remains cytoplasmically
Until calcinuerin dephosphorylates
And into the nucleus it translocates.
Now it’s hippocampal neurons, the excitatory kind,
That the Martin team has come to find
That in TTX, with activity nixed:
In the PSD, CRTC1 remains transfixed.
But with GABA suppressed by bicuculline…
To the nucleus! The location is redefined!
Fig 1. Immunostaining of microtubule-associated protein 2 (MAP2; red), CRTC1 (green), and Hoechst nuclear die (blue) pretreated in the sodium channel blocker TTX or the GABAa receptor antagonist Bicuculline (BIC). BIC application disinhibits cell firing and causes CRTC1 to translocate to the cell nucleus. From Ch’ng et al., 2012, Figure 2A.
Blocking AMPA receptors (and NMDA too)
Suggests that this trick requires Glu.
And with calcium voltage channels blocked
CRTC1’s position remained locked.
But should calcineurin act constitutively,
To the nucleus CRTC1 flees.
Fig 2. TOP: Ratio of CRTC1 cytoplasmic to nuclear immunofluorescence in each pretreatment regiment: TTX (tetrodotoxin), BIC (bicuculline, GABAa receptor agonist), APV (NMDA receptor agonist), NIM (nimodipine; L-type voltage gated calcium channel agonist). Figure illustrates the necessity of calcium influx and glutamate signalling via the NMDA receptor (AMPA receptor data not shown) for BIC induced CRTC1 translocation. BOTTOM: Effect of constitutively active calcineurin on CRTC1 nuclear translocation in basal, TTX, and BIC preps. From Ch’ng et al., 2012, Figure 2A and 5B.
“Now”, they said, “this chemistry is nice,
But lets pump some current into this slice!”
So in the Schaffer collateral, presynaptically,
They stimulated at 5Hz repeatedly.
And after immunostaining, what was observed?
To the nucleus was CRTC1 reserved.
Fig 3. Immunohistochemistry of CRTC1 in acute hippocampal slices. Stimulating with 5 trains of 5Hz stimulation for 30 seconds with 30 second intervals in the alveus (antidromic) and Schaffer collaterals (orthodromic). Orthodromic stimulation, mimicking the signalling that would induce long-term potentiation, caused nuclear translocation of CRTC1. R, stratum radiatum (Schaffer collaterals) and P, stratum pyramidale. From Ch’ng et al., 2012, Figure 3B.
“Maybe,” they thought, “that was too much,
As transmitter is released in a tiny puff.”
So targeting but a single branch with UV light
They uncaged glutamate, and to their delight,
Showed again that CRTC1
Was on its synapto-nuclear run.
Fig 4. TOP: Cultured hippocampal neurons transduced with lentivirus expressing GFP and pretreated with UV-light sensitive caged glutamate. Left column shoes living cells (green is GFP) and right column shows fixation and staining for MAP2 (green), CRTC1 (red), and Hoechst nuclear dye (blue). Insets show targeted site of glutamate uncaging, marked by dotted red box. Far right insets show cell soma for each stain and merged imaged. BOTTOM: Ratio of nuclear to cytoplasimic immunofluorescence of CRTC1. From Ch’ng et al., 2012, Figure 4Bi.
In summary, lets review
The discovery by this clever crew;
In the hippocampus, when glutamate binds,
Calcium enters and calcineurin finds
CRTC1 so that it may release
The protein from its synaptic leash.
Once in the nucleus, to CREB it binds
And transcription will be redefined.
And since these changes have been shown to be
Necessary for plasticity,
We can say we’ve stepped forward in our quest to know,
What makes synapses strengthen and memories grow.
Please join us for the latest installment of the 2012-2013 Neuroscience Seminar series as Kelsey Martin discusses the molecular roots of memory storage at 4 pm on Tuesday, November 20th in the CNCB Large Conference Room.
Kyle Fischer is a first year in the neurosciences Ph.D. program. He’s completing his first rotation in the lab of Dr. Jill Leutgeb and preparing for the inevitable use of poetic verse as the required format in all peer-reviewed publication.
Ch’ng T., Uzgil B., Lin P., Avliyakulov N., O’Dell T. & Martin K. (2012). Activity-Dependent Transport of the Transcriptional Coactivator CRTC1 from Synapse to Nucleus, Cell, 150 (1) 207-221. DOI: 10.1016/j.cell.2012.05.027