【Abstract】

 

Deciphering the intricate and interactive relationship between the information encoded in the genome and the ongoing synaptic activity is critical for understanding the molecular and cellular signaling underlying long-term memory formation and maintenance of long-lasting changes within the brain. To systematically dissect this question, we investigated the molecular basis of the signaling from synapses to the nucleus and from the nucleus to the synapses, which crucially determines the persistence of synaptic plasticity. We thus uncovered an activity-dependent protein kinase cascade CaMKK-CaMKIV that critically controls the amplitude and time course of phosphorylation of a nuclear transcription factor CREB downstream of synaptic activity, thereby activating a plethora of adaptive transcriptional responses within an active neuronal circuit. We also identified a novel “inverse” synaptic tagging mechanism in which one of CREB’s target gene, Arc, acts as a brake that helps weaken the non-potentiated synapses during the maintenance phase of synaptic plasticity. In genomic parlance, Arc’s rapid induction following strong physiological stimuli is dictated by a potent synaptic activity-responsive element (SARE) present in its enhancer/promoter region, which strikingly harbors a unique cluster of binding sites for CREB, MEF2 and SRF/TCF. Based on this discovery, we created a synthetic promoter E-SARE which now allows to map, label, record and manipulate active neuronal ensembles in various areas of the brain in vivo. Recently, we designed a new set of genetically encoded Ca2+ indicators (GECIs), such as R-CaMP2, which are molecular spies of neuronal activity with most desirable properties such as fast speed and signal linearity. This was largely achieved by exchanging the M13 sequence of classical GECIs with a sensitive CaM-binding sequence engineered based on neuronal CaMKKs. These efforts collectively start to illuminate key molecular and cellular events that are essential in neuronal coding and information processing in active neuronal circuits and systems in vivo.