Neurogranin/RC3 is a neural-specific Ca ²⁺ -sensitive calmodulin (CaM)-binding protein whose CaM-binding affinity is modulated by phosphorylation and oxidation. Here we show that deletion of the Ng gene in mice did not result in obvious developmental or neuroanatomical abnormalities but caused an impairment of spatial learning and changes in hippocampal short- and long-term plasticity (paired-pulse depression, synaptic fatigue, long-term potentiation induction). These deficits were accompanied by a decreased basal level of the activated Ca ²⁺ /CaM-dependent kinase II (CaMKII) (≈60% of wild type). Furthermore, hippocampal slices of the mutant mice displayed a reduced ability to generate activated CaMKII after stimulation of protein phosphorylation and oxidation by treatments with okadaic acid and sodium nitroprusside, respectively. These results indicate a central role of Ng in the regulation of CaMKII activity with decisive influences on synaptic plasticity and spatial learning.

In neurons, neurogranin (Ng) binds calmodulin (CaM), and its binding affinity is reduced by increasing Ca²⁺, phosphorylation by PKC, or oxidation by oxidants. Ng concentration in the hippocampus of adult mice varied broadly (Ng^+/+, ∼160-370 and Ng^+/-, ∼70-230 pmol/mg); the level in Ng^+/+mice is one of the highest among all neuronal CaM-binding proteins. Among Ng^+/-mice, but less apparent in Ng^+/+, a significant relationship existed between their hippocampal levels of Ng and performances in the Morris water maze. Ng^-/-mice performed poorly in this task; they also displayed deficits in high-frequency-induced long-term potentiation (LTP) in area CA1 of hippocampal slices, whereas low-frequency-induced long-term depression was enhanced. Thus, compared with Ng^+/+mice, the frequency-response curve of Ng^-/-shifted to the right. Paired-pulse facilitation and synaptic fatigue during prolonged stimulation at 10 Hz (900 pulses) were unchanged in Ng^-/-slices, indicating their normal presynaptic function. Measurements of Ca²⁺transients in CA1 pyramidal neurons after weak and strong tetanic stimulations (100 Hz, 400 and 1000 msec, respectively) revealed a significantly greater intracellular Ca²⁺([Ca²⁺]_i) response in Ng^+/+compared with Ng^-/-mice, but the decay time constants did not differ. The diminished Ca²⁺dynamics in Ng^-/-mice are a likely cause of their decreased propensity to undergo LTP. Thus, Ng may promote a high [Ca²⁺]_iby a “mass-action” mechanism; namely, the higher the Ng concentration, the more Ng-CaM complexes will be formed, which effectively raises [Ca²⁺]_iat any given Ca²⁺influx. This mechanism provides potent signal amplification in enhancing synaptic plasticity as well as learning and memory.

FUSE-binding protein (FBP) binds the single-stranded far upstream element of active c-myc genes, possesses potent transcription activation and repression domains, and is necessary for c-myc expression. A novel 60 kDa protein, the FBP interacting repressor (FIR), blocked activator-dependent, but not basal, transcription through TFIIH. Recruited through FBP's nucleic acid–binding domain, FIR formed a ternary complex with FBP and FUSE. FIR repressed a c-myc reporter via the FUSE. The amino terminus of FIR contained an activator-selective repression domain capable of acting in cis or even in trans in vivo and in vitro. The repression domain of FIR targeted only TFIIH's p89/XPB helicase, required at several stages in transcription, but not factors required for promoter selection. Thus, FIR locks TFIIH in an activation-resistant configuration that still supports basal transcription.