Studies on various forms of synaptic plasticity have demonstrated a link between mRNA translation, learning and memory. Like memory, synaptic plasticity includes an early phase which depends on modification of pre-existing proteins, and a late phase that requires transcription and synthesis of new proteins 1,2 . Activation of post-synaptic targets appears to trigger the transcription of plasticityrelated genes. The new mRNAs are either translated in the soma or transported to synapses before translation. GCN2, a key protein kinase, regulates the initiation of translation. We now report a unique feature of hippocampal slices from GCN2 -/-mice: in CA1, a single 100 Hz train induces a strong and sustained long-term potentiation (late-LTP or L-LTP), which is transcription and translation dependent. In contrast, stimulation that elicits late-LTP in wild type slices, such as four 100 Hz trains or forskolin, fails to evoke L-LTP in GCN2 -/-slices. This aberrant synaptic plasticity is mirrored in the behavior of GCN2 -/-mice in the Morris water maze: after weak training, their spatial memory is enhanced, but it is impaired after more intense training. Activated GCN2 stimulates mRNA translation of ATF4, a CREB antagonist. Accordingly, in the hippocampus of GCN2 -/-mice, the expression of ATF4 is reduced and CREB activity is increased. Our study provides genetic, physiological, behavioral and molecular evidence that GCN2 regulates synaptic plasticity, as well as learning and memory through modulation of the ATF4/CREB pathway.Translation of eukaryotic mRNAs is primarily regulated at the level of initiation 3 . Binding of the initiator tRNA, Met-tRNA i Met , to the 40S subunit is facilitated by the initiation factor 2 (eIF2) which forms a ternary complex with GTP and Met-tRNA i Met . Although phosphorylation

In this report, we provide direct demonstration that the neurotrophin nerve growth factor (NGF) is released in the extracellular space in an activity-dependent manner in its precursor form (proNGF) and that it is in this compartment that its maturation and degradation takes place because of the coordinated release and the action of proenzymes and enzyme regulators. This converting protease cascade and its endogenous regulators (including tissue plasminogen activator, plasminogen, neuroserpin, precursor matrix metalloproteinase 9, and tissue inhibitor metalloproteinase 1) are colocalized in neurons of the cerebral cortex and released upon neuronal stimulation. We also provide evidence that this mechanism operates in in vivo conditions, as the CNS application of inhibitors of converting and degrading enzymes lead to dramatic alterations in the tissue levels of either precursor NGF or mature NGF. Pathological alterations of this cascade in the CNS might cause or contribute to a lack of proper neuronal trophic support in conditions such as cerebral ischemia, seizure and Alzheimer's disease or, conversely, to excessive local production of neurotrophins as reported in inflammatory arthritis pain. matrix metalloproteinase 9 ͉ neuroserpin ͉ plasminogen ͉ tissue plasminogen activator ͉ plasmin T he neurotrophin family of growth factors plays a critical role in neuronal survival and differentiation (1, 2). They are produced and liberated in an activity-dependent manner (3) and are responsible for maintaining neuronal phenotype in the adult CNS (4), including the regulation of the steady-state number of synapses (5). These actions are normally attributed to mature neurotrophins, although recently, a biological role for precursor neurotrophin molecules also has been proposed (6, 7). Despite this wealth of knowledge, it is not clear whether these neurotrophins convert to their mature and biologically active form intracellularly or extracellularly, nor is it clear whether, upon their activity-dependent release into the CNS, they are in their mature or precursor form. For example, nerve growth factor (NGF) has been proposed to be released in its mature form (8-10), whereas in the case of BDNF, recent experimental data suggests that the precursor form of BDNF was released and then was processed extracellularly to elicit longterm potentiation (11).These issues are of functional significance because recent in vitro studies with cells transfected with furin-resistant mutated forms of precursor NGF (proNGF) have shown that unprocessed proNGF interacts preferentially with p75 neurotrophin receptor instead of the high-affinity NGF receptor, TrkA, facilitating an apoptotic mechanism in embryonic cells of the peripheral nervous system (7). Moreover, it has been proposed that, in the adult CNS, proNGF expression is up-regulated after CNS lesions, probably contributing to cell death through p75 neurotrophin receptor and sortilin (12, 13). However, other authors have provided evidence suggesting a neurotrophic role for proNGF, albeit to a...

We previously reported that the precursor form of nerve growth factor (pro-NGF) and not mature NGF is liberated in the CNS in an activity-dependent manner, and that its maturation and degradation occur in the extracellular space by the coordinated action of proteases.Here, we present evidence of diminished conversion of pro-NGF to its mature form and of greater NGF degradation in Alzheimer disease (AD) brain samples compared with controls. These alterations of the NGF metabolic pathway likely resulted in the increased pro-NGF levels. The pro-NGF was largely in a peroxynitrited form in the AD samples. Intrahippocampal injection of amyloid-beta oligomers provoked similar upregulation of pro-NGF in naive rats that was accompanied by evidence of microglial activation (CD40), increased levels of inducible nitric oxide synthase, and increased activity of the NGF-degrading enzyme matrix metalloproteinase 9. The elevated inducible nitric oxide synthase provoked the generation of biologically inactive, peroxynitrite-modified pro-NGF in amyloid-beta oligomer-injected rats. These parameters were corrected by minocycline treatment. Minocycline also diminished altered matrix metalloproteinase 9, inducible nitric oxide synthase, and microglial activation (CD40); improved cognitive behavior; and normalized pro-NGF levels in a transgenic mouse AD model. The effects of amyloid-beta amyloid CNS burden on NGF metabolism may explain the paradoxical upregulation of pro-NGF in AD accompanied by atrophy of forebrain cholinergic neurons.

Nerve growth factor (NGF)-dependent cholinergic basal forebrain neurons degenerate during the progression of Alzheimer disease (AD). Elevated proNGF and reduced levels of the TrkA high-affinity NGF receptor occur in prodromal and advanced stages of AD. We recently described a protease cascade responsible for the conversion of proNGF to mature NGF (mNGF) in which matrix metalloproteinase 9 (MMP-9) degrades mNGF in the extracellular space. To determine whether this proteolytic cascade is altered during the progression of AD, we examined human frontal and parietal cortex tissue from aged subjects with a clinical diagnosis of AD, mild cognitive impairment (MCI) or no cognitive impairment (NCI). The analysis demonstrated greater MMP-9 activity in both AD and MCI compared to NCI brain samples (p < 0.01), which supports the notion that a metabolic failure in the NGF-maturation/degradation pathway may be associated with an exacerbated degradation of mNGF in the cerebral cortex in early AD. Moreover, there were inverse correlations between Global Cognitive Score and Mini-Mental State Examination score and MMP-9 activity. These findings suggest that a reduction in mNGF as a consequence of MMP-9-mediated degradation may in part underlie the pathogenesis of cognitive deficits in MCI and AD.

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