Jessica L. Banko

The Translation Repressor 4E-BP2 Is Critical for eIF4F Complex Formation, Synaptic Plasticity, and Memory in the Hippocampus

Long-lasting synaptic plasticity and memory requires mRNA translation, yet little is known as to how this process is regulated. To explore the role that the translation repressor 4E-BP2 plays in hippocampal long-term potentiation (LTP) and learning and memory, we examined 4E-BP2 knock-out mice. Interestingly, genetic elimination of 4E-BP2 converted early-phase LTP to late-phase LTP (L-LTP) in the Schaffer collateral pathway, likely as a result of increased eIF4F complex formation and translation initiation. A critical limit for activity-induced translation was revealed in the 4E-BP2 knock-out mice because L-LTP elicited by traditional stimulation paradigms was obstructed. Moreover, the 4E-BP2 knock-out mice also exhibited impaired spatial learning and memory and conditioned fear-associative memory deficits. These results suggest a crucial role for proper regulation of the eIF4F complex by 4E-BP2 during LTP and learning and memory in the mouse hippocampus.

Regulation of eukaryotic initiation factor 4E by converging signaling pathways during metabotropic glutamate receptor-dependent long-term depression.

Long-term depression (LTD) is an activity-dependent decrease in synaptic efficacy that can be induced in hippocampal area CA1 by pharmacological application of the selective group I metabotropic glutamate receptor (mGluR) agonist 3,5-diyhroxyphenylglycine (DHPG). Recent work has demonstrated that DHPG-induced LTD recruits at least two signal transduction pathways known to couple to translation, the mitogen-activated protein kinase kinase (MEK)–extracellular signal-regulated kinase (ERK) signaling pathway and the phosphoinositide 3-kinase (PI3K)–Akt–mammalian target of rapamycin (mTOR) signaling pathway. However, it remains unclear which translation factors are engaged by these two signaling pathways during mGluR-LTD. In this study, we investigated whether the group I mGluRs couple to the cap-dependent translation proteins: Mnk1, eIF4E, and 4E-BP. We found that both the MEK–ERK and PI3K–mTOR signaling pathways are critical for the DHPG-induced regulation of these translation factors. Furthermore, we demonstrate that increasing eIF4F complex availability via the genetic elimination of 4E-BP2 can enhance the degree of LTD achieved by DHPG application in an ERK-dependent manner. Our results provide direct evidence that cap-dependent translation is engaged during mGluR-LTD and demonstrate that the MEK–ERK and PI3K–mTOR signaling pathways converge to regulate eIF4E activity after induction of DHPG-LTD.

NMDA receptor activation results in PKA‐ and ERK‐dependent Mnk1 activation and increased eIF4E phosphorylation in hippocampal area CA1

Protein synthesis is essential for the stabilization of glutamate receptor‐dependent forms of long‐lasting hippocampal synaptic plasticity and for the consolidation of memory, but the signal transduction mechanisms that regulate translation factors during these processes are not well understood. As a first step towards understanding how translation is activated during synaptic plasticity, we investigated how the eukaryotic initiation factor 4E (eIF4E), a rate‐limiting mRNA cap‐binding protein, and its kinase, Mnk1, are regulated by protein kinase C (PKC), cAMP‐dependent protein kinase (PKA) and N‐methyl‐d‐aspartate (NMDA) receptor activation in hippocampal area CA1. We found that treatment of mouse hippocampal slices with either phorbol ester, to activate PKC, or forskolin, to activate PKA, resulted in activation of Mnk1 and increased eIF4E phosphorylation that was dependent on extracellular signal‐regulated kinase (ERK). Similarly, brief treatment of hippocampal slices with NMDA resulted in activation of Mnk1 and increased phosphorylation of eIF4E. The NMDA‐induced activation of Mnk1 and increased phosphorylation of eIF4E were dependent on PKA and ERK, but not PKC, and were present in synaptoneurosome preparations. Immunohistochemical analysis revealed that the PKA‐ and ERK‐dependent increases in Mnk1 activation induced by NMDA also occured in dendrites. These findings identify a specific regulatory pathway that can couple NMDA receptor activation to translation initiation factors in the hippocampus, and may represent a mechanism for triggering dendritic protein synthesis during long‐term potentiation and long‐term memory formation.

Reelin supplementation enhances cognitive ability, synaptic plasticity, and dendritic spine density

Apolipoprotein receptors belong to an evolutionarily conserved surface receptor family that has intimate roles in the modulation of synaptic plasticity and is necessary for proper hippocampal-dependent memory formation. The known lipoprotein receptor ligand Reelin is important for normal synaptic plasticity, dendritic morphology, and cognitive function; however, the in vivo effect of enhanced Reelin signaling on cognitive function and synaptic plasticity in wild-type mice is unknown. The present studies test the hypothesis that in vivo enhancement of Reelin signaling can alter synaptic plasticity and ultimately influence processes of learning and memory. Purified recombinant Reelin was injected bilaterally into the ventricles of wild-type mice. We demonstrate that a single in vivo injection of Reelin increased activation of adaptor protein Disabled-1 and cAMP-response element binding protein after 15 min. These changes correlated with increased dendritic spine density, increased hippocampal CA1 long-term potentiation (LTP), and enhanced performance in associative and spatial learning and memory. The present study suggests that an acute elevation of in vivo Reelin can have long-term effects on synaptic function and cognitive ability in wild-type mice.

Inhibition of the interactions between eukaryotic initiation factors 4E and 4G impairs long-term associative memory consolidation but not reconsolidation

Considerable evidence indicates that the general blockade of protein synthesis prevents both the initial consolidation and the postretrieval reconsolidation of long-term memories. These findings come largely from studies of drugs that block ribosomal function, so as to globally interfere with both cap-dependent and -independent forms of translation. Here we show that intra-amygdala microinfusions of 4EGI-1, a small molecule inhibitor of cap-dependent translation that selectively disrupts the interaction between eukaryotic initiation factors (eIF) 4E and 4G, attenuates fear memory consolidation but not reconsolidation. Using a combination of behavioral and biochemical techniques, we provide both in vitro and in vivo evidence that the eIF4E–eIF4G complex is more stringently required for plasticity induced by initial learning than for that triggered by reactivation of an existing memory.

Activation of exchange protein activated by cyclic-AMP enhances long-lasting synaptic potentiation in the hippocampus

cAMP is a critical second messenger implicated in synaptic plasticity and memory in the mammalian brain. Substantial evidence links increases in intracellular cAMP to activation of cAMP-dependent protein kinase (PKA) and subsequent phosphorylation of downstream effectors (transcription factors, receptors, protein kinases) necessary for long-term potentiation (LTP) of synaptic strength. However, cAMP may also initiate signaling via a guanine nucleotide exchange protein directly activated by cAMP (Epac). The role of Epac in hippocampal synaptic plasticity is unknown. We found that in area CA1 of mouse hippocampal slices, activation of Epac enhances maintenance of LTP without affecting basal synaptic transmission. The persistence of this form of LTP requires extracellular signal-regulated protein kinase (ERK) and new protein synthesis, but not transcription. Because ERK is involved in translational control of long-lasting plasticity and memory, our data suggest that Epac is a crucial link between cAMP and ERK during some forms of protein synthesis-dependent LTP. Activation of Epac represents a novel signaling pathway for rapid regulation of the stability of enduring forms of LTP and, perhaps, of hippocampus- dependent long-term memories.

Behavioral alterations in mice lacking the translation repressor 4E-BP2

The requirement for de novo protein synthesis during multiple forms of learning, memory and behavior is well-established; however, we are only beginning to uncover the regulatory mechanisms that govern this process. In order to determine how translation initiation is regulated during neuroplasticity we engineered mutant C57Bl/6J mice that lack the translation repressor eukaryotic initiation factor 4E-binding protein 2 (4E-BP2) and have previously demonstrated that 4E-BP2 plays a critical role in hippocampus-dependent synaptic plasticity and memory. Herein, we examined the 4E-BP2 knockout mice in a battery of paradigms to address motor activity and motor skill learning, anxiety and social dominance behaviors, working memory and conditioned taste aversion. We found that the 4E-BP2 knockout mice demonstrated altered activity in the rotating rod test, light/dark exploration test, spontaneous alternation T-maze and conditioned taste aversion test. The information gained from these studies builds a solid foundation for future studies on the specific role of 4E-BP2 in various types of behavior, and for a broader, more detailed examination of the mechanisms of translational control in the brain.

Pre-MCI and MCI: neuropsychological, clinical, and imaging features and progression rates.

OBJECTIVE To compare clinical, imaging, and neuropsychological characteristics and longitudinal course of subjects with pre-mild cognitive impairment (pre-MCI), who exhibit features of MCI on clinical examination but lack impairment on neuropsychological examination, to subjects with no cognitive impairment (NCI), nonamnestic MCI (naMCI), amnestic MCI (aMCI), and mild dementia. METHODS For 369 subjects, clinical dementia rating sum of boxes (CDR-SB), ApoE genotyping, cardiovascular risk factors, parkinsonism (UPDRS) scores, structural brain MRIs, and neuropsychological testing were obtained at baseline, whereas 275 of these subjects received an annual follow-up for 2-3 years. RESULTS At baseline, pre-MCI subjects showed impairment on tests of executive function and language, higher apathy scores, and lower left hippocampal volumes (HPCV) in comparison to NCI subjects. Pre-MCI subjects showed less impairment on at least one memory measure, CDR-SB and UPDRS scores, in comparison to naMCI, aMCI and mild dementia subjects. Follow-up over 2-3 years showed 28.6% of pre-MCI subjects, but less than 5% of NCI subjects progressed to MCI or dementia. Progression rates to dementia were equivalent between naMCI (22.2%) and aMCI (34.5%) groups, but greater than for the pre-MCI group (2.4%). Progression to dementia was best predicted by the CDR-SB, a list learning and executive function test. CONCLUSION This study demonstrates that clinically defined pre-MCI has cognitive, functional, motor, behavioral and imaging features that are intermediate between NCI and MCI states at baseline. Pre-MCI subjects showed accelerated rates of progression to MCI as compared to NCI subjects, but slower rates of progression to dementia than MCI subjects.

Phosphorylation Dynamics Regulate Hsp27-Mediated Rescue of Neuronal Plasticity Deficits in Tau Transgenic Mice

Molecular chaperones regulate the aggregation of a number of proteins that pathologically misfold and accumulate in neurodegenerative diseases. Identifying ways to manipulate these proteins in disease models is an area of intense investigation; however, the translation of these results to the mammalian brain has progressed more slowly. In this study, we investigated the ability of one of these chaperones, heat shock protein 27 (Hsp27), to modulate tau dynamics. Recombinant wild-type Hsp27 and a genetically altered version of Hsp27 that is perpetually pseudo-phosphorylated (3×S/D) were generated. Both Hsp27 variants interacted with tau, and atomic force microscopy and dynamic light scattering showed that both variants also prevented tau filament formation. However, extrinsic genetic delivery of these two Hsp27 variants to tau transgenic mice using adeno-associated viral particles showed that wild-type Hsp27 reduced neuronal tau levels, whereas 3×S/D Hsp27 was associated with increased tau levels. Moreover, rapid decay in hippocampal long-term potentiation (LTP) intrinsic to this tau transgenic model was rescued by wild-type Hsp27 overexpression but not by 3×S/D Hsp27. Because the 3×S/D Hsp27 mutant cannot cycle between phosphorylated and dephosphorylated states, we can conclude that Hsp27 must be functionally dynamic to facilitate tau clearance from the brain and rescue LTP; however, when this property is compromised, Hsp27 may actually facilitate accumulation of soluble tau intermediates.

Defining Mild Cognitive Impairment: Impact of Varying Decision Criteria on Neuropsychological Diagnostic Frequencies and Correlates

OBJECTIVE To examine the impact of varying decision criteria on neuropsychological diagnostic frequencies and on their correlates. DESIGN Descriptive and correlational study. SETTING Florida Alzheimers Disease Research Center. PARTICIPANTS A sample of 373 individuals with comprehensive baseline analyses participating in a longitudinal study of cognitive decline and early Alzheimer disease. MEASUREMENTS Mild cognitive impairment (MCI) diagnoses were made on the basis of four sets of decision criteria created by crossing two approaches: varying the number of impaired test results required for a diagnosis within any domain (1 test versus 2) and varying the performance level required to determine impairment (1.5 or 2 standard deviations [SDs] below the normative mean) for any test. RESULTS Under each criteria set, single-domain amnestic MCI was the most frequent MCI diagnosis. MCI global and subtype diagnosis frequencies were inversely related to the stringency of the criteria. The single test-1.5 SD criterion identified the largest number of cases as qualifying for an MCI diagnosis, and the two test-2.0 SD cutoff identified the fewest. Across all sets of criteria, the authors found significant positive associations between neuropsychological diagnoses and Clinical Dementia Rating score categories. Significant relationships between diagnoses and both apolipoprotein E (APOE) genotype and magnetic resonance imaging ratings of medial temporal atrophy (MTA) application were found only for the two test-1.5 SD and two test-2.0 SD cutoffs. CONCLUSION MCI diagnosis frequencies are substantively affected by the stringency of the criteria, but the relative rankings of MCI subtype diagnoses are fairly consistent regardless of the stringency of the criteria. Significant associations of neuropsychological diagnoses with independent markers such as APOE genotype and MTA are only found with more stringent criteria, suggesting that a coherent network of associations reflecting cognitive decline occurs with more restrictive definitions for impairment.