Frédéric Bernard

A TAT–DEF–Elk-1 Peptide Regulates the Cytonuclear Trafficking of Elk-1 and Controls Cytoskeleton Dynamics

The transcription factor Elk-1 plays a key role in cell differentiation, proliferation and apoptosis. This role is thought to arise from its phosphorylation by activated extracellular signal-regulated kinases (ERKs), a critical posttranslational event for the transcriptional activity of the ternary complex composed of Elk-1 and a dimer of serum response factor (SRF) at the serum response element (SRE) regulatory site of transcription. In addition to its nuclear localization, Elk-1 is found in the dendrites and soma of neuronal cells and recent evidence implicate a cytoplasmic proapoptotic function of Elk-1, via its association with the mitochondrial permeability transition pore complex. Thus, the nuclear versus cytoplasmic localization of Elk-1 seems to be crucial for its biological function. In this study we show that the excitatory neurotransmitter, glutamate, induces an ERK-dependent Elk-1 activation and nuclear relocalization. We demonstrate that Elk-1 phosphorylation on Ser383/389 has a dual function and triggers both Elk-1 nuclear translocation and SRE-dependent gene expression. Mutating these sites into inactive residues or using a synthetic penetrating peptide (TAT–DEF–Elk-1), which specifically interferes with the DEF docking domain of Elk-1, prevents Elk-1 nuclear translocation without interfering with ERK nor MSK1 (mitogen- and stress-activated protein kinase 1), a CREB kinase downstream from ERK- activation. This results in a differential regulation of glutamate-induced IEG regulation when compared with classical inhibitors of the ERK pathway. Using the TAT–DEF–Elk-1 peptide or the dominant-negative version of Elk-1, we show that Elk-1 phosphorylation controls dendritic elongation, SRF and Actin expression levels as well as cytoskeleton dynamics.

Contributions of frontal and medial temporal regions to verbal episodic memory: A PET study

Using PET, subtraction and correlation analysis were jointly employed to determine the specific and complementary contributions of frontal and medial temporal regions to verbal episodic encoding and retrieval processes. Subtraction analysis highlighted prefrontal rCBF increases which were predominantly left-sided during intentional encoding and exclusively right-sided during retrieval, the latter being moreover associated with bilateral precuneus activation. However, significant correlation between rCBF values obtained during intentional encoding and performance scores obtained during retrieval concerned, among other regions, the left parahippocampal gyrus, which indicated that the higher the neuronal activity in this medial temporal region during encoding, the better the retrieval performance.

Reliving lifelong episodic autobiographical memories via the hippocampus: A correlative resting PET study in healthy middle‐aged subjects

We aimed at identifying the cerebral structures whose synaptic function subserves the recollection of lifetimes episodic autobiographical memory (AM) via autonoetic consciousness. Twelve healthy middle‐aged subjects (mean age: 59 years ± 2.5) underwent a specially designed cognitive test to assess the ability to relive richly detailed episodic autobiographical memories from five time periods using the Remember/Know procedure. We computed an index of episodicity (number of Remember responses justified by the recall of specific events and details) and an index of retrieval spontaneity, and additionally an index of semanticized memories (number of Know responses). The regional cerebral blood flow (rCBF) was measured in the resting state, with H2O15 as part of an activation PET study. The indexes were correlated with blood flow using volumes of interest in frontotemporal regions, including hippocampus and voxel‐wise analyses in SPM. With both analyses, significant correlations were mainly found between the index of episodicity and rCBF in the medial temporal lobe, including hippocampus, across the five time periods (unlike the index of semanticized memories) and between the spontaneity index and rCBF in the prefrontal areas. These results highlight, in healthy subjects, the distinct role of these two structures in AM retrieval and support the view that the hippocampus is needed for reexperiencing detailed episodic memories no matter how old they are.

Functional Parcellation of the Lateral Mesencephalus

The mesencephalic locomotor region (MLR), which includes the pedunculopontine nucleus (PPN) and the cuneiform nucleus (CN), has been recently identified as a key structure for locomotion and gait control in mammals. However, the function and the precise anatomy of the MLR remain unclear in humans. To study the lateral mesencephalus, we used fMRI in 15 right-handed healthy volunteers performing two tasks: imagine walking in a hallway and imagine an object moving along the same hallway. Both tasks were performed at two different speeds: normal and 30% faster. We identified two distinct networks of cortical activation: one involving motor/premotor cortices and the cerebellum for the walking task and the other involving posterior parietal and dorsolateral prefrontal cortices for the object moving task. In the lateral mesencephalus, we found that two different but anatomically connected parts of the MLR were activated during the fast condition of each task. The CN and the dorsal part of the PPN were activated during the fast imaginary walking task, whereas the ventral part of the PPN and the ventral part of the reticular formation were activated while subjects were imagining the object moving fast. Our data suggest that the lateral mesencephalus participates in different aspects of gait in humans, with the CN and dorsal PPN controlling motor aspects of locomotion and the ventral PPN being involved in integrating sensory information.

Neural correlates of age-related verbal episodic memory decline: A PET study with combined subtraction/correlation analysis

Using PET, we have determined the neural substrates of age-related verbal episodic memory decline. Twelve young and twelve older healthy volunteers (mean age; 22 and 59 years, respectively) were scanned while performing encoding and retrieval tasks. Retrieval performance was lower in old than in young subjects. The PET data were analyzed using a combined subtraction/correlation approach. Classic subtraction disclosed prefrontal rCBF increases common to both groups, distributed bilaterally during encoding and exclusively right-sided during retrieval, without between-group differences. The correlation analysis between PET activity during encoding and subsequent retrieval performance revealed significant correlations for the left hippocampal region in both groups, but for the right inferior frontal gyrus in the older subjects only. Thus, lower performance in older subjects during an episodic retrieval task may reflect a combination of (i) subtle encoding dysfunction, evidenced by more widespread activity-performance correlations and (ii) less efficient retrieval, as evidenced by unaltered activation pattern (as revealed by the classic subtraction method) despite reduced performance. These exploratory findings suggest the aged brain may be unable to compensate for reduced efficiency of right prefrontal cortex by additional left frontal activation.

Counteraction of axonal growth inhibitory properties of Semaphorin 3A and myelin-associated proteins by a synthetic neurotrophic compound

One of the reasons for the lack of nerve regeneration in the CNS is the formation of a glial scar over‐expressing multiple inhibitory factors including myelin‐associated proteins and members of the Semaphorin family. Innovative therapeutic strategies must stimulate axon extension across the lesion site despite this inhibitory molecular barrier. We recently developed a synthetic neurotrophic compound combining an ω‐alkanol with a retinol‐like cycle (3‐(15‐hydroxy‐pentadecyl)‐2,4,4,‐trimethyl‐cyclohexen‐2‐one (tCFA15)). Here, we demonstrate that tCFA15 is able to promote cortical axon outgrowth in vitro even in the presence of the inhibitory Semaphorin 3A and myelin extracts. This growth‐promoting effect is selectively observed in axons and requires multiple growth‐associated intracellular pathways. Our results illustrate the potential use of synthetic neurotrophic compounds to promote nerve regeneration by counteracting the axonal growth inhibition triggered by glial scar‐associated inhibitory factors.

When Higher Activations Reflect Lower Deactivations: A PET Study in Alzheimer’s Disease during Encoding and Retrieval in Episodic Memory

The aim of the present study was to explore the cerebral substrates of episodic memory disorders in Alzheimer’s disease (AD) and investigate patients’ hyperactivations frequently reported in the functional imaging literature. It remains unclear whether some of these hyperactivations reflect real increased activity or deactivation disturbances in the default mode network (DMN). Using positron emission tomography (15O-H2O), cerebral blood flow was measured in 11 AD patients and 12 healthy elderly controls at rest and during encoding and stem-cued recall of verbal items. Subtractions analyses between the target and control conditions were performed and compared between groups. The average signal was extracted in regions showing hyperactivation in AD patients versus controls in both contrasts. To determine whether hyperactivations occurred in regions that were activated or deactivated during the memory tasks, we compared signal intensities between the target conditions versus rest. Our results showed reduced activation in AD patients compared to controls in several core episodic memory regions, including the medial temporal structures, during both encoding and retrieval. Patients also showed hyperactivations compared to controls in a set of brain areas. Further analyses conducted on the signal extracted in these areas indicated that most of these hyperactivations actually reflected a failure of deactivation. Indeed, whereas almost all of these regions were significantly more activated at rest than during the target conditions in controls, only one region presented a similar pattern of deactivation in patients. Altogether, our findings suggest that hyperactivations in AD must be interpreted with caution and may not systematically reflect increased activity. Although there has been evidence supporting the existence of genuine compensatory mechanisms, dysfunction within the DMN may be responsible for part of the apparent hyperactivations reported in the literature on AD.

pH is an intracellular effector controlling differentiation of oligodendrocyte precursors in culture via activation of the ERK1/2 pathway.

We reported previously that onset of oligodendrocyte precursor cell (OPC) differentiation is accompanied by an increase in intracellular pH (pHi). We show that OPC differentiation is dependent primarily on a permissive pHi value. The highest differentiation levels were observed for pHi values around 7.15 and inhibition of differentiation was observed at slightly more acidic or alkaline values. Clamping the pHi of OPCs at 7.15 caused a transient activation of ERK1/2 that was not observed at more acidic or alkaline values. Furthermore, inhibition of ERK activation with the UO126 compound totally prevented OPC differentiation in response to pHi shift. These results indicate that pHi, acting through the ERK1/2 pathway, is a key determinant for oligodendrocyte differentiation. We also show that this pHi pathway is involved in the process of retinoic acid‐induced OPC differentiation.