Past events

The allele E4 of apolipoprotein E (apoE4), the most prevalent genetic risk factor for Alzheimer’s disease, is associated with elevated levels of brain amyloid. This led to the suggestion that the pathological effects of apoE4 are mediated via synergistic pathological interactions with amyloid β (Aβ). We have recently shown that activation of the amyloid cascade by inhibition of the Aβ-degrading enzyme neprilysin in brains of apoE3 and apoE4 mice results in the isoform specific degeneration in apoE4 mice, of hippocampal CA1 neurons and of entorhinal and septal neurons. This is accompanied by the accumulation of intracellular Aβ and apoE and by pronounced cognitive deficits in the ApoE4 mice. We presently investigated the cellular mechanisms underlying the apoE4 dependent Aβ mediated neurodegeneration of CA1 and septal neurons and their neuronal specificity. Confocal microscopy kinetic studies revealed that the accumulated Aβ in CA1 neurons of apoE4 mice co-localizes with lysosomes and is associated with lysosomal activation and subsequent apoptotic neuronal cell death. Furthermore the accumulated Aβ is oligomerized. In contrast the degeneration of septal neurons is not associated with oligomerization of the accumulated Aβ. Instead intracellular Aβ in septal neurons co-localizes with the apoE receptor LRP whose levels are specifically elevated in these cells. These findings suggest that the apoE4 dependent Aβ mediated neurodegeneration is related, in CA1 but not in septal neurons, to oligomerization of the accumulated Aβ. In addition, neurodegeneration of CA1 but not of septal neurons is associated with inflammatory activation suggesting that the brain area specificity of the effects of apoE4 and Aβ are also related to brain area specific non neuronal mechanisms such as inflammation. Neuronal plasticity experiments revealed that apoE4 inhibits synaptogenesis and neurogenesis and stimulates apoptosis in hippocampal neurons of apoE4 mice that have been exposed to an enriched environment. These effects are also associated with the specific accumulation of apoE4 and oligomerized Aβ in the affected neurons. Additional experiments revealed that apoE4 up-regulates the expression of inflammation-related genes following i.c.v injection of LPS and that this effect is also associated with the accumulation of intra neuronal Aβ in hippocampal neurons. These findings suggest that the impaired neuronal plasticity and hyper inflammatory effects of apoE4 may also be mediated via cross talk interactions of apoE4 with the amyloid cascade.

Circadian rhythms in locomotor activity are an example of a well-characterized behavior for which the molecular and neurobiological bases are not yet completely understood. These rhythms are self-sustained 24 hours rhythms that underlie most physiological and behavioral processes. The central circadian clock, which is situated in the brain, is responsible for daily rhythms in locomotor activity that persist even after weeks in constant darkness (DD). Peripheral clocks are spread trough the fly body and regulate a plethora of physiological functions that include: olfaction, detoxification and immunity. All these clocks keep time trough complex transcriptional-translational feedback loops that include the proteins CLK, CYC, PER and TIM. My research focuses on the study of the molecular basis of the circadian clock. In particular, I am interested in the contribution of the different molecular interactions and processes to the generation of robust 24hs rhythms. In this context, I have recently demonstrated that transcriptional speed of the clock gene PER is a determinant of the circadian period and that translational regulation by miRNAs is part of the central circadian clock.

Never before have such intense experimental efforts been focused on neuronal circuits of the size of few hundred thousands neurons whose functions are relatively well defined. The extraordinarily powerful new genetic tools and 3D reconstruction methods, combined with modern multi-electrode arrays, telemetry, two-photon imaging and photo-activation are starting to shed bright light on the intricacies of these circuits, and in particular of the cortical column. But without tools that integrate all this different types of data, one cannot expect to gain a comprehensive understanding on how these circuits perform specific sensory-motor or cognitive functions. As in any other complex system, a modeling study is essential if we are to ever say that we understand how this system works. I will describe several attempts in my group to begin building detailed models of the cortical column, highlighting that, at both circuit level and at the level of individual neurons, models should capture experimental variability and that the building of these models should become automated. I will demonstrate how these models could be used to fruitfully guide new experiments and discuss were all this new integrated "simulation-driven brain research agenda" might lead to.

Although prominent theories of vision have emphasized dissociations between two visual streams specialized for perception and action, in some situations, the two streams must interact. One such situation is the performance of actions upon remembered objects. Neuropsychological evidence from two patients with occipitotemporal lesions suggests that while immediate actions can be performed using only the dorsal vision-for-action stream, delayed actions require integrity of the ventral vision-for-perception stream. My lab has investigated the interactions between the two streams during delayed grasping using functional magnetic resonance imaging and transcranial magnetic stimulation. Our results suggest that delayed actions re-recruit information about object properties such as shape, size and orientation from the ventral stream and early visual areas at the time the delayed action is performed

It is frequently proposed that conscious perceptual decisions are produced by recurrent interactions among multiple brain areas. Sensory stimuli, which are close to psychophysical threshold or perceptually bistable, induce fluctuating percepts in the face of constant sensory input. Thus, these stimuli provide ideal tools for probing the intrinsic neural mechanisms underlying perceptual decisions, in the absence of extrinsic stimulus changes. I will present human neuroimaging (MEG and fMRI) studies, in which we used this approach for probing the large-scale neural mechanisms underlying decisions about the presence or absence of simple visual features. Our results suggest that neural population activity in parietal, prefrontal, and premotor areas reflects the decision process, and that population activity in extrastriate ventral visual cortex reflects perception. Further, cooperative and competitive long- range interactions, across multiple levels of the cortical processing hierarchy, both seem to underlie simple perceptual decisions.

The off-line processing of acquired sensory information in the mammalian cortex is an example for the unique way biology creates to compute and store information which guides behavior. The relatively short temporal phase in the process (i.e. hours following acquisition) is defined biochemically by its sensitivity to protein synthesis inhibitors. Until recently this negative definition of molecular consolidation did not reveal the details of the endogenous processes taking place, minutes to hours, in the neurons and circuit underlying a given memory. We use taste learning paradigms in order to study this process of molecular consolidation in the gustatory cortex. Recent results, from our laboratory, obtained from genetic, pharmacological, biochemical, electrophysiological and behavioral studies demonstrate that translational control, at the initiation and elongation phases of translation, plays a key role in the process of molecular consolidation. Moreover, this spatially and temporally regulated translation control modifies both general and synaptic protein expression that is crucial for memory stabilization. We propose a model to explain the interplay between regulation of initiation and elongation phases of translation and demonstrate that in certain situations cognitive enhancement can be achieved.

A major challenge for biology is to develop new ways of determining how proteins operate in complexes in cells. This requires molecularly focused methods for dynamic interrogation and manipulation. An attractive approach is to use light as both input and output to probe molecular machines in cells. While there has been significant progress in optical detection of protein function, little advance has been made in remote control of any kind, including optical methods. As part of our efforts in the NIH Nanomedicine Development Center for the Optical Control of Biological Function, we are developing methods for rapidly switching on and off with light the function of select proteins in cells. The strategies are broadly applicable across protein classes. Our approach has been to synthesize Photoswitched Tethered Ligands (PTLs), which are attached in a site directed manner to a protein of interest. The site of attachment is designed into the protein to be at a precise distance from a binding site for the ligand. The geometric precision has two important consequences. First, light of two different wavelengths is used to isomerize the linker in such a way that the ligand can only bind in one of the sites, thus making it possible to toggle binding on and off with light. Second, native proteins are not affected by the PTL and remain insensitive to light, since the PTL does not attach. This means that a specific protein in a cell, a tissue and even in an intact freely behaving organism, can have its biochemical signaling turned on and off by remote optical control. The switching is very fast, taking place in ~1 millisecond, i.e. at the rate of the fastest nerve impulse. I will describe how we used our light-gated kaintate-type glutamate receptor, LiGluR, to study vertebrate locomotion. We used intersectional optogenetics in larval zebrafish to identify a new class of neurons that provide an important modulatory drive to swim behavior.