A novel peptide derived from vascular endothelial growth factor prevents amyloid beta aggregation and toxicity.

Amyloid-ß oligomers (Aßo) are the most pathologically relevant Aß species in Alzheimer's disease (AD), because they induce early synaptic dysfunction that leads to learning and memory impairments. In contrast, increasing VEGF (Vascular Endothelial Growth Factor) brain levels have been shown to improve learning and memory processes, and to alleviate Aß-mediated synapse dysfunction. Here, we designed a new peptide, the blocking peptide (BP), which is derived from an Aßo-targeted domain of the VEGF protein, and investigated its effect on Aß-associated toxicity. Using a combination of biochemical, 3D and ultrastructural imaging, and electrophysiological approaches, we demonstrated that BP strongly interacts with Aßo and blocks Aß fibrillar aggregation process, leading to the formation of Aß amorphous aggregates. BP further impedes the formation of structured Aßo and prevents their pathogenic binding to synapses. Importantly, acute BP treatment successfully rescues long-term potentiation (LTP) in the APP/PS1 mouse model of AD, at an age when LTP is highly impaired in hippocampal slices. Moreover, BP is also able to block the interaction between Aßo and VEGF, which suggests a dual mechanism aimed at both trapping Aßo and releasing VEGF to alleviate Aßo-induced synaptic damage. Our findings provide evidence for a neutralizing effect of the BP on Aß aggregation process and pathogenic action, highlighting a potential new therapeutic strategy.

A critical role for VEGF and VEGFR2 in NMDA receptor synaptic function and fear-related behavior.

Vascular endothelial growth factor (VEGF) is known to be required for the action of antidepressant therapies but its impact on brain synaptic function is poorly characterized. Using a combination of electrophysiological, single-molecule imaging and conditional transgenic approaches, we identified the molecular basis of the VEGF effect on synaptic transmission and plasticity. VEGF increases the postsynaptic responses mediated by the N-methyl-D-aspartate type of glutamate receptors (GluNRs) in hippocampal neurons. This is concurrent with the formation of new synapses and with the synaptic recruitment of GluNR expressing the GluN2B subunit (GluNR-2B). VEGF induces a rapid redistribution of GluNR-2B at synaptic sites by increasing the surface dynamics of these receptors within the membrane. Consistently, silencing the expression of the VEGF receptor 2 (VEGFR2) in neural cells impairs hippocampal-dependent synaptic plasticity and consolidation of emotional memory. These findings demonstrated the direct implication of VEGF signaling in neurons via VEGFR2 in proper synaptic function. They highlight the potential of VEGF as a key regulator of GluNR synaptic function and suggest a role for VEGF in new therapeutic approaches targeting GluNR in depression.

Early divergence of magnocellular and parvocellular functional subsystems in the embryonic primate visual system.

In both human and Old World primates visual information is conveyed by two parallel pathways: the magnocellular (M) and parvocellular (P) streams that project to separate layers of the lateral geniculate nucleus and are involved primarily in motion and color/form discrimination. The present study provides evidence that retinal ganglion cells in the macaque monkey embryo diverge into M and P subtypes soon after their last mitotic division and that optic axons project directly and selectively to either the M or P moieties of the developing lateral geniculate nucleus. Thus, initial M projections from the eyes overlap only in prospective layers 1 and 2, whereas initial P projections overlap within prospective layers 3-6. We suggest that the divergence of the M and P pathways requires developmental mechanisms different from those underlying competition-driven segregation of initially intermixed eye-specific domains in the primate visual system.

Specificity of retinal ganglion cell projections in the embryonic rhesus monkey.

Recent studies dealing with the organization of retinal projections in the developing rhesus monkey brain have revealed a high degree of developmental specificity. This is demonstrated by the ingrowth patterns of the initial contingents of crossed and uncrossed fibers that form the primordial optic tract as well as by the adult-like nasotemporal retinal decussation pattern evident even before the period of ganglion cell death. On the basis of these observations, it is suggested that early generated retinal fibers are guided through the optic chiasm by a transiently expressed decussation signal, and that later generated fibers utilize retinal position-dependent cues to innervate the appropriate hemisphere. Furthermore, the first retinal fibers to arrive at the dorsal lateral geniculate nucleus invade only the presumed parvocellular layers. Thus, the initial innervation of the lateral geniculate nucleus appears to reflect the birth order of retinal ganglion cell classes. It is suggested that the high degree of precision evident in the macaque monkey nasotemporal retinal decussation pattern relates to the adultlike distribution of callosal projection neurons in the developing striate cortex of the primate.

Organization of pioneer retinal axons within the optic tract of the rhesus monkey.

Retinal ganglion cell axons must make a decision at the embryonic optic chiasm to grow into the appropriate optic tract. To gain insight into the cues that play a role in sorting out the crossed from the uncrossed optic axons, we investigated the sequence of their initial ingrowth in rhesus monkey embryos. Two carbocyanine dyes, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate and 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide, were placed, respectively, into the left and right retinas to identify the course of uncrossed and crossed retinal axons through the optic chiasm and tract. Our results show that at embryonic day 36 the most advanced retinal projections are uncrossed. At this age the leading crossed axons are just reaching the chiasmatic midline, whereas the uncrossed fibers have already entered the optic tract. This indicates that the pathfinding of these pioneer uncrossed fibers does not require the presence of retinal axons from the opposite eye. At subsequent stages of development (embryonic days 40 and 42) there is a clear partial segregation of the uncrossed and crossed retinal axons within the optic tract: the uncrossed-component course is in the deeper portion of the optic tract, whereas the crossed component lies in a more superficial region. Thus, the spatial organization of retinal axons within the primordial optic tract reflects the sequential addition of the uncrossed and crossed retinal fibers. The orderly and sequential ingrowth of these pioneer retinal axons indicates that specific chiasmatic cues are expressed early in development and that such pioneer fibers may serve as guides for the later-arriving retinal fibers.

Transient cortical pathways in the pyramidal tract of the neonatal ferret.

Anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was used to study transient axons from the visual cortex in the pyramidal tract. Injections at birth restricted to the visual cortex labeled axons in the vicinity of the pontine nuclei. Two to eight days after birth, axons from the occipital cortex were found posterior to the pontine nucleus, their caudalmost stable target. Transient corticospinal axons from the presumptive primary visual cortex did not grow caudal to the pyramidal decussation. Innervation of more distal targets preceded innervation of proximal targets. Innervation of the pontine nucleus is initiated around 68 hours after birth, when the transient extension in the medullary pyramidal tract has attained its maximum caudal extent. Innervation of the superior colliculus begins 9 days after birth. Retrograde tracers were used to follow the developmental changes in the cortical distribution of the parent neurons giving rise to axons in the pyramidal tract. In the adult, labeled neurons following injection of retrograde tracer in the pyramidal tract occupied less than a third of the neocortex and were centred on the anterior part of the coronal and spleniocruciate gyri. In the immature brain, labeled neurons covered more than two-thirds of the neocortex. Areal density measurements in the neonate showed that peak labeling was centred in the anterior coronal and spleniocruciate gyri, where corticospinal cells in the adult are located. There was a marked rostral-caudal gradient so that labeled neurons were very scarce towards the occipital pole. These results, showing transient neocortical axons in the pyramidal tract in a carnivore, suggest that this may be a common feature of mammalian development. The finding that the adult pattern of corticospinal projections does not emerge from a uniform distribution is discussed with respect to the areal specification of cortical connectivity.

Segregation of callosal and association pathways during development in the visual cortex of the primate.

The segregation of callosal and association pathways in the developing visual cortex of the monkey was studied using the retrograde tracers fast blue and diamidino yellow. Quantitative analysis of the laminar distribution of labeled callosal and association neurons made it possible to reveal the shifting pattern of connections that characterizes the development of these two pathways. In the adult, callosal neurons are restricted to supragranular layers, where they are concentrated at the bottom of layer 3. Association neurons are located both in infra- and supragranular layers. Supragranular layer association neurons are concentrated in layer 2, with limited spread into layer 3 so that there is little overlap with callosal neurons. In the immature brain, callosal neurons are characterized by a tangential distribution that is more widespread than in the adult, while their laminar distribution undergoes little developmental change. Association neurons show two types of changes in their laminar distribution: (1) in the early fetus, there is a large excess of association neurons in supragranular layers, the adult distribution being achieved some time after birth; and (2) during maturation there is a selective elimination of at least 50% of the projections originating from the lower part of layers 2/3. Hence, the adult radial segregation of association and callosal pathways is achieved in part by regressive phenomena. The developmental reduction of bihemispheric projections is largely independent of changes in the organization of association neurons. Quantitative analysis of the morphology and spatial location of neurons sending axon collaterals to both hemispheres suggests that they constitute a subset of callosal neurons and that their frequency is determined by factors that regulate directly this population. These results are discussed with respect to the specification of visual cortical pathways during ontogenesis.

Incidence of visual cortical neurons which have axon collaterals projecting to both cerebral hemispheres during prenatal primate development.

Fast blue was injected massively in extrastriate cortex of one hemisphere Diamidino yellow in area 17 of the other hemisphere, in adult and prenatal cynomolgus monkeys. After a suitable survival period the brains were processed for fluorescent dyes. Counts were made of the total number of labeled neurons and of those neurons which were labeled by both dyes and which project therefore to both hemispheres by means of bifurcating axon collaterals. At 122 and 135 days after conception (E122 and E135), shortly after cortico-cortical pathways are established, double-labeled neurons constituted 0.45% and 0.46% of the total population of labeled neurons in area V2. In V2 in the adult the range of values of double-labeled neurons was 0.03-0.08%.