Iftach Dolev

Amyloid-beta as a positive endogenous regulator of release probability at hippocampal synapses.

Accumulation of cerebral amyloid-β peptide (Aβ) is essential for developing synaptic and cognitive deficits in Alzheimers disease. However, the physiological functions of Aβ, as well as the primary mechanisms that initiate early Aβ-mediated synaptic dysfunctions, remain largely unknown. Here we examine the acute effects of endogenously released Aβ peptides on synaptic transfer at single presynaptic terminals and synaptic connections in rodent hippocampal cultures and slices. Increasing extracellular Aβ by inhibiting its degradation enhanced release probability, boosting ongoing activity in the hippocampal network. Presynaptic enhancement mediated by Aβ was found to depend on the history of synaptic activation, with lower impact at higher firing rates. Notably, both elevation and reduction in Aβ levels attenuated short-term synaptic facilitation during bursts in excitatory synaptic connections. These observations suggest that endogenous Aβ peptides have a crucial role in activity-dependent regulation of synaptic vesicle release and might point to the primary pathological events that lead to compensatory synapse loss in Alzheimers disease.

Spike bursts increase amyloid-β 40/42 ratio by inducing a presenilin-1 conformational change

Accumulated genetic evidence suggests that attenuation of the ratio between cerebral amyloid-β Aβ40 and Aβ42 isoforms is central to familial Alzheimers disease (FAD) pathogenesis. However, FAD mutations account for only 1–2% of Alzheimers disease cases, leaving the experience-dependent mechanisms regulating Aβ40/42 an enigma. Here we explored regulation of Aβ40/42 ratio by temporal spiking patterns in the rodent hippocampus. Spike bursts boosted Aβ40/42 through a conformational change in presenilin1 (PS1), the catalytic subunit of γ-secretase, and subsequent increase in Aβ40 production. Conversely, single spikes did not alter basal PS1 conformation and Aβ40/42. Burst-induced PS1 conformational shift was mediated by means of Ca2+-dependent synaptic vesicle exocytosis. Presynaptic inhibition in vitro and visual deprivation in vivo augmented synaptic and Aβ40/42 facilitation by bursts in the hippocampus. Thus, burst probability and transfer properties of synapses represent fundamental features regulating Aβ40/42 by experience and may contribute to the initiation of the common, sporadic Alzheimers disease.

Basal GABA Regulates GABABR Conformation and Release Probability at Single Hippocampal Synapses

Presynaptic GABA(B) receptor (GABA(B)R) heterodimers are composed of GB(1a)/GB(2) subunits and critically influence synaptic and cognitive functions. Here, we explored local GABA(B)R activation by integrating optical tools for monitoring receptor conformation and synaptic vesicle release at individual presynaptic boutons of hippocampal neurons. Utilizing fluorescence resonance energy transfer (FRET) spectroscopy, we detected a wide range of FRET values for CFP/YFP-tagged GB(1a)/GB(2) receptors that negatively correlated with release probabilities at single synapses. High FRET of GABA(B)Rs associated with low release probability. Notably, pharmacological manipulations that either reduced or increased basal receptor activation decreased intersynapse variability of GB(1a)/GB(2) receptor conformation. Despite variability along axons, presynaptic GABA(B)R tone was dendrite specific, having a greater impact on synapses at highly innervated proximal branches. Prolonged neuronal inactivity reduced basal receptor activation, leading to homeostatic augmentation of release probability. Our findings suggest that local variations in basal GABA concentration are a major determinant of GB(1a)/GB(2) conformational variability, which contributes to heterogeneity of neurotransmitter release at hippocampal synapses.

Intraneuronal amyloid‐β plays a role in mediating the synergistic pathological effects of apoE4 and environmental stimulation

The allele E4 of apolipoprotein E4 (apoE4), which is the most prevalent genetic risk factor of Alzheimer’s disease (AD), inhibits synaptogenesis and neurogenesis and stimulates apoptosis in brains of apoE4 transgenic mice that have been exposed to an enriched environment. In the present study, we investigated the hypothesis that the brain activity‐dependent impairments in neuronal plasticity, induced by apoE4, are mediated via the amyloid cascade. Importantly, we found that exposure of mice transgenic for either apoE4, or the Alzheimer’s disease benign allele apoE3, to an enriched environment elevates similarly the hippocampal levels of amyloid‐β peptide (Aβ) and apoE of these mice, but that the degree of aggregation and spatial distribution of Aβ in these mice are markedly affected by the apoE genotype. Accordingly, environmental stimulation triggered the formation of extracellular plaque‐like Aβ deposits and the accumulation of intra‐neuronal oligomerized Aβ specifically in brains of apoE4 mice. Further experiments revealed that hippocampal dentate gyrus neurons are particularly susceptible to apoE4 and environmental stimulation and that these neurons are specifically enriched in both oligomerized Aβ and apoE. These findings show that the impairments in neuroplasticity which are induced by apoE4 following environmental stimulation are associated with the accumulation of intraneuronal Aβ and suggest that oligomerized Aβ mediates the synergistic pathological effects of apoE4 and environmental stimulation.

The nucleation growth and reversibility of Amyloid-β deposition in vivo

The amyloid-beta (Abeta) peptide is a major constituent of the brain senile plaques that characterize Alzheimers disease (AD). Converging observations led to the formulation of the amyloid hypothesis whereby the accumulation of soluble aggregates and insoluble Abeta deposits is the primary event in AD pathogenesis. Furthermore, the apoE4 isoform of apolipoprotein E, a major prevalent genetic risk factor of AD, is associated with increased Abeta deposition. To investigate the initial stages of the amyloid cascade in vivo and how this is affected by apoE4, we studied the effects of prolonged inhibition and subsequent reactivation of the Abeta-degrading enzyme, neprilysin, on aggregation and deposition of Abeta in apoE transgenic and control mice. The results revealed that Abeta deposition in vivo is initiated by aggregation of Abeta42, which is followed by reversible deposition of both Abeta42 and Abeta40, along with growth of the deposits, and by their subsequent irreversible fibrillization. The initiation of Abeta42 deposition is accelerated isoform-specifically by apoE4, whereas the growth and dissolution of the Abeta deposits as well as their fibrillization are similarly stimulated by the various apoE isoforms. Interestingly, Abeta deposition was associated with increased gliosis, which may reflect early pathological interactions of beta with the brains parenchyma.

Activation of the Amyloid Cascade by Intracerebroventricular Injection of the Protease Inhibitor Phosphoramidon

We presently investigated the pathological effects of prolonged inhibition of brain β-amyloid (Aβ) degradation in vivo. The results obtained revealed that intracerebroventricular injection of the protease inhibitor phosphoramidon into wild-type mice for up to a month elevated the soluble and deposited brain Aβ levels and concomitantly induced the neurodegeneration of distinct hippocampal neurons as well as neuroinflammation. These findings reproduce pathological effects associated with the initial stages of the amyloid cascade and provide a novel model system for studying their underlying mechanisms.

Neuron-specific susceptibility to apolipoprotein E4.

The article by Schonheit et al. in this issue (2006) describes he effects of apoE4 on dendritic shape and geometry in he human hippocampus and discusses whether these effects re due to developmental differences in dendritic tree comlexity, or they appear in patients with Alzheimer’s disease AD) because of diminished capacity to withstand ageand D-related pathological changes. To minimize the possiility of secondary effects of AD pathology being exerted n the neuronal dendrites, the authors chose to focus on arvalbumin-positive GABAergic hippocampal interneurons f non-demented subjects with minimal AD pathology. Their tudy showed that, relative to apoE3, neither apoE4 nor apoE2 ignificantly affects dendritic morphology. This suggests that he dendritic tree of GABAergic hippocampal neurons is ot developmentally affected nor is it affected at a young ge by apoE4. The differentiation between clinically normal ubjects with mild AD pathology and normal aging is not learcut; the presently studied cases may thus contain both reclinical AD and normal aging subjects. However, previus studies revealed that the degeneration of parvalbuminontaining GABAergic interneurons in AD occurs late in the isease and is subsequent to the loss of other neuronal sysems [32], whereas other reports showed no effect on such eurons in AD [8,28]. These findings and results presented y Schonheit et al. in their present article both suggest that the arvalbumin-containing GABAergic neurons are relatively esistant to the pathological effects of AD and apoE4. The length and arborization of neuronal dendrites and heir plastic remodeling in AD are more severely affected in poE4 AD patients than in those who lack apoE4. The magitude of this effect is brain-area specific. For example, in

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deactivation pattern of MCI subjects was heterogeneous varying both according to their level of impairment and APOE-4 status. AD patients showed significantly less deactivation than OC in medial parietal and cingulate/retrosplenial cortices (z 5.32, p 0.0001). Comparing OC and AD subgroups, we found evidence of a hierarchy of deactivation decreasing in the following order: OC APOE-4 non-carriers OC APOE-4 carriers AD APOE-4 non-carriers AD APOE-4 carriers. Conclusions: The present study demonstrates that the pattern of task-related fMRI deactivation is remarkably disrupted in AD patients, particularly in AD APOE-4 carriers. Furthermore, consistent with PET findings, APOE-4 was associated with impaired parietal and posterior cingulate deactivation even in cognitively intact older individuals at risk for AD.