Samuel Frere

APP Homodimers Transduce an Amyloid-β-Mediated Increase in Release Probability at Excitatory Synapses

Accumulation of amyloid-β peptides (Aβ), the proteolytic products of the amyloid precursor protein (APP), induces a variety of synaptic dysfunctions ranging from hyperactivity to depression that are thought to cause cognitive decline in Alzheimers disease. While depression of synaptic transmission has been extensively studied, the mechanisms underlying synaptic hyperactivity remain unknown. Here, we show that Aβ40 monomers and dimers augment release probability through local fine-tuning of APP-APP interactions at excitatory hippocampal boutons. Aβ40 binds to the APP, increases the APP homodimer fraction at the plasma membrane, and promotes APP-APP interactions. The APP activation induces structural rearrangements in the APP/Gi/o-protein complex, boosting presynaptic calcium flux and vesicle release. The APP growth-factor-like domain (GFLD) mediates APP-APP conformational changes and presynaptic enhancement. Thus, the APP homodimer constitutes a presynaptic receptor that transduces signal from Aβ40 to glutamate release. Excessive APP activation may initiate a positive feedback loop, contributing to hippocampal hyperactivity in Alzheimers disease.

Spatially resolved recording of transient fluorescence‐lifetime effects by line‐scanning TCSPC

We present a technique that records transient changes in the fluorescence lifetime of a sample with spatial resolution along a one‐dimensional scan. The technique is based on scanning the sample with a high‐frequency pulsed laser beam, detecting single photons of the fluorescence light, and building up a photon distribution over the distance along the scan, the arrival times of the photons after the excitation pulses and the time after a stimulation of the sample. The maximum resolution at which lifetime changes can be recorded is given by the line scan period. Transient lifetime effects can thus be resolved at a resolution of about one millisecond. We demonstrate the technique for recording photochemical and nonphotochemical chlorophyll transients in plants and transient changes in free Ca2+ in cultured neurons. Microsc. Res. Tech. 77:216–224, 2014.

Alzheimer's disease‐causing proline substitutions lead to presenilin 1 aggregation and malfunction

Do different neurodegenerative maladies emanate from the failure of a mutual protein folding mechanism? We have addressed this question by comparing mutational patterns that are linked to the manifestation of distinct neurodegenerative disorders and identified similar neurodegeneration‐linked proline substitutions in the prion protein and in presenilin 1 that underlie the development of a prion disorder and of familial Alzheimers disease (fAD), respectively. These substitutions were found to prevent the endoplasmic reticulum (ER)‐resident chaperone, cyclophilin B, from assisting presenilin 1 to fold properly, leading to its aggregation, deposition in the ER, reduction of γ‐secretase activity, and impaired mitochondrial distribution and function. Similarly, reduced quantities of the processed, active presenilin 1 were observed in brains of cyclophilin B knockout mice. These discoveries imply that reduced cyclophilin activity contributes to the development of distinct neurodegenerative disorders, propose a novel mechanism for the development of certain fAD cases, and support the emerging theme that this disorder can stem from aberrant presenilin 1 function. This study also points at ER chaperones as targets for the development of counter‐neurodegeneration therapies.

Alzheimer’s Disease: From Firing Instability to Homeostasis Network Collapse

Alzheimers disease (AD) starts from pure cognitive impairments and gradually progresses into degeneration of specific brain circuits. Although numerous factors initiating AD have been extensively studied, the common principles underlying the transition from cognitive deficits to neuronal loss remain unknown. Here we describe an evolutionarily conserved, integrated homeostatic network (IHN) that enables functional stability of central neural circuits and safeguards from neurodegeneration. We identify the critical modules comprising the IHN and propose a central role of neural firing in controlling the complex homeostatic network at different spatial scales. We hypothesize that firing instability and impaired synaptic plasticity at early AD stages trigger a vicious cycle, leading to dysregulation of the whole IHN. According to this hypothesis, the IHN collapse represents the major driving force of the transition from early memory impairments to neurodegeneration. Understanding the core elements of homeostatic control machinery, the reciprocal connections between distinct IHN modules, and the role of firing homeostasis in this hierarchy has important implications for physiology and should offer novel conceptual approaches for AD and other neurodegenerative disorders.

Multiporous Supramolecular Microspheres for Artificial Photosynthesis

Artificial photosynthesis shows a promising potential for sustainable supply of nutritional ingredients. While most studies focus on the assembly of the light-sensitive chromophores to 1-D architectures in an artificial photosynthesis system, other supramolecular morphologies, especially bioinspired ones, which may have more efficient light-harvesting properties, have been far less studied. Here, MCpP-FF, a bioinspired building block fabricated by conjugating porphyrin and diphenylalanine, was designed to self-assemble into nanofibers-based multiporous microspheres. The highly organized aromatic moieties result in extensive excitation red-shifts and notable electron transfer, thus leading to a remarkable attenuated fluorescence decay and broad-spectrum light sensitivity of the microspheres. Moreover, the enhanced photoelectron production and transfer capability of the microspheres are demonstrated, making them ideal candidates for sunlight-sensitive antennas in artificial photosynthesis. These properties induce a high turnover frequency of NADH, which can be used to produce bioproducts in biocatalytic reactions. In addition, the direct electron transfer makes external mediators unnecessary, and the insolubility of the microspheres in water allows their easy retrieval for sustainable applications. Our findings demonstrate an alternative to design new platforms for artificial photosynthesis, as well as a new type of bioinspired, supramolecular multiporous materials.

Sensory Deprivation Triggers Synaptic and Intrinsic Plasticity in the Hippocampus

Hippocampus, a temporal lobe structure involved in learning and memory, receives information from all sensory modalities. Despite extensive research on the role of sensory experience in cortical map plasticity, little is known about whether and how sensory experience regulates functioning of the hippocampal circuits. Here, we show that 9 ± 2 days of whisker deprivation during early mouse development depresses activity of CA3 pyramidal neurons by several principal mechanisms: decrease in release probability, increase in the fraction of silent synapses, and reduction in intrinsic excitability. As a result of deprivation-induced presynaptic inhibition, CA3-CA1 synaptic facilitation was augmented at high frequencies, shifting filtering properties of synapses. The changes in the AMPA-mediated synaptic transmission were accompanied by an increase in NR2B-containing NMDA receptors and a reduction in the AMPA/NMDA ratio. The observed reconfiguration of the CA3-CA1 connections may represent a homeostatic adaptation to augmentation in synaptic activity during the initial deprivation phase. In adult mice, tactile disuse diminished intrinsic excitability without altering synaptic facilitation. We suggest that sensory experience regulates computations performed by the hippocampus by tuning its synaptic and intrinsic characteristics.

Targeting PTEN interactions for Alzheimer's disease

Depression of AMPA receptor–mediated synaptic currents and impairment of long-term potentiation, triggered by amyloid-β, are the hallmarks of Alzheimers pathophysiology. These dysfunctions are now linked to upregulated PDZ domain–dependent PTEN translocation to spines, contributing to cognitive deficits in model mice.

Calcium Imaging Using Transient Fluorescence-Lifetime Imaging by Line-Scanning TCSPC

We present a technique that records transient changes in the concentration of free Ca2+ in live neurons with spatial resolution along a one-dimensional scan. The technique is based on recording fluorescence lifetime changes of a Ca2+ probe by multi-dimensional TCSPC. The sample is scanned with a high-frequency pulsed laser beam, single photons of the fluorescence light are detected, and a photon distribution over the distance along the scan, the arrival times of the photons after the excitation pulses and the time after a periodical stimulation of the sample is built up. The maximum resolution at which lifetime changes can be recorded is given by the line scan period. Transient lifetime effects can thus be resolved at a resolution of about 1 ms.

Recording Ca ++ Transients in Neurons by TCSPC FLIM

Multi-dimensional time-correlated single photon counting (TCSPC) techniques are able to record Ca++ transients in live neurons via the fluorescence lifetime changes of a Ca++-sensitive dye. The technique is based on periodical stimulation of the sample, raster scanning, or line scanning with a high-frequency pulsed laser, and multi-dimensional TCSPC. The recording process builds up a photon distribution over the spatial coordinates of the scan, the times of the photons in the laser pulse period, and the times of the photons in the stimulation period. The transient-time resolution is about 40 ms for raster scanning and about 1 ms for line scanning. We demonstrate the technique for electrical stimulation of cultured neurons incubated with Oregon Green Bapta.

SECRETED AMYLOID PRECURSOR PROTEIN IS A GABABR1A LIGAND THAT SUPPRESSES SYNAPTIC VESICLE RELEASE

homogenisation and fractionation of occipital and temporal brain sections. The effects of synthetic monomers and oligomers of amylin were assessed in neurons derived from induced pluripotent stem cells (iPSCs). Western blotting, ELISA and mesoscale analysis were used to measure the effects of amylin on Aß production and degradation and on markers of autophagy. Human occipital grey matter was also used to investigate IAPP (amylin precursor) gene expression by RT-PCR. Results: Immunohistochemistry demonstrated amylin deposits in the vasculature of brain tissue. Investigation of effects of amylin on iPSC derived neurons demonstrated an increase in extracellular Ab levels. Mesoscale assay of levels of sAPPa and sAPPb in media demonstrated no increase in either soluble fragment. This result implies that amylin increases extracellular Aß without influencing cleavage of APP. Our investigation of the potential mechanisms causing increased extracellular Aß suggests amylin may impair autophagy. RT-PCR data demonstrated expression of IAPP gene in human brain fractions and this was upregulated in AD cases. Conclusions: Our results confirm deposition of amylin in cerebral vasculature and provide new data to suggest amylin may be produced by brain tissue. Amylin appears to contribute to AD by increasing cerebral Ab via impairment of autophagy-mediated Aß degradation. The results of this study provide further evidence for amylin as a link between T2D and AD.