Removal of AMPA receptors (AMPARs) from synapses is preceded by transient endocytosis of extrasynaptic AMPARs

AMPA receptors (AMPARs) are dynamically regulated at synapses, but the time course and location of their exocytosis and endocytosis are not known. Therefore, we have used ecliptic pHluorin-tagged glutamate receptor 2 to visualize changes in AMPAR surface expression in real time. We show that synaptic and extrasynaptic AMPARs respond very differently to NMDA receptor activation; there is a rapid internalization of extrasynaptic AMPARs that precedes the delayed removal of synaptic AMPARs

Maturation of a recurrent excitatory neocortical circuit by experience-dependent unsilencing of newly-formed dendritic spines

SummaryLocal recurrent excitatory circuits are ubiquitous in neocortex, yet little is known about their development or architecture. Here we introduce a quantitative technique for efficient single-cell resolution circuit mapping using 2-photon (2P) glutamate uncaging and analyze experience-dependent neonatal development of the layer 4 barrel cortex local excitatory circuit. We show that sensory experience specifically drives a 3-fold increase in connectivity at postnatal day (P) 9, producing a highly recurrent network. A profound dendritic spinogenesis occurs concurrent with the connectivity increase, but this is not experience dependent. However, in experience-deprived cortex, a much greater proportion of spines lack postsynaptic AMPA receptors (AMPARs) and synaptic connectivity via NMDA receptors (NMDARs) is the same as in normally developing cortex. Thus we describe a approach for quantitative circuit mapping and show that sensory experience sculpts an intrinsically developing template network, which is based on NMDAR-only synapses, by driving AMPARs into newly formed silent spines

Synaptic alterations associated with disrupted sensory encoding in a mouse model of tauopathy

This is the final version. Available on open access from Oxford University Press via the DOI in this recordData availability: The data and analysis code used for this study are available from the corresponding author, upon reasonable request.Synapse loss is currently the best biological correlate of cognitive decline in Alzheimer’s disease and other tauopathies. Synapses seem to be highly vulnerable to tau-mediated disruption in neurodegenerative tauopathies. However, it is unclear how and when this leads to alterations in function related to the progression of tauopathy and neurodegeneration. We used the well-characterized rTg4510 mouse model of tauopathy at 5–6 months and 7–8 months of age, respectively, to study the functional impact of cortical synapse loss. The earlier age was used as a model of prodromal tauopathy, with the later age corresponding to more advanced tau pathology and presumed progression of neurodegeneration. Analysis of synaptic protein expression in the somatosensory cortex showed significant reductions in synaptic proteins and NMDA and AMPA receptor subunit expression in rTg4510 mice. Surprisingly, in vitro whole-cell patch clamp electrophysiology from putative pyramidal neurons in layer 2/3 of the somatosensory cortex suggested no functional alterations in layer 4 to layer 2/3 synaptic transmission at 5–6 months. From these same neurons, however, there were alterations in dendritic structure, with increased branching proximal to the soma in rTg4510 neurons. Therefore, in vivo whole-cell patch clamp recordings were utilized to investigate synaptic function and integration in putative pyramidal neurons in layer 2/3 of the somatosensory cortex. These recordings revealed a significant increase in the peak response to synaptically driven sensory stimulation-evoked activity and a loss of temporal fidelity of the evoked signal to the input stimulus in rTg4510 neurons. Together, these data suggest that loss of synapses, changes in receptor expression and dendritic restructuring may lead to alterations in synaptic integration at a network level. Understanding these compensatory processes could identify targets to help delay symptomatic onset of dementia.Medical Research Council (MRC)Alzheimer’s Research UKElizabeth Blackwell Institute, University of BristolWellcome Trus

Design principles of hair-like structures as biological machines

Hair-like structures are prevalent throughout biology and frequently act to sense or alter interactions with an organism's environment. The overall shape of a hair is simple: a long, filamentous object that protrudes from the surface of an organism. This basic design, however, can confer a wide range of functions, owing largely to the flexibility and large surface area that it usually possesses. From this simple structural basis, small changes in geometry, such as diameter, curvature and inter-hair spacing, can have considerable effects on mechanical properties, allowing functions such as mechanosensing, attachment, movement and protection. Here, we explore how passive features of hair-like structures, both individually and within arrays, enable diverse functions across biology. Understanding the relationships between form and function can provide biologists with an appreciation for the constraints and possibilities on hair-like structures. Additionally, such structures have already been used in biomimetic engineering with applications in sensing, water capture and adhesion. By examining hairs as a functional mechanical unit, geometry and arrangement can be rationally designed to generate new engineering devices and ideas

Corticosterone Alters AMPAR Mobility and Facilitates Bidirectional Synaptic Plasticity

Background: The stress hormone corticosterone has the ability both to enhance and suppress synaptic plasticity and learning and memory processes. However, until today there is very little known about the molecular mechanism that underlies the bidirectional effects of stress and corticosteroid hormones on synaptic efficacy and learning and memory processes. In this study we investigate the relationship between corticosterone and AMPA receptors which play a critical role in activity-dependent plasticity and hippocampal-dependent learning. Methodology/Principal Findings: Using immunocytochemistry and live cell imaging techniques we show that corticosterone selectively increases surface expression of the AMPAR subunit GluR2 in primary hippocampal cultures via a glucocorticoid receptor and protein synthesis dependent mechanism. In agreement, we report that corticosterone also dramatically increases the fraction of surface expressed GluR2 that undergo lateral diffusion. Furthermore, our data indicate that corticosterone facilitates NMDAR-invoked endocytosis of both synaptic and extra-synaptic GluR2 under conditions that weaken synaptic transmission. Conclusion/Significance: Our results reveal that corticosterone increases mobile GluR2 containing AMPARs. The enhanced lateral diffusion properties can both facilitate the recruitment of AMPARs but under appropriate conditions facilitate the loss of synaptic AMPARs (LTD). These actions may underlie both the facilitating and suppressive effects of corticosteroid hormones on synaptic plasticity and learning and memory and suggest that these hormones accentuate synaptic efficacy

Altered synapse stability in the early stages of tauopathy

Synapse loss is a key feature of dementia, but it is unclear whether synaptic dysfunction precedes degenerative phases of the disease. Here, we show that even before any decrease in synapse density, there is abnormal turnover of cortical axonal boutons and dendritic spines in a mouse model of tauopathy-associated dementia. Strikingly, tauopathy drives a mismatch in synapse turnover; postsynaptic spines turn over more rapidly, whereas presynaptic boutons are stabilized. This imbalance between pre- and post-synaptic stability coincides with reduced synaptically driven neuronal activity in pre-degenerative stages of the disease

Photo-antagonism of the GABAA receptor

Neurotransmitter receptor trafficking is fundamentally important for synaptic transmission and neural network activity. GABAA receptors and inhibitory synapses are vital components of brain function, yet much of our knowledge regarding receptor mobility and function at inhibitory synapses is derived indirectly from using recombinant receptors, antibody-tagged native receptors and pharmacological treatments. Here we describe the use of a set of research tools that can irreversibly bind to and affect the function of recombinant and neuronal GABAA receptors following ultraviolet photoactivation. These compounds are based on the competitive antagonist gabazine and incorporate a variety of photoactive groups. By using site-directed mutagenesis and ligand-docking studies, they reveal new areas of the GABA binding site at the interface between receptor β and α subunits. These compounds enable the selected inactivation of native GABAA receptor populations providing new insight into the function of inhibitory synapses and extrasynaptic receptors in controlling neuronal excitation

Fast Inhibition of Glutamate-Activated Currents by Caffeine

Background: Caffeine stimulates calcium-induced calcium release (CICR) in many cell types. In neurons, caffeine stimulates CICR presynaptically and thus modulates neurotransmitter release. Methodology/Principal Findings: Using the whole-cell patch-clamp technique we found that caffeine (20 mM) reversibly increased the frequency and decreased the amplitude of miniature excitatory postsynaptic currents (mEPSCs) in neocortical neurons. The increase in mEPSC frequency is consistent with a presynaptic mechanism. Caffeine also reduced exogenously applied glutamate-activated currents, confirming a separate postsynaptic action. This inhibition developed in tens of milliseconds, consistent with block of channel currents. Caffeine (20 mM) did not reduce currents activated by exogenous NMDA, indicating that caffeine block is specific to non-NMDA type glutamate receptors. Conclusions/Significance: Caffeine-induced inhibition of mEPSC amplitude occurs through postsynaptic block of non-NMDA type ionotropic glutamate receptors. Caffeine thus has both pre and postsynaptic sites of action at excitatory synapses

Inhibitors of Bcl-2 protein family deplete ER Ca2+ stores in pancreatic acinar cells

Physiological stimulation of pancreatic acinar cells by cholecystokinin and acetylcholine activate a spatial-temporal pattern of cytosolic [Ca+2] changes that are regulated by a coordinated response of inositol 1,4,5-trisphosphate receptors (IP3Rs), ryanodine receptors (RyRs) and calcium-induced calcium release (CICR). For the present study, we designed experiments to determine the potential role of Bcl-2 proteins in these patterns of cytosolic [Ca+2] responses. We used small molecule inhibitors that disrupt the interactions between prosurvival Bcl-2 proteins (i.e. Bcl-2 and Bcl-xl) and proapoptotic Bcl-2 proteins (i.e. Bax) and fluorescence microfluorimetry techniques to measure both cytosolic [Ca+2] and endoplasmic reticulum [Ca+2]. We found that the inhibitors of Bcl-2 protein interactions caused a slow and complete release of intracellular agonist-sensitive stores of calcium. The release was attenuated by inhibitors of IP3Rs and RyRs and substantially reduced by strong [Ca2+] buffering. Inhibition of IP3Rs and RyRs also dramatically reduced activation of apoptosis by BH3I-2′. CICR induced by different doses of BH3I-2′ in Bcl-2 overexpressing cells was markedly decreased compared with control. The results suggest that Bcl-2 proteins regulate calcium release from the intracellular stores and suggest that the spatial-temporal patterns of agonist-stimulated cytosolic [Ca+2] changes are regulated by differential cellular distribution of interacting pairs of prosurvival and proapoptotic Bcl-2 proteins