Late-phase, protein synthesis-dependent long-term potentiation in hippocampal CA1 pyramidal neurones with destabilized microtubule networks

A TrkB and TrkC partial agonist restores deficits in synaptic function and promotes activity-dependent synaptic and microglial transcriptomic changes in a late-stage Alzheimer's mouse model

INTRODUCTION

Tropomyosin related kinase B (TrkB) and C (TrkC) receptor signaling promotes synaptic plasticity and interacts with pathways affected by amyloid beta (Aβ) toxicity. Upregulating TrkB/C signaling could reduce Alzheimer's disease (AD)-related degenerative signaling, memory loss, and synaptic dysfunction.

METHODS

PTX-BD10-2 (BD10-2), a small molecule TrkB/C receptor partial agonist, was orally administered to aged London/Swedish-APP mutant mice (APPL/S) and wild-type controls. Effects on memory and hippocampal long-term potentiation (LTP) were assessed using electrophysiology, behavioral studies, immunoblotting, immunofluorescence staining, and RNA sequencing.

RESULTS

In APPL/S mice, BD10-2 treatment improved memory and LTP deficits. This was accompanied by normalized phosphorylation of protein kinase B (Akt), calcium-calmodulin-dependent kinase II (CaMKII), and AMPA-type glutamate receptors containing the subunit GluA1; enhanced activity-dependent recruitment of synaptic proteins; and increased excitatory synapse number. BD10-2 also had potentially favorable effects on LTP-dependent complement pathway and synaptic gene transcription.

DISCUSSION

BD10-2 prevented APPL/S/Aβ-associated memory and LTP deficits, reduced abnormalities in synapse-related signaling and activity-dependent transcription of synaptic genes, and bolstered transcriptional changes associated with microglial immune response.

HIGHLIGHTS

Small molecule modulation of tropomyosin related kinase B (TrkB) and C (TrkC) restores long-term potentiation (LTP) and behavior in an Alzheimer's disease (AD) model. Modulation of TrkB and TrkC regulates synaptic activity-dependent transcription. TrkB and TrkC receptors are candidate targets for translational therapeutics. Electrophysiology combined with transcriptomics elucidates synaptic restoration. LTP identifies neuron and microglia AD-relevant human-mouse co-expression modules.

A TrkB and TrkC partial agonist restores deficits in synaptic function and promotes activity-dependent synaptic and microglial transcriptomic changes in a late-stage Alzheimer's mouse model

Introduction

TrkB and TrkC receptor signaling promotes synaptic plasticity and interacts with pathways affected by amyloid-β (Aβ)-toxicity. Upregulating TrkB/C signaling could reduce Alzheimer's disease (AD)-related degenerative signaling, memory loss, and synaptic dysfunction.

Methods

PTX-BD10-2 (BD10-2), a small molecule TrkB/C receptor partial agonist, was orally administered to aged London/Swedish-APP mutant mice (APP L/S ) and wild-type controls (WT). Effects on memory and hippocampal long-term potentiation (LTP) were assessed using electrophysiology, behavioral studies, immunoblotting, immunofluorescence staining, and RNA-sequencing.

Results

Memory and LTP deficits in APP L/S mice were attenuated by treatment with BD10-2. BD10-2 prevented aberrant AKT, CaMKII, and GLUA1 phosphorylation, and enhanced activity-dependent recruitment of synaptic proteins. BD10-2 also had potentially favorable effects on LTP-dependent complement pathway and synaptic gene transcription.

Conclusions

BD10-2 prevented APP L/S /Aβ-associated memory and LTP deficits, reduced abnormalities in synapse-related signaling and activity-dependent transcription of synaptic genes, and bolstered transcriptional changes associated with microglial immune response.

Microtubules as Regulators of Neural Network Shape and Function: Focus on Excitability, Plasticity and Memory

Neuronal microtubules (MTs) are complex cytoskeletal protein arrays that undergo activity-dependent changes in their structure and function as a response to physiological demands throughout the lifespan of neurons. Many factors shape the allostatic dynamics of MTs and tubulin dimers in the cytosolic microenvironment, such as protein–protein interactions and activity-dependent shifts in these interactions that are responsible for their plastic capabilities. Recently, several findings have reinforced the role of MTs in behavioral and cognitive processes in normal and pathological conditions. In this review, we summarize the bidirectional relationships between MTs dynamics, neuronal processes, and brain and behavioral states. The outcomes of manipulating the dynamicity of MTs by genetic or pharmacological approaches on neuronal morphology, intrinsic and synaptic excitability, the state of the network, and behaviors are heterogeneous. We discuss the critical position of MTs as responders and adaptative elements of basic neuronal function whose impact on brain function is not fully understood, and we highlight the dilemma of artificially modulating MT dynamics for therapeutic purposes.

Measuring mRNA translation in neuronal processes and somata by tRNA-FRET

In neurons, the specific spatial and temporal localization of protein synthesis is of great importance for function and survival. Here, we visualized tRNA and protein synthesis events in fixed and live mouse primary cortical culture using fluorescently-labeled tRNAs. We were able to characterize the distribution and transport of tRNAs in different neuronal sub-compartments and to study their association with the ribosome. We found that tRNA mobility in neural processes is lower than in somata and corresponds to patterns of slow transport mechanisms, and that larger tRNA puncta co-localize with translational machinery components and are likely the functional fraction. Furthermore, chemical induction of long-term potentiation (LTP) in culture revealed up-regulation of mRNA translation with a similar effect in dendrites and somata, which appeared to be GluR-dependent 6 h post-activation. Importantly, measurement of protein synthesis in neurons with high resolutions offers new insights into neuronal function in health and disease states.

Role of microtubules in late-associative plasticity of hippocampal Schaffer collateral-CA1 synapses in mice

The microtubule network represents a key scaffolding structure that forms part of the neuronal cytoskeleton and contributes to biomolecule exchange within neurons. However, researchers have not determined whether an intact microtubule network is required for late associative plasticity. Therefore, the late associative plasticity of field excitatory postsynaptic potentials from two synaptic inputs was analyzed. Synaptic potentiation was induced through alternating tetanization of hippocampal Schaffer-collateral CA1 synaptic populations in acute slices prepared from young-adult C57BL/6 mice. Vincristine was applied to depolymerize microtubules. Vincristine did not alter the phosphorylation levels of plasticity-related pre- or postsynaptic proteins but reduced the level of a protein marker of the ER-Golgi intermediate compartment (ERGIC-53/p58). Vincristine did not alter the magnitude or maintenance of the synaptic potentiation evoked by repeated tetanization (3 × 100 stimuli at 100 Hz) of one synaptic population. However, this synaptic potentiation was sensitive to the coapplication of a protein synthesis inhibitor, such as rapamycin, anisomycin or cycloheximide, indicating that protein synthesis has become essential in depolymerized microtubules during the first hour of the synaptic potentiation. The application of vincristine up to a 70 stimuli, 100 Hz tetanization of a second synaptic input prevented the transformation of short-term potentiation into long-term potentiation (LTP), further indicating that intact microtubules are required for the late associative properties of synaptic plasticity. Therefore, activity-dependent synaptic plasticity does not rely on microtubules within the first two hours after tetanization; however, the associative interaction of independent synaptic inputs relies on their proper function. In addition, either new protein synthesis or microtubule-based processes are sufficient to stabilize LTP within the first 3 h after tetanization, and a deficit in synaptic plasticity is only observable when both processes are blocked.

The Synaptic Theory of Memory: A Historical Survey and Reconciliation of Recent Opposition

Trettenbrein (2016) has argued that the concept of the synapse as the locus of memory is outdated and has made six critiques of this concept. In this article, we examine these six critiques and suggest that the current theories of the neurobiology of memory and the empirical data indicate that synaptic activation is the first step in a chain of cellular and biochemical events that lead to memories formed in cell assemblies and neural networks that rely on synaptic modification for their formation. These neural networks and their modified synaptic connections can account for the cognitive basis of learning and memory and for memory deterioration in neurological disorders. We first discuss Hebb's (1949) theory that synaptic change and the formation of cell assemblies and phase sequences can link neurophysiology to cognitive processes. We then examine each of Trettenbrein's (2016) critiques of the synaptic theory in light of Hebb's theories and recent empirical data. We examine the biochemical basis of memory formation and the necessity of synaptic modification to form the neural networks underlying learning and memory. We then examine the use of Hebb's theories of synaptic change and cell assemblies for integrating neurophysiological and cognitive conceptions of learning and memory. We conclude with an examination of the applications of the Hebb synapse and cell assembly theories to the study of the neuroscience of learning and memory, the development of computational models of memory and the construction of "intelligent" robots. We conclude that the synaptic theory of memory has not met its demise, but is essential to our understanding of the neural basis of memory, which has two components: synaptic plasticity and intrinsic plasticity.

Epigenetic regulation and chromatin remodeling in learning and memory

Understanding the underlying mechanisms of memory formation and maintenance has been a major goal in the field of neuroscience. Memory formation and maintenance are tightly controlled complex processes. Among the various processes occurring at different levels, gene expression regulation is especially crucial for proper memory processing, as some genes need to be activated while some genes must be suppressed. Epigenetic regulation of the genome involves processes such as DNA methylation and histone post-translational modifications. These processes edit genomic properties or the interactions between the genome and histone cores. They then induce structural changes in the chromatin and lead to transcriptional changes of different genes. Recent studies have focused on the concept of chromatin remodeling, which consists of 3D structural changes in chromatin in relation to gene regulation, and is an important process in learning and memory. In this review, we will introduce three major epigenetic processes involved in memory regulation: DNA methylation, histone methylation and histone acetylation. We will also discuss general mechanisms of long-term memory storage and relate the epigenetic control of learning and memory to chromatin remodeling. Finally, we will discuss how epigenetic mechanisms can contribute to the pathologies of neurological disorders and cause memory-related symptoms.

Membrane-tethered AKT kinase regulates basal synaptic transmission and early phase LTP expression by modulation of post-synaptic AMPA receptor level

The serine/threonine kinase AKT/PKB plays a fundamental role in a wide variety of neuronal functions, including neuronal cell development, axonal growth, and synaptic plasticity. Multiple evidence link AKT signaling pathways to regulation of late phase long-term synaptic plasticity, synaptogenesis, and spinogenesis, as well as long-term memory formation. Nevertheless, the downstream effectors mediating the effects of AKT on early phase long-term potentiation (eLTP) are currently unknown. Here we report that using different regimes of pharmacological activation and inhibition of AKT activity in acute hippocampal slices, we found that AKT regulates the post-synaptic expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA) receptors affecting solely the expression of eLTP, with no effect on its induction and maintenance. We further show that both maintenance of basal synaptic activity and expression of eLTP require plasma membrane tethering by activated AKT and that basal synaptic activity may be regulated via the direct effects of AKT1 on the expression level of post-synaptic AMPA receptors bypassing the canonical AKT signaling. Finally, we establish that eLTP expression requires the involvement of both the canonical AKT signaling pathways and the direct effect of AKT1 on AMPA receptor activity/expression in the post-synaptic membrane. © 2016 Wiley Periodicals, Inc.

Ca(2+)- and CaMKII-mediated processes in early LTP

Learning methods determine the degree of stimulation of engrams encoding information to be memorised. More enriching modes of learning allow more enduring long-term potentiation of the synapses associated with these memories. The additional activity causes a prolonged increase in [Ca2+] in the dendritic spine of the postsynaptic neuron. This allows Ca(2+)-mediated molecular pathways to bring about cytoskeletal remodeling, posttranslational modifications, and protein trafficking. These processes contribute to early long-term potentiation of the synapses, strengthening the memory they store and lead to improved performance on tests of memory recall.

Conditional ablation of raptor or rictor has differential impact on oligodendrocyte differentiation and CNS myelination

During CNS development, oligodendrocytes, the myelinating glia of the CNS, progress through multiple transitory stages before terminating into fully mature cells. Oligodendrocyte differentiation and myelination is a tightly regulated process requiring extracellular signals to converge to elicit specific translational and transcriptional changes. Our lab has previously shown that the protein kinases, Akt and mammalian Target of Rapamycin (mTOR), are important regulators of CNS myelination in vivo. mTOR functions through two distinct complexes, mTOR complex 1 (mTORC1) and mTORC2, by binding to either Raptor or Rictor, respectively. To establish whether the impact of mTOR on CNS myelination results from unique functions of mTORC1 or mTORC2 during CNS myelination, we conditionally ablated either Raptor or Rictor in the oligodendrocyte lineage, in vivo. We show that Raptor (mTORC1) is a positive regulator of developmental CNS mouse myelination when mTORC2 is functional, whereas Rictor (mTORC2) ablation has a modest positive effect on oligodendrocyte differentiation, and very little effect on myelination, when mTORC1 is functional. Also, we show that loss of Raptor in oligodendrocytes results in differential dysmyelination in specific areas of the CNS, with the greatest impact on spinal cord myelination.

Stable mossy fiber long-term potentiation requires calcium influx at the granule cell soma, protein synthesis, and microtubule-dependent axonal transport

The synapses formed by the mossy fiber (MF) axons of hippocampal dentate gyrus granule neurons onto CA3 pyramidal neurons exhibit an intriguing form of experience-dependent synaptic plasticity that is induced and expressed presynaptically. In contrast to most other CNS synapses, long-term potentiation (LTP) at the MF-CA3 synapse is readily induced even during blockade of postsynaptic glutamate receptors. Furthermore, blocking voltage-gated Ca(2+) channels prevents MF-LTP, supporting an involvement of presynaptic Ca(2+) signaling via voltage-gated Ca(2+) channels in MF-LTP induction. We examined the contribution of activity in both dentate granule cell somata and MF terminals to MF-LTP. We found that the induction of stable MF-LTP requires tetanization-induced action potentials not only at MF boutons, but also at dentate granule cell somata. Similarly, blocking Ca(2+) influx via voltage-gated Ca(2+) channels only at the granule cell soma was sufficient to disrupt MF-LTP. Finally, blocking protein synthesis or blocking fast axonal transport mechanisms via disruption of axonal tubulin filaments resulted in decremental MF-LTP. Collectively, these data suggest that-in addition to Ca(2+) influx at the MF terminals-induction of MF synaptic plasticity requires action potential-dependent Ca(2+) signaling at granule cell somata, protein synthesis, and fast axonal transport along MFs. A parsimonious interpretation of these results is that somatic activity triggers protein synthesis at the soma; newly synthesized proteins are then transported to MF terminals, where they contribute to the stabilization of MF-LTP. Finally, the present data imply that synaptic plasticity at the MF-CA3 synapse can be affected by local modulation of somatic and presynaptic Ca(2+) channel activity.