Photoactivatable CaMKII induces synaptic plasticity in single synapses

Rem2 interacts with CaMKII at synapses and restricts long-term potentiation in hippocampus

Synaptic plasticity, the process whereby neuronal connections are either strengthened or weakened in response to stereotyped forms of stimulation, is widely believed to represent the molecular mechanism that underlies learning and memory. The holoenzyme calcium/calmodulin-dependent protein kinase II (CaMKII) plays a well-established and critical role in the induction of a variety of forms of synaptic plasticity such as long-term potentiation (LTP), long-term depression (LTD) and depotentiation. Previously, we identified the GTPase Rem2 as a potent, endogenous inhibitor of CaMKII. Here, we report that knock out of Rem2 enhances LTP at the Schaffer collateral to CA1 synapse in hippocampus, consistent with an inhibitory action of Rem2 on CaMKII in vivo. Further, re-expression of WT Rem2 rescues the enhanced LTP observed in slices obtained from Rem2 conditional knock out (cKO) mice, while expression of a mutant Rem2 construct that is unable to inhibit CaMKII in vitro fails to rescue increased LTP. In addition, we demonstrate that CaMKII and Rem2 interact in dendritic spines using a 2pFLIM-FRET approach. Taken together, our data lead us to propose that Rem2 serves as a brake on synaptic potentiation via inhibition of CaMKII activity. Further, the enhanced LTP phenotype we observe in Rem2 cKO slices reveals a previously unknown role for Rem2 in the negative regulation of CaMKII function.

Closed-loop experiments and brain machine interfaces with multiphoton microscopy

In the field of neuroscience, the importance of constructing closed-loop experimental systems has increased in conjunction with technological advances in measuring and controlling neural activity in live animals. We provide an overview of recent technological advances in the field, focusing on closed-loop experimental systems where multiphoton microscopy-the only method capable of recording and controlling targeted population activity of neurons at a single-cell resolution in vivo-works through real-time feedback. Specifically, we present some examples of brain machine interfaces (BMIs) using in vivo two-photon calcium imaging and discuss applications of two-photon optogenetic stimulation and adaptive optics to real-time BMIs. We also consider conditions for realizing future optical BMIs at the synaptic level, and their possible roles in understanding the computational principles of the brain.

Studying CaMKII: Tools and standards

The Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a ubiquitous mediator of cellular Ca2+ signals with both enzymatic and structural functions. Here, we briefly introduce the complex regulation of CaMKII and then provide a comprehensive overview of the expanding toolbox to study CaMKII. Beyond a variety of distinct mutants, these tools now include optical methods for measurement and manipulation, with the latter including light-induced inhibition, stimulation, and sequestration. Perhaps most importantly, there are now three mechanistically distinct classes of specific CaMKII inhibitors, and their combined use enables the interrogation of CaMKII functions in a manner that is powerful and sophisticated yet also accessible. This review aims to provide guidelines for the interpretation of the results obtained with these tools, with careful consideration of their direct and indirect effects.

Molecular tools for recording and intervention of neuronal activity

Observing the activity of neural networks is critical for the identification of learning and memory processes, as well as abnormal activities of neural circuits in disease, particularly for the purpose of tracking disease progression. Methodologies for describing the activity history of neural networks using molecular biology techniques first utilized genes expressed by active neurons, followed by the application of recently developed techniques including optogenetics and incorporation of insights garnered from other disciplines, including chemistry and physics. In this review, we will discuss ways in which molecular biological techniques used to describe the activity of neural networks have evolved along with the potential for future development.

Alpha-Asarone Ameliorates Neurological Dysfunction of Subarachnoid Hemorrhagic Rats in Both Acute and Recovery Phases via Regulating the CaMKII-Dependent Pathways

Early brain injury (EBI) is the leading cause of poor prognosis for patients suffering from subarachnoid hemorrhage (SAH), particularly learning and memory deficits in the repair phase. A recent report has involved calcium/calmodulin-dependent protein kinase II (CaMKII) in the pathophysiological process underlying SAH-induced EBI. Alpha-asarone (ASA), a major compound isolated from the Chinese medicinal herb Acorus tatarinowii Schott, was proven to reduce secondary brain injury by decreasing CaMKII over-phosphorylation in rats' model of intracerebral hemorrhage in our previous report. However, the effect of ASA on SAH remains unclear, and the role of CaMKII in both acute and recovery stages of SAH needs further investigation. In this work, we first established a classic SAH rat model by endovascular perforation and intraperitoneally administrated different ASA doses (10, 20, and 40 mg/kg) 2 h after successful modeling. Then, the short- and long-term neurobehavioral performances were blindly evaluated to confirm ASA's efficacy against SAH. Subsequently, we explored ASA's therapeutic mechanism in both acute and recovery stages using histopathological examination, TUNEL staining, flow cytometry, Western-blot, double-immunofluorescence staining, and transmission electron microscopy (TEM) observation. Finally, KN93, a selective CaMKII inhibitor, was applied in oxyhemoglobin-damaged HT22 cells to explore the role of CaMKII in ASA's neuroprotective effect. The results demonstrated that ASA alleviated short- and long-term neurological dysfunction, reduced mortality and seizure rate within 24 h, and prolonged 14-day survival in SAH rats. Histopathological examination showed a reduction of neuronal damage and a restoration of the hippocampal structure after ASA treatment in both acute and recovery phases of SAH. In the acute stage, the Western-blot and flow cytometer analyses showed that ASA restored E/I balance, reduced calcium overload and CaMKII phosphorylation, and inhibited mitochondrion-involved apoptosis, thus preventing neuronal damage and apoptosis underlying EBI post-SAH. In the recovery stage, the TEM observation, double-immunofluorescence staining, and Western-blot analyses indicated that ASA increased the numbers of synapses and enhanced synaptic plasticity in the ipsilateral hippocampi, probably by promoting NR2B/CaMKII interaction and activating subsequent CREB/BDNF/TrkB signaling pathways. Furthermore, KN93 notably reversed ASA's neuroprotective effect on oxyhemoglobin-damaged HT22 cells, confirming CaMKII a potential target for ASA's efficacy against SAH. Our study confirmed for the first time that ASA ameliorated the SAH rats' neurobehavioral deterioration, possibly via modulating CaMKII-involved pathways. These findings provided a promising candidate for the clinical treatment of SAH and shed light on future drug discovery against SAH.

Rem2 interacts with CaMKII at synapses and restricts long-term potentiation in hippocampus

Synaptic plasticity, the process whereby neuronal connections are either strengthened or weakened in response to stereotyped forms of stimulation, is widely believed to represent the molecular mechanism that underlies learning and memory. The holoenzyme CaMKII plays a well-established and critical role in the induction of a variety of forms of synaptic plasticity such as long-term potentiation (LTP), long-term depression (LTD) and depotentiation. Previously, we identified the GTPase Rem2 as a potent, endogenous inhibitor of CaMKII. Here, we report that knock out of Rem2 enhances LTP at the Schaffer collateral to CA1 synapse in hippocampus, consistent with an inhibitory action of Rem2 on CaMKII in vivo. Further, re-expression of WT Rem2 rescues the enhanced LTP observed in slices obtained from Rem2 conditional knock out (cKO) mice, while expression of a mutant Rem2 construct that is unable to inhibit CaMKII in vitro fails to rescue increased LTP. In addition, we demonstrate that CaMKII and Rem2 interact in dendritic spines using a 2pFLIM-FRET approach. Taken together, our data lead us to propose that Rem2 serves as a brake on runaway synaptic potentiation via inhibition of CaMKII activity. Further, the enhanced LTP phenotype we observe in Rem2 cKO slices reveals a previously unknown role for Rem2 in the negative regulation of CaMKII function.

Understanding neural circuit function through synaptic engineering

Synapses are a key component of neural circuits, facilitating rapid and specific signalling between neurons. Synaptic engineering - the synthetic insertion of new synaptic connections into in vivo neural circuits - is an emerging approach for neural circuit interrogation. This approach is especially powerful for establishing causality in neural circuit structure-function relationships, for emulating synaptic plasticity and for exploring novel patterns of circuit connectivity. Contrary to other approaches for neural circuit manipulation, synaptic engineering targets specific connections between neurons and functions autonomously with no user-controlled external activation. Synaptic engineering has been successfully implemented in several systems and in different forms, including electrical synapses constructed from ectopically expressed connexin gap junction proteins, synthetic optical synapses composed of presynaptic photon-emitting luciferase coupled with postsynaptic light-gated channels, and artificial neuropeptide signalling pathways. This Perspective describes these different methods and how they have been applied, and examines how the field may advance.

Optical mapping and optogenetics in cardiac electrophysiology research and therapy: a state-of-the-art review

State-of-the-art innovations in optical cardiac electrophysiology are significantly enhancing cardiac research. A potential leap into patient care is now on the horizon. Optical mapping, using fluorescent probes and high-speed cameras, offers detailed insights into cardiac activity and arrhythmias by analysing electrical signals, calcium dynamics, and metabolism. Optogenetics utilizes light-sensitive ion channels and pumps to realize contactless, cell-selective cardiac actuation for modelling arrhythmia, restoring sinus rhythm, and probing complex cell-cell interactions. The merging of optogenetics and optical mapping techniques for 'all-optical' electrophysiology marks a significant step forward. This combination allows for the contactless actuation and sensing of cardiac electrophysiology, offering unprecedented spatial-temporal resolution and control. Recent studies have performed all-optical imaging ex vivo and achieved reliable optogenetic pacing in vivo, narrowing the gap for clinical use. Progress in optical electrophysiology continues at pace. Advances in motion tracking methods are removing the necessity of motion uncoupling, a key limitation of optical mapping. Innovations in optoelectronics, including miniaturized, biocompatible illumination and circuitry, are enabling the creation of implantable cardiac pacemakers and defibrillators with optoelectrical closed-loop systems. Computational modelling and machine learning are emerging as pivotal tools in enhancing optical techniques, offering new avenues for analysing complex data and optimizing therapeutic strategies. However, key challenges remain including opsin delivery, real-time data processing, longevity, and chronic effects of optoelectronic devices. This review provides a comprehensive overview of recent advances in optical mapping and optogenetics and outlines the promising future of optics in reshaping cardiac electrophysiology and therapeutic strategies.

Meta-reinforcement learning via orbitofrontal cortex

The meta-reinforcement learning (meta-RL) framework, which involves RL over multiple timescales, has been successful in training deep RL models that generalize to new environments. It has been hypothesized that the prefrontal cortex may mediate meta-RL in the brain, but the evidence is scarce. Here we show that the orbitofrontal cortex (OFC) mediates meta-RL. We trained mice and deep RL models on a probabilistic reversal learning task across sessions during which they improved their trial-by-trial RL policy through meta-learning. Ca2+/calmodulin-dependent protein kinase II-dependent synaptic plasticity in OFC was necessary for this meta-learning but not for the within-session trial-by-trial RL in experts. After meta-learning, OFC activity robustly encoded value signals, and OFC inactivation impaired the RL behaviors. Longitudinal tracking of OFC activity revealed that meta-learning gradually shapes population value coding to guide the ongoing behavioral policy. Our results indicate that two distinct RL algorithms with distinct neural mechanisms and timescales coexist in OFC to support adaptive decision-making.

Controlling the interaction between CaMKII and Calmodulin with a photocrosslinking unnatural amino acid

Using unnatural amino acid mutagenesis, we made a mutant of CaMKII that forms a covalent linkage to Calmodulin upon illumination by UV light. Like wild-type CaMKII, the L308BzF mutant stoichiometrically binds to Calmodulin, in a calcium-dependent manner. Using this construct, we demonstrate that Calmodulin binding to CaMKII, even under these stochiometric conditions, does not perturb the CaMKII oligomeric state. Furthermore, we were able to achieve activation of CaMKII L308BzF by UV-induced binding of Calmodulin, which, once established, is further insensitive to calcium depletion. In addition to the canonical auto-inhibitory role of the regulatory segment, inter-subunit crosslinking in the absence of CaM indicates that kinase domains and regulatory segments are substantially mobile in basal conditions. Characterization of CaMKIIL308BzF in vitro, and its expression in mammalian cells, suggests it could be a promising candidate for control of CaMKII activity in mammalian cells with light.

Binding of CaMKII to the NMDA receptor is sufficient for long-term potentiation

Learning and the underlying long-lasting increases in glutamatergic synapse strength [called long-term potentiation (LTP)] require both Ca2+ influx through NMDA-type glutamate receptors (NMDARs) and the kinase CaMKII. New evidence now suggests that CaMKII can induce LTP purely by binding to the NMDAR subunit GluN2B and does not require the catalytic activity of the kinase.

Activity-dependent organization of prefrontal hub-networks for associative learning and signal transformation

Associative learning is crucial for adapting to environmental changes. Interactions among neuronal populations involving the dorso-medial prefrontal cortex (dmPFC) are proposed to regulate associative learning, but how these neuronal populations store and process information about the association remains unclear. Here we developed a pipeline for longitudinal two-photon imaging and computational dissection of neural population activities in male mouse dmPFC during fear-conditioning procedures, enabling us to detect learning-dependent changes in the dmPFC network topology. Using regularized regression methods and graphical modeling, we found that fear conditioning drove dmPFC reorganization to generate a neuronal ensemble encoding conditioned responses (CR) characterized by enhanced internal coactivity, functional connectivity, and association with conditioned stimuli (CS). Importantly, neurons strongly responding to unconditioned stimuli during conditioning subsequently became hubs of this novel associative network for the CS-to-CR transformation. Altogether, we demonstrate learning-dependent dynamic modulation of population coding structured on the activity-dependent formation of the hub network within the dmPFC.

Optogenetic activation of visual thalamus generates artificial visual percepts

The lateral geniculate nucleus (LGN), a retinotopic relay center where visual inputs from the retina are processed and relayed to the visual cortex, has been proposed as a potential target for artificial vision. At present, it is unknown whether optogenetic LGN stimulation is sufficient to elicit behaviorally relevant percepts, and the properties of LGN neural responses relevant for artificial vision have not been thoroughly characterized. Here, we demonstrate that tree shrews pretrained on a visual detection task can detect optogenetic LGN activation using an AAV2-CamKIIα-ChR2 construct and readily generalize from visual to optogenetic detection. Simultaneous recordings of LGN spiking activity and primary visual cortex (V1) local field potentials (LFPs) during optogenetic LGN stimulation show that LGN neurons reliably follow optogenetic stimulation at frequencies up to 60 Hz and uncovered a striking phase locking between the V1 LFP and the evoked spiking activity in LGN. These phase relationships were maintained over a broad range of LGN stimulation frequencies, up to 80 Hz, with spike field coherence values favoring higher frequencies, indicating the ability to relay temporally precise information to V1 using light activation of the LGN. Finally, V1 LFP responses showed sensitivity values to LGN optogenetic activation that were similar to the animal's behavioral performance. Taken together, our findings confirm the LGN as a potential target for visual prosthetics in a highly visual mammal closely related to primates.

Effects of vortioxetine on hippocampal-related cognitive impairment induced in rats by androgen deprivation as a model of prostate cancer treatment

Advances in prostate cancer treatment have significantly improved survival, but quality of life for survivors remains an under-studied area of research. Androgen deprivation therapy (ADT) is a foundational treatment for advanced prostate cancer and is used as an adjuvant for prolonged periods in many high-risk, localized tumors. More than half of patients treated with ADT experience debilitating cognitive impairments in domains such as spatial learning and working memory. In this study, we investigated the effects of androgen deprivation on hippocampal-mediated cognition in rats. Vortioxetine, a multimodal antidepressant, has been shown to improve cognition in depressed patients. Thus, we also tested the potential efficacy of vortioxetine in restoring impaired cognition after ADT. We further investigated mechanisms that might contribute to these effects, measuring changes in the circuitry and gene expression within the dorsal hippocampus. ADT via surgical castration induced impairments in visuospatial cognition on the novel object location test and attenuated afferent-evoked local field potentials recorded in the CA1 region of the dorsal hippocampus. Chronic dietary administration of vortioxetine effectively reversed these deficits. Castration significantly altered gene expression in the hippocampus, whereas vortioxetine had little effect. Pathway analysis revealed that androgen depletion altered pathways related to synaptic plasticity. These results suggest that the hippocampus may be vulnerable to ADT, contributing to cognitive impairment in prostate cancer patients. Further, vortioxetine may be a candidate to improve cognition in patients who experience cognitive decline after androgen deprivation therapy for prostate cancer and may do so by restoring molecular and circuit-level plasticity-related mechanisms compromised by ADT.

Synaptic memory and CaMKII

Ca2+/calmodulin-dependent protein kinase II (CaMKII) and long-term potentiation (LTP) were discovered within a decade of each other and have been inextricably intertwined ever since. However, like many marriages, it has had its up and downs. Based on the unique biochemical properties of CaMKII, it was proposed as a memory molecule before any physiological linkage was made to LTP. However, as reviewed here, the convincing linkage of CaMKII to synaptic physiology and behavior took many decades. New technologies were critical in this journey, including in vitro brain slices, mouse genetics, single-cell molecular genetics, pharmacological reagents, protein structure, and two-photon microscopy, as were new investigators attracted by the exciting challenge. This review tracks this journey and assesses the state of this marriage 40 years on. The collective literature impels us to propose a relatively simple model for synaptic memory involving the following steps that drive the process: 1) Ca2+ entry through N-methyl-d-aspartate (NMDA) receptors activates CaMKII. 2) CaMKII undergoes autophosphorylation resulting in constitutive, Ca2+-independent activity and exposure of a binding site for the NMDA receptor subunit GluN2B. 3) Active CaMKII translocates to the postsynaptic density (PSD) and binds to the cytoplasmic C-tail of GluN2B. 4) The CaMKII-GluN2B complex initiates a structural rearrangement of the PSD that may involve liquid-liquid phase separation. 5) This rearrangement involves the PSD-95 scaffolding protein, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), and their transmembrane AMPAR-regulatory protein (TARP) auxiliary subunits, resulting in an accumulation of AMPARs in the PSD that underlies synaptic potentiation. 6) The stability of the modified PSD is maintained by the stability of the CaMKII-GluN2B complex. 7) By a process of subunit exchange or interholoenzyme phosphorylation CaMKII maintains synaptic potentiation in the face of CaMKII protein turnover. There are many other important proteins that participate in enlargement of the synaptic spine or modulation of the steps that drive and maintain the potentiation. In this review we critically discuss the data underlying each of the steps. As will become clear, some of these steps are more firmly grounded than others, and we provide suggestions as to how the evidence supporting these steps can be strengthened or, based on the new data, be replaced. Although the journey has been a long one, the prospect of having a detailed cellular and molecular understanding of learning and memory is at hand.

LTP induction by structural rather than enzymatic functions of CaMKII

Learning and memory are thought to require hippocampal long-term potentiation (LTP), and one of the few central dogmas of molecular neuroscience that has stood undisputed for more than three decades is that LTP induction requires enzymatic activity of the Ca2+/calmodulin-dependent protein kinase II (CaMKII)1-3. However, as we delineate here, the experimental evidence is surprisingly far from conclusive. All previous interventions inhibiting enzymatic CaMKII activity and LTP4-8 also interfere with structural CaMKII roles, in particular binding to the NMDA-type glutamate receptor subunit GluN2B9-14. Thus, we here characterized and utilized complementary sets of new opto-/pharmaco-genetic tools to distinguish between enzymatic and structural CaMKII functions. Several independent lines of evidence demonstrated LTP induction by a structural function of CaMKII rather than by its enzymatic activity. The sole contribution of kinase activity was autoregulation of this structural role via T286 autophosphorylation, which explains why this distinction has been elusive for decades. Directly initiating the structural function in a manner that circumvented this T286 role was sufficient to elicit robust LTP, even when enzymatic CaMKII activity was blocked.

LOV2-based photoactivatable CaMKII and its application to single synapses: Local Optogenetics

Optogenetic techniques offer a high spatiotemporal resolution to manipulate cellular activity. For instance, Channelrhodopsin-2 with global light illumination is the most widely used to control neuronal activity at the cellular level. However, the cellular scale is much larger than the diffraction limit of light (<1 μm) and does not fully exploit the features of the "high spatial resolution" of optogenetics. For instance, until recently, there were no optogenetic methods to induce synaptic plasticity at the level of single synapses. To address this, we developed an optogenetic tool named photoactivatable CaMKII (paCaMKII) by fusing a light-sensitive domain (LOV2) to CaMKIIα, which is a protein abundantly expressed in neurons of the cerebrum and hippocampus and essential for synaptic plasticity. Combining photoactivatable CaMKII with two-photon excitation, we successfully activated it in single spines, inducing synaptic plasticity (long-term potentiation) in hippocampal neurons. We refer to this method as "Local Optogenetics", which involves the local activation of molecules and measurement of cellular responses. In this review, we will discuss the characteristics of LOV2, the recent development of its derivatives, and the development and application of paCaMKII.

Mechanical regulation of synapse formation and plasticity

Dendritic spines are small protrusions arising from dendrites and constitute the major compartment of excitatory post-synapses. They change in number, shape, and size throughout life; these changes are thought to be associated with formation and reorganization of neuronal networks underlying learning and memory. As spines in the brain are surrounded by the microenvironment including neighboring cells and the extracellular matrix, their protrusion requires generation of force to push against these structures. In turn, neighboring cells receive force from protruding spines. Recent studies have identified BAR-domain proteins as being involved in membrane deformation to initiate spine formation. In addition, forces for dendritic filopodium extension and activity-induced spine expansion are generated through cooperation between actin polymerization and clutch coupling. On the other hand, force from expanding spines affects neurotransmitter release from presynaptic terminals. Here, we review recent advances in our understanding of the physical aspects of synapse formation and plasticity, mainly focusing on spine dynamics.

The potential therapeutic roles of Rho GTPases in substance dependence

Rho GTPases family are considered to be molecular switches that regulate various cellular processes, including cytoskeleton remodeling, cell polarity, synaptic development and maintenance. Accumulating evidence shows that Rho GTPases are involved in neuronal development and brain diseases, including substance dependence. However, the functions of Rho GTPases in substance dependence are divergent and cerebral nuclei-dependent. Thereby, comprehensive integration of their roles and correlated mechanisms are urgently needed. In this review, the molecular functions and regulatory mechanisms of Rho GTPases and their regulators such as GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs) in substance dependence have been reviewed, and this is of great significance for understanding their spatiotemporal roles in addictions induced by different addictive substances and in different stages of substance dependence.

Mechanical transmission at spine synapses: Short-term potentiation and working memory

Do dendritic spines, which comprise the postsynaptic component of most excitatory synapses, exist only for their structural dynamics, receptor trafficking, and chemical and electrical compartmentation? The answer is no. Simultaneous investigation of both spine and presynaptic terminals has recently revealed a novel feature of spine synapses. Spine enlargement pushes the presynaptic terminals with muscle-like force and augments the evoked glutamate release for up to 20 min. We now summarize the evidence that such mechanical transmission shares critical features in common with short-term potentiation (STP) and may represent the cellular basis of short-term and working memory. Thus, spine synapses produce the force of learning to leave structural traces for both short and long-term memories.

Role of miR-219a-5p in regulating NMDAR in nonylphenol-induced synaptic plasticity damage

Nonylphenol (NP) is a typical environmental endocrine disruptor with estrogenic effects. It serves as an emulsifier and as the main ingredient of detergents and has become an increasingly common pollutant in both fresh and salt water, vegetables, and fruits. This study aimed to clarify whether NP exposure could lead to cognitive dysfunction and synaptic plasticity impairment, and also explore the mechanism of microRNA (miR)- 219a-5p regulation of N-methyl-D-aspartate receptor (NMDAR) in NP-induced synaptic plasticity impairment in vivo and in vitro. In vivo, 30 male Sprague-Dawley rats were randomly divided into 2 groups: blank control group (pure corn oil) and NP-exposed group [NP 80 mg/(kg·d)], with 15 rats in each group. In vitro, the extracted hippocampal neurons were divided into 6 groups: blank control group, mimics NC group, miR-219 mimics group, NP group (70 μmol/L NP), NP + mimics NC group, and NP + miR-219 mimics group. In vivo, the content of NP in hippocampal tissues after 90 days of NP exposure was significantly higher in the NP-stained group than in the blank control group. NP exposure could lead to a decrease in the ability to learn and memory, ability to remember, and space spatial memory ability in rats. The dendrites in the NP-stained group were disordered, with few dendritic spines and significantly decreased dendritic spine density. The postsynaptic densities were loosely arranged, the thickness and length of the postsynaptic densities shortened, and the length and width of the synaptic gap increased. Glutamine (Glu) and γ-aminobutyric acid (GABA) contents in hippocampal tissues decreased in the NP-stained group. The expression of miR-219a-5p mRNA decreased in the NP-stained group after 3 months of NP exposure. The expression of NMDAR1, NMDAR2A, NMDAR2B, nerve growth-associated protein (GAP-43), and Ca/calmodulin-dependent kinase II (CaMKII) mRNA/proteins decreased in the NP-stained group. In vitro, NMDAR protein expression decreased, while GAP-43 and CaMKII protein expression increased in the miR-219 mimics group compared with the control group. The expression levels of NMDAR and GAP-43 and CaMKII proteins were higher in the NP + miR-219 mimics group compared with the NP group. The levels of neurotransmitters Glu and GABA decreased in the NP and NP + mimics NC groups compared with the blank group. Shortened synaptic active band length, decreased thickness of postsynaptic densities, and shortened length of postsynaptic densities were observed in the NP, NP + mimics NC, and NP + miR-219 mimics groups compared with the blank control group. In vivo, NP exposure reduced learning memory capacity and neurotransmitter content in rats and caused a decrease in dendritic spine density and synaptic number density and a decrease in miR-219a-5p expression. In vitro, high expression of miR-219a-5p inhibited the expression of NMDAR, thus reducing the effect of NP on synaptic plasticity impairment in hippocampal neurons. Our study provided a scientific basis for the prevention of cognitive impairment owing to NP exposure and the development of targeted drug treatment strategies.

Subanesthetic dose of S-ketamine improved cognitive dysfunction via the inhibition of hippocampal astrocytosis in a mouse model of post-stroke chronic stress

Post-stroke chronic stress (PSCS) is generally associated with the poorer recovery and more pronounced cognitive dysfunction. Recent evidence has implied that S-ketamine can reduce suicidal ideation in treatment-resistant depression. In this current study, we aimed to investigate whether the administration of S-ketamine ameliorated cognitive deficits under PSCS conditions, which was established by a model combining middle cerebral artery occlusion (MCAO) and chronic restraint stress. Our data suggested that mice exposed to PSCS exhibited depression-like behavior and cognitive impairment, which coincided with astrocytosis as indicated by increased GFAP-positive cells and impairment of long-time potentiation (LTP) in the hippocampal CA1. Subanesthetic doses (10 mg/kg) of S-ketamine have significantly mitigated depression-like behaviors, cognitive deficits and LTP impairment, reduced astrocytosis, excessive GABA, and inflammatory factors, including NLRP3 and IL-18 in astrocytes in the CA1. Besides, neuroprotective effects induced by S-ketamine administration were found in vitro but could be partially reversed by an agonist of the NLRP3 nigericin. Our current data also suggests that the subanesthetic doses of S-ketamine improved cognitive dysfunction via the inhibition of hippocampal astrocytosis in a mouse model of PSCS.

Unraveling the mysteries of dendritic spine dynamics: Five key principles shaping memory and cognition

Recent research extends our understanding of brain processes beyond just action potentials and chemical transmissions within neural circuits, emphasizing the mechanical forces generated by excitatory synapses on dendritic spines to modulate presynaptic function. From in vivo and in vitro studies, we outline five central principles of synaptic mechanics in brain function: P1: Stability - Underpinning the integral relationship between the structure and function of the spine synapses. P2: Extrinsic dynamics - Highlighting synapse-selective structural plasticity which plays a crucial role in Hebbian associative learning, distinct from pathway-selective long-term potentiation (LTP) and depression (LTD). P3: Neuromodulation - Analyzing the role of G-protein-coupled receptors, particularly dopamine receptors, in time-sensitive modulation of associative learning frameworks such as Pavlovian classical conditioning and Thorndike's reinforcement learning (RL). P4: Instability - Addressing the intrinsic dynamics crucial to memory management during continual learning, spotlighting their role in "spine dysgenesis" associated with mental disorders. P5: Mechanics - Exploring how synaptic mechanics influence both sides of synapses to establish structural traces of short- and long-term memory, thereby aiding the integration of mental functions. We also delve into the historical background and foresee impending challenges.

In vivo three- and four-photon fluorescence microscopy using a 1.8 µm femtosecond fiber laser system

Multiphoton microscopy has enabled us to image cellular dynamics in vivo. However, the excitation wavelength for imaging with commercially available lasers is mostly limited between 0.65-1.04 µm. Here we develop a femtosecond fiber laser system that produces ∼150 fs pulses at 1.8 µm. Our system starts from an erbium-doped silica fiber laser, and its wavelength is converted to 1.8 µm using a Raman shift fiber. The 1.8 µm pulses are amplified with a two-stage Tm:ZBLAN fiber amplifier. The final pulse energy is ∼1 µJ, sufficient for in vivo imaging. We successfully observe TurboFP635-expressing cortical neurons at a depth of 0.7 mm from the brain surface by three-photon excitation and Clover-expressing astrocytes at a depth of 0.15 mm by four-photon excitation.

A Review: Research Progress of Neural Probes for Brain Research and Brain-Computer Interface

Neural probes, as an invasive physiological tool at the mesoscopic scale, can decipher the code of brain connections and communications from the cellular or even molecular level, and realize information fusion between the human body and external machines. In addition to traditional electrodes, two new types of neural probes have been developed in recent years: optoprobes based on optogenetics and magnetrodes that record neural magnetic signals. In this review, we give a comprehensive overview of these three kinds of neural probes. We firstly discuss the development of microelectrodes and strategies for their flexibility, which is mainly represented by the selection of flexible substrates and new electrode materials. Subsequently, the concept of optogenetics is introduced, followed by the review of several novel structures of optoprobes, which are divided into multifunctional optoprobes integrated with microfluidic channels, artifact-free optoprobes, three-dimensional drivable optoprobes, and flexible optoprobes. At last, we introduce the fundamental perspectives of magnetoresistive (MR) sensors and then review the research progress of magnetrodes based on it.

Early posttraumatic CSF1R inhibition via PLX3397 leads to time- and sex-dependent effects on inflammation and neuronal maintenance after traumatic brain injury in mice

BACKGROUND

There is a need for early therapeutic interventions after traumatic brain injury (TBI) to prevent neurodegeneration. Microglia/macrophage (M/M) depletion and repopulation after treatment with colony stimulating factor 1 receptor (CSF1R) inhibitors reduces neurodegeneration. The present study investigates short- and long-term consequences after CSF1R inhibition during the early phase after TBI.

METHODS

Sex-matched mice were subjected to TBI and CSF1R inhibition by PLX3397 for 5 days and sacrificed at 5 or 30 days post injury (dpi). Neurological deficits were monitored and brain tissues were examined for histo- and molecular pathological markers. RNAseq was performed with 30 dpi TBI samples.

RESULTS

At 5 dpi, CSF1R inhibition attenuated the TBI-induced perilesional M/M increase and associated gene expressions by up to 50%. M/M attenuation did not affect structural brain damage at this time-point, impaired hematoma clearance, and had no effect on IL-1β expression. At 30 dpi, following drug discontinuation at 5 dpi and M/M repopulation, CSF1R inhibition attenuated brain tissue loss regardless of sex, as well as hippocampal atrophy and thalamic neuronal loss in male mice. Selected gene markers of brain inflammation and apoptosis were reduced in males but increased in females after early CSF1R inhibition as compared to corresponding TBI vehicle groups. Neurological outcome in behaving mice was almost not affected. RNAseq and gene set enrichment analysis (GSEA) of injured brains at 30 dpi revealed more genes associated with dendritic spines and synapse function after early CSF1R inhibition as compared to vehicle, suggesting improved neuronal maintenance and recovery. In TBI vehicle mice, GSEA showed high oxidative phosphorylation, oxidoreductase activity and ribosomal biogenesis suggesting oxidative stress and increased abundance of metabolically highly active cells. More genes associated with immune processes and phagocytosis in PLX3397 treated females vs males, suggesting sex-specific differences in response to early CSF1R inhibition after TBI.

CONCLUSIONS

M/M attenuation after CSF1R inhibition via PLX3397 during the early phase of TBI reduces long-term brain tissue loss, improves neuronal maintenance and fosters synapse recovery. Overall effects were not sex-specific but there is evidence that male mice benefit more than female mice.

E2F4DN Transgenic Mice: A Tool for the Evaluation of E2F4 as a Therapeutic Target in Neuropathology and Brain Aging

E2F4 was initially described as a transcription factor with a key function in the regulation of cell quiescence. Nevertheless, a number of recent studies have established that E2F4 can also play a relevant role in cell and tissue homeostasis, as well as tissue regeneration. For these non-canonical functions, E2F4 can also act in the cytoplasm, where it is able to interact with many homeostatic and synaptic regulators. Since E2F4 is expressed in the nervous system, it may fulfill a crucial role in brain function and homeostasis, being a promising multifactorial target for neurodegenerative diseases and brain aging. The regulation of E2F4 is complex, as it can be chemically modified through acetylation, from which we present evidence in the brain, as well as methylation, and phosphorylation. The phosphorylation of E2F4 within a conserved threonine motif induces cell cycle re-entry in neurons, while a dominant negative form of E2F4 (E2F4DN), in which the conserved threonines have been substituted by alanines, has been shown to act as a multifactorial therapeutic agent for Alzheimer’s disease (AD). We generated transgenic mice neuronally expressing E2F4DN. We have recently shown using this mouse strain that expression of E2F4DN in 5xFAD mice, a known murine model of AD, improved cognitive function, reduced neuronal tetraploidization, and induced a transcriptional program consistent with modulation of amyloid-β (Aβ) peptide proteostasis and brain homeostasis recovery. 5xFAD/E2F4DN mice also showed reduced microgliosis and astrogliosis in both the cerebral cortex and hippocampus at 3-6 months of age. Here, we analyzed the immune response in 1 year-old 5xFAD/E2F4DN mice, concluding that reduced microgliosis and astrogliosis is maintained at this late stage. In addition, the expression of E2F4DN also reduced age-associated microgliosis in wild-type mice, thus stressing its role as a brain homeostatic agent. We conclude that E2F4DN transgenic mice represent a promising tool for the evaluation of E2F4 as a therapeutic target in neuropathology and brain aging.

Spinal Sirtuin 3 Contributes to Electroacupuncture Analgesia in Mice with Chronic Constriction Injury–Induced Neuropathic Pain

BACKGROUND

Electroacupuncture (EA) is a traditional Chinese therapeutic technique that has a beneficial effect on neuropathic pain; however, the specific mechanism remains unclear. In this study, we investigated whether EA inhibits spinal Ca/calmodulin-dependent protein kinase II (CaMKIIα) phosphorylation through Sirtuin 3 (SIRT3) protein, thus relieving neuropathic pain.

MATERIALS AND METHODS

We used wild-type and SIRT3 knockout (SIRT3^-/-) mice and used chronic constriction injury (CCI) as a pain model. We performed Western blotting, immunostaining, von Frey, and Hargreaves tests to gather biochemical and behavioral data. Downregulation and overexpression and spinal SIRT3 protein were achieved by intraspinal injection of SIRT3 small interfering RNA and intraspinal injection of lentivirus-SIRT3. To test the hypothesis that CaMKIIα signaling was involved in the analgesic effects of EA, we expressed CaMKIIα-specific designer receptors exclusively activated by designer drugs (DREADDs) in the spinal dorsal horn (SDH) of mice.

RESULTS

These results showed that the mechanical and thermal hyperalgesia induced by CCI was related to the decreased spinal SIRT3 expression in the SDH of mice. A significant reduction of mechanical and thermal thresholds was found in the SIRT3^-/- mice. SIRT3 overexpression or EA treatment alleviated CCI-induced neuropathic pain and prevented the spinal CaMKIIα phosphorylation. Most importantly, EA increased the expression of spinal SIRT3 protein in the SDH. Downregulation of spinal SIRT3 or CaMKIIα Gq-DREADD activation inhibited the regulatory effect of EA on neuropathic pain.

CONCLUSION

Our results showed that CaMKIIα phosphorylation was inhibited by spinal SIRT3 in neuropathic pain and that EA attenuated CCI-induced neuropathic pain mainly by upregulating spinal SIRT3 expression in the SDH of mice.

A biopolymer-gated ionotronic junctionless oxide transistor array for spatiotemporal pain-perception emulation in nociceptor network

Capable of reflecting the location and intensity of external harmful stimuli, a nociceptor network is of great importance for receiving pain-perception information. However, the hardware-based implementation of a nociceptor network through the use of a transistor array remains a great challenge in the area of brain-inspired neuromorphic applications. Herein, a simple ionotronic junctionless oxide transistor array with pain-perception abilities is successfully realized due to a coplanar-gate proton-coupling effect in sodium alginate biopolymer electrolyte. Several important pain-perception characteristics of nociceptors are emulated, such as a pain threshold, the memory of prior injury, and sensitization behavior due to pathway alterations. In particular, a good graded pain-perception network system has been successfully established through coplanar capacitance and resistance. More importantly, clear polarity reversal of Lorentz-type spatiotemporal pain-perception emulation can be finally realized in our projection-dependent nociceptor network. This work may provide new avenues for bionic medical machines and humanoid robots based on these intriguing pain-perception abilities.

Chronic neuronal excitation leads to dual metaplasticity in the signaling for structural long-term potentiation

Synaptic plasticity is long-lasting changes in synaptic currents and structure. When neurons are exposed to signals that induce aberrant neuronal excitation, they increase the threshold for the induction of long-term potentiation (LTP), known as metaplasticity. However, the metaplastic regulation of structural LTP (sLTP) remains unclear. We investigate glutamate uncaging/photoactivatable (pa)CaMKII-dependent sLTP induction in hippocampal CA1 neurons after chronic neuronal excitation by GABA_A receptor antagonists. We find that the neuronal excitation decreases the glutamate uncaging-evoked Ca²⁺ influx mediated by GluN2B-containing NMDA receptors and suppresses sLTP induction. In addition, single-spine optogenetic stimulation using paCaMKII indicates the suppression of CaMKII signaling. While the inhibition of Ca²⁺ influx is protein synthesis independent, the paCaMKII-induced sLTP suppression depends on it. Our findings demonstrate that chronic neuronal excitation suppresses sLTP in two independent ways (i.e., dual inhibition of Ca²⁺ influx and CaMKII signaling). This dual inhibition mechanism may contribute to robust neuronal protection in excitable environments.

Mechanical actions of dendritic-spine enlargement on presynaptic exocytosis

Synaptic transmission involves cell-to-cell communication at the synaptic junction between two neurons, and chemical and electrical forms of this process have been extensively studied. In the brain, excitatory glutamatergic synapses are often made on dendritic spines that enlarge during learning^1-5. As dendritic spines and the presynaptic terminals are tightly connected with the synaptic cleft⁶, the enlargement may have mechanical effects on presynaptic functions⁷. Here we show that fine and transient pushing of the presynaptic boutons with a glass pipette markedly promotes both the evoked release of glutamate and the assembly of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins^8-12-as measured by Förster resonance transfer (FRET) and fluorescence lifetime imaging-in rat slice culture preparations¹³. Both of these effects persisted for more than 20 minutes. The increased presynaptic FRET was independent of cytosolic calcium (Ca²⁺), but dependent on the assembly of SNARE proteins and actin polymerization in the boutons. Notably, a low hypertonic solution of sucrose (20 mM) had facilitatory effects on both the FRET and the evoked release without inducing spontaneous release, in striking contrast with a high hypertonic sucrose solution (300 mM), which induced exocytosis by itself¹⁴. Finally, spine enlargement induced by two-photon glutamate uncaging enhanced the evoked release and the FRET only when the spines pushed the boutons by their elongation. Thus, we have identified a mechanosensory and transduction mechanism¹⁵ in the presynaptic boutons, in which the evoked release of glutamate is enhanced for more than 20 min.

Imaging intracellular protein interactions/activity in neurons using 2-photon fluorescence lifetime imaging microscopy

Through the decades, 2-photon fluorescence microscopy has allowed visualization of microstructures, such as synapses, with high spatial resolution in deep brain tissue. However, signal transduction, such as protein activity and protein-protein interaction in neurons in tissues and in vivo, has remained elusive because of the technical difficulty of observing biochemical reactions at the level of subcellular resolution in light-scattering tissues. Recently, 2-photon fluorescence microscopy combined with fluorescence lifetime imaging microscopy (2pFLIM) has enabled visualization of various protein activities and protein-protein interactions at submicrometer resolution in tissue with a reasonable temporal resolution. Thus far, 2pFLIM has been extensively applied for imaging kinase and small GTPase activation in dendritic spines of hippocampal neurons in slice cultures. However, it has been recently applied to various subcellular structures, such as axon terminals and nuclei, and has increased our understanding of spatially organized molecular dynamics. One of the future directions of 2pFLIM utilization is to combine various optogenetic tools for manipulating protein activity. This combination allows the activation of specific proteins with light and visualization of its readout as the activation of downstream molecules. Here, we have introduced the recent application of 2pFLIM for neurons and present the utilization of a new optogenetic tool in combination with 2pFLIM.

AMPA receptor trafficking and LTP: Carboxy-termini, amino-termini and TARPs

AMPA receptors (AMPARs) are fundamental elements in excitatory synaptic transmission and synaptic plasticity in the CNS. Long term potentiation (LTP), a form of synaptic plasticity which contributes to learning and memory formation, relies on the accumulation of AMPARs at the postsynapse. This phenomenon requires the coordinated recruitment of different elements in the AMPAR complex. Based on recent research reviewed herein, we propose an updated AMPAR trafficking and LTP model which incorporates both extracellular as well as intracellular mechanisms. This article is part of the special Issue on 'Glutamate Receptors - AMPA receptors'.

Temporal pattern and synergy influence activity of ERK signaling pathways during L-LTP induction

Long-lasting long-term potentiation (L-LTP) is a cellular mechanism of learning and memory storage. Studies have demonstrated a requirement for extracellular signal-regulated kinase (ERK) activation in L-LTP produced by a diversity of temporal stimulation patterns. Multiple signaling pathways converge to activate ERK, with different pathways being required for different stimulation patterns. To answer whether and how different temporal patterns select different signaling pathways for ERK activation, we developed a computational model of five signaling pathways (including two novel pathways) leading to ERK activation during L-LTP induction. We show that calcium and cAMP work synergistically to activate ERK and that stimuli given with large intertrial intervals activate more ERK than shorter intervals. Furthermore, these pathways contribute to different dynamics of ERK activation. These results suggest that signaling pathways with different temporal sensitivities facilitate ERK activation to diversity of temporal patterns.

The Next Frontier for Designing Switchable Proteins: Rational Enhancement of Kinetics

Designing proteins that can switch between active (ON) and inactive (OFF) conformations in response to signals such as ligand binding and incident light has been a tantalizing endeavor in protein engineering for over a decade. While such designs have yielded novel biosensors, therapeutic agents, and smart biomaterials, the response times (times for switching ON and OFF) of many switches have been too slow to be of practical use. Among the defining properties of such switches, the kinetics of switching has been the most challenging to optimize. This is largely due to the difficulty of characterizing the structures of transient states, which are required for manipulating the height of the effective free energy barrier between the ON and OFF states. We share our perspective of the most promising new experimental and computational strategies over the past several years for tackling this next frontier for designing switchable proteins.

Spatiotemporal Regulation of Transcript Isoform Expression in the Hippocampus

Proper development and plasticity of hippocampal neurons require specific RNA isoforms to be expressed in the right place at the right time. Precise spatiotemporal transcript regulation requires the incorporation of essential regulatory RNA sequences into expressed isoforms. In this review, we describe several RNA processing strategies utilized by hippocampal neurons to regulate the spatiotemporal expression of genes critical to development and plasticity. The works described here demonstrate how the hippocampus is an ideal investigative model for uncovering alternate isoform-specific mechanisms that restrict the expression of transcripts in space and time.

The role of CaMKII autophosphorylation for NMDA receptor-dependent synaptic potentiation

Potentiation of glutamatergic synaptic transmission is thought to underlie memory. The induction of this synaptic potentiation relies on activation of NMDA receptors which allows for calcium influx into the post-synapse. A key mechanistic question for the understanding of synaptic potentiation is what signaling is activated by the calcium influx. Here, I review evidences that at mature synapses the elevated calcium levels activate primarily calcium/calmodulin-dependent kinase II (CaMKII) and cause its autophophorylation. CaMKII autophosphorylation leads to calcium-independent activity of the kinase, so that kinase signaling can outlast NMDA receptor-dependent calcium influx. Prolonged CaMKII signaling induces downstream signaling for AMPA receptor trafficking into the post-synaptic density and causes structural enlargement of the synapse. Interestingly, however, CaMKII autophosphorylation does not have such an essential role in NMDA receptor-dependent synaptic potentiation in early postnatal development and in adult dentate gyrus, where neurogenesis occurs. Additionally, in old age memory-relevant NMDA receptor-dependent synaptic plasticity appears to be due to generation of multi-innervated dendritic spines, which does not require CaMKII autophosphorylation. In conclusion, CaMKII autophosphorylation has a conditional role in the induction of NMDA receptor-dependent synaptic potentiation.

Finding functions of phase separation in the presynapse

Synapses are the basic units of neuronal communication. Understanding how synapses assemble and function is therefore essential to understanding nervous systems. Decades of study have identified many molecular components and functional mechanisms of synapses. Recently, an additional level of synaptic protein organization has been identified: phase separation. In the presynapse, components of the central active zone and a synaptic vesicle-clustering factor have been shown to form liquid-liquid phase-separated condensates or hydrogels. New in vivo functional studies have directly tested how phase separation impacts both synapse formation and function. Here, we review this emerging evidence for in vivo functional roles of phase separation at the presynapse and discuss future functional studies necessary to understand its complexity.

Photoactivatable CaMKII: Rewiring the Brain, One Synapse at a Time

A recent article by Shibata et al. introduces the engineered photoactivatable enzyme paCaMKII. Activation of this new tool is sufficient to induce long-term potentiation (LTP) of hippocampal synapses in slice culture and in intact animals, thereby expanding the existing toolkit for light-induced modification of brain connectivity at the synaptic level.