Mechanosensitivity of NMDA receptors in cultured mouse central neurons

Study on Improving the Modulatory Effect of Rhythmic Oscillations by Transcranial Magneto-Acoustic Stimulation

In hippocampus, synaptic plasticity and rhythmic oscillations reflect the cytological basis and the intermediate level of cognition, respectively. Transcranial ultrasound stimulation (TUS) has demonstrated the ability to elicit changes in neural response. However, the modulatory effect of TUS on synaptic plasticity and rhythmic oscillations was insufficient in the present studies, which may be attributed to the fact that TUS acts mainly through mechanical forces. To enhance the modulatory effect on synaptic plasticity and rhythmic oscillations, transcranial magneto-acoustic stimulation (TMAS) which induced a coupled electric field together with TUS's ultrasound field was applied. The modulatory effect of TMAS and TUS with a pulse repetition frequency of 100 Hz were compared. TMAS/TUS were performed on C57 mice for 7 days at two different ultrasound intensities (3 W/cm2 and 5 W/cm [Formula: see text]. Behavioral tests, long-term potential (LTP) and local field potentials in vivo were performed to evaluate TUS/TMAS modulatory effect on cognition, synaptic plasticity and rhythmic oscillations. Protein expression based on western blotting were used to investigate the under- lying mechanisms of these beneficial effects. At 5 W/cm2, TMAS-induced LTP were 113.4% compared to the sham group and 110.5% compared to TUS. Moreover, the relative power of high gamma oscillations (50-100Hz) in the TMAS group ( 1.060±0.155 %) was markedly higher than that in the TUS group ( 0.560±0.114 %) and sham group ( 0.570±0.088 %). TMAS significantly enhanced the synchronization of theta and gamma oscillations as well as theta-gamma cross-frequency coupling. Whereas, TUS did not show relative enhancements. TMAS provides enhanced effect for modulating the synaptic plasticity and rhythmic oscillations in hippocampus.

Single-neuron mechanical perturbation evokes calcium plateaus that excite and modulate the network

Mechanical stimulation is a promising means to non-invasively excite and modulate neuronal networks with a high spatial resolution. Despite the thorough characterization of the initiation mechanism, whether or how mechanical responses disperse into non-target areas remains to be discovered. Our in vitro study demonstrates that a single-neuron deformation evokes responses that propagate to about a third of the untouched neighbors. The responses develop via calcium influx through mechanosensitive channels and regeneratively propagate through the neuronal ensemble via gap junctions. Although independent of action potentials and synapses, mechanical responses reliably evoke membrane depolarizations capable of inducing action potentials both in the target and neighbors. Finally, we show that mechanical stimulation transiently potentiates the responding assembly for further inputs, as both gain and excitability are transiently increased exclusively in neurons that respond to a neighbor's mechanical stimulation. The findings indicate a biological component affecting the spatial resolution of mechanostimulation and point to a cross-talk in broad-network mechanical stimulations. Since giga-seal formation in patch-clamp produces a similar mechanical stimulus on the neuron, our findings inform which neuroscientific questions could be reliably tackled with patch-clamp and what recovery post-gigaseal formation is necessary.

Machine learning analysis predicts a person's sex based on mechanical but not thermal pain thresholds

Sex differences in pain perception have been extensively studied, but precision medicine applications such as sex-specific pain pharmacology have barely progressed beyond proof-of-concept. A data set of pain thresholds to mechanical (blunt and punctate pressure) and thermal (heat and cold) stimuli applied to non-sensitized and sensitized (capsaicin, menthol) forearm skin of 69 male and 56 female healthy volunteers was analyzed for data structures contingent with the prior sex structure using unsupervised and supervised approaches. A working hypothesis that the relevance of sex differences could be approached via reversibility of the association, i.e., sex should be identifiable from pain thresholds, was verified with trained machine learning algorithms that could infer a person's sex in a 20% validation sample not seen to the algorithms during training, with balanced accuracy of up to 79%. This was only possible with thresholds for mechanical stimuli, but not for thermal stimuli or sensitization responses, which were not sufficient to train an algorithm that could assign sex better than by guessing or when trained with nonsense (permuted) information. This enabled the translation to the molecular level of nociceptive targets that convert mechanical but not thermal information into signals interpreted as pain, which could eventually be used for pharmacological precision medicine approaches to pain. By exploiting a key feature of machine learning, which allows for the recognition of data structures and the reduction of information to the minimum relevant, experimental human pain data could be characterized in a way that incorporates "non" logic that could be translated directly to the molecular pharmacological level, pointing toward sex-specific precision medicine for pain.

Experimental and theoretical probe on mechano- and chemosensory integration in the insect antennal lobe

In nature, olfactory signals are delivered to detectors—for example, insect antennae—by means of turbulent air, which exerts concurrent chemical and mechanical stimulation on the detectors. The antennal lobe, which is traditionally viewed as a chemosensory module, sits downstream of antennal inputs. We review experimental evidence showing that, in addition to being a chemosensory structure, antennal lobe neurons also respond to mechanosensory input in the form of wind speed. Benchmarked with empirical data, we constructed a dynamical model to simulate bimodal integration in the antennal lobe, with model dynamics yielding insights such as a positive correlation between the strength of mechanical input and the capacity to follow high frequency odor pulses, an important task in tracking odor sources. Furthermore, we combine experimental and theoretical results to develop a conceptual framework for viewing the functional significance of sensory integration within the antennal lobe. We formulate the testable hypothesis that the antennal lobe alternates between two distinct dynamical regimes, one which benefits odor plume tracking and one which promotes odor discrimination. We postulate that the strength of mechanical input, which correlates with behavioral contexts such being mid-flight versus hovering near a flower, triggers the transition from one regime to the other.

PIEZO channels and newcomers in the mammalian mechanosensitive ion channel family

Many ion channels have been described as mechanosensitive according to various criteria. Most broadly defined, an ion channel is called mechanosensitive if its activity is controlled by application of a physical force. The last decade has witnessed a revolution in mechanosensory physiology at the molecular, cellular, and system levels, both in health and in diseases. Since the discovery of the PIEZO proteins as prototypical mechanosensitive channel, many proteins have been proposed to transduce mechanosensory information in mammals. However, few of these newly identified candidates have all the attributes of bona fide, pore-forming mechanosensitive ion channels. In this perspective, we will cover and discuss new data that have advanced our understanding of mechanosensation at the molecular level.

Deletion of Grin1 in mouse megakaryocytes reveals NMDA receptor role in platelet function and proplatelet formation

The process of proplatelet formation (PPF) requires coordinated interaction between megakaryocytes (MKs) and the extracellular matrix (ECM), followed by a dynamic reorganization of the actin and microtubule cytoskeleton. Localized fluxes of intracellular calcium ions (Ca2+) facilitate MK-ECM interaction and PPF. Glutamate-gated N-methyl-D--aspartate receptor (NMDAR) is highly permeable to Ca2+. NMDAR antagonists inhibit MK maturation ex vivo, however there is no in vivo data. Using the Cre-loxP system, we generated a platelet lineage-specific knockout mouse model of reduced NMDAR function in MKs and platelets (Pf4-Grin1-/- mice). Effects of NMDAR deletion were examined using well-established assays of platelet function and production in vivo and ex vivo. We found that Pf4-Grin1-/- mice had defects in megakaryopoiesis, thrombopoiesis and platelet function, which manifested as reduced platelet counts, lower rates of platelet production in the immune model of thrombocytopenia, and a prolonged tail bleeding time. Platelet activation was impaired to a range of agonists associated with reduced Ca2+ responses, including metabotropic-like, and defective platelet spreading. MKs showed reduced colony and proplatelet formation. Impaired reorganization of intracellular F-actin and α-tubulin was identified as the main cause of reduced platelet function and production. Pf4-Grin1-/- MKs also had lower levels of transcripts encoding crucial ECM elements and enzymes, suggesting NMDAR signaling is involved in ECM remodeling. In summary, we provide the first genetic evidence that NMDAR plays an active role in platelet function and production. NMDARs regulate PPF through the mechanism that involves MK-ECM interaction and cytoskeletal reorganization. Our results suggest that NMDAR helps guide PPF in vivo.

A review on magnetic and spintronic neurostimulation: challenges and prospects

In the treatment of neurodegenerative, sensory and cardiovascular diseases, electrical probes and arrays have shown quite a promising success rate. However, despite the outstanding clinical outcomes, their operation is significantly hindered by non-selective control of electric fields. A promising alternative is micromagnetic stimulation (μMS) due to the high permeability of magnetic field through biological tissues. The induced electric field from the time-varying magnetic field generated by magnetic neurostimulators is used to remotely stimulate neighboring neurons. Due to the spatial asymmetry of the induced electric field, high spatial selectivity of neurostimulation has been realized. Herein, some popular choices of magnetic neurostimulators such as microcoils (μcoils) and spintronic nanodevices are reviewed. The neurostimulator features such as power consumption and resolution (aiming at cellular level) are discussed. In addition, the chronic stability and biocompatibility of these implantable neurostimulator are commented in favor of further translation to clinical settings. Furthermore, magnetic nanoparticles (MNPs), as another invaluable neurostimulation material, has emerged in recent years. Thus, in this review we have also included MNPs as a remote neurostimulation solution that overcomes physical limitations of invasive implants. Overall, this review provides peers with the recent development of ultra-low power, cellular-level, spatially selective magnetic neurostimulators of dimensions within micro- to nano-range for treating chronic neurological disorders. At the end of this review, some potential applications of next generation neuro-devices have also been discussed.

Short-Term Efficacy of Transcranial Focused Ultrasound to the Hippocampus in Alzheimer’s Disease: A Preliminary Study

Preclinical studies have suggested that low-intensity transcranial focused ultrasound (tFUS) may have therapeutic potential for Alzheimer’s disease (AD) by opening the blood–brain barrier (BBB), reducing amyloid pathology, and improving cognition. This study investigated the effects of tFUS on BBB opening, regional cerebral metabolic rate of glucose (rCMRglu), and cognitive function in AD patients. Eight patients with AD received image-guided tFUS to the right hippocampus immediately after intravenous injection of microbubble ultrasound contrast agents. Patients completed magnetic resonance imaging (MRI), ¹⁸F-fluoro-2-deoxyglucose positron emission tomography (PET), and cognitive assessments before and after the sonication. No evidence of transient BBB opening was found on T1 dynamic contrast-enhanced MRI. However, immediate recall (p = 0.03) and recognition memory (p = 0.02) were significantly improved on the verbal learning test. PET image analysis demonstrated increased rCMRglu in the right hippocampus (p = 0.001). In addition, increases of hippocampal rCMRglu were correlated with improvement in recognition memory (Spearman’s ρ = 0.77, p = 0.02). No adverse event was observed. Our results suggest that tFUS to the hippocampus of AD patients may improve rCMRglu of the target area and memory in the short term, even without BBB opening. Further larger sham-controlled trials with loger follow-up are warranted to evaluate the efficacy and safety of tFUS in patients with AD.

Neuromechanobiology: An Expanding Field Driven by the Force of Greater Focus

The brain processes information by transmitting signals through highly connected and dynamic networks of neurons. Neurons use specific cellular structures, including axons, dendrites and synapses, and specific molecules, including cell adhesion molecules, ion channels and chemical receptors to form, maintain and communicate among cells in the networks. These cellular and molecular processes take place in environments rich of mechanical cues, thus offering ample opportunities for mechanical regulation of neural development and function. Recent studies have suggested the importance of mechanical cues and their potential regulatory roles in the development and maintenance of these neuronal structures. Also suggested are the importance of mechanical cues and their potential regulatory roles in the interaction and function of molecules mediating the interneuronal communications. In this review, the current understanding is integrated and promising future directions of neuromechanobiology are suggested at the cellular and molecular levels. Several neuronal processes where mechanics likely plays a role are examined and how forces affect ligand binding, conformational change, and signal induction of molecules key to these neuronal processes are indicated, especially at the synapse. The disease relevance of neuromechanobiology as well as therapies and engineering solutions to neurological disorders stemmed from this emergent field of study are also discussed.

Regulation of the NMDA receptor by its cytoplasmic domains: (How) is the tail wagging the dog?

Excitatory neurotransmission mediated by N-methyl-d-aspartate receptors (NMDARs) is critical for synapse development, function, and plasticity in the brain. NMDARs are tetra-heteromeric cation-channels that mediate synaptic transmission and plasticity. Extensive human studies show the existence of genetic variants in NMDAR subunits genes (GRIN genes) that are associated with neurodevelopmental and neuropsychiatric disorders, including autism spectrum disorders (ASD), epilepsy (EP), intellectual disability (ID), attention deficit hyperactivity disorder (ADHD), and schizophrenia (SCZ). NMDAR subunits have a unique modular architecture with four semiautonomous domains. Here we focus on the carboxyl terminal domain (CTD), also known as the intracellular C-tail, which varies in length among the glutamate receptor subunits and is the most diverse domain in terms of amino acid sequence. The CTD shows no sequence homology to any known proteins but encodes short docking motifs for intracellular binding proteins and covalent modifications. Our review will discuss the many important functions of the CTD in regulating NMDA membrane and synaptic targeting, stabilization, degradation targeting, allosteric modulation and metabotropic signaling of the receptor. This article is part of the special issue on 'Glutamate Receptors - NMDA Receptors'.

Aβ Oligomers Dysregulate Calcium Homeostasis by Mechanosensitive Activation of AMPA and NMDA Receptors

Alzheimer's disease, which is the most common form of dementia, is characterized by the aggregation of the amyloid β peptide (Aβ) and by an impairment of calcium homeostasis caused by excessive activation of glutamatergic receptors (excitotoxicity). Here, we studied the effects on calcium homeostasis caused by the formation of Aβ oligomeric assemblies. We found that Aβ oligomers cause a rapid influx of calcium ions (Ca2+) across the cell membrane by rapidly activating extrasynaptic N-methyl-d-aspartate (NMDA) receptors and, to a lower extent, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. We also observed, however, that misfolded oligomers do not interact directly with these receptors. Further experiments with lysophosphatidylcholine and arachidonic acid, which cause membrane compression and stretch, respectively, indicated that these receptors are activated through a change in membrane tension induced by the oligomers and transmitted mechanically to the receptors via the lipid bilayer. Indeed, lysophosphatidylcholine is able to neutralize the oligomer-induced activation of the NMDA receptors, whereas arachidonic acid activates the receptors similarly to the oligomers with no additive effects. An increased rotational freedom observed for a fluorescent probe embedded within the membrane in the presence of the oligomers also indicates a membrane stretch. These results reveal a mechanism of toxicity of Aβ oligomers in Alzheimer's disease through the perturbation of the mechanical properties of lipid membranes sensed by NMDA and AMPA receptors.

Intrinsic plasticity of cerebellar stellate cells is mediated by NMDA receptor regulation of voltage-gated Na+ channels

KEY POINTS

We show that NMDA receptors (NMDARs) elicit a long-term increase in the firing rates of inhibitory stellate cells of the cerebellum NMDARs induce intrinsic plasticity through a Ca²⁺ - and CaMKII-dependent pathway that drives shifts in the activation and inactivation properties of voltage-gated Na⁺ (Na_v ) channels An identical Ca²⁺ - and CaMKII-dependent signalling pathway is triggered during whole-cell recording which lowers the action potential threshold by causing a hyperpolarizing shift in the gating properties of Na_v channels. Our findings open the more general possibility that NMDAR-mediated intrinsic plasticity found in other cerebellar neurons may involve similar shifts in Na_v channel gating.

ABSTRACT

Memory storage in the mammalian brain is mediated not only by long-lasting changes in the efficacy of neurotransmitter receptors but also by long-term modifications to the activity of voltage-gated ion channels. Activity-dependent plasticity of voltage-gated ion channels, or intrinsic plasticity, is found throughout the brain in virtually all neuronal types, including principal cells and interneurons. Although intrinsic plasticity has been identified in neurons of the cerebellum, it has yet to be studied in inhibitory cerebellar stellate cells of the molecular layer which regulate activity outflow from the cerebellar cortex by feedforward inhibition onto Purkinje cells. The study of intrinsic plasticity in stellate cells has been particularly challenging as membrane patch breakthrough in electrophysiology experiments unintentionally triggers changes in spontaneous firing rates. Using cell-attached patch recordings to avoid disruption, we show that activation of extrasynaptic N-methyl-d-aspartate receptors (NMDARs) elicits a long-term increase in the firing properties of stellate cells by stimulating a rise in cytosolic Ca²⁺ and activation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). An identical signalling pathway is triggered during whole-cell recording which lowers the action potential threshold by causing a hyperpolarizing shift in the gating properties of voltage-gated sodium (Na_v ) channels. Together, our findings identify an unappreciated role of Na_v channel-dependent intrinsic plasticity in cerebellar stellate cells which, in concert with non-canonical NMDAR signalling, provides the cerebellum with an unconventional mechanism to fine-tune motor behaviour.

Magnetic Vortex Nanodiscs Enable Remote Magnetomechanical Neural Stimulation

Magnetic nanomaterials in magnetic fields can serve as versatile transducers for remote interrogation of cell functions. In this study, we leveraged the transition from vortex to in-plane magnetization in iron oxide nanodiscs to modulate the activity of mechanosensory cells. When a vortex configuration of spins is present in magnetic nanomaterials, it enables rapid control over their magnetization direction and magnitude. The vortex configuration manifests in near zero net magnetic moment in the absence of a magnetic field, affording greater colloidal stability of magnetic nanomaterials in suspensions. Together, these properties invite the application of magnetic vortex particles as transducers of externally applied minimally invasive magnetic stimuli in biological systems. Using magnetic modeling and electron holography, we predict and experimentally demonstrate magnetic vortex states in an array of colloidally synthesized magnetite nanodiscs 98-226 nm in diameter. The magnetic nanodiscs applied as transducers of torque for remote control of mechanosensory neurons demonstrated the ability to trigger Ca²⁺ influx in weak (≤28 mT), slowly varying (≤5 Hz) magnetic fields. The extent of cellular response was determined by the magnetic nanodisc volume and magnetic field conditions. Magnetomechanical activation of a mechanosensitive cation channel TRPV4 (transient receptor potential vanilloid family member 4) exogenously expressed in the nonmechanosensitive HEK293 cells corroborated that the stimulation is mediated by mechanosensitive ion channels. With their large magnetic torques and colloidal stability, magnetic vortex particles may facilitate basic studies of mechanoreception and its applications to control electroactive cells with remote magnetic stimuli.

The Redox Biology of Excitotoxic Processes: The NMDA Receptor, TOPA Quinone, and the Oxidative Liberation of Intracellular Zinc

This special issue of Frontiers in Neuroscience-Neurodegeneration celebrates the 50th anniversary of John Olney's seminal work introducing the concept of excitotoxicity as a mechanism for neuronal cell death. Since that time, fundamental research on the pathophysiological activation of glutamate receptors has played a central role in our understanding of excitotoxic cellular signaling pathways, leading to the discovery of many potential therapeutic targets in the treatment of acute or chronic/progressive neurodegenerative disorders. Importantly, excitotoxic signaling processes have been found repeatedly to be closely intertwined with oxidative cellular cascades. With this in mind, this review looks back at long-standing collaborative efforts by the authors linking cellular redox status and glutamate neurotoxicity, focusing first on the discovery of the redox modulatory site of the N-methyl-D-aspartate (NMDA) receptor, followed by the study of the oxidative conversion of 3,4-dihydroxyphenylalanine (DOPA) to the non-NMDA receptor agonist and neurotoxin 2,4,5-trihydroxyphenylalanine (TOPA) quinone. Finally, we summarize our work linking oxidative injury to the liberation of zinc from intracellular metal binding proteins, leading to the uncovering of a signaling mechanism connecting excitotoxicity with zinc-activated cell death-signaling cascades.

Functional interactions of ion channels with the actin cytoskeleton: does coupling to dynamic actin regulate NMDA receptors?

Synapses are enriched in the cytoskeletal protein actin, which determines the shape of the pre- and postsynaptic compartments, organizes the neurotransmitter release machinery, and provides a framework for trafficking of components. In the postsynaptic compartment, interactions with actin or its associated proteins are also critical for the localization and activity of synaptic neurotransmitter receptors and ion channels. Actin binding proteins, including spectrin and α-actinin, serve as molecular linkages between the actin cytoskeleton and a diverse collection of receptors, including the NMDA receptor (NMDAR) and voltage-gated Na⁺ channels. The actin cytoskeleton can regulate neurotransmitter receptors and ion channels by controlling their trafficking and localization at the synapse and by directly gating receptor channel opening. We highlight evidence that synaptic actin couples physically and functionally to the NMDAR and supports its activity. The molecular mechanisms by which actin regulates NMDARs are only just emerging, and recent advancements in light and electron microscopy-based imaging techniques should aide in elucidating these mechanisms.

Life with Bacterial Mechanosensitive Channels, from Discovery to Physiology to Pharmacological Target

General principles in biology have often been elucidated from the study of bacteria. This is true for the bacterial mechanosensitive channel of large conductance, MscL, the channel highlighted in this review. This channel functions as a last-ditch emergency release valve discharging cytoplasmic solutes upon decreases in osmotic environment. Opening the largest gated pore, MscL passes molecules up to 30 Å in diameter; exaggerated conformational changes yield advantages for study, including in vivo assays. MscL contains structural/functional themes that recur in higher organisms and help elucidate how other, structurally more complex, channels function. These features of MscL include (i) the ability to directly sense, and respond to, biophysical changes in the membrane, (ii) an α helix ("slide helix") or series of charges ("knot in a rope") at the cytoplasmic membrane boundary to guide transmembrane movements, and (iii) important subunit interfaces that, when disrupted, appear to cause the channel to gate inappropriately. MscL may also have medical applications: the modality of the MscL channel can be changed, suggesting its use as a triggered nanovalve in nanodevices, including those for drug targeting. In addition, recent studies have shown that the antibiotic streptomycin opens MscL and uses it as one of the primary paths to the cytoplasm. Moreover, the recent identification and study of novel specific agonist compounds demonstrate that the channel is a valid drug target. Such compounds may serve as novel-acting antibiotics and adjuvants, a way of permeabilizing the bacterial cell membrane and, thus, increasing the potency of commonly used antibiotics.

Neurons differentiate magnitude and location of mechanical stimuli

Neuronal activity can be modulated by mechanical stimuli. To study this phenomenon quantitatively, we mechanically stimulated rat cortical neurons by shear stress and local indentation. Neurons show 2 distinct responses, classified as transient and sustained. Transient responses display fast kinetics, similar to spontaneous neuronal activity, whereas sustained responses last several minutes before returning to baseline. Local soma stimulations with micrometer-sized beads evoke transient responses at low forces of ∼220 nN and pressures of ∼5.6 kPa and sustained responses at higher forces of ∼360 nN and pressures of ∼9.2 kPa. Among the neuronal compartments, axons are highly susceptible to mechanical stimulation and predominantly show sustained responses, whereas the less susceptible dendrites predominantly respond transiently. Chemical perturbation experiments suggest that mechanically evoked responses require the influx of extracellular calcium through ion channels. We propose that subtraumatic forces/pressures applied to neurons evoke neuronal responses via nonspecific gating of ion channels.

Remembering Mechanosensitivity of NMDA Receptors

An increase in post-synaptic Ca²⁺ conductance through activation of the ionotropic N-methyl-D-aspartate receptor (NMDAR) and concomitant structural changes are essential for the initiation of long-term potentiation (LTP) and memory formation. Memories can be initiated by coincident events, as occurs in classical conditioning, where the NMDAR can act as a molecular coincidence detector. Binding of glutamate and glycine, together with depolarization of the postsynaptic cell membrane to remove the Mg²⁺ channel pore block, results in NMDAR opening for Ca²⁺ conductance. Accumulating evidence has implicated both force-from-lipids and protein tethering mechanisms for mechanosensory transduction in NMDAR, which has been demonstrated by both, membrane stretch and application of amphipathic molecules such as arachidonic acid (AA). The contribution of mechanosensitivity to memory formation and consolidation may be to increase activity of the NMDAR leading to facilitated memory formation. In this review we look back at the progress made toward understanding the physiological and pathological role of NMDA receptor channels in mechanobiology of the nervous system and consider these findings in like of their potential functional implications for memory formation. We examine recent studies identifying mechanisms of both NMDAR and other mechanosensitive channels and discuss functional implications including gain control of NMDA opening probability. Mechanobiology is a rapidly growing area of biology with many important implications for understanding form, function and pathology in the nervous system.

A New Integrative Theory of Brain-Body-Ecosystem Medicine: From the Hippocratic Holistic View of Medicine to Our Modern Society

Humans are increasingly aware that their fate will depend on the wisdom they apply in interacting with the ecosystem. Its health is defined as the condition in which the ecosystem can deliver and continuously renew its fundamental services. A healthy ecosystem allows optimal interactions between humans and the other biotic/abiotic components, and only in a healthy ecosystem can humans survive and efficiently reproduce. Thus, both the human and ecosystem health should be considered together in view of their interdependence. The present article suggests that this relationship could be considered starting from the Hippocrates (460 BC-370 BC) work "On Airs, Waters, and Places" to derive useful medical and philosophical implications for medicine which is indeed a topic that involves scientific as well as philosophical concepts that implicate a background broader than the human body. The brain-body-ecosystem medicine is proposed as a new more complete approach to safeguarding human health. Epidemiological data demonstrate that exploitation of the environment resulting in ecosystem damage affects human health and in several instances these diseases can be detected by modifications in the heart-brain interactions that can be diagnosed through the analysis of changes in heart rate variability.

Opioid receptor-mediated changes in the NMDA receptor in developing rat and chicken

The use of opioids during pregnancy has been associated with neurodevelopmental toxicity in exposed children, leading to cognitive and behavioural deficits later in life. The N-methyl-D-aspartate receptor (NMDAR) subunit GluN2B plays critical roles in cerebellar development, and methadone has been shown to possess NMDAR antagonist effect. Consequently, we wanted to explore if prenatal opioid exposure affected GluN2B subunit expression and NMDAR function in rat and chicken cerebellum. Pregnant rats were exposed to methadone (10 mg/kg/day) or buprenorphine (1 mg/kg/day) for the whole period of gestation, using an osmotic minipump. To further examine potential effects of prenatal opioid exposure in a limited time window, chicken embryos were exposed to a 20 mg/kg dose of methadone or morphine on embryonic days 13 and 14. Western blot analysis of cerebella isolated from 14 days old rat pups exposed to buprenorphine showed significantly lower level of the GluN2B subunit, while the opioid exposed chicken embryo cerebellar GluN2B expression remained unaffected at embryonic day 17. However, we observed increased NMDA/glycine-induced calcium influx in cerebellar granule neurone cultures from opioid exposed chicken embryos. We conclude that prenatal opioid exposure leads to opioid receptor-dependent reduction in the postnatal expression of GluN2B in rat cerebella, and increase in NMDA-induced calcium influx in chicken embryo cerebella.

Transcranial Low-Intensity Pulsed Ultrasound Modulates Structural and Functional Synaptic Plasticity in Rat Hippocampus

Plasticity of synaptic structure and function play an essential role in neuronal development, cognitive functions, and degenerative diseases. Recently, low-intensity pulsed ultrasound (LIPUS) stimulation has been reported as a promising technology for neuromodulation. However, the effect of LIPUS stimulation on the structural and functional synaptic plasticity in rat hippocampus has not yet been addressed. The aim of this study was to investigate whether LIPUS stimulation could affect the dendritic structure, electrophysiological properties, and expression level of glutamate receptors GluN2A, GluN2B, and GluR1 subunits in rat hippocampus. Transcranial LIPUS was delivered to CA1 of the intact hippocampus of rats ( n = 40 ) for 10 days (10 min/day) with the following parameters: fundamental frequency of 0.5 MHz, pulse repetition frequency (PRF) of 500 Hz, peak negative pressure of 0.42 MPa, and I_spta of 360 mW/cm². The effect of LIPUS on dendritic structure, electrophysiological properties, and the expression of neurotransmitter receptors was measured using Golgi staining, electrophysiological recording, and western blotting, respectively. Golgi staining and electrophysiological recordings showed that LIPUS stimulation significantly increased the density of dendritic spines (0.72 ± 0.17 versus 0.94 ± 0.19 spines/ [Formula: see text], ) and the frequency of spontaneous excitatory postsynaptic current (0.37 ± 0.14 versus 1.77 ± 0.37 Hz, ) of CA1 hippocampal neurons. Furthermore, the western blotting analysis demonstrated a significant increase in the expression level of GluN2A ( ). The results illustrated the effect of LIPUS on the dendritic structure, function, and neurotransmitter receptors, which may provide a powerful tool for treating neurodegenerative diseases.

The Emerging Role of Mechanics in Synapse Formation and Plasticity

The regulation of synaptic strength forms the basis of learning and memory, and is a key factor in understanding neuropathological processes that lead to cognitive decline and dementia. While the mechanical aspects of neuronal development, particularly during axon growth and guidance, have been extensively studied, relatively little is known about the mechanical aspects of synapse formation and plasticity. It is established that a filamentous actin network with complex spatiotemporal behavior controls the dendritic spine shape and size, which is thought to be crucial for activity-dependent synapse plasticity. Accordingly, a number of actin binding proteins have been identified as regulators of synapse plasticity. On the other hand, a number of cell adhesion molecules (CAMs) are found in synapses, some of which form transsynaptic bonds to align the presynaptic active zone (PAZ) with the postsynaptic density (PSD). Considering that these CAMs are key components of cellular mechanotransduction, two critical questions emerge: (i) are synapses mechanically regulated? and (ii) does disrupting the transsynaptic force balance lead to (or exacerbate) synaptic failure? In this mini review article, I will highlight the mechanical aspects of synaptic structures-focusing mainly on cytoskeletal dynamics and CAMs-and discuss potential mechanoregulation of synapses and its relevance to neurodegenerative diseases.

Effects of Low Intensity Focused Ultrasound on Liposomes Containing Channel proteins

The ability to reversibly and non-invasively modulate region-specific brain activity in vivo suggests Low Intensity Focused Ultrasound (LIFU) as potential therapeutics for neurological dysfunctions such as epilepsy and Parkinson's disease. While in vivo studies provide evidence of the bioeffects of LIFU on neuronal activity, they merely hint at potential mechanisms but do not fully explain how this technology achieves these effects. One potential hypothesis is that LIFU produces local membrane depolarization by mechanically perturbing the neuronal cell membrane, or activating channels or other proteins embedded in the membrane. Proteins that sense mechanical perturbations of the membrane, such as those gated by membrane tension, are prime candidates for activating in response to LIFU and thus leading to the neurological responses that have been measured. Here we use the bacterial mechanosensitive channel MscL, which has been purified and reconstituted in liposomes, to determine how LIFU may affect the activation of this membrane-tension gated channel. Two bacterial voltage-gated channels, KvAP and NaK2K F92A channels were also studied. Surprisingly, the results suggest that ultrasound modulation and membrane perturbation does not induce channel gating, but rather induces pore formation at the membrane protein-lipid interface. However, in vesicles with high MscL mechanosensitive channel concentrations, apparent decreases in pore formation are observed, suggesting that this membrane-tension-sensitive protein may serve to increase the elasticity of the membrane, presumably because of expansion of the channel in the plane of the membrane independent of channel gating.

How lipids contribute to ion channel function, a fat perspective on direct and indirect interactions

Membrane lipid composition and remodeling influence the function of ion channels. Polyunsaturated fatty acids (PUFAs) and their derivatives modulate ion channel function; whether this effect occurs directly by binding to the protein or indirectly through alteration of membranes' mechanical properties has been difficult to distinguish. There are a large number of studies addressing the effect of fatty acids; recent structural and functional analyses have identified binding sites and provided further evidence for the role of the plasma membrane in ion channel function. Here, we review cation channels that do not share a common topology or lipid-binding signature sequence, but for which there are recent compelling data that support both direct and indirect modulation by PUFAs or their derivatives.

Medium-retaining Petri dish insert to grow and image cultured cells

BACKGROUND

Microscope chambers that accept glass coverslips with cultured cells are often used to monitor intracellular Ca²⁺ concentration ([Ca²⁺]_i) during cell superfusion. Unfortunately, the experimental maneuvers associated with the coverslip installation in these chambers (medium removal and re-application) trigger unintended [Ca²⁺]_i elevations.

NEW METHOD

To prevent these [Ca²⁺]_i elevations, a Petri dish insert has been constructed. The insert features a superfusion-optimized well to grow cell cultures. After this insert is removed from the Petri dish, the well retains the medium. This feature allows the inserts to be installed in microscope chambers while keeping the cells submerged at all times.

RESULTS

These inserts were used to test the impact of a transient medium removal from the well (an equivalent of a coverslip removal from the medium) on [Ca²⁺]_i in primary murine cortical neurons and astrocytes, and in HEK-293 cells. In all of these models, the medium removal/re-application caused a micromolar [Ca²⁺]_i spike. While in neurons this spike was caused by a Ca²⁺ influx, in astrocytes and HEK-293 cells, it was caused by a Ca²⁺ release from intracellular stores. After the spike, a subpopulation of neurons failed to restore low [Ca²⁺]_i; in 24% of the astrocytes, the spike triggered [Ca²⁺]_i oscillations. However, prior to the spike, [Ca²⁺]_i was low and uniform in all these cells.

COMPARISON WITH EXISTING METHOD(S)

The new method avoids the artificially-induced [Ca²⁺]_i elevations that take place during the handling of glass coverslips with cultured cells.

CONCLUSIONS

The new method allows monitoring [Ca²⁺]_i without disturbing the basal [Ca²⁺]_i levels.

Functional roles of three cues that provide nonsynaptic modes of communication in the brain: electromagnetic field, oxygen, and carbon dioxide

The integrative actions of the brain depend on the exchange of information among its computational elements. Hence, this phenomenon plays the key role in driving the complex dynamics of the central nervous system, in which true computations interact with noncomputational dynamical processes to generate brain representations of the body and of the body in the external world, and hence the finalistic behavior of the organism. In this context, it should be pointed out that, besides the intercellular interactions mediated by classical electrochemical signals, other types of interactions, namely, "cues" and "coercions," also appear to be exploited by the system to achieve its function. The present review focuses mainly on cues present in the environment and on those produced by cells of the body, which "pervade" the brain and contribute to its dynamics. These cues can also be metabolic substrates, and, in most cases, they are of fundamental importance to brain function and the survival of the entire organism. Three of these highly pervasive cues will be analyzed in greater detail, namely, oxygen, carbon dioxide, and electromagnetic fields (EMF). Special emphasis will be placed on EMF, since several authors have suggested that these highly pervasive energy fluctuations may play an important role in the global integrative actions of the brain; hence, EMF signaling may transcend classical connectionist models of brain function. Thus the new concept of "broadcasted neuroconnectomics" has been introduced, which transcends the current connectomics view of the brain.

Chimeric Glutamate Receptor Subunits Reveal the Transmembrane Domain Is Sufficient for NMDA Receptor Pore Properties but Some Positive Allosteric Modulators Require Additional Domains

NMDA receptors are ligand-gated ion channels that underlie transmission at excitatory synapses and play an important role in regulating synaptic strength and stability. Functional NMDA receptors require two copies of the GluN1 subunit coassembled with GluN2 (and/or GluN3) subunits into a heteromeric tetramer. A diverse array of allosteric modulators can upregulate or downregulate NMDA receptor activity. These modulators include both synthetic compounds and endogenous modulators, such as cis-unsaturated fatty acids, 24(S)-hydroxycholesterol, and various neurosteroids. To evaluate the structural requirements for the formation and allosteric modulation of NMDA receptor pores, we have replaced portions of the rat GluN1, GluN2A, and GluN2B subunits with homologous segments from the rat GluK2 kainate receptor subunit. Our results with these chimeric constructs show that the NMDA receptor transmembrane domain is sufficient to account for most pore properties, but that regulation by some allosteric modulators requires additional cytoplasmic or extracellular domains.

SIGNIFICANCE STATEMENT

Glutamate receptors mediate excitatory synaptic transmission by forming cation channels through the membrane that open upon glutamate binding. Although many compounds have been identified that regulate glutamate receptor activity, in most cases the detailed mechanisms that underlie modulation are poorly understood. To identify what parts of the receptor are essential for pore formation and sensitivity to allosteric modulators, we generated chimeric subunits that combined segments from NMDA and kainate receptors, subtypes with distinct pharmacological profiles. Surprisingly, our results identify separate domain requirements for allosteric potentiation of NMDA receptor pores by pregnenolone sulfate, 24(S)-hydroxycholesterol, and docosahexaenoic acid, three endogenous modulators derived from membrane constituents. Understanding where and how these compounds act on NMDA receptors should aid in designing better therapeutic agents.

Long-term alcohol exposure elicits hippocampal nonsynaptic epileptiform activity changes associated with expression and functional changes in NKCC1, KCC2 co-transporters and Na+/K+-ATPase

Nonsynaptic mechanism changes, particularly the enhancement of NKCC1 expression in the dentate gyrus (DG) after 4weeks of ethanol consumption, motivate the present work, in which rats were submitted to a period of chronic consumption (12weeks). Four groups of six animals (6-week-old male Wistar rats) were formed, including the control (C), ethanol 1 (E1), ethanol 2 (E2) and ethanol 3 (E3) groups. The rats in the E1, E2 and E3 groups were treated daily with a 30% v/v solution of ethanol, administered via oral gavage (1.0, 2.0 and 3.0g/kg, respectively). Nonsynaptic epileptiform activities (NEA) were induced by means of the zero-Ca2+ and high-K+ model using hippocampal slices and were recorded in the DG. The presence of NKCC1, KCC2, α1-Na+/K+-ATPase and GFAP immunoreactivity was analyzed. The results demonstrate that alcohol consumption changes NEA, and these changes are more prominent at the lower dosage. An increase in the DC shifts associated with epileptiform discharges was present with the low dose. This increase was correlated with the increment of NKCC1 expression. Confocal microscopy images indicate the NKCC1 increase was pronounced in the initial axonal segment of granule cells. The blockage of these cotransporters during NEA induction with bumetanide suppressed the DC shift increase and diminished all parameters of NEA that were quantified for all groups treated with ethanol. Therefore, the increase in NKCC1 expression and the effective activity of this cotransporter, which were observed in the treated groups, suggest that drugs that act for block NKCC1 represent promising strategies for diminishing the effects of alcohol damage on the brain.

Mechanical stress activates NMDA receptors in the absence of agonists

While studying the physiological response of primary rat astrocytes to fluid shear stress in a model of traumatic brain injury (TBI), we found that shear stress induced Ca2+ entry. The influx was inhibited by MK-801, a specific pore blocker of N-Methyl-D-aspartic acid receptor (NMDAR) channels, and this occurred in the absence of agonists. Other NMDA open channel blockers ketamine and memantine showed a similar effect. The competitive glutamate antagonists AP5 and GluN2B-selective inhibitor ifenprodil reduced NMDA-activated currents, but had no effect on the mechanically induced Ca2+ influx. Extracellular Mg2+ at 2 mM did not significantly affect the shear induced Ca2+ influx, but at 10 mM it produced significant inhibition. Patch clamp experiments showed mechanical activation of NMDAR and inhibition by MK-801. The mechanical sensitivity of NMDARs may play a role in the normal physiology of fluid flow in the glymphatic system and it has obvious relevance to TBI.

The multi-facet aspects of cell sentience and their relevance for the integrative brain actions: role of membrane protein energy landscape

Several ion channels can be randomly and spontaneously in an open state, allowing the exchange of ion fluxes between extracellular and intracellular environments. We propose that the random changes in the state of ion channels could be also due to proteins exploring their energy landscapes. Indeed, proteins can modify their steric conformation under the effects of the physicochemical parameters of the environments with which they are in contact, namely, the extracellular, intramembrane and intracellular environments. In particular, it is proposed that the random walk of proteins in their energy landscape is towards attractors that can favor the open or close condition of the ion channels and/or intrinsic activity of G-protein-coupled receptors. The main aspect of the present proposal is that some relevant physicochemical parameters of the environments (e.g. molecular composition, temperature, electrical fields) with which some signaling-involved plasma membrane proteins are in contact alter their conformations. In turn, these changes can modify their information handling via a modulatory action on their random walk towards suitable attractors of their energy landscape. Thus, spontaneous and/or signal-triggered electrical activities of neurons occur that can have emergent properties capable of influencing the integrative actions of brain networks. Against this background, Cook’s hypothesis on ‘cell sentience’ is developed by proposing that physicochemical parameters of the environments with which the plasma-membrane proteins of complex cellular networks are in contact fulfill a fundamental role in their spontaneous and/or signal-triggered activity. Furthermore, it is proposed that a specialized organelle, the primary cilium, which is present in most cells (also neurons and astrocytes), could be of peculiar importance to pick up chemical signals such as ions and transmitters and to detect physical signals such as pressure waves, thermal gradients, and local field potentials.

Regulation of Piezo2 Mechanotransduction by Static Plasma Membrane Tension in Primary Afferent Neurons

The Piezo2 channel is a newly identified mammalian mechanical transducer that confers rapidly adapting mechanically activated (RA-MA) currents in primary afferent neurons. The Piezo2 channels sense rapid membrane displacement, but it is not clear whether they are sensitive to osmotic swelling, which slowly increases static plasma membrane tension (SPMT). Here, we show that SPMT exerts a profound impact on the mechanical sensitivity of RA-MA channels in primary afferent neurons. RA-MA currents are greatly enhanced, and the mechanical threshold was reduced in both primary afferent neurons of rat dorsal root ganglia (DRG) and HEK293 cells heterologously expressing Piezo2 when these cells undergo osmotic swelling to increase SPMT. Osmotic swelling switches the kinetics of RA-MA currents to the slowly adapting type in both cultured DRG neurons and HEK293 cells heterologously expressing Piezo2. The potentiation of RA-MA currents is abolished when cultured DRG neurons are treated with cytochalasin D, an actin filament disruptor that prevents SPMT of cultured DRG neurons from an increase by osmotic swelling. Osmotic swelling significantly increases DRG neuron mechano-excitability such that a subthreshold mechanical stimulus can result in action potential firing. Behaviorally, the mechanical hind paw withdrawal threshold in rats is reduced following the injection of a hypotonic solution, but this osmotic effect is abolished when cytochalasin D or Gd(3+) is co-administered with the hypo-osmotic solution. Taken together, our findings suggest that Piezo2-mediated mechanotransduction is regulated by SPMT in primary afferent neurons. Because SPMT can be changed by multiple biological factors, our findings may have broad implications in mechanical sensitivity under physiological and pathological conditions.

Osmotic Edema Rapidly Increases Neuronal Excitability Through Activation of NMDA Receptor-Dependent Slow Inward Currents in Juvenile and Adult Hippocampus

Cellular edema (cell swelling) is a principal component of numerous brain disorders including ischemia, cortical spreading depression, hyponatremia, and epilepsy. Cellular edema increases seizure-like activity in vitro and in vivo, largely through nonsynaptic mechanisms attributable to reduction of the extracellular space. However, the types of excitability changes occurring in individual neurons during the acute phase of cell volume increase remain unclear. Using whole-cell patch clamp techniques, we report that one of the first effects of osmotic edema on excitability of CA1 pyramidal cells is the generation of slow inward currents (SICs), which initiate after approximately 1 min. Frequency of SICs increased as osmolarity decreased in a dose-dependent manner. Imaging of real-time volume changes in astrocytes revealed that neuronal SICs occurred while astrocytes were still in the process of swelling. SICs evoked by cell swelling were mainly nonsynaptic in origin and NMDA receptor-dependent. To better understand the relationship between SICs and changes in neuronal excitability, recordings were performed in increasingly physiological conditions. In the absence of any added pharmacological reagents or imposed voltage clamp, osmotic edema induced excitatory postsynaptic potentials and burst firing over the same timecourse as SICs. Like SICs, action potentials were blocked by NMDAR antagonists. Effects were more pronounced in adult (8-20 weeks old) compared with juvenile (P15-P21) mice. Together, our results indicate that cell swelling triggered by reduced osmolarity rapidly increases neuronal excitability through activation of NMDA receptors. Our findings have important implications for understanding nonsynaptic mechanisms of epilepsy in relation to cell swelling and reduction of the extracellular space.

Molecular Mechanoneurobiology: An Emerging Angle to Explore Neural Synaptic Functions

Neural synapses are intercellular asymmetrical junctions that transmit biochemical and biophysical information between a neuron and a target cell. They are very tight, dynamic, and well organized by many synaptic adhesion molecules, signaling receptors, ion channels, and their associated cytoskeleton that bear forces. Mechanical forces have been an emerging factor in regulating axon guidance and growth, synapse formation and plasticity in physiological and pathological brain activity. Therefore, mechanical forces are undoubtedly exerted on those synaptic molecules and modulate their functions. Here we review current progress on how mechanical forces regulate receptor-ligand interactions, protein conformations, ion channels activation, and cytoskeleton dynamics and discuss how these regulations potentially affect synapse formation, stabilization, and plasticity.

Compliant intracortical implants reduce strains and strain rates in brain tissue in vivo

OBJECTIVE

The objective of this research is to characterize the mechanical interactions of (1) soft, compliant and (2) non-compliant implants with the surrounding brain tissue in a rodent brain. Understanding such interactions will enable the engineering of novel materials that will improve stability and reliability of brain implants.

APPROACH

Acute force measurements were made using a load cell in n = 3 live rats, each with 4 craniotomies. Using an indentation method, brain tissue was tested for changes in force using established protocols. A total of 4 non-compliant, bare silicon microshanks, 3 non-compliant polyvinyl acetate (PVAc)-coated silicon microshanks, and 6 compliant, nanocomposite microshanks were tested. Stress values were calculated by dividing the force by surface area and strain was estimated using a linear stress-strain relationship. Micromotion effects from breathing and vascular pulsatility on tissue stress were estimated from a 5 s interval of steady-state measurements. Viscoelastic properties were estimated using a second-order Prony series expansion of stress-displacement curves for each shank.

MAIN RESULTS

The distribution of strain values imposed on brain tissue for both compliant nanocomposite microshanks and PVAc-coated, non-compliant silicon microshanks were significantly lower compared to non-compliant bare silicon shanks. Interestingly, step-indentation experiments also showed that compliant, nanocomposite materials significantly decreased stress relaxation rates in the brain tissue at the interface (p < 0.05) compared to non-compliant silicon and PVAc-coated silicon materials. Furthermore, both PVAc-coated non-compliant silicon and compliant nanocomposite shanks showed significantly reduced (by 4-5 fold) stresses due to tissue micromotion at the interface.

SIGNIFICANCE

The results of this study showed that soft, adaptive materials reduce strains and strain rates and micromotion induced stresses in the surrounding brain tissue. Understanding the material behavior at the site of tissue contact will help to improve neural implant design.

Synaptosomal membrane-based Langmuir-Blodgett films: a platform for studies on γ-aminobutyric acid type A receptor binding properties

In this work we used Langmuir-Blodgett films (LB) as model membranes to study the effect of molecular packing on the flunitrazepam (FNZ) accessibility to the binding sites at the GABAA receptor (GABAA-R). Ligand binding data were correlated with film topography analysis by atomic force microscopy images (AFM) and SDS-PAGE. Langmuir films (LF) were prepared by the spreading of synaptosomal membranes (SM) from bovine brain cortex at the air-water interface. LBs were obtained by the transference, at 15 or 35 mN/m constant surface pressure (π), of one (LB15/1c and LB35/1c) or two (LB35/2c) LFs to a film-free hydrophobic alkylated substrate (CONglass). Transference was performed in a serial manner, which allowed the accumulation of a great number of samples. SDS-PAGE clearly showed a 55 kDa band characteristic of GABAA-R subunits. Detrended fluctuation analysis of topographic data from AFM images exhibited a single slope value (self-similarity parameter α) in CONglass and a discontinuous slope change in the α value at an autocorrelation length of ∼100 nm in all LB samples, supporting the LF transference to the substrate. AFM images of CONglass and LB15/1c exhibited roughness and average heights that were similar between measurements and significantly lower than those of LB35/1c and LB35/2c, suggesting that the substrate coverage in the latter was more stable than in LB15/1c. While [(3)H]FNZ binding in LB15/1c did not reach saturation, in LB35/1c the binding kinetics became sigmoid with a binding affinity lower than in the SM suspension. Our results highlight the π dependence of both binding and topological data and call to mind the receptor mechanosensitivity. Thus, LB films provide a tool for bionanosensing GABAA-R ligand binding as well as GABAA-R activity modulation induced by the environmental supramolecular organization.

Cellular biomechanics of central nervous system injury

With a growing interest in how the brain responds and remodels itself following a traumatic injury, this chapter outlines the major organizing principles of how to study these injuries in the laboratory and extend these findings back into the clinic. A new repertoire of models is available to examine the response of isolated circuits of the brain in vitro, and to study precisely how mechanical forces applied to even small regions of these circuits can disrupt the entire circuit dysfunction. We review the existing knowledge garnered from these models and our current understanding of mechanically sensitive receptors and channels activated immediately following trauma. In turn, we point to the emergence of in silico models of network function that will lead to an improved understanding of the principles for the remodeling of circuit structure after traumatic, possibly pointing out new biological rules for circuit reassembly that would help guide new therapies for reconstructing brain circuits after trauma.

Red blood cells of sickle cell disease patients exhibit abnormally high abundance of N-methyl D-aspartate receptors mediating excessive calcium uptake

Recently we showed that N-methyl D-aspartate receptors (NMDARs) are expressed in erythroid precursors (EPCs) and present in the circulating red blood cells (RBCs) of healthy humans, regulating intracellular Ca(2+) in these cells. This study focuses on investigating the possible role of NMDARs in abnormally high Ca(2+) permeability in the RBCs of patients with sickle cell disease (SCD). Protein levels of the NMDAR subunits in the EPCs of SCD patients did not differ from those in EPCs of healthy humans. However, the number and activity of the NMDARs in circulating SCD-RBCs was substantially up-regulated, being particularly high during haemolytic crises. The number of active NMDARs correlated negatively with haematocrit and haemoglobin levels in the blood of SCD patients. Calcium uptake via these non-selective cation channels was induced by RBC treatment with glycine, glutamate and homocysteine and was facilitated by de-oxygenation of SCD-RBCs. Oxidative stress and RBC dehydration followed receptor stimulation and Ca(2+) uptake. Inhibition of the NMDARs with an antagonist memantine caused re-hydration and largely prevented hypoxia-induced sickling. The EPCs of SCD patients showed higher tolerance to memantine than those of healthy subjects. Consequently, NMDARs in the RBCs of SCD patients appear to be an attractive target for pharmacological intervention.

Single-neuron NMDA receptor phenotype influences neuronal rewiring and reintegration following traumatic injury

Alterations in the activity of neural circuits are a common consequence of traumatic brain injury (TBI), but the relationship between single-neuron properties and the aggregate network behavior is not well understood. We recently reported that the GluN2B-containing NMDA receptors (NMDARs) are key in mediating mechanical forces during TBI, and that TBI produces a complex change in the functional connectivity of neuronal networks. Here, we evaluated whether cell-to-cell heterogeneity in the connectivity and aggregate contribution of GluN2B receptors to [Ca(2+)]i before injury influenced the functional rewiring, spontaneous activity, and network plasticity following injury using primary rat cortical dissociated neurons. We found that the functional connectivity of a neuron to its neighbors, combined with the relative influx of calcium through distinct NMDAR subtypes, together contributed to the individual neuronal response to trauma. Specifically, individual neurons whose [Ca(2+)]i oscillations were largely due to GluN2B NMDAR activation lost many of their functional targets 1 h following injury. In comparison, neurons with large GluN2A contribution or neurons with high functional connectivity both independently protected against injury-induced loss in connectivity. Mechanistically, we found that traumatic injury resulted in increased uncorrelated network activity, an effect linked to reduction of the voltage-sensitive Mg(2+) block of GluN2B-containing NMDARs. This uncorrelated activation of GluN2B subtypes after injury significantly limited the potential for network remodeling in response to a plasticity stimulus. Together, our data suggest that two single-cell characteristics, the aggregate contribution of NMDAR subtypes and the number of functional connections, influence network structure following traumatic injury.

Pressure modulation of Ras-membrane interactions and intervesicle transfer

Proteins attached to the plasma membrane frequently encounter mechanical stresses, including high hydrostatic pressure (HHP) stress. Signaling pathways involving membrane-associated small GTPases (e.g., Ras) have been identified as critical loci for pressure perturbation. However, the impact of mechanical stimuli on biological outputs is still largely terra incognita. The present study explores the effect of HHP on the membrane association, dissociation, and intervesicle transfer process of N-Ras by using a FRET-based assay to obtain the kinetic parameters and volumetric properties along the reaction path of these processes. Notably, membrane association is fostered upon pressurization. Conversely, depending on the nature and lateral organization of the lipid membrane, acceleration or retardation is observed for the dissociation step. In addition, HHP can be inferred as a positive regulator of N-Ras clustering, in particular in heterogeneous membranes. The susceptibility of membrane interaction to pressure raises the idea of a role of lipidated signaling molecules as mechanosensors, transducing mechanical stimuli to chemical signals by regulating their membrane binding and dissociation. Finally, our results provide first insights into the influence of pressure on membrane-associated Ras-controlled signaling events in organisms living under extreme environmental conditions such as those that are encountered in the deep sea and sub-seafloor environments, where pressures reach the kilobar (100 MPa) range.

Magnetic nanoparticle-based approaches to locally target therapy and enhance tissue regeneration in vivo

Magnetic-based systems utilizing superparamagnetic nanoparticles and a magnetic field gradient to exert a force on these particles have been used in a wide range of biomedical applications. This review is focused on drug targeting applications that require penetration of a cellular barrier as well as strategies to improve the efficacy of targeting in these biomedical applications. Another focus of this review is regenerative applications utilizing tissue engineered scaffolds prepared with the aid of magnetic particles, the use of remote actuation for release of bioactive molecules and magneto-mechanical cell stimulation, cell seeding and cell patterning.

The mechanobiology of brain function

All cells are influenced by mechanical forces. In the brain, force-generating and load-bearing proteins twist, turn, ratchet, flex, compress, expand and bend to mediate neuronal signalling and plasticity. Although the functions of mechanosensitive proteins have been thoroughly described in classical sensory systems, the effects of endogenous mechanical energy on cellular function in the brain have received less attention, and many working models in neuroscience do not currently integrate principles of cellular mechanics. An understanding of cellular-mechanical concepts is essential to allow the integration of mechanobiology into ongoing studies of brain structure and function.

Acid-sensing ion channels in pathological conditions

Acid-sensing ion channels (ASICs), a novel family of proton-gated amiloride-sensitive cation channels, are expressed primarily in neurons of peripheral sensory and central nervous systems. Recent studies have shown that activation of ASICs, particularly the ASIC1a channels, plays a critical role in neuronal injury associated with neurological disorders such as brain ischemia, multiple sclerosis, and spinal cord injury. In normal conditions in vitro, ASIC1a channels desensitize rapidly in the presence of a continuous acidosis or following a preexposure to minor pH drop, raising doubt for their contributions to the acidosis-mediated neuronal injury. It is now known that the properties of ASICs can be dramatically modulated by signaling molecules or biochemical changes associated with pathological conditions. Modulation of ASICs by these molecules can lead to dramatically enhanced and/or prolonged activities of these channels, thus promoting their pathological functions. Understanding of how ASICs behave in pathological conditions may help define new strategies for the treatment and/or prevention of neuronal injury associated with various neurological disorders.

Dendritic filopodia, Ripped Pocket, NOMPC, and NMDARs contribute to the sense of touch in Drosophila larvae

BACKGROUND

Among the Aristotelian senses, the subcellular and molecular mechanisms involved in the sense of touch are the most poorly understood.

RESULTS

We demonstrate that specialized sensory neurons, the class II and class III multidendritic (md) neurons, are gentle touch sensors of Drosophila larvae. Genetic silencing of these cells significantly impairs gentle touch responses, optogenetic activation of these cells triggers behavioral touch-like responses, and optical recordings from these neurons show that they respond to force. The class III neurons possess highly dynamic dendritic protrusions rich in F-actin. Genetic manipulations that alter actin dynamics indicate that the actin-rich protrusions (termed sensory filopodia) on the class III neurons are required for behavioral sensitivity to gentle touch. Through a genome-wide RNAi screen of ion channels, we identified Ripped Pocket (rpk), No Mechanoreceptor Potential C (nompC), and NMDA Receptors 1 and 2 (Nmdars) as playing critical roles in both behavioral responses to touch and in the formation of the actin-rich sensory filopodia. Consistent with this requirement, reporters for rpk and nompC show expression in the class III neurons. A genetic null allele of rpk confirms its critical role in touch responses.

CONCLUSIONS

Output from class II and class III md neurons of the Drosophila larvae is necessary and sufficient for eliciting behavioral touch responses. These cells show physiological responses to force. Ion channels in several force-sensing gene families are required for behavioral sensitivity to touch and for the formation of the actin-rich sensory filopodia.

Electrolyte therapy reduces spike-and-wave discharges in the WAG/Rij rat model of absence epilepsy

Although antiepileptic drugs are often effective in the control of seizures, some patients show little or no improvement. As alternative treatments, different dietary modifications were shown to be beneficial for patients with poor tolerance for AEDS. Previous reports have shown that rice-based oral electrolyte hydration therapy is effective in seizure control in patients with refractory absence seizures. In the present study, using an animal model of absence epilepsy, we showed that the occurrence of spike-and-wave discharges significantly decreases upon switching to electrolyte therapy. We also showed that consumption of solution with the same osmolarity as rice-based oral electrolyte solution leads to a decrease in the number of spike-and-wave discharges per hour. We suggest that the antiepileptic effect of rice-based oral electrolyte hydration therapy can be at least in part due to hyperosmolarity of the ingested solution.

Mechanosensitivity of nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are heteropentameric ligand-gated ion channels that mediate excitatory neurotransmission at the neuromuscular junction (NMJ) and other peripheral and central synapses. At the NMJ, acetylcholine receptors (AChRs) are constantly exposed to mechanical stress resulting from muscle contraction. It is therefore of interest to understand if their function is influenced by mechanical stimuli. In this study, patch-clamp recordings showed that AChR channel activity was enhanced upon membrane stretching in both cultured Xenopus muscle cells and C2C12 myotubes. To examine how this property is physiologically regulated, effects of membrane-intrinsic and membrane-extrinsic factors on AChRs expressed in HEK293T cells were studied. As in muscle cells, AChR single channel currents recorded under cell-attached configuration were significantly increased-without change in current amplitude-when negative pressure was applied through the patch pipette. GsMTx-4, a peptide toxin that blocks mechanically activated cation channels, inhibited this effect on AChRs. The mechanosensitivity decreased when cells were treated with MβCD, latrunculin A or cytochalasin D, but increased when exposed to lysophosphatidylcholine, indicating contributions from both membrane lipids and the cytoskeleton. Rapsyn, which binds to AChRs and mediates their cytoskeletal interaction in muscle, suppressed AChR mechanosensitivity when co-expressed in HEK293T cells, but this influence of rapsyn was impaired following the deletion of rapsyn's AChR-binding domain or upon cytoskeletal disruption by cytochalasin D. These results suggest a mechanism for regulating AChR's mechanosensitivity through its cytoskeletal linkage via rapsyn, which may serve to protect the receptors and sarcolemmal integrity under high mechanical stress encountered by the NMJ.

In vitro stretch injury induces time- and severity-dependent alterations of STEP phosphorylation and proteolysis in neurons

Striatal-enriched tyrosine phosphatase (STEP) has been identified as a component of physiological and pathophysiological signaling pathways mediated by N-methyl-d-aspartate (NMDA) receptor/calcineurin/calpain activation. Activation of these pathways produces a subsequent change in STEP isoform expression or activation via dephosphorylation. In this study, we evaluated changes in STEP phosphorylation and proteolysis in dissociated cortical neurons after sublethal and lethal mechanical injury using an in vitro stretch injury device. Sublethal stretch injury produces minimal changes in STEP phosphorylation at early time points, and increased STEP phosphorylation at 24 h that is blocked by the NMDA-receptor antagonist APV, the calcineurin-inhibitor FK506, and the sodium channel blocker tetrodotoxin. Lethal stretch injury produces rapid STEP dephosphorylation via NR2B-containing NMDA receptors, but not calcineurin, and a subsequent biphasic phosphorylation pattern. STEP(61) expression progressively increases after sublethal stretch with no change in calpain-mediated STEP(33) formation, while lethal stretch injury results in STEP(33) formation via a NR2B-containing NMDA receptor pathway within 1 h of injury. Blocking calpain activation in the initial 30 min after stretch injury increases the ratio of active STEP in cells and blocks STEP(33) formation, suggesting that STEP is an early substrate of calpain after mechanical injury. There is a strong correlation between the amount of STEP(33) formed and the degree of cell death observed after lethal stretch injury. In summary, these data demonstrate that previously characterized pathways of STEP regulation via the NMDA receptor are generally conserved in mechanical injury, and suggest that calpain-mediated cleavage of STEP(33) should be further examined as an early marker of neuronal fate after stretch injury.