Single mechanically-gated cation channel currents can trigger action potentials in neocortical and hippocampal pyramidal neurons

The timing of sleep spindles is modulated by the respiratory cycle in humans

OBJECTIVE

Coupling of sleep spindles with cortical slow waves and hippocampus sharp-waves ripples is crucial for sleep-related memory consolidation. Recent literature evidenced that nasal respiration modulates neural activity in large-scale brain networks. In rodents, this respiratory drive strongly varies according to vigilance states. Whether sleep oscillations are also respiration-modulated in humans remains open. In this work, we investigated the influence of breathing on sleep spindles during non-rapid-eye-movement sleep in humans.

METHODS

Full night polysomnography of twenty healthy participants were analysed. Spindles and slow waves were automatically detected during N2 and N3 stages. Spindle-related sigma power as well as spindle and slow wave events were analysed according to the respiratory phase.

RESULTS

We found a significant coupling between both slow and fast spindles and the respiration cycle, with enhanced sigma activity and occurrence probability of spindles during the middle part of the expiration phase. A different coupling was observed for slow waves negative peaks which were rather distributed around the two respiration phase transitions.

CONCLUSION

Our findings suggest that breathing cycle influences the dynamics of brain activity during non-rapid-eye-movement sleep.

SIGNIFICANCE

This coupling may enable sleep spindles to synchronize with other sleep oscillations and facilitate information transfer between distributed brain networks.

Mechanical stimulation and electrophysiological monitoring at subcellular resolution reveals differential mechanosensation of neurons within networks

A growing consensus that the brain is a mechanosensitive organ is driving the need for tools that mechanically stimulate and simultaneously record the electrophysiological response of neurons within neuronal networks. Here we introduce a synchronized combination of atomic force microscopy, high-density microelectrode array and fluorescence microscopy to monitor neuronal networks and to mechanically characterize and stimulate individual neurons at piconewton force sensitivity and nanometre precision while monitoring their electrophysiological activity at subcellular spatial and millisecond temporal resolution. No correlation is found between mechanical stiffness and electrophysiological activity of neuronal compartments. Furthermore, spontaneously active neurons show exceptional functional resilience to static mechanical compression of their soma. However, application of fast transient (∼500 ms) mechanical stimuli to the neuronal soma can evoke action potentials, which depend on the anchoring of neuronal membrane and actin cytoskeleton. Neurons show higher responsivity, including bursts of action potentials, to slower transient mechanical stimuli (∼60 s). Moreover, transient and repetitive application of the same compression modulates the neuronal firing rate. Seemingly, neuronal networks can differentiate and respond to specific characteristics of mechanical stimulation. Ultimately, the developed multiparametric tool opens the door to explore manifold nanomechanobiological responses of neuronal systems and new ways of mechanical control.

KCNJ2 inhibition mitigates mechanical injury in a human brain organoid model of traumatic brain injury

Traumatic brain injury (TBI) strongly correlates with neurodegenerative disease. However, it remains unclear which neurodegenerative mechanisms are intrinsic to the brain and which strategies most potently mitigate these processes. We developed a high-intensity ultrasound platform to inflict mechanical injury to induced pluripotent stem cell (iPSC)-derived cortical organoids. Mechanically injured organoids elicit classic hallmarks of TBI, including neuronal death, tau phosphorylation, and TDP-43 nuclear egress. We found that deep-layer neurons were particularly vulnerable to injury and that TDP-43 proteinopathy promotes cell death. Injured organoids derived from C9ORF72 amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD) patients displayed exacerbated TDP-43 dysfunction. Using genome-wide CRISPR interference screening, we identified a mechanosensory channel, KCNJ2, whose inhibition potently mitigated neurodegenerative processes in vitro and in vivo, including in C9ORF72 ALS/FTD organoids. Thus, targeting KCNJ2 may reduce acute neuronal death after brain injury, and we present a scalable, genetically flexible cerebral organoid model that may enable the identification of additional modifiers of mechanical stress.

Blood pressure pulsations modulate central neuronal activity via mechanosensitive ion channels

The transmission of the heartbeat through the cerebral vascular system causes intracranial pressure pulsations. We discovered that arterial pressure pulsations can directly modulate central neuronal activity. In a semi-intact rat brain preparation, vascular pressure pulsations elicited correlated local field oscillations in the olfactory bulb mitral cell layer. These oscillations did not require synaptic transmission but reflected baroreceptive transduction in mitral cells. This transduction was mediated by a fast excitatory mechanosensitive ion channel and modulated neuronal spiking activity. In awake animals, the heartbeat entrained the activity of a subset of olfactory bulb neurons within ~20 milliseconds. Thus, we propose that this fast, intrinsic interoceptive mechanism can modulate perception-for example, during arousal-within the olfactory bulb and possibly across various other brain areas.

The neurovascular unit of capillary blood vessels in the rat nervous system. A rapid-Golgi electron microscopy study

We describe a pericapillary organ in the rat forebrain and cerebellar cortex. It consists of a series of tripartite synapses with synaptic extensions enveloped by astrocytic endfeet that are linked to the capillary wall by synaptic extensions. Reciprocal specializations of the pericyte-capillary blood vessel (CBV) with such specialized synapses suggest a mechanoreceptor role. In Golgi-impregnated and 3D reconstructions of the cerebral cortex and thalamus, a series of TSs appear to be sequentially ordered in a common dendrite, paralleled by synaptic outgrowths termed golf club synaptic extensions (GCE) opposed to a longitudinal crest (LC) from the capillary basal lamina (BL). Our results show that, in the cerebellar cortex, afferent fibers and interneurons display microanatomical structures that strongly suggest an interaction with the capillary wall. Afferent mossy fiber (MF) rosettes and ascending granule cell axons and their dendrites define the pericapillary passage interactions that are entangled by endfeet. The presence of mRNA of the mechanosensitive channel Piezo1 in the MF rosettes, together with the surrounding end-feet and the capillary wall form mechanosensory units. The ubiquity of such units to modulate synaptic transmission is also supported by Piezo1 mRNA expressing pyramidal isocortical and thalamic neurons. This scenario suggests that ascending impulses to the cerebellar and cortical targets are presynaptically modulated by the reciprocal interaction with the mechanosensory pericapillary organ that ultimately modulates the vasomotor response.

Human neural network activity reacts to gravity changes in vitro

During spaceflight, humans experience a variety of physiological changes due to deviations from familiar earth conditions. Specifically, the lack of gravity is responsible for many effects observed in returning astronauts. These impairments can include structural as well as functional changes of the brain and a decline in cognitive performance. However, the underlying physiological mechanisms remain elusive. Alterations in neuronal activity play a central role in mental disorders and altered neuronal transmission may also lead to diminished human performance in space. Thus, understanding the influence of altered gravity at the cellular and network level is of high importance. Previous electrophysiological experiments using patch clamp techniques and calcium indicators have shown that neuronal activity is influenced by altered gravity. By using multi-electrode array (MEA) technology, we advanced the electrophysiological investigation covering single-cell to network level responses during exposure to decreased (micro-) or increased (hyper-) gravity conditions. We continuously recorded in real-time the spontaneous activity of human induced pluripotent stem cell (hiPSC)-derived neural networks in vitro. The MEA device was integrated into a custom-built environmental chamber to expose the system with neuronal cultures to up to 6 g of hypergravity on the Short-Arm Human Centrifuge at the DLR Cologne, Germany. The flexibility of the experimental hardware set-up facilitated additional MEA electrophysiology experiments under 4.7 s of high-quality microgravity (10–6 to 10–5 g) in the Bremen drop tower, Germany. Hypergravity led to significant changes in activity. During the microgravity phase, the mean action potential frequency across the neural networks was significantly enhanced, whereas different subgroups of neurons showed distinct behaviors, such as increased or decreased firing activity. Our data clearly demonstrate that gravity as an environmental stimulus triggers changes in neuronal activity. Neuronal networks especially reacted to acute changes in mechanical loading (hypergravity) or de-loading (microgravity). The current study clearly shows the gravity-dependent response of neuronal networks endorsing the importance of further investigations of neuronal activity and its adaptive responses to micro- and hypergravity. Our approach provided the basis for the identification of responsible mechanisms and the development of countermeasures with potential implications on manned space missions.

Respiratory entrainment of units in the mouse parietal cortex depends on vigilance state

Synchronous oscillations are essential for coordinated activity in neuronal networks and, hence, for behavior and cognition. While most network oscillations are generated within the central nervous system, recent evidence shows that rhythmic body processes strongly influence activity patterns throughout the brain. A major factor is respiration (Resp), which entrains multiple brain regions at the mesoscopic (local field potential) and single-cell levels. However, it is largely unknown how such Resp-driven rhythms interact or compete with internal brain oscillations, especially those with similar frequency domains. In mice, Resp and theta (θ) oscillations have overlapping frequencies and co-occur in various brain regions. Here, we investigated the effects of Resp and θ on neuronal discharges in the mouse parietal cortex during four behavioral states which either show prominent θ (REM sleep and active waking (AW)) or lack significant θ (NREM sleep and waking immobility (WI)). We report a pronounced state-dependence of spike modulation by both rhythms. During REM sleep, θ effects on unit discharges dominate, while during AW, Resp has a larger influence, despite the concomitant presence of θ oscillations. In most states, unit modulation by θ or Resp increases with mean firing rate. The preferred timing of Resp-entrained discharges (inspiration versus expiration) varies between states, indicating state-specific and different underlying mechanisms. Our findings show that neurons in an associative cortex area are differentially and state-dependently modulated by two fundamentally different processes: brain-endogenous θ oscillations and rhythmic somatic feedback signals from Resp.

Mechanotransduction in hippocampal neurons operates under localized low picoNewton forces

There is growing evidence suggesting that mechanical properties of CNS neurons may play an important regulatory role in cellular processes. Here, we employ an oscillatory optical tweezers (OOT) to exert a local indentation with forces in the range of 5–50 pN. We found that single local indentation above a threshold of 13 ± 1 pN evokes a transient intracellular calcium change, whereas repeated mechanical stimulations induce a more sustained and variable calcium response. Importantly, neurons were able to differentiate the magnitude of mechanical stimuli. Chemical perturbation and whole-cell patch clamp recordings suggest that mechanically evoked response requires the influx of extracellular calcium through transmembrane ion channels. Moreover, we observed a mechanically evoked activation of the CAMKII and small G protein RhoA. These results all together suggest that mechanical signaling among developed neurons fully operates in neuronal networks under physiological conditions.

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Hippocampal neurons are mechanically adapted to forces in the picoNewton range

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Localized compressive mechanical stimuli in the range of 13–50 pN lead to Ca²⁺ influx

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The evoked Ca²⁺ neuronal response is differentiated by the magnitude of the force

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The subsequent mechanotransduction pathway involves the activation of CAMKII and RhoA

Mechanosensation in traumatic brain injury

Traumatic brain injury (TBI) is distinct from other neurological disorders because it is induced by a discrete event that applies extreme mechanical forces to the brain. This review describes how the brain senses, integrates, and responds to forces under both normal conditions and during injury. The response to forces is influenced by the unique mechanical properties of brain tissue, which differ by region, cell type, and sub-cellular structure. Elements such as the extracellular matrix, plasma membrane, transmembrane receptors, and cytoskeleton influence its properties. These same components also act as force-sensors, allowing neurons and glia to respond to their physical environment and maintain homeostasis. However, when applied forces become too large, as in TBI, these components may respond in an aberrant manner or structurally fail, resulting in unique pathological sequelae. This so-called "pathological mechanosensation" represents a spectrum of cellular responses, which vary depending on the overall biomechanical parameters of the injury and may be compounded by repetitive injuries. Such aberrant physical responses and/or damage to cells along with the resulting secondary injury cascades can ultimately lead to long-term cellular dysfunction and degeneration, often resulting in persistent deficits. Indeed, pathological mechanosensation not only directly initiates secondary injury cascades, but this post-physical damage environment provides the context in which these cascades unfold. Collectively, these points underscore the need to use experimental models that accurately replicate the biomechanics of TBI in humans. Understanding cellular responses in context with injury biomechanics may uncover therapeutic targets addressing various facets of trauma-specific sequelae.

Ultrasound neuromodulation depends on pulse repetition frequency and can modulate inhibitory effects of TTX

Ultrasound is gaining traction as a neuromodulation method due to its ability to remotely and non-invasively modulate neuronal activity with millimeter precision. However, there is little consensus about optimal ultrasound parameters required to elicit neuromodulation and how specific parameters drive mechanisms that underlie ultrasound neuromodulation. We address these questions in this work by performing a study to determine effective ultrasound parameters in a transgenic mouse brain slice model that enables calcium imaging as a quantitative readout of neuronal activity for ultrasound neuromodulation. We report that (1) calcium signaling increases with the application of ultrasound; (2) the neuronal response rate to ultrasound is dependent on pulse repetition frequency (PRF); and (3) ultrasound can reversibly alter the inhibitory effects of tetrodotoxin (TTX) in pharmacological studies. This study offers mechanistic insight into the PRF dependence of ultrasound neuromodulation and the nature of ultrasound/ion channel interaction.

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.

Tale of tail current

The largest biomass of channel proteins is located in unicellular organisms and bacteria that have no organs. However, orchestrated bidirectional ionic currents across the cell membrane via the channels are important for the functioning of organs of organisms, and equally concern both fauna or flora. Several ion channels are activated in the course of action potentials. One of the hallmarks of voltage-dependent channels is a 'tail current' - deactivation as observed after prior and sufficient activation predominantly at more depolarized potentials e.g. for Kv while upon hyperpolarization for HCN α subunits. Tail current also reflects the timing of channel closure that is initiated upon termination of stimuli. Finally, deactivation of currents during repolarization could be a selective estimate for given channel as in case of HERG, if dedicated long and more depolarized 'tail pulse' is used. Since from a holding potential of e.g. -70 mV are often a family of outward K⁺ currents comprising I_A and I_K are simultaneously activated in native cells.

Magnetic Entropy as a Proposed Gating Mechanism for Magnetogenetic Ion Channels

Magnetically sensitive ion channels would allow researchers to better study how specific brain cells affect behavior in freely moving animals; however, recent reports of "magnetogenetic" ion channels based on biogenic ferritin nanoparticles have been questioned because known biophysical mechanisms cannot explain experimental observations. Here, we reproduce a weak magnetically mediated calcium response in HEK cells expressing a previously published TRPV4-ferritin fusion protein. We find that this magnetic sensitivity is attenuated when we reduce the temperature sensitivity of the channel but not when we reduce the mechanical sensitivity of the channel, suggesting that the magnetic sensitivity of this channel is thermally mediated. As a potential mechanism for this thermally mediated magnetic response, we propose that changes in the magnetic entropy of the ferritin particle can generate heat via the magnetocaloric effect and consequently gate the associated temperature-sensitive ion channel. Unlike other forms of magnetic heating, the magnetocaloric mechanism can cool magnetic particles during demagnetization. To test this prediction, we constructed a magnetogenetic channel based on the cold-sensitive TRPM8 channel. Our observation of a magnetic response in cold-gated channels is consistent with the magnetocaloric hypothesis. Together, these new data and our proposed mechanism of action provide additional resources for understanding how ion channels could be activated by low-frequency magnetic fields.