Structural heterogeneity of the ion and lipid channel TMEM16F.

Transmembrane protein 16 F (TMEM16F) is a Ca2+-activated homodimer which functions as an ion channel and a phospholipid scramblase. Despite the availability of several TMEM16F cryogenic electron microscopy (cryo-EM) structures, the mechanism of activation and substrate translocation remains controversial, possibly due to restrictions in the accessible protein conformational space. In this study, we use atomic force microscopy under physiological conditions to reveal a range of structurally and mechanically diverse TMEM16F assemblies, characterized by variable inter-subunit dimerization interfaces and protomer orientations, which have escaped prior cryo-EM studies. Furthermore, we find that Ca2+-induced activation is associated to stepwise changes in the pore region that affect the mechanical properties of transmembrane helices TM3, TM4 and TM6. Our direct observation of membrane remodelling in response to Ca2+ binding along with additional electrophysiological analysis, relate this structural multiplicity of TMEM16F to lipid and ion permeation processes. These results thus demonstrate how conformational heterogeneity of TMEM16F directly contributes to its diverse physiological functions.

Unfolding and identification of membrane proteins in situ.

Single-molecule force spectroscopy (SMFS) uses the cantilever tip of an atomic force microscopy (AFM) to apply a force able to unfold a single protein. The obtained force-distance curve encodes the unfolding pathway, and from its analysis it is possible to characterize the folded domains. SMFS has been mostly used to study the unfolding of purified proteins, in solution or reconstituted in a lipid bilayer. Here, we describe a pipeline for analyzing membrane proteins based on SMFS, which involves the isolation of the plasma membrane of single cells and the harvesting of force-distance curves directly from it. We characterized and identified the embedded membrane proteins combining, within a Bayesian framework, the information of the shape of the obtained curves, with the information from mass spectrometry and proteomic databases. The pipeline was tested with purified/reconstituted proteins and applied to five cell types where we classified the unfolding of their most abundant membrane proteins. We validated our pipeline by overexpressing four constructs, and this allowed us to gather structural insights of the identified proteins, revealing variable elements in the loop regions. Our results set the basis for the investigation of the unfolding of membrane proteins in situ, and for performing proteomics from a membrane fragment.

The dual action of glioma-derived exosomes on neuronal activity: synchronization and disruption of synchrony.

Seizures represent a frequent symptom in gliomas and significantly impact patient morbidity and quality of life. Although the pathogenesis of tumor-related seizures is not fully understood, accumulating evidence indicates a key role of the peritumoral microenvironment. Brain cancer cells interact with neurons by forming synapses with them and by releasing exosomes, cytokines, and other small molecules. Strong interactions among neurons often lead to the synchronization of their activity. In this paper, we used an in vitro model to investigate the role of exosomes released by glioma cell lines and by patient-derived glioma stem cells (GSCs). The addition of exosomes released by U87 glioma cells to neuronal cultures at day in vitro (DIV) 4, when neurons are not yet synchronous, induces synchronization. At DIV 7-12 neurons become highly synchronous, and the addition of the same exosomes disrupts synchrony. By combining Ca2+ imaging, electrical recordings from single neurons with patch-clamp electrodes, substrate-integrated microelectrode arrays, and immunohistochemistry, we show that synchronization and de-synchronization are caused by the combined effect of (i) the formation of new neuronal branches, associated with a higher expression of Arp3, (ii) the modification of synaptic efficiency, and (iii) a direct action of exosomes on the electrical properties of neurons, more evident at DIV 7-12 when the threshold for spike initiation is significantly reduced. At DIV 7-12 exosomes also selectively boost glutamatergic signaling by increasing the number of excitatory synapses. Remarkably, de-synchronization was also observed with exosomes released by glioma-associated stem cells (GASCs) from patients with low-grade glioma but not from patients with high-grade glioma, where a more variable outcome was observed. These results show that exosomes released from glioma modify the electrical properties of neuronal networks and that de-synchronization caused by exosomes from low-grade glioma can contribute to the neurological pathologies of patients with brain cancers.

2D MXene interfaces preserve the basal electrophysiology of targeted neural circuits.

Neural interfaces enable the monitoring of the state of the brain and its composite cell networks, as well as stimulate them to treat nervous disorders. In addition to their highly efficient charge transduction and stability during operation, the neural electrodes should avoid altering the physiological properties of targeted neuronal tissues. Two-dimensional (2D) MXene materials integrate the advantages of metallic conductivity, high specific-surface area and surface functionality in aqueous dispersions, showing promising potential in neural interface applications. Here, we apply uncoated Ti3C2Tx MXene to interface neuronal development. The impacts of the uncoated Ti3C2Tx MXene interface on neuronal development and neuronal microcircuit activity were tested for the first time. Compared to the standard neuronal culture with a poly-L-ornithine coated coverslip, uncoated Ti3C2Tx MXene surfaces did not affect the cell morphology, density, neuron ratios, maturation or the compositions of the neuronal network. Moreover, calcium imaging, spontaneous postsynaptic currents (sPSCs) and also miniature postsynaptic currents (mPSCs) were recorded to demonstrate that Ti3C2Tx MXene interfaces preserved the basal physiology of neuronal activity. The ability to interface neuronal circuit development without altering neuronal signaling properties enables the construction of MXene-based neural prosthetic devices for neuroscience research, diagnosis, and therapies.

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.

CNG channel structure, function, and gating: a tale of conformational flexibility.

Cyclic nucleotide-gated (CNG) channels are key to the signal transduction machinery of certain sensory modalities both in vertebrate and invertebrate organisms. They translate a chemical change in cyclic nucleotide concentration into an electrical signal that can spread through sensory cells. Despite CNG and voltage-gated potassium channels sharing a remarkable amino acid sequence homology and basic architectural plan, their functional properties are dramatically different. While voltage-gated potassium channels are highly selective and require membrane depolarization to open, CNG channels have low ion selectivity and are not very sensitive to voltage. In the last few years, many high-resolution structures of intact CNG channels have been released. This wealth of new structural information has provided enormous progress toward the understanding of the molecular mechanisms and driving forces underpinning CNG channel activation. In this review, we report on the current understanding and controversies surrounding the gating mechanism in CNG channels, as well as the deep intertwining existing between gating, the ion permeation process, and its modulation by membrane voltage. While the existence of this powerful coupling was recognized many decades ago, its direct structural demonstration, and ties to the CNG channel inherent pore flexibility, is a recent achievement.

Molecular Mechanisms of the Blockage of Glioblastoma Motility.

Glioblastoma (GBM) is the most common and lethal brain tumor. GBM has a remarkable degree of motility and is able to infiltrate the healthy brain. In order to perform a rationale-based drug-repositioning study, we have used known inhibitors of two small Rho GTPases, Rac1 and Cdc42, which are upregulated in GBM and are involved in the signaling processes underlying the orchestration of the cytoskeleton and cellular motility. The selected inhibitors (R-ketorolac and ML141 for Cdc42 and R-ketorolac and EHT 1864 for Rac1) have been successfully employed to reduce the infiltration propensity of GBM in live cell imaging studies. Complementarily, all-atom simulations have elucidated the molecular basis of their inhibition mechanism, identifying the binding sites targeted by the inhibitors and dissecting their impact on the small Rho GTPases' function. Our results demonstrate the potential of targeting the Rac1 and Cdc42 proteins with small molecules to contrast GBM infiltration growth and supply precious information for future drug discovery studies aiming to fight GBM and other infiltrative cancer types.

Biomimetic 3D bacterial cellulose-graphene foam hybrid scaffold regulates neural stem cell proliferation and differentiation.

Neural stem cell (NSC)-based therapy is a promising candidate for treating neurodegenerative diseases and the preclinical researches call an urgent need for regulating the growth and differentiation of such cells. The recognition that three-dimensional culture has the potential to be a biologically significant system has stimulated an extraordinary impetus for scientific researches in tissue engineering and regenerative medicine. Here, A novel scaffold for culturing NSCs, three-dimensional bacterial cellulose-graphene foam (3D-BC/G), which was prepared via in situ bacterial cellulose interfacial polymerization on the skeleton surface of porous graphene foam has been reported. 3D-BC/G not only supports NSC growth and adhesion, but also maintains NSC stemness and enhances their proliferative capacity. Further phenotypic analysis indicated that 3D-BC/G induces NSCs to selectively differentiate into neurons, forming a neural network in a short amount of time. The scaffold has good biocompatibility with primary cortical neurons enhancing the neuronal network activities. To explore the underlying mechanisms, RNA-Seq analysis to identify genes and signaling pathways was performed and it suggests that 3D-BC/G offers a more promising three-dimensional conductive substrate for NSC research and neural tissue engineering, and the repertoire of gene expression serves as a basis for further studies to better understand NSC biology.

3D Free-Standing Ordered Graphene Network Geometrically Regulates Neuronal Growth and Network Formation.

The control of cell-microenvironment interactions plays a pivotal role in constructing specific scaffolds for tissue engineering. Here, we fabricated a 3D free-standing ordered graphene (3D-OG) network with a precisely defined pattern. When primary cortical cells are cultured on 3D-OG scaffolds, they form well-defined 3D connections. Astrocytes have a more ramified shape similar to that seen in vivo because of the nanosized ripples and wrinkles on the surface of graphene skeleton. Neurons have axons and dendrites aligned along the graphene skeleton, allowing the formation of neuronal networks with highly controlled connections. Neuronal networks have higher electrical activity with functional signaling over a long distance along the graphene skeleton. Our study, for the first time, investigated the geometrical cues on ordered neuronal growth and network formation with the support of graphene in 3D, which therefore advanced the development of customized scaffolds for brain-machine interfaces or neuroprosthetic devices.

Calcium flares and compartmentalization in rod photoreceptors.

Rod photoreceptors are composed of a soma and an inner segment (IS) connected to an outer segment (OS) by a thin cilium. OSs are composed of a stack of ∼800 lipid discs surrounded by the plasma membrane where phototransduction takes place. Intracellular calcium plays a major role in phototransduction and is more concentrated in the discs, where it can be incorporated and released. To study calcium dynamics in rods, we used the fluorescent calcium dye CaSiR-1 AM working in the near-infrared (NIR) (excitation at 650 and emission at 664 nm), an advantage over previously used dyes. In this way, we investigated calcium dynamics with an unprecedented accuracy and most importantly in semidark-adapted conditions. We observed light-induced drops in [Ca2+]i with kinetics similar to that of photoresponses recorded electrophysiologically. We show three properties of the rods. First, intracellular calcium and key proteins have concentrations that vary from the OS base to tip. At the OS base, [Ca2+]i is ∼80 nM and increases up to ∼200 nM at the OS tip. Second, there are spontaneous calcium flares in healthy and functional rod OSs; these flares are highly localized and are more pronounced at the OS tip. Third, a bright flash of light at 488 nm induces a drop in [Ca2+]i at the OS base but often a flare at the OS tip. Therefore, rod OSs are not homogenous structures but have a structural and functional gradient, which is a fundamental aspect of transduction in vertebrate photoreceptors.

Textured nanofibrils drive microglial phenotype.

Microglia are highly plastic cells that change their properties in response to their microenvironment. By using immunofluorescence, live-cell imaging, electrophysiological recordings and RNA sequencing, we investigated the regulation of modified bacterial cellulose (mBC) nanofibril substrates on microglial properties. We demonstrate that mBC substrates induce ramified microglia with constantly extending and retracting processes, reminiscent of what is observed in vivo. Patch-clamp recordings show that microglia acquire a more negative resting membrane potential and have increased inward rectifier K+ currents, caused by an upregulation of Kir2.1 channels. Transcriptome analysis shows upregulation of genes involved in the immune response and downregulation of genes linked to cell adhesion and cell motion. Furthermore, Arp2/3 complex activation and integrin-mediated signaling modulate microglial morphology and motility. Our studies demonstrate that mBC nanofibril substrates modulate microglial phenotype, paving the way for a microglia-material interface that may be very valuable for anti-neuroinflammatory drug screening.

Mechanosensitivity is an essential component of phototransduction in vertebrate rods.

Photoreceptors are specialized cells devoted to the transduction of the incoming visual signals. Rods are able also to shed from their tip old disks and to synthesize at the base of the outer segment (OS) new disks. By combining electrophysiology, optical tweezers (OTs), and biochemistry, we investigate mechanosensitivity in the rods of Xenopus laevis, and we show that 1) mechanosensitive channels (MSCs), transient receptor potential canonical 1 (TRPC1), and Piezo1 are present in rod inner segments (ISs); 2) mechanical stimulation-of the order of 10 pN-applied briefly to either the OS or IS evokes calcium transients; 3) inhibition of MSCs decreases the duration of photoresponses to bright flashes; 4) bright flashes of light induce a rapid shortening of the OS; and 5) the genes encoding the TRPC family have an ancient association with the genes encoding families of protein involved in phototransduction. These results suggest that MSCs play an integral role in rods' phototransduction.

Rac1 Promotes Cell Motility by Controlling Cell Mechanics in Human Glioblastoma.

The failure of existing therapies in treating human glioblastoma (GBM) mostly is due to the ability of GBM to infiltrate into healthy regions of the brain; however, the relationship between cell motility and cell mechanics is not well understood. Here, we used atomic force microscopy (AFM), live-cell imaging, and biochemical tools to study the connection between motility and mechanics in human GBM cells. It was found thatRac1 inactivation by genomic silencing and inhibition with EHT 1864 reduced cell motility, inhibited cell ruffles, and disrupted the dynamics of cytoskeleton organization and cell adhesion. These changes were correlated with abnormal localization of myosin IIa and a rapid suppression of the phosphorylation of Erk1/2. At the same time, AFM measurements of the GBM cells revealed a significant increase in cell elasticity and viscosity following Rac1 inhibition. These results indicate that mechanical properties profoundly affect cell motility and may play an important role in the infiltration of GBM. It is conceivable that the mechanical characters might be used as markers for further surgical and therapeutical interventions.

Mechanisms of malignancy in glioblastoma cells are linked to mitochondrial Ca2+ uniporter upregulation and higher intracellular Ca2+ levels.

Glioblastoma (GBM) is one of the most malignant brain tumours and, despite advances in treatment modalities, it remains largely incurable. Ca2+ regulation and dynamics play crucial roles in different aspects of cancer, but they have never been investigated in detail in GBM. Here, we report that spontaneous Ca2+ waves in GBM cells cause unusual intracellular Ca2+ ([Ca2+]i) elevations (>1 µM), often propagating through tumour microtubes (TMs) connecting adjacent cells. This unusual [Ca2+]i elevation is not associated with the induction of cell death and is concomitant with overexpression of mitochondrial Ca2+ uniporter (MCU). We show that MCU silencing decreases proliferation and alters [Ca2+]i dynamics in U87 GBM cells, while MCU overexpression increases [Ca2+]i elevation in human astrocytes (HAs). These results suggest that changes in the expression level of MCU, a protein involved in intracellular Ca2+ regulation, influences GBM cell proliferation, contributing to GBM malignancy.This article has an associated First Person interview with the first author of the paper.

Electrophysiological Changes During Early Steps of Retinitis Pigmentosa.

Purpose: The rhodopsin mutation P23H is responsible for a significant portion of autosomal-dominant retinitis pigmentosa, a disorder characterized by rod photoreceptor death. The mechanisms of toxicity remain unclear; previous studies implicate destabilization of P23H rhodopsin during light exposure, causing decreased endoplasmic reticulum (ER) exit and ER stress responses. Here, we probed phototransduction in Xenopus laevis rods expressing bovine P23H rhodopsin, in which retinal degeneration is inducible by light exposure, in order to examine early physiological changes that occur during retinal degeneration. Methods: We recorded single-cell and whole-retina responses to light stimuli using electrophysiology. Moreover, we monitored morphologic changes in rods after different periods of light exposure. Results: Initially, P23H rods had almost normal photoresponses, but following a brief light exposure varying from 4 to 32 photoisomerizations per disc, photoresponses became irreversibly prolonged. In intact retinas, rods began to shed OS fragments after a rod-saturating exposure of 12 minutes, corresponding to approximately 10 to 100 times more photoisomerizations. Conclusions: Our results indicate that in P23H rods light-induced degeneration occurs in at least two stages, the first involving impairment of phototransduction and the second involving initiation of morphologic changes.

A Fully 3D Interconnected Graphene-Carbon Nanotube Web Allows the Study of Glioma Infiltration in Bioengineered 3D Cortex-Like Networks.

Currently available 3D assemblies based on carbon nanotubes (CNTs) lag far behind their 2D CNT-based bricks and require major improvements for biological applications. By using Fe nanoparticles confined to the interlamination of graphite as catalyst, a fully 3D interconnected CNT web is obtained through the pores of graphene foam (GCNT web) by in situ chemical vapor deposition. This 3D GCNT web has a thickness up to 1.5 mm and a completely geometric, mechanical and electrical interconnectivity. Dissociated cortical cells cultured inside the GCNT web form a functional 3D cortex-like network exhibiting a spontaneous electrical activity that is closer to what is observed in vivo. By coculturing and fluorescently labeling glioma and healthy cortical cells with different colors, a new in vitro model is obtained to investigate malignant glioma infiltration. This model allows the 3D trajectories and velocity distribution of individual infiltrating glioma to be reconstructed with an unprecedented precision. The model is cost effective and allows a quantitative and rigorous screening of anticancer drugs. The fully 3D interconnected GCNT web is biocompatible and is an ideal tool to study 3D biological processes in vitro representing a pivotal step toward precise and personalized medicine.

Bottom Up Ethics - Neuroenhancement in Education and Employment.

Neuroenhancement involves the use of neurotechnologies to improve cognitive, affective or behavioural functioning, where these are not judged to be clinically impaired. Questions about enhancement have become one of the key topics of neuroethics over the past decade. The current study draws on in-depth public engagement activities in ten European countries giving a bottom-up perspective on the ethics and desirability of enhancement. This informed the design of an online contrastive vignette experiment that was administered to representative samples of 1000 respondents in the ten countries and the United States. The experiment investigated how the gender of the protagonist, his or her level of performance, the efficacy of the enhancer and the mode of enhancement affected support for neuroenhancement in both educational and employment contexts. Of these, higher efficacy and lower performance were found to increase willingness to support enhancement. A series of commonly articulated claims about the individual and societal dimensions of neuroenhancement were derived from the public engagement activities. Underlying these claims, multivariate analysis identified two social values. The Societal/Protective highlights counter normative consequences and opposes the use enhancers. The Individual/Proactionary highlights opportunities and supports use. For most respondents these values are not mutually exclusive. This suggests that for many neuroenhancement is viewed simultaneously as a source of both promise and concern.

The permeation mechanism of organic cations through a CNG mimic channel.

Several channels, ranging from TRP receptors to Gap junctions, allow the exchange of small organic solute across cell membrane. However, very little is known about the molecular mechanism of their permeation. Cyclic Nucleotide Gated (CNG) channels, despite their homology with K+ channels and in contrast with them, allow the passage of larger methylated and ethylated ammonium ions like dimethylammonium (DMA) and ethylammonium (EA). We combined electrophysiology and molecular dynamics simulations to examine how DMA interacts with the pore and permeates through it. Due to the presence of hydrophobic groups, DMA enters easily in the channel and, unlike the alkali cations, does not need to cross any barrier. We also show that while the crystal structure is consistent with the presence of a single DMA ion at full occupancy, the channel is able to conduct a sizable current of DMA ions only when two ions are present inside the channel. Moreover, the second DMA ion dramatically changes the free energy landscape, destabilizing the crystallographic binding site and lowering by almost 25 kJ/mol the binding affinity between DMA and the channel. Based on the results of the simulation the experimental electron density maps can be re-interpreted with the presence of a second ion at lower occupancy. In this mechanism the flexibility of the channel plays a key role, extending the classical multi-ion permeation paradigm in which conductance is enhanced by the plain interaction between the ions.

Cell Mechanotransduction With Piconewton Forces Applied by Optical Tweezers.

Mechanical stresses are always present in the cellular environment and mechanotransduction occurs in all cells. Although many experimental approaches have been developed to investigate mechanotransduction, the physical properties of the mechanical stimulus have yet to be accurately characterized. Here, we propose a mechanical stimulation method employing an oscillatory optical trap to apply piconewton forces perpendicularly to the cell membrane, for short instants. We show that this stimulation produces membrane indentation and induces cellular calcium transients in mouse neuroblastoma NG108-15 cells dependent of the stimulus strength and the number of force pulses.

Corrigendum: Cell Mechanotransduction With Piconewton Forces Applied by Optical Tweezers.

[This corrects the article on p. 130 in vol. 12, PMID: 29867363.].