Ca2+ binding to synapsin I regulates resting Ca2+ and recovery from synaptic depression in nerve terminals.

Synapsin I (SynI) is a synaptic vesicle (SV)-associated phosphoprotein that modulates neurotransmission by controlling SV trafficking. The SynI C-domain contains a highly conserved ATP binding site mediating SynI oligomerization and SV clustering and an adjacent main Ca2+ binding site, whose physiological role is unexplored. Molecular dynamics simulations revealed that the E373K point mutation irreversibly deletes Ca2+ binding to SynI, still allowing ATP binding, but inducing a destabilization of the SynI oligomerization interface. Here, we analyzed the effects of this mutation on neurotransmitter release and short-term plasticity in excitatory and inhibitory synapses from primary hippocampal neurons. Patch-clamp recordings showed an increase in the frequency of miniature excitatory postsynaptic currents (EPSCs) that was totally occluded by exogenous Ca2+ chelators and associated with a constitutive increase in resting terminal Ca2+ concentrations. Evoked EPSC amplitude was also reduced, due to a decreased readily releasable pool (RRP) size. Moreover, in both excitatory and inhibitory synapses, we observed a marked impaired recovery from synaptic depression, associated with impaired RRP refilling and depletion of the recycling pool of SVs. Our study identifies SynI as a novel Ca2+ buffer in excitatory terminals. Blocking Ca2+ binding to SynI results in higher constitutive Ca2+ levels that increase the probability of spontaneous release and disperse SVs. This causes a decreased size of the RRP and an impaired recovery from depression due to the failure of SV reclustering after sustained high-frequency stimulation. The results indicate a physiological role of Ca2+ binding to SynI in the regulation of SV clustering and trafficking in nerve terminals.

Optofluidic Flow Cytometer with In-Plane Spherical Mirror for Signal Enhancement.

Statistical analysis of the properties of single microparticles, such as cells, bacteria or plastic slivers, has attracted increasing interest in recent years. In this regard, field flow cytometry is considered the gold standard technique, but commercially available instruments are bulky, expensive, and not suitable for use in point-of-care (PoC) testing. Microfluidic flow cytometers, on the other hand, are small, cheap and can be used for on-site analyses. However, in order to detect small particles, they require complex geometries and the aid of external optical components. To overcome these limitations, here, we present an opto-fluidic flow cytometer with an integrated 3D in-plane spherical mirror for enhanced optical signal collection. As a result, the signal-to-noise ratio is increased by a factor of six, enabling the detection of particle sizes down to 1.5 µm. The proposed optofluidic detection scheme enables the simultaneous collection of particle fluorescence and scattering using a single optical fiber, which is crucial to easily distinguishing particle populations with different optical properties. The devices have been fully characterized using fluorescent polystyrene beads of different sizes. As a proof of concept for potential real-world applications, signals from fluorescent HEK cells and Escherichia coli bacteria were analyzed.

The epilepsy-associated protein TBC1D24 is required for normal development, survival and vesicle trafficking in mammalian neurons.

Mutations in the Tre2/Bub2/Cdc16 (TBC)1 domain family member 24 (TBC1D24) gene are associated with a range of inherited neurological disorders, from drug-refractory lethal epileptic encephalopathy and DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation, seizures) to non-syndromic hearing loss. TBC1D24 has been implicated in neuronal transmission and maturation, although the molecular function of the gene and the cause of the apparently complex disease spectrum remain unclear. Importantly, heterozygous TBC1D24 mutation carriers have also been reported with seizures, suggesting that haploinsufficiency for TBC1D24 is significant clinically. Here we have systematically investigated an allelic series of disease-associated mutations in neurons alongside a new mouse model to investigate the consequences of TBC1D24 haploinsufficiency to mammalian neurodevelopment and synaptic physiology. The cellular studies reveal that disease-causing mutations that disrupt either of the conserved protein domains in TBC1D24 are implicated in neuronal development and survival and are likely acting as loss-of-function alleles. We then further investigated TBC1D24 haploinsufficiency in vivo and demonstrate that TBC1D24 is also crucial for normal presynaptic function: genetic disruption of Tbc1d24 expression in the mouse leads to an impairment of endocytosis and an enlarged endosomal compartment in neurons with a decrease in spontaneous neurotransmission. These data reveal the essential role for TBC1D24 at the mammalian synapse and help to define common synaptic mechanisms that could underlie the varied effects of TBC1D24 mutations in neurological disease.

A pH-sensitive closed-loop nanomachine to control hyperexcitability at the single neuron level.

Epilepsy affects 1% of the general population and 30% of patients are resistant to antiepileptic drugs. Although optogenetics is an efficient antiepileptic strategy, the difficulty of illuminating deep brain areas poses translational challenges. Thus, the search of alternative light sources is strongly needed. Here, we develop pH-sensitive inhibitory luminopsin (pHIL), a closed-loop chemo-optogenetic nanomachine composed of a luciferase-based light generator, a fluorescent sensor of intracellular pH (E2GFP), and an optogenetic actuator (halorhodopsin) for silencing neuronal activity. Stimulated by coelenterazine, pHIL experiences bioluminescence resonance energy transfer between luciferase and E2GFP which, under conditions of acidic pH, activates halorhodopsin. In primary neurons, pHIL senses the intracellular pH drop associated with hyperactivity and optogenetically aborts paroxysmal activity elicited by the administration of convulsants. The expression of pHIL in hippocampal pyramidal neurons is effective in decreasing duration and increasing latency of pilocarpine-induced tonic-clonic seizures upon in vivo coelenterazine administration, without affecting higher brain functions. The same treatment is effective in markedly decreasing seizure manifestations in a murine model of genetic epilepsy. The results indicate that pHIL represents a potentially promising closed-loop chemo-optogenetic strategy to treat drug-refractory epilepsy.

Nanoactuator for Neuronal Optoporation.

Light-driven modulation of neuronal activity at high spatial-temporal resolution is becoming of high interest in neuroscience. In addition to optogenetics, nongenetic membrane-targeted nanomachines that alter the electrical state of the neuronal membranes are in demand. Here, we engineered and characterized a photoswitchable conjugated compound (BV-1) that spontaneously partitions into the neuronal membrane and undergoes a charge transfer upon light stimulation. The activity of primary neurons is not affected in the dark, whereas millisecond light pulses of cyan light induce a progressive decrease in membrane resistance and an increase in inward current matched to a progressive depolarization and action potential firing. We found that illumination of BV-1 induces oxidation of membrane phospholipids, which is necessary for the electrophysiological effects and is associated with decreased membrane tension and increased membrane fluidity. Time-resolved atomic force microscopy and molecular dynamics simulations performed on planar lipid bilayers revealed that the underlying mechanism is a light-driven formation of pore-like structures across the plasma membrane. Such a phenomenon decreases membrane resistance and increases permeability to monovalent cations, namely, Na+, mimicking the effects of antifungal polyenes. The same effect on membrane resistance was also observed in nonexcitable cells. When sustained light stimulations are applied, neuronal swelling and death occur. The light-controlled pore-forming properties of BV-1 allow performing "on-demand" light-induced membrane poration to rapidly shift from cell-attached to perforated whole-cell patch-clamp configuration. Administration of BV-1 to ex vivo retinal explants or in vivo primary visual cortex elicited neuronal firing in response to short trains of light stimuli, followed by activity silencing upon prolonged light stimulations. BV-1 represents a versatile molecular nanomachine whose properties can be exploited to induce either photostimulation or space-specific cell death, depending on the pattern and duration of light stimulation.

Modulation of Mechanosensitive Potassium Channels by a Membrane-targeted Nongenetic Photoswitch.

Mechanosensitive ion channels are present in the plasma membranes of all cells. They play a fundamental role in converting mechanical stimuli into biochemical signals and are involved in several physiological processes such as touch sensation, hearing, and blood pressure regulation. This protein family includes TWIK-related arachidonic acid-stimulated K+ channel (TRAAK), which is specifically implicated in the maintenance of the resting membrane potential and in the regulation of a variety of important neurobiological functions. Dysregulation of these channels has been linked to various diseases, including blindness, epilepsy, cardiac arrhythmia, and chronic pain. For these reasons, mechanosensitive channels are targets for the treatment of several diseases. Here, we propose a new approach to investigate TRAAK ion channel modulation that is based on nongenetic photostimulation. We employed an amphiphilic azobenzene, named Ziapin2. In the dark, Ziapin2 preferentially dwells in the plasma membrane, causing a thinning of the membrane. Upon light irradiation, an isomerization occurs, breaking the dimers and inducing membrane relaxation. To study the effect of Ziapin2 on the mechanosensitive channels, we expressed human TRAAK (hTRAAK) channels in HEK293T cells. We observed that Ziapin2 insertion in the membrane is able per se to recruit hTRAAK, permitting the exit of K+ ions outside the cells with a consequent hyperpolarization of the cell membrane. During light stimulation, membrane relaxation induces hTRAAK closure, generating a consistent and compensatory depolarization. These results add information to the Ziapin2 mechanism and suggest that membrane deformation can be a tool for the nonselective modulation of mechanosensitive channels.

A membrane intercalating metal-free conjugated organic photosensitizer for bacterial photodynamic inactivation.

Photodynamic inhibition (PDI) of bacteria represents a powerful strategy for dealing with multidrug-resistant pathogens and infections, as it exhibits minimal development of antibiotic resistance. The PDI action stems from the generation of a triplet state in the photosensitizer (PS), which subsequently transfers energy or electrons to molecular oxygen, resulting in the formation of reactive oxygen species (ROS). These ROS are then able to damage cells, eventually causing bacterial eradication. Enhancing the efficacy of PDI includes the introduction of heavy atoms to augment triplet generation in the PS, as well as membrane intercalation to circumvent the problem of the short lifetime of ROS. However, the former approach can pose safety and environmental concerns, while achieving stable membrane partitioning remains challenging due to the complex outer envelope of bacteria. Here, we introduce a novel PS, consisting of a metal-free donor-acceptor thiophene-based conjugate molecule (BV-1). It presents several advantageous features for achieving effective PDI, namely: (i) it exhibits strong light absorption due to the conjugated donor-acceptor moieties; (ii) it exhibits spontaneous and stable membrane partitioning thanks to its amphiphilicity, accompanied by a strong fluorescence turn-on; (iii) it undergoes metal-free intersystem crossing, which occurs preferentially when the molecule resides in the membrane. All these properties, which we rationalized via optical spectroscopies and calculations, enable the effective eradication of Escherichia coli, with an inhibition concentration that is below that of current state-of-the-art treatments. Our approach holds significant potential for the development of new PS for controlling bacterial infections, particularly those caused by Gram-negative bacteria.

Membrane Order Effect on the Photoresponse of an Organic Transducer.

Non-genetic photostimulation, which allows for control over cellular activity via the use of cell-targeting phototransducers, is widely used nowadays to study and modulate/restore biological functions. This approach relies on non-covalent interactions between the phototransducer and the cell membrane, thus implying that cell conditions and membrane status can dictate the effectiveness of the method. For instance, although immortalized cell lines are traditionally used in photostimulation experiments, it has been demonstrated that the number of passages they undergo is correlated to the worsening of cell conditions. In principle, this could impact cell responsivity against exogenous stressors, including photostimulation. However, these aspects have usually been neglected in previous experiments. In this work, we investigated whether cell passages could affect membrane properties (such as polarity and fluidity). We applied optical spectroscopy and electrophysiological measurements in two different biological models: (i) an epithelial immortalized cell line (HEK-293T cells) and (ii) liposomes. Different numbers of cell passages were compared to a different morphology in the liposome membrane. We demonstrated that cell membranes show a significant decrease in ordered domains upon increasing the passage number. Furthermore, we observed that cell responsivity against external stressors is markedly different between aged and non-aged cells. Firstly, we noted that the thermal-disordering effect that is usually observed in membranes is more evident in aged cells than in non-aged ones. We then set up a photostimulation experiment by using a membrane-targeted azobenzene as a phototransducer (Ziapin2). As an example of a functional consequence of such a condition, we showed that the rate of isomerization of an intramembrane molecular transducer is significantly impaired in aged cells. The reduction in the photoisomerization rate translates in cells with a sustained reduction of the Ziapin2-related hyperpolarization of the membrane potential and an overall increase in the molecule fluorescence. Overall, our results suggest that membrane stimulation strongly depends on membrane order, highlighting the importance of cell passage during the characterization of the stimulation tools. This study can shine light on the correlation between aging and the development of diseases driven by membrane degradation as well as on the different cell responsivities against external stressors, such as temperature and photostimulation.

Skeletal muscle cells opto-stimulation by intramembrane molecular transducers.

Optical stimulation and control of muscle cell contraction opens up a number of interesting applications in hybrid robotic and medicine. Here we show that recently designed molecular phototransducer can be used to stimulate C2C12 skeletal muscle cells, properly grown to exhibit collective behaviour. C2C12 is a skeletal muscle cell line that does not require animal sacrifice Furthermore, it is an ideal cell model for evaluating the phototransducer pacing ability due to its negligible spontaneous activity. We study the stimulation process and analyse the distribution of responses in multinuclear cells, in particular looking at the consistency between stimulus and contraction. Contractions are detected by using an imaging software for object recognition. We find a deterministic response to light stimuli, yet with a certain distribution of erratic behaviour that is quantified and correlated to light intensity or stimulation frequency. Finally, we compare our optical stimulation with electrical stimulation showing advantages of the optical approach, like the reduced cell stress.

Insight on the Intracellular Supramolecular Assembly of DTTO: A Peculiar Example of Cell-Driven Polymorphism.

The assembly of supramolecular structures within living systems is an innovative approach for introducing artificial constructs and developing biomaterials capable of influencing and/or regulating the biological responses of living organisms. By integrating chemical, photophysical, morphological, and structural characterizations, it is shown that the cell-driven assembly of 2,6-diphenyl-3,5-dimethyl-dithieno[3,2-b:2',3'-d]thiophene-4,4-dioxide (DTTO) molecules into fibers results in the formation of a "biologically assisted" polymorphic form, hence the term bio-polymorph. Indeed, X-ray diffraction reveals that cell-grown DTTO fibers present a unique molecular packing leading to specific morphological, optical, and electrical properties. Monitoring the process of fiber formation in cells with time-resolved photoluminescence, it is established that cellular machinery is necessary for fiber production and a non-classical nucleation mechanism for their growth is postulated. These biomaterials may have disruptive applications in the stimulation and sense of living cells, but more crucially, the study of their genesis and properties broadens the understanding of life beyond the native components of cells.

Resonant Enhancement of Polymer-Cell Optostimulation by a Plasmonic Metasurface.

Organic semiconductors have shown great potential as efficient bioelectronic materials. Specifically, photovoltaic polymers such as the workhorse poly(thiophene) derivatives, when stimulated with visible light, can depolarize neurons and generate action potentials, an effect that has been also employed for rescuing vision in blind rats. In this context, however, the coupling of such materials with optically resonant structures to enhance those photodriven biological effects is still in its infancy. Here, we employ the optical coupling between a nanostructured metasurface and poly(3-hexylthiophene) (P3HT) to improve the bioelectronic effects occurring upon photostimulation at the abiotic-biotic interface. In particular, we designed a spectrally tuned aluminum metasurface that can resonate with P3HT, hence augmenting the effective field experienced by the polymer. In turn, this leads to an 8-fold increase in invoked inward current in cells. This enhanced activation strategy could be useful to increase the effectiveness of P3HT-based prosthetic implants for degenerative retinal disorders.

Interactions Between 2D Materials and Living Matter: A Review on Graphene and Hexagonal Boron Nitride Coatings.

Two-dimensional material (2DM) coatings exhibit complex and controversial interactions with biological matter, having shown in different contexts to induce bacterial cell death and contribute to mammalian cell growth and proliferation in vitro and tissue differentiation in vivo. Although several reports indicate that the morphologic and electronic properties of the coating, as well as its surface features (e.g., crystallinity, wettability, and chemistry), play a key role in the biological interaction, these kinds of interactions have not been fully understood yet. In this review, we report and classify the cellular interaction mechanisms observed in graphene and hexagonal boron nitride (hBN) coatings. Graphene and hBN were chosen as study materials to gauge the effect of two atomic-thick coatings with analogous lattice structure yet dissimilar electrical properties upon contact with living matter, allowing to discern among the observed effects and link them to specific material properties. In our analysis, we also considered the influence of crystallinity and surface roughness, detailing the mechanisms of interaction that make specific coatings of these 2DMs either hostile toward bacterial cells or innocuous for mammalian cells. In doing this, we discriminate among the material and surface properties, which are often strictly connected to the 2DM production technique, coating deposition and post-processing method. Building on this knowledge, the selection of 2DM coatings based on their specific characteristics will allow to engineer desired functionalities and devices. Antibacterial coatings to prevent biofouling, biocompatible platforms suitable for biomedical applications (e.g., wound healing, tissue repairing and regeneration, and novel biosensing devices) could be realized in the next future. Overall, a clear understanding on how the 2DM coating's properties may modulate a specific bacterial or cellular response is crucial for any future innovation in the field.

Graphene Nanoplatelets Render Poly(3-Hydroxybutyrate) a Suitable Scaffold to Promote Neuronal Network Development.

The use of composite biomaterials as innovative bio-friendly neuronal interfaces has been poorly developed so far. Smart strategies to target neuro-pathologies are currently exploiting the mixed and complementary characteristics of composite materials to better design future neural interfaces. Here we present a polymer-based scaffold that has been rendered suitable for primary neurons by embedding graphene nanoplatelets (GnP). In particular, the growth, network formation, and functionality of primary neurons on poly(3-hydroxybutyrate) [P(3HB)] polymer supports functionalized with various concentrations of GnP were explored. After growing primary cortical neurons onto the supports for 14 days, all specimens were found to be biocompatible, revealing physiological growth and maturation of the neuronal network. When network functionality was investigated by whole patch-clamp measurements, pure P(3HB) led to changes in the action potential waveform and reduction in firing frequency, resulting in decreased neuronal excitability. However, the addition of GnP to the polymer matrix restored the electrophysiological parameters to physiological values. Interestingly, a low concentration of graphene was able to promote firing activity at a low level of injected current. The results indicate that the P(3HB)/GnP composites show great potential for electrical interfacing with primary neurons to eventually target central nervous system disorders.

Neuroinflammation induces synaptic scaling through IL-1ß-mediated activation of the transcriptional repressor REST/NRSF.

Neuroinflammation is associated with synapse dysfunction and cognitive decline in patients and animal models. One candidate for translating the inflammatory stress into structural and functional changes in neural networks is the transcriptional repressor RE1-silencing transcription factor (REST) that regulates the expression of a wide cluster of neuron-specific genes during neurogenesis and in mature neurons. To study the cellular and molecular pathways activated under inflammatory conditions mimicking the experimental autoimmune encephalomyelitis (EAE) environment, we analyzed REST activity in neuroblastoma cells and mouse cortical neurons treated with activated T cell or microglia supernatant and distinct pro-inflammatory cytokines. We found that REST is activated by a variety of neuroinflammatory stimuli in both neuroblastoma cells and primary neurons, indicating that a vast transcriptional change is triggered during neuroinflammation. While a dual activation of REST and its dominant-negative splicing isoform REST4 was observed in N2a neuroblastoma cells, primary neurons responded with a pure full-length REST upregulation in the absence of changes in REST4 expression. In both cases, REST upregulation was associated with activation of Wnt signaling and increased nuclear translocation of ß-catenin, a well-known intracellular transduction pathway in neuroinflammation. Among single cytokines, IL-1ß caused a potent and prompt increase in REST transcription and translation in neurons, which promoted a delayed and strong synaptic downscaling specific for excitatory synapses, with decreased frequency and amplitude of spontaneous synaptic currents, decreased density of excitatory synaptic connections, and decreased frequency of action potential-evoked Ca2+ transients. Most important, the IL-1ß effects on excitatory transmission were strictly REST dependent, as conditional deletion of REST completely occluded the effects of IL-1ß activation on synaptic transmission and network excitability. Our results demonstrate that REST upregulation represents a new pathogenic mechanism for the synaptic dysfunctions observed under neuroinflammatory conditions and identify the REST pathway as therapeutic target for EAE and, potentially, for multiple sclerosis.

Hydrogenated Graphene Improves Neuronal Network Maturation and Excitatory Transmission.

Graphene is regarded as a viable bio-interface for neuroscience due to its biocompatibility and electrical conductivity, which would contribute to efficient neuronal network signaling. Here, monolayer graphene grown via chemical vapor deposition is treated with remote hydrogen plasma to demonstrate that hydrogenated graphene (HGr) fosters improved cell-to-cell communication with respect to pristine graphene in primary cortical neurons. When transferred to polyethylene terephthalate, HGr exhibits higher wettability than graphene (water contact angle of 83.7° vs 40.7°), while preserving electrical conductivity (≈3 kΩ â–¡-1 ). A rich and mature network is observed to develop onto HGr. The intrinsic excitability and firing properties of neurons plated onto HGr appears unaltered, while the basic passive and active membrane properties are fully preserved. The formation of excitatory synaptic connections increases in HGr with respect to pristine graphene, leading to a doubled miniature excitatory postsynaptic current frequency. This study supports the use of hydrogenation for tailoring graphene into an improved neuronal interface, indicating that wettability, more than electrical conductivity, is the key parameter to be controlled. The use of HGr can bring about a deeper understanding of neuronal behavior on artificial bio-interfaces and provide new insight for graphene-based biomedical applications.