Voltage-driven polyelectrolyte complexation inside a nanopore

We have investigated how a pair of oppositely charged macromolecules can be driven by an electric field to form a polyelectrolyte complex inside a nanopore. To observe and isolate an individual complex pair, a model protein nanopore, embedded in artificial phospholipid membrane, allowing compartmentalization (cis/trans) is employed. A polyanion in the cis and a polycation in the trans compartments are subjected to electrophoretic capture by the pore. We find that the measured ionic current across the pore has a distinguishable signature of complex formation, which is different from the signature of the passage of individual molecules through the pore. The ionic current signature allows us to detect the interaction between the two oppositely charged macromolecules and thus, enables us to measure the lifetime of the complex inside the nanopore. After showing that we can isolate a complex pair in the nanopore, we studied the effects of molecular identity on the nature of interaction in different complex pairs. In contrast to the irreversible conductance state of the alpha hemolysin (alpha HL) channel in the complexation of poly-styrene sulfonate (PSS) and poly L lysine (PLL), a reversible conductance state is observed during complexation between single stranded DNA (ssDNA) and PLL. This suggests that there is a weak interaction between ssDNA and PLL, when compared to the interaction in a PSS PLL complex. Analysis of the PSS-PLL complexation events and its lifetime inside the nanopore supports a four step mechanism: (i) The polyanion is captured by the pore, (ii) the polyanion starts threading through the pore. (iii) The polycation is captured, a complex pair is formed in the pore, and the polyanion slides along the polycation. (iv) The complex pair can be pulled through the pore into the trans compartment or it can dissociate. Additionally, we have developed a simple theoretical model, which describes the lifetime of the complex inside the pore. The observed reversible two-state conductance across alpha HL channel during ssDNA PLL complexation, is described as the binding/unbinding of PLL during the translocation of ssDNA. This enables us to evaluate the apparent rate constants for association/dissociation and equilibrium dissociation constants for the interaction of PLL with ssDNA. This work throws light on the behavior of polyelectrolyte complexes in an electric field and enhances our understanding of the electrical aspects of inter-macromolecular interactions, which plays an extremely important role in the organization of macromolecules in the crowded and confined cellular environment.

Optoelectronic synapse using monolayer MoS2 field effect transistors

Optical data sensing, processing and visual memory are fundamental requirements for artificial intelligence and robotics with autonomous navigation. Traditionally, imaging has been kept separate from the pattern recognition circuitry. Optoelectronic synapses hold the special potential of integrating these two fields into a single layer, where a single device can record optical data, convert it into a conductance state and store it for learning and pattern recognition, similar to the optic nerve in human eye. In this work, the trapping and de-trapping of photogenerated carriers in the MoS2/SiO2 interface of a n-channel MoS2 transistor was employed to emulate the optoelectronic synapse characteristics. The monolayer MoS2 field effect transistor (FET) exhibits photo-induced short-term and long-term potentiation, electrically driven long-term depression, paired pulse facilitation (PPF), spike time dependent plasticity, which are necessary synaptic characteristics. Moreover, the device’s ability to retain its conductance state can be modulated by the gate voltage, making the device behave as a photodetector for positive gate voltages and an optoelectronic synapse at negative gate voltages.

Network synchronization and synchrony propagation: emergent elements of inspiration

SUMMARYThe preBötzinger Complex (preBötC) – the kernel of breathing rhythmogenesis in mammals – is a non-canonical central pattern generator with undetermined mechanisms. We assessed preBötC network dynamics under respiratory rhythmic and nonrhythmic conditions in vitro. In each cycle under rhythmic conditions, an inspiratory burst emerges as (presumptive) preBötC rhythmogenic neurons transition from aperiodic uncorrelated population spike activity to become increasingly synchronized during preinspiration, triggering bursts; burst activity subsides and the cycle repeats. In a brainstem slice in nonrhythmic conditions, antagonizing GABAA receptors can initiate this periodic synchronization and consequent rhythm coincident with inducing a higher conductance state in nonrhythmogenic preBötC output neurons. Furthermore, when input synchrony onto these neurons was weak, preBötC activity failed to propagate to motor nerves. Our analyses uncover a dynamic reorganization of preBötC network activity – underpinning intricate cyclic neuronal interactions leading to network synchronization and its efficient propagation – correlated with and, we postulate, essential to, rhythmicity.

Synaptic plasticity through activation of GluA3-containing AMPA-receptors

Excitatory synaptic transmission is mediated by AMPA-type glutamate receptors (AMPARs). In CA1 pyramidal neurons of the hippocampus two types of AMPARs predominate: those that contain subunits GluA1 and GluA2 (GluA1/2), and those that contain GluA2 and GluA3 (GluA2/3). Whereas subunits GluA1 and GluA2 have been extensively studied, the contribution of GluA3 to synapse physiology has remained unclear. Here we show in mice that GluA2/3s are in a low-conductance state under basal conditions, and although present at synapses they contribute little to synaptic currents. When intracellular cyclic AMP (cAMP) levels rise, GluA2/3 channels shift to a high-conductance state, leading to synaptic potentiation. This cAMP-driven synaptic potentiation requires the activation of both protein kinase A (PKA) and the GTPase Ras, and is induced upon the activation of β-adrenergic receptors. Together, these experiments reveal a novel type of plasticity at CA1 hippocampal synapses that is expressed by the activation of GluA3-containing AMPARs.