Non-photonic sensing of membrane-delimited reactive species with a Na+ channel protein containing selenocysteine

Selenomethionine mis-incorporation and redox-dependent voltage-gated sodium channel gain of function

Selenomethionine (SeMet) readily replaces methionine (Met) residues in proteins during translation. Long-term dietary SeMet intake results in the accumulation of the amino acid in tissue proteins. Despite the high rates of SeMet incorporation in proteins and its stronger susceptibility to oxidation compared to Met, little is known about the effect of SeMet mis-incorporation on electrical excitability and ion channels. Fast inactivation of voltage-gated sodium (NaV ) channels is essential for exact action potential shaping with even minute impairment of inactivation resulting in a plethora of adverse phenotypes. Met oxidation of the NaV channel inactivation motif (Ile-Phe-Met) and further Met residues causes a marked loss of inactivation. Here, we examined the impact of SeMet mis-incorporation on the function of NaV channels. While extensive SeMet incorporation into recombinant rat NaV 1.4 channels preserved their normal function, it greatly sensitized the channels to mild oxidative stress, resulting in loss of inactivation and diminished maximal current, both reversible by dithiothreitol-induced reduction. SeMet incorporation similarly affected human NaV 1.4, NaV 1.2, NaV 1.5, and NaV 1.7. In mouse dorsal root ganglia (DRG) neurons, 1 day of SeMet exposure exacerbated the oxidation-mediated broadening of action potentials. SeMet-treated DRGs also exhibited a stronger increase in the persistent NaV current in response to oxidation. SeMet incorporation in NaV proteins coinciding with oxidative insults may therefore result in hyperexcitability pathologies, such as cardiac arrhythmias and neuropathies, like congenital NaV channel gain-of-function mutations.

Continuous live cell imaging using dark field microscopy

The optical observation of live cell behavior is critical for biological and biomedical studies. The development of techniques for long-term cell and tissue imaging is, however, hindered by phototoxicity induced by excited fluorophores. We, herein, propose a methodology to capture live cell behavior using dark field microscopy (DM). Since the light intensity of DM is merely ∼0.1% of bright-field microscopy (BM) and ∼0.5% of fluorescence microscopy (FM), it allows super long and frequent live cell imaging. Our results demonstrate that continuous exposure to DM light for 48 h brings about no observable effect on the growth rate of 3T3 fibroblasts and HepG2 hepatoma cells, indicating minimum photo-toxicity. Moreover, DM images show contrast comparable to FM, which does not depend on the probes and staining efficiency. We, therefore, conclude that the proposed approach is suitable for long-term live cell imaging with super-high temporal resolution.

Surface vs. core N/S/Se-heteroatom doping of carbon nanodots produces divergent yet consistent optical responses to reactive oxygen species

Carbon nanodots (CNDs) have attracted substantial scientific curiosity because of their intriguing stimuli-responsive optical properties. However, one obstacle to the more widespread use of CNDs as transducers for e.g., biodetection systems is incomplete knowledge regarding the underlying chemical changes responsible for this responsiveness, and how these chemical features can be engineered via the precursors chosen for CND synthesis. This study demonstrates that the precursor's functional groups play a key role in directing N/S/Se heteroatom dopants either towards the surface of the CNDs, towards the aromatic core, or towards small organic fluorophores in the core. Divergent optical properties, which were consistent amongst groups of CNDs prepared with similar precursors, were obtained including either a decrease or increase of fluorescence intensity in the presence of hydrogen peroxide. Moreover, CNDs were identified with orthogonal responsiveness to radical (hydroxyl radicals, ˙OH; down to 2.5 μM) vs. non-radical oxidants (H₂O₂; down to 50 μM), which suggests that control of the chemistry of CNDs via the choice of precursor could yield probes that are specific to certain sub-species of reactive oxygen species or entirely different molecules altogether, based on the way they chemically-modify the surface (respond faster) and core functional groups (respond slower) associated with chromophores/fluorophores of which the CNDs are composed.

Membrane potential manipulation with visible flash lamp illumination of targeted microbeads

The electrical membrane potential (V_m) is a key dynamical variable of excitable membranes. Despite the tremendous success of optogenetic methods to modulate V_m with light, there are some shortcomings, such as the need of genetic manipulation and limited time resolution. Direct optical stimulation of gold nanoparticles targeted to cells is an attractive alternative because the absorbed energy heats the membrane and, thus, generates capacitive current sufficient to trigger action potentials [1, Carvalho-de-Souza et al., 2015]. However, focused laser light is required and precise location and binding of the nanoparticles cannot be assessed with a conventional microscope. We therefore examined a complementary method to manipulate V_m in a spatio-temporal fashion by non-focused visible flashlight stimulation (Xenon discharge lamp, 385-485 nm, ∼500 μs) of superparamagnetic microbeads. Flashlight stimulation of single beads targeted to cells resulted in transient inward currents under whole-cell patch-clamp control. The waveform of the current reflected the first time derivative of the local temperature induced by the absorbed light and subsequent heat dissipation. The maximal peak current as well as the temperature excursion scaled with the proximity to the plasma membrane. Transient illumination of light-absorbing beads, targeted to specific cellular sites via protein-protein interaction or direct micromanipulation, may provide means of rapid and spatially confined heating and electrical cell stimulation.