Light-triggered modulation of cellular electrical activity by ruthenium diimine nanoswitches

Cell Networks in Endocrine/Neuroendocrine Gland Function

Reproduction, growth, stress, and metabolism are determined by endocrine/neuroendocrine systems that regulate circulating hormone concentrations. All these systems generate rhythms and changes in hormone pulsatility observed in a variety of pathophysiological states. Thus, the output of endocrine/neuroendocrine systems must be regulated within a narrow window of effective hormone concentrations but must also maintain a capacity for plasticity to respond to changing physiological demands. Remarkably most endocrinologists still have a "textbook" view of endocrine gland organization which has emanated from 20^th century histological studies on thin 2D tissue sections. However, 21^st -century technological advances, including in-depth 3D imaging of specific cell types have vastly changed our knowledge. We now know that various levels of multicellular organization can be found across different glands, that organizational motifs can vary between species and can be modified to enhance or decrease hormonal release. This article focuses on how the organization of cells regulates hormone output using three endocrine/neuroendocrine glands that present different levels of organization and complexity: the adrenal medulla, with a single neuroendocrine cell type; the anterior pituitary, with multiple intermingled cell types; and the pancreas with multiple intermingled cell types organized into distinct functional units. We give an overview of recent methodologies that allow the study of the different components within endocrine systems, particularly their temporal and spatial relationships. We believe the emerging findings about network organization, and its impact on hormone secretion, are crucial to understanding how homeostatic regulation of endocrine axes is carried out within endocrine organs themselves. © 2022 American Physiological Society. Compr Physiol 12:3371-3415, 2022.

Synthesis of unsymmetric perylenediimide dye molecule and its photochemical properties on lipid membrane

Optical manipulation of cellular function is one of the important targets in chemical biology and medicine. To achieve manipulation of cellular function using small molecules, photochemical reaction, such as photo-isomerization and photo-induced electron transfer, is one of the most promising reactions. Especially, photo-induced electron transfer process may be the crucial for their further development of photo-functional agents in living cells. However, such molecules, which enable the modification of cellular function, are limited and the further development is necessary. Herein, we synthesized a novel unsymmetric perylenediimide dye and investigated the cellular staining upon the addition in the cell culture medium. Furthermore, we observed the fluorescence quenching upon the addition of ascorbic acid as electron donor and report the preliminary results to manipulate Ca²⁺ concentration in living cell line upon 488-nm light photoirradiation.

Tunable Membrane Potential Reconstituted in Giant Vesicles Promotes Permeation of Cationic Peptides at Nanomolar Concentrations

We investigate the influence of membrane potential on the permeation of cationic peptides. Therefore, we employ a microfluidic chip capable of capturing giant unilamellar vesicles (GUVs) in physical traps and fast exchange of chemical compounds. Control experiments with calcein proved that the vesicle membranes' integrity is not affected by the physical traps and applied shear forces. Combined with fluorescence correlation spectroscopy, permeation of fluorescently labeled peptides across vesicle membranes can be measured down to the nanomolar level. With the addition of a lipophilic ruthenium(II) complex Ru(C17)22+, GUVs consisting of mixed acyl phospholipids are prepared with a negative membrane potential, resembling the membrane asymmetry in cells. The membrane potential serves as a driving force for the permeation of cationic cell-penetrating peptides (CPPs) nonaarginine (Arg9) and the human immunodeficiency virus trans-activator of transcription (TAT) peptide already at nanomolar doses. Hyperpolarization of the membrane by photo-oxidation of Ru(C17)22+ enhances permeation significantly from 55 to 78% for Arg9. This specific enhancement for Arg9 (cf. TAT) is ascribed to the higher affinity of the arginines to the phosphoserine head groups. On the other hand, permeation is decreased by introducing an additional negative charge in close proximity to the N-terminal arginine residue when changing the fluorophore. In short, with the capability to reconstitute membrane potential as well as shear stress, our system is a suitable platform for modeling the membrane permeability of pharmaceutics candidates. The results also highlight the membrane potential as a major cause of discrepancies between vesicular and cellular studies on CPP permeation.

Nongenetic Optical Methods for Measuring and Modulating Neuronal Response

The ability to probe and modulate electrical signals sensitively at cellular length scales is a key challenge in the field of electrophysiology. Electrical signals play integral roles in regulating cellular behavior and in controlling biological function. From cardiac arrhythmias to neurodegenerative disorders, maladaptive phenotypes in electrophysiology can result in serious and potentially deadly medical conditions. Understanding how to monitor and to control these behaviors precisely and noninvasively represents an important step in developing next-generation therapeutic devices. As we develop a deeper understanding of neural network formation, electrophysiology has the potential to offer fundamental insights into the inner working of the brain. In this Perspective, we explore traditional methods for examining neural function, discuss recent genetic advances in electrophysiology, and then focus on the latest innovations in optical sensing and stimulation of action potentials in neurons. We emphasize nongenetic optical methods, as these provide high spatiotemporal resolution and can be achieved with minimal invasiveness.

Exciton Localization on Ru-Based Photosensitizers Induced by Binding to Lipid Membranes

The characterization of electronic properties of metal complexes embedded in membrane environments is of paramount importance to develop efficient photosensitizers in optogenetic applications. Molecular dynamics and QM/MM simulations together with quantitative wave function analysis reveal a directional electronic redistribution of the exciton formed upon excitation of [Ru(bpy)₂(bpy-C17)]²⁺ when going from water to a lipid bilayer, despite the fact that the media influence neither the metal-to-ligand charge-transfer character nor the excitation energy of the absorption spectra. When the photosensitizer is embedded into the DOPC lipid membrane, exciton population is mainly located in the bypyridyl sites proximal to the positively charged surface of the bilayer due to electrostatic interactions. This behavior shows that the electronic structure of metal complexes can be controlled through the binding to external species, underscoring the crucial role of the environment in directing the electronic flow upon excitation and thus helping rational tuning of optogenetic agents.

A One Donor-Two Acceptor Lipid Bilayer FRET Assay Based on Asymmetrically Labeled Liposomes

The fusion of two opposing membranes is essential in biological functions such as fertilization, viral entry, membrane trafficking and synaptic transmission. Before the membrane bilayers are fully connected, at some stage a hemifusion intermediate-when the outer leaflets are merged but not the inner leaflets-is formed. However, the position of hemifusion in the energy landscape and the duration of it vary and have not been fully mapped out. To date, there has not been a way to differentiate lipid mixing of the two leaflets directly in a single experiment. Herein we demonstrate labeling of the outer and inner leaflets with different fluorophores, which can be distinguished by their fluorescence lifetimes. As a proof of concept, the asymmetrically labeled liposomes were used as acceptor liposomes in a novel one donor-two acceptor Förster resonance energy transfer (FRET) assay to monitor membrane fusion reactions mediated by the synaptic proteins soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) in microfluidic devices. Initial hemifusion was clearly indicated by the acceptor fluorescence lifetime originating solely from FRET acceptors on the outer leaflet (Oregon Green 488; τ_Fl ∼ 4.8 ns). Progression to full fusion was then indicated by the significantly increasing lifetime contribution from acceptors on the inner leaflet (nitrobenzoxadiazole; τ_Fl ∼ 6.7 ns). The new labeling strategy creates many possibilities in the design of bulk and single-molecule experiments.

Synthetically tuneable biomimetic artificial photosynthetic reaction centres that closely resemble the natural system in purple bacteria

Porphyrin-based photosynthetic reaction centre (PRC) mimics, ZnPQ-Q2HP-C60 and MP2Q-Q2HP-C60 (M = Zn or 2H), designed to have a similar special-pair electron donor and similar charge-separation distances, redox processes and photochemical reaction rates to those in the natural PRC from purple bacteria, have been synthesised and extensive photochemical studies performed. Mechanisms of electron-transfer reactions are fully investigated using femtosecond and nanosecond transient absorption spectroscopy. In benzonitrile, all models show picosecond-timescale charge-separations and the final singlet charge-separations with the microsecond-timescale. The established lifetimes are long compared to other processes in organic solar cells or other organic light harvesting systems. These rigid, synthetically flexible molecules provide the closest mimics to the natural PRC so far synthesised and present a future direction for the design of light harvesters with controllable absorption, redox, and kinetics properties.

Gram Scale Synthesis of Benzophenanthroline and Its Blue Phosphorescent Platinum Complex

The design, synthesis, and characterization of 12-phenylbenzo[f][1,7]phenanthroline, Bzp, is reported. Its use as a fluorine-free ligand for sky blue phosphorescence is demonstrated in a cyclometalated platinum complex, BzpPtDpm. BzpPtDpm phosphoresces at the same wavelength as its analogous 4,6-difluorophenylpyridine complex at both room temperature (466 nm) and 77 K (458 nm). Finally, production of a conformationally restricted derivative of BzpPtDpm with greatly increased quantum yield (46%) validates the versatility of the synthetic route.

Optical control of neuronal firing via photoinduced electron transfer in donor-acceptor conjugates

A rationally designed donor–acceptor conjugate efficiently generates a photoinduced charge-separated state in a cellular environment, achieving photoinduction of neuronal firing.

A series of porphyrin–fullerene linked molecules has been synthesized to evaluate the effects of substituents and molecular structures on their charge-separation yield and the lifetime of a final charge-separated state in various hydrophilic environments. The selected high-performance molecule effectively achieved depolarization in a plasma cell membrane by visible light as well as two-photon excitation using a near-infrared light laser. Moreover, it was revealed that the depolarization can trigger neuronal firing in rat hippocampal neurons, demonstrating the potential and versatility for controlling cell functions using light.