Single carbon nanotube-based reversible regulation of biological motor activity

Trans-scale thermal signaling in biological systems

Biochemical reactions in cells serve as the endogenous source of heat, maintaining a constant body temperature. This process requires proper control; otherwise, serious consequences can arise due to the unwanted but unavoidable responses of biological systems to heat. This review aims to present a range of responses to heat in biological systems across various spatial scales. We begin by examining the impaired thermogenesis of malignant hyperthermia in model mice and skeletal muscle cells, demonstrating that the progression of this disease is caused by a positive feedback loop between thermally driven Ca2+ signaling and thermogenesis at the subcellular scale. After we explore thermally driven force generation in both muscle and non-muscle cells, we illustrate how in vitro assays using purified proteins can reveal the heat-responsive properties of proteins and protein assemblies. Building on these experimental findings, we propose the concept of 'trans-scale thermal signaling'.

Intracellular Heat Transfer and Thermal Property Revealed by Kilohertz Temperature Imaging with a Genetically Encoded Nanothermometer

Despite improved sensitivity of nanothermometers, direct observation of heat transport inside single cells has remained challenging for the lack of high-speed temperature imaging techniques. Here, we identified insufficient temperature resolution under short signal integration time and slow sensor kinetics as two major bottlenecks. To overcome the limitations, we developed B-gTEMP, a nanothermometer based on the tandem fusion of mNeonGreen and tdTomato fluorescent proteins. We visualized the propagation of heat inside intracellular space by tracking the temporal variation of local temperature at a time resolution of 155 μs and a temperature resolution 0.042 °C. By comparing the fast in situ temperature dynamics with computer-simulated heat diffusion, we estimated the thermal diffusivity of live HeLa cells. The present thermal diffusivity in cells was about 1/5.3 of that of water and much smaller than the values reported for bulk tissues, which may account for observations of heterogeneous intracellular temperature distributions.

A highly-sensitive genetically encoded temperature indicator exploiting a temperature-responsive elastin-like polypeptide

Genetically encoded temperature indicators (GETIs) allow for real-time measurement of subcellular temperature dynamics in live cells. However, GETIs have suffered from poor temperature sensitivity, which may not be sufficient to resolve small heat production from a biological process. Here, we develop a highly-sensitive GETI, denoted as ELP-TEMP, comprised of a temperature-responsive elastin-like polypeptide (ELP) fused with a cyan fluorescent protein (FP), mTurquoise2 (mT), and a yellow FP, mVenus (mV), as the donor and acceptor, respectively, of Förster resonance energy transfer (FRET). At elevated temperatures, the ELP moiety in ELP-TEMP undergoes a phase transition leading to an increase in the FRET efficiency. In HeLa cells, ELP-TEMP responded to the temperature from 33 to 40 °C with a maximum temperature sensitivity of 45.1 ± 8.1%/°C, which was the highest ever temperature sensitivity among hitherto-developed fluorescent nanothermometers. Although ELP-TEMP showed sensitivity not only to temperature but also to macromolecular crowding and self-concentration, we were able to correct the output of ELP-TEMP to achieve accurate temperature measurements at a subcellular resolution. We successfully applied ELP-TEMP to accurately measure temperature changes in cells induced by a local heat spot, even if the temperature difference was as small as < 1 °C, and to visualize heat production from stimulated Ca²⁺ influx in live HeLa cells induced by a chemical stimulation. Furthermore, we investigated temperatures in the nucleus and cytoplasm of live HeLa cells and found that their temperatures were almost the same within the temperature resolution of our measurement. Our study would contribute to better understanding of cellular temperature dynamics, and ELP-TEMP would be a useful GETI for the investigation of cell thermobiology.

From isolated structures to continuous networks: A categorization of cytoskeleton-based motile engineered biological microstructures

As technology at the small scale is advancing, motile engineered microstructures are becoming useful in drug delivery, biomedicine, and lab-on-a-chip devices. However, traditional engineering methods and materials can be inefficient or functionally inadequate for small-scale applications. Increasingly, researchers are turning to the biology of the cytoskeleton, including microtubules, actin filaments, kinesins, dyneins, myosins, and associated proteins, for both inspiration and solutions. They are engineering structures with components that range from being entirely biological to being entirely synthetic mimics of biology and on scales that range from isotropic continuous networks to single isolated structures. Motile biological microstructures trace their origins from the development of assays used to study the cytoskeleton to the array of structures currently available today. We define 12 types of motile biological microstructures, based on four categories: entirely biological, modular, hybrid, and synthetic, and three scales: networks, clusters, and isolated structures. We highlight some key examples, the unique functionalities, and the potential applications of each microstructure type, and we summarize the quantitative models that enable engineering them. By categorizing the diversity of motile biological microstructures in this way, we aim to establish a framework to classify these structures, define the gaps in current research, and spur ideas to fill those gaps. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Nanotechnology Approaches to Biology > Cells at the Nanoscale Biology-Inspired Nanomaterials > Protein and Virus-Based Structures Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Characterization and Biodistribution Analysis of Oxygen-Doped Single-Walled Carbon Nanotubes Used as in Vivo Fluorescence Imaging Probes

Single-walled carbon nanotubes (SWCNTs) show strong fluorescence in the 1000-1700 nm second near-infrared (NIR-II) wavelength range and are considered promising candidates for angiographic imaging probes. Oxygen-doped SWCNTs coated with phospholipid-polyethylene glycol (o-SWCNT-PEG) show exceptional potential, as they emit fluorescence at ∼1300 nm through excitation with 980 nm light. Here, with the aim of putting o-SWCNTs to practical use as an angiographic agent in animal experiments, the retention time after intravenous administration in the vasculature of mice and the biodistribution were studied. To provide bio affinity, the o-SWCNTs were coated with phospholipid polyethylene glycol. The intravenously injected o-SWCNT-PEG circulated within the vasculature for 3 h and cleared within 1 day. There was prominent fluorescence and Raman signals from the SWCNTs in the liver and spleen early in the experiment; the signals remained for 1 month. No apparent abnormalities in weight or appearance were observed after 2 months, suggesting low toxicity of o-SWCNT-PEG. These characteristics of o-SWCNT-PEG would make it useful as an angiographic imaging probe in the NIR-II wavelength range.

Local heating of molecular motors using single carbon nanotubes

Temperature globally affects all chemical processes and biomolecules in living cells. Elevating the temperature of an entire cell accelerates so many biomolecular reactions simultaneously that it is difficult to distinguish the various mechanisms involved. The ability to localize temperature changes to the nanometer range within a cell could provide a powerful new tool for regulating biomolecular activity at the level of individual molecules. The search for a nanoheater for biological research has prompted experiments with carbon nanotubes (CNTs), which have the highest conductivity of any known material. The adsorption of skeletal muscle myosin molecules along the length of single multi-walled CNTs (~10 μm) has allowed researchers to observe the ATP-driven sliding of fluorescently labeled actin filaments. In one study, red-laser irradiation focused on one end of a myosin-coated CNT was used to heat myosin motors locally without directly heating the surrounding water; this laser irradiation instantly accelerated the actin-filament sliding speeds from ~6 to ~12 μm/s in a reversible manner, indicating a local, real-time heating of myosin motors by approximately Δ12 K. Calculation of heat transfer using the finite element method, based on the estimated temperature along a single CNT with a diameter of 170 nm, indicated a high thermal conductivity of ~1540 Wm^-1K^-1 in solution, consistent with values measured in vacuum in earlier studies. Temperature distribution indicated by half-decrease distances was ~3660 nm along the length of the CNT and ~250 nm perpendicular to the length. These results suggest that single-CNT-based heating at the nanometer- or micrometer-range could be used to regulate various biomolecules in many areas of biological, physical, and chemical research.