Trans-scale thermal signaling in biological systems

Abstract

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'.

Keywords: ATPase; fluorescence microscopy; heat-induced calcium release; microheating; type 1 ryanodine receptor. Abbreviations: [Ca2+]i, intracellular Ca2+ concentration; CICR, Ca2+-induced Ca2+ release; ER, endoplasmic reticulum; FDB, flexor digitorum brevis; HEK293 cell, human embryonic kidney 293 cell; HICR, heat-induced Ca2+ release; IP3R, inositol 1,4,5-trisphosphate receptor; MH, malignant hyperthermia; RCC, rapid cooling contracture; RyR1, type 1 ryanodine receptor; SERCA, sarco/endoplasmic reticulum Ca2+-ATPase; SR, sarcoplasmic reticulum; TRP, transient receptor potential; WT, wild type.

Graphical Abstract

Fig. 1

Schematic illustration of the Ca ²⁺ flow pathway related to the type 1 ryanodine receptor in skeletal muscle cells. When the neuromuscular junction receives a neurotransmitter such as acetylcholine, the dihydropyridine receptor (DHPR), a type of voltage-dependent Ca²⁺ channel, is activated by the depolarization of the transverse tubule (T-tubule) membrane. The direct interaction between DHPRs and the type 1 ryanodine receptors (RyR1s) converts the signal of depolarization into Ca²⁺ release from the sarcoplasmic reticulum through RyR1, which is termed depolarization-induced Ca²⁺ release (DICR). Then, Ca²⁺-induced Ca²⁺ release (CICR) is induced, where the opening of RyR1 is followed by the rapid release of Ca²⁺ from the sarcoplasmic reticulum into the cytoplasm. Muscle contraction and Ca²⁺ uptake from the cytosol into the lumen of SR, which involve contractile proteins (such as actin and myosin) and sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA), respectively, are accompanied by ATP hydrolysis and thermogenesis. These reactions in skeletal muscle are proposed as the heat sources that trigger heat-induced Ca²⁺ release (HICR) through RyR1. Wavy arrows, heat conduction.

Fig. 2

Acceleration of an increase in temperature during malignant hyperthermia in R2509C mice. (A, B) Changes in rectal temperature of WT (A) and R2509C (B) mice upon exposure to 1.5–2.0 vol% isoflurane. n = 5 and 15 in WT and R2509C mice, respectively (shown in different colors). Inset, a photograph of the R2509C-RyR1 heterozygous mouse, which responded with full body contractions as reflected in the arching of the back and extension of the legs. (C) Shift of the rate of rectal temperature elevation at the early (from 38 °C to 39 °C), middle (from 39 °C to 40 °C), and late (from 40 °C to 41 °C) stages. Statistical significance was determined by a paired t test. Early vs. middle, n = 15 and P = 1.4 × 10⁻⁵; middle vs late, n = 12 and P = 1.7 × 10⁻⁵. Figures are reproduced with modifications from Yamazawa et al. (16).

Fig. 3

Fluorescence imaging of [Ca ²⁺ ] _i and temperature of the sarcoplasmic reticulum in flexor digitorum brevis muscle fibers. (A) Phase contrast (left) and fluorescence (center and right) images of flexor digitorum brevis (FDB) muscles isolated from WT (center) or R2509C (left and right) mice. The fluorescence images show immunostained myosin heavy chains (MHC), nicotinic acetylcholine receptors stained with α-bungarotoxin (αBTX), and nuclei stained with 4′,6-diamidino-2-phenylindole (DAPI). (B) Chemical structure of ERthermAC (left), and a fluorescence image of FDB fiber isolated from R2509C mice following staining with ERthermAC. (C) Time course of changes in the relative fluorescence intensity of Cal-520 (top) and ERthermAC (bottom) in WT (left) and R2509C-Het (right) cells. Gray lines represent individual cells. Thick colored lines represent average responses. Figures are reproduced with modifications from Kriszt et al. and Tsuboi et al. (19,22).

Fig. 4

Investigation of the heat sensitivities of RyR1 MH mutants. (A) Illustration depicting the experimental design. (B) Fluorescence images capturing heat-induced Ca²⁺ bursts observed in fluo-4-loaded HEK293 cells expressing wild type (WT) RyR1 or R164C mutants. (C) The positive feedback loop at the SR membrane, involving local temperature elevation due to cellular thermogenesis by SERCA and thermally driven Ca²⁺ release via RyR1 mutants, is closed by heat-induced Ca²⁺ release (HICR), leading to the progression of malignant hyperthermia. Figures have been reproduced with modifications from Oyama et al. (21).

Fig. 5

Schematic illustrations of the in vitro motility assays that are combined with the IR laser-based local heating. (A) Reconstituted thin filaments from actin filaments and troponin (Tn)-tropomyosin (Tm) complexes interacted with heavy meromyosin (HMM; α-chymotrypsin proteolytic fragment of myosin II) attached to the glass surface. Local temperature within the field of view of the fluorescence microscopy was directly increased by a focused IR laser beam (λ = 1,455 nm), which elicits thin filament movements (as shown by orange arrows). (B) Reconstituted filaments from actin filaments and drebrin E interacted with HMM attached to the glass surface. In both assays, the sliding velocity was dependent on the distance from the heat source, i.e. the temperature. Figures are reproduced with modifications from Ishii et al. and Kubota et al. (42,44).

Fig. 6

Schematic illustration of the proposed trans-scale thermal signaling in muscle, as a representative of biological systems. Diseases, such as malignant hyperthermia, are caused at the scale of organisms when the thermal management at the molecular scale is impaired.