Mechano-Signal Transduction Pathways of the Diaphragmatic Muscle and Role of Cytoskeleton

Abstract

Mechanotransduction, also referred to as mechano-signal transduction, is a biophysical process wherein cells perceive and respond to mechanical stimuli by converting them into biochemical signals that initiate specific cellular responses. This mechanism is fundamental to the development and growth, and proper functioning of mechanically active tissues, such as the diaphragm-a respiratory muscle vital for breathing in mammals. In vivo, the diaphragm is subjected to transdiaphragmatic pressure, and therefore, its muscle fibers are subjected to mechanical forces not only in the direction of the muscle fibers but also in the direction transverse to the fibers. Previous research conducted in our laboratory uncovered that stretching the diaphragm in either the longitudinal or transverse direction activates distinct mechanotransduction pathways. This indicates that signaling pathways in the diaphragm muscle are regulated in an anisotropic manner. In this review paper, we discussed the underlying mechanisms that regulate the anisotropic signaling pathways in the diaphragmatic muscle, emphasizing the mechanical role of cytoskeletal proteins in this context. Furthermore, we explored the regulatory mechanisms governing mechanosensitive gene transcription, including microRNAs (mechanomiRs), within the diaphragm muscle. Finally, we examined potential links between anisotropic signaling in the diaphragm muscle and various skeletal muscle disorders.

Keywords: anisotropic gene regulation; cytoskeletal proteins; mechanical stretch; mechanomiRs; mechanotransduction; skeletal muscle.

Figure 1

Cytoskeletal protein arrangements and their possible force transmission pathways in skeletal muscles. This schematic picture presents 2 pathways for force transmission. Pathway 1 includes the structural elements that are situated along the muscle fibers. These structural elements are collagen, merosin m-chain (M), dystroglycans B-subunit (DB), dystroglycans A-subunit (DA), sarco-glycan complex (α, β, γ, δ, ε), dystrophin (DY), actin, and z-disks. Pathway 2 consists of structural elements situated in the transverse plane to the muscle fibers. Structural elements in this pathway include collagen, merosin m-chain, integrin A-subunit (IA), integrin B-subunit (IB), talin (T), vinculin (V), desmin (DE), and z-disks. Merosin is visualized in pathway 1, along muscle fibers, and in pathway 2, transverse to muscle fibers. J Appl Physiol 94: 2524–2533, 2003 [17].

Figure 2

The proposed model of the anisotropic regulation of gene transcriptions in the diaphragmatic muscle. In this model, when the diaphragm is mechanically loaded in the transverse direction, the force activates AP-1, Elk-1, or p90RSK signaling proteins. In contrast, when the diaphragm is mechanically loaded in the axial (longitudinal) direction, the force activates NF-κB transcription factors through Akt or PTK signaling pathways. Subsequently, these distinct directional dependents signaling pathways trigger gene transcriptions.

Figure 3

Genome-wide expression profile of mechanomiRs in diaphragm muscle by anisotropic regulation. Longitudinal or transverse stretch was applied to mouse left hemidiaphragm for 15 minutes. The right hemidiaphragm was treated as a control with no stretch. Immediately after stretch, total RNA was isolated from stretched and non-stretched diaphragm and used in miRNA microarray analyses to determine differentially regulated mechanomiRs. (A,B) The scatter plot shows log₁₀-transformed signal intensities for each probe labeled with Cy3 for unstretched (control) and Cy5 for LS (A) or TS (B) diaphragm. Each dot represents one miRNA probe. (C,D) Data on the heat map show differentially expressed mechanomiRs in the diaphragm in response to LS (C) or TS (D). (E) Venn diagram shows up (red)- and down (blue)-regulated mechanomiRs (1.5-fold) after corresponding stretch. (F) Percentage of differentially expressed mechanomiRs to the total number of miRNAs in the array. (G) Percentage of differentially expressed mechanomiRs based on their genomic location. Asterisk (*) denotes the less abundant miRNA strand. Mohamed et al., 2015 J. Biol. Chem. 290 (41) 24986–25011 [61].

Figure 4

Anisotropic regulation of mechanomiRs in the diaphragm muscle of mdm dystrophic mice. Longitudinal or transverse stretch was applied to the dystrophic mouse’s left hemidiaphragm for 15 minutes. The unstretched right hemidiaphragm was treated as a control. Immediately after stretch, total RNA was isolated from stretched and non-stretched diaphragm and used in miRNA microarray analyses to determine differentially regulated mechanomiRs. (A,B) The scatter plot shows log₁₀-transformed signal intensities for each probe labeled with Cy3 for unstretched (control) and Cy5 for LS (A) or TS (B) diaphragm. Each dot represents one miRNA probe. (C,D) Data on the heat map show mechanomiRs differentially expressed in the diaphragm in response to LS (C) or TS (D). (E) Venn diagram shows up (red)- and down (blue)-regulated mechanomiRs (>1.5-fold) after stretch. (F) Percentage of differentially expressed mechanomiRs to the total number of miRNAs in the array. (G) Percentage of differentially expressed mechanomiRs based on their genomic location. Asterisk (*) denotes the less abundant miRNA strand. Mohamed et al., 2015 J. Biol. Chem. 290 (41) 24986–25011 [61].