In a recent study published in Communications Biology, researchers explored the intricacies between life and sound, investigating the impact of acoustic stimulation on cell behavior.
Sound waves induce diverse cellular responses that could inform cell manipulation applications such as living tissue engineering, artificial tissue culture, biotechnology, and regenerative medicine.
In particular, acoustic stimulation prevents preadipocytes from maturing into adipocytes or fat cells, which may have consequences for metabolic disorders such as obesity. The findings question the conventional understanding of sound perception by living beings. The study suggests that cells, not only the brain, respond to sounds.
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Introduction
Eukaryotic cells use mechanosensory systems to respond to mechanical stimuli in their surroundings. Sound is a compressional mechanical wave communicating oscillating and fluctuating pressure through material.
Sound transmits primarily through bone and soft tissues in living tissues, creating a complex acoustic environment. However, sound waves have not been researched extensively as sources of cellular stimulation. There is limited data on cell-level responses to audible ranges of acoustic waves.
About the Study
In the present study, researchers explored cellular and genetic changes induced by acoustic stimulation, focusing on mechanosensitive genes and adipocyte differentiation. They investigated cellular responses to the physiological range of acoustic stimulations.
The researchers established a direct sound emission system using a vibrational transducer from polyether ether ketone (PEEK) plastic to generate acoustic waves in a culture medium.
The transducer transmitted acoustic waves at frequencies of 440 Hz and 14 kHz and white noise to murine C2C12 myoblasts at an intensity of 100 Pa, as measured by a hydrophone.
The team performed continuous sound emission over two hours and 24 hours, after which they extracted ribonucleic acid (RNA) to identify sound-sensitive genes by RNA sequencing.
Quantitative polymerase chain reaction (PCR) yielded time-course patterns of sound-sensitive gene responses. Gene annotation analysis revealed various molecular, cellular, and body/tissue-level activities affected by acoustic stimuli.
The team investigated sound- and cell-related factors that regulate gene responses. They also explored mechanisms underlying the effects of sound on cellular activities, particularly concerning mechanosensitive tissues like adipose tissue.
The researchers investigated the expression levels of two genes, CCAAT/enhancer-binding protein α (CEBPA) and peroxisome proliferator-activated receptor γ (PPARΓ), to determine the impact of acoustic stimuli on adipocyte differentiation.
Chemiluminescence assays quantified the extent of prostaglandin E2 (PGE2) released into the culture medium to reduce lipid accumulation and compared the effects with acoustic stimulations. Confocal microscopy revealed cell structural details.
Results
The study demonstrates that cells detect mechanical stimuli, like sound, and respond to acoustic stimulation. Adhesive stromal cells and their offshoot cells, such as myoblasts, fibroblasts, adipocytes, and osteoblasts, are sensitive to sound.
In contrast, epithelial and neuroblastoma cells are insensitive, probably due to their less migratory and sticky nature. These cellular alterations are linked to cytoskeletal systems that control cell adhesion and migration. In the present study, all sound types boosted cell adhesion areas by 15% to 20% within an hour.
Acoustic waves inhibit adipocyte development by lowering the expression of CEBPA and PPARγ, which are mechanosensitive and early-response markers.
Over three days, two-hourly acoustic stimulations reduced lipid accumulation by 13-15%, comparable to the effects of 1000 ng/ml PGE2 and in line with confocal microscopy findings.
Upon adding comparable amounts of PGE2 to the media, researchers noticed gene responses identical to those seen after acoustic stimulation but without sound emissions.
L161.982, a selective antagonist of PGE2’s EP4 receptor, blocked the impact of acoustic waves on the genes. The findings suggest that sound-induced genetic changes depend on PGE2 signaling via EP4.
Acoustic transduction pathways commence at focal adhesion areas and activate gene sets. Sound transduction relies heavily on focal adhesion kinase (FAK). FAK is implicated in mechanosensing and governs cell migration by Y397 phosphorylation in moving fibroblasts. Y15, a selective inhibitor of Y397 phosphorylation, inhibited cell growth at a 2.0 µM concentration.
The FAK pathway regulates the expression of genes, including connective tissue growth factor (CTGF) and prostaglandin-endoperoxide synthase 2 (PTGS2). CTGF activation could trigger intracellular signaling pathways that enhance cell adhesion and migration.
PTGS2 activation could increase PGE2 production for cell growth and induce genetic changes similar to those observed after acoustic stimulation.
RNA sequencing revealed 42 early and 145 late response genes. After two hours of sound exposure, most gene activity reverted to baseline, indicating a transient response to acoustic stimulation.
Meanwhile, upon re-emission of sound 24 hours later, most genes reacted again, revealing the recurrent nature of sound-induced gene responses. The findings suggest that frequency, intensity, and waveform are crucial in determining gene responses.
Conclusion
Based on the study findings, sound waves, particularly those at precise frequencies and intensities, might impact cellular responses and gene activity, underlining sound's potential as a non-invasive tool for cellular manipulation.
Sound-induced gene expression alterations are transitory. Acoustic stimulation can inhibit adipocyte development, which could have implications for controlling obesity and metabolic illnesses.
Acoustic signals transduce from focal adhesions and trigger PTGS2 responses through FAK phosphorylation to boost PGE2 activities against its EP4 receptors for gene set activation.
Subsequently, the cells increase their adhesion areas and strengthen focal adhesions. The findings emphasize the link between life and sound, which suggests evolutionary adaptations.
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