Tumor Biology: Cells under pressure
Packed like sardines, most cells in our body must operate in confined spaces. Neighboring cells, the extracellular fluid that surrounds cells, and the extracellular matrix that provides physical support to tissues, can all restrict the growth and motion of cells. This is exacerbated in tumors. As the cancer cells in the tumor multiply, they gradually push back the surrounding tissue, and in turn, experience varying degrees of compression or 'solid stress' (Nia et al., 2017). Increasing evidence suggests that the physical interactions between cancer cells and their microenvironment affect the signals that regulate their growth and spread.
Much of our knowledge about solid stress and its impact on tumor development stems from research on three-dimensional aggregates of cancer cells grown in the laboratory. When these aggregates – which are also known as multicellular tumor spheroids – multiply in a confined environment, such as a hydrogel, their growth is restricted (Helmlinger et al., 1997). These observations are somewhat intuitive, since cell division is an inherently physical process during which cells increase in volume and undergo striking morphological changes – both of which require space (Nam and Chaudhuri, 2018; Zlotek-Zlotkiewicz et al., 2015; Son et al., 2012).
It has been shown that restricting or reducing the volume of a cell through increased osmotic pressure – which draws water from the cell – stops them from multiplying (Nam et al., 2019; Delarue et al., 2014). Likewise, compression along one axis can suppress the growth of cells and even induce programmed cell death in tumor spheroids (Cheng et al., 2009). When applied transiently to single cells, it may also reverse the malignant phenotype, with the cells displaying behaviors of normal cells (Ricca et al., 2018).
However, full three-dimensional control of uniform compression of tumor spheroids has not been achieved to date. Now, in eLife, Giovanni Cappello, Pierre Recho and colleagues from the Université Grenoble Alpes, the Université de Lyon and the Collège de France – including Monika Dolega as first author – report a new way to study the impact of solid stress on tumor spheroids (Dolega et al., 2021).
Dolega et al. applied two strategies to compare the impact of solid stress on both single cells and tumor spheroids. To do so, they exerted osmotic pressure using osmolytes of varying sizes: small dextran molecules measuring less than 5 nm in diameter (following the standard approach), and large ones with a diameter of 15 nm or more (representing the new approach). The small dextran molecules were able to infiltrate the extracellular matrix of the spheroids, thereby applying osmotic pressure on the single cells within. However, the 15 nm dextran molecules were too big to enter the extracellular matrix: instead, the osmotic pressure acted on the entire spheroid, resulting in a global compression of the cells by the extracellular matrix (Figure 1).
By keeping the magnitude of pressure the same, Dolega et al. discovered that global compression decreased the volume of the tumor spheroids significantly more than the osmotic compression of single cells, with cells within the spheroid growing and migrating more slowly. Comparable results were also observed in single cells encapsulated in an extracellular matrix-like hydrogel. There, global compression reduced the growth and migration of the cells while osmotic compression did not. These findings suggest that solid stress in the form of global compression does indeed regulate the growth and spread of a tumor.
Dolega et al. have developed a robust approach to probe the impact of solid stress on cells in three-dimensional microenvironments, and the different outcomes observed for osmotic compression of single cells and global compression of tumor spheroids draws attention to the possibility that the extracellular matrix can act as a pressure sensor that might regulate cell behavior. It remains unclear if cells sense solid stress through the same pathways that are implicated in osmotic compression, including stretch-activated ion channels, or through alternate pathways, perhaps involving the cytoskeleton and cell-matrix adhesions (Nam et al., 2019). Probing these mechanisms will help advance our understanding of how solid stress regulates cell behavior in a wide range of contexts, from embryonic development to cancer.
Compressive stress inhibits proliferation in tumor spheroids through a volume limitationBiophysical Journal 107:1821–1828.https://doi.org/10.1016/j.bpj.2014.08.031
Solid stress inhibits the growth of multicellular tumor spheroidsNature Biotechnology 15:778–783.https://doi.org/10.1038/nbt0897-778
Solid stress and elastic energy as measures of tumour mechanopathologyNature Biomedical Engineering 1:0004.https://doi.org/10.1038/s41551-016-0004
Optical volume and mass measurements show that mammalian cells swell during mitosisJournal of Cell Biology 211:765–774.https://doi.org/10.1083/jcb.201505056
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- Version of Record published: April 23, 2021 (version 1)
© 2021, Indana and Chaudhuri
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- Physics of Living Systems
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- Physics of Living Systems
Postoperative knee instability is one of the major reasons accounting for unsatisfactory outcomes, as well as a major failure mechanism leading to total knee arthroplasty (TKA) revision. Nevertheless, subjective knee instability is not well defined clinically, plausibly because the relationships between instability and implant kinematics during functional activities of daily living remain unclear. Although muscles play a critical role in supporting the dynamic stability of the knee joint, the influence of joint instability on muscle synergy patterns is poorly understood. Therefore, this study aimed to understand the impact of self-reported joint instability on tibiofemoral kinematics and muscle synergy patterns after TKA during functional gait activities of daily living.
Tibiofemoral kinematics and muscle synergy patterns were examined during level walking, downhill walking, and stair descent in eight self-reported unstable knees after TKA (3M:5F, 68.9 ± 8.3 years, body mass index [BMI] 26.1 ± 3.2 kg/m2, 31.9 ± 20.4 months postoperatively), and compared against 10 stable TKA knees (7M:3F, 62.6 ± 6.8 years, 33.9 ± 8.5 months postoperatively, BMI 29.4 ± 4.8 kg/m2). For each knee joint, clinical assessments of postoperative outcome were performed, while joint kinematics were evaluated using moving video-fluoroscopy, and muscle synergy patterns were recorded using electromyography.
Our results reveal that average condylar A-P translations, rotations, as well as their ranges of motion were comparable between stable and unstable groups. However, the unstable group exhibited more heterogeneous muscle synergy patterns and prolonged activation of knee flexors compared to the stable group. In addition, subjects who reported instability events during measurement showed distinct, subject-specific tibiofemoral kinematic patterns in the early/mid-swing phase of gait.
Our findings suggest that accurate movement analysis is sensitive for detecting acute instability events, but might be less robust in identifying general joint instability. Conversely, muscle synergy patterns seem to be able to identify muscular adaptation associated with underlying chronic knee instability.
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.