Cells must constantly sense and respond to physical forces from their environment in order to migrate, differentiate, and proliferate. These processes are critical for diverse cell functions, including growth, wound healing, and immune responses. The cytoskeleton - a complex and dynamic network of protein filaments - acts as a cell’s structural framework, determining its shape, organising its intracellular components, and enabling its movement and division. While the pivotal role of cytoskeleton in responding to external forces is well recognised, the mechanisms by which these mechanical signals are transmitted to the nucleus to influence cell behaviour have remained unclear.
The research team have evidenced an ATR-RASSF1A signalling cascade that is activated by mechanical forces, allowing mechanical coupling of the cytoskeleton with the nuclear envelope. This newly uncovered pathway stabilises nuclear actin, protecting the nucleus during cell motility and compression.
Experimental findings
To investigate how cells respond to mechanical forces, the team applied direct mechanical stretch and hyperosmotic stress to mimic the physiological forces experienced by cells. These conditions led to the recruitment of ATR to the nuclear envelope and subsequent elevation of phosphorylated RASSF1A (pRASSF1A), a well-known tumour-suppressor protein that acts as a central Hippo pathway scaffold.
Further experiments showed that ATR recruitment promoted the polymerisation of nuclear actin into filamentous actin (F-actin) through Filamin-A. This process was dependent on ATR-mediated phosphorylation of RASSF1A. In the absence of either ATR or RASSF1A, Filamin-A recruitment and resultant nuclear actin polymerisation were lost, providing strong evidence for a mechanically induced ATR-RASSF1A signalling cascade that regulates nuclear actin stability.
Eric O'Neill, Professor of Cell and Molecular Biology at the University of Oxford, who led this investigation commented:
“Mechanical signalling by cells is important for health and normal physiology. Any problem in communicating external information of a cell’s environment to the genome can cause major problem during development but also in normal functions, particularly of soft tissues like the brain. We have identified a key mechanism through which mechanical force is sensed by the envelope that surrounds the genomic material in the nucleus. This signal is then converted into physical manipulation of the nucleus and can influence how the cell then behaves.”
Genetic Links to Neurological Disorders
To understand the physiological relevance of this cascade, the team used genome-wide association studies (GWAS) datasets to look into phenotypic traits associated with genetic variations in pathway-associated genes. They found that single nucleotide polymorphisms (SNPs) in this pathway were consistently associated with neurological traits, body fat distribution, and cancer risk. Notable associations were seen for SNPs in RASSF1 with schizophrenia, neuroticism, and depression, suggesting an important role for this cascade in mental health. Variations in cortex thickness were also associated with this pathway.
Notably, analysis of disease signatures revealed a schizophrenia-associated mutation in RASSF1A which alters RASSF1A’s structure, restricting its phosphorylation by ATR and thereby disabling nuclear mechanotransduction. This common germline mutation is present in ~11% in Europeans, rising to as high as 17% in some populations, such as in Finland.
This mutation was shown to associate with body fat and neurological traits, including neuroticism, nervousness, and anxiety, with trends towards significance for low mood and depression. MRI imaging of individuals carrying the minor allele also showed structural changes in key brain regions linked to neurological disorders. Based on these data, the research team believe that this mutation may represent a low-penetrance, psychiatric-prone syndrome, characterized by chronic failures in mechanotransduction.
"Defects is signal transduction means the cell is blind to its environment and starts making proteins is shouldn't, leading to disease and problems. Importantly, we have identified a new syndrome and a genetic basis for schizophrenia, anxiety and depression as well as cancer in 1/10 people from a common mutation in RASSF1A (alanine 133 to serine)." - Eric O'Neill, Lead Author
In vivo data from a RASSF1A-mutant mouse model support this theory, showing increased cancer risk, regional brain volume changes, and an increased association with body fat.
Future Directions
Further work is needed to elucidate the precise consequences of mechanoresponsive F-actin on genome regulation, cell fate, and migration, but these data suggest that defective mechanically regulated pathways are implicated in brain function and may explain some complex disorders with multiple seemingly unrelated clinical problems. As F-actin is increasingly recognised for its fundamental roles in nuclear architecture, DNA regulation, and cell fate decisions, this newly identified pathway is likely to have a wide-ranging implications in mechanobiology and will inform future research.
The paper, ‘ATR-hippo drives force signaling to nuclear F-actin and links mechanotransduction to neurological disorders’, is out now in Science Advances.