Our anatomy is essentially a system of compartments within compartments; take, for example, the simplest anatomical unit – the cell. A phospholipid bilayer compartmentalises the cell and houses other membrane-bound organelles such as the nucleus and mitochondria. More enigmatic are so-called membrane-less organelles, or biomolecular condensates, whose function and principles of formation are much less clear.
Paraspeckles are a prominent model condensate that is present within the mammalian nucleus. Paraspeckles are built upon the long noncoding RNA NEAT1 that serves as a scaffold for essential proteins SFPQ, NONO and FUS to bind. When this specific arrangement of RNA and proteins is satisfied, a paraspeckle is born; a biomolecular condensate partitioned from its surrounding nucleoplasm through the process of liquid-liquid phase separation. Once formed, paraspeckles are major deciders of cell fate. Specifically, via the mechanisms of macromolecule sequestration and genome re-organisation, paraspeckles can alter gene regulation, thereby regulating signalling and myriad other processes involved in homeostasis.
In this study we explore the dynamics of cellular paraspeckles under conditions of hypo-osmotic stress; a stress that impacts mechanobiology (cell mechanical perturbation) as well macromolecule and biochemical interactions (RNA-RNA & RNA-protein dependent paraspeckle formation). Using two different cell lines, quantitative microscopy and quantitative PCR, we observe dynamic changes in cell and nuclear volumes when placed within hypo-osmolarity medium, as well as a dramatic effect on paraspeckle formation. Specifically, we detected a complete ablation of paraspeckles and NEAT1 within 2 hours of hypo-osmotic culture. Using microscopy we see loss of NEAT1 lncRNA FISH foci, but no loss of paraspeckle protein staining, suggesting the phenomenon is RNA-driven. We also identified paraspeckles rapidly recovering following the return to iso-osmotic conditions and will be exploring the functional consequences of this in follow up experiments. In contrast to the results with paraspeckles, these effects were not observed for nuclear speckles, a different nuclear condensate. Overall, our results speak to the importance of biomolecular condensates and lncRNA in cell homeostasis and hormesis.