Almost all organisms on Earth are subjected to a 24-hour oscillatory behaviour due to their adaptation to the daily pattern of sunlight. In mammals, those oscillations emerge from a molecular clock contained in almost every cell of their body, which can work independently but is controlled by the suprachiasmatic nucleus (SCN), a part of the brain that is sensitive to light. Recent studies have estimated that around the 43% of the mouse protein coding genes have a time-dependent expression somewhere in the organism and that this is cell-type dependent. Our project aims to study until what extent the clock of different cell types can be set on time and maintained beyond the instructions received by the SCN.
MECHADIAN aims to study the impact of the cell mechanical environment into their circadian clock. Until recently, it was believed that in peripheral tissues the clock is set on time by the pace dictated by the SCN which, although autonomous in its ability to oscillate, is regulated by the reception of light by the optical nerves. The SCN then would send signals to the rest of the cells of the body in either endocrine or neurocrine ways. However, studies have demonstrated that changes in the activity of some tissues are able to affect the timing of other cells bypassing the SCN: the liver is set in time not only by the SCN but also by the intake of food and the circadian timing of the muscular system gets affected by exercise.
We hypothesise that the clock of cells in peripheral tissues may be sensitive to, not only chemical but also mechanical, changes in their environment; and we aimed to a) study the impact of the mechanical microenvironment on the cellular clock, b) discover a possible influence of circadian oscillations on the mechanical behaviour of the cells and c) set the basis to characterise the interplay between mechanics and the clock during stem cell differentiation. We focused on the intracellular master clock of NIH3T3 fibroblasts, taking as a reference REVERBA, one of the main proteins that direct the circadian clock at the cell level.
We have developed an automatized method to measure population-wise properties of microscopy images of cells displaying REVERBA oscillations and observed that NIH3T3 fibroblasts show a robust circadian expression of REVERBA in a cell density-dependent manner. We have demonstrated that the effect of cell density on the circadian clock does not reside in differences in biochemical signalling but in mechanosensing. This mechanosensing ultimately controls the localization of certain transcriptional coactivators, whose enrichment in the nucleus in their active form breaks the fine regulation of the circadian clock.
This study could have a big impact in the field of chronomedicine, which addresses the treatment of pathologies considering the timing factor, given both the circadian behaviour and the importance of mechanosensing in the regulation of stem cells and their derived organs and tissues.
