TICK-TOCK-HEDGEHOG: Hedgehog signalling controls the liver circadian clock and vice versa

The Hedgehog signalling (HH-) pathway presents an evolutionary highly conserved cascade. Being one of the crucial modulators of embryogenesis and tissue differentiation its role in adult healthy organs like liver is often neglected. Interestingly, recent analyses identified the hedgehog signalling as a putative modulator of the circadian clock, which, at the molecular level, is organised by different self-regulatory transcriptional-translational feedback loops, comprising the so cold Core Clock Genes (CCGs). As an extensive biological process the clock is tightly linked to the elements of cell cycle control and intracellular signalling pathways, so the hedgehog-clock association seems to be reasonable. However, so far no data have been reported.

Our aim was, therefore, to analyse the liver specific interaction of the HH-signalling and the circadian clock, using the transgenic knock-out mice, displaying a conditional hepatocyte specific deletion of Smo (Smo-KO), an essential transducer within the HH-signalling. There, the deletion of Smo has been induced by doxycycline application at the age of 8 weeks. We analysed the expression of CCGs in the Smo deficient hepatocytes. At defined time points, some of the major components of the circadian rhythm displayed alterations in their expression levels, compared to non-transgenic littermates. Additionally, to answer the question to which extent the changes in the hepatic hedgehog pathway lead to an overall alteration of mice behavior, we observed respected features of the animals in metabolic cages prior to and after knock out manifestation. While, compared to wild type state, no changes could be observed with respect to the energy expenditure, the knock out animals showed a slight decrease in the food intake rate, which correlated with a slightly decreased physical activity. Further, the fact that the action of the HH-pathway is driven by the Gli transcription factors, gave us the impetus to perform Gli in-vitro knock-down experiments which also showed altered expression levels of some CCGs similar to those seen in-vivo.

A putative transcription factor binding site prediction allows us to conclude that in adult liver the HH signalling might be closely connected with the circadian clock via transcriptional control. However, the precise interactions of HH-signalling with the clock regulatory circuit still have to be elucidated, requiring further investigations.