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Hedgehog morphogen gradient interpretation

Final Activity Report Summary - HHGI (Hedgehog morphogen gradient interpretation)

The Hedgehog (Hh) pathway is an evolutionarily conserved signal transduction pathway recognised as a key mediator of many fundamental processes in embryonic development. It has also been implicated in a number of human diseases, ranging from different types of cancer to developmental defects. Hh acts as a morphogen, activating different target genes depending on its extracellular concentration. A central component of the Hh pathway is the Hh receptor Patched (Ptc). Ptc controls the expression of Hh target genes by repressing Smoothened (Smo), an essential transducer of the Hh signal. Hh binding to Ptc abrogates this inhibitory effect, leading to the activation of the pathway. Although Ptc plays a central role in the Hh pathway, it is still not known how it functions, how it represses Smo and how the binding of Hh alleviates this repression. Furthermore, little is known about the mechanism that allows cells to measure and respond to different concentrations of Hh.

We have followed a novel experimental approach by expressing forms of the Hh receptor Ptc, in vivo, tagged with GFP and at physiologically relevant levels. This work has led to the demonstration that Ptc modulates its own stability posttranscriptionally, as the more Ptc protein is present in a cell the higher is its degradation rate. We have shown that such modulation of Ptc stability is essential for cells to measure correctly the extracellular concentration of Hh. We have shown previously that cells interpret Hh concentration by measuring the ratio of active (unbound to Hh) to inactive (bound to Hh) Ptc, this being the first demonstration that a morphogen gradient is read by comparing the ratio of active to inactive receptor.

Our new results open a new level of pathway activity regulation, as the total amount of Ptc present at the cell membrane is important in order to read the ratio between bound and unbound Ptc correctly. In the near future we will aim to understand the mechanism that allows cells to measure Ptc at the cell membranes and target it for degradation. The understanding of the mechanism that allow cells to measure and respond to distinct Hh concentrations is not only relevant during normal development but also for our understanding of mechanisms of diseases that involve Hh signalling.