Fig. (11).

Cell biological agenda for HP signalling. Following secretion through a still not fully understood pathway, HPs must recognize their target cells and complex sugars of the glycosaminoglycan (GAG) family are probably involved raising the "sugar code" issue. Internalization is not well understood and probably involves a local an transient destabilization of the membrane followed by rapid repair. Within the cells, HPs have translational and transcriptional targets that need to be identified and an epigenetic activity that remains to be fully characterized. HP signaling in the best known cases synergizes with other signaling pathways that might vary depending on the situation (type of HP, type of target cell, age, etc). Finally, in the case of Engrailed and Otx2, internalization is followed by the translation of mitochondrial mRNAs and ATP synthesis and secretion and it is not known whether this can be generalized to all other transducing HPs.