Recombinant Mouse Hip Protein

Hedgehog signaling proteins act as mitogens, morphogens, or inducing factors in many different cell types during embryonic development. They aid in growth, patterning, and morphogenesis in both vertebrates and insects (1). Hip (Hedgehog-interacting protein) is a type I transmembrane protein identified for its ability to bind biologically active Sonic Hedgehog. It is comprised of 700 aa, and includes a hydrophobic signal sequence, two EGF-like domains near the C-terminus, and a 22 amino acid membrane-spanning region at the C-terminal end (2). Hip has only been identified in vertebrates and binds all three mammalian Hedgehogs: sonic (Shh), desert (Dhh), and Indian (Ihh). Like the Hedgehog receptor Patched, Hip is a transcriptional target of Hedgehog signaling (2). Unlike Patched, Hip’s ability to bind hedgehogs is not involved in transducing a signal intracellularly, rather it regulates the availability of Hedgehog ligand extracellularly (3). Transgenic mice overexpressing Hip in proliferating chondrocytes display skeletal defects similar to those observed in Ihh mutant mice. These results indicate that Hip is involved in attenuating Hedgehog signaling (2). The expression pattern of Hip correlates with its ability to interact with all three mammalian Hedgehogs. It is expressed in a variety of organs, adjacent with sites of hedgehog expression. For instance, Shh is expressed in the epithelium of the lung, and Hip is found in the underlying lung mesenchyme (2). In fact, Hip knock-out mice exhibit neonatal lethality with respiratory failure due to defective branching morphogenesis. This phenotype correlates with altered expression of Shh markers suggesting an increase in Shh signaling (3). Interestingly, other developmental mechanisms that rely on normal Shh signaling, such as dorsal-ventral patterning of the neural tube, development of the somites, and organ laterality appeared histologically normal in Hip^-/- mice (3). Mouse and human Hip share 94% amino acid identity through the entire protein sequence (4). R&D Systems’ recombinant Hedgehog-interacting protein is a potent antagonist of Shh signaling in vitro.