Molecular genetic studies on mammalian brain morphogenesis

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Inositol 1, 4, 5-trisphosphate (IP_3) is Produced through the activation of phosphoinositide (PI) tumover by the physiological action of hormones, neurotransmitters and growth factors, and functions as an intracellular second messenger. IP_3 binds to a specific IP_3 receptor that releases Ca^<2+> from intracellular store sites such as the endoplasmic reticulum (ER). The increase in [Ca^<2+>]_i probably modulates functions of various Ca_<2+>1-associated proteins, leading to a variety of cellular responses to extracellular stimuli.The IP_3 receptor was originally identified as the glycosylated and phosphorylated protein P400 present in the cerebellum from normal mice but not from cerebellar Purkinje-celldeficient mutant mice. We purified the protein from mouse cerebellum, prepared monoclonal antibodies, and subcellularly localized the receptor mostly at the ER of Purkinje cells. Current recording experiments by reconstitution of the purified receptors in planner lipid bilayer represented that the receptor has an IP_3-induced cation selective ion channel activity that is enhanced in the presence of ATP. We isolated the cDNA from a cerebellar cDNA library. The structural analysis and functional expression of the IP_3 receptor cDNA indicated that the functional IP_3 receptor complex is a homotetramer : the pretomer (2749 amino acids) has a large N-terminal cytoplasmic region (83%) with a ligand (IP_3) -binding site (N-terminal 650 amino acids) and modulation sites for cAMP-dependent phosphorylation and ATP-binding, and a short C-teffninal transmembrane region involved in forming a Ca^<2+> channel pore. This characteristic structure is well-conserved in the recently-cloned Drosophila receptor homologue, as well. From regional distribution of the IP_3 receptor, we will discuss the functional role of IP_3/Ca^<2+> signalling in the central nervous system.