Na(v)2/NaG is a putative sodium channel, whose physiological role has long been an enigma. We generated Na(v)2 gene-deficient mice by inserting the lacZ gene. Analysis of the targeted mice allowed us to identify Na(v)2-producing cells by examining the lacZ expression. Besides in the lung, heart, dorsal root ganglia, and Schwann cells in the peripheral nervous system, Na(v)2 was expressed in neurons and ependymal cells in restricted areas of the CNS, particularly in the circumventricular organs, which are involved in body-fluid homeostasis. Under water-depleted conditions, c-fos expression was markedly elevated in neurons in the subfornical organ and organum vasculosum laminae terminalis compared with wild-type animals, suggesting a hyperactive state in the Na(v)2-null mice. Moreover, the null mutants showed abnormal intakes of hypertonic saline under both water- and salt-depleted conditions. These findings suggest that the Na(v)2 channel plays an important role in the central sensing of body-fluid sodium level and regulation of salt intake behavior.

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Synaptotagmin-like protein 4-a (Slp4-a)/granuphilin-a was originally identified as a protein specifically associated with insulin-containing vesicles in pancreatic ␤-cells (Wang, J., Takeuchi, T., Yokota, H., and Izumi, T. (1999) J. Biol. Chem. 274, 28542-28548). Previously, we showed that the N-terminal Slp homology domain of Slp4-a interacts with the GTP-bound form of Rab3A, Rab8, and Rab27A both in vitro and in intact cells (Kuroda, T. S., Fukuda, M., Ariga, H., and Mikoshiba, K. (2002) J. Biol. Chem. 277, 9212-9218). How Slp4-a⅐Rab complex controls regulated secretion, and which Rab isoforms dominantly interact with Slp4-a in vivo, however, have remained unknown. In this study, we showed by immunocytochemistry and subcellular fractionation that three Rabs, Rab3A, Rab8, and Rab27A, and Slp4-a are endogenously expressed in neuroendocrine PC12 cells and localized on dense-core vesicles, and we discovered that the Slp4-a⅐Rab8 and Slp4-a⅐Rab27A complexes, but not Slp4-a⅐Rab3A complexes, are formed on dense-core vesicles in PC12 cells, although the majority of Rab8 is present in the cell body and is free of Slp4-a. We further showed that expression of Rab27A, but not of Rab8, promotes high KCl-dependent secretion of neuropeptide Y (NPY) in PC12 cells, whereas expression of Slp4-a, but not of an Slp4-a mutant incapable of Rab27A binding, inhibits NPY secretion in PC12 cells. In contrast, expression of Slp3-a, but not of Slp3-b lacking an N-terminal Rab27A-binding domain, promotes NPY secretion. These findings suggest that the Slp family controls regulated dense-core vesicle exocytosis via binding to Rab27A.The synaptotagmin-like protein (Slp) 1 family was originally identified as proteins that share homology to tandem C2 domains of the synaptotagmin (Syt) family (1), and it was subsequently defined as proteins that contain a unique N-terminal Slp homology domain (SHD) (2) and C-terminal tandem C2 domains (known as the C2A domain and C2B domain), which are putative Ca 2ϩ -binding motifs (2). To date, five members of the Slp family (Slp1/Jfc1, Slp2-a, Slp3-a, Slp4-a/granuphilin-a, and Slp5) have been described in the mouse and human (1-5), and several alternative splicing isoforms have been identified in Slp2, Slp3, and Slp4 (2, 4).The SHD consists of two conserved domains, referred to as SHD1 and SHD2 (2). The SHD1 and SHD2 of Slp3-a, Slp4-a, and Slp5 are separated by a sequence containing two zincfinger motifs, whereas Slp1 and Slp2-a lack such zinc-finger motifs, and their SHD1 and SHD2 are linked (2). The SHD has also been found in other proteins, including Slac2-a (Slp homologue lacking C2 domains-a)/melanophilin (2, 6, 7), Slac2-b/ KIAA0624 (8, 9), and Slac2-c/MyRIP (10). 2 We recently discovered that the SHD of the Slp family and Slac2 functions as an effector domain for specific Rab (7, 9), a small GTP-binding protein believed to be essential for membrane trafficking in eukaryotic cells (reviewed in Refs. 11-13). All SHDs directly interact with the GTP-bound form of Rab27A (and possibly Rab27B), both in vitro...