Histidine decarboxylase (HDC) synthesizes histamine from histidine in mammals. To evaluate the role of histamine, we generated HDC-deficient mice using a gene targeting method. The mice showed a histamine deficiency and lacked histaminesynthesizing activity from histidine. These HDC-deficient mice are viable and fertile but exhibit a decrease in the numbers of mast cells while the remaining mast cells show an altered morphology and reduced granular content. The amounts of mast cell granular proteases were tremendously reduced. The HDCdeficient mice provide a unique and promising model for studying the role of histamine in a broad range of normal and disease processes. ß

Thyroid hormone receptors are encoded by the TR␣ (NR1A1) and TR␤ (NR1A2) loci. These genes are transcribed into multiple variants whose functions are unclear. Analysis by gene inactivation in mice has provided new insights into the functional complexity of these products. Different strategies designed to modify the TR␣ locus have led to strikingly different phenotypes. In order to analyze the molecular basis for these alterations, we generated mice devoid of all known isoforms produced from the TR␣ locus (TR␣ 0/0 ). These mice are viable and exhibit reduced linear growth, bone maturation delay, moderate hypothermia, and reduced thickness of the intestinal mucosa. Compounding TR␣ 0 and TR␤ ؊ mutations produces viable TR␣ 0/0 ␤ ؊/؊ mice, which display a more severe linear growth reduction and a more profound hypothermia as well as impaired hearing. A striking phenotypic difference is observed between TR␣ 0/0 and the previously described TR␣ ؊/؊ mice, which retain truncated TR⌬␣ isoforms arising from a newly described promoter in intron 7. The lethality and severe impairment of the intestinal maturation in TR␣ ؊/؊ mice are rescued in TR␣ 0/0 animals. We demonstrate that the TR⌬␣ protein isoforms, which are natural products of the TR␣ locus, are the key determinants of these phenotypical differences. These data reveal the functional importance of the non-T3-binding variants encoded by the TR␣ locus in vertebrate postnatal development and homeostasis.

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We have previously shown that protein kinase C-␤ (PKC-␤) is required for activation of tyrosinase (Park, H. Y., Russakovsky, V., Ohno, S., and Gilchrest, B. A. (1993) J. Biol. Chem. 268, 11742-11749), the rate-limiting enzyme in melanogenesis. We now examine its mechanism of activation in human melanocytes. In vivo phosphorylation experiments revealed that tyrosinase is phosphorylated through the PKC-dependent pathway and that introduction of PKC-␤ into nonpigmented human melanoma cells lacking PKC-␤ lead to the phosphorylation and activation of tyrosinase. Preincubation of intact melanosomes with purified active PKC-␤ in vitro increased tyrosinase activity 3-fold. By immunoelectron microscopy, PKC-␤ but not PKC-␣ was closely associated with tyrosinase on the outer surface of melanosomes. Western blot analysis confirmed the association of PKC-␤ with melanosomes. Only the cytoplasmic (extramelanosomal) domain of tyrosinase, which contains two serines but no threonines, was phosphorylated by the serine/threonine kinase PKC-␤. These two serines at positions 505 and 509 both are present in the C-terminal peptide generated by trypsin digestion of tyrosinase. Co-migration experiments comparing synthetic peptide standards of all three possible phosphorylated tryptic peptides, a diphosphopeptide and two monophosphopeptides, to tyrosinase-phosphorylated in intact melanocytes by PKC-␤ and then subjected to trypsin digestion revealed that both serine residues are phosphorylated by PKC-␤. We conclude that PKC-␤ activates tyrosinase directly by phosphorylating serine residues at positions 505 and 509 in the cytoplasmic domain of this melanosome-associated protein.

Melanocytes synthesize melanin and transfer it to keratinocytes via dendritic processes. Keratinocytes are known to produce constitutively several factors, including endothelin-1 (ET-1), that together affect melanocyte proliferation, migration, melanogenesis, and dendrite formation. After ultraviolet (UV) irradiation, synthesis and secretion of ET-1 are up-regulated in keratinocytes. Because UV irradiation of skin is known to be associated with increased melanocyte dendricity, and because medium conditioned by UV-irradiated keratinocytes (UV-KCM) induces melanocyte dendricity to a greater degree than does baseline keratinocyte-conditioned medium (KCM), we investigated whether ET-1 promotes melanocyte dendricity. ET-1, originally recognized as a vasoconstrictive peptide, has recently been shown to stimulate melanocyte proliferation and tyrosinase activity. We now report that ET-1 supplementation of cultured melanocytes significantly increases the percentage of dendritic melanocytes, as well as dendrite length, in a dose-dependent manner. Moreover, UV-KCM was found to contain over 25-fold more ET-1 than KCM, and ET-1 supplementation of KCM induced melanocyte dendricity comparable to that induced by UV-KCM. Further, melanocyte dendricity induced by UV-KCM was significantly inhibited by the addition of anti-ET-1 monoclonal antibody to the medium, suggesting that the UV-KCM effect on melanocyte dendricity is mediated largely through ET-1. Our findings suggest that in the skin, ET-1 of keratinocyte origin promotes melanocyte dendricity in response to UV irradiation.