The natriuretic peptides are hormones that can stimulate natriuretic, diuretic, and vasorelaxant activity in vivo, presumably through the activation of two known cell surface receptor guanylyl cyclases (ANPR-A and ANPR-B). Although atrial natriuretic peptide (ANP) and, to a lesser extent, brain natriuretic peptide (BNP) are efficient activators of the ANPR-A guanylyl cyclase, neither hormone can significantly stimulate ANPR-B. A member of this hormone family, C-type natriuretic peptide (CNP), potently and selectively activated the human ANPR-B guanylyl cyclase. CNP does not increase guanosine 3',5'-monophosphate accumulation in cells expressing human ANPR-A. The affinity of CNP for ANPR-B is 50- or 500-fold higher than ANP or BNP, respectively. This ligand-receptor pair may be involved in the regulation of fluid homeostasis by the central nervous system.

Complementary DNA clones encoding two distinct tumor necrosis factor receptors were isolated from a mouse macrophage cDNA library. The cDNA for murine tumor necrosis factor receptor type 1 (mTNF-Rl) predicts a mature polypeptide of 425 amino acids that is 64% identical to its human counterpart, whereas the cDNA of murine tumor necrosis factor receptor type 2 (mTNF-R2) predicts a mature protein of 452 amino acids that is 62% identical to human tumor necrosis factor receptor type 2. The two murine tumor necrosis factor receptors have limited sequence homology (-20% identity) in their extracellular regions but no apparent similarity in their cytoplasmic portions. Northern (RNA) analysis indicates a single 2.6-kilobase (kb) transcript for mTNF-R1;,a 3.6-kb and a more predominant 4.5-kb transcript are observed for mTNF-R2. A human cell line transfected with either mTNF-R1 or mTNF-R2 expression vectors specifically bound I-labeled recombinant murine tumor necrosis factor a (TNF-a). Although mTNF-R1 had a similar affinity for both recombinant murine TNF-a and human TNF-a, mTNF-R2 showed strong specificity for recombinant murine TNF-a. This result suggests that the various activities of human tumor necrosis factor a reported in mice or in murine cell lines are probably mediated by mTNF-R1. and 75-kDa were identified (17)(18)(19). Studies with anti-55-kDa receptor (human type 1) antibodies suggest that this receptor is involved in several TNF-mediated processes, such as cytotoxicity, resistance to chlamidiae, and synthesis of prostaglandin E2 (17, 18, 20). Although the cDNAs for both human proteins have recently been cloned (21-23), the biological activity mediated by each has yet to be demonstrated through expression of the cloned genes.Similarly, studies conducted on mouse cell lines have also suggested the presence of more than one receptor for TNF. While the reported range of Kd values for binding of recombinant murine TNF-a (mTNF-a) to a number of mouse cell lines varies only slightly, the variability in the affinity of recombinant human TNF-a (hTNF-a) binding seemed to imply that murine cells possess at least two distinct receptors (24-27). Here we describe the cloning and expression of the murine homologs of the 55-kDa and 75-kDa human TNF

Transforming growth factor alpha (TGF-alpha) is produced by and required for the growth of epithelial cells and is angiogenic in vivo. Since epidermal hyperplasia and angiogenesis are hallmarks of psoriasis, TGF-alpha gene expression was analyzed in epidermal biopsies of normal and psoriatic skin. TGF-alpha messenger RNA and protein are much more abundant in lesional psoriatic epidermis than in normal-appearing skin of psoriatic patients or in normal epidermis. In contrast, messenger RNA levels of transforming growth factor beta 1 (TGF-beta 1), which inhibits epithelial cell growth, are not significantly different in normal, uninvolved, and lesional psoriatic epidermis. Thus, psoriatic epidermal hyperplasia may involve increased expression of a keratinocyte mitogen (TGF-alpha) rather than deficient expression of a growth inhibitor (TGF-beta 1).

Glial-cell-line-derived neurotrophic factor (GDNF) and neurturin (NTN) are two structurally related, potent survival factors for sympathetic, sensory and central nervous system neurons. GDNF mediates its actions through a multicomponent receptor system composed of a ligand-binding glycosyl-phosphatidylinositol (GPI)-linked protein (designated GDNFR-alpha) and the transmembrane protein tyrosine kinase Ret. In contrast, the mechanism by which the NTN signal is transmitted is not well understood. Here we describe the identification and tissue distribution of a GPI-linked protein (designated NTNR-alpha) that is structurally related to GDNFR-alpha. We further demonstrate that NTNR-alpha binds NTN (K[d] approximately 10 pM) but not GDNF with high affinity; that GDNFR-alpha binds to GDNF but not NTN with high affinity; and that cellular responses to NTN require the presence of NTNR-alpha. Finally, we show that NTN, in the presence of NTNR-alpha, induces tyrosine-phosphorylation of Ret, and that NTN, NTNR-alpha and Ret form a physical complex on the cell surface. These findings identify Ret and NTNR-alpha as signalling and ligand-binding components, respectively, of a receptor for NTN and define a novel family of receptors for neurotrophic and differentiation factors composed of a shared transmembrane protein tyrosine kinase and a ligand-specific GPI-linked protein.

Hepatocyte growth factor (HGF) is a potent mitogen for parenchymal liver, epithelial and endothelial cells. Structurally, it has similarities to kringle‐containing serine proteases, although it does not possess proteolytic activity. A structure‐activity relationship study of human HGF was performed by functional analysis of HGF substitution and deletion variants. Analysis of HGF variants was accomplished by defining their ability to induce DNA synthesis on hepatocytes in primary culture and to compete with wild‐type HGF for binding to a soluble form of the HGF receptor. Three groups of variants were made: (i) substitutions at the cleavage site, (ii) substitutions within the protease‐like domain and (iii) deletions of the beta‐chain and/or kringle domains. Our results show that: (i) single‐chain HGF is a zymogen‐like promitogen in that cleavage into a two‐chain form is required for biological activity, however, the single chain form of HGF still retains substantial receptor binding capacity; (ii) certain mutations in the protease‐like domain result in variants that are completely defective for mitogenic activity, yet exhibit apparent receptor binding affinities similar to wild‐type HGF (Kd approximately 50–70 pM); and (iii) a variant containing the N‐terminal 272 residues of mature HGF showed only a 4‐fold increase in Kd when compared with wild‐type HGF indicating that a primary receptor binding determinant is located within this sequence.