The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfor...

Amyloid ␤ peptide (A␤), the principal proteinaceous component of amyloid plaques in brains of Alzheimer's disease patients, is derived by proteolytic cleavage of the amyloid precursor protein (APP). Proteolytic cleavage of APP by a putative ␣-secretase within the A␤ sequence precludes the formation of the amyloidogenic peptides and leads to the release of soluble APPs␣ into the medium. By overexpression of a disintegrin and metalloprotease (ADAM), classified as ADAM 10, in HEK 293 cells, basal and protein kinase C-stimulated ␣-secretase activity was increased severalfold. The proteolytically activated form of ADAM 10 was localized by cell surface biotinylation in the plasma membrane, but the majority of the proenzyme was found in the Golgi. These results support the view that APP is cleaved both at the cell surface and along the secretory pathway. Endogenous ␣-secretase activity was inhibited by a dominant negative form of ADAM 10 with a point mutation in the zinc binding site. Studies with purified ADAM 10 and A␤ fragments confirm the correct ␣-secretase cleavage site and demonstrate a dependence on the substrate's conformation. Our results provide evidence that ADAM 10 has ␣-secretase activity and many properties expected for the proteolytic processing of APP. Increases of its expression and activity might be beneficial for the treatment of Alzheimer's disease.The amyloid precursor protein (APP) is a type I transmembrane glycoprotein constitutively expressed in many types of mammalian cells. APP is the precursor of the amyloid ␤ peptide (A␤), the principal proteinaceous component of amyloid plaques in brains of AlzheimerЈs disease patients (1, 2). The A␤ sequence includes 28 amino acids of the extracellular and 12-15 residues of the membrane-spanning region of APP. A␤ is derived by proteolytic processing of the precursor protein from as yet not identified proteases, the ␤-secretase cleaving at the N terminus, and the ␥-secretase cleaving at the C terminus.The major proteolytic pathway of APP is the constitutive secretory pathway that involves cleavage by a putative ␣-secretase within the A␤ sequence at the cell surface (3-5) and in the trans-Golgi network (6-9). Soluble N-terminal APP (APPs␣) fragments of 105-125 kDa (10) are released into the extracellular medium. The membrane-bound 10-kDa C-terminal fragment (p10) produced by ␣-secretase cleavage of APP contains only part of the amyloidogenic A␤ and can be further cleaved by the ␥-secretase to yield a secreted 3-kDa fragment (p3) (11). Because this pathway does not produce intact A␤, it is nonamyloidogenic and cannot lead to Alzheimer's disease pathology.There has been intensive interest in the secretase enzymes that cleave APP in relation to the pathology of AlzheimerЈs disease. The putative ␣-secretase cleavage site has been precisely determined (12): in human embryonic kidney cells (HEK 293) transfected with various constructs of APP, the soluble form ends at Gln-15 of A␤, and the N terminus of the cleaved C-terminal fragment begins at Leu-17. It ...

Biochemical, epidemiological, and genetic findings demonstrate a link between cholesterol levels, processing of the amyloid precursor protein (APP), and Alzheimer's disease. In the present report, we identify the ␣-secretase ADAM 10 (a disintegrin and metalloprotease) as a major target of the cholesterol effects on APP metabolism. Treatment of various peripheral and neural cell lines with either the cholesterol-extracting agent methyl-␤-cyclodextrin or the hydroxymethyl glutaryl-CoA reductase inhibitor lovastatin resulted in a drastic increase of secreted ␣-secretase cleaved soluble APP. This strong stimulatory effect was in the range obtained with phorbol esters and was further increased in cells overexpressing ADAM 10. In cells overexpressing APP, the increase of ␣-secretase activity resulted in a decreased secretion of A␤ peptides. Several mechanisms were elucidated as being the basis of enhanced ␣-secretase activity: increased membrane fluidity and impaired internalization of APP were responsible for the effect observed with methyl-␤-cyclodextrin; treatment with lovastatin resulted in higher expression of the ␣-secretase ADAM 10. Our results demonstrate that cholesterol reduction promotes the nonamyloidogenic ␣-secretase pathway and the formation of neuroprotective ␣-secretase cleaved soluble APP by several mechanisms and suggest approaches to prevention of or therapy for Alzheimer's disease.A myloid-␤ peptides (A␤), the principal proteinaceous components of amyloid plaques in brains of Alzheimer's disease (AD) patients, are derived from proteolytic cleavage of the amyloid precursor protein (APP), a type I integral membrane protein that is ubiquitously expressed. Both during and after its transport through the secretory pathway to the surface of cultured cells, a fraction of APP molecules undergoes specific endoproteolytic cleavage, most frequently by a scission between amino acids 16 and 17 of the A␤ region (1). This principal secretory cleavage is effected by (a) protease(s) designated as ␣-secretase(s). Soluble N-terminal APP fragments of 105-125 kDa are released into vesicle lumens and from the cell surface; similar species are readily detected in human plasma and cerebrospinal fluid (2). Recently, evidence has been provided that members of the ADAM family (a disintegrin and metalloprotease) act as ␣-secretases (3-5). For ADAM 10, basal and protein kinase C-stimulated ␣-secretase activity and many properties expected for the proteolytic processing of APP have been found (4).The stimulation of ␣-secretase activity and an increase of ␣-secretase cleaved soluble APP (APPs␣) might be beneficial for the treatment of AD for several reasons. In principle, proteolytic cleavage of APP within the A␤ sequence precludes the formation of the amyloid peptides derived from alternative proteolysis of APP with the ␤-secretase cleaving at the N terminus and the ␥-secretase(s) at the C terminus of A␤ peptides (for a recent review of APP processing, see ref. 6). On the other hand, APPs␣ has trophic effects on cerebral neuro...