Identification of peptides from brain and pituitary of Cpe(fat)/Cpe(fat) mice.

Cpe(fat)/Cpe(fat) mice have a naturally occurring point mutation within the carboxypeptidase E gene that inactivates this enzyme, leading to an accumulation of many neuroendocrine peptides containing C-terminal basic residues. These processing intermediates can be readily purified on an anhydrotrypsin affinity resin. Using MS to obtain molecular mass and partial sequence information, more than 100 peptides have been identified. These peptides represent fragments of 16 known secretory pathway proteins, including proenkephalin, proopiomelanocortin, protachykinins A and B, chromogranin A and B, and secretogranin II. Many of the identified peptides represent previously uncharacterized fragments of the precursors. For example, 12 of the 13 chromogranin B-derived peptides found in the present study have not been previously reported. Of these 13 chromogranin B-derived peptides, only five contain consensus cleavage sites for prohormone convertases at both the C and N termini. Two distinct chromogranin B-derived peptides result from cleavage at Trp-Trp bonds, a site not typically associated with neuropeptide processing. An RIA was used to confirm that one of these peptides, designated WE-15, exists in wild-type mouse brain, thus validating the approach to identify peptides in Cpe(fat)/Cpe(fat) mice. These "orphan" peptides are candidate ligands for orphan G protein-coupled receptors. In addition, the general technique of using affinity chromatography to isolate endogenous substrates from a mutant organism lacking an enzyme should be applicable to a wide range of enzyme-substrate systems.

ProSAAS processing in mouse brain and pituitary.

ProSAAS is a newly discovered protein with a neuroendocrine distribution generally similar to that of prohormone convertase 1 (PC1), a peptide-processing endopeptidase. Several proSAAS-derived peptides were previously identified in the brain and pituitary of the Cpe(fat)/Cpe(fat) mouse based on the accumulation of C-terminally extended peptides due to the absence of enzymatically active carboxypeptidase E, a peptide-processing exopeptidase. In the present study, antisera against different regions of proSAAS were used to develop radioimmunoassays and examine the processing profile of proSAAS in wild type and Cpe(fat)/Cpe(fat) mouse tissues following gel filtration and reverse phase high performance liquid chromatography. In wild type mouse brain and pituitary, the majority of proSAAS is processed into smaller peptides. These proSAAS-derived peptides elute from the reverse-phase column in the same positions as synthetic peptides that correspond to little SAAS, PEN, and big LEN. Mass spectrometry revealed the presence of peptides with the expected molecular masses of little SAAS and big LEN in the fractions containing immunoreactive peptides. The processing of proSAAS is slightly impaired in Cpe(fat)/Cpe(fat) mice, relative to wild-type mice, leading to the accumulation of partially processed peptides. One of these peptides, the C-terminally extended form of PEN, is known to inhibit PC1 activity and this could account for the reduction in enzymatically active PC1 seen in Cpe(fat)/Cpe(fat) mice. The observation that little SAAS and big LEN are the major forms of these peptides produced in mouse brain and pituitary raises the possibility that these peptides function as neurotransmitters or hormones.

Impaired prohormone convertases in Cpe(fat)/Cpe(fat) mice.

A spontaneous point mutation in the coding region of the carboxypeptidase E (CPE) gene results in a loss of CPE activity that correlates with the development of late onset obesity (Nagert, J. K., Fricker, L. D., Varlamov, O., Nishina, P. M., Rouille, Y., Steiner, D. F., Carroll, R. J., Paigen, B. J., and Leiter, E. H. (1995) Nat. Genet. 10, 135-142). Examination of the level of neuropeptides in these mice showed a decrease in mature bioactive peptides as a result of a decrease in both carboxypeptidase and prohormone convertase activities. A defect in CPE is not expected to affect endoproteolytic processing. In this report we have addressed the mechanism of this unexpected finding by directly examining the expression of the major precursor processing endoproteases, prohormone convertases PC1 and PC2 in Cpe(fat) mice. We found that the levels of PC1 and PC2 are differentially altered in a number of brain regions and in the pituitary. Since these enzymes have been implicated in the generation of neuroendocrine peptides (dynorphin A-17, beta-endorphin, and alpha- melanocyte-stimulating hormone) involved in the control of feeding behavior and body weight, we compared the levels of these peptides in Cpe(fat) and wild type animals. We found a marked increase in the level of dynorphin A-17, a decrease in the level of alpha-melanocyte-stimulating hormone, and an alteration in the level of C-terminally processed beta-endorphin. These results suggest that the impairment in the level of these and other peptides involved in body weight regulation is mainly due to an alteration in carboxypeptidase and prohormone convertase activities and that this may lead to the development of obesity in these animals.

Defective prodynorphin processing in mice lacking prohormone convertase PC2.

Prodynorphin, a multifunctional precursor of several important opioid peptides, is expressed widely in the CNS. It is processed at specific single and paired basic sites to generate various biologically active products. Among the prohormone convertases (PCs), PC1 and PC2 are expressed widely in neuroendocrine tissues and have been proposed to be the major convertases involved in the biosynthesis of hormonal and neural peptides. In this study we have examined the physiological involvement of PC2 in the generation of dynorphin (Dyn) peptides in mice lacking active PC2 as a result of gene disruption. Enzymological and immunological assays were used to confirm the absence of active PC2 in these mice. The processing profiles of Dyn peptides extracted from brains of these mice reveal a complete lack of Dyn A-8 and a substantial reduction in the levels of Dyn A-17 and Dyn B-13. Thus, PC2 appears to be involved in monobasic processing, leading to the generation of Dyn A-8, Dyn A-17, and Dyn B-13 from prodynorphin under physiological conditions. Brains of heterozygous mice exhibit only half the PC2 activity of wild-type mice; however, the levels of Dyn peptides in these mice are similar to those of wild-type mice, suggesting that a 50% reduction in PC2 activity is not sufficient to significantly reduce prodynorphin processing. The disruption of the PC2 gene does not lead to compensatory up-regulation in the levels of other convertases with similar substrate specificity because we find no significant changes in the levels of PC1, PC5/PC6, or furin in these mice as compared with wild-type mice. Taken together, these results support a critical role for PC2 in the generation of Dyn peptides.

The C-terminal region of proSAAS is a potent inhibitor of prohormone convertase 1.

ProSAAS is a recently discovered 26-kDa neuroendocrine protein that was previously found to inhibit prohormone convertase (PC) 1 and not PC2. In the present study, the specificity of proSAAS toward other members of the prohormone convertase family was determined. Two microm proSAAS selectively inhibits PC1 but not furin, PACE4, PC5A, or PC7. The PC1 inhibitory region of proSAAS was mapped to an 8-12-residue region near the C terminus that includes a critical Lys-Arg sequence. Synthetic peptides corresponding to this region are competitive inhibitors of PC1 with apparent K(i) values of 14-40 nm. The inhibition becomes more effective with incubation time, indicating that the inhibitor is slow binding. A fusion protein containing the inhibitory region of proSAAS linked to the C terminus of glutathione S-transferase binds the 71-kDa form but not the 85-kDa form of PC1. This binding, which occurs at pH 5.5 and not at pH 7.4, is stable to incubation at room temperature for 1 h in the presence or absence of 0.5% Triton X-100 and/or 0.5 m NaCl. The removal of Ca(2+) with chelating agents partially releases the bound PC1. High concentrations of the inhibitory peptide quantitatively release the bound PC1. Taken together, these data support the proposal that proSAAS functions as an endogenous inhibitor of PC1.

Identification and characterization of proSAAS, a granin-like neuroendocrine peptide precursor that inhibits prohormone processing.

Five novel peptides were identified in the brains of mice lacking active carboxypeptidase E, a neuropeptide-processing enzyme. These peptides are produced from a single precursor, termed proSAAS, which is present in human, mouse, and rat. ProSAAS mRNA is expressed primarily in brain and other neuroendocrine tissues (pituitary, adrenal, pancreas); within brain, the mRNA is broadly distributed among neurons. When expressed in AtT-20 cells, proSAAS is secreted via the regulated pathway and is also processed at paired-basic cleavage sites into smaller peptides. Overexpression of proSAAS in the AtT-20 cells substantially reduces the rate of processing of the endogenous prohormone proopiomelanocortin. Purified proSAAS inhibits prohormone convertase 1 activity with an IC(50) of 590 nM but does not inhibit prohormone convertase 2. Taken together, proSAAS may represent an endogenous inhibitor of prohormone convertase 1.

Cloning, expression, and characterization of human metalloprotease 1: a novel member of the pitrilysin family of metalloendoproteases.

A novel cDNA, designated human metalloendoprotease 1 (hMP1), was identified on the basis of homology to known metalloendoproteases of the pitrilysin family. The full-length MP1 codes for a protein with an open reading frame of 1038 amino acids. The N-terminal region contains the HXXEH(X)76E catalytic domain that is conserved in the members of pitrilysin family, namely insulin-degrading enzyme and NRD convertase. The hMP1 mRNA is expressed in a number of cell lines and tissues as a single species of about 3.4 kb. The expression of hMP1 mRNA is higher in muscle and heart than in brain, pancreas, liver, lung, and placenta. The full-length hMP1 was expressed in the baculovirus system and purified to homogeneity using isoelectrofocusing and ion-exchange chromatography. The enzyme exhibited a neutral pH optimum and high sensitivity to thiol reagents. HMP1 was inactivated by 1,10-phenanthroline, a specific inhibitor of Zn(+2)-dependent metalloproteases. The enzyme was not inhibited by agents that inhibit neutral metalloendoproteases of the thermolysin family such as thimet endo-oligopeptidase, enkephalinase, or angiotensin-converting enzyme. HMP1 cleaved a prodynorphin-derived peptide, leumorphin, N-terminal to Arg in the monobasic processing site, as evidenced by MALDI-TOF mass spectrometry. However, the enzyme did not exhibit strict monobasic cleavage specificity, as peptide substrates with amino acid substitutions around the monobasic site was cleaved efficiently by hMP1. Taken together, these results suggest that hMP1 is a novel member of the metalloendoprotease superfamily with ubiquitous distribution that could play a broad role in general cellular regulation.