Peptide-amidating Enzymes Are Expressed in the Stellate Epithelial Cells of the Thymic Medulla

Abstract

SUMMARY C-terminal amidation is a post-translational processing step necessary to convey biological activity to a large number of regulatory peptides. In this study we have demonstrated that the peptidyl-glycine ␣ -amidating monooxygenase enzyme complex (PAM) responsible for this activity is located in the medullary stellate epithelial cells of the thymus and in cultured epithelial cells bearing a medullary phenotype, using Northern blot, immunocytochemistry, in situ hybridization, and enzyme assays. Immunocytochemical localization revealed a granular pattern in the cytoplasm of the stellate cells, which were also positive for cytokeratins and a B-lymphocyte-associated antigen. The presence of PAM activity in medium conditioned by thymic epithelial cell lines suggests that PAM is a secreted product of these cells. Among the four epithelial cell lines examined, there was a direct correlation between PAM activity and content of oxytocin, an amidated peptide. Taken together, these data provide convincing evidence that thymic epithelial cells have the capacity to generate amidated peptides that may influence T-cell differentiation and suggest that the amidating enzymes could play an important role in the regulation of thymic physiology.

Medullary Thymic Epithelium: A Mosaic of Epithelial “Self”?

“…If expression of differentiated epithelial gene products reflects differentiation of medullary TE along different lineage-restricted pathways, these proteins should be heterogeneously expressed with different gene products associated with distinct subsets of medullary thymic epithelium. Consistent with this hypothesis, thymic expression of PAM is restricted to small scattered foci of medullary TE (14), as are cells expressing high molecular weight keratins associated with terminally differentiated keratinocytes (24).…”

“…The association of numerous neuroendocrine peptides such as vasopressin and oxytocin with thymic epithelial cells in situ (13), and the production of some of these hormones by thymic epithelial cell lines in vitro (14), has led to the consideration of the thymus as an endocrine organ. Additional proteins that are normally associated with other tissues/cell types and have been localized to the thymus at the mRNA level include insulin (15), interphotoreceptor binding protein (16), and myelin basic protein (17), as well as the precursor of peptide amidating enzymes necessary to generate biologically active regulatory peptide hormones (peptidyl-glycine a-amidating monooxygenase [PAM]; reference 14). Because of the diversity of these “tissue-specific” molecules detected within the thymus and the observation that thymic and hepatic CRP expression appears to be similarly regulated (12), thymic expression of these genes may reflect a general physiological property of medullary thymic epithelium.…”

Ontogenesis and functional aspects of oxytocin and vasopressin gene expression in the thymus network

“…This difference in the delay of OT appearance in the thymus and in the brain suggests a difference in the processing between hypothalamic neurons and TEC. Such discrepancy does not concern the late steps of OT processing since peptidyl-glycine-a-amidating monooxygenase (PAM), the last enzyme in the processing of neurohypophysial peptides (Bradbury et al, 1982), is present in its active form in rat thymic TEC (Martínez et al, 1998). It may be related to the difference in the precursor processing in association with neurohormone secretion in the hypothalamo-neurohypophysial axis, and in association with self-antigen presentation in the thymus during induction of central T-cell self-tolerance.…”

Fetal development of regulatory mechanisms for body fluid homeostasis

“…www.bjournal.com.br an amidating monooxygenase, which is present in an active form in thymic epithelial cells (39). Among all animals, AVP mRNA expression was significantly negatively correlated with pituitary AVP content.…”