Carboxypeptidase E, an essential element of the regulated secretory pathway, is expressed and partially co-localized with chromogranin A in chicken thymus

Although the functions of hormones and neuropeptides in the thymus have been extensively studied, we still do not know whether these intra-thymic humoral elements are released in a stimulated manner via the regulated secretory pathway or in a constitutive manner. Carboxypeptidase E (CpE) and chromogranin A (CgA) are functional and structural hallmarks of the regulated secretory pathway in (neuro)endocrine cells. Whereas we have previously shown a CgA-positive neuroendocrine population in the chicken thymus, the current study assesses the expression of CpE in the thymus, both at the mRNA and the protein level. Our immunohistochemical studies provide evidence for the co-existence of CgA and CpE in identical neuroendocrine cells in the thymus. CpE and CgA dual-positive cells have primarily been found in the transition zone between the cortex and medulla of the thymus, an area known to contain numerous arterioles and to be innervated by the autonomic nervous system. Our findings suggest that the diffuse neuroendocrine system serves as a relay for nervous stimuli delivered by the sympathetic and/or parasympathetic nervous system. Thus, these newly defined neuroendocrine cells might play an important role in the immuno-neuro-endocrine cross-talk in the thymus, potentially enabling thymopoiesis to be fine-tuned via the regulated secretory pathway by a variety of physical and environmental factors.