Eric Vandersmissen

In Vitro Stimulation of the Prepubertal Rat Gonadotropin-Releasing Hormone Pulse Generator by Leptin and Neuropeptide Y through Distinct Mechanisms

Leptin may act as a negative feedback signal to the brain in the control of appetite through suppression of neuropeptide Y (NPY) secretion and stimulation of cocaine- and amphetamine-regulated transcript (CART), a new anorectic peptide. We aimed at studying whether leptin, NPY, and CART have related effects on the hypothalamic control of the pituitary-gonadal system and the developmental changes in NPY and CART effects. Using retrochiasmatic hypothalamic explants from prepubertal 15-day-old male rats, the GnRH interpulse interval (mean ± sd : 62 ± 5 min) was significantly reduced by 10−7 m of leptin (46 ± 3.3 min) as well as 10−7 m of NPY (47 ± 4.4 min) and 10−6 m of CART (46 ± 2.7 min), whereas the GnRH pulse amplitude was not affected. The stimulatory effects of different NPY receptor agonists [human PYY 3–36, porcine NPY 13–36, human (D-Trp 32) NPY, porcine (Leu 31 Pro 34) NPY, human pancreatic polypeptide (PP)], as well as the absent effects of rat PP were consistent with the involvement of the Y5-rec...

Cocaine and Amphetamine-Regulated-Transcript Peptide Mediation of Leptin Stimulatory Effect on the Rat Gonadotropin-Releasing Hormone Pulse Generator In Vitro

Pulsatile gonadotropin‐releasing hormone (GnRH) secretion was studied in vitro using explants of the retrochiasmatic hypothalamus from prepubertal male and female rats. Leptin caused a dose‐dependent reduction of the GnRH interpulse interval in both sexes. We studied the effects of cocaine‐ and amphetamine‐regulated transcript (CART) since this peptide was shown recently to mediate the anorectic effects of leptin in the hypothalamus. CART caused a reduction of the GnRH interpulse interval. This effect was prevented using an anti‐CART antiserum which could partially overcome leptin stimulatory effects as well. Using hypothalamic explants from Zucker rats homozygous for the leptin receptor mutation ( fa/fa), GnRH pulse frequency was not affected by leptin, while a significant acceleration was caused by the CART‐peptide. In conclusion, leptin involves the hypothalamic CART‐peptide to stimulate the prepubertal GnRH pulse generator in vitro.

Leptin effects on pulsatile gonadotropin releasing hormone secretion from the adult rat hypothalamus and interaction with cocaine and amphetamine regulated transcript peptide and neuropeptide Y

Leptin may act as a negative feedback signal to the hypothalamic control of appetite through suppression of neuropeptide Y (NPY) secretion and stimulation of cocaine and amphetamine regulated transcript (CART). We aimed at studying the effects of leptin, CART and NPY on the hypothalamic control of the pituitary-gonadal system. Pulsatile gonadotropin-releasing hormone (GnRH) secretion was studied in vitro using retrochiasmatic hypothalamic explants from adult rats. In the female, GnRH pulse amplitude was significantly increased by leptin (10(-7) M) and CART (10(-6) M) irrespective of the estrus cycle phase while no such effects were seen in the male. The GnRH interpulse interval was not affected in both sexes. Passive immunoneutralization against CART caused a reduction in GnRH pulse amplitude in the female. A slight but significant increase in GnRH pulse amplitude was caused by NPY (10(-7) M) in the female. However, GnRH pulse amplitude was not affected by a Y5-receptor antagonist (10(-6) M) while the interpulse interval was significantly increased as shown previously in the male. The increase in GnRH pulse amplitude caused by leptin was totally prevented by coincubation with an anti-CART antiserum whereas it was not affected by coincubation with the NPY Y5-receptor antagonist (10(-7) M). In conclusion, leptin and NPY show separate permissive effects on GnRH secretion in the adult rat hypothalamus. In both sexes, NPY is prominently involved in the control of the frequency of pulsatile GnRH secretion through the Y5 receptor subtype. Leptin causes a female-specific facilitatory effect on GnRH pulse amplitude which is mediated by CART and which occurs irrespective of the estrus cycle phase.

Identification of neurotensin-related peptides in human thymic epithelial cell membranes and relationship with major histocompatibility complex class I molecules

This study shows the expression at the cell surface of human thymic epithelial cells (TEC) of a neurotensin (NT)-like immunoreactivity. NT radio-immunoassay (RIA) revealed that cultured human TEC contain +/-5 ng immunoreactive (ir) NT/10(6) cells, of which 5% is associated with plasma cell membranes. HPLC analysis of NT-ir present in human TEC showed a major peak of NT-ir corresponding to NT1-13. NT-ir was not detected in the supernatant of human TEC cultures. Using an affinity column prepared with a anti-MHC class I monoclonal antibody, NT-ir-related peptides were retained on the column and eluted together with MHC class I-related proteins. According to the elution time on HPLC of these peptides, they correspond to intact NT1-13, as well as to smaller fragments of NT1-13.

Cryptocrine signaling in the thymus network and T cell education to neuroendocrine self-antigens

Both during phylogeny and ontogeny the thymus appears as a nodal point between the two major systems of cell-to-cell signaling, the neuroendocrine and immune systems. This review presents the experimental observations which support a dual role in T cell selection played by the thymic repertoire of neuroendocrine polypeptide precursors. Through the mode of cryptocrine intercellular signaling thymic neuroendocrine-related precursors synthesized in thymic epithelial cells have been shown to influence the early steps in T cell differentiation. In addition, thymic neuroendocrine-related polypeptides are a source of self-antigens which are presented by the major histocompatibility system of the thymic epithelium. Preliminary data also suggest that the intrathymic T cell education to neuroendocrine self-antigens is not strictly superimposible to the antigen presentation by dedicated presenting cells. Insulin-like growth factor-II (IGF-II) was identified as one dominant member of the insulin family expressed by thymic epithelial and nurse cells. The intrathymic presentation of IGF-II or IGF-II derived self-antigens is under current investigation. If further confirmed, the central tolerogenic properties of IGF-II could be considered in the elaboration of a strategy for an efficient and safe prevention of insulin-dependent diabetes.

Cellular and Molecular Aspects of Thymic T-Cell Education in Neuroendocrine Self Principles: Implications for Autoimmunitya

Abstract: Thymic epithelial and nurse cells from different species express a repertoire of neuroendocrine polypeptide precursors. This repertoire exerts a dual role in T‐lymphocyte selection according to their status either as cryptocrine signals or as neuroendocrine self‐antigens of the peptide sequences that are processed from those precursors then presented to pre‐T cells. Thymic neuroendocrine self‐antigens correspond to peptide sequences highly conserved throughout evolution of their family. Though thymic MHC class I molecules are involved in the processing of thymic neuroendocrine self‐antigens, preliminary data show that their presentation to pre‐T cells is not allelically restricted. Thymic T‐cell education in neuroendocrine families also implies that the structure of a given family may be presented to pre‐T cells. Our studies have evidenced the homology between thymic neuroendocrine‐related self‐antigens and dominant T‐cell epitopes of peripheral neuroendocrine signals (neuroendocrine autoantigens). The biochemical difference between neuroendocrine autoantigens and homologous thymic self‐antigens might explain the opposite immune responses evoked by those two types of antigens (activation and memory induction vs. tolerogenic effect). Altogether, these studies support the therapeutic use of thymic neuroendocrine self‐antigens in reprogramming the immunological self‐tolerance that is broken in autoimmune endocrine diseases like insulin‐dependent diabetes type I. As recently stated by P. M. Allen in an important review, the fate of developing T lymphocytes in the thymus is influenced by the numerous types of peptidic interactions within the thymic cellular environment. 1 To define the precise nature of thymic cells and naturally occurring biochemical peptide signals involved in positive and negative selection of immature T cells has become a prominent objective for the future research efforts in thymic physiology. This paper will try to show how thymic neuroendocrine‐related peptides synthesized and processed within the thymic microenvironment indeed can play a role both in the development of the peripheral T‐cell repertoire and in the death of randomly rearranged, self‐reactive T cells.

Developmental and evolutionary aspects of thymic T-cell education to neuroendocrine self

Thymic epithelial cells, including nurse cells (TECs/TNCs), from various species synthesize neuroendocrine-related precursors belonging to neurohypophysial, tachykinin and insulin hormone families. The thymic repertoire of neuroendocrine-related polypeptides illustrates at the molecular level the paradoxical role of the thymus in both T cell positive and negative selection. On the one hand, these precursors are a source of signals which interact with neuroendocrine-type receptors expressed by target pre-T cells according to the cryptocrine type of cell-to-cell signaling. On the other hand, the same precursors constitute a source of self-antigens which are presented to pre-T cells by the thymic major histocompatibility complex system. Basically, the model of thymic T cell education to neuroendocrine self was established by the identification in TECs/TNCs of immunoreactive (ir) oxytocin as the self-antigen of the neurohypophysial family. Nevertheless, through the expression in TECs/TNCs of ir-neurokinin A and ir-insulin-like growth factor-II, the model also applies to the tachykinin and insulin hormone families.

Thymic neuroendocrine self peptides and T cell selection.

Our previous studies have shown that the thymic epithelial cells (TEC) of different animal species were the site for synthesis of polypeptide precursors belonging to the neurohypophysial (NHP), tachykinin (TK), and insulin neuroendocrine families1,2,3,4,5. However, at least in basal conditions, cultured human TEC do not secrete NHP-related peptides, neurokinin A (NKA) nor insulin-like growth factor 2 (IGF2); the existence of a classical secretory pathway in the thymic epithelium may thus be questioned. We also failed to detect immunoreactive (ir) thymic NHP-related peptides in classical secretory granules and a very elegant recent study has demonstrated that ir oxytocin (OT), the dominant thymic NHP-related peptide, was located diffusely in the cytosol and in clear vacuoles of murine TEC6. The term cryptocrine has been introduced in the word-list of Endocrinology to describe this particular type of cell-to-cell signaling in specialized microenvironments constituted by large “nursing” epithelial cells (like TEC/TNC in the thymus, or Sertoli cells in the testis) enclosing cell populations that migrate and differentiate at their very close contact (respectively, T cells and spermatids)7. In the general evolution of cell-to-cell communication, the cryptocrine type of signaling is located at a rather primitive step, between intercellular adhesion and paracrine exchanges of soluble signals. Moreover, in the thymus, the cryptocrine stage is closely associated with the presentation of the self molecular structure to the developing T cell system. Therefore, the thymus appears as one crucial meeting point for the two major systems of intercellular communication: therein, the endocrine system may influence the early steps of the immune response, whereas the immune system is educated in self neuroendocrine principles8. We would like to present here our experimental arguments that permit to transpose at the level of the thymic repertoire of neuroendocrine-related peptides the dual physiological role of this primary lymphoid organ in T cell positive and negative selection.