NMB receptor

Bombesin is an endogenous peptide involved in a wide spectrum of physiological activities ranging from satiety, control of circadian rhythm and thermoregulation in the central nervous system, to stimulation of gastrointestinal hormone release, activation of macrophages and effects on development in peripheral tissues. Actions of the peptide are mediated through the two high affinity G-protein coupled receptors BB1R and BB2R. Under pathophysiological conditions, these receptors are overexpressed in many different types of tumors, such as prostate cancer, breast cancer, small and non-small cell lung cancer and pancreatic cancer. This observation has been used for designing cell markers, but it has not been yet exploited for therapeutical purposes. Despite the enormous biological interest of the peptide, little is known about the stereochemical features that contribute to their activity. On the one hand, mutagenesis studies identified a few receptor residues important for high bombesin affinity and on the other, a few studies focused on the relevance of diverse residues of the peptide for receptor activation. Models of the peptide bound to BB1R and BB2R can be helpful to improve our understanding of the stereochemical features granting bombesin activity. Accordingly, the present study describes the computational process followed to construct such models by means of Steered Molecular Dynamics, using models of the peptide and its receptors. Present results provide new insights into the structure-activity relationships of bombesin and its receptors, as well as render an explanation for the differential binding affinity observed towards BB1R and BB2R. Finally, these models can be further exploited to help for designing novel small molecule peptidomimetics with improved pharmacokinetics profile.

Cushing's disease (CD) results from autonomous adrenocorticotropic hormone (ACTH) secretion by corticotroph adenomas, leading to excessive cortisol production, ultimately affecting morbidity and mortality. Pasireotide is the only FDA approved tumor directed treatment for CD, but it is effective in only about 25% of patients, and is associated with a high rate of hyperglycemia. Neuromedin B (NMB), a member of the bombesin-like peptide family, regulates endocrine secretion and cell proliferation. Here, we assessed NMB and NMB receptor (NMBR) expression in human corticotroph adenomas and the effects of NMBR antagonist PD168368 on murine and human corticotroph tumors.

To investigate NMB and NMBR expression, real-time qPCR and immunostaining on human pathological specimens of corticotroph, non-functional and somatotroph adenomas were performed. The effects of PD168368 on hormone secretion and cell proliferation were studied in vitro, in vivo and in seven patient-derived corticotroph adenoma cells. NMB and NMBR were expressed in higher extent in human corticotroph adenomas compared with non-functional or somatotroph adenomas.

In murine AtT-20 cells, PD168368 reduced proopiomelanocortin (Pomc) mRNA/protein expression and ACTH secretion as well as cell proliferation. In mice with tumor xenografts, tumor growth, ACTH and corticosterone were downregulated by PD168368. In patient-derived adenoma cells, PD168368 reduced POMC mRNA expression in four out of seven cases and ACTH secretion in two out of five cases. A PD168368-mediated cyclin E suppression was also identified in AtT-20 and patient-derived cells.

NMBR antagonist represents a potential treatment for CD and its effect may be mediated by cyclin E suppression.

Respiratory chemoreceptor activity encoding arterial PCO₂ and PO₂ is a critical determinant of ventilation. Currently, the relative importance of several putative chemoreceptor mechanisms for maintaining eupneic breathing and respiratory homeostasis is debated. Transcriptomic and anatomical evidence suggest that bombesin-related peptide Neuromedin-B (Nmb) expression identifies chemoreceptor neurons in the retrotrapezoid nucleus (RTN) that mediate the hypercapnic ventilatory response, but functional support is missing. In this study, we generated a transgenic Nmb-Cre mouse and used Cre-dependent cell ablation and optogenetics to test the hypothesis that RTN ^Nmb neurons are necessary for the CO₂-depedent drive to breathe in adult male and female mice. Selective ablation of ∼95% of RTN ^Nmb neurons causes compensated respiratory acidosis due to alveolar hypoventilation, as well as profound breathing instability and respiratory-related sleep disruption. Following RTN ^Nmb lesion, mice were hypoxemic at rest and were prone to severe apneas during hyperoxia, suggesting that oxygen-sensitive mechanisms, presumably the peripheral chemoreceptors, compensate for the loss of RTN ^Nmb neurons. Interestingly, ventilation following RTN ^Nmb -lesion was unresponsive to hypercapnia, but behavioral responses to CO₂ (freezing and avoidance) and the hypoxia ventilatory response were preserved. Neuroanatomical mapping shows that RTN ^Nmb neurons are highly collateralized and innervate the respiratory-related centers in the pons and medulla with a strong ipsilateral preference. Together, this evidence suggests RTN ^Nmb neurons are dedicated to the respiratory effects of arterial PCO₂/pH and maintain respiratory homeostasis in intact conditions and suggest that malfunction of these neurons could underlie the etiology of certain forms of sleep-disordered breathing in humans.Significance Statement:Respiratory chemoreceptors stimulate neural respiratory motor output to regulate arterial PCO₂ and PO₂, thereby maintaining optimal gas exchange. Neurons in the retrotrapezoid nucleus (RTN) that express the bombesin-related peptide Neuromedin-B are proposed to be important in this process, but functional evidence has not been established. Here, we developed a transgenic mouse model and demonstrated that RTN neurons are fundamental for respiratory homeostasis and mediate the stimulatory effects of CO₂ on breathing. Our functional and anatomical data indicate that Nmb-expressing RTN neurons are an integral component of the neural mechanisms that mediate CO₂-dependent drive to breathe and maintain alveolar ventilation. This work highlights the importance of the interdependent and dynamic integration of CO₂- and O₂-sensing mechanisms in respiratory homeostasis of mammals.