The Effect And Mechanism Of Neuregulin 4 And Calcitonin On Regulating Energy Homeostasis Through The Hypothalamus

Obesity is often closely related to the development of a variety of chronic diseases,such as metabolic diseases and cardiovascular diseases.Energy homeostasis is the basic requirement of living organisms and depends on the balance between energy intake and energy consumption.The malfunction of the regulatory system that maintains energy homeostasis can lead to fat accumulation and body weight gain.It is well known that the hypothalamus is the central regulator of energy balance.Many peripheral hormones such as leptin and insulin are able to regulate energy balance and glucolipid metabolism by acting on the hypothalamus.Neuregulin 4(Nrg4)and calcitonin are also small molecule hormones secreted by peripheral tissues,but the exact mechanism by which they regulate energy balance through the hypothalamus remains unclear.Part Ⅰ.The effect and mechanism of neuregulin 4 on regulating energy homeostasis through the hypothalamusNrg4,a member of the epidermal growth factor family,is mainly expressed in brown adipose tissue.Nrg4 is closely associated with obesity and improves glucolipid metabolism through peripheral metabolic tissues.Its tyrosine kinase receptor ErbB4 is abundantly expressed in the hypothalamus.This part of the study investigates the central effects and mechanisms of Nrg4-ErbB4 signaling in regulating energy balance.Nrg4 and ErbB4 mRNA expression levels in various metabolic tissues were measured by real-time quantitative PCR,and it was found that Nrg4 was mainly expressed in brown adipose tissue while ErbB4 was mainly expressed in the hypothalamus.Immunofluorescence staining of c-Fos on brain tissue sections and western blotting of hypothalamic p-ErbB4 revealed that peripheral Nrg4 could act on hypothalamic ErbB4 and activate PVN neurons.Lateral ventricular injection of Nrg4 recombinant protein(rNrg4)ameliorated energy imbalance and metabolic disorders in HFD-fed mice.Inhibition of PVN ErbB4-expressing neurons by chemical genetic techniques could block the central appetite suppressing effects of rNrg4.ErbB4 was expressed in hypothalamic PVN oxytocin(Oxt)neurons.We found that inhibition of ErbB4 in PVN Oxt neurons exacerbated the poor metabolic status of HFD-fed mice.Moreover,double immunofluorescence staining of Oxt and c-Fos indicated that Nrg4 treatment activated PVN Oxt neurons.Serum Oxt levels were measured by enzyme-linked immunosorbent assay(ELISA),and it was found that inhibition of ErbB4 in Oxt neurons could eliminate the effect of increased serum Oxt levels induced by rNrg4 treatment.Also Oxt release assay confirmed that Nrg4-ErbB4 signaling stimulated PVN Oxt neurons to release oxytocin,and oxytocin supplementation somewhat ameliorated the energy imbalance and metabolic disturbance caused by ErbB4 inhibition.Taken together,Nrg4 acts on PVN Oxt neurons via ErbB4 to promote Oxt release,which in turn regulates energy homeostasis and glucolipid metabolism homeostasis.Part Ⅱ.The effect and mechanism of calcitonin on regulating energy homeostasis through the hypothalamusCalcitonin,a circulating hormone,is produced by parafollicular cells of the thyroid gland.Calcitonin is closely associated with obesity,and salmon calcitonin(sCT)treatment ameliorates disorders of glucolipid metabolism and energy imbalance in obese rats.Calcitonin receptor(Calcr),a seven-transmembrane G protein-coupled receptor,is abundantly expressed in the brain,especially in the hypothalamus.This part of the study investigates the role and mechanism of calcitonin in regulating energy balance through the hypothalamus.The expression of Calcr in the hypothalamic arcuate nucleus was detected by immunofluorescence staining in different metabolic states,and it was found that the expression of Calcr in the arcuate nucleus was significantly decreased in the HFDfeeding and fasting states.Body weight and food intake of HFD-fed mice were significantly reduced after lateral ventricular injection of sCT,while energy expenditure was significantly increased.The co-staining of AgRP and c-Fos in the arcuate nucleus of HFD-fed mice was significantly reduced after intraperitoneal injection of sCT,while the expression of AgRP in the paraventricular nucleus was significantly reduced and the expression of c-Fos was significantly increased,and the expression of pStat3 in the arcuate nucleus was significantly increased.Taken together,calcitonin can inhibit AgRP expression and the activity of AgRP neurons through Stat3 pathway,which in turn suppresses appetite and promotes energy expenditure.