Involvement Of Central Leptin Signaling In Obesity-related Disordered Breathing

The prevalence of obesity is apparently increasing worldwide and it is also rising in young people.Obesity-related disordered breathing has severely threatened public health.At present,accumulated studies demonstrate that the cause of obesity-related disordered breathing is not merely associated with the increase in respiratory muscle-derived mechanical load in obese patients,but also the reduced central respiratory driving force,leptin signaling pathway abnormality or leptin resistance.However,the underlying mechanism remains incompletely understood.Leptin,a small molecule hormone produced mainly by white fat,plays multiple physiological roles through activation of leptin receptor-b（Lep Rb）and the downstream pathways.Of much interest,leptin signaling system has an obvious role in facilitation of breathing and amplification of central respiratory driving force.However,molecular neural mechanism is still to be underpinned.We have found that obese Zucker rats with leptin receptor deficiency exhibited hypoventilation during wakefulness and the hypercapnic ventilatory response（HCVR）was significantly attenuated,whereas the simple obese group with the matched weight and normal leptin receptor function manifested hypoventilation during exposure to room air without impaired HCVR,primarily due to excessive respiratory load.So far it is little known as to whether leptin signaling plays an important role in clinical obesity patients with obesity-related respiratory disorders.Accumulating evidence indicates that leptin signaling in the retrotrapezoid nucleus（RTN）contributed to regulation of breathing.RTN,as well as nucleus tractus solitarius（NTS）,functions as central respiratory chemoreceptor to control breathing.TASK-2 channels and G-protein coupled receptors 4 have been considered to mediate p H-sensitive responses in the RTN.The mechanism responsible for regulation of breathing by leptin signaling in the RTN remains elusive.Hence,using whole body plethysmography,molecular technique and histological methods,the present study would attempt to address the following issues in high fat diet-fed mice:1)Effect of diet induced obesity（DIO）on breathing;2)Mechanism underlying the role of leptin signaling in the RTN in obesity-related disordered breathing.The study would be expected to provide therapy strategy and molecular targets for obesity-related disordered breathing.Objective:To reveal the central mechanism underlying the role of leptin signaling in obesity-related disordered breathing.Methods:The experiment was randomly divided into DIO group and Lean group with male C57 mice aged 4-week years old.The DIO group was fed with high fat diet for 20 weeks,so the Lean group with standard chow diet,the final age was 24 weeks.Body weight was measured weekly.The whole body plethysmography was used to measure tidal volume（TV）,breathing frequency（BF）and minute ventilation（MV）.High concentration of CO₂ was inhaled to activate the central respiratory chemoreceptor.Leptin concentrations in serum and cerebrospinal fluid were measured by ELISA kit.Immunofluorescence was performed for counting the number of CO₂-activated neurons in the RTN.Using Western blot to quantitatively analyze the changes of downstream signaling pathway of Lep Rb in the RTN.Results:1.Body weight,the ratio of abdominal fat and body weight,leptin concentration in serum and cerebrospinal fluid in DIO group was significantly higher than that in Lean group（P<0.05～0.0001）.2.During exposure to room air（21%O₂）,TV and MV were moderately reduced in DIO group when compared with Lean group（P<0.05～0.0001）.During exposure to 2%,5%and 8%CO₂,BF,TV and MV increased gradually in both groups.But TV and MV were considerably reduced in DIO group relative to Lean group（P<0.05～0.0001）.3.During exposure to 5%CO₂,there was a negative correlation between MV with body weight and serum leptin level（P<0.05～0.01）.During exposure to 8%CO₂,a strong negative correlation between MV with body weight and serum leptin level（P<0.0001）.There was no significant correlation between cerebrospinal fluid leptin level and MV.4.Immunofluorescence results showed that the number of CO₂-activated RTN neurons was far less in DIO group compared with Lean group（P<0.05）.5.Quantitative analysis by Western blot showed that no significant difference was found in expression of Lep Rs in the RTN between DIO group and Lean group.Expressions of p STAT3,p AKT and p ERK proteins were significantly reduced（P<0.05）,whereas the SOCS3 protein was unregulated.6.TASK-2 channel expression in RTN in DIO group were significantly lower than that in Lean group.Conclusions:Obesity induced by high-fat diet produces basic hypoventilation and blunted HCVR.These effects are closely associated with leptin receptor-b dysfunction in the RTN,as well as reduced TASK-2 channel protein that determines p H sensitivity of RTN neurons.