Mild hypothermia ameliorates muscle wasting in septic rats associated with hypothalamic AMPK-induced autophagy and neuropeptides

Highlights

• Sepsis decreases hypothalamic AMPK-induced autophagy and neuropeptides.

• Muscle wasting is associated with hypothalamic autophagy and neuropeptides.

• Mild hypothermia ameliorates muscle wasting during sepsis.

• Mild hypothermia reverses hypothalamic AMPK-induced autophagy and neuropeptides.

Abstract

Sepsis, always developing muscle wasting, contributes to serious complications and mortality. Mild hypothermia has been reported to have protective effects on the prognosis of septic patients. However, the underlying mechanisms remain unclear. We therefore hypothesized that mild hypothermia could ameliorate muscle wasting during sepsis and whether it was associated with hypothalamus AMPK-induced autophagy and neuropeptides. Adult male Sprague-Dawley rats were intraperitoneally injected with lipopolysaccharide (LPS) (5 mg/kg) or saline. Mild hypothermia was instantly induced at 33 °C for 3h after LPS injected. Meanwhile, the control and sepsis groups were simultaneously placed on the thermal mattress to maintain the a normal temperature in control group whatever the changes induced by anesthesia. Twenty-four hours after injection, skeletal muscle and hypothalamus tissues were obtained. Muscle wasting was measured by the mRNA expression of two muscle atrophic genes, muscle ring finger 1 (MuRF-1) and muscle atrophy F-box (MAFbx), as well as 3-methylhistidine (3-MH) and tyrosine release. Hypothalamic AMPK-induced autophagy markers and neuropeptides expression were also detected. Results showed that LPS administration significantly decreased hypothalamic AMPK-induced autophagy together with muscle wasting. Also, increased hypothalamic neuropeptides, proopiomelanocortin (POMC), cocaine and amphetamine-related transcript (CART) and neuro-peptides Y (NPY) and decreased agouti-related protein (AgRP) were observed. Mild hypothermia significantly increased hypothalamic AMPK-induced autophagy and ameliorated LPS-induced muscle wasting, and attenuated the alteration of neuropeptides, POMC, CART and NPY. In conclusion, mild hypothermia could alleviate muscle wasting by LPS injection, which was associated with reversing the level of hypothalamic AMPK-induced autophagy and the alteration of neuropeptides. These results suggested that mild hypothermia could be a potential treatment concept and a novel mechanism in management of muscle wasting in critically ill patients.

Introduction

Sepsis, presenting a hypermetabolic state associated with systemic inflammatory response, contributes to serious complications and mortality in the intensive care unit (ICU). Among the hypermetabolic state, acute muscle wasting is the significant and important metabolic change in the early stage of sepsis [1]. The recently research reported by Zudin found muscle wasting occurred early and rapidly during the first week of critical illness regardless of nutrition strategies and the severe muscle wasting interfered with recovery from primary disease, which was closely associated with the prognosis [2]. To counteract the hypercatabolism, EPaNIC and EDEN trials employed different nutrition strategies in critical patients, finally failing to show the effects of early parental feeding or full enteral feeding [3], [4]. What's more, too much exogenous protein may aggravate hypercatabolism and increase serious complications in early critical illness [3]. Several researches attempting to attenuate the sustained hypercatabolism during sepsis showed little effects, including hydrocortison, insuin, activated protein C and estrogen [5], [6], [7], [8], [9].

Among potential approaches, Mild hypothermia (MH) could be an efficacious one. Induced MH decreased the mortality in nineteen consecutive patients with septic adult respiratory distress syndrome [10]. The induction of MH decreased the demand for vasopressors after resuscitation and fever control in patients with septic shock and improved myocardial function in patients with cardiogenic shock [11], [12]. Septic animal mode also showed a consistent protective effect of MH. Prompt MH significantly increased the survival duration of septic rats maintained at 34 °C throughout the experiment compared with those maintained at 38 °C [13]. The research reported by Chisholm K showed sepsis was associated with brain dysfunction and influenced cortical oxygenation and mitochondrial function, and that MH could be protective [14]. Induced MH could reversed low oxygen consumption and ameliorate mitochondrial function in skeletal muscle [15]. Whereas, the potential role of MH on alleviating hypermetabolic state and muscle wasting remains uncertain.

Hypothalamus is the regulatory center of temperature, metabolism and food intake. In previous studies, we found that the arcuate nucleus (ARC) of hypothalamus exerted a critical role in the regulation of acute skeletal muscle wasting during sepsis [7], [8], [9], [16]. Two major populations of neural in the ARC, AgRP and POMC neurons, play a pivotal role in calorie intake and energy expenditure [17], [18]. The former neuron co-expresses orexigenic peptides NPY and AgRP, which promote food intake and decrease metabolism by acting as an inverse agonist of the type-4 melanocortin receptor (MC4-R). Whereas the other neuron co-expresses the anorexigenic POMC and CARTPT/CART. POMC could be pyrolysed into α-mela-nocytestimulating hormone (α-MSH), binding to MC3-R and MC4-R to derive anorectic effects [19]. Recent studies also provide the evidence that hypothalamus autophagy is closely involved in the regulation of food intake [20], [21]. The specific loss of autophagy in hypothalamic POMC decreases α-MSH levels and elevates adiposity, which is consistent with increased food intake [21], [22]. Emerging evidence suggests that Hypothalamic AMP-activated protein kinase (AMPK) plays critical roles in the regulation of energy balance and AMPK knockdown in the ARC would decrease autophagic activity and up-regulate POMC expression, leading to a reduction in food intake and body weight [23]. And in sepsis shock model, Santos found the inactivation of hypothalamic AMPK would lead to metabolic complications [24]. Moreover, in brain trauma model, MH could increase the level of autophagy in pallium to attenuate brain dysfunction [25].

We therefore hypothesized that MH might augment hypothalamus AMPK-induced autophagy and neuropeptides to exert beneficial effects on hypermetabolic and muscle wasting during sepsis. In the present study, we induced MH in rats exposed to lipopoly-saccharide (LPS) infusion, a well-characterized experimental model of sepsis and examined the level of AMPK-induced autophagy, neuropeptides and the therapeutic effects on muscle wasting.