Short-term caloric restriction exerts neuroprotective effects following mild traumatic brain injury by promoting autophagy and inhibiting astrocyte activation

Highlights

• Caloric restriction (CR) can ameliorate mild traumatic brain injury (mTBI).

• We demonstrated improved cognitive dysfunction in a mouse model of mTBI after CR.

• CR decreased the number of GFAP-positive astrocytes in the hippocampus after mTBI.

• CR increased LC3b and Beclin1 expression and decreased mTOR expression.

• Short-term CR may alleviate mTBI by promoting autophagy and suppressing astrocytes.

Abstract

Cognitive deficits may occur after mild traumatic brain injury (mTBI), but effective treatment modalities are presently unavailable. Caloric restriction (CR) has beneficial effects on neurodegenerative diseases and brain injury. However, the underlying mechanisms have not yet been clearly defined. Therefore, the aim of the present study was to investigate the short-term effects of CR treatment on cognitive function in mice after mTBI. Forty-five 12-week-old C57/BL6 mice were subjected to closed-head mTBI using a weight drop device. The mice were then randomly divided into three groups according to their diet for 30 days: the normal calorie group (mTBI + NC group, n = 15), the caloric restriction group (mTBI + CR group, n = 15), and the high energy group (mTBI + HE group, n = 15). After 30 days, the Morris water maze test was performed to evaluate learning abilities. Nissl staining, immunohistochemistry, and western blotting were used to monitor pathological changes and changes in autophagy-associated proteins in the hippocampus. The average escape latency was significantly shorter in the mTBI + CR group than in the mTBI + NC and mTBI + HE groups, and the number of target platform crossings in the mTBI + CR group was significantly higher than in the other two groups. In the hippocampus, the expression of GFAP and mTOR was increased in the mTBI + HE group and decreased in the mTBI + CR group. Conversely, the expression of LC3B was decreased in the mTBI + HE group and increased in the mTBI + CR group. Our findings suggest that short-term CR after mTBI may ameliorate cognitive dysfunction induced by mTBI by increasing the level of autophagy and suppressing astrocyte activation.

Introduction

Traumatic brain injury (TBI), with trauma related primary and secondary mechanisms of neuronal damage is a leading cause of mortality and long-term disability [1], [2]. Approximately 75–90% of the 1.7 million TBI-related emergency room visits in the US each year are a result of mild TBI (mTBI) [3]. Patients with mTBI, unlike those with moderate and severe TBI, may show cognitive impairment with a lack of obvious tissue lesions in the brain [4], [5]. Moreover, some patients with mTBI still have measurable cognitive impairment after 1 year [6]. However, the pathology underlying mTBI is poorly understood and treatment modalities are essentially absent [7]. Thus, it is extremely urgent to investigate the mechanisms underlying mTBI-induced cognitive impairment in order to mitigate the sequelae of mTBI.

Autophagy, which is the principal mechanism for bulk degradation of superfluous or aberrant cytoplasmic components, has been implicated both clinically and experimentally in the delayed response to TBI [8]. Induction of autophagy after TBI may serve to eliminate aberrant cellular components, thus maintaining cellular homeostasis. Emerging data suggest that autophagy flux may be either increased or decreased after TBI. Therefore, autophagy may play either a beneficial or detrimental functional role after injury. However, it appears that a relatively mild injury could lead to upstream activation of autophagy flux as a protective mechanism [9]. Previous studies have indicated that several key molecular components participate in the initiation, progression and completion of autophagy, such as the mammalian target of rapamycin (mTOR), which inhibits autophagy, and Beclin1 and light chain (LC) 3, which promote it [10].

The underlying biological mechanisms of caloric restriction (CR), which is a dietary intervention that provides reduced energy intake without compromising nutrient adequacy, has been investigated for decades as the most robust and reliable experimental strategy for extending the longevity of laboratory animals [11]. Recent studies suggest that CR protects the central nervous system from neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease and Huntington's disease [12]. Numerous hypothetical mechanisms have been proposed to explain the neuroprotective effects of CR, including autophagy. Although the molecular mechanisms are still obscure, it has been proposed that CR protects the brain after TBI in a rat model [13] by suppressing microglial activation [14] and elevating brain derived neurotrophic factor (BDNF) [15], [16]. Therefore, the work described here was designed to test the hypothesis that CR can improve spatial memory in a mouse model of TBI and to begin to identify underlying molecular mechanisms that may lead to improved outcomes.