Aluminum trichloride caused hippocampal neural cells death and subsequent depression-like behavior in rats via the activation of IL-1β/JNK signaling pathway

Highlights

• AlCl₃ induced hippocampal neural cells apoptosis and necroptosis.

• AlCl₃ induced apoptosis and necroptosis via IL-1β/JNK signaling pathway.

• High doses of AlCl₃ are more likely to induce neural cells necroptosis.

Abstract

Aluminum (Al) is an inorganic pollutant that induces nerve cells apoptosis and necroptosis, thereby causing depression and neurodegenerative diseases. IL-1β/JNK signaling pathway can regulate apoptosis and necroptosis. However, it remains unclear whether IL-1β/JNK signaling pathway is involving in the regulation of Al-induced hippocampal neural cells apoptosis and necroptosis. To investigate the mechanism of Al on neural cells apoptosis and necroptosis, rats were orally exposed to different doses of AlCl₃ for 90 days. The open-field test results showed that AlCl₃ caused depressive behavior in rats. Histopathological evidence showed that AlCl₃ induced hippocampal neural cells apoptosis and necrosis. Moreover, Bax/Bcl-2 mRNA expression ratio, caspase-3 activity and mRNA expression and TUNEL positive rates were upregulated, meanwhile, TNF-α mRNA and protein expression levels, TNFR1, RIP1, RIP3 and MLKL proteins levels were increased, while caspase-8 protein level was decreased in the hippocampus of Al-exposed groups. These results proved that AlCl₃ induced hippocampal neural cells apoptosis and necroptosis. Combined with histopathology and correlation analysis, we deduced that hippocampal neural cells were more likely to undergo necroptosis at high doses (450 mg/kg) of AlCl₃, while <150 mg/kg AlCl₃ tended to induce apoptosis. Finally, AlCl₃ increased the proteins level of IL-1β, IL-1RI, IL-1RAcP, JNK and p-JNK, indicating that AlCl₃ activated IL-1β/JNK signaling pathway. However, the application of IL-1 receptor antagonist (IL-1Ra) inhibited the phosphorylation of JNK and the related genes expression of apoptosis and necroptosis caused by AlCl₃. Thus, we concluded that AlCl₃ induced hippocampal neural cells death and depression-like behavior in rats by activating IL-1β/JNK signaling pathway.

Introduction

Aluminum (Al) is a cumulative environmental pollutant that pollutes water, air and food and endangers human and animal health (Boullemant, 2011; Dobranskyte et al., 2004; Zhang et al., 2018). Although Al pollution has been widely concerned and controlled in recent years, neurotoxic effects of Al exposure to human beings still cannot be ignored, especially for dialysis patients, Al factory workers and nearby residents, who are more likely to be exposed to large doses of Al from dialysis drugs, Al dust and contaminated water (Klotz et al., 2017; Wang et al., 2016). Al exposure leads to accumulation of Al in the hippocampus, resulting in neurotoxicity, which leads to neuron degeneration and death, abnormal behavior, learning and memory dysfunction, and eventually induces depression and neurodegenerative diseases (Bhattacharjee et al., 2014; Cao et al., 2017). Depression currently affects 300 million sufferers (Hollos et al., 2018) and is projected to become the second leading contributory factor of disability in the world by 2030 (Eshel and Roiser, 2010). Beyond that, nearly 50 million people worldwide were reported to suffer from neurodegenerative diseases, which are expected to increase to 114 million by 2050, placing a heavy burden on society and families (Park, 2010; Prince et al., 2015). Increasingly severe Al pollution, depression and neurodegenerative diseases have become global public health problems. Therefore, it is necessary and urgent to further reveal the neurotoxic mechanism of Al.

Massive death of nerve cells is a pathological feature of depression and neurodegenerative diseases (Arrazola et al., 2019; Liu et al., 2014). Apoptosis and necroptosis are two forms of programmed cell death (PCD), which is regulated by genes. Apoptosis removes unnecessary, aged or damaged cells and plays a major role in physiological cell death (Christofferson and Yuan, 2010). Apoptosis also occurs as a defense mechanism, such as in immune response or cell destruction by disease or noxious substances. Necroptosis, by contrast, is considered as a pathological lesion that can be regulated (Zhang et al., 2017). Studies have shown that neural cells apoptosis and necroptosis play a prominent role in neurotoxicity and neurodegenerative processes induced by Al (Fu et al., 2003; Zhang, 2018). Moreover, previous studies have focused on the mechanism of Al-induced apoptosis of cultured cortical neurons (Fu et al., 2003), as well as the morphological changes of Al-induced neuronal necrosis in vitro (Zhang, 2018). However, few studies have elucidated the molecular mechanisms by which Al regulates apoptosis and necroptosis of hippocampal neural cells at gene and protein levels in vivo. Therefore, the molecular mechanism of Al-induced neural cells apoptosis and necroptosis in vivo remains to be further elucidated.

It is well documented that IL-1β is crucial in the regulation of necroptosis and apoptosis (Pasparakis and Vandenabeele, 2015). IL-1β initiates a downstream signaling cascade by acting on its specific receptor and receptor accessory protein (IL-1RI and IL-1RAcP) which activates c-Jun N-terminal kinase (JNK) signal (Weber et al., 2010). Activated JNK can regulate the expression of apoptotic target genes (pro-apoptotic protein Bax and anti-apoptotic protein Bcl-2) and then activate a caspase cascade to mediate apoptosis (Chen et al., 2018; Mao et al., 2008). JNK has been reported to regulate apoptotic and necroptosis (Dhanasekaran and Reddy, 2017). Sustained JNK activation also can mediate the expression of TNF-α (Das et al., 2009). Under apoptotic competent conditions, TNF-α binding to TNFR1 recruits the receptor-interacting protein kinase (RIP) 1 and activates caspase-8 which inhibits the activation of RIP1 and RIP3, and initiates the caspase cascade to induce apoptosis (Christofferson and Yuan, 2010; Micheau and Tschopp, 2003). Whereas necroptosis competent conditions, TNF-α binding to TNFR1 induces the activation of RIP1, RIP3, and mixed lineage kinase domain-like protein (MLKL) which are essential for executing necroptosis (Christofferson and Yuan, 2010; Declercq et al., 2009; Sun et al., 2012). Apoptosis and necroptosis are the main mechanisms of Al-induced neural cells death and degeneration. In addition, our previous studies have confirmed that Al increases cytokine IL-1β, initiating the IL-1 signaling (Zhang et al., 2018). Nevertheless, it is still unclear whether the IL-1β/JNK signaling pathway regulates Al-induced neural cells apoptosis and necroptosis.

Therefore, this study aims to investigate whether IL-1β/JNK signaling pathway is involved in the regulation of Al-induced apoptosis and necroptosis in the hippocampus. Moreover, the death form (apoptosis or necroptosis) of hippocampal neural cells induced by different doses of Al was further determined. Our study provides new insights into the mechanism by which Al induces cell death and may contribute to the discovery of therapeutic targets and drugs for neurodegenerative diseases and depression.