Effects of chronic hypoxia on the expression of seladin-1/Tuj1 and the number of dark neurons of hippocampus

Highlights

• Chronic hypoxia induces signs of Alzheimer’s diseases.

• proinflamatory cytokines such as TNF-α and IL-1β can be used to Alzheimer’s diseases diagnosis in preclinical stage.

• Seladin-1 and Tuj1 down-regulate in hypoxia-AD.

Abstract

Background

There are evidences showing the relation between chronic hypoxia and Alzheimer’s disease (AD) as a metabolic neurodegenerative disease. This study was designed to evaluate the effects of chronic hypoxia on factors which characterized in AD to introduce a new model of AD-dementia.

Methods and materials

Twenty-four male rats were randomly divided in three groups: Control group (Co), Sham group (Sh), Hypoxia induction group (Hx, exposed to hypoxic chamber [oxygen 8% and nitrogen 92%] for 30 days, 4 h/day). Spatial learning and memory were analyzed using the Morris water maze task. At day 30 after hypoxia period, animals were sacrificed and serum was gathered for pro-inflammatory cytokines (interleukin-1β and tumor necrosis factor) measurements and brains were used for molecular and histopathological investigations.

Results

According to behavioral studies, a significant impairment was seen in Hx group (P < 0.05). TNF-α and IL-1β showed a significant enhanced in Hx group comparing with Co group and Sh group (P < 0.05). As well, the gene expression of seladin-1, Tuj1 and the number of seladin-1+, Tuj1+neurons significantly decreased and also the mean number of dark neurons significantly increased in CA1 and CA3 regions of hippocampus.

Conclusions

In this study, a new model of AD was developed which showed the underlying mechanisms of AD and its relations with chronic hypoxia. Hypoxia for 30 days decreased seladin-1, Tuj1 expression, increased the number of dark neurons, and also induced memory impairment. These results indicated that chronic hypoxia mediated the dementia underlying AD and AD-related pathogenesis in rat.

Introduction

Hypoxia can damage the blood-brain-barrier by opening tight junctions and increasing its permeability for unicellular and cellular components such as pro-inflammatory cytokines like interleukin-1β (IL-1β), IL-6, IL-10 and tumor necrosis factor (TNF-α) (Heneka et al., 2015a; Zhang and Le, 2010). Inflammatory processes include the proliferation and activity of astrocytes and glial cells as well as the strong activation of the complement system and enhance expression of cytokines (Holmes et al., 2009). A chronic inflammatory neurological condition that occurs following injury to nervous tissue, the disease has been reported in neurodegenerative diseases such as Alzheimer's (Hampel, 2012; Holmes et al., 2009). Neuroinflammation, as an advanced manufacturing process of proinflammatory cytokines (Honarvar et al., 2016), in its acute phase is beneficial to combate pathogens and tissue repair (Zhu et al., 2017). However, chronic neuroinflammatory responses that are triggered by the immune response and inflammatory responses can mediate brain neurodegeneration (Dong and Benveniste, 2001; Heneka et al., 2015b).

There are clinical evidences that hypoxic conditions promote cognitive dysfunction and Alzheimer's disease can increase risk factors for cardiovascular disease (de la Torre, 2012). AD is a dynamic metabolic and neurodegenerative disorder characterized by cerebrovascular and blood–brain barrier (BBB) dysfunction (Di Marco et al., 2015; Zhang and Le, 2010). In addition to neuronal loss, mainly in the cortex and hippocampus (Abolhassani et al., 2017), a progressive loss of memory (dementia) and decline in cognitive functions have been reported (Ashaari et al., 2018; Zhu et al., 2017). Pathological features of AD include accumulation of intracellular neurofibrillary tangles (consist of hyper-phosphorylated microtubule-associated protein called tau), acceleration of extracellular beta amyloid plaques (Aβ) (produced from the amyloid precursor protein [APP]) and up-regulation of β-site APP cleaving enzyme 1 (BACE1)(Bell and Zlokovic, 2009; Liu and Le, 2014). Aβs, which are surrounded by active astrocytes and microglia, initiate inflammatory reactions (Pimplikar et al., 2010; Swerdlow, 2007). Also, it has been shown that seladin-1 as a neuroprotective gene is down-regulated in AD-affected brain regions (Hassanzadeh et al., 2016). Reduction of Seladin-1 due to impaired insulin signaling may have an important role in the AD pathogenesis (Kazkayasi et al., 2016). Seladin-1 is founded in the central nervous system of mammalian (Greeve et al., 2000) and many functions have been attributed to this protein such as balance in plasma membrane cholesterol, regulation of cellular responses against oxidative stress, apoptotic signaling, and inflammatory reaction control (Bloch, 1983). Plus, Tuj1 is a specific factor in neurons that dramatically decreases when neurons are harmed and can be used in neuroscience research to prove neuronal damage (Zhu et al., 2018).

To identify the common causes of neurodegenerative diseases such as AD, it is thought that not only genetic factors but also various environmental exposures result in psychiatric disorders, decreased cognitive performance and biological aging (Biswal et al., 2016) and these multiply the risk of AD (Liu et al., 2016; Lochhead et al., 2017). Existing evidence indicates that AD is strongly induced by the environmental factors including aging, diet and nutrition, insulin deficiency, diabetes and brain trauma (Kazkayasi et al., 2016). Among all of the environmental exposures, chronic hypoxia has been considered recently (Zhang and Le, 2010). Hypoxia, loss of oxygen pressure in the atmosphere which leads to reduce cerebral oxygen supply, can cause neurodegenerative and morphological changes in the hippocampus and induce memory impairment (Kadar et al., 1994).

Animal models are being used as valuable tools for investigating new therapeutic approaches to treatment of human diseases in different fields such as neuroscience. As well, these models are offered for studying the pathological events involved in the processes of disease. Due to the lack of researches to complete understanding the etiology of AD, we need to establish animal models (Benedikz et al., 2009). However, all the available models for AD have limitations and just illustrate one or some of the characteristics of this neurologic disorder. Additionally, no natural models of AD have been introduced yet and thus most of the studies are performed using animal models with the disease phenotypes which activated by manipulation or transgenic animal models (Kaushal et al., 2016).

In this regards, the present study tries to determine association between exposing to global chronic hypoxia and AD dementia features such as memory deficits, neuronal damage in the hippocampus and expression of aging related proteins to introduce a new model of Alzheimer’s disease (AD) dementia.