Insulin protects against Aβ-induced spatial memory impairment, hippocampal apoptosis and MAPKs signaling disruption

Highlights

• Intra-hippocampal insulin treatment protects against Aβ-induced memory impairment.

• Intra-hippocampal insulin treatment prevents Aβ-induced hippocampal apoptosis.

• Intra-hippocampal insulin treatment prevents Aβ-induced hippocampal P38 activation.

• Intra-hippocampal insulin treatment prevents Aβ-induced hippocampal ERK activation.

Abstract

Alzheimer disease (AD) is a progressive neurodegenerative disease characterized by extracellular deposits of beta amyloid (Aβ) and neuronal loss particularly in the hippocampus. Accumulating evidences have implied that insulin signaling impairment plays a key role in the pathology of AD; as much as it is considered as type 3 Diabetes. MAPKs are a group of signaling molecules which are involved in pathobiology of AD. Therefore this study was designed to investigate if intrahippocampal insulin hinders Aβ-related memory deterioration, hippocampal apoptosis and MAPKs signaling alteration induced by Aβ. Adult male Sprague-Dawely rats weighing 250–300 g were used in this study. The canules were implanted bilaterally into CA1 region. Aβ25-35 was administered during first 4 days after surgery (5 μg/2.5 μL/daily). Insulin treatment (0.5 or 6 mU) was done during days 4–9. The animal's learning and memory capability was assessed on days 10–13 using Morris water maze. After finishing of behavioral studies the hippocampi was isolated and the amount of hippocampal cleaved caspase 3 (the landmark of apoptosis) and the phosphorylated (activated) forms of P38, JNK and ERK was analyzed by western blot. The results showed that insulin in 6 but not 0.5 mU reversed the memory loss induced by Aβ25-35. Western blot analysis revealed that Aβ25-35 induced elevation of caspase-3 and all 3 MAPks subfamily activity, while insulin in 6 mu restored ERK and P38 activation but has no effect on JNK. This study disclosed that intrahippocampal insulin treatment averts not only Aβ-induced memory deterioration but also hippocampal caspase-3, ERK and P38 activation.

Introduction

For several decades insulin-independent uptake of glucose, led scientists to regard brain as an insulin-independent tissue; however the discovery of insulin and its receptor within the brain revolutionized that old misconception (Havrankova et al., 1978a, Havrankova et al., 1978b). Since then numerous evidences have verified the expression of insulin receptors (IRs) in various brain structures (van Houten et al., 1979). Distribution of IRs in brain structures is not ubiquitous and some regions like the hippocampus have higher IR density (Hill et al., 1986). Insulin has been shown to play various important roles in central nervous system (CNS). It has also been shown that impaired insulin signaling is involved in pathology of Alzheimer Disease (AD); in a way that this disease is named as “Diabetes type-3” (reviewed in (Cholerton et al., 2013, Ghasemi et al., 2013)). Even some trial studies suggest that intranasal insulin treatment might be helpful in treating AD (Freiherr et al., 2013, Reger et al., 2006, Reger et al., 2008). Intranasal administration of insulin has been shown to achieve direct CNS delivery because it travels through olfactory nerve (both extracellularly and intracellularly), while has no systemic effect (Chapman et al., 2013).

AD is one of the most prevalent neurodegenerative diseases estimating to affect about 30 million people in the world (Holtzman et al., 2011). This disease is characterized by accumulation of extracellular senile plaques and intracellular neurofibrillary tangles (NFT) along with neuronal loss particularly in the hippocampus (Serrano-Pozo et al., 2011). Hippocampus is one of the most important brain structures playing a pivotal role in spatial learning and memory (Li et al., 2012) and is the most important part of the brain affected by AD pathology (Fellgiebel and Yakushev, 2011).

MAPKs are a group of serine–threonine kinases playing roles in a variety of cellular activities and are divided into 3 main subgroups; extracellular signal-regulated kinases (ERKs), Jun N-terminal kinases (JNKs) and P38 MAPK (Jin et al., 2006). Theses kinases are assumed to contribute in the development of AD as there is a relationship between beta amyloid (Aβ) and them (reviewed in (Ghribi et al., 2003, Kim and Choi, 2010)). For instance MAPKs are reported to take part in tau hyperphosphorylation which in turn participates in AD pathology (Ferrer et al., 2001, Ferrer et al., 2003, Puig et al., 2004). MAPKs are also illustrated to play roles in APP metabolism and Aβ production (Cho et al., 2007, Colombo et al., 2009). Additionally they are documented to contribute in apoptosis and LTP disruption caused by Aβ (Ramin et al., 2011, Wang et al., 2004). Then MAPKs are considered to be engaged in AD pathology.

Considering the protective effects of insulin against AD, the present study aimed to clarify if intra-hippocampal insulin administration is protective against Aβ-induced spatial memory impairment, hippocampal apoptosis and MAPKs activity alteration.