Altered arginine metabolism in Alzheimer's disease brains

Abstract

L-arginine is a semi-essential amino acid with a number of bioactive metabolites. Accumulating evidence suggests the implication of altered arginine metabolism in the pathogenesis of Alzheimer's disease (AD). The present study systematically compared the metabolic profile of L-arginine in the superior frontal gyrus, hippocampus, and cerebellum from AD (mean age 80 years) and normal (mean age 80 or 60 years) cases. The activity and protein expression of nitric oxide synthase and arginase were altered with AD and age in a region-specific manner. There were also AD- and age-related changes in the tissue concentrations of L-arginine and its downstream metabolites (L-citrulline, L-ornithine, agmatine, putrescine, spermidine, spermine, glutamate, γ-aminobutyric acid, and glutamine) in a metabolite- or region-specific manner. These findings demonstrate that arginine metabolism is dramatically altered in diverse regions of AD brains, thus meriting further investigation to understand its role in the pathogenesis and/or progression of the disease.

Introduction

Alzheimer's disease (AD) is a progressive neurodegenerative condition with memory loss as an early prognostic sign and aging as a major risk factor. Its cardinal histopathological features consist of aggregates of hyperphosphorylated tau and amyloid beta (Aβ), which form intracellular neurofibrillary tangles (NFTs) and extracellular senile plaques (SPs), respectively, in the affected brain regions. Although Aβ plays a central and causative role in the disease development (Hardy and Higgins, 1992, Selkoe, 2011), a growing body of evidence suggests the involvement of arginine metabolism in AD pathogenesis (Law et al., 2001, Malinski, 2007, Yi et al., 2009).

L-arginine is a semi-essential amino acid that can be metabolized to form a number of bioactive molecules (Fig. 1; Wu and Morris, 1998). Nitric oxide (NO) is a gaseous signaling molecule produced by NO synthase (NOS). NO derived from neuronal NOS (nNOS) plays an important role in synaptic plasticity and learning and memory (Feil and Kleppisch, 2008, Susswein et al., 2004, Zhou and Zhu, 2009), whereas endothelial NOS (eNOS)-derived NO is a key factor for the stabilization and regulation of the vascular microenvironment (de la Torre, 2012, Forstermann and Sessa, 2012). In AD brains, NFTs and SPs are associated with reduced capillary expression of eNOS (Jeynes and Provias, 2009, Provias and Jeynes, 2008). There is evidence suggesting that eNOS-derived NO can directly modulate the production of Aβ and protect against increases in Aβ (Austin et al., 2010). Because of its nature as a free radical, however, an excessive amount of NO, particularly that derived from inducible NOS (iNOS), leads to neurotoxicity and neurodegeneration (Law et al., 2001, Malinski, 2007). It has been reported that NO produced in response to Aβ triggers mitochondrial fission, synaptic loss, and neuronal damage (Cho et al., 2009).

L-ornithine is the arginase-mediated metabolite of L-arginine, with urea as the by-product (Fig. 1). It can be metabolized by ornithine decarboxylase (ODC) to produce the polyamines putrescine, spermidine, and spermine (Fig. 1), which are essential for cells to grow and to function in an optimal manner (Alm and Oredsson, 2009, Igarashi and Kashiwagi, 2010, Wallace, 2009, Wallace et al., 2003). In AD brains, there is altered arginase I and arginase II messenger RNA (mRNA) expression, ODC protein expression, and polyamine tissue concentrations (Colton et al., 2006, Hansmannel et al., 2010, Morrison and Kish, 1995, Morrison et al., 1998). Moreover, the arginase II allele rs742869 is associated with an increased risk of earlier onset AD (Hansmannel et al., 2010). L-ornithine can also be channeled to produce glutamate that can be further metabolized to generate γ-aminobutyric acid (GABA) and glutamine by glutamic acid decarboxylase and glutamine synthase (GS), respectively (Fig. 1; Tapiero et al., 2002, Wiesinger, 2001, Wu and Morris, 1998). Previous research has reported decreased glutamate and GABA levels in AD brains and increased GS level in lumbar cerebrospinal fluid of AD patients (Ellison et al., 1986, Tumani et al., 1999).

Agmatine is the product of arginine decarboxylase (ADC) (Wu and Morris, 1998), and is present in mammalian brain (Li et al., 1994). Agmatine inhibits nNOS and iNOS, but stimulates eNOS, hence it has an important role in regulating NO production (Halaris and Plietz, 2007, Joshi et al., 2007, Santhanam et al., 2007, Satriano, 2003). As agmatine can itself be metabolized by agmatinase to form putrescine (Fig. 1) and induce antizyme, a small regulatory protein that inhibits ODC and down-regulates polyamine uptake (Satriano, 2003), it has an important role in controlling cellular polyamine content. Moreover, agmatine is considered a novel putative neurotransmitter (Reis and Regunathan, 2000), and recent evidence suggests its involvement in learning and memory processes (Leitch et al., 2011, Liu et al., 2008b, Liu et al., 2009a, Rushaidhi et al., 2013, Seo et al., 2011). To the best of our knowledge, there is no previous research on how agmatine changes in AD brains.

Metabolomics refers to the analysis of the component small molecules produced by a biological system and is playing an increasingly prominent role in efforts at biomarker identification for AD (Trushina and Mielke, 2013, Trushina et al., 2013). As described previously, L-arginine is versatile metabolically but its involvement in AD is largely based on scattered information from a single pathway. Hence, it is essential to understand how the arginine metabolic profile changes in AD brains. In the present study, we systematically compared NOS and arginase activity and protein expression, as well as the tissue concentrations of L-arginine and its 9 downstream metabolites in postmortem superior frontal gyrus, hippocampus, and cerebellum from AD cases (mean age 80 years) and normal control cases (mean age 60 or 80 years). This experimental design allowed us to assess the effects of AD, as well as advanced aging, on arginine metabolism in the brain. Since the hippocampus and superior frontal gyrus are more vulnerable in AD, whereas the cerebellum is less affected (Braak and Braak, 1991, Serrano-Pozo et al., 2011), we investigated whether the L-arginine metabolic profile changes are region-specific.