Redox dysregulation, immuno-inflammatory alterations and genetic variants of BDNF and MMP-9 in schizophrenia: Pathophysiological and phenotypic implications

Abstract

Background

Although a clear mechanism underlying the pathophysiology of schizophrenia (SZ) remains elusive, oxidative stress, inflammatory syndrome and immune activation have become an attractive hypothesis for explaining the pathophysiology of SZ. Data from prior studies on the role of matrix metalloproteinase 9 (MMP-9) and brain-derived neurotrophic factor (BDNF) single nucleotide polymorphisms (SNPs) in SZ are contradictory. We aimed to investigate whether oxidative stress, inflammatory and immune activation markers as well as MMP-9 levels may be implicated in SZ pathogenesis. The association of MMP-9 and BDNF SNPs with the clinical expression of SZ was examined.

Subjects and methods

Ninety-four subjects were recruited, including 44 SZ patients and 50 healthy controls. Serum levels of thiobarbituric acid reactive substances (TBARS), protein carbonyl content (PCC), nitrite, C-reactive protein (CRP), interleukin 6 (IL-6), tumor necrosis factor alpha (TNF-α), Beta-2 microglobulin (Β2M), complement component 3 (C3), C4 and MMP-9 were measured. The MMP-9 − 1562C > T and BDNF196G > A SNPs were genotyped using polymerase chain reaction-restriction fragment length polymorphism assay. Psychopathology was assessed using the positive and negative syndrome scale (PANSS).

Results

SZ patients showed significantly higher TBARS, PCC, nitrite, CRP, IL-6, TNF-α, Β2M, C3 and MMP-9 levels than controls. In distinguishing SZ patients from healthy controls, CRP and MMP-9 yielded similar discriminatory performance, and both perform better than IL-6, Β2M, C3, nitrite, TBARS, PCC, TNF-α and C4. The MMP-9 − 1562C > T SNP genotypes distribution didn't differ significantly between controls and SZ patients. As compared to controls, SZ patients harbor a significantly higher frequency of the BDNF196GG genotype and a lower frequency of the BDNF196GA/AA genotype. Patients carrying the MMP-9 − 1562CC or BDNF196GG genotype revealed a significantly higher PANSS than those carrying MMP-9 − 1562CT/TT or BDNF196GA/AA genotype. Male gender and the MMP-9 − 1562CC genotype were identified as independent predictive factors for higher PANSS.

Conclusions

Redox dysregulation and alterations in the immuno-inflammatory pathways are major culprits in the pathogenesis of SZ. MMP-9 and BDNF SNPs are associated with the clinical phenotype of SZ and, thus, may be a useful marker predicting the phenotypic expression and prognosis of SZ patients.

Introduction

Schizophrenia (SZ) is an astringent, chronic and weakening neuropsychiatric disorder (Tandon et al., 2009). It represents the fifth driving reason of incapacitation worldwide and influences around 1% of the general population around the world (MacDonald and Schulz, 2009).

Currently, there is a restored interest in immune/inflammatory changes (Leza et al., 2015) and their associated oxidative consequences (Emiliani et al., 2014) as key pathophysiological mechanisms in SZ. It has been demonstrated that variation in oxidant status is highly interactive with the state of inflammation and cell-mediated immune activation, supporting the hypothesis that changes in the immune/inflammatory-related pathways might be implicated in the pathophysiological mechanisms of SZ (Kunz et al., 2011). Oxidative stress due to an imbalance between radical-generating and radical-scavenging systems has been identified as a possible element in the neuropathological process of SZ (Fendri et al., 2006), via lipid peroxidation and protein modification (Bošković et al., 2011). Accumulation of lipid peroxides and oxidized proteins in the brain potentiates neurodegeneration, deregulates synaptic plasticity and impairs cognitive function (Martínez-Cengotitabengoa et al., 2012). Importantly, immune-mediated pathogenesis has been shown to result in excessive free radicals production, leading to increased oxidative stress (Ng et al., 2008). Furthermore, it has been hypothesized that inflammation and oxidative stress reciprocally induce each other in a positive feedback manner (Bitanihirwe and Woo, 2011). Although the “starting point” of inflammatory and oxidative stress abnormalities remains unclear, several hypotheses with regard to the pathophysiology of SZ have been postulated, including activated microglia (Monji et al., 2009), impaired antioxidant defenses (Yao and Keshavan, 2011), redox dysregulation (Do et al., 2009) and impaired glutathione synthesis (Gysin et al., 2007). In a previous meta-analysis, immuno-inflammatory mediators, such as cytokines, which can tip the redox balance into a pro-oxidant state, have been found to be altered in patients with SZ (Miller et al., 2011). Interestingly, in tune with the T-helper 1 (Th1)/Th2 imbalance theory of SZ, it is conceivable that β2-microglobulin (β2M) might play a role in the pathogenesis of SZ (Chittiprol et al., 2009). Curiously, the complement system has been shown to be activated in SZ, presumably through the alternative pathway (Sória et al., 2012).

A compelling confirmation has proposed a critical role of extracellular proteolysis in neuroplasticity and synaptic remodeling (Lepeta and Kaczmarek, 2015). The matrix metalloproteinases (MMPs) are a vast group of extracellularly acting endopeptidases (Sternlicht and Werb, 2001). MMP-9 impacts hippocampal and prefrontal cortical function and is possibly involved in SZ, a condition in which prefrontal cortex impairment is one of the most prevalent pathological findings (Bunney and Bunney, 2000). In hippocampal slices from rats and mice, MMP-9 has been shown to act upstream of integrin receptor activation to modulate hippocampal synaptic physiology. Alternatively, it is possible that MMP-9 may bind directly to synaptic integrins, which in turn optimally anchor or position them to cleave other target substrates, and consequently, modulating hippocampal synaptic function. MMP-9 may also contribute to synaptic plasticity by enabling structural synaptic remodeling which may occur via MMP-9 cleavage of matrix or adhesion proteins that, together, normally provide the structural scaffolding that maintains synaptic architecture (Nagy et al., 2006). In brains from rats, tissue inhibitor of matrix metalloproteinases 1 (TIMP-1) has been found to abolish MMP-9-dependent long-lasting long-term potentiation in the prefrontal cortex, engaging TIMP-1-controlled proteolysis as an extracellular mechanism of late long-term potentiation (Okulski et al., 2007). Eminently, the limited number of studies examined the role of serum MMP-9 levels in the pathogenesis of SZ present conflicting results (Devanarayanan et al., 2016, Domenici et al., 2010, Niitsu et al., 2014, Yamamori et al., 2013). Several single nucleotide polymorphisms (SNPs) of the human MMP-9 gene have been recognized. Interestingly, the promoter − 1562C > T SNP was shown to exert a functional effect on gene transcription, with the T allele showing a higher transcriptional activity than the C allele, which appeared to be due to preferential binding of a putative transcription repressor protein to the C allelic promoter (Zhang et al., 1999). Several studies have fixated on the association between MMP-9 genetic variants and pathogenesis of SZ. Especially, the role of the promoter − 1562C > T SNP – has been broadly studied, with strikingly contrasting results (Bienkowski et al., 2015, Groszewska et al., 2011, Han et al., 2011, Rybakowski et al., 2009).

Accumulating evidence proposed that a deficit in neurotrophin supply to cortical neurons may be an underlying factor in the pathophysiology of SZ (Gorski et al., 2003). Brain-derived neurotrophic factor (BDNF) is a neurotrophin that regulates neuronal differentiation, growth and survival during brain development (Poo, 2001). A prevalent SNP (196G > A) in exon XIIIA of the BDNF gene, producing an amino acid substitution of Val to Met at codon 66, has been shown to drastically alter the intracellular trafficking and packaging of the pro-BDNF precursor peptide and, thus, regulate the secretion of the mature BDNF peptide (Notaras et al., 2015). Additionally, it has been demonstrated that the Val 66 to Met substitution disrupts interactions with intracellular transport molecules such as sortilin, which plays an essential role in sorting BDNF-containing vesicles to the regulatory release pathway, influencing the release of BDNF at the synapse (Chen et al., 2005). It has also been acknowledged that the trafficking of BDNF mRNA to dendrites is disrupted by the Val 66 to Met substitution, with the RNA transport molecule, translin, binding at the site in which the Val 66 to Met substitution happens (Chiaruttini et al., 2009). Notwithstanding numerous groups endeavoring to examine the association between the 196G > A BDNF SNP and susceptibility to SZ, conflicting results remain (Golimbet et al., 2008, Lu et al., 2012, Skibinska et al., 2008, Zakharyan et al., 2011, Zhang et al., 2012, Zhou et al., 2010).

Although there are increasing data to implicate the oxidative stress, inflammatory and immune activation markers as well as MMP-9 in SZ pathogenesis, their clinical significance remains incompletely understood. In an attempt to shed some light on the association of these factors with SZ pathogenesis, the present study was designed to explore their involvement in SZ pathogenesis. Furthermore, in an attempt to decrease the uncertainty of the association of MMP-9 and BDNF genetic variants with the clinical variability of SZ phenotype, we investigated the relationship of these variants with the clinical symptoms of SZ to provide more conclusive evidence regarding their association with various clinical expression of SZ.