Brain injury and inflammation genes common to a number of neurological diseases and the genes involved in the genesis of GABAnergic neurons are altered in monoamine oxidase B knockout mice

Abstract

Monoamine oxidase B (MAO B) oxidizes trace amine phenylethylamine (PEA), and neurotransmitters serotonin and dopamine in the brain. We reported previously that PEA levels increased significantly in all brain regions, but serotonin and dopamine levels were unchanged in MAO B knockout (KO) mice. PEA and dopamine are both synthesized from phenylalanine by aromatic L-amino acid decarboxylase in dopaminergic neurons in the striatum. A high concentration of PEA in the striatum may cause dopaminergic neuronal death in the absence of MAO B. We isolated the RNA from brain tissue of MAO B KO mice (2-month old) and age-matched wild type (WT) male mice and analyzed the altered genes by Affymetrix microarray. Differentially expressed genes (DEGs) in MAO B KO compared to WT mice were analyzed by Partek Genomics Suite, followed by Ingenuity Pathway Analysis (IPA) to assess their functional relationships. DEGs in MAO B KO mice are involved in brain inflammation and the genesis of GABAnergic neurons. The significant DEGs include four brain injury or inflammation genes (upregulated: Ido1, TSPO, AVP, Tdo2), five gamma-aminobutyric acid (GABA) receptors (down-regulated: GABRA2, GABRA3, GABRB1, GABRB3, GABRG3), five transcription factors related to adult neurogenesis (upregulated: Wnt7b, Hes5; down-regulated: Pax6, Tcf4, Dtna). Altered brain injury and inflammation genes in MAO B knockout mice are involved in various neurological disorders: attention deficit hyperactive disorder, panic disorder, obsessive compulsive disorder, autism, amyotrophic lateral sclerosis, Parkinson's diseases, Alzheimer's disease, bipolar affective disorder. Many were commonly involved in these disorders, indicating that there are overlapping molecular pathways.

Keywords: GABAnergic neuron; Inflammation; Monoamine oxidase B; Phenylethylamine.

Figure 1. Altered expression of genes linked to neurological disorders in 2-month old MAO B KO mice.

The altered genes linked to attention deficit hyperactive disorder, panic disorder, obsessive compulsive disorder, autism, amyotrophic lateral sclerosis, Parkinson’s disease, Alzheimer’s disease, and bipolar affective disorder are displayed. RNA was isolated from the brain tissue of 2-month old MAO B KO and age-matched WT mice of the same genetic background and converted to cDNA to generate probes for hybridization. Gene expression levels were determined using Affymetrix GeneChip Array. Partek Genomics Suite was used for statistical analysis of microarray data to identify differentially expressed genes (DEGs) between MAO B KO and WT mice. DEGs are defined as MAO B KO vs WT [false discovery rate (FDR) < 0.05, fold-change (FC) >1.5]. Ingenuity Pathways Analysis (IPA) annotation database was used to identify DEGs linked to various neurological disorders. Numerous altered genes are commonly related to these neurological disorders, indicating that there are overlapping molecular pathways.

Figure 2. Gene interaction network map generated by Ingenuity Pathway Analysis (IPA) for altered genes in 2-month old MAO B KO mice.

The DEG data were further analyzed with IPA to integrate the microarray data with known disease pathways to generate a map of gene expression relevant to neurological diseases. The analysis shows the interaction network between molecules in the dataset. The interaction network map for attention deficit hyperactive disorder, panic disorder, and obsessive compulsive disorder is shown. The green color indicates decreased expression while the red color denotes increased expression. The intensity of color corresponds to fold-change. The lines connecting the GABA receptor (GABR) subunits indicate the protein-to-protein interactions.

Figure 3. Regulatory networks and pathways generated by Ingenuity Pathway Analysis (IPA) for altered genes in 2-month old MAO B KO mice.

The IPA analysis revealed the potential pathways of upstream regulatory networks or downstream functions involving the DEGs. A network diagram depicting the possible signal transduction pathways involving inflammation-related genes, GABA receptors, and transcription factors pertinent to neuronal stem cells, which are listed in Tables 1 through 3, is shown. The other genes altered between the MAO B KO mice and WT mice were also included. Small molecules and other genes (without altered expression) were added to complete the pathway. The broken line indicates an indirect relation whereas the filled line denotes a direct protein-to-protein interaction.