Transglutaminase-catalyzed post-translational modifications of proteins in the nervous system and their possible involvement in neurodegenerative diseases

Proteomics analysis of the brain from a Gaucher disease mouse identifies pathological pathways including a possible role for transglutaminase 1

Gaucher disease (GD) is a lysosomal storage disorder (LSD) caused by the defective activity of acid β-glucosidase (GCase) which results from mutations in GBA1. Neurological forms of GD (nGD) can be generated in mice by intra-peritoneal injection of conduritol B-epoxide (CBE) which irreversibly inhibits GCase. Using this approach, a number of pathological pathways have been identified in mouse brain by RNAseq. However, unlike transcriptomics, proteomics gives direct information about protein expression which is more likely to provide insight into which cellular pathways are impacted in disease. We now perform non-targeted, mass spectrometry-based quantitative proteomics on brains from mice injected with 50 mg/kg body weight CBE for 13 days. Of the 5038 detected proteins, 472 were differentially expressed between control and CBE-injected mice of which 104 were selected for further analysis based on higher stringency criteria. We also compared these proteins with differentially expressed genes (DEGs) identified by RNAseq. Some lysosomal proteins were up-regulated as was interferon signaling, whereas levels of ion channel related proteins and some proteins associated with neurotransmitter signaling were reduced, as was cholesterol metabolism. One protein, transglutaminase 1 (TGM1), which is elevated in a number of neurodegenerative diseases, was absent from the control group but was found at high levels in CBE-injected mice, and located in the extracellular matrix (ECM) in layer V of the cortex and intracellularly in Purkinje cells in the cerebellum. Together, the proteomics data confirm previous RNAseq data and add additional mechanistic understanding about cellular pathways that may play a role in nGD pathology.

Neuronal and Endothelial Transglutaminase-2 Expression during Experimental Autoimmune Encephalomyelitis and Multiple Sclerosis

Transglutiminase-2 (TG2) is a multifunctional enzyme that has been implicated in the pathogenesis of experimental autoimmune encephalomyelitis (EAE) and multiple sclerosis (MS) using global knockout mice and TG2 selective inhibitors. Previous studies have identified the expression of TG2 in subsets of macrophages-microglia and astrocytes after EAE. The aims of the current investigation were to examine neuronal expression of TG2 in rodent models of chronic-relapsing and non-relapsing EAE and through co-staining with intracellular and cell death markers, provide insight into the putative role of TG2 in neuronal pathology during disease progression. Here we report that under normal physiological conditions there is a low basal expression of TG2 in the nucleus of neurons, however following EAE or MS, robust induction of cytoplasmic TG2 occurs in most neurons surrounding perivascular lesion sites. Importantly, TG2-positive neurons also labeled for phosphorylated Extracellular signal-regulated kinase 1/2 (ERK1/2) and the apoptotic marker cleaved caspase-3. In white and gray matter lesions, high levels of TG2 were also found within the vasculature and endothelial cells as well as in tissue migrating pericytes or fibroblasts, though rarely did TG2 colocalize with cells identified with glial cell markers (astrocytes, oligodendrocytes and microglia). TG2 induction occurred concurrently with the upregulation of the blood vessel permeability factor and angiogenic molecule Vascular Endothelial Growth Factor (VEGF). Extracellular TG2 was found to juxtapose with fibronectin, within and surrounding blood vessels. Though molecular and pharmacological studies have implicated TG2 in the induction and severity of EAE, the cell autonomous functions of this multifunctional enzyme during disease progression remains to be elucidated.

Autism spectrum disorders are associated with an elevated autoantibody response to tissue transglutaminase-2

We report that a significant number of autistic children have serum levels of IgA antibodies above normal to the enzyme tissue transglutaminase II (TG2), and that expression of these antibodies to TG2 is linked to the (HLA)-DR3, DQ2 and DR7, DQ2 haplotypes. TG2 is expressed in the brain, where it has been shown to be important in cell adhesion and synaptic stabilization. Thus, these children appear to constitute a subpopulation of autistic children who fall within the autism disease spectrum, and for whom autoimmunity may represent a significant etiological component of their autism.

Human tissue transglutaminase is inhibited by pharmacologic and chemical acetylation

Human tissue transglutaminase (TGM2) is implicated in the pathogenesis of several neurodegenerative disorders including Alzheimer's, Parkinson's and expanded polyglutamine (polyQ) diseases. TGM2 promotes formation of soluble and insoluble high molecular weight aggregates by catalyzing a covalent linkage between peptide-bound Q residues in polyQ proteins and a peptide-bound Lys residue. Therapeutic approaches to modulate the activity of TGM2 are needed to proceed with studies to test the efficacy of TGM2 inhibition in disease processes. We investigated whether acetylation of Lys-residues by sulfosuccinimidyl acetate (SNA) or aspirin (ASA) would alter the crosslinking activity of TGM2. Acetylation by either SNA and/or ASA resulted in a loss of >90% of crosslinking activity. The Lys residues that were critical for inhibition were identified by mass spectrometry as Lys(444), Lys(468), and Lys(663). Hence, acetylation of Lys-residues may modulate the enzymatic function of TGM2 in vivo and offer a novel approach to treatment of TGM2 mediated disorders.

Changes in the spinal cord proteome of an amyotrophic lateral sclerosis murine model determined by differential in-gel electrophoresis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by loss of motor neurons resulting in progressive paralysis. To date, more than 140 different mutations in the gene encoding CuZn-superoxide dismutase (SOD1) have been associated with ALS. Several transgenic murine models exist in which various mutant SOD1s are expressed. We used DIGE to analyze the changes in the spinal cord proteome induced by expression of the unstable SOD1 truncation mutant G127insTGGG (G127X) in mice. Unlike mutants used in most other models, G127X lacks SOD activity and is present at low levels, thus reducing the risk of overexpression artifacts. The mice were analyzed at their peak body weights just before onset of symptoms. Variable importance plot analysis showed that 420 of 1,800 detected protein spots contributed significantly to the differences between the groups. By MALDI-TOF MS analysis, 54 differentially regulated proteins were identified. One spot was found to be a covalently linked mutant SOD1 dimer, apparently analogous to SOD1-immunoreactive bands migrating at double the molecular weight of SOD1 monomers previously detected in humans and mice carrying mutant SOD1s and in sporadic ALS cases. Analyses of affected functional pathways and the subcellular representation of alterations suggest that the toxicity exerted by mutant SODs induces oxidative stress and affects mitochondria, cellular assembly/organization, and protein degradation.

Dissecting the mechanisms of tissue transglutaminase-induced cross-linking of alpha-synuclein: implications for the pathogenesis of Parkinson disease

Tissue transglutaminase (tTG) has been implicated in the pathogenesis of Parkinson disease (PD). However, exactly how tTG modulates the structural and functional properties of alpha-synuclein (alpha-syn) and contributes to the pathogenesis of PD remains unknown. Using site-directed mutagenesis combined with detailed biophysical and mass spectrometry analyses, we sought to identify the exact residues involved in tTG-catalyzed cross-linking of wild-type alpha-syn and alpha-syn mutants associated with PD. To better understand the structural consequences of each cross-linking reaction, we determined the effect of tTG-catalyzed cross-linking on the oligomerization, fibrillization, and membrane binding of alpha-syn in vitro. Our findings show that tTG-catalyzed cross-linking of monomeric alpha-syn involves multiple cross-links (specifically 2-3). We subjected tTG-catalyzed cross-linked monomeric alpha-syn composed of either wild-type or Gln --> Asn mutants to sequential proteolysis by multiple enzymes and peptide mapping by mass spectrometry. Using this approach, we identified the glutamine and lysine residues involved in tTG-catalyzed intramolecular cross-linking of alpha-syn. These studies demonstrate for the first time that Gln(79) and Gln(109) serve as the primary tTG reactive sites. Mutating both residues to asparagine abolishes tTG-catalyzed cross-linking of alpha-syn and tTG-induced inhibition of alpha-syn fibrillization in vitro. To further elucidate the sequence and structural basis underlying these effects, we identified the lysine residues that form isopeptide bonds with Gln(79) and Gln(109). This study provides mechanistic insight into the sequence and structural basis of the inhibitory effects of tTG on alpha-syn fibrillogenesis in vivo, and it sheds light on the potential role of tTG cross-linking on modulating the physiological and pathogenic properties of alpha-syn.