Role of transglutaminase 2 in brain ageing

Abstract

Transglutaminases (TGs) are a nine members protein family, among which tissue transglutaminase (TG2) is unique for its multiple enzyme activities, ubiquitous expression, cellular localizations and physiological roles. Primary and best studied TG2 function is the Ca+2-dependent transamidating activity, which can result in protein cross-linking between a glutamine and a lysine residue (Iismaa et al., 2009). It is found throughout the body and highly expressed in many tissue, including the nervous one (Thomazy & Fesus, 1989; Chen & Mehta, 1999; Fesus & Piacentini, 2002; Siegel & Khosla, 2007). Within the brain, the enzyme is present in many regions including frontal and temporal cortex, hippocampus and cerebellum (Appelt et al., 1996; Johnson et al., 1997; Kim et al., 1999; Lesort et al., 1999, 2000; Andringa et al., 2004; Wilhelmus et al., 2008). It is primarily neuronal, but can be found in glia as well (Wilhelmus et al., 2008). Originally regarded as a cytosolic protein, TG2 was later found in other cell compartments, including the nucleus, mitochondria, endoplasmic reticulum, as well as in the extracellular space (Lesort et al., 1998; Piacentini et al., 2002; Verderio et al. 2003; Rodolfo et al., 2004; Mishra et al., 2006; Zemskov et al. 2006; Malorni et al., 2009; Szegezdi et al., 2009; Piacentini et al., 2011, Caccamo et al., 2012; Gundemir et al., 2012; Piacentini et al., 2014). Given the ubiquitous expression and vast array of enzymatic and non-enzymatic TG2 activities, it is not surprising that this protein is intimately involved in the regulation of numerous functions, including cell survival and apoptosis (Fesus & Szondy, 2005; Mehta et al., 2006; Verma & Mehta, 2007), autophagy (Mastroberardino & Piacentini, 2010), inflammation (Caccamo et al., 2005b, c; Takano et al., 2010) and oxidative stress processes (Fesus & Szondy 2005; Caccamo et al., 2012). The observed increase of mRNA levels during normal aging and TG2 role in apoptosis, autophagy, ROS imbalance e inflammation create a direct link with the ageing process. Thus it is certain that there are grounds for TG2 to become a new therapeutic target in many age-related diseases in which TG2 contribution has been long described: for instance, cardiovascular disease (Sane et al., 2007), cancer pathology (Budillon et al., 2013), hepatic disease (Nardacci et al., 2003) and age-related neurodegenerative disease, including HD, PD, AD (Jeitner et al., 2009). Given its involvement in neural cell homeostatic pathways, my PhD project aimed at characterizing the TG2 in vivo contribution to mechanisms such as autophagy and antioxidant defense, and how these processes are modulated in ii the different brain areas and during normal ageing. This was achieved by studying a knockout mouse model, generated by De Laurenzi & Melino in 2001, carrying a homologous recombination to replace the region of the TG2 gene encoding its catalytic site with the neomycin resistance gene. The main results obtained from the research and described in this thesis can be summarized as follows: 1. Our data provided in vivo evidence of a negative modulatory role of TG2 in the induction phase of autophagy, so far only supported by in vitro literature (Akar et al., 2007; Ozpolat et al., 2007; Luciani et al., 2010) Noteworthy, induction of autophagy in TG2-/- mouse brain appears not to be followed by successful completion of the process. This statement is mainly supported by electron microscopic findings concerning (i) accumulation in TG2-/- mouse brain of mitochondria/lipofuscin aggregates, index of decreased lysosomal degradation, (ii) presence of intracellular “myelin figures”, reminiscent of residual autophagic bodies, and (iii) deranged Golgi apparatus, sign of lysosomal disturbed biogenesis. 2. Our ultrastructural results highlighted profound alterations of the mitochondrial compartment subsequent to TG2 ablation, consistent with the role of the enzyme in stabilizing complex I and II of the electron respiratory chain (Malorni et al., 2009). Mitochondrial impairment likely leads to superoxide anion leakage, whose accumulation appear aggravated by the decreased expression of its major scavenger SOD2. 3. Our data pointed out that there are brain region-based differences in coping with TG2 ablation. Indeed, neocortex and, partly, the hippocampal formation, regions where ROS detoxifying enzymes are downregulated, are oxidative stress prone areas. By contrast, cerebellar area, long known to be a more protected region of the brain in many neurodegenerative diseases, displays less damaged mitochondria in TG2-/- mice. This could relate to an efficient activation of the antioxidant systems – indeed we detected higher expression of H2O2 metabolizing enzymes CAT and GPx 1/2- and/or to a higher autophagic flux. 4. Response to cellular stress caused by TG2 ablation apparently involves peroxisomes, which are biogenetically induced, possibly to cope with mitochondrial deficiency, as it occurs in other pathophysiological situations. 5. Our data clearly show that any attempt by the cell to counteract cell injury consequent to TG2 ablation, is subject to progressive loss of efficiency when iii senescence ensues. When comparatively analyzing results from 12- and 24-month-old mice, a worsening of antioxidant defences and autophagic efficiency, associated with ultrastructural damage is detected, even in regions (cerebellum) more resistant to insult. Our study, by providing novel insights into previously suggested TG2 roles in the brain, and especially by highlighting new functions of the protein in the complex organ, opens the way to future mechanistic investigations, which will hopefully lead to therapeutic applications.