HSPB5 (αB-crystallin) confers protection against paraquat-induced oxidative stress at the organismal level in a tissue-dependent manner

摘要

Oxidative stress is one of the major and continuous stresses, an organism encounters during its lifetime. Tissues such as the brain, liver and muscles are more prone to damage by oxidative stress due to their metabolic activity, differences in physiological and adaptive processes. One of the defence mechanisms against continuous oxidative stress is a set of small heat shock proteins. αB-Crystallin/HSPB5, a small heat shock protein, gets upregulated under stress and acts as a molecular chaperone. In addition to acting as a molecular chaperone, HSPB5 is shown to have a role in other cytoprotective functions such as inhibition of apoptosis, prevention of oxidative stress and stabilisation of cytoskeletal system. Such protection in vivo, at the organism level, particularly in a tissue-dependent manner, has not been investigated. We have expressed HSPB5 in fat body (liver), neurons and specifically in dopaminergic and motor neurons in Drosophila and investigated its protective effect against paraquat-induced oxidative stress. We observed that expression of HSPB5 in neurons and fat body confers protection against paraquat-induced oxidative stress. Expression in dopaminergic neurons showed a higher protective effect. Our results clearly establish the protective ability of HSPB5 in vivo; the extent of protection, however, varies depending on the tissue in which it is expressed. Interestingly, neuronal expression of HSPB5 resulted in an improvement in negative geotropic behaviour, whereas specific expression in muscle tissue did not show such a beneficial effect. Keywords: αB-Crystallin, Small heat shock proteins, Oxidative stress, Tissue-dependence, HSPB5, Drosophila IntroductionOxidative stress is a major stress encountered by living organisms. It is generated by both intrinsic (metabolic processes) and extrinsic factors (pollutants, drugs, chemicals, radiation, etc.) (Cui et al. 2012; Liguori et al. 2018). Thus, chronic oxidative stress leads to a gradual accumulation of toxic substances, the failure of cell functions, changes in cell signalling, mitochondrial dysfunction, ER stress and inflammation, which accelerate the biological ageing process leading to diseases such as neurodegeneration and cancer (Cui et al. 2012; Pham-Huy et al. 2008). Cells have evolutionarily developed efficient defence mechanisms to protect against oxidative stress. One of the important defence systems that cells have evolved is the upregulation of heat shock proteins (HSPs). Among the HSPs, small heat shock proteins (sHsps) constitute the first layer of defence. They are ATP-independent chaperones; they bind to unfolded proteins and prevent them from further denaturation and aggregation (Kappe et al. 2003). The small heat shock proteins exist as oligomers with subunit molecular masses ranging from 12 to 43?kDa. All the small heat shock proteins are shown to contain a stretch of 80–100 conserved amino acids towards the C-terminal region called the “α-crystallin domain” (ACD) flanked by N-terminal domain (NTD) and C-terminal extension (CTE) (Franck et al. 2004; Kriehuber et al. 2010). The human genome is shown to contain genes for 10 small heat shock proteins (Kappe et al. 2003). αB-Crystallin/HSPB5, one of the 10 sHsps, was first identified in the eye lens (de Jong et al. 1993; Groenen et al. 1994; Ingolia and Craig 1982). It was also identified in non-lenticular tissues such as the brain, muscles, heart, liver, adipose and kidney (Bhat and Nagineni 1989; Iwaki et al. 1990). It was shown to prevent the aggregation of other proteins like a molecular chaperone (Horwitz 1992). Studies from our laboratory have shown that structural perturbation of HSPB5/αB-crystallin significantly increases the exposed surface hydrophobicity and its chaperone-like activity (Raman and Rao 1994). In addition to functioning as a molecular chaperone in preventing protein aggregation, it also performs other functions such as inhibition of apoptosis (Adhikari et al. 2011; Kamradt et al. 2001) metal-binding, especially to Cu~(+2) (Ahmad et al. 2008), modulation of cytoskeletal dynamics (Perng et al. 1999; Singh et al. 2007), modulation of cell cycle and prevention of oxidative stress (McGreal et al. 2012).The extent of susceptibility to oxidative stress varies with tissue, largely due to varying intrinsic ROS levels and adaptive processes (Leeuwenburgh et al. 1997; Surai et al. 1996). The brain is vulnerable to oxidative insults due to multiple factors: high oxygen consumption (20%), abundance of redox-active metals such as Cu~(+2), Fe~(+2) and Zn~(+2) (Halliwell 1992) and high concentration of oxidisable polyunsaturated fatty acids (Cobley et al. 2018; Wang and Michaelis 2010). Neurons are more vulnerable to oxidative damage due to greater metabolic demands associated with structural complexity (long dendritic projections and axonal branching) and functions such as synaptic transmission, propagation of action potential and molecular trafficking, and modest antio