Toxicogenomics reveals neuronal specific and non-specific responses to cadmium possibly involved in neurodegeneration

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease affecting the voluntary motor nervous system. Genetic factors play a major role in the familial form, represented by 10% of cases. Whereas, in the remaining 90% sporadic cases a multifactorial origin is supposed, in which environmental factors may be involved. Particularly, the involvement of metal toxicity is proposed, based on epidemiologic data and on the increasing number of case reports of ALS-like syndromes associated to metal exposure. Among metals, cadmium is of particular concern for its heavy anthropogenic release (~30,000 tons/year) and for its chemical properties. Cd exposure can primarily occur through food, drinking water, air particles, cosmetics and cigarette smoking. Once assumed, even at very low concentrations, Cd is accumulated with a long biological half-life (> 26 years). Cd accumulation contributes to oxidative stress and alteration of divalent ions homeostasis, primarily Zn2+ and Ca2+, both crucial for neuronal signaling. Studies of neurodegenerative diseases mainly rely on the use of animal models and on stem cells. Already ten years ago in a Nature Report the validity of neurodegenerative disease animal models was questioned, and direction towards the reduction of transgenic models is mandatory. Human neuroblastoma SH-SY5Y is a dopaminergic cell line that reproduces biochemical and morphological properties of neurons, thus being often used as an in vitro model for human neurons in neurotoxicity or neuroprotection studies in experimental disease research. We have applied a toxicogenomic approach to evidence deregulated pathways in SH-SY5Y cells exposed to cadmium to unravel neuronal specific and non-specific responses to the toxic metal. A whole genome analysis by cDNA microarray was performed in SH-SY5Y cells exposed to non-cytotoxic concentrations of cadmium (10 and 20 mM for 48 h). Very interestingly, microarray analysis evidenced that the top up-regulated genes were enriched in pathways related to the mineral absorption biological process in which metallothioneins (MT), and heme oxygenase-1 (HMOX-1) are among the highest up-regulated genes. MT are low molecular weight proteins that bind essential metal ions and sequester toxic metals, thus representing an important broad mechanisms of metal homeostasis. MT expression was visualized in SH-SY5Y cells by Western blot analysis revealing a dose-dependent trend of the protein, validating microarray data. The increased HMOX-1 expression has just been related [1] to the progression of several diseases including neurodegeneration. Another remarkable result concerns NEU4 that was found in our cell samples as the top down-regulated gene upon cadmium treatment. This gene encodes for a neuraminidase specifically expressed in brain that regulates neuronal function by catabolizing brain gangliosides. Sialidase NEU4 has been shown to be localized in the outer mitochondrial membrane and involved in different cellular processes, such as apoptosis, neuronal differentiation and tumorigenesis [2]. A dramatic decrease of NEU4 expression level was previously observed prior to apoptotic neurodegeneration. Overall our results evidence specific neuronal responses to cadmium and suggest the possible mechanisms triggered by this toxic metal in neurodegeneration. The authors acknowledge the European Commission, the Mistral University Research Center, and UniMIB for funding (2016- ATE-0411 to CU). References [1] Waza, A. A., Hamid, Z., Ali, S. et al. (2018). Inflamm Res 67, 579-588. doi:10.1007/s00011-018-1151-x [2] Bigi, A., Tringali, C., Forcella, M. et al. (2013). Glycobiology 23, 1499-1509. doi:10.1093/glycob/cwt078