Challenging the Metallothionein (MT) Gene of Biomphalaria glabrata: Unexpected Response Patterns Due to Cadmium Exposure and Temperature Stress

Abstract

Metallothioneins (MTs) are low-molecular-mass, cysteine-rich, metal binding proteins. In most animal species, they are involved in metal homeostasis and detoxification, and provide protection from oxidative stress. Gastropod MTs are highly diversified, exhibiting unique features and adaptations like metal specificity and multiplications of their metal binding domains. Here, we show that the MT gene of Biomphalaria glabrata, one of the largest MT genes identified so far, is composed in a unique way. The encoding for an MT protein has a three-domain structure and a C-terminal, Cys-rich extension. Using a bioinformatic approach involving structural and in silico analysis of putative transcription factor binding sites (TFBs), we found that this MT gene consists of five exons and four introns. It exhibits a regulatory promoter region containing three metal-responsive elements (MREs) and several TFBs with putative involvement in environmental stress response, and regulation of gene expression. Quantitative real-time polymerase chain reaction (qRT-PCR) data indicate that the MT gene is not inducible by cadmium (Cd) nor by temperature challenges (heat and cold), despite significant Cd uptake within the midgut gland and the high Cd tolerance of metal-exposed snails.

Cadmium Pathways in Snails Follow a Complementary Strategy between Metallothionein Detoxification and Auxiliary Inactivation by Phytochelatins

“…We compare two species of gastropods, Arion vulgaris and Biomphalaria glabrata, and their response to Cd exposure. Arion vulgaris possesses an inducible Cd-selective MT (AvMT1) [39], while the second species relies on a non-selective MT that is hardly inducible by Cd exposure [37,40]. We provide evidence that in snails, PCs are ready to take over protection against Cd impact, if the detoxification capacity of MTs becomes deficient due to lacking metal binding selectivity or oversaturation of their binding capacity.…”

“…However, the efficiency of Cd detoxification may depend on the degree of their Cd selectivity. Some snails like marine littorinids and terrestrial helicids possess highly Cd-selective MTs with a high response and binding capacity against Cd exposure [21,23,36], whereas other species, particularly from freshwater environments, express poorly responsive, non-selective MTs that bind Cd 2+ ions with a much lower degree of efficiency [37]. In some of these species like Lymnaea stagnalis, apparently both detoxification systems (MTs and PCs), co-exist [24,38].…”

Cadmium Uptake, MT Gene Activation and Structure of Large-Sized Multi-Domain Metallothioneins in the Terrestrial Door Snail Alinda biplicata (Gastropoda, Clausiliidae)

“…Intron loss can be a sign for pseudogenization [46], meaning that the respective gene is not transcribed into mRNA any more, which normally serves as a template for translation into the respective protein. [47]; H.p., Helix pomatia [13]; B.g., Biomphalaria glabrata [48]; A.b., Alinda biplicata (this study); T.m., Tremella mesenterica [49]; C.v., Crassostrea virginica [50]; T.p., Tetrahymena pyriformis [51].…”

“…All these features argue against the pseudogenization of the two intronless MT genes. Overall, the internal architecture of MT genes can be very variable ( Figure 3), and three different kinds of MT gene structures can be distinguished: (i) a typical exon/intron structure as known for many other genes, too [13,[47][48][49][50]; (ii) a deviated exon/intron structure with a large exon consisting of repeat sequences that sometimes represent multiple domains [47]; and (iii) real intronless genes where introns are not present anymore [51] (this study). In addition, the length of the introns can vary to a great extent, mostly in contrast to the rather uniform size of exons ( Figure 3).…”

Effects of Vitamin D Receptor, Metallothionein 1A, and 2A Gene Polymorphisms on Toxicity of the Peripheral Nervous System in Chronically Lead-Exposed Workers

“…[27] Metallothioneins (MTs) are cysteine-rich, metal-binding, low-molecular-weight proteins [28,29]. MT proteins have multiple functions, such as free radical scavenging and the detoxification of metal(loid)s including cadmium, arsenic, mercury, and lead [30][31][32][33][34]. The human MT genes consist of four subfamilies, classified as MT1-MT4.…”