Coping with cadmium exposure in various ways: the two Helicid snails Helix pomatia and Cantareus aspersus share the metal transcription factor‐2, but differ in promoter organization and transcription of their Cd‐metallothionein genes

Abstract

Gastropods are able to withstand fluctuating availabilities of nonessential trace elements such as cadmium by induction of Cd-specific metallothionein isoform (Cd-MT) expression. As in other species, the induction mechanism involves the binding of metal-regulatory transcription factors (MTF-1 or MTF-2) to metal responsive elements (MREs) in the MT promoter regions. Cd-dependent transcription of Cd-MT genes was assessed by quantitative real time PCR in two helicid gastropods, Helix pomatia and Cantareus aspersus, over a period of eight days. The promoter regions of the Cd-MT genes of the two species were sequenced and compared regarding the position of MREs and other relevant potential transcription factor binding sites (TFBs). Cd-MT gene transcription is induced after Cd exposure in Helix pomatia and Cantareus aspersus, showing a transient peak in Helix pomatia, contrasting with a persistent induction rate in Cantareus aspersus. Since the existence of MTF-2 was verified in both species, differing transcription patterns of Cd-MT genes must be due to functional differences in their metal-responsive promoter regions. Both promoters contain a proximal cluster of three MREs overlapping with TFBs for the transcriptional regulator Sp1. In contrast to Cantareus aspersus, however, the Cd-MT gene of Helix pomatia hosts an additional distal MRE overlapping with a Sp1 binding site and a CACCC box. Inhibitory effects of MRE overlapping Sp1 binding sites were observed in other MT genes. We therefore suggest that transient Cd-MT transcription upon Cd(2+) exposure in Helix pomatia may be the result of an inhibitory action of the distal MRE cluster.

Cantareus aspersus metallothionein metal binding abilities: The unspecific CaCd/CuMT isoform provides hints about the metal preference determinants in metallothioneins

“…Two of these three isoforms correspond to the paralogs of the well-characterized, metal specific H. pomatia isoforms: the CaCdMT, upregulated by Cd and devoted to Cd detoxification in the midgut gland [29], and the CaCuMT, constitutively expressed in rhogocytes, and involved in internal Cu balance and metabolism, mainly in relation to copper respiratory pigments [16]. The third isoform was natively isolated as a mixed Cd and Cu complex, and for this reason named CaCd/CuMT, but the poor level of expression and the lack of metal response of its encoding gene suggested a residual role, if any, in the snail metabolism of metals, either physiological or xenobiotic.…”

Regulatory Plasticity of Earthworm wMT-2 Gene Expression

“…Zn could therefore be the limiting factor for Cd detoxification via MTs in injured animals. In most invertebrates like earthworms, however, MTF-1 could not be identified [10,11]. In general, earthworms do not accumulate Zn [32,33], but Zn can induce wMT-2 expression [33], yet the underlying mechanism is lacking.…”

“…A metal responsive transcription factor 1 (MTF-1), which binds to metal responsive elements (MREs) activating MTs in vertebrates [9], has not been elucidated in most invertebrates [10], although in L. rubellus three MREs have been found in the wMT-2 promotor region [11]. One putative transcriptional activator of earthworm MT has recently been identified as the cAMP responsive element binding protein (CREB) [12].…”

Cadmium stress stimulates tissue turnover in Helix pomatia: increasing cell proliferation from metal tolerance to exhaustion in molluscan midgut gland

“…The Cd detoxifying role of Cd-MT in Roman snails has been documented by several pieces of evidence. First, transcription of the Cd-MT gene and Cd-MT synthesis are inducible by Cd 2+ but barely by other metal ions, resulting in Cd-MT concentrations depending primarily on increasing Cd contents in the midgut gland Höckner et al 2009). This has also been confirmed by the present study showing a steady increase of Cd-MT Fig.…”