Field study reveals morphological and neuroendocrine correlates of seasonal breeding in female water voles, Arvicola terrestris.

Seasonally breeding mammals display timely physiological switches between reproductive activity and sexual rest, which ensure synchronisation of births at the most favourable time of the year. These switches correlate with seasonal changes along the hypothalamo-pituitary-gonadal axis, but they are primarily orchestrated at the hypothalamic level through environmental control of KISS1-dependent GnRH release. Our field study shows that births of fossorial water voles, Arvicola terrestris, are concentrated between March and October, which indicates the existence of an annual reproductive cycle in this species. Monthly field monitoring for over a year further reveals dramatic seasonal changes in the morphology of the ovary, uterus and lateral scent glands, which correlate with the reproductive status. Finally, we demonstrate seasonal variation in kisspeptin expression within the hypothalamic arcuate nucleus. Altogether, this study demonstrates a marked rhythm of seasonal breeding in the water vole and we speculate that this is governed by seasonal changes in photoperiod.

Seasonal diet-based resistance to anticoagulant rodenticides in the fossorial water vole (Arvicola amphibius).

Anticoagulant rodenticides (AR) resistance has been defined as "a major loss of efficacy due to the presence of a strain of rodent with a heritable and commensurately reduced sensitivity to the anticoagulant". The mechanism that supports this resistance has been identified as based on mutations in the Vkorc1 gene leading to severe resistance in rats and mice. This study evaluates the validity of this definition in the fossorial water vole and explores the possibility of a non-genetic diet-based resistance in a strict herbivorous rodent species. Genetic support was explored by sequencing the Vkorc1 gene and the diet-based resistance was explored by the dosing of vitamins K in liver of voles according to seasons. From a sample of 300 voles, only 2 coding mutations, G71R and S149I, were detected in the Vkorc1 gene in the heterozygous state with low allele frequencies (0.5-1%). These mutations did not modify the sensitivity to AR, suggesting an absence of genetic Vkorc1-based resistance in the water vole. On the contrary, vitamin K1 was shown to be 5 times more abundant in the liver of the water vole compared to rats. This liver concentration was shown to seasonally vary, with a trough in late winter and a peak in late spring/early summer related to the growth profile of grass. This increase in concentration might be responsible for the increased resistance of water voles to AR. This study highlights a non-genetic, diet-related resistance mechanism in rodents to AR. This diet-based resistance might explain the different evolution of the Vkorc1 gene in the fossorial water vole compared to rats and mice.

Water vole management - Could anticoagulant rodenticides stereochemistry mitigate the ecotoxicity issues associated to their use?

Cyclic water vole population explosions can be controlled in some European countries with anticoagulant rodenticides leading sometimes to wildlife poisonings due to the toxin's tissue persistence. Here, we analyzed the pharmacokinetics of rodenticide residues in voles and we explored potential ways of improving the mass application of these agents based on the concept of stereoisomers. We demonstrated the dramatic persistence of bromadiolone in vole tissues with a hepatic half-life of about 10-30 days, while the tissue persistence of chlorophacinone is rather short with a hepatic half-life of about one day. The dramatic persistence of bromadiolone is due to the trans-isomer group (the major compound in bromadiolone), while the cis-isomer group has a short half-life. Because of resistance to chlorophacinone, the cis-bromadiolone isomers may constitute an excellent compromise between efficacy and ecotoxicological risk to control voles. A mathematical model is proposed to favor the development of baits mixed with cis-isomer groups.

Structural Investigation of the Vitamin K Epoxide Reductase (VKORC1) Binding Site with Vitamin K.

The vitamin K epoxide reductase (VKORC1) enzyme is of primary importance in many physiological processes, i.e., blood coagulation, energy metabolism, and arterial calcification prevention, due to its role in the vitamin K cycle. Indeed, VKORC1 catalyzes reduction of vitamin K epoxide to quinone and then to hydroquinone. However, the three-dimensional VKORC1 structure remains experimentally undetermined, because of the endoplasmic reticulum membrane location of this enzyme. Here we present a molecular modeling investigation of the VKORC1 enzymatic site structure and function, supported by in vitro enzymatic assays. Four VKORC1 mutants were designed in silico (F55G, F55Y, N80G, and F83G) based on a previous study that identified residues F55, N80, and F83 as being crucial for vitamin K epoxide binding. F55G, N80G, and F83G nonconservative mutants were all predicted to be inactive by molecular modeling analyses. However, the F55Y conservative mutant was expected to be active compared to wild-type VKORC1. In vitro enzymatic assays performed on recombinant proteins assessed our molecular modeling hypotheses and led us to describe the role of accurate VKORC1 active site residues with respect to VKORC1. Residues F55, N80, and F83 appeared to act in a concerted manner to keep vitamin K epoxide close to the C135 catalytic residue. Residues F55 and N80 prevent naphthoquinone head rotation away from the active site, assisted by residue F83 that prevents vitamin K from sliding outside the enzymatic pocket, through hydrophobic tail stabilization. Our results thus highlighted the specific functions of VKORC1 catalytic pocket residues and evidenced the ability of our structural model to predict biological effects of VKORC1 mutations.