Co-ordinated induction of beta-carotene cleavage enzyme and retinal reductase in the duodenum of the developing chicks

Evolutionary Interpretations of Nicotinic Acetylcholine Receptor Targeting Venom Effects by a Clade of Asian Viperidae Snakes

Ecological variability among closely related species provides an opportunity for evolutionary comparative studies. Therefore, to investigate the origin and evolution of neurotoxicity in Asian viperid snakes, we tested the venoms of Azemiops feae, Calloselasma rhodostoma, Deinagkistrodon acutus, Tropidolaeums subannulatus, and T. wagleri for their relative specificity and potency upon the amphibian, lizard, bird, rodent, and human α-1 (neuromuscular) nicotinic acetylcholine receptors. We utilised a biolayer interferometry assay to test the binding affinity of these pit viper venoms to orthosteric mimotopes of nicotinic acetylcholine receptors binding region from a diversity of potential prey types. The Tropidolaemus venoms were much more potent than the other species tested, which is consistent with the greater prey escape potential in arboreal niches. Intriguingly, the venom of C. rhodostoma showed neurotoxic binding to the α-1 mimotopes, a feature not known previously for this species. The lack of prior knowledge of neurotoxicity in this species is consistent with our results due to the bias in rodent studies and human bite reports, whilst this venom had a greater binding affinity toward amphibian and diapsid α-1 targets. The other large terrestrial species, D. acutus, did not display any meaningful levels of neurotoxicity. These results demonstrate that whilst small peptide neurotoxins are a basal trait of these snakes, it has been independently amplified on two separate occasions, once in Azemiops and again in Tropidolaemus, and with Calloselasma representing a third possible amplification of this trait. These results also point to broader sources of novel neuroactive peptides with the potential for use as lead compounds in drug design and discovery.

Ancient Diversification of Three-Finger Toxins in Micrurus Coral Snakes

Coral snakes, most notably the genus Micrurus, are the only terrestrial elapid snakes in the Americas. Elapid venoms are generally known for their potent neurotoxicity which is usually caused by Three-Finger Toxin (3FTx) proteins. These toxins can have a wide array of functions that have been characterized from the venom of other elapids. We examined publicly available sequences from Micrurus 3FTx to show that they belong to 8 monophyletic clades that diverged as deep in the 3FTx phylogenetic tree as the other clades with characterized functions. Functional residues from previously characterized clades of 3FTx are not well conserved in most of the Micrurus toxin clades. We also analyzed the patterns of selection on these toxins and find that they have been diversifying at different rates, with some having undergone extreme diversifying selection. This suggests that Micrurus 3FTx may contain a previously underappreciated functional diversity that has implications for the clinical outcomes of bite victims, the evolution and ecology of the genus, as well as the potential for biodiscovery efforts focusing on these toxins.

The venom-gland transcriptome of the eastern coral snake (Micrurus fulvius) reveals high venom complexity in the intragenomic evolution of venoms

BACKGROUND

Snake venom is shaped by the ecology and evolution of venomous species, and signals of positive selection in toxins have been consistently documented, reflecting the role of venoms as an ecologically critical phenotype. New World coral snakes (Elapidae) are represented by three genera and over 120 species and subspecies that are capable of causing significant human morbidity and mortality, yet coral-snake venom composition is poorly understood in comparison to that of Old World elapids. High-throughput sequencing is capable of identifying thousands of loci, while providing characterizations of expression patterns and the molecular evolutionary forces acting within the venom gland.

RESULTS

We describe the de novo assembly and analysis of the venom-gland transcriptome of the eastern coral snake (Micrurus fulvius). We identified 1,950 nontoxin transcripts and 116 toxin transcripts. These transcripts accounted for 57.1% of the total reads, with toxins accounting for 45.8% of the total reads. Phospholipases A(2) and three-finger toxins dominated expression, accounting for 86.0% of the toxin reads. A total of 15 toxin families were identified, revealing venom complexity previously unknown from New World coral snakes. Toxins exhibited high levels of heterozygosity relative to nontoxins, and overdominance may favor gene duplication leading to the fixation of advantageous alleles. Phospholipase A(2) expression was uniformly distributed throughout the class while three-finger toxin expression was dominated by a handful of transcripts, and phylogenetic analyses indicate that toxin divergence may have occurred following speciation. Positive selection was detected in three of the four most diverse toxin classes, suggesting that venom diversification is driven by recurrent directional selection.

CONCLUSIONS

We describe the most complete characterization of an elapid venom gland to date. Toxin gene duplication may be driven by heterozygote advantage, as the frequency of polymorphic toxin loci was significantly higher than that of nontoxins. Diversification among toxins appeared to follow speciation reflecting species-specific adaptation, and this divergence may be directly related to dietary shifts and is suggestive of a coevolutionary arms race.

The retinol dehydrogenase Rdh10 localizes to lipid droplets during acyl ester biosynthesis

Rdh10 catalyzes the first step of all-trans-retinoic acid biogenesis physiologically, conversion of retinol into retinal. We show that Rdh10 associates predominantly with mitochondria/mitochondrial-associated membrane (MAM) in the absence of lipid droplet biosynthesis, but also locates with lipid droplets during acyl ester biosynthesis. Targeting to lipid droplets requires the 32 N-terminal residues, which include a hydrophobic region followed by a net positive charge. Targeting to mitochondria/MAM and/or the stability of Rdh10 require both the N-terminal and the 48 C-terminal hydrophobic residues. Rdh10 behaves similarly to cellular retinol-binding protein, type 1, which also localizes to mitochondria/MAM before lipid droplet synthesis, and associates with lipid droplets during acyl ester synthesis (Jiang, W., and Napoli, J. L. (2012) Biochem. Biophys. Acta 1820, 859-8692). LRAT, an ER protein, also associates with lipid droplets upon acyl ester biosynthesis. Colocalization of Rdh10, Crbp1, and LRAT on lipid droplets suggests a metabolon that mediates retinol homeostasis.

Candidate genes for carotenoid coloration in vertebrates and their expression profiles in the carotenoid-containing plumage and bill of a wild bird

Carotenoid-based coloration has attracted much attention in evolutionary biology owing to its role in honest, condition-dependent signalling. Knowledge of the genetic pathways that regulate carotenoid coloration is crucial for an understanding of any trade-offs involved. We identified genes with potential roles in carotenoid coloration in vertebrates via (i) carotenoid uptake (SR-BI, CD36), (ii) binding and deposition (StAR1, MLN64, StAR4, StAR5, APOD, PLIN, GSTA2), and (iii) breakdown (BCO2, BCMO1). We examined the expression of these candidate loci in carotenoid-coloured tissues and several control tissues of the red-billed quelea (Quelea quelea), a species that exhibits a male breeding plumage colour polymorphism and sexually dimorphic variation in bill colour. All of the candidate genes except StAR1 were expressed in both the plumage and bill of queleas, indicating a potential role in carotenoid coloration in the quelea. However, no differences in the relative expression of any of the genes were found among the quelea carotenoid phenotypes, suggesting that other genes control the polymorphic and sexually dimorphic variation in carotenoid coloration observed in this species. Our identification of a number of potential carotenoid genes in different functional categories provides a critical starting point for future work on carotenoid colour regulation in vertebrate taxa.

Three cases of cerebellar hypoplasia and vitamin a deficiency: a case report and a possible pathophysiology

Studies in animal models have established that intra-uterine vitamin A deficiency can hinder hindbrain formation; however, reports of such a phenomenon in humans had not been published until recently, when our group presented the case of an infant diagnosed with pontocerebellar hypoplasia and vitamin A deficiency. We currently report the cases of 3 infants with cerebellar hypoplasia and hypovitaminosis A, whose vitamin A consumption was determined to be adequate, and whose mothers had no such deficiency. We suggest a possible pathophysiology whereby a mutation in the gene coding for cytoplasmic retinol-binding protein II, which is expressed both in the placenta and the yolk sac (during fetal development) and in the absorptive intestinal cells, can cause vitamin A deficiency, forming hindbrain anomalies. Validation of our hypothesis will require further research, including fetal vitamin A measurements and hindbrain examination in cytoplasmic retinol-binding protein II knockout animals.

Developmental changes in gene expressions of β-carotene cleavage enzyme and retinoic acid synthesizing enzymes in the chick duodenum

Vitamin A is derived from provitamin A carotenoids, mainly beta-carotene, by beta-carotene 15,15'-monooxygenase (BCMO1; EC 1.13.11.21). We previously reported that chick duodenal BCMO1 activity increased abruptly just after hatching. In this study, we further investigated mechanisms and physiological roles of the postnatal induction of BCMO1 expression in the chick duodenum. We showed that BCMO1 mRNA levels increased in the chick duodenum during postnatal period after hatching, but remain unchanged in the chick liver throughout the perinatal period. Serum hydrocortisone (HC) levels were also increased after hatching. Moreover, HC-administered chicks showed an enhancement of duodenal BCMO1 mRNA during the perinatal period. We further analyzed the developmental gene expression patterns of three types of retinoic acid (RA) synthesizing enzymes in the chick duodenum. Among them, retinal dehydrogenase 1 (RALDH1) mRNA levels in the chick duodenum increased during the postnatal period, indicating a similar developmental expression pattern to that of BCMO1. These results suggest that the postnatal induction of BCMO1 gene expression in the chick duodenum may be caused by the elevation of serum HC levels and may contribute to the RALDH1-mediated RA synthetic pathway.

Vitamin A nutrition of growing cockatiel chicks (Nymphicus hollandicus)

The experiments examined the physiological response of growing cockatiel chicks to varying levels of dietary vitamin A (VA) or beta-carotene and the rate of liver VA uptake. Adult cockatiels breeding pairs (n=10 pairs) were fed a VA-deficient diet for approximately 90 days prior to onset of egg laying. Breeding pairs were then allowed to feed their chicks diets containing either 0 IU VA/kg, 4000 IU VA/kg, or 2.4 mg beta-carotene/kg. After 5 weeks, chicks fed 0 IU VA developed poor feathering, facial dermatitis and reduced body weight (p<0.05). Liver VA was higher in chicks fed 4,000 IU VA or 2.4 mg beta-carotene vs. those fed 0 IU VA (p<0.05). Duodenal beta-actin and 15,15'-dioxygenase mRNA expression was similar to that of growing chickens, and greatest for cockatiel chicks fed 0 IU VA (p<0.01). Chicks fed 0 IU VA had keratinization of the bursa and oral mucosa, and reduced bursa development and lymphocyte density (p<0.05). Finally, when chicks fed 0 IU VA were orally gavaged with 20 IU VA/g body weight, maximal liver retinol uptake occurred between 0 and 24 h and reached a plateau at 36 h. These data demonstrate that VA deficiency can be prevented with 4,000 IU VA/kg diet or 2.4 mg beta-carotene/kg diet, although beta-carotene conversion to VA may be lower in cockatiels than chickens.

California Ground Squirrel (Spermophilus beecheyi) Defenses Against Rattlesnake Venom Digestive and Hemostatic Toxins

Previous studies have shown that some mammals are able to neutralize venom from snake predators. California ground squirrels (Spermophilus beecheyi) show variation among populations in their ability to bind venom and minimize damage from northern Pacific rattlesnakes (Crotalus oreganus), but the venom toxins targeted by resistance have not been investigated. Four California ground squirrel populations, selected for differences in local density or type of rattlesnake predators, were assayed for their ability to neutralize digestive and hemostatic effects of venom from three rattlesnake species. In Douglas ground squirrels (S. b. douglasii), we found that animals from a location where snakes are common showed greater inhibition of venom metalloprotease and hemolytic activity than animals from a location where snakes are rare. Effects on general proteolysis were not different. Douglas ground squirrels also reduced the metalloprotease activity of venom from sympatric northern Pacific rattlesnakes (C. o. oreganus) more than the activity of venom from allopatric western diamondback rattlesnakes (C. atrox), but enhanced the fibrinolysis of sympatric venom almost 1.8 times above baseline levels. Two Beechey ground squirrel (S. b. beecheyi) populations had similar inhibition of venoms from northern and southern Pacific rattlesnakes (C. o. helleri), despite differences between the populations in the locally prevalent predator. However, the venom toxins inhibited by Beechey squirrels varied among venom from Pacific rattlesnake subspecies, and between these venoms and venom from allopatric western diamondback rattlesnakes. Blood plasma from Beechey squirrels showed highest inhibition of metalloprotease activity of northern Pacific rattlesnake venom, general proteolytic activity and hemolysis of southern Pacific rattlesnake venom, and hemolysis by allopatric western diamondback venom. These results reveal previously cryptic variation in venom activity against resistant prey that suggests reciprocal adaptation at the molecular level.

California ground squirrel (Spermophilus beecheyi) defenses against rattlesnake venom digestive and hemostatic toxins

Previous studies have shown that some mammals are able to neutralize venom from snake predators. California ground squirrels (Spermophilus beecheyi) show variation among populations in their ability to bind venom and minimize damage from northern Pacific rattlesnakes (Crotalus oreganus), but the venom toxins targeted by resistance have not been investigated. Four California ground squirrel populations, selected for differences in local density or type of rattlesnake predators, were assayed for their ability to neutralize digestive and hemostatic effects of venom from three rattlesnake species. In Douglas ground squirrels (S. b. douglasii), we found that animals from a location where snakes are common showed greater inhibition of venom metalloprotease and hemolytic activity than animals from a location where snakes are rare. Effects on general proteolysis were not different. Douglas ground squirrels also reduced the metalloprotease activity of venom from sympatric northern Pacific rattlesnakes (Crotalus oreganus oreganus) more than the activity of venom from allopatric western diamondback rattlesnakes (C. atrox), but enhanced fibrinolysis of sympatric venom almost 1.8 times above baseline levels. Two Beechey ground squirrel (S. b. beecheyi) populations had similar inhibition of venoms from northern and southern Pacific rattlesnakes (C. o. helleri), despite differences between the populations in the locally prevalent predator. However, the venom toxins inhibited by Beechey squirrels did vary among venom from Pacific rattlesnake subspecies, and between these venoms and venom from allopatric western diamondback rattlesnakes. Blood plasma from Beechey squirrels showed highest inhibition of metalloprotease activity of northern Pacific rattlesnake venom, general proteolytic activity and hemolysis of southern Pacific rattlesnake venom, and hemolysis by allopatric western diamondback venom. These results reveal previously cryptic variation in venom activity against resistant prey that suggests reciprocal adaptation at the molecular level.

Identification of beta-carotene 15, 15'-monooxygenase as a peroxisome proliferator-activated receptor target gene

Beta-carotene 15,15'-monooxygenase (BCM) catalyzes the first step of vitamin A biosynthesis from provitamin A carotenoids. We wished to determine the factors underlying the transcriptional regulation of this gene. After cloning of the 40 kilobase pair (kbp) mouse Bcm gene and determination of its genomic organization, analysis of the 2 kb 5'-flanking region showed several putative transcription factor binding sites including TATA box, a peroxisome proliferator response element (PPRE), AP2, and bHLH. The 2 kb fragment drove specific luciferase gene expression in vitro only in cell lines that express BCM (TC7, PF11, and monkey retinal pigment epithelium). Nucleotides -41 to +163, and -60 to +163 drove basal and specific Bcm transcriptional activity, respectively. Site-directed mutagenesis and gel shift experiments demonstrate that PPRE was essential for Bcm promoter specificity and that the peroxisome proliferator activated receptor (PPAR) gamma (PPARgamma) specifically binds to this element. Furthermore, cotransfection experiments and pharmacological treatments in vitro, using the specific PPARgamma agonists LY17883 and ciglitazone, demonstrate that the PPRE element confers peroxisome proliferator responsiveness via the PPARgamma and retinoid X receptor-alpha heterodimer. Treatment of mice with the PPARalpha/gamma agonist WY14643 increases BCM protein expression in liver. Thus PPAR is a key transcription factor for the transcriptional regulation of the Bcm gene, suggesting a broader function for PPARs in the regulation of carotenoid metabolism metabolism that is consistent with their established role in neutral lipid metabolism and transport.

Anhydrolutein in the zebra finch: a new, metabolically derived carotenoid in birds

Many birds acquire carotenoid pigments from the diet that they deposit into feathers and bare parts to develop extravagant sexual coloration. Although biologists have shown interest in both the mechanisms and function of these colorful displays, the carotenoids ingested and processed by these birds are poorly described. Here we document the carotenoid-pigment profile in the diet, blood and tissue of captive male and female zebra finches (Taeniopygia guttata). Dietary carotenoids including: lutein; zeaxanthin; and beta-cryptoxanthin were also present in the plasma, liver, adipose tissue and egg-yolk. These were accompanied in the blood and tissues by a fourth pigment, 2',3'-anhydrolutein, that was absent from the diet. To our knowledge, this is the first reported documentation of anhydrolutein in any avian species; among animals, it has been previously described only in human skin and serum and in fish liver. We also identified anhydrolutein in the plasma of two closely related estrildid finch species (Estrilda astrild and Sporaeginthus subflavus). Anhydrolutein was the major carotenoid found in zebra finch serum and liver, but did not exceed the concentration of lutein and zeaxanthin in adipose tissue or egg yolk. Whereas the percent composition of zeaxanthin and beta-cryptoxanthin were similar between diet and plasma, lutein was comparatively less abundant in plasma than in the diet. Lutein also was proportionally deficient in plasma from birds that circulated a higher percentage of anhydrolutein. These results suggest that zebra finches metabolically derive anhydrolutein from dietary sources of lutein. The production site and physiological function of anhydrolutein have yet to be determined.