Vitellogenesis in the lizard Lacerta vivipara jacquin. I. Purification and partial characterization of plasma vitellogenin

The physiology of lipid storage and use in reptiles

Lipid metabolism is central to understanding whole-animal energetics. Reptiles store most excess energy in lipid form, mobilise those lipids when needed to meet energetic demands, and invest lipids in eggs to provide the primary source of energy to developing embryos. Here, I review the mechanisms by which non-avian reptiles store, transport, and use lipids. Many aspects of lipid absorption, transport, and storage appear to be similar to birds, including the hepatic synthesis of lipids from glucose substrates, the transport of triglycerides in lipoproteins, and the storage of lipids in adipose tissue, although adipose tissue in non-avian reptiles is usually concentrated in abdominal fat bodies or the tail. Seasonal changes in fat stores suggest that lipid storage is primarily for reproduction in most species, rather than for maintenance during aphagic periods. The effects of fasting on plasma lipid metabolites can differ from mammals and birds due to the ability of non-avian reptiles to reduce their metabolism drastically during extended fasts. The effect of fasting on levels of plasma ketones is species specific: β-hydroxybutyrate concentration may rise or fall during fasting. I also describe the process by which the bulk of lipids are deposited into oocytes during vitellogenesis. Although this process is sometimes ascribed to vitellogenin-based transport in reptiles, the majority of lipid deposition occurs via triglycerides packaged in very-low-density lipoproteins (VLDLs), based on physiological, histological, biochemical, comparative, and genomic evidence. I also discuss the evidence for non-avian reptiles using 'yolk-targeted' VLDLs during vitellogenesis. The major physiological states - feeding, fasting, and vitellogenesis - have different effects on plasma lipid metabolites, and I discuss the possibilities and potential problems of using plasma metabolites to diagnose feeding condition in non-avian reptiles.

Multihormonal control of vitellogenesis in lower vertebrates

The comparative approach on how and when vitellogenesis occurs in the diverse reproductive strategies displayed by aquatic and terrestrial lower vertebrates is presented in this chapter; moreover, attention has been paid to the multihormonal control of hepatic vitellogenin synthesis as it is related to seasonal changes and to vitellogenin use by growing oocytes. The hormonal mechanisms regulating vitellogenin synthesis are also considered, and the effects of environmental estrogens on the feminization process in wildlife and humans have been reported. It is then considered how fundamental nonmammalian models appear to be, for vitellogenesis research, addressed to clarifying the yolkless egg and the evolution of eutherian viviparity.

Immunodissection of yolk lipovitellin (LV1) demonstrates the existence of different LV1-domains and suggests a complex family of vitellogenin genes in the lizard Anolis pulchellus

In the tropical lizard Anolis pulchellus vitellogenin (Vtg), the plasma precursor of egg-yolk proteins, consists of a family of five polypeptides ranging between 116 and 226 kDa. In addition, lipovitellin-LV1, the main yolk protein, resolves electrophoretically into several forms. This suggests that yolk LV1 is a mixture of similar but distinct peptides that presumably correspond to the LV1 domains of the plasma Vtgs. Here, we test the hypothesis that each component of yolk LV1 is distinct enough to be antigenically different and that these components correspond to cleavage products of plasma Vtg polypeptides. Our experimental approach is based on adsorptions of a polyclonal antiserum against purified LV1 with membrane-immobilized Vtgs. This immunochemical design permitted us to dissect sub-populations of LV1 antibodies that specifically reacted with one of the plasma Vtgs. The presence of unique epitopes in the LV1 components directly indicates differences among the structures of their plasma Vtg precursors and supports the idea of a heterogeneous multigene Vtg family in Anolis. These results also show the potential of this immunoadsorption approach for the identification, and even separation of proteins sharing a high degree of similarity in their molecular structures.

Vitellogenin and yolk protein processing in Bothrops jararaca (Wied), a tropical venomous snake

The plasma vitellogenin of Bothrops jararaca is composed of two subunits. The larger subunit (160 kDa) is phosphate rich and carbohydrate poor, while the smaller (110 kDa) is highly glycosylated and less phosphorylated. As in other vertebrates, the vitellogenin of B. jararaca is synthesized in the liver under estrogen control. The newly synthesized vitellogenin molecule is a 270 kDa polypeptide. This polypeptide originates the two subunits of the plasma vitellogenin by proteolytic cleavage. In the eggs of B. jararaca six main yolk polypeptides have been detected (113, 107, 104, 72, 27.2 and 20.7 kDa). Using phosphoprotein staining we have shown that the 72 kDa polypeptide is the larger phosvitin so far described in a vertebrate egg yolk. The 107 kDa yolk polypeptide also seems to be phosphorylated, but to a lesser extent than the phosvitin. The 104 kDa vitellin originates from the larger vitellogenin subunit while the 113 kDa vitellin originates from the smaller vitellogenin subunit. Based on these results we propose a general scheme for vitellogenin and vitellin processing in B. jararaca.

The reproductive cycle of the female American alligator (Alligator mississippiensis)

We examined ovarian and oviducal gross morphology and collected blood samples from wild female alligators in central Florida during most of the year. Females with vitellogenic follicles were observed throughout the year, although ovaries containing follicles larger than 15 mm were seen only during the spring and early summer (March-June). We detected a poor relationship between female body size and the number of vitellogenic follicles whereas body size was significantly correlated with clutch size. Plasma samples were analyzed for (1) estradiol-17beta (E2), testosterone (T), and corticosterone by specific radioimmunoassays; (2) vitellogenin by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis; and (3) total protein, phosphorus, and calcium by spectrophotometric assays. Reproductively active females showed elevated plasma concentrations of E2 during the fall (September-November) and spring (March-May) whereas non-reproductively active females exhibited basal levels with no apparent peaks. Vitellogenin was detected in the plasma during the same months that plasma E2 concentrations were elevated. Elevated plasma vitellogenin and E2 were not correlated with plasma total protein but were correlated with plasma calcium concentration. During late vitellogenesis, plasma T concentrations were elevated in reproductively active females coincident with a period of intense courtship and mating. Corticosterone plasma concentrations exhibit no significant monthly variation, nor apparent changes during various stages of reproductive activity although plasma concentrations were elevated during late gravidity. Our data suggest that female reproductive activity begins in the fall with an increase in plasma E2 concentration in September or October and vitellogenesis in October. Ovarian activity slows during winter and reactivates with the onset of spring.

Purification, partial characterization and peptide sequences of vitellogenin from a reptile, the tuatara (Sphenodon punctatus)

Vitellogenin (Vg), a major precursor to egg yolk proteins, was purified from plasma of an estradiol-treated female tuatara (Sphenodon punctatus) by MgCl2-EDTA precipitation and DEAE-cellulose chromatography. The amino acid composition of tuatara Vg is similar to that of other vertebtate Vgs and contains a large proportion of serine (13.7 mol/100 mol of total amino acid). The amino acid sequences of the N-terminus of mature Vg (33 residues) and of several trypsin- and CNBr-generated peptides were determined. Six peptide sequences obtained from tuatara Vg could be aligned with Vg sequences from other vertebrates. Reduced and non-reduced forms of tuatara Vg have the same apparent molecular mass (approximately 218 kDa) when resolved by SDS-PAGE, indicating that inter-chain disulfide bonds are not a feature of the molecule in this species. Western blot analysis with anti-tuatara Vg antiserum indicated that at least some epitopes are shared among Vgs of turtle, alligator and tuatara.

Synthesis of vitellogenin polypeptides and deposit of yolk proteins in Anolis pulchellus

The recognition of liver and serum polypeptides in Anolis pulchellus by a polyclonal antibody against S1-lipovitellin confirmed their identity as vitellogenins (Vtg) and demonstrated their structural relationship to yolk lipoproteins. In vivo labeling demonstrated active synthesis of Vtg by vitellogenic females since intracellular incorporation of [3H]-leucine was detected at short periods of label in all five Anolis Vtg forms. Time course analysis of 3H-Vtg levels indicated a 1 hr lag phase between synthesis and secretion. On the other hand, 32P-Vtg appears to be rapidly secreted from the liver into the blood since label was detected simultaneously in both compartments. After 2 hr intracellular 32P-Vtg levels reached a plateau. Decreasing 32P-Vtg levels in the blood were observed several hours after injection. In growing oocytes 32P was detected in yolk phosphoproteins ranging from 37,000 to 75,000 in molecular weight. Based on these results together with previous published data we conclude that in tropical anole the yolk phosphoproteins appear to be derived from the larger highly phosphorylated Vtg forms according to the typical vertebrate Vtg precursor-product relationship. However, the main component of yolk lipovitellin is synthesized in the liver as an independent lipoprotein (Vtg-116) which is taken up by growing oocytes without major proteolytic modifications. This novel mode of lipovitellin biosynthesis and deposit in reptiles has not been reported previously.

Identification of a 130-kDa albumin in tuatara (Sphenodon) and detection of a novel albumin polymorphism

Electrophoretic, immunochemical, and protein sequence analyses were performed on plasma albumin of the tuatara (Sphenodon), a rare reptile endemic to New Zealand. The analyses revealed that, unlike other terrestrial vertebrates, tuatara do not seem to possess a 60- to 75-kDa plasma albumin. The common form of plasma albumin in this genus has an apparent molecular mass of 130 kDa, making it by far the largest albumin reported for any terrestrial vertebrate. Starch gel electrophoresis of samples from tuatara on 24 of the 30 islands inhabited by this genus resolved two forms of the 130-kDa albumin (albumins A and C). A third albumin of approximately 170 kDa (albumin B), reflecting a novel alloalbuminemia, was found in tuatara in three geographically isolated populations. Albumin A appears to be restricted to populations at the southern extremity of the tuatara's distribution, while albumin C was found in all but four (southern) populations. Possible explanations for the origin and distribution of these albumins are discussed.

Identification of vitellogenin in the little skate (Raja erinacea)

1. Vitellogenin was isolated from mature female skates by selective precipitation with MgCl2/EDTA followed by chromatography on DEAE-cellulose columns. 2. A single monomer of approximately 205 kDa was identified on 6.0% SDS-PAGE gels. 3. In addition, isolation of yolk proteins with ammonium sulfate yielded proteins of 94 and 38 kDa (putative phosvitins) and putative lipovitellins of ca 105, 91 and 67 kDa. 4. In vivo phosphate incorporation in female and male skates implanted with estradiol indicated that vitellogenin was phosphorylated. 5. Total protein phosphate incorporation was significantly higher in females than male skates. 6. In male skates treated with estradiol, phosphate incorporation increased from 2 days after implantation to a maximum at approximately 11 days after implantation. 7. Determination of the rate of disappearance of 32P-labeled protein suggests a half-life of ca 200 hr in normal female skate plasma.

An ultrastructural study of differentiation of pyriform cells and their contribution to oocyte growth in representative squamata

An electron microscopic study of the differentiation of pyriform cells and their contribution to oocyte growth in three lizards (Tarentola mauritanica, Cordylus wittifer, Platysaurus intermedius) and one colubrid snake (Coluber viridiflavus) revealed that pyriform cells differentiate from small follicle cells via intermediate cells after establishing an intercellular bridge with the oocyte (see also Hubert: Bull Soc Zool Fr 102:151-158, 1977; Filosa et al: J Embryol Exp Morphol 54:5-15 1979; Klosterman: J Morphol 192:125-144, 1987). Once differentiated, pyriform cells display ultrastructural features indicative of synthetic activity, including abundant ribosomes, Golgi membranes, vacuoles, mitochondria, and lipid droplets. These cellular components extend to the apex of the cell at the level of the intercellular bridge, suggesting that constituents of pyriform cells may be transferred to the oocyte. Furthermore, we demonstrate for the first time that pyriform cells incorporate exogenous yolk. The yolk is segregated inside maturing yolk granules that form in the pyriform cell in the same manner as described for vitellogenic oocytes in non-mammalian vertebrates (see Wallace: Developmental Biology, A Comprehensive Synthesis 127-177, 1985). It is the first clear evidence that pyriform cells and the oocyte may fulfill similar vitellogenic functions. The establishment of an intercellular bridge may represent a crucial event in the development of an integrated system in which pyriform cells and oocyte cooperate.

Plasma vitellogenin and 17 beta-estradiol levels during the annual reproductive cycle of Podarcis s. sicula Raf

Plasma vitellogenin and 17 beta-estradiol concentration were determined during the annual reproductive cycle of the female lizard Podarcis s. sicula Raf. living around Naples. Plasma vitellogenin was purified from estrogenized males for characterization and to raise specific immune serum. Using ELISA, plasma vitellogenin titers were determined in relation to ovary weight; plasma 17 beta-estradiol was measured by RIA method. Native vitellogenin was present as two polypeptide bands: alpha and beta. The electrophoretic patterns, studied in normal male and estrogenized male and female, showed vitellogenin to be a protein present in female and in estrogenized male plasma but not in normal males. Lizard monomeric VTG, determined by SDS-PAGE, was about 200 kDa. Correlations between seasonal ovarian weight variations and plasma vitellogenin and 17 beta-estradiol suggest that ovarian development in Podarcis depends on plasma vitellogenin synthesis, which in turn relies on plasma estradiol levels. The two ovulatory waves observed in this study coincided with the two peak values of plasma vitellogenin and 17 beta-estradiol.

Vitellogenesis in Anolis pulchellus: induction of VTG-like protein in liver explants from male and immature lizards

Vitellogenin (VTG) synthesis has been described as an ideal system to study the hormonal regulation of gene expression. In Xenopus the molecular aspects of this control have been analyzed; however, in other non-mammalian species such as reptiles, very few studies approaching this level have been undertaken. We report on the induction by estradiol-17 beta of VTG-like proteins in liver explants from adult males and immature male and female lizards (A. pulchellus). A concentration of 10(-7) M was optimum for adult males while a higher concentration (10(-6) M) is required for the immature animals. No differences were observed in the hormonal induction in male and female immature animals, suggesting that there are no sexual distinctions in the liver at this stage. The effect of the hormone in male liver appears to be primarily on mRNA synthesis, since increases in 3H-uridine incorporation in total RNA were prevented by addition of 1 microgram/ml of the RNA polymerase II inhibitor alpha-Amanitin; however, rRNA synthesis was also increased as observed by agarose gel analysis. A 48 hr lag period was required for the detection of the intracellular as well as the secreted VTG-like protein. Electrophoretical analysis of the secretory products revealed the induction of a group of phosphoproteins immunologically related to yolk lipovitellin whose molecular weights range from 116,000 to 200,000.

Cold entrainment of the annual cycle of ovarian activity in the lizard Lacerta vivipara: thermoperiodic rhythm versus hibernation

A constant warm thermoperiod maintains ovarian quiescence in the lizard Lacerta vivipara, whereas a 4-month artificial hibernation rapidly induces synchronized vitellogenesis after transfer to warmth. The present study examined the possibility of a thermoperiodic regulation of the ovarian cycle and the formal properties of an internal temporal program. These questions were addressed using 24-hr thermoperiodic conditions that combined a long or a short thermophase (6 or 2 hr of basking) with a warm (19-21 degrees C), a cool (5-15 degrees C), or a cold (3-7 degrees C) cryophase. Lizards were exposed to the natural photocycle or to LD 12:12. Occurrence and timing of vitellogenesis completion were monitored using immunodetection of plasma vitellogenin and laparotomies. Cold remained stimulatory when given intermittently with a 24-hr periodicity. However, under long-thermophase conditions, lizards responded poorly to cool cryophases but fully to cold ones (72.7-100% vitellogenesis). Thus a certain amount of cold must be provided during each 24-hr cycle in order to be effective through the succession of thermocycles. Reduction of the daily heat input from 6 to 2 hr modulated the stimulating effects of cold cryophases: The median date for the beginning of vitellogenesis occurred 1 month earlier, but the number of responding females decreased from 100% to 40%. The thermoperiodic regulation of the ovarian cycle also relies upon a precise heat-cold balance per nycthemeral unit. This ensures the entrainment of an internal rhythm, since the timing of reproductive responses varies with the date of transfer from the inhibitory warm thermoperiod to the inducing thermoperiod (long thermophase, cold cryophase). At least half the females started vitellogenesis within 1-2 months after a late transfer (winter solstice) instead of 6 months after an early one (autumn equinox), and the median date for onset differed by 1 month between the two groups. However, autumn transfer was the only one to induce a group response in close agreement with the natural timing.

Vitellogenin diversity in tropical lizards (Anolis pulchellus): identification and partial characterization

1. Three size-classes of vitellogenin polypeptides were detected by electrophoretic and radiolabeling techniques in 32Pi-labeled plasma of vitellogenic female lizards but not in male animals. Based on their apparent Mr, the polypeptides were designated as VTG-226-201K, VTG-169-153K and VTG-116-123K. 2. Structural differences were found between VTG-169-153K and VTG-116K by partial proteolysis with S. aureus V8 protease and radiolabeling techniques. 3. Autoradiography of a 3-10% native gradient gel revealed three different species of VTG in 32Pi-labeled plasma of stimulated males: VTG-I (Mr = 850,000), VTG-II (Mr = 750,000) and VTG-III (Mr = 610,000). By 2D PAGE, it was shown that VTG-169K and VTG-116K are components of VTG-I, while VTG-153K and VTG-116K comprise VTG-III. These results suggest an oligomeric structure for native VTG. 4. Amino acid analysis, 32Pi incorporation, electrophoretical behavior and Mr estimation demonstrated homology between VTG-116K and the S1-lipovitellin from the lizard egg. 5. These results strongly indicate an unusual multiplicity of VTG forms in tropical lizards when compared to other egg-laying vertebrates.

Exogenous vitellogenesis and micropinocytosis in the lizard, Podarcis sicula, treated with follicle-stimulating hormone

The regulation of oocyte growth and of exogenous vitellogenesis by micropinocytosis has been studied in lizard Podarcis sicula kept at 28 degrees during the winter stasis and stimulated by follicle-stimulating hormone (FSH). Under these experimental conditions, oocyte auxocytosis as well as vitellogenesis is stimulated, while the follicular hierarchy is preserved. At the ultrastructural level, the flow of exogenous yolk precursors toward the oocyte increases and the pathway taken by them through intercellular spaces and zona pellucida is the same as that taken by peroxidase (tracer). Yolk precursor endocytosis is found only in oocytes greater than 1500 microns in diameter and takes place through the formation of several coated pits and vesicles. It is suggested that membrane receptors necessary for micropinocytosis are available only in such oocytes. Last, a different permeability of the ovarian follicle to exogenous yolk precursors during the different stages of oocyte growth and endovarian control of vitellogenesis are suggested.

Relationship between estradiol-17 beta seasonal profile and annual vitellogenin content of liver, fat body, plasma, and ovary in the frog (Rana esculenta)

The seasonal plasma estradiol-17 beta (E2-17 beta) profile and annual vitellogenin content of liver, fat body, plasma, and ovary were investigated in Rana esculenta. Concomitant with the increase in E2-17 beta, vitellogenin peaked in liver, plasma, and ovary during autumn and winter, while it remained at a relatively high concentration in fat body during spring. In vitro experiments showed that E2-17 beta (10(-9) M) is ineffective in inducing vitellogenin production in fat body, but is effective in inducing vitellogenin production in liver. As fat bodies do not produce the vitellogenin they contain, we suggest that fat bodies are involved in the transfer of vitellogenin to the ovary.

Vitellogenesis in the lizard Lacerta vivipara jacquin. II. Vitellogenin synthesis during the reproductive cycle and its control by ovarian steroids

An analysis of the development of the vitellogenic process following artificial hibernation in the lizard Lacerta vivipara was undertaken. For that purpose, organ weights (ovaries, oviducts, liver, fat bodies) and plasma concentrations of total proteins, calcium, and estrogens were monitored. The induction of the vitellogenic growth of 2-5 oocytes per ovary was characterized by a rapid increase in calcemia (from 2.4-2.6 mM to 4-10 mM), and in oviduct and liver weights. During the active and continuous phase of vitellus incorporation (congruent to 3 weeks, follicle diameter 1.6-2.0 mm to greater than 5 mm) the developments of ovaries and oviducts were positively correlated, liver weight and calcemia remained elevated (respectively, 1.2-2.2 times and 2.5-3.5 times the previtellogenic values). Ovulation was preceded by a significant rise in calcemia and followed by a decrease in liver weight, but no modification of oviduct mass. Plasma concentration in total proteins (50-60 mg/ml) was not modified during the entire process. Plasma estrogens were difficult to measure in this small species. Levels of estradiol-17 beta were very often below the assay sensitivity (less than 0.3-0.6 ng/ml), never above 2 ng/ml, and very variable among individuals. No correlation with vitellogenin production could be established. Therefore, the abilities of different ovarian steroids to induce vitellogenin synthesis were tested in vivo. To reduce the rise of plasma estradiol titer (observed during a 4-week experiment), the steroids were implanted in ovariectomized lizards for a short time (5 days). The vitellogenic response was assessed by measuring the distribution of the 32P radioactivity between the acidoprecipitable plasma fraction and the plasma vitellogenin recognized by the lizard antivitellogenin serum. Plasma titers of estradiol-17 beta were monitored. The estrone potencies could not be determined as this treatment involved an important rise in estradiol level. Progesterone, delta 4, testosterone, and 5 alpha-androstanediol were unable to stimulate vitellogenin synthesis. Estradiol-17 beta was the only effective steroid. It was further demonstrated that the estradiol-induced hypercalcemia, hyperproteinemia, and liver growth in ovariectomized lizards were dependent upon the total amount of estrogen injected.