Purification, partial characterization, and radioimmunoassay of prolactin and growth hormone from the Bennett's wallaby

Growth axis maturation is linked to nutrition, growth and developmental rate

Maturation of the mammalian growth axis is thought to be linked to the transition from fetal to post-natal life at birth. However, in an altricial marsupial, the tammar wallaby (Macropus eugenii), this process occurs many months after birth but at a time when the young is at a similar developmental stage to that of neonatal eutherian mammals. Here we manipulate growth rates and demonstrate in slow, normal and fast growing tammar young that nutrition and growth rate affect the time of maturation of the growth axis. Maturation of GH/IGF-I axis components occurred earlier in fast growing young, which had significantly increased hepatic GHR, IGF1 and IGFALS expression, plasma IGF-I concentrations, and significantly decreased plasma GH concentrations compared to age-matched normal young. These data support the hypothesis that the time of maturation of the growth axis depends on the growth rate and maturity of the young, which can be accelerated by changing their nutritional status.

Enhanced Yield and Homogeneity of Buffalo Growth Hormone by an Improved Chromatographic Protocol

Loss of buffalo Growth Hormone (buGH) in the various side fractions of standard buGH purification protocol has been determined quantitatively by direct binding ELISA and qualitatively by SDS-PAGE and Western blot analysis. Accounting result indicated that there was a considerable loss of buGH in the side fractions. An alternative protocol to prevent loss and to obtain a high yield of buGH has been developed by introducing anion exchange chromatography, QAE-Sephadex. This has resulted in a simple, reproducible three-step protocol. In this protocol, an extract obtained at 250 mM (NH4)2 SO4, pH 5.5, was loaded onto the QAE-Sephadex column in 0.1 M NH4 HCO3. At this salt concentration, the bulk of the buGH came as QAE unbound fraction. Some amount of buGH, together with contaminating proteins, was bound to QAE-Sephadex and these could be eluted with 1 M KCl. The immunopotency of the enriched buGH preparation "QUB" (QAE unbound fraction) in a direct binding ELISA was similar to that of the semi-pure buGH (ECS/APECS) preparation obtained using the standard protocol, but the yield was 4 times higher. The SDS-PAGE data showed that the banding pattern of standard semi-pure buGH and QUB were quite similar and QUB can be loaded onto the Sephacryl S-200 gel filtration chromatography to yield a highly purified buGH. SDS-PAGE and Western blot analyses showed the major band of buGH in QUB at the same position as in the case of standard buGH. It has also been demonstrated here that it is possible to separate buffalo prolactin (buPRL) and buGH on QAE-Sephadex.

Radioimmunoassay of prolactin for the meerkat (Suricata suricatta), a cooperatively breeding carnivore

We report the development and validation of a highly specific heterologous radioimmunoassay (RIA) to measure meerkat prolactin (PRL) by using rabbit antiserum to human prolactin and canine [125I]iodo-PRL. Dilutions of meerkat pituitary standard and plasma gave parallel inhibition curves in the assay. Gel filtration of meerkat pituitary extracts and canine [125I]iodo-PRL run separately on a Sephadex G-100 generated identical peaks of activity, and Western blot analysis of meerkat pituitary extract with the human prolactin antiserum used in the RIA gave a molecular weight similar to canine prolactin (21kDa). We carried out a biological validation of the prolactin assay by administering three different doses each of sulpiride and cabergoline to adult male meerkats. Increasing doses of sulpiride and cabergoline caused substantial increases and decreases, respectively, in the plasma prolactin of the study animals as expected. Activation of the stress response in meerkats by capture and ketamine hydrochloride anesthesia caused short-term but significant increases in prolactin levels in individuals bled repeatedly. The RIA developed and described here was able to determine plasma concentrations of prolactin in all animals sampled. We conclude, however, that it will be important in all future studies to confine blood sampling times to 4-7 min after capture/administration of anesthesia to avoid the confounding effects of the stress response on prolactin levels.

The prolactin receptor from the brushtail possum (Trichosurus vulpecula): cDNA cloning, expression and functional analysis

A full length, prolactin receptor cDNA clone has been isolated from the brushtail possum (Trichosurus vulpecula). This clone encodes a 625 amino acid protein which shares 60-70 and 54% sequence identity with prolactin receptor (long form) sequences from mammalian and avian species, respectively. Sequence similarity was highest in the extra-cellular, hormone-binding domain and in specific regions of the intracellular domain which regulates prolactin receptor signalling in cells. Prolactin receptor mRNA was detected in a wide range of possum tissues and in the mammary gland the PRL-R gene was differentially expressed during lactation with peak mRNA levels being detected during the first 6 days of lactation and after day 115 throughout late lactation. This pattern of PRL-R mRNA expression in the mammary gland is similar to that observed for circulating prolactin in the lactating possum. In CHO cells transiently transfected with the possum prolactin receptor, expression of a beta-lactoglobulin promoter/reporter gene construct was increased 3-fold by adding prolactin. The possum prolactin receptor is therefore capable of binding ovine prolactin and activating the Jak2/Stat5 signalling cascade. This provides evidence for the highly conserved nature of the prolactin signalling pathway in mammalian evolution.

Cloning and sequence analysis of a pituitary prolactin cDNA from the brushtail possum (Trichosurus vulpecula)

Overlapping cDNA partial clones of pituitary prolactin from the marsupial brushtail possum (Trichosurus vulpecula) were isolated and sequenced. The nucleotide and deduced amino acid sequences showed high sequence identity with pig prolactin (84.3 and 92.5%, respectively) and all of the expected structural features of a quadruped prolactin. A prolactin gene tree was constructed and rates of evolution calculated for possum along with several mammalian and nonmammalian prolactins. Possum prolactin was most closely linked to the prolactins of eutherian mammals but branched from the main mammalian line well before the eutherian prolactins. The prolactin/GH family shows variable rates of evolution ranging from 0.3 substitutions/amino acid site/year x 10(9) for pig prolactin to 7.0 substitutions/ amino acid site/year x 10(9) for the mouse. Since divergence from the eutherian mammals, possum prolactin has shown a slow rate of evolution (0.2 substitutions/ amino acid site/year x 10(9)). As expected, the prolactin gene was expressed in the possum pituitary gland but not in the liver, lung, kidney, heart, or mammary gland.

cDNA cloning of growth hormone from the brushtail possum (Trichosurus vulpecula)

The full-length nucleotide sequence for the cDNA of growth hormone (GH) from the marsupial brushtail possum (Trichosurus vulpecula) was determined using reverse transcription followed by polymerase chain reaction (RT-PCR) and the 5'/3' rapid amplification of cDNA ends (RACE) technique. Sequence information showed that the possum GH cDNA was 831 base pairs (bp) in length, including the 5'untranslated region (60 bp), the signal peptide (75 bp), the mature protein (573 bp, including stop codon), and the 3' untranslated region (123 bp). At both the nucleotide and the amino acid level (deduced from nucleotide sequence), there was a high degree of sequence identity with pig and horse. These species were similar at 82.8 and 83.0% of bases at the nucleotide level and 91.6 and 91.1% at the amino acid level, respectively. Northern analysis showed that GH mRNA is present in the pituitary gland and was similar in size to that seen in other mammals (approximately equal to 0.9 kb). Analysis of molecular evolution of GH in the possum indicated that the rate of evolution is relatively slow (0.4 substitutions/ amino acid site/year x 10(9) and typical of that seen for nonprimate mammals, which exhibit rates ranging between 0.2 and 1.3 substitutions/amino acid site/year x 10(9).

Heterogeneity in buffalo pituitary prolactin

The Ellis procedure of serial extraction of gonadotropins and growth hormone (GH) followed by alkaline ethanol extraction was adopted to process freshly frozen buffalo pituitaries. The procedure after slight modification was found very useful as more than 2 mg of GH free immunoreactive prolactin (PRL) could be isolated from each gram of wet pituitary tissue. Further, the biochemical purity and immunobiological potency of the extracted PRL, designated as P-I, was comparable with that of the highly purified samples of homologous and heterologous PRLs. No non-PRL protein was detectable in P-I. Micro-heterogeneity with regard to size, charge, co- and post-translational modifications was also investigated under different conditions of extraction and at different stages of purification. Immunological and biological potencies were compared in homologous competitive enzyme linked immunosorbent assay (ELISA) developed for buffalo PRL and in rat Nb2 lymphoma proliferation assay respectively. Structural heterogeneity was observed in all the preparations checked including fresh pituitary homogenate and highly purified hormone. Nevertheless a 25 K species corresponding to the hormone monomer was always the only paramount form comprising more than 90% of the total PRL protein in all the samples including P-I. Similar size forms were observed in all preparations and were found to be equivalents of monomers, dimers, covalent-and non-covalent multimers, disulphide bridged forms and cleaved fragments. Other sibling species identified were glycosylated PRL, charge isoforms and forms that perhaps differed in their extractability from the pituitary tissue. Strong apparent size heterogeneity was displayed by the monomeric buffalo PRL. In light of these observations and the information on the structural and functional significance and the consequences of polymeric forms, the use of a heterogeneous PRL (P-I) as a reference hormone is recommended for a valid assay.