Evolution of three neuropeptides isolated from the brain of sturgeon
Date
2018-07-23
Authors
Lescheid, David William
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Abstract
In vertebrates the brain superimposes control on fundamental processes such as
reproduction and growth. Neuropeptides secreted from the brain initiate a cascade of
events that affect these processes. In this thesis three neuropeptides are examined to
determine their structures and patterns in the context of vertebrate evolution.
Reproduction in vertebrates is controlled by the neuropeptide gonadotropin-releasing
hormone, GnRH, a decapeptide belonging to a peptide family of twelve known
members. One common theme in vertebrates is that there is usually more than one form of
GnRH in the brain of a single species; often each form of GnRH has a separate location in
the brain and therefore, an implied distinct function. In this thesis, the brain of Siberian
sturgeon, Acipenser gueldenstaedti, initially was examined for GnRH using reversed-phase
high performance liquid chromatography, HPLC, and radioimmunoassay, RIA,
with specific antisera and was shown to contain mammalian (m)GnRH by chemical
sequence analysis and by accurate determination of the molecular mass. In addition,
another form of GnRH, termed chicken (c)GnRH-II, was found in the sturgeon brain.
This is the first report to show that the primary structure of GnRH is identical in an
evolutionarily-ancient fish and in mammals including humans. Further, the second form of
GnRH, cGnRH-11, was identified for the first time in the brain of adult stumptail monkeys
(Macaca speciosa) as well as in adult and fetal rhesus monkey (Macaca mulatta) brains.
This study implies that at least two forms of GnRH are found in the brain of most
vertebrate species including mammals.
In cartilaginous fish that evolved earlier than sturgeon, the same HPLC and RIA
methods were used to demonstrate that regions of the brain and pituitary of skate. Raja
canebensis, also contained cGnRH-II but dogfish (df)GnRH rather than mGnRH. By the
same criteria, teleost fish like whitefish (Prosopium williamsoni), platyfish (Xiphophorus
maculatus), green swordtail (Xiphophorus hellerei) and sablefish (Anoplomia fimbria)
were shown to have cGnRH-II and salmon (s)GnRH, as well as one or two more
immunoreactive variants of GnRH with novel or seabream (sb)GnRH-like properties,
within their brain. The identity of at least three types of immunoreactive GnRH molecules
in the brain of these fish species suggests that three forms of GnRH in the brain is an early
condition in teleost evolution.
Ancestral sturgeon emerged at a branch point between the bony fish lineage and the
tetrapod lineage and therefore, it is useful to compare the neuropeptide structures found in
their brain with those both in fish and more evolutionarily-advanced vertebrates. Several
tetrapod species were examined to determine if the forms of GnRH found in the sturgeon
brain had been retained in their evolution. In contrasts to studies in our laboratory and by
others showing that most amphibians, reptiles and birds contain two forms of GnRH, the
present research shows that the brain of the green anole lizard, Anolis carolinensis,
contained only cGnRH-II within its brain. In addition, my HPLC and RIA studies showed
that only mGnRH was present in the brain of guinea pig, hamster and rat suggesting that
there are some species which function with only one form of GnRH in their brain. Also,
there were no distinguishable forms of GnRH in a human placenta, demonstrating that the
type(s) of GnRH might be tissue-specific.
Two neuropeptides associated with growth also were isolated from the sturgeon
brain. A cDNA encoding growth hormone-releasing factor, GRF, and pituitary adenylate
cyclase-activating polypeptide, PACAP, was isolated and sequenced using the polymerase
chain reaction, PCR, and other molecular biology methods. In contrast to mammals where
GRF and PACAP are encoded on separate genes, in sturgeon, GRF and PACAP are
encoded in tandem on a single mRNA.
In this thesis, I establish the structure of GnRH, GRF, and PACAP in sturgeon, a
species that evolved near a critical branching point between bony fish and tetrapods. These
structures are used as a focal point for comparison to those of other vertebrates. This
comparative evolutionary approach is an important step toward understanding the evolution
of these important neuropeptides as well as enhancing our knowledge of general principles
in the endocrine systems controlling reproduction and growth.
Description
Keywords
Fishes, Physiology, Neuropeptides, Pathophysiology, Neurobiology