System Of Hagfish Research Articles

Glycoprotein hormone in the pituitary of hagfish and its evolutionary implications

The pituitary gland is present in all vertebrates, from agnathans (jawless vertebrates) to mammals, but not in invertebrates. Reproduction in gnathostomes (jawed vertebrates) is controlled by two pituitary gonadotropins (GTHs), luteinizing hormone and follicle-stimulating hormone, which are part of the pituitary glycoprotein hormone (GPH) family. Hagfishes, which lack both jaws and vertebrae, are considered the most primitive vertebrate known, living or extinct. Accordingly, they are of particular importance in understanding the evolution of the pituitary GPHs and their functions related to vertebrate reproduction. Nevertheless, key elements of the reproductive endocrine system in hagfish have yet to be elucidated. Our current report has revealed the first identification of a functional GPH composed of two subunits that possess gonadotropic action at the pituitary of brown hagfish. It seems most likely that an ancestral GPH gave rise to only one GTH in hagfish pituitary and that multiplicity of GPHs arose later during the early evolution of gnathostomes. This paper briefly summarizes the latest findings on the hagfish GPH from an evolutionary point of view.

EVOLUTIONARY ORIGIN OF A FUNCTIONAL GONADOTROPIN IN THE PITUITARY OF THE MOST PRIMITIVE VERTEBRATE, HAGFISH

Hagfish, which lack both jaws and vertebrae, are considered themost primitive vertebrate known, living or extinct. Hagfish havelong been the enigma of vertebrate evolution not only becauseof their evolutionary position, but also because of our lack ofknowledge on fundamental processes. Key elements of the re-productive endocrine system in hagfish have yet to be elucidated.Here, the presence and identity of a functional glycoprotein hor-mone (GPH) have been elucidated from the brown hagfish Para-myxine atami. The hagfish GPH consists of two subunits, α and β,which are synthesized and colocalized in the same cells of theadenohypophysis. The cellularand transcriptional activitiesof hag-fish GPHα and -β were significantly correlated with the develop-mental stages of the gonad. The purified native GPH induced thereleaseofgonadalsexsteroidsinvitro.Fromourphylogeneticanal-ysis, we propose that ancestral glycoprotein α-subunit 2 (GPA2)and β-subunit 5 (GPB5) gave rise to GPHα and GPHβ of the verte-brate glycoprotein hormone family, respectively. The identifiedhagfishGPHαand-βsubunitsappeartobethetypicalgnathostomeGPHαand-βsubunitsbasedonthesequenceandphylogeneticanal-yses.WehypothesizethattheidentityofasinglefunctionalGPHofthehagfish,hagfishGTH,providescriticalevidencefortheexistenceof a pituitary-gonadal system in the earliest divergent vertebratethat likely evolved from an ancestral, prevertebrate exclusivelyneuroendocrine mechanism by gradual emergence of a previouslyundescribed control level, the pituitary, which is not found inthe Protochordates.

Evolutionary Origin of a Functional Gonadotropin in the Pituitary of the most Primitive Vertebrate, Hagfish

Hagfish, which lack both jaws and vertebrae, are considered themost primitive vertebrate known, living or extinct. Hagfish havelong been the enigma of vertebrate evolution not only becauseof their evolutionary position, but also because of our lack ofknowledge on fundamental processes. Key elements of the re-productive endocrine system in hagfish have yet to be elucidated.Here, the presence and identity of a functional glycoprotein hor-mone (GPH) have been elucidated from the brown hagfish Para-myxine atami. The hagfish GPH consists of two subunits, α and β,which are synthesized and colocalized in the same cells of theadenohypophysis. The cellularand transcriptional activitiesof hag-fish GPHα and -β were significantly correlated with the develop-mental stages of the gonad. The purified native GPH induced thereleaseofgonadalsexsteroidsinvitro.Fromourphylogeneticanal-ysis, we propose that ancestral glycoprotein α-subunit 2 (GPA2)and β-subunit 5 (GPB5) gave rise to GPHα and GPHβ of the verte-brate glycoprotein hormone family, respectively. The identifiedhagfishGPHαand-βsubunitsappeartobethetypicalgnathostomeGPHαand-βsubunitsbasedonthesequenceandphylogeneticanal-yses.WehypothesizethattheidentityofasinglefunctionalGPHofthehagfish,hagfishGTH,providescriticalevidencefortheexistenceof a pituitary-gonadal system in the earliest divergent vertebratethat likely evolved from an ancestral, prevertebrate exclusivelyneuroendocrine mechanism by gradual emergence of a previouslyundescribed control level, the pituitary, which is not found inthe Protochordates.

Evolutionary origin of a functional gonadotropin in the pituitary of the most primitive vertebrate, hagfish

Hagfish, which lack both jaws and vertebrae, are considered the most primitive vertebrate known, living or extinct. Hagfish have long been the enigma of vertebrate evolution not only because of their evolutionary position, but also because of our lack of knowledge on fundamental processes. Key elements of the reproductive endocrine system in hagfish have yet to be elucidated. Here, the presence and identity of a functional glycoprotein hormone (GPH) have been elucidated from the brown hagfish Paramyxine atami. The hagfish GPH consists of two subunits, alpha and beta, which are synthesized and colocalized in the same cells of the adenohypophysis. The cellular and transcriptional activities of hagfish GPHalpha and -beta were significantly correlated with the developmental stages of the gonad. The purified native GPH induced the release of gonadal sex steroids in vitro. From our phylogenetic analysis, we propose that ancestral glycoprotein alpha-subunit 2 (GPA2) and beta-subunit 5 (GPB5) gave rise to GPHalpha and GPHbeta of the vertebrate glycoprotein hormone family, respectively. The identified hagfish GPHalpha and -beta subunits appear to be the typical gnathostome GPHalpha and -beta subunits based on the sequence and phylogenetic analyses. We hypothesize that the identity of a single functional GPH of the hagfish, hagfish GTH, provides critical evidence for the existence of a pituitary-gonadal system in the earliest divergent vertebrate that likely evolved from an ancestral, prevertebrate exclusively neuroendocrine mechanism by gradual emergence of a previously undescribed control level, the pituitary, which is not found in the Protochordates.

Distribution of gonadotropin‐releasinghormone immunoreactivity in the brain of the pacific hagfish, Eptatretus stouti (craniata: Myxinoidea)

The distribution of gonadotropin-releasing hormone (GnRH)-like immunoreactivity in the brain of a myxinoid, the Pacific hagfish (Eptatretus stouti), was investigated via immunohistochemistry, including the use of six different antisera. In the diencephalon, immunoreactive cell bodies were found in two systems: the infundibular hypothalamus, a neuromodulatory nucleus with diffuse projections of varicose fibers to most areas of the brain, and a primarily preoptic system of putatively hypophysiotropic neurons that projects to the neurohypophysis. Some potential neurovascular and CSF contacts were also identified. These findings are consistent with those of similar studies in other craniates and suggest that a preoptic hypophysiotropic system may be present in all craniates. We therefore tentatively accept the homology of this system in hagfish and vertebrates. The homology of the distributed hypothalamic system is more dubious. It may be homologous to a caudal GnRH system of modulatory neurons found in many vertebrates. Antiserum PBL-49 displays a differential affinity for the two systems, indicating that the two systems differ in the amount or identity of the immunoreactive substance. We suggest that the two systems have distinct functions in hagfish. The primitive function of GnRH-like molecules in craniates may have thus been both neuromodulatory and hypophysiotropic. These findings also indicate that the brain-pituitary axis of hagfish is more similar to that of vertebrates than has been previously suggested.