Primordial germ-cell development in zebrafish

Germ cell transplantation as a potential biotechnological approach to fish reproduction

Although the use of germ cell transplantation has been relatively well established in mammals, the technique has only been adapted for use in fish after entering the 2000s. During the last decade, several different approaches have been developed for germ cell transplantation in fish using recipients of various ages and life stages, such as blastula-stage embryos, newly hatched larvae and sexually mature specimens. As germ cells can develop into live organisms through maturation and fertilization processes, germ cell transplantation in fish has opened up new avenues of research in reproductive biotechnology and aquaculture. For instance, the use of xenotransplantation in fish has lead to advances in the conservation of endangered species and the production of commercially valuable fish using surrogated recipients. Further, this could also facilitate the engineering of transgenic fish. However, as is the case with mammals, knowledge regarding the basic biology and physiology of germline stem cells in fish remains incomplete, imposing a considerable limitation on the application of germ cell transplantation in fish. Furthering our understanding of germline stem cells would contribute significantly to advances regarding germ cell transplantation in fish.

Identification of a heritable model of testicular germ cell tumor in the zebrafish

Germ cell tumors (GCTs) affect infants, children, and adults and are the most common cancer type in young men. Progress in understanding the molecular basis of GCTs has been hampered by a lack of suitable animal models. Here we report the identification of a zebrafish model of highly penetrant, heritable testicular GCT isolated as part of a forward genetic screen for cancer susceptibility genes. The mutant line develops spontaneous testicular tumors at a median age of 7 months, and pedigree analysis indicates dominant inheritance of the GCT susceptibility trait. The zebrafish model exhibits disruption of testicular tissue architecture and the accumulation of primitive, spermatogonial-like cells with loss of spermatocytic differentiation. Radiation treatment leads to apoptosis of the tumor cells and tumor regression. The GCT-susceptible line can serve as a model for understanding the mechanisms regulating germ cells in normal development and disease and as a platform investigating new therapeutic approaches for GCTs.

A "block and rescue" pharmacogenetic approach to dissecting a biochemical pathway controlling germ cell migration

Specific small molecule inhibitors of the de novo cholesterol synthesis pathway (statins) and the protein prenylation pathway were used to study their effect on germ cell migration. Hydroxymethylglutaryl-Coenzyme A reductase (HMGCoAR) catalyzes the rate-limiting step in the mevalonate pathway that produces isoprenoids and cholesterol. Pharmacological HMGCoAR inhibition by statins alters zebrafish development and germ cell migration. These effects were completely blocked by prior injection of mevalonate, the product of HMGCoAR activity, or the prenylation precursors farnesol and geranylgeraniol. Finally, pharmacological inhibition of geranylgeranyl transferase I activity, an enzyme downstream from mevalonate synthesis and responsible for the transfer of a lipid to target proteins, resulted in abnormal germ cell migration. These data together with new data from Drosophila demonstrate that protein prenylation is an evolutionarily conserved pathway mediating germ cell migration. Further, pharmacological block-and-rescue approach provided detailed information about the elements of isoprenoid biosynthesis that contribute to germ cell migration. A key question raised by this work is the identity of the prenylated protein which facilitates proper germ cell migration. Work from other laboratories suggests that germ cell migration might be a general model for the long-range migration of other cell types including cancer metastasis.

Has2 is required upstream of Rac1 to govern dorsal migration of lateral cells during zebrafish gastrulation

The large extracellular polysaccharide Hyaluronan (HA) and its synthesizing enzymes (Has) have been implicated in regulating the migratory potential of metastatic cancer cells. Here, we analyze the roles of zebrafish Has2 in normal development. Antisense morpholino oligonucleotide (MO)-mediated knockdown of zebrafish Has2 leads to the loss of HA, and severe migratory defects during gastrulation, somite morphogenesis and primordial germ cell migration. During gastrulation, ventrolateral cells of has2 morphant embryos fail to develop lamellipodia and to migrate dorsally, resulting in a blockage of dorsal convergence, whereas extension of the dorsal axis is normal. The effect is cell autonomous, suggesting that HA acts as an autocrine signal to stimulate the migration of HA-generating cells. Upon ectopic expression in axial cells, has2 causes the formation of supernumerary lamellipodia and a blockage of axis extension. Epistasis analyses with constitutively active and dominant-negative versions of the small GTPase Rac1 suggest that HA acts by Rac1 activation, rather than as an essential structural component of the extracellular matrix. Together, our data provide evidence that convergence and extension are separate morphogenetic movements of gastrulation. In addition, they suggest that the same HA pathways are active to auto-stimulate cell migration during tumor invasion and vertebrate embryogenesis.

Generation of live fry from intraperitoneally transplanted primordial germ cells in rainbow trout

Germ cell transplantation has tremendous applications in transgenic animal production, assisted reproductive technology, and germline stem cell research. Here, we report for the first time the production of individuals from intraperitoneally transplanted primordial germ cells (PGCs) in animals. To trace the behavior of exogenous PGCs in recipients, PGCs visualized by a green fluorescent protein gene were used as donors. The PGCs prepared from the genital ridges of hatching embryos were transplanted into recipients at various developmental stages. The PGCs injected into the peritoneal cavities of hatching embryos had the ability to migrate toward, and to colonize, the genital ridges of recipient embryos. Furthermore, donor-derived PGCs proliferated and differentiated into mature eggs and sperm in the allogenic gonads; the resulting gametes produced live fry, showing the donor-derived phenotype, through fertilization. Combined with in vitro culture, genetic modification, and cryopreservation of PGCs, this technique provides new approaches for fish bioengineering.