Study of the potential spermatogonial stem cell compartment in dogfish testis, Scyliorhinus canicula L

Characterization of gonadotropins and their receptors in a chondrichthyan, Scyliorhinus canicula, fills a gap in the understanding of their coevolution

In Gnathostomes, reproduction is mainly controlled by the hypothalamic-pituitary-gonadal (HPG) axis, with the involvement of the pituitary gonadotropic hormones (GTH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which activate their cognate receptors, FSHR and LHR, expressed in gonads. Each GTH consists of a common α subunit and of a specific FSHβ or LHβ subunit. Chondrichthyes (holocephalans and elasmobranchs) is a sister group of bony vertebrates. This position is highly favorable for the understanding of the evolution of endocrine regulations of reproduction among gnathostomes. Surprisingly, the characterization of gonadotropins and their receptors is still limited in chondrichthyes. In the present study, GTH and GTHR sequences have been identified from several chondrichthyan genomes, and their primary structures were analyzed relative to human orthologs. 3D models of GTH/GTHR interaction were built, highlighting the importance of the receptor hinge region for ligand recognition. Functional hormone-receptor interactions have been studied in HEK cells using the small-spotted catshark (Scyliorhinus canicula) recombinant proteins and showed that LHR was specifically activated by LH whereas FSHR was activated by both FSH and LH. Expression profiles of GTHs and their receptors were explored by real-time PCR, in situ hybridization and immunohistochemistry during spermatogenesis, along the male genital tract and other tissues, as well as in some female tissues for comparison. Tissue-expression analyses showed that the highest levels were observed for fshr transcripts in testis and ovary and for lhr in specific extragonadal tissues. The two receptors were expressed at all stages of spermatogenesis by both germ cells and somatic cells, including undifferentiated spermatogonia, spermatocytes, spermatids, somatic precursors and Sertoli cells; differentiated Leydig cells being absent in the testis of S. canicula. Receptors were also expressed by the lymphomyeloid epigonal tissue and the testicular tubules. These results, suggest a wide range of gonadotropin-regulated functions in Elasmobranchs, as well as functional redundancy during spermatogenesis. These extended functions are discussed in an evolutionary context in which the specificity of gonadotropin signaling must have contributed to the evolution of gonadal cells' morphology and function.

Characterization and enrichment of spermatogonial stem cells of common carp (Cyprinus carpio)

Spermatogenesis is a systematically organized process that ensures uninterrupted sperm production in which the spermatogonial stem cells (SSCs) play a crucial role. However, the existing absence of teleost-specific molecular markers for SSCs presents a notable challenge. Herein we characterized phenotypically the spermatogonial stem cells using specific molecular markers and transmission electron microscopy. Moreover, we also describe a simple method to suppress common carp spermatogenesis using the combination of Busulfan and thermo-chemical treatment, and finally, we isolate and enrich the undifferentiated spermatogonial fraction. Our results showed that C-kit, GFRα1, and POU2 proteins were expressed by germ cells, meanwhile, undifferentiated spermatogonial populations preferentially expressed GFRα1 and POU2. Moreover, the combination of high temperature (35 °C) and Busulfan (40 mg/kg/BW) effectively suppressed the spermatogenesis of common carp males. Additionally, the amh expression analysis showed differences between the control (26 °C) when compared to 35 °C with a single or two Busulfan doses, confirming that the testes were depleted by the association of Busulfan at high temperatures. In an attempt to isolate the undifferentiated spermatogonial fraction, we used the Percoll discontinuous density gradient. Thus, we successfully dissociated the carp whole testes in different cellular fractions; subsequently, we isolated and enriched the undifferentiated spermatogonial population. Therefore, our results suggest that probably both GFRα-1 and POU2 are highly conserved factors expressed in common carp germinative epithelium and that these molecules were well conserved along the evolutionary process. Furthermore, the enriched undifferentiated spermatogonial population developed here can be used in further germ cell transplantation experiments to preserve and propagate valued and endangered fish species.

Nesfatin-1 in a reptile: its role and hormonal regulation in wall lizard testis

Nesfatin-1 is a pleiotropic hormone implicated in various physiological functions including reproduction. Studies though limited, have established an important role of the peptide in regulation of testicular functions in mammals and fishes. However, role of nesfatin-1 in regulation of spermatogenesis and testicular steroidogenesis remains completely unexplored in reptiles. Therefore, present study aimed to develop an insight into reproductive phase-dependent testicular expression, function and regulation of nucb2/nesfatin-1 in a reptile, Hemidactylus flaviviridis. Expression of nucb2/nesfatin-1 in testis of wall lizard varied significantly depending upon reproductive phase, being highest in the active phase while lowest during regressed phase. Further, in vitro treatment of wall lizard testis with nesfatin-1 showed a concentration- and time-dependent stimulatory effect of the peptide on expression of cell proliferation and differentiation markers like scf, c-kit and pcna suggesting a spermatogenic role of nesfatin-1 in wall lizard. Also, nesfatin-1 stimulated the anti-apoptotic marker, bcl-2 while inhibited the apoptotic marker, caspase-3, suggesting its role as an inhibitor of apoptosis of testicular cells. Further, treatment with nesfatin-1 resulted in significantly higher expression of star along with a concomitant increase in testosterone production by the lizard testis. The present study also demonstrates hormonal regulation of testicular nucb2/nesfatin-1 wherein follicle-stimulating hormone (FSH) inhibited while sex steroids like dihydrotestosterone (DHT) and 17β-estradiol-3-benzoate (E2) stimulated the mRNA expression of nesfatin-1. Observations from the current study for the first time provide comprehensive evidence of spermatogenic and steroidogenic role of nesfatin-1 as well as its hormonal regulation in the testis of a reptile, H. flaviviridis.

Comparative Proteome Analysis of Four Stages of Spermatogenesis in the Small-Spotted Catshark (Scyliorhinus canicula), Using High-Resolution NanoLC-ESI-MS/MS

Spermatogenesis is a highly specialized process of cell proliferation and differentiation leading to the production of spermatozoa from spermatogonial stem cells. Due to its testicular anatomy, Scyliorhinus canicula is an interesting model to explore stage-based changes in proteins during spermatogenesis. The proteomes of four testicular zones corresponding to the germinative niche and to spermatocysts (cysts) with spermatogonia (zone A), cysts with spermatocytes (zone B), cysts with young spermatids (zone C), and cysts with late spermatids (zone D) have been analyzed by nanoLC-ESI-MS/MS. Gene ontology and KEGG annotations were also performed. A total of 3346 multiple protein groups were identified. Zone-specific protein analyses highlighted RNA-processing, chromosome-related processes, cilium organization, and cilium activity in zones A, D, C, and D, respectively. Analyses of proteins with zone-dependent abundance revealed processes related to cellular stress, ubiquitin-dependent degradation by the proteasome, post-transcriptional regulation, and regulation of cellular homeostasis. Our results also suggest that the roles of some proteins, such as ceruloplasmin, optineurin, the pregnancy zone protein, PA28β or the Culling-RING ligase 5 complex, as well as some uncharacterized proteins, during spermatogenesis could be further explored. Finally, the study of this shark species allows one to integrate these data in an evolutionary context of the regulation of spermatogenesis. Mass spectrometry data are freely accessible via iProX-integrated Proteome resources (https://www.iprox.cn/) for reuse purposes.

Biological Sound vs. Anthropogenic Noise: Assessment of Behavioural Changes in Scyliorhinus canicula Exposed to Boats Noise

Despite the growing interest in human-made noise effects on marine wildlife, few studies have investigated the potential role of underwater noise on elasmobranch species. In this study, twelve specimens of small-spotted catshark (Scyliorhinus canicula) were exposed to biological and anthropogenic sounds in order to assess their behavioural changes in response to prey acoustic stimuli and to different amplitude levels of shipping noise. The sharks, individually held in aquariums, were exposed to four experimental acoustic conditions characterized by different spectral (Hz) components and amplitude (dB re 1 µPa) levels. The swimming behaviour and spatial distribution of sharks were observed. The results highlighted significant differences in swimming time and in the spatial use of the aquarium among the experimental conditions. When the amplitude levels of biological sources were higher than those of anthropogenic sources, the sharks' swimming behaviour was concentrated in the bottom sections of the aquarium; when the amplitude levels of anthropogenic sources were higher than biological ones, the specimens increased the time spent swimming. Moreover, their spatial distribution highlighted a tendency to occupy the least noisy sections of the aquarium. In conclusion, this study highlighted that anthropogenic noise is able to affect behaviour of catshark specimens and the impact depends on acoustic amplitude levels.

Simple, once-off mapping of various, recurrent immunostaining patterns of proliferating cell nuclear antigen in spermatogonia at the immature pole of the testis of adult wild-caught blue shark, Prionace glauca: Correlations with changes in testicular status

This study was a single time-point mapping of various immunostaining patterns revealed with the proliferating cell nuclear antigen (PCNA) PC10 antibody in spermatogonia at the immature pole of the testis of the Blue shark (Prionace glauca). Scattered in the stroma of the germinal ridge that demarcates the immature pole's outer boundary were nests of variously immunoreactive A-spermatogonia, each flanked by a fusiform cell. Spermatocysts were assembled from niche-derived stromal cells, displaced A-progenitors, and their progeny, which showed one of two main immunostaining patterns (i.e., an uneven light brown/globular and homogeneous dark [hod] brown appearance). The testes of wild-caught Prionace showed two conditions, namely, extensive multinucleate cell death (MNC) near the mitosis-meiosis transition or an early recovery phase from the latter showing vacuolated areas. Both the proportion of cysts with immature B^hod -spermatogonia and the frequency of mitotic figures in such cysts in the early recovery testis condition were significantly higher than the comparable parameters in MNC testis condition. Moreover, the post-MNC recovery phase revealed a decrease in the proportion of immature cysts with uneven light brown/globular-like spermatogonia. The protracted spread of a cell cycle signal in an anatomically discrete, syncytially connected spermatogonial clone manifests as different PCNA immunoreactivities.

Low salinity negatively affects early larval development of Nile tilapia, Oreochromis niloticus: insights from skeletal muscle and molecular biomarkers

The present study evaluated the effects of low salinity on the early larval development of Oreochromis niloticus, specifically histological damage to white muscle, morphology of the yolk-sac surface and trunk area, and molecular expression of apoptosis and cell proliferation biomarkers. Newly hatched larvae were submitted to four salinity treatments for a period of 48 or 72 h, in duplicate: (S0) freshwater, (S2) 2 g l-1, (S4) 4 g l-1, and (S6) 6 g l-1NaCl. Larval development was examined using histology, electron microscopy, enzyme-linked immunosorbent assay (ELISA), and morphometry. At the yolk-sac surface, larvae of S4 and S6 displayed alterations to the apical opening of chloride cells that may be related to osmotic expenditure caused by the increased salinity. Caspase-3 expression did not differ significantly among treatments, however significantly lower proliferating cell nuclear antigen (PCNA) expression (P < 0.05) suggested minor cell proliferation in larvae of S4 and S6 compared with S0 and S2. Furthermore, there was a significant reduction in both trunk area and percentage of normal white muscle fibres (WF) in larvae of S4 and S6. Vacuolated areas and myofibrils concentrated at the cell periphery and found in the white muscle from larvae exposed to saline environments suggested disturbance to muscle development. Oedema and mononuclear infiltrate were also observed in the white muscle of S4 and S6 larvae. Together these results indicated that treatments with 4 and 6 g l-1 NaCl may cause osmoregulation expenditure, morphological alterations to the yolk-sac surface and histological damage to skeletal muscle that negatively affected the early larval development of O. niloticus.

Influence of salinity on spermatogenesis in adult Nile tilapia (Oreochromis niloticus) testis

Continental waters salinisation is a global threat that has grown because of climate change and human activities, but little is known about how and what biological tracts are affected. The aim of this study was to investigate the influence of different water salinities on the expression of HSP70, PCNA and caspase-3 during spermatogenesis of Nile tilapia. Adult males were submitted to four salinity treatments: (S₀) fresh water, (S₇) 7 g L^-1, (S₁₄) 14 g L^-1, and (S₂₁) 21 g L^-1 for 1, 4, and 9 days. All specimens were in spermatogenic activity and the highest values of the gonadosomatic index (GSI) occurred in the S₀ and S₇. In the morphometric analysis, spermatocytes were the most frequent germ cell detected in all treatments (>50%) and spermatids achieved about 20% of the testicular proportion, with few variations among treatments. Spermatozoa were significantly reduced only in S14 compared to S7. Leydig cells were significantly increased in S₁₄ when compared to S7 but plasma concentrations of 11-KT showed no significant difference among treatments. ELISA assay showed higher testicular expression of HSP70 at 1 day in all groups, followed by a significant decrease at days 4 and 9 in S₁₄ and S₂₁. The expression of PCNA was significantly lower while the activity of caspase-3 was higher in S₁₄ and S₂₁ when compared to S₀ and S₇. These results indicate that higher salinities in S₁₄ and S₂₁ interfere with the relationship between testicular HSP70, PCNA, and caspase-3, but with few effects over spermatogenesis dynamics of Nile tilapia.

Multiple Sources for Sertoli Cells and Two Sertoli Phenotypes in the Adult Elasmobranch Testis: Insight from Two Species Belonging to Different Orders

Findings presented here for the chondrichthyans Prionace glauca and Isurus oxyrinchus show that the assembly of the spermatocyst and it's attached collecting duct are seamlessly connected developmental phenomena. The cyst's somatic cell component (i.e., Sertoli cells, SCs) and the duct's constituent cells derive both from a common precursor cell type (typically a large oblong cell) found among the A-spermatogonia in the folliculogenic region. Novel findings show that the co-developing collecting duct itself serves also as a source of either normal-looking or basophilic atypical SCs (aSCs), depending on whether the duct-cyst transition remains mitotically active and open, or is sealed. The aSCs arise from accumulating slender basophilic cells at the duct-cyst interface after which the newly formed cyst is sealed. Quantitative analysis of the latter in P. glauca revealed a correlation between the appearance of this aSC in immature cysts and the degenerated testicular condition that displays a gradient of multinucleate cell (MNC) death among spermatogonial cysts. Findings seem to implicate these aSCs in the life-death balance in mature spermatogonial cohorts downstream in the spermatogenic sequence, rather than in newly formed cysts that exhibit low rates of apoptosis. Photomicrographs of developing spermatogonial cysts showing several aSCs interspersed among cytoplasmically linked spermatogonia that are proliferating or have died, seem to suggest that these small SCs may be involved in confining MNC death to a given cyst region under conditions of subthreshold levels of apoptosis such as to ensure cyst recovery in immature spermatogonial cysts. Anat Rec, 301:1944-1954, 2018. © 2018 Wiley Periodicals, Inc.

Expression profiling of c-kit and its impact after esiRNA silencing during gonadal development in catfish

Receptor, c-Kit is a member of a family of growth factor receptors that have tyrosine kinase activity, and are involved in the transduction of growth regulatory signals across plasma membrane by activation of its ligand, kitl/scf. The present study analyzed mRNA and protein expression profiles of c-kit in the gonads of catfish, Clarias gariepinus, using real time PCR, in situ hybridization and immunohistochemistry. Tissue distribution analysis revealed higher expression mainly in the catfish gonads. Ontogeny studies showed minimal expression during early developmental stages and highest during 50-75 days post hatch, and the dimorphic expression in gonads decreased gradually till adulthood, which might suggest an important role for this gene around later stages of sex differentiation and gonadal development. Expression of c-kit was analyzed at various phases of gonadal cycle in both male and female, which showed minimal expression during the resting phase, and higher expression during the pre-spawning phase in male compared to females. In vitro and in vivo induction using human chorionic gonadotropin elevated the expression of c-kit indicating the regulatory influence of hypothalamo-hypophyseal axis. In vivo transient gene silencing using c-kit-esiRNA in adult catfish during gonadal recrudescence showed a decrease in c-kit expression, which affected the expression levels of germ cell meiotic marker sycp3, as well as several factors and steroidogenic enzyme genes that are involved in germ cell development. Decrease in the levels of 11-ketotestosterone and testosterone in serum were also observed after esiRNA silencing. The findings suggests that c-kit has an important role in the process of germ cell proliferation, development and maturation during gonadal development and recrudescence in catfish.

The nanos1 gene was duplicated in early Vertebrates and the two paralogs show different gonadal expression profiles in a shark

Nanos are RNA-binding proteins playing crucial roles in germ cell development and maintenance. Based on phylogenetic and synteny analyses, this study reveals that nanos1 gene has undergone multiple duplications and gene copies losses in Vertebrates. Chondrichthyan species display two nanos1 genes (named nanos1A/1B), which were both retrieved in some Osteichthyes at basal positions in Sarcopterygii and Actinopterygii lineages. In contrast, Teleosts have lost nanos1A but duplicated nanos1B leading to the emergence of two ohnologs (nanos1Ba/1Bb), whereas Tetrapods have lost nanos1B gene. The two successive nanos gene duplications may result from the second and third whole genome duplication events at the basis of Vertebrates and Teleosts respectively. The expression profiles of nanos1A and nanos1B paralogs were characterized in the dogfish, Scyliorhinus canicula. Nanos1A was strongly expressed in brain and also localized in all germ cell types in the polarized testis. In contrast, nanos1B was detected in testis with the highest expression in the germinative zone. In addition, Nanos1B protein was predominantly located in the nuclei of male germinal cells. In the ovary, both paralogs were detected in germinal and somatic cells. Our study opens new perspectives concerning the complex evolution of nanos1 paralogs and their potential distinct roles in Vertebrates gonads.

Stay Connected: A Germ Cell Strategy

Germ cells develop as a cyst of interconnected sibling cells in a broad range of organisms in both sexes. A well-established function of intercellular connectivity is to transport cytoplasmic materials from 'nurse' cells to oocytes, a critical process for developing functional oocytes in ovaries of many species. However, there are situations where connectivity exists without a nursing mechanism, and the biological meaning of such connectivity remains obscure. In this review, we summarize current knowledge on the formation of intercellular connectivity, and discuss its meaning by visiting multiple examples of germ cell connectivity observed in evolutionarily distant species.

Production of fertile sperm from in vitro propagating enriched spermatogonial stem cells of farmed catfish, Clarias batrachus

Spermatogenesis is a highly co-ordinated and complex process. In vitro propagation of spermatogonial stem cells (SSCs) could provide an avenue in which to undertake in vivo studies of spermatogenesis. Very little information is known about the SSC biology of teleosts. In this study, collagenase-treated testicular cells of farmed catfish (Clarias batrachus, popularly known as magur) were purified by Ficoll gradient centrifugation followed by magnetic activated cell sorting using Thy1.2 (CD90.2) antibody to enrich for the spermatogonial cell population. The sorted spermatogonial cells were counted and gave ~3 × 106 cells from 6 × 106 pre-sorted cells. The purified cells were cultured in vitro for >2 months in L-15 medium containing fetal bovine serum (10%), carp serum (1%) and other supplements. Microscopic observations depicted typical morphological SSC features, bearing a larger nuclear compartment (with visible perinuclear bodies) within a thin rim of cytoplasm. Cells proliferated in vitro forming clumps/colonies. mRNA expression profiling by qPCR documented that proliferating cells were Plzf + and Pou2+, indicative of stem cells. From 60 days onwards of cultivation, the self-renewing population differentiated to produce spermatids (~6 × 107 on day 75). In vitro-produced sperm (2260 sperm/SSC) were free swimming in medium and hence motile (non-progressive) in nature. Of those, 2% were capable of fertilizing and generated healthy diploid fingerlings. Our documented evidence provides the basis for producing fertile magur sperm in vitro from cultured magur SSCs. Our established techniques of SSC propagation and in vitro sperm production together should trigger future in vivo experiments towards basic and applied biology research.

Maintenance of potential spermatogonial stem cells in vitro by GDNF treatment in a chondrichthyan model (Scyliorhinus canicula L.)

Previous work in dogfish, Scyliorhinus canicula, has identified the testicular germinative area as the spermatogonial stem cell niche. In the present study, an in vitro co-culture system of spermatogonia and somatic cells from the germinative area was developed. Long-term maintenance of spermatogonia has been successful, and addition of GDNF has promoted the development of clones of spermatogonia expressing stem cell characteristics such as alkaline phosphatase activity and has allowed maintenance of self-renewal in spermatogonia for at least 5 mo under culture conditions, notably by decreasing cell apoptosis. Furthermore, clones of spermatogonia expressed the receptor of GDNF, GFRalpha1, which is consistent with the effect of GDNF on cells despite the lack of identification of a GDNF sequence in the dogfish's transcriptome. However, a sequence homologous to artemin has been identified, and in silico analysis supports the hypothesis that artemin could replace GDNF in the germinative area in dogfish. This study, as the first report on long-term in vitro maintenance of spermatogonia in a chondrichthyan species, suggests that the GFRalpha1 signaling function in self-renewal of spermatogonial stem cells is probably conserved in gnathostomes.

Spermatogonial stem cell quest: nanos2, marker of a subpopulation of undifferentiated A spermatogonia in trout testis

Continuous or cyclic production of spermatozoa throughout life in adult male vertebrates depends on a subpopulation of undifferentiated germ cells acting as spermatogonial stem cells (SSCs). What makes these cells self-renew or differentiate is barely understood, in particular in nonmammalian species, including fish. In the highly seasonal rainbow trout, at the end of the annual spermatogenetic cycle, tubules of the spawning testis contain only spermatozoa, with the exception of scarce undifferentiated spermatogonia that remain on the tubular wall and that will support the next round of spermatogenesis. Taking advantage of this model, we identified putative SSCs in fish testis using morphological, molecular, and functional approaches. In all stages, large spermatogonia with ultrastructural characteristics of germinal stem cells were found, isolated or in doublet. Trout homologues of SSC and/or immature progenitor markers in mammals-nanos2 and nanos3, pou2, plzf, and piwil2-were preferentially expressed in the prepubertal testis and in the undifferentiated A spermatogonia populations purified by centrifugal elutriation. This expression profile strongly suggests that these genes are functionally conserved between fish and mammals. Moreover, transplantation into embryonic recipients of the undifferentiated spermatogonial cells demonstrated their high "stemness" efficiency in terms of migration into gonads and the ability to give functional gametes. Interestingly, we show that nanos2 expression was restricted to a subpopulation of undifferentiated spermatogonia (less than 20%) present as isolated cells or in doublet in the juvenile and in the maturing trout testis. In contrast, nanos2 transcript was detected in all the undifferentiated spermatogonia remaining in the spawning testis. Plzf was also immunodetected in A-Spg from spawning testis, reinforcing the idea that these cells are stem cells. From those results, we hypothesize that the subset of undifferentiated A spermatogonia expressing nanos2 transcript are putative SSC in trout.

Characterization of spermatogonial markers in the mature testis of the dogfish (Scyliorhinus canicula L.)

In dogfish, spermatogenesis progresses from a restricted germinative zone, which lines the dorsal testicular vessel. Single spermatogonia (A(s)), including the spermatogonial stem cells (SSCs), produce successively paired (A(p)), undifferentiated (A(u4) to A(u512)), and differentiated (A(d1) to A(d8)) spermatogonia and preleptotene (PL) spermatocytes through 13 mitoses. Dogfish spermatogonial subpopulations present classical morphological characteristics but cannot be distinguished on the basis of molecular markers. This characterization has been initiated in mammals despite the difficulty to separate each spermatogonial subpopulation. For instance, both glial cell-derived neurotrophic factor family receptor alpha 1 (GFRα1) and promyelocytic leukemia zinc finger protein (PLZF) are markers of undifferentiated spermatogonia, whereas receptor tyrosine kinase C-kit is a marker of differentiated spermatogonia. The aim of this study is to characterize spermatogonial markers and to differentiate several spermatogonial subpopulations. Dogfish cDNA sequences have been identified and validated by phylogenetic analyses for gfrα1, plzf, pou2, as well as for high-mobility group box proteins 2 and 3 (hmgb2 and 3) and for mini-chromosome maintenance protein 6 (mcm6). We have used the anatomical advantage of the polarized dogfish testis to analyze the expression of those markers by RT-PCR and in situ hybridization. gfrα1, pou2, and plzf have been detected in the testicular germinative zone, suggesting that spermatogonial markers are relatively well conserved among vertebrates but with a less restricted expression for plzf. Moreover, hmgb3 and mcm6 have been identified as new markers of differentiated spermatogonia. Finally, this first molecular characterization of spermatogonial subpopulations in a chondrichthyan model will be useful for further studies on the SSC niche evolution.

Coordination of spermatogenic processes in the testis: lessons from cystic spermatogenesis

A common observation in the vertebrate testis is that new germ cell clones enter spermatogenesis proper before previously formed clones have completed their development. The extent to which the developmental advance of any given germ cell clone in any phase of spermatogenesis is dependent on that of neighboring clones and/or on the coordinating influence of associated Sertoli cells in the immediate vicinity or of others further away remains unclear. This review presents an overall synthesis of findings in an ancient vertebrate, the spiny dogfish shark and shows that, even at this phyletic level, the developmental advance of a given germ cell clone is the outcome of various processes emanating from its spatiotemporal relationship with (1) its own complement of Sertoli cells in the anatomically distinct spermatocyst and (2) Sertoli cells associated with other germ cell clones that lie upstream or downstream in the spermatogenic progression and that secrete, among others, androgen and estrogen destined for target sites upstream. Analysis of the protracted spermatogenic cycle shows the coordination in space and time of spermatogenic and steroidogenic events. Furthermore, the natural withdrawal of pituitary gonadotropin support in the dogfish causes a distinct and highly ordered gradient of apoptosis among the spermatogonial generations; this in turn is a major contributing factor to the cyclic nature of sperm production observed in this lower vertebrate. Because of the simplicity of their testicular organization, their cystic spermatogenesis and their phylogenetic position, cartilaginous fishes constitute a valid vertebrate reference system for comparative analysis with higher vertebrates.

Isolation of enriched carp spermatogonial stem cells from Labeo rohita testis for in vitro propagation

The in vitro culture system for spermatogonial stem cells (SSCs) is a powerful tool for exploring molecular mechanisms of male gametogenesis and gene manipulation. Very little information is available for fish SSC biology. Our aim was to isolate highly pure SSCs from the testis of commercially important farmed carp, Labeo rohita. The minced testis of L. rohita was dissociated with collagenase. Dissociated cells purified by two-step Ficoll gradient centrifugation followed by magnetic activated cell sorting (MACS) using Thy1.2 (CD90.2) antibody dramatically heightened recovery rate for spermatogonial cells. The purified cells were cultured in vitro conditions for more than two months in L-15 media containing 10% fetal bovine serum (FBS), 1% carp serum, and other nutrients. The proliferative cells were dividing as validated by 5-bromo-2'-deoxyuridine (BrdU) incorporation assay and formed colonies/clumps with the typical characteristics of SSCs A majority of enriched cell population represented a Vasa(+), Pou5f1/pou5f1(+), Ssea-1(+), Tra-1-81(+), plzf(+), Gfrα1/gfrα1(-), and c-Kit/c-kit(-) as detected by immunocytochemical and/or quantitative real-time polymerase chain reaction (RT-PCR) analyses. Thus, Thy1(+) SSCs were enriched with greater efficiency from the mixed population of testicular cells of L. rohita. A population of enriched spermatogonial cells could be cultured in an undifferentiated state. The isolated SSCs could provide avenue for undertaking research on basic and applied reproductive biology.

Cloning of cDNA and prediction of peptide structure of Plzf expressed in the spermatogonial cells of Labeo rohita

The promyelocytic leukemia zinc finger (Plzf) gene containing an evolutionary conserved BTB (bric-a-brac/tramtrack/broad complex) domain plays a key role in self-renewal of mammalian spermatogonial stem cells (SSCs) via recruiting transcriptional co-repressors. Little is known about the function of Plzf in vertebrate, especially in fish species. To gain better understanding of its role in fishes, we have cloned Plzf from the testis of Labeo rohita (rohu), a commercially important freshwater carp. The full-length cDNA contains an open reading frame (ORF) of 2004bp translatable to 667 amino acids (aa) containing a conserved N-terminal BTB domain and C-terminal C(2)H(2)-zinc finger motifs. L. rohita Plzf, which is phylogenetically related to Danio rerio counterpart, abundantly expressed in spermatogonial stem cells (SSCs). A three-dimensional (3D) model of BTB domain of Plzf protein was constructed by homology modeling approach. Molecular docking on this 3D structure established a homo-dimer between two BTB domains creating a charged pocket containing conserved aa residues: L33, C34, D35 and R49. Thus, Plzf of SSC is structurally and possibly functionally conserved. The conserved aa residues in the cleft resulting from Plzf BTB self-association are likely to be the binding platform for interaction with recruited co-repressor peptides. The identified Plzf could be the first step towards exploring its role in rohu SSC behavior.

Stage-specific gene expression during spermatogenesis in the dogfish (Scyliorhinus canicula)

In the dogfish testis, the cystic arrangement and polarization of germ cell stages make it possible to observe all stages of spermatogenesis in a single transverse section. By taking advantage of the zonation of this organ, we have used suppressive subtractive libraries construction, real-time PCR, and in situ hybridization to identify 32 dogfish genes showing differential expressions during spermatogenesis. These include homologs of genes already known to be expressed in the vertebrate testis, but found here to be specifically expressed either in pre-meiotic and/or meiotic zones (ribosomal protein S8, high-mobility group box 3, ubiquitin carboxyl-terminal esterase L3, 20beta-hydroxysteroid dehydrogenase, or cyclophilin B) or in post-meiotic zone (speriolin, Soggy, zinc finger protein 474, calreticulin, or phospholipase c-zeta). We also report, for the first time, testis-specific expression patterns for dogfish genes coding for A-kinase anchor protein 5, ring finger protein 152, or F-box only protein 7. Finally, the study highlights the differential expression of new sequences whose identity remains to be assessed. This study provides the first molecular characterization of spermatogenesis in a chondrichthyan, a key species to gain insight into the evolution of this process in gnathostomes.

Proteomic analysis of the spermatogonial stem cell compartment in dogfish Scyliorhinus canicula L

In the dogfish (Scyliorhinus canicula L.) the testicular germinative zone (GZ), composed of large isolated spermatogonia surrounded by elongating pre-Sertoli cells, is located between the albuginea and the ventrolateral intratesticular vessel. During the spermatogenic wave, cysts radiate in maturational order forming distinct testicular zones. In this study, soluble proteins of the GZ and of the zone containing cysts with spermatocytes were separated by two-dimensional electrophoresis. Gel images were matched and then evaluated for GZ-specific proteins. From the1400 protein spots identified, 680 were found to be apparently specific to this zone. Using MALDI-TOF/TOF mass spectrometry, de novo sequences were obtained for 33 proteins out of the 169 selected for identification by mass spectrometry, but only 16 of these 169 proteins were identified. One of them, proteasome subunit alpha-6, was analyzed further by immunohistochemistry. This study demonstrates the utility of the dogfish as a model for proteome analysis of the spermatogonial stem cell niche, even if it remains restricted by the lack of genomic data available on Elasmobranchs.

The Dogfish Scyliorhinus canicula: A Reference in Jawed Vertebrates

INTRODUCTIONDue to their large size and long generation times, chondrichthyans have been largely ignored by geneticists. However, their key phylogenetic position makes them ideal subjects to study the molecular bases of the important morphological and physiological innovations that characterize jawed vertebrates. Such analyses are crucial to understanding the origin of the complex genetic mechanisms unraveled in osteichthyans. The small spotted dogfish Scyliorhinus canicula, a representative of the largest order of extant sharks, presents a number of advantages in this context. Due to its relatively small size among sharks, its abundance, and easy maintenance, the dogfish has been an important model in comparative anatomy and physiology for more than a century. Recently, revived interest has occurred with the development of large-scale transcriptomic and genomic resources, together with the establishment of facilities allowing massive egg and embryo production. These new tools open the way to molecular analyses of the elaborate physiological and sensory systems used by sharks. They also make it possible to take advantage of unique characteristics of these species, such as organ zonation, in analyses of cell proliferation and differentiation. Finally, they offer important perspectives to evolutionary developmental biology that will provide a better understanding of the origin and diversifications of jawed vertebrates. The dogfish whole-genome sequence, which may shortly become accessible, should establish this species as an essential shark reference, complementary to other chondrichthyan models. These analyses are likely to reveal an organism of an underestimated complexity, far from the primitive prototypical gnathostome anticipated in gradistic views.