Expression cloning of camelid nanobodies specific for Xenopus embryonic antigens

Developmental biology relies heavily on the use of conventional antibodies, but their production and maintenance involves significant effort. Here we use an expression cloning approach to identify variable regions of llama single domain antibodies (known as nanobodies), which recognize specific embryonic antigens. A nanobody cDNA library was prepared from lymphocytes of a llama immunized with Xenopus embryo lysates. Pools of bacterially expressed cDNAs were sib-selected for the ability to produce specific staining patterns in gastrula embryos. Three different nanobodies were isolated: NbP1 and NbP3 stained yolk granules, while the reactivity of NbP7 was predominantly restricted to the cytoplasm and the cortex. The isolated nanobodies recognized specific protein bands in immunoblot analysis. A reverse proteomic approach identified NbP1 target antigen as EP45/Seryp, a serine protease inhibitor. Given the unique stability of nanobodies and the ease of their expression in diverse systems, we propose that nanobody cDNA libraries represent a promising resource for molecular markers for developmental biology.

Hyalin is a cell adhesion molecule involved in mediating archenteron-blastocoel roof attachment

The US National Institutes of Health has designated the sea urchin embryo as a model organism because around 25 discoveries in this system have led to insights into the physiology of higher organisms, including humans. Hyalin is a large glycoprotein in the hyaline layer of sea urchin embryos that functions to maintain general adhesive relationships in the developing embryo. It consists of the hyalin repeat domain that has been identified in organisms as diverse as bacteria, worms, flies, mice, sea urchins and humans. Here we show, using a polyclonal antibody raised against the 11.6 S species of hyalin, that it localizes at the tip of the archenteron and on the roof of the blastocoel exactly where these two structures bond in an adhesive interaction that has been of interest for over a century. In addition, the antibody blocks the interaction between the archenteron tip and blastocoel roof. These results, in addition to other recent findings from this laboratory that will be discussed, suggest that hyalin is involved in mediating this cellular interaction. This is the first demonstration that suggests that hyalin functions as a cell adhesion molecule in many organisms, including humans.

The term cell epitope PG-2 is expressed in primordial germ cells and in hypoblast cells of the gastrulating rabbit embryo

Rapid progress in the functional analysis of germline segregation has been made recently using the mouse as an experimental and molecular model. However, comparative vertebrate embryology suggests that the time point and mode of germline segregation may vary between mammalian species to a greater extent than hitherto suspected. Therefore, we started to make use of the monoclonal antibody PG-2 specific for primordial germ cells (PGCs) of the rabbit as an opportunity to investigate the early phases of germ cell formation in a mammalian species other than the mouse. Using immunohistochemistry on whole mount preparations and frozen sections we describe the typical mitochondrial labelling of PGCs in the posterior part of the primitive streak at 7.0 days post conception (d.p.c.) and the subsequent distribution of labelled PGCs at early somite stages (8 d.p.c.) within a bilobed area that flanks the posterior margin of the embryo. At these later stages, PGCs were found close to, and within, the yolk sac epithelium but they were still within the confines of the embryo as defined by the peripheral margin in the epiblast/ectoderm layer. Interestingly, cells expressing the PG-2 epitope in an atypical, finely granulated intracellular pattern were found in the hypoblast layer, but not in the epiblast, at the primitive streak stage. This atypical expression pattern may be interpreted as a sign of cells gradually losing the PG-2 epitope and this, in turn, may indicate that PGC progenitors are allocated to the hypoblast layer before appearing in the mesoderm compartment of the primitive streak. These results raise the question as to whether the germline in the rabbit is separated during early blastocyst stages, i.e. rather earlier than in the mouse.

Localization and characterization of blastocoelic extracellular matrix antigens in early sea urchin embryos and evidence for their proteolytic modification during gastrulation

Previously, results were presented showing a spatiotemporal expression of matrix metalloproteases consistent with a role in remodeling the blastocoelic extracellular matrix (bECM) of the gastrulating sea urchin embryo [35]. In the present work, we provide evidence suggesting that the bECM is in fact the substrate for developmentally regulated proteolysis. Monoclonal antibody (mAb) LG11C7 was generated against testicular tissue of the sea urchin, Strongylocentrotus purpuratus, and recognizes extracellular matrix antigens overlying the perivisceral epithelium. Indirect immunofluorescence microscopy shows that mAb LG11C7 cross-reacts with components of the basal lamina lining the blastocoeles of early embryos and Western immunoblots of detergent extracts indicate that it recognizes gastrula-stage antigens with M(r)s of 158, 68, and 37 kDa. Glycosidase treatments reveal that the embryonal antigens contain multiple N-linked oligosaccharides. Developmental studies employing immunoprecipitations and Western blot analyses of staged embryonal detergent extracts show that the 68-kDa antigen appears between 18 and 24 h after fertilization and is accompanied by a substantial increase in the 37-kDa antigen. Thus, the appearances of the 68- and 37-kDa antigens occur during the blastula-gastrula transition, and their spatiotemporal expression is similar to that of the matrix metalloproteases reported previously. The appearance of the 68-kDa antigen and the increase in the 37-kDa antigen may be blocked by exposing the embryos to the metalloprotease inhibitor 1,10-phenanthroline, which also blocks gastrulation reversibly. These results suggest (1) that the 68- and 37-kDa antigens are products of developmentally regulated proteolysis of a basal laminar glycoprotein, and (2) that this proteolysis is required for the cell-cell/cell-matrix interactions and morphogenetic movements associated with normal gastrulation in the sea urchin embryo.

Visceral endoderm-1 (VE-1): an antigen marker that distinguishes anterior from posterior embryonic visceral endoderm in the early post-implantation mouse embryo

We describe here an antigen marker, designated VE-1, that is detected early in gastrulation (approximately E6.5 through approximately E7.25) in the anterior visceral endoderm overlying the embryonic ectoderm opposite the primitive streak. The antibody-positive domain extends from the embryonic-extraembryonic junction to the distal tip of the embryo, and laterally around approximately one-third of the circumference of the egg cylinder. Analysis of embryos at earlier stages indicates that VE-1 is first expressed shortly after implantation, at approximately E5.0, in the visceral endoderm on one side of the embryo and thus is the earliest molecular marker of A-P asymmetry in the post-implantation mouse embryo described to date. Although VE-1 was detected with a polyclonal antiserum raised against a 24 amino acid polypeptide sequence of FGF2, we provide evidence that the VE-1 antigen is not FGF2. The data reported here are the first to provide molecular evidence that A-P polarity in the mouse embryo is established by E5.0 and that the visceral endoderm has A-P polarity.

Inhibition of yolk sac function in late gastrulation rat conceptuses as a cause of teratogenesis: an in vivo/in vitro study

A rabbit anti-rat yolk sac antiserum, administered by intraperitoneal injection at 0.25 mL/100 g bodyweight into 8.5-day pregnant rats, resulted in a resorption incidence of 20%, and growth retardation and malformation of all surviving fetuses at term. In all subsequent experiments, pregnant rats either received this same dose of antiserum at 8.5 days or were untreated. When 9.5-day rat conceptuses were cultured for 48 h in a medium containing serum from a 9.5-day pregnant rat that had been treated with antiserum, development was severely abnormal, regardless of whether conceptuses were explanted from antiserum-treated or untreated dams. In contrast, culture for 48 h in serum from untreated 9.5-day pregnant rats resulted in normal growth and development of conceptuses explanted from untreated dams, and in slight growth retardation and dysmorphogenesis in 9.5-day conceptuses explanted from antiserum-treated dams. In the former, development was similar to that attained by 11.5-day conceptuses from untreated dams; in the latter, development was appreciably better than in 11.5-day conceptuses from antiserum-treated dams. These results indicate that the critical period of exposure of embryos in utero to teratogenic antiserum after administration to the 8.5-day pregnant rat is longer than 24 h and that a significant insult is delivered to the conceptus before 9.5 days ea. Using 9.4-day ea conceptuses in culture, pinocytosis (uptake of 125I-labelled polyvinylpyrrolidone) by the yolk sac, and its inhibition by antiserum, was demonstrated. No pinocytic activity was evident in the embryo itself. These data support the hypothesis that anti-yolk sac antiserum exerts its teratogenic action by inhibiting the nutritional function of the yolk sac during early organogenesis.

Cell proliferation in the gastrulating chick embryo: a study using BrdU incorporation and PCNA localization

Cell proliferation in the gastrulating chick embryo was assessed using two independent techniques which mark cells in S phase of the mitotic cycle: nuclear incorporation of bromodeoxyuridine (BrdU) detected immunocytochemically and immunolocalization of proliferating cell nuclear antigen (PCNA). Computer-reconstructed maps were produced showing the distribution of labelled nuclei in the primitive streak and the cell layers. These distributions were also normalized to take into account regional differences in cell density across the embryo. Results from a 2 hour pulse of BrdU indicated that although cells at caudal levels of the primitive streak showed the highest incorporation, this region showed a similar proportion of labelled cells to the surrounding caudal regions of the epiblast and mesoderm when normalized for cell density. The entire caudal third of the embryo showed the highest proportion of cells in S phase. Cells of Hensen's node showed a relatively low rate of incorporation and, although the chordamesoderm cells showed many labelled nuclei, this appeared to be a reflection of a high cell density in this region. Combining this result with results from a 4 hour pulse of BrdU permitted mapping of cell generation time across the entire embryo. Generation times ranged from a low value of approximately 2 hours at caudal levels of both the epiblast and mesoderm, to an upper value of approximately 10 hours in the rostral regions of the primitive streak, in the mid-lateral levels of the epiblast and in the chordamesoderm rostral to Hensen's node. Cells at caudal regions of the primitive streak showed a generation time of approximately 5 hours. Taking into account that cells are generally considered to be continuously moving through the primitive streak, we conclude that cell division, as judged by generation time, is greatly reduced during transit through this region, despite the presence there of cells in S phase and M phase. Immunocytochemical localization of PCNA-positive nuclei gave generally similar distributions to those obtained with BrdU incorporation, confirming that this endogenous molecule is a useful S-phase marker during early embryogenesis. Mid-levels and caudal levels of the primitive streak showed the highest numbers of positive nuclei, and the highest proportion of labelling after cell density was accounted for. As with BrdU incorporation, the highest proportions of PCNA-positive nuclei were found towards the caudal regions of the epiblast and mesoderm. These results suggest that the differential growth of the caudal region of the embryo at this time is a direct consequence of elevated levels of cell proliferation in this region.(ABSTRACT TRUNCATED AT 400 WORDS)

The L5 epitope: an early marker for neural induction in the chick embryo and its involvement in inductive interactions

The pattern of expression of the carbohydrate epitope L5 was studied during early development of the chick neuroepithelium. Immunoreactivity first appears during gastrulation, at mid-primitive streak stage, and persists until at least 3.5 days of development. The epitope is expressed on all the components of the developing nervous system, both central and peripheral. In immunoblots, the antibody recognises a major component of about Mr 500,000 and several more minor components of lower molecular mass. If a Hensen's node from a donor embryo is transplanted into the area opaca of a host embryo, L5 immunoreactivity appears in the epiblast surrounding the graft. If hybridoma cells secreting the antibody are grafted together with Hensen's node into a host chick embryo, the induction of a supernumerary nervous system is inhibited. We suggest that the L5 epitope is an early and general marker for neural induction and that it may be involved directly in inductive interactions.

Developmental patterning of the carbohydrate antigen FC10.2 during early embryogenesis in the chick

An oligosaccharide antigen (FC10.2), formerly described only in mammalian cells and secreted glycoproteins, has been detected and found to display striking temporal and spatial patterning in the chick during early embryonic development. This antigen is expressed on type 1 chains, which are isomers of oligosaccharides of the poly-N-acetyllactosamine series (type 2 chains). Immunoreactivities before and after neuraminidase treatment of serial sections of chick embryos during the first 17 stages of development indicate that the FC10.2 structure occurs predominantly in the sialylated form (S-FC10.2). The FC10.2 and S-FC10.2 antigens are prominent markers of the primordial germ cells, being strongly expressed by these cells from the pre-primitive streak stage onwards. S-FC10.2 is also a clear marker of the pronephric duct from its first appearance. Initially present over the entire apical surface of the ectoderm, antigenicity diminishes in an antero-posterior direction as neurulation proceeds. A unique pattern for a carbohydrate antigen is displayed by cells of the primitive streak; antigenicity is lost with de-epithelialisation and ingression, but is regained in a pericellular distribution on the mesoderm cells that emerge from the primitive streak. Thereafter, successive changes in expression and distribution of FC10.2 and S-FC10.2 are features of mesodermal tissues, particularly during somitogenesis. These antigens are prominent components of the extracellular matrix around the notochord and sclerotome cells. They are also prominent posteriorly in the subectodermal region, ceasing abruptly at the lateral limits of the embryo proper. Although no absolute correlations can yet be made, several features of the distribution of these antigens suggest that they may be integral components of, or ligands for, cell adhesion molecules.

Developmental expression of regionally specific cell surface antigens in the Xenopus gastrula

Molecular markers for specific cell lineages would be useful in studies of cellular differentiation. To isolate such markers monoclonal antibodies (MoABs) were raised against plasma membranes isolated from gastrulating Xenopus embryos. Those antibodies that recognized subsets of cells within the embryo were selected by indirect immunofluorescence. The analysis of eight such MoAbs is presented. Western blot analysis showed that all but one MoAb recognized a complex pattern of glycoconjugates associated with glycoproteins. All the antigens recognized by the MoAbs were maternal in origin and displayed similar spatial patterns of pregastrular expression. This pattern of immunoreactivity at the apical surface was inherited passively during cleavage by the resulting superficial blastomeres suggesting that ectodermal specific markers of maternal origin are pre-localized to the cortical ooplasm in mature oocytes. We suggest that these maternal components may be specific glycosyl transferases. Three different patterns of expression were observed during gastrulation as exemplified by MoAbs 1F10C1, 3A4D1, and 6F10B6. MoAb 6F10B6 was specific for both neural and non-neural epithelium. MoAb 3A4D1 was specific for non-neural epidermis. MoAb 1F10C1 appeared to recognize a protein epitope on an extracellular component expressed by the superficial and involuting epithelial cells. The pattern of expression for the 1F10C1 antigen suggests that it may play a role in facilitating the movement of the involuting cells during gastrulation.

Ontogeny and characterization of mesenchyme antigens of the sea urchin embryo

A monoclonal antibody, Sp12, binds to cortical granules, the hyaline layer, and skeletogenic, chromogenic, and blastocoelar mesenchyme of sea urchin eggs and embryos. Adult urchins also express Sp12 antigens in the dermal layer of the test and spines. Antigen is expressed on the surface of primary mesenchyme cells after they have entered the blastocoel, and by two secondary mesenchyme derivatives--the blastocoelar cells after they have been released from the tip of the archenteron, and the pigment cells in prism stage embryos. Immunogold localizations show antigen on the surfaces of mesenchyme, within membrane bounded vesicles, and associated with the Golgi apparatus. Western blots of antigens immunoprecipitated from seven developmental stages reveal twelve antigens ranging in Mr from 35 k to 240 k. Most of these antigens appear, disappear or change Mr over the first five days of development. Characterizations of this complex array of antigens show that the epitope recognized by Sp12 is eliminated by proteolytic enzymes and endoglycosidase F, while immunoreactivity is only reduced by periodate oxidation. As well, calcium magnesium free seawater extracts a subset of antigens different from that retained by crude membrane preparations. It is proposed that the mesenchyme of sea urchin embryos produces a family of developmentally regulated cell surface and extracellular matrix glycoproteins which all exhibit a carbohydrate epitope recognized by Sp12.

[Water-soluble proteins in early amphibian embryos. III. Immunoelectrophoretic analysis of the antigenic changes in the ectoderm of the early gastrula and neural plate during development]

20 water-soluble antigen have been identified with the help of rabbit antisera to extracts of the early gastrula ectoderm and neural plate in Rana temporaria. All of them were also found in the early blastula embryos and unfertilized eggs. The identified antigens are characterized by a definite embryospecificity. As the development proceeds, the concentration of these antigens in the embryonic tissues decreases until the complete disappearance of corresponding immunoelectrophoretic reactions. By this characteristic all antigens under study are subdivided into four groups: I--five antigens identified at the early developmental stages only (until hatching, stage 29); II--nine antigens present up to stages 33--35; III--three antigens followed up to stages 39--40 (well formed tadpole); IV--three antigens were found at all developmental stages under study up to stages 45--47. 11 out of 20 identified antigens have antigenic similarity with the proteins of blood serum of adult amphibians. Besides, the early gastrula ectoderm contains antigens similar with those of the brain of adult amphibians.