Receptor-mediated transport and deposition of complement component C3 into developing chicken oocytes.

Immunological resistance of the chick embryo is dependent upon IgG present in the yolk of the layed egg. Here we show that complement factor 3 (C3), a key component of the humoral complement system, is a yolk component of chicken eggs. C3 is transported into oocytes by LR8-mediated endocytosis. LR8 also binds and transports other major yolk components such as vitellogenin, very-low-density lipoprotein, and alpha(2)-macroglobulin. Expression studies of LR8 during chicken development and oocyte maturation, in combination with studies on the uptake of individual yolk components, suggest the following model for oocyte maturation in the chicken: all oocytes present in the ovary contain high levels of LR8 mRNA and protein long before the onset of oocyte maturation. Selected oocytes gain access to yolk precursors, and LR8 binds, internalizes, and deposits the major yolk components in the ratio of their relative abundance in the accessible pool.

Transglutaminase factor XIIIA in the cartilage of developing avian long bones.

Previously, we showed that mRNA for transglutaminase factor XIIIA (FXIIIA) is up-regulated in the hypertrophic zone of the growth plate of the chicken tibiotarsus, a well-characterized model of long bone development. In the present study, we have studied the distribution of the FXIIIA protein and of transglutaminase enzymatic activity in this growth plate, as well as in the cartilage of the epiphysis, which includes that of the articular surface. By immunohistochemical analysis, the protein is detected in the zone of maturation, where it is mostly intracellular, and in the hypertrophic zone, where it is present both intracellularly and in the extracellular matrix. The intracellular enzyme is mostly a zymogen, as determined with an antibody specific for the activation peptide. Externalization of FXIIIA is accompanied by enzyme activation. To study the pattern of transglutaminase activity, a synthetic transglutaminase substrate, rhodamine-conjugated tetrapeptide (Pro-Val-Lys-Gly), was used for pulse labeling in organ cultures. Intensive incorporation of the fluorescent substrate was observed throughout the hypertrophic zone and in the cells surrounding the forming blood vessels. The patterns of FXIIIA immunostaining and substrate incorporation overlap almost completely. The cartilaginous factor XIIIA is different from the plasma form in that, both intracellularly and extracellularly, it exists as a monomer, as determined by Western analysis, whereas the plasma form of FXIII is a tetrameric complex composed of both A and B subunits. We also identified FXIIIA and transglutaminase activity within the articular and condylar regions of the tarsus, suggesting a possible involvement of mechanical pressure and/or stress in the production of the molecule and subsequent cross-linking of the cartilage matrix. Thus, transglutaminases, in particular FXIIIA, are involved in the formation of long bones through its activity both in the hypertrophic region of the growth plate and in the formation of articular/epiphyseal cartilages.

Chicken coagulation factor XIIIA is produced by the theca externa and stabilizes the ovarian follicular wall.

Development of the follicle in egg-laying species such as the chicken is regulated by systemic factors as well as by the highly orchestrated interplay of differentially expressed genes within this organ. Differential mRNA display analysis of defined phases of follicle development resulted in the characterization of coagulation factor XIIIA. It is expressed and produced by cells of the theca externa in a highly regulated manner during distinct growth phases of the follicle. Transcripts for factor XIIIA are already detectable at the beginning of follicle development and peak at the end of phase 2. Protein levels, however, still increase during phase 3, peak shortly after ovulation, and persist until the postovulatory tissue is completely resorbed. Factor XIIIA is secreted as a monomer into the extracellular matrix of the theca externa and is not associated with factor XIIIB as is the case in plasma. Our data suggest that, due to its transglutaminase activity, factor XIIIA stabilizes the follicular wall by cross-linking matrix components. Thus, coagulation factor XIIIA might play a key role in coping with the massive mechanical stress exerted by the large amount of yolk accumulating during the rapid growth phase of the oocyte.