Characterization of an antigen present on testicular cells and preimplantation embryos whose expression is modified by the t12 haplotype

The T-locus - inspiration and distraction?

The T/t locus was a major focus of study by mouse geneticists during the 20th century. In the 70s, as the study of cell surface antigens controlling transplantation antigens was taking off, several laboratories hypothesized that alleles of this locus would control cell surface antigens important for embryonic development. One such antigen, the embryonal carcinoma F9 antigen was said to be an example. Other antigens were described on sperm and embryos that were said to be controlled by alleles at the T/t complex. These findings were later found to be false. The history of the findings and their refutation is described.

Mate choice for genetic compatibility in the house mouse

In house mice, genetic compatibility is influenced by the t haplotype, a driving selfish genetic element with a recessive lethal allele, imposing fundamental costs on mate choice decisions. Here, we evaluate the cost of genetic incompatibility and its implication for mate choice in a wild house mice population. In laboratory reared mice, we detected no fertility (number of embryos) or fecundity (ability to conceive) costs of the t, and yet we found a high cost of genetic incompatibility: heterozygote crosses produced 40% smaller birth litter sizes because of prenatal mortality. Surprisingly, transmission of t in crosses using +/t males was influenced by female genotype, consistent with postcopulatory female choice for + sperm in +/t females. Analysis of paternity patterns in a wild population of house mice showed that +/t females were more likely than +/+ females to have offspring sired by +/+ males, and unlike +/+ females, paternity of their offspring was not influenced by +/t male frequency, further supporting mate choice for genetic compatibility. As the major histocompatibility complex (MHC) is physically linked to the t, we investigated whether females could potentially use variation at the MHC to identify male genotype at the sperm or individual level. A unique MHC haplotype is linked to the t haplotype. This MHC haplotype could allow the recognition of t and enable pre- and postcopulatory mate choice for genetic compatibility. Alternatively, the MHC itself could be the target of mate choice for genetic compatibility. We predict that mate choice for genetic compatibility will be difficult to find in many systems, as only weak fertilization biases were found despite an exceptionally high cost of genetic incompatibility.

t-complex-associated embryonic surface antigen homologous to mLAMP-1. I. Biochemical and molecular analyses

Previously we described an embryonic cell surface glycoprotein, ESGp, associated with the t-embryonic lethal alleles of the mouse t complex. This antigen is expressed on the cell surface of both early mouse embryos and embryonal carcinoma (EC) cell lines. The antigen is localized to areas of cell-cell contact in EC lines and redistributes to the outer edges of the blastomeres during compaction, thereby indicating a potential role in embryonic cell-cell interaction. We now report that this t-complex-associated ESGp is homologous to the mouse lysosomal-associated membrane protein-1 (LAMP-1). Limited protein sequence analyses of the amino terminal and an internal peptide indicate considerable homology with the LAMP-1 protein. Biochemical parameters such as protein core size, sulfation and phosphorylation status, and resistance to proteolysis also demonstrate homology. While we detect only a single message with a mouse LAMP-1 cDNA probe via Northern blotting, Southern analyses indicate the existence of at least two homologous LAMP-1 genes. Additionally, we present evidence suggesting that ESGp/LAMP-1 serves as a substrate which may be differentially glycosylated by the activities of the gene products of the different t-lethal alleles.

t-complex-associated embryonic surface antigen homologous to mLAMP-1. II. Expression and distribution analyses

We have previously demonstrated that a mouse t-complex-associated antigen (ESGp) is present on the cell surface, specifically at areas of cell-cell contact, of cleavage stage embryos and embryonal carcinoma cell lines. In the accompanying paper, we document isolation, purification, and partial sequencing of this molecule. Amino-terminal and internal peptide fragments are highly homologous to the mouse lysosomal-associated membrane protein-1 (mLAMP-1). We also demonstrate considerable similarity, though not necessarily identity, between these two antigens through both biochemical and molecular analyses. In this report we use immunological techniques to compare the expression and distribution of these antigens in various cell lines as well as mouse tissues and preimplantation stage embryos. The data presented indicate that there are: (1) different antigenic forms of the ESGp/ mLAMP-1 homologues and (2) changes in the molecular composition and expression of these forms during embryogenesis and differentiation. It is obvious from these studies that mLAMP-1 is not strictly a static molecule whose sole function is to protect the lumenal side of the lysosomal membrane but rather is a dynamic molecule of potential import in many other cellular and organismal functions.

Characterization of a developmentally regulated mouse embryonic antigen

A mammalian embryonic cell surface glycoprotein (ESGp), whose expression and biochemical structure seem to be developmentally regulated, has been isolated and characterized. The molecule expressed in two cell through morula stage mouse embryos has a molecular weight, by electrophoretic analyses, of 90 kDa. At the blastocyst stage, however, the molecule migrates as a broad, heterogeneous band ranging from 90 to 110 kDa. Evidence obtained from studies of embryonal carcinoma (EC) cells indicates that this band is actually a composite of three distinct molecules (molecular weight 90, 95, and 105 to 110 kDa), each of which is synthesized uniquely by one of the different cell types of the blastocyst: the embryonic ectoderm and visceral and parietal endoderms, respectively. A survey of various mouse tissues and cell lines has revealed that undifferentiated cells express the low molecular weight form (90 kDa) characteristic of embryonic ectoderm, whereas differentiated cells and adult tissues express the high molecular weight form (110 kDa) characteristic of parietal endoderm. Only the EC visceral endoderm cell analogues have been shown to express the intermediate molecule (95 kDa). In embryos, the antigen is uniformly distributed over the cell surface during early cleavage stages (two to eight cell); just before compaction, however, it seems to redistribute and becomes polarized at the outside exposed edges of blastomeres. In cultured EC cells, ESGp is found only in areas of cell-to-cell contact; free-standing surfaces of cells are negative for expression. It is possible, therefore, that ESGp may be involved in the intercellular adhesion of both EC cells and compacting embryos.

Genetic analysis of the T/t complex

The T/t-complex has held considerable interest for immunologists, primarily because of its close genetic linkage to the major histocompatibility complex (MHC) on mouse chromosome 17. This interest has been heightened recently with the discovery that the MHC is fully contained within the t-complex and that two regions of the MHC, Qa and K, contain t-lethal genes. For a long time, T/t has been an enigmatic system, mainly because classical genetic analysis was not possible. Here the system is defined, recent information is presented, and our understanding of the mouse data to available information about the human MHC is correlated.