Expression of TAPA-1 in preimplantation mouse embryos

Tetraspanins in cell stemness and cancer initiation: markers or active players?

Tetraspanins mark stem cells and tumor initiating cells. Recent studies in adipose development, intestinal crypt remodeling, and muscle stem cells shed new light on the contribution of tetraspanins and their associated partners in cell fate determination. These studies reveal that these partnerships actively help guide precursor cell fate.

Proteolytic cleavage of the cell surface protein p160 is required for detachment of the fertilization envelope in the sea urchin

Sea urchin eggs secrete a serine protease activity, CGSP1, at fertilization that is essential for the block to polyspermy. Several targets of this proteolytic activity on the plasma membrane were identified here using a cell surface biotinylation approach. Amino acid microsequencing of one of these proteins led to the identification of a 4.75-kb cDNA clone from a Strongylocentrotus purpuratus ovary cDNA library that encodes a 160-kDa protein called p160. This protein contains five CUB domains and a putative transmembrane domain suggesting that p160 is an integral membrane protein with protein-protein interaction motifs facing the extracellular matrix of the egg. Whole-mount immunolocalization studies demonstrate that p160 is on the surface of the egg, enriched at the tips of microvilli. The protein is removed at fertilization in a protease-dependent manner, and functional assays suggest that p160 serves to link the plasma membrane to the vitelline layer until fertilization. Thus, p160 is a key candidate for a vitelline-layer linker protein, the selective proteolysis of which functions in the block to polyspermy in the sea urchin egg.

Genomic organization and embryonic expression of Igsf8, an immunoglobulin superfamily member implicated in development of the nervous system and organ epithelia

Igsf8 is an immunoglobulin protein that binds to the tetraspanin molecules, CD81 and CD9. We describe the genomic organization of mouse and human Igsf8, and reveal a dynamic expression pattern during embryonic and fetal development. Igsf8 is first expressed at E9.5 in a ventral domain of the neural tube, with dorsal expression apparent at E10.5. We show that the ventral, but not the dorsal, domain of neural tube expression is dependent on Shh signaling. From E11.5, Igsf8 is expressed at the lateral edge of the ventricular zone, in early postmitotic neuroblasts, and in dorsal root and cranial ganglia. Igsf8 is also expressed in the branchial arches, dorsal pancreatic primordium, neural retina, olfactory epithelium, gut, kidney, and lung.

Development of human lymphohematopoiesis defined by CD34 and CD81 expression

Human blood cells, except for erythrocytes and platelets, express CD81, a member of the transmembrane 4 superfamily (TM4SF). CD81 is also expressed on most of human immature hematopoietic cells, CD34+ cells, which are divided into three populations according to the expression of CD34 and CD81; CD34+CD81+, CD34+CD81(High) and CD34(Low)CD81+. Myeloid and lymphoid progenitors exist in the CD34+CD81+ population, and megakaryocytic progenitors are only in CD34(Low)CD81+ population. Erythroid and multipotential progenitors are shared by CD34+CD81+ and CD34(Low)CD81+ populations, but multipotential progenitors in the CD34+CD81+ population have already lost most of their myeloid potential. NK cells and mast cells can be generated from all three populations. Long-term repopulating (LTR) lymphohematopoietic stem cells are present in the CD34+CD81+ population. Based on these findings, we propose a model for the development of CD34+CD81+ lymphohematopoietic stem cells. Along the differentiation cascade from CD34+CD81+ lymphohematopoietic stem cells, there appear to be pathways to CD34(Low)CD81 + or CD34+CD81(High) cells, even if they are indirect. CD34(Low)CD81+ pathways define the loss of LTR ability, and lymphoid and myeloid potentials, whereas CD34+CD81(High) pathways represent the exclusive commitment to NK cells and mast cells.

Fertilin beta and other ADAMs as integrin ligands: insights into cell adhesion and fertilization

One of the most important cell-cell interactions is that of the sperm with the egg. This interaction, which begins with cell adhesion and culminates with membrane fusion, is mediated by multiple molecules on the gametes. One of the best-characterized of these molecules is fertilin beta, a ligand on mammalian sperm and one of the first ADAMs (A Disintegrin and A Metalloprotease domain) to be identified. Fertilin beta (also known as ADAM2) participates in sperm-egg membrane binding, and it has long been hypothesized that this function is achieved through the interaction of the disintegrin domain of fertilin beta with an integrin on the egg surface. There are now approximately 30 members of the ADAM family and, to date, five different ADAMs (fertilin beta, ADAM9, ADAM12, ADAM15, ADAM23) have been described to interact with integrins (specifically alpha(6)beta(1), alpha(v)beta(3), alpha(9)beta(1), alpha(v)beta(5), and/or alpha(5)beta(1)). This field will be discussed with respect to what is known about specific ADAMs and the integrins with which they interact, and what the implications are for sperm-egg interactions and for integrin function. These data will also be discussed in the context of recent knockout studies, which show that eggs lacking the alpha(6) integrin subunit can be fertilized, and eggs lacking the integrin-associated tetraspanin protein CD9 fail to fertilize. Key issues in cell adhesion that pertain to gametes and fertilization will also be highlighted.

Differential expression of murine CD81 highlighted by new anti-mouse CD81 monoclonal antibodies

We describe the use of a soluble CD81-Fc fusion protein to screen for novel monoclonal antibody (MAb) reactive with the extracellular loops of murine CD81 (TAPA-1). Two such MAbs, Eat1 and Eat2 (for Extracellular Anti-TAPA1), were used to assess the expression and function of CD81 on murine lymphocytes. Although CD81 is expressed uniformly on all human lymphocytes, murine CD81 was found to be expressed at much higher levels on resting B cells than on resting T cells. This was particularly evident when staining with the new MAbs, Eat1 and Eat2. The molecule is also functionally active on B cells, as Eat1 and Eat2 induce homotypic adhesion of B lymphocytes. Stimulated B cells undergo early apoptotic events in the presence of Eat2, as shown by binding of Annexin V-fluorescein isothiocyanate (FITC). Polyclonal activation of murine T cells also induces higher level CD81 expression, and many immortalized murine T-cell lines express high levels of the protein. In contrast to human CD81, which is expressed equally on all thymocytes, murine CD81 is induced during thymic development, being expressed at high levels on CD4+CD8+ thymocytes, in contrast to other subsets of thymocytes. Finally, murine dendritic cells, splenic macrophages, and non-killer (NK) cells all express high levels of CD81. We conclude that CD81 is differentially expressed in the murine immune system, and is involved in regulating the adhesion and activation of murine B cells.

CD81 (TAPA-1): a molecule involved in signal transduction and cell adhesion in the immune system

CD81 (TAPA-1) is a widely expressed cell-surface protein involved in an astonishing variety of biologic responses. It has been cloned independently several times for different functional effects and is reported to influence adhesion, morphology, activation, proliferation, and differentiation of B, T, and other cells. On B cells CD81 is part of a complex with CD21, CD19, and Leu13. This complex reduces the threshold for B cell activation via the B cell receptor by bridging Ag specific recognition and CD21-mediated complement recognition. Similarly on T cells CD81 associates with CD4 and CD8 and provides a costimulatory signal with CD3. In fetal thymic organ culture, mAb to CD81 block maturation of CD4-CD8- thymocytes, and expression of CD81 on CHO cells endows those cells with the ability to support T cell maturation. However, CD81-deficient mice express normal numbers and subsets of T cells. These mice do exhibit diminished antibody responses to protein antigens. CD81 is also physically and functionally associated with several integrins. Anti-CD81 can activate integrin alpha 4 beta 1 (VLA-4) on B cells, facilitating their adhesion to tonsilar interfollicular stroma. Similarly, anti-CD81 can activate alpha L beta 2 (LFA-1) on human thymocytes. CD81 can also affect cognate B-T cell interactions because anti-CD81 increases IL-4 synthesis by T cells responding to antigen presented by B cells but not by monocytes. The tetraspanin superfamily (or TM4SF) includes CD81, CD9, CD37, CD53, CD63, CD82, CD151, and an increasing number of additional proteins. Like CD81, several tetraspanins are involved in cell adhesion, motility, and metastasis, as well as cell activation and signal transduction.

Normal lymphocyte development but delayed humoral immune response in CD81-null mice

CD81 is a cell surface molecule expressed on many cell types and associated with the CD19/CD21/Leu13 signal-transducing complex on B cells. A recent report implies that CD81 expression on thymic stromal cells is important in the maturation of thymocytes from CD4-CD8- to CD4+CD8+. However, we have produced CD81-null mice by gene targeting, and find that they undergo normal development of thymocytes and express normal numbers of T cells. B cells are also found in normal numbers in the spleen, blood, and peritoneal cavity of CD81-null mice, but they express a lower level of CD19 compared to heterozygous littermates. Finally, early antibody responses to the protein antigen ovalbumin are weaker in CD81-null mice compared to their heterozygous littermates. This is consistent with the proposed role of the CD19/CD21/CD81-signaling complex in lowering the threshold for B cell responses. These results show that CD81 is not required for maturation of T cells, but is important for optimal expression of CD19 on B cells and optimal stimulation of antibody production.

Pleiotropic effects of the mouse lethal yellow (Ay) mutation explained by deletion of a maternally expressed gene and the simultaneous production of agouti fusion RNAs

Heterozygosity for the mouse lethal yellow (Ay) mutation leads to obesity, increased tumor susceptibility and increased activity of the agouti coat color gene; homozygosity for Ay results in embryonic death around the time of implantation. Although these pleiotropic effects have not been separated by recombination, previous studies have suggested that the dominant and recessive effects result from distinct genetic lesions. Here we use a combination of genomic and cDNA cloning experiments to demonstrate that the Ay mutation is caused by a 120 kb deletion which lies centromere-proximal to the agouti coat color gene. The deletion removes coding but not 5′ untranslated sequences for a ubiquitously expressed gene predicted to encode a protein similar in sequence to an RNA-binding protein, which we named Merc, for maternally expressed hnRNP C-related gene, but have renamed Raly, since the gene is nearly identical to one reported recently by Michaud et al. (Gene Dev. 7, 1203–1213, 1993). The Ay deletion results in the splicing of Merc/Raly 5′ untranslated sequences to agouti protein-coding sequences, which suggests that ectopic expression of the normal agouti protein by the Ay fusion RNA is responsible for the pleiotropic effects associated with heterozygosity for Ay. We find that Merc/Raly RNA is present in the unfertilized egg and is also transcribed in preimplantation embryos. Using a PCR-based assay to determine the genotype of individual embryos from an Ay/a × Ay/a intercross, we show that, in the absence of zygotic Merc/Raly expression, Ay/Ay embryos develop to the blastocyst stage, but do not hatch from the zona pellucida or form trophoblastic outgrowths. Injection of a Merc/Raly antisense oligonucleotide into non-mutant embryos blocks development prior to the blastocyst stage, and can be rescued by coinjection of a Merc/Raly transgene. These results suggest that maternal expression of Merc/Raly plays an important role in preimplantation development and that its deletion of is sufficient to explain Ay-associated embryonic lethality.