ALCAM (CD166): its role in hematopoietic and endothelial development

A critical role for the endothelium of yolk sac and dorsal aorta has been shown in embryonic hematopoiesis. A stromal cell line derived from yolk sac, YSCL-72, has been chosen to search for a novel molecule associated with embryonic hematopoiesis. Analysis between YSCL-72 and an adult aorta-derived endothelial cell line, EOMA, demonstrated that activated leukocyte cell adhesion molecule (ALCAM, or CD166) was specifically expressed in YSCL-72 but not in EOMA. Immunohistochemical study showed that ALCAM was expressed in the endothelium of yolk sac and dorsal aorta but not in adult aorta. ALCAM-transfected EOMA cells supported development of hematopoietic progenitor cells compared with vector-transfected EOMA cells, suggesting that ALCAM appeared to be crucial for hematopoiesis. In addition, ALCAM was found to be involved in capillary tube formation and hemangioblast differentiation. Taken together with these findings, ALCAM is highly associated not only with embryonic hematopoiesis but also vasculoangiogenesis.

Regulation of the association between PSTPIP and CD2 in murine T cells

Prominent in T cells and natural killer cells, CD2 binding protein 1 (CD2BP1) plays an important role in CD2-mediated adhesion and signal transduction. In the current study, we investigated CD2 and PSTPIP (proline, serine, threonine phosphatase interacting protein, murine homologue of CD2BP1) interactions in purified mouse splenic T cells. PSTPIP associated with CD2 in both resting and activated T cells. Following various stimuli, such as concanavalin A, anti-TCRbeta, anti-CD3epsilon, anti-CD3epsilon/phorbol myristate acetate (PMA), IL-2, or PMA/ionomycin, PSTPIP and CD2 expression, as well as their association, increased in a time-dependent fashion. While PSTPIP expression and CD2 expression were comparable across most groups, the PSTPIP-CD2 association stimulated by anti-CD3epsilon alone was significantly greater than with other stimuli. Stimulation by anti-CD3epsilon plus anti-CD28 induced even greater PSTPIP-CD2 association than anti-CD3epsilon treatment alone, indicating that CD28 initiated signals are involved in regulating this interaction. There was no direct association between CD3epsilon or CD28 and PSTPIP. Tyrosine phosphorylated PSTPIP bound poorly to CD2 compared to dephosphorylated PSTPIP, and protein tyrosine phosphatase was shown to affect both phosphorylation of PSTPIP and the CD2-PSTPIP association. In addition to CD2, PSTPIP associated with CD4, CD8, CD54, and CD62L. CD2 and CD4 ligation reciprocally regulated their association with PSTPIP. These findings indicate that T cell activation, particularly through the CD3 and CD28 signal transduction pathways, regulates PSTPIP-CD2 interactions. PSTPIP likely has additional broader effects through interactions with CD4, CD8, CD54, and CD62L, and this may influence T cell responses to antigen.

PTEN 2, a Golgi-associated testis-specific homologue of the PTEN tumor suppressor lipid phosphatase

The tumor suppressor PTEN is a phosphatidylinositol phospholipid phosphatase, which indirectly down-regulates the activity of the protein kinase B/Akt survival kinases. Examination of sequence data bases revealed the existence of a highly conserved homologue of PTEN. This homologue, termed PTEN 2, contained an extended amino-terminal domain having four potential transmembrane motifs, a lipid phosphatase domain, and a potential lipid-binding C2 domain. Transcript analysis demonstrated that PTEN 2 is expressed only in testis and specifically in secondary spermatocytes. In contrast to PTEN, PTEN 2 was localized to the Golgi apparatus via the amino-terminal membrane-spanning regions. Molecular modeling suggested that PTEN 2 is a phospholipid phosphatase with potential specificity for the phosphate at the 3 position of inositol phosphates. Enzymatic analysis of PTEN 2 revealed substrate specificity that is similar to PTEN, with a preference for the dephosphorylation of the phosphatidylinositol 3,5-phosphate phospholipid, a known mediator of vesicular trafficking. Together, these data suggest that PTEN 2 is a Golgi-localized, testis-specific phospholipid phosphatase, which may contribute to the terminal stages of spermatocyte differentiation.

Caenorhabditis elegans PIAK, a phospholipid-independent kinase that activates the AKT/PKB survival kinase

Phospholipid-dependent kinase 1 (PDK 1) is a 3'-phospholipid-responsive serine/threonine kinase that plays a critical role in cell survival by phosphorylating and activating the anti-apoptotic AKT/PKB kinase. While PDK 1 is clearly an important component of the cell survival machinery, the potential for phospholipid-independent activation of the AKT/PKB survival pathway has not been extensively examined at the molecular level. We have identified a second form of PDK 1 in the nematode Caenorhabditis elegans that we have termed PIAK (phospholipid-independent AKT/PKB kinase). PIAK is highly homologous to C. elegans and mammalian PDK 1 with the exception that the novel kinase lacks a phospholipid binding pleckstrin homology domain. The domain structure of PIAK suggests that it might be a phospholipid-independent kinase, and PIAK phosphorylates mammalian AKT/PKB at the activating Thr(308) residue in the presence of the phosphatidylinositol (PI) 3-kinase inhibitors as well as in the absence of growth factors. In addition, PIAK is capable of inducing the phospholipid-independent, AKT/PKB-induced phosphorylation of the AFX-type forkhead transcription factor, resulting in its cytoplasmic localization. Because the nuclear localization of this transcription factor induces an apoptotic state, this PIAK-mediated cytoplasmic sequestration allows for cell survival. Finally, PIAK activity appears to be induced by various inhibitors of cell cycle G(1) progression. These data suggest an alternate, phosphatidylinositol 3-kinase-independent mechanism for the activation of the AKT/PKB survival pathway that may be utilized during periods of cellular quiescence.

CrkL is an adapter for Wiskott-Aldrich syndrome protein and Syk

Wiskott-Aldrich syndrome (WAS) and X-linked thrombocytopenia are caused by mutations of the WAS protein (WASP) gene. WASP may be involved in the regulation of podosome, an actin-rich dynamic cell adhesion structure formed by various types of cells. The molecular links between WASP and podosomes or other cell adhesion structures are unknown. Platelets express an SH2-SH3 adapter molecule, CrkL, that can directly associate with paxillin, which is localized in podosomes. The hypothesis that CrkL binds to WASP was, therefore, tested. Results from coprecipitation experiments using anti-CrkL and GST-fusion proteins suggest that CrkL binds to WASP through its SH3 domain and that the binding was not affected by WASP tyrosine phosphorylation. The binding of GST-fusion SH3 domain of PSTPIP1 in vitro was also not affected by WASP tyrosine phosphorylation, suggesting that the binding of the SH3 domains to WASP is not inhibited by tyrosine phosphorylation of WASP. Anti-CrkL also coprecipitates a 72-kd protein, which was identified as syk tyrosine kinase, critical for collagen induced-platelet activation. CrkL immunoprecipitates contain kinase-active syk, as evidenced by an in vitro kinase assay. Coprecipitation experiments using GST-fusion CrkL proteins suggest that both SH2 and SH3 domains of CrkL are involved in the binding of CrkL to syk. WASP, CrkL, syk, and paxillin-like Hic-5 incorporated to platelet cytoskeleton after platelet aggregation. Thus, CrkL is a novel molecular adapter for WASP and syk and may potentially transfer these molecules to the cytoskeleton through association with cytoskeletal proteins such as Hic-5.

Selective expression of murine prostate stem cell antigen in fetal and adult tissues and the transgenic adenocarcinoma of the mouse prostate model of prostate carcinogenesis

Prostate stem cell antigen (PSCA) is a GPI-anchored membrane protein whose expression is reportedly up-regulated in a majority of human prostate cancers, including advanced stages and metastases. In this study, we investigate the expression pattern of the murine orthologue of PSCA by in situ hybridization in fetal and adult mouse tissues. Murine PSCA is expressed during fetal development in the urogenital sinus, skin, and gastrointestinal tract. The expression in these tissues is restricted to the most superficial cell layer. In the adult mouse, expression is highest in the mucosal lining of the urinary tract. In the normal adult prostate, expression of PSCA is detected exclusively in the secretory epithelium. Examination of PSCA during carcinogenesis of the murine prostate in the transgenic adenocarcinoma of the mouse prostate model showed a markedly increased expression in areas of neoplasia. The transgenic adenocarcinoma of the mouse prostate model may represent a valuable model for the study of PSCA as a potential target for immunotherapy of prostate cancer, despite potential differences in the pattern of expression between mice and humans.

Cytoskeletal protein PSTPIP1 directs the PEST-type protein tyrosine phosphatase to the c-Abl kinase to mediate Abl dephosphorylation

A search for c-Abl interacting proteins resulted in the recovery of PSTPIP1, originally identified as a binding protein of the PEST-type protein tyrosine phosphatases (PTP). PSTPIP1 was phosphorylated by c-Abl, and growth factor-induced PSTPIP1 phosphorylation was diminished in Abl null fibroblasts. PSTPIP1 was able to bridge c-Abl to the PEST-type PTPs. Several experiments suggest that the PEST-type PTPs negatively regulate c-Abl activity: c-Abl was hyperphosphorylated in PTP-PEST-deficient cells; disruption of the c-Abl-PSTPIP1-PEST-type PTP ternary complex by overexpression of PSTPIP1 mutants increased c-Abl phosphotyrosine content; and PDGF-induced c-Abl kinase activation was prolonged in PTP-PEST-deficient cells. Dephosphorylation of c-Abl by PEST-type PTP represents a novel mechanism by which c-Abl activity is regulated.

Analysis of PDZ domain-ligand interactions using carboxyl-terminal phage display

PDZ domains mediate protein-protein interactions at specialized subcellular sites, such as epithelial cell tight junctions and neuronal post-synaptic densities. Because most PDZ domains bind extreme carboxyl-terminal sequences, the phage display method has not been amenable to the study of PDZ domain binding specificities. For the first time, we demonstrate the functional display of a peptide library fused to the carboxyl terminus of the M13 major coat protein. We used this library to analyze carboxyl-terminal peptide recognition by two PDZ domains. For each PDZ domain, the library provided specific ligands with sub-micromolar binding affinities. Synthetic peptides and homology modeling were used to dissect and rationalize the binding interactions. Our results establish carboxyl-terminal phage display as a powerful new method for mapping PDZ domain binding specificity.

Interaction of the tumor suppressor PTEN/MMAC with a PDZ domain of MAGI3, a novel membrane-associated guanylate kinase

PTEN/MMAC is a phosphatase that is mutated in multiple human tumors. PTEN/MMAC dephosphorylates 3-phosphorylated phosphatidylinositol phosphates that activate AKT/protein kinase B (PKB) kinase activity. AKT/PKB is implicated in the inhibition of apoptosis, and cell lines and tumors with mutated PTEN/MMAC show increased AKT/PKB kinase activity and resistance to apoptosis. PTEN/MMAC contains a PDZ domain-binding site, and we show here that the phosphatase binds to a PDZ domain of membrane-associated guanylate kinase with inverted orientation (MAGI) 3, a novel inverted membrane-associated guanylate kinase that localizes to epithelial cell tight junctions. Importantly, MAGI3 and PTEN/MMAC cooperate to modulate the kinase activity of AKT/PKB. These data suggest that MAGI3 allows for the juxtaposition of PTEN/MMAC to phospholipid signaling pathways involved with cell survival.

WECHE: a novel hematopoietic regulatory factor

Previously, we described AGM-derived endothelial cell lines that either inhibited or permitted the development of erythroid or B cells. We utilized a differential gene expression method to isolate a chemokine, termed WECHE, from one of these cell lines. WECHE inhibited the formation of erythroid cells but had no effect on either myeloid or B cell formation. WECHE repressed BFU-E development from either mouse fetal liver or bone marrow progenitor cells but had no effect on colony formation induced by IL-3 or IL-7. WECHE reduced HPP-CFC production from fetal liver-derived stem cells. WECHE hindered the growth of yolk sac-derived endothelial cells. WECHE was also chemotactic for bone marrow cells. Thus, WECHE is a novel chemokine that regulates hematopoietic differentiation.

Multicolor immunofluorescence and flow cytometry utilizing cascade blue to purify murine hematopoietic stem cells from fetal liver and bone marrow

BACKGROUND

Here we demonstrate the utility of cascade blue (CB), to purify hematopoietic stem cells by flow cytometry. Multicolor immunofluorescence and the sensitivity (signal-to-noise) of the fluorochromes are essential for the identification and isolation of rare stem cell populations.

METHODS

We isolated hematopoietic stem cells utilizing a 407 nm laser line to excite CB and propidium iodide (PI) in combination with FITC, PE, and Red670 which were excited at 488 nm.

RESULTS

CB is maximally excited using a 407 nm laser line, when compared to UV or 413 nm excitation. The increase in sensitivity of CB at 407 nm can be contributed to higher absorption of CB and a reduction of autofluorescence at this excitation wavelength (Ropp et al.: Cytometry 21: 309-317, 1995).

CONCLUSIONS

Despite the fact that the CB antibody conjugate has a tendency to adhere specifically to a B cell subpopulation in bone marrow, we nevertheless could purify stem cells by using CB for the detection and elimination of lineage positive cells. Isolated stem cells from mouse fetal liver (Lin-CD34(+)Sca-1(+)c-Kit(high)) and adult bone marrow (Lin-CD34(-/low)Sca-1(+)c-Kit(+)) were transplanted into lethally irradiated mice, and the sorted stem cells had the ability to efficiently repopulate all mature hematopoietic lineages in recipient mice.

Protein tyrosine phosphatase-PEST regulates focal adhesion disassembly, migration, and cytokinesis in fibroblasts

In this article, we show that, in transfected COS-1 cells, protein tyrosine phosphatase (PTP)-PEST translocates to the membrane periphery following stimulation by the extracellular matrix protein fibronectin. When plated on fibronectin, PTP-PEST (-/-) fibroblasts display a strong defect in motility. 3 h after plating on fibronectin, the number and size of vinculin containing focal adhesions were greatly increased in the homozygous PTP-PEST mutant cells as compared with heterozygous cells. This phenomenon appears to be due in part to a constitutive increase in tyrosine phosphorylation of p130(CAS), a known PTP-PEST substrate, paxillin, which associates with PTP-PEST in vitro, and focal adhesion kinase (FAK). Another effect of this constitutive hyperphosphorylation, consistent with the focal adhesion regulation defect, is that (-/-) cells spread faster than the control cell line when plated on fibronectin. In the PTP-PEST (-/-) cells, an increase in affinity for the SH2 domains of Src and Crk towards p130(CAS) was also observed. In (-/-) cells, we found a significant increase in the level of tyrosine phosphorylation of PSTPIP, a cleavage furrow-associated protein that interacts physically with all PEST family members. An effect of PSTPIP hyperphosphorylation appears to be that some cells remain attached at the site of the cleavage furrow for an extended period of time. In conclusion, our data suggest PTP-PEST plays a dual role in cell cytoskeleton organization, by promoting the turnover of focal adhesions required for cell migration, and by directly or indirectly regulating the proline, serine, threonine phosphatase interacting protein (PSTPIP) tyrosine phosphorylation level which may be involved in regulating cleavage furrow formation or disassembly during normal cell division.

PSTPIP 2, a second tyrosine phosphorylated, cytoskeletal-associated protein that binds a PEST-type protein-tyrosine phosphatase

Although cytoskeletal regulation is critical to cell function during interphase and mitosis, the components of the cytoskeleton involved with its control are only beginning to be elucidated. Recently, we reported the identification of a cytoskeletal-associated protein, proline-serine-threonine phosphatase-interacting protein (PSTPIP), whose level of tyrosine phosphorylation was controlled by PEST-type protein-tyrosine phosphatases (PTPs) bound to a novel protein interaction site in the PSTPIP predicted coiled-coil domain. We also showed that the PSTPIP SH3 domain interacts with the Wiskott-Aldrich syndrome protein (WASP), a cytoskeletal regulatory protein, in a manner modulated by tyrosine phosphorylation. Here we describe the identification of PSTPIP 2, a widely expressed protein that is related to PSTPIP. PSTPIP 2 lacks an SH3 domain but contains a region predicted to bind to PEST-type PTPs, and structure-function analyses demonstrate that PSTPIP 2 interacts with the proline-rich C terminus of the PEST-type PTP hematopoietic stem cell factor in a manner similar to that previously demonstrated for PSTPIP. Confocal microscopy revealed that PSTPIP 2 colocalizes with PSTPIP in F actin-rich regions. PSTPIP 2 was found to be efficiently phosphorylated in v-Src-transfected or pervanadate-treated cells at two tyrosines conserved in PSTPIP, but in contrast to PSTPIP, tyrosine phosphorylated PSTPIP 2 was only weakly dephosphorylated in the presence of PTP HSCF. Finally, analysis of oligomer formation demonstrated that PSTPIP and PSTPIP 2 formed homo- but not heterodimers. These data suggest that a family of tyrosine phosphorylated, PEST PTP binding proteins may be implicated in cytoskeletal regulation.

Hematopoietic stem cell maintenance and differentiation are supported by embryonic aorta-gonad-mesonephros region-derived endothelium

Hematopoietic stem cells are capable of extensive self-renewal and expansion, particularly during embryonic growth. Although the molecular mechanisms involved with stem cell maintenance remain mysterious, it is now clear that an intraembryonic location, the aorta-gonad-mesonephros (AGM) region, is a site of residence and, potentially, amplification of the definitive hematopoietic stem cells that eventually seed the fetal liver and adult bone marrow. Because several studies suggested that morphologically defined hematopoietic stem/progenitor cells in the AGM region appeared to be attached in clusters to the ventrally located endothelium of the dorsal aorta, we derived cell lines from this intraembryonic site using an anti-CD34 antibody to select endothelial cells. Analysis of two different AGM-derived CD34(+) cell lines revealed that one, DAS 104-8, efficiently induced fetal-liver hematopoietic stem cells to differentiate down erythroid, myeloid, and B-lymphoid pathways, but it did not mediate self-renewal of these pluripotent cells. In contrast, a second cell line, DAS 104-4, was relatively inefficient at the induction of hematopoietic differentiation. Instead, this line provoked the expansion of early hematopoietic progenitor cells of the lin-CD34(+)Sca-1(+)c-Kit+ phenotype and was proficient at maintaining fetal liver-derived hematopoietic stem cells able to competitively repopulate the bone marrow of lethally irradiated mice. These data bolster the hypothesis that the endothelium of the AGM region acts to mediate the support and differentiation of hematopoietic stem cells in vivo.

Tyrosine phosphorylation regulates the SH3-mediated binding of the Wiskott-Aldrich syndrome protein to PSTPIP, a cytoskeletal-associated protein

Wiskott-Aldrich syndrome is an X-linked hematopoietic disease that manifests itself in platelet deficiency and a compromised immune system. Analysis of hematopoietic cells from affected individuals reveals that mutations in the Wiskott-Aldrich syndrome protein (WASP) result in structural and functional abnormalities in the cell cortex, consistent with the suggestion that WASP is involved with regulation of the actin-rich cortical cytoskeleton. Here we report that WASP interacts with a recently described cytoskeletal-associated protein, PSTPIP, a molecule that is related to the Schizosaccharomyces pombe cleavage furrow regulatory protein, CDC15p. This association is mediated by an interaction between the PSTPIP SH3 domain and two polyproline-rich regions in WASP. Co-expression of PSTPIP with WASP in vivo results in a loss of WASP-induced actin bundling activity and co-localization of the two proteins, which requires the PSTPIP SH3 domain. Analysis of tyrosine phosphorylation of PSTPIP reveals that two sites are modified in response to v-Src co-transfection or pervanadate incubation. One of these tyrosines is found in the SH3 domain poly-proline recognition site, and mutation of this tyrosine to aspartate or glutamate to mimic this phosphorylation state results in a loss of WASP binding in vitro and a dissolution of co-localization in vivo. In addition, PSTPIP that is tyrosine phosphorylated in the SH3 domain interacts poorly with WASP in vitro. These data suggest that the PSTPIP and WASP interaction is regulated by tyrosine phosphorylation of the PSTPIP SH3 domain, and this binding event may control aspects of the actin cytoskeleton.

Identification of a novel polyproline recognition site in the cytoskeletal associated protein, proline serine threonine phosphatase interacting protein

Protein-protein interactions are often mediated by the recognition of proline-rich domains by SH3 or WW modules. Previously, we demonstrated that the PEST-type protein-tyrosine phosphatase, PTP HSCF (hematopoietic stem cell fraction), bound to a novel cytoskeletal associated protein, proline serine threonine phosphatase interacting protein (PST PIP), via an interaction between the proline-rich COOH terminus of the PTP and a site within the putative coiled-coil domain of PST PIP. Here we describe a more detailed analysis of this interaction. Earlier data suggested that the NH2 terminus of PST PIP was important for binding to the phosphatase, and deletion of the NH2-terminal 50 amino acids of the PST PIP resulted in an apparently misfolded protein that was incapable of binding PTP HSCF. To examine the region involved with binding to PTP HSCF, alanine-scanning mutants were produced at intervals throughout PST PIP. This analysis demonstrated that a tryptophan at position 232 was essential for binding in vitro. Transfection experiments demonstrated that the Trp232 mutant protein was capable of association with the cortical cytoskeleton but was not bound to PTP HSCF in vivo. Alanine scanning of a peptide derived from the COOH-terminal proline-rich domain of PTP HSCF revealed that a subset of prolines, as well as other residues, was required for efficient binding to PST PIP, and introduction of alanines at some of these positions in the protein resulted in decreased binding to PST PIP in vitro and in vivo. Analysis of in vivo tyrosine phosphorylation of the Trp232 mutant of PST PIP in the presence of v-Src revealed that this protein was phosphorylated more efficiently than the wild-type molecule. Thus, the interaction between PTP HSCF and PST PIP is mediated by a novel site in the cytoskeletal associated protein which interacts with residues within the proline-rich COOH terminus of the phosphatase.

PSTPIP: a tyrosine phosphorylated cleavage furrow-associated protein that is a substrate for a PEST tyrosine phosphatase

We have investigated proteins which interact with the PEST-type protein tyrosine phosphatase, PTP hematopoietic stem cell fraction (HSCF), using the yeast two-hybrid system. This resulted in the identification of proline, serine, threonine phosphatase interacting protein (PSTPIP), a novel member of the actin- associated protein family that is homologous to Schizosaccharomyces pombe CDC15p, a phosphorylated protein involved with the assembly of the actin ring in the cytokinetic cleavage furrow. The binding of PTP HSCF to PSTPIP was induced by a novel interaction between the putative coiled-coil region of PSTPIP and the COOH-terminal, proline-rich region of the phosphatase. PSTPIP is tyrosine phosphorylated both endogenously and in v-Src transfected COS cells, and cotransfection of dominant-negative PTP HSCF results in hyperphosphorylation of PSTPIP. This dominant-negative effect is dependent upon the inclusion of the COOH-terminal, proline-rich PSTPIP-binding region of the phosphatase. Confocal microscopy analysis of endogenous PSTPIP revealed colocalization with the cortical actin cytoskeleton, lamellipodia, and actin-rich cytokinetic cleavage furrow. Overexpression of PSTPIP in 3T3 cells resulted in the formation of extended filopodia, consistent with a role for this protein in actin reorganization. Finally, overexpression of mammalian PSTPIP in exponentially growing S. pombe results in a dominant-negative inhibition of cytokinesis. PSTPIP is therefore a novel actin-associated protein, potentially involved with cytokinesis, whose tyrosine phosphorylation is regulated by PTP HSCF.

A novel protein-tyrosine phosphatase related to the homotypically adhering kappa and mu receptors

Here we describe a novel member of the receptor-like protein-tyrosine phosphatases (PTPs) termed PTP lambda, which is homologous to the homotypically adherent PTPs kappa and mu. Murine PTP lambda contains MAM, IgG, fibronectin type III, and dual phosphatase domains. As has been demonstrated for PTPs kappa and mu, PTP lambda mediates homotypic adhesion in vitro, and PTP lambda is associated with beta catenin in kidney epithelial cells. The extracellular domain of PTP lambda is proteolytically processed in cell culture as well as in vivo. Northern blot analysis reveals that PTP lambda is expressed throughout embryonic development and is predominately found in adult brain, lung, and kidney. In situ hybridization to 15.5-day old rat embryos reveals that PTP lambda is expressed in a variety of embryonic neuronal sites as well as in the esophagus, lung bronchiolar epithelium, kidney glomerular epithelium, olfactory epithelium, and various cartilagenous sites. Analysis of neonatal brain demonstrates expression in cells of the hippocampus, cortex, and the substantia nigra. Finally, immunohistochemical analysis reveals expression of this PTP on specific neurons of the spinal cord as well as on isolated cortical neurons.

Hematopoiesis: wandering progenitor cells

The generation of mice that are chimeric for expression of alpha 4 integrin has revealed a critical role for this adhesion molecule, specifically in postnatal lymphopoiesis.

Characterization of a novel member of the macrophage mannose receptor type C lectin family

The recognition of a diversity of carbohydrates by the various calcium dependent (type C) lectin family members has been shown to be critical for a variety of processes ranging from cell adhesion to antigen presentation. Examination of the expressed sequence tag (EST) data base for novel type C lectins using E-selectin as a probe resulted in the identification of a distantly related short polypeptide sequence containing many of the conserved residues found in these carbohydrate-binding proteins. Cloning of the full-length murine cDNA containing this region revealed that this protein is a novel member of the family that includes the macrophage mannose, the phospholipase A2, and the DEC 205 receptors, with a cysteine-rich domain, a fibronectin type 2 domain, eight type C lectin domains, a transmembrane domain, and a short cytoplasmic carboxyl terminus. Genomic Southern analysis suggests that this is a conserved protein, and examination of a human homologue revealed a high degree of sequence homology with the murine form. Northern blot analysis revealed expression of a large transcript in a number of different human and murine tissues and tumor cells and an alternatively spliced smaller transcript with a divergent 5' sequence was expressed specifically in the human fetal liver. Analysis of the genomic structure revealed that the gene encoding this lectin was interrupted by a large number of introns, and the intron structure was similar to the macrophage mannose receptor gene. Finally, in situ hybridization analysis demonstrated that the transcript encoding this lectin was found in a number of highly endothelialized sites as well as in chondrocytes in cartilaginous regions of the embryo.

A novel protein tyrosine phosphatase expressed in lin(lo)CD34(hi)Sca(hi) hematopoietic progenitor cells

Stem cells are capable of extensive self-renewal in the absence of differentiation. The maintenance of this undifferentiated state occurs despite the fact that this cell is exposed to a milieu that is rich in a variety of growth and differentiation factors. A unifying feature of such hematopoietic factors is that they mediate their effects through the phosphorylation of tyrosine residues by various cellular kinases. Therefore, one mechanism that might inhibit such differentiation signals in the self-renewing stem cell is the dephosphorylation of tyrosine residues by protein tyrosine phosphatases (PTPs). We have thus investigated the types of tyrosine phosphatases expressed by murine embryonic lin(lo)CD34hiSca(hi) hematopoietic progenitor cells by using a consensus polymerase chain reaction (PCR) approach. Although many known tyrosine phosphatases were detected using this method, a novel PTP related to the previously described PTP PEST type enzymes, murine PTP PEP and murine/human PTP PEST, was also observed. Cloning of the full-length cDNA encoding this enzyme showed that it was indeed a novel new member of this family, with an amino terminal tyrosine phosphatase domain followed by a region rich in serine, threonine, and proline. The carboxy terminus of this novel PTP contained a short sequence that was homologous to a region of the murine PTP PEP that was involved with nuclear localization. Bacterial expression of the phosphatase domain showed that this enzyme could efficiently dephosphorylate tyrosines in vitro. Analysis of the expression of the novel nuclear PTP by quantitative PCR showed that the transcript disappeared as the lin(lo)CD34hiSca(hi) cells differentiated in the presence of interleukin-1, interleukin-3, erythropoietin, and granulocyte-macrophage colony-stimulating factor. In agreement with its potential role in the hematopoietic progenitor cell, this novel PTP was expressed at a barely detectable level in a very limited subset of adult tissues. However, analysis of several murine hematopoietic progenitor cell lines, but not of a differentiated T-cell line, showed a high level of expression of the novel PTP. These data suggest that this novel phosphatase may play a critical role in the maintenance of the undifferentiated state of the hematopoietic stem cell.