Homeodomain proteins of the Meis subfamily are expressed dynamically in several organs during embryogenesis and exert potent regulatory activity through their interaction with Hox proteins and other transcription factors. Here we show that Meis1 is expressed in the hematopoietic stem cell (HSC) compartment in the fetal liver, and in the primary sites of definitive hematopoiesis, including the aorta-gonad-mesonephros (AGM) mesenchyme, the hemogenic embryonic arterial endothelium, and hematopoietic clusters within the aorta, vitelline, and umbilical arteries. We inactivated the Meis1 gene in mice and found that Meis1 mutant mice die between embryonic days 11.5 and 14.5, showing internal hemorrhage, liver hypoplasia, and anemia. In Meis1 mutant mouse fetal liver and AGM, HSC compartments are severely underdeveloped and colony-forming potential is profoundly impaired. AGM mesenchymal cells expressing Runx1, an essential factor for definitive HSC specification, are almost absent in mutant mice. In addition, hematopoietic clusters in the dorsal aorta, vitelline, and umbilical arteries are reduced in size and number. These results show a requirement for Meis1 in the establishment of definitive hematopoiesis in the mouse embryo. Meis1 mutant mice also displayed complete agenesis of the megakaryocyte lineage and localized defects in vascular patterning, which may cause the hemorrhagic phenotype.

Proteolysis is central to the regulation of a wide variety of physiological and pathological processes. The matrix metalloproteinases (MMP) 1 constitute an endopeptidase family that includes collagenases, gelatinases, stromelysins, and membrane-type MMP, with a broad spectrum of proteolytic activities toward extracellular matrix (ECM) components (1-3). The proteolytic activity of the matrix metalloproteinases is controlled by their expression as proenzymes that are processed to active forms through proteolysis, as well as by specific physiological tissue inhibitors (TIMP). MMP are believed to mediate many biological processes in which tissue remodeling is implicated, such as embryo implantation and morphogenesis, cell migration, metastasis, tumor invasion, and wound healing (3). Human stromelysin-3 (hST-3, MMP-11) was first described in fibroblasts surrounding neoplastic cells in both primary and metastatic breast tumors, and classified as an MMP on the basis of sequence homology (4). High ST-3 expression has been correlated with increased local tumor aggressiveness (5), and high ST-3 RNA levels are predictive of recurrence in breast carcinoma (6). Recent evidence suggests that hST-3 expression promotes tumor formation in nude mice (7). hST-3 may thus represent a local factor contributing to tumor cell survival and implantation by ECM remodeling. Putative mature forms of hST-3 nevertheless appear unable to degrade any major ECM component (8,9). hST-3 thus may not be considered an ECM degrading enzyme.MMP proteolytic activity on substrates other than matrix components have been reported; shedding activities on tumor necrosis factor-␣ (10, 11), Fas ligand (12), and L-selectin (13) have been ascribed to MMP. Several insulin-like growth factorbinding proteins (IGFBP) have been described as MMP substrates. MMP-1, MMP-2, and MMP-3 degrade and , and TIMP-1 inhibits proteolytic cleavage of IGFBP-3 in rat pregnant serum (16). IGFBP proteolysis may represent a mechanism for tissue-specific regulation of IGF bioavailability, either inhibiting and/or enhancing IGF activity in many cell types (17)(18)(19)(20).Identification of new hST-3 substrates is a necessary step for the understanding of its physiological relevance. To date, hST-3 proteolytic activity has been described only for the nonspecific MMP substrates ␤-casein and ␣ 2 -macroglobulin. When physiologically relevant substrates were sought among tumor cell line-derived secretory products, the unique major hST-3 target molecule was ␣1-protease inhibitor (8). Recent evidence suggests that hST-3 in vivo may contribute to tumor cell survival rather than to tumor invasion (7); this effect on cell viability may thus be mediated by regulating the activity of survival factors such as IGF-I or -II. As a part of a larger study of the relationship between hST-3 and IGF axis on tumor cell proliferation and survival, we tested whether hST-3 might act as an IGFBP-1 protease, thus controlling IGF-I activity at the cellular level.Our data show that (i) IGFBP-1 is a substrate for hS...

Telomere length must be tightly regulated in highly proliferative tissues, such as the lymphohematopoietic system. Under steady-state conditions, the levels and functionality of hematopoietic-committed or multipotent progenitors were not affected in late-generation telomerase-deficient mice (mTerc ؊/؊ ) with critically short telomeres. Evaluation of self-renewal potential of mTerc ؊/؊ day-12 spleen colonyforming units demonstrated no alteration as compared with wildtype progenitors. However, the replating ability of mTerc ؊/؊ granulocyte-macrophage CFUs (CFU-GMs) was greatly reduced as compared with wildtype CFU-GMs, indicating a diminished capacity of late-generation mTerc ؊ IntroductionEukaryotic chromosomes are capped by a special structure, the telomere, that in all vertebrates consists of tandem repeats of the DNA sequence TTAGGG and of associated proteins. Telomeres guarantee chromosome integrity by preventing illegitimate recombination, degradation, and end fusions. 1,2 Telomere shortening occurs in each replication cycle and is proposed to mediate replicative senescence in human cells in culture, as well as the aging process. 3,4 Telomere maintenance involves a ribonucleoprotein with reverse-transcriptase activity, called telomerase. 5,6 Telomerase is active during human embryonic development and is downregulated immediately after birth. 7,8 In adults, most normal somatic cells lack detectable telomerase activity, whereas cells from germline tissues and most tumors express high levels of telomerase activity. 9 Telomerase activity is also detected in normal human somatic tissues containing cells with self-renewal capacity, such as those of the lymphohematopoietic system 10 and the skin epithelium. 11 Hematopoiesis requires self-renewal of stem cells, as well as proliferation and differentiation of the committed progenitors. This process demands an extraordinary replicative capacity in certain cell types, especially those of the immune system. Telomeres in blood cells from bone marrow (BM) transplant recipients are shorter than those in cells from the BM donor, 12,13 suggesting that the additional cell divisions in the stem cell compartment required for BM regeneration result in a measurable decline in telomere length. Analysis of human BM cells showed that, in vitro, telomerase activity is repressed in quiescent stem cells, expressed at low levels in cycling stem cells, and up-regulated following cytokine stimulation. 10,14,15 Moreover, cytokine-induced differentiation of CD34 ϩ cells results in a decrease in telomerase activity. 16 In murine fetal liver and adult BM, results based on single-cell analysis 17 showed that the majority of long-term reconstituting BM hematopoietic stem cells (HSCs) and transiently self-renewing multipotent progenitors exhibit telomerase activity.Mice genetically deficient for the mouse telomerase RNA (mTerc) gene lack telomerase activity and show telomere shortening at a rate of 4 to 5 kilobases (kb) per mouse generation. 7,18 This shortening is accompanied by an increase in...