IntroductionHematopoietic development is controlled by an intricate network of finely tuned transcriptional programs. Consequently, a perturbation of the transcription factors involved can block differentiation. This developmental roadblock cooperates with mutations in pathways that signal growth and/or survival to cause acute leukemias. 1 A prime example for such a mechanism is mixed lineage leukemia. In this disease, the gene for the histone methyltransferase MLL participates in chromosomal translocations that eventually create maturation-blocking and therefore leukemogenic MLL fusion proteins. 2,3 These protein chimeras consist of N-terminal portions of MLL joined to a variety of mostly unrelated fusion partners that replace the original MLL C-terminus, including the methyltransferase domain (http://atlasgeneticsoncology.org/Genes/ MLL.html). MLL fusion proteins are aberrant transcription factors that ectopically activate genes important for hematopoietic development like the abdominal-type Hox genes Hoxa7 and Hoxa9 and their dimerization partner Meis1. [4][5][6][7] Despite intensive study, little is known about the biological function of MLL fusion partners in normal cells, and it is mostly unclear how these proteins activate the oncogenic potential of MLL. In the rare cases where MLL is joined to a cytoplasmatic protein, domains introduced by the partner force a dimerization of the fusion that is crucial for oncogenic activity. 8,9 The overwhelming majority of leukemias with MLL rearrangement, however, involves nuclear proteins as translocation partners. The data available seem to indicate a role of some nuclear MLL partners in transcriptional control and histone modification. Support for this speculation comes from the detection of direct protein-protein interactions of the homologous MLL fusion partners AF4 and AF5q31 that both bind to ENL and the closely related AF9. 10,11 ENL in turn interacts with histone H3. 11 In addition, another MLL fusion partner, AF10, recruits the histone H3 lysine 79-specific methyltransferase DOT1L that introduces a dimethyl mark during transcriptional elongation. 12 The same modification is also a hallmark of genes activated by MLL-ENL. 13 Finally, the proteins CBX8 (chromobox 8) and BCoR (BCL6 corepressor) that are involved in chromatin-dependent gene repression have also been found to associate with ENL and AF9. [14][15][16] In order to learn more about the biological function of a classical MLL fusion partner, we identified proteins associated with the "Eleven Nineteen Leukemia" protein (ENL) originally discovered as an MLL fusion partner in the recurrent translocation t(11;19). Here, we describe the purification and analysis of ENL-associated proteins (EAPs) by tandem immunoprecipitation of ENL. This protein assembly contains several other MLL fusion partners, positive transcription elongation factor b (pTEFb), DOT1L, and polycomb group proteins. The composition of EAP suggests that ENL works in a new unit of transcriptional regulation that coordinates transcriptional elo...

Lymphatic vessels develop from specialized endothelial cells in preexisting blood vessels, but the molecular signals that regulate this separation are unknown. Here we identify a failure to separate emerging lymphatic vessels from blood vessels in mice lacking the hematopoietic signaling protein SLP-76 or Syk. Blood-lymphatic connections lead to embryonic hemorrhage and arteriovenous shunting. Expression of slp-76 could not be detected in endothelial cells, and blood-filled lymphatics also arose in wild-type mice reconstituted with SLP-76 -deficient bone marrow. These studies reveal a hematopoietic signaling pathway required for separation of the two major vascular networks in mammals.Mammals have two circulatory systems, a closed blood vasculature and an open lymphatic vasculature, that operate in parallel but develop in series (1,2). Although derived from venous endothelial precursors, lymphatic vessels do not communicate with blood vessels except at a single point where the thoracic duct empties into the subclavian vein (1-3). Recent studies have identified specific transcription factors and growth factors required to regulate the development † To whom correspondence should be addressed.

The structural integrity of the heart is maintained by the end-to-end connection between the myocytes called the intercalated disc. The intercalated disc contains different junctional complexes that enable the myocardium to function as a syncytium. One of the junctional complexes, the zonula adherens or adherens junction, consists of the cell adhesion molecule, N-cadherin, which mediates strong homophilic cell-cell adhesion via linkage to the actin cytoskeleton. To determine the function of N-cadherin in the working myocardium, we generated a conditional knockout containing loxP sites flanking exon 1 of the N-cadherin (Cdh2) gene. Using a cardiac-specific tamoxifen-inducible Cre transgene, N-cadherin was deleted in the adult myocardium. Loss of N-cadherin resulted in disassembly of the intercalated disc structure, including adherens junctions and desmosomes. The mutant mice exhibited modest dilated cardiomyopathy and impaired cardiac function, with most animals dying within two months after tamoxifen administration. Decreased sarcomere length and increased Z-line thickness were observed in the mutant hearts consistent with loss of muscle tension because N-cadherin was no longer available to anchor myofibrils at the plasma membrane. Ambulatory electrocardiogram monitoring captured the abrupt onset of spontaneous ventricular tachycardia, confirming that the deaths were arrhythmic in nature. A significant decrease in the gap junction protein, connexin 43, was observed in the N-cadherin-depleted hearts. This animal model provides the first demonstration of the hierarchical relationship of the structural components of the intercalated disc in the working myocardium, thus establishing N-cadherin's paramount importance in maintaining the structural integrity of the heart.

Stem cell-based cellular cardiomyoplasty represents a promising therapy for myocardial infarction. Noninvasive imaging techniques would allow the evaluation of survival, migration, and differentiation status of implanted stem cells in the same subject over time. This review describes methods for cell visualization using several corresponding noninvasive imaging modalities, including magnetic resonance imaging, positron emission tomography, single-photon emission computed tomography, and bioluminescent imaging. Reporter-based cell visualization is compared with direct cell labeling for short- and long-term cell tracking.