In animal societies, chemical communication plays an important role in conflict and cooperation. For ants, cuticular hydrocarbon (CHC) blends produced by non-nestmates elicit overt aggression. We describe a sensory sensillum on the antennae of the carpenter ant Camponotus japonicus that functions in nestmate discrimination. This sensillum is multiporous and responds only to non-nestmate CHC blends. This suggests a role for a peripheral recognition mechanism in detecting colony-specific chemical signals.

Summary The genetic programs that promote retention of self-renewing leukemia stem cells (LSCs) at the apex of cellular hierarchies in acute myeloid leukemia (AML) are not known. In a mouse model of human AML, LSCs exhibit variable frequencies that correlate with the initiating MLL oncogene and are maintained in a self-renewing state by a transcriptional sub-program more akin to that of embryonic stem cells (ESCs) than adult stem cells. The transcription/chromatin regulatory factors Myb, Hmgb3 and Cbx5 are critical components of the program and suffice for Hoxa/Meis-independent immortalization of myeloid progenitors when co-expressed, establishing the cooperative and essential role of an ESC-like LSC maintenance program ancillary to the leukemia initiating MLL/Hox/Meis program. Enriched expression of LSC maintenance and ESC-like program genes in normal myeloid progenitors and poor prognosis human malignancies links the frequency of aberrantly self-renewing progenitor-like cancer stem cells to prognosis in human cancer.

Oncogenic mutations of the MLL histone methyltransferase confer an unusual ability to transform non-self-renewing myeloid progenitors into leukemia stem cells (LSCs) by mechanisms that remain poorly defined. Misregulation of Hox genes is likely to be critical for LSC induction and maintenance but alone it does not recapitulate the phenotype and biology of MLL leukemias, which are clinically heterogeneouspresumably reflecting differences in LSC biology and/or frequency. TALE (three-amino-acid loop extension) class homeodomain proteins of the Pbx and Meis families are also misexpressed in this context, and we thus employed knockout, knockdown, and dominant-negative genetic techniques to investigate the requirements and contributions of these factors in MLL oncoprotein-induced acute myeloid leukemia. Our studies show that induction and maintenance of MLL transformation requires Meis1 and is codependent on the redundant contributions of Pbx2 and Pbx3. Meis1 in particular serves a major role in establishing LSC potential, and determines LSC frequency by quantitatively regulating the extent of self-renewal, differentiation arrest, and cycling, as well as the rate of in vivo LSC generation from myeloid progenitors. Thus, TALE proteins are critical downstream effectors within an essential homeoprotein network that serves a rate-limiting regulatory role in MLL leukemogenesis.[Keywords: Leukemia stem cells; MLL; Meis1; Pbx; TALE homeodomain proteins] Supplemental material is available at http://www.genesdev.org.

Despite advances in defining the critical molecular determinants for leukemia stem cell (LSC) generation and maintenance, little is known about the roles of microRNAs in LSC biology. Here, we identify microRNAs that are differentially expressed in LSC-enriched cell fractions (c-kit + ) in a mouse model of MLL leukemia.Members of the miR-17 family were notably more abundant in LSCs compared with their normal counterpart granulocyte-macrophage progenitors and myeloblast precursors. Expression of miR-17 family microRNAs was substantially reduced concomitant with leukemia cell differentiation and loss of self-renewal, whereas forced expression of a polycistron construct encoding miR-17-19b miRNAs significantly shortened the latency for MLL leukemia development. Leukemias expressing increased levels of the miR-17-19b construct displayed a higher frequency of LSCs, more stringent block of differentiation, and enhanced proliferation associated with reduced expression of p21, a cyclin-dependent kinase inhibitor previously implicated as a direct target of miR-17 microRNAs. Knockdown of p21 in MLL-transformed cells phenocopied the overexpression of the miR-17 polycistron, including a significant decrease in leukemia latency, validating p21 as a biologically relevant and direct in vivo target of the miR-17 polycistron in MLL leukemia. Expression of c-myc, a crucial upstream regulator of the miR-17 polycistron, correlated with miR-17-92 levels, enhanced self-renewal, and LSC potential. Thus, microRNAs quantitatively regulate LSC self-renewal in MLL-associated leukemia in part by modulating the expression of p21, a known regulator of normal stem cell function. Cancer Res; 70(9); 3833-42. ©2010 AACR.