Characterization of myocyte enhancer factor 2 (MEF2) expression in B and T cells: MEF2C is a B cell-restricted transcription factor in lymphocytes.

Our studies examined the expression and DNA binding activity of myocyte enhancer factor 2 (MEF2A-D) transcription factors in lymphopoietic tissues, cell lines, and primary lymphocytes. Our analyses demonstrate that mef2C expression is restricted to B cells within the lymphocyte lineage. Using in situ hybridization, mef2C is detected in foci in fetal liver and postnatal thymic medulla, and both mef2B and mef2C are expressed in areas of the postnatal spleen and lymph node that also express kappa light chain (Ckappa), a B cell-specific marker. Reverse transcriptase-PCR (RT-PCR) analyses demonstrate that all mef2 family members are expressed in B cell lines, and all except mef2C are expressed in T cell lines. Immunoblot analyses of cell lines and primary thymic and splenic lymphocytes show that MEF2C and MEF2D proteins are expressed in B cells and that MEF2D is expressed in T cells; however, MEF2A protein is not detected in lymphocytes. Electrophoretic mobility shift assays (EMSA) demonstrate that B cell lines have MEF2C-containing, MEF2-specific DNA binding complexes whereas T cells do not. Our data is the first to describe mef2C expression in the lymphocyte lineage, and this finding suggests possible roles for MEF2C activity in B cell development and function.

A comparison between improvers and non-improvers among children with anemia enrolled in the WIC program.

OBJECTIVES: To investigate differences between children ages one to five enrolled in the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) in the state of Texas whose anemia status improved and those whose anemia status did not improve. METHODS: The study involved administration of a survey to a randomly selected stratified sample of parents and guardians of WIC children residing in the eleven public health regions in Texas. Subjects volunteered to complete the survey during a scheduled WIC appointment. A total of 459 usable surveys were completed. Fifty-one percent of the surveys were completed by parents/guardians of children classified as anemia improvers and 49% were completed by parents/guardians of children classified as anemia non-improvers. RESULTS AND CONCLUSIONS: A majority of the respondents were the child's parent (97%). Most of the children were Hispanic/Latino (77%). More non-improvers than improvers were of young age (p < 0.01), had a parent/guardian who was 18-30 years of age (p < 0.01), lived in a household with four or more other children (p = 0.05) and were from households with a very low income (p = 0.03). Compared to improvers, non-improvers ate fewer snacks (p = 0.01) and a greater percentage never consumed dried fruits (p < 0.01). Knowledge level of the parents/guardians was similar for the two groups. A high percentage of parents/guardians of non-improvers incorrectly believed that their child's condition had improved. WIC staff and other public health educators can utilize these findings to identify those children with anemia who are at higher risk for non-improvement.

Transformation of undifferentiated Thy-1lo B220+ thymic lymphoid cells by the Abelson murine leukemia virus.

Intrathymic injection of the Abelson murine leukemia virus (A-MuLV) results in transformation of immature T and B lymphoid cells. In this report we demonstrate that the concentration of A-MuLV injected into murine thymi influences the selection of the transformation target. Thus, concentrated A-MuLV gives rise to Thy-1+ B220- thymomas. In contrast, dilute virus induces B220+ thymomas that also express low levels of Thy-1 (Thy-1lo), a phenotype that is similar to marrow-derived progenitor B-lymphoid cells (pro-B cells) that are highly susceptible to A-MuLV transformation in vitro. However, rare B220+ lymphoid cells isolated from normal adult thymi were not transformed by A-MuLV in vitro, while B220+ cells isolated from bone marrow were highly susceptible to transformation by A-MuLV. The Thy-1lo B220+ population in the primary thymomas had not rearranged TCRgamma, TCRbeta, or Igkappa genes, but contained subpopulations that assembled Ig DJ(H) or VDJ(H) genes and were therefore similar to transformed pro- and pre-B cells obtained from A-MuLV infected fetal liver and adult bone marrow, respectively. However, unlike A-MuLV-transformed pro- and pre-B cells, many (40-70%) of the Thy-1lo B220+ transformed thymoma cells had not rearranged Igh genes, and therefore appear to represent undifferentiated lymphoid cells. We conclude that A-MuLV may transform an undifferentiated lymphoid target in the thymus.

Mist1: a novel basic helix-loop-helix transcription factor exhibits a developmentally regulated expression pattern.

Basic helix-loop-helix (bHLH) proteins often belong to a family of transcription factors that bind to the DNA target sequence -CANNTG- (E-box) that is present in the promoter or enhancer regions of numerous developmentally regulated genes. In this study, we report the isolation and initial characterization of a novel bHLH factor, termed Mist1, that was identified by virtue of its ability to interact with E-box regulatory elements in a yeast "one-hybrid" screening procedure. Northern analysis revealed that Mist1 transcripts are expressed in several adult tissues, including stomach, liver, lung, and spleen but no expression is detected in the heart, brain, kidney, or testis. During mouse embryogenesis, Mist1 mRNA is first observed at E10.5 in the primitive gut and in the developing lung bud. Expression persists through E16.5 and remains restricted primarily to the epithelial lining. Mist1 also is detected in skeletal muscle tissues beginning at E12.5, persisting throughout all embryonic stages examined although in older embryos and in the adult expression becomes severely reduced. At later developmental times, Mist1 transcripts also are found in the pancreas, submandibular gland, and adult spleen. As predicted, the Mist1 protein is nuclear and binds efficiently to E-box sites as a homodimer. Mist1 also is capable of binding to E-box elements when complexed as a heterodimer with the widely expressed E-proteins, E12 and E47. Surprisingly, although Mist1 binds to E-boxes in vivo, the Mist1 protein lacks a functional transcription activation domain. These observations suggest that Mist1 may function as a unique regulator of gene expression in several different embryonic and postnatal cell lineages.

In vitro preselection of gene-trapped embryonic stem cell clones for characterizing novel developmentally regulated genes in the mouse.

We have developed an in vitro gene trap screen for novel murine genes that allows one to determine, prior to making chimeric or transgenic animals, if these genes are expressed in one or more specific embryonic tissues. Totipotent embryonic stem (ES) cells are infected with a retroviral gene trap construct encoding a selectable lacZ/neo fusion gene, which is expressed only if the gene trap inserts within an active transcription unit. G418-resistant ES cell clones are induced to differentiate in vitro, and neurons, glia, myocytes, and chondrocytes are screened for expression of beta-galactosidase (beta-gal). cDNAs of the gene trap transcripts are obtained by 5' rapid amplification of cDNA ends and are sequenced to determine if they represent novel genes. In situ hybridization analyses show that trapped genes are expressed in vivo within the cell types that express beta-gal in vitro. Gene traps and their wild-type alleles are characterized in terms of copy number, alternate splicing of their transcripts, and the proportion of endogenous mRNA sequence that is replaced by lacZ/neo in the hybrid gene trap transcript. This approach, which we term "in vitro preselection," is more economical than standard in vivo gene trap screening because tissue-specific expression of probable knockout alleles is verified before transgenic animals are generated. These results also highlight the utility of ES cell differentiation in vitro as a method with which to study the molecular mechanisms regulating the specification and commitment of a variety of cell and tissue types.

Mrj encodes a DnaJ-related co-chaperone that is essential for murine placental development.

We have identified a novel gene in a gene trap screen that encodes a protein related to the DnaJ co-chaperone in E. coli. The gene, named Mrj (mammalian relative of DnaJ) was expressed throughout development in both the embryo and placenta. Within the placenta, expression was particularly high in trophoblast giant cells but moderate levels were also observed in trophoblast cells of the chorion at embryonic day 8.5, and later in the labyrinth which arises from the attachment of the chorion to the allantois (a process called chorioallantoic fusion). Insertion of the ROSAbetageo gene trap vector into the Mrj gene created a null allele. Homozygous Mrj mutants died at mid-gestation due to a failure of chorioallantoic fusion at embryonic day 8.5, which precluded formation of the mature placenta. At embryonic day 8.5, the chorion in mutants was morphologically normal and expressed the cell adhesion molecule beta4 integrin that is known to be required for chorioallantoic fusion. However, expression of the chorionic trophoblast-specific transcription factor genes Err2 and Gcm1 was significantly reduced. The mutants showed no abnormal phenotypes in other trophoblast cell types or in the embryo proper. This study indicates a previously unsuspected role for chaperone proteins in placental development and represents the first genetic analysis of DnaJ-related protein function in higher eukaryotes. Based on a survey of EST databases representing different mouse tissues and embryonic stages, there are 40 or more DnaJ-related genes in mammals. In addition to Mrj, at least two of these genes are also expressed in the developing mouse placenta. The specificity of the developmental defect in Mrj mutants suggests that each of these genes may have unique tissue and cellular activities.