Mlx, a new Max-like bHLHZip family member: the center stage of a novel transcription factors regulatory pathway?

The Myc proto-oncogene family members have been identified as the cellular homologs of the transforming oncogene of avian retroviruses. They encode central regulators of mammalian cell proliferation and apoptosis, and they associate with the bHLHZip protein Max to bind specific DNA sequences and regulate the expression of genes important for cell cycle progression. The other family members, Mad1, Mxi1, Mad3, Mad4 and Rox (Mnt) antagonize their activities. The Mads and Rox compete with Myc in heterodimerizing with Max and in binding to the same specific target sequences. These Mads:Max and Rox:Max dimers repress transcription through binding to the mSIN3 corepressor protein and by tethering histone deacetylase-containing complexes to the DNA. In a screen for Rox interactors we isolated Mlx, a bHLHZip protein previously identified in a screen for Mad1 interactors. In the present work we extend the known dimerization partners of Mlx by demonstrating its ability to interact with Rox. Moreover, we show that contrary to previous reports Mlx is able to homodimerize and to bind E-box sequences at low concentration levels. The possible role of Mlx in an emerging regulatory pathway and acting parallel to the Max driven network is discussed.

MID2, a homologue of the Opitz syndrome gene MID1: similarities in subcellular localization and differences in expression during development

The B-box family is an expanding new family of genes encoding proteins involved in diverse cellular functions such as developmental patterning and oncogenesis. A member of this protein family, MID1, is the gene responsible for the X-linked form of Opitz G/BBB syndrome, a developmental disorder characterized by defects of the midline structures. We now report the identification of MID2, a new transcript closely related to MID1. MID2 maps to Xq22 in human and to the syntenic region on the mouse X chromosome. The two X-linked genes share the same domains, the same exon-intron organization, a high degree of similarity at the protein level and the same subcellular localization, both being confined to the cytoplasm in association to micro-tubular structures. The expression pattern studied by RNA in situ hybridization in mouse revealed that Mid2 is expressed early in development and the highest level of expression is detected in the heart, unlike Mid1 for which no expression was detected in the developing heart. Together, these data suggest that midin and MID2 have a similar biochemical function but a different physiological role during development.

Functional characterization of the Opitz syndrome gene product (midin): evidence for homodimerization and association with microtubules throughout the cell cycle

Opitz syndrome (OS) is a multiple congenital anomaly manifested by abnormal closure of midline structures. The gene responsible for the X-linked form of this disease, MID1, encodes a protein (midin) that contains a RING, two B-boxes, a coiled-coil (the so-called tripartite motif) and an RFP-like domain. The tripartite motif is characteristic of a family of proteins, named the B-box family, involved in cell proliferation and development. Since the subcellular compartmentalization and the ability to form multiprotein structures both appear to be crucial for the function of this family of proteins, we have studied these properties on the wild-type and mutated forms of midin. We found that endogenous midin is associated with microtubules throughout the cell cycle, co-localizing with cytoplasmic fibres in interphase and with the mitotic spindle and midbodies during mitosis and cytokinesis. Immunoprecipitation experiments demonstrated the ability of the tripartite motif to mediate midin homodimerization, consistent with the evidence, obtained by gel filtration analysis, that midin exists in the form of large protein complexes. Functional characterization of altered forms of midin, resulting from mutations found in OS patients, revealed that association with microtubules is compromised, while the ability to homodimerize and form multiprotein complexes is retained. We suggest that midin is involved in the formation of multiprotein structures acting as anchor points to microtubules and that impaired association with these cytoskeletal structures causes OS developmental defects.

Rox, a novel bHLHZip protein expressed in quiescent cells that heterodimerizes with Max, binds a non-canonical E box and acts as a transcriptional repressor

Proteins of the Myc and Mad family are involved in transcriptional regulation and mediate cell differentiation and proliferation. These molecules share a basic-helix-loop-helix leucine zipper domain (bHLHZip) and bind DNA at the E box (CANNTG) consensus by forming heterodimers with Max. We report the isolation, characterization and mapping of a human gene and its mouse homolog encoding a new member of this family of proteins, named Rox. Through interaction mating and immunoprecipitation techniques, we demonstrate that Rox heterodimerizes with Max and weakly homodimerizes. Interestingly, bandshift assays demonstrate that the Rox-Max heterodimer shows a novel DNA binding specificity, having a higher affinity for the CACGCG site compared with the canonical E box CACGTG site. Transcriptional studies indicate that Rox represses transcription in both human HEK293 cells and yeast. We demonstrate that repression in yeast is through interaction between the N-terminus of the protein and the Sin3 co-repressor, as previously shown for the other Mad family members. ROX is highly expressed in quiescent fibroblasts and expression markedly decreases when cells enter the cell cycle. Moreover, ROX expression appears to be induced in U937 myeloid leukemia cells stimulated to differentiate with 12-O-tetradecanoylphorbol-13-acetate. The identification of a novel Max-interacting protein adds an important piece to the puzzle of Myc/Max/Mad coordinated action and function in normal and pathological situations. Furthermore, mapping of the human gene to chromosome 17p13.3 in a region that frequently undergoes loss of heterozygosity in a number of malignancies, together with the biochemical and expression features, suggest involvement of ROX in human neoplasia.

Characterization of a cluster of sulfatase genes on Xp22.3 suggests gene duplications in an ancestral pseudoautosomal region

An obligatory crossing-over event between the X and Y chromosomes in mammals occurs at each male meiosis within the 2.6 Mb of DNA defining the pseudoautosomal region (PAR). Genes located within or near the human PAR have homologous copies on the X and Y chromosomes, escape X inactivation and appear to be highly divergent throughout evolution. We have characterized the genomic structure of two genes from a recently identified cluster of sulfatase genes (ARSD and ARSE) located in the Xp22.3 region, and of their homologs on the Y chromosome. Our results indicate that the ARSD and ARSE genes from within this cluster have a conserved genomic organization, shared also by another Xp22.3 gene, STS, but completely different from that of all the other sulfatase genes. Sequence analysis of the Y-linked homologs indicate that they represent truncated pseudogenes. Sequence identity values between the X and Y copies of each gene is on average 91%, significantly higher than the values obtained by comparing different members of the family. FISH mapping experiments performed in several primate species revealed an identical localization of the X-linked copies to that in man, but different localizations of the Y homologs. Together, our data indicate that the cluster of sulfatase genes on human Xp22.3 was created through duplication events which probably occurred in an ancestral PAR, and support the view that the PAR has undergone multiple changes during recent mammalian evolution.

Enhanced neuropeptide Y release in the hippocampus is associated with chronic seizure susceptibility in kainic acid treated rats

We measured the release of neuropeptide Y (NPY) from hippocampal slices of rats at various times after limbic seizures induced by a subcutaneous injection of 12 mg/kg kainic acid (KA). Two days after KA, 100 mM KCl induced a 1.6 +/- 0.2-fold increase in NPY release compared to saline-injected rats (P < 0.05), while spontaneous and 50 mM KCl-induced release were unchanged. Thirty days after KA, the spontaneous and 100 mM KCl-induced efflux of NPY was enhanced 2-fold on average (P < 0.01) compared to controls, while no significant differences were found using 50 mM KCl. Tissue concentration of NPY was raised 2.2 +/- 0.2 times (P < 0.01) 30 days after KA. Thirty days after KA, the rats showed enhanced susceptibility to tonic-clonic seizures, assessed using a normally subconvulsive dose of pentylenetetrazol (PTZ; 30 mg/kg). A selective antibody (Ab) raised against NPY in a rabbit was infused bilaterally for three days in the CA3 area and dentate gyrus (DG) of the dorsal hippocampus of rats treated 30 days before with KA. This significantly reduced (P < 0.05) the number of animals with tonic-clonic seizures induced by 30 mg/kg PTZ, compared to KA treated rats which received the inactivated Ab. The Ab was ineffective in naive rats injected with a full convulsive dose of PTZ (55 mg/kg). The present results show that neuronal release of NPY is enhanced in the hippocampus after limbic seizures induced in rats by KA. This effect persists for at least 30 days and may contribute to the chronically enhanced susceptibility to seizures after injection of this toxin.