Mutations in the gene encoding the lamin B receptor produce an altered nuclear morphology in granulocytes (Pelger-Huët anomaly)

Pelger-Hu?t anomaly (PHA; OMIM *169400) is an autosomal dominant disorder characterized by abnormal nuclear shape and chromatin organization in blood granulocytes. Affected individuals show hypolobulated neutrophil nuclei with coarse chromatin. Presumed homozygous individuals have ovoid neutrophil nuclei, as well as varying degrees of developmental delay, epilepsy and skeletal abnormalities. Homozygous offspring in an extinct rabbit lineage showed severe chondrodystrophy, developmental anomalies and increased pre- and postnatal mortality. Here we show, by carrying out a genome-wide linkage scan, that PHA is linked to chromosome 1q41-43. We identified four splice-site, two frameshift and two nonsense mutations in LBR, encoding the lamin B receptor. The lamin B receptor (LBR), a member of the sterol reductase family, is evolutionarily conserved and integral to the inner nuclear membrane; it targets heterochromatin and lamins to the nuclear membrane. Lymphoblastoid cells from heterozygous individuals affected with PHA show reduced expression of the lamin B receptor, and cells homozygous with respect to PHA contain only trace amounts of it. We found that expression of the lamin B receptor affects neutrophil nuclear shape and chromatin distribution in a dose-dependent manner. Our findings have implications for understanding nuclear envelope-heterochromatin interactions, the pathogenesis of Pelger-like conditions in leukemia, infection and toxic drug reactions, and the evolution of neutrophil nuclear shape.     

Identification of SLC39A4, a gene involved in acrodermatitis enteropathica

We have characterized the human gene SLC39A4, which encodes a protein with features characteristic of a ZIP zinc transporter. The chromosomal location and expression of SLC39A4, together with mutational analysis of eight families affected with acrodermatitis enteropathica, suggest that SLC39A4 is centrally involved in the pathogenesis of this condition.     

A bisexually reproducing all-triploid vertebrate

Green toads are common in the Palaearctic region, where they have differentiated into several taxa. The toads exist with variable amounts of ploidy, similar to other anuran species or reptiles. In vertebrate biology, the very rare occurrence of triploidy is coupled with infertility or unisexuality, or requires the coexistence of individuals of different ploidy in a reproductive community. The reproduction of naturally occurring triploids has been reported to occur only through parthenogenesis, gynogenesis or hybridogenesis. The bisexual reproduction of pure triploids has been considered to be impossible because of the problem of equally distributing three chromosome sets in meiosis. Here we report geographically isolated populations of green toads (Bufo viridis complex) that are all-triploid and reproduce bisexually.     

Involvement of chemokine receptors in breast cancer metastasis

Breast cancer is characterized by a distinct metastatic pattern involving the regional lymph nodes, bone marrow, lung and liver. Tumour cell migration and metastasis share many similarities with leukocyte trafficking, which is critically regulated by chemokines and their receptors. Here we report that the chemokine receptors CXCR4 and CCR7 are highly expressed in human breast cancer cells, malignant breast tumours and metastases. Their respective ligands CXCL12/SDF-1alpha and CCL21/6Ckine exhibit peak levels of expression in organs representing the first destinations of breast cancer metastasis. In breast cancer cells, signalling through CXCR4 or CCR7 mediates actin polymerization and pseudopodia formation, and subsequently induces chemotactic and invasive responses. In vivo, neutralizing the interactions of CXCL12/CXCR4 significantly impairs metastasis of breast cancer cells to regional lymph nodes and lung. Malignant melanoma, which has a similar metastatic pattern as breast cancer but also a high incidence of skin metastases, shows high expression levels of CCR10 in addition to CXCR4 and CCR7. Our findings indicate that chemokines and their receptors have a critical role in determining the metastatic destination of tumour cells.     

Trans-complex formation by proteolipid channels in the terminal phase of membrane fusion

SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) and Rab-GTPases, together with their cofactors, mediate the attachment step in the membrane fusion of vesicles. But how bilayer mixing--the subsequent core process of fusion--is catalysed remains unclear. Ca2+/calmodulin controls this terminal process in many intracellular fusion events. Here we identify V0, the membrane-integral sector of the vacuolar H+-ATPase, as a target of calmodulin on yeast vacuoles. Between docking and bilayer fusion, V0 sectors from opposing membranes form complexes. V0 trans-complex formation occurs downstream from trans-SNARE pairing, and depends on both the Rab-GTPase Ypt7 and calmodulin. The maintenance of existing complexes and completion of fusion are independent of trans-SNARE pairs. Reconstituted proteolipids form sealed channels, which can expand to form aqueous pores in a Ca2+/calmodulin-dependent fashion. V0 trans-complexes may therefore form a continuous, proteolipid-lined channel at the fusion site. We propose that radial expansion of such a protein pore may be a mechanism for intracellular membrane fusion.     

The bacterial conjugation protein TrwB resembles ring helicases and F1-ATPase

The transfer of DNA across membranes and between cells is a central biological process; however, its molecular mechanism remains unknown. In prokaryotes, trans-membrane passage by bacterial conjugation, is the main route for horizontal gene transfer. It is the means for rapid acquisition of new genetic information, including antibiotic resistance by pathogens. Trans-kingdom gene transfer from bacteria to plants or fungi and even bacterial sporulation are special cases of conjugation. An integral membrane DNA-binding protein, called TrwB in the Escherichia coli R388 conjugative system, is essential for the conjugation process. This large multimeric protein is responsible for recruiting the relaxosome DNA-protein complex, and participates in the transfer of a single DNA strand during cell mating. Here we report the three-dimensional structure of a soluble variant of TrwB. The molecule consists of two domains: a nucleotide-binding domain of alpha/beta topology, reminiscent of RecA and DNA ring helicases, and an all-alpha domain. Six equivalent protein monomers associate to form an almost spherical quaternary structure that is strikingly similar to F1-ATPase. A central channel, 20 A in width, traverses the hexamer.     

Chromatic sensitivity of ganglion cells in the peripheral primate retina

Visual abilities change over the visual field. For example, our ability to detect movement is better in peripheral vision than in foveal vision, but colour discrimination is markedly worse. The deterioration of colour vision has been attributed to reduced colour specificity in cells of the midget, parvocellular (PC) visual pathway in the peripheral retina. We have measured the colour specificity (red-green chromatic modulation sensitivity) of PC cells at eccentricities between 20 and 50 degrees in the macaque retina. Here we show that most peripheral PC cells have red-green modulation sensitivity close to that of foveal PC cells. This result is incompatible with the view that PC pathway cells in peripheral retina make indiscriminate connections ('random wiring') with retinal circuits devoted to different spectral types of cone photoreceptors. We show that selective cone connections can be maintained by dendritic field anisotropy, consistent with the morphology of PC cell dendritic fields in peripheral retina. Our results also imply that postretinal mechanisms contribute to the psychophysically demonstrated deterioration of colour discrimination in the peripheral visual field.     

An agrin minigene rescues dystrophic symptoms in a mouse model for congenital muscular dystrophy

Congenital muscular dystrophy is a heterogeneous and severe, progressive muscle-wasting disease that frequently leads to death in early childhood. Most cases of congenital muscular dystrophy are caused by mutations in LAMA2, the gene encoding the alpha2 chain of the main laminin isoforms expressed by muscle fibres. Muscle fibre deterioration in this disease is thought to be caused by the failure to form the primary laminin scaffold, which is necessary for basement membrane structure, and the missing interaction between muscle basement membrane and the dystrophin-glycoprotein complex (DGC) or the integrins. With the aim to restore muscle function in a mouse model for this disease, we have designed a minigene of agrin, a protein known for its role in the formation of the neuromuscular junction. Here we show that this mini-agrin-which binds to basement membrane and to alpha-dystroglycan, a member of the DGC-amends muscle pathology by a mechanism that includes agrin-mediated stabilization of alpha-dystroglycan and the laminin alpha5 chain. Our data provides in vivo evidence that a non-homologous protein in combination with rational protein design can be used to devise therapeutic tools that may restore muscle function in human muscular dystrophies.