A nuclear-mitochondrial DNA interaction affecting hearing impairment in mice

The pathophysiologic pathways and clinical expression of mitochondrial DNA (mtDNA) mutations are not well understood. This is mainly the result of the heteroplasmic nature of most pathogenic mtDNA mutations and of the absence of clinically relevant animal models with mtDNA mutations. mtDNA mutations predisposing to hearing impairment in humans are generally homoplasmic, yet some individuals with these mutations have severe hearing loss, whereas their maternal relatives with the identical mtDNA mutation have normal hearing. Epidemiologic, biochemical and genetic data indicate that nuclear genes are often the main determinants of these differences in phenotype. To identify a mouse model for maternally inherited hearing loss, we screened reciprocal backcrosses of three inbred mouse strains, A/J, NOD/LtJ and SKH2/J, with age-related hearing loss (AHL). In the (A/J x CAST/Ei) x A/J backcross, mtDNA derived from the A/J strain exerted a significant detrimental effect on hearing when compared with mtDNA from the CAST/Ei strain. This effect was not seen in the (NOD/LtJ x CAST/Ei) x NOD/LtJ and (SKH2/J x CAST/Ei) x SKH2/J backcrosses. Genotyping revealed that this effect was seen only in mice homozygous for the A/J allele at the Ahl locus on mouse chromosome 10. Sequencing of the mitochondrial genome in the three inbred strains revealed a single nucleotide insertion in the tRNA-Arg gene (mt-Tr) as the probable mediator of the mitochondrial effect. This is the first mouse model with a naturally occurring mtDNA mutation affecting a clinical phenotype, and it provides an experimental model to dissect the pathophysiologic processes connecting mtDNA mutations to hearing loss.     

The prolactin releasing peptides: RF-amide peptides

Although dopamine is considered the major hypothalamic controller of prolactin release from the anterior pituitary gland, there is evidence that a yet to be discovered prolactin releasing factor (PRF) also exists in brain. Recently, two peptides were isolated, products of the same prohormone, that were reported to have significant prolactin-releasing activity. These peptides, called prolactin releasing peptides, are not accepted by all investigators to be in fact PRFs. Instead, it appears that their widespread distribution in brain and the presence of receptors for the peptides in sites unrelated to neuroendocrine function are the basis for a variety of central nervous system action including activation of the autonomic nervous system. Thus, these peptides may not be PRFs, but instead neuroactive agents that are involved in many brain circuits with divergent functions.     

Role for sperm in spatial patterning of the early mouse embryo

Despite an apparent lack of determinants that specify cell fate, spatial patterning of the mouse embryo is evident early in development. The axis of the post-implantation egg cylinder can be traced back to organization of the pre-implantation blastocyst. This in turn reflects the organization of the cleavage-stage embryo and the animal-vegetal axis of the zygote. These findings suggest that the cleavage pattern of normal development may be involved in specifying the future embryonic axis; however, how and when this pattern becomes established is unclear. In many animal eggs, the sperm entry position provides a cue for embryonic patterning, but until now no such role has been found in mammals. Here we show that the sperm entry position predicts the plane of initial cleavage of the mouse egg and can define embryonic and abembryonic halves of the future blastocyst. In addition, the cell inheriting the sperm entry position acquires a division advantage and tends to cleave ahead of its sister. As cell identity reflects the timing of the early cleavages, these events together shape the blastocyst whose organization will become translated into axial patterning after implantation. We present a model for axial development that accommodates these findings with the regulative nature of mouse embryos.     

The core of the motor domain determines the direction of myosin movement

Myosins constitute a superfamily of at least 18 known classes of molecular motors that move along actin filaments. Myosins move towards the plus end of F-actin filaments; however, it was shown recently that a certain class of myosin, class VI myosin, moves towards the opposite end of F-actin, that is, in the minus direction. As there is a large, unique insertion in the myosin VI head domain between the motor domain and the light-chain-binding domain (the lever arm), it was thought that this insertion alters the angle of the lever-arm switch movement, thereby changing the direction of motility. Here we determine the direction of motility of chimaeric myosins that comprise the motor domain and the lever-arm domain (containing an insert) from myosins that have movement in the opposite direction. The results show that the motor core domain, but neither the large insert nor the converter domain, determines the direction of myosin motility.     

Regulatory defects in liver and intestine implicate abnormal hepcidin and Cybrd1 expression in mouse hemochromatosis

Individuals with hereditary hemochromatosis suffer from systemic iron overload due to duodenal hyperabsorption. Most cases arise from a founder mutation in HFE (845G-->A; ref. 2) that results in the amino-acid substitution C282Y and prevents the association of HFE with beta2-microglobulin. Mice homozygous with respect to a null allele of Hfe (Hfe-/-) or homozygous with respect to the orthologous 882G-->A mutation (Hfe(845A/845A)) develop iron overload that recapitulates hereditary hemochromatosis in humans, confirming that hereditary hemochromatosis arises from loss of HFE function. Much work has focused on an exclusive role for the intestine in hereditary hemochromatosis. HFE deficiency in intestinal crypt cells is thought to cause intestinal iron deficiency and greater expression of iron transporters such as SLC11A2 (also called DMT1, DCT1 and NRAMP2) and SLC11A3 (also called IREG1, ferroportin and MTP1; ref. 3). Published data on the expression of these transporters in the duodenum of HFE-deficient mice and humans are contradictory. In this report, we used a custom microarray to assay changes in duodenal and hepatic gene expression in Hfe-deficient mice. We found unexpected alterations in the expression of Slc39a1 (mouse ortholog of SLC11A3) and Cybrd1, which encode key iron transport proteins, and Hamp (hepcidin antimicrobial peptide), a hepatic regulator of iron transport. We propose that inappropriate regulatory cues from the liver underlie greater duodenal iron absorption, possibly involving the ferric reductase Cybrd1.     

A defective response to Hedgehog signaling in disorders of cholesterol biosynthesis

Smith-Lemli-Opitz syndrome (SLOS), desmosterolosis and lathosterolosis are human syndromes caused by defects in the final stages of cholesterol biosynthesis. Many of the developmental malformations in these syndromes occur in tissues and structures whose embryonic patterning depends on signaling by the Hedgehog (Hh) family of secreted proteins. Here we report that response to the Hh signal is compromised in mutant cells from mouse models of SLOS and lathosterolosis and in normal cells pharmacologically depleted of sterols. We show that decreasing levels of cellular sterols correlate with diminishing responsiveness to the Hh signal. This diminished response occurs at sterol levels sufficient for normal autoprocessing of Hh protein, which requires cholesterol as cofactor and covalent adduct. We further find that sterol depletion affects the activity of Smoothened (Smo), an essential component of the Hh signal transduction apparatus.     

A class of compact dwarf galaxies from disruptive processes in galaxy clusters

Dwarf galaxies have attracted increased attention in recent years, because of their susceptibility to galaxy transformation processes within rich galaxy clusters. Direct evidence for these processes, however, has been difficult to obtain, with a small number of diffuse light trails and intra-cluster stars being the only signs of galaxy disruption. Furthermore, our current knowledge of dwarf galaxy populations may be very incomplete, because traditional galaxy surveys are insensitive to extremely diffuse or compact galaxies. Aware of these concerns, we recently undertook an all-object survey of the Fornax galaxy cluster. This revealed a new population of compact members, overlooked in previous conventional surveys. Here we demonstrate that these 'ultra-compact' dwarf galaxies are structurally and dynamically distinct from both globular star clusters and known types of dwarf galaxy, and thus represent a new class of dwarf galaxy. Our data are consistent with the interpretation that these are the remnant nuclei of disrupted dwarf galaxies, making them an easily observed tracer of galaxy disruption.