Infantile cerebral and cerebellar atrophy is associated with a mutation in the MED17 subunit of the transcription preinitiation mediator complex

Primary microcephaly of postnatal onset is a feature of many neurological disorders, mostly associated with mental retardation, seizures, and spasticity, and it typically carries a grave prognosis. Five infants from four unrelated families of Caucasus Jewish origin presented soon after birth with spasticity, epilepsy, and profound psychomotor retardation. Head circumference percentiles declined, and brain MRI disclosed marked cereberal and cerebellar atrophy with severe myelination defect. A search for a common homozygous region revealed a 2.28 Mb genomic segment on chromosome 11 that encompassed 16 protein-coding genes. A missense mutation in one of them, MED17, segregated with the disease state in the families and was carried by four of 79 anonymous Caucasus Jews. A corresponding mutation in the homologous S.cerevisiae gene SRB4 inactivated the protein, according to complementation assays. Screening of MED17 in additional patients with similar clinical and radiologic findings revealed four more patients, all homozygous for the p.L371P mutation and all originating from Caucasus Jewish families. We conclude that the p. L371P mutation in MED17 is a founder mutation in the Caucasus Jewish community and that homozygosity for this mutation is associated with infantile cerebral and cerebellar atrophy with poor myelination.     

Cooperative interactions at the SLP‐76 complex are critical for actin polymerization

T‐cell antigen receptor (TCR) engagement induces formation of multi‐protein signalling complexes essential for regulating T‐cell functions. Generation of a complex of SLP‐76, Nck and VAV1 is crucial for regulation of the actin machinery. We define the composition, stoichiometry and specificity of interactions in the SLP‐76, Nck and VAV1 complex. Our data reveal that this complex can contain one SLP‐76 molecule, two Nck and two VAV1 molecules. A direct interaction between Nck and VAV1 is mediated by binding between the C‐terminal SH3 domain of Nck and the VAV1 N‐terminal SH3 domain. Disruption of the VAV1:Nck interaction deleteriously affected actin polymerization. These novel findings shed new light on the mechanism of actin polymerization after T‐cell activation.     

HIV-1 Vpr-induced apoptosis is cell cycle dependent and requires Bax but not ANT

The HIV-1 accessory protein viral protein R (Vpr) causes G2 arrest and apoptosis in infected cells. We previously identified the DNA damage-signaling protein ATR as the cellular factor that mediates Vpr-induced G2 arrest and apoptosis. Here, we examine the mechanism of induction of apoptosis by Vpr and how it relates to induction of G2 arrest. We find that entry into G2 is a requirement for Vpr to induce apoptosis. We investigated the role of the mitochondrial permeability transition pore by knockdown of its essential component, the adenine nucleotide translocator. We found that Vpr-induced apoptosis was unaffected by knockdown of ANT. Instead, apoptosis is triggered through a different mitochondrial pore protein, Bax. In support of the idea that checkpoint activation and apoptosis induction are functionally linked, we show that Bax activation by Vpr was ablated when ATR or GADD45alpha was knocked down. Certain mutants of Vpr, such as R77Q and I74A, identified in long-term nonprogressors, have been proposed to inefficiently induce apoptosis while activating the G2 checkpoint in a normal manner. We tested the in vitro phenotypes of these mutants and found that their abilities to induce apoptosis and G2 arrest are indistinguishable from those of HIV-1NL4-3 vpr, providing additional support to the idea that G2 arrest and apoptosis induction are mechanistically linked.     

Unraveling functional and structural interactions between transmembrane domains IV and XI of NhaA Na+/H+ antiporter of Escherichia coli

A functionally important, interface domain between transmembrane segments (TMSs) IV and XI of the NhaA Na+/H+ antiporter of Escherichia coli has been unraveled. Scanning by single Cys replacements identified new mutations (F136C, G125C, and A137C) that cluster in one face of TMS IV and increase dramatically the Km of the antiporter. Whereas G125C, in addition, causes a drastic alkaline shift to the pH dependence of the antiporter, G338C alleviates the pH control of NhaA. Scanning by double Cys replacements (21 pairs of one replacement per TMS) identified genetically eight pairs of residues that showed very strong negative complementation. Cross-linking of the double mutants identified six double mutants (T132C/G338C, D133C/G338C, F136C/S342C, T132C/S342C, A137C/S342C, and A137C/G338C) of which pronounced intramolecular cross-linking defined an interface domain between the two TMSs. Remarkably, cross-linking by a short and rigid reagent (N,N'-o-phenylenedimaleimide) revived the Li+/H+ antiport activity, whereas a shorter reagent (1,2-ethanediyl bismethanethiosulfonate) revived both Na+/H+ and Li+/H+ antiporter activities and even the pH response of the dead mutant T132C/G338C. Hence, cross-linking at this position restores an active conformation of NhaA.     

Mitochondrial deoxyribonucleoside triphosphate pools in thymidine kinase 2 deficiency

Deficiency of mitochondrial thymidine kinase (TK2) is associated with mitochondrial DNA (mtDNA) depletion and manifests by severe skeletal myopathy in infancy. In order to elucidate the pathophysiology of this condition, mitochondrial deoxyribonucleoside triphosphate (dNTP) pools were determined in patients' fibroblasts. Despite normal mtDNA content and cytochrome c oxidase (COX) activity, mitochondrial dNTP pools were imbalanced. Specifically, deoxythymidine triphosphate (dTTP) content was markedly decreased, resulting in reduced dTTP:deoxycytidine triphosphate ratio. These findings underline the importance of balanced mitochondrial dNTP pools for mtDNA synthesis and may serve as the basis for future therapeutic interventions.     

Oocyte activation after intracytoplasmic injection with sperm frozen without cryoprotectants results in live offspring from inbred and hybrid mouse strains

Re-establishment of mouse strains used for mutagenesis and transgenesis has been hindered by difficulties in freezing sperm. The use of intracytoplasmic sperm injection (ICSI) enables the production of embryos for the restoration of mouse lines using sperm with reduced quality. By using ICSI, simplified sperm-freezing methods such as snap freezing can be explored. We examined the capacity of embryos from the inbred C57Bl/6J and 129Sv/ImJ mouse strains, commonly used for transgenic and N-ethyl-N-nitrosourea mutagenesis purposes to develop to blastocysts in vitro and to term following ICSI with sperm frozen without cryoprotectant. The results were compared to F1 (C57BlxCBA) hybrid embryos. Following freezing, sperm were immotile but could fertilize oocytes at similar rates to fresh sperm. However, embryo development in vitro to the blastocyst stage was reduced in all three strains. No pups were born from C57Bl/6J or 129Sv/ImJ embryos obtained from frozen sperm following transfer to foster females, and only a limited number of F1 embryos developed to term. Activation of oocytes injected with frozen sperm with 1.7 mM Sr2+ (SrCl2) did result in the birth of pups in all three strains. We conclude that the inability of sperm frozen without cryoprotectants to effectively activate oocytes may affect embryo development to term and can be overcome by strontium activation. This may become an effective strategy for sperm preservation and the restoration of most popular strains used for genetic modifications.     

Iron Uptake in Ustilago maydis: Tracking the Iron Path

In this study, we monitored and compared the uptake of iron in the fungus Ustilago maydis by using biomimetic siderophore analogs of ferrichrome, the fungal native siderophore, and ferrioxamine B (FOB), a xenosiderophore. Ferrichrome-iron was taken up at a higher rate than FOB-iron. Unlike ferrichrome-mediated uptake, FOB-mediated iron transport involved an extracellular reduction mechanism. By using fluorescently labeled siderophore analogs, we monitored the time course, as well as the localization, of iron uptake processes within the fungal cells. A fluorescently labeled ferrichrome analog, B9-lissamine rhodamine B, which does not exhibit fluorescence quenching upon iron binding, was used to monitor the entry of the compounds into the fungal cells. The fluorescence was found intracellularly 4 h after the application and later was found concentrated in two to three vesicles within each cell. The fluorescence of the fluorescently labeled FOB analog CAT18, which is quenched by iron, was visualized around the cell membrane after 4 h of incubation with the ferrated (nonfluorescent) compounds. This fluorescence intensity increased with time, demonstrating fungal iron uptake from the siderophores, which remained extracellular. We here introduce the use of fluorescent biomimetic siderophores as tools to directly track and discriminate between different pathways of iron uptake in cells.