PAXX and Xlf interplay revealed by impaired CNS development and immunodeficiency of double KO mice.

The repair of DNA double-stranded breaks (DNAdsb) through non-homologous end joining (NHEJ) is a prerequisite for the proper development of the central nervous system and the adaptive immune system. Yet, mice with Xlf or PAXX loss of function are viable and present with very mild immune phenotypes, although their lymphoid cells are sensitive to ionizing radiation attesting for the role of these factors in NHEJ. In contrast, we show here that mice defective for both Xlf and PAXX are embryonically lethal owing to a massive apoptosis of post-mitotic neurons, a situation reminiscent to XRCC4 or DNA Ligase IV KO conditions. The development of the adaptive immune system in Xlf-/-PAXX-/- E18.5 embryos is severely affected with the block of B- and T-cell maturation at the stage of IgH and TCRß gene rearrangements, respectively. This damaging phenotype highlights the functional nexus between Xlf and PAXX, which is critical for the completion of NHEJ-dependent mechanisms during mouse development.

Transport activities and expression patterns of glycine transporters 1 and 2 in the developing murine brain stem and spinal cord.

Glycine serves as a neurotransmitter in spinal cord and brain stem, where it activates inhibitory glycine receptors. In addition, it serves as an essential co-agonist of excitatory N-methyl-d-aspartate receptors. In the central nervous system, extracellular glycine concentrations are regulated by two specific glycine transporters (GlyTs), GlyT1 and GlyT2. Here, we determined the relative transport activities and protein levels of GlyT1 and GlyT2 in membrane preparations from mouse brain stem and spinal cord at different developmental stages. We report that early postnatally (up to postnatal day P5) GlyT1 is the predominant transporter isoform responsible for a major fraction of the GlyT-mediated [(3)H]glycine uptake. At later stages (≥ P10), however, the transport activity and expression of GlyT2 increases, and in membrane fractions from adult mice both GlyTs contribute about equally to glycine uptake. These alterations in the activities and expression profiles of the GlyTs suggest that the contributions of GlyT1 and GlyT2 to the regulation of extracellular glycine concentrations at glycinergic synapses changes during development.

Glutamate residue 90 in the predicted transmembrane domain 2 is crucial for cation flux through channelrhodopsin 2.

Channelrhodopsin 2 (ChR2) is a microbial-type rhodopsin with a putative heptahelical structure that binds all-trans-retinal. Blue light illumination of ChR2 activates an intrinsic leak channel conductive for cations. Sequence comparison of ChR2 with the related ChR1 protein revealed a cluster of charged amino acids within the predicted transmembrane domain 2 (TM2), which includes glutamates E90, E97 and E101. Charge inversion substitutions of these residues significantly altered ChR2 function as revealed by two-electrode voltage-clamp recordings of light-induced currents from Xenopus laevis oocytes expressing the respective mutant proteins. Specifically, replacement of E90 by lysine or alanine resulted in differential effects on H(+)- and Na(+)-mediated currents. Our results are consistent with this glutamate side chain within the proposed TM2 contributing to ion flux through and the cation selectivity of ChR2.