A Point Mutation in Translation Initiation Factor 2B Leads to a Continuous Hyper Stress State in Oligodendroglial-Derived Cells

Background

Mutations in eukaryotic translation initiation factor 2B (eIF2B) cause Childhood Ataxia with CNS Hypomyelination (CACH), also known as Vanishing White Matter disease (VWM). The disease is manifested by loss of brain myelin upon physiological stress. In a previous study, we showed that fibroblasts isolated from CACH/VWM patients are hypersensitive to pharmacologically-induced endoplasmic reticulum (ER) stress. Since brain cells from affected individuals are not available for research, we wished to assess the effect of eIF2B mutation on oligodendroglial-derived cells.

Methodology/Principal Findings

A rat oligodendroglial-derived cell line was used for a stable knock-down of eIF2B5 followed by stable expression of mutated eIF2B5(R195H) cDNA. In response to a pharmacological ER-stress agent, eIF2B5(R195H) expressing cells exhibited heightened ER-stress response demonstrated by hyper induction of ATF4, GADD34, Bip, PDIA1, PDIA3, PDIA4 and PDIA6 proteins. Moreover, even in the absence of a pharmacological stress agent, eIF2B5(R195H)-expressing cells exhibited high basal levels of ATF4, GADD34 and ER-associated Bip, PDIA1 and PDIA3.

Significance

The data provide evidence that oligodendroglial-derived cells expressing a mutated eIF2B constantly use their stress response mechanism as an adaptation mean in order to survive. The current study is the first to demonstrate the effects of eIF2B5 mutation on ER homeostasis in oligodendroglial-derived cells.     

Heterogeneous adaptive trajectories of small populations on complex fitness landscapes

Background

Small populations are thought to be adaptively handicapped, not only because they suffer more from deleterious mutations but also because they have limited access to new beneficial mutations, particularly those conferring large benefits.

Methodology/Principal Findings

Here, we test this widely held conjecture using both simulations and experiments with small and large bacterial populations evolving in either a simple or a complex nutrient environment. Consistent with expectations, we find that small populations are adaptively constrained in the simple environment; however, in the complex environment small populations not only follow more heterogeneous adaptive trajectories, but can also attain higher fitness than the large populations. Large populations are constrained to near deterministic fixation of rare large-benefit mutations. While such determinism speeds adaptation on the smooth adaptive landscape represented by the simple environment, it can limit the ability of large populations from effectively exploring the underlying topography of rugged adaptive landscapes characterized by complex environments.

Conclusions

Our results show that adaptive constraints often faced by small populations can be circumvented during evolution on rugged adaptive landscapes.     

A specific and rapid neural signature for parental instinct

Darwin originally pointed out that there is something about infants which prompts adults to respond to and care for them, in order to increase individual fitness, i.e. reproductive success, via increased survivorship of one's own offspring. Lorenz proposed that it is the specific structure of the infant face that serves to elicit these parental responses, but the biological basis for this remains elusive. Here, we investigated whether adults show specific brain responses to unfamiliar infant faces compared to adult faces, where the infant and adult faces had been carefully matched across the two groups for emotional valence and arousal, as well as size and luminosity. The faces also matched closely in terms of attractiveness. Using magnetoencephalography (MEG) in adults, we found that highly specific brain activity occurred within a seventh of a second in response to unfamiliar infant faces but not to adult faces. This activity occurred in the medial orbitofrontal cortex (mOFC), an area implicated in reward behaviour, suggesting for the first time a neural basis for this vital evolutionary process. We found a peak in activity first in mOFC and then in the right fusiform face area (FFA). In mOFC the first significant peak (p<0.001) in differences in power between infant and adult faces was found at around 130 ms in the 10-15 Hz band. These early differences were not found in the FFA. In contrast, differences in power were found later, at around 165 ms, in a different band (20-25 Hz) in the right FFA, suggesting a feedback effect from mOFC. These findings provide evidence in humans of a potential brain basis for the "innate releasing mechanisms" described by Lorenz for affection and nurturing of young infants. This has potentially important clinical applications in relation to postnatal depression, and could provide opportunities for early identification of families at risk.     

Polymorphisms within the Novel Type 2 Diabetes Risk Locus MTNR1B Determine β-Cell Function

Background

Very recently, a novel type 2 diabetes risk gene, i.e., MTNR1B, was identified and reported to affect fasting glycemia. Using our thoroughly phenotyped cohort of subjects at an increased risk for type 2 diabetes, we assessed the association of common genetic variation within the MTNR1B locus with obesity and prediabetes traits, namely impaired insulin secretion and insulin resistance.

Methodology/Principal Findings

We genotyped 1,578 non-diabetic subjects, metabolically characterized by oral glucose tolerance test, for five tagging single nucleotide polymorphisms (SNPs) covering 100% of common genetic variation (minor allele frequency >0.05) within the MTNR1B locus (rs10830962, rs4753426, rs12804291, rs10830963, rs3781638). In a subgroup (N = 513), insulin sensitivity was assessed by hyperinsulinemic-euglycemic clamp, and in a further subgroup (N = 301), glucose-stimulated insulin secretion was determined by intravenous glucose tolerance test. After appropriate adjustment for confounding variables and Bonferroni correction for multiple comparisons, none of the tagging SNPs was reliably associated with measures of adiposity. SNPs rs10830962, rs4753426, and rs10830963 were significantly associated with higher fasting plasma glucose concentrations (p<0.0001) and reduced OGTT- and IVGTT-induced insulin release (p≤0.0007 and p≤0.01, respectively). By contrast, SNP rs3781638 displayed significant association with lower fasting plasma glucose levels and increased OGTT-induced insulin release (p<0.0001 and p≤0.0002, respectively). Moreover, SNP rs3781638 revealed significant association with elevated fasting- and OGTT-derived insulin sensitivity (p≤0.0021). None of the MTNR1B tagging SNPs altered proinsulin-to-insulin conversion.

Conclusions/Significance

In conclusion, common genetic variation within MTNR1B determines glucose-stimulated insulin secretion and plasma glucose concentrations. Their impact on β-cell function might represent the prevailing pathomechanism how MTNR1B variants increase the type 2 diabetes risk.     

Effect of sleep stage on breathing in children with central hypoventilation.

The early literature suggests that hypoventilation in infants with congenital central hypoventilation syndrome (CHS) is less severe during rapid eye movement (REM) than during non-REM (NREM) sleep. However, this supposition has not been rigorously tested, and subjects older than infancy have not been studied. Given the differences in anatomy, physiology, and REM sleep distribution between infants and older children, and the reduced number of limb movements during REM sleep, we hypothesized that older subjects with CHS would have more severe hypoventilation during REM than NREM sleep. Nine subjects with CHS, aged (mean +/- SD) 13 +/- 7 yr, were studied. Spontaneous ventilation was evaluated by briefly disconnecting the ventilator under controlled circumstances. Arousal was common, occurring in 46% of REM vs. 38% of NREM trials [not significant (NS)]. Central apnea occurred during 31% of REM and 54% of NREM trials (NS). Although minute ventilation declined precipitously during both REM and NREM trials, hypoventilation was less severe during REM (drop in minute ventilation of 65 +/- 23%) than NREM (drop of 87 +/- 16%, P = 0.036). Despite large changes in gas exchange during trials, there was no significant change in heart rate during either REM or NREM sleep. We conclude that older patients with CHS frequently have arousal and central apnea, in addition to hypoventilation, when breathing spontaneously during sleep. The hypoventilation in CHS is more severe during NREM than REM sleep. We speculate that this may be due to increased excitatory inputs to the respiratory system during REM sleep.     

Ruffling of metalloporphyrins bound to IsdG and IsdI, two heme-degrading enzymes in Staphylococcus aureus

IsdG and IsdI are paralogous proteins that are intracellular components of a complex heme uptake system in Staphylococcus aureus. IsdG and IsdI were shown previously to reductively degrade hemin. Crystal structures of the apoproteins show that these proteins belong to a newly identified heme degradation family distinct from canonical eukaryotic and prokaryotic heme oxygenases. Here we report the crystal structures of an inactive N7A variant of IsdG in complex with Fe(3+)-protoporphyrin IX (IsdG-hemin) and of IsdI in complex with cobalt protoporphyrin IX (IsdI-CoPPIX) to 1.8 A or better resolution. These structures show that the metalloporphyrins are buried into similar deep clefts such that the propionic acids form salt bridges to two Arg residues. His(77) (IsdG) or His(76) (IsdI), a critical residue required for activity, is coordinated to the Fe(3+) or Co(3+) atoms, respectively. The bound porphyrin rings form extensive steric interactions in the binding cleft such that the rings are highly distorted from the plane. This distortion is best described as ruffled and places the beta- and delta-meso carbons proximal to the distal oxygen-binding site. In the IsdG-hemin structure, Fe(3+) is pentacoordinate, and the distal side is occluded by the side chain of Ile(55). However, in the structure of IsdI-CoPPIX, the distal side of the CoPPIX accommodates a chloride ion in a cavity formed through a conformational change in Ile(55). The chloride ion participates in a hydrogen bond to the side chain amide of Asn(6). Together the structures suggest a reaction mechanism in which a reactive peroxide intermediate proceeds with nucleophilic oxidation at the beta- or delta-meso carbon of the hemin.     

rac regulates its effector phospholipase Cgamma2 through interaction with a split pleckstrin homology domain

Several isoforms of phospholipase C (PLC) are regulated through interactions with Ras superfamily GTPases, including Rac proteins. Interestingly, of two closely related PLCgamma isoforms, only PLCgamma(2) has previously been shown to be activated by Rac. Here, we explore the molecular basis of this interaction as well as the structural properties of PLCgamma(2) required for activation. Based on reconstitution experiments with isolated PLCgamma variants and Rac2, we show that an unusual pleckstrin homology (PH) domain, designated as the split PH domain (spPH), is both necessary and sufficient to effect activation of PLCgamma(2) by Rac2. We also demonstrate that Rac2 directly binds to PLCgamma(2) as well as to the isolated spPH of this isoform. Furthermore, through the use of NMR spectroscopy and mutational analysis, we determine the structure of spPH, define the structural features of spPH required for Rac interaction, and identify critical amino acid residues at the interaction interface. We further discuss parallels and differences between PLCgamma(1) and PLCgamma(2) and the implications of our findings for their respective signaling roles.     

T2384, a novel antidiabetic agent with unique peroxisome proliferator-activated receptor gamma binding properties

The nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARgamma) plays central roles in adipogenesis and glucose homeostasis and is the molecular target for the thiazolidinedione (TZD) class of antidiabetic drugs. Activation of PPARgamma by TZDs improves insulin sensitivity; however, this is accompanied by the induction of several undesirable side effects. We have identified a novel synthetic PPARgamma ligand, T2384, to explore the biological activities associated with occupying different regions of the receptor ligand-binding pocket. X-ray crystallography studies revealed that T2384 can adopt two distinct binding modes, which we have termed "U" and "S", interacting with the ligand-binding pocket of PPARgamma primarily via hydrophobic contacts that are distinct from full agonists. The different binding modes occupied by T2384 induced distinct patterns of coregulatory protein interaction with PPARgamma in vitro and displayed unique receptor function in cell-based activity assays. We speculate that these unique biochemical and cellular activities may be responsible for the novel in vivo profile observed in animals treated systemically with T2384. When administered to diabetic KKAy mice, T2384 rapidly improved insulin sensitivity in the absence of weight gain, hemodilution, and anemia characteristics of treatment with rosiglitazone (a TZD). Moreover, upon coadministration with rosiglitazone, T2384 was able to antagonize the side effects induced by rosiglitazone treatment alone while retaining robust effects on glucose disposal. These results are consistent with the hypothesis that interactions between ligands and specific regions of the receptor ligand-binding pocket might selectively trigger a subset of receptor-mediated biological responses leading to the improvement of insulin sensitivity, without eliciting less desirable responses associated with full activation of the receptor. We suggest that T2384 may represent a prototype for a novel class of PPARgamma ligand and, furthermore, that molecules sharing some of these properties would be useful for treatment of type 2 diabetes.     

Prion protein attenuates excitotoxicity by inhibiting NMDA receptors

It is well established that misfolded forms of cellular prion protein (PrP [PrP(C)]) are crucial in the genesis and progression of transmissible spongiform encephalitis, whereas the function of native PrP(C) remains incompletely understood. To determine the physiological role of PrP(C), we examine the neurophysiological properties of hippocampal neurons isolated from PrP-null mice. We show that PrP-null mouse neurons exhibit enhanced and drastically prolonged N-methyl-d-aspartate (NMDA)-evoked currents as a result of a functional upregulation of NMDA receptors (NMDARs) containing NR2D subunits. These effects are phenocopied by RNA interference and are rescued upon the overexpression of exogenous PrP(C). The enhanced NMDAR activity results in an increase in neuronal excitability as well as enhanced glutamate excitotoxicity both in vitro and in vivo. Thus, native PrP(C) mediates an important neuroprotective role by virtue of its ability to inhibit NR2D subunits.