Biological fractionation of lithium isotopes by cellular Na+/H+ exchangers unravels fundamental transport mechanisms.

Lithium (Li) has a wide range of uses in science, medicine, and industry, but its isotopy is underexplored, except in nuclear science and in geoscience. 6Li and 7Li isotopic ratio exhibits the second largest variation on earth's surface and constitutes a widely used tool for reconstructing past oceans and climates. As large variations have been measured in mammalian organs, plants or marine species, and as 6Li elicits stronger effects than natural Li (∼95% 7Li), a central issue is the identification and quantification of biological influence of Li isotopes distribution. We show that membrane ion channels and Na+-Li+/H+ exchangers (NHEs) fractionate Li isotopes. This systematic 6Li enrichment is driven by membrane potential for channels, and by intracellular pH for NHEs, where it displays cooperativity, a hallmark of dimeric transport. Evidencing that transport proteins discriminate between isotopes differing by one neutron opens new avenues for transport mechanisms, Li physiology, and paleoenvironments.

Renal Ischemia Tolerance Mediated by eIF5A Hypusination Inhibition Is Regulated by a Specific Modulation of the Endoplasmic Reticulum Stress.

Through kidney transplantation, ischemia/reperfusion is known to induce tissular injury due to cell energy shortage, oxidative stress, and endoplasmic reticulum (ER) stress. ER stress stems from an accumulation of unfolded or misfolded proteins in the lumen of ER, resulting in the unfolded protein response (UPR). Adaptive UPR pathways can either restore protein homeostasis or can turn into a stress pathway leading to apoptosis. We have demonstrated that N1-guanyl-1,7-diamineoheptane (GC7), a specific inhibitor of eukaryotic Initiation Factor 5A (eIF5A) hypusination, confers an ischemic protection of kidney cells by tuning their metabolism and decreasing oxidative stress, but its role on ER stress was unknown. To explore this, we used kidney cells pretreated with GC7 and submitted to either warm or cold anoxia. GC7 pretreatment promoted cell survival in an anoxic environment concomitantly to an increase in xbp1 splicing and BiP level while eiF2α phosphorylation and ATF6 nuclear level decreased. These demonstrated a specific modulation of UPR pathways. Interestingly, the pharmacological inhibition of xbp1 splicing reversed the protective effect of GC7 against anoxia. Our results demonstrated that eIF5A hypusination inhibition modulates distinctive UPR pathways, a crucial mechanism for the protection against anoxia/reoxygenation.

How Does Our Knowledge on the Na+/H+ Exchanger NHE1 Obtained by Biochemical and Molecular Analyses Keep up With Its Recent Structure Determination?

Na+/H+ exchangers are membrane transporters conserved in all living systems and therefore are assumed to be amongst the most ancestral molecular devices that equipped the first protocells. Following the cloning and sequencing of its gene, the mammalian NHE1, that regulates pH and volume in all cells, has been thoroughly scrutinized by molecular and biochemical analyses. Those gave a series of crucial clues concerning its topology, dimeric organization, pharmacological profile, regulation, and the role of key amino acids. Recently thanks to cryogenic Electron Microscopy (Cryo-EM) the long-awaited molecular structures have been revealed. With this information in mind we will challenge the robustness of the earlier conclusions and highlight how the new information enriches our understanding of this key cellular player. At the mechanistic level, we will pinpoint how the NHE1 3D structures reveal that the previously identified amino acids and regions are organized to coordinate transported cations, and shape the allosteric transition that makes NHE1 able to sense intracellular pH and be regulated by signaling pathways.

Intracellular pH Control by Membrane Transport in Mammalian Cells. Insights Into the Selective Advantages of Functional Redundancy.

Intracellular pH is a vital parameter that is maintained close to neutrality in all mammalian cells and tissues and acidic in most intracellular compartments. After presenting the main techniques used for intracellular an vesicular pH measurements we will briefly recall the main molecular mechanisms that affect and regulate intracellular pH. Following this we will discuss the large functional redundancy found in the transporters of H+ or acid-base equivalents. For this purpose, we will use mathematical modeling to simulate cellular response to persistent and/or transient acidification, in the presence of different transporters, single or in combination. We will also test the presence or absence of intracellular buffering. This latter section will highlight how modeling can yield fundamental insight into deep biological questions such as the utility of functional redundancy in natural selection.

Drosophila ALS regulates growth and metabolism through functional interaction with insulin-like peptides.

In metazoans, factors of the insulin family control growth, metabolism, longevity, and fertility in response to environmental cues. In Drosophila, a family of seven insulin-like peptides, called Dilps, activate a common insulin receptor. Some Dilp peptides carry both metabolic and growth functions, raising the possibility that various binding partners specify their functions. Here we identify dALS, the fly ortholog of the vertebrate insulin-like growth factor (IGF)-binding protein acid-labile subunit (ALS), as a Dilp partner that forms a circulating trimeric complex with one molecule of Dilp and one molecule of Imp-L2, an IgG-family molecule distantly related to mammalian IGF-binding proteins (IGFBPs). We further show that dALS antagonizes Dilp function to control animal growth as well as carbohydrate and fat metabolism. These results lead us to propose an evolutionary perspective in which ALS function appeared prior to the separation between metabolic and growth effects that are associated with vertebrate insulin and IGFs.

SMHS1 is involved in oxidative/glycolytic-energy metabolism balance of muscle fibers.

With the aim of finding important mediators of muscle atrophy, we cloned SMHS1, a novel gene that was found to be upregulated in rat soleus muscle atrophied by restriction of activity. The SMHS1 amino acid sequence shares 65% similarity with RTP801-which is a cellular stress response protein regulated by HIF-1-but SMHS1 expression was demonstrated to be independent of HIF-1. SMHS1 was found to be mainly expressed in skeletal muscle, and comparisons of its expression in atrophied versus hypertrophied muscles and in oxidative versus glycolytic muscles suggested that SMHS1 contributes to the muscle energy metabolism phenotypes.

Linolenic acid prevents neuronal cell death and paraplegia after transient spinal cord ischemia in rats.

PURPOSE: Spinal cord ischemia is a devastating complication of thoracic and thoracoabdominal aortic surgery. Recent studies have suggested a neuroprotective effect of polyunsaturated fatty acids against cerebral ischemia. We investigated the effect of linolenic acid (LIN) in a rat model of spinal cord ischemia. METHODS: Rats were subjected to cross-clamping of the aortic arch and left subclavian artery for 14 minutes. Groups were as follows: sham operation (n = 15); ischemia (n = 15), receiving only vehicle; LIN A (n = 15), receiving LIN before clamping; and LIN B (n = 15), receiving LIN at onset of reperfusion. Neurologic status was assessed daily for 7 days. Spinal cords were harvested for histopathologic analysis, TUNEL staining, and immunohistochemistry for Bax, heat shock protein 70 (HSP70), and nuclear factor-kappaB. RESULTS: Ischemic rats had severe and definitive paraplegia. LIN-treated rats had significantly better neurologic function. Histopathologic analysis disclosed severe neuronal necrosis in the lumbar gray matter of ischemic rats, whereas most of the LIN-treated rats sustained mild to moderate injury. LIN reduced the loss of motor neurons at 7 days (LIN A, 17 +/- 6, and LIN B, 15 +/- 7, versus ischemia, 6 +/- 2 per section; P <.05). LIN prevented apoptotic neuronal cell death, Bax immunoreactivity of the pro-apoptotic protein Bax, and the nuclear transcription factor NF-kappaB. Nuclear HSP70 immunoreactivity was noted exclusively in motor neurons from LIN-treated rats and not in motor neurons from ischemic rats. CONCLUSION: These results suggest that LIN can induce protection against ischemia in the spinal cord, thereby preventing both necrosis and apoptosis of motor neurons.