Astrocytes and neurons communicate via a monocarboxylic acid shuttle

Since formulation of the Astrocyte-Neuron Lactate Shuttle (ANLS) hypothesis in 1994, the hypothesis has provoked criticism and debate. Our review does not criticise, but rather integrates experimental data characterizing proton-linked monocarboxylate transporters (MCTs) into the ANLS. MCTs have wide substrate specificity and are discussed to be in protein complex with a proton donor (PD). We particularly focus on the proton-driven transfer of L-lactic acid (L-lacH) and pyruvic acid (pyrH), were PDs link MCTs to a flow of energy. The precise nature of the PD predicts the activity and catalytic direction of MCTs. By doing so, we postulate that the MCT4•phosphoglycerate kinase complex exports and at the same time in the same astrocyte, MCT1•carbonic anhydrase II complex imports monocarboxylic acids. Similarly, neuronal MCT2 preferentially imports pyrH. The repertoire of MCTs in astrocytes and neurons allows them to communicate via monocarboxylic acids. A change in imported pyrH/L-lacH ratio in favour of L-lacH encodes signals stabilizing the transit of glucose from astrocytes to neurons. The presented astrocyte neuron communication hypothesis has the potential to unite the community by suggesting that the exchange of monocarboxylic acids paves the path of glucose provision

IC 3599 did it again: A second outburst of the X-ray transient Seyfert 1.9 Galaxy

We report on the Swift discovery of a second high-amplitude (factor 100) outburst of the Seyfert 1.9 galaxy IC 3599, and discuss implications for outburst scenarios. Swift detected this active galactic nucleus (AGN) again in February 2010 in X-rays at a level of (1.50\plm0.11)$\times 10^{36}$ W (0.2-2.0 keV), which is nearly as luminous as the first outburst detected with ROSAT in 1990. Optical data from the Catalina sky survey show that the optical emission was already bright two years before the Swift X-ray high-state. Our new Swift observations performed between 2013 and 2015 show that IC 3599 is currently again in a very low X-ray flux state. This repeat optical and X-ray outburst, and the long optical duration, suggest that IC 3599 is likely not a tidal disruption event (TDE). Instead, variants of AGN-related variability are explored. The data are consistent with an accretion disk instability around a black hole of mass on the order 10$^6$--10$^7$ M$_{\odot}$; a value estimated using several different methods.Comment: Accepted for publication in ApJ Letter, 6 pages, 1 table, 3 figure

How do the environmental extremes of Siberian permafrost soils shape the composition of the bacterial soil community?

Microbial communities in permafrost soils of the Siberian Arctic are exposed to extreme environmental conditions. The soils are frozen throughout the entire year except for the short summer period, when thawing of the uppermost 20 to 50 cm of the permafrost sediment allows for the formation of a so-called active layer. Active layers show steep temperature gradients between 10 to 18 °C near the surface and 0 to 1 °C near the permafrost table. Additionally, seasonal freezing and thawing processes lead to the formation of patterns of low-centered polygons. Low-centered polygons determine a pronounced small-scale heterogeneity with regard to their physical and chemical properties between the elevated polygon rims and the depressed polygon centers.Within the active layer of a polygon rim, vertical profiles of potential methane oxidation rates in respond to different temperatures indicated a shift in the temperature optimum from 21 °C near the surface to 4 °C near the permafrost table [1]. This temperature shift could not be shown in samples of the polygon center. Based on these results we used 16S rDNA clone libraries as well as in-situ cell counting to compare the bacterial, in particular the methane oxidizing, community near the surface and near the permafrost table in samples of the polygon rim. The phylogenetic analyses show that the composition of the bacterial community near the surface is significantly different from the bacterial community near the permafrost table. The results also show that bacterial diversity and abundance in Siberian permafrost soils are comparably high as in temperate terrestrial environments.[1] Liebner. S. and Wagner, D. (in press) Abundance, distribution and potential activity of methane oxidizing bacteria in permafrost soils from the Lena Delta, Siberia. Environmental Microbiology doi: 10.1111/j.1462-2920.2006.01120.