In This Issue

ENVIRONMENTAL SCIENCES, ECOLOGY, SUSTAINABILITY SCIENCE

River dams decrease U.S. water flow diversity

In the past century, over 75,000 dams exceeding 2 m in height have been built along streams and rivers in the United States, an average of one dam every 70 km of stream or river. At the local level, dams have negative effects on sediment flux, temperature regimes, and river continuity. LeRoy Poff et al. studied the effects of dams in medium-sized U.S. rivers on a regional scale and found that dams decreased the natural diversity of flow patterns in different regions. The authors divided the United States into 16 regions and statistically analyzed daily water flow measurements collected over the past 100 years from 186 gauges in rivers with dams and 317 gauges in free-flowing rivers. Comparison of pre- and postdam measurements showed that the magnitude and timing, and to a lesser extent duration and frequency, of low and high flows became more similar in different regions. Comparable changes were not observed in free-flowing rivers. Because the unique water flow dynamics of each river underlie its characteristic ecosystem, Poff et al. suggest that regional dam re-regulation mimicking natural flows and conservation of natural rivers will be important steps to preserve the diversity of U.S. river ecosystems. — F.A.H.

“Homogenization of regional river dynamics by dams and global biodiversity implications” by N. LeRoy Poff, Julian D. Olden, David M. Merritt, and David M. Pepin (see pages 5732–5737)

ENVIRONMENTAL SCIENCES, ECOLOGY

Global warming and novel and disappearing climates

Climate is a principal influence in species distributions and ecosystem function, and the disappearance of existing climates, or the development of future climates not found at present, could have profound ecological consequences. John Williams et al. attempted to forecast future distributions of novel and disappearing climates, using both the A2 and B1 emissions scenarios from the Intergovernmental Panel on Climate Change recent assessment report as a framework. In the high-end (A2) scenario, up to 39% and 48% of the terrestrial surface may experience a novel or disappearing climate, respectively. Climate changes in the low-end (B1) scenario would affect up to 20% of the surface in both cases. Novel climates are projected for tropical and subtropical regions, with the highest dissimilarities occurring in the Amazonian and Indonesian rainforests. In contrast, disappearing climates are predicted to be concentrated in tropical mountains, such as the Peruvian and Colombian Andes, and the poleward regions of continents, areas previously characterized as biodiversity hotspots. Williams et al. note that these novel and disappearing climates should increase species extinctions, promote the formation of novel species associations, and challenge ecological forecasting attempts. — N.Z.

“Projected distributions of novel and disappearing climates by 2100 AD” by John W. Williams, Stephen T. Jackson, and John E. Kutzbach (see pages 5738–5742)

CELL BIOLOGY

Hepatitis C virus production depends on lipoprotein secretion

Infected liver hepatocytes are the only cells that secrete the hepatitis C virus (HCV), which circulates in the blood in complex with triglyceride-rich, very low-density lipoproteins (VLDLs). Hua Huang et al. found that the restriction of HCV production to hepatocytes depends on the synthesis and secretion of VLDL, which is also uniquely produced in the liver. The authors purified membrane vesicles containing the HCV replication complex from cultured human hepatoma cells that harbor a subgenomic replicon of the virus. These vesicles were highly enriched in proteins that are required for the assembly of VLDL: apolipoproteins B and E and microsomal triglyceride transfer protein. An inhibitor and small interfering RNAs targeting these proteins, which inhibited VLDL assembly, blocked the secretion of HCV from the human hepatoma cells. These results closely link HCV production and VLDL assembly, which suggests that agents that block VLDL secretion in the liver could be used to treat hepatitis C infection, according to the authors. — F.A.

“Hepatitis C virus production by human hepatocytes dependent on assembly and secretion of very low-density lipoproteins” by Hua Huang, Fang Sun, David M. Owen, Weiping Li, Yan Chen, Michael Gale, Jr., and Jin Ye (see pages 5848–5853)

IMMUNOLOGY

Lipid deficiency effect on natural killer T cells

Although constituting <1% of immune cells, invariant natural killer T (iNKT) cells help defend against microbial infections and tumor growth. As with other lymphocytes, iNKT cells undergo positive selection in the thymus, where endogenous lipid antigen is presented to the cells. Previous research suggested that this antigen was the glycosphingolipid isoglobotrihexosylceramide (iGb3). However, two separate studies by Anneliese Speak et al. and Stefan Porubsky et al. found little physiological evidence to support iGb3's role in thymic iNKT selection. Speak et al. assayed human and mouse tissues for the presence of the lipid and detected iGb3 in the mouse dorsal root ganglion but not in human or mouse thymus or dendritic cells. Porubsky et al. generated mice lacking iGb3 synthase, the only enzyme required for the synthesis of iGb3, and these mice displayed normal numbers of iNKT cells in the thymus and spleen. Activation of iNKT and dendritic cells in the mutant animals was also similar to that observed in wild-type mice. The exclusion of iGb3 as the iNKT cell-selecting lipid in the thymus opens the possibility that other endogenous lipid ligands play this role. — F.A.

“Implications for invariant natural killer T cell ligands due to the restricted presence of isoglobotrihexosylceramide in mammals” by Anneliese O. Speak, Mariolina Salio, David C. A. Neville, Josette Fontaine, David A. Priestman, Nick Platt, Tanya Heare, Terry D. Butters, Raymond A. Dwek, Francois Trottein, Mark A. Exley, Vincenzo Cerundolo, and Frances M. Platt (see pages 5971–5976)

and

“Normal development and function of invariant natural killer T cells in mice with isoglobotrihexosylceramide (iGb3) deficiency” by Stefan Porubsky, Anneliese O. Speak, Bruno Luckow, Vincenzo Cerundolo, Frances M. Platt, and Hermann-Josef Gröne (see pages 5977–5982)

MEDICAL SCIENCES

Exercise benefit for rare neuromuscular disease

Abnormal protein accumulation is a hallmark feature of neurodegenerative diseases such as Alzheimer's disease, but in recent years, the phenomenon has been linked to a family of rare neuromuscular diseases as well. Desmin-related myopathy, which leads to progressive muscle atrophy, weakness, and cardiomyopathy, is characterized by the intracellular accumulation of the intermediate filament desmin and other proteins in skeletal and heart muscle. Because previous research has shown that exercise reduces amyloid accumulation and improves cognitive function in mouse models of neurodegenerative diseases, Alina Maloyan et al. investigated the therapeutic potential of voluntary exercise in mice that overexpress αB-crystallin (CryAB), a heat-shock protein that normally prevents desmin aggregation, in the heart. The authors found that 6 months of voluntary exercise in these mice led to 100% survival at a time when all unexercised CryAB mutant mice had died. Exercise also reduced protein aggregation in these mice through a mechanism involving the enzyme neprilysin. The results suggest that exercise and, perhaps, modulation of neprilysin may provide therapeutic avenues for treating this progressive neuromuscular disease, as well as related cardiac diseases. — M.M.

“Exercise reverses preamyloid oligomer and prolongs survival in αB-crystallin-based desmin-related cardiomyopathy” by Alina Maloyan, James Gulick, Charles G. Glabe, Rakez Kayed, and Jeffrey Robbins (see pages 5995–6000)