Loss of ACTN3 gene function alters mouse muscle metabolism and shows evidence of positive selection in humans

More than a billion humans worldwide are predicted to be completely deficient in the fast skeletal muscle fiber protein alpha-actinin-3 owing to homozygosity for a premature stop codon polymorphism, R577X, in the ACTN3 gene. The R577X polymorphism is associated with elite athlete status and human muscle performance, suggesting that alpha-actinin-3 deficiency influences the function of fast muscle fibers. Here we show that loss of alpha-actinin-3 expression in a knockout mouse model results in a shift in muscle metabolism toward the more efficient aerobic pathway and an increase in intrinsic endurance performance. In addition, we demonstrate that the genomic region surrounding the 577X null allele shows low levels of genetic variation and recombination in individuals of European and East Asian descent, consistent with strong, recent positive selection. We propose that the 577X allele has been positively selected in some human populations owing to its effect on skeletal muscle metabolism.     

Functional connectivity in the retina at the resolution of photoreceptors

To understand a neural circuit requires knowledge of its connectivity. Here we report measurements of functional connectivity between the input and ouput layers of the macaque retina at single-cell resolution and the implications of these for colour vision. Multi-electrode technology was used to record simultaneously from complete populations of the retinal ganglion cell types (midget, parasol and small bistratified) that transmit high-resolution visual signals to the brain. Fine-grained visual stimulation was used to identify the location, type and strength of the functional input of each cone photoreceptor to each ganglion cell. The populations of ON and OFF midget and parasol cells each sampled the complete population of long- and middle-wavelength-sensitive cones. However, only OFF midget cells frequently received strong input from short-wavelength-sensitive cones. ON and OFF midget cells showed a small non-random tendency to selectively sample from either long- or middle-wavelength-sensitive cones to a degree not explained by clumping in the cone mosaic. These measurements reveal computations in a neural circuit at the elementary resolution of individual neurons.     

New protein fold revealed by a 2.3-A resolution crystal structure of nerve growth factor

Nerve growth factor (NGF) is a member of an expanding family of neurotrophic factors (including brain-derived neurotrophic factor and the neurotrophins) that control the development and survival of certain neuronal populations both in the peripheral and in the central nervous systems. Its biological effects are mediated by a high-affinity ligand-receptor interaction and a tyrosine kinase signalling pathway. A potential use for NGF and its relatives in the treatment of neurological disorders such as Alzheimer's disease and Parkinson's disease requires an understanding of the structure-function relationships of NGF. NGF is a dimeric molecule, with 118 amino acids per protomer. We report the crystal structure of the murine NGF dimer at 2.3-A resolution, which reveals a novel protomer structure consisting of three antiparallel pairs of beta strands, together forming a flat surface. Two subunits associate through this surface, thus burying a total of 2,332 A. Four loop regions, which contain many of the variable residues observed between different NGF-related molecules, may determine the different receptor specificities. A clustering of positively charged side chains may provide a complementary interaction with the acidic low-affinity NGF receptor. The structure provides a model for rational design of analogues of NGF and its relatives and for testing the NGF-receptor recognition determinants critical for signal transduction.     

Smooth muscle alpha-action is a transformation-sensitive marker for mouse NIH 3T3 and Rat-2 cells

Heteroploid mouse NIH 3T3 fibroblasts and several rat fibroblast strains (Rat-1, Rat-2 and REF-52) are cell lines of special interest in the field of carcinogenesis because of their extensive use as normal cells in transformation assays for putative cancer-causing genes. Exposure of these cells to carcinogenic chemicals or oncogenic DNA produces anchorage-independent cells with retracted cytoplasms that lack actin cables. All human fibroblast strains, normal and transformed, synthesize two electrophoretic forms of actin (beta- and gamma-actin). In contrast, we discovered that early-passage mouse and rat strains synthesize abundant amounts of each of the three electrophoretic forms of actin (alpha-, beta- and gamma-actin) but mouse and rat cancer cells express only beta- and gamma-actins. We now show that in NIH 3T3 and Rat-2 fibroblasts a third actin, the smooth muscle alpha isoform, is abundantly co-expressed with beta- and gamma-actin. In every instance tested following transformation to tumorigenicity, the accumulation of alpha-actin messenger RNA and alpha-actin synthesis was greatly inhibited. Shutdown of alpha-actin expression thus appears to be a reproducible transformation-sensitive marker in rodent fibroblasts.