1. Chromogranins Concentrate Catecholamines
Monica S. Montesinos, J. David Machado, Marcial Camacho, Jesica Diaz, Yezer G. Morales, Diego Alvarez de la Rosa, Emilia Carmona, Agustin Castañeyra, O. Humberto Viveros, Daniel T. O’Connor, Sushil K. Mahata, and Ricardo Borges
The function of chromogranins, a major protein colocalized with catecholamines in dense-core vesicles and chromaffin granules, has been elusive. Montesinos et al. have addressed this mystery and another: how catecholamines are maintained at such high levels in vesicles. The authors used patch amperometry to detect the release of single chromaffin granules from wild-type and chromogranin-A null mouse cells. Because this technique allows discrete, precise measurements of different components of release—formation and expansion of the fusion pore as well as granule and quantal size—it can indicate which component is altered bymutation. In chromogranin-A knock-outs, the quantal size of chromaffin granules was lower, and granules emptied more quickly after fusion compared to controls. Addition of a catecholamine precursor increased quantal size in controls but not knock-outs, suggesting that chromaffin granules from knock-outs could not sequester additional catecholamine. These results strongly suggest that chromogranins help sequester catecholamines in, and slow release from, chromaffin granules.
2. Brn3b Specifies and Suppresses Retinal Cell Fates
Feng Qiu, Haisong Jiang, and Mengqing Xiang
During development, multipotent progenitor cells become committed to a specific fate through expression of cell-specific transcription factors. This week, Qiu et al. present evidence that the transcription factor Brn3b specifies retinal ganglion cell fate and also represses expression of transcription factors that specify alternative fates. The authors used microarrays to find differences in gene expression between Brn3b null and heterozygous mice. They found that many of the most highly upregulated genes in null mice encoded transcription factors known to specify other retinal cell types. Examination of cell-type-specific markers by immunostaining suggested that Brn3b null retinas had fewer ganglion cells and many more amacrine and horizontal cells than controls. On the other hand, overexpression of Brn3b in embryonic retinas increased the proportion of ganglion cells and reduced the number of amacrine and horizontal cells. Thus Brn3b appears to specify retinal ganglion cell fate but also prevents transdifferentiation into other retinal cell types.
3. Adaptation to Curves Affects Perception of Faces
Hong Xu, Peter Dayan, Richard M. Lipkin, and Ning Qian
Are faces perceived holistically or as sums of their parts? Many studies have suggested the former, but experiments reported in this issue suggest that the parts are also perceived individually. To address the question, Xu et al. examined the ability of adaptation to low-level features (curves) to influence perception of high-level features (facial expressions) and vice versa. As shown previously, adaptation to simple curves made human subjects perceive subsequently viewed lines as curving in the opposite direction, and adaptation to sad faces caused subjects to view neutral faces as happy. Additionally, adaptation to concave frown-like curves made subjects perceive neutral faces—cartoons or photographs—as happy. In other words, adaptation to low-level stimuli affected higher-level perception. But the effect of adaptation to curves was smaller than that of adaptation to faces, even for cartoon faces whose mouths were identical to the adapting curve, suggesting that additional holistic components contribute to facial perception.
4. Seizures Disrupt Astrocytic Domain Structure
Nancy Ann Oberheim, Guo-Feng Tian, Xiaoning Han, Weiguo Peng, Takahiro Takano, Bruce Ransom, and Maiken Nedergaard
It was recently discovered that astrocytes tile the brain in nonoverlapping domains that parcel the neuropil into discrete areas infiltrated by single astrocytes. Oberheim et al. now show that this organization is disrupted in mouse models of epilepsy. One week after seizures were induced by ferrous chloride, reactive gliosis appeared, and two types of reactive astrocytes were identified: palisading astrocytes immediately surrounded the injection site and were associated with neurons that had decreased dendritic number, width, and spine density, whereas hypertrophic astrocytes were farther from the site and surrounded neurons that had thicker dendrites with more spines than controls. Both astrocytic types had thicker, longer processes, which were significantly more interdigitated than normal. These changes were also seen in mice that underwent seizures due to kainate injection or genetic mutation, but not in mice that were successfully treated with antiseizure medication or those that showed gliosis due to a non-seizure-inducing mutation.
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