In this weeks Journal, Regan et al. used bacterial artificial chromosome (BAC) transgenes to visualize the expression pattern of two glutamate transporters in mice, glutamateaspartate transporter (GLAST) and glutamate transporter-1 (GLT-1). These two transporters are primarily expressed in astroglia and do the heavy lifting in terms of glutamate uptake in the embryonic and adult brain, respectively. The BAC transgene containing the promoter sequence for GLAST encoded the red fluorescent protein Discosoma red, whereas the BAC containing GLT-1 encoded green fluorescent protein. The mice expressed endogenous transporters, and the fluorescent reporters revealed transporter expression patterns, allowing the authors to study endogenous transporter activity in cells of interest. GLAST promoter activity was widespread early in development but tapered off after birth, with some interesting exceptions such as radial glia and white matter oligodendrocytes. In contrast, GLT-1 promoter activity was present in virtually all astrocytes, but it was higher in forebrain than spinal cord. Most cell types expressed only one of these two transporters.

Signaling between secreted Slit proteins and their Robo receptors are known to affect axon branching and guidance. However, this week, Ma and Tessier-Lavigne show that how it works depends on the end of the cell at which youre looking. In the peripheral branches of trigeminal sensory neurons, arborizations were altered in the ophthalmic projection of mice lacking Slit2/Slit3 or Robo1/Robo2. Although branches below the eye appeared normal, the arbor above the eye was significantly smaller, apparently due to loss of
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