Cellular/Molecular
Coupling Binding to Gating in the GABAA Receptor
J. Glen Newell, Ross A. McDevitt, and Cynthia Czajkowski
(see pages 11226-11235)
Agonist binding leads to channel opening in ligand-gated channels, but what are the underlying structural rearrangements in the protein? This week, Newell et al. used scanning cysteine mutagenesis in GABAA receptor channels to investigate this question. They focused on the segment between Ile154 and Asp163 of the β2 subunit, a region predicted to be critical for channel gating. They analyzed the function of mutant β2 subunits expressed with α subunits in oocytes and the effects of modification of the cysteine residues by N-biotinylaminoethyl methanethiosulfonate (MTSEA-biotin). Agonists, antagonists, and the allosteric modulator pentobarbital slowed the reaction of MTSEA-biotin with cysteines at positions 160 and 163, suggesting that these amino acids are near the ligand-binding site. The E155C mutation caused spontaneous openings (i.e., in the absence of agonist); these openings were blocked by reaction with MTSEA-biotin. Thus the authors suggest that the region forms a protein hinge that couples binding to channel opening.
Development/Plasticity/Repair
Transcriptional Regulation and Myelin Paranodes
Cherie Southwood, Chris He, James Garbern, John Kamholz, Edgardo Arroyo, and Alexander Gow
(see pages 11215-11225)
Homeodomain proteins are well known as transcriptional regulators during development, but their role in postnatal tissue and differentiated cells is not so clear. One member of this family, Nkx6-2, is expressed in oligodendrocytes, suggesting a possible role in myelination or axoglial interactions. Southwood et al. explored this possibility in mice by replacing the coding region of Nkx6-2 with lacZ, which revealed the expected expression in oligodendrocytes. The ultrastructural effects were readily apparent in Nkx6-2 null mice, particularly at paranodal axoglial junctions, where abnormal “vermicular-like processes” grew underneath the myelin. Accordingly, the authors investigated proteins mediating microtubule dynamics and found reduced expression of stathmin 1, a paranodal microtubule-destabilizing protein. The adhesion molecules neurofascin and contactin were upregulated in oligodendrocytes of Nkx6-2 null mice. Although behavioral abnormalities were not obvious, additional testing revealed locomotor deficits and slowed conduction velocities, consistent with disruption of axoglial junctions in mice lacking Nkx6-2.
Behavioral/Systems/Cognitive
“Change Blindness” Starring the First Author
James Cavanaugh and Robert H. Wurtz
(see pages 11236-11243)
In this week's Journal, Cavanaugh and Wurtz dissect a curious phenomenon, “change blindness.” We all, perhaps unknowingly, experience this effect hundreds of times each day. For example, when a new feature appears in our visual field, our attention shifts. During the eye movements (saccades) that redirect central vision, we are blind to changes in the visual scene. In the true spirit of experimentalism, the first author, along with another human and two monkeys, served as a subject to test this phenomenon. Change detection was improved and sped up by attentional cues. The authors hypothesized that a coordinated mechanism is responsible for saccadic movement and attentional shifts. Accordingly, stimulation of the superior colliculus (SC) in monkeys (the first author graciously declined we presume) resulted in the same task improvement produced by a cue. The authors conclude that the SC not only signals saccade initiation and targeting but also signals the cortex to facilitate visual information processing.
Change blindness is the inability to see large changes in a visual scene that occur during some type of visual distraction. Elements of a scene can even disappear without detection unless one is paying particular attention to these elements. Change blindness therefore gives us a milieu that we can use to explore the neuronal mechanisms that underlie visual spatial attention. For a demonstration on change blindness, see ftp://lsr-ftp.nei.nih.gov/web/jc/cb_demo.htm. Also see the article by Cavanaugh and Wurtz for details.
Neurobiology of Disease
Testing Transepithelial Prion Protein Transport In Vitro
Ravi Shankar Mishra, Subhabrata Basu, Yaping Gu, Xiu Luo, Wen-Quan Zou, Richa Mishra, Ruliang Li, Shu G. Chen, Pierluigi Gambetti, Hisashi Fujioka, and Neena Singh
(see pages 11280-11290)
The discovery of the “mad cow” variant of Creutzfeldt-Jakob disease (CJD) as a foodborne illness not only created a worldwide scare but also focused attention on the mechanisms of transmission of the infective agent, the scrapie prion protein (PrPSc). How does this proteinase-resistant peptide make its way across the highly selective intestinal epithelial cell layer? One view is that this occurs via lymphoid tissue. In this issue, Mishra et al. examined the ability of PrPSc, isolated from normal and CJD-infected human brain tissue, to migrate directly across the epithelial-like layer of Caco-2 cells in vitro. After treatment with digestive enzymes or proteinase K, they were surprised to find PrPSc associated with ferritin, a protein clearly abundant in muscle tissue associated with food. The PrPSc and ferritin complex formed vesicular structures that appear to cross the Caco-2 cell layer via a receptor or transporter. Because ferritin is similar across species, this piggyback arrangement may be important in allowing the protease-resistant core of PrPSc to cross gut epithelium.
