Cellular/Molecular
Visualizing APP/PS1 Interactions
A scanning electron microscope image of a new hair cell generated in the mature guinea pig cochlea by Math1 gene therapy.
Amyloid Precursor Protein Associates with a Nicastrin-Dependent Docking Site on the Presenilin 1–γ-Secretase Complex in Cells Demonstrated by Fluorescence Lifetime Imaging
Oksana Berezovska, Pavan Ramdya, Jesse Skoch, Michael S. Wolfe, Brian J. Bacskai, and Bradley T. Hyman
(see pages 4560-4566)
The cleavage of the membrane protein amyloid precursor protein (APP) to the plaque-forming β-amyloid protein (Aβ) is one of the hallmarks of Alzheimer's disease. Thus, the enzymatic steps in APP cleavage may be potential therapeutic targets. The γ-secretase responsible for final cleavage of APP is actually a complex of at least four proteins, including presenilin (PS), mutations of which can cause early onset Alzheimer's. The active site for γ-secretase activity is thought to reside on PS1, although the interaction between substrate (APP) and the γ-secretase complex is not yet well understood. Berezovska et al. approached this question in cell lines using two fluorescence resonance energy transfer (FRET)-based assays: fluorescence lifetime imaging (FLIM) and photobleach dequenching. These assays rely on changes in fluorescence (or fluorescent lifetime in the case of FLIM) that occur when fluorescently labeled donor and acceptor molecules are tightly associated (within ≈10 nm). Although PS1 and APP were abundantly colocalized in perinuclear compartments, APP and PS1 were most closely associated near the cell surface, the site of APP cleavage. Genetic or pharmacologic inhibition of γ-secretase enzymatic activity did not prevent the tight association. Notably, nicastrin, another known molecular component of γ-secretase, was required for the docking, as it is for cleavage. The authors suggest that the APP docking site on γ-secretase is separate from the PS1 active site, thus providing another potential target for blocking Aβ production.
Development/Plasticity/Repair
New Hair Cells do Math1
Math1 Gene Transfer Generates New Cochlear Hair Cells in Mature Guinea Pigs In Vivo
Kohei Kawamoto, Shin-Ichi Ishimoto, Ryosei Minoda, Douglas E. Brough, and Yehoash Raphael
(see pages 4395-4400)
Hair cells of the inner ear are vulnerable to many everyday dangers, such as loud music, infections, and even aging. Once these sensory cells are lost from the cochlea, they are not regenerated, thus dooming many of us to hearing loss. That may eventually change in light of a report from Kawamoto et al. that suggests that gene therapy may generate new hair cells. Previous studies have shown that developing hair cells express the transcription factor Math1, and that in vitro overexpression of Math1 causes nonsensory epithelial precursor cells to differentiate into hair cells. In the current work, the transcription factor was overexpressed in guinea pig cochlea in vivo via viral-mediated gene transfer. Within 2 months, each cochlea generated 2–10 new hair cells, which expressed Math1. Encouragingly, the new hair cells seemed to attract innervation by auditory neurons, crucial to functional recovery of hearing. Thus nonsensory cells in the mature cochlea retain the competence to form new hair cells. Can 10 new hair cells per cochlea make a functional difference? Probably not, but it's an encouraging start.
Behavioral/Systems/Cognitive
Oscillating with and without Connexins
Deformation of Network Connectivity in the Inferior Olive of Connexin 36-Deficient Mice Is Compensated by Morphological and Electrophysiological Changes at the Single Neuron Level
Chris I. De Zeeuw, Edilzh Chorev, Anna Devor, Yait Manor, Ruben S. Van Der Giessen, Marcel T. De Jeu, Casper C. Hoogenraad, Jan Bijman, Tom J. H. Ruigrok, Pim French, Dick Jaarsma, Werner M. Kistler, Carola Meier, Elisabeth Petrasch-Parwez, Rolf Dermietzel, Goran Sohl, Martin Gueldenagel, Klaus Willecke, and Yosi Yarom
(see pages 4700-4711)
Generally, neuroscientists consider compensatory mechanisms to be a troublesome complication of gene knock-out technology. After all, it complicates our analysis. However, as DeZeeuw et al. document this week, such compensation may also tell us interesting things about networks. Inferior olive (IO) neurons are the origin of the powerful climbing fiber input to Purkinje cells. Dendritic gap junctions between IO neurons have been proposed to control their oscillating activity, although another recent study in the Journal [Long MA, Deans MR, Paul DL, Connors BW (2002) Rhythmicity without synchrony in the electrically uncoupled inferior olive. J Neurosci 22: 10898–10905] found that rhythmic behavior was retained in knock-outs of connexin 36 (Cx36), the connexin highly expressed in the IO. DeZeeuw et al. now report that IO neurons in mutant mice have thickened dendrites and “gap junction-like” structures but lack functional gap junctions. The neurons also had modified intrinsic electrical properties that were controlled by membrane potential rather than by the behavior of neighboring cells. The voltage-dependent oscillations appeared to result from a decrease in leak conductance and hyperexcitability in the subthreshold voltage range, seemingly attributable to hyper-polarization-sensitive membrane conductances such as Ih and low-threshold calcium channels. Manipulation of these conductances in wild-type neurons using the “dynamic clamp” technique mimicked the mutant rhythmic activity. It seems clear that IO neurons can maintain their intrinsic rhythmicity without gap junctions. Whether rapid electrical coupling provided by gap junctions tempers the intrinsic excitability in a way that ensures greater synchronization among IO neurons should be interesting to explore.
