12 February 2007. The fate of neurons is to be spread thin. Their axons and outlying synapses depend on a constant flow of materials to and from the cell body, making efficient transport a survival skill. Two papers published this week in PNAS examine the transport problem, one in the context of neurodegenerative disorders characterized by a “dying back” of axons, and the other defining a new player in the docking of transport vesicles to motor proteins.
In the first study, Scott Brady and colleagues of the University of Illinois at Chicago continue their dissection of the effects of different neurodegenerative perpetrators on the mechanics of axonal transport. The current contribution uses the group’s isolated squid axoplasm system to look at the effects of the Parkinson disease-causing toxin 1-methyl-4-phenylpyridinium (MPP+). The squid preparation provides a cell-free soup of extruded axoplasm that maintains both anterograde and retrograde transport capabilities. Because the system has no cell membrane, the investigators can directly apply various proteins or chemicals and assess the effects on transport in both directions. Previous work has shown how polyglutamine expanded proteins and Aβ both directly affect transport in this system (see ARF related news story and ARF SfN coverage).
First author Gerardo Morfini worked with Rodolfo Llinas and colleagues at the New York University School of Medicine to demonstrate that MPP+ treatment significantly increased dynein-dependent retrograde vesicle transport (away from synapses) and slightly decreased kinesin-1 mediated anterograde transport (toward synapses). Using specific enzyme inhibitors, they showed that the axonal effects depended on the activation of caspase 3 by the protein kinase Cδ isoform.
Those changes had ramifications for synapses. When the investigators injected MPP+ into the presynaptic region of intact squid giant synapses, they saw a decrease in vesicle abundance at the synapses. An accompanying paper (Serulle et al., 2007) describes the structural and functional study of the MPP+ injected synapses, which show diminished neurotransmission due to the scarcity of vesicles available for release.
The starvation of synapses by increased transport out, and decreased transport in, could lead to the type of neuronal death observed in Parkinson disease, the researchers speculate. “We propose that PD and other neurodegenerative diseases exhibiting dying-back neuropathology represent a previously undescribed category of neurological diseases characterized by dysfunction of vesicle transport and associated with the loss of synaptic function,” they write.
In the second paper, Yanmin Yang from Stanford University in California looks at the mechanism of retrograde transport, and specifically at how dynein motor proteins couple with vesicular cargoes. First author Jia-Jia Liu and coworkers identify a protein they call retrolinkin as the receptor that tethers cargoes to the dynein adaptor protein BPAG1n4 in sensory neurons. The role of retrolinkin in retrograde transport appears analogous to that discovered for the amyloid precursor protein in anterograde transport, where it was recently shown to link vesicles to the kinesin motor (see ARF related news story).
The investigators initially identified retrolinkin in a yeast two-hybrid screen for BPAG1n4-interacting proteins. They showed retrolinkin was a transmembrane protein localized in endosomal vesicles, and that it directly bound BPAG1n4 via a cytoplasmic domain. A fragment of that cytoplasmic domain acted as a dominant-negative inhibitor of dynein-mediated vesicular transport in cells.
Further studies using immunoelectron microscopy revealed that in normal mice, dynein occurred on the surface of retrolinkin-associated vesicles, but not on vesicles from BPAG1 knockout mice. These mice are severely deficient in retrograde transport, and also suffer from degeneration of sensory neurons (Guo et al., 1995). While the current work focuses on sensory neurons, the site of most BPAG1n4 expression, it may spur the hunt for other cell-specific vesicle adaptors or receptors that function critically in the health and well-being of other types of neurons.—Pat McCaffrey.
Morfini G, Pigino G, Opalach K, Serulle Y, Moreira JE, Sugimori M, Llinas RR, Brady ST. 1-Methyl-4-phenylpyridinium affects fast axonal transport by activation of caspase and protein kinase C. Proc Natl Acad Sci U S A. 2007 Feb 7; [Epub ahead of print] Abstract
Liu JJ, Ding J, Wu C, Bhagavatula P, Cui B, Chu S, Mobley WC, Yang Y. Retrolinkin, a membrane protein, plays an important role in retrograde axonal transport. Proc Natl Acad Sci U S A. 2007 Feb 7; [Epub ahead of print] Abstract