The cytoskeleton is the major intrinsic determinant of the shape of a neuron and is responsible for the asymmetric distribution of organelles within the cytoplasm and many other cytoskeletal-dependent processes. The neurological diseases that display cytoskeletal abnormalities include Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), parkinson’s, infantile spinal muscular atrophy (SPA), giant axonal neuropathy (GAN), and hereditary sensory motor neuropathy (HSMN). We have been using model systems to gain knowledge of the pathogenic course of the neurodegenerative diseases that are associated with cytoskeletal deregulations. We employ a wide range of techniques to conduct thorough molecular and biochemical examinations, and to link molecular defects to cell biological processes. We focus on two main areas of investigation: How do cytoskeletal components interact with each other to maintain integrations of neuronal structure and function? How do cytoskeletal abnormalities mediate neurodegeneration and cell death?
An aberrant regulation on cytoskeletal components in GAN disorder
Among all of the above-mentioned devastating diseases, the generalized cytoskeletal abnormalities found in GAN appear to be especially prominent. For this reason, we consider this severe and progressive neuropathy as an ideal disease model for my lab to imitate the exploration on correlations between aberrant cytoskeleton and neurological diseases. With a line of evidence, my lab demonstrates that gigaxonin specifically mediates protein degradations of the cytoskeletal proteins via ubiquitin-proteasome dependent mechanism (JBC, 2002; Nature, 2005, Curr Biol, 2005; HMG, 2006). Our findings reveal that, rather than being a direct cytoskeletal organizer, gigaxonin acts actively as distinct regulator of cytoskeletons that could represent a general target for other neurodegenerative disorders with cytoskeletal alterations. The studies on the gigaxonin-mediated UPS pathway suggest a novel activity of the ubiquitination cascade, or may mark the existence of a derivative UPS pathway. In fact, the GAN-associated mutations tested in our in vitro assays could disrupt completely or impairs substantially the interactions between gigaxonin and those cytoskeletal related proteins. We found that accumulation of the tubulin chaperon protein is the direct cause of dramatic reduction of microtubule density observed in GAN samples (Curr Biol, 2005). It is probably due to tubulin misfolding subsequently leading to reduced microtubule assembly. Likewise, high levels of microtubule binding proteins result in pathogenic alterations of microtubule structure and dynamics and are proved to be sufficient to cause axonal degeneration and cell death (Nature, 2005; HMG, 2006). The identification on importance of precise regulation of cytoskeletal components has profound implications for understanding many neurological disorders featuring cytoskeleton pathology.
Axonal transport and sensory neuron degeneration
Neurons possess extraordinary long axons which may require a more complex system to meet the unusual transport challenges that are far beyond those of nonneuronal cells. Impaired axonal transport has been long implicated as a mechanism underlying axonal degeneration and neuronal death. How do axonal transport systems coordinate and exchange cargoes for transport? How do pathogenic factors exert their effects on transport systems? My lab is interested in unraveling the fundamental mechanisms of axonal transport.
BPAG1 (Bullous Pemphigoid Antigen 1) null mouse is an interesting neurological mutant that features cytoskeletal disorganization and severely disrupted axonal transport with heavily accumulated vesicles and other membranous organelles in sensory neurons. This mutant is invaluable for conducting transport studies. The fourth neural isoform of BPAG1 (BPAG1n4, ~600kDa) recently characterized in my lab appears to be particularly interesting. We found that BPAG1n4 interacts directly with dynactin / dynein, the retrograde molecular motor complex, through its unique ERM1 domain (Liu et al., JCB, 2003). Using dominant negative manner with ERM1 domain to disrupt the BPAG1n4-dynactin interaction, the impaired retrograde transport in cultured DRG neurons (dorsal root ganglia) recapitulates the transport phenotype observed in the BPAG1 null neurons. My lab has also recently tracked our effort on continuing uncovering other interactor of BPAG1n4. After a series studies, we identified a novel vesicle protein, which we named as retrolinkin. Retrolinkin is a type II transmembrane protein of endosomal vesicles (PNAS, 2007). We found that retrolinkin recruits the ERM2 domain of BPAG1n4 to the surface of endosomal vesicle surface. Disrupted interaction between BPAG1n4 and retrolinkin severs motor proteins from vesicle cargoes causing severe defect in retrograde axonal transport, culminated in neurodegeneration and cell death (PNAS, 2007). In addition to ERM1 and ERM2 domains, BPAG1n4 also contains microtubule binding site at the C-terminus which is also required for its function in the retrograde transport. Thus, BPAG1n4 has been defined as the first protein orchestrating all three transport components: vesicular cargos, molecular motors, and the cytoskeletal rails, to facilitate retrograde axonal transport. The identification of the endosomal membrane receptor protein, retrolinkin, may open a new field for defining vesicle adaptors / receptors responsible for cargo selection and specificity.
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