research
Summary: Our lab investigates cellular transport pathways that are essential for brain and muscle development and compromised in Alzheimer's disease. We combine cutting edge techniques from structural biology, biophysics, biochemistry, neuroscience and cell biology to establish the molecular mechanism for positioning of the nucleus in the cell by motor protein complexes along microtubules. Furthermore, we investigate how these cellular transport pathways are affected by associated neuromuscular and neurodegenerative diseases, such as Alzheimer's disease and spinal muscular atrophy, the most common genetic cause of death in infants. Our research is funded by the National Institute of Health grant R01 GM144578.
Mutations in Bicaudal D2 cause neuromuscular disease: The dynein adapter Bicaudal D2 (BicD2) facilitates three cellular transport pathways, which are essential for brain and muscle development. The significance of these pathways is underscored by the fact that BicD2 mutations cause devastating brain development and neuromuscular diseases including spinal muscular atrophy, which is the most common genetic cause of death in infants. We have established that several mutations change the affinity of BicD2 to distinct cargoes and result in defects of the associated transport pathways. We propose that some of these mutations cause a coiled-coil registry shift in BicD2, which is a vertical displacement of the alpha-helices, that remodels the surface of BicD2 and changes its cargo selectivity. A coiled-coil registry shift would be a novel disease mechanism that may apply to other coiled coils, which are the most common tertiary structure in proteins. Read more 1 + 2.
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Role of tau protein in Alzheimer's disease: In normal brain physiology, the microtubules associated protein tau binds and regulates microtubules transport in the axons of neurons by interacting with both kinesin and dynein motor complexes. However, in Alzheimer's disease, tau becomes hyperphosphorylated which lowers its affinity to microtubules and alters its interactions with the molecular machineries of microtubules transport. Several human disease mutations of tau puts patients at increased risk for Alzheimer's disease. We investigate how such Alzheimer's disease causing mutations and hyperphoshorylation alter cellular transport pathways that are key for the survival of healthy neurons. Results will help devise therapies for Alzheimer's disease.​​
Bi-directional transport of the cell nucleus by dynein and kinesin-1 motors along microtubules facilitates fundamental processes in brain and muscle development: The cell nucleus is bi-directionally transported and positioned in a cell cycle specific manner, a process that is essential for differentiation of stem cells that give rise to the majority of neurons and glia cells in the brain. Yet, it is unknown how teams of opposing motor complexes collaborate to achieve correct timing, directionality and velocity of transport. The nuclear pore complex protein Nup358 provides recruitment sites at the cell nucleus for the opposite polarity motor complexes dynein and kinesin-1, which can bind simultaneously and facilitate bi-directional positioning of the nucleus along microtubules. Our study serves as a model system to understand how cargo adapters regulate the motility and directionality of cargo transported bi-directionally by both dynein and kinesin-1. Read more.
Cargo selection and activation mechanism of the dynein adapter Bicaudal D2: Nup358, a protein of the nuclear pore complex, facilitates a nuclear positioning pathway that is essential for neuromuscular and brain development. Nup358 interacts with the dynein adaptor Bicaudal D2 (BicD2), which in turn recruits the dynein machinery to position the nucleus. We showed that a minimal Nup358 domain activates dynein/dynactin/BicD2 for processive motility on microtubules. We identified a Nup358 α-helix, which formed the core of the Nup358-BicD2 interface. Mutations in this region of Nup358 decreased the Nup358/BicD2 interaction, resulting in decreased dynein recruitment and impaired motility. BicD2 thus recognizes Nup358 through a ‘cargo recognition α-helix,’ a structural feature that may stabilize BicD2 in its activated state and promote processive dynein motility. Read more
Single-particle cryo-electron microscopy is a cutting-edge structural biology method that allows us to determine the structures of proteins at a resolution where individual atoms can be visualized. Our lab is combining it with other structural biology methods such as X-ray crystallography, computational methods, NMR spectroscopy, circular dichroism spectroscopy, multi-angle light scattering, binding assays and small-angle X-ray scattering to analyze the structures of motor protein complexes that are important for positioning of the nucleus in the cell.
A coiled-coil registry shift may activate the dynein adapter Bicaudal D2 for dynein recruitment: Dynein adaptors such as Bicaudal D2 (BicD2) recognize cargoes and link them to dynein. In the absence of cargo, BicD2 is autoinhibited and cannot recruit dynein. Our research has established mechanistic insights into activation. Based on our X-ray structures, M.D. simulations and other data, we propose that binding of cargo induces a coiled-coil registry shift in BicD2, i.e. a vertical displacement of the two helices against each other by one helical turn, which activates BicD2 for dynein recruitment. Activation of dynein adapters such as BicD2 is a key regulatory step for transport, as adapters are required to activate dynein for processive transport. Dynein facilitates a vast number of cellular transport events that are critical for chromosome segregation, signal transmission at synapses, as well as essential for brain and muscle development.
Nucleo-cytoplasmic transport. Nuclear pore complexes consist of 30 proteins, and facilitate the selective exchange of macromolecules between the nucleus and the cytosol. Their transport channel is arguably the largest and most complex transport conduit in the eukaryotic kingdom. A central question of nuclear transport is: how can the huge protein scaffold of the nuclear pore complex adjust the diameter of its transport channel from 10 to 50 nm to accommodate cargoes of different sizes, including ribosomal subunits and viruses? To address this question, we have determined the protein structures of portions of the channel nups. Based on these structures, we have proposed a 'ring cycle hypothesis' for dilating and constricting the transport channel of the nuclear pore complex from 10-50 nm . This mechanism can help us understand how large cargo and viruses, such as HIV, cross the nuclear pore complex.