Erika L.F. Holzbaur

Mutant dynactin in motor neuron disease.

Impaired axonal transport in motor neurons has been proposed as a mechanism for neuronal degeneration in motor neuron disease. Here we show linkage of a lower motor neuron disease to a region of 4 Mb at chromosome 2p13. Mutation analysis of a gene in this interval that encodes the largest subunit of the axonal transport protein dynactin showed a single base-pair change resulting in an amino-acid substitution that is predicted to distort the folding of dynactins microtubule-binding domain. Binding assays show decreased binding of the mutant protein to microtubules. Our results show that dysfunction of dynactin-mediated transport can lead to human motor neuron disease.

Differential Regulation of Dynein and Kinesin Motor Proteins by Tau

Dynein and kinesin motor proteins transport cellular cargoes toward opposite ends of microtubule tracks. In neurons, microtubules are abundantly decorated with microtubule-associated proteins (MAPs) such as tau. Motor proteins thus encounter MAPs frequently along their path. To determine the effects of tau on dynein and kinesin motility, we conducted single-molecule studies of motor proteins moving along tau-decorated microtubules. Dynein tended to reverse direction, whereas kinesin tended to detach at patches of bound tau. Kinesin was inhibited at about a tenth of the tau concentration that inhibited dynein, and the microtubule-binding domain of tau was sufficient to inhibit motor activity. The differential modulation of dynein and kinesin motility suggests that MAPs can spatially regulate the balance of microtubule-dependent axonal transport.

Disruption of Dynein/Dynactin Inhibits Axonal Transport in Motor Neurons Causing Late-Onset Progressive Degeneration

To test the hypothesis that inhibition of axonal transport is sufficient to cause motor neuron degeneration such as that observed in amyotrophic lateral sclerosis (ALS), we engineered a targeted disruption of the dynein-dynactin complex in postnatal motor neurons of transgenic mice. Dynamitin overexpression was found to disassemble dynactin, a required activator of cytoplasmic dynein, resulting in an inhibition of retrograde axonal transport. Mice overexpressing dynamitin demonstrate a late-onset progressive motor neuron degenerative disease characterized by decreased strength and endurance, motor neuron degeneration and loss, and denervation of muscle. Previous transgenic mouse models of ALS have shown abnormalities in microtubule-based axonal transport. In this report, we describe a mouse model that confirms the critical role of disrupted axonal transport in the pathogenesis of motor neuron degenerative disease.

Cytoplasmic dynein and dynactin in cell division and intracellular transport.

Since the initial discovery of cytoplasmic dynein, it has become apparent that this microtubule-based motor is involved in several cellular functions including cell division and intracellular transport. Another multisubunit complex, dynactin, may be required for most, if not all, cytoplasmic dynein-driven activities and may provide clues to dyneins functional diversity. Recent genetic and biochemical findings have illuminated the cellular roles of dynein and dynactin and provided insight into the functional mechanism of this complex motor.

Affinity Chromatography Demonstrates a Direct Binding between Cytoplasmic Dynein and the Dynactin Complex

We used affinity chromatography to probe for a direct binding interaction between cytoplasmic dynein and dynactin. Purified cytoplasmic dynein was found to bind to an affinity column of p150, the largest polypeptide in the dynactin complex. To test the specificity of the interaction, we loaded rat brain cytosol onto the p150 affinity column and observed that cytoplasmic dynein from cytosol was specifically retained on the column. Preincubation of the p150 affinity matrix with excess exogenous dynein intermediate chain resulted in a significant reduction of dynein binding, suggesting that p150 may be interacting with dynein via this polypeptide. Therefore we constructed an affinity column of recombinant dynein intermediate chain and observed that dynactin was retained from rat brain cytosol. These results demonstrate that the native dynein and dynactin complexes are capable of direct in vitro interaction mediated by a direct binding of the dynein intermediate chain to the p150component of the dynactin complex. We have mapped the site of this interaction to the amino-terminal region of p150, which is predicted to form an α-helical coiled-coil. Regulation of the dynein-dynactin interaction may prove to be key in the control mechanism for cytoplasmic dynein-mediated vesicular transport.

Autophagosomes initiate distally and mature during transport toward the cell soma in primary neurons

Autophagosome biogenesis and maturation in primary neurons is a constitutive process that is spatially and temporally regulated along the axon.

Dynein binds to β-catenin and may tether microtubules at adherens junctions

Interactions between microtubule and actin networks are thought to be crucial for mechanical and signalling events at the cell cortex. Cytoplasmic dynein has been proposed to mediate many of these interactions. Here, we report that dynein is localized to the cortex at adherens junctions in cultured epithelial cells and that this localization is sensitive to drugs that disrupt the actin cytoskeleton. Dynein is recruited to developing contacts between cells, where it localizes with the junctional proteins β-catenin and E-cadherin. Microtubules project towards these early contacts and we hypothesize that dynein captures and tethers microtubules at these sites. Dynein immunoprecipitates with β-catenin, and biochemical analysis shows that dynein binds directly to β-catenin. Overexpression of β-catenin disrupts the cellular localization of dynein and also dramatically perturbs the organization of the cellular microtubule array. In cells overexpressing β-catenin, the centrosome becomes disorganized and microtubules no longer appear to be anchored at the cortex. These results identify a novel role for cytoplasmic dynein in capturing and tethering microtubules at adherens junctions, thus mediating cross-talk between actin and microtubule networks at the cell cortex.

Optineurin is an autophagy receptor for damaged mitochondria in parkin-mediated mitophagy that is disrupted by an ALS-linked mutation

Significance In mitophagy, damaged mitochondria recruit parkin to ubiquitinate proteins on the outer mitochondrial membrane, targeting mitochondria for autophagosome engulfment and degradation. However, the proteins involved in mediating autophagosome formation to degrade damaged and ubiquitinated mitochondria remain unknown. We used live cell imaging to demonstrate that optineurin is actively recruited to parkin-labeled ubiquitinated mitochondria and is stabilized by its ubiquitin binding domain. Optineurin binds the autophagosome protein LC3 (microtubule-associated protein light chain 3), and this binding recruits autophagosome assembly around damaged mitochondria. We find that the E478G optineurin mutation, causative for the neurodegenerative disease amyotrophic lateral sclerosis, disrupts autophagosome recruitment. As mutations in parkin are linked to Parkinson’s disease, this study indicates that defects in a single mitochondrial degradation pathway lead to neurodegenerative diseases with distinct pathologies. Mitophagy is a cellular quality control pathway in which the E3 ubiquitin ligase parkin targets damaged mitochondria for degradation by autophagosomes. We examined the role of optineurin in mitophagy, as mutations in optineurin are causative for amyotrophic lateral sclerosis (ALS) and glaucoma, diseases in which mitochondrial dysfunction has been implicated. Using live cell imaging, we demonstrate the parkin-dependent recruitment of optineurin to mitochondria damaged by depolarization or reactive oxygen species. Parkin’s E3 ubiquitin ligase activity is required to ubiquitinate outer mitochondrial membrane proteins, allowing optineurin to stably associate with ubiquitinated mitochondria via its ubiquitin binding domain; in the absence of parkin, optineurin transiently localizes to damaged mitochondrial tips. Following optineurin recruitment, the omegasome protein double FYVE-containing protein 1 (DFCP1) transiently localizes to damaged mitochondria to initialize autophagosome formation and the recruitment of microtubule-associated protein light chain 3 (LC3). Optineurin then induces autophagosome formation around damaged mitochondria via its LC3 interaction region (LIR) domain. Depletion of endogenous optineurin inhibits LC3 recruitment to mitochondria and inhibits mitochondrial degradation. These defects are rescued by expression of siRNA-resistant wild-type optineurin, but not by an ALS-associated mutant in the ubiquitin binding domain (E478G), or by optineurin with a mutation in the LIR domain. Optineurin and p62/SQSTM1 are independently recruited to separate domains on damaged mitochondria, and p62 is not required for the recruitment of either optineurin or LC3 to damaged mitochondria. Thus, our study establishes an important role for optineurin as an autophagy receptor in parkin-mediated mitophagy and demonstrates that defects in a single pathway can lead to neurodegenerative diseases with distinct pathologies.

Processive bidirectional motion of dynein-dynactin complexes in vitro

Cytoplasmic dynein is the primary molecular motor responsible for transport of vesicles, organelles, proteins and RNA cargoes from the periphery of the cell towards the nucleus along the microtubule cytoskeleton of eukaryotic cells. Dynactin, a large multi-subunit activator of dynein, docks cargo to the motor and may enhance dynein processivity. Here, we show that individual fluorescently labelled dynein–dynactin complexes exhibit bidirectional and processive motility towards both the plus and minus ends of microtubules. The dependence of this activity on substrate ATP concentration, nucleotide analogues and inhibitors suggests that bidirectional motility is an active energy-transduction property of dynein–dynactin motor mechano-chemistry. The unique motility characteristics observed may reflect the flexibility of the dynein structure that leads to an enhanced ability to navigate around obstacles in the cell.

Axonal Transport: Cargo-Specific Mechanisms of Motility and Regulation

Axonal transport is essential for neuronal function, and many neurodevelopmental and neurodegenerative diseases result from mutations in the axonal transport machinery. Anterograde transport supplies distal axons with newly synthesized proteins and lipids, including synaptic components required to maintain presynaptic activity. Retrograde transport is required to maintain homeostasis by removing aging proteins and organelles from the distal axon for degradation and recycling of components. Retrograde axonal transport also plays a major role in neurotrophic and injury response signaling. This review provides an overview of axonal transport pathways and discusses their role in neuronal function.