Biology subjects

Holzbaur, E. L. F.

Publications and source records attributed to Holzbaur, E. L. F..

5 recordsLinked to original sources

bioRxiv2021

Structural Basis for Membrane Recruitment of ATG16L1 by WIPI2 in Autophagy

Autophagy is a cellular process that degrades cytoplasmic cargo by engulfing it in a double membrane vesicle, known as the autophagosome, and delivering it to the lysosome. The ATG12-5-16L1 complex is responsible for conjugating members of the ubiquitin-like ATG8 protein family to phosphatidylethanolamine in the growing autophagosomal membrane, known as the phagophore. ATG12-5-16L1 is recruited to the phagophore by a subset of the phosphatidylinositol 3-phosphate-binding seven bladed {beta}-propeller WIPI proteins. We determined the crystal structure of WIPI2d in complex with the WIPI2 interacting region (W2IR) of ATG16L1 comprising residues 207-230 at 1.85 [A] resolution. The structure shows that the ATG16L1 W2IR adopts an alpha helical conformation and binds in an electropositive and hydrophobic groove between WIPI2 {beta}-propeller blades 2 and 3. Mutation of residues at the interface reduces or blocks the recruitment of ATG12-5-16L1 and the conjugation of the ATG8 protein LC3B to synthetic membranes. Interface mutants show a decrease in starvation-induced autophagy. Comparisons across the four human WIPIs suggest that WIPI1 and 2 belong to a W2IR-binding subclass responsible for localizing ATG12-5-16L1 and driving ATG8 lipidation, whilst WIPI3 and 4 belong to a second W34IR-binding subclass responsible for localizing ATG2, and so directing lipid supply to the nascent phagophore. The structure provides a framework for understanding the regulatory node connecting two central events in autophagy initiation, the action of the autophagic PI 3-kinase complex on the one hand, and ATG8 lipidation on the other.

cell biology↗

bioRxiv2021

ALS and FTD-associated missense mutations in TBK1 differentially disrupt mitophagy

TANK-binding kinase 1 (TBK1) is a multi-functional kinase with an essential role in mitophagy, the selective clearance of damaged mitochondria. More than 90 distinct mutations in TBK1 are linked to amyotrophic lateral sclerosis (ALS) and fronto-temporal dementia (FTD), including missense mutations that disrupt the ability of TBK1 to dimerize, associate with the mitophagy receptor optineurin (OPTN), auto-activate, or catalyze phosphorylation. We investigated how ALS-associated mutations in TBK1 affect Parkin-dependent mitophagy using imaging to dissect the molecular mechanisms involved in clearing damaged mitochondria. Some mutations cause severe dysregulation of the pathway, while others induce limited disruption. Mutations that abolish either TBK1 dimerization or kinase activity were insufficient to fully inhibit mitophagy, while mutations that reduced both dimerization and kinase activity were more disruptive. Ultimately, both TBK1 recruitment and OPTN phosphorylation at S177 are necessary for engulfment of damaged mitochondra by autophagosomal membranes. Surprisingly, we find that ULK1 activity contributes to the phosphorylation of OPTN in the presense of either WT- or kinase inactive TBK1. In primary neurons, TBK1 mutants induce mitochondrial stress under basal conditions; network stress is exacerbated with further mitochondrial insult. Our study further refines the model for TBK1 function in mitophagy, demonstrating that some ALS-linked mutations likely contribute to disease pathogenesis by inducing mitochondrial stress or inhibiting mitophagic flux. Other TBK1 mutations exhibited much less impact on mitophagy in our assays, suggesting that cell-type specific effects, cumulative damage, or alternative TBK1-dependent pathways such as innate immunity and inflammation also factor into the development of ALS in affected individuals. SIGNIFICANCE STATEMENTMissense mutations in TANK-binding kinase 1 (TBK1) have various biophysical and biochemical effects on the molecule, and are associated with the neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and fronto-temporal dementia (FTD). TBK1 plays an essential role in clearing damaged mitochondria. Here, we investigate the impact of 10 ALS-linked TBK1 mutations on the critical early stage of mitophagy. We find that both TBK1 recruitment and kinase activity contribute to the clearance of the damaged mitochondria. Furthermore, in neurons, expression of TBK1 mutants alone affects mitochondrial network health. Our investigation utilizes disease-linked mutations to further refine the current model of mitophagy, identifying crosstalk between the regulatory kinases TBK1 and ULK1, and providing new insights into the roles of TBK1 in neurodegenerative pathogenesis.

cell biology↗

bioRxiv2021

Kinesin-4 Motor Teams Effectively Navigate Dendritic Microtubule Arrays Through Track Switching and Regulation of Microtubule Dynamics

Microtubules establish the directionality of intracellular transport by kinesins and dynein through their polarized assembly, but it remains unclear how directed transport occurs along microtubules organized with mixed polarity. We investigated the ability of the plus-end directed kinesin-4 motor KIF21B to navigate mixed polarity microtubules in mammalian dendrites. Reconstitution assays with recombinant KIF21B and engineered microtubule bundles or extracted neuronal cytoskeletons indicate that nucleotide- independent microtubule binding regions of KIF21B modulate microtubule dynamics and promote directional switching on antiparallel microtubules. Optogenetic recruitment of KIF21B to organelles in live neurons resulted in unidirectional transport in axons but bi-directional transport with a net retrograde bias in dendrites; microtubule dynamics and the secondary microtubule binding regions are required for this net directional bias. We propose a model in which cargo-bound KIF21B motors coordinate nucleotide- sensitive and insensitive microtubule binding sites to achieve net retrograde movement along the dynamic mixed polarity microtubule arrays of dendrites.

cell biology↗

bioRxiv2020

Cega: A Single Particle Segmentation Algorithm to Identify Moving Particles in a Noisy System

Improvements to particle tracking algorithms are required to effectively analyze the motility of biological molecules in complex or noisy systems. A typical single particle tracking (SPT) algorithm detects particle coordinates for trajectory assembly. However, particle detection filters fail for datasets with low signal-to-noise levels. When tracking molecular motors in complex systems, standard techniques often fail to separate the fluorescent signatures of moving particles from background noise. We developed an approach to analyze the motility of kinesin motor proteins moving along the microtubule cytoskeleton of extracted neurons using the Kullback-Leibler (KL) divergence to identify regions where there are significant differences between models of moving particles and background signal. We tested our software on both simulated and experimental data and found a noticeable improvement in SPT capability and a higher identification rate of motors as compared to current methods. This algorithm, called Cega, for find the object, produces data amenable to conventional blob detection techniques that can then be used to obtain coordinates for downstream SPT processing. We anticipate that this algorithm will be useful for those interested in tracking moving particles in complex in vitro or in vivo environments.

cell biology↗

bioRxiv2020

Sequential dynein effectors regulate axonal autophagosome motility in a maturation-dependent pathway

Autophagy is a degradative pathway required to maintain neuronal homeostasis. Neuronal autophagosomes form constitutively at the axon terminal and mature via lysosomal fusion during dynein-mediated transport to the soma. How the dynein-autophagosome interaction is regulated during maturation is unknown. Here, we identify a series of handoffs between dynein effectors as autophagosomes transit along the axons of primary neurons. In the distal axon, JIP1 initiates autophagosomal transport, while autophagosomes in the mid-axon require HAP1 and Huntingtin for motility. We demonstrate that HAP1 is a bonafide dynein activator, binding the dynein-dynactin complex via canonical and noncanonical interactions. JIP3 is found on most axonal autophagosomes but specifically regulates the transport of acidified autolysosomes. Inhibiting autophagosomal transport disrupts maturation, while inhibiting autophagosomal maturation perturbs the association and function of dynein effectors. Thus maturation and transport are tightly linked. These results describe a novel maturation-based dynein effector handoff on neuronal autophagosomes that is key to autophagosomal motility, cargo degradation, and the maintenance of axonal health. SummaryNeuronal autophagosomes form in the distal axon and mature via fusion with lysosomes during their dynein-driven transport to the soma. Dynein is regulated on autophagosomes by distinct effector proteins--JIP1, HAP1, and JIP3--depending on location and autophagosomal maturity. In this sequential pathway, transport and maturation state are tightly linked to maintain neuronal health.

cell biology↗