Biology subjects

Hyman, A. A.

Publications and source records attributed to Hyman, A. A..

6 recordsLinked to original sources

bioRxiv2018

Kinetically distinct phases of tau on microtubules regulate kinesin motors and severing enzymes

Tau is an intrinsically disordered protein, which diffuses on microtubules. In neurodegenerative diseases collectively termed tauopathies, tau malfunction and its detachment from axonal microtubules is correlated with microtubule degradation. It is known that tau can protect microtubules from microtubule-degrading enzymes, such as katanin. However, how tau can fulfill such regulative function is still unclear. Using in vitro reconstitution, we here show that tau molecules on microtubules cooperatively form islands of an ordered layer with regulatory qualities distinct from a comparably dense layer of diffusible tau. These islands shield the microtubules from katanin and kinesin-1 but are penetrable by kinesin-8 which causes the islands to disassemble. Our results indicate a new phase of tau, constituting an adjustable protective sheath around microtubules.

biophysics↗

bioRxiv2017

Rab5 and Alsin regulate stress-activated cytoprotective signaling on mitochondria

Mitochondrial stress response is essential for cell survival, and damaged mitochondria are a hallmark of neurodegenerative diseases. It is thus fundamental to understand how mitochondria relay information within the cell. Here, by investigating mitochondrial-endosome contact sites we made the surprising observation that the small GTPase Rab5 translocates from early endosomes to the outer mitochondrial membrane upon oxidative stress. This is accompanied by an increase in Rab5-positive endosomes in contact with mitochondria. Interestingly, activation of Rab5 on mitochondria depend on the Rab5-GEF ALS2/Alsin, which is encoded by a gene mutated in amyotrophic lateral sclerosis (ALS). Alsin-/- human induced pluripotent stem cell-derived spinal motor neurons cannot relocate Rab5 to mitochondria and display increased susceptibility to oxidative stress. These findings define a novel pathway whereby Alsin catalyzes assembly of the Rab5 endocytic machinery on mitochondria. Defects in stress-sensing by endosomes could be crucial for mitochondrial quality control during the onset of ALS.

cell biology↗

bioRxiv2017

Phosphatase PP2A and microtubule pulling forces disassemble centrosomes during mitotic exit

Centrosomes are major microtubule-nucleating organelles that facilitate chromosome segregation and cell division in metazoans. Centrosomes comprise centrioles that organize a micron-scale mass of protein called pericentriolar material (PCM) from which microtubules nucleate. During each cell cycle, PCM accumulates around centrioles through phosphorylation-mediated assembly of PCM scaffold proteins. During mitotic exit, PCM swiftly disassembles by an unknown mechanism. Here, we used Caenorhabditis elegans embryos to determine the mechanism and importance of PCM disassembly in dividing cells. We found that the phosphatase PP2A and its regulatory subunit SUR-6 (PP2ASUR-6), together with cortically directed microtubule pulling forces, actively disassemble PCM. In embryos depleted of these activities, [~]25% of PCM persisted from one cell cycle into the next, resulting in cytokinetic furrow ingression errors, excessive centrosome accumulation, and embryonic death. Purified PP2ASUR-6 could dephosphorylate the major PCM scaffold protein SPD-5 in vitro. Our data suggest that PCM disassembly occurs through a combination of dephosphorylation of PCM components and catastrophic rupture of the PCM scaffold.

cell biology↗

bioRxiv2017

Local Nucleation Of Microtubule Bundles Through Tubulin Concentration Into A Condensed Tau Phase

Non-centrosomal microtubule bundles play important roles in cellular organization and function. Although many diverse proteins are known that can bundle microtubules, biochemical mechanisms by which cells could locally control the nucleation and formation of microtubule bundles are understudied. Here, we demonstrate that concentration of tubulin into a condensed, liquid-like compartment composed of the unstructured neuronal protein tau is sufficient to nucleate microtubule bundles. We show that under conditions of macro-molecular crowding, tau forms liquid drops. Tubulin partitions into these drops, efficiently increasing tubulin concentration and driving the nucleation of microtubules. These growing microtubules form bundles enclosed in a liquid sheath of tau. Our data suggest that condensed compartments of microtubule bundling proteins could promote the local formation of microtubule bundles in neurons by acting as non-centrosomal microtubule nucleation centers, and that liquid-like tau encapsulation could provide both stability and plasticity to long axonal microtubule bundles.

cell biology↗

bioRxiv2016

Parallel adaptation to higher temperatures in divergent clades of the nematode Pristionchus pacificus

Studying the effect of temperature on fertility is particularly important in the light of ongoing climate change. We need to know if organisms can adapt to higher temperatures and, if so, what are the evolutionary mechanisms behind such adaptation. Such studies have been hampered by the lack different populations of sufficient sizes with which to relate the phenotype of temperature tolerance to the underlying genotypes. Here, we examined temperature adaptation in populations of the nematode Pristionchus pacificus, in which individual strains are able to successfully reproduce at 30{degrees}C. Analysis of the frequency of heat tolerant strains in different temperature zones on La Reunion supports that this trait is subject to natural selection. Reconstruction of ancestral states along the phylogeny of highly differentiated P. pacificus clades suggests that heat tolerance evolved multiple times independently. This is further supported by genome wide association studies showing that heat tolerance is a polygenic trait and that different loci are used by individual P. pacificus clades to develop heat tolerance. More precisely, analysis of allele frequencies indicated that most genetic markers that are associated with heat tolerance are only polymorphic in individual clades. While in some P. pacificus clades, parallel evolution of heat tolerance can be explained by ancestral polymorphism or by gene flow across clades, we observe at least one clearly distinct and independent scenario where heat tolerance emerged by de novo mutation. Thus, temperature tolerance evolved at least two times independently in the evolutionary history of this species. Our data suggest that studies of wild populations of P. pacificus will reveal distinct cellular mechanisms driving temperature adaptation.

evolutionary biology↗

bioRxiv2016

The centrosome is a selective phase that nucleates microtubules by concentrating tubulin

Centrosomes are non-membrane-bound compartments that nucleate microtubule arrays. They consist of nanometer-scale centrioles surrounded by a micron-scale, dynamic assembly of protein called the pericentriolar material (PCM). To study how PCM forms a spherical compartment that nucleates microtubules, we reconstituted PCM-dependent microtubule nucleation in vitro using recombinant C.elegans proteins. We found that macromolecular crowding drives phase separation of the key PCM scaffold protein SPD-5 into spherical droplets that morphologically and dynamically resemble in vivo PCM. These SPD-5 droplets recruited the microtubule polymerase ZYG-9 (XMAP215 homologue) and the microtubule-stabilizing protein TPXL-1 (TPX2 homologue). Together, these three proteins concentrated tubulin ~4- fold over background, which was sufficient to reconstitute nucleation of microtubule asters in vitro. Our results suggest that in vivo PCM is a selective phase that organizes microtubule arrays through localized concentration of tubulin by microtubule effector proteins.\n\nOne Sentence SummaryPhase separation of C. elegans centrosome proteins drive the formation of micron-sized microtubule organizing centers.

cell biology↗