Amyloid-like Self-Assembly of a Cellular Compartment.

Most vertebrate oocytes contain a Balbiani body, a large, non-membrane-bound compartment packed with RNA, mitochondria, and other organelles. Little is known about this compartment, though it specifies germline identity in many non-mammalian vertebrates. We show Xvelo, a disordered protein with an N-terminal prion-like domain, is an abundant constituent of Xenopus Balbiani bodies. Disruption of the prion-like domain of Xvelo, or substitution with a prion-like domain from an unrelated protein, interferes with its incorporation into Balbiani bodies in vivo. Recombinant Xvelo forms amyloid-like networks in vitro. Amyloid-like assemblies of Xvelo recruit both RNA and mitochondria in binding assays. We propose that Xenopus Balbiani bodies form by amyloid-like assembly of Xvelo, accompanied by co-recruitment of mitochondria and RNA. Prion-like domains are found in germ plasm organizing proteins in other species, suggesting that Balbiani body formation by amyloid-like assembly could be a conserved mechanism that helps oocytes function as long-lived germ cells.

Dividing cells regulate their lipid composition and localization.

Although massive membrane rearrangements occur during cell division, little is known about specific roles that lipids might play in this process. We report that the lipidome changes with the cell cycle. LC-MS-based lipid profiling shows that 11 lipids with specific chemical structures accumulate in dividing cells. Using AFM, we demonstrate differences in the mechanical properties of live dividing cells and their isolated lipids relative to nondividing cells. In parallel, systematic RNAi knockdown of lipid biosynthetic enzymes identified enzymes required for division, which highly correlated with lipids accumulated in dividing cells. We show that cells specifically regulate the localization of lipids to midbodies, membrane-based structures where cleavage occurs. We conclude that cells actively regulate and modulate their lipid composition and localization during division, with both signaling and structural roles likely. This work has broader implications for the active and sustained participation of lipids in basic biology.

A human organoid system that self-organizes to recapitulate growth and differentiation of a benign mammary tumor.

As 3D culture has become central to investigation of tissue biology, mammary epithelial organoids have emerged as powerful tools for investigation of epithelial cell polarization and carcinogenesis. However, most current protocols start from single cells suspended in Matrigel, which can also restrict cell differentiation and behavior. Here, we show that the noncancerous mammary cell line HMT-3522 S1, when allowed to spontaneously form cell aggregates ("spheroids") in medium without Matrigel, switches to a collective growth mode that recapitulates many attributes of "usual ductal hyperplasia" (UDH), a common benign mammary lesion. Interestingly, these spheroids undergo a complex maturation process reminiscent of embryonic development: solid-cell cords form their own basement membrane, grow on the surface of initially homogeneous cell aggregates, and form asymmetric lumina lined by two distinct cell types that express basal and luminal cytokeratins. This sequence of events provides a cellular mechanism that explains how the characteristic crescent-shaped, asymmetrical lumina form in UDH. Our results suggest that HMT-3522 S1 spheroids are useful as an in vitro model system to study UDH biology, glandular lumen formation, and stem cell biology of the mammary gland.

Proteomics of phosphorylation and protein dynamics during fertilization and meiotic exit in the Xenopus egg.

Fertilization releases the meiotic arrest and initiates the events that prepare the egg for the ensuing developmental program. Protein degradation and phosphorylation are known to regulate protein activity during this process. However, the full extent of protein loss and phosphoregulation is still unknown. We examined absolute protein and phosphosite dynamics of the fertilization response by mass spectrometry-based proteomics in electroactivated eggs. To do this, we developed an approach for calculating the stoichiometry of phosphosites from multiplexed proteomics that is compatible with dynamic, stable, and multisite phosphorylation. Overall, the data suggest that degradation is limited to a few low-abundance proteins. However, this degradation promotes extensive dephosphorylation that occurs over a wide range of abundances during meiotic exit. We also show that eggs release a large amount of protein into the medium just after fertilization, most likely related to the blocks to polyspermy. Concomitantly, there is a substantial increase in phosphorylation likely tied to calcium-activated kinases. We identify putative degradation targets and components of the slow block to polyspermy. The analytical approaches demonstrated here are broadly applicable to studies of dynamic biological systems.

The Atg1-Tor pathway regulates yolk catabolism in Drosophila embryos.

Yolk provides an important source of nutrients during the early development of oviparous organisms. It is composed mainly of vitellogenin proteins packed into membrane-bound compartments called yolk platelets. Catabolism of yolk is initiated by acidification of the yolk platelet, leading to the activation of Cathepsin-like proteinases, but it is unknown how this process is triggered. Yolk catabolism initiates at cellularization in Drosophila melanogaster embryos. Using maternal shRNA technology we found that yolk catabolism depends on the Tor pathway and on the autophagy-initiating kinase Atg1. Whereas Atg1 was required for a burst of spatially regulated autophagy during late cellularization, autophagy was not required for initiating yolk catabolism. We propose that the conserved Tor metabolic sensing pathway regulates yolk catabolism, similar to Tor-dependent metabolic regulation on the lysosome.

Human cytomegalovirus UL50 and UL53 recruit viral protein kinase UL97, not protein kinase C, for disruption of nuclear lamina and nuclear egress in infected cells.

Herpesvirus nucleocapsids traverse the nuclear envelope into the cytoplasm in a process called nuclear egress that includes disruption of the nuclear lamina. In several herpesviruses, a key player in nuclear egress is a complex of two proteins, whose homologs in human cytomegalovirus (HCMV) are UL50 and UL53. However, their roles in nuclear egress during HCMV infection have not been shown. Based largely on transfection studies, UL50 and UL53 have been proposed to facilitate disruption of the nuclear lamina by recruiting cellular protein kinase C (PKC), as occurs with certain other herpesviruses, and/or the viral protein kinase UL97 to phosphorylate lamins. To investigate these issues during HCMV infection, we generated viral mutants null for UL50 or UL53. Correlative light electron microscopic analysis of null mutant-infected cells showed the presence of intranuclear nucleocapsids and the absence of cytoplasmic nucleocapsids. Confocal immunofluorescence microscopy revealed that UL50 and UL53 are required for disruption of the nuclear lamina. A subpopulation of UL97 colocalized with the nuclear rim, and this was dependent on UL50 and, to a lesser extent, UL53. However, PKC was not recruited to the nuclear rim, and its localization was not affected by the absence of UL50 or UL53. Immunoprecipitation from cells infected with HCMV expressing tagged UL53 detected UL97 but not PKC. In summary, HCMV UL50 and UL53 are required for nuclear egress and disruption of nuclear lamina during HCMV infection, and they recruit UL97, not PKC, for these processes. Thus, despite the strong conservation of herpesvirus nuclear egress complexes, a key function can differ among them.

Dopamine receptor D3 regulates endocytic sorting by a Prazosin-sensitive interaction with the coatomer COPI.

Macromolecules enter cells by endocytosis and are sorted to different cellular destinations in early/sorting endosomes. The mechanism and regulation of sorting are poorly understood, although transitions between vesicular and tubular endosomes are important. We found that the antihypertensive drug Prazosin inhibits endocytic sorting by an off-target perturbation of the G protein-coupled receptor dopamine receptor D(3) (DRD3). Prazosin is also a potent cytokinesis inhibitor, likely as a consequence of its effects on endosomes. Prazosin stabilizes a normally transient interaction between DRD3 and the coatomer COPI, a complex involved in membrane transport, and shifts endosomal morphology entirely to tubules, disrupting cargo sorting. RNAi depletion of DRD3 alone also inhibits endocytic sorting, indicating a noncanonical role for a G protein-coupled receptor. Prazosin is a powerful tool for rapid and reversible perturbation of endocytic dynamics.

Tankyrase-1 polymerization of poly(ADP-ribose) is required for spindle structure and function.

Poly(ADP-ribose) (PAR) is a large, negatively charged post-translational modification that is produced by polymerization of NAD+ by PAR polymerases (PARPs). There are at least 18 PARPs in the human genome, several of which have functions that are unknown. PAR modifications are dynamic; PAR structure depends on the balance between synthesis and hydrolysis by PAR glycohydrolase2. We previously found that PAR is enriched in vertebrate somatic-cell mitotic spindles and demonstrated a requirement for PAR in the assembly of Xenopus egg extract spindles. Here, we knockdown all characterized PARPs using RNA interference (RNAi), and identify tankyrase-1 as the PARP that is required for mitosis. Tankyrase-1 localizes to mitotic spindle poles, to telomeres and to the Golgi apparatus. Tankyrase-1 RNAi was recently shown to result in mitotic arrest, with abnormal chromosome distributions and spindle morphology observed--data that is interpreted as evidence of post-anaphase arrest induced by failure of telomere separation6. We show that tankyrase-1 RNAi results in pre-anaphase arrest, with intact sister-chromatid cohesion. We also demonstrate a requirement for tankyrase-1 in the assembly of bipolar spindles, and identify the spindle-pole protein NuMA as a substrate for covalent modification by tankyrase-1.

Human Cytomegalovirus Nuclear Egress Complex Subunit, UL53, Associates with Capsids and Myosin Va, but Is Not Important for Capsid Localization towards the Nuclear Periphery.

After herpesviruses encapsidate their genomes in replication compartments (RCs) within the nuclear interior, capsids migrate to the inner nuclear membrane (INM) for nuclear egress. For human cytomegalovirus (HCMV), capsid migration depends at least in part on nuclear myosin Va. It has been reported for certain herpesviruses that the nucleoplasmic subunit of the viral nuclear egress complex (NEC) is important for this migration. To address whether this is true for HCMV, we used mass spectrometry and multiple other methods to investigate associations among the HCMV NEC nucleoplasmic subunit, UL53, myosin Va, major capsid protein, and/or capsids. We also generated complementing cells to derive and test HCMV mutants null for UL53 or the INM NEC subunit, UL50, for their importance for these associations and, using electron microscopy, for intranuclear distribution of capsids. We found modest associations among the proteins tested, which were enhanced in the absence of UL50. However, we found no role for UL53 in the interactions of myosin Va with capsids or the percentage of capsids outside RC-like inclusions in the nucleus. Thus, UL53 associates somewhat with myosin Va and capsids, but, contrary to reports regarding its homologs in other herpesviruses, is not important for migration of capsids towards the INM.

Reassembly of contractile actin cortex in cell blebs.

Contractile actin cortex is involved in cell morphogenesis, movement, and cytokinesis, but its organization and assembly are poorly understood. During blebbing, the membrane detaches from the cortex and inflates. As expansion ceases, contractile cortex re-assembles under the membrane and drives bleb retraction. This cycle enabled us to measure the temporal sequence of protein recruitment to the membrane during cortex reassembly and to explore dependency relationships. Expanding blebs were devoid of actin, but proteins of the erythrocytic submembranous cytoskeleton were present. When expansion ceased, ezrin was recruited to the membrane first, followed by actin, actin-bundling proteins, and, finally, contractile proteins. Complete assembly of the contractile cortex, which was organized into a cagelike mesh of filaments, took approximately 30 s. Cytochalasin D blocked recruitment of actin and alpha-actinin, but had no effect on membrane association of ankyrin B and ezrin. Ezrin played no role in actin nucleation, but was essential for tethering the membrane to the cortex. The Rho pathway was important for cortex assembly in blebs.

Improper Fit in American Youth Football Helmets Across One Competitive Season.

Improper helmet fit is related to sport-related concussion symptomology. The objective of this study was to determine the prevalence of improperly fit helmets in American youth tackle football players across one competitive season. Four recreation leagues including 147 players (45.2 ± 14.7 cm, 147.5 ± 9.0 kg), aged 7-12 years, participated in pre-season and post-season data collection timepoints. Participant and league demographics were collected at pre-season. Helmet fit was assessed at pre- and post-season using a 13-item checklist. A helmet was defined as improperly fit if failed to comply with or more of the checklist items. Most players (84%) rented helmets from the league. At preseason, 71.4% of helmets, and at post-season 79.6%, were improperly fit with no significant change over time (p = 0.14). Of the 105 improperly fit helmets at the start of the season, 61% were still considered improperly fit at post season. The 11-12 year old age group had significantly more improperly fit helmets than the 7-10 year old age group at post-season (p = 0.033), but not pre-season (p = 0.655). American youth football players depend on the league to fit their helmet. Most players did not meet at least one checklist criteria. Helmets improperly fit at preseason were still not fit at post.

Graded actin filament polarity is the organization of oriented actomyosin II filament bundles required for fibroblast polarization.

Actomyosin II filament assemblies in cells are required for shaping the cell body and forming the cell rear during morphological polarization and triggering of migration. However, precise steps in myosin II-based mechanisms are unknown in this event; one reason is due to lack of information on the organization of the actin filament substrate for myosin II. Whilst muscle sarcomeric-like contraction drives cell tension in stationary nonmuscle cells, alternative nonsarcomeric modes of myosin II force-generation power forwards movement of the cell body in already migrating cells. Which one contributes to initial cell shape change has not previously been experimentally sought in any polarizing cell. Sarcomeric and nonsarcomeric-based force require completely different types of organization and filament polarity in the actin substrate for myosin II, and these can only currently be distinguished by labour-intensive submicron analysis and electron microscopy. For the first time in any polarizing cell using such analysis we have identified that oriented actomyosin II filament bundles, required for fibroblast polarization, are nonsarcomeric and are organized with graded filament polarity. As this actin organization is similar to the organization in already migrating fibroblasts, we conclude that graded filament polarity is a pivotal myosin II substrate coordinating initial cell shape change and triggering of migration.

An inner centromere protein that stimulates the microtubule depolymerizing activity of a KinI kinesin.

Mitosis requires precise control of microtubule dynamics. The KinI kinesin MCAK, a microtubule depolymerase, is critical for this regulation. In a screen to discover previously uncharacterized microtubule-associated proteins, we identified ICIS, a protein that stimulates MCAK activity in vitro. Consistent with this biochemical property, blocking ICIS function in Xenopus extracts with antibodies caused excessive microtubule growth and inhibited spindle formation. Prior to anaphase, ICIS localized in an MCAK-dependent manner to inner centromeres, the chromosomal region located in between sister kinetochores. From Xenopus extracts, ICIS coimmunoprecipitated MCAK and the inner centromere proteins INCENP and Aurora B, which are thought to promote chromosome biorientation. By immunoelectron microscopy, we found that ICIS is present on the surface of inner centromeres, placing it in an ideal location to depolymerize microtubules associated laterally with inner centromeres. At inner centromeres, MCAK-ICIS may destabilize these microtubules and provide a mechanism that prevents kinetochore-microtubule attachment errors.

Self- and actin-templated assembly of Mammalian septins.

Septins are polymerizing GTPases required for cytokinesis and cortical organization. The principles by which they are targeted to, and assemble at, specific cell regions are unknown. We show that septins in mammalian cells switch between a linear organization along actin bundles and cytoplasmic rings, approximately 0.6 microm in diameter. A recombinant septin complex self-assembles into rings resembling those in cells. Linear organization along actin bundles was reconstituted by adding an adaptor protein, anillin. Perturbation of septin organization in cells by expression of a septin-interacting fragment of anillin or by septin depletion via siRNA causes loss of actin bundles. We conclude that septins alone self-assemble into rings, that adaptor proteins recruit septins to actin bundles, and that septins help organize these bundles.

Fascin-mediated propulsion of Listeria monocytogenes independent of frequent nucleation by the Arp2/3 complex.

Actin-dependent propulsion of Listeria monocytogenes is thought to require frequent nucleation of actin polymerization by the Arp2/3 complex. We demonstrate that L. monocytogenes motility can be separated into an Arp2/3-dependent nucleation phase and an Arp2/3-independent elongation phase. Elongation-based propulsion requires a unique set of biochemical factors in addition to those required for Arp2/3-dependent motility. We isolated fascin from brain extracts as the only soluble factor required in addition to actin during the elongation phase for this type of movement. The nucleation reaction assembles a comet tail of branched actin filaments directly behind the bacterium. The elongation-based reaction generates a hollow cylinder of parallel bundles that attach along the sides of the bacterium. Bacteria move faster in the elongation reaction than in the presence of Arp2/3, and the rate is limited by the concentration of G-actin. The biochemical and structural differences between the two motility reactions imply that each operates through distinct biochemical and biophysical mechanisms.

Integrated proteogenetic analysis reveals the landscape of a mitochondrial-autophagosome synapse during PARK2-dependent mitophagy.

The PINK1 protein kinase activates the PARK2 ubiquitin ligase to promote mitochondrial ubiquitylation and recruitment of ubiquitin-binding mitophagy receptors typified by OPTN and TAX1BP1. Here, we combine proximity biotinylation of OPTN and TAX1BP1 with CRISPR-Cas9-based screens for mitophagic flux to develop a spatial proteogenetic map of PARK2-dependent mitophagy. Proximity labeling of OPTN allowed visualization of a "mitochondrial-autophagosome synapse" upon mitochondrial depolarization. Proximity proteomics of OPTN and TAX1BP1 revealed numerous proteins at the synapse, including both PARK2 substrates and autophagy components. Parallel mitophagic flux screens identified proteins with roles in autophagy, vesicle formation and fusion, as well as PARK2 targets, many of which were also identified via proximity proteomics. One protein identified in both approaches, HK2, promotes assembly of a high-molecular weight complex of PINK1 and phosphorylation of ubiquitin in response to mitochondrial damage. This work provides a resource for understanding the spatial and molecular landscape of PARK2-dependent mitophagy.

Life and times of a cellular bleb.

Blebs are spherical cellular protrusions that occur in many physiological situations. Two distinct phases make up the life of a bleb, each of which have their own biology and physics: expansion, which lasts approximately 30 s, and retraction, which lasts approximately 2 min. We investigate these phases using optical microscopy and simple theoretical concepts, seeking information on blebbing itself, and on cytomechanics in general. We show that bleb nucleation depends on pressure, membrane-cortex adhesion energy, and membrane tension, and test this experimentally. Bleb growth occurs through a combination of bulk flow of lipids and delamination from the cell cortex via the formation and propagation of tears. In extreme cases, this can give rise to a traveling wave around the cell periphery, known as "circus movement." When growth stalls, an actin cortex reforms under the bleb membrane, and retraction starts, driven by myosin-II. Using flicker spectroscopy, we find that retracting blebs are fivefold more rigid than expanding blebs, an increase entirely explained by the properties of the newly formed cortical actin mesh. Finally, using artificially nucleated blebs as pressure sensors, we show that cells rounded up in mitosis possess a substantial intracellular pressure.

XRHAMM functions in ran-dependent microtubule nucleation and pole formation during anastral spindle assembly.

BACKGROUND: The regulated assembly of microtubules is essential for bipolar spindle formation. Depending on cell type, microtubules nucleate through two different pathways: centrosome-driven or chromatin-driven. The chromatin-driven pathway dominates in cells lacking centrosomes. RESULTS: Human RHAMM (receptor for hyaluronic-acid-mediated motility) was originally implicated in hyaluronic-acid-induced motility but has since been shown to associate with centrosomes and play a role in astral spindle pole integrity in mitotic systems. We have identified the Xenopus ortholog of human RHAMM as a microtubule-associated protein that plays a role in focusing spindle poles and is essential for efficient microtubule nucleation during spindle assembly without centrosomes. XRHAMM associates both with gamma-TuRC, a complex required for microtubule nucleation and with TPX2, a protein required for microtubule nucleation and spindle pole organization. CONCLUSIONS: XRHAMM facilitates Ran-dependent, chromatin-driven nucleation in a process that may require coordinate activation of TPX2 and gamma-TuRC.

Cell-free extract systems and the cytoskeleton: preparation of biochemical experiments for transmission electron microscopy.

Cell-free systems can be used to reconstitute complex actin or microtubule-based phenomena. For example, a number of extracts support actin-dependent propulsion of Listeria monocytogenes, whereas Xenopus laevis extracts support formation of a microtubule-based meiotic spindle. Working in vitro opens these complex processes to biochemical dissection. Here, we describe methods to view these in vitro preparations by thin-section electron microscopy.

Electron microscopy of microtubule cytoskeleton assembly in vitro.

Cell-free cytoplasm isolated from meiotic Xenopus egg extracts reconstitutes microtubule phenomena in vitro. These crude extracts assemble bipolar meiotic spindles and are readily fractionated for biochemical assays, providing a good tool to dissect molecular mechanism. We developed techniques for immunoelectron microscopy of microtubule structures assembled in perfusion chambers and in solution.