Inherited nuclear pore substructures template post-mitotic pore assembly

Nuclear envelope assembly during late mitosis includes rapid formation of several thousand complete nuclear pore complexes (NPCs). This efficient use of NPC components (nucleoporins or "NUPs") is essential for ensuring immediate nucleocytoplasmic communication in each daughter cell. We show that octameric subassemblies of outer and inner nuclear pore rings remain intact in the mitotic endoplasmic reticulum (ER) after NPC disassembly during prophase. These "inherited" subassemblies then incorporate into NPCs during post-mitotic pore formation. We further show that the stable subassemblies persist through multiple rounds of cell division and the accompanying rounds of NPC mitotic disassembly and post-mitotic assembly. De novo formation of NPCs from newly synthesized NUPs during interphase will then have a distinct initiation mechanism. We postulate that a yet-to-be-identified modification marks and "immortalizes" one or more components of the specific octameric outer and inner ring subcomplexes that then template post-mitotic NPC assembly during subsequent cell cycles.

HDAC6 mediates an aggresome-like mechanism for NLRP3 and pyrin inflammasome activation

Inflammasomes are supramolecular complexes that play key roles in immune surveillance. This is accomplished by the activation of inflammatory caspases, which leads to the proteolytic maturation of interleukin 1β (IL-1β) and pyroptosis. Here, we show that nucleotide-binding domain, leucine-rich repeat, and pyrin domain-containing protein 3 (NLRP3)- and pyrin-mediated inflammasome assembly, caspase activation, and IL-1β conversion occur at the microtubule-organizing center (MTOC). Furthermore, the dynein adapter histone deacetylase 6 (HDAC6) is indispensable for the microtubule transport and assembly of these inflammasomes both in vitro and in mice. Because HDAC6 can transport ubiquitinated pathological aggregates to the MTOC for aggresome formation and autophagosomal degradation, its role in NLRP3 and pyrin inflammasome activation also provides an inherent mechanism for the down-regulation of these inflammasomes by autophagy. This work suggests an unexpected parallel between the formation of physiological and pathological aggregates.

Dynamics of Auxilin 1 and GAK in clathrin-mediated traffic

Clathrin-coated vesicles lose their clathrin lattice within seconds of pinching off, through the action of the Hsc70 “uncoating ATPase.” The J- and PTEN-like domain–containing proteins, auxilin 1 (Aux1) and auxilin 2 (GAK), recruit Hsc70. The PTEN-like domain has no phosphatase activity, but it can recognize phosphatidylinositol phosphate head groups. Aux1 and GAK appear on coated vesicles in successive transient bursts, immediately after dynamin-mediated membrane scission has released the vesicle from the plasma membrane. These bursts contain a very small number of auxilins, and even four to six molecules are sufficient to mediate uncoating. In contrast, we could not detect auxilins in abortive pits or at any time during coated pit assembly. We previously showed that clathrin-coated vesicles have a dynamic phosphoinositide landscape, and we have proposed that lipid head group recognition might determine the timing of Aux1 and GAK appearance. The differential recruitment of Aux1 and GAK correlates with temporal variations in phosphoinositide composition, consistent with a lipid-switch timing mechanism.

Abstract 1854: Novel pateamine analogs to target the translation initiation factor eIF4A in chronic lymphocytic leukemia

The viability of chronic lymphocytic leukemia (CLL) is critically dependent upon staving off death by apoptosis, a hallmark of CLL pathophysiology. The overexpression of the Bcl-2 family proteins likely play a major role in the apoptosis blockade in CLL, and has been an effective target of CLL therapy. The recognition that Mcl-1, a major component of the anti-apoptotic response, is intrinsically short-lived and must be continually resynthesized suggested a novel therapeutic approach. Pateamine A (PatA), a macrolide marine natural product, inhibits cap-dependent translation by binding to the initiation factor eIF4A. We have previously reported the first total synthesis of PatA. Mechanistic studies suggested that binding of eIF4A to PatA caused the stalling of initiation complexes on mRNA, halting the translation initiation process. In this study, we demonstrated that a synthetic derivative of PatA, des-methyl des-amino PatA (DMDAPatA), blocked mRNA translation, reduced Mcl-1 protein and initiated apoptosis in CLL cells. This action was synergistic with the Bcl-2 antagonist ABT-199, by a mechanism to inhibit the two parallel arms of apoptosis control in CLL. However, avid binding to human plasma proteins limited DMDAPatA potency, precluding further development. To address this, we synthesized a new series of 27 PatA analogs with modifications on various regions of PatA, and screened their toxicity against the primary CLL cells. Any modifications on the side chain, or the rigid binding domain of the PatA macrolide ring led to complete loss of activity. Rather, introduction of an amino group at either the C2 or C3 positions of the flexible region of the macrocycle retained the activity and reduced plasma protein binding, likely through a lowered lipophilicity. We identified three new leads with potent inhibition of proteins synthesis and strong CLL cytotoxicity. They also exhibited greater selectivity towards CLL cells over normal lymphocytes comparing to the parental compound PatA. To gain structural insights into the interaction of the PatA analogs with eIF4A, a homology model of the human eIF4A1 was generated using the closed conformation of the eIF4A3 structure (PDB ID: 2HYI) as a template. The predicted PatA binding site is located at the interface of the N-terminal domain and the C-terminal domain, in between the RNA and ATP binding sites. In silico docking analysis of the PatA analogs to eIF4A correlated with their structure-activity relationships and suggested that these compounds may act by stabilizing the closed conformation of eIF4A. Thus, these novel PatA analogs hold promise for application to cancers within the appropriate biological context, such as CLL.

Citation Format: Rong Chen, Mingzhao Zhu, Rajan R. Chaudhari, Omar Robles, Yuling Chen, Wesley Skillern, Qun Qin, William Wierda, Shuxing Zhang, Kenneth G. Hull, Daniel Romo, William Plunkett. Novel pateamine analogs to target the translation initiation factor eIF4A in chronic lymphocytic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1854.

Dynamics of phosphoinositide conversion in clathrin-mediated endocytic traffic

Vesicular carriers transport proteins and lipids from one organelle to another, recognizing specific identifiers for the donor and acceptor membranes. Two important identifiers are phosphoinositides and GTP-bound GTPases, which provide well-defined but mutable labels. Phosphatidylinositol and its phosphorylated derivatives are present on the cytosolic faces of most cellular membranes. Reversible phosphorylation of its headgroup produces seven distinct phosphoinositides. In endocytic traffic, phosphatidylinositol-4,5-biphosphate marks the plasma membrane, and phosphatidylinositol-3-phosphate and phosphatidylinositol-4-phosphate mark distinct endosomal compartments. It is unknown what sequence of changes in lipid content confers on the vesicles their distinct identity at each intermediate step. Here we describe 'coincidence-detecting' sensors that selectively report the phosphoinositide composition of clathrin-associated structures, and the use of these sensors to follow the dynamics of phosphoinositide conversion during endocytosis. The membrane of an assembling coated pit, in equilibrium with the surrounding plasma membrane, contains phosphatidylinositol-4,5-biphosphate and a smaller amount of phosphatidylinositol-4-phosphate. Closure of the vesicle interrupts free exchange with the plasma membrane. A substantial burst of phosphatidylinositol-4-phosphate immediately after budding coincides with a burst of phosphatidylinositol-3-phosphate, distinct from any later encounter with the phosphatidylinositol-3-phosphate pool in early endosomes; phosphatidylinositol-3,4-biphosphate and the GTPase Rab5 then appear and remain as the uncoating vesicles mature into Rab5-positive endocytic intermediates. Our observations show that a cascade of molecular conversions, made possible by the separation of a vesicle from its parent membrane, can label membrane-traffic intermediates and determine their destinations.

Recruitment dynamics of ESCRT-III and Vps4 to endosomes and implications for reverse membrane budding

The ESCRT machinery mediates reverse membrane scission. By quantitative fluorescence lattice light-sheet microscopy, we have shown that ESCRT-III subunits polymerize rapidly on yeast endosomes, together with the recruitment of at least two Vps4 hexamers. During their 3–45 s lifetimes, the ESCRT-III assemblies accumulated 75–200 Snf7 and 15–50 Vps24 molecules. Productive budding events required at least two additional Vps4 hexamers. Membrane budding was associated with continuous, stochastic exchange of Vps4 and ESCRT-III components, rather than steady growth of fixed assemblies, and depended on Vps4 ATPase activity. An all-or-none step led to final release of ESCRT-III and Vps4. Tomographic electron microscopy demonstrated that acute disruption of Vps4 recruitment stalled membrane budding. We propose a model in which multiple Vps4 hexamers (four or more) draw together several ESCRT-III filaments. This process induces cargo crowding and inward membrane buckling, followed by constriction of the nascent bud neck and ultimately ILV generation by vesicle fission.

Miro1-mediated mitochondrial positioning shapes intracellular energy gradients required for cell migration

The ratio of ATP:ADP is highest at perinuclear sites, where mitochondria are dense, and dissipates toward the periphery. Miro1 positions mitochondria toward the cortical cytoskeleton. Deletion of Miro1 results in perinuclear clustering of mitochondria, altering intracellular ATP:ADP gradients, and impairs energy-expensive cell migratory processes.

It has long been postulated, although never directly demonstrated, that mitochondria are strategically positioned in the cytoplasm to meet local requirements for energy production. Here we show that positioning of mitochondria in mouse embryonic fibroblasts (MEFs) determines the shape of intracellular energy gradients in living cells. Specifically, the ratio of ATP to ADP was highest at perinuclear areas of dense mitochondria and gradually decreased as more-peripheral sites were approached. Furthermore, the majority of mitochondria were positioned at the ventral surface of the cell, correlating with high ATP:ADP ratios close to the ventral membrane, which rapidly decreased toward the dorsal surface. We used cells deficient for the mitochondrial Rho-GTPase 1 (Miro1), an essential mediator of microtubule-based mitochondrial motility, to study how changes in mitochondrial positioning affect cytoplasmic energy distribution and cell migration, an energy-expensive process. The mitochondrial network in Miro1^−/− MEFs was restricted to the perinuclear area, with few mitochondria present at the cell periphery. This change in mitochondrial distribution dramatically reduced the ratio of ATP to ADP at the cell cortex and disrupted events essential for cell movement, including actin dynamics, lamellipodia protrusion, and membrane ruffling. Cell adhesion status was also affected by changes in mitochondrial positioning; focal adhesion assembly and stability was decreased in Miro1^−/− MEFs compared with Miro1^+/+ MEFs. Consequently Miro1^−/− MEFs migrated slower than control cells during both collective and single-cell migration. These data establish that Miro1-mediated mitochondrial positioning at the leading edge provides localized energy production that promotes cell migration by supporting membrane protrusion and focal adhesion stability.

Abstract B21: Novel pateamine A analogs to target pro-survival proteins in chronic lymphocytic leukemia

Introduction: Dysregulation of protein translation is a common feature of cancer. Because the great majority of oncoproteins turn-over rapidly, thus have an acute need for mRNA translation compared to housekeeping proteins, protein synthesis is a promising avenue to explore for targeted cancer therapy. The marine natural product pateamine A (PatA) inhibits cap-dependent translation initiation by binding to the eukaryotic initiation factor 4A (eIF4A). We have previously synthesized a family of PatA-based small molecules. The lead compound, des-methyl, des-amino pateamine A (DMDAPatA), is more stable and easier to synthesize than the parent natural product, and demonstrated anti-cancer activities in both cell lines and animal models. In this study, we extended the investigations of the activities of DMDAPatA in B cell malignancies, focusing on the primary chronic lymphocytic leukemia (CLL) cells. We propose to target the biological rewiring of the CLL cells, namely the overexpression of short-lived pro-survival proteins that keep these cells from undergoing apoptosis. We hypothesize that a transient inhibition of translation will cause a lethal decrease in those labile pro-survival proteins in CLL cells that will initiate apoptosis.

Experimental Procedures: Primary CLL cells were used to screen the PatA analogs for their anti-leukemia activity. Cell viability was analyzed by flow cytometry. Protein translation was measured by tritium labeled leucine incorporation. Protein expression was determined by immunoblotting.

Results: Our data showed that DMDAPatA blocked protein synthesis in the CLL cells. This action reduced the short-lived anti-apoptotic proteins Mcl-1 and XIAP, and initiated apoptosis in as short as 4 hr. The levels of the more stable anti-apoptotic proteins Bcl-2 and Bcl-XL were not affected. This is consistent with the selective activity of translation inhibitors toward proteins with rapid turn-over rates. Since DMDAPatA depleted Mcl-1 without affecting Bcl-2, and the Bcl-2 antagonist ABT-199 inhibits Bcl-2 activity but spares Mcl-1, the combination of DMDAPatA and ABT-199 targeted both arms of apoptosis control and killed CLL cells synergistically. Dose reduction index analysis demonstrated mutual potentiation of ABT-199 and DMDAPatA. However, in the presence of human plasma proteins, DMDAPatA lost its potency substantially. A plasma protein binding analysis showed over 99% binding in human plasma, which may hamper its development as a therapeutic agent. To address this issue, we synthesized a new series of PatA analogues with the goal of improving the physical properties and CLL potency. We found that any alteration on the binding domain of PatA, either on eastern half of the macrolide ring or the side chain, resulted in dramatic loss of activity toward the CLL cells. The scaffolding domain on the western side of the ring was more tolerable to modifications which made it feasible to improvement. We proposed to introduce an amino group at either the C2 (alpha-amino variant) or C3 position (beta-amino variant) of the macrolide ring to lower lipophilicity and reduce binding to plasma proteins. This strategy led to the identification of three new leads with potent inhibition of protein synthesis. They exhibited greater cytotoxic potency toward CLL cells than DMDAPatA and lower human plasma protein binding. They also showed better selectivity towards CLL cells over normal lymphocytes compared to the parental compound PatA, demonstrating improved therapeutic index.

Conclusions: We have successfully synthesized three new PatA analogs with potent anti-leukemia activities. These compounds hold promise for application to cancers with the right biological context, such as CLL, in which the leukemia cells are addicted to the sustained expression of short-lived oncoproteins for survival.

Citation Format: Rong Chen, Mingzhao Zhu, Yuling Chen, Wesley Skillern, Qun Qin, William G. Wierda, Kenneth G. Hull, Daniel Romo, William Plunkett. Novel pateamine A analogs to target pro-survival proteins in chronic lymphocytic leukemia. [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr B21.

Membrane dynamics of dividing cells imaged by lattice light-sheet microscopy

Lattice light-sheet microscopy is used to examine two problems in membrane dynamics—molecular events in clathrin-coated pit formation and changes in cell shape during cell division. This methodology sets a new standard for imaging membrane dynamics in single cells and multicellular assemblies.

Membrane remodeling is an essential part of transferring components to and from the cell surface and membrane-bound organelles and for changes in cell shape, which are particularly critical during cell division. Earlier analyses, based on classical optical live-cell imaging and mostly restricted by technical necessity to the attached bottom surface, showed persistent formation of endocytic clathrin pits and vesicles during mitosis. Taking advantage of the resolution, speed, and noninvasive illumination of the newly developed lattice light-sheet fluorescence microscope, we reexamined their assembly dynamics over the entire cell surface and found that clathrin pits form at a lower rate during late mitosis. Full-cell imaging measurements of cell surface area and volume throughout the cell cycle of single cells in culture and in zebrafish embryos showed that the total surface increased rapidly during the transition from telophase to cytokinesis, whereas cell volume increased slightly in metaphase and was relatively constant during cytokinesis. These applications demonstrate the advantage of lattice light-sheet microscopy and enable a new standard for imaging membrane dynamics in single cells and multicellular assemblies.

Abstract 1765: Creating novel translation inhibitors to target pro-survival proteins in chronic lymphocytic leukemia

Many aspects of malignant phenotypes are associated with dysregulation of translation control. Due to the fact that the great majority of oncoproteins turnover rapidly, and have an acute need for mRNA translation compared to housekeeping proteins, protein synthesis is a promising avenue to explore for targeted cancer therapy. We propose to target the biological rewiring that is characteristic of B cell malignancies, namely the overexpression of short-lived pro-survival proteins that keep these cells from undergoing apoptosis. We hypothesize that transient inhibition of translation will cause a lethal decrease in those labile pro-survival proteins in CLL cells initiating apoptosis. Further, this will provide selectivity toward the malignancy over normal cells and may prove to be a more general strategy for overcoming drug resistance. The marine natural product pateamine A (PatA) inhibits cap-dependent translation initiation by binding to the eukaryotic initiation factor 4A (eIF4A) and stalls the initiation complex on mRNA. We have synthesized a family of PatA-based small molecules to test their potency for inducing apoptosis in primary CLL cells. Our lead compound, DMDAPatA, is more stable and easier to synthesize than the parent natural product, and demonstrated anti-leukemia activity in primary CLL cells with an IC50 of 2.5 μM after 24 h of incubation. This derivative was equally active in samples from patients with poor prognostic characteristics such as advanced Rai stage, the unmutated status of the IgVH gene, or the over-expression of the ZAP70 protein, suggesting its ability to bypass resistance to conventional CLL therapy. Tritium labeled leucine incorporation measurements confirmed inhibition of translation by DMDAPatA that occurred as soon as 4 h, preceding the full activation of apoptosis. Reduction of the short-lived anti-apoptotic proteins Mcl-1 and XIAP were observed by 4 h, and persisted over 24 h, while the levels of the more stable proteins Bcl-2 and Bcl-XL were not affected. This is consistent with the selective activity of translation inhibitors toward proteins with rapid turnover rates. Diminished levels of Mcl-1 and XIAP reduced the anti-apoptotic capacity such that the pro-apoptotic proteins became more prevalent to initiate cell death. Since DMDAPatA depleted Mcl-1 without affecting Bcl-2, and ABT-199 inhibits Bcl-2 activity but spares Mcl-1, the combination of DMDAPatA and ABT-199 targeted both arms of apoptosis control and killed CLL cells synergistically. Dose reduction index analysis demonstrated mutual potentiation of ABT-199 and DMDAPatA. Thus, these studies provide the first proof-of-mechanism investigations of PatA analogs for CLL therapeutics. DMDAPatA appeared to be highly plasma protein bound compared to PatA. New derivatives are being synthesized to reduce plasma protein binding and increase potency.

Citation Format: Rong Chen, Mingzhao Zhu, Yuling Chen, Wesley Skillern, William G. Wierda, Ken Hull, Daniel Romo, William Plunkett. Creating novel translation inhibitors to target pro-survival proteins in chronic lymphocytic leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1765. doi:10.1158/1538-7445.AM2015-1765