Observation of temporal variations in seismic anisotropy within an active fault‐zone revealed from the Taiwan Chelungpu‐fault Drilling Project Borehole Seismic Array

Temporal fault-zone observations are important to better understand the evolution of fault structure and stress configuration. However, long-term monitoring in the fault-zone is rare after a large earthquake. Here, we use seismic data in the fault-zone at 1-km depth from the Taiwan Chelungpu-fault Drilling Project to study long-term anisotropy after the 1999 Mw7.6 Chi-Chi earthquake. The direct S-wave splitting measurements resolve the overall weak anisotropy in the shallow crust. In order to resolve fault damage zone anisotropy, we perform coda cross-correlation technique for 794 microearthquakes between 2007 and 2013. We estimate the temporal change in background shear-wave velocity, fast shear-wave polarization direction (FSP), and strength of anisotropy (Aani) in the fault damage zone. We show the average FSP direction is N93°E with a significant Aani of about 12%, likely due to the pervasive vertical microcracks created after the earthquake. Temporal variations of anisotropy exhibit seasonal variation with periodicity every 9 to 12 months that correlates with rainfall events. Furthermore, long-term anisotropy shows a gradual rotation of FSP direction of about 15° during the first 4 years of observation. At the same time, the strength of anisotropy reduced from 17 to 10 % and shear-wave velocity increased, suggesting the fault healed after the earthquake. This study reports in-situ evidence for two key observations: (1) long-term, fault-zone healing after a major earthquake, and (2) modulation of 1-km deep fault-zone properties by surficial hydrologic processes. These observations may provide constraints on the response of the fault damage zone in the interseismic period

Ruei-Jiun Hung's Quick Files

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Direct Redox Regulation of F-Actin Assembly and Disassembly by MICAL

How guidance cues present outside of cells exert their precise effects on the internal actin cytoskeleton is poorly understood. Such effects are critical for diverse cellular behaviors including polarity, morphology, adhesion, motility, process elongation, navigation, and connectivity. Semaphorins, for example, are one of the largest families of these guidance signals and play critical roles in neurobiology, angiogenesis, immunology, and cancer. One interesting characteristic of the Semaphorins is that they inhibit the movement of cells (and their membranous processes) through their ability to disrupt actin cytoskeletal organization. However, despite considerable progress in the identification of Semaphorin receptors and their signal transduction pathways, the molecules linking them to the precise control of the actin cytoskeleton have remained mysterious. During my graduate studies, I sought to better understand a family of unusual proteins called the MICALs (which includes one Drosophila Mical and three vertebrate MICALs), which associate with the Semaphorin cell-surface receptor Plexin and are important for Semaphorins to exert their effects. Nothing was known, however, regarding the specific role of the MICALs in these Semaphorin-dependent events. Not long after I began my graduate work, my colleagues and I noticed that Mical was necessary for proper actin cytoskeletal organization and sufficient to reorganize the actin cytoskeleton in vivo. Therefore, to better understand the role that Mical plays in actin cytoskeletal rearrangements, I took a biochemical approach, and purified the Mical protein. Utilizing biochemical and imaging approaches with purified proteins, I found that Mical directly binds to actin filaments (F-actin) and is able to induce the rapid disassembly of F-actin. Thus, my results revealed that Mical is a novel F-actin disassembly factor that provides a molecular conduit through which F-actin disassembly can be precisely achieved in response to Semaphorins. So I next wondered how Mical induces F-actin disassembly. Interestingly, the MICALs belong to a class of flavoprotein monooxygenase/hydroxylase enzymes that associate with flavin adenine dinucleotide (FAD) and use the co-enzyme nicotinamide adenine dinucleotide phosphate (NADPH) in oxidationreduction (Redox) reactions. Although MICALs have no known substrate/s, my in vivo and in vitro results revealed that Mical employs its Redox region to bind F-actin and disassembles filaments in an NADPH-dependent manner. Moreover, this Mical-treated actin failed to repolymerize even after removal of Mical, indicating that Mical stably modifies actin to alter polymerization. Mass spectrometric analyses revealed that F-actin subunits were directly modified by Mical on their conserved pointed-end that is critical for filament assembly. Specifically, Mical post-translationally oxidized a conserved amino acid (Methionine 44) within a region of actin that is critical for actin-actin contacts, simultaneously severing filaments and decreasing polymerization. Thus, my thesis observations reveal a novel and specific oxidation dependent signaling mechanism that selectively regulates actin dynamics and cellular behaviors

Enhanced Production of the Mical Redox Domain for Enzymology and F-actin Disassembly Assays

To change their behaviors, cells require actin proteins to assemble together into long polymers/filaments—and so a critical goal is to understand the factors that control this actin filament (F-actin) assembly and stability. We have identified a family of unusual actin regulators, the MICALs, which are flavoprotein monooxygenase/hydroxylase enzymes that associate with flavin adenine dinucleotide (FAD) and use the co-enzyme nicotinamide adenine dinucleotide phosphate (NADPH) in Redox reactions. F-actin is a specific substrate for these MICAL Redox enzymes, which oxidize specific amino acids within actin to destabilize actin filaments. Furthermore, this MICAL-catalyzed reaction is reversed by another family of Redox enzymes (SelR/MsrB enzymes)—thereby revealing a reversible Redox signaling process and biochemical mechanism regulating actin dynamics. Interestingly, in addition to the MICALs’ Redox enzymatic portion through which MICALs covalently modify and affect actin, MICALs have multiple other domains. Less is known about the roles of these other MICAL domains. Here we provide approaches for obtaining high levels of recombinant protein for the Redox only portion of Mical and demonstrate its catalytic and F-actin disassembly activity. These results provide a ground state for future work aimed at defining the role of the other domains of Mical — including characterizing their effects on Mical’s Redox enzymatic and F-actin disassembly activity

F-actin dismantling through a redox-driven synergy between Mical and cofilin.

Numerous cellular functions depend on actin filament (F-actin) disassembly. The best-characterized disassembly proteins, the ADF (actin-depolymerizing factor)/cofilins (encoded by the twinstar gene in Drosophila), sever filaments and recycle monomers to promote actin assembly. Cofilin is also a relatively weak actin disassembler, posing questions about mechanisms of cellular F-actin destabilization. Here we uncover a key link to targeted F-actin disassembly by finding that F-actin is efficiently dismantled through a post-translational-mediated synergism between cofilin and the actin-oxidizing enzyme Mical. We find that Mical-mediated oxidation of actin improves cofilin binding to filaments, where their combined effect dramatically accelerates F-actin disassembly compared with either effector alone. This synergism is also necessary and sufficient for F-actin disassembly in vivo, magnifying the effects of both Mical and cofilin on cellular remodelling, axon guidance and Semaphorin-Plexin repulsion. Mical and cofilin, therefore, form a redox-dependent synergistic pair that promotes F-actin instability by rapidly dismantling F-actin and generating post-translationally modified actin that has altered assembly properties

Transcriptional and functional motifs defining renal function revealed by single-nucleus RNA sequencing

Recent advances in single-cell sequencing provide a unique opportunity to gain novel insights into the diversity, lineage, and functions of cell types constituting a tissue/organ. Here, we performed a single-nucleus study of the adult Drosophila renal system, consisting of Malpighian tubules and nephrocytes, which shares similarities with the mammalian kidney. We identified 11 distinct clusters representing renal stem cells, stellate cells, regionally specific principal cells, garland nephrocyte cells, and pericardial nephrocytes. Characterization of the transcription factors specific to each cluster identified fruitless (fru) as playing a role in stem cell regeneration and Hepatocyte nuclear factor 4 (Hnf4) in regulating glycogen and triglyceride metabolism. In addition, we identified a number of genes, including Rho guanine nucleotide exchange factor at 64C (RhoGEF64c), Frequenin 2 (Frq2), Prip, and CG1093 that are involved in regulating the unusual star shape of stellate cells. Importantly, the single-nucleus dataset allows visualization of the expression at the organ level of genes involved in ion transport and junctional permeability, providing a systems-level view of the organization and physiological roles of the tubules. Finally, a cross-species analysis allowed us to match the fly kidney cell types to mouse kidney cell types and planarian protonephridia, knowledge that will help the generation of kidney disease models. Altogether, our study provides a comprehensive resource for studying the fly kidney