Edward R. Kuczmarski

The nucleoskeleton: lamins and actin are major players in essential nuclear functions

The nucleoskeleton is composed of many interacting structural proteins that provide the framework for DNA replication, transcription and a variety of other nuclear functions. For example, the type-V intermediate filament proteins, the lamins, and their associated proteins (e.g. Lap2alpha) play important roles in DNA replication and transcription. Furthermore, actin, actin-related proteins and other actin-associated proteins likewise appear to be important in nuclear functions because they are components of chromatin-remodeling complexes and are involved in mRNA synthesis, processing and transport. Newly described nuclear proteins that contain both actin- and lamin-binding domains could be involved in regulating molecular crosstalk between these two types of nucleoskeletal proteins. This range of activities might help to explain why genetic defects in some of the nucleoskeletal proteins contribute to an ever-expanding list of human diseases.

Keratin 8 Phosphorylation by Protein Kinase C δ Regulates Shear Stress-mediated Disassembly of Keratin Intermediate Filaments in Alveolar Epithelial Cells

Phosphorylation of keratin intermediate filaments (IF) is known to affect their assembly state and organization; however, little is known about the mechanisms regulating keratin phosphorylation. In this study, we demonstrate that shear stress, but not stretch, causes disassembly of keratin IF in lung alveolar epithelial cells (AEC) and that this disassembly is regulated by protein kinase C δ-mediated phosphorylation of keratin 8 (K8) Ser-73. Specifically, in AEC subjected to shear stress, keratin IF are disassembled, as reflected by their increased solubility. In contrast, AEC subjected to stretch showed no changes in the state of assembly of IF. Pretreatment with the protein kinase C (PKC) inhibitor, bisindolymaleimide, prevents the increase in solubility of either K8 or its assembly partner K18 in shear-stressed AEC. Phosphoserine-specific antibodies demonstrate that K8 Ser-73 is phosphorylated in a time-dependent manner in shear-stressed AEC. Furthermore, we showed that shear stress activates PKC δ and that the PKC δ peptide antagonist, δ V1-1, significantly attenuates the shear stress-induced increase in keratin phosphorylation and solubility. These data suggested that shear stress mediates the phosphorylation of serine residues in K8, leading to the disassembly of IF in alveolar epithelial cells. Importantly, these data provided clues regarding a molecular link between mechanically induced signal transduction and alterations in cytoskeletal IF.

Intermediate filaments : versatile building blocks of cell structure

Cytoskeletal intermediate filaments (IF) are organized into a dynamic nanofibrillar complex that extends throughout mammalian cells. This organization is ideally suited to their roles as response elements in the subcellular transduction of mechanical perturbations initiated at cell surfaces. IF also provide a scaffold for other types of signal transduction that together with molecular motors ferries signaling molecules from the cell periphery to the nucleus. Recent insights into their assembly highlight the importance of co-translation of their precursors, the hierarchical organization of their subunits in the formation of unit-length filaments (ULF) and the linkage of ULF into mature apolar IF. Analyses by atomic force microscopy reveal that mature IF are flexible and can be stretched to over 300% of their length without breaking, suggesting that intrafilament subunits can slide past one another when exposed to mechanical stress and strain. IF also play a role in the organization of organelles by modulating their motility and providing anchorage sites within the cytoplasm.

Vimentin Intermediate Filaments Modulate the Motility of Mitochondria

The vimentin N-terminal domain contains the sequence responsible for the interaction with mitochondria. The interaction of vimentin intermediate filaments with mitochondria causes the inhibition of their movements and contributes to their anchoring in cytoplasm.

Inroads into the Structure and Function of Intermediate Filament Networks

Although intermediate filaments are one of three major cytoskeletal systems of vertebrate cells, they remain the least understood with respect to their structure and function. This is due in part to the fact that they are encoded by a large gene family which is developmentally regulated in a cell and tissue type specific fashion. This article is in honor of Ueli Aebi. It highlights the studies on IF that have been carried out by our laboratory for more than 40 years. Many of our advances in understanding IF are based on conversations with Ueli which have taken place during adventurous and sometimes dangerous hiking and biking trips throughout the world.

Intermediate Filaments Play a Pivotal Role in Regulating Cell Architecture and Function

Intermediate filaments (IFs) are composed of one or more members of a large family of cytoskeletal proteins, whose expression is cell- and tissue type-specific. Their importance in regulating the physiological properties of cells is becoming widely recognized in functions ranging from cell motility to signal transduction. IF proteins assemble into nanoscale biopolymers with unique strain-hardening properties that are related to their roles in regulating the mechanical integrity of cells. Furthermore, mutations in the genes encoding IF proteins cause a wide range of human diseases. Due to the number of different types of IF proteins, we have limited this short review to cover structure and function topics mainly related to the simpler homopolymeric IF networks composed of vimentin, and specifically for diseases, the related muscle-specific desmin IF networks.

Insights into the mechanical properties of epithelial cells: the effects of shear stress on the assembly and remodeling of keratin intermediate filaments

The effects of shear stress on the keratin intermediate filament (KIF) cytoskeleton of cultured human alveolar epithelial (A549) cells have been investigated. Under normal culture conditions, immunofluorescence revealed a delicate network of fine tonofibrils containing KIFs, together with many nonfilamentous, keratin‐containing “particles,” mostly containing either keratin 8 (K8) or 18 (K18), but not both. Triton X‐100 extracted ~10% of the cellular keratin, and this was accompanied by a loss of the particles but not the KIFs. Shear stress dramatically reduced the soluble keratin component and transformed the fine bundles of KIFs into thicker, “wavy” tonofibrils. Both effects were accompanied by the disappearance of most keratin particles and by increased phosphorylation of K8 and K18 on serine residues 73 and 33, respectively. The particles that remained after shearing were phosphorylated and were closely associated with KIFs. We suggest that keratin particles constitute a reservoir of protein that can be recruited into KIFs under flow, creating a more robust cytoskeleton able to withstand shear forces more effec‐tively.—Flitney, E. W., Kuczmarski, E. R., Adam, S. A., Goldman, R. D. Insights into the mechanical properties of epithelial cells: the effects of shear stress on the assembly and remodeling of keratin intermediate filaments. FASEB J. 23, 2110–2119 (2009)

Dictyostelium discoideum myosin: isolation and characterization of cDNAs encoding the essential light chain.

We used an antibody specific for Dictyostelium discoideum myosin to screen a lambda gt11 cDNA expression library to obtain cDNA clones which encode the Dictyostelium essential myosin light chain (EMLC). The amino acid sequence predicted from the sequence of the cDNA clone showed 31.5% identity with the amino acid sequence of the chicken EMLC. Comparisons of the Dictyostelium EMLC, a nonmuscle cell type, with EMLC sequences from similar MLCs of skeletal- and smooth-muscle origin, showed distinct regions of homology. Much of the observed homology was localized to regions corresponding to consensus Ca2+-binding of E-F hand domains. Southern blot analysis suggested that the Dictyostelium genome contains a single gene encoding the EMLC. Examination of the pattern of EMLC mRNA expression showed that a significant increase in EMLC message levels occurred during the first few hours of development, coinciding with increased actin expression and immediately preceding the period of maximal chemotactic activity.

Myosin specific phosphatases isolated from Dictyostelium discoideum.

SummaryUsing native myosin phosphorylated on either the heavy chain or the light chain, we have isolated myosin phosphatases from extracts of vegetativeDictyostelium amoeba. Two phosphatases were resolved by DEAE-cellulose chromatography. One of these phosphatases removed phosphate from heavy chain or light chain at approximately the same rate. The other phosphatase appeared to be much more specific for phosphorylated myosin heavy chain. Although these enzymes removed phosphate from other phosphoprotein substrates such as histone or casein, they did so at a much lower rate. Both enzymes required magnesium for activity, but appeared to be independent of calcium.

Partial purification of two myosin heavy chain kinases from Dictyostelium discoideum.

SummaryMyosin heavy chain kinase activity was identified in the high speed supernate of lysedDictyostelium amoebae and was precipated by 30–50% ammonium sulphate. In low ionic strength buffer, the activity bound tightly to a Cibacron Blue Sepharose column and eluted as a single peak with 1.0m NaCl. Gel filtration chromatography resolved the kinase into two activities, each of which phosphorylated the tail portion of purifiedDictyostelium myosin. One of these activities phosphorylated both serine and threonine residues of the heavy chain, while the other activity only phosphorylated threonine residues. Peptide mapping studies indicated thatin vivo andin vitro phosphorylation sites were identical. The heavy chain kinases required Mg2+ for activity but were unaffected by Ca2+ or calmodulin. The heavy chain kinases did not phosphorylateDictyostelium light chain, and also did not phosphorylate myosins from striated, smooth, or other nonmuscle sources.