Intracompartmental sorting of essential myosin light chains: molecular dissection and in vivo monitoring by epitope tagging

Localization of the Lys, Asp, Glu, Leu tetrapeptide receptor to the Golgi complex and the intermediate compartment in mammalian cells

The carboxyl-terminal Lys-Asp-Glu-Leu (KDEL), or a closely-related sequence, is important for ER localization of both lumenal as well as type II membrane proteins. This sequence functions as a retrieval signal at post-ER compartment(s), but the exact compartment(s) where the retrieval occurs remains unresolved. With an affinity-purified antibody against the carboxyl-terminal sequence of the mammalian KDEL receptor, we have investigated its subcellular localization using immunogold labeling on thawed cryosections of different tissues, such as mouse spermatids and rat pancreas, as well as HeLa, Vero, NRK, and mouse L cells. We show that rab1 is an excellent marker of the intermediate compartment, and we use this marker, as well as budding profiles of the mouse hepatitis virus (MHV) in cells infected with this virus, to identify this compartment. Our results demonstrate that the KDEL receptor is concentrated in the intermediate compartment, as well as in the Golgi stack. Lower but significant labeling was detected in the rough ER. In general, only small amounts of the receptor were detected on the trans side of the Golgi stack, including the trans-Golgi network (TGN) of normal cells and tissues. However, some stress conditions, such as infection with vaccinia virus or vesicular stomatitis virus, as well as 20 degrees C or 43 degrees C treatment, resulted in a significant shift of the distribution towards the trans-TGN side of the Golgi stack. This shift could be quantified in HeLa cells stably expressing a TGN marker. No significant labeling was detected in structures distal to the TGN under all conditions tested. After GTP gamma S treatment of permeabilized cells, the receptor was detected in the beta-COP-containing buds/vesicles that accumulate after this treatment, suggesting that these vesicles may transport the receptor between compartments. We propose that retrieval of KDEL-containing proteins occurs at multiple post-ER compartments up to the TGN along the exocytotic pathway, and that within this pathway, the amounts of the receptor in different compartments varies according to physiological conditions.

Segregated assembly of muscle myosin expressed in nonmuscle cells

Skeletal muscle myosin cDNAs were expressed in a simian kidney cell line (COS) and a mouse myogenic cell line to investigate the mechanisms controlling early stages of myosin filament assembly. An embryonic chicken muscle myosin heavy chain (MHC) cDNA was linked to constitutive promoters from adenovirus or SV40 and transiently expressed in COS cells. These cells accumulate hybrid myosin molecules composed of muscle MHCs and endogenous, nonmuscle, myosin light chains. The muscle myosin is found associated with a Triton insoluble fraction from extracts of the COS cells by immunoprecipitation and is detected in 2.4 +/- 0.8-micron-long filamentous structures distributed throughout the cytoplasm by immunofluorescence microscopy. These structures are shown by immunoelectron microscopy to correspond to loosely organized bundles of 12-16-nm-diameter myosin filaments. The muscle and nonmuscle MHCs are segregated in the transfected cells; the endogenous nonmuscle myosin displays a normal distribution pattern along stress fibers and does not colocalize with the muscle myosin filament bundles. A similar assembly pattern and distribution are observed for expression of the muscle MHC in a myogenic cell line. The myosin assembles into filament bundles, 1.5 +/- 0.6 micron in length, that are distributed throughout the cytoplasm of the undifferentiated myoblasts and segregated from the endogenous nonmuscle myosin. In both cell lines, formation of the myosin filament bundles is dependent on the accumulation of the protein. In contrast to these results, the expression of a truncated MHC that lacks much of the rod domain produces an assembly deficient molecule. The truncated MHC is diffusely distributed throughout the cytoplasm and not associated with cellular stress fibers. These results establish that the information necessary for the segregation of myosin isotypes into distinct cellular structures is contained within the primary structure of the MHC and that other factors are not required to establish this distribution.

Remodelling of cardiomyocyte cytoarchitecture visualized by three-dimensional (3D) confocal microscopy

The break-down and reassembly of myofibrils in long-term cultures of adult rat cardiomyocytes was investigated by a novel combination of confocal laser scanning microscopy and three-dimensional image reconstruction, referred to as FTCS, to visualize the morphological changes these cells undergo in culture. FTCS is discussed as an alternative imaging mode to low-magnification scanning electron microscopy. The three-dimensional shape of the cells are correlated with the assembly state of myofibrils in different stages. Based on immunofluorescence and confocal laser scanning microscopy it was shown that myofibrils are degraded within a few days after plating and that newly assembled myofibrils are predominantly confined to the continuous area in the perinuclear region close to the membrane in contact with the substratum. The localization of myofibrils along the cell's vertical axis has been investigated both by optical sectioning using confocal light microscopy and by physical sectioning followed by transmission electron microscopy. Based on the distribution of myofibrillar proteins we propose a model of myofibrillar growth locating the putative assembly sites to a region concentric around the nuclei. We provide evidence that the cell shape is dominated by the myofibrillar apparatus.

Effect of okadaic acid on protein phosphorylation patterns of chicken myogenic cells with special reference to creatine kinase

Okadaic acid and other agents affecting cellular phosphorylation and dephosphorylation processes profoundly changed the phosphoprotein pattern of 32Pi-labelled chicken embryonic skeletal muscle cells. The phosphorylation states of proteins in the lower molecular weight range were especially increased. Immunoprecipitation of cellular extracts with anti-creatine kinase antibodies enabled us to identify creatine kinase (CK) phosphoproteins. B-CK was phosphorylated after treating the cultures with 1-oleoyl-2-acetyl-sn-glycerol, dibutyryl-cAMP, okadiac acid and combinations thereof, but not with 1,2-dioleoyl-sn-glycerol. M-CK was also shown to be phosphorylated. The results indicated that in vivo, CK isoforms in muscle are subjected to control mediated by phosphorylation and dephosphorylation processes.

Sarcomeric myosin heavy chain expressed in nonmuscle cells forms thick filaments in the presence of substoichiometric amounts of light chains

Central to the function of myosin is its ability to assemble into thick filaments which interact precisely and specifically with other myofibrillar proteins. We have established a novel experimental system for studying myofibrillogenesis using transient transfections of COS cells, a monkey kidney cell line. We have expressed both full-length rat alpha cardiac myosin heavy chain (MHC) and a truncated heavy meromyosin-like alpha MHC (sHMM) and shown that immunoreactive MHC proteins of the expected sizes were detected in lysates of transfected cells. Surprisingly, the full-length MHC formed large spindle-shaped structures throughout the cytoplasm of transfected cells as determined by immunofluorescence microscopy. The structures were not found in cells expressing the sHMM construct, indicating that their formation required an MHC rod. The spindle-shaped structures ranged in length from approximately 1 micron to over 20 microns in length and were birefringent suggesting that they are ordered arrays of thick filaments. This was confirmed by electron microscopic analysis of the transfected cells which revealed arrays of filamentous structures approximately 12 nm in diameter at their widest point. In addition, the vast majority of transfected MHC did not associate with the endogenous nonmuscle myosin light chains, demonstrating that myosin thick filaments can form in the absence of stoichiometric amounts of myosin light chains.

Identification of a program of contractile protein gene expression initiated upon skeletal muscle differentiation

The functional diversity of skeletal muscle is largely determined by the combinations of contractile protein isoforms that are expressed in different fibers. Just how the developmental expression of this large array of genes is regulated to give functional phenotypes is thus of great interest. In the present study, we performed a comprehensive analysis of contractile protein isoform mRNA profiles in skeletal muscle systems representing each generation of fiber formed: primary, secondary, and regenerating fibers. We find that in each system examined there is a common pattern of isoform gene expression during early differentiation for 5 of the 6 gene families we have investigated: myosin light chain (MLC)1, MLC2, tropomyosin, troponin (Tn)C, and TnI. We suggest that the common isoform patterns observed together represent a genetic program of skeletal muscle differentiation that is independent of the mature fiber phenotype and is found in all newly formed myotubes. Within each of these contractile protein gene families the program is independent of the isoforms of myosin heavy chain (MHC) expressed. The maintenance of such a program may reflect a specific requirement of the initial differentiation process.

Contractile protein dynamics of myofibrils in paired adult rat cardiomyocytes

The purpose of this study was to determine how quickly contractile proteins are incorporated into the myofibrils of freshly isolated cardiomyocytes and to determine whether there are regions of the cells that are more dynamic than others in their ability to incorporate the proteins. Paired cardiomyocytes joined at intercalated discs and single cells were isolated from adult rats, and microinjected 3 hours later with fluorescently labeled actin, alpha-actinin, myosin light chains and vinculin. The cells were fixed and permeabilized at various period, 5 seconds and longer, after microinjection. Actin became incorporated throughout the I-Bands in as short a time as 5 seconds. The free edges of the cells, which were formerly intercalated discs, exhibited concentrations of actin greater than that incorporated in the I-Bands. This extra concentration of actin was not detected, however, at intact intercalated discs connecting paired cells. Alpha-actinin was incorporated immediately into Z-Bands and intercalated discs. Vinculin, also, was localized at the Z-Bands and at intercalated discs, but in contrast to alpha-actinin, there was a higher concentration of vinculin in the region of the intact intercalated discs. Both alpha-actinin and vinculin were concentrated at the free ends of the cells that were formerly parts of intercalated discs. Myosin light chains were observed to incorporate into the A-Bands in periods as short as 5 seconds. These results suggest that the myofibrils of adult cardiomyocytes may be capable of rapid isoform transitions along the length of the myofibrils. The rapid accumulation of fluorescent actin, alpha-actinin, and vinculin in membrane sites that were previously parts of intercalated discs, may reflect the response to locomotory activity that is initiated in these areas as cells spread in culture. A similar response after an injury in the intact heart could allow repair to occur.

Expression of desmin cDNA in PtK2 cells results in assembly of desmin filaments from multiple sites throughout the cytoplasm

The assembly of intermediate filaments into a cytoplasmic network was studied by microinjecting into the nuclei and cytoplasms of PtK2 cells, plasmids that contained a full length desmin cDNA and an RSV promoter. Immunofluorescence was used to monitor the expression of desmin and its integration into the cells' vimentin intermediate filament network. We found that the expressed desmin co-localized with filaments of vimentin just as it does with fluorescently labelled desmin is microinjected into the cytoplasm of PtK2 cells. As early as two hours after microinjection of the plasmids, small discrete dots and short fragments of desmin could be detected throughout the cytoplasm of the cells. This initial distribution of desmin was superimposed on the filamentous pattern of vimentin in the cells. At 8 hours after microinjection of the plasmids, some of the desmin was present in long filaments that were coincident with vimentin filaments. By 18 hours, most of the desmin was in a filamentous network co-localizing with vimentin. There was no indication that desmin assembly began in the perinuclear region and proceeded toward the cell periphery. In some cells, excessively high levels of desmin were expressed. In these cases, overexpression led to clumping of desmin filaments as well as to an accumulation of diffusely distributed desmin protein in the center of the cells. This effect was apparent at approximately 18 hours after introduction of the plasmid. The native vimentin filaments in such cells were also aggregated around the nucleus, co-localizing with desmin. The microtubule networks in all injected cells appeared normal; microtubules were extended in typical arrays out to the periphery of the cells.