Sarcoplasmic Granules Research Articles

P.18.12 Histological and magnetic resonance imaging findings of Vastus Lateralis in Myotonic Dystrophies: Do they match up?

In myotonic dystrophy type 1 (DM1), MRI shows a characteristic pattern of skeletal muscle involvement, and data are usually in line with the clinical presentation. In contrast, myotonic dystrophy type 2 (DM2) patients often show discrepancies between their clinical phenotype and MRI. Systematic data on MRI abnormalities and their histological correlates are not available to date. We analyzed Vastus Lateralis biopsies, corresponding 3T MRI, and clinical data in 1 DM1 (f, 150–200 CTGs; MIRS IV; ages at biopsy/MRI: 49/50 y) and 6 DM2 patients (f/m:3/3). In DM2, mean ages at biopsy and subsequent MRI examinations were 53.7+/−12.2 y and 57.7+/−13.3 y, respectively. 3 female DM2 patients had pathological MRI results indicative of fatty degeneration: 2 patients showed moderate fatty changes, 1 patient had a complete fibroadipose tissue replacement. The corresponding histological findings included increased fiber size variation, type I fiber predominance, increased numbers of internal nuclei, numerous pyknotic nuclear clumps, acid phosphatase-positive sarcoplasmic granules in normal appearing and/or degenerative fibers, and widespread fatty tissue replacement in all 3 patients, and results correlated well with the extent of MRI changes. Further 3 DM2 patients had normal MRI, however also showed histological abnormalities which were milder and included type I fiber predominance, type II fiber atrophy, an increase of internal nuclei, and numerous pyknotic nuclear clumps but no fatty degeneration. In DM1, MRI of Vastus Lateralis was normal. The histological findings were in line and showed only mild variations in fiber size and few nuclear clumps. In DM, histological and imaging findings correlated well with the clinical data on muscle strength of knee extensors. We conclude that MRI most reliably reflected fatty tissue replacement in vivo. In contrast, changes in fiber type distribution, abnormal position of myonuclei, and numerous nuclear clumps did not show MRI correlates.

MRNA cycles through hypoxia-induced stress granules in live Drosophila embryonic muscles

In some myopathies, hypoxia can be the result of pathologic effects like muscle necrosis and abnormal blood flow. At the molecular level, the consequence of hypoxic conditions is not yet fully understood. Under stress conditions, many housekeeping gene mRNAs are translationally silenced, while translation of other mRNAs increases. Alterations to the pool of mRNAs available for translation lead to the formation of so-called stress granules containing both mRNAs and proteins. Stress granule formation and dynamics have been investigated using cells in culture, but have not yet been examined in vivo. In Drosophila embryonic muscles, we found that hypoxia induces the formation of sarcoplasmic granules containing the established stress granule markers RIN and dFMR1. Upon restoration of normoxia, the observed granules were decreased in size, indicating that their formation might be reversible. Employing photobleaching approaches, we found that a cytoplasmic reporter mRNA rapidly shuttles in and out of the granules. Hence, stress granules are highly dynamic complexes and not simple temporary storage sites. Although mRNA rapidly cycles through the granules, its movement throughout the muscle is, remarkably, spatially restricted by the presence of yet undefined myofiber domains. Our results suggest that in hypoxic muscles mRNA remains highly mobile; however, its movement throughout the muscle is restricted by certain boundaries. The development of this Drosophila hypoxia model makes it possible to study the formation and dynamics of stress granules and their associated mRNAs and proteins in a living organism.

Localization of Salmon Cardiac Peptide (sCP) in the Heart of Salmon (Salmo salar L.)

We have previously cloned and characterized a novel cardiac hormone from the salmon (Salmo salar) which has a uniquely heart-specific distribution and a low structural similarity with any other known natriuretic peptides. Specific antibodies were raised in goat against the salmon cardiac peptide. For localization and quantification, four different methods were applied: immunohistochemistry (avidin–biotin peroxidase), transmission electron microscopy, cryoimmunoelectron microscopy (protein A-gold), and a specific radioimmunoassay. Both atrial and ventricular myocytes stained immunohistochemically. The staining was similar in all myocytes and no specific myoendocrine cells were found. Within a single myocyte, both atrial and ventricular, the staining was stronger near the nucleus. Transmission electron microscopy revealed that both the atrium and the ventricle contained small sarcoplasmic granules of similar type with a diameter of 100 to 200 nm and an electron-dense core with a clear halo. The granules were typical vesicles which can be found in secretory cells utilizing the regulatory pathway. The highest number of granules was found near the nucleus, but granules were located also near the Golgi apparatus, between myofilament bundles, and in subsarcolemmal positions. Gold particles, conjugated to antibodies raised against the salmon cardiac peptide, were deposited on similar sarcoplasmic granules found in transmission electron microscopy. Among the sarcoplasmic granules with gold particles there were granules which did not show any cardiac peptide immunoreactivity. A significantly (Student's t test, P < 0.05) higher concentration of cardiac peptide was found in the heart atrium than in the ventricle, 16.2 ± 3.5 pmol/mg tissue (n = 8) and 4.5 ± 1.7 pmol/mg tissue (n = 8), respectively. The findings show that the salmon cardiac peptide is localized in secretory granules in both compartments of the heart. The morphology of the granules suggests that both the atrium and the ventricle utilize the regulatory pathway to release salmon cardiac peptide.

Fine structure of rat intrafusal muscle fibers. The equatorial region.

An ultrastructural study has been undertaken on the equatorial (sensory) region of the rat muscle spindle. Two kinds of intrafusal muscle fibers, a nuclear bag fiber and a nuclear chain fiber, have been identified in this region on the basis of fiber diameter, nuclear disposition, and M-band appearance. The large-diameter nuclear bag fiber contains an aggregation of tightly packed vesicular nuclei, while the small-diameter nuclear chain fiber contains a single row of elongated, well-separated nuclei. Both muscle fibers contain an attenuated peripheral cylinder of myofilaments surrounding a central core of sarcoplasm. Elements of the sarcotubular system, dilatations of the sarcoplasmic reticulum, and the presence of other sarcoplasmic organelles and inclusions are considerably more abundant in the nuclear chain fiber than in the nuclear bag fiber. Leptomeric organelles and membrane-bounded sarcoplasmic granules are present in both intrafusal fiber types and may be situated between the myofibrils or in intimate association with the sarcolemma. The functional significance of some of these structural findings is discussed.

The ultrastructural effects of triparanol on rat atrial muscle.

The atrial musculature of rats given the cholesterol inhibitor triparanol (MER/29) (250 mg/kg daily) for 8 days was examined under the electron microscope and compared with that from untreated animals. The sarcoplasmic core of muscle fibers from animals given triparanol exhibited a new formation of sarcoplasmic granules which displayed a crystalline latticework with opaque lines approximately 40–60 A separated by clear spaces 50–70 A. They were partially or completely surrounded by a membrane. The crystalline bodies in cardiac muscle fibers were not as numerous as those observed in adrenocortical, testicular interstitial, or luteal cells as reported earlier by the investigators.

THE INFLUENCE OF GROWTH HORMONE ON THE PROTEIN COMPOSITION OF RAT MUSCLE

SUMMARY 1. The effect of 15 days' treatment with purified pituitary growth hormone (GH) on the composition of the protein of the quadriceps muscle was studied in adult female rats. 2. The muscle protein was divided into three fractions, the sarcoplasmic protein, the myofibrillar protein, and collagen and elastin. No differences were found in the proportions of the three fractions in normal and in GH-treated rats. 3. There are two components of muscle with adenosine triphosphatase (ATP-ase) activity: the myofibrils and the sarcoplasmic granules. These two activities and the total ATP-ase activity were assayed in homogenates from GH-treated and control rats. The only alteration observed in the rats receiving GH treatment was a diminution in the amount of myofibrillar ATP-ase per gram of fresh tissue.