Exercise running and tetracycline as means to enhance skeletal muscle stem cell performance after external fixation

Prolonged limb immobilization, which is often the outcome of injury and illness, results in the atrophy of skeletal muscles. The basis of muscle atrophy needs to be better understood in order to allow development of effective countermeasures. The present study focused on determining whether skeletal muscle stem cells, satellite cells, are directly affected by long-term immobilization as well as on investigating the potential of pharmacological and physiological avenues to counterbalance atrophy-induced muscle deterioration. We used external fixation (EF), as a clinically relevant model, to gain insights into the relationships between muscle degenerative and regenerative conditions to the myogenic properties and abundance of bona fide satellite cells. Rats were treated with tetracycline (Tet) through the EF period, or exercise trained on a treadmill for 2 weeks after the cessation of the atrophic stimulus. EF induced muscle mass loss; declined expression of the muscle specific regulatory factors (MRFs) Myf5, MyoD, myogenin, and also of satellite cell numbers and myogenic differentiation aptitude. Tet enhanced the expression of MRFs, but did not prevent the decline of the satellite cell pool. After exercise running, however, muscle mass, satellite cell numbers (enumerated through the entire length of myofibers), and myogenic differentiation aptitude (determined by the lineal identity of clonal cultures of satellite cells) were re-gained to levels prior to EF. Together, our results point to Tet and exercise running as promising and relevant approaches for enhancing muscle recovery after atrophy.     

Adipocyte Stiffness Increases with Accumulation of Lipid Droplets

Adipogenesis and increase in fat tissue mass are mechanosensitive processes and hence should be influenced by the mechanical properties of adipocytes. We evaluated subcellular effective stiffnesses of adipocytes using atomic force microscopy (AFM) and interferometric phase microscopy (IPM), and we verified the empirical results using finite element (FE) simulations. In the AFM studies, we found that the mean ratio of stiffnesses of the lipid droplets (LDs) over the nucleus was 0.83 ± 0.14, from which we further evaluated the ratios of LDs over cytoplasm stiffness, as being in the range of 2.5 to 8.3. These stiffness ratios, indicating that LDs are stiffer than cytoplasm, were verified by means of FE modeling, which simulated the AFM experiments, and provided good agreement between empirical and model-predicted structural behavior. In the IPM studies, we found that LDs mechanically distort their intracellular environment, which again indicated that LDs are mechanically stiffer than the surrounding cytoplasm. Combining these empirical and simulation data together, we provide in this study evidence that adipocytes stiffen with differentiation as a result of accumulation of LDs. Our results are relevant to research of adipose-related diseases, particularly overweight and obesity, from a mechanobiology and cellular mechanics perspectives.     

Prolonged Lag in Population Outbreak of an Invasive Mussel: A Shifting-Habitat Model

Biological invasions pose a great threat to the integrity of natural communities. Some invasive species demonstrate a population explosion shortly after arrival while in other cases a prolonged lag between arrival and population outbreak is evident. This paper describes a case of a prolonged lag and explores the possible mechanism for this lag. The Red Sea mussel Brachidontes pharaonis, a Lessepsian migrant, was first recorded in the Mediterranean seven years after the opening of the Suez Canal in 1869. Since then it spread along the Israeli coast and as far northwest as Sicily. Studies conducted in the late 1970s, when B. pharaonis was still rare, predicted that it would not establish dense populations along the Israeli coast and would not outcompete the indigenous mussel Mytilaster minimus, although it has strong negative effects on survival and growth of the native species. It was attributed to the invader's low intrinsic rate of increase relative to that of the native species, and to strong density-independent mortality generated by exposure to high wave action and sedimentation. In contrast to these predictions, we found massive formations of B. pharaonis beds after lag of about 120 years. We looked for distributional patterns that may explain this lag and found no south–north gradient but a strong habitat-dependent colonization. Most apparent are dense B. pharaonis mussel beds (density >300 per 100cm²) on rocky platforms where mussel beds were absent in the past. These platforms lack the vermetid rim that is typical to this formation. In platforms protected by a biogenic rim, sediment accumulation is high and perennial algae flourish. None of the mussel species form beds in such habitats. We suggest that the delayed formation of B. pharaonis beds along the Israeli coast is a consequence of a recent shift in habitat conditions on some platforms. It is possible that receding of the biogenic rim at the edge of these platforms allowed more effective washing, reduced sediment accumulation, and reduced perennial algae cover making platforms more suitable for the mussels. Lower density-independent mortality allowed B. pharaonis to dominate on such platforms over the indigenous species. On beachrock, a habitat previously dominated by M. minimus, we recorded a rapid shift in numerical domination to B. pharaonis (from 1:7 to 1.4:1 Brachidontes/Mytilaster individuals) over a period of 4 years (1995–1999). This is probably a result of saturation of the habitat by B. pharaonis recruits originating from the established populations on platforms. Salinity changes and a potential genetic shift may also have contributed to the invasive mussel outbreak.     

Reproduction and the unusual condition of hermaphroditism in Sarcophyton glaucum (Octocorallia,Alcyoniidae) in KwaZulu-Natal,South Africa

The genus Sarcophyton is an abundant soft coral on the marginal, high-latitude reefs in KwaZulu-Natal. A 2-year study on reproduction in the most common species,S. glaucum, revealed that gametogenesis in male and female colonies takes 9–10 and 16–18 mo, respectively, in this gonochoric, seasonal, broadcast spawner. Gametogenesis and spawning are synchronised in and between colonies, and with other common soft corals on the reefs. Spawning occurs annually in March between full and new moon with the release of spermsacs and the mature oocytes, the smaller oocytes being retained for further development. Thus far, the reproductive attributes of S. glaucum are comparable with other studies on the genus. However, the KwaZulu-Natal population of S. glaucum manifested the unusual attribute of hermaphroditism in 9% of the colonies, these having Stage I and II spermaries and predominantly Stage III and IV oocytes in their polyps. Small numbers of Stage I and II oocytes indicated that such colonies may be simultaneous hermaphrodites but this could not be confirmed. The complex reproductive strategy and associated measure of hermaphroditism in S. glaucum on KwaZulu-Natal reefs are discussed in terms of the species’ recruitment success in this marginal, high-latitude environment.     

Estimating cell depth from somatic mutations

The depth of a cell of a multicellular organism is the number of cell divisions it underwent since the zygote, and knowing this basic cell property would help address fundamental problems in several areas of biology. At present, the depths of the vast majority of human and mouse cell types are unknown. Here, we show a method for estimating the depth of a cell by analyzing somatic mutations in its microsatellites, and provide to our knowledge for the first time reliable depth estimates for several cells types in mice. According to our estimates, the average depth of oocytes is 29, consistent with previous estimates. The average depth of B cells ranges from 34 to 79, linearly related to the mouse age, suggesting a rate of one cell division per day. In contrast, various types of adult stem cells underwent on average fewer cell divisions, supporting the notion that adult stem cells are relatively quiescent. Our method for depth estimation opens a window for revealing tissue turnover rates in animals, including humans, which has important implications for our knowledge of the body under physiological and pathological conditions.     

Biochemical composition, metabolism, and amino acid transport in planula-larvae of the soft coral Heteroxenia fuscescens

We determined the monthly percentage of biochemical components in planulae of the soft coral Heteroxenia fuscescens, for a 3-year period, and evaluated the findings in relation to seasonal fluctuations in water temperature. We determined the biochemical profile and metabolic rate of aging planulae and examined the possible absorption of dissolved organic material (DOM) from the water by the planulae. Our study is the first to present a long-term biochemical profile of planulae. They contained an average of 2.2% ash, 51.5% lipid, 33.6% protein, and 1.3% carbohydrate. Calculation of the average energetic content of a planula revealed a value of 1. 63 J planula(-1). Significant seasonal differences in planulae weight were noted between the summer and the other seasons. A significant decrease (41%) from the initial weight, 0.029 mg, took place in the planulae dry weight within 15 days. Significant decreases over time were also found in lipid (50%) and carbohydrate (83%) concentration but not in protein (20%). Metabolic rates of a planula was 0.06 microl O(2) planula(-1) hr(-1). The study shows for the first time that a soft coral planulae can take up dissolved free amino acids from seawater. Even though each of the amino acids was initially present at equimolar concentrations, there was a much faster uptake for the neutral, nonpolar amino acids, than for polar and basic ones. The potential contribution to the metabolic demand of planulae, from the uptake of amino acids, is estimated to be 11%. It is suggested that this uptake does not appear to be due to energetic considerations, but may have a more significant impact on their nitrogen budget.     

Hormonal changes affect the bone and bone marrow cells in a rat model

In this study, we used a rat model to investigate the effects of gonad hormones and replacement therapy on bone structure and the immune system. In the first phase of the study, 3- and 11-month-old F344 rats underwent ovariectomy (OVX) or were sham operated. Three months later, severe osteopenia was histologically observed in OVX rats of both age groups. The changes in the bone marrow structure of OVX rats included deterioration of cancellous bone that was associated with a remarkable increase of adipocyte cells. Furthermore, differential analyses for the expression of cell surface antigens by lymph-myeloid cells was studied using flow cytometry (FACS). The number of myeloid cells expressing ED-9(+) or CD-44(+) was similar in both age groups, and unaffected by OVX. However, an augmentation of T-lymphoid cells expressing CD4(+), CD5(+), or both, were observed with age, as well as after OVX. In the second phase of the study, 11-month-old rats were divided into five experimental groups: sham-operated, OVX, and OVX treated with sustained-release pellets of 17beta-estradiol (OVX-E), progesterone (OVX-P), or both (OVX-E/P). Hormone replacement therapy maintained low physiological levels, and rats were tested 12 weeks after treatment initiation. Administration of 17beta-E, with or without the addition of progesterone, prevented the rise of T lymphoid cells observed in OVX rats, whereas progesterone alone had no effect. In agreement with findings from the first phase, neither OVX nor replacement therapy affected the myeloid cells expression of ED-9 or CD-44. In summary, the cellular changes in the bone marrow of OVX rats were associated with an increase in adipocytes that was correlated with bone atrophy. An augmentation of T-lymphopoiesis was noted with increase in age or after OVX. This increase was reversed to baseline levels by 17beta-E treatment.