The actions of the chalones

CELLULAR FEEDBACK NETWORKS AND THEIR RESILIENCE AGAINST MUTATIONS

Many tissues undergo a steady turnover, where cell divisions are on average balanced with cell deaths. Cell fate decisions such as stem cell (SC) differentiations, proliferations, or differentiated cell (DC) deaths, may be controlled by cell populations through cell-to-cell signaling. Here, we examine a class of mathematical models of turnover in SC lineages to understand engineering design principles of control (feedback) loops, that may operate in such systems. By using ordinary differential equations that describe the co-dynamics of SCs and DCs, we study the effect of different types of mutations that interfere with feedback present within cellular networks. For instance, we find that mutants that do not participate in feedback are less dangerous in the sense that they will not rise from low numbers, whereas mutants that do not respond to feedback signals could rise and replace the wild-type population. Additionally, we asked if different feedback networks can have different degrees of resilience against such mutations. We found that all minimal networks, that is networks consisting of exactly one feedback loop that is sufficient for homeostatic stability of the wild-type population, are equally vulnerable. Mutants with a weakened/eliminated feedback parameter might expand from lower numbers and either enter unlimited growth or reach an equilibrium with an increased number of SCs and DCs. Therefore, from an evolutionary viewpoint, it appears advantageous to combine feedback loops, creating redundant feedback networks. Interestingly, from an engineering prospective, not all such redundant systems are equally resilient. For some of them, any mutation that weakens/eliminates one of the loops will lead to a population growth of SCs. For others, the population of SCs can actually shrink as a result of “cutting” one of the loops, thus slowing down further unwanted transformations.

Myostatin as a mediator of sarcopenia versus homeostatic regulator of muscle mass: insights using a new mass spectrometry-based assay

BACKGROUND

Myostatin is a protein synthesized and secreted by skeletal muscle that negatively regulates muscle mass. The extent to which circulating myostatin levels change in the context of aging is controversial, largely due to methodological barriers.

METHODS

We developed a specific and sensitive liquid chromatography with tandem mass spectrometry (LC-MS/MS) assay to measure concentrations of myostatin and two of its key inhibitors, follistatin-related gene (FLRG) protein and growth and serum protein-1 (GASP-1) in 80 younger (<40 years), 80 older (>65 years), and 80 sarcopenic older women and men.

RESULTS

Older women had 34 % higher circulating concentrations of myostatin than younger women. Per unit of lean mass, both older and sarcopenic older women had >23 % higher myostatin levels than younger women. By contrast, younger men had higher myostatin concentrations than older men with and without sarcopenia. Younger men had approximately twofold higher concentrations of myostatin than younger women; however, older women and sarcopenic older women had significantly higher relative myostatin levels than the corresponding groups of men. In both sexes, sarcopenic older subjects had the highest concentrations of FLRG. Circulating concentrations of myostatin exhibited positive, but not robust, correlations with relative muscle mass in both sexes.

CONCLUSIONS

Our data suggest that myostatin may contribute to the higher prevalence of sarcopenia in women but acts as a homeostatic regulator of muscle mass in men. Moreover, this new LC-MS/MS-based approach offers a means to determine the extent to which myostatin serves as a biomarker of muscle health in diverse conditions of muscle loss and deterioration.

What controls smooth muscle phenotype?

Enzyme-dispersed smooth muscle cells from the adult pig aortic media in the first few days of primary culture are in the contractile phenotype and do not divide when challenged with 5% WBS. After 6--8 days the isolated cells spontaneously undergo a change in phenotype where contraction cannot be stimulated and the cells respond to mitogens in WBS by logarithmic growth. The change in phenotype is reversible if the cells are seeded sufficiently densely (5 x 10(4) to 1 x 10(5)/ml) that a confluent monolayer results after less than 1 week of proliferation, but is irreversible if the cells are seeded sparsely (1 x 10(3) to 5 x 10(3)/ml) and take more than 2 weeks of proliferation to reach confluence. When the cells are seeded so densely (10(6)/ml) that a confluent monolayer is present from day 1, the cells do not undergo a change in phenotype but remain indefinitely in the contractile state. The spontaneous modulation of phenotype of isolated smooth muscle cells can be inhibited by a confluent monolayer of contractile smooth muscle or endothelial cells in co-culture with the sparsely seeded smooth muscle such that the two cell layers are not in contact but bathed by the same nutrient medium. Smooth muscle modulation can also be inhibited by a factor extracted from pig and rabbit aortic tissue, and its effects mimicked by commercially available sodium heparin at 50 units/ml.

The effects of fractions (chalones) obtained from lymphoid organs on the immune response in vivo

Aqueous extracts of calf and pig lymphoid organs were prepared and fractionated by means of gel filtration, ion exchange chromatography, and isoelectric focusing. These fractions, which had been previously assessed on mitogen-stimulated mouse spleen lymphocytes and other cells in vitro, were tested for their in vivo activity on humoral (haemolytic PFC in mice) and on cell-mediated immunity (skin allograft survival in mice, lymph node weight assay in rats, and systemic GvH-reaction in mice). None of these several fractions elicited either biologically significant or reproducible inhibitory effects. In particular, two fractions, a high and a small molecular weight fraction which were strongly inhibitory in vitro, remained without any chalone-like activity in these in vivo assays. Our results therefore failed to support the existence of a lymphocyte chalone.

The effects of fractions (chalones) obtained from lymphoid organs on lymphocyte proliferation in vitro

Aqueous extracts of lymphoid organs were prepared and fractionated by means of gel filtration, ion exchange chromatography, and isoelectric focusing. A protein-containing fraction with a molecular weight of approximately 80,000--90,000 and isoelectric points of 7.6 and 5.3--6.2 was isolated and shown to inhibit reproducibly both thymidine incorporation and proliferation of concanavalin A-stimulated mouse spleen lymphocytes in vitro. This effect appeared specific since proliferation of mastocytoma P-815 and leukemia L-1210 cells remained unaffected. A small molecular weight fraction (500 to 10,000 daltons) was also found to inhibit lymphocyte proliferation in vitro but was without apparent specificity for cell type.

Characterization of a G1 inhibitor from old JB-1 ascites tumor fluid. Interaction with polyions and ion exchangers

In most experimental ascites tumors the growth rate decreases with increasing age and cell number. This decrease is caused by a prolongation of the cell cycle and an increasing accumulation of noncycling cells in resting (or quiescent) G1 and G2 compartments. In cell-free ascitic fluid from the JB-1 ascites tumor in the plateau phase of growth, low molecular weight substances have been found which reversibly and specifically arrest JB-1 cells in G1 and G2. In order to characterize the JB-1 G1 inhibitor we have investigated the effect of ion exchangers and polyions on the activity of this inhibitor assayed in vitro by means of a partially synchronized JB-1 cell population analyzed by flow microfluorometry. The results indicate that polyanions and cation exchangers (immobilized polyanions) bind and abolish the G1-inhibitory activity. From this it is suggested that the G1 inhibitor is of a basic or polycationic nature. Since anion exchangers (immobilized polycations) are without effect on this activity it was surprising to find that polycations also neutralize the activity. The results indicate that this occurs by blocking an anionic G2-inhibitor receptor on the cell, thus preventing the polycationic G1 inhibitor from being bound to this receptor.

Some aspects of melanin biology: 1950-1975

Recent advances in the biology of mammalian pigmentation are reviewed. The multicellular epidermal melanin unit (melanocyte and associated pool of keratinocytes) rather than the melanocyte alone forms the focal point for melanin metabolism within mammalian epidermis. Within an epidermal melanin unit, melanosomes are synthesized by melanocytes and transferred to keratinocytes where they are degraded as they ascend to the epidermal surface. During the past 25 years, technical advances in biology and biochemistry have frosted a multidisciplinary approach to research on mammalian pigmentation. Emphasizing this perspective, we have examined the current state of knowledge of the form and function of epidermal melanin units from the levels of biologic organization ranging from the molecules relevant to melanin synthesis through the skin as a totally intergrated system. To an unusual degree, advances in melanin pigmentation have resulted from the integration of clinical medicine and basic science.

Employment of synchronized cells and flow microfluorometry in investigations on the JB-1 ascites tumour chalones

In most experimental ascites tumours the growth rate decreases with increasing age and cell number. This decrease is caused by a prolongation of the cell cycle and an increasing accumulation of non-cycling cells in resting (or quiescent) G1 and G2 compartments. In cell-free ascitic fluid from the JB-1 ascites tumour in the plateau phase of growth lowmolecular-weight substances have been found which reversibly and specifically arrest JB-1 cells in G1 and G2. The present paper describes an in-vitro model for testing the effect of the humoral growth inhibitors contained in the ascitic fluid. The test system is based on synchronized JB-1 cells analysed by flow-through cytofluorometry. Addition to the synchronous cells of a ultrafiltrate (less than 50000 Daltons) of the JB-1 ascitic fluid was found to induce a complete, but temporary arrest of the cells at the G1-S border.

Studies on the regulation of lymphocyte production in the murine thymus and some effects of a crude thymus extract

Cell proliferation in the murine thymus was studied in vivo under normal conditions and from 0 to 24 hr after a single injection of a water-soluble extract from mouse thymus, mouse spleen, and mouse skin. The thymus extract reduced during the first 24 hr the mitiotic activity 40%; the spleen extract had a weaker inhibitory effect. The skin extract had no such effect. The thymus extract and spleen extract inhibited the flux of cells into the S phase 0-8 hr after the injection of the extract. Initial labelling index was also reduced in this period. Eight hours after injection of the thymus or spleen extracts the inhibited cells initiated DNA synthesis. The rate of progression of blast cells through the cell cycle was normal 24 hr after the injection of the extracts. It was deduced from the analysis that the thymus extract inhibits processes triggering G0/G1 cells into DNA synthesis, the inhibition of G2 efflux being of minor importance. Finally a model for the regulation of proliferating thymic blast cells and the emigration of small lymphocytes from the thymus is proposed.