Tensional Homeostasis in Single Fibroblasts

Adherent cells generate forces through acto-myosin contraction to move, change shape, and sense the mechanical properties of their environment. They are thought to maintain defined levels of tension with their surroundings despite mechanical perturbations that could change tension, a concept known as tensional homeostasis. Misregulation of tensional homeostasis has been proposed to drive disorganization of tissues and promote progression of diseases such as cancer. However, whether tensional homeostasis operates at the single cell level is unclear. Here, we directly test the ability of single fibroblast cells to regulate tension when subjected to mechanical displacements in the absence of changes to spread area or substrate elasticity. We use a feedback-controlled atomic force microscope to measure and modulate forces and displacements of individual contracting cells as they spread on a fibronectin-patterned atomic-force microscope cantilever and coverslip. We find that the cells reach a steady-state contraction force and height that is insensitive to stiffness changes as they fill the micropatterned areas. Rather than maintaining a constant tension, the fibroblasts altered their contraction force in response to mechanical displacement in a strain-rate-dependent manner, leading to a new and stable steady-state force and height. This response is influenced by overexpression of the actin crosslinker α-actinin, and rheology measurements reveal that changes in cell elasticity are also strain- rate-dependent. Our finding of tensional buffering, rather than homeostasis, allows cells to transition between different tensional states depending on how they are displaced, permitting distinct responses to slow deformations during tissue growth and rapid deformations associated with injury.     

Characterization of Cholesterol Homeostasis in Telomerase-immortalized Tangier Disease Fibroblasts Reveals Marked Phenotype Variability

We compared the consequences of an ABCA1 mutation that produced an apparent lack of atherosclerosis (Tangier family 1, N935S) with an ABCA1 mutation with functional ABCA1 knockout that was associated with severe atherosclerosis (Tangier family 2, Leu⁵⁴⁸:Leu⁵⁷⁵-End), using primary and telomerase-immortalized fibroblasts. Telomerase-immortalized Tangier fibroblasts of family 1 (TT1) showed 30% residual cholesterol efflux capacity in response to apolipoprotein A-I, whereas telomerase-immortalized Tangier fibroblasts of family 2 (TT2) showed only 20%. However, there were a number of secondary differences that were often stronger and may help to explain the more rapid development of atherosclerosis in family 2. First, the total cellular cholesterol content increase was 2–3-fold and 3–5-fold in TT1 and TT2 cells, respectively. The corresponding increase in esterified cholesterol concentration was 10- and 40-fold, respectively. Second, 24-, 25-, and 27-hydroxycholesterol concentrations were moderately increased in TT1 cells, but were increased as much as 200-fold in TT2 cells. Third, cholesterol biosynthesis was moderately decreased in TT1 cells, but was markedly decreased in TT2 cells. Fourth, potentially atheroprotective LXR-dependent SREBP1c signaling was normal in TT1, but was rather suppressed in TT2 cells. Cultivated primary Tangier fibroblasts were characterized by premature aging in culture and were associated with less obvious biochemical differences. In summary, these results may help to understand the differential atherosclerotic susceptibility in Tangier disease and further demonstrate the usefulness of telomerase-immortalized cells in studying this cellular phenotype. The data support the contention that side chain-oxidized oxysterols are strong suppressors of cholesterol biosynthesis under specific pathological conditions in humans.     

Small Molecules Enable Cardiac Reprogramming of Mouse Fibroblasts with a Single Factor,Oct4

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Neuronal Ubiquitin Homeostasis

Neurons have highly specialized intracellular compartments that facilitate the development and activity of the nervous system. Ubiquitination is a post-translational modification that controls many aspects of neuronal function by regulating protein abundance. Disruption of this signaling pathway has been demonstrated in neurological disorders such as Parkinson’s disease, Amyotrophic Lateral Sclerosis and Angleman Syndrome. Since many neurological disorders exhibit ubiquitinated protein aggregates, the loss of neuronal ubiquitin homeostasis may be an important contributor of disease. This review discusses the mechanisms utilized by neurons to control the free pool of ubiquitin necessary for normal nervous system development and function as well as new roles of protein ubiquitination in regulating the synaptic activity.     

Epidermal Tissue Homeostasis

Abstract. Toad epidermis is a suitable model for studies on tissue homeostasis because cell pool size, influx into and efflux from the cell pool can be easily determined. the cell pool size was obtained by cell counting on photomicrographs, the influx (cell birth rate) was assessed by the metaphase-arrest technique, and the efflux (cell loss by moulting) assessed by counting the number of cells in the corneal layer and recording of intermoult periods. the importance of the methods for assessing these parameters per square unit of skin surface is emphasized.
These parameters were studied in eight groups of ten adult male toads sacrificed at various hours of the day. There were minor variations in the cell birth rate, fluctuating around a mean of 26 cells/mm²/hr (obtained at the metaphase collection period from 11.00-14.00 hours). By summation of the cell productions during the eight metaphase collection periods of 3 hr, and extrapolation to an intermoult period (time between two moults), a calculated cell production of about 6340 cells/mm² in 10.3 days was obtained, whereas the cell loss at each moult was only 2370 cells/mm². Thus the cell production rate exceeds the rate of cell loss through moults by a factor of 2.7. Arguments are presented that the 'surplus' of cells produced cannot be permanently accommodated within the living epidermis. Consequently a cell deletion rate beyond that by moulting of about 4000 cells/mm² in 10.3 days or 16 cells/mm²/hr can be calculated.
These results are discussed in relation to current concepts of tissue homeostatic mechanism(s). the results are consistent with the hypothesis that controlled cell deletion may be a tissue homeostatic mechanism complementary to controlled cell divisions.     

Metal Acquisition and Homeostasis in Fungi

Transition metals, particularly iron, zinc and copper, have multiple biological roles and are essential elements in biological processes. Among other micronutrients, these metals are frequently available to cells in only limited amounts, thus organisms have evolved highly regulated mechanisms to cope and to compete with their scarcity. The homeostasis of such metals within the animal hosts requires the integration of multiple signals producing depleted environments that restrict the growth of microorganisms, acting as a barrier to infection. As the hosts sequester the necessary transition metals from invading pathogens, some, as is the case of fungi, have evolved elaborate mechanisms to allow their survival and development to establish infection. Metalloregulatory factors allow fungal cells to sense and to adapt to the scarce metal availability in the environment, such as in host tissues. Here we review recent advances in the identification and function of molecules that drive the acquisition and homeostasis of iron, copper and zinc in pathogenic fungi.     

Epidermal Tissue Homeostasis.

Abstract. It has previously been shown that in toad epidermis the cell birth rate (K_b) exceeds the rate of cell loss through moulting (K_b) and that the ‘surplus’ of cells seems to be removed in a controlled manner. Assuming that the epidermis is non-expanding, a K_b/K_d ratio > 1 indicates that cell deletion additional to desquamation takes place. In normal toads this ratio is 2-3. Following implantation of hydrocortisone pellets into intact toads (release rate, 18 μ/g toad/d), the K_b/K_d ratio, over a period of 14 d of hormone treatment, had increased to about 7, due mainly to an increased K_b and to a lesser extent to a decreased K_d. No change in the epidermal cell pool size had taken place. It was previously shown that, following removal of the pars distalis of the pituitary gland, the K_b/K_d ratio decreased with time, due to a decreasing K_b and an increasing K_d, eventually leading to a decreased epidermal cell pool size. In this paper it is shown that, in pars distalisectomized toads with hydrocortisone pellets implanted, the K_b/K_d ratio is restored to control levels by a restoration of the K_b as well as the K_d. the results differ from those of previous studies in which ACTH or adrenocortico-steroids were administered discontinuously (by injection). Thus, by experimental manipulation, different K_b/K_d ratios can be obtained: low (< 1, pars distalis ablation), medium (2-3, normal toads) and high (7, hydrocortisone implantation). the potentiality of this unique situation in analysing the important question of how the ‘surplus’ cells are deleted is discussed.     

Gradients in Planarian Regeneration and Homeostasis

Planarian regeneration was one of the first models in which the gradient concept was developed. Morphological studies based on the analysis of the regeneration rates of planarian fragments from different body regions, the generation of heteromorphoses, and experiments of tissue transplantation led T.H. Morgan (1901) and C.M Child (1911) to postulate different kinds of gradients responsible for the regenerative process in these highly plastic animals. However, after a century of research, the role of morphogens in planarian regeneration has yet to be demonstrated. This may change soon, as the sequencing of the planarian genome and the possibility of performing gene functional analysis by RNA interference (RNAi) have led to the isolation of elements of the bone morphogenetic protein (BMP), Wnt, and fibroblast growth factor (FGF) pathways that control patterning and axial polarity during planarian regeneration and homeostasis. Here, we discuss whether the actions of these molecules could be based on morphogenetic gradients.Freshwater planarians are bilaterally symmetrical metazoans of the phylum Platyhelminthes. These animals are unsegmented, acoelomate, and possess well-defined anteroposterior (AP) and dorsoventral (DV) axes. Along the AP axis, we can distinguish an anterior cephalic region containing the brain and, usually, a pair of eyespots, a central region with a pharynx and a ventral mouth opening, and a posterior tail region (Fig. 1A). Planarians are best known for their ability to regenerate complete animals from tiny fragments of their own bodies in 1 wk (for review, see Saló and Baguñá 2002; Reddien and Sánchez-Alvarado 2004; Saló 2006; Sánchez-Alvarado 2006). This ability has attracted the interest of many scientists since long ago (Pallas 1774; Johnson 1822; Morgan 1901). Planarian regeneration requires the production of new tissue from the unique proliferative and pluripotent stem cells known as neoblasts (Handberg-Thorsager et al. 2008). After amputation, neoblasts close to the wound proliferate, giving rise to the regenerative blastema, defined as the unpigmented tissues where the missing tissues will differentiate (Fig. 1B–E). Remarkably, planarian pieces cut at any level along any of its axes can regenerate a whole worm, perfectly proportionate in only a few days (Fig. 1F). The process of tissue regeneration in the wound region from proliferating neoblasts was termed epimorphosis. In addition, a repatterning of the whole organism is required to recover a complete and proportionate regenerated planarian. This process of remodeling old tissues was termed morphallaxis (Morgan 1901). Together, with the initial studies on planarian regeneration, the first hypotheses suggesting a role of morphogenetic gradients in this process were proposed based on the observation of a differential regenerative capacity along the AP axis (Morgan 1901; Child 1911; Huxley and de Beer 1934).Open in a separate windowFigure 1.Regenerative capacity of freshwater planarians. (A) Schmidtea mediterranea planarian (top left). (e) Eyespots, (ph) pharynx. Bar, 1 mm. (B–E) Tail pieces at various stages of regeneration (top right). The white tissue in the most anterior tip is the regenerative blastema. Two small eyespots are evident within it after 5 d of regeneration. (F) Planarians display unique regenerative capacities, as any small fragments from almost anywhere can regenerate a new organism in 2 wk. In this diagram, we summarize the main types of planarian regeneration: (1) Terminal regeneration: After transverse sectioning, the anterior end (red line) will regenerate the missing head, whereas the posterior end (green line) will regenerate the missing tail. This indicates that the remaining tissue is polarized and knows what is missing. (2) Lateral regeneration: After longitudinal sectioning (blue line), the old tissue regenerates the missing lateral half. (3) Intercalary regeneration: After joining two distal pieces produced by transverse sections, planarians intercalate the missing region. In that case, cells from each piece participate equally in the production of an intercalary blastema (Saló and Baguñà 1985).     

神经退行性疾病中的乙酰化内稳态

近年来对神经退行性疾病机制的研究发现,乙酰化和去乙酰化在这一过程扮演了重要角色。组蛋白乙酰化酶(histone acetylase,HAT)和组蛋白去乙酰化酶(histone deacetylase,HDAC)两大家族分别催化组蛋白的乙酰化和去乙酰化,两者相互拮抗,维持体内乙酰化内稳态的平衡。乙酰化内稳态的概念就是在这样的基础上提出的。在神经退行性疾病的发病过程中,组蛋白乙酰基转移酶含量下降,乙酰化内稳态被打破,影响了神经细胞内重要基因的转录,从而导致了神经细胞功能失调甚至死亡。该文主要介绍HAT和HDAC两大家族在神经退行性疾病中的作用机制,以及针对乙酰化内稳态平衡机制的治疗策略。     

Integrating Protein Homeostasis Strategies in Prokaryotes

Bacterial cells are frequently exposed to dramatic fluctuations in their environment, which cause perturbation in protein homeostasis and lead to protein misfolding. Bacteria have therefore evolved powerful quality control networks consisting of chaperones and proteases that cooperate to monitor the folding states of proteins and to remove misfolded conformers through either refolding or degradation. The levels of the quality control components are adjusted to the folding state of the cellular proteome through the induction of compartment specific stress responses. In addition, the activities of several quality control components are directly controlled by these stresses, allowing for fast activation. Severe stress can, however, overcome the protective function of the proteostasis network leading to the formation of protein aggregates, which are sequestered at the cell poles. Protein aggregates are either solubilized by AAA+ chaperones or eliminated through cell division, allowing for the generation of damage-free daughter cells.     

Maintaining Homeostasis by Decision-Making

Living organisms need to maintain energetic homeostasis. For many species, this implies taking actions with delayed consequences. For example, humans may have to decide between foraging for high-calorie but hard-to-get, and low-calorie but easy-to-get food, under threat of starvation. Homeostatic principles prescribe decisions that maximize the probability of sustaining appropriate energy levels across the entire foraging trajectory. Here, predictions from biological principles contrast with predictions from economic decision-making models based on maximizing the utility of the endpoint outcome of a choice. To empirically arbitrate between the predictions of biological and economic models for individual human decision-making, we devised a virtual foraging task in which players chose repeatedly between two foraging environments, lost energy by the passage of time, and gained energy probabilistically according to the statistics of the environment they chose. Reaching zero energy was framed as starvation. We used the mathematics of random walks to derive endpoint outcome distributions of the choices. This also furnished equivalent lotteries, presented in a purely economic, casino-like frame, in which starvation corresponded to winning nothing. Bayesian model comparison showed that—in both the foraging and the casino frames—participants’ choices depended jointly on the probability of starvation and the expected endpoint value of the outcome, but could not be explained by economic models based on combinations of statistical moments or on rank-dependent utility. This implies that under precisely defined constraints biological principles are better suited to explain human decision-making than economic models based on endpoint utility maximization.     

Endocrine Aspects of Homeostasis

SYNOPSIS. Recent findings in endocrine research have greatlyincreased our understanding of the relationship between hormonesand homeostasis. The present paper reviews selected major advancesin such areas as neuropeptides, peptide biosynthesis in endocrineand neuronal cells, peptide receptors and intracellular pathwaysin target cells, "new" peptide hormones, and evolutionary considerationsof peptide hormones. Further understanding of hormone interactionsand of relationships between nervous, endocrine, and immunesystems has added to the growing complexity of the mechanismsof fine tuning and regulating our internal environment. Moreover,discovery of the same or similar peptides throughout the courseof evolution, i.e., from unicellular organisms through vertebrates,has led to a new unifying theory of intercellular communication.Endocrine aspects of homeostasis is an expanding and excitingfield of biology.     

Cell Growth and Size Homeostasis in Silico

Cell growth in size is a complex process coordinated by intrinsic and environmental signals. In a research work performed by a different group, size distributions of an exponentially growing population of mammalian cells were used to infer cell-growth rate in size. The results suggested that cell growth was neither linear nor exponential, but subject to size-dependent regulation. To explain the observed growth pattern, we built a mathematical model in which growth rate was regulated by the relative amount of mRNA and ribosomes in a cell. Under the growth model and a stochastic division rule, we simulated the evolution of a population of cells. Both the sampled growth rate and size distribution from this in silico population agreed well with experimental data. To explore the model space, alternative growth models and division rules were studied. This work may serve as a starting point to understand the mechanisms behind cell growth and size regulation using predictive models.