Myosin-V is a mechanical ratchet

Myosin-V is a linear molecular motor that hydrolyzes ATP to move processively toward the plus end of actin filaments. Motion of this motor under low forces has been studied recently in various single-molecule assays. In this paper we show that myosin-V reacts to high forces as a mechanical ratchet. High backward loads can induce rapid and processive backward steps along the actin filament. This motion is completely independent of ATP binding and hydrolysis. In contrast, forward forces cannot induce ATP-independent forward steps. We can explain this pronounced mechanical asymmetry by a model in which the strength of actin binding of a motor head is modulated by the lever arm conformation. Knowledge of the complete force-velocity dependence of molecular motors is important to understand their function in the cellular environment.     

Modelling survivorship kinetics: a two-parameter model.

A mathematical model of survivorship kinetics is presented. It takes into account both deterministic and stochastic aspects of survival curves. Earlier reports [Piantanelli: Arch Gerontol Geriatr 1986;5:107-118; Piantanelli: Ann NY Acad Sci 1988; 521:99-109] described a model capable of making distinct predictions on the mean and standard deviation of an index of physiological function, fitting data even in the tail of survivorship curves, and accounting for the selection of the cohort at advanced ages. However, it contains four parameters whose biological interpretation is unclear. In the present paper we propose a modification of the model which maintains the main characteristics of the previous one and, in addition, results in two significant improvements. First, the number of free parameters is reduced to only two, making much easier both their estimation and interpretation, particularly when the model is applied to data from various animal groups manipulated in different ways. Second, it is possible to relate the parameters to well-defined deterministic and stochastic factors: specifically, a deterministic component describing the environmental and genetic influence on physiological functions, and a stochastic component representing the fluctuating interactions of the living organism and its environment.     

A coarse-grained model for a nanometer-scale molecular pump

A theory for a nanometer-scale pump based on the ratchet concept is presented. A lattice gas model with a set of moves that satisfy hydrodynamic equations is used to describe an asymmetric nanometer channel connecting two reservoirs of fluid. The channel, which is coupled to an external oscillatory (or stochastic) driving force, pumps fluid from one reservoir to the other. The frequency of the external driving force, the fluid density, and the channel dimensions are used to control the fluid flow. We observe a nonmonotonic behavior of the flow with respect to some model parameters and discuss the efficiency of the device.     

Molecularly targeted therapies for hepatocellular carcinoma: sorafenib as a stepping stone

Since sorafenib, a multikinase inhibitor targeting angiogenesis of hepatocellular carcinoma (HCC), demonstrated survival benefits in recent clinical trials, it has changed the treatment paradigm and become the standard first-line treatment for patients with advanced HCC. However, disease stabilization with sorafenib lasts a few months, possibly due to the development of resistance, and thus the survival advantage was modest, even in patients with preserved liver function. Furthermore, there is currently no biomarker for monitoring the response or resistance to sorafenib. Currently, various kinds of molecularly targeted agents have been developed and are being evaluated in clinical trials. There are several steps required to improve the outcome from sorafenib therapy. First, a reliable predictive and prognostic biomarker is urgently needed. Second, a compelling indication of sorafenib treatment for HCC needs more clinical studies and consensus. Third, the actual benefits of sorafenib to patients with advanced liver dysfunction should be clarified and a more effective strategy for targeted therapy needs to be developed, for example, using a combination of targeted agents acting on different pathways or different levels of a key pathway. Finally, sorafenib could be used with other treatment modalities, such as local ablation or transarterial chemoembolization, to synergize efficacy. Based on the successful introduction of sorafenib, future studies should focus on plans to further improve the outcome of HCC patients by overcoming resistance and maximizing the efficacy of molecularly targeted therapy.     

Myosin V processivity: multiple kinetic pathways for head-to-head coordination

Myosin V, a double-headed molecular motor, transports organelles within cells by walking processively along actin, a process that requires coordination between the heads. To understand the mechanism underlying this coordination, processive runs of single myosin V molecules were perturbed by varying nucleotide content. Contrary to current views, our results show that the two heads of a myosin V molecule communicate, not through any one mechanism but through an elaborate system of cooperative mechanisms involving multiple kinetic pathways. These mechanisms introduce redundancy and safeguards that ensure robust processivity under differing physiologic demands.     

Analysis of urea nitrogen and creatinine kinetics in hemodialysis: comparison of a variable-volume two-compartment model with a regional blood flow model and investigation of an appropriate solute kinetics model for clinical application

To investigate an appropriate solute kinetics model for clinical application, we analyzed urea nitrogen (UN) and creatinine (Cr) kinetics by a variable-volume two-compartmental model (2CM) and a regional blood flow model (RBF) in 44 hemodialysis patients with varying proportions of first compartmental volume and regional volume (p(1)). Solute kinetics could not be solved in some of the patients with higher p(1) values, and there were more solution failures by the RBF than by the 2CM. The solute generation rate (g) and solute distribution volume in the dry state (V(D)) increased with increases in p(1) in both models, but there were some differences between the two models. When g was normalized by V(D), it became relatively constant, irrespective of the p(1) value or model used (0.133 +/- 0.029 mg/min/l by the 2CM and 0.132 +/- 0.029 mg/min/l by the RBF for UN; 0.0200 +/- 0.0049 mg/min/l by the 2CM and 0.0198 +/- 0.0048 mg/min/l by the RBF for Cr). The intercompartmental mass transfer coefficient (K(c); liters/min) calculated by the 2CM decreased as p(1) increased (K(c) = -1.77.p(1) + 1.16, p < 0.0001, R = 0.999 for UN; K(c) = -0.847.p(1) + 0.556, p < 0.0001, R = 1.000 for Cr). The systemic blood flow (Q(sys); liters/min) calculated by the RBF also decreased as p(1) increased (Q(sys) = -11.1.p(1) + 6.21, p < 0.0005, R = 1.000 for UN; Q(sys) = -5.22.p(1) + 2.90, p < 0.001, R = 0.999 for Cr). Since the RBF more frequently failed to solve the solute kinetics and since there was a difference in its Q(sys) values for UN and Cr, the 2CM was considered to be a superior model. When p(1) was extremely low, the 2CM could be transformed into a modified variable-volume one-compartment model (1CM) which presented a similar g/V(D) (0.133 +/- 0.029 for UN; 0.0200 +/- 0.0048 for Cr). This modified 1CM was considered to satisfy appropriate conditions for clinical application, since it is simpler than the 2CM and provides useful information on the dialysis dose.     

The peptide-catalyzed stereospecific synthesis of tetroses: a possible model for prebiotic molecular evolution

Using a water-based prebiotic model of sugar synthesis involving glycolaldehyde self-condensation, we demonstrate that homochiral L-dipeptide catalysts lead to the stereospecific syntheses of tetroses. The asymmetric effect is largest for erythrose, which may reach a D-enantiomeric excess of >80% with L-Val-L-Val catalyst. Based on results obtained with various peptides, we propose a possible catalytic-reaction intermediate, consisting of an imidazolidinone ring formed between the two nitrogen atoms of the peptide catalyst and the C1 of one glycolaldehyde molecule. The study was motivated by the premise that exogenous material, such as the nonracemic amino acids found in meteorites, could have participated in the terrestrial evolution of molecular asymmetry by stereospecific catalysis. Because peptides might have formed readily on the early Earth, it is possible that their catalytic contribution was relevant in the prebiotic processes that preceded the onset of life.     

Caenorhabditis elegans muscle: a genetic and molecular model for protein interactions in the heart

The nematode Caenorhabditis elegans has become established as a major experimental organism with applications to many biomedical research areas. The body wall muscle cells are a useful model for the study of human cardiomyocytes and their homologous structures and proteins. The ability to readily identify mutations affecting these proteins and structures in C elegans and to be able to rigorously characterize their genotypes and phenotypes at the cellular and molecular levels permits mechanistic studies of the responsible interactions relevant to the inherited human cardiomyopathies. Future work in C elegans muscle holds great promise in uncovering new mechanisms in the pathogenesis of these cardiac disorders.     

Compartmental analysis of calcium kinetics in man: application of a four-compartmental model

⁴⁵Ca kinetic data was obtained in seven normal subjects and in 22 pathologic conditions. The specific radioactivities of the serum were measured over a period of 10–14 days. They were carried out by liquid scintillation counting after purification of the serum calcium by cationic exchange resin chromatography. The data was automatically analyzed by a program that determines the fitness of different possible multiexponential solutions. This program showed that the best solution is the sum of four exponential functions. The exchangeable calcium pool and the amounts transferred have been calculated according to a pure mammillary model. The observed mean normal values, recal-culated for the standard body surface (1.73 sq m), were: (1) total exchangeable pool, 5.52g; (2) accessible (central) compartment I = 1.07 g; (3) the three peripheral compartments II = 0.70 g; III = 1.60 g; IV = 2.14 g; (4) the transfer rates, I–II = 29 g/day; I–III = 6.4 g/day; I–IV = 1.1 g/day; (5) total turnover, 0.75 g/day; and (6) accretion, 420 mg/day. Significant deviations from the normal of one or several parameters of this model were found in hyperparathyroidism (eight cases), hypoparathyroidism (four cases), osteoporosis (six cases), osteomalacia (two cases), and bone metastasis (two cases). Dependency of the various parameters of the model on each other was tested by correlation analysis. The validity of the compartmental analysis is discussed.     

Acute promyelocytic leukemia: a model of molecular target based therapy

Leukemia, a group of hematological malignancies characterized by clonal expansion of hematopoietic cells with uncontrolled proliferation, decreased apoptosis and blocked differentiation, is one of the most notorious enemies of mankind which accounts for some 300,000 new cases and 222,000 deaths each year worldwide. Leukemia can be divided into acute or chronic, lymphoid or myeloid types, based on the disease progression and hematopoietic lineages involved 5. The responses of leukemia to therapies differ from one type or subtype to another. Hence, to improve the clinical outcome, the therapeutic strategies should be disease pathogenesis-based and individualized. The close collaboration between bench and bedside may not only shed new lights on leukemogenesis, gain insights into therapeutic mechanisms, but also provide opportunities for designing more rational therapies. The development of curative approaches for acute promyelocytic leukemia (APL) may serve as a paradigm.     

Iodothyronine kinetics in the rabbit: an experimental model

Turnover studies of thyroxine (T4), 3,5,3'-tri-iodothyronine (T3) and 3,3',5'-tri-iodothyronine (rT3) have been performed in the rabbit. A novel modification of a conventional radioimmunoassay has been used to measure specific 125I-labelled iodothyronines in small volumes of plasma in the presence of other 125I-labelled metabolites. Kinetic analysis of plasma disappearance of tracer was performed by a new theoretical approach. For T4 the mean (+/- S.D.) plasma concentration, clearance and production rates were 34 +/- 12 nmol/l, 109 +/- 19 ml/kg per day and 3.7 +/- 1.4 nmol/kg per day respectively (n = 9). For T3 the corresponding values were 2.04 +/- 0.42 nmol/l, 1.52 +/- 0.29 litres/kg per day and 3.07 +/- 0.76 nmol/kg per day (n = 8), and for rT3 0.12 +/- 0.04 nmol/l, 5.7 +/- 1.7 litres/kg per day and 0.69 +/- 0.23 nmol/kg per day (n = 8). The combination of these two new methodologies affords a simple and convenient means of studying iodothyronine metabolism under normal and abnormal conditions. The techniques employed may be generally applied to turnover studies of other compounds of physiological interest which can be measured by radioimmunoassay.     

Diabetic Rats Supplemented with L-Glutamine: A Study of Immunoreactive Myosin-V Myenteric Neurons and the Proximal Colonic Mucosa

We studied the neuronal density and size of myenteric neurons and the epithelial cell proliferation and crypt depth of the proximal colon in diabetic Wistar rats after supplementing them with L-glutamine (1%). The animals were divided into five groups: untreated normoglycemic (UN), L-glutamine-treated normoglycemic (NG), untreated diabetic (UD), and L-glutamine-treated diabetics 4 days (DG4) and 45 days (DG45) days after the onset of diabetes. We observed a reduction of 52.7% and 50.44% in the neuronal density of the proximal colon of the UD group compared to the UN and NG groups, respectively (P < 0.05). The neuronal density found for the DG4 (32.8%) and DG45 (28.6%) groups was higher than that of the UD group (P > 0.05). There were no significant differences (P > 0.05) when the data relative to the area of the myenteric neuron cell bodies, metaphasic index, and crypt depth in the proximal colon were compared among experimental groups.     

Application of a molecular enzyme kinetic model for aging cells and tissues

According to the membrane hypothesis of aging (MHA), cellular senescence is attributable to a life-long, cross-linking action of oxygen-free radicals in the cell plasma membrane, resulting in a continuous decrease of the passive ion permeabilities. The consequent increase in the intracellular potassium content is accompanied by a considerable condensation of the intracellular mass (i.e., by loss of water). MHA suggested that an age-dependent increase in the physical density of the intracellular mass can underly the well-known age-dependent decreases of the macromolecular synthetic processes, the enzymic turnover rates, etc. MHA was partly based on a molecular enzyme kinetic model (MEKM) suggesting that environmental factors can substantially influence the enzyme catalysis and regulation through collisional coupling. However, the possible quantitative ranges of alterations in enzyme activities have not been estimated. This paper concludes, using principal features of the two models, that known age-dependent changes in the membrane lipid fluidity and intracellular density may result in even a 10-fold overall decrease in the enzyme activities (characterized by kcat and k-1) during the life.