Statistically accurate estimation of hormone concentrations and associated uncertainties: methodology, validation, and applications

We describe a data reduction procedure to assign statistically accurate estimates of unknown hormone concentrations, with associated uncertainties, based on experimental uncertainties in sample replicates and the fitted calibration curve. Three mathematical calibration curve functions are considered. The one providing optimal statistical characterization of reference calibrators is chosen for unknown evaluation. Experimental error is addressed by assigning and propagating uncertainty estimates for each measured response (including zero-dose responses) by an empirically determined discrete uncertainty profile and by propagating calibration curve uncertainty. Discrete uncertainty profiles account for both response precision (replicability) and accuracy (deviation from predicted calibration curves) without relying on assumed theoretical response variance-assay response relations. The validity of assigning variable response weighting by this procedure was assessed by Monte Carlo simulations based on chemiluminescence growth hormone calibration curves. Much-improved accuracy and estimated precision are achieved for unknown hormone concentrations, particularly extremely low concentrations, by using this variable response weighting procedure.

Long-term monitoring of circadian rhythms in c-fos gene expression from suprachiasmatic nucleus cultures

BACKGROUND

The AP-1 family of transcription factors has been implicated in the control of the expression of many genes in response to environmental signals. Previous studies have provided temporal profiles for c-fos expression by taking measurements from many animals at several points in time, but these studies provide limited information about dynamic changes in expression. Here, we have devised a method of continuously measuring c-fos expression.

RESULTS

A transgenic mouse line expressing the human c-fos promoter linked to the firefly luciferase reporter gene (fos/luc) was generated to continuously monitor c-fos gene expression. A second transgenic mouse line expressing luciferase under the control of the cytomegalovirus promoter (CMV/luc) served as a control. Luminescence originating from identifiable brain regions was imaged from fos/luc brain slice cultures. Expression of the fos/luc transgene accurately reflected transcriptional responses of the endogenous c-fos gene. Dynamic changes in fos/luc expression in suprachiasmatic nuclei (SCN) explant cultures were monitored continuously, and luminescence showed almost 24 hour rhythms lasting up to five circadian cycles. In contrast, bioluminescence monitored from CMV/luc SCN explant cultures was not rhythmic.

CONCLUSION

The fos/luc transgenic mouse will be useful for long-term, non-invasive monitoring of c-fos transcriptional responses to the changing cellular environment. Circadian rhythms in c-fos expression can be monitored non-invasively in real time from the SCN, clearly demonstrating that c-fos transcription is regulated by the circadian clock.

Aging alters feed-forward and feedback linkages between LH and testosterone in healthy men

To discern the effect of aging on coordinate luteinizing hormone (LH) and testosterone secretion, we sampled healthy older men (age 62-74 yr, n = 11) and young controls (age 21-34 yr, n = 13) every 2.5 min overnight. Deconvolution analysis and cross-correlation were used to relate serum LH concentrations to calculated testosterone secretion rates (feed-forward stimulation), as well as serum testosterone concentrations to computed LH secretion rates (feedback inhibition). Despite statistically similar mean serum LH and testosterone concentrations in the young and older men, older individuals had diminished feed-forward stimulation of LH concentrations on calculated testosterone secretion rates, as well as delayed feedback inhibition of testosterone concentrations on computed LH secretion rates.

Effects of synergistic signaling by phytochrome A and cryptochrome1 on circadian clock-regulated catalase expression

Persistent oscillation in constant conditions is a defining characteristic of circadian rhythms. However, in plants transferred into extended dark conditions, circadian rhythms in mRNA abundance commonly damp in amplitude over two or three cycles to a steady state level of relatively constant, low mRNA abundance. In Arabidopsis, catalase CAT3 mRNA oscillations damp rapidly in extended dark conditions, but unlike catalase CAT2 and the chlorophyll a/b binding protein gene CAB, in which the circadian oscillations damp to low steady state mRNA abundance, CAT3 mRNA oscillations damp to high steady state levels of mRNA abundance. Mutational disruption of either phytochrome- or cryptochrome-mediated light perception prevents damping of the oscillations in CAT3 mRNA abundance and reveals strong circadian oscillations that persist for multiple cycles in extended dark conditions. Damping of CAT3 mRNA oscillations specifically requires phytochrome A but not phytochrome B and also requires the cryptochrome1 blue light receptor. Therefore, we conclude that synergistic signaling mediated through both phytochrome A and cryptochrome1 is required for damping of circadian CAT3 mRNA oscillations in extended dark conditions.

Quantitative analysis of Drosophila period gene transcription in living animals

To determine the in vivo regulatory pattern of the clock gene period (per), the authors recently developed transgenic Drosophila carrying a luciferase cDNA fused to the promoter region of per. They have now carried out noninvasive, high time-resolution experiments allowing high-throughput monitoring of circadian bioluminescence rhythms in individual living adults for several days. This immediately solved several problems (resulting directly from individual asynchrony within a population) that have accompanied previous biochemical experiments in which groups of animals were sacrificed at each time point. Furthermore, the authors have developed numerical analysis methods for automatically determining rhythmicity associated with bioluminescence records from single flies. This has revealed some features of per gene transcription that were previously unappreciated and provides a general strategy for the analysis of rhythmic time series in the study of molecular rhythms.

Circadian rhythms in mouse suprachiasmatic nucleus explants on multimicroelectrode plates

The suprachiasmatic nucleus (SCN) of the mammalian hypothalamus functions as a circadian pacemaker. This study used multimicroelectrode plates to measure extracellular action potential activity simultaneously from multiple sites within the cultured mouse SCN. Neurons within the isolated mouse SCN expressed a circadian rhythm in spontaneous firing rate for weeks in culture.

Conditional circadian dysfunction of the Arabidopsis early-flowering 3 mutant

Photoperiodic responses, such as the daylength-dependent control of reproductive development, are associated with a circadian biological clock. The photoperiod-insensitive early-flowering 3 (elf3) mutant of Arabidopsis thaliana lacks rhythmicity in two distinct circadian-regulated processes. This defect was apparent only when plants were assayed under constant light conditions. elf3 mutants retain rhythmicity in constant dark and anticipate light/dark transitions under most light/dark regimes. The conditional arrhythmic phenotype suggests that the circadian pacemaker is intact in darkness in elf3 mutant plants, but the transduction of light signals to the circadian clock is impaired.

Novel features of drosophila period Transcription revealed by real-time luciferase reporting

The rapid turnover of luciferase and the sensitive, non-invasive nature of its assay make this reporter gene uniquely situated for temporal gene expression studies. To determine the in vivo regulatory pattern of the Drosophila clock gene period (per), we generated transgenic strains carrying a luciferase cDNA fused to the promoter region of the per gene. This has allowed us to monitor circadian rhythms of bioluminescence from pacemaker cells within the head for several days in individual living adults. These high time-resolution experiments permitted neuronal per transcription and opens the door to vastly simplified experiments in general chronobiology and studies of temporally regulated transcription in a wide range of experimental systems.

Thermodynamics of interaction of the fusion-inhibiting peptide Z-D-Phe-L-Phe-Gly with dioleoylphosphatidylcholine vesicles: direct calorimetric determination

The binding of the fusion-inhibiting peptide Z-D-Phe-L-Phe-Gly to unilamellar lipid vesicles of dioleoylphosphatidylcholine (DOPC) was investigated by isothermal titration calorimetry (ITC). The peptide Z-D-Phe-L-Phe-Gly is known to inhibit fusion of myxo- and paramyxoviruses with cells as well as cell-cell and vesicle-vesicle fusion in model systems. Calorimetric titrations conducted over a range of temperatures permitted characterization of the thermodynamics of the interaction of Z-D-Phe-L-Phe-Gly with model DOPC lipid membranes. Simultaneous global analysis of 15 ITC binding curves acquired at four different temperatures allowed determination of the equilibrium site association constant (K), stoichiometry of binding (n), binding enthalpy change (delta H), and heat capacity change of binding (delta Cp) in a single set of experiments. The binding affinity and enthalpy change per mole of DOPC bound at 25 degrees C was log K = 2.463 +/- 0.075 and delta H = -1.07 +/- 0.12 kcal/mol DOPC while the binding heat capacity change per mole of DOPC bound was delta Cp = -20.3 +/- 2.8 cal/(K.mol DOPC) with a temperature dependence (from 10-45 degrees C) of d(delta Cp)/dT = 0.37 +/- 0.18 cal/(K2.mol DOPC). A temperature-independent binding stoichiometry was determined to be n = 5.56 +/- 0.33 DOPC molecules per Z-D-Phe-L-Phe-Gly. A comparison of these results with previous peptide-lipid binding studies is discussed as is their relevance to a current model of the interaction of fusion-inhibiting peptides with phospholipid membranes.

The regulation of circadian period by phototransduction pathways in Arabidopsis

Transgenic Arabidopsis plants expressing a luciferase gene fused to a circadian-regulated promoter exhibited robust rhythms in bioluminescence. The cyclic luminescence has a 24.7-hour period in white light but 30- to 36-hour periods under constant darkness. Either red or blue light shortened the period of the wild type to 25 hours. A phytochrome-deficient mutation lengthened the period in continuous red light but had little effect in continuous blue light, whereas seedlings carrying mutations that activate light-dependent pathways in darkness maintained shorter periods in constant darkness. These results suggest that both phytochrome- and blue light-responsive photoreceptor pathways control the period of the circadian clock.

Enhanced sensitivity growth hormone (GH) chemiluminescence assay reveals lower postglucose nadir GH concentrations in men than women

Modifications were made to a commercially available human (h) GH chemiluminescence assay (Nichols Luma Tag hGH assay), which improved its sensitivity to 0.002 micrograms/L. The results of this assay had a high correlation with those of the Nichols hGH immunoradiometric assay (IRMA; r = 0.91; P < 0.001). The addition of recombinant hGH-binding protein (0.1-10 nmol/L) to standards and serum samples caused a dose-responsive reduction in measured GH in both the chemiluminescence assay and the IRMA; at physiological concentrations of hGH-binding protein, a 10-20% reduction was observed. Fifteen normal young adults (nine men and six women) underwent a standard 100-g oral glucose tolerance test, and plasma GH was measured from 30 min before until 5 h after glucose ingestion. GH was measurable in all samples with the chemiluminescence assay, but fell below the sensitivity of the IRMA in 59% of the samples. There was no difference between baseline or peak glucose levels in male and female subjects, but serum GH concentrations (mean +/- SD) measured by the enhanced sensitivity chemiluminescence assay were lower in male than female subjects at both baseline (0.12 +/- 0.08 vs. 2.3 +/- 2.3 micrograms/L; P < 0.01) and the postglucose GH nadir (0.029 +/- 0.014 vs. 0.25 +/- 0.23 micrograms/L; P < 0.01). The high correlation between baseline and nadir GH (r = 0.82; P < 0.001) and the equivalent fractional decline in mean GH levels in men and women after glucose administration (67 +/- 17% vs. 84 +/- 8%; P = 0.06) suggest that the lower GH levels in men after glucose treatment are due to lower baseline values and not to a greater suppressive effect of glucose.

Two-dimensional differential scanning calorimetry: simultaneous resolution of intrinsic protein structural energetics and ligand binding interactions by global linkage analysis

A general theoretical development for the design and analysis of two-dimensional thermal stability surfaces of proteins is presented. The surfaces are generated from multiple excess heat capacity profiles (<delta Cp> vs T) obtained at varying concentrations of an interacting ligand. The energetics of both the intrinsic protein stability and the protein-ligand interaction are simultaneously resolved by employing statistical thermodynamic models in global linkage analysis. This formalism allows resolution of the intrinsic protein folding-unfolding parameters (enthalpy, entropy, and heat capacity changes) as well as the ligand interaction parameters (binding stoichiometry, enthalpy, entropy, and heat capacity changes). The theory has been applied to the case of ribonuclease A and its interaction with cytidine-2'-monophosphate. The accuracy of the thermodynamic parameters obtained by this approach compares within error with those parameters that can be obtained by direct measurements.

Role of sn-1-saturated,sn-2-polyunsaturated phospholipids in control of membrane receptor conformational equilibrium: effects of cholesterol and acyl chain unsaturation on the metarhodopsin I in equilibrium with metarhodopsin II equilibrium

The effect of phospholipid bilayer acyl chain packing free volume on the equilibrium concentration of the form of photolyzed rhodopsin which initiates visual signal transduction, metarhodopsin II (meta II), is examined in reconstituted systems formed from the saturated phospholipid dimyristoylphosphatidylcholine (DMPC) and in the polyunsaturated phospholipid sn-1-palmitoyl-sn-2-arachidonoylphosphatidylcholine (PAPC) with and without 30 mol% cholesterol. The extent of meta II formation is determined from both flash photolysis measurements and rapidly acquired absorbance spectra. Equilibrium and dynamic properties of the lipid bilayer are characterized by the dynamic fluorescence properties of 1,6-diphenyl-1,3,5-hexatriene (DPH). DPH orientational properties are characterized by fv, a parameter which reflects the volume available for probe reorientation in the bilayer, relative to that available in an unhindered, isotropic environment [Straume, M., & Litman, B. J. (1987) Biochemistry 26, 5121-5126]. The metarhodopsin I in equilibrium with meta II equilibrium constant, Keq has a linear relationship with fv for rhodopsin in PAPC vesicles with and without cholesterol as well as for rhodopsin in DMPC vesicles, and these two correlation lines have different slopes. The correlations between Keq and fv in PAPC and DMPC systems are compared with a similar correlation in the native rod outer segment disk membrane and one reported previously in an egg phosphatidylcholine (egg PC) system [Mitchell, D. C., Straume, M., Miller, J. L., & Litman, B. J. (1990) Biochemistry 29, 9143-9149].(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of metarhodopsin formation by cholesterol-induced ordering of bilayer lipids

The effect of lipid ordering on the kinetics and extent of metarhodopsin II (meta II) formation was evaluated in bovine rhodopsin which had been reconstituted into phosphatidylcholine vesicles containing 0, 15, and 30 mol% cholesterol. The rate of establishment of the dynamic equilibrium between metarhodopsin I (meta I) and the two kinetically distinguished forms of meta II in the branched meta II model [meta IIfast and meta IIslow; Straume, M., Mitchell, D. C., Miller, J. L., & Litman, B. J. (1990) Biochemistry (preceding paper in this issue)] is derived from kinetic measurements of rhodopsin photolysis in these vesicle systems at several temperatures. Values of the meta I in equilibrium with meta IItotal equilibrium constant, Keq, are calculated from the derived model-dependent rate constants, and are shown to be equivalent to those derived from rapidly acquired absorbance spectra. The presence of 30 mol% cholesterol reduces Keq by approximately 50% between 10 and 37 degrees C. Analysis of the model-dependent parameters in terms of delta H and delta S reveals that cholesterol raises the free energy of meta IIslow, relative to meta I, by increasing delta H whereas it raises the relative free energy of meta IIfast by making delta S meta IIfast relative to meta I less positive. The reduction in Keq by both temperature and cholesterol is found to be directly correlated with a parameter that reflects the free volume available for molecular motion in the hydrophobic core of the bilayer [Straume, M., & Litman, B. J. (1988) Biochemistry 27, 7723-7733].(ABSTRACT TRUNCATED AT 250 WORDS)

Interconversion of metarhodopsins I and II: a branched photointermediate decay model

Flash photolysis experiments designed to monitor the establishment of the metarhodopsin I to metarhodopsin II equilibrium are interpreted according to a branched model in which two spectrally indistinguishable but kinetically distinguishable forms of metarhodopsin II are postulated to exist in equilibrium with a common pool of metarhodopsin I. This interpretation arises from the consistent requirement for at least three exponentials for a valid description of the observed growth of absorbance at 380 nm following bleaching of bovine rhodopsin in rod outer segment disk membranes. Analysis of the 380-nm transient absorbance data permitted direct determination of the five physically interpretable individual rate constants of the model. This analysis represents a more explicit interpretation of kinetic data than that employed in earlier experiments of this kind, which involved estimating only apparent rates and apparent amplitudes of discrete multiexponential functions. The 380-nm absorbance contributions of all relevant species contributing to the observed dynamic absorbance change were accounted for simultaneously during nonlinear least-squares estimation of the model rate parameters. Analysis of deconvoluted equilibrium spectra acquired from samples identical with those used in the kinetics experiments confirmed the metarhodopsin I-metarhodopsin II equilibrium constants, Keq, derived from the dynamic analyses. It is shown that Keq varies from 1.28 at 10 degrees C to 7.3 at 37 degrees C and that approximately 90% of the metarhodopsin II present is in the form of metarhodopsin IIslow over the temperature range 10-37 degrees C. A physical interpretation of this decay model is discussed in the context of a distribution of metarhodopsin II structural and energetic states.

Resolvability of free energy changes for oxygen binding and subunit association by human hemoglobin

Probability distributions of the free energy changes for oxygen binding, subunit association, and quaternary enhancement by human hemoglobin were obtained from Monte Carlo simulations performed on two independent sets of variable protein concentration equilibrium oxygen-binding data. Uncertainties in unliganded and fully liganded dimer to tetramer association free energy changes (0 delta G'2 and 4 delta G'2) were accounted for in the simulations. Distributions of the dimer to tetramer association free energy changes for forming singly and triply liganded tetramers (1 delta G'2 and 3 delta G'2) are well defined and quite symmetric, whereas that for forming doubly liganded tetramers (2 delta G'2) is poorly defined and highly asymmetric. The distribution of the dimer stepwise oxygen-binding free-energy change (delta g'2i) is well defined and quite symmetric as are those of the tetramer stepwise oxygen-binding free-energy changes for binding the first and last oxygens to tetramers (delta g'41 and delta g'44). Distributions of the intermediate tetramer stepwise oxygen-binding free-energy changes (delta g'42 and delta g'43) are poorly defined and highly asymmetric, but are compensatory in that their sum (delta g'4[2 + 3]) is again well defined and nearly symmetric. Distributions of the free energy changes corresponding to the tetramer product Adair oxygen binding constants (delta G'4i) are well defined and quite symmetric for i = 1, 3, 4 but not for i = 2. The distribution of delta g'44 - delta g'2i (the quaternary enhancement free energy change) is relatively narrow, nearly symmetric, and confined to the negative free-energy domain. This suggests that the quaternary enhancement free energy change (a) may be resolved with good confidence from this data and (b) is finite and negative under the conditions of these experiments. Our results also suggest two different four-state combinatorial switch models that provide accurate characterization of hemoglobin's functional behavior.

Equilibrium and dynamic bilayer structural properties of unsaturated acyl chain phosphatidylcholine-cholesterol-rhodopsin recombinant vesicles and rod outer segment disk membranes as determined from higher order analysis of fluorescence anisotropy decay

Limited-frequency phase-modulation fluorometry of diphenylhexatriene (DPH) and trimethylammonium-diphenylhexatriene (TMA-DPH) was used to characterize the equilibrium and dynamic lipid structural properties of (1) reconstituted egg phosphatidylcholine (egg PC)-rhodopsin vesicles varying in rhodopsin content from 0 to approximately 1 mol %, (2) reconstituted PC-cholesterol-rhodopsin vesicles containing approximately 1 mol % rhodopsin and 0, approximately 15, or approximately 30 mol % cholesterol with egg PC, DOPC (di-18:1-PC), or PAPC (16:0,20:4-PC) as the phospholipid constituent, and (3) native bovine rod outer segment disk membranes. Experiments were conducted at 37, 25, 15, and 5 degrees C. Fluorescence lifetime analysis was performed by fitting the data to a constrained, discrete, biexponential model. Rotational depolarization properties were considered by a model requiring a single rotational diffusion coefficient and capable of producing orthogonal, bimodal orientational distributions for DPH and unimodal distributions for TMA-DPH [Straume, M., & Litman, B. J. (1987) Biochemistry 26, 5113-5120]. Unbleached rhodopsin reduced mean fluorophore lifetimes in proportion to the amount of protein present in PC vesicles as a result of probe-to-retinal energy transfer by (1) redistributing the relative lifetime contributions in favor of the short lifetime population and (2) reducing the lifetimes of each derived population. Lifetimes were increased by cholesterol and by reduction of the temperature, but the relative proportions of derived short- and long-lifetime populations were not affected. TMA-DPH lifetimes were more sensitive (in a relative manner) than were those of DPH. These observations are interpreted in terms of cholesterol and reduced temperature each inhibiting water penetrability into these bilayers, with a greater effect occurring in the headgroup and interfacial regions (probed by TMA-DPH) than in the hydrophobic bilayer interior (probed by DPH). Diunsaturated DOPC-rhodopsin recombinants were more resistant to temperature-dependent lifetime changes than were mixed-chain egg PC or PAPC vesicles. This suggests less favorable interaction of rhodopsin with diunsaturated PCs than with mixed-chain PCs. Lifetimes in disk membranes exhibited this same temperature dependence although DPH in disks had lifetimes longer than those seen in recombinant vesicles. TMA-DPH lifetimes in disks were more similar to those observed in cholesterol-containing recombinants. It would therefore appear that the large proportion of small, charged (at pH 7) phosphatidylethanolamine and phosphatidylserine headgroups present in disks reduces water penetrability into the d

Three-state combinatorial switch models as applied to the binding of oxygen by human hemoglobin

We have generated a series of all 6561 unique, discrete three-state combinatorial switch models to describe the partitioning of the cooperative oxygen-binding free change among the 10 variously ligated forms of human hemoglobin tetramers. These models were inspired by the experimental observation of Smith and Ackers that the cooperative free energy of the intersubunit contact regions of the 10 possible ligated forms of human hemoglobin tetramers can be represented by a particular distribution of three distinct energy levels [Smith, F. R., & Ackers, G. K. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 5347-5351]. A statistical thermodynamic formulation accounting for both dimer-tetramer equilibria and ligand binding properties of hemoglobin solutions as a function of oxygen and protein concentrations was utilized to exhaustively test these thermodynamic models. In this series of models each of the 10 ligated forms of the hemoglobin tetramer can exist in one, and only one, of three possible energy levels; i.e., each ligated form was assumed to be associated with a discrete energy state. This series of models includes all possible ways that the 10 ligation states of hemoglobin can be distributed into three distinct cooperative energy levels. The mathematical models, as presented here, do not permit equilibria between energy states to exist for any of the 10 unique ligated forms of hemoglobin tetramers. These models were analyzed by nonlinear least-squares estimation of the free energy parameters characteristic of this statistical thermodynamic development.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of cholesterol on equilibrium and dynamic bilayer structure of unsaturated acyl chain phosphatidylcholine vesicles as determined from higher order analysis of fluorescence anisotropy decay

The influence of cholesterol on equilibrium and dynamic bilayer structure in minimally to highly unsaturated phosphatidylcholine (PC) vesicles has been examined by characterization of the dynamic fluorescence properties of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH). Large, unilamellar egg PC, palmitoyloleoyl-PC (POPC), dioleoyl-PC (DOPC), palmitoylarachidonoyl-PC (PAPC), and palmitoyldocosahexaenoyl-PC (P-22:6-PC) vesicles containing no cholesterol or approximately 15 or 30 mol % cholesterol have been examined. Equilibrium and dynamic DPH orientational properties were analyzed according to an orthogonal, bimodal orientational distribution function [Straume, M., & Litman, B.J. (1987) Biochemistry (preceding paper in this issue)]. The same mathematical formalism was applied to TMA-DPH except that probe orientational probability was permitted only in the distribution peak aligned parallel to the bilayer normal. TMA-DPH fluorescence lifetimes were consistently increased by incorporation of cholesterol into these vesicles. Greater acyl chain unsaturation and increasing temperature each promoted reduction of lifetimes in the presence or absence of cholesterol. DPH lifetimes were much less sensitive than those of TMA-DPH to changes in composition or temperature. This behavior is consistent with reduced water penetrability into liquid-crystalline bilayers as cholesterol content is increased and as acyl chain unsaturation and temperature are reduced. Cholesterol also induces substantial equilibrium ordering of the bilayer both at the hydrophobic core and at the bilayer-water interface. DPH orientational distributions were shifted in favor of alignment parallel to the acyl side chains. The distributions of both probes were narrowed in response to incorporation of cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)