Decoding Supramolecular Packing Patterns from Computed Anisotropic Deformability Maps of Molecular Crystals

The ability to encode and embed desired mechanical properties into active pharmaceutical ingredient solid forms would significantly advance drug development. In recent years, computational methods, particularly dispersion-corrected density functional theory (DFT), have come of age, opening the possibility of reliably predicting and rationally engineering the mechanical response of molecular crystals. Here, many-body dispersion and Tkatchenko-Scheffler dispersion-corrected DFT were used to calculate the elastic constants of a series of archetypal systems, including paracetamol and aspirin polymorphs and model hydrogen-bonded urea and π-π-bound benzene crystals, establishing their structure-mechanics relations. Both methods showed semiquantitative and excellent qualitative agreement with experiment. The calculations revealed that the plane of maximal Young's modulus generally coincides with extended H-bond or π-π networks, showing how programmable supramolecular packing dictates the mechanical behavior. In a pharmaceutical setting, these structure-mechanics relations can steer the molecular design of solid forms with improved physicochemical and compression properties.

A Piezoelectric Ionic Cocrystal of Glycine and Sulfamic Acid

Cocrystallization of two or more molecular compounds can dramatically change the physicochemical properties of a functional molecule without the need for chemical modification. For example, coformers can enhance the mechanical stability, processability, and solubility of pharmaceutical compounds to enable better medicines. Here, we demonstrate that amino acid cocrystals can enhance functional electromechanical properties in simple, sustainable materials as exemplified by glycine and sulfamic acid. These coformers crystallize independently in centrosymmetric space groups when they are grown as single-component crystals but form a noncentrosymmetric, electromechanically active ionic cocrystal when they are crystallized together. The piezoelectricity of the cocrystal is characterized using techniques tailored to overcome the challenges associated with measuring the electromechanical properties of soft (organic) crystals. The piezoelectric tensor of the cocrystal is mapped using density functional theory (DFT) computer models, and the predicted single-crystal longitudinal response of 2 pC/N is verified using second-harmonic generation (SHG) and piezoresponse force microscopy (PFM). The experimental measurements are facilitated by polycrystalline film growth that allows for macroscopic and nanoscale quantification of the longitudinal out-of-plane response, which is in the range exploited in piezoelectric technologies made from quartz, aluminum nitride, and zinc oxide. The large-area polycrystalline film retains a damped response of ≥0.2 pC/N, indicating the potential for application of such inexpensive and eco-friendly amino acid-based cocrystal coatings in, for example, autonomous ambient-powered devices in edge computing.

Density functional theory predictions of the mechanical properties of crystalline materials

The DFT-predicted mechanical properties of crystalline materials are crucial knowledge for their screening, design, and exploitation.

Terahertz spectroscopy of 2,4,6-trinitrotoluene molecular solids from first principles

We present a computational analysis of the terahertz spectra of the monoclinic and the orthorhombic polymorphs of 2,4,6-trinitrotoluene. Very good agreement with experimental data is found when using density functional theory that includes Tkatchenko–Scheffler pair-wise dispersion interactions. Furthermore, we show that for these polymorphs the theoretical results are only weakly affected by many-body dispersion contributions. The absence of dispersion interactions, however, causes sizable shifts in vibrational frequencies and directly affects the spatial character of the vibrational modes. Mode assignment allows for a distinction between the contributions of the monoclinic and orthorhombic polymorphs and shows that modes in the range from 0 to ca. 3.3 THz comprise both inter- and intramolecular vibrations, with the former dominating below ca. 1.5 THz. We also find that intramolecular contributions primarily involve the nitro and methyl groups. Finally, we present a prediction for the terahertz spectrum of 1,3,5-trinitrobenzene, showing that a modest chemical change leads to a markedly different terahertz spectrum.

Use of Crystal Structure Informatics for Defining the Conformational Space Needed for Predicting Crystal Structures of Pharmaceutical Molecules

Determining the range of conformations that a flexible pharmaceutical-like molecule could plausibly adopt in a crystal structure is a key to successful crystal structure prediction (CSP) studies. We aim to use conformational information from the crystal structures in the Cambridge Structural Database (CSD) to facilitate this task. The conformations produced by the CSD Conformer Generator are reduced in number by considering the underlying rotamer distributions, an analysis of changes in molecular shape, and a minimal number of molecular ab initio calculations. This method is tested for five pharmaceutical-like molecules where an extensive CSP study has already been performed. The CSD informatics-derived set of crystal structure searches generates almost all the low-energy crystal structures previously found, including all experimental structures. The workflow effectively combines information on individual torsion angles and then eliminates the combinations that are too high in energy to be found in the solid state, reducing the resources needed to cover the solid-state conformational space of a molecule. This provides insights into how the low-energy solid-state and isolated-molecule conformations are related to the properties of the individual flexible torsion angles.

Generation of crystal structures using known crystal structures as analogues

This analysis attempts to answer the question of whether similar molecules crystallize in a similar manner. An analysis of structures in the Cambridge Structural Database shows that the answer is yes - sometimes they do, particularly for single-component structures. However, one does need to define what we mean by similar in both cases. Building on this observation we then demonstrate how this correlation between shape similarity and packing similarity can be used to generate potential lattices for molecules with no known crystal structure. Simple intermolecular interaction potentials can be used to minimize these potential lattices. Finally we discuss the many limitations of this approach.

Report on the sixth blind test of organic crystal structure prediction methods

The sixth blind test of organic crystal structure prediction (CSP) methods has been held, with five target systems: a small nearly rigid molecule, a polymorphic former drug candidate, a chloride salt hydrate, a co-crystal and a bulky flexible molecule. This blind test has seen substantial growth in the number of participants, with the broad range of prediction methods giving a unique insight into the state of the art in the field. Significant progress has been seen in treating flexible molecules, usage of hierarchical approaches to ranking structures, the application of density-functional approximations, and the establishment of new workflows and `best practices' for performing CSP calculations. All of the targets, apart from a single potentially disordered Z' = 2 polymorph of the drug candidate, were predicted by at least one submission. Despite many remaining challenges, it is clear that CSP methods are becoming more applicable to a wider range of real systems, including salts, hydrates and larger flexible molecules. The results also highlight the potential for CSP calculations to complement and augment experimental studies of organic solid forms.

Hydrogen bonding at C=Se acceptors in selenoureas, selenoamides and selones

In recent years there has been considerable interest in chalcogen and hydrogen bonding involving Se atoms, but a general understanding of their nature and behaviour has yet to emerge. In the present work, the hydrogen-bonding ability and nature of Se atoms in selenourea derivatives, selenoamides and selones has been explored using analysis of the Cambridge Structural Database andab initiocalculations. In the CSD there are 70 C=Se structures forming hydrogen bonds, all of them selenourea derivatives or selenoamides. Analysis of intramolecular geometries andab initiopartial charges show that this bonding stems from resonance-induced Cδ+=Seδ−dipoles, much like hydrogen bonding to C=S acceptors. C=Se acceptors are in many respects similar to C=S acceptors, with similar vdW-normalized hydrogen-bond lengths and calculated interaction strengths. The similarity between the C=S and C=Se acceptors for hydrogen bonding should inform and guide the use of C=Se in crystal engineering.

van der Waals dispersion interactions in molecular materials: beyond pairwise additivity

van der Waals (vdW) dispersion interactions are a key ingredient in the structure, stability, and response properties of many molecular materials and essential for us to be able to understand and design novel intricate molecular systems. Pairwise-additive models of vdW interactions are ubiquitous, but neglect their true quantum-mechanical many-body nature. In this perspective we focus on recent developments and applications of methods that can capture collective and many-body effects in vdW interactions. Highlighting a number of recent studies in this area, we demonstrate both the need for and usefulness of explicit many-body treatments for obtaining qualitative and quantitative accuracy for modelling molecular materials, with applications presented for small-molecule dimers, supramolecular host-guest complexes, and finally stability and polymorphism in molecular crystals.

The integration of solid-form informatics into solid-form selection

OBJECTIVES

To demonstrate how the use of structural informatics during drug development assists with the assessment of the risk of polymorphism and the selection of a commercial solid form.

METHODS

The application of structural chemistry knowledge derived from the hundreds of thousands of crystal structures contained in the Cambridge Structural Database to drug candidates is described. Examples given show the comparison of intermolecular geometries to database-derived statistics, the use of Full Interaction Maps to assess polymorph stability and the calculation of hydrogen bond propensities to provide assurance of a stable solid form. The software tools used are included in the Cambridge Structural Database System and the Solid Form Module of Mercury.

KEY FINDINGS

The early identification of an unusual supramolecular motif in the development phase of maraviroc led to further experimental work to find the most stable polymorph. Analyses of two polymorphs of a pain candidate drug demonstrated how consideration of molecular conformation and intermolecular interactions were used for the assessment of relative stability. Informatics analysis confirmed that the solid form of crizotinib, a monomorphic system, had a low risk of polymorphism.

CONCLUSIONS

The application of informatics-based assessment of new chemical entities complements experimental studies and provides a deeper understanding of the qualities of the structure. The information provided by structural analyses is incorporated into the assessment of risk. Informatics techniques are quick to apply and are straightforward to use, allowing an assessment of progressing drug candidates.

Sixth blind test of organic crystal-structure prediction methods

Over the past 15 years progress in predicting crystal structures of small organic molecules has been charted by a series of blind tests hosted by the Cambridge Crystallographic Data Centre. This letter announces a sixth blind test to take place between September 2014 and August 2015, giving details of the target systems and the revised procedure. We hope that as many methods as possible will be assessed and benchmarked in this new blind test.

Role of dispersion interactions in the polymorphism and entropic stabilization of the aspirin crystal

Aspirin has been used and studied for over a century but has only recently been shown to have an additional polymorphic form, known as form II. Since the two observed solid forms of aspirin are degenerate in terms of lattice energy, kinetic effects have been suggested to determine the metastability of the less abundant form II. Here, first-principles calculations provide an alternative explanation based on free-energy differences at room temperature. The explicit consideration of many-body van der Waals interactions in the free energy demonstrates that the stability of the most abundant form of aspirin is due to a subtle coupling between collective electronic fluctuations and quantized lattice vibrations. In addition, a systematic analysis of the elastic properties of the two forms of aspirin rules out mechanical instability of form II as making it metastable.

The Gas-phase Structure of the Hexasilsesquioxane Si6O9(OSiMe3)6

The equilibrium molecular structure of the hexasilsesquioxane, Si6O9(OSiMe3)6, has been determined in the gas phase by electron diffraction. With OSi-containing substituents on the cage silicon atoms, this molecule closely resembles the moiety that if reproduced in a periodic manner would yield a zeolite-type structure. Semi-empirical molecular-dynamics (SE-MD) calculations were used to give amplitudes of vibration, vibrational distance corrections (differences between interatomic distances in the equilibrium structure and the vibrationally averaged distances that are given directly by the diffraction data) and anharmonic constants. A number of different SE-MD methods were tested, and their results are compared. The inclusion of d-type orbitals in the SE-MD method is crucial for obtaining accurate vibrational quantities for Si6O9(OSiMe3)6, with the PM6 and MNDO/D methods both giving acceptable values

Understanding the role of vibrations, exact exchange, and many-body van der Waals interactions in the cohesive properties of molecular crystals

The development and application of computational methods for studying molecular crystals, particularly density-functional theory (DFT), is a large and ever-growing field, driven by their numerous applications. Here we expand on our recent study of the importance of many-body van der Waals interactions in molecular crystals [A. M. Reilly and A. Tkatchenko, J. Phys. Chem. Lett. 4, 1028 (2013)], with a larger database of 23 molecular crystals. Particular attention has been paid to the role of the vibrational contributions that are required to compare experiment sublimation enthalpies with calculated lattice energies, employing both phonon calculations and experimental heat-capacity data to provide harmonic and anharmonic estimates of the vibrational contributions. Exact exchange, which is rarely considered in DFT studies of molecular crystals, is shown to have a significant contribution to lattice energies, systematically improving agreement between theory and experiment. When the vibrational and exact-exchange contributions are coupled with a many-body approach to dispersion, DFT yields a mean absolute error (3.92 kJ/mol) within the coveted "chemical accuracy" target (4.2 kJ/mol). The role of many-body dispersion for structures has also been investigated for a subset of the database, showing good performance compared to X-ray and neutron diffraction crystal structures. The results show that the approach employed here can reach the demanding accuracy of crystal-structure prediction and organic material design with minimal empiricism.

Predicting anisotropic displacement parameters using molecular dynamics: density functional theory plus dispersion modelling of thermal motion in benzophenone

The benefits of combining experimental and computational methods have been demonstrated by a study of the dynamics and solid-state structure of α-benzophenone. Dispersion-corrected and -uncorrected density functional theory molecular dynamics simulations were used to obtain displacement parameters, with the dispersion-corrected simulations showing good agreement with the experimental neutron and X-ray diffraction values. At 70 K, quantum-nuclear effects resulted in poor values for the hydrogen atoms, but the heavy-atom values still show excellent agreement, suggesting that molecular dynamics simulations can be a useful tool for determining displacement parameters where experimental data are poor or limited.

Seamless and Accurate Modeling of Organic Molecular Materials

The near endless possibilities for assembling molecular materials has long posed a difficult challenge for theory. All crystal-structure prediction methods acknowledge the crucial contribution of van der Waals or dispersion interactions, but few go beyond a pairwise additive description of dispersion, ignoring its many-body nature. Here we use two databases to show how a many-body approach to dispersion can seamlessly model both solid and gas-phase interactions within the coveted "chemical accuracy" benchmark, while the underlying pairwise approach fails for solid-state interactions due to the absence of many-body polarization and energy contributions. Our results show that recently developed methods that treat the truly collective nature of dispersion interactions are able to reach the accuracy required for predicting molecular materials, when coupled with nonempirical density functionals.

Temperature- and pressure-dependent densities, self-diffusion coefficients, and phase behavior of monoacid saturated triacylglycerides: toward molecular-level insights into processing

Using molecular-dynamics (MD) simulations the densities and self-diffusion coefficients of a range of liquid monoacid triacylglycerides (TAGs) have been studied as a function of temperature and, for the first time, pressure. While offset by their ambient properties, the response of the TAGs to temperature and pressure is qualitatively similar. Application of pressure was found to significantly increase densities and reduce diffusion of the TAG molecules, suggesting that it may have as much a role in processing and crystallizing TAGs as supercooling does. A solution of glycerol tripalmitate and glycerol trihexanoate was also studied, showing that application of pressure should lead to a significant decrease in the saturation point of the solution, which is an important consideration for processing TAGs. Different solid/liquid interfaces of glycerol tripalmitate have also been investigated. Although crystal growth could not be observed, dissolution of one interface was seen in the MD simulations. The results suggest that over moderate distances the melting of TAGs may be cooperative in nature, rather than involving dissolution of individual TAG molecules.

Using molecular-dynamics simulations to understand and improve the treatment of anharmonic vibrations. I. Study of positional parameters

Molecular-dynamics-derived numerical probability density functions (PDFs) have been used to illustrate the effect of different models for thermal motion on the parameters refined in a crystal structure determination. Specifically, anharmonic curved or asymmetric PDFs have been modelled using the traditional harmonic approximation and the anharmonic Gram-Charlier series treatment. The results show that in cases of extreme anharmonicity the mean and covariance matrix of the harmonic treatment can deviate significantly from physically meaningful values. The use of a Gram-Charlier anharmonic PDF gives means and covariance matrices closer to the true (numerically determined) anharmonic values. The physical significance of the maxima of the anharmonic distributions (the most probable or mode positions) is also discussed. As the data sets used for the modelling process are theoretical in origin, these most probable positions can be compared to equilibrium positions that represent the system at the bottom of its potential-energy surface. The two types of position differ significantly in some cases but the most probable position is still worthy of report in crystal structure determinations.

Using molecular-dynamics simulations to understand and improve the treatment of anharmonic vibrations. II. Developing and assessing new Debye-Waller factors

Two new anharmonic forms for the Debye-Waller factor, aimed at modelling curvilinear and asymmetric motion, have been introduced. These forms permit the refinement of structures with these types of anharmonic motion using a small number of additional parameters. Molecular-dynamics-derived numerical probability density functions (PDFs) have been used to assess the merit of these new functions in real space. The comparison is favourable particularly for the curvilinear PDF based on a parabolic coordinate system change of a trivariate Gaussian distribution. The initial results also suggest that high-order even terms from the Gram-Charlier series may be important for modelling methyl-group libration. The molecular-dynamics data sets provide useful insights into the nature of anharmonic thermal motion. Addressing the problem in real space allows intuitive PDFs to be developed but numerical methods may be necessary for these methods to be implemented in refinement programs as an analytical Debye-Waller factor cannot always be obtained.

What makes the huge 31P-31P coupling constants in S(PF2)2 and Se(PF2)2 vary so much with temperature?

The enormous temperature dependence of the (2)J(PP) coupling constants in S(PF(2))(2) and Se(PF(2))(2) has been explained by a theoretical investigation of their conformations and NMR coupling constants. In contrast, the coupling in O(PF(2))(2) is almost invariant. Gas electron diffraction data for S(PF(2))(2) have been reinterpreted. The results show that two conformers, with C(s) and C(2v) symmetry, exist for the S and Se compounds. The C(s) and C(2v) conformers have very different (2)J(PP) coupling constants (-12.6 and 395.2 Hz for S(PF(2))(2) at B3LYP/aug-cc-pVQZ) and thermal interconversion of these conformers explains the experimental behavior.

Gene transfer: manipulating and monitoring function in cells and tissues

1. The ectopic expression of genes has proven to be an extremely valuable tool for biologists. The most widely used systems involve electrically or chemically mediated transfer of genes to immortalized cell lines and, at the other end of the spectrum, transgenic animal models. As would be expected, there are compromises to be made when using either of these broad approaches. Immortalized cell lines have limited "physiological relevance" and transgenic approaches are costly and out of the reach of many laboratories. There is also significant time required for the de novo generation of a transgenic animal. 2. As a viable alternative to these approaches, we describe the use of recombinant adenovirus and Sindbis virus to deliver genes to cells and tissues. 3. We exemplify this approach with studies from our laboratories: (i) an investigation of Ca2+ handling deficits in cardiac myocytes of hypertrophied hearts using infection with recombinant adenovirus encoding either green fluorescent protein (GFP) or the sarcoplasmic/endoplasmic reticulum calcium-ATPase (Serca2a); (ii) a study of the mechanism of macrophage/microglial migration by infection of embryonic phagocytes with a GFP-encoding virus and coculture with brain slices to then track the movement of labelled cells; and (iii) we are also exploiting the natural tropism of the Sindbis virus to label neurons in hippocampal brain slices in culture to resolve high-resolution structure and to map neuronal connectivity. 4. Further development of these approaches should open new avenues of investigation for the study of physiology in a range of cells and tissues.

Restoration of calcium handling properties of adult cardiac myocytes from hypertrophied hearts

Reductions in cardiac sarcoplasmic reticulum calcium-ATPase (Serca2a) levels are thought to underlie the prolonged calcium (Ca(2+)) transients and consequent reduced contractile performance seen in human cardiac hypertrophy and heart failure. In freshly isolated cardiac myocytes from rats with monocrotaline-induced right ventricular hypertrophy we found reduced sarcoplasmic reticulum Serca2a expression and prolonged Ca(2+)transients, characteristic of hypertrophic cardiac disease. Modulation of intracellular Ca(2+)levels, Ca(2+) kinetics or Ca(2+)sensitivity is the focus of many current therapeutic approaches to improve contractile performance in the hypertrophic or failing heart. However, the functional effects of increasing Serca2a expression on Ca(2+) handling properties in myocytes from an animal model of cardiac hypertrophy are largely unknown. Here, we describe enhancement of the deficient Ca(2+) handling properties evident in myocytes from hypertrophied hearts following adenoviral-mediated transfer of the human Serca2a gene to these myocytes. These results highlight the importance of Serca2a deficiencies in the hypertrophic phenotype of cardiac muscle and suggest a simple, effective approach for manipulation of normal cardiac function.

Genetically targeted calcium sensors enhance the study of organelle function in living cells

1. Understanding the regulation of calcium (Ca2+), the most common of the mineral ions within the human body, has always been of extreme interest to physiologists. While the importance of Ca2+ in contributing to physiological events through regulation of levels has been significantly established, seldom is consideration given to the intricacies of this ion and its mechanics in producing such diverse physiological responses in different regions of the cell. 2. The present review will summarize new methodologies used in our laboratories for the study of two major intracellular organelles, mitochondria and the nucleus. These techniques are based predominantly on the use of molecular biological approaches to both create and then target protein-based sensor molecules to specific intracellular locations. 3. The regulation of Ca2+ in the mitochondria and nucleus is of particular interest to us because of the central involvement of these organelles in: (i) cardiac cell responses during ischaemia/reperfusion; and (ii) the control of gene expression, respectively.

Dexamethasone inhibits endotoxin-induced changes in calcium and contractility in rat isolated papillary muscle

This study investigates whether endotoxin-induced contractile dysfunction is associated with a defect in the modulation of calcium homeostasis and the potential mechanisms involved. Treatment of rats in vivo with endotoxin significantly decreased the magnitude of contractile transients in electrically stimulated left ventricular papillary muscle isolated after an equilibration period of 6 hours. Although no significant difference was found in the peak intracellular calcium concentration ([Ca2+]i) between the endotoxin-treated and control groups, resting [Ca2+]i) was significantly elevated in the endotoxin-treated group, producing a smaller Ca2+ transient (basal-peak difference) in this group. Pretreatment of rats with dexamethasone prevented the endotoxin-induced decrease in peak tension and inhibited the elevation in resting [Ca2+]i, with a resultant maintenance of Ca2+ transient magnitude. Similar observations were made during stimulation of the muscles by the beta-adrenoceptor agonist, isoprenaline. These results show that endotoxin-induced reduction of cardiac contractile performance is mediated, at least in part, by elevating resting [Ca2+]i, and a glucocorticoid protected from these negative effects. While endotoxin reduces the magnitude of the Ca2+ transient it does not alter peak [Ca2+]i availability. Further investigation is required to determine whether endotoxin decreases contractile performance by reducing the sensitivity of cardiac myofilaments to calcium.

Structural requirements for PAK activation by Rac GTPases

The Rho family GTPases, Rac1 and Rac2, regulate a variety of cellular functions including cytoskeletal reorganization, the generation of reactive oxygen species, G1 cell cycle progression and, in concert with Ras, oncogenic transformation. Among the many putative protein targets identified for Rac (and/or Cdc42), the Ser/Thr kinase p21-activated kinase (PAK) is a prime candidate for mediating some of Rac's cellular effects. This report shows that Rac1 binds to and stimulates the kinase activity of PAK1 approximately 2- and 4-5-fold, respectively, better than Rac2. Mutational analysis was employed to determine the structural elements on Rac and PAK that are important for optimal binding and activation. The most notable difference between the highly homologous Rac isomers is the composition of their C-terminal polybasic domains. Mutation of these six basic residues in Rac1 to neutral amino acids dramatically decreased the ability of Rac1 to bind PAK1 and almost completely abolished its ability to stimulate PAK activity. Moreover, replacing the highly charged polybasic domain of Rac1 with the less charged domain of Rac2 (and vice versa) completely reversed the PAK binding/activation properties of the two Rac isomers. Thus, polybasic domain differences account for the disparate abilities of Rac1 and Rac2 to activate PAK. PAK proteins also contain a basic region, consisting of three contiguous lysine residues (Lys66-Lys67-Lys68), which lies outside of the previously identified Cdc42/Rac-binding domain. Mutation of these Lys residues to neutral residues decreased PAK binding to activated Rac1 and Rac2 (but not Cdc42) and greatly reduced PAK1 activation by Rac1, Rac2, and Cdc42 proteins in vivo. In contrast, mutation of lysines 66-68 to basic Arg residues did not decrease (and in some cases enhanced) the ability of Rac1, Rac2, and Cdc42 to bind and activate PAK1. Our studies suggest that the polybasic domain of Rac is a novel effector domain that may allow the two Rac isomers to activate different effector proteins. In addition, our results indicate that a basic region in PAK is required for PAK activation and that binding of Rac/Cdc42 to PAK is not sufficient for kinase activation.

A GTPase-independent mechanism of p21-activated kinase activation. Regulation by sphingosine and other biologically active lipids

p21-activated kinases (PAKs) are serine/threonine kinases that have been identified as targets for the small GTPases Rac and Cdc42. PAKs have been implicated in cytoskeletal regulation, stimulation of mitogen-activated protein kinase cascades, and in control of the phagocyte NADPH oxidase. Membrane targeting of PAK1 induced increased kinase activity in a GTPase-independent manner, suggesting that other mechanisms for PAK regulation exist. We observed concentration- and time-dependent activation of PAK1 by sphingosine and several related long chain sphingoid bases but not by ceramides or a variety of other lipids. Although phospholipids were generally ineffective, phosphatidic acid and phosphatidylinositol also had stimulatory effects on PAK1. Lipid stimulation induced a similar level of PAK1 activity as did stimulation by GTPases, and the patterns of PAK1 autophosphorylation determined after partial tryptic digestion and two-dimensional peptide analysis were similar with each class of activator. Lipid stimulation of PAK1 activity was dependent upon intact PAK kinase activity, as indicated by studies with a kinase-dead PAK1 mutant. Treatment of COS-7 cells expressing wild type PAK1 with sphingosine, fumonisin B, or sphingomyelinase, all of which are able to elevate the levels of free sphingosine, induced increased activity of PAK1 as determined using a p47(phox) peptide substrate. Studies using PAK1 mutants suggest that lipids act at a site overlapping or identical to the GTPase-binding domain on PAK. The inactive sphingosine derivative N,N-dimethylsphingosine was an effective inhibitor of PAK1 activation in response to either sphingosine or Cdc42. Our results demonstrate a novel GTPase-independent mechanism of PAK activation and, additionally, suggest that PAK(s) may be important mediators of the biological effects of sphingolipids.

Regulation of renal tubular sodium transport by angiotensin II and atrial natriuretic factor

1. The effects of angiotensin II (AngII) on water and electrolyte transport are biphasic and dose-dependent, such that low concentrations (10(-12) to 10(-9) mol/L) stimulate reabsorption and high concentrations (10(-7) to 10(-6) mol/L) inhibit reabsorption. Similar dose-response relationships have been obtained for luminal and peritubular addition of AngII. 2. The cellular responses to AngII are mediated via AT1 receptors coupled via G-regulatory proteins to several possible signal transduction pathways. These include the inhibition of adenylyl cyclase, activation of phospholipases A2, C or D and Ca2+ release in response to inositol-1,4,5,-triphosphate or following Ca2+ channel opening induced by the arachidonic acid metabolite 5,6,-epoxy-eicosatrienoic acid. In the brush border membrane, transduction of the AngII signal involves phospholipase A2, but does not require second messengers. 3. Angiotensin II affects transepithelial sodium transport by modulation of Na+/H+ exchange at the luminal membrane and Na+/HCO3 cotransport, Na+/K(+)-ATPase activity and K+ conductance at the basolateral membrane. 4. Atrial natriuretic factor (ANF) does not appear to affect proximal tubular sodium transport directly, but acts via specific receptors on the basolateral and brush border membranes to raise intracellular cGMP levels and inhibit AngII-stimulated transport. 5. It is concluded that there is a receptor-mediated action of ANF on proximal tubule reabsorption acting via elevation of cGMP to inhibit AngII-stimulated sodium transport. This effect is exerted by peptides delivered at both luminal and peritubular sides of the epithelium and provides a basis for the modulation by ANF of proximal glomerulotubular balance. The evidence reviewed supports the concept that in the proximal tubule, AngII and ANF act antagonistically in their roles as regulators of extracellular fluid volume.

Biphasic effect of angiotensin II on intracellular sodium concentration in rat proximal tubules

Intracellular Na+ concentration ([Na+]i) was determined using ratiometric measurement of the Na(+)-sensitive fluorescent probe, sodium-binding benzofuran isophthalate (SBFI). Angiotensin II (ANG II, 10(-11)-10(-7) M), applied to the basolateral membrane of rat isolated proximal convoluted tubules, induced a rapid and reversible dose-dependent increase in [Na+]i, which was initiated within 300 ms. A maximal response was observed over the range 10(-9)-10(-7) M ANG II, with an average increase in [Na+]i of 7.4 +/- 1.0 mM. At higher concentrations (10(-6)-10(-5) M) ANG II decreased [Na+]i compared with control (14.2 +/- 0.6 mM). The increase in [Na+]i induced by 10(-9) M ANG II was attenuated by inhibiting the Na+/H+ antiporter with clonidine, whereas HOE-694, a specific blocker of the NHE-1 isoform of the Na+/H+ exchanger, had no effect. The increase in [Na+]i induced by 10(-9) M ANG II was enhanced by inhibition of the Na(+)-HCO3- cotransporter with hydrogen-4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, with an average increase in [Na+]i of 17.1 +/- 6.6 mM. The data provide direct, high time resolution measurements of the effects of ANG II on [Na+]i in the proximal tubule and support the proposition that an increase in transepithelial Na+ reabsorption by ANG II involves stimulation of both an Na+/H+ exchanger and the Na(+)-HCO3- cotransporter.