Distortion of proximodistal information causes JNK-dependent apoptosis in Drosophila wing.

Distinct and evolutionarily conserved signal-transduction cascades mediate the survival or death of cells during development. The c-Jun amino-terminal kinases (JNKs) of the mitogen-activated protein kinase superfamily are involved in apoptotic signalling in various cultured cells. However, the role of the JNK pathway in development is less well understood. In Drosophila, Decapentaplegic (Dpp; a homologue of transforming growth factor-beta) and Wingless (Wg; a Wnt homologue) proteins are secretory morphogens that act cooperatively to induce formation of the proximodistal axis of appendages. Here we show that either decreased Dpp signalling in the distal wing cells or increased Dpp signalling in the proximal wing cells causes apoptosis. Inappropriate levels of Dpp signalling lead to aberrant morphogenesis in the respective wing zones, and these apoptotic zones are also determined by the strength of the Wg signal. Our results indicate that distortion of the positional information determined by Dpp and Wg signalling gradients leads to activation of the JNK apoptotic pathway, and the consequent induction of cell death thereby maintains normal morphogenesis.     

Integrin cytoplasmic tyrosine motif is required for outside-in alphaIIbbeta3 signalling and platelet function.

Integrins not only bind adhesive ligands, they also act as signalling receptors. Both functions allow the integrin alphaIIbbeta3 to mediate platelet aggregation. Platelet agonists activate alphaIIbbeta3 (inside-out signalling) to allow the binding of soluble fibrinogen. Subsequent platelet aggregation leads to outside-in alphaIIbbeta3 signalling, which results in calcium mobilization, tyrosine phosphorylation of numerous proteins including beta3 itself, increased cytoskeletal reorganisation and further activation of alphaIIbbeta3. Thus, outside-in signals enhance aggregation, although the mechanisms and functional consequences of specific signalling events remain unclear. Here we describe a mouse that expresses an alphaIIbbeta3 in which the tyrosines in the integrin cytoplasmic tyrosine motif have been mutated to phenylalanines. These mice are selectively impaired in outside-in alphaIIbbeta3 signalling, with defective aggregation and clot-retraction responses in vitro, and an in vivo bleeding defect which is characterized by a pronounced tendency to rebleed. These data provide evidence for an important role of outside-in signalling in platelet physiology. Furthermore, they identify the integrin cytoplasmic tyrosine motif as a key mediator of beta-integrin signals and a potential target for new therapeutic agents.     

Structure of Cdc42 in complex with the GTPase-binding domain of the 'Wiskott-Aldrich syndrome' protein.

The Rho-family GTP-hydrolysing proteins (GTPases), Cdc42, Rac and Rho, act as molecular switches in signalling pathways that regulate cytoskeletal architecture, gene expression and progression of the cell cycle. Cdc42 and Rac transmit many signals through GTP-dependent binding to effector proteins containing a Cdc42/Rac-interactive-binding (CRIB) motif. One such effector, the Wiskott-Aldrich syndrome protein (WASP), is postulated to link activation of Cdc42 directly to the rearrangement of actin. Human mutations in WASP cause severe defects in haematopoletic cell function, leading to clinical symptoms of thrombocytopenia, immunodeficiency and eczema. Here we report the solution structure of a complex between activated Cdc42 and a minimal GTPase-binding domain (GBD) from WASP. An extended amino-terminal GBD peptide that includes the CRIB motif contacts the switch I, beta2 and alpha5 regions of Cdc42. A carboxy-terminal beta-hairpin and alpha-helix pack against switch II. The Phe-X-His-X2-His portion of the CRIB motif and the alpha-helix appear to mediate sensitivity to the nucleotide switch through contacts to residues 36-40 of Cdc42. Discrimination between the Rho-family members is likely to be governed by GBD contacts to the switch I and alpha5 regions of the GTPases. Structural and biochemical data suggest that GBD-sequence divergence outside the CRIB motif may reflect additional regulatory interactions with functional domains that are specific to individual effectors.     

基于麦克斯韦电路理论的方环散射体

不同于传统数值计算方法,麦克斯韦电路(MC)理论融合了场的理论和路的理论,使二者高效地结合在一起.运用麦克斯韦电路理论分析方环散射体,研究方环散射体的MC求解方法以及MC的宽带特性,求出其散射电流,数值实验结果与矩量法结果吻合良好.研究还发现,MC可以在一个较宽的频带上得到准确电流解,因此,比其他数值方法更加高效和实用.     

Induction and organization of Ca2+ waves by enteric neural reflexes.

The motility of the gastrointestinal tract consists of local, non-propulsive mixing (pendular or segmental) and propulsive (peristaltic) movements. It is generally considered that mixing movements are produced by intrinsic pacemakers which generate rhythmic contractions, and peristalsis by intrinsic excitatory and inhibitory neural reflex pathways, but the relationship between mixing and peristalsis is poorly understood. Peristalsis is compromised in mice lacking interstitial cells of Cajal, suggesting that these pacemaker cells may also be involved in neural reflexes. Here we show that mixing movements within longitudinal muscle result from spontaneously generated waves of elevated internal calcium concentration which originate from discrete locations (pacing sites), spread with anisotropic conduction velocities in al directions, and terminate by colliding with each other or with adjacent neurally suppressed regions. Excitatory neural reflexes control the spread of excitability by inducing new pacing sites and enhancing the overall frequency of pacing, whereas inhibitory reflexes suppress the ability of calcium waves to propagate. We provide evidence that the enteric nervous system organizes mixing movements to generate peristalsis, linking the neural regulation of pacemakers to both types of gut motility.     

Mechanical and biochemical characterization of the contraction elicited by a calcium-independent myosin light chain kinase in chemically skinned smooth muscle

Summary The contraction induced by a Ca²⁺-independent myosin light chain kinase (MLCK-) was characterized in terms of isometric force (F_o), immediate elastic recoil (SE), unloaded shortening velocity (V_us), shortening under a constant load and ATPase activity of chemically skinned smooth muscle preparations. These parameters were compared to those measured in a Ca²⁺-induced contraction to assess the nature of cross bridge interaction in the MLCK-induced contraction. F_o developed in chicken gizzard fibers as well as SE were similar in contractions elicited by either agent. V_us in the contraction induced by MLCK-(0.36 mg/ml) was similar though averaged 39.3±8.9% less than V_us induced by Ca²⁺ (1.6x10^–6M) in the control fibers. Addition of Ca²⁺ (1.6x10^–6M) to a contraction induced by MLCK-resulted in small increases in both F_o and V_us. Shortening under a constant load was similar for both types of contractions. The contraction induced by MLCK-was accompanied by an increased rate of ATP hydrolysis. The MLCK-induced contraction is thus kinetically similar though not identical to a contraction induced by Ca²⁺. We conclude that with respect to actin-myosin interaction, MLCK- and Ca²⁺-induced contractions are similar.     

Effect of testosterone on the kinetics of the development of suppressor cells in adjuvant arthritis

Summary The induction of unresponsiveness to mycobacterial adjuvant took a longer time in male DA rats than in female rats. A shift in the induction time of unresponsiveness in males toward the female type was brought about by castration, but could be reverted to the male type by the application of testosterone. The transfer study revealed that cells capable of preventing arthritis required a longer incubation time for their development in males than in females. This suggests that testosterone inhibits the development of suppressor cells in adjuvant arthritis.Acknowledgments. We are grateful to The Naito Foundation Research Grant for 1983.     

Velocity and myosin phosphorylation transients in arterial smooth muscle: effects of agonist diffusion

Summary Transients in myoplasmic [Ca²⁺] and in phosphorylation of the 20,000 dalton light chain of myosin have been reported following stimulation of vascular smooth muscle by various agonists. Since these transients are rapid compared with the time required to attain a steady-state stress, agonist diffusion rates may be a significant limitation in activation. The purpose of this study was to estimate the effect of agonist diffusion rates on the time course of activation as assessed by mechanical measurements of stress development and isotonic shortening velocities and by determinations of the time course of myosin phosphorylation. The approach was to measure these parameters in K⁺-stimulated preparations of the swine carotid media of varying thicknesses and to estimate the theoretical contributions imposed by diffusion rates and the presence of a diffusion boundary layer surrounding the tissue. The results show that the time course of parameters which are tissue averages such as stiffness, active stress, and myosin phosphorylation is dominated by agonist diffusion rates. The sequence of events involved in excitation-contraction coupling including agonist actions on the cell membrane, Ca²⁺ release, activation of myosin light chain kinase, and cross-bridge phosphorylation appear to be very rapid events compared with stress development. Estimates of unloaded or lightly loaded shortening velocities which are not simple tissue averages appear to provide an imporoved estimate of activation rates.Supported by a grant from the National Institutes of Health 5-PO1-HL19242. K. E. Kamm was supported by a National Heart, Lung, and Blood Institute Research Service Award HL-05957.