Actin cytoskeleton is crucial to support spermatogenesis in the mammalian testis. However, the molecular mechanism(s) underlying changes of actin cytoskeletal organization in response to cellular events that take place across the seminiferous epithelium (e.g., self-renewal of spermatogonial stem cells, germ cell differentiation, meosis, spermiogenesis, spermiation) at specific stages of the epithelial cycle of spermatogenesis remain largely unexplored. This, at least in part, is due to the lack of suitable study models to identify the crucial regulatory proteins and to investigate how these proteins work in concert to support actin dynamics. Much of the information on the role of actin binding proteins in the literature, namely the actin bundling proteins, actin nucleation proteins and motor proteins, are either findings based on genetic models or morphological analyses. While this information is helpful to delineate the function of these proteins to support spermatogenesis, they are not helpful to identify the regulatory signaling proteins, the signaling pathways and the cascade of events to modulate actin cytoskeleton dynamics. Recent studies based on the use of toxicant models, both in vitro and in vivo, however, have bridged this gap by identifying putative regulatory and signaling proteins of actin cytoskeleton. Herein, we summarize and critically evaluate these findings. We also provide a hypothetical model by which actin cytoskeletal dynamics in Sertoli cells are regulated, which in turn supports spermatid transport across the seminiferous epithelium, and at the blood-testis barrier (BTB) during the epithelial cycle of spermatogenesis. 相似文献
Abstract: The novel Cα‐tetrasubstituted α‐amino acid Cα‐methyl, Cα‐cyclohexylglycine was prepared by hydrogenation of its Cα‐methyl, Cα‐phenylglycine precursor. Terminally protected homodi‐, homotri‐, and homotetrapeptides from Cα‐methyl, Cα‐cyclohexylglycine and co‐oligopeptides to the pentamer level in combination with Gly or α‐aminoisobutyric acid residues were prepared by solution methods and fully characterized. The results of a conformational analysis, performed by use of Fourier transform infrared (FT‐IR) spectrophotomet absorption, 1H NMR, and X‐ray diffraction techniques, support the contention that this Cα‐methylated, Cβ‐trisubstituted aliphatic α‐amino acid is an effective β‐turn and 310‐helix inducer in tri‐ and longer peptides as its Cα‐methyl valine parent compound, but partially divergent from the corresponding aromatic Cα‐methyl, Cα‐diphenylmethylglycine residue, known to promote folded and fully extended structures to a significant extent in these oligomers. 相似文献
Abstract: The N‐terminal 1–34 segment of parathyroid hormone (PTH) is fully active in vitro and in vivo and it can reproduce all biological responses in bone characteristic of the native intact PTH. Recent studies have demonstrated that N‐terminal fragments presenting the principal activating domain such as PTH(1–11) and PTH(1–14) with helicity‐enhancing substitutions yield potent analogues with PTH(1–34)‐like activity. To further investigate the role of α‐helicity on biological potency, we designed and synthesized by solid‐phase methodology the following hPTH(1–11) analogues substituted at positions 1 and/or 3 by the sterically hindered and helix‐promoting Cα‐tetrasubstituted α‐amino acids α‐amino isobutyric acid (Aib), 1‐aminocyclopentane‐1‐carboxylic acid (Ac5c) and 1‐aminocyclohexane‐1‐carboxylic acid (Ac6c): Ac5c‐V‐Aib‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH2 ( I ); Aib‐V‐Ac5c‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH2 ( II ); Ac6c‐V‐Aib‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH2 ( III ); Aib‐V‐Ac6c‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH2 ( IV ); Aib‐V‐Aib‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH2 ( V ); S‐V‐Aib‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH2 ( VI ), S‐V‐Ac5c‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH2 ( VII ); Ac5c‐V‐S‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH2 ( VIII ); Ac6c‐V‐S‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH2 ( IX ); Ac5c‐V‐Ac5c‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH2 ( X ); Ac6c‐V‐Ac6c‐E‐I‐Q‐L‐M‐H‐Q‐R‐NH2 ( XI ). All analogues were biologically evaluated and conformationally characterized in 2,2,2‐trifluoroethanol (TFE) solution by circular dichroism (CD). Analogues I – V , which cover the full range of biological activity observed in the present study, were further conformationally characterized in detail by nuclear magnetic resonance (NMR) and computer simulations studies. The results of ligand‐stimulated cAMP accumulation experiments indicated that analogues I and II are active, analogues III , VI and VII are very weakly active and analogues IV , V , VIII–XI are inactive. The most potent analogue, I exhibits biological activity 3500‐fold higher than that of the native PTH(1–11) and only 15‐fold weaker than that of the native sequence hPTH(1–34). Remarkably, the two most potent analogues, I and II , and the very weakly active analogues, VI and VII , exhibit similar helix contents. These results indicate that the presence of a stable N‐terminal helical sequence is an important but not sufficient condition for biological activity. 相似文献
Nature has mastered the art of creating complex structures through self-assembly of simpler building blocks. Adapting such a bottom-up view provides a potential route to the fabrication of novel materials. However, this approach suffers from the lack of a sufficiently detailed understanding of the noncovalent forces that hold the self-assembled structures together. Here we demonstrate that nature can indeed guide us, as we explore routes to helicity with achiral building blocks driven by the interplay between two competing length scales for the interactions, as in DNA. By characterizing global minima for clusters, we illustrate several realizations of helical architecture, the simplest one involving ellipsoids of revolution as building blocks. In particular, we show that axially symmetric soft discoids can self-assemble into helical columnar arrangements. Understanding the molecular origin of such spatial organisation has important implications for the rational design of materials with useful optoelectronic applications. 相似文献
Several well‐controlled polystyrene‐block‐poly(styrene‐alt‐maleic anhydride) (PS‐b‐P(St‐alt‐MA)) functionalized block copolymers (BCs) and their sodium salt ionomers with different block ratios and molecular weights are synthesized through two‐step reversible addition–fragmentation chain transfer (RAFT) polymerization. The atomic force microscopy (AFM) images and differential scanning calorimetry (DSC) results reveal that the prepared BCs and their ionomers microphase separate into various nanostructures depending on the block ratios. By taking advantage of the formation of amphiphilic nanostructures via the microphase, the block ionomers serve as nucleation agents to improve the crystallization rate of poly(ethylene terephthalate) (PET). The crystallization behavior of PET upon the addition of block ionomers is investigated isothermally and non‐isothermally by DSC. The results show that the block ionomers can effectively accelerate the crystallization rate of PET, which strongly depends on the molecular weights and block ratios
The mechanisms by which amorphous silica dissolves have proven elusive because noncrystalline materials lack the structural order that allows them to be studied by the classical terrace, ledge, kink-based models applied to crystals. This would seem to imply amorphous phases have surfaces that are disordered at an atomic scale so that the transfer of SiO(4) tetrahedra to solution always leaves the surface free energy of the solid unchanged. As a consequence, dissolution rates of amorphous phases should simply scale linearly with increasing driving force (undersaturation) through the higher probability of detaching silica tetrahedra. By examining rate measurements for two amorphous SiO(2) glasses we find, instead, a paradox. In electrolyte solutions, these silicas show the same exponential dependence on driving force as their crystalline counterpart, quartz. We analyze this enigma by considering that amorphous silicas present two predominant types of surface-coordinated silica tetrahedra to solution. Electrolytes overcome the energy barrier to nucleated detachment of higher coordinated species to create a periphery of reactive, lesser coordinated groups that increase surface energy. The result is a plausible mechanism-based model that is formally identical with the classical polynuclear theory developed for crystal growth. The model also accounts for reported demineralization rates of natural biogenic and synthetic colloidal silicas. In principle, these insights should be applicable to materials with a wide variety of compositions and structural order when the reacting units are defined by the energies of their constituent species. 相似文献
The RAFT polymerization of styrene in a solvent consisting of a water/alcohol mixture with different water contents is performed and the solvent effects on polymerization kinetics, polymer chain propagation, and polymer particle growth are evaluated. It is found that the solvent affects the RAFT polymerization kinetics greatly, and the apparent polymerization rate constant (Kpapp) increases with an increase in the water content of the water/alcohol mixture. In addition, RAFT polymerization in a water/alcohol mixture with a higher water content affords better control of the polydispersity index (PDI) of the synthesized polymers. Furthermore, the solvent also exerts a great influence on the growth of the polymer particles. Hollow particles are formed either at the initial polymerization with low monomer conversion or in the solvent with a low water content, whereas solid polymer particles are produced either at high monomer conversion or in the solvent with a high water content.