An exhaustive compilation on chemistry of triazolopyrimidine: A journey through decades

The triazolopyrimidine scaffold represents one of the privileged structure in chemistry, and there has been an increase in number of studies utilizing this scaffold and its derivatives. Optimization of synthetic protocols such as aza-Wittig reaction, [3 + 2] cycloaddition reaction along with previous methods including condensation with 1,3-dicarbonyl substrates and oxidation of aminopyrimidine Schiff bases have been performed to obtain desired triazolopyrimidines. The triazolopyrimidine ring has been extensively used as a template in medicinal chemistry for its diverse pharmacological properties. Several medicinally active molecules possessing triazolopyrimidine scaffold, either fused or coupled with other heterocycles, have been reported in the literature, highlighting the significance of this nucleus. Interestingly, the unique triazolopyrimidine scaffold also exhibits an impressive potential as a ligand for the synthesis of several metal complexes with significant biological potential. Literature provides enough evidence of exhaustive exploration of this scaffold as a ligand for the chelates of platinum, ruthenium and other metals. This review aims to be a comprehensive and general summary of the different triazolopyrimidine syntheses, their use as ligands for the synthesis and development of metal complexes as medicinal agents and their main biological activities.     

Structural studies of N-terminal mutants of Connexin 32 using (1)H NMR spectroscopy

Microtubules (MTs) control cell replication, material transport and motion in eukaryotic cells, but MT role in several pathologies is still unknown. These functions are related to the MT physico-chemical properties and MT formation mode starting from tubulin molecules. This study describes a new method, based on the computer aided analysis of the electron paramagnetic resonance (EPR) spectra of selected spin probes to obtain structural and dynamical information on tubulins and MTs and the kinetics of MTs formation promoted by guanosine-5'-triphosphate (GTP). It was found that tubulin and MTs avoid radical quenching caused by ethylene glycol tetraacetic acid (EGTA). MT formation showed different kinetics as a function of tubulin concentration. At 5 mg/mL of tubulin, MTs were formed in 8 min. These results are also useful for getting information on MT-drug interactions.     

The search and prime hypothesis for growth cone turning

In the fields of axonal and dendritic guidance, there is now a significant accumulation of knowledge of how extracellular signaling molecules activate their cognate growth cone receptors. Relatively little is known about the subsequent activation of intracellular signaling pathways and actin reorganization, and very little is known about how microtubules (MTs) reorganize during growth cone turning. I hypothesize that dynamic MTs are required in order to catalyze the polarized actin assembly necessary for growth cone turning, that MTs and actin filaments promote each other's assembly through positive feedback, that MT stability is enhanced further through the formation of membrane-associated MT attachment sites, and that these MT stabilization events subsequently accelerate axonal/dendritic shaft formation.     

Detyrosination of alpha tubulin does not stabilize microtubules in vivo [published erratum appears in J Cell Biol 1990 Sep;111(3):1325-6]

The relationship between alpha tubulin detyrosination and microtubule (MT) stability was examined directly in cultured fibroblasts by experimentally converting the predominantly tyrosinated MT array to a detyrosinated (Glu) array and then assaying MT stability. MTs in mouse Swiss 3T3 cells displayed an increase in Glu immunostaining fluorescence approximately 1 h after microinjecting antibodies to the tyrosinating enzyme, tubulin tyrosine ligase. Detyrosination progressed to virtual completion after 12 h and persisted for 30-35 h before tyrosinated subunits within MTs were again detected. The stability of these experimentally detyrosinated MTs was tested by first injecting either biotinylated or Xrhodamine-labeled tubulin and then measuring bulk turnover by hapten-mediated immunocytochemistry or fluorescence recovery after photobleaching, respectively. By both methods, turnover was found to be similarly rapid, possessing a half time of approximately 3 min. As a final test of MT stability, the level of acetylated tubulin staining in antibody-injected cells was compared with that observed in adjacent, uninjected cells and also with the staining observed in cells whose MTs had been stabilized with taxol. Although intense Glu staining was observed in both injected and taxol-treated cells, increased acetylated tubulin staining was observed only in the taxol-stabilized MTs, indicating that the MTs were not stabilized by detyrosination. Together, these results demonstrated clearly that detyrosination does not directly confer stability on MTs. Therefore, the stable MTs observed in these and other cell lines must have arisen by another mechanism, and may have become posttranslationally modified after their stabilization.     

Direct Regulation of Microtubule Dynamics by KIF17 Motor and Tail Domains

KIF17 is a kinesin-2 family motor that interacts with EB1 at microtubule (MT) plus-ends and contributes to MT stabilization in epithelial cells. The mechanism by which KIF17 affects MTs and how its activity is regulated are not yet known. Here, we show that EB1 and the KIF17 autoinhibitory tail domain (KIF17-Tail) interacted competitively with the KIF17 catalytic motor domain (K370). Both EB1 and KIF17-Tail decreased the K_0.5MT of K370, with opposing effects on MT-stimulated ATPase activity. Importantly, K370 had independent effects on MT dynamic instability, resulting in formation of long MTs without affecting polymerization rate or total polymer mass. K370 also inhibited MT depolymerization induced by dilution in vitro and by nocodazole in cells, suggesting that it acts by protecting MT plus-ends. Interestingly, KIF17-Tail bound MTs and tubulin dimers, delaying initial MT polymerization in vitro and MT regrowth in cells. However, neither EB1 nor KIF17-Tail affected K370-mediated MT polymerization or stabilization significantly in vitro, and EB1 was dispensable for MT stabilization by K370 in cells. Thus, although EB1 and KIF17-Tail may coordinate KIF17 catalytic activity, our data reveal a novel and direct role for KIF17 in regulating MT dynamics.     

Effect on microtubule dynamics of XMAP230, a microtubule-associated protein present in Xenopus laevis eggs and dividing cells [published erratum appears in J Cell Biol 1995 Mar;128(5):following 988]

The reorganization from a radial [corrected] interphase microtubule (MT) network into a bipolar spindle at the onset of mitosis involves a dramatic change in MT dynamics. Microtubule-associated proteins (MAPs) and other factors are thought to regulate MT dynamics both in interphase and in mitosis. In this study we report the purification and functional in vitro characterization of a 230-KD MAP from Xenopus egg extract (XMAP230). This protein is present in eggs, oocytes, testis and a Xenopus tissue culture cell line. It is apparently absent from non- dividing cells in which an immunologically related 200-kD protein is found. XMAP230 is composed of two isoforms with slightly different molecular masses and pIs. It is localized to interphase MTs, dissociates from MTs at the onset of prophase and specifically binds to spindle MTs during metaphase and anaphase. The dissociation constant of XMAP230 is 500 nM, the stoichiometry of binding to MTs is between 1:8 and 1:4, and the in vivo concentration is approximately 200 nM. Both isoforms are phosphorylated and have reduced affinity for microtubules in mitotic extracts. Analysis of the effect of XMAP230 on MT dynamics by video microscopy shows that it increases the growth rate, decreases the shrinking rate of MTs and strongly suppresses catastrophes. These results suggest that in vivo, XMAP230 participates in the control of the MT elongation rate, stabilizes MTs and locally modulates MT dynamics during mitosis.     

The formin mDia2 stabilizes microtubules independently of its actin nucleation activity

A critical microtubule (MT) polarization event in cell migration is the Rho/mDia-dependent stabilization of a subset of MTs oriented toward the direction of migration. Although mDia nucleates actin filaments, it is unclear whether this or a separate activity of mDia underlies MT stabilization. We generated two actin mutants (K853A and I704A) in a constitutively active version of mDia2 containing formin homology domains 1 and 2 (FH1FH2) and found that they still induced stable MTs and bound to the MT TIP proteins EB1 and APC, which have also been implicated in MT stabilization. A dimerization-impaired mutant of mDia2 (W630A) also generated stable MTs in cells. We examined whether FH1FH2mDia2 had direct activity on MTs in vitro and found that it bound directly to MTs, stabilized MTs against cold- and dilution-induced disassembly, and reduced the rates of growth and shortening during MT assembly and disassembly, respectively. These results indicate that mDia2 has a novel MT stabilization activity that is separate from its actin nucleation activity.     

Functional homologs of fungal metallothionein genes from Arabidopsis.

Metallothioneins (MTs) are cysteine-rich proteins required for heavy metal tolerance in animals and fungi. Two cDNAs encoding proteins with homology to animal and fungal MTs have been isolated from Arabidopsis. The genes represented by these cDNAs are referred to as MT1 and MT2. When expressed in an MT-deficient (cup1 delta) mutant of yeast, both MT1 and MT2 complemented the cup1 delta mutation, providing a high level of resistance to CuSO4 and moderate resistance to CdSO4. Although the MT-deficient yeast was not viable in the presence of either 300 microM CuSO4 or 5 microM CdSO4, cells expressing MT1 were able to grow in medium supplemented with 3 mM CuSO4 and 10 microM CdSO4, and those expressing MT2 grew in the presence of 3 mM CuSO4 and 100 microM CdSO4. In plants, MT1 mRNA was more abundant in roots and dark-grown seedlings than in leaves. In contrast, MT2 mRNA accumulated more in leaves than in either roots or darkgrown seedlings. MT2 mRNA was strongly induced in seedlings by CuSO4, but only slightly by CdSO4 or ZnSO4. However, MT1 mRNA was induced by CuSO4 in excised leaves that were submerged in medium. These results indicated that Arabidopsis MT genes are involved in copper tolerance. Plants also synthesized metal binding phytochelatins (poly[gamma-glutamylcysteine]glycine) when exposed to heavy metals. The results presented here argue against the hypothesis that phytochelatins are the sole molecules involved in heavy metal tolerance in plants. We conclude that Arabidopsis MT1 and MT2 are functional homologs of yeast MT.     

Torque generation by one of the motor subunits of heterotrimeric kinesin-2

Heterotrimeric kinesin-2 motors [1] and [2] transport intraflagellar transport (IFT)-particles from the base to the tip of the axoneme to assemble and maintain cilia [3], [4], [5], [6], [7], [8], [9] and [10]. These motors are distinct in containing two non-identical motor subunits together with an accessory subunit [1], [11], [12], [13], [14] and [15]. We evaluated the significance of this organization by comparing purified wild type kinesin-2 holoenzymes that support IFT in vivo, with mutant trimers containing only one type of motor domain that do not support IFT in vivo. In motility assays, wild type kinesin-2 moved microtubules (MTs) at a rate intermediate between the rates supported by the two mutants. Interestingly, one of the mutants, but not the other mutant or the wild type protein, was observed to drive a persistent counter-clock-wise rotation of the gliding MTs. Thus one of the two motor domains of heterotrimeric kinesin-2 exerts torque as well as axial force as it moves along a MT, which may allow kinesin-2 to control its circumferential position around a MT doublet within the cilium.     

Cdc42, dynein, and dynactin regulate MTOC reorientation independent of Rho-regulated microtubule stabilization

In migrating adherent cells such as fibroblasts and endothelial cells, the microtubule-organizing center (MTOC) reorients toward the leading edge [1-3]. MTOC reorientation repositions the Golgi toward the front of the cell [1] and contributes to directional migration [4]. The mechanism of MTOC reorientation and its relation to the formation of stabilized microtubules (MTs) in the leading edge, which occurs concomitantly with MTOC reorientation [3], is unknown. We show that serum and the serum lipid, lysophosphatidic acid (LPA), increased Cdc42 GTP levels and triggered MTOC reorientation in serum-starved wounded monolayers of 3T3 fibroblasts. Cdc42, but not Rho or Rac, was both sufficient and necessary for LPA-stimulated MTOC reorientation. MTOC reorientation was independent of Cdc42-induced changes in actin and was not blocked by cytochalasin D. Inhibition of dynein or dynactin blocked LPA- and Cdc42-stimulated MTOC reorientation. LPA also stimulates a Rho/mDia pathway that selectively stabilizes MTs in the leading edge [5, 6]; however, activators and inhibitors of MTOC reorientation and MT stabilization showed that each response was regulated independently. These results establish an LPA/Cdc42 signaling pathway that regulates MTOC reorientation in a dynein-dependent manner. MTOC reorientation and MT stabilization both act to polarize the MT array in migrating cells, yet these processes act independently and are regulated by separate Rho family GTPase-signaling pathways.     

Impact-Free Measurement of Microtubule Rotations on Kinesin and Cytoplasmic-Dynein Coated Surfaces

Knowledge about the three-dimensional stepping of motor proteins on the surface of microtubules (MTs) as well as the torsional components in their power strokes can be inferred from longitudinal MT rotations in gliding motility assays. In previous studies, optical detection of these rotations relied on the tracking of rather large optical probes present on the outer MT surface. However, these probes may act as obstacles for motor stepping and may prevent the unhindered rotation of the gliding MTs. To overcome these limitations, we devised a novel, impact-free method to detect MT rotations based on fluorescent speckles within the MT structure in combination with fluorescence-interference contrast microscopy. We (i) confirmed the rotational pitches of MTs gliding on surfaces coated by kinesin-1 and kinesin-8 motors, (ii) demonstrated the superiority of our method over previous approaches on kinesin-8 coated surfaces at low ATP concentration, and (iii) identified MT rotations driven by mammalian cytoplasmic dynein, indicating that during collective motion cytoplasmic dynein side-steps with a bias in one direction. Our novel method is easy to implement on any state-of-the-art fluorescence microscope and allows for high-throughput experiments.     

Centrosomal and non-centrosomal microtubules.

While microtubule (MT) arrays in cells are often focused at the centrosome, a variety of cell types contain a substantial number of non-centrosomal MTs. Epithelial cells, neurons, and muscle cells all contain arrays of non-centrosomal MTs that are critical for these cells' specialized functions. There are several routes by which non-centrosomal MTs can arise, including release from the centrosome, cytoplasmic assembly, breakage or severing, and stabilization from non-centrosomal sites. Once formed, MTs that are not tethered to the centrosome must be organized, which can be accomplished by means of self-organization or by capture and nucleation of MTs where they are needed. The presence of free MTs requires stabilization of minus ends, either by MT-associated proteins or by an end-capping complex. Although some of the basic elements of free MT formation and organization are beginning to be understood, a great deal of work is still necessary before we have a complete picture of how non-centrosomal MT arrays are assembled in specific cell types.     

Reduction in Microtubule Dynamics In Vitro by Brain Microtubule-Associated Proteins and by a Microtubule-Associated Protein-2 Second Repeated Sequence Analogue

Abstract: Microtubule-associated protein (MAP) binding to assembled microtubules (MTs) can be reduced by the addition of polyglutamate without significant MT depolymerization or interference with MT elongation reactions. Ensuing polymer length redistribution in MAP-depleted MTs occurs on a time scale characteristic of that observed with MAP-free MTs. The redistribution phase occurs even in the absence of mechanical shearing and without appreciable effects from end-to-end annealing, as indicated by the time course of incremental changes in polymer length and MT number concentration. We also observed higher rates of MT length redistribution when the [MAP]/[tubulin] ratio was decreased. Together, these results demonstrate that MT length redistribution rates are greatly influenced by MAP content, and the data are compatible with the dynamic instability model. We also found that a peptide analogue corresponding to the second repeated sequence in the MT-binding region of MAP-2 can also markedly retard MT length redistribution kinetics, a finding that accords with the ability of this peptide to promote tubulin polymerization in the absence of MAPs and to displace MAP-2 from MTs. These results provide further evidence that MAPs can modulate MT assembly/disassembly dynamics and that peptide analogues can mimic the action of intact MAPs without the need for three contiguous repeated sequences in the MT-binding region.     

Centrosome fragments and microtubules are transported asymmetrically away from division plane in anaphase

Spinning disc confocal microscopy of LLCPK1 cells expressing GFP-tubulin was used to demonstrate that microtubules (MTs) rapidly elongate to the cell cortex after anaphase onset. Concurrently, individual MTs are released from the centrosome and the centrosome fragments into clusters of MTs. Using cells expressing photoactivatable GFP-tubulin to mark centrosomal MT minus ends, a sevenfold increase in MT release in anaphase is documented as compared with metaphase. Transport of both individually released MTs and clusters of MTs is directionally biased: motion is directed away from the equatorial region. Clusters of MTs retain centrosomal components at their focus and the capacity to nucleate MTs. Injection of mRNA encoding nondegradable cyclin B blocked centrosome fragmentation and the stimulation of MT release in anaphase despite allowing anaphase-like chromosome segregation. Biased MT release may provide a mechanism for MT-dependent positioning of components necessary for specifying the site of contractile ring formation.     

Morphologically distinct microtubule ends in the mitotic centrosome of Caenorhabditis elegans

During mitosis, the connections of microtubules (MTs) to centrosomes and kinetochores are dynamic. From in vitro studies, it is known that the dynamic behavior of MTs is related to the structure of their ends, but we know little about the structure of MT ends in spindles. Here, we use high-voltage electron tomography to study the centrosome- and kinetochore-associated ends of spindle MTs in embryonic cells of the nematode, Caenorhabditis elegans. Centrosome-associated MT ends are either closed or open. Closed MT ends are more numerous and are uniformly distributed around the centrosome, but open ends are found preferentially on kinetochore-attached MTs. These results have structural implications for models of MT interactions with centrosomes.     

Cytoplasmic linker protein-170 enhances spreading and phagocytosis in activated macrophages by stabilizing microtubules

Activation of macrophages causes increased cell spreading, increased secretion of cytokines and matrix metalloproteinases, and enhanced phagocytosis. The intracellular mechanisms driving the up-regulation of these activities have not been completely clarified. We observe that classical activation of murine resident peritoneal or RAW 264.7 macrophages with a combination of IFN-gamma and LPS induces an increase in stabilized cytoplasmic microtubules (MTs), measured with an anti-acetylated alpha-tubulin Ab. We examined the mechanism of this MT stabilization and find that macrophage activation causes redistribution of the MT plus-end tracking protein, cytoplasmic linker protein-170 (CLIP-170). CLIP-170 is localized at the distal plus-ends of MTs in resting macrophages, but accumulates along the length of MTs in IFN-gamma/LPS-activated cells. A direct involvement of CLIP-170 in MT stabilization has not been thoroughly established. In this study, we show that expression of a mutant CLIP-170 chimeric protein (dominant-negative CLIP-170-GFP), lacking the MT-binding domain, prevents MT stabilization in activated RAW 264.7 macrophages. Furthermore, we find enhanced CLIP-170 association with MTs and MT stabilization by treating resting macrophages with okadaic acid, implicating the protein phosphatase 2A in CLIP-170 binding and MT stabilization in RAW 264.7 cells. Finally, we observed enhanced cell spreading and phagocytosis in both IFN-gamma/LPS-activated and okadaic acid-treated resting RAW 264.7 cells, which are markedly reduced in activated cells expressing dominant-negative CLIP-170-GFP. These results identify CLIP-170 as a key regulator of MT stabilization and establish a prominent role for stabilized MTs in cell spreading and phagocytosis in activated macrophages.     

Stable isotope labeling by amino acids in cell culture-based liquid chromatography–mass spectrometry assay to measure microtubule dynamics in neuronal cell cultures

Microtubules (MTs) are highly dynamic polymers composed of α- and β-tubulin heterodimers. Dysregulation of MT dynamics in neurons may be a contributing factor in the progression of various neurodegenerative diseases. We developed a stable isotope labeling by amino acids in cell culture (SILAC)-based liquid chromatography–mass spectrometry (LC–MS) method to measure the fraction of [¹³C₆]leucine-labeled α-tubulin-derived surrogate peptides. Using this approach, we measured the time course of incorporation of [¹³C₆]leucine label into the MT and dimer pools isolated from cycling cells and rat primary hippocampal neurons. We found that the MT pool is in rapid equilibrium with the dimer pool in the cycling cells, consistent with rapid MT polymerization/depolymerization during cell proliferation. Conversely, in neurons, we found that labeling of the MT pool was rapid, whereas the dimer pool was delayed. These results suggest that newly synthesized α-tubulin is first incorporated into MTs or complexes that co-sediment with MTs and that appearance of labeled α-tubulin in the dimer pool may be a consequence of MT depolymerization or breakdown. Our results demonstrate that a SILAC-based approach can be used to measure MT dynamics and may have utility for exploring MT dysregulation in various models of neurodegenerative disease.     

Actin-capping protein promotes microtubule stability by antagonizing the actin activity of mDia1

In migrating fibroblasts, RhoA and its effector mDia1 regulate the selective stabilization of microtubules (MTs) polarized in the direction of migration. The conserved formin homology 2 domain of mDia1 is involved both in actin polymerization and MT stabilization, and the relationship between these two activities is unknown. We found that latrunculin A (LatA) and jasplakinolide, actin drugs that release mDia1 from actin filament barbed ends, stimulated stable MT formation in serum-starved fibroblasts and caused a redistribution of mDia1 onto MTs. Knockdown of mDia1 by small interfering RNA (siRNA) prevented stable MT induction by LatA, whereas blocking upstream Rho or integrin signaling had no effect. In search of physiological regulators of mDia1, we found that actin-capping protein induced stable MTs in an mDia1-dependent manner and inhibited the translocation of mDia on the ends of growing actin filaments. Knockdown of capping protein by siRNA reduced stable MT levels in proliferating cells and in starved cells stimulated with lysophosphatidic acid. These results show that actin-capping protein is a novel regulator of MT stability that functions by antagonizing mDia1 activity toward actin filaments and suggest a novel form of actin–MT cross-talk in which a single factor acts sequentially on actin and MTs.     

Synthesis and SAR of 6-chloro-4-fluoroalkylamino-2-heteroaryl-5-(substituted)phenylpyrimidines as anti-cancer agents

The synthesis and SAR of a series of 6-chloro-4-fluoroalkylamino-2-heteroaryl-5-(substituted)phenylpyrimidines as anti-cancer agents are described. This series of 2-heteroarylpyrimidines was developed by modifying a series of anti-tumor [1,2,4]triazolo[1,5-a]pyrimidines and 2-cyanoaminopyrimidines we reported earlier. For the 2-heteroaryl group, the best activity is obtained when the heteroaryl group has a nitrogen atom at the ortho-position to the pyrimidyl core. The structure–activity relationship for the rest of the molecule in this 2-heteroarylpyrimidine series mimics that of the [1,2,4]triazolo[1,5-a]pyrimidine series. Like triazolopyrimidines and 2-cyanoaminopyrimidines, the 2-heteroarylpyrimidines retain the capability to overcome multidrug resistance due to Pgp. Mechanism of action studies showed that the lead compounds behaved in the same manner as triazolopyrimidines and 2-cyanoaminopyrimidines. The lead compounds in this series are more potent than the corresponding triazolopyrimidines in vitro and in vivo. Compound 21 (PTI-868) showed tumor growth inhibition in several nude mouse xenograft models, and was selected to advance to preclinical development.     

Metallothionein Induction in Neonatal Rat Primary Astrocyte Cultures Protects Against Methylmercury Cytotoxicity

Abstract: Metallothionein (MT) protein and mRNA levels were monitored following exposure of rat neonatal primary astrocyte cultures to methylmercury (MeHg). MT-I and MT-II mRNAs were probed on northern blots with an [α-³²P]dCTP-labeled synthetic cDNA probe specific for rat MT mRNA. MT-I and MT-II mRNAs were detected in untreated cells, suggesting constitutive MT expression in these cells. The probes hybridize to a single mRNA with a size appropriate for MT, ∼550 and 350 bp for MT-I and MT-II, respectively. Expression of MT-I and MT-II mRNA in astrocyte monolayers exposed to 2 × 10⁻⁶ M MeHg for 6 h was increased over MT-I and MT-II mRNA levels in controls. Western blot analysis revealed a time-dependent increase in MT protein synthesis through 96 h of exposure to MeHg. Consistent with the constitutive expression of MTs at both the mRNA level and the protein level, we have also demonstrated a time-dependent increase in MT immunoreactivity in astrocytes exposed to MeHg. The cytotoxic effects of MeHg were measured by the rate of astrocytic d -[³H]aspartate uptake. Preexposure of astrocytes to CdCl₂, a potent inducer of MTs, completely reversed the inhibitory effect of MeHg on d -[³H]aspartate uptake that occurs in MeHg-treated astrocytes with constitutive MT levels. Associated with CdCl₂ treatment was a time-dependent increase in astrocytic MT levels. In summary, astrocytes constitutively express MTs; treatment with MeHg increases astrocytic MT expression, and increased MT levels (by means of CdCl₂ pretreatment) attenuate MeHg-induced toxicity. Increased MT expression may represent a generalized response to heavy metal exposure, thus protecting astrocytes and perhaps also, indirectly, juxtaposed neurons from the neurotoxic effects of heavy metals.