[Show abstract][Hide abstract] ABSTRACT: A reusable silicon-based transfer agent (1) has been designed, synthesized, and validated for effective room-temperature palladium-catalyzed cross-coupling reactions (CCRs) of aryl and heteroaryl chlorides with readily accessible aryl lithium reagents. The crystalline, bench-stable siloxane transfer agent (1) is easily prepared via a one-step protocol. Importantly, this "green" CCR protocol circumvents prefunctionalization, isolation of organometallic cross-coupling partners, and/or stoichiometric waste aside from LiCl. DFT calculations support a σ-bond metathesis mechanism during transmetalation and lead to insights on the importance of the CF3 groups.
Full-text · Article · Feb 2016 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: Dendritic spines represent the major postsynaptic input of excitatory synapses. Loss of spines and changes in their morphology correlate with cognitive impairment in Alzheimer’s disease (AD) and are thought to occur early during pathology. Therapeutic intervention at a preclinical stage of AD to modify spine changes might thus be warranted. To follow the development and to potentially interfere with spine changes over time, we established a long term ex vivo model from organotypic cultures of the hippocampus from APP transgenic and control mice. The cultures exhibit spine loss in principal hippocampal neurons, which closely resembles the changes occurring in vivo, and spine morphology progressively changes from mushroom-shaped to stubby. We demonstrate that spine changes are completely reversed within few days after blocking amyloid-β (Aβ) production with the gamma-secretase inhibitor DAPT. We show that the microtubule disrupting drug nocodazole leads to spine loss similar to Aβ expressing cultures and suppresses DAPT-mediated spine recovery in slices from APP transgenic mice. Finally, we report that epothilone D (EpoD) at a subnanomolar concentration, which slightly stabilizes microtubules in model neurons, completely reverses Aβ-induced spine loss and increases thin spine density. Taken together the data indicate that Aβ causes spine changes by microtubule destabilization and that spine recovery requires microtubule polymerization. Moreover, our results suggest that a low, subtoxic concentration of EpoD is sufficient to reduce spine loss during the preclinical stage of AD.
epothilone; Alzheimer's disease; dendritic spine; microtubules; amyloid beta
Full-text · Article · Jan 2016 · Neuropharmacology
[Show abstract][Hide abstract] ABSTRACT: Human immunodeficiency virus type 1 (HIV-1) infection causes a progressive depletion of CD4+ T cells. Despite its importance for HIV-1 pathogenesis, the precise mechanisms underlying CD4+ T-cell depletion remain incompletely understood. Here we make the surprising observation that antibody-dependent cell-mediated cytotoxicity (ADCC) mediates the death of uninfected bystander CD4+ T cells in cultures of HIV-1-infected cells. While HIV-1-infected cells are protected from ADCC by the action of the viral Vpu and Nef proteins, uninfected bystander CD4+T cells bind gp120 shed from productively infected cells and are efficiently recognized by ADCC-mediating antibodies. Thus, gp120 shedding represents a viral mechanism to divert ADCC responses towards uninfected bystander CD4+ T cells. Importantly, CD4-mimetic molecules redirect ADCC responses from uninfected bystander cells to HIV-1-infected cells; therefore, CD4-mimetic compounds might have therapeutic utility in new strategies aimed at specifically eliminating HIV-1-infected cells.
[Show abstract][Hide abstract] ABSTRACT: Importance:
An HIV-1 cure remains elusive due to the persistence of long-lived latently infected cells. An HIV-1 cure strategy, termed "shock and kill", aims to reactivate HIV-1 expression in latently infected cells and subsequently eliminate the reactivated cells through immune-mediated killing. While recent research efforts have focused on reversing HIV-1 latency, it remains unclear whether pre-existing immune responses within HIV-1(+) individuals can efficiently eliminate the reactivated cells. HIV-1-specific antibodies can potentially eliminate cells reactivated from latency via Fc effector functions by recruiting innate immune cells. Our study highlights the potential role that antibody-dependent cellular cytotoxicity might play in anti-latency cure approaches.
Full-text · Article · Dec 2015 · Journal of Virology
[Show abstract][Hide abstract] ABSTRACT: The design, synthesis, and validation of a new bifunctional aldehyde linchpin for Type II anion relay chemistry have been achieved. For this linchpin, the initial nucleophilic addition proceeds under Felkin-Anh control to generate the syn-alkoxide, which undergoes a 1,4-Brook rearrangement to relay the negative charge, thus leading to the formation of a dithiane-stabilized carbanion. Subsequent trapping with an electrophile furnishes a tricomponent adduct with an embedded propionate subunit, a ubiquitous structural motif found in polyketides. The utility of this new linchpin is demonstrated with the construction of a potential C16-C29 fragment for the synthesis of rhizopodin, an actin-binding macrolide.
[Show abstract][Hide abstract] ABSTRACT: A highly convergent, stereocontrolled total synthesis of the architecturally complex marine sponge metabolite (-)-enigmazole A has been achieved. Highlights include an unprecedented late-stage large-fragment Petasis-Ferrier union/rearrangement, a multicomponent Type I Anion Relay Chemistry (ARC) tactic, and a dithiane-epoxide union in conjunction with an oxazole-directed stereoselective reduction.
No preview · Article · Dec 2015 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: Rett syndrome (RTT) is a rare X-linked neurodevelopmental disorder, characterized by normal post-natal development followed
by a sudden deceleration in brain growth with progressive loss of acquired motor and language skills, stereotypic hand movements
and severe cognitive impairment. Mutations in the methyl-CpG-binding protein 2 (MECP2) cause more than 95% of classic cases. Recently, it has been shown that the loss of Mecp2 from glia negatively influences
neurons in a non-cell-autonomous fashion, and that in Mecp2-null mice, re-expression of Mecp2 preferentially in astrocytes significantly improved locomotion and anxiety levels, restored
respiratory abnormalities to a normal pattern and greatly prolonged lifespan compared with globally null mice. We now report
that microtubule (MT)-dependent vesicle transport is altered in Mecp2-deficient astrocytes from newborn Mecp2-deficient mice compared with control wild-type littermates. Similar observation has
been made in human MECP2 p.Arg294* iPSC-derived astrocytes. Importantly, administration of Epothilone D, a brain-penetrant MT-stabilizing natural
product, was found to restore MT dynamics in Mecp2-deficient astrocytes and in MECP2 p.Arg294* iPSC-derived astrocytes in vitro. Finally, we report that relatively low weekly doses of Epothilone D also partially reversed the impaired exploratory behavior
in Mecp2308/y male mice. These findings represent a first step toward the validation of an innovative treatment for RTT.
No preview · Article · Nov 2015 · Human Molecular Genetics
[Show abstract][Hide abstract] ABSTRACT: The design, synthesis, and validation of new highly effective bifunctional linchpins for type II anion relay chemistry (ARC) has been achieved. The mechanistically novel negative-charge migration that comprises the Brook rearrangement is now initiated by a stabilized tetrahedral intermediate, which is generated by nucleophilic addition to a Weinreb amide, rather than by a simple oxyanion that is generated from an epoxide. As a result, the linchpin preserves the carbonyl functionality in the ARC adducts, thus permitting access to functionally complex systems in a single flask without the need for further chemical manipulations. This tactic was validated with the one-pot preparation of monoprotected 1,3-diketones as well as pyran and spiroketal scaffolds, depending on the choice of nucleophile, electrophile, and work-up conditions.
No preview · Article · Nov 2015 · Angewandte Chemie International Edition
[Show abstract][Hide abstract] ABSTRACT: Background
Dendritic simplification, a key feature of the neurodegenerative triad of Alzheimer’s disease (AD) in addition to spine changes and neuron loss, occurs in a region-specific manner. However, it is unknown how changes in dendritic complexity are mediated and how they relate to spine changes and neuron loss.
To investigate the mechanisms of dendritic simplification in an authentic CNS environment we employed an ex vivo model, based on targeted expression of enhanced green fluorescent protein (EGFP)-tagged constructs in organotypic hippocampal slices of mice. Algorithm-based 3D reconstruction of whole neuron morphology in different hippocampal regions was performed on slices from APPSDL-transgenic and control animals. We demonstrate that induction of dendritic simplification requires the combined action of amyloid beta (Aβ) and human tau. Simplification is restricted to principal neurons of the CA1 region, recapitulating the region specificity in AD patients, and occurs at sites of Schaffer collateral input. We report that γ-secretase inhibition and treatment with the NMDA-receptor antagonist, CPP, counteract dendritic simplification. The microtubule-stabilizing drug epothilone D (EpoD) induces simplification in control cultures per se. Similar morphological changes were induced by a phosphoblocking tau construct, which also increases microtubule stability. In fact, low nanomolar concentrations of naturally secreted Aβ decreased phosphorylation at S262 in a cellular model, a site which is known to directly modulate tau-microtubule interactions.
The data provide evidence that dendritic simplification is mechanistically distinct from other neurodegenerative events and involves microtubule stabilization by dendritic tau, which becomes dephosphorylated at certain sites. They imply that treatments leading to an overall decrease of tau phosphorylation might have a negative impact on neuronal connectivity.
Full-text · Article · Nov 2015 · Molecular Neurodegeneration
[Show abstract][Hide abstract] ABSTRACT: An update on the literature covering the akuammiline family of alkaloids is presented. This chapter begins with a summary of new akuammiline alkaloids reported since 2000 and is followed by an overview of new reported bioactivities of akuammiline alkaloids since 2000. The remainder of the chapter comprises a comprehensive review of the synthetic chemistry that has been reported in the last 50. years concerning akuammiline alkaloids and their structural motifs.
[Show abstract][Hide abstract] ABSTRACT: A total synthesis of (-)-secu'amamine A has been achieved exploiting Type II Anion Relay Chemistry (ARC) to provide the full linear carbon and nitrogen skeleton in a single flask with the requisite stereochemistry and functionality. A mechanistic rationale is also proposed to account for the stereochemical outcome of the key aldol reaction leading to the advanced aza tricyclic core.
[Show abstract][Hide abstract] ABSTRACT: A convergent total synthesis of the architecturally complex indole diterpenoid (-)-nodulisporic acid D has been achieved. Key synthetic transformations include vicinal difunctionalization of an advanced α,β-unsaturated aldehyde to form the E,F-trans-fused 5,6-ring system of the eastern hemisphere and a cascade cross-coupling/indolization protocol leading to the CDE multisubstituted indole core.
Full-text · Article · Jun 2015 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: The synthesis of a C(1)-C(24) advanced southern hemisphere fragment towards the total synthesis of spirastrellolide E has been achieved. Highlights of the route include a highly convergent Type I Anion Relay Chemistry (ARC) tactic for fragment assembly, in conjunction with a directed, regioselective gold-catalyzed alkyne functionalization to generate the central unsaturated [6,6]-spiroketal.
No preview · Article · Jun 2015 · Tetrahedron Letters
[Show abstract][Hide abstract] ABSTRACT: Die Tedanolide sind biologisch aktive Polyketide, deren Makrolaktonring aus einem primären Alkohol aufgebaut ist. Da Polyketidumwandlungen nur sekundäre Alkohole erzeugen, postulierte Taylor, dass das Tedanolid-Lakton aus einer intramolekularen Umesterung hervorgegangen ist. Um diese Hypothese zu prüfen und das biologische Profil des vermuteten Vorläufers aller Mitglieder der Tedanolidfamilie zu untersuchen, unternahmen wir die Synthese von Desepoxyisotedanolid und dessen biologische Evaluierung im Vergleich zum Desepoxytedanolid. Die biologischen Experimente deckten ein zweites Zielprotein für Desepoxytedanolid auf und erbrachten Hinweise, dass die vorgeschlagene Umesterung dem produzierenden Mikroorganismus einen evolutionären Vorteil erbringt.
No preview · Article · Apr 2015 · Angewandte Chemie
[Show abstract][Hide abstract] ABSTRACT: Synthetic analysis of spirastrellolide E envisioned to entail a cross-metathesis union of the northern and southern hemispheres followed by a Sharpless epoxidation/methylation sequence to achieve the C(22,23) stereogenicity leads to the design of a C(1)-C(23) advanced southern hemisphere exploiting a gold-catalyzed directed spiroketalization as a key step. Stereochemical analysis of this strategic transformation provides insight on the impact of the directing group carbinol stereogenicity on the reaction efficiency and, in turn, permits the conversion of the minor isomer of the spiroketal precursor to the requisite congener for successful spiroketalization.
[Show abstract][Hide abstract] ABSTRACT: Protocols have been achieved that permit facile introduction of s-tetrazine into unprotected peptides and the protein, thioredoxin, between two cysteine sulfhydryl groups (i.e., staple), followed by photochemical release (i.e., unstaple) and regeneration of the peptide/protein upon removal of the cyano groups from the derived bisthiocyanate. The S,S-tetrazine macrocycles in turn provide a convenient handle for probe introduction by exploiting the inverse electron demand Diels-Alder reactivity of the tetrazine.
No preview · Article · Mar 2015 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: The HIV-1 envelope (Env) mediates viral entry into host cells. To enable the direct imaging of conformational dynamics within Env we introduced fluorophores into variable regions of the gp120 subunit and measured single-molecule fluorescence resonance energy transfer (smFRET) within the context of native trimers on the surface of HIV-1 virions. Our observations revealed unliganded HIV-1 Env to be intrinsically dynamic, transitioning between three distinct pre-fusion conformations, whose relative occupancies were remodeled by receptor CD4 and antibody binding. The distinct properties of neutralization-sensitive and neutralization-resistant HIV-1 isolates support a dynamics-based mechanism of immune evasion and ligand recognition.
[Show abstract][Hide abstract] ABSTRACT: The
evolution of an enantioselective total synthesis of (+)-18-epi-latrunculol A, a congener of the marine-sponge-derived
latrunculins A and B, is reported. Key steps include a late-stage
Mitsunobu macrolactonization to construct the 16-membered macrolactone,
a mild Carreira alkynylation to unite the northern and southern hemispheres,
a diastereoselective, acid-mediated δ-hydroxy enone cyclization/equilibration
sequence, and a functional-group-tolerant cross-metathesis to access
the enone cyclization precursor.
Preview · Article · Sep 2014 · The Journal of Organic Chemistry