A "Clickable" Photoconvertible Small Fluorescent Molecule as a Minimalist Probe for Tracking Individual Biomolecule Complexes.

Photoconvertible fluorophores can enable the visualization and tracking of a specific biomolecules, complexes, and cellular compartments with precise spatiotemporal control. The field of photoconvertible probes is dominated by fluorescent protein variants, which can introduce perturbations to the target biomolecules due to their large size. Here, we present a photoconvertible small molecule, termed CPX, that can be conjugated to any target through azide-alkyne cycloaddition ("click" reaction). To demonstrate its utility, we have applied CPX to study (1) trafficking of biologically relevant synthetic vesicles and (2) intracellular processes involved in transmission of α-synuclein (αS) pathology. Our results demonstrate that CPX can serve as a minimally perturbing probe for tracking the dynamics of biomolecules.

Transmission of α-synuclein seeds in neurodegenerative disease: recent developments.

Cell-to-cell transmission of proteopathic alpha-synuclein (α-syn) seeds is increasingly thought to underlie the progression of neurodegenerative diseases including Parkinson's disease, dementia with Lewy bodies, multiple system atrophy, and related synucleinopathies. As such, it is important to understand the chemical and biological relationships between cells and pathological aggregates of α-syn. This brief review updates our understanding of the templated spread of α-syn pathology in neurodegenerative disease from the perspective of proteopathic α-syn seeds, including how these seeds are processed by cells as well as their effects on cellular function. Recent advances in understanding the conformations of α-syn seeds are highlighted, and the possible structural basis for the observed heterogeneity of synucleinopathies is discussed. Finally, we propose the possibility that some known risk factors for synucleinopathies may in fact potentiate the cell-to-cell transmission of α-syn pathology via imbalances in interrelated cell biological processes.

Selective imaging of internalized proteopathic α-synuclein seeds in primary neurons reveals mechanistic insight into transmission of synucleinopathies.

Direct cell-to-cell transmission of proteopathic α-synuclein (α-syn) aggregates is thought to underlie the progression of neurodegenerative synucleinopathies. However, the specific intracellular processes governing this transmission remain unclear because currently available model systems are limited. For example, in cell culture models of α-syn-seeded aggregation, it is difficult to discern intracellular from extracellular exogenously applied α-syn seed species. Herein, we employed fluorescently labeled α-syn preformed fibrils (pffs) in conjunction with the membrane-impermeable fluorescence quencher trypan blue to selectively image internalized α-syn seeds in cultured primary neurons and to quantitatively characterize the concentration dependence, time course, and inhibition of pff uptake. To study the long-term fates of exogenous α-syn pffs in neurons, we developed a pff species labeled at amino acid residue 114 with the environmentally insensitive fluorophore BODIPY or the pH-sensitive dye pHrodo red. We found that pffs are rapidly trafficked along the endolysosomal pathway, where most of the material remains for days. We also found that brief pharmacological perturbation of lysosomes shortly after the pff treatment causes aberrations in intracellular processing of pff seeds concomitant with an increased rate of inclusion formation via recruitment of endogenous α-syn to a relatively small number of exogenous seeds. Our results validate a quantitative assay for pff uptake in primary neurons, implicate lysosomal processing as the major fate of internalized proteopathic seeds, and suggest lysosomal integrity as a significant rate-determining step in the transmission of α-syn pathology. Further, lysosomal processing of transmitted seeds may represent a new therapeutic target to combat the spread of synucleinopathies.

Comparison of strategies for non-perturbing labeling of α-synuclein to study amyloidogenesis.

Characterization of the amyloidogenic Parkinson's disease protein α-synuclein (αS) has proven difficult due to its structural plasticity. Here, we present a number of complementary methods to site-specifically introduce fluorescent probes to examine αS fibril formation and cellular uptake. By using various combinations of conventional Cys modification, amber codon suppression, transferase mediated N-terminal modification, and native chemical ligation, several variants of singly- and doubly-labeled αS were produced. We validated the nonperturbative nature of the label by a combination of in vitro aggregation kinetics measurements and imaging of the resulting fibrils. The labeled αS can then be used to monitor conformational changes during fibril formation or cellular uptake of αS fibrils in models of disease propagation.

Correction: Comparison of strategies for non-perturbing labeling of α-synuclein to study amyloidogenesis.

Correction for 'Comparison of strategies for non-perturbing labeling of α-synuclein to study amyloidogenesis' by Conor M. Haney, et al., Org. Biomol. Chem., 2016, 14, 1584-1592.

Studies of Thioamide Effects on Serine Protease Activity Enable Two-Site Stabilization of Cancer Imaging Peptides.

Thioamide substitutions in peptides can be used as fluorescence quenchers in protease sensors and as stabilizing modifications of hormone analogs. To guide these applications in the context of serine proteases, we here examine the cleavage of several model substrates, scanning a thioamide between the P3 and P3' positions, and identify perturbing positions for thioamide substitution. While all serine proteases tested were affected by P1 thioamidation, certain proteases were also significantly affected by other thioamide positions. We demonstrate how these findings can be applied by harnessing the combined P3/P1 effect of a single thioamide on kallikrein proteolysis to protect two key positions in a neuropeptide Y-based imaging probe, increasing its serum half-life to >24 h while maintaining potency for binding to Y1 receptor expressing cells. Such stabilized peptide probes could find application in imaging cell populations in animal models or even in clinical applications such as fluorescence-guided surgery.

Designing a norepinephrine optical tracer for imaging individual noradrenergic synapses and their activity in vivo.

Norepinephrine is a monoamine neurotransmitter with a wide repertoire of physiological roles in the peripheral and central nervous systems. There are, however, no experimental means to study functional properties of individual noradrenergic synapses in the brain. Development of new approaches for imaging synaptic neurotransmission is of fundamental importance to study specific synaptic changes that occur during learning, behavior, and pathological processes. Here, we introduce fluorescent false neurotransmitter 270 (FFN270), a fluorescent tracer of norepinephrine. As a fluorescent substrate of the norepinephrine and vesicular monoamine transporters, FFN270 labels noradrenergic neurons and their synaptic vesicles, and enables imaging synaptic vesicle content release from specific axonal sites in living rodents. Combining FFN270 imaging and optogenetic stimulation, we find heterogeneous release properties of noradrenergic synapses in the somatosensory cortex, including low and high releasing populations. Through systemic amphetamine administration, we observe rapid release of cortical noradrenergic vesicular content, providing insight into the drug's effect.

Deconstructing the Iboga Alkaloid Skeleton: Potentiation of FGF2-induced Glial Cell Line-Derived Neurotrophic Factor Release by a Novel Compound.

Modulation of growth factor signaling pathways in the brain represents a new experimental approach to treating neuropsychiatric disorders such as depression, anxiety, and addiction. Neurotrophins and growth factors exert synaptic, neuronal, and circuit level effects on a wide temporal range, which suggests a possibility of rapid and lasting therapeutic effects. Consequently, identification of small molecules that can either enhance the release of growth factors or potentiate their respective pathways will provide a drug-like alternative to direct neurotrophin administration or viral gene delivery and thus represents an important frontier in chemical biology and drug design. Glial cell line-derived neurotrophic factor (GDNF), in particular, has been implicated in marked reduction of alcohol consumption in rodent addiction models, and the natural product ibogaine, a substance used traditionally in ritualistic ceremonies, has been suggested to increase the synthesis and release of GDNF in the dopaminergic system in rats. In this report, we describe a novel iboga analog, XL-008, created by unraveling the medium size ring of the ibogamine skeleton, and its ability to induce release of GDNF in C6 glioma cells. Additionally, XL-008 potentiates the release of GDNF induced by fibroblast growth factor 2 (FGF2), another neurotrophin implicated in major depressive disorder, increasing potency more than 2-fold (from 7.85 ± 2.59 ng/mL to 3.31 ± 0.98 ng/mL) and efficacy more than 3-fold. The GDNF release by both XL-008 and the FGF2/XL-008 mixture was found to be mediated through the MEK and PI3K signaling pathways but not through PLCγ in C6 glioma cells.

Fluorescent false neurotransmitter reveals functionally silent dopamine vesicle clusters in the striatum.

Neurotransmission at dopaminergic synapses has been studied with techniques that provide high temporal resolution, but cannot resolve individual synapses. To elucidate the spatial dynamics and heterogeneity of individual dopamine boutons, we developed fluorescent false neurotransmitter 200 (FFN200), a vesicular monoamine transporter 2 (VMAT2) substrate that selectively traces monoamine exocytosis in both neuronal cell culture and brain tissue. By monitoring electrically evoked Ca(2+) transients with GCaMP3 and FFN200 release simultaneously, we found that only a small fraction of dopamine boutons that exhibited Ca(2+) influx engaged in exocytosis, a result confirmed with activity-dependent loading of the endocytic probe FM1-43. Thus, only a low fraction of striatal dopamine axonal sites with uptake-competent VMAT2 vesicles are capable of transmitter release. This is consistent with the presence of functionally 'silent' dopamine vesicle clusters and represents, to the best of our knowledge, the first report suggestive of presynaptically silent neuromodulatory synapses.

Visualizing neurotransmitter secretion at individual synapses.

To advance understanding of the brain, the ability to measure both nerve cell electrical spiking and chemical neurotransmission with high spatial resolution is required. In comparison to the development of voltage sensors and Ca(2+) indicator dyes over the past several decades, high resolution imaging of neurotransmitter (NT) release at single synapses has not been possible. In this Viewpoint, we discuss two recent developments toward this goal, namely, the design of fluorescent false neurotransmitters (FFNs) and optical neurotransmitter sensors.

APP+, a fluorescent analogue of the neurotoxin MPP+, is a marker of catecholamine neurons in brain tissue, but not a fluorescent false neurotransmitter.

We have previously introduced fluorescent false neurotransmitters (FFNs) as optical reporters that enable visualization of individual dopaminergic presynaptic terminals and their activity in the brain. In this context, we examined the fluorescent pyridinium dye 4-(4-dimethylamino)phenyl-1-methylpyridinium (APP+), a fluorescent analogue of the dopaminergic neurotoxin MPP+, in acute mouse brain tissue. APP+ is a substrate for the dopamine transporter (DAT), norepinephrine transporter (NET), and serotonin transporter (SERT), and as such represented a candidate for the development of new FFN probes. Here we report that APP+ labels cell bodies of catecholaminergic neurons in the midbrain in a DAT- and NET-dependent manner, as well as fine dopaminergic axonal processes in the dorsal striatum. APP+ destaining from presynaptic terminals in the dorsal striatum was also examined under the conditions inducing depolarization and exocytotic neurotransmitter release. Application of KCl led to a small but significant degree of destaining (approximately 15% compared to control), which stands in contrast to a nearly complete destaining of the new generation FFN agent, FFN102. Electrical stimulation of brain slices at 10 Hz afforded no significant change in the APP+ signal. These results indicate that the majority of the APP+ signal in axonal processes originates from labeled organelles including mitochondria, whereas only a minor component of the APP+ signal represents the releasable synaptic vesicular pool. These results also show that APP+ may serve as a useful probe for identifying catecholaminergic innervations in the brain, although it is a poor candidate for the development of FFNs.

New fluorescent substrate enables quantitative and high-throughput examination of vesicular monoamine transporter 2 (VMAT2).

Vesicular monoamine transporter 2 (VMAT2) is an essential component of the monoaminergic neurotransmission system in the brain as it transports monoamine neurotransmitters from the neuronal cytosol into the synaptic vesicles and thus contributes to modulation of neurotransmitter release. Considering the continuing interest in VMAT2 as a drug target, as well as a target for the design of imaging probes, we have developed a fluorescent substrate well suited for the study of VMAT2 in cell culture. Herein, we report the synthesis and characterization of a new fluorescent probe, FFN206, as an excellent VMAT2 substrate capable of detecting VMAT2 activity in intact cells using fluorescence microscopy, with subcellular localization to VMAT2-expressing acidic compartments without apparent labeling of other organelles. VMAT2 activity can also be measured via microplate reader. The apparent Km of FFN206 at VMAT2 was found to be 1.16 ± 0.10 µM, similar to that of dopamine. We further report the development and validation of a cell-based fluorescence assay amenable to high-throughput screening (HTS) using VMAT2-transfected HEK cells (Z'-factor of 0.7-0.8), enabling rapid identification of VMAT2 inhibitors and measurement of their inhibition constants over a broad range of affinities. FFN206 thus represents a new tool for optical examination of VMAT2 function in cell culture.

Control of absolute helicity in single-stranded abiotic metallofoldamers.

Described is the design, synthesis, and characterization of abiotic, single-stranded metallofoldamers that adopt helical secondary structures upon metal complexation. The absolute helicity is determined by stereocenters at the ends of the structures and is enforced by the steric influence and hydrogen bonding ability of esters in the backbone of the foldamer. Folding of the structures is characterized in the solid state by X-ray crystallography and in solution by specific rotation, CD spectroscopy, and NMR spectroscopy.