Patrick McNutt

Embryonic Stem Cell-Derived Neurons are a Novel, Highly Sensitive Tissue Culture Platform for Botulinum Research

There are no pharmacological treatments to rescue botulinum neurotoxin (BoNT)-mediated paralysis of neuromuscular signaling. In part, this failure can be attributed to the lack of a cell culture model system that is neuron-based, allowing detailed elucidation of the mechanisms underlying BoNT pathogenesis, yet still compatible with modern cellular and molecular approaches. We have developed a method to derive highly enriched, glutamatergic neurons from suspension-cultured murine embryonic stem (ES) cells. Hypothesizing that ES cell-derived neurons (ESNs) might comprise a novel platform to investigate the neurotoxicology of BoNTs, we evaluated the susceptibility of ESNs to BoNT/A and BoNT/E using molecular and functional assays. ESNs express neuron-specific proteins, develop synapses and release glutamate in a calcium-dependent manner under depolarizing conditions. They express the BoNT substrate SNARE proteins SNAP25, VAMP2 and syntaxin, and treatment with BoNT/A and BoNT/E holotoxin results in proteolysis of SNAP25 within 24 h with EC50s of 0.81 and 68.6 pM, respectively. Intoxication with BoNT/A results in the functional inhibition of potassium-induced, calcium-dependent glutamate release. ESNs remain viable and susceptible to intoxication for up to 90 days after plating, enabling longitudinal screens exploring toxin-specific mechanisms underlying persistence of synaptic blockade. The evidence suggests that derived neurons are a novel, biologically relevant model system that combines the verisimilitude of primary neurons with the genetic tractability and scalable expansion of a continuous cell line, and thus should significantly accelerate BoNT research and drug discovery while dramatically decreasing animal use.

Longitudinal RNA sequencing of the deep transcriptome during neurogenesis of cortical glutamatergic neurons from murine ESCs

Using paired-end RNA sequencing, we have quantified the deep transcriptional changes that occur during differentiation of murine embryonic stem cells into a highly enriched population of glutamatergic cortical neurons. These data provide a detailed and nuanced account of longitudinal changes in the transcriptome during neurogenesis and neuronal maturation, starting from mouse embryonic stem cells and progressing through neuroepithelial stem cell induction, radial glial cell formation, neurogenesis, neuronal maturation and cortical patterning. Understanding the transcriptional mechanisms underlying the differentiation of stem cells into mature, glutamatergic neurons of cortical identity has myriad applications, including the elucidation of mechanisms of cortical patterning; identification of neurogenic processes; modeling of disease states; detailing of the host cell response to neurotoxic stimuli; and determination of potential therapeutic targets. In future work we anticipate correlating changes in longitudinal gene expression to other cell parameters, including neuronal function as well as characterizations of the proteome and metabolome. In this data article, we describe the methods used to produce the data and present the raw sequence read data in FASTQ files, sequencing run statistics and a summary flatfile of raw counts for 22,164 genes across 31 samples, representing 3-5 biological replicates at each timepoint. We propose that this data will be a valuable contribution to diverse research efforts in bioinformatics, stem cell research and developmental neuroscience studies.

Progression of ocular sulfur mustard injury: development of a model system

Exposure of tissues to sulfur mustard (SM) results in the formation of protein and nucleotide adducts that disrupt cellular metabolism and cause cell death. Subsequent pathologies involve a significant proinflammatory response, disrupted healing, and long‐term defects in tissue architecture. Following ocular exposure, acute corneal sequelae include epithelial erosions, necrosis, and corneal inflammation. Longer term, a progressive injury becomes distributed throughout the anterior chamber, which ultimately causes a profound remodeling of corneal tissues. In many cases, debilitating and vision‐threatening injuries reoccur months to years after the initial exposure. Preliminary data in humans suffering from chronic epithelial lesions suggest that thymosin β4 (Tβ4) may be a viable candidate to mitigate acute or long‐term ocular SM injury. To evaluate therapeutic candidates, we have developed a rabbit ocular exposure model system. In this paper, we report molecular, histological, ultrastructural, and clinical consequences of rabbit ocular SM injury, which can be used to assess Tβ4 efficacy, including timepoints at which Tβ4 will be assessed for therapeutic utility.

Morphological and functional differentiation in BE(2)-M17 human neuroblastoma cells by treatment with Trans-retinoic acid

BackgroundImmortalized neuronal cell lines can be induced to differentiate into more mature neurons by adding specific compounds or growth factors to the culture medium. This property makes neuronal cell lines attractive as in vitro cell models to study neuronal functions and neurotoxicity. The clonal human neuroblastoma BE(2)-M17 cell line is known to differentiate into a more prominent neuronal cell type by treatment with trans-retinoic acid. However, there is a lack of information on the morphological and functional aspects of these differentiated cells.ResultsWe studied the effects of trans-retinoic acid treatment on (a) some differentiation marker proteins, (b) types of voltage-gated calcium (Ca2+) channels and (c) Ca2+-dependent neurotransmitter ([3H] glycine) release in cultured BE(2)-M17 cells. Cells treated with 10 μM trans-retinoic acid (RA) for 72 hrs exhibited marked changes in morphology to include neurite extensions; presence of P/Q, N and T-type voltage-gated Ca2+ channels; and expression of neuron specific enolase (NSE), synaptosomal-associated protein 25 (SNAP-25), nicotinic acetylcholine receptor α7 (nAChR-α7) and other neuronal markers. Moreover, retinoic acid treated cells had a significant increase in evoked Ca2+-dependent neurotransmitter release capacity. In toxicity studies of the toxic gas, phosgene (CG), that differentiation of M17 cells with RA was required to see the changes in intracellular free Ca2+ concentrations following exposure to CG.ConclusionTaken together, retinoic acid treated cells had improved morphological features as well as neuronal characteristics and functions; thus, these retinoic acid differentiated BE(2)-M17 cells may serve as a better neuronal model to study neurobiology and/or neurotoxicity.

Cultivation and characterization of limbal epithelial stem cells on contact lenses with a feeder layer: toward the treatment of limbal stem cell deficiency.

Purpose: Limbal epithelial sheets are used to promote corneal surface reconstruction after the detection of limbal epithelial stem cell deficiency. The aim of this study was to evaluate a novel combination of limbal stem cells (LSCs) maintained on contact lenses (CLs) in the presence of a 3T3 feeder cell layer regarding preservation of stem cell phenotype and the potential use for future in vivo transplantation. Methods: Limbal epithelial cells were isolated from rabbit cornea and cultured with 3T3 cells on CLs. The preservation of LSC phenotype was determined using p63&agr; and ABCG2 immunostaining, whereas epithelial differentiation was evaluated using CK3 and CK19. The colony-forming assay was used to determine the percentage of LSCs in cultures. Finally, CLs seeded with PKH26-labeled LSCs were transferred to rabbit eyes after performing a surgical keratectomy, and the transition and phenotype of labeled cells on the corneal surface were evaluated in whole-mount corneas. Results: Proliferation of individual limbal cells was observed on CLs with a 3T3 feeder cell layer, showing holoclone formation and retention of viable stem or progenitor cell phenotype. Finally, a higher transition of cultivated cells after a dual sequential CL transplantation to the ocular surface was observed, showing the preservation of the LSC phenotype in the corneal surface. Conclusions: Limbal cells cultivated on a CL carrier overlaying a 3T3 feeder layer are mitotically active and retain the LSC phenotype. This novel technique of using CLs as a carrier offers an easily manipulable and nonimmunogenic method for transferring LSCs for ocular surface reconstruction in patients with limbal epithelial stem cell deficiency.

High yield derivation of enriched glutamatergic neurons from suspension-cultured mouse ESCs for neurotoxicology research

BackgroundRecently, there has been a strong emphasis on identifying an in vitro model for neurotoxicity research that combines the biological relevance of primary neurons with the scalability, reproducibility and genetic tractability of continuous cell lines. Derived neurons should be homotypic, exhibit neuron-specific gene expression and morphology, form functioning synapses and consistently respond to neurotoxins in a fashion indistinguishable from primary neurons. However, efficient methods to produce neuronal populations that are suitable alternatives to primary neurons have not been available.MethodsWith the objective of developing a more facile, robust and efficient method to generate enriched glutamatergic neuronal cultures, we evaluated the neurogenic capacity of three mouse embryonic stem cell (ESC) lines (R1, C57BL/6 and D3) adapted to feeder-independent suspension culture. Neurogenesis and neuronal maturation were characterized as a function of time in culture using immunological, genomic, morphological and functional metrics. The functional responses of ESNs to neurotropic toxins with distinctly different targets and mechanisms of toxicity, such as glutamate, α-latrotoxin (LTX), and botulinum neurotoxin (BoNT), were also evaluated.ResultsSuspension-adapted ESCs expressed markers of pluripotency through at least 30 passages, and differentiation produced 97×106 neural progenitor cells (NPCs) per 10-cm dish. Greater than 99% of embryonic stem cell-derived neurons (ESNs) expressed neuron-specific markers by 96 h after plating and rapidly developed complex axodendritic arbors and appropriate compartmentalization of neurotypic proteins. Expression profiling demonstrated the presence of transcripts necessary for neuronal function and confirmed that ESN populations were predominantly glutamatergic. Furthermore, ESNs were functionally receptive to all toxins with sensitivities and responses consistent with primary neurons.ConclusionsThese findings demonstrate a cost-effective, scalable and flexible method to produce a highly enriched glutamatergic neuron population. The functional characterization of pathophysiological responses to neurotropic toxins and the compatibility with multi-well plating formats were used to demonstrate the suitability of ESNs as a discovery platform for molecular mechanisms of action, moderate-throughput analytical approaches and diagnostic screening. Furthermore, for the first time we demonstrate a cell-based model that is sensitive to all seven BoNT serotypes with EC50 values comparable to those reported in primary neuron populations. These data providing compelling evidence that ESNs offer a neuromimetic platform suitable for the evaluation of molecular mechanisms of neurotoxicity.

Architectural and biochemical expressions of mustard gas keratopathy: preclinical indicators and pathogenic mechanisms.

A subset of victims of ocular sulfur mustard (SM) exposure develops an irreversible, idiotypic keratitis with associated secondary pathologies, collectively referred to as mustard gas keratopathy (MGK). MGK involves a progressive corneal degeneration resulting in chronic ocular discomfort and impaired vision for which clinical interventions have typically had poor outcomes. Using a rabbit corneal vapor exposure model, we previously demonstrated a clinical progression with acute and chronic sequelae similar to that observed in human casualties. However, a better understanding of the temporal changes that occur during the biphasic SM injury is crucial to mechanistic understanding and therapeutic development. Here we evaluate the histopathologic, biochemical and ultrastructural expressions of pathogenesis of the chronic SM injury over eight weeks. We confirm that MGK onset exhibits a biphasic trajectory involving corneal surface regeneration over the first two weeks, followed by the rapid development and progressive degeneration of corneal structure. Preclinical markers of corneal dysfunction were identified, including destabilization of the basal corneal epithelium, basement membrane zone abnormalities and stromal deformation. Clinical sequelae of MGK appeared abruptly three weeks after exposure, and included profound anterior edema, recurring corneal erosions, basement membrane disorganization, basal cell necrosis and stromal degeneration. Unlike resolved corneas, MGK corneas exhibited frustrated corneal wound repair, with significantly elevated histopathology scores. Increased lacrimation, disruption of the basement membrane and accumulation of pro-inflammatory mediators in the aqueous humor provide several mechanisms for corneal degeneration. These data suggest that the chronic injury is fundamentally distinct from the acute lesion, involving injury mechanisms that operate on different time scales and in different corneal tissues. Corneal edema appears to be the principal pathology of MGK, in part resulting from persistent necrosis of the basal corneal epithelium and deterioration of the basement membrane. The findings also provide a potential explanation as to why administration of anti-inflammatories transiently delays, but does not prevent, the development of MGK sequelae.

Alpha-latrotoxin rescues SNAP-25 from BoNT/A-mediated proteolysis in embryonic stem cell-derived neurons.

The botulinum neurotoxins (BoNTs) exhibit zinc-dependent proteolytic activity against members of the core synaptic membrane fusion complex, preventing neurotransmitter release and resulting in neuromuscular paralysis. No pharmacologic therapies have been identified that clinically relieve botulinum poisoning. The black widow spider venom α-latrotoxin (LTX) has the potential to attenuate the severity or duration of BoNT-induced paralysis in neurons via the induction of synaptic degeneration and remodeling. The potential for LTX to antagonize botulinum poisoning was evaluated in embryonic stem cell-derived neurons (ESNs), using a novel screening assay designed around the kinetics of BoNT/A activation. Exposure of ESNs to 400 pM LTX for 6.5 or 13 min resulted in the nearly complete restoration of uncleaved SNAP-25 within 48 h, whereas treatment with 60 mM K+ had no effect. Time-lapse imaging demonstrated that LTX treatment caused a profound increase in Ca2+ influx and evidence of excitotoxicity, though ESNs remained viable 48 h after LTX treatment. This is the first instance of a cell-based treatment that has shown the ability to eliminate BoNT activity. These data suggest that LTX treatment may provide the basis for a new class of therapeutic approach to BoNT intoxication and may contribute to an improved understanding of long-term mechanisms of BoNT intoxication and recovery. They further demonstrate that ESNs are a novel, responsive and biologically relevant model for LTX research and BoNT therapeutic drug discovery.

Novel Application of Stem Cell-Derived Neurons to Evaluate the Time- and Dose-Dependent Progression of Excitotoxic Injury

Glutamate receptor (GluR)-mediated neurotoxicity is implicated in a variety of disorders ranging from ischemia to neural degeneration. Under conditions of elevated glutamate, the excessive activation of GluRs causes internalization of pathologic levels of Ca2+, culminating in bioenergetic failure, organelle degradation, and cell death. Efforts to characterize cellular and molecular aspects of excitotoxicity and conduct therapeutic screening for pharmacologic inhibitors of excitogenic progression have been hindered by limitations associated with primary neuron culture. To address this, we evaluated glutamate-induced neurotoxicity in highly enriched glutamatergic neurons (ESNs) derived from murine embryonic stem cells. As of 18 days in vitro (DIV 18), ESNs were synaptically coupled, exhibited spontaneous network activity with neurotypic mEPSCs and expressed NMDARs and AMPARs with physiological current:voltage behaviors. Addition of 0.78–200 μM glutamate evoked reproducible time- and dose-dependent metabolic failure in 6 h, with a calculated EC50 value of 0.44 μM at 24 h. Using a combination of cell viability assays and electrophysiology, we determined that glutamate-induced toxicity was specifically mediated by NMDARs and could be inhibited by addition of NMDAR antagonists, increased extracellular Mg2+ or substitution of Ba2+ for Ca2+. Glutamate treatment evoked neurite fragmentation and focal swelling by both immunocytochemistry and scanning electron microscopy. Presentation of morphological markers of cell death was dose-dependent, with 0.78–200 μM glutamate resulting in apoptosis and 3000 μM glutamate generating a mixture of necrosis and apoptosis. Addition of neuroprotective small molecules reduced glutamate-induced neurotoxicity in a dose-dependent fashion. These data indicate that ESNs replicate many of the excitogenic mechanisms observed in primary neuron culture, offering a moderate-throughput model of excitotoxicity that combines the verisimilitude of primary neurons with the flexibility and scalability of cultured cells. ESNs therefore offer a physiologically relevant platform that exhibits characteristic NMDAR responses, and appears suitable to evaluate molecular mechanisms of glutamate-induced excitotoxicity and screen for candidate therapeutics.

Structural, morphological, and functional correlates of corneal endothelial toxicity following corneal exposure to sulfur mustard vapor.

PURPOSE Sulfur mustard (SM) is a highly reactive vesicant that causes severe ocular injuries. Following exposure to moderate or high doses, a subset of victims develops a chronic injury known as mustard gas keratopathy (MGK) involving a keratitis of unknown etiopathogenesis with secondary keratopathies such as persistent epithelial lesions, corneal neovascularization, and progressive corneal degeneration. This study was designed to determine whether SM exposure evokes acute endothelial toxicity and to determine whether endothelial pathologies were specifically observed in MGK corneas as opposed to healed corneas. METHODS Corneas of New Zealand white rabbits were exposed to SM vapor, and the corneal endothelium was evaluated at 1 day and 8 weeks using scanning electron microscopy (SEM), transmission electron microscopy (TEM), in vivo confocal microscopy (IVM), and fluorescent microscopy. Barrier function was measured by uptake of a fluorescent dye injected into the anterior chamber. RESULTS A centripetal endothelial injury at 1 day was observed by SEM, TEM, IVM, and fluorescent microscopy. In vivo confocal microscopy revealed additional cytotoxicity between 1 and 13 days. In contrast to healed corneas, which appeared similar to sham-exposed naive eyes at 8 weeks, MGK corneas exhibited significant evidence of continued pathological changes in the endothelium. CONCLUSIONS Endothelial toxicity occurs at the right time and with the appropriate pathophysiology to contribute to MGK. Based on these findings, we propose a model that explains the relationships among SM dose, the biphasic progression, and the various clinical trajectories of corneal SM injury and that provides a mechanism for temporal variations in MGK onset. Finally, we discuss the implications for the management of SM casualties.