Anna Ruth Cliffe

Nicotine, the principal addictive component of cigarettes, is linked to cognitive decline and neurodegenerative alterations, likely through oxidative stress and impaired iron regulation in neurons. Yet, underlying molecular pathways remain unclear. This study examined the role of pulmonary neuroendocrine cells (PNECs) in smoke-induced neural changes. Using human pluripotent stem cells, we generated induced PNECs (iPNECs) to overcome culture limitations and performed mechanistic analyses. We found that nicotine exposure stimulates iPNECs to secrete exosomes enriched with serotransferrin, an iron-binding glycoprotein. Neurons internalizing these exosomes displayed elevated levels of transferrin receptor 1 (TFR1), divalent metal transporter 1, and duodenal cytochrome b, associated with ferritin accumulation, oxidative stress, and adenosine triphosphate depletion. Inhibition of TFR1 alleviated these effects. Furthermore, nicotine-triggered exosomes increased α-synuclein expression in neurons in a manner consistent with stress- and vulnerability-associated signatures observed in human lungs and nicotine-exposed mice, highlighting PNEC-derived exosomal signaling that may contribute to neuronal dysfunction.

Nociceptive afferent neurons within the dorsal root ganglion (DRG) detect and relay painful peripheral stimuli, and the malfunctioning of this process leads to sustained pain states. Animal model studies have been invaluable for demonstrating the importance of the DRG nociceptor in pain sensation and the development of related analgesic targets. However, a human in vitro model of nociception is essential to confirming the relevance of preclinical findings for therapeutic drug development. We characterized the nociceptive properties of differentiated cells from the human DRG‐derived immortalized cell line HD10.6 and developed their use into an in vitro model of human nociceptive signaling and therapy. Within differentiated HD10.6 cells, we confirmed the abundance and function of machinery linked with pain sensation, including key ion channels (TRPV1, NaV1.7) and afferent peptides (CGRP, Substance P), by immunofluorescence and calcium influx assays. Through whole‐cell patch clamp, including current clamp and voltage clamp, we recorded the baseline electrophysiological parameters of differentiated HD10.6 cells. We further found that differentiated HD10.6 cells express the mu opioid receptor 1 protein, and that mu agonist DAMGO blocks depolarization‐evoked calcium influx in a naloxone‐reversible fashion. Importantly, excitation and peripheral sensitization were induced within HD10.6 cells in response to an inflammatory cocktail, mirroring nociceptors in a pain state during and after tissue damage or inflammation. HD10.6 cells were also cultured into dual‐chambered microfluidic devices to mirror the physiological anatomy of the nociceptor. Within this system, genetic therapy adeno‐associated‐virus was successfully taken up by the peripheral terminals and transported to the soma.

Herpes simplex virus-1 (HSV-1) establishes a latent infection in neurons, periodically reactivating to cause disease. Neuronal conditions, including immune signaling, during initial HSV-1 infection, impact later reactivation. Type I interferon (IFNα) exposure during initial infection results in promyelocytic leukemia nuclear-body (PML-NB) formation and subsequent restriction of reactivation, via mechanisms that were unknown. Here we find that PML-NB formation results in the recruitment of histone chaperones to the viral genome and increased enrichment of the repressive heterochromatin mark, histone H3 lysine 9 tri-methylation (H3K9me3), and its reader, ATRX (alpha-thalassemia/mental retardation, X-linked). ATRX is highly abundant in neurons and prevents reactivation from H3K9me3-bound latent genomes by remaining associated with viral chromatin. Therefore, we demonstrate how immune signaling during initial infection results in an epigenetic memory on HSV-1 genomes and identify ATRX as a neuronal restriction factor against HSV-1 reactivation, elucidating a new potential target for inhibiting HSV-1 reactivation and subsequent human disease.

Herpes simplex virus 1 (HSV-1) establishes a latent infection of neurons, where it persists for life with the capability to reactivate. During latent infection, heterochromatin is deposited onto the viral genome to presumably permit viral gene silencing. Repressive histone modifications associated with the latent genome include the constitutive heterochromatin modifications, H3K9me2/3, and the facultative heterochromatin modification, H3K27me3. The role of these different types during entry into latency and as substrates for reactivation is unknown. H3K27me3 is associated with Polycomb silencing, and a second modification also associated with Polycomb silencing, especially in pluripotent cells and during early development, is H2AK119ub1. Here we found that H2AK119ub1 is enriched on a sub-population of latent HSV genomes. We examined the contribution of both H2AK119ub1 and H3K7me3 deposition to HSV-1 gene silencing during entry into latency. We found that H2AK119ub1 was deposited prior to H3K27me3 and that only inhibition of H2AK119ub1 deposition plays a subtle role in latency establishment. Importantly, we found that inhibiting the enzymatic activity of Polycomb Repressive Complex 1 (PRC1) that deposits H2AK119ub1 during latency establishment prevented later reactivation. In contrast, inhibiting the activity of PRC2, which deposits H3K27me3, did not impact reactivation. Together, these data demonstrate the heterogenic nature of the epigenetic structure of latent HSV genomes and provide evidence that those associated with H2AK119ub1 are the template for reactivation.

Nociceptive afferent neurons within the dorsal root ganglion (DRG) detect and relay painful stimuli from the periphery to the brain, and the malfunctioning of this process leads to sustained pain states. Animal model studies have been invaluable for demonstrating the importance of the DRG nociceptor in pain sensation and the development of related analgesic targets. However, there are functional biological differences between human and animal model nociceptors. Therefore, a complementary in vitro model of human nociception is critical to confirming the relevance of preclinical findings for therapeutic drug development. We characterized the nociceptive properties of differentiated cells from the human DRG-derived immortalized cell line HD10.6. Within differentiated HD10.6 cells, we documented the abundance and localization of nociceptive machinery central to regulating excitability and linked with pain sensation including ion channels TRPV1 and NaV1.7 and afferent peptides CGRP and Substance P. Using calcium influx imaging assays, we confirmed the electrical functionality of TRPV1 and NaV1.7 in HD10.6 cells, and through whole-cell patch clamp, we found similar baseline electrophysiological parameters of HD10.6 cells to those previously observed in human patient DRGs. Further, we found that differentiated HD10.6 cells express the mu opioid receptor 1 protein, and DAMGO, a mu agonist, blocks depolarization-evoked calcium influx in a naloxone-reversible fashion. Importantly, using an inflammatory cocktail, excitation and peripheral sensitization are induced within HD10.6 cells, mirroring nociceptors in a pain state during or after tissue damage or inflammation. Finally, HD10.6 cells were also cultured into dual-chambered microfluidic devices to mirror the biological anatomy of the nociceptor. Within this system, we demonstrated the uptake of adeno-associated-virus (AAV) by the peripheral terminals and AAV transport to the soma. Altogether, we have developed the use of HD10.6 cells to create a system of human nociceptive signaling on a chip to study human nociceptor physiology and intervention. Perspective: There are essential differences between human and animal model nociceptors. Here, we develop a physiological model of “nociceptive signaling on a chip” using human-derived nociceptors to ultimately enhance the translatability of preclinical afferent signaling research to the human patient.

Herpes simplex virus-1 (HSV-1) establishes a lifelong latent infection in neurons and reactivation from this latent state is the cause of recurrent oral and ocular infections, herpes simplex keratitis, and encephalitis. Neuronal conditions during initial HSV-1 infection have a long-term impact on latency, modulating how responsive latent genomes are to reactivation and, therefore, their ability to cause disease. Type I interferon (IFNα) exposure during initial infection results in promyelocytic leukemia nuclear-body (PML-NB) formation and a more restrictive form of HSV-1 latency. Here we demonstrate that IFNα induced PML-NBs recruit histone chaperones to the viral genome and promote the deposition of the repressive heterochromatin mark, histone H3 lysine 9 tri-methylation (H3K9me3), and its reader, ATRX (alpha-thalassemia/mental retardation, X-linked). This work reveals the mechanism by which immune signaling during initial infection induces an epigenetic memory on HSV-1 genomes that is maintained during latency and inhibits reactivation. ATRX is highly abundant in neurons and is essential for maintaining cellular heterochromatin during neuronal stress. Here we find that ATRX prevents transcription, and subsequent reactivation, from H3K9me3-bound latent genomes by remaining associated with viral chromatin following stress-induced phosphorylation of histone H3. This indicates that H3K9me3-associated viral genomes are refractory to reactivation when read by ATRX. This work demonstrates that ATRX acts as a neuronal restriction factor against HSV-1 reactivation, elucidating a new potential target for inhibiting HSV-1 reactivation and subsequent human disease.

Although viruses subvert innate immune pathways for their replication, there is evidence they can also co-opt antiviral responses for their benefit. The ubiquitous human pathogen, Herpes simplex virus-1 (HSV-1), encodes a protein (UL12.5) that induces the release of mitochondrial nucleic acid into the cytosol, which activates immune-sensing pathways and reduces productive replication in nonneuronal cells. HSV-1 establishes latency in neurons and can reactivate to cause disease. We found that UL12.5 is required for HSV-1 reactivation in neurons and acts to directly promote viral lytic gene expression during initial exit from latency. Further, the direct activation of innate immune-sensing pathways triggered HSV-1 reactivation and compensated for a lack of UL12.5. Finally, we found that the induction of HSV-1 lytic genes during reactivation required intact RNA- and DNA-sensing pathways, demonstrating that HSV-1 can respond to and active antiviral nucleic acid–sensing pathways to reactivate from a latent infection.

Herpes simplex virus-1 (HSV-1) establishes a latent infection in peripheral neurons and periodically reactivates in response to a stimulus to permit transmission. In vitro models using primary neurons are invaluable to studying latent infection because they use bona fide neurons that have undergone differentiation and maturation in vivo . However, culture conditions in vitro should remain as close to those in vivo as possible. This is especially important when considering minimizing cell stress, as it is a well-known trigger of HSV reactivation. We recently developed an HSV-1 model system that requires neurons to be cultured for extended lengths of time. Therefore, we sought to refine culture conditions to optimize neuronal health and minimize secondary effects on latency and reactivation. Here, we demonstrate that culturing primary neurons under conditions closer to physiological oxygen concentrations (5% oxygen) results in cultures with features consistent with reduced stress. Furthermore, culture in these lower oxygen conditions diminishes the progression to full HSV-1 reactivation despite minimal impacts on latency establishment and earlier stages of HSV-1 reactivation. We anticipate that our findings will be useful for the broader microbiology community as they highlight the importance of considering physiological oxygen concentration in studying host-pathogen interactions.

Summary: Although viruses subvert innate immune pathways for their replication, there is evidence they can also co-opt anti-viral responses for their benefit. The ubiquitous human pathogen, Herpes Simplex Virus-1 (HSV-1), encodes a protein (UL12.5) that induces the release of mitochondrial nucleic acid into the cytosol, which activates immune sensing pathways and reduces productive replication in non-neuronal cells. HSV-1 establishes latency in neurons and can reactivate to cause disease. We found that UL12.5 is required for HSV-1 reactivation in neurons and acts to directly promote viral lytic gene expression during initial exit from latency. Further, the direct activation of innate immune sensing pathways triggered HSV reactivation and compensated for a lack of UL12.5. Finally, we found that the induction of HSV-1 lytic genes during reactivation required intact RNA and DNA sensing pathways, demonstrating that HSV-1 can both respond to and active antiviral nucleic acid sensing pathways to reactivate from a latent infection.

The fate of herpesvirus genomes following entry into different cell types is thought to regulate the outcome of infection. For the Herpes simplex virus 1 (HSV-1), latent infection of neurons is characterized by association with repressive heterochromatin marked with Polycomb silencing-associated lysine 27 methylation on histone H3 (H3K27me). However, whether H3K27 methylation plays a role in repressing lytic gene expression in non-neuronal cells is unclear. To address this gap in knowledge, and with consideration that the fate of the viral genome and outcome of HSV-1 infection could be heterogeneous, we developed an assay to quantify the abundance of histone modifications within single viral genome foci of infected fibroblasts. Using this approach, combined with bulk epigenetic techniques, we were unable to detect any role for H3K27me3 during HSV-1 lytic infection of fibroblasts. By contrast, we could detect the lesser studied H3K27me2 on a subpopulation of viral genomes, which was consistent with a role for H3K27 demethylases in promoting lytic gene expression. In addition, viral genomes co-localized with the H3K27me2 reader protein PHF20L1, and this association was enhanced by inhibition of the H3K27 demethylases UTX and JMJD3. Notably, targeting of H3K27me2 to viral genomes was enhanced following infection with a transcriptionally defective virus in the absence of Promyelocytic leukemia nuclear bodies. Collectively, these studies implicate a role for H3K27me2 in fibroblast-associated HSV genome silencing in a manner dependent on genome sub-nuclear localization and transcriptional activity. IMPORTANCE: Investigating the potential mechanisms of gene silencing for DNA viruses in different cell types is important to understand the differential outcomes of infection, particularly for viruses like herpesviruses that can undergo distinct types of infection in different cell types. In addition, investigating chromatin association with viral genomes informs on the mechanisms of epigenetic regulation of DNA processes. However, there is a growing appreciation for heterogeneity in the outcome of infection at the single cell, and even single viral genome, level. Here we describe a novel assay for quantifying viral genome foci with chromatin proteins and show that a portion of genomes are targeted for silencing by H3K27me2 and associate with the reader protein PHF20L1. This study raises important questions regarding the mechanism of H3K27me2-specific targeting to viral genomes, the contribution of epigenetic heterogeneity to herpesvirus infection, and the role of PHF20L1 in regulating the outcome of DNA virus infection.