ATR signaling controls the bystander responses of human chondrosarcoma cells by promoting RAD51-dependent DNA repair

PURPOSE

Radiation-induced bystander effect (RIBE) frequently is seen as DNA damage in unirradiated bystander cells, but the repair processes initiated in response to that DNA damage are not well understood. RIBE-mediated formation of micronuclei (MN), a biomarker of persistent DNA damage, was previously observed in bystander normal fibroblast (AG01522) cells, but not in bystander human chondrosarcoma (HTB94) cells. The molecular mechanisms causing this disparity are not clear. Herein, we investigate the role of DNA repair in the bystander responses of the two cell lines.

METHODS

Cells were irradiated with X-rays and immediately co-cultured with un-irradiated cells using a trans-well insert system in which they share the same medium. The activation of DNA damage response (DDR) proteins was detected by immunofluorescence staining or Western blotting. MN formation was examined by the cytokinesis-block MN assay, which is a robust method to detect persistent DNA damage.

RESULTS

Immunofluorescent foci of γH2AX and 53BP1, biomarkers of DNA damage and repair, revealed a greater capacity for DNA repair in HTB94 cells than in AG01522 cells in both irradiated and bystander populations. Autophosphorylation of ATR at the threonine 1989 site was expressed at a greater level in HTB94 cells compared to AG01522 cells at the baseline and in response to hydroxyurea treatment or exposure to 1 Gy of X-rays. An inhibitor of ATR, but not of ATM, promoted MN formation in bystander HTB94 cells. In contrast, no effect of either inhibitor was observed in bystander AG01522 cells, indicating that ATR signaling might be a pivotal pathway to preventing the MN formation in bystander HTB94 cells. Supporting this idea, we found an ATR-dependent increase in the fractions of bystander HTB94 cells with pRPA2 S33 and RAD51 foci. A blocker of RAD51 facilitated MN formation in bystander HTB94 cells.

CONCLUSION

Our results indicate that HTB94 cells were likely more efficient in DNA repair than AG01522 cells, specifically via ATR signaling, which inhibited the bystander signal-induced MN formation. This study highlights the significance of DNA repair efficiency in bystander cell responses.

Easy and Reproducible Low-Density Primary Culture using Frozen Stock of Embryonic Hippocampal Neurons

Neuronal culture is a valuable system for evaluating synaptic functions and drug screenings. In particular, a low-density culture of primary hippocampal neurons allows the study of individual neurons or subcellular components. We have shown subcellular protein localization within a neuron by immunocytochemistry, neuronal polarity, synaptic morphology, and its developmental change using a low-density primary hippocampal culture. Recently, ready-to-use frozen stocks of neurons have become commercially available. These frozen stocks of neurons reduce the time needed to prepare animal experiments and also contribute to the reduction of the number of animals used. Here, we introduce a reproducible low-density primary culture method using a 96-well plate. We used a commercially available frozen stock of neurons from the rat embryonic hippocampus. The neurons can be stably cultured long-term without media changes by reducing the growth of glial cells at particular timepoints. This high-throughput assay using low-density culture allows reproducible imaging-based evaluations of synaptic plasticity.

Single-nucleus RNA sequencing reveals the shared mechanisms inducing cognitive impairment between COVID-19 and Alzheimer’s disease

Alzheimer’s disease (AD)-like cognitive impairment, a kind of Neuro-COVID syndrome, is a reported complication of SARS-CoV-2 infection. However, the specific mechanisms remain largely unknown. Here, we integrated single-nucleus RNA-sequencing data to explore the potential shared genes and pathways that may lead to cognitive dysfunction in AD and COVID-19. We also constructed ingenuity AD-high-risk scores based on AD-high-risk genes from transcriptomic, proteomic, and Genome-Wide Association Studies (GWAS) data to identify disease-associated cell subtypes and potential targets in COVID-19 patients. We demonstrated that the primary disturbed cell populations were astrocytes and neurons between the above two dis-eases that exhibit cognitive impairment. We identified significant relationships between COVID-19 and AD involving synaptic dysfunction, neuronal damage, and neuroinflammation. Our findings may provide new insight for future studies to identify novel targets for preventive and therapeutic interventions in COVID-19 patients.

Distinct but overlapping roles of LRRTM1 and LRRTM2 in developing and mature hippocampal circuits

LRRTMs are postsynaptic cell adhesion proteins that have region-restricted expression in the brain. To determine their role in the molecular organization of synapses in vivo, we studied synapse development and plasticity in hippocampal neuronal circuits in mice lacking both Lrrtm1 and Lrrtm2. We found that LRRTM1 and LRRTM2 regulate the density and morphological integrity of excitatory synapses on CA1 pyramidal neurons in the developing brain but are not essential for these roles in the mature circuit. Further, they are required for long-term-potentiation in the CA3-CA1 pathway and the dentate gyrus, and for enduring fear memory in both the developing and mature brain. Our data show that LRRTM1 and LRRTM2 regulate synapse development and function in a cell-type and developmental-stage-specific manner, and thereby contribute to the fine-tuning of hippocampal circuit connectivity and plasticity.

LRRTMs Organize Synapses through Differential Engagement of Neurexin and PTPσ

Presynaptic neurexins (Nrxs) and type IIa receptor-type protein tyrosine phosphatases (RPTPs) organize synapses through a network of postsynaptic ligands. We show that leucine-rich-repeat transmembrane neuronal proteins (LRRTMs) differentially engage the protein domains of Nrx but require its heparan sulfate (HS) modification to induce presynaptic differentiation. Binding to the HS of Nrx is sufficient for LRRTM3 and LRRTM4 to induce synaptogenesis. We identify mammalian Nrx1γ as a potent synapse organizer and reveal LRRTM4 as its postsynaptic ligand. Mice expressing a mutant form of LRRTM4 that cannot bind to HS show structural and functional deficits at dentate gyrus excitatory synapses. Through the HS of Nrx, LRRTMs also recruit PTPσ to induce presynaptic differentiation but function to varying degrees in its absence. PTPσ forms a robust complex with Nrx, revealing an unexpected interaction between the two presynaptic hubs. These findings underscore the complex interplay of synapse organizers in specifying the molecular logic of a neural circuit.

Low-Density Primary Hippocampal Neuron Culture

The ability to probe the structure and physiology of individual nerve cells in culture is crucial to the study of neurobiology, and allows for flexibility in genetic and chemical manipulation of individual cells or defined networks. Such ease of manipulation is simpler in the reduced culture system when compared to the intact brain tissue. While many methods for the isolation and growth of these primary neurons exist, each has its own limitations. This protocol describes a method for culturing low-density and high-purity rodent embryonic hippocampal neurons on glass coverslips, which are then suspended over a monolayer of glial cells. This 'sandwich culture' allows for exclusive long-term growth of a population of neurons while allowing for trophic support from the underlying glial monolayer. When neurons are of sufficient age or maturity level, the neuron coverslips can be flipped-out of the glial dish and used in imaging or functional assays. Neurons grown by this method typically survive for several weeks and develop extensive arbors, synaptic connections, and network properties.

Role of LRRTMs in synapse development and plasticity

Leucine-rich-repeat transmembrane neuronal proteins (LRRTMs) are a family of four synapse organizing proteins critical for the development and function of excitatory synapses. The genes encoding LRRTMs and their binding partners, neurexins and HSPGs, are strongly associated with multiple psychiatric disorders. Here, we review the literature covering their structural features, expression patterns in the developing and adult brains, evolutionary origins, and discovery as synaptogenic proteins. We also discuss their role in the development and plasticity of excitatory synapses as well as their disease associations.

Early-stage development of human induced pluripotent stem cell-derived neurons

Recent advances in human induced pluripotent stem cells (hiPSCs) offer new possibilities for biomedical research and clinical applications. Differentiated neurons from hiPSCs are expected to be useful for developing novel methods of treatment for various neurological diseases. However, the detailed process of functional maturation of hiPSC-derived neurons (hiPS neurons) remains poorly understood. This study analyzes development of hiPS neurons, focusing specifically on early developmental stages through 48 hr after cell seeding; development was compared with that of primary cultured neurons derived from the rat hippocampus. At 5 hr after cell seeding, neurite formation occurs in a similar manner in both neuronal populations. However, very few neurons with axonal polarization were observed in the hiPS neurons even after 48 hr, indicating that hiPS neurons differentiate more slowly than rat neurons. We further investigated the elongation speed of axons and found that hiPS neuronal axons were slower. In addition, we characterized the growth cones. The localization patterns of skeletal proteins F-actin, microtubule, and drebrin were similar to those of rat neurons, and actin depolymerization by cytochalasin D induced similar changes in cytoskeletal distribution in the growth cones between hiPS neurons and rat neurons. These results indicate that, during the very early developmental stage, hiPS neurons develop comparably to rat hippocampal neurons with regard to axonal differentiation, but the growth of axons is slower.

Spikar, a novel drebrin-binding protein, regulates the formation and stabilization of dendritic spines

Dendritic spines are small, actin-rich protrusions on dendrites, the development of which is fundamental for the formation of neural circuits. The actin cytoskeleton is central to dendritic spine morphogenesis. Drebrin is an actin-binding protein that is thought to initiate spine formation through a unique drebrin-actin complex at postsynaptic sites. However drebrin overexpression in neurons does not increase the final density of dendritic spines. In this study, we have identified and characterized a novel drebrin-binding protein, spikar. Spikar is localized in cell nuclei and dendritic spines, and accumulation of spikar in dendritic spines directly correlates with spine density. A reporter gene assay demonstrated that spikar acts as a transcriptional co-activator for nuclear receptors. We found that dendritic spine, but not nuclear, localization of spikar requires drebrin. RNA-interference knockdown and overexpression experiments demonstrated that extranuclear spikar regulates dendritic spine density by modulating de novo spine formation and retraction of existing spines. Unlike drebrin, spikar does not affect either the morphology or function of dendritic spines. These findings indicate that drebrin-mediated postsynaptic accumulation of spikar regulates spine density, but is not involved in regulation of spine morphology.

Selective reduction of drebrin and actin in dendritic spines of hippocampal neurons by activation of 5-HT(2A) receptors

Abnormal architecture of dendritic spines is associated with neurodevelopmental and neurodegenerative diseases. The 5-HT(2A) receptor is a potential therapeutic target for mental illnesses and it is functionally and genetically associated with many types of psychiatric disorders. It has been reported that 5-HT(2A) receptor activation alters spine architecture. Although actin cytoskeleton has a key role in the regulation of spine architecture, it is not clarified whether 5-HT(2A)+ receptor activation affect the actin cytoskeleton in dendritic spines. In the present study, we examined the effect of 5-HT(2A) receptor activation on the actin cytoskeleton in dendritic spines of mature hippocampal neurons in low-density culture. Immunocytochemical analysis showed that 15 min exposure of 5-HT(2A) receptor agonist (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) significantly decreased the cluster densities of drebrin (control, 37.0±6.9 per 100 μm, DOI, 12.5±2.9) and F-actin (control, 18.3±4.9; DOI, 7.7±2.1) at dendritic spines without any detectable changes in the cluster densities of synapsin I and PSD-95. At the same time period DOI exposure did not affect spine architecture (spine density: control, 38.3±5.1 per 100 μm; DOI, 25.6±3.5; spine length: control, 1.99±0.18; DOI, 2.00±0.29; spine width: control, 0.72±0.06; DOI, 0.77±0.11). Thus, it is indicated that decrease of drebrin and F-actin can occur at the dendritic spines without morphological changes. Together our data suggest that 5-HT(2A) receptors activation is involved in the regulation of distribution of cytoskeleton in the dendritic spines.