The recombination activation gene 1 (Rag1) is expressed in a subset of zebrafish olfactory neurons but is not essential for axon targeting or amino acid detection

Rag2-/- accelerates lipofuscin accumulation in the brain: Implications for human stem cell brain transplantation studies

Immunodeficient mice are widely used in human stem cell transplantation research. Recombination activating gene 1 (Rag1) deletion results in immunodeficiency and leads to accelerated aging in zebrafish with increased cytosolic accumulation of lipofuscin (LF). Unlike zebrafish, mammals have two homologs, Rag1 and Rag2, that regulate adaptive immunity. Currently, little is known if and how Rag1-/- and Rag2-/- may impact aging and LF accumulation in immunodeficient mouse brains and how this may confound results in human neural cell transplantation studies. Here, we demonstrate that in Rag2-/- mouse brains, LF appears early, spreads broadly, emits strong autofluorescence, and accumulates with age. LF is found in various types of glial cells, including xenografted human microglia. Surprisingly, in Rag1-/- mouse brains, LF autofluorescence is seen at much older ages compared with Rag2-/- brains. This study provides direct evidence that Rag2-/- expedites LF occurrence and sets a context for studies using aged immunodeficient mice.

Modulation of olfactory signal detection in the olfactory epithelium: focus on the internal and external environment, and the emerging role of the immune system

Detection and discrimination of odorants by the olfactory system plays a pivotal role in animal survival. Olfactory-based behaviors must be adapted to an ever-changing environment. Part of these adaptations includes changes of odorant detection by olfactory sensory neurons localized in the olfactory epithelium. It is now well established that internal signals such as hormones, neurotransmitters, or paracrine signals directly affect the electric activity of olfactory neurons. Furthermore, recent data have shown that activity-dependent survival of olfactory neurons is important in the olfactory epithelium. Finally, as olfactory neurons are directly exposed to environmental toxicants and pathogens, the olfactory epithelium also interacts closely with the immune system leading to neuroimmune modulations. Here, we review how detection of odorants can be modulated in the vertebrate olfactory epithelium. We choose to focus on three cellular types of the olfactory epithelium (the olfactory sensory neuron, the sustentacular and microvillar cells) to present the diversity of modulation of the detection of odorant in the olfactory epithelium. We also present some of the growing literature on the importance of immune cells in the functioning of the olfactory epithelium, although their impact on odorant detection is only just beginning to be unravelled.

Stimulus-specific behavioral responses of zebrafish to a large range of odors exhibit individual variability

BACKGROUND

Odor-driven behaviors such as feeding, mating, and predator avoidance are crucial for animal survival. The neural pathways processing these behaviors have been well characterized in a number of species, and involve the activity of diverse brain regions following stimulation of the olfactory bulb by specific odors. However, while the zebrafish olfactory circuitry is well understood, a comprehensive characterization linking odor-driven behaviors to specific odors is needed to better relate olfactory computations to animal responses.

RESULTS

Here, we used a medium-throughput setup to measure the swimming trajectories of 10 zebrafish in response to 17 ecologically relevant odors. By selecting appropriate locomotor metrics, we constructed ethograms systematically describing odor-induced changes in the swimming trajectory. We found that adult zebrafish reacted to most odorants using different behavioral programs and that a combination of a few relevant behavioral metrics enabled us to capture most of the variance in these innate odor responses. We observed that individual components of natural food and alarm odors do not elicit the full behavioral response. Finally, we show that zebrafish blood elicits prominent defensive behaviors similar to those evoked by skin extract and activates spatially overlapping olfactory bulb domains.

CONCLUSION

Altogether, our results highlight a prominent intra- and inter-individual variability in zebrafish odor-driven behaviors and identify a small set of waterborne odors that elicit robust responses. Our behavioral setup and our results will be useful resources for future studies interested in characterizing innate olfactory behaviors in aquatic animals.

Immunization Elicits Antigen-Specific Antibody Sequestration in Dorsal Root Ganglia Sensory Neurons

The immune and nervous systems are two major organ systems responsible for host defense and memory. Both systems achieve memory and learning that can be retained, retrieved, and utilized for decades. Here, we report the surprising discovery that peripheral sensory neurons of the dorsal root ganglia (DRGs) of immunized mice contain antigen-specific antibodies. Using a combination of rigorous molecular genetic analyses, transgenic mice, and adoptive transfer experiments, we demonstrate that DRGs do not synthesize these antigen-specific antibodies, but rather sequester primarily IgG1 subtype antibodies. As revealed by RNA-seq and targeted quantitative PCR (qPCR), dorsal root ganglion (DRG) sensory neurons harvested from either naïve or immunized mice lack enzymes (i.e., RAG1, RAG2, AID, or UNG) required for generating antibody diversity and, therefore, cannot make antibodies. Additionally, transgenic mice that express a reporter fluorescent protein under the control of Igγ1 constant region fail to express Ighg1 transcripts in DRG sensory neurons. Furthermore, neural sequestration of antibodies occurs in mice rendered deficient in neuronal Rag2, but antibody sequestration is not observed in DRG sensory neurons isolated from mice that lack mature B cells [e.g., Rag1 knock out (KO) or μMT mice]. Finally, adoptive transfer of Rag1-deficient bone marrow (BM) into wild-type (WT) mice or WT BM into Rag1 KO mice revealed that antibody sequestration was observed in DRG sensory neurons of chimeric mice with WT BM but not with Rag1-deficient BM. Together, these results indicate that DRG sensory neurons sequester and retain antigen-specific antibodies released by antibody-secreting plasma cells. Coupling this work with previous studies implicating DRG sensory neurons in regulating antigen trafficking during immunization raises the interesting possibility that the nervous system collaborates with the immune system to regulate antigen-mediated responses.

Identification and Characterization of the V(D)J Recombination Activating Gene 1 in Long-Term Memory of Context Fear Conditioning

An increasing body of evidence suggests that mechanisms related to the introduction and repair of DNA double strand breaks (DSBs) may be associated with long-term memory (LTM) processes. Previous studies from our group suggested that factors known to function in DNA recombination/repair machineries, such as DNA ligases, polymerases, and DNA endonucleases, play a role in LTM. Here we report data using C57BL/6 mice showing that the V(D)J recombination-activating gene 1 (RAG1), which encodes a factor that introduces DSBs in immunoglobulin and T-cell receptor genes, is induced in the amygdala, but not in the hippocampus, after context fear conditioning. Amygdalar induction of RAG1 mRNA, measured by real-time PCR, was not observed in context-only or shock-only controls, suggesting that the context fear conditioning response is related to associative learning processes. Furthermore, double immunofluorescence studies demonstrated the neuronal localization of RAG1 protein in amygdalar sections prepared after perfusion and fixation. In functional studies, intra-amygdalar injections of RAG1 gapmer antisense oligonucleotides, given 1 h prior to conditioning, resulted in amygdalar knockdown of RAG1 mRNA and a significant impairment in LTM, tested 24 h after training. Overall, these findings suggest that the V(D)J recombination-activating gene 1, RAG1, may play a role in LTM consolidation.

Impaired sense of smell and altered olfactory system in RAG-1(-∕-) immunodeficient mice

Immune deficiencies are often associated with a number of physical manifestations including loss of sense of smell and an increased level of anxiety. We have previously shown that T and B cell-deficient recombinase activating gene (RAG-1)(-∕-) knockout mice have an increased level of anxiety-like behavior and altered gene expression involved in olfaction. In this study, we expanded these findings by testing the structure and functional development of the olfactory system in RAG-1 (-∕-) mice. Our results show that these mice have a reduced engagement in different types of odors and this phenotype is associated with disorganized architecture of glomerular tissue and atrophy of the main olfactory epithelium. Most intriguingly this defect manifests specifically in adult age and is not due to impairment in the patterning of the olfactory neuron staining at the embryo stage. Together these findings provide a formerly unreported biological evidence for an altered function of the olfactory system in RAG-1 (-∕-) mice.

Physiological significance of recombination-activating gene 1 in neuronal death, especially optic neuropathy

Although the transcription factor nuclear factor-κB (NF-κB) is known to regulate cell death and survival, its precise role in cell death within the central nervous system remains unknown. We previously reported that mice with a homozygous deficiency for NF-κBp50 spontaneously develop optic neuropathy. The aim of the present study was to demonstrate the expression and activation of the proapoptotic factor(s) that mediate optic neuropathy in p50-deficient mice. Recombination-activating gene (Rag) 1 is known to activate the recombination of immunoglobulin V(D)J. In this study, experiments with genetically engineered mice revealed the involvement of Rag1 expression in apoptosis of Brn3a-positive retinal ganglion cells, and also demonstrated the specific effect of p50 deficiency on the activation of Rag1 gene transcription. Furthermore, genetic analysis of murine neuronal stem-like cells clarified the biological significance of Rag1 in N-methyl-D-aspartate-induced neuronal apoptosis. We also detected the apoptosis-regulating factors Bax and cleaved caspase 3, 8 and 9 in HEK293 cells transfected-molecule of Rag1, and a human histological examination revealed the expression of Rag1 in retinal ganglion cells. The results of the present study indicate that Rag1 plays a role in optic neuropathy as a proapoptotic candidate in p50-deficient mice. This finding may lead to new therapeutic targets in optic neuropathy.

The alarm response in zebrafish: innate fear in a vertebrate genetic model

The alarm response is an antipredator behavior displayed by many fish species and was first described 70 years ago. It is triggered through the olfactory system by substances released from injured skin and is characterized by dramatic, measurable changes in locomotion as well as physiology. We propose that this is an ideal time to revisit this response and to utilize it as an assay for understanding how neural circuits mediate innate fear. A suitable organism for these studies is the zebrafish, a genetic model with a rapidly expanding toolkit for molecular manipulation of the nervous system. Individual neurons mediating the response, ranging from receptor neurons to those in higher brain centers, should first be identified. New tools, specifically transgenic lines that allow spatial and temporal control of neural activity, provide a way to define and test the role of specific neurons, while genetic screens provide a route to identifying individual molecules essential for a normal response. Optical recording, which has proven successful in studies of information processing in the bulb, will provide valuable insights into neural circuitry function during the alarm response. When carried out on mutants, physiological analysis can provide insight into aspects of signal processing that are essential for normal behavior. The alarm response thus provides a paradigm to examine innate fear in a vertebrate system, enabling analysis at multiple levels from genes to the entire neural circuit. Additionally, the context dependency of the response can be utilized to investigate attention and decision making.

Impaired social recognition memory in recombination activating gene 1-deficient mice

The recombination activating genes (RAGs) encode two enzymes that play key roles in the adaptive immune system. RAG1 and RAG2 mediate VDJ recombination, a process necessary for the maturation of B- and T-cells. Interestingly, RAG1 is also expressed in the brain, particularly in areas of high neural density such as the hippocampus, although its function is unknown. We tested evidence that RAG1 plays a role in brain function using a social recognition memory task, an assessment of the acquisition and retention of conspecific identity. In a first experiment, we found that RAG1-deficient mice show impaired social recognition memory compared to mice wildtype for the RAG1 allele. In a second experiment, by breeding to homogenize background genotype, we found that RAG1-deficient mice show impaired social recognition memory relative to heterozygous or RAG2-deficient littermates. Because RAG1 and RAG2 null mice are both immunodeficient, the results suggest that the memory impairment is not an indirect effect of immunological dysfunction. RAG1-deficient mice show normal habituation to non-socially derived odors and habituation to an open-field, indicating that the observed effect is not likely a result of a general deficit in habituation to novelty. These data trace the origin of the impairment in social recognition memory in RAG1-deficient mice to the RAG1 gene locus and implicate RAG1 in memory formation.

Developing T lymphocytes are uniquely sensitive to a lack of topoisomerase III alpha

All organisms possess at least one type IA DNA topoisomerase. These topoisomerases function as part of a DNA structure-specific "dissolvasome," also known as the RTR complex, which has critical functions in faithful DNA replication, recombination, and chromosome segregation. In humans, the heteromeric RTR complex consists of RMI1, RMI2, the Bloom's syndrome gene product (BLM), and topoisomerase 3A (TOP3A) proteins. Here, we describe the identification and characterization of two deleterious mutations in the zebrafish top3a gene that reveal an unexpected tissue-specific requirement of top3a function in developing thymocytes. Deficiency in top3a activates a p53-dependent check-point but does not affect VDJ recombination. Our results suggest that TOP3A could be a candidate gene involved in human primary immunodeficiency syndromes.

Neurogenesis

For more than a decade, the zebrafish has proven to be an excellent model organism to investigate the mechanisms of neurogenesis during development. The often cited advantages, namely external development, genetic, and optical accessibility, have permitted direct examination and experimental manipulations of neurogenesis during development. Recent studies have begun to investigate adult neurogenesis, taking advantage of its widespread occurrence in the mature zebrafish brain to investigate the mechanisms underlying neural stem cell maintenance and recruitment. Here we provide a comprehensive overview of the tools and techniques available to study neurogenesis in zebrafish both during development and in adulthood. As useful resources, we provide tables of available molecular markers, transgenic, and mutant lines. We further provide optimized protocols for studying neurogenesis in the adult zebrafish brain, including in situ hybridization, immunohistochemistry, in vivo lipofection and electroporation methods to deliver expression constructs, administration of bromodeoxyuridine (BrdU), and finally slice cultures. These currently available tools have put zebrafish on par with other model organisms used to investigate neurogenesis.

Characterization of rag1 mutant zebrafish leukocytes

Background

Zebrafish may prove to be one of the best vertebrate models for innate immunology. These fish have sophisticated immune components, yet rely heavily on innate immune mechanisms. Thus, the development and characterization of mutant and/or knock out zebrafish are critical to help define immune cell and immune gene functions in the zebrafish model. The use of Severe Combined Immunodeficient (SCID) and recombination activation gene 1 and 2 mutant mice has allowed the investigation of the specific contribution of innate defenses in many infectious diseases. Similar zebrafish mutants are now being used in biomedical and fish immunology related research. This report describes the leukocyte populations in a unique model, recombination activation gene 1^-/- mutant zebrafish (rag1 mutants).

Results

Differential counts of peripheral blood leukocytes (PBL) showed that rag1 mutants had significantly decreased lymphocyte-like cell populations (34.7%) compared to wild-types (70.5%), and significantly increased granulocyte populations (52.7%) compared to wild-types (17.6%). Monocyte/macrophage populations were similar between mutants and wild-types, 12.6% and 11.3%, respectively. Differential leukocyte counts of rag1 mutant kidney hematopoietic tissue showed a significantly reduced lymphocyte-like cell population (8%), a significantly increased myelomonocyte population (57%), 34.8% precursor cells, and 0.2% thrombocytes, while wild-type hematopoietic kidney tissue showed 29.4% lymphocytes/lymphocyte-like cells, 36.4% myelomonocytes, 33.8% precursors and 0.5% thrombocytes.

Flow cytometric analyses of kidney hematopoietic tissue revealed three leukocyte populations. Population A was monocytes and granulocytes and comprised 34.7% of the gated cells in rag1 mutants and 17.6% in wild-types. Population B consisted of hematopoietic precursors, and comprised 50% of the gated cells for rag1 mutants and 53% for wild-types. Population C consisted of lymphocytes and lymphocyte-like cells and comprised 7% of the gated cells in the rag1 mutants and 26% in the wild-types.

Reverse transcriptase polymerase chain reaction (RT-PCR) assays demonstrated rag1 mutant kidney hematopoietic tissue expressed mRNA encoding Non-specific Cytotoxic cell receptor protein-1 (NCCRP-1) and Natural Killer (NK) cell lysin but lacked T cell receptor (TCR) and immunoglobulin (Ig) transcript expression, while wild-type kidney hematopoietic tissue expressed NCCRP-1, NK lysin, TCR and Ig transcript expression.

Conclusion

Our study demonstrates that in comparison to wild-type zebrafish, rag1 mutants have a significantly reduced lymphocyte-like cell population that likely includes Non-specific cytotoxic cells (NCC) and NK cells (and lacks functional T and B lymphocytes), a similar macrophage/monocyte population, and a significantly increased neutrophil population. These zebrafish have comparable leukocyte populations to SCID and rag 1 and/or 2 mutant mice, that possess macrophages, natural killer cells and neutrophils, but lack T and B lymphocytes. Rag1 mutant zebrafish will provide the platform for remarkable investigations in fish and innate immunology, as rag 1 and 2 mutant mice did for mammalian immunology.