Early postnatal respiratory viral infection alters hippocampal neurogenesis, cell fate, and neuron morphology in the neonatal piglet

Adult neurogenesis and the microbiota-gut-brain axis in farm animals: underestimated and understudied parameters for improving welfare in livestock farming

Welfare in commercial livestock farming is becoming increasingly important in current agriculture research. Unfortunately, there is a lack of understanding about the neuronal mechanisms that underlie well-being on an individual level. Neuroplasticity in the hippocampus, the subventricular zone (SVZ), the olfactory bulb (OB) and the hypothalamus may be essential regulatory components in the context of farm animal behaviour and welfare that may be altered by providing environmental enrichment (EE). The importance of pre-and probiotics as a form of EE and the microbiota-gut-brain axis (MGBA) has come under the spotlight in the last 20 years, particularly in the contexts of research into stress and of stress resilience. However, it could also be an important regulatory system for animal welfare in livestock farming. This review aims to present a brief overview of the effects of EE on physiology and behaviour in farm animals and briefly discusses literature on behavioural flexibility, as well as inter-individual stress-coping styles and their relationship to animal welfare. Most importantly, we will summarise the literature on different forms of neural plasticity in farm animals, focusing on neurogenesis in various relevant brain regions. Furthermore, we will provide a brief outlook connecting these forms of neuroplasticity, stress, EE, the MGBA and welfare measures in modern livestock farming, concentrating on pigs.

Neural Injury and Repair in a Novel Neonatal Mouse Model of Listeria Monocytogenes Meningoencephalitis

To improve the therapy of neonatal central nervous system infections, well-characterized animal models are urgently needed. The present study analyzes neuropathological alterations with particular focus on neural injury and repair in brains of neonatal mice with Listeria monocytogenes (LM) meningitis/meningoencephalitis using a novel nasal infection model. The hippocampal formation and frontal cortex of 14 neonatal mice with LM meningitis/meningoencephalitis and 14 uninfected controls were analyzed by histology, immunohistochemistry, and in situ tailing for morphological alterations. In the dentate gyrus of the hippocampal formation of mice with LM meningitis/meningoencephalitis, an increased density of apoptotic neurons visualized by in situ tailing (p = 0.04) and in situ tailing plus immunohistochemistry for activated Caspase-3 (p < 0.0001) was found. A decreased density of dividing cells stained with an anti-PCNA-antibody (p < 0.0001) and less neurogenesis visualized by anti-calretinin (p < 0.0001) and anti-calbindin (p = 0.01) antibodies were detected compared to uninfected controls. The density of microglia was higher in LM meningitis (p < 0.0001), while the density of astrocytes remained unchanged. Infiltrating monocytes and neutrophilic granulocytes likely contributed to tissue damage. In conclusion, in the brains of LM-infected mice a strong immune response was observed which led to neuronal apoptosis and an impaired neural regeneration. This model appears very suitable to study therapies against long-term sequelae of neonatal LM meningitis.

The effects of early-life immune activation on microglia-mediated neuronal remodeling and the associated ontogeny of hippocampal-dependent learning in juvenile rats

Many neurodevelopmental disorders and associated learning deficits have been linked to early-life immune activation or ongoing immune dysregulation (Laskaris et al., 2016; O'Connor et al., 2014; Frick et al., 2013). Neuroscientists have begun to understand how the maturation of neural circuits allows for the emergence of cognitive and learning behaviors; yet we know very little about how these developing neural circuits are perturbed by certain events, including risk-factors such as early-life immune activation and immune dysregulation. To answer these questions, we examined the impact of early-life immune activation on the emergence of hippocampal-dependent learning in juvenile male and female rats using a well-characterized hippocampal-dependent learning task and we investigated the corresponding, dynamic multicellular interactions in the hippocampus that may contribute to these learning deficits. We found that even low levels of immune activation can result in hippocampal-depedent learning deficits days later, but only when this activation occurs during a sensitive period of development. The initial immune response and associated cytokine production in the hippocampus resolved within 24 h, several days prior to the observed learning deficit, but notably the initial immune response was followed by altered microglial-neuronal communication and synapse remodeling that changed the structure of hippocampal neurons during this period of juvenile brain development. We conclude that immune activation or dysregulation during a sensitive period of hippocampal development can precipitate the emergence of learning deficits via a multi-cellular process that may be initiated by, but not the direct result of the initial cytokine response. SIGNIFICANCE STATEMENT: Many neurodevelopmental disorders have been linked to early-life immune activation or immune dysregulation; however, very little is known about how dynamic changes in neuroimmune cells mediate the transition from normal brain function to the early stages of cognitive disorders, or how changes in immune signaling are subsequently integrated into developing neuronal networks. The current experiments examined the consequences of immune activation on the cellular and molecular changes that accompany the emergence of learning deficits during a sensitive period of hippocampal development. These findings have the potential to significantly advance our understanding of how early-life immune activation or dysregulation can result in the emergence of cognitive and learning deficits that are the largest source of years lived with disability in humans.

Immunostaining for NeuN Does Not Show all Mature and Healthy Neurons in the Human and Pig Brain: Focus on the Hippocampus

Neuronal nuclei (NeuN) is a neuron-specific nuclear protein, reported to be stably expressed in most postmitotic neurons of the vertebrate nervous system. Reduced staining has been interpreted by some to indicate loss of cell viability in human studies, while others suggest this may be because of changes in the antigenicity of the target epitope. Preliminary studies in our laboratory found low immunostaining for the NeuN antibody on formalin fixed and paraffin embedded (FFPE) human brain tissue. We report on the techniques and results used to enhance the staining for NeuN in that tissue. In parallel, we stained NeuN in piglet brain tissue, sourced from an experimental model where methodological parameters, including those for tissue fixation and storage, were tightly controlled. In human FFPE brain tissue, we were unable to enhance NeuN immunostaining to a degree sufficient for cell counting. In contrast, we found consistently high levels of staining in the piglet brain tissue. We conclude that processes used for fixation and storage of human FFPE brain tissue are responsible for the reduced staining. These results emphasize that a cautionary approach should be taken when interpreting NeuN staining outcomes in human FFPE brain tissue.

A history of juvenile mild malaria exacerbates chronic stress-evoked anxiety-like behavior, neuroinflammation, and decline of adult hippocampal neurogenesis in mice

Children residing in high malaria transmission regions are particularly susceptible to malaria. This early-life window is also a critical period for development and maturation of the nervous system, and inflammatory insults during this period may evoke a persistent increase in vulnerability for psychopathology. We employed a two-hit model of juvenile mild malaria and a two-week chronic unpredictable mild stress (CUMS) regime, commencing 60 days post-parasite clearance, to assess whether a history of juvenile infection predisposed the mice towards mood-related behavioral alterations and neurocognitive deficits. We showed that adult mice with a history of juvenile malaria (A-H/JMAL) exhibited heightened CUMS-associated anxiety-like behavior, with no observable change in cognitive behavior. In contrast, mice with a history of adult malaria did not exhibit such enhanced stress vulnerability. At baseline, A-H/JMAL mice showed increased activated microglia within the hippocampal dentate gyrus subfield. This was accompanied by a decrease in proliferating neuronal progenitors, with total number of immature hippocampal neurons unaltered. This neuroinflammatory and neurogenic decline was further exacerbated by CUMS. At day-14 post-CUMS, hippocampi of A-H/JMAL mice showed significantly higher microglial activation, and a concomitant decrease in progenitor proliferation and number of immature neurons. Taken together, these results suggest that a history of juvenile mild malaria leaves a neuroinflammatory mark within the hippocampal niche, and this may contribute to a heightened stress response in adulthood. Our findings lend credence to the idea that the burden of malaria in early-life results in sustained CNS changes that could contribute to increased vulnerability to adult-onset neuronal insults.

Effects of inflammation on the developing respiratory system: Focus on hypoglossal (XII) neuron morphology, brainstem neurochemistry, and control of breathing

Breathing is fundamental to life and any adverse change in respiratory function can endanger the health of an organism or even be fatal. Perinatal inflammation is known to adversely affect breathing in preterm babies, but lung infection/inflammation impacts all stages of life from birth to death. Little is known about the role of inflammation in respiratory control, neuronal morphology, or neural function during development. Animal models of inflammation can provide understanding and insight into respiratory development and how inflammatory processes alter developmental phenotype in addition to providing insight into new treatment modalities. In this review, we focus on recent work concerning the development of neurons, models of perinatal inflammation with an emphasis on two common LPS-based models, inflammation and its impact on development, and current and potential treatments for inflammation within the respiratory control circuitry of the mammalian brainstem. We have also discussed models of inflammation in adults and have specifically focused on hypoglossal motoneurons (XII) and neurons of the nucleus tractus solitarii (nTS) as these nuclei have been studied more extensively than other brainstem nuclei participating in breathing and airway control. Understanding the impact of inflammation on the developmental aspects of respiratory control and breathing pattern is critical to addressing problems of cardiorespiratory dysregulation in disease and this overview points out many gaps in our current knowledge.

Dietary Iron Deficiency Impaired Peripheral Immunity but Did Not Alter Brain Microglia in PRRSV-Infected Neonatal Piglets

During the postnatal period the developing brain is vulnerable to insults including nutrient insufficiency and infection that may lead to disrupted development and cognitive dysfunction. Since iron deficiency (ID) often presents with immunodeficiency, the objective of this study was to investigate peripheral viremia and inflammation as well as brain microglial phenotype and function when ID and respiratory infection occur simultaneously in a neonatal piglet model. On postnatal day 2 (PD 2) male and female piglets were assigned to one of four treatments and fed either control or ID milk replacer. On PD 8 half the pigs on each diet were inoculated with either vehicle or porcine reproductive and respiratory syndrome virus (PRRSV; P-129). Blood samples were collected prior to inoculation (PD 7) and repeated once weekly. Rectal temperature, feeding score, and sickness behavior were measured daily until PD 28. Hematocrit, hemoglobin, and serum iron were reduced by ID but not PRRSV infection. PRRSV-infected piglets displayed viremia by PD 14; however, those fed control diet had lower viral titer on PD 28, while circulating virus remained elevated in those fed an ID diet, suggesting that ID either impaired immune function necessary for viral clearance or increased viral replication. ID piglets infected with PRRSV displayed reduced sickness behavior compared to those fed control diet on PD 13-15 and 18-20. While ID piglet sickness behavior progressively worsened, piglets fed control diet displayed improved sickness score after PD 21. Microglia isolated from PRRSV piglets had increased MHCII expression and phagocytic activity ex vivo compared to uninfected piglets. ID did not alter microglial activation or phagocytic activity. Similarly, microglial cytokine expression was increased by PRRSV but unaffected by ID, in stark contrast to peripheral blood mononuclear cell (PBMC) cytokine expression, which was increased by infection and generally decreased by ID. Taken together, these data suggest that ID decreases peripheral immune function leading to increased viremia, but immune activity in the brain is protected from acute ID.

Altered Hippocampal Gene Expression and Morphology in Fetal Piglets following Maternal Respiratory Viral Infection

Maternal infection during pregnancy increases the risk of neurobehavioral problems in offspring. Evidence from rodent models indicates that the maternal immune response to infection can alter fetal brain development, particularly in the hippocampus. However, information on the effects of maternal viral infection on fetal brain development in gyrencephalic species is limited. Thus, the objective of this study was to assess several effects of maternal viral infection in the last one-third of gestation on hippocampal gene expression and development in fetal piglets. Pregnant gilts were inoculated with porcine reproductive and respiratory syndrome virus (PRRSV) at gestational day (GD) 76 and the fetuses were removed by cesarean section at GD 111 (3 days before anticipated parturition). The gilts infected with PRRSV had elevated plasma interleukin-6 levels and developed transient febrile and anorectic responses lasting approximately 21 days. Despite having a similar overall body weight, fetuses from the PRRSV-infected gilts had a decreased brain weight and altered hippocampal gene expression compared to fetuses from control gilts. Notably, maternal infection caused a reduction in estimated neuronal numbers in the fetal dentate gyrus and subiculum. The number of proliferative Ki-67+ cells was not altered, but the relative integrated density of GFAP+ staining was increased, in addition to an increase in GFAP gene expression, indicating astrocyte-specific gliosis. Maternal viral infection caused an increase in fetal hippocampal gene expression of the inflammatory cytokines TNF-α and IFN-γ and the myelination marker myelin basic protein. MHCII protein, a classic monocyte activation marker, was reduced in microglia, while expression of the MHCII gene was decreased in hippocampal tissue of the fetuses from PRRSV-infected gilts. Together, these data suggest that maternal viral infection at the beginning of the last trimester results in a reduction in fetal hippocampal neurons that is evident 5 weeks after infection, when fetal piglets are near full term. The neuronal reduction was not accompanied by pronounced neuroinflammation at GD 111, indicating that any activation of classic neuroinflammatory pathways by maternal viral infection, if present, is mostly resolved by parturition.

Simple and reproducible approaches for the collection of select porcine ganglia

BACKGROUND

The anatomy and physiology of the pig nervous system is more similar to humans compared to traditional rodent models. This makes the pig an attractive model to answer questions relating to human health and disease. Yet the technical and molecular tools available to pig researchers are limited compared to rodent researchers.

NEW METHOD

We developed simple and rapid methods to isolate the trigeminal, nodose (distal vagal), and dorsal root ganglia from neonatal pigs. We selected these ganglia due to their broad applicability to basic science researchers and clinicians.

RESULTS

Use of these methods resulted in reproducible isolation of all three types of ganglia as validated by histological examination.

COMPARISON WITH EXISTING METHOD(S)

There are currently no methods that describe a step-by-step protocol to isolate these porcine ganglia.

CONCLUSIONS

In conclusion, these methods for ganglia collection will facilitate and accelerate future neuroscience investigations in pig models of human disease.

Neurological Complications of Respiratory Disease

The respiratory and central nervous systems are intimately connected. Ventilatory control is strictly regulated by central mechanisms in a complex process that involves central and peripheral chemoreceptors, baroreceptors, the cardiovascular system, and specific areas of the brain responsible for autonomic control. Disorders of the lung and respiratory system can interfere with these mechanisms and temporarily or permanently disrupt this complex network resulting in mild to severe neurological sequelae. This article explores the wide variety of neurological problems resulting from respiratory dysfunction, with emphasis on its pathophysiology, clinical features, prognosis, and long-term outcome.

Maternal viral infection during pregnancy elicits anti-social behavior in neonatal piglet offspring independent of postnatal microglial cell activation

Maternal infection during pregnancy increases risk for neurodevelopmental disorders and reduced stress resilience in offspring, but the mechanisms are not fully understood. We hypothesized that piglets born from gilts infected with a respiratory virus during late gestation would exhibit aberrant microglia activity, cognitive deficits and reduced sociability. Pregnant gilts were inoculated with porcine reproductive and respiratory syndrome virus (PRRSV; 5×10⁵ TCID₅₀ of live PRRSV) or saline at gestational day 76. Gilts infected with PRRSV exhibited fever (p<0.01) and reduced appetite (p<0.001) for 2weekspost-inoculation and were PRRSV-positive at parturition. Piglets born from infected and control gilts were weaned at postnatal day (PD) 1 and assigned to two groups. Group 1 was challenged with lipopolysaccharide (LPS, 5μg/kg body weight i.p.) or saline on PD 14 and tissues were collected. Group 2 was tested in a T-maze task to assess spatial learning and in a 3-chamber arena with unfamiliar conspecifics to assess social behavior from PD 14-27. Microglia (CD11b⁺ CD45^low) isolated from Group 2 piglets at PD 28 were challenged ex vivo with LPS; a subset of cells was analyzed for MHCII expression. Maternal infection did not affect offspring circulating TNFα, IL-10, or cortisol levels basally or 4h post-LPS challenge. While performance in the T-maze task was not affected by maternal infection, both sociability and preference for social novelty were decreased in piglets from infected gilts. There was no effect of maternal infection on microglial MHCII expression or LPS-induced cytokine production. Taken together, these results suggest the reduced social behavior elicited by maternal infection is not due to aberrant microglia activity postnatally.

Peripheral viral infection induced microglial sensome genes and enhanced microglial cell activity in the hippocampus of neonatal piglets

Although poorly understood, early-life infection is predicted to affect brain microglial cells, making them hypersensitive to subsequent stimuli. To investigate this, we assessed gene expression in hippocampal tissue obtained from a previously published study reporting increased microglial cell activity and reduced hippocampal-dependent learning in neonatal piglets infected with porcine reproductive and respiratory syndrome virus (PRRSV), a virus that induces interstitial pneumonia. Infection altered expression of 455 genes, of which 334 were up-regulated and 121 were down-regulated. Functional annotation revealed that immune function genes were enriched among the up-regulated differentially expressed genes (DEGs), whereas calcium binding and synaptic vesicle genes were enriched among the down-regulated DEGs. Twenty-six genes encoding part of the microglia sensory apparatus (i.e., the sensome) were up-regulated (e.g., IL1R1, TLR2, and TLR4), whereas 15 genes associated with the synaptosome and synaptic receptors (e.g., NPTX2, GABRA2, and SLC5A7) were down-regulated. As the sensome may foretell microglia reactivity, we next inoculated piglets with culture medium or PRRSV at PD 7 and assessed hippocampal microglia morphology and function at PD 28 when signs of infection were waning. Consistent with amplification of the sensome, microglia from PRRSV piglets had enhanced responsiveness to chemoattractants, increased phagocytic activity, and secreted more TNFα in response to lipopolysaccharide and Poly I:C. Immunohistochemical staining indicated PRRSV infection increased microglia soma length and length-to-width ratio. Bipolar rod-like microglia not evident in hippocampus of control piglets, were present in infected piglets. Collectively, this study suggests early-life infection alters the microglia sensome as well as microglial cell morphology and function.