Sarah M. Clark

Fluoxetine rescues deficient neurogenesis in hippocampus of the Ts65Dn mouse model for Down syndrome.

The Ts65Dn mouse, an adult model of Down syndrome displays behavioral deficits consistent with a dysfunctional hippocampus, similar to that seen with DS. In looking for mechanisms underlying these performance deficits, we have assessed adult neurogenesis in the dentate gyrus of Ts65Dn. Under untreated conditions, Ts65Dn mice (2-5 months old) showed markedly fewer BrdU-labeled cells than euploid animals. Chronic antidepressant treatment for over 3 weeks with the serotonin selective reuptake inhibitor, fluoxetine, increased neurogenesis in the Ts65Dn to comparable levels seen in the euploid by augmenting both proliferation and survival of BrdU-labeled cells in the subgranular layer and granule cell layer of the hippocampus, respectively.

Quantification of Brain Maturation and Growth Patterns in C57BL/6J Mice via Computational Neuroanatomy of Diffusion Tensor Images

Diffusion Tensor magnetic resonance imaging and computational neuroanatomy are used to quantify postnatal developmental patterns of C57BL/6J mouse brain. Changes in neuronal organization and myelination occurring as the brain matures into adulthood are examined, and a normative baseline is developed, against which transgenic mice may be compared in genotype-phenotype studies. In early postnatal days, gray matter-based cortical and hippocampal structures exhibit high water diffusion anisotropy, presumably reflecting the radial neuronal organization. Anisotropy drops rapidly within a week, indicating that the underlying brain tissue becomes more isotropic in orientation, possibly due to formation of a complex randomly intertwined web of dendrites. Gradual white matter anisotropy increase implies progressively more organized axonal pathways, likely reflecting the myelination of axons forming tightly packed fiber bundles. In contrast to the spatially complex pattern of tissue maturation, volumetric growth is somewhat uniform, with the cortex and the cerebellum exhibiting slightly more pronounced growth. Temporally, structural growth rates demonstrate an initial rapid volumetric increase in most structures, gradually tapering off to a steady state by about 20 days. Fiber maturation reaches steady state in about 10 days for the cortex, to 30-40 days for the corpus callosum, the hippocampus, and the internal and external capsules.

Environmental neurotoxin-induced progressive model of parkinsonism in rats

Exposure to a number of drugs, chemicals, or environmental factors can cause parkinsonism. Epidemiologic evidence supports a causal link between the consumption of flour made from the washed seeds of the plant Cycas micronesica by the Chamorro population of Guam and the development of amyotrophic lateral sclerosis/parkinsonism dementia complex.

Sleep alterations in an environmental neurotoxin-induced model of parkinsonism

Parkinsons disease (PD) is classically defined as a motor disorder resulting from decreased dopamine production in the basal ganglia circuit. In an attempt to better diagnose and treat PD before the onset of severe motor dysfunction, recent attention has focused on the early, non-motor symptoms, which include but are not limited to sleep disorders such as excessive daytime sleepiness (EDS) and REM behavioral disorder (RBD). However, few animal models have been able to replicate both the motor and non-motor symptoms of PD. Here, we present a progressive rat model of parkinsonism that displays disturbances in sleep/wake patterns. Epidemiological studies elucidated a link between the Guamanian variant of Amyotrophic Lateral Sclerosis/Parkinsonism Dementia Complex (ALS/PDC) and the consumption of flour made from the washed seeds of the plant Cycas micronesica (cycad). Our study examined the effects of prolonged cycad consumption on sleep/wake activity in male, Sprague-Dawley rats. Cycad-fed rats exhibited an increase in length and/or number of bouts of rapid eye movement (REM) sleep and Non-REM (NREM) sleep at the expense of wakefulness during the active period when compared to control rats. This hypersomnolent behavior suggests an inability to maintain arousal. In addition, cycad-fed rats had significantly fewer orexin cells in the hypothalamus. Our results reveal a novel rodent model of parkinsonism that includes an EDS-like syndrome that may be associated with a dysregulation of orexin neurons. Further characterization of this early, non-motor symptom, may provide potential therapeutic interventions in the treatment of PD.

Defective hematopoietic stem cell and lymphoid progenitor development in the Ts65Dn mouse model of Down syndrome: potential role of oxidative stress.

AIMS Down Syndrome (DS), a genetic disease caused by a triplication of chromosome 21, is characterized by increased markers of oxidative stress. In addition to cognitive defects, patients with DS also display hematologic disorders and increased incidence of infections and leukemia. Using the Ts65Dn mouse model of DS, the goal of this study was to examine hematopoietic stem and lymphoid progenitor cell function in DS. RESULTS Analysis of hematopoietic progenitor populations showed that Ts65Dn mice possessed fewer functional hematopoietic stem cells and a significantly decreased percentage of bone marrow lymphoid progenitors. Increased reactive oxygen species and markers of oxidative stress were detected in hematopoietic stem cell populations and were associated with a loss of quiescence. Bone marrow progenitor populations expressed diminished levels of the IL-7Rα chain, which was associated with decreased proliferation and increased apoptosis. Modulating oxidative stress in vitro suggested that oxidative stress selectively leads to decreased IL-7Rα expression, and inhibits the survival of IL-7Rα-expressing hematopoietic progenitors, potentially linking increased reactive oxygen species and immunopathology. INNOVATION The study results identify a link between oxidative stress and diminished IL-7Rα expression and function. Further, the data suggest that this decrease in IL-7Rα is associated with defective hematopoietic development in Down Syndrome. CONCLUSION The data suggest that hematopoietic stem and lymphoid progenitor cell defects underlie immune dysfunction in DS and that increased oxidative stress and reduced cytokine signaling may alter hematologic development in Ts65Dn mice.

Immune status influences fear and anxiety responses in mice after acute stress exposure

Significant evidence suggests that exposure to traumatic and/or acute stress in both mice and humans results in compromised immune function that in turn may affect associated brain processes. Additionally, recent studies in mouse models of immune deficiency have suggested that adaptive immunity may play a role during traumatic stress exposure and that impairments in lymphocyte function may contribute to increased susceptibility to various psychogenic stressors. However, rodent studies on the relationship between maladaptive stress responses and lymphocyte deficiency have been complicated by the fact that genetic manipulations in these models may also result in changes in CNS function due to the expression of targeted genes in tissues other than lymphocytes, including the brain. To address these issues we utilized mice with a deletion of recombination-activating gene 2 (Rag2), which has no confirmed expression in the CNS; thus, its loss should result in the absence of mature lymphocytes without altering CNS function directly. Stress responsiveness of immune deficient Rag2(-/-) mice on a BALB/c background was evaluated in three different paradigms: predator odor exposure (POE), fear conditioning (FC) and learned helplessness (LH). These models are often used to study different aspects of stress responsiveness after the exposure to an acute stressor. In addition, immunoblot analysis was used to assess hippocampal BDNF expression under both stressed and non-stressed conditions. Subsequent to POE, Rag2(-/-) mice exhibited a reduced acoustic startle response compared to BALB/c mice; no significant differences in behavior were observed in either FC or LH. Furthermore, analysis of hippocampal BDNF indicated that Rag2(-/-) mice have elevated levels of the mature form of BDNF compared to BALB/c mice. Results from our studies suggest that the absence of mature lymphocytes is associated with increased resilience to stress exposure in the POE and does not affect behavioral responses in the FC and LH paradigms. These findings indicate that lymphocytes play a specific role in stress responsiveness dependent upon the type, nature and intensity of the stressor.

Defective thymic progenitor development and mature T-cell responses in a mouse model for Down syndrome

In addition to archetypal cognitive defects, Down syndrome (DS) is characterized by altered lymphocyte development and function, including premature thymic involution and increased incidence of infections. However, the potential mechanisms for these changes have not been fully elucidated. The current study used the Ts65Dn mouse model of DS to assess deficiencies in T‐cell development and possible molecular alterations. Ts65Dn mice exhibited premature thymic involution and a threefold to fourfold decrease in the number and proportion of immature, double‐negative thymocyte progenitors. In addition, there were twofold fewer double‐positive and CD4 single‐positive thymocytes in Ts65Dn thymuses. Reflecting this deficient thymic function, there were fewer naive T cells in the spleen and polyclonal stimulation of peripheral T cells exhibited a marked reduction in proliferation, suggesting a senescent phenotype. In contrast, B‐cell progenitors were unchanged in the bone marrow of Ts65Dn mice, but in the spleen, there were decreased transitional and follicular B cells and these cells proliferated less upon antigen receptor stimulus but not in response to lipopolysaccharide. As a potential mechanism for diminished thymic function, immature thymocyte populations expressed diminished levels of the cytokine receptor interleukin‐7Rα, which was associated with decreased proliferation and increased apoptosis. Increased oxidative stress and inhibition of the Notch pathway were identified as possible mediators of decreased interleukin‐7Rα expression in Ts65Dn mice. The data suggest that immature thymocyte defects underlie immune dysfunction in DS and that increased oxidative stress and reduced cytokine signalling may alter lymphocyte development in Ts65Dn mice.

CD4(+) T cells confer anxiolytic and antidepressant-like effects, but enhance fear memory processes in Rag2(-/-) mice.

Abstract Accumulating evidence supports a role of T cells in behavioral stress responsiveness. Our laboratory previously reported that lymphocyte deficient Rag2−/− mice on a BALB/c background display resilience to maladaptive stress responses when compared with immune competent mice in the predator odor exposure (POE) paradigm, while exhibiting similar behavior in a cued fear-conditioning (FC) paradigm. In the present study, Rag2−/− mice on a C57BL/6 background were assessed in the same behavioral paradigms, as well as additional tests of anxiety and depressive-like behavior. Furthermore, the effects of naïve CD4+  T cells were evaluated by adoptive transfer of functional cells from nonstressed, wild-type donors to Rag2−/− mice. Consistent with our prior results, Rag2−/− mice displayed an attenuated startle response after POE. Nevertheless, reconstitution of Rag2−/− mice with CD4+  T cells did not modify startle reactivity. Additionally, in contrast with our previous findings, Rag2−/− mice showed attenuated fear responses in the FC paradigm compared to wild-type mice and reconstitution with CD4+  T cells promoted fear learning and memory. Notably, reconstitution with CD4+  T cells had anxiolytic and antidepressant-like effects in Rag2−/− mice that had not been previously stressed, but had no effect after POE. Taken together, our results support a role of CD4+  T cells in emotionality, but also indicate that they may promote fear responses by enhancing learning and memory processes.

Dissociation between sickness behavior and emotionality during lipopolysaccharide challenge in lymphocyte deficient Rag2(-/-) mice.

Inflammatory diseases are highly associated with affective disorders including depression and anxiety. While the role of the innate immune system on emotionality has been extensively studied, the role of adaptive immunity is less understood. Considering that chronic inflammatory conditions are mediated largely by maladaptive lymphocyte function, the role of these cells on brain function and behavior during inflammation warrants investigation. In the present study we employed mice deficient in lymphocyte function and studied behavioral and inflammatory responses during challenge with bacterial lipopolysaccharides (LPS). Rag2(-/-) mice lacking mature lymphocytes were susceptible to death under sub-septic (5 mg/kg) doses of LPS and survived only to moderate (1 mg/kg) doses of LPS. Under these conditions, they displayed attenuated TNF-alpha responses and behavioral symptoms of sickness when compared with immunocompetent mice. Nevertheless, Rag2(-/-) mice had protracted motivational impairments after recovery from sickness suggesting a specific function for lymphocytes on the re-establishment of motivational states after activation of the innate immune system. The behavioral impairments in Rag2(-/-) mice were paralleled by an elevation in plasma corticosterone after behavioral tests. These results provide evidence that the absence of adaptive immunity may be associated with emotional deficits during inflammation and suggest that depressive states associated with medical illness may be mediated in part by impaired lymphocyte responses.

Expansion of brain T cells in homeostatic conditions in lymphopenic Rag2(-/-) mice.

The concept of the brain as an immune privileged organ is rapidly evolving in light of new findings outlining the sophisticated relationship between the central nervous and the immune systems. The role of T cells in brain development and function, as well as modulation of behavior has been demonstrated by an increasing number of studies. Moreover, recent studies have redefined the existence of a brain lymphatic system and the presence of T cells in specific brain structures, such as the meninges and choroid plexus. Nevertheless, much information is needed to further the understanding of brain T cells and their relationship with the central nervous system under non-inflammatory conditions. In the present study we employed the Rag2(-/-) mouse model of lymphocyte deficiency and reconstitution by adoptive transfer to study the temporal and anatomical expansion of T cells in the brain under homeostatic conditions. Lymphopenic Rag2(-/-) mice were reconstituted with 10 million lymphoid cells and studied at one, two and four weeks after transfer. Moreover, lymphoid cells and purified CD4(+) and CD8(+) T cells from transgenic GFP expressing mice were used to define the neuroanatomical localization of transferred cells. T cell numbers were very low in the brain of reconstituted mice up to one week after transfer and significantly increased by 2weeks, reaching wild type values at 4weeks after transfer. CD4(+) T cells were the most abundant lymphocyte subtype found in the brain followed by CD8(+) T cells and lastly B cells. Furthermore, proliferation studies showed that CD4(+) T cells expand more rapidly than CD8(+) T cells. Lymphoid cells localize abundantly in meningeal structures, choroid plexus, and circumventricular organs. Lymphocytes were also found in vascular and perivascular spaces and in the brain parenchyma across several regions of the brain, in particular in structures rich in white matter content. These results provide proof of concept that the brain meningeal system, as well as vascular and perivascular spaces, are homing sites of lymphocytes and suggest the possibility of a brain specific T cell subtype.