Inbal Goshen

Immune modulation of learning, memory, neural plasticity and neurogenesis

Over the past two decades it became evident that the immune system plays a central role in modulating learning, memory and neural plasticity. Under normal quiescent conditions, immune mechanisms are activated by environmental/psychological stimuli and positively regulate the remodeling of neural circuits, promoting memory consolidation, hippocampal long-term potentiation (LTP) and neurogenesis. These beneficial effects of the immune system are mediated by complex interactions among brain cells with immune functions (particularly microglia and astrocytes), peripheral immune cells (particularly T cells and macrophages), neurons, and neural precursor cells. These interactions involve the responsiveness of non-neuronal cells to classical neurotransmitters (e.g., glutamate and monoamines) and hormones (e.g., glucocorticoids), as well as the secretion and responsiveness of neurons and glia to low levels of inflammatory cytokines, such as interleukin (IL)-1, IL-6, and TNFα, as well as other mediators, such as prostaglandins and neurotrophins. In conditions under which the immune system is strongly activated by infection or injury, as well as by severe or chronic stressful conditions, glia and other brain immune cells change their morphology and functioning and secrete high levels of pro-inflammatory cytokines and prostaglandins. The production of these inflammatory mediators disrupts the delicate balance needed for the neurophysiological actions of immune processes and produces direct detrimental effects on memory, neural plasticity and neurogenesis. These effects are mediated by inflammation-induced neuronal hyper-excitability and adrenocortical stimulation, followed by reduced production of neurotrophins and other plasticity-related molecules, facilitating many forms of neuropathology associated with normal aging as well as neurodegenerative and neuropsychiatric diseases.

Brain interleukin-1 mediates chronic stress-induced depression in mice via adrenocortical activation and hippocampal neurogenesis suppression

Several lines of evidence implicate the pro-inflammatory cytokine interleukin-1 (IL-1) in the etiology and pathophysiology of major depression. To explore the role of IL-1 in chronic stress-induced depression and some of its underlying biological mechanisms, we used the chronic mild stress (CMS) model of depression. Mice subjected to CMS for 5 weeks exhibited depressive-like symptoms, including decreased sucrose preference, reduced social exploration and adrenocortical activation, concomitantly with increased IL-1β levels in the hippocampus. In contrast, mice with deletion of the IL-1 receptor type I (IL-1rKO) or mice with transgenic, brain-restricted overexpression of IL-1 receptor antagonist did not display CMS-induced behavioral or neuroendocrine changes. Similarly, whereas in wild-type (WT) mice CMS significantly reduced hippocampal neurogenesis, measured by incorporation of bromodeoxyuridine (BrdU) and by doublecortin immunohistochemistry, no such decrease was observed IL-1rKO mice. The blunting of the adrenocortical activation in IL-1rKO mice may play a causal role in their resistance to depression, because removal of endogenous glucocorticoids by adrenalectomy also abolished the depressive-like effects of CMS, whereas chronic administration of corticosterone for 4 weeks produced depressive symptoms and reduced neurogenesis in both WT and IL-1rKO mice. The effects of CMS on both behavioral depression and neurogenesis could be mimicked by exogenous subcutaneous administration of IL-1β via osmotic minipumps for 4 weeks. These findings indicate that elevation in brain IL-1 levels, which characterizes many medical conditions, is both necessary and sufficient for producing the high incidence of depression found in these conditions. Thus, procedures aimed at reducing brain IL-1 levels may have potent antidepressive actions.

A dual role for interleukin-1 in hippocampal-dependent memory processes

Ample research demonstrates that pathophysiological levels of the pro-inflammatory cytokine interleukin-1 (IL-1) produces detrimental effects on memory functioning. However, recent evidence suggests that IL-1 may be required for the normal physiological regulation of hippocampal-dependent memory. To substantiate the physiological role of IL-1 in learning and memory we examined the induction of IL-1 gene expression following a learning experience, and the effects of IL-1 signaling blockade, by either genetic or pharmacological manipulations, on memory functioning. We show that IL-1 gene expression is induced in the hippocampus 24h following fear-conditioning in wild type mice, but not in two mouse strains with impaired IL-1 signaling. Moreover, we report that mice with transgenic over-expression of IL-1 receptor antagonist restricted to the CNS (IL-1raTG) display impaired hippocampal-dependent and intact hippocampal-independent memory in the water maze and fear-conditioning paradigms. We further demonstrate that continuous administration of IL-1ra via osmotic minipumps during prenatal development disrupt memory performance in adult mice, suggesting that IL-1 plays a critical role not only in the formation of hippocampal-dependent memory but also in normal hippocampal development. Finally, we tested the dual role of IL-1 in memory by intracerebroventricular (ICV) administration of different doses of IL-1beta and IL-1ra following learning, providing the first systematic evidence that the involvement of IL-1 in hippocampal-dependent memory follows an inverted U-shaped pattern, i.e., a slight increase in brain IL-1 levels can improve memory, whereas any deviation from the physiological range, either by excess elevation in IL-1 levels or by blockade of IL-1 signaling, results in impaired memory.

Interleukin-1 (IL-1): A central regulator of stress responses

Ample evidence demonstrates that the pro-inflammatory cytokine interleukin-1 (IL-1), produced following exposure to immunological and psychological challenges, plays an important role in the neuroendocrine and behavioral stress responses. Specifically, production of brain IL-1 is an important link in stress-induced activation of the hypothalamus-pituitary-adrenal axis and secretion of glucocorticoids, which mediate the effects of stress on memory functioning and neural plasticity, exerting beneficial effects at low levels and detrimental effects at high levels. Furthermore, IL-1 signaling and the resultant glucocorticoid secretion mediate the development of depressive symptoms associated with exposure to acute and chronic stressors, at least partly via suppression of hippocampal neurogenesis. These findings indicate that whereas under some physiological conditions low levels of IL-1 promote the adaptive stress responses necessary for efficient coping, under severe and chronic stress conditions blockade of IL-1 signaling can be used as a preventive and therapeutic procedure for alleviating stress-associated neuropathology and psychopathology.

Brain Interleukin-1 Is Involved in Spatial Memory and Passive Avoidance Conditioning

Within the brain, the inflammatory cytokine interleukin-1 (IL-1) mediates illness-associated neural, neuroendocrine, and behavioral responses; however, its role in normal neurobehavioral processes is not clear. To examine the role of IL-1 signaling in memory, we infused Long-Evans rats intracerebroventricularly with IL-1beta (10 ng/rat), IL-1 receptor antagonist (IL-1ra, 100 microg/rat), or saline immediately following a learning task and tested memory functioning 1-8 days later. In the Morris water maze (MWM), IL-1ra caused memory impairment in the hippocampus-dependent, spatial version, whereas IL-1beta had no effect. Neither IL-1beta nor IL-1ra influenced the hippocampus-independent, nonspatial version of the MWM. In the passive avoidance response, which also depends on hippocampal functioning, IL-1ra caused memory impairment, and IL-1beta caused memory improvement. These results suggest that IL-1 signaling within the hippocampus plays a critical role in learning and memory processes.

Depression induces bone loss through stimulation of the sympathetic nervous system

Major depression is associated with low bone mass and increased incidence of osteoporotic fractures. However, causality between depression and bone loss has not been established. Here, we show that mice subjected to chronic mild stress (CMS), an established model of depression in rodents, display behavioral depression accompanied by impaired bone mass and structure, as portrayed by decreases in trabecular bone volume density, trabecular number, and trabecular connectivity density assessed in the distal femoral metaphysis and L3 vertebral body. Bone remodeling analysis revealed that the CMS-induced skeletal deficiency is accompanied by restrained bone formation resulting from reduced osteoblast number. Antidepressant therapy, which prevents the behavioral responses to CMS, completely inhibits the decrease in bone formation and markedly attenuates the CMS-induced bone loss. The depression-triggered bone loss is associated with a substantial increase in bone norepinephrine levels and can be blocked by the β-adrenergic antagonist propranolol, suggesting that the sympathetic nervous system mediates the skeletal effects of stress-induced depression. These results define a linkage among depression, excessive adrenergic activity, and reduced bone formation, thus demonstrating an interaction among behavioral responses, the brain, and the skeleton, which leads to impaired bone structure. Together with the common occurrence of depression and bone loss in the aging population, the present data implicate depression as a potential major risk factor for osteoporosis and the associated increase in fracture incidence.

Reversible modulations of neuronal plasticity by VEGF

Neurons, astrocytes, and blood vessels are organized in functional “neurovascular units” in which the vasculature can impact neuronal activity and, in turn, dynamically adjust to its change. Here we explored different mechanisms by which VEGF, a pleiotropic factor known to possess multiple activities vis-à-vis blood vessels and neurons, may affect adult neurogenesis and cognition. Conditional transgenic systems were used to reversibly overexpress VEGF or block endogenous VEGF in the hippocampus of adult mice. Importantly, this was done in settings that allowed the uncoupling of VEGF-promoted angiogenesis, neurogenesis, and memory. VEGF overexpression was found to augment all three processes, whereas VEGF blockade impaired memory without reducing hippocampal perfusion or neurogenesis. Pertinent to the general debate regarding the relative contribution of adult neurogenesis to memory, we found that memory gain by VEGF overexpression and memory impairment by VEGF blockade were already evident at early time points at which newly added neurons could not yet have become functional. Surprisingly, VEGF induction markedly increased in vivo long-term potentiation (LTP) responses in the dentate gyrus, and VEGF blockade completely abrogated LTP. Switching off ectopic VEGF production resulted in a return to a normal memory and LTP, indicating that ongoing VEGF is required to maintain increased plasticity. In summary, the study not only uncovered a surprising role for VEGF in neuronal plasticity, but also suggests that improved memory by VEGF is primarily a result of increasing plasticity of mature neurons rather than the contribution of newly added hippocampal neurons.

Interleukin-1 antagonizes morphine analgesia and underlies morphine tolerance.

Pain sensitivity reflects a balance between pain facilitatory and inhibitory systems. To characterize the relationships between these systems we examined the interactions between the analgesic effects of morphine and the anti‐analgesic effects of the pro‐inflammatory cytokine interleukin‐1 (IL‐1). We report that administration of a neutral dose of IL‐1β abolished morphine analgesia in mice, whereas acute or chronic blockade of IL‐1 signaling by various IL‐1 blockers (IL‐1 receptor antagonist (IL‐1ra), α‐melanocyte‐stimulating hormone, or IL‐1 tri‐peptide antagonist) significantly prolonged and potentiated morphine analgesia. Morphine‐induced analgesia was also extended in strains of mice genetically impaired in IL‐1 signaling (mice with transgenic over‐expression of IL‐1 receptor antagonist, deletion of the IL‐1 receptor type I, or deletion of the IL‐1 receptor accessory protein). The finding that IL‐1 produces a marked anti‐analgesic effect, suggests that it may also be involved in the development of opiate tolerance. Indeed, genetic or pharmacological blockade of IL‐1 signaling prevented the development of tolerance following repeated morphine administration. Moreover, acute administration of IL‐1ra in wild type mice, either immediately following the cessation of acute morphine analgesia, or following the development of chronic morphine tolerance, re‐instated the analgesia, suggesting that blockade of the IL‐1 system unmasks the analgesic effect of morphine. These findings suggest that morphine produces an IL‐1‐mediated homeostatic response, which serves to limit the duration and extent of morphine analgesia and which underlies the development of tolerance.

Intrahippocampal Transplantation of Transgenic Neural Precursor Cells Overexpressing Interleukin-1 Receptor Antagonist Blocks Chronic Isolation-Induced Impairment in Memory and Neurogenesis

The proinflammatory cytokine interleukin-1 (IL-1) within the brain is critically involved in mediating the memory impairment induced by acute inflammatory challenges and psychological stress. However, the role of IL-1 in memory impairment and suppressed neurogenesis induced by chronic stress exposure has not been investigated before now. We report here that mice that were isolated for 4 weeks displayed a significant elevation in hippocampal IL-1β levels concomitantly with body weight loss, specific impairment in hippocampal-dependent memory, and decreased hippocampal neurogenesis. To examine the causal role of IL-1 in these effects, we developed a novel approach for long-term delivery of IL-1 receptor antagonist (IL-1ra) into the brain, using transplantation of neural precursor cells (NPCs), obtained from neonatal mice with transgenic overexpression of IL-1ra (IL-1raTG) under the glial fibrillary acidic protein promoter. Four weeks following intrahippocampal transplantation of IL-1raTG NPCs labeled with PKH-26, the transplanted cells were incorporated within the dentate gyrus and expressed mainly astrocytic markers. IL-1ra levels were markedly elevated in the hippocampus, but not in other brain regions, by 10 days and for at least 4 weeks post-transplantation. Transplantation of IL-1raTG NPCs completely rescued the chronic isolation-induced body weight loss, memory impairment, and suppressed hippocampal neurogenesis, compared with isolated mice transplanted with WT cells or sham operated. The transplantation had no effect in group-housed mice. These findings elucidate the role of IL-1 in the pathophysiology of chronic isolation and suggest that transplantation of IL-1raTG NPCs may provide a useful therapeutic procedure for IL-1-mediated memory disturbances in chronic inflammatory and neurological conditions.

Impairment of interleukin-1 (IL-1) signaling reduces basal pain sensitivity in mice: genetic, pharmacological and developmental aspects

The cytokine interleukin‐1 (IL‐1) has been implicated in modulation of pain perception under various inflammatory conditions. The present study examined the hypothesis that IL‐1 signaling is also involved in pain sensitivity under normal, non‐inflammatory states, using three mouse models of impaired IL‐1 signaling: targeted deletion of the IL‐1 receptor type I or the IL‐1 receptor accessory protein, and transgenic over‐expression of IL‐1 receptor antagonist within the brain and spinal cord. Thermal and mechanical pain sensitivity was assessed using the paw‐flick, hot‐plate, and von Frey tests. All mutant strains displayed significantly lower pain sensitivity, compared with their respective wild‐type control strains, and with their parent strains (C57BL/6, CBA and 129), in all tests. In contrast, mice with targeted deletion of the p55 or p75 TNF receptor, or of interleukin‐18, displayed normal or higher pain sensitivity compared to their respective controls. To differentiate between developmental vs. on‐going effects of IL‐1, mice were chronically treated with IL‐1 receptor antagonist (IL‐1ra) via osmotic micropumps, either in adulthood or prenatally (throughout the last 2 weeks of gestation). Adult mice that were treated with IL‐1ra either in adulthood or in utero, displayed lower pain sensitivity, similar to mice with impaired IL‐1 signaling. These findings suggest that basal pain sensitivity is genetically, developmentally and tonically influenced by IL‐1 signaling.