Strongly compromised inflammatory response to brain injury in interleukin-6-deficient mice

Injury to the central nervous system (CNS) elicits an inflammatory response involving activation of microglia, brain macrophages, and astrocytes, processes likely mediated by the release of proinflammatory cytokines. In order to determine the role of interleukin-6 (IL-6) during the inflammatory response in the brain following disruption of the blood-brain barrier (BBB), we examined the effects of a focal cryo injury to the fronto-parietal cortex in interleukin-6-deficient (IL-6-/-) and normal (IL-6+/+) mice. In IL-6+/+ mice, brain injury resulted in the appearance of brain macrophages and reactive astrocytes surrounding the lesion site. In addition, expression of granulocyte-macrophage colony-stimulating factor (GM-CSF) and metallothionein-I+II (MT-I+II) were increased in these cells, while the brain-specific MT-III was only moderately upregulated. In IL-6-/- mice, however, the response of brain macrophages and reactive astrocytes was markedly depressed and the number of NSE positive neurons was reduced. Brain damage-induced GM-CSF and MT-I+II expression were also markedly depressed compared to IL-6+/+ mice. In contrast, MT-III immunoreactivity was markedly increased in brain macrophages and astrocytes. In situ hybridization analysis indicates that MT-I+II but not MT-III immunoreactivity reflect changes in the messenger levels. The number of cell divisions was similar in IL-6+/+ and IL-6-/- mice. The present results demonstrate that IL-6 is crucial for the recruitment of myelo-monocytes and activation of glial cells following brain injury with disrupted BBB. Furthermore, our results suggest IL-6 is important for neuroprotection and the induction of GM-CSF and MT expression. The opposing effect of IL-6 on MT-I+II and MT-III levels in the damaged brain suggests MT isoform-specific functions.     

Adult neurogenesis in serotonin transporter deficient mice

Summary Serotonin (5-HT) is a regulator of morphogenetic activities during early brain development and neurogenesis, including cell proliferation, migration, differentiation, and synaptogenesis. The 5-HT transporter (5-HTT, SLC6A4) mediates high-affinity reuptake of 5-HT into presynaptic terminals and thereby fine-tunes serotonergic neurotransmission. Inactivation of the 5-HTT gene in mice reduces 5-HT clearance resulting in persistently increased concentrations of synaptic 5-HT. In the present study, we investigated the effects of elevated 5-HT levels on adult neurogenesis in the hippocampus of 5-HTT deficient mice, including stem cell proliferation, survival, and differentiation. Using an in vivo approach, we showed an increase in proliferative capacity of hippocampal adult neural stem cells in aged 5-HTT knockout mice (∼14.5 months) compared to wildtype controls. In contrast, in vivo and additional in vitro analyses of younger adult 5-HTT knockout mice (∼7 weeks and ∼3.0 months) did not reveal significant changes in proliferation of neural stem cells or survival of newborn cells. We showed that the cellular fate of newly generated cells in 5-HTT knockout mice is not different with respect to the total number and percentage of neurons or glial cells from wildtype controls. Our findings indicate that elevated synaptic 5-HT concentration throughout early development and later life of 5-HTT deficient mice does not induce adult neurogenesis in adult mice, but that elevated 5-HT levels in aged mice influence stem cell proliferation. First two authors contributed equally to this work     

Mu-opioid receptor knockout mice show diminished food-anticipatory activity

We have previously suggested that during or prior to activation of anticipatory behaviour to a coming reward, mu-opioid receptors are activated. To test this hypothesis schedule induced food-anticipatory activity in mu-opioid receptor knockout mice was measured using running wheels. We hypothesized that mu-knockout mice show little food-anticipatory activity. In wildtype mice we observed that food-anticipatory activity increased proportional to reduced food intake levels during daily scheduled food access, and thus reflects the animal's physiological need for food. mu-Knockout mice do not adjust their schedule induced running wheel behaviour prior to and during feeding time in the same way as wildtype mice; rather than showing more running wheel activity before than during feeding, they showed an equal amount of activity before and during feeding. As food-anticipatory activity is dependent on the mesolimbic dopamine system and mu-opioid receptors regulate dopaminergic activity, these data suggest a change in the dopamine system's activity in mu-knockout mice. As we observed that mu-knockout mice tended to show a stronger locomotor activity response than wildtype mice to the indirect dopamine agonist d-amphetamine, it appears that the dopaminergic system per se is intact and sensitive to activation. We found no differences in the expression of pro-opiomelanocortin, a precursor of endogenous endorphin, in the arcuate nucleus between mu-knockout mice and wildtype mice during restricted feeding, showing that the mu-opioid receptor does not regulate endogenous endorphin levels. These data overall suggest a role for mu-opioid receptors in adapting reward related behaviour to the requirements of the environment.     

VEGF-overexpressing transgenic mice show enhanced post-ischemic neurogenesis and neuromigration

New neurons are generated continuously in the subventricular zone and dentate gyrus of the adult brain. Neuropathologic processes, including cerebral ischemia, can enhance neurogenesis, as can growth factors and other physiologic stimuli. Vascular endothelial growth factor (VEGF) is an angiogenic and neuroprotective growth factor that can promote neurogenesis, but it is unknown whether VEGF can enhance migration of newborn neurons toward sites of ischemic injury, where they might be able to replace neurons that undergo ischemic death. In the present study we produced permanent focal cerebral ischemia in transgenic (Tg) mice that overexpress VEGF. Cell proliferation and neurogenesis were assessed with bromodeoxyuridine (Brdu) labeling and immunostaining for cell type-specific markers. In VEGF-Tg mice, brains examined 7-28 days after cerebral ischemia showed markedly increased subventricular zone (SVZ) neurogenesis, chains of neuroblasts extending from the SVZ to the peri-infarct cortex, and an increase in the number of newly generated cortical neurons at 14-28 days after ischemia. In concert with these effects, VEGF overexpression reduced infarct volume and improved postischemic motor function. These findings provide evidence that VEGF increases SVZ neurogenesis and neuromigration, consistent with a possible role in repair. Our data suggest that in addition to its neuroprotective effects, which are associated with improved outcome in the acute phase after cerebral ischemia, VEGF enhances postischemic neurogenesis, which could provide a therapeutic target for more chronic brain repair.     

Reelin-deficient mice show impaired neurogenesis and increased stroke size

Reelin (Reln) is a protein involved in migration of newborn neurons during development. Reln mutations produce the reeler phenotype in mice, which is characterized by a defect in brain lamination, and autosomal recessive lissencephaly in humans. Reln expression persists in adult brain, but little is known about its function. We used reeler mice to investigate the effects of Reln deficiency on neurogenesis and the response to injury in the adult brain. Newborn neurons were decreased in number in the dentate gyrus and rostral migratory stream of reeler, compared to wild-type, mice. This was due, at least in part, to impaired cell migration. In addition, reeler mice showed increased susceptibility to ischemic brain injury. Cerebral infarcts from middle cerebral artery occlusion were larger in reeler than in wild-type mice, and associated neurobehavioral abnormalities were more severe. The brains of reeler mice also showed larger excitotoxic lesions after the intracerebral injection of N-methyl-D-aspartate. Finally, despite the fact that reeler mice had larger cerebral infarcts, the ischemia-induced enhancement of neurogenesis observed in wild-type mice was attenuated. These findings suggest that, in addition to its neurodevelopmental effects, Reln deficiency continues to influence neurogenesis and ischemic neuronal injury in the adult brain.     

Homozygote inositol transporter knockout mice show a lithium-like phenotype

Objective:  Lithium inhibits inositol monophosphatase and also reduces inositol transporter function. To determine if one or more of these mechanisms might underlie the behavioral effects of lithium, we studied inositol transporter knockout mice. We previously reported that heterozygous knockout mice with reduction of 15–37% in brain inositol had no abnormalities of pilocarpine sensitivity or antidepressant-like behavior in the Porsolt forced swim test. We now report on studies of homozygous inositol transporter knockout mice.
Methods:  Homozygote knockout mice were rescued by 2% inositol supplementation to the drinking water of the dam mice through pregnancy and lactation. Genotyping was carried out by polymerase chain reaction followed by agarose electrophoresis. Brain free myo -inositol levels were determined gas-chromatographically. Motor activity and coordination were assessed by the rotarod test. Behavior of the mice was studied in lithium–pilocarpine seizure models for lithium action and in the Porsolt forced swim test model for depression.
Results:  In homozygote knockout mice, free inositol levels were reduced by 55% in the frontal cortex and by 60% in the hippocampus. There were no differences in weight or motor coordination by the rotarod test. They behaved similarly to lithium-treated animals in the model of pilocarpine seizures and in the Porsolt forced swimming test model of depression.
Conclusions:  Reduction of brain inositol more than 15–37% may be required to elicit lithium-like neurobehavioral effects.     

Polychlorinated biphenyls induce proinflammatory cytokine release and dopaminergic dysfunction: protection in interleukin-6 knockout mice

Proinflammatory cytokines are not only important mediators of brain development, but also pose an increased risk for neurodegeneration following exposure to neurotoxicants or trauma. We have used the ubiquitous environmental and occupational neurotoxicant polychlorinated biphenyls (PCBs) to investigate the putative role of inflammatory agents in mediating processes involved in basal ganglia dysfunctions. PCBs induced inflammatory responses in C57BL/6 adult male mice, significantly elevating serum levels of IL-6 (31%), IL-1beta (71%) and TNF-alpha (22%) and significantly reducing striatal dopamine (DA, 21%), tyrosine hydroxylase (TH, 26%), dopamine transporter (DAT, 39%), and synaptophysin (29%) concentrations. We also exposed mice deficient in the proinflammatory cytokine interleukin-6 (IL-6-/-) to PCBs, to explore the role of this specific cytokine in mediating PCB-induced DA neurodegeneration. Not only did the PCB-treated IL-6-/- mice exhibit a decrease in serum levels of IL-1beta and TNF-alpha, but they were also protected from PCB-induced striatal dopaminergic dysfunction, displaying no signs of toxicant-induced reductions in DA levels, or TH, DAT or synaptophysin expression. Taken together, these results suggest that: (1) PCB exposure results in a peripheral inflammatory response associated with striatal terminal degeneration; and (2) the absence of IL-6 prevents PCB-induced dopaminergic losses in the striatum.     

Increased neurogenesis and brain-derived neurotrophic factor in neurokinin-1 receptor gene knockout mice

It has previously been shown that chronic treatment with antidepressant drugs increases neurogenesis and levels of brain-derived neurotrophic factor in the hippocampus. These changes have been correlated with changes in learning and long-term potentiation and may contribute to the therapeutic efficacy of antidepressant drug treatment. Recently, antagonists at the neurokinin-1 receptor, the preferred receptor for the neuropeptide substance P, have been shown to have antidepressant activity. Mice with disruption of the neurokinin-1 receptor gene are remarkably similar both behaviourally and neurochemically to mice maintained chronically on antidepressant drugs. We demonstrate here that there is a significant elevation of neurogenesis but not cell survival in the hippocampus of neurokinin-1 receptor knockout mice. Neurogenesis can be increased in wild-type but not neurokinin-1 receptor knockout mice by chronic treatment with antidepressant drugs which preferentially target noradrenergic and serotonergic pathways. Hippocampal levels of brain-derived neurotrophic factor are also two-fold higher in neurokinin-1 receptor knockout mice, whereas cortical levels are similar. Finally, we examined hippocampus-dependent learning and memory but found no clear enhancement in neurokinin-1 receptor knockout mice. These data argue against a simple correlation between increased levels of neurogenesis or brain-derived neurotrophic factor and mnemonic processes in the absence of increased cell survival. They support the hypothesis that increased neurogenesis, perhaps accompanied by higher levels of brain-derived neurotrophic factor, may contribute to the efficacy of antidepressant drug therapy.     

Amyloid precursor protein knockout mice show age-dependent deficits in passive avoidance learning

Amyloid precursor protein (APP) is involved in the pathogenesis of Alzheimer's disease (AD), but its role in cognition has been relatively little studied. APP knockout (KO) animals have been described previously and show deficits in grip strength, reduced locomotor activity and impaired learning and memory in a conditioned avoidance test and the Morris water-maze. In order to further investigate the in vivo function of APP and its proteolytic derivatives, we tested APP KO mice and age-matched wild type controls at two different ages, 3 and 8 months, in a range of behavioural tests measuring neuromuscular, locomotor and cognitive functions. These tests included the acquisition of a passive avoidance response as a measure of long-term memory of an aversive experience, and spontaneous alternation in a Y-maze, regarded as a measure of spatial short-term memory. The absence of APP expression in APP KO mice was confirmed at the protein level using hippocampal tissue in Western blotting. APP KO mice displayed deficits in forelimb grip strength and locomotor activity in agreement with previous studies. In the Y-maze test used for spontaneous alternation behaviour, APP KO animals did not exhibit reduced alternation rates. On the other hand, in the passive avoidance test, APP KO mice showed an age-related deficit in retention of memory for an aversive experience. The results suggest that APP and/or its proteolytic derivatives may play a role in long-term memory in adult brain and/or may be required during the development and maintenance of neuronal networks involved in this type of memory.     

Adult neurogenesis and neuroplasticity

After cerebral strokes and traumatic brain injuries (TBIs), there is a striking amount of neurological recovery in the following months and years, despite often-permanent structural damage. Though the mechanisms underlying such recovery are not fully understood, properties of plasticity of the central nervous system (CNS), such as the reorganization of the pre-existing network and axonal sprouting have been implicated in the recovery. With the recent evidences that neurogenesis occurs in the adult brain, and neural stem cells (NSCs) reside in the adult CNS, the involvement of newly generated neuronal cells in the recovery following injury to the CNS remains to be established. Neurogenesis is increased bilaterally in the dentate gyrus (DG) and the subventicular zone (SVZ) after cerebral strokes and TBIs, and new neuronal cells are generated at the sites of injury, where they replace some of the degenerated nerve cells. Newly generated neuronal cells at the sites of injury may represent an attempt by the CNS to regenerate itself after injury, whereas the increased neurogenesis in the DG and SVZ would also contribute to the CNS plasticity. Thus, injury-induced neurogenesis may contribute to the recovery and plasticity of the CNS.     

Adult brain neurogenesis and depression

The waning and waxing of neurogenesis in brain areas such as the dentate gyrus is proposed as a key factor in the descent into and recovery from clinical depression, respectively. A decrease in neurogenesis could occur due to genetic factors, stress (especially because of the involvement of adrenal corticoids), and/or a decline in serotonergic neurotransmission. An increase in neurogenesis could be brought about by several factors, but especially those that activate the serotonin 5-HT(1A) receptor. The possible interaction of immune system factors, especially the proinflammatory cytokines, with adult brain neurogenesis is discussed.     

成年海马神经再生与抑郁

重症抑郁是一种严重的精神障碍,其终生患病率超过15%,主要的行为变化是情绪低落、消极观念、无助、活力和注意力下降,自杀比例约15%,是全球第四大致残因素[1].     

Adult hippocampal neurogenesis and aging

The demographic changes in the foreseeable future stress the need for research on successful cognitive aging. Advancing age constitutes a primary risk factor for disease of the central nervous system most notably neurodegenerative disorders. The hippocampus is one of the brain regions that is prominently affected by neurodegeneration and functional decline even in what is still considered “normal aging”. Plasticity is the basis for how the brain adapts to changes over time. The discovery of adult hippocampal neurogenesis has added a whole new dimension to research on structural plasticity in the adult and aging hippocampus. In this article, we briefly summarize and discuss recent findings on the regulation of adult neurogenesis with relevance to aging. Aging is an important co-variable for many regulatory mechanisms affecting adult neurogenesis but so far, only few studies have specifically addressed this interaction. We hypothesize that adult neurogenesis contributes to a neural reserve, i.e. the maintained potential for structural plasticity that allows compensation in situations of functional losses with aging. As such we propose that adult neurogenesis might contribute to the structural correlates of successful aging.     

Adult neurogenesis from reprogrammed astrocytes

The details of adult neurogenesis,including environmental triggers,region specificity,and species homology remain an area of intense investigation.Slowing or halting age-related cognitive dysfunction,or restoring neurons lost to disease or injury represent just a fraction of potential therapeutic applications.New neurons can derive from stem cells,pluripotent neural progenitor cells,or non-neuronal glial cells,such as astrocytes.Astrocytes must be epigeneticallyreprogrammedto become neurons,which can occur both naturally in vivo,and via artificial exogenous treatments.While neural progenitor cells are localized to a few neurogenic zones in the adult brain,astrocytes populate almost every brain structure.In this review,we will summarize recent research into neurogenesis that arises from conversion of post-mitotic astrocytes,detail the genetic and epigenetic pathways that regulate this process,and discuss the possible clinical relevance in supplementing stem-cell neurogenic therapies.     

Adult neurogenesis and Parkinson's disease

Parkinson's disease is a neurodegenerative disorder characterized by a progressive neuronal loss affecting preferentially the dopaminergic neurons of the nigrostriatal projection. Transplantation of fetal dopaminergic precursor cells has provided the proof of principle that a cell replacement therapy can ameliorate clinical symptoms in affected patients. Recent years have provided evidence for the existence of neural stem cells with the potential to produce new neurons, particularly of a dopaminergic phenotype, in the adult mammalian brain. Such stem cells have been identified in so called neurogenic brain areas, where neurogenesis is constitutively ongoing, but also in primarily non-neurogenic areas, such as the midbrain and the striatum, where neurogenesis does not occur under normal physiological conditions. We review here presently published evidence to evaluate the concept that endogenous neural stem cells may have the potential to be instrumentalized for a non-invasive cell replacement therapy with autologous neurons to repair the damaged nigrostriatal dopaminergic projection in Parkinson's disease.     

Adult neurogenesis in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of age-related dementia, characterized by progressive memory loss and cognitive disturbances. The hippocampus, where adult hippocampal neurogenesis (AHN), a relatively novel form of brain plasticity that refers to the birth of new neurons, occurs, is one of the first brain regions to be affected in AD patients. Recent studies showed that AHN persists throughout life in humans, but it drops sharply in AD patients. Next questions to consider would be whether AHN impairment is a contributing factor to learning and memory impairment in AD and whether restoring AHN could ameliorate or delay cognitive dysfunction. Here, we outline and discuss the current knowledge about the state of AHN in AD patients, AHN impairment as a potentially relevant mechanism underlying memory deficits in AD, therapeutic potential of activating AHN in AD, and the mechanisms of AHN impairment in AD.     

CB1 receptor knockout mice show similar behavioral modifications to wild-type mice when enkephalin catabolism is inhibited

Behavioral and biochemical studies have suggested a functional link between the endogenous cannabinoid and opioid systems. Different hypotheses have been proposed to explain the interactions between opioid and cannabinoid systems such as a common pathway stimulating the dopaminergic system, a facilitation of signal-transduction- and/or a cannabinoid-induced enhancement of opioid peptide release. However, at this time, all the studies have been performed with exogenous agonists (delta-9-tetrahydrocannabinol or morphine), leading to a generally excessive stimulation of receptors normally stimulated by endogenous effectors (anandamide or opioid peptides) in various brain structures. To overcome this problem, we have measured various behavioral responses induced by the stimulation of the endogenous opioid system using the dual inhibitor of enkephalin-degrading enzymes, RB101, in CB1 receptor knockout mice. Thus, analgesia, locomotor activity, anxiety and antidepressant-like effects were measured after RB101 administration (80 and 120 mg/kg i.p. or 10 mg/kg, i.v.) in CB1 receptor knockout mice and their wild-type littermates. In all the experiments, inhibition of enkephalin catabolism produced similar modifications in behavior observed in CB1 knockout and wild-type mice. These results suggest limited physiological interaction between cannabinoid and opioid systems.     

Excessive novelty-induced c-Fos expression and altered neurogenesis in the hippocampus of GluA1 knockout mice

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluA1 subunit-deficient (GluA1-/-) mice display novelty-induced hyperactivity, cognitive and social defects and may model psychiatric disorders, such as schizophrenia and depression/mania. We used c-Fos expression in GluA1-/- mice to identify brain regions responsible for novelty-induced hyperlocomotion. Exposure to a novel cage for 2 h significantly increased c-Fos expression in many brain regions in both wild-type and knockout mice. Interestingly, the clearest genotype effect was observed in the hippocampus and its main input region, the entorhinal cortex, where the novelty-induced c-Fos expression was more strongly enhanced in GluA1-/- mice. Their novelty-induced hyperlocomotion partly depended on the activity of AMPA receptors, as it was diminished by the AMPA receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulphonamide (NBQX) and unaffected by the AMPA receptor potentiator 2,3-dihydro-1,4-benzodioxin-6-yl-1-piperidinylmethanone (CX546). The hyperlocomotion of GluA1-/- mice was normalised to the level of wild-type mice within 5-6 h, after which their locomotion followed normal circadian rhythm and was not affected by acute or chronic treatments with the selective serotonin reuptake inhibitor escitalopram. We propose that hippocampal dysfunction, as evidenced by the excessive c-Fos response to novelty, is the major contributor to novelty-induced hyperlocomotion in GluA1-/- mice. Hippocampal dysfunction was also indicated by changes in proliferation and survival of adult-born dentate gyrus cells in the knockout mice. These results suggest focusing on the functions of hippocampal formation, such as novelty detection, when using the GluA1-/- mouse line as a model for neuropsychiatric and cognitive disorders.     

Neuroaxonal dystrophy in PLA2G6 knockout mice

The PLA2G6 gene encodes group VIA calcium‐independent phospholipase A₂ (iPLA₂β), which belongs to the PLA₂ superfamily that hydrolyses the sn‐2 ester bond in phospholipids. In the nervous system, iPLA₂β is essential for remodeling membrane phospholipids in axons and synapses. Mutated PLA2G6 causes PLA2G6‐associated neurodegeneration (PLAN) including infantile neuroaxonal dystrophy (INAD) and adult‐onset dystonia‐parkinsonism (PARK14), which have unique clinical phenotypes. In the PLA2G6 knockout (KO) mouse, which is an excellent PLAN model, specific membrane degeneration takes place in neurons and their axons, and this is followed by axonal spheroid formation. These pathological findings are similar to those in PLAN. This review details the evidence that membrane degeneration of mitochondria and axon terminals is a precursor to spheroid formation in this disease model. From a young age before the onset, many mitochondria with damaged inner membranes appear in PLA2G6 KO mouse neurons. These injured mitochondria move anterogradely within the axons, increasing in the distal axons. As membrane degeneration progresses, the collapse of the double membrane of mitochondria accompanies axonal injury near impaired mitochondria. At the axon terminals, the membranes of the presynapses expand irregularly from a young age. Over time, the presynaptic membrane ruptures, causing axon terminal degeneration. Although these processes occur in different degenerating membranes, both contain tubulovesicular structures, which are a specific ultrastructural marker of INAD. This indicates that two unique types of membrane degeneration underlie PLAN pathology. We have shown a new pathological mechanism whereby axons degenerate due to defective maintenance and rupture of both the inner mitochondrial and presynaptic membranes. This degeneration mechanism could possibly clarify the pathologies of PLAN, Parkinson disease and neurodegeneration with iron accumulation (NBIA), which are assumed to be due to the primary degeneration of axons.     

Adult neurogenesis—a reality check

It is established beyond doubt that new neurons are born in discrete areas of the adult brain throughout the lifetime of most mammals. Recent findings have shed new light on the regional limitations, regulation, and possible function of adult neurogenesis. This article aims to look critically at the existence and relevance of adult neurogenesis under physiological conditions, based on recent advances in the field. We also evaluate the therapeutic potential of adult neurogenesis and what is realistic to expect from the future. We conclude that, to date, little is known with certainty about why new neurons are generated in the adult brain. Until there is more causal evidence at hand, assumptions about the potential functions of new neurons remain hypothetical. Provided we learn how to safely regulate proliferation, migration, and proper maturation of new neurons, endogenous neurogenesis could be a promising source of new cells for replacement therapies.