'Unpredictable chronic mild stress does not exacerbate memory impairment or altered neuronal and glial plasticity in the hippocampus of middle-aged vitamin D deficient mice'.

Vitamin D deficiency is a worldwide health concern, especially in the elderly population. Much remains unknown about the relationship between vitamin D deficiency (VDD), stress-induced cognitive dysfunctions and depressive-like behaviour. In this study, 4-month-old male C57Bl/6J mice were fed with control or vitamin D free diet for 6 months, followed by unpredictable chronic stress (UCMS) for 8 weeks. VDD induced cognitive impairment and reduced grooming behaviour, but did not induce depressive-like behaviour. While UCMS in vitamin D sufficient mice induced expected depressive-like phenotype and impairments in the contextual fear memory, chronic stress did not manifest as an additional risk factor for memory impairments and depressive-like behaviour in VDD mice. In fact, UCMS restored self-care behaviour in VDD mice. At the histopathological level, VDD mice exhibited cell loss in the granule cell layer, reduced survival of newly generated cells, accompanied with an increased number of apoptotic cells and alterations in glial morphology in the hippocampus; however, these effects were not exacerbated by UCMS. Interestingly, UCMS reversed VDD induced loss of microglial cells. Moreover, tyrosine hydroxylase levels decreased in the striatum of VDD mice, but not in stressed VDD mice. These findings indicate that long-term VDD in adulthood impairs cognition but does not augment behavioural response to UCMS in middle-aged mice. While VDD caused cell loss and altered glial response in the DG of the hippocampus, these effects were not exacerbated by UCMS and could contribute to mechanisms regulating altered stress response.

Pharmacological approach for targeting dysfunctional brain plasticity: Focus on neural cell adhesion molecule (NCAM).

Brain plasticity refers to the ability of the brain to undergo functionally relevant adaptations in response to external and internal stimuli. Alterations in brain plasticity have been associated with several neuropsychiatric disorders, and current theories suggest that dysfunctions in neuronal circuits and synaptogenesis have a major impact in the development of these diseases. Among the molecules that regulate brain plasticity, neural cell adhesion molecule (NCAM) and its polysialylated form PSA-NCAM have been of particular interest for years because alterations in NCAM and PSA-NCAM levels have been associated with memory impairment, depression, autistic spectrum disorders and schizophrenia. In this review, we discuss the roles of NCAM and PSA-NCAM in the regulation of brain plasticity and, in particular, their roles in the mechanisms of depression. We also demonstrate that the NCAM-mimetic peptides FGL and Enreptin are able to restore disrupted neuronal plasticity. FGL peptide has also been demonstrated to ameliorate the symptoms of depressive-like behavior in NCAM-deficient mice and therefore, may be considered a new drug candidate for the treatment of depression as well as other neuropsychiatric disorders with disrupted neuroplasticity.

Neuroprotective and memory enhancing properties of a dual agonist of the FGF receptor and NCAM.

The fibroblast growth factor receptor (FGFR) plays a vital role in the development of the nervous system regulating a multitude of cellular processes. One of the interaction partners of the FGFR is the neural cell adhesion molecule (NCAM), which is known to play an important role in neuronal development, regeneration and synaptic plasticity. Thus, simultaneous activation of FGFR- and NCAM-mediated signaling pathways may be expected to affect processes underlying neurodegenerative diseases. We here report the identification of a peptide compound, Enreptin, capable of interacting with both FGFR and NCAM. We demonstrate that this dual specificity agonist induces phosphorylation of FGFR and differentiation and survival of primary neurons in vitro, and that these effects are inhibited by abrogation of both NCAM and FGFR signaling pathways. Furthermore, Enreptin crosses the blood-brain barrier after subcutaneous administration, enhances long-term memory in normal mice and ameliorates memory deficit in mice with induced brain inflammation. Moreover, Enreptin reduces cognitive impairment and neuronal death induced by Aß25-35 in a rat model of Alzheimer's disease, and reduces the mortality rate and clinical signs of experimental autoimmune encephalomyelitis in rats. Thus, Enreptin is an attractive candidate for the treatment of neurological diseases.

Enhanced Cognition and Neurogenesis in miR-146b Deficient Mice.

The miR-146 family consists of two microRNAs (miRNAs), miR-146a and miR-146b, which are both known to suppress a variety of immune responses. Here in this study, we show that miR-146b is abundantly expressed in neuronal cells, while miR-146a is mainly expressed in microglia and astroglia of adult mice. Accordingly, miR-146b deficient (Mir146b-/-) mice exhibited anxiety-like behaviors and enhanced cognition. Characterization of cellular composition of Mir146b-/- mice using flow cytometry revealed an increased number of neurons and a decreased abundancy of astroglia in the hippocampus and frontal cortex, whereas microglia abundancy remained unchanged. Immunohistochemistry showed a higher density of neurons in the frontal cortex of Mir146b-/- mice, enhanced hippocampal neurogenesis as evidenced by an increased proliferation, and survival of newly generated cells with enhanced maturation into neuronal phenotype. No microglial activation or signs of neuroinflammation were observed in Mir146b-/- mice. Further analysis demonstrated that miR-146b deficiency is associated with elevated expression of glial cell line-derived neurotrophic factor (Gdnf) mRNA in the hippocampus, which might be at least in part responsible for the observed neuronal expansion and the behavioral phenotype. This hypothesis is partially supported by the positive correlation between performance of mice in the object recognition test and Gdnf mRNA expression in Mir146b-/- mice. Together, these results show the distinct function of miR-146b in controlling behaviors and provide new insights in understanding cell-specific function of miR-146b in the neuronal and astroglial organization of the mouse brain.

Paradoxical attenuation of neuroinflammatory response upon LPS challenge in miR-146b deficient mice.

The miR-146 family consists of two microRNAs (miRNAs), miR-146a and miR-146b (miR-146a/b), both of which are known to suppress immune responses in a variety of conditions. Here, we studied how constitutive deficiency of miR-146b (Mir146b-/-) affects lipopolysaccharide (LPS)-induced neuroinflammation in mice. Our experiments demonstrated that miR-146b deficiency results in the attenuation of LPS-induced neuroinflammation, as it was evidenced by the reduction of sickness behavior, a decrease in the inflammatory status of microglia, and the loss of morphological signs of microglial activation in the hippocampus. Gene expression analysis revealed that LPS-induced upregulation of hippocampal pro-inflammatory cytokines is attenuated in Mir146b-/- mice, compared to wild-type (WT) mice. In addition, reduced expression of the NF-κB nuclear protein p65, reduced miR-146 family target TLR4 expression and relatively stronger upregulation of miR-146a was found in Mir146b-/- mice as compared to WT mice upon LPS challenge. Compensatory upregulation of miR-146a can explain the attenuation of the LPS-induced neuroinflammation. This was supported by experiments conducted with miR-146a/b deficient mice (Mir146a/b-/-), which demonstrated that additional deletion of the miR-146a led to the restoration of LPS-induced sickness behavior and proinflammatory cytokines. Our experiments also showed that the observed upregulation of miR-146a in Mir146b-/- mice is due to the overexpression of a miR-146a transcription inducer, interferon regulatory factor 7 (Irf7). Altogether, our results show the existence of crosstalk between miR-146a and mir-146b in the regulation of LPS-induced neuroinflammation.

Development of depression-like behavior and altered hippocampal neurogenesis in a mouse model of chronic neuropathic pain.

Chronic-pain patients often suffer from depression. In rodent models of neuropathic pain, animals develop depression-like and anxiety behaviors, indicating a relationship between chronic pain and affective disorders. However, the underlying neurobiological mechanisms linking chronic pain and depression are not yet fully understood. Neurogenesis in the hippocampus is a fundamental process related to brain plasticity. Reduced neurogenesis has been associated with the development of mood disorders and cognitive impairments. The current study aims to elucidate the underlying long-term changes in brain plasticity induced by neuropathic pain in mice at a time point when depression-like behavior has already developed. Furthermore, our focus is set on alterations in neurogenesis in the hippocampus. We found that manifestation of anxiety- and depressive-like behavior as well as cognitive impairment co-occur with decreased survival of newly generated cells but not with impaired proliferative activity or reduced number of immature neurons in the dentate gyrus area of the hippocampus. Moreover, we detected an impairment of differentiation of newly generated cells into mature calbindin-positive neurons, accompanied with a shift towards increased differentiation into astroglial cells. These findings indicate that a reduction in mature functional neurons, rather than reduced proliferation or neuronal progenitor cells, are the long-term changes in hippocampal plasticity that manifest in neuropathic pain conditions after depression-like behavior has developed.

Cocaine-induced changes in behaviour and DNA methylation in rats are influenced by inter-individual differences in spontaneous exploratory activity.

BACKGROUND: Individual differences in behavioural traits influence susceptibility to addictive disorders. Drug addiction involves changes in gene expression, proposed to occur via DNA methylation (DNAm). AIMS: To investigate DNAm changes in reward-related brain structures (nucleus accumbens (NAc), lateral habenula (LHb)) in response to cocaine exposure in rats differing in spontaneous exploratory activity. METHODS: Rats were observed in the exploration box and categorised as high- (HE) or low explorers (LE). Rats were administered vehicle or cocaine (12 mg/kg, i.p.) for 7 days, followed by a 14-day withdrawal period and cocaine challenge (7 mg/kg); horizontal locomotor activity was recorded. Brain tissue was dissected after 24 h; we analysed messenger RNA (mRNA) and activity levels of epigenetic DNA modifiers (DNMTs and TETs) as well as mRNA and promoter methylation levels at selected genes previously linked to addictive behaviours. RESULTS: The cocaine challenge dose stimulated locomotor activity in both LE- and HE rats only when administered after a repeated cocaine schedule, suggesting development of behavioural sensitisation. Quantitative polymerase chain reaction analyses demonstrated higher basal expression of Dnmt3a, Tet2 and Tet3 in the LHb of HE- vs. LE rats, and we observed differential effects of cocaine exposure on the expression and activity of epigenetic DNA modifiers in the NAc and LHb of HE- and LE rats. Furthermore, cocaine exposure differentially altered promoter methylation levels of A2AR, Ppp1cc, and Taar7b in the NAc and LHb of HE- and LE rats. CONCLUSIONS: DNAm might play a role in the HE- and LE phenotypes as well as mediate behavioural effects of LE- and HE rats in response to drugs of abuse.

Effects of the drug combination memantine and melatonin on impaired memory and brain neuronal deficits in an amyloid-predominant mouse model of Alzheimer's disease.

OBJECTIVES: Alzheimer's disease (AD) is a neurodegenerative disorder with no cure. Limited treatment options available today do not offer solutions to slow or stop any of the suspected causes. The current medications used for the symptomatic treatment of AD include memantine and acetylcholine esterase inhibitors. Some studies suggest that melatonin could also be used in AD patients due to its sleep-improving properties. METHODS: In this study, we evaluated whether a combination of memantine with melatonin, administered for 32 days in drinking water, was more effective than either drug alone with respect to Aß aggregates, neuroinflammation and cognition in the double transgenic APP/PS1 (5xFAD) mouse model of AD. KEY FINDINGS: In this study, chronic administration of memantine with melatonin improved episodic memory in the object recognition test and reduced the number of amyloid aggregates and reactive microgliosis in the brains of 5xFAD mice. Although administration of memantine or melatonin alone also reduced the number of amyloid aggregates and inflammation in brain, this study shows a clear benefit of the drug combination, which had a significantly stronger effect in this amyloid-dominant mouse model of AD. CONCLUSION: Our data suggest considerable potential for the use of memantine with melatonin in patients with AD.

Depression-like behaviour in neural cell adhesion molecule (NCAM)-deficient mice and its reversal by an NCAM-derived peptide, FGL.

The neural cell adhesion molecule (NCAM) plays a pivotal role in brain plasticity. Brain plasticity itself has a crucial role in the development of depression. The aim of this study was to analyze whether NCAM-deficient (NCAM(-/-)) mice exhibit depression-like behaviour and whether a peptide termed FGL, derived from the NCAM binding site for the fibroblast growth factor (FGF) receptor, is able to reverse the depression-like signs in NCAM(-/-) mice. Our study showed that NCAM(-/-) mice demonstrated increased freezing time in the tail-suspension test and reduced preference for sucrose consumption in the sucrose preference test, reduced adult neurogenesis in the dentate gyrus and reduced levels of the phosphorylated cAMP response element-binding protein (pCREB) in the hippocampus. FGL administered acutely or repeatedly reduced depression-like behaviour in NCAM(-/-) mice without having an effect on their wild-type littermates. Repeated administration of FGL enhanced survival of the newly born neurons in NCAM(-/-) mice and increased the levels of pCREB in both NCAM(+/+) and NCAM(-/-) mice. In conclusion, our data demonstrate that NCAM deficiency in mice results in a depression-like phenotype which can be reversed by the acute or repeated administration of FGL. The results also suggest a role of the deficit in NCAM signalling through the FGF receptor in depression.

Alterations in the polysialylated neural cell adhesion molecule and retinal ganglion cell density in mice with diabetic retinopathy.

AIM: To investigate the impact of polysialylated neural cell adhesion molecule (PSA-NCAM) on the survival of retinal ganglion cells (RGCs) in the experimentally induced diabetes in mice. METHODS: Diabetes was induced in 2.5 months old Swiss Webster mice by intraperitoneal injection of streptozotocin (STZ, 90 mg/kg) once daily for two consecutive days. Examination of the proteins of interest in the retinas from diabetic mice at 2mo after diabetes induction was performed using immunohistochemistry and Western blot analysis. RGCs were counted in the wholemounted retinas, and Brn3a marker was used. RESULTS: Examination of retinas from diabetic mice at 2mo after diabetes induction revealed a considerable reduction in RGC density. Our experiments also demonstrated a redistribution of PSA-NCAM in the retina of diabetic animals. PSA-NCAM immunoreactivity was diminished in the inner part of the retina where RGCs were located. In contrast, an enhanced PSA-NCAM immunoreactivity was detected in the outer layers of the retina. PSA-NCAM signal was co-localized with glial fibrillary acidic protein immunoreactivity in the Müller cell branches. Previous studies have shown that matrix metalloproteinase-9 (MMP-9) is responsible for the reduction in PSA-NCAM levels in neuronal cells. The reduced levels of PSA-NCAM in inner layers (nerve fiber layer, ganglion cell layer) were accompanied by the increased expression of MMP-9. In contrast, in the outer retinal layers, the expression of MMP-9 was much less pronounced. CONCLUSION: MMP-9 induces PSA-NCAM shedding in the inner part of the retina and the decreased level of PSA-NCAM in the inner part of the retina might be, at least in part, responsible for the loss of RGCs in diabetic mice.

Early post-natal, low-level lead exposure increases the number of PSA-NCAM expressing cells in the dentate gyrus of adult rat hippocampus.

Although lead is widely known as a potent neurotoxin, the effect of lead exposure on the expression of the polysialic acid linked neural cell adhesion molecule (PSA-NCAM) remains unclear. We exposed Wistar rat pups to 0.2% lead acetate from postnatal day (PND) 1 to PND 30. This exposure protocol resulted in pup blood lead levels, which increased to 29.3+/-5.0 mg/dl on PND 15, and subsequently rose to 34.2+/-5.8 mg/dl at weaning. Corresponding brain tissue lead levels were 456+/-23 ng/g on PND 15 and 781+/-87 ng/g on PND 30. Animals were sacrificed on PND 80, when the blood and brain lead concentrations did not differ from those of the control group. Lead exposure induced a significant increase in the total number of PSA-NCAM expressing cells, compared to the control group (p<0.01), and did not change the proportion of cells co-expressing PSA-NCAM with glial or neuronal markers (calbindin, TuJ1, GFAP). These results suggest that early post-natal lead exposure induces persistent changes in the number of PSA-NCAM expressing cells, which could be, at least, partly the basis of impairments in the learning and memory formation, which follows low-level lead exposure.

Developmental lead exposure impairs contextual fear conditioning and reduces adult hippocampal neurogenesis in the rat brain.

The effects of developmental lead exposure on the emotional reactivity, contextual fear conditioning and neurogenesis in the dentate gyrus of 60-80 days-old rats were studied. Wistar rat pups were exposed to 0.2% lead acetate via their dams' drinking water from postnatal day (PND) 1 to PND 21 and directly via drinking water from weaning until PND 30. At PND 60 and 80 the level of anxiety and contextual fear conditioning were studied, respectively. At PND 80 all animals received injections of BrdU to determine the effects of Pb on the generation of new cells in the dentate gyrus of hippocampus and on their survival and differentiation patterns. The results of the present study demonstrate that developmental lead exposure induces persistent increase in the level of anxiety and inhibition of contextual fear conditioning. Developmental lead exposure reduced generation of new cells in the dentate gyrus and altered the pattern of differentiation of BrdU-positive cells into mature neurons. A lower proportion of BrdU-positive cells co-expressed with the marker for mature neurons, calbindin. In contrast, the proportions of young not fully differentiated neurons and proportions of astroglial cells, generated from newly born cells, were increased in lead-exposed animals. Our results demonstrate that developmental lead exposure induces persistent inhibition of neurogenesis and alters the pattern of differentiation of newly born cells in the dentate gyrus of rat hippocampus, which could, at least partly, contribute to behavioral and cognitive impairments observed in adulthood.

PSA modification of NCAM supports the survival of injured retinal ganglion cells in adulthood.

Neural cell adhesion molecule (NCAM) is known as the cell surface glycoprotein, and it belongs to the immunoglobulin superfamily of adhesion molecules. Polysialic acid (PSA) is a carbohydrate attached to NCAM via either of two specific sialyltransferases: ST8SiaII and ST8SiaIV. Polysialylated neural cell adhesion molecule (PSA-NCAM) mediates cell interactions, plays a role in axon growth, migration, synaptic plasticity during development and cell regeneration. Some evidence has shown that PSA-NCAM supports the survival of neurons. It was demonstrated that PSA-NCAM is present in abundance in the retina during development and in adulthood. The aim of this study was to investigate whether PSA-NCAM promotes retinal ganglion cell (RGC) survival in transgenic mice with deficiencies in sialyltransferases or NCAM or after the administration of endoneuraminidase (Endo-N). RGC injury was induced by intravitreal administration of kainic acid (KA). These studies showed that injection of Endo-N after 14 days enhances the toxicity of KA to RGCs in wild-type (WT) mice by 18%. In contrast, in knockout mice (ST8SiaII-/-, ST8SiaIV-/-, NCAM-/-), survival of RGCs after KA injury did not change. Deficiencies of either ST8SiaII or ST8SiaIV did not influence the level of PSA-NCAM in the adult retina, however, in neonatal animals, decreased levels of PSA-NCAM were observed. In knockout ST8SiaII-/- adults, a reduced number of RGCs was detected, whereas in contrast, increased numbers of RGCs were noted in NCAM-/- mice. In conclusion, these data demonstrate that PSA-NCAM supports the survival of injured RGCs in adulthood. However, the role of PSA-NCAM in the adult retina requires further clarification.

Schizophrenia-like phenotype of polysialyltransferase ST8SIA2-deficient mice.

Posttranslational modification of the neural cell adhesion molecule (NCAM) by polysialic acid (polySia) is crucial for nervous system development and brain plasticity. PolySia attachment is catalyzed by the polysialyltransferases (polySTs) ST8SIA2 and ST8SIA4, two enzymes with distinct but also common functions during neurodevelopment and in the adult brain. A growing body of evidence links aberrant levels of NCAM and polySia as well as variation in the ST8SIA2 gene to neuropsychiatric disorders, including schizophrenia. To investigate whether polyST deficiency might cause a schizophrenia-like phenotype, St8sia2 (-/-) mice, St8sia4 (-/-) mice and their wildtype littermates were assessed neuroanatomically and subjected to tests of cognition and sensorimotor functions. St8sia2 (-/-) but not St8sia4 (-/-) mice displayed enlarged lateral ventricles and a size reduction of the thalamus accompanied by a smaller internal capsule and a highly disorganized pattern of fibers connecting thalamus and cortex. Reduced levels of the vesicular glutamate transporter VGLUT2 pointed towards compromised glutamatergic thalamocortical input into the frontal cortex of St8sia2 (-/-) mice. Both polyST-deficient lines were impaired in short- and long-term recognition memory, but only St8sia2 (-/-) mice displayed impaired working memory and deficits in prepulse inhibition. Furthermore, only the St8sia2 (-/-) mice exhibited anhedonic behavior and increased sensitivity to amphetamine-induced hyperlocomotion. These results reveal that reduced polysialylation in St8sia2 (-/-) mice leads to pathological brain development and schizophrenia-like behavior. We therefore propose that genetic variation in ST8SIA2 has the potential to confer a neurodevelopmental predisposition to schizophrenia.

Partial reduction in neural cell adhesion molecule (NCAM) in heterozygous mice induces depression-related behaviour without cognitive impairment.

The neural cell adhesion molecule (NCAM) plays an important role in brain plasticity. Using mice deficient in all isoforms of NCAM we have previously demonstrated that constitutive deficiency in the NCAM gene (NCAM-/-) resulted in cognitive impairment, anhedonic behaviour and a reduced ability to cope with stress. This was accompanied by reduced basal phosphorylation of the fibroblast growth factor receptor 1 (FGFR1) and reduced phosphorylation of calcium-calmodulin kinase (CaMK) II and IV and cAMP response element binding protein (CREB). The present study was aimed to investigate how partial deficiency in NCAM in mice (NCAM+/-) affected phenotype. We found that NCAM+/- mice showed a longer period of immobility in the tail suspension test, increased latency to feed in the novelty-suppressed feeding test and reduced preference for sucrose in sucrose preference test. Both NCAM+/- and NCAM-/- mice showed reduced extinction of contextual fear. In contrast to NCAM-/- mice, NCAM+/- mice did not demonstrate memory impairment in either object recognition or contextual fear conditioning tests. Levels of phosphorylated FGFR1 in the hippocampus and prefrontal/frontal cortex of NCAM+/- mice were partially reduced and no changes in the phosphorylation of CaMKII, CaMKIV or CREB in the hippocampus were found. We conclude that a constitutive partial reduction in NCAM proteins results in a behavioural phenotype related to depression without impairment in cognitive functions, also affecting the level of FGFR1 phosphorylation without major alterations in CaMKII and CaMKIV intracellular signalling. Partial reduction in FGFR1 phosphorylation might explain the observed behavioural phenotype in NCAM+/- mice.

Dysregulated CREB signaling pathway in the brain of neural cell adhesion molecule (NCAM)-deficient mice.

The neural cell adhesion molecule (NCAM) mediates cell-cell interactions and plays an important role in processes associated with neural plasticity, including learning and memory formation. It has been shown that mice deficient in all isoforms of NCAM (NCAM-/- mice) demonstrate impairment in long-term plasticity at multiple hippocampal synapses, disrupted spatial learning, and impaired contextual and auditory-cued fear conditioning. The formation of long-term memory is associated with activation of transcription factor CREB (cAMP response element binding protein). The aims of this study were to investigate NCAM-mediated signaling transduction pathways and the levels of the phosphorylated (Ser133) active form of the CREB in the brain structures (the pre- and frontal cortex, basolateral amygdala, and hippocampus) involved in the memory formation in NCAM-deficient mice. Immunohistochemical analysis revealed reduced levels of pCREB in the prefrontal cortex (PFC), frontal cortex (FC), CA3 subregion of the hippocampus (CA3) and basolateral nucleus of amygdala (BLA) in NCAM-/- mice. NCAM-/- mice had also reduced levels of the phosphorylated CaMKII and CaMKIV in PFC/FC and the hippocampus, which are the downstream signaling molecules of NCAM. The levels of non-phosphorylated kinases did not differ from those seen in the wild-type mice. These results provide evidence that NCAM deficiency results in the dysregulation of CREB-mediated signaling pathways in the brain regions, which is related to the formation of memory.