Monika Jürgenson

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.

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.

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.

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.

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.

Behavioral profile of mice with impaired cognition in the elevated plus-maze due to a deficiency in neural cell adhesion molecule.

The elevated plus-maze (EPM) test is one of the most used tests for screening levels of anxiety in rodents. In the present study, we studied how impaired cognition due to a deficiency in the neural cell adhesion molecule (NCAM) could affect the behavior of mice in the EPM task. NCAM-knockout mice demonstrated impaired learning in both object-recognition and fear-conditioning tasks. Analysis of the behavior of mice in the EPM task using a minute-by-minute method revealed a profound influence of genotype. Wild-type mice demonstrated quick learning of the aversive properties of the open arms during the first few minutes of a single EPM task, whereas NCAM-/- mice were unable to learn the aversive properties of the open arms of EPM. Wild-type mice also demonstrated habituation to the EPM task in a test/retest paradigm whereas NCAM-knockout mice failed to habituate during the second EPM presentation. Our data show that the anxiolytic-like behavior of NCAM-knockout mice is not just related to levels of innate anxiety but also to their inability to recognize potential danger associated with the open arms of the EPM task.

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 Aβ25-35 in a rat model of Alzheimers 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.

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.

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.

Depression-like phenotype in NCAM-deficient mice

Depression is highly prevalent complex, heterogeneous disorder with possible serious physical, mental and socioeconomical consequences. There are several theories trying to explain the genesis of depression, one novel, links the depression with reduced brain plasticity. Neural cell adhesion (NCAM) plays important roles in the development of central nervous system and in the structural and functional plasticity in the CNS throughout life. NCAM is a member of the immunoglobulin superfamily of adhesion molecules is characterized by several immunoglobulin (Ig)-like domains. If the reduced neuronal plasticity is an important factor predisposing to the development of psychiatric disorders and depression in particular, than animals with reduced brain plasticity due to deficiency in NCAM should serve as a valuable model of depression. Mice (c56/BL strain) with genetic deficiency for NCAMs were subjected to two tests widely used for the assessment of depression-like phenotype: tails suspension test and sucrose preference test. NCAMdeficient mice demonstrated higher immobility time in the tail suspension test and lower preference for sucrose solution in sucrose consumption test as compared with their wild type littermates. The increased immobility time in tail suppression test was reversed by amitriptyline and citalopram treatment. Furthermore, a peptide with NCAMmimetic actions, FGL was also able to rescue “depression-like” phenotype in NCAM-deficient mice. Analysis of hippocampal neurogenesis in NCAMdeficient mice revealed a reduced survival and differentiation of the newly born cells into calbindinpositive granule neurons. At the same time point, larger proportion of young postmitotic neurons expressing Tuj1 was found in NCAM−/− mice. No changes in the proportion of BrdUpositive cells, which have been differentiated into glial phenotype, were found. In conclusion, NCAM-deficient mice demonstrate “depression-like” behaviour associated with reduced hippocampal neurogenesis and might serve as a genetic model of depression.