Neuropeptide Y immunoreactive neurons in murine trisomy 16 cortical cultures. Plasticity of expression and differentiation

Altered distribution of hippocampal interneurons in the murine Down Syndrome model Ts65Dn

Down Syndrome, with an incidence of one in 800 live births, is the most common genetic alteration producing intellectual disability. We have used the Ts65Dn model, that mimics some of the alterations observed in Down Syndrome. This genetic alteration induces an imbalance between excitation and inhibition that has been suggested as responsible for the cognitive impairment present in this syndrome. The hippocampus has a crucial role in memory processing and is an important area to analyze this imbalance. In this report we have analysed, in the hippocampus of Ts65Dn mice, the expression of synaptic markers: synaptophysin, vesicular glutamate transporter-1 and isoform 67 of the glutamic acid decarboxylase; and of different subtypes of inhibitory neurons (Calbindin D-28k, parvalbumin, calretinin, NPY, CCK, VIP and somatostatin). We have observed alterations in the inhibitory neuropil in the hippocampus of Ts65Dn mice. There was an excess of inhibitory puncta and a reduction of the excitatory ones. In agreement with this observation, we have observed an increase in the number of inhibitory neurons in CA1 and CA3, mainly interneurons expressing calbindin, calretinin, NPY and VIP, whereas parvalbumin cell numbers were not affected. These alterations in the number of interneurons, but especially the alterations in the proportion of the different types, may influence the normal function of inhibitory circuits and underlie the cognitive deficits observed in DS.

Altered expression of neuropeptides in the primary somatosensory cortex of the Down syndrome model Ts65Dn

Down syndrome is the most common genetic disorder associated with mental retardation. Subjects and mice models for Down syndrome (such as Ts65Dn) show defects in the formation of neuronal networks in both the hippocampus and the cerebral cortex. The principal neurons display alterations in the morphology, density and distribution of dendritic spines in the cortex as well as in the hippocampus. Several evidences point to the possibility that the atrophy observed in principal neurons could be mediated by changes in their inhibitory inputs and, in fact, an imbalance between excitation and inhibition has been observed in Ts65Dn mice in these regions, which are crucial for learning and information processing. These animals have an increased density of interneurons in the primary somatosensory cortex, especially of those expressing calretinin and calbindin D-28k. Here, we have analysed the expression and distribution of several neuropeptides in the primary somatosensory cortex of Ts65Dn mice in order to investigate whether these subpopulations of interneurons are affected. We have observed an increase in the total density of somatostatin expressing interneurons and of those expressing VIP in layer IV in Ts65Dn mice. The typology of the somatostatin and VIP interneurons was unaltered as attested by the pattern of co-expression with other markers. Somatostatin immunoreactive neurons co-express mainly D-28k calbindin and VIP expressing interneurons maintain its pattern of co-expression with calcium binding proteins. These alterations, in case they were also present in subjects with Down syndrome, could be related to their impairment in cognitive profile and could be involved in the neurological defects observed in this disorder.

Neonatal mice of the Down syndrome model, Ts65Dn, exhibit upregulated VIP measures and reduced responsiveness of cortical astrocytes to VIP stimulation

The Ts65Dn segmental mouse model of Down syndrome (DS) possesses a triplication of the section of chromosome 16 that is most homologous to the human chromosome 21 that is trisomic in DS. This model exhibits many of the characteristics of DS including small size, developmental delays, and a decline of cholinergic systems and cognitive function with age. Recent studies have shown that vasoactive intestinal peptide (VIP) systems are upregulated in aged Ts65Dn mice and that VIP dysregulation during embryogenesis is followed by the hypotonia and developmental delays as seen in both DS and in Ts65Dn mice. Additionally, astrocytes from aged Ts65Dn brains do not respond to VIP stimulation to release survival-promoting substances. To determine if VIP dysregulation is age-related in Ts65Dn mice, the current study examined VIP and VIP receptors (VPAC-1 and VPAC-2) in postnatal day 8 Ts65Dn mice. VIP and VPAC-1 expression was significantly increased in the brains of trisomic mice compared with wild-type mice. VIP-binding sites were also significantly increased in several brain areas of young Ts65Dn mice, especially in the cortex, caudate/putamen, and hippocampus. Further, in vitro treatment of normal neurons with conditioned medium from VIP-stimulated Ts65Dn astrocytes from neonatal mice did not enhance neuronal survival. This study indicates that VIP anomalies are present in neonatal Ts65Dn mice, a defect occurs in the signal transduction mechanism of the VPAC-1 VIP receptor, cortical astrocytes from neonatal brains are dysfunctional, and further, that VIP dysregulation may play a significant role in DS.

Vasoactive intestinal peptide in the brain of a mouse model for Down syndrome

The most common genetic cause of mental retardation is Down syndrome, trisomy of chromosome 21, which is accompanied by small stature, developmental delays, and mental retardation. In the Ts65Dn segmental trisomy mouse model of Down syndrome, the section of mouse chromosome 16 most homologous to human chromosome 21 is trisomic. This model exhibits aspects of Down syndrome including growth restriction, delay in achieving developmental milestones, and cognitive dysfunction. Recent data link vasoactive intestinal peptide malfunction with developmental delays and cognitive deficits. Blockage of vasoactive intestinal peptide during rodent development results in growth and developmental delays, neuronal dystrophy, and, in adults, cognitive dysfunction. Also, vasoactive intestinal peptide is elevated in the blood of newborn children with autism and Down syndrome. In the current experiments, vasoactive intestinal peptide binding sites were significantly increased in several brain areas of the segmental trisomy mouse, including the olfactory bulb, hippocampus, cortex, caudate/putamen, and cerebellum, compared with wild-type littermates. In situ hybridization for VIP mRNA revealed significantly more dense vasoactive intestinal peptide mRNA in the hippocampus, cortex, raphe nuclei, and vestibular nuclei in the segmental trisomy mouse compared with wild-type littermates. In the segmental trisomy mouse cortex and hippocampus, over three times as many vasoactive intestinal peptide-immunopositive cells were visible than in wild-type mouse cortex. These abnormalities in vasoactive intestinal peptide parameters in the segmental trisomy model of Down syndrome suggest that vasoactive intestinal peptide may have a role in the neuropathology of Down-like cognitive dysfunction.

Glial-neurotrophic mechanisms in Down syndrome

Complex interactions and interconnectivity between neurons are hallmarks of normal neuronal differentiation and development. Neurons also interact with other cell types, notably glia, and rely on substances released by glia for their normal function. A deficit in glial response may disturb this critical neuronal-glial-neuronal interaction in Down syndrome (DS), leading to loss of neurons and other defects of development, and contribute to cognitive limitation and early onset of Alzheimer disease. The hypothesis this paper will discuss is that normal neural development involves an activity-dependent release of substances from neurons, and that these substances act upon glia cells which in turn release substances that influence neurons to promote their survival and development. This glial influence affects cortical neurons and also the subcortical cholinergic neurons that project to the cerebral and hippocampal cortices to maintain cortical neuronal excitability and activity. The neuronal activity stimulates glial secretion of sustaining substances, in a reciprocally interactive cycle. Some aspect of this "virtuous cycle" is deficient in Down syndrome. The result is a small but slowly increasing deficit in activity-dependent support by glia cells which produces a gradually increasing abnormality of cortical and subcortical, perhaps especially cholinergic, function.

On the cause of mental retardation in Down syndrome: extrapolation from full and segmental trisomy 16 mouse models

Down syndrome (DS, trisomy 21, Ts21) is the most common known cause of mental retardation. In vivo structural brain imaging in young DS adults, and post-mortem studies, indicate a normal brain size after correction for height, and the absence of neuropathology. Functional imaging with positron emission tomography (PET) shows normal brain glucose metabolism, but fewer significant correlations between metabolic rates in different brain regions than in controls, suggesting reduced functional connections between brain circuit elements. Cultured neurons from Ts21 fetuses and from fetuses of an animal model for DS, the trisomy 16 (Ts16) mouse, do not differ from controls with regard to passive electrical membrane properties, including resting potential and membrane resistance. On the other hand, the trisomic neurons demonstrate abnormal active electrical and biochemical properties (duration of action potential and its rates of depolarization and repolarization, altered kinetics of active Na(+), Ca(2+) and K(+) currents, altered membrane densities of Na(+) and Ca(2+) channels). Another animal model, the adult segmental trisomy 16 mouse (Ts65Dn), demonstrates reduced long-term potentiation and increased long-term depression (models for learning and memory related to synaptic plasticity) in the CA1 region of the hippocampus. Evidence suggests that the abnormalities in the trisomy mouse models are related to defective signal transduction pathways involving the phosphoinositide cycle, protein kinase A and protein kinase C. The phenotypes of DS and its mouse models do not involve abnormal gene products due to mutations or deletions, but result from altered expression of genes on human chromosome 21 or mouse chromosome 16, respectively. To the extent that the defects in signal transduction and in active electrical properties, including synaptic plasticity, that are found in the Ts16 and Ts65Dn mouse models, are found in the brain of DS subjects, we postulate that mental retardation in DS results from such abnormalities. Changes in timing and synaptic interaction between neurons during development can lead to less than optimal functioning of neural circuitry and signaling then and in later life.

In vitro 1H and 31P NMR spectroscopic evidence of multiple aberrant biochemical pathways in murine trisomy 16 brain development

Nuclear magnetic resonance (NMR) spectroscopy was used to evaluate cytosolic compounds and membrane phospholipids simultaneously in trisomy 16 (Ts16) and euploid (control) murine brain at fetal day 15 in order to examine the cellular biochemistry that underlies the neurodevelopmental consequences of chromosome triplication in this model of Down syndrome (DS). Proton NMR spectroscopic analysis of brain tissue extracts demonstrated decreased levels of choline and increased levels of myo-inositol (MI) in Ts16 brains compared with control. These data are consistent with the cholinergic deficits and elevated MI levels previously described in Ts16. Compared with euploid brains. Ts16 brains also possess higher levels of creatine, adenosine, and tyrosine. Increased levels of MI and creatine, compounds that are localized to glia, imply abnormalities in the trophic environment of Ts16 brain. Phosphorus NMR spectroscopic analysis of extracts further revealed elevated levels of anionic phospholipid membrane components, such as phosphatidylinositol (PtdIno) and phosphatidylethanolamine, in Ts16 brains. Since these compounds are confined to the inner leaflet of the membrane, the findings suggest that membrane composition is altered specifically in the cytosolic bilayer at this stage. Together our proton and phosphorus NMR spectroscopic results indicate that multiple biochemical pathways are affected in Ts16 brain development. Understanding the effects of these aberrations may elucidate the processes that lead to neural dysfunction and Alzheimer's disease (AD) neuropathology in DS individuals.

Long-term basal forebrain cholinergic-rich grafts derived from trisomy 16 mice do not develop beta-amyloid pathology and neurodegeneration but demonstrate neuroinflammatory responses

Patients with Down syndrome (human trisomy 21) develop neuropathological and cholinergic functional defects characteristic of Alzheimer's disease, which has been attributed to the location of the Alzheimer beta-amyloid precursor protein on chromosome 21. Due to the partial genetic homology between mouse chromosome 16 and human chromosome 21, murine trisomy 16 was used as a model to study functional links between increased expression of the amyloid precursor protein, neurodegeneration and neuroinflammatory responses. Basal forebrain cholinergic-rich tissue derived from trisomy 16 mice at embryonic age of day 16 was transplanted into the lateral ventricle of adult normal mice. At 1, 3, 6, 9 and 12 months after transplantation, the grafts were characterized by immunocytochemistry, molecular biological analysis, and stereological methods. Grafts survived up to one year and still demonstrated immunoreactivity for cholinergic, GABAergic and astroglial cells. Though a 1.5-fold neuronal over-expression of amyloid precursor protein was detected in brains from trisomy 16 embryos by Northern analysis, beta-amyloid deposits were found neither in control nor trisomic grafts. Detailed stereological analysis of trisomic grafts did not reveal any neurodegeneration or morphological changes of cholinergic and GABAergic neurons during the course of graft maturation up to one year, as compared to grafts derived from euploid tissue. However, both euploid and trisomic grafts demonstrated a strong infiltration with T- and B-lymphocytes and a significant micro- and astroglial activation (hypertrophic astrocytes) within and around the grafts. These observations further suggest that the trisomy 16-induced neurodegeneration is seemingly due to a lack of neuron supporting factors which are provided by either the metabolic interaction of trisomic graft with surrounding healthy host tissue or by cells of the immune system infiltrating the graft.

Somatostatin expression in TS16 mouse brain cultures

Somatostatin expression in trisomy 16 mouse neuronal cultures has been studied to investigate the effects of the presence of an extra copy of the pre-pro-somatostatin (ppSS) gene on mouse chromosome 16. The immunoreactivity for somatostatin (SS) was considered in mixed cultures of neurons and glia cells and in neuron-enriched cultures as well as that for neuropeptide Y, glutamic acid decarboxylase, and gamma-enolase immunoreactivity the genes of which are not present on mouse chromosome 16. ppSS and pre-pro-neuropeptide Y (ppNPY) mRNA expression was evaluated and SS immunoreactivity in neurons analyzed by a morphometrical study. The extra copy of the ppSS gene resulted in a significantly increased level of the transcript in trisomic cultures, whereas the expression of the other neuropeptides did not differ. The absence of glial cells in these cultures reduced the number of SS-positive neurons making their number comparable in the trisomic and control cultures. Thus, in spite of higher expression of the ppSS mRNA in trisomic cultures, the determination of this peptidergic phenotype was influenced by the presence of neuroglial cells.

Cerebral cortical astroglia from the trisomy 16 mouse, a model for down syndrome, produce neuronal cholinergic deficits in cell culture

Trisomy 21 (Down syndrome) is associated with a high incidence of Alzheimer disease and with deficits in cholinergic function in humans. We used the trisomy 16 (Ts16) mouse model for Down syndrome to identify the cellular basis for the cholinergic dysfunction. Cholinergic neurons and cerebral cortical astroglia, obtained separately from Ts16 mouse fetuses and their euploid littermates, were cultured in various combinations. Choline acetyltransferase activity and cholinergic neuron number were both depressed in cultures in which both neurons and glia were derived from Ts16 fetuses. Cholinergic function of normal neurons was significantly down-regulated by coculture with Ts16 glia. Conversely, neurons from Ts16 animals could express normal cholinergic function when grown with normal glia. These observations indicate that astroglia may contribute strongly to the abnormal cholinergic function in the mouse Ts16 model for Down syndrome. The Ts16 glia could lack a cholinergic supporting factor present in normal glia or contain a factor that down-regulates cholinergic function.

Immortalized neural cells from trisomy 16 mice as models for Alzheimer's disease

The trisomy 16 mouse (Ts16) is a general accepted animal model for both Downs syndrome (DS) and Alzheimer's Disease (AD). However, the efficacy of this model is severely hampered by the fact that Ts16 is lethal after about 18-20 days of gestation. Chimeras, long-term tissue culture and neural transplantation of Ts16 material have previously been used to overcome this limitation presented by death in utero of the Ts16. In this paper we describe a new strategy to overcome this limitation, i.e. immortalization of primary cells from Ts16 mice with retrovirus-mediated gene transfer of a temperature sensitive immortalizing oncogene. By this method we have obtained a total of 21 stable cell lines from Ts16 hippocampus, Ts16 cortex, normal hippocampus, and normal cortex. So far, two of the cell lines have been karyotyped and as expected, the cell line immortalized from Ts16 embryos has retained three copies of chromosome 16. We are currently characterizing these cell lines with respect to expression of APP, T-antigen, Nestin, GFAP, NF and Map-2. Moreover, the processing and secretion of APP fragments are being investigated by immunoblotting. In summary, we have immortalized CNS cells from Ts16 mice and we expect that these cell lines will be useful as in vitro and in vivo models for studying various aspects of the pathology of Alzheimer's disease.