Impaired Ca-signaling in astrocytes from the Ts16 mouse model of Down syndrome

Astroglial and microglial pathology in Down syndrome: Focus on Alzheimer's disease

Down syndrome (DS) arises from the triplication of human chromosome 21 and is considered the most common genetic cause of intellectual disability. Glial cells, specifically astroglia and microglia, display pathological alterations that might contribute to DS neuropathological alterations. Further, in middle adulthood, people with DS develop clinical symptoms associated with premature aging and Alzheimer's disease (AD). Overexpression of the amyloid precursor protein (APP) gene, encoded on chromosome 21, leads to increased amyloid-β (Aβ) levels and subsequent formation of Aβ plaques in the brains of individuals with DS. Amyloid-β deposition might contribute to astroglial and microglial reactivity, leading to neurotoxic effects and elevated secretion of inflammatory mediators. This review discusses evidence of astroglial and microglial alterations that might be associated with the AD continuum in DS.

Spatially and optically tailored 3D printing for highly miniaturized and integrated microfluidics

Traditional 3D printing based on Digital Light Processing Stereolithography (DLP-SL) is unnecessarily limiting as applied to microfluidic device fabrication, especially for high-resolution features. This limitation is due primarily to inherent tradeoffs between layer thickness, exposure time, material strength, and optical penetration that can be impossible to satisfy for microfluidic features. We introduce a generalized 3D printing process that significantly expands the accessible spatially distributed optical dose parameter space to enable the fabrication of much higher resolution 3D components without increasing the resolution of the 3D printer. Here we demonstrate component miniaturization in conjunction with a high degree of integration, including 15 μm × 15 μm valves and a 2.2 mm × 1.1 mm 10-stage 2-fold serial diluter. These results illustrate our approach's promise to enable highly functional and compact microfluidic devices for a wide variety of biomolecular applications.

Astrocytes in Down Syndrome Across the Lifespan

Down Syndrome (DS) is the most common genetic cause of intellectual disability in which delays and impairments in brain development and function lead to neurological and cognitive phenotypes. Traditionally, a neurocentric approach, focusing on neurons and their connectivity, has been applied to understanding the mechanisms involved in DS brain pathophysiology with an emphasis on how triplication of chromosome 21 leads to alterations in neuronal survival and homeostasis, synaptogenesis, brain circuit development, and neurodegeneration. However, recent studies have drawn attention to the role of non-neuronal cells, especially astrocytes, in DS. Astrocytes comprise a large proportion of cells in the central nervous system (CNS) and are critical for brain development, homeostasis, and function. As triplication of chromosome 21 occurs in all cells in DS (with the exception of mosaic DS), a deeper understanding of the impact of trisomy 21 on astrocytes in DS pathophysiology is warranted and will likely be necessary for determining how specific brain alterations and neurological phenotypes emerge and progress in DS. Here, we review the current understanding of the role of astrocytes in DS, and discuss how specific perturbations in this cell type can impact the brain across the lifespan from early brain development to adult stages. Finally, we highlight how targeting, modifying, and/or correcting specific molecular pathways and properties of astrocytes in DS may provide an effective therapeutic direction given the important role of astrocytes in regulating brain development and function.

Astrocytes in rare neurological conditions: Morphological and functional considerations

Astrocytes are a population of central nervous system (CNS) cells with distinctive morphological and functional characteristics that differ within specific areas of the brain and are widely distributed throughout the CNS. There are mainly two types of astrocytes, protoplasmic and fibrous, which differ in morphologic appearance and location. Astrocytes are important cells of the CNS that not only provide structural support, but also modulate synaptic activity, regulate neuroinflammatory responses, maintain the blood-brain barrier, and supply energy to neurons. As a result, astrocytic disruption can lead to widespread detrimental effects and can contribute to the pathophysiology of several neurological conditions. The characteristics of astrocytes in more common neuropathologies such as Alzheimer's and Parkinson's disease have significantly been described and continue to be widely studied. However, there still exist numerous rare neurological conditions in which astrocytic involvement is unknown and needs to be explored. Accordingly, this review will summarize functional and morphological changes of astrocytes in various rare neurological conditions based on current knowledge thus far and highlight remaining neuropathologies where astrocytic involvement has yet to be investigated.

Do Astrocytes Play a Role in Intellectual Disabilities?

Neurodevelopmental disorders, including those involving intellectual disability, are characterized by abnormalities in formation and functions of synaptic circuits. Traditionally, research on synaptogenesis and synaptic transmission in health and disease focused on neurons, however, a growing number of studies have highlighted the role of astrocytes in this context. Tight structural and functional interactions of astrocytes and synapses indeed play important roles in brain functions, and the repertoire of astroglial regulations of synaptic circuits is large and complex. Recently, genetic studies of intellectual disabilities have underscored potential contributions of astrocytes in the pathophysiology of these disorders. Here we review how alterations of astrocyte functions in disease may interfere with neuronal excitability and the balance of excitatory and inhibitory transmission during development, and contribute to intellectual disabilities.

The control of brain mitochondrial energization by cytosolic calcium: the mitochondrial gas pedal

This review focuses on problems of the intracellular regulation of mitochondrial function in the brain via the (i) supply of mitochondria with ADP by means of ADP shuttles and channels and (ii) the Ca(2+) control of mitochondrial substrate supply. The permeability of the mitochondrial outer membrane for adenine nucleotides is low. Therefore rate dependent concentration gradients exist between the mitochondrial intermembrane space and the cytosol. The existence of dynamic ADP gradients is an important precondition for the functioning of ADP shuttles, for example CrP-shuttle. Cr at mM concentrations instead of ADP diffuses from the cytosol through the porin pores into the intermembrane space. The CrP-shuttle isoenzymes work in different directions which requires different metabolite concentrations mainly caused by dynamic ADP compartmentation. The ADP shuttle mechanisms alone cannot explain the load dependent changes in mitochondrial energization, and a complete model of mitochondrial regulation have to account the Ca(2+) -dependent substrate supply too. According to the old paradigmatic view, Ca(2+) (cyt) taken up by the mitochondrial Ca(2+) uniporter activates dehydrogenases within the matrix. However, recently it was found that Ca(2+) (cyt) at low nM concentrations exclusively activates the state 3 respiration via aralar, the mitochondrial glutamate/aspartate carrier. At higher Ca(2+) (cyt) (> 500 nM), brain mitochondria take up Ca(2+) for activation of substrate oxidation rates. Since brain mitochondrial pyruvate oxidation is only slightly influenced by Ca(2+) (cyt) , it was proposed that the cytosolic formation of pyruvate from its precursors is tightly controlled by the Ca(2+) dependent malate/aspartate shuttle. At low (50-100 nM) Ca(2+) (cyt) the pyruvate formation is suppressed, providing a substrate limitation control in neurons. This so called "gas pedal" mechanism explains why the energy metabolism of neurons in the nucleus suprachiasmaticus could be down-regulated at night but activated at day as a basis for the circadian changes in Ca(2+) (cyt) . It also could explain the energetic disadvantages caused by altered Ca(2+) (cyt) at mitochondrial diseases and neurodegeneration.

In vivo restoration of physiological levels of truncated TrkB.T1 receptor rescues neuronal cell death in a trisomic mouse model

Imbalances in neurotrophins or their high-affinity Trk receptors have long been reported in neurodegenerative diseases. However, a molecular link between these gene products and neuronal cell death has not been established. In the trisomy 16 (Ts16) mouse there is increased apoptosis in the cortex, and hippocampal neurons undergo accelerated cell death that cannot be rescued by administration of brain-derived neurotrophic factor (BDNF). Ts16 neurons have normal levels of the TrkB tyrosine kinase receptor but an upregulation of the TrkB.T1 truncated receptor isoform. Here we show that restoration of the physiological level of the TrkB.T1 receptor by gene targeting rescues Ts16 cortical cell and hippocampal neuronal death. Moreover, it corrects resting Ca2+ levels and restores BDNF-induced intracellular signaling mediated by full-length TrkB in Ts16 hippocampal neurons. These data provide a direct link between neuronal cell death and abnormalities in Trk neurotrophin receptor levels.

Apolipoprotein E isoforms increase intracellular Ca2+ differentially through a omega-agatoxin IVa-sensitive Ca2+-channel

Apolipoprotein E (apoE) is the major apolipoprotein in the brain and is known for its important role in plasticity and neurodegeneration. We show that apoE dose-dependently increases intracellular free Ca2+ in rat hippocampal astrocytes and neurons. This effect varies with isoforms in the order E4 > E3 > E2. It is insensitive to blockade of action potentials by tetrodotoxin or inhibition of binding of apoE by heparinase, by the LRP ligand lactoferrin and by low density lipoprotein. ApoE evoked Ca2+-increases are blocked in zero [Ca]o and by the Ca-channel antagonists nickel and omega-Agatoxin-IVa but not by nifedipine and omega-Conotoxin-GVIa, demonstrating an isoform-specific activation of P/Q type Ca2+-channels. This novel mechanism is discussed with respect to Alzheimer's disease, that is linked for most cases to the apoE epsilon-allelic variation (epsilon4 > epsilon3 > epsilon2).

Mouse Models of Cognitive Disorders in Trisomy 21: A Review

Trisomy 21 (TRS21) is the most frequent genetic cause of mental retardation. Although the presence of an extra copy of HSA21 is known to be at the origin of the syndrome, we do not know which 225 HSA21 genes have an effect on cognitive processes. Mouse models of TRS21 have been developed using syntenies between HSA21 and MMU16, MMU10 and MMU17. Available mouse models carry extra fragments of MMU16 or of HSA21 that cover all of HSA21 (chimeric HSA21) or MMU16 (Ts16); some carry large parts of MMU16 (Ts65Dn, Ts1Cje, Ms1Cje), while others have reduced contiguous fragments covering the D21S17-ETS2 region or single transfected genes. This offers a nest design strategy for deciphering cognitive (learning, memory and exploration) and associated brain abnormalities involving each of these chromosomal regions. This review confirms the crucial but not exclusive contribution of the D21S17-ETS2 region encompassing 16 genes to cognitive disorders.

Minoranomalien der Hornhaut bei der murinen Trisomie�16

BACKGROUND

The prevalence of human Down's syndrome is about 1:700. Investigations using animal models are therefore of clinical relevance for understanding its etiopathogenesis. No corneal changes have been reported with transgenic murine trisomy 16.

METHODS

A total of 20 fetal mice (n=40 eyes) with experimentally induced trisomy 16 were investigated from day 18 of pregnancy in order to determine whether visible developmental disorders of the cornea occur. All specimen were investigated microscopically in serial sections.

RESULTS

In addition to disturbances in systemic development, the transgenic mouse fetuses showed high rates of malformation of the eyes. Developmental and differentiation disorders of the corneal epithelial cell layers and structural disturbances of the corneal parenchyma were found. Our findings are the first demonstration of developmental disorders of the cornea in mouse fetuses with trisomy 16. These minor anomalies of the cornea could well have resulted in keratoconus if the animals had survived.

CONCLUSIONS

Our findings in transgenic mouse fetuses with trisomy 16 correspond to the clinical pattern of Down's syndrome in humans. Disturbed development of lids and lenses have a high prevalence, whereas corneal hypoplasia is found less often.

Altered astrocyte calcium homeostasis and proliferation in theTs65Dn mouse, a model of Down syndrome

Genes from the Down syndrome (DS) critical region of human chromosome 21, which contribute to the pathology of DS, are also found on mouse chromosome 16. Several animal models of DS with triplication of genes from the DS critical region have been generated, including mouse trisomy 16 (Ts16) and a partial trisomic mouse, Ts65Dn. Using computer-assisted imaging of fura-2 fluorescence, we found an elevation of intracellular cytoplasmic calcium in cortical astrocytes from neonatal Ts65Dn mouse brain, similar to that observed previously in embryonic Ts16 astrocytes. Furthermore, astrocytes from both Ts65Dn and Ts16 cortex fail to respond to the anti-proliferative actions of glutamate. These results suggest that defective regulation of cell proliferation and cellular calcium can result from triplication of DS critical region genes.

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.

Digitonin-permeabilization of astrocytes in culture monitored by trypan blue exclusion and loss of S100B by ELISA

The present protocol details a procedure to permeabilize astrocytes in cultures with digitonin as well as to discuss some data about factors that interfere in permeabilization, particularly divalent cations and nucleotides. Two methods to assess astrocyte permeabilization are described: trypan blue exclusion and ELISA for S100B, a specific protein expressed by these cells. Digitonin-permeabilization of astrocytes has been used to investigate intracellular pools of Ca(2+), internal stores of metabolites, phosphoinositide hydrolysis, and recently we standardized a procedure to study protein phosphorylation (Brain Res. 853 (2000) 32-40). A short incubation time (10 min) with 30 microM digitonin permeabilized at least 75% of cells. A range of media with different ionic nature can be used in cell permeabilization without affecting significantly the extent of permeabilization, but calcium and ATP of the order of 10(-5) M induced a partial resealing which deserves to be considered in assays of permeabilized preparations of astrocytes.

Evidence for an interferon-related inflammatory reaction in the trisomy 16 mouse brain leading to caspase-1-mediated neuronal apoptosis

The trisomy of human chromosome 21 (Down syndrome) is the leading genetic cause of learning difficulties in children, and predisposes this population to the early onset of the neurodegeneration of Alzheimer's disease. Down syndrome is associated with increased interferon (IFN) sensitivity resulting in unexpectedly high levels of IFN inducible gene products including Fas, complement factor C3, and neuronal HLA I which could result in a damaging inflammatory reaction in the brain. Consistent with this possibility, we report here that the trisomy 16 mouse fetus has significantly increased whole brain IFN-gamma and Fas receptor immunoreactivity and that cultured whole brain trisomy 16 mouse neurons have increased basal levels of caspase 1 activity and altered homeostasis of intracellular calcium and pH. The trisomic neurons also showed a heightened sensitivity to the increase in both Fas receptor levels and caspase 1 activity we observed when IFN-gamma was added to the neuron culture media. Because of the autoregulatory nature of IFN activity, and the IFN inducing capability of caspase-1-activated cytokine activity, our data argue in favor of the possibility of an interferon-mediated, self-perpetuating, inflammatory response in the trisomy brain that could subserve the loss of neuron viability seen in this trisomy 16 mouse model for Down syndrome.

Molecular markers of oxidative stress vulnerability

In astrocyte primary cultures of trisomy 16 mice, an animal model for Down's syndrome, protein oxidation was 50% higher than in diploid littermates. Exposure to 10 microM H2O2 or 50 microM kainic acid incremented protein oxidation in trisomic but not in diploid cultures. Studies on stress response genes showed that metallothionein (MT) level was 2-3 times higher in trisomy 16 than in diploid cultures. Kainic acid or H2O2 exposure increased the MT protein level in diploid cultures but failed to increase it in trisomy 16 mouse beyond its elevated basal level. The reduced responsiveness of MT to simulated oxidative stress may result in insufficient removal of ROS, which could partially explain the further increase of protein oxidation in trisomy 16 cultures. In contrast, Pb exposure increased MT in trisomy 16 and diploid primary cultures to a similar extent. The similar metal responsiveness of MT in both phenotypes indicated that MT in trisomic glial cultures was not yet maximally stimulated. The flawed redox sensitivity in trisomy 16 mouse suggests possible alterations in the binding activity of ROS-sensitive transcription factors on the MT promoter.

GFAP phosphorylation studied in digitonin-permeabilized astrocytes: standardization of conditions

Cycles of assembly/disassembly of the intermediate filaments of astrocytes are modulated by the phosphorylation of glial fibrillary acidic protein (GFAP). The sites on GFAP are localized at the N-terminal where they are phosphorylated by cAMP-dependent and Ca(2+)-dependent protein kinases. Phosphorylation of GFAP has been investigated in brain slices, astrocyte cultures, cytoskeletal fractions and purified systems. Here we describe a different approach to study GFAP phosphorylation. We show that permeabilization of astrocytes in culture with digitonin allows direct access to the systems phosphorylating GFAP. Conditions for the permeabilization were established with an assay based on the exclusion of Trypan blue. Incubation of permeabilized cells with cAMP and Ca(2+) increased the phosphorylation state of GFAP. Immunocytochemistry with anti-GFAP showed that permeabilized astrocytes retained their typical flat, fibroblast morphology and exhibited well preserved glial filaments. On incubation with cAMP the filaments apparently condensed to form long processes. The results suggest the approach of studying structural changes in glial filaments in parallel to protein phosphorylation, in the presence of specific modulators of protein kinases and phosphatases has considerable potential.

Neuronal apoptosis in mouse trisomy 16: mediation by caspases

Hippocampal neurons from the trisomy 16 (Ts16) mouse, a potential animal model of Down's syndrome (trisomy 21) and neurodegenerative disorders such as Alzheimer's disease (AD), die at an accelerated rate in vitro. Here, we present evidence that the accelerated neuronal death in Ts16 occurs by apoptosis, as has been reported for neurons in AD. First, the nuclei of dying Ts16 neurons are pyknotic and undergo DNA fragmentation, as revealed by terminal transferase-mediated dUTP nick end-labeling. Second, the accelerated death of Ts16 neurons is prevented by inhibitors of the caspase family of proteases, which are thought to act at a late, obligatory step in the apoptosis pathway. In the presence of maximally effective concentrations of caspase inhibitors, Ts16 neuron survival was indistinguishable from that of control neurons. These results suggest that overexpression of one or more genes on mouse chromosome 16 leads to caspase-mediated apoptosis in Ts16 neurons.

Altered Ca2+ signaling and mitochondrial deficiencies in hippocampal neurons of trisomy 16 mice: a model of Down's syndrome

It has been suggested that augmented nerve cell death in neurodegenerative diseases might result from an impairment of mitochondrial function. To test this hypothesis, we investigated age-dependent changes in neuronal survival and glutamate effects on Ca2+ homeostasis and mitochondrial energy metabolism in cultured hippocampal neurons from diploid and trisomy 16 (Ts16) mice, a model of Down's syndrome. Microfluorometric techniques were used to measure survival rate, [Ca2+]i level, mitochondrial membrane potential, and NAD(P)H autofluorescence. We found that Ts16 neurons die more than twice as fast as diploid neurons under otherwise identical culture conditions. Basal [Ca2+]i levels were elevated in Ts16 neurons. Moreover, in comparison to diploid neurons, Ts16 neurons showed a prolonged recovery of [Ca2+]i and mitochondrial membrane potential after brief glutamate application. Glutamate evoked an initial NAD(P)H decrease that was found to be extended in Ts16 neurons in comparison to diploid neurons. Furthermore, for all age groups tested, glutamate failed to cause a subsequent NAD(P)H overshoot in Ts16 cultures in contrast to diploid cultures. In the presence of cyclosporin A, an inhibitor of the mitochondrial membrane permeability transition, NAD(P)H increase was observed in both diploid and Ts16 neurons. The results support the hypothesis that Ca2+ impairs mitochondrial energy metabolism and may play a role in the pathogenesis of neurodegenerative changes in neurons from Ts16 mice.

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.