Neurochemical characterization of embryonic brain development in trisomy 19 (Ts19) mice: implications of selective deficits observed for abnormal neural development in aneuploidy

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.

Impaired cholinergic function in cell lines derived from the cerebral cortex of normal and trisomy 16 mice

Murine trisomy 16 is an animal model of human Down's syndrome. We have successfully established permanently growing cell lines from the cerebral cortex of normal and trisomy 16 foetal mice using an original procedure. These lines, named CNh (derived from a normal animal) and CTb (derived from a trisomic foetus), express neuronal markers. Considering that Down's syndrome exhibits cholinergic deficits, we examined cholinergic function in these lines, using incorporation of [3H]-choline and fractional release studies. After 1, 3 and 5 min of [3H]-choline incubation, CTb cell uptake was lower by approximately 50% compared to controls. Hemicholinium-3 significantly reduced the incorporation of [3H]-choline in both CNh and CTb cells at high concentration (10 microM), suggesting high-affinity choline transport. However, CTb cells exhibited greater sensitivity to the blocker. For fractional release experiments, the cells were stimulated by K+ depolarization, glutamate or nicotine. When depolarized, CTb cells showed a 68% reduction in fractional release of [3H]-acetylcholine compared to CNh cell line, and a 45% reduction when stimulated by nicotine. Interestingly, glutamate induced similar levels of release in both cell types. The results indicate the existence of cholinergic dysfunction in CTb cells when compared to CNh, similar to that reported for primary cultures of trisomy 16 brain tissue (Fiedler et al. 1994, Brain Res., 658, 27-32). Thus, the CTb cell line may serve as a model for the study of Down's syndrome pathophysiology.

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.

Alterations of central noradrenergic transmission in Ts65Dn mouse, a model for Down syndrome

Mice with segmental trisomy 16 (Ts65Dn) which have triplication of a region of mouse chromosome 16 homologous to the Down syndrome critical region in human chromosome 21, are used as a model for Down syndrome. Functioning of the central beta-noradrenergic transmission was studied in Ts65Dn mice. Binding analysis in cerebral cortex revealed no change in the number of beta-adrenoceptors and a slight reduction of affinity. The beta-adrenoceptor transduction was assessed by analyzing cAMP formation in the cerebral cortex, hippocampus and cerebellar cortex under basal conditions and after stimulation with isoprenaline and forskolin. Basal production of cAMP was significantly reduced in hippocampus and cerebellar cortex of Ts65Dn mice compared to control, but not in cerebellum. After phosphodiesterase inhibition, net increments in cAMP accumulation were similar in both groups of mice. Stimulation of cAMP production by isoprenaline (10 microM) and forskolin (10 microM) was much higher in hippocampus than in cerebral cortex of either group. In both areas, but not in cerebellum, the stimulatory responses were consistently and significantly smaller in Ts65Dn than in control mice. Concentration-response curves for isoprenaline and forskolin were generated in the cerebral cortex. Emax responses were lower in trisomic than in control mice; however, in Ts65Dn mice the slope of the response curve to isoprenaline was markedly depressed whereas that to forskolin was similar to control. It is concluded that Ts65Dn mice show severe deficiencies in the synaptic transmission of the central beta-noradrenergic system, which are selective for specific brain areas.

Septal cell lines derived from the trisomy 16 mouse: generation, characterization, and response to NGF

The trisomy 16 mouse is a genetic model of Down syndrome. Clonal cell lines were developed from trisomic as well as euploid embryonic mouse septal cells by introduction of thermolabile large T antigen mutant of SV 40. The cell lines underwent morphological differentiation at the non-permissive temperature and in response to a differentiating agent. Immunocytochemical staining indicated that cells of neuronal lineage were immortalized. The addition of beta-nerve growth factor (100 ng/ml) increased the survival rate of a trisomy cell line in differentiated state, as measured by Trypan blue exclusion. These cell lines may prove useful in studies of neuronal abnormalities in this mouse model of Down syndrome.

Regional alteration of cholinergic function in central neurons of trisomy 16 mouse fetuses, an animal model of human trisomy 21 (Down syndrome)

The trisomy-16 (TS16) mouse is considered to be a model of human trisomy 21 (Down syndrome) because of genetic homology between mouse chromosome 16 and human chromosome 21. We examined cholinergic function of brain and spinal cord tissue and in cultured neurons from TS16 mouse compared with that of age matched controls. Mean acetylcholinesterase activity in both tissue types did not differ between trisomic and control conditions. Acetylcholine (ACh) synthesis, measured as choline O-acetyltratransferase (acetyl-CoA) activity, was reduced to 67% of control in TS16 brain but not in TS16 spinal cord. Steady-state accumulation of ACh precursor, [3H]choline, was measured in primary cell cultures. Steady-state choline uptake was reduced to 35% and to 61% in neurons of TS16 brain and spinal cord, respectively, when compared with controls. Kinetics experiments in TS16 brain cells showed a 50% reduction of the maximal velocity of choline uptake when compared to controls. Further, the ACh release induced by KCl depolarization in TS16 spinal cord neurons did not differ from control neurons but was reduced in TS16 brain neurons. This effect cannot be explained solely by a reduction in ACh synthesis. The results indicate that the TS16 condition in mice significantly modified the cholinergic function in brain, and to a lesser degree in spinal cord, suggesting that the higher gene dosage inherent to the trisomic condition affects cholinergic neurons in different regions of the central nervous system in a differential fashion.

Altered postnatal development of the visual cortex in trisomy 19 mice

The development of the visual cortex was studied in 30 Trisomy 19 (Ts19) mice aged 1-16 days postpartum and their euploid littermates. Morphogenesis of the Ts19 visual cortex, though delayed in development, followed the regular sequence observed in control littermates. Early morphogenetic events, such as obliteration of the ventricular lumen, disappearance of the ventricular zone, formation of a visible apical dendrite, as well as disappearance of both migrating neurons and the columnar organization of bipolar preneurons were delayed by 1 day; maturation steps occurring later such as appearance and disappearance of perikaryal basophilia were delayed by 2 days. Myelination of the white matter was similarly retarded by 2 days. The fronto-occipital length of the cerebral hemispheres and the thickness of the visual cortex were decreased by about 20%, consistent with a hypoplasia of the Ts19 neocortex. Unlike in the cerebral cortex of human Ts21, morphometric analysis of the visual cortex of Ts19 mice did not give any indication of a selective deficit in a particular neuron population; the increased cell density and the reduced nuclear volume observed during early postnatal development are attributable to a maturational delay. The relevance of these results with respect to the mechanisms underlying neuropathological alterations in human Ts21 is discussed.

Mouse fetal trisomy 13 and hypotrophy of the spinal cord: effect on calbindin-D28k and calretinin expressed by neurons of the spinal cord and dorsal root ganglia

Trisomy 13 was detected in 10% of mouse embryos obtained from pregnant females which were doubly heterozygous for Robertsonian chromosomes involving chromosome 13. The developing dorsal root ganglia and spinal cords were examined in trisomy 13 and littermate control mice between days 12 and 18 of gestation (E12-18). The overall size of the dorsal root ganglia and number of ganglion cells within a given ganglion were not altered, but the number of neurons immunoreactive for calbindin and calretinin was reduced. The trisomic spinal cord was reduced in size with neurons lying in a tightly compact distribution in the gray matter. In trisomic fetuses, the extent of the neuropil of the spinal cord was reduced, and may represent a diminished field of interneuronal connectivity, due to reduced arborization of dendritic processes of the neurons present, particularly of calbindin-immunostained neurons. Furthermore, the subpopulation of calretinin-immunoreactive neurons and axons was also reduced in developing trisomic gray and white matter, respectively. Thus, overexpression of genes on mouse chromosome 13 exerts a deleterious effect on the development of neuropil, affecting both dendritic and axonal arborization in the trisomy 13 mouse. The defect of calbindin or calretinin expression by subsets of dorsal root ganglion or spinal cord neurons may result from deficient cell-to-cell interactions with targets which are hypoplastic.

Foliation of the cerebellar vermis in trisomy 19 mice

The development of the folial pattern was studied in the cerebellar vermis of 32 trisomy 19 (Ts19) mice aged 1-16 days postpartum and their euploid littermates. In the Ts19 cerebellum, fissures were formed in the regular sequence observed in control littermates, but their appearance was delayed by about 2 days. Fissure number increased until day 6 in euploid controls and in Ts19 mice, remaining constant thereafter. In Ts19 cerebella, fissure number and fissure depth were reduced significantly; there were 30% fewer Purkinje cells and the cross-sectional areas of the external germinal layer and of the total cerebellar vermis were decreased, reflecting a permanent hypoplasia. Both in Ts19 and control mice, a temporal and quantitative relationship was observed between fissure formation and the expansion of the external germinal layer, whereas the increase in fissure depth was found to correlate with the growth of the whole cerebellar vermis. Determination of the surface folding index revealed that only during fissure formation, the expansion of the cerebellar surface exceeded that of the cerebellar volume. The present study does not give any indication that foliation and histogenesis of the cerebellum are differentially affected by trisomy.

Postnatal development of the locus coeruleus in trisomy 19 mice: morphological and morphometric study

To study the effect of trisomy upon a brain region that is generated very early during development, the locus coeruleus (LC) has been examined morphologically and morphometrically in 23 Trisomy 19 (Ts19) mice and their chromosomally balanced control littermates aged 2-18 days postpartum. Gross morphological alterations of the Ts19 LC could neither be observed by light nor by electron microscopy. The LC was properly located. Ultrastructural features indicating increasing protein synthesis such as nucleolus-like bodies and a rise in the amount of granular endoplasmic reticulum and in the size of the nucleoli have been observed both in Ts19 and control mice. Maturation of the LC was delayed in Ts19. Morphometric studies on the volume, cell number, and cell density revealed that, apart from a 2-day delay in development, the Ts19 LC was of normal size. The present study supports the observation that the noradrenergic system is not affected in the Ts19 CNS. Taking into account that the cerebellum of Ts19 mice is markedly hypoplastic, the results indicate a differential pathogenic effect of trisomy upon different neural systems.

Developmental expression of the gene encoding growth-associated protein 43 (Gap43) in the brains of normal and aneuploid mice

The gene encoding growth-associated protein 43 (Gap43), a neuronal phosphoprotein associated with axonal outgrowth and synaptic plasticity, is located on mouse chromosome 16 (MMU16). We examined the developmental expression of Gap43 in normal, trisomy 16 (Ts16), and trisomy 19 (Ts19) mouse brain using northern blot analysis and in situ hybridization as a first step toward understanding the neurobiologic consequences of increased gene dosage on brain development. Gap43 expression was detected by in situ hybridization throughout the mesencephalon, rhombencephalon, spinal cord, and first branchial arch in whole embryos as early as day 10 of gestation (E10). By E15, Gap43 expression was localized to cells in the retina, the olfactory bulbs, and anterior olfactory structures, the cortical plate, the basal telencephalon, diencephalon, midbrain, hindbrain, and spinal cord. Northern blot analysis detected a three-fold increase in Gap43 mRNA levels in the brains of normal mice between E12-E18. At E15, Gap43 mRNA levels were increased 35-40% in Ts16 mouse brain and decreased 10% in Ts19 mouse brain, relative to euploid littermate controls. Using in situ hybridization we found that overexpression of Gap43 occurred in the diencephalon, medial and lateral basal telencephalon, and cortical plate region in Ts16 mice relative to littermate controls. Thus, the degree of overexpression of Gap43 mRNA in Ts16 mice is consistent with that expected from gene dosage effects.

Developmental profiles of gangliosides in trisomy 19 mice

The ganglioside composition of the cerebrum, cerebellum, brainstem, liver, heart, and spleen was analyzed quantitatively in trisomy 19 (Ts19) mice aged 4 to 12 days postpartum. The developmental profiles of cerebral gangliosides were similar in Ts19 mice and control littermates: Total ganglioside-sialic acid as well as the proportions of the individual gangliosides GD1a and GM1 increased with age, while the percentages of GQ1b and GT1b decreased during development. Both the accretion of the total ganglioside content and the development of the individual ganglioside fractions were delayed by 2-3 days in the Ts19 telencephalon. Likewise, the shift from the b- to the a-pathway of ganglioside synthesis was retarded. Ganglioside development was equally delayed in the cerebellum and the brainstem of Ts19 mice. Since in Ts19 mice, morphogenesis of several brain regions is similarly delayed by 2 days, these results confirm the usefulness of gangliosides as biochemical markers for brain maturation. In contrast to brain gangliosides, the ganglioside composition of the Ts19 livers was clearly distinguished from that of control livers. Total ganglioside-bound sialic acid was increased by 35-50% in Ts19 livers. This elevation in ganglioside content not explicable by a simple delay in development was mainly due to an increase in GD3 and fraction 2, which is likely to contain GD1a and GD1b. In contrast, GM2 which increased considerably with age in control mice persisted on a low level in Ts19 livers. Comparable alterations of the ganglioside pattern were neither observed in the spleen nor in the heart of Ts19 mice. The data presented give additional evidence that ganglioside synthesis in the liver is under a different regulation mechanism than that in the brain, heart, and spleen.

Electrical membrane properties of cultured dorsal root ganglion neurons from trisomy 19 mouse fetuses: a comparison with the trisomy 16 mouse fetus, a model for Down syndrome

Because of synteny between mouse chromosome 16 and human chromosome 21, murine trisomy 16 (Ts16) has been considered an animal model for Down syndrome. Indeed, previous investigations have demonstrated that action potentials of cultured dorsal root ganglion (DRG) neurons from human trisomy 21 (Down syndrome) or mouse Ts16 fetuses show increased depolarization and repolarization rates, and decreased spike duration, compared to control neurons. In order to determine the specificity of these changes, we studied the electrical membrane properties of DRG neurons in culture from trisomy 19 (Ts19) and control fetal mice, using the whole cell patch-pipette recording technique. We found no significant differences in action potential parameters and passive membrane properties between Ts19 and control neurons. These findings support the notion that the alterations previously reported in Ts16 DRG neurons are specific, and not a general consequence of genetic imbalance imposed by autosomal trisomies.

Selective retardation of the development of the basal forebrain cholinergic and pontine catecholaminergic nuclei in the brain of trisomy 16 mouse, an animal model of Down's syndrome

Brain development, examined at embryonic day 17, was retarded in murine trisomy 16 (Ts16). Ts16 is considered to serve as a model of the human trisomy 21 (Down's syndrome) by virtue of the presence in the mouse chromosome 16 of a set of genes located in humans in the segment of chromosome 21 that is requisite to produce the phenotypic features of Down's syndrome when present in triplicate. In addition to a reduction in brain size and cortical thickness, we observed a severe reduction throughout the brain in the density of muscarinic receptors, assessed by autoradiographic detection of specifically bound tritiated N-methyl-scopolamine, and by the failure of the development of the differentiated pattern of receptor distribution in the brainstem. The effect of gene dosage was also examined on specific neuronal populations. The distribution of acetylcholine esterase (AChE)-, tyrosine hydroxylase (TH)- and 5-hydroxytryptamine (5-HT)-positive cells in the trisomic brain was similar to that observed in chromosomally balanced littermates. On the other hand, the number of AChE-positive cells was 60-70% of the estimates in littermate controls in regions containing the septum, the vertical and horizontal limbs of the diagonal band and the basal forebrain cholinergic nuclei. Similarly, the number of TH-positive cells was reduced by about 30% in the pons. In contrast, in the trisomic foetuses the number of TH-positive cells in the mesencephalon and the diencephalon was similar to that in littermate controls, while that of 5-HT-positive cells in the mesencephalic nuclei was only slightly affected, if at all. Ts16 results, therefore, in a selective retardation of some neuronal systems, and this may lead to a perturbation of brain development. Furthermore, the systems whose development was retarded selectively are those which in Down's syndrome adults exhibit pronounced deficits of cells that--in case the murine Ts16 is a valid model--may also involve developmental disorders.

Altered placental morphology associated with murine trisomy 16 and murine trisomy 19

The morphology of placentas from trisomy 16 and trisomy 19 mouse conceptuses aged 12 to 18 gestational days was studied at the light microscopic level. Comparisons were made with placentas from normal littermate animals. Trisomy 16 placentas showed marked changes from normal: 1) the junctional zone showed little indication of normal morphologic differentiation throughout gestation; 2) clusters of germinal trophoblast cells persisted in the labyrinth throughout gestation, whereas these cells disappeared by gestational day 16 in the normal littermate placentas; 3) the labyrinth was reduced in size in the trisomic placentas, and the differentiation of the interhemal membranes was delayed. The size of the labyrinths from trisomy 19 placentas appeared to be decreased, but otherwise the placentas appeared to have normal morphology. These observations and others from the literature show that placental development is affected by the presence of a trisomic genome, and that different trisomies influence the development of the placenta differently. For trisomy 16, we propose that the striking changes of the junctional zone may be associated with the trisomy 16-related gene dosage effect for alpha- and beta-interferon cell surface receptors. Because of the homology for this and other genes on mouse chromosome 16 with genes on human chromosome 21, findings related to the altered development of the trisomy 16 mouse may be relevant to understanding some of the phenotypic variations associated with human trisomy 21, the Down syndrome.

Neurogenesis of the basal forebrain in euploid and trisomy 16 mice: an animal model for developmental disorders in Down syndrome

The neurogenesis and early histochemical differentiation of the basal forebrain in trisomy 16 fetal mice and their euploid littermates were examined by combining [3H]thymidine autoradiography with acetylcholinesterase histochemistry. Neurons of the basal forebrain were being born between embryonic day 11 and 15 in both chromosomally normal (euploid) and aneuploid mice. In euploid littermate controls, neurogenesis proceeded along a caudal to rostral gradient with the peak on embryonic day 11 for caudal portions and embryonic day 13 for rostral portions of the basal forebrain. In contrast, in trisomy 16 mice, rostral sections exhibited a peak of neurogenesis on embryonic day 11, 2 days earlier than in their euploid littermate controls. Hypocellularity of the basal forebrain region was noted in trisomy 16 mice; particularly dramatic was the reduction of the population of cells that expressed acetylcholinesterase. This reduction in cell number in the trisomics was not accompanied by a reduction in cell size or by a dramatic change in the distribution of residual neurons when compared to that of euploid littermate controls. Since trisomy 16 mice do not survive the perinatal period, we examined the pattern of acetylcholinesterase expression in normal C57B1/6J mice from embryonic day 16 to postnatal day 5 to determine the postnatal disposition of these neurons. Already at embryonic day 16, fibers staining for acetylcholinesterase penetrated the striatal anlage, in their course towards targets in the cerebral cortices. By postnatal day 5, the previously expansive distribution of basal forebrain neurons had become consolidated in a more ventral and rostral position by the extensive outgrowth of the striatal neurons, a pattern resembling that seen in adult animals.

Neuroanatomical localization and quantification of amyloid precursor protein mRNA by in situ hybridization in the brains of normal, aneuploid, and lesioned mice

Amyloid precursor protein mRNA was localized in frozen sections from normal and experimentally lesioned adult mouse brain and from normal and aneuploid fetal mouse brain by in situ hybridization with a 35S-labeled mouse cDNA probe. The highest levels of hybridization in adult brain were associated with neurons, primarily in telencephalic structures. The dense labeling associated with hippocampal pyramidal cells was reduced significantly when the cells were eliminated by injection of the neurotoxin ibotenic acid but was not affected when electrolytic lesions were placed in the medial septum. Since the gene encoding amyloid precursor protein has been localized to mouse chromosome 16, we also examined the expression of this gene in the brains of mouse embryos with trisomy 16 and trisomy 19 at 15 days of gestation. RNA gel blot analysis and in situ hybridization showed a marked increase in amyloid precursor protein mRNA in the trisomy 16 mouse head and brain when compared with euploid littermates or with trisomy 19 mice.