Neuronal dysfunction in Down syndrome: contribution of neuronal models in cell culture

Effects of a Tripeptide on Mitogen-Activated Protein Kinase and Glycogen Synthase Kinase Activation in a Cell Line Derived from the Foetal Hippocampus of a Trisomy 16 Mouse: an Animal Model of Down Syndrome

Down syndrome (DS) is a developmental disorder that results from the trisomy of chromosome 21. DS patients show several abnormalities including cognitive deficits. Here, we show enhanced activation of the extracellular signal-regulated kinase (ERK), a kinase that critically regulates synaptic plasticity and memory, in a hippocampal cell line derived from trisomy 16 mouse foetus. In addition, these cells show enhanced activation of p38 mitogen-activated protein kinase (p38 MAPK). The hyper-activation of ERK and p38 MAPK is significantly reduced by a small peptide, Gly-Pro-Glu (GPE), derived from insulin-like growth factor-1. In addition, the trisomic cells show reduced level of inhibitory phosphorylation of glycogen synthase kinase-3β (GSK-3β), which is enhanced by GPE. Furthermore, the trisomic cells do not show ERK activation in response to KCl depolarization or forskolin treatment. Importantly, ERK activation by these stimuli is observed after GPE treatment of the cells. These results suggest that GPE may help reduce aberrant signalling in the trisomic neurons by affecting MAPK and GSK-3β activation.

Advances in understanding the association between Down syndrome and Hirschsprung disease (DS-HSCR)

The clinical association between Trisomy 21 (Down syndrome) and aganglionosis (Hirschsprung disease; DS-HSCR) is well-established, being of the order of 5% and remains the most common congenital association with Hirschsprung disease. However, little consensus exists as to the possible etiologic and genetic factors influencing this association. Recent research has identified a number of levels at which development of the enteric nervous system is potentially affected in Trisomy 21. These include a decreased central pool of available neuroblasts for migration into the enteric nervous system, abnormal neuroblast type, poor synaptic nerve function and early germline gene-related influences on the migrating neuroblasts due to genetic mutations of a number of important developmental genes, and possible somatic mutations resulting from alterations in the local tissue microenvironment. In this paper, we review available evidence for this association. In addition, we provide evidence of both germline and somatic gene mutations suggesting causation. Although the picture is complex, recent associations between specific RET proto-oncogene variations have been shown to be significant in Down syndrome patients with Hirschsprung disease, as they probably interfere with vital RET functions in the development of the autonomic and enteric nervous systems, increasing the risk of disturbed normal function. In addition, we explore potential role of other facilitatory influence of other susceptibility genes as well as potential other chromosome 21 gene actions and the microenvironment on the Down syndrome gastro-intestinal tract. The various ways in which trisomy of chromosome influences the enteric nervous system are becoming clearer. The sum of these effects influences the outcome of surgery in Down syndrome patients with Hirschsprung Disease.

Investigation of copy number variations on chromosome 21 detected by comparative genomic hybridization (CGH) microarray in patients with congenital anomalies

Background

The clinical features of Down syndrome vary among individuals, with those most common being congenital heart disease, intellectual disability, developmental abnormity and dysmorphic features. Complex combination of Down syndrome phenotype could be produced by partially copy number variations (CNVs) on chromosome 21 as well. By comparing individual with partial CNVs of chromosome 21 with other patients of known CNVs and clinical phenotypes, we hope to provide a better understanding of the genotype-phenotype correlation of chromosome 21.

Methods

A total of 2768 pediatric patients sample collected at the Genetics Laboratory at Oklahoma University Health Science Center were screened using CGH Microarray for CNVs on chromosome 21.

Results

We report comprehensive clinical and molecular descriptions of six patients with microduplication and seven patients with microdeletion on the long arm of chromosome 21. Patients with microduplication have varied clinical features including developmental delay, microcephaly, facial dysmorphic features, pulmonary stenosis, autism, preauricular skin tag, eye pterygium, speech delay and pain insensitivity. We found that patients with microdeletion presented with developmental delay, microcephaly, intrauterine fetal demise, epilepsia partialis continua, congenital coronary anomaly and seizures.

Conclusion

Three patients from our study combine with four patients in public database suggests an association between 21q21.1 microduplication of CXADR gene and patients with developmental delay. One patient with 21q22.13 microdeletion of DYRK1A shows association with microcephaly and scoliosis. Our findings helped pinpoint critical genes in the genotype-phenotype association with a high resolution of 0.1 Mb and expanded the clinical features observed in patients with CNVs on the long arm of chromosome 21.

Nucleic Acid-Based Theranostics for Tackling Alzheimer's Disease

Nucleic acid-based technologies have received significant interest in recent years as novel theranostic strategies for various diseases. The approval by the United States Food and Drug Administration (FDA) of Nusinersen, an antisense oligonucleotide drug, for the treatment of spinal muscular dystrophy highlights the potential of nucleic acids to treat neurological diseases, including Alzheimer's disease (AD). AD is a devastating neurodegenerative disease characterized by progressive impairment of cognitive function and behavior. It is the most common form of dementia; it affects more than 20% of people over 65 years of age and leads to death 7-15 years after diagnosis. Intervention with novel agents addressing the underlying molecular causes is critical. Here we provide a comprehensive review on recent developments in nucleic acid-based theranostic strategies to diagnose and treat AD.

Knockdown of Myo-Inositol Transporter SMIT1 Normalizes Cholinergic and Glutamatergic Function in an Immortalized Cell Line Established from the Cerebral Cortex of a Trisomy 16 Fetal Mouse, an Animal Model of Human Trisomy 21 (Down Syndrome)

The Na⁺/myo-inositol cotransporter (SMIT1) is overexpressed in human Down syndrome (DS) and in trisomy 16 fetal mice (Ts16), an animal model of the human condition. SMIT1 overexpression determines increased levels of intracellular myo-inositol, a precursor of phophoinositide synthesis. SMIT1 is overexpressed in CTb cells, an immortalized cell line established from the cerebral cortex of a Ts16 mouse fetus. CTb cells exhibit impaired cytosolic Ca²⁺ signals in response to glutamatergic and cholinergic stimuli (increased amplitude and delayed time-dependent kinetics in the decay post-stimulation), compared to our CNh cell line, derived from the cerebral cortex of a euploid animal. Considering the role of myo-inositol in intracellular signaling, we normalized SMIT1 expression in CTb cells using specific mRNA antisenses. Forty-eight hours post-transfection, SMIT1 levels in CTb cells reached values comparable to those of CNh cells. At this time, decay kinetics of Ca²⁺ signals induced by either glutamate, nicotine, or muscarine were accelerated in transfected CTb cells, to values similar to those of CNh cells. The amplitude of glutamate-induced cytosolic Ca²⁺ signals in CTb cells was also normalized. The results suggest that SMIT1 overexpression contributes to abnormal cholinergic and glutamatergic Ca²⁺ signals in the trisomic condition, and knockdown of DS-related genes in our Ts16-derived cell line could constitute a relevant tool to study DS-related neuronal dysfunction.

Restrained Phosphatidylcholine Synthesis in a Cellular Model of Down's Syndrome is Associated with the Overexpression of Dyrk1A

Aberrant formation of the cerebral cortex could be attributed to the lack of suitable substrates that direct the migration of neurons. Previous work carried out at our laboratory has shown that oleic acid is a neurotrophic factor. In order to characterize the effect of oleic acid in a cellular model of Down's syndrome (DS), here, we used immortalized cell lines derived from the cortex of trisomy Ts16 and euploid mice. We report that in the plasma membrane of euploid cells, an increase in phosphatidylcholine concentrations occurs in the presence of oleic acid. However, in trisomic cells, oleic acid failed to increase phosphatidylcholine incorporation into the plasma membrane. Gene expression analysis of trisomic cells revealed that the phosphatidylcholine biosynthetic pathway was deregulated. Taken together, these results suggest that the overdose of specific genes in trisomic lines delays differentiation in the presence of oleic acid. The dual-specificity tyrosine (Y) phosphorylation-regulated kinase 1A (DYRK1A) gene is located on human chromosome 21. DYRK1A contributes to intellectual disability and the early onset of Alzheimer's disease in DS patients. Here, we explored the potential role of Dyrk1A in the reduction of phosphatidylcholine concentrations in trisomic cells in the presence of oleic acid. The downregulation of Dyrk1A by small interfering RNA (siRNA) in trisomic cells returned phosphatidylcholine concentrations up to similar levels to those of euploid cells in the presence of oleic acid. Thus, our results highlight the role of Dyrk1A in brain development through the modulation of phosphatidylcholine location, levels and synthesis.

Overexpressed Down Syndrome Cell Adhesion Molecule (DSCAM) Deregulates P21-Activated Kinase (PAK) Activity in an In Vitro Neuronal Model of Down Syndrome: Consequences on Cell Process Formation and Extension

In humans, Down syndrome (DS) is caused by the presence of an extra copy of autosome 21. The most striking finding in DS patients is intellectual disability and the onset of Alzheimer's disease (AD)-like neuropathology in adulthood. Gene overdose is most likely to underlie both developmental impairments, as well as altered neuronal function in DS. Lately, the disruption of cellular signaling and regulatory pathways has been implicated in DS pathophysiology, and many of such pathways may represent common targets for diverse DS-related genes, which could in turn represent attractive therapeutical targets. In this regard, one DS-related gene Down Syndrome Cell Adhesion Molecule (DSCAM), has important functions in neuronal proliferation, maturation, and synaptogenesis. p21-associated kinases (PAKs) appear as a most interesting possibility for study, as DSCAM is known to regulate the PAKs pathway. Hence, in DS, overexpressed DSCAM could deregulate PAKs activity and affect signaling pathways that regulate synaptic plasticity such as dendritic spine dynamics and axon guidance and growth. In the present work, we used an immortalized cell line derived from the cerebral cortex of an animal model of DS such as the trisomy 16 (Ts16) fetal mouse (named CTb), and a similar cell line established from a normal littermate (named CNh), to study the effect of DSCAM in the PAKs pathway. The present study shows that DSCAM is overexpressed in CTb cells by approximately twofold, compared to CNh cells. Congruently, PAK1, as well as its downstream effectors LIMK and cofilin, stay phosphorylated for longer periods after DSCAM activation in the CTb cells, leading to an altered actin dynamics, expressed as an increased basal F/G ratio and reduced neurite growth, in the trisomic condition. The present work presents the correlation between DSCAM gene overexpression and a dysregulation of the PAK pathway, resulting in altered morphological parameters of neuronal plasticity in the trisomic cell line, namely decreased number and length of processes.

Calretinin-Immunoreactive Hypoinnervation in Down Syndrome (DS): Report of an Infant with Very Short-Segment Hirschsprung Disease and Comparison to Biopsy Findings in 20 Normal Infants and 11 Infants with DS and Chronic Constipation

In Down syndrome (DS) constipation is common, and the incidence of Hirschsprung disease (HD) is 1-2%. Rectal suction biopsies (RSBs) in DS may show discordant features; calretinin immunoreactivity (CRir) often helps resolve discrepancies. We report a case of unequivocal very short-segment HD (vsHD) in an infant with DS who had aganglionosis with abnormal acetylcholine esterase (AChE) activity in 3 RSBs. The CRir patterns were scanty positive rather than the expected absent CRir innervation in the lamina propria (LP). The resection specimen was grossly typical for short-segment HD, with a 5.5-cm, narrow but normally ganglionated segment proximal to the verified very short distal anganglionic zone. Unequivocal calretinin hypoinnervation was limited to the distal 2 cm, substantiating the warning of Kapur that small numbers of CRir nerves in the LP do not exclude a diagnosis of vsHD. We evaluated RSBs from 11 DS and 20 randomly selected normal infants <6 months of age with chronic constipation. The normal infants had abundant mucosal calretinin innervation and AChE histochemistry. We observed variable CRir hypoinnervation in RSBs in DS infants (including 6/7 with "normal" original diagnosis and 1/4 with HD). Our findings caution against overdependence on "normal" calretinin immunohistochemistry and suggest that AChE may be more reliable than CRir in the context of DS. An unknown number of patients with DS may have enteric nervous system disorders functionally similar to HD, which are possibly related to abnormal or imbalanced autonomic innervation, of which distal calretinin hypoinnervation is one manifestation, despite the presence of ganglia.

Developmental and maintenance defects in Rett syndrome neurons identified by a new mouse staging system in vitro

Rett Syndrome (RTT) is a neurodevelopmental disorder associated with intellectual disability, mainly caused by loss-of-function mutations in the MECP2 gene. RTT brains display decreased neuronal size and dendritic arborization possibly caused by either a developmental failure or a deficit in the maintenance of dendritic arbor structure. To distinguish between these two hypotheses, the development of Mecp2-knockout mouse hippocampal neurons was analyzed in vitro. Since a staging system for the in vitro development of mouse neurons was lacking, mouse and rat hippocampal neurons development was compared between 1–15 days in vitro (DIV) leading to a 6-stage model for both species. Mecp2-knockout hippocampal neurons displayed reduced growth of dendritic branches from stage 4 (DIV4) onwards. At stages 5–6 (DIV9-15), synapse number was lowered in Mecp2-knockout neurons, suggesting increased synapse elimination. These results point to both a developmental and a maintenance setback affecting the final shape and function of neurons in RTT.

Overexpression of DYRK1A inhibits choline acetyltransferase induction by oleic acid in cellular models of Down syndrome

Histological brain studies of individuals with DS have revealed an aberrant formation of the cerebral cortex. Previous work from our laboratory has shown that oleic acid acts as a neurotrophic factor and induces neuronal differentiation. In order to characterize the effects of oleic acid in a cellular model of DS, immortalized cell lines derived from the cortex of trisomy Ts16 (CTb) and normal mice (CNh) were incubated in the absence or presence of oleic acid. Oleic acid increased choline acetyltransferase expression (ChAT), a marker of cholinergic differentiation in CNh cells. However, in trisomic cells (CTb line) oleic acid failed to increase ChAT expression. These results suggest that the overdose of specific genes in trisomic lines delays differentiation in the presence of oleic acid by inhibiting acetylcholine production mediated by ChAT. The dual-specificity tyrosine (Y) phosphorylation-regulated kinase 1A (DYRK1A) gene is located on human chromosome 21 and encodes a proline-directed protein kinase. It has been proposed that DYRK1A plays a prominent role in several biological functions, leading to mental retardation in DS patients. Here we explored the potential role of DYRK1A in the modulation of ChAT expression in trisomic cells and in the signaling pathways of oleic acid. Down-regulation of DYRK1A by siRNA in trisomic CTb cells rescued ChAT expression up to levels similar to those of normal cells in the presence of oleic acid. In agreement with these results, oleic acid was unable to increase ChAT expression in neuronal cultures of transgenic mice overexpressing DYRK1A. In summary, our results highlight the role played by DYRK1A in brain development through the control of ChAT expression. In addition, the overexpression of DYRK1A in DS models prevented the neurotrophic effect of oleic acid, a fact that may account for mental retardation in DS patients.

Role of tau protein in neuronal damage in Alzheimer's disease and Down syndrome

Neurodegenerative disorders constitute a growing concern worldwide. Their incidence has increased steadily, in particular among the elderly, a high-risk population that is becoming an important segment of society. Neurodegenerative mechanisms underlie many ailments such as Parkinson's disease, Huntington's disease, Alzheimer's disease (AD) and Down syndrome (DS, trisomy 21). Interestingly, there is increasing evidence suggesting that many such diseases share pathogenic mechanisms at the cellular and subcellular levels. These include altered protein misfolding, impaired autophagy, mitochondrial dysfunction, membrane damage, and altered axonal transport. Regarding AD and DS, the first common link comes from observations that DS patients undergo AD-like pathology early in adulthood. Also, the gene encoding for the amyloid precursor protein is present in human autosome 21 and in murine chromosome 16, an animal model of DS. Important functions related to preservation of normal neuronal architecture are impaired in both conditions. In particular, the stable assembly of microtubules, which is critical for the cytoskeleton, is impaired in AD and DS. In this process, tau protein plays a pivotal role in controlling microtubule stability. Abnormal tau expression and hyperphosphorylation are common features in both conditions, yet the mechanisms leading to these phenomena remain obscure. In the present report we review possible common mechanisms that may alter tau expression and function, in particular in relation to the effect of certain overexpressed DS-related genes, using cellular models of human DS. The latter contributes to the identification of possible therapeutic targets that could aid in the treatment of both AD and DS.

Altered voltage dependent calcium currents in a neuronal cell line derived from the cerebral cortex of a trisomy 16 fetal mouse, an animal model of Down syndrome

Human Down syndrome (DS) is determined by the trisomy of autosome 21 and is expressed by multiple abnormalities, being mental retardation the most striking feature. The condition results in altered electrical membrane properties (EMPs) of fetal neurons, which are qualitatively identical to those of trisomy 16 fetal mice (Ts16), an animal model of the human condition. Ts16 hippocampal cultured neurons reportedly exhibit increased voltage-dependent calcium currents (I (Ca)) amplitude. Since Ts16 animals are unviable, we have established immortalized cell lines from the cerebral cortex of Ts16 (named CTb) and normal littermates (named CNh). Using the whole-cell patch-clamp technique, we have now studied I (Ca) in CTb and CNh cells. Current activation occurs at -40 mV in both cell lines (V (holding) = -80 mV). Trisomic cells exhibited a 2.4 fold increase in the maximal Ca(2+) current density compared to normal cells (CNh = -6.3 ± 0.77 pA/pF, n = 18; CTb = -16.4 ± 2.423 pA/pF; P < 0.01, n = 13). Time dependent kinetics for activation and inactivation did not differ between the two cell types. However, steady state inactivation studies revealed a 15 mV shift toward more depolarized potentials in the trisomic condition, suggesting that altered voltage dependence of inactivation may underlie the increased current density. Further, the total charge movement across the membrane is increased in CTb cells, in agreement with that expected by the potential sensitivity shift. These results indicate that CTb cells present altered Ca(2+) currents, similar to those of Ts16 primary cultured central neurons. The CTb cell line represents a model for studying DS-related impairments of EMPs.

Down syndrome and the enteric nervous system

Down syndrome (DS) is the most common chromosomal abnormality occurring in humans. Up to 77% of DS children have associated gastrointestinal (GI) abnormalities, which may be structural or functional in nature. Functional disturbances may, in turn, affect the outcome of corrective surgical procedures, prompting to caution. It is becoming clear that the processes affecting the enteric nervous system (ENS) in DS not only affect the micro-anatomy but also nerve function, and there is some histological evidence of ENS variations in both human and DS animal models. This suggests that developmental disorders of the ENS are probably fundamental to the functional GI disturbances encountered in patients with DS. The anomalous brain development, function and resulting intellectual impairment associated with DS appears to result from the genetic imbalance created by the trisomy of chromosome 21. The possible links between the brain, GI and ENS involvement are not as yet entirely clear. Neurotropic factors affecting brain development during embryogenesis are probably interlinked with ENS development, but the precise mechanism of how this occurs has yet to be established. This study explores what is known about the ENS dysfunction in DS and reviews the possible importance of chromosome 21 located and other genes in its etiology. Functional motor disturbances of the esophagus and colon are not uncommon and may be congenital or acquired in nature. The most prominent of these include esophageal dysmotility syndromes (e.g. achalasia, gastroesophageal reflux, dysphagia) as well as a higher incidence of chronic constipation and Hirschsprung's disease (HSCR) (2-15%) occurring in association with DS. Chromosome 21 itself is thought to be the site of a modifier gene for HSCR. Recently identified candidate genetic mechanisms provide unique insights into the genetic background of the neurological and cognitive disorders associated with DS. Although the role of the triplicated chromosome 21 and genetic dosage remain important, the additional role of other chromosome 21 genes in the etiology of ENS developmental anomalies remains undetermined and requires ongoing research.

Strategies for present and future mental retardation diagnosis

Mental retardation (MR) is a highly heterogeneous condition with a prevalence of 1–3% in the general population. The psychosocial burden on families with mentally handicapped children is extensive. In addition, the accompanying expenses with mental handicaps are considerable. In this review a comprehensive strategy to systematically identify the causative genetic defect in patients with mental retardation is proposed. This strategy is a combination of routinely used and recently developed approaches, such as direct DNA sequencing, single nucleotide polymorphism arrays and expression profiling, to establish a molecular diagnosis in MR patients. Finally, it will be described how these mutations can be studied in different model systems, which can eventually be used to elucidate the neurobiological basis of MR and to facilitate possible therapeutic intervention.