Early Appearance of Dendritic Alterations in Neocortical Pyramidal Neurons of the Ts65Dn Model of Down Syndrome.

Down syndrome (DS), which is due to triplication of chromosome 21, is constantly associated with intellectual disability (ID). ID can be ascribed to both neurogenesis impairment and dendritic pathology. These defects are replicated in the Ts65Dn mouse, a widely used model of DS. While neurogenesis impairment in DS is a fetal event, dendritic pathology occurs after the first postnatal months. Neurogenesis alterations across the life span have been extensively studied in the Ts65Dn mouse. In contrast, there is scarce information regarding dendritic alterations at early life stages in this and other models, although there is evidence for dendritic alterations in adult mouse models. Thus, the goal of the current study was to establish whether dendritic alterations are already present in the neonatal period in Ts65Dn mice. In Golgi-stained brains, we quantified the dendritic arbors of layer II/III pyramidal neurons in the frontal cortex of Ts65Dn mice aged 2 (P2) and 8 (P8) days and their euploid littermates. In P2 Ts65Dn mice, we found a moderate hypotrophy of the apical and collateral dendrites but a patent hypotrophy of the basal dendrites. In P8 Ts65Dn mice, the distalmost apical branches were missing or reduced in number, but there were no alterations in the collateral and basal dendrites. No genotype effects were detected on either somatic or dendritic spine density. This study shows dendritic branching defects that mainly involve the basal domain in P2 Ts65Dn mice and the apical but not the other domains in P8 Ts65Dn mice. This suggests that dendritic defects may be related to dendritic compartment and age. The lack of a severe dendritic pathology in Ts65Dn pups is reminiscent of the delayed appearance of patent dendritic alterations in newborns with DS. This similarly highlights the usefulness of the Ts65Dn model for the study of the mechanisms underlying dendritic alterations in DS and the design of possible therapeutic interventions.

Treatment with the flavonoid 7,8-Dihydroxyflavone: a promising strategy for a constellation of body and brain disorders.

Flavonoids have long been known to exert benefits in various health problems. Among them, the BDNF mimetic 7,8-Dihydroxyflavone (7,8-DHF) is emerging as a potential treatment for a constellation of brain and body pathologies. During the past 10 years, more than 180 preclinical studies have explored the efficacy of 7,8-DHF in animal models of different pathologies. The current review intends to be an exhaustive survey of these studies. By providing detailed information on the rationale of the experimental design and outcome of treatment, we will give the reader tools to critically interpret the achievement obtained so far. If we put together each individual piece of this complex mosaic, a picture emerges that is full of promise regarding the potential usefulness of 7,8-DHF for human treatment. Much has been done so far and we believe that the time is now ripe to move from the bench to the bedside, in order to establish whether supplementation with 7,8-DHF may serve as therapy or, at least, as adjuvant for the treatment of pathologies affecting brain and body functioning.

Obstructive sleep apneas naturally occur in mice during REM sleep and are highly prevalent in a mouse model of Down syndrome.

STUDY OBJECTIVES: The use of mouse models in sleep apnea study is limited by the belief that central (CSA) but not obstructive sleep apneas (OSA) occur in rodents. We aimed to develop a protocol to investigate the presence of OSAs in wild-type mice and, then, to apply it to a validated model of Down syndrome (Ts65Dn), a human pathology characterized by a high incidence of OSAs. METHODS: In a pilot study, nine C57BL/6J wild-type mice were implanted with electrodes for electroencephalography (EEG), neck electromyography (nEMG), and diaphragmatic activity (DIA), and then placed in a whole-body-plethysmographic (WBP) chamber for 8 h during the rest (light) phase to simultaneously record sleep and breathing activity. CSA and OSA were discriminated on the basis of WBP and DIA signals recorded simultaneously. The same protocol was then applied to 12 Ts65Dn mice and 14 euploid controls. RESULTS: OSAs represented about half of the apneic events recorded during rapid-eye-movement-sleep (REMS) in each experimental group, while the majority of CSAs were found during non-rapid eye movement sleep. Compared with euploid controls, Ts65Dn mice had a similar total occurrence rate of apneic events during sleep, but a significantly higher occurrence rate of OSAs during REMS, and a significantly lower occurrence rate of CSAs during NREMS. CONCLUSIONS: Mice physiologically exhibit both CSAs and OSAs. The latter appear almost exclusively during REMS, and are highly prevalent in Ts65Dn. Mice may, thus, represent a useful model to accelerate the understanding of the pathophysiology and genetics of sleep-disordered breathing and to help the development of new therapies.

Early appearance of developmental alterations in the dendritic tree of the hippocampal granule cells in the Ts65Dn model of Down syndrome.

Down syndrome (DS), a genetic condition caused by triplication of chromosome 21, is characterized by alterations in various cognitive domains, including hippocampus-dependent memory functions, starting from early life stages. The major causes of intellectual disability in DS are prenatal neurogenesis alterations followed by impairment of dendritic development in early infancy. While there is evidence that the Ts65Dn mouse, the most widely used model of DS, exhibits dendritic alterations in adulthood, no studies are available regarding the onset of dendritic pathology. The goal of the current study was to establish whether this model exhibits early dendritic alterations in the hippocampus, a region whose function is severely damaged in DS. To this purpose, in Golgi-stained brains, we evaluated the dendritic arborization and dendritic spines of the granule cells of the hippocampal dentate gyrus in Ts65Dn mice aged 8 (P8) and 15 (P15) days. While P15 Ts65Dn mice exhibited a notably hypotrophic dendritic arbor and a reduced spine density, P8 mice exhibited a moderate reduction in the number of dendritic ramifications and no differences in spine density in comparison with their euploid counterparts. Both in P8 and P15 mice, spines were longer and had a longer neck, suggesting possible alterations in synaptic function. Moreover, P8 and P15 Ts65Dn mice had more thin spines and fewer stubby spines in comparison with euploid mice. Our study provides novel evidence on the onset of dendritic pathology, one of the causes of intellectual disability in DS, showing that it is already detectable in the dentate gyrus of Ts65Dn pups. This evidence strengthens the suitability of this model of DS as a tool to study dendritic pathology in DS and to test the efficacy of early therapeutic interventions aimed at ameliorating hippocampal development and, therefore, memory functions in children with DS.

Neonatal therapy with clenbuterol and salmeterol restores spinogenesis and dendritic complexity in the dentate gyrus of the Ts65Dn model of Down syndrome.

Down syndrome (DS), a neurodevelopmental disorder caused by triplication of chromosome 21, is characterized by intellectual disability. In DS, defective neurogenesis causes an overall reduction in the number of neurons populating the brain and defective neuron maturation causes dendritic hypotrophy and reduction in the density of dendritic spines. No effective therapy currently exists for the improvement of brain development in individuals with DS. Drug repurposing is a strategy for identifying new medical use for approved drugs. A drug screening campaign showed that the ß2-adrenergic receptor (ß2-AR) agonists clenbuterol hydrochloride (CLEN) and salmeterol xinafoate (SALM) increase the proliferation rate of neural progenitor cells from the Ts65Dn model of DS. The goal of the current study was to establish their efficacy in vivo, in the Ts65Dn model. We found that, at variance with the in vitro experiments, treatment with CLEN or SALM did not restore neurogenesis in the hippocampus of Ts65Dn mice treated during the postnatal (P) period P3-P15. In Ts65Dn mice treated with CLEN or SALM, however, dendritic spine density and dendritic arborization of the hippocampal granule cells were restored and the lowest dose tested here (0.01 mg/kg/day) was sufficient to elicit these effects. CLEN and SALM are used in children as therapy for asthma and, importantly, they pass the blood-brain barrier. Our study suggests that treatment with these ß2-AR agonists may be a therapy of choice in order to correct dendritic development in DS but is not suitable to rescue neurogenesis.

Neonatal treatment with cyclosporine A restores neurogenesis and spinogenesis in the Ts65Dn model of Down syndrome.

Down syndrome (DS), a genetic condition due to triplication of chromosome 21, is characterized by reduced proliferation of neural progenitor cells (NPCs) starting from early life stages. This defect is worsened by a reduction of neuronogenesis (accompanied by an increase in astrogliogenesis) and dendritic spine atrophy. Since this triad of defects underlies intellectual disability, it seems important to establish whether it is possible to pharmacologically correct these alterations. In this study, we exploited the Ts65Dn mouse model of DS in order to obtain an answer to this question. In the framework of an in vitro drug-screening campaign of FDA/EMA-approved drugs, we found that the immunosuppressant cyclosporine A (CSA) restored proliferation, acquisition of a neuronal phenotype, and maturation of neural progenitor cells (NPCs) from the subventricular zone (SVZ) of the lateral ventricle of Ts65Dn mice. Based on these findings, we treated Ts65Dn mice with CSA in the postnatal period P3-P15. We found that treatment fully restored NPC proliferation in the SVZ and in the subgranular zone of the hippocampal dentate gyrus, and total number of hippocampal granule cells. Moreover, CSA enhanced development of dendritic spines on the dendritic arbor of the granule cells whose density even surpassed that of euploid mice. In hippocampal homogenates from Ts65Dn mice, we found that CSA normalized the excessive levels of p21, a key determinant of proliferation impairment. Results show that neonatal treatment with CSA restores the whole triad of defects of the trisomic brain. In DS CSA treatment may pose caveats because it is an immunosuppressant that may cause adverse effects. However, CSA analogues that mimic its effect without eliciting immunosuppression may represent practicable tools for ameliorating brain development in individuals with DS.

Neuroanatomical alterations and synaptic plasticity impairment in the perirhinal cortex of the Ts65Dn mouse model of Down syndrome.

Down syndrome (DS), a genetic condition due to triplication of Chromosome 21, is characterized by numerous neurodevelopmental alterations and intellectual disability. Individuals with DS and DS mouse models are impaired in several memory domains, including hippocampus-dependent declarative (spatial, in rodents) memory and visual recognition memory, a form of memory in which the perirhinal cortex (PRC) plays a fundamental role. The anatomo-functional substrates of hippocampus-dependent memory impairment have been largely elucidated in the Ts65Dn mouse model of DS. In contrast, there is a lack of corresponding information regarding visual recognition memory. Therefore, we deemed it of interest to examine at both an anatomical and functional level the PRC of Ts65Dn mice. We found that the PRC of adult (1.5-3.5month-old) Ts65Dn mice exhibited diffused hypocellularity and neurons with a reduced spine density. No difference between Ts65Dn and euploid mice was detected in the abundance of glutamatergic and GABAergic terminals. We examined brain slices for long-term potentiation (LTP), a form of synaptic plasticity involved in long-term memory. Theta burst stimulation of intracortical fibers was used in order to elicit LTP in the superficial layers of the PRC. We found that in trisomic slices LTP had a similar time-course but a reduced magnitude in comparison with euploid slices. While exposure to the GABAA receptor antagonist picrotoxin had no effect on LTP magnitude, exposure to the GABAB receptor antagonist CGP55845 caused an increase in LTP magnitude that became even larger than in euploid slices. Western blot analysis showed increased levels of the G-protein-activated inwardly rectifying K+ channel 2 (GIRK2) in the PRC of Ts65Dn mice, consistent with triplication of the gene coding for GIRK2. This suggests that the reduced magnitude of LTP may be caused by GIRK2-dependent exaggerated GABAB receptor-mediated inhibition. Results provide novel evidence for anatomo-functional alterations in the PRC of Ts65Dn mice. These alterations may underlie trisomy-due impairment in visual recognition memory.

Long-term effect of neonatal inhibition of APP gamma-secretase on hippocampal development in the Ts65Dn mouse model of Down syndrome.

Neurogenesis impairment is considered a major determinant of the intellectual disability that characterizes Down syndrome (DS), a genetic condition caused by triplication of chromosome 21. Previous evidence obtained in the Ts65Dn mouse model of DS showed that the triplicated gene APP (amyloid precursor protein) is critically involved in neurogenesis alterations. In particular, excessive levels of AICD (amyloid precursor protein intracellular domain) resulting from APP cleavage by gamma-secretase increase the transcription of Ptch1, a Sonic Hedgehog (Shh) receptor that keeps the mitogenic Shh pathway repressed. Previous evidence showed that neonatal treatment with ELND006, an inhibitor of gamma-secretase, reinstates the Shh pathway and fully restores neurogenesis in Ts65Dn pups. In the framework of potential therapies for DS, it is extremely important to establish whether the positive effects of early intervention are retained after treatment cessation. Therefore, the goal of the current study was to establish whether early treatment with ELND006 leaves an enduring trace in the brain of Ts65Dn mice. Ts65Dn and euploid pups were treated with ELND006 in the postnatal period P3-P15 and the outcome of treatment was examined at ~one month after treatment cessation. We found that in treated Ts65Dn mice the pool of proliferating cells in the hippocampal dentate gyrus (DG) and total number of granule neurons were still restored as was the number of pre- and postsynaptic terminals in the stratum lucidum of CA3, the site of termination of the mossy fibers from the DG. Accordingly, patch-clamp recording from field CA3 showed functional normalization of the input to CA3. Unlike in field CA3, the number of pre- and postsynaptic terminals in the DG of treated Ts65Dn mice was no longer fully restored. The finding that many of the positive effects of neonatal treatment were retained after treatment cessation provides proof of principle demonstration of the efficacy of early inhibition of gamma-secretase for the improvement of brain development in DS.

Inhibition of APP gamma-secretase restores Sonic Hedgehog signaling and neurogenesis in the Ts65Dn mouse model of Down syndrome.

Neurogenesis impairment starting from early developmental stages is a key determinant of intellectual disability in Down syndrome (DS). Previous evidence provided a causal relationship between neurogenesis impairment and malfunctioning of the mitogenic Sonic Hedgehog (Shh) pathway. In particular, excessive levels of AICD (amyloid precursor protein intracellular domain), a cleavage product of the trisomic gene APP (amyloid precursor protein) up-regulate transcription of Ptch1 (Patched1), the Shh receptor that keeps the pathway repressed. Since AICD results from APP cleavage by γ-secretase, the goal of the current study was to establish whether treatment with a γ-secretase inhibitor normalizes AICD levels and restores neurogenesis in trisomic neural precursor cells. We found that treatment with a selective γ-secretase inhibitor (ELND006; ELN) restores proliferation in neurospheres derived from the subventricular zone (SVZ) of the Ts65Dn mouse model of DS. This effect was accompanied by reduction of AICD and Ptch1 levels and was prevented by inhibition of the Shh pathway with cyclopamine. Treatment of Ts65Dn mice with ELN in the postnatal period P3-P15 restored neurogenesis in the SVZ and hippocampus, hippocampal granule cell number and synapse development, indicating a positive impact of treatment on brain development. In addition, in the hippocampus of treated Ts65Dn mice there was a reduction in the expression levels of various genes that are transcriptionally regulated by AICD, including APP, its origin substrate. Inhibitors of γ-secretase are currently envisaged as tools for the cure of Alzheimer's disease because they lower ßamyloid levels. Current results provide novel evidence that γ-secretase inhibitors may represent a strategy for the rescue of neurogenesis defects in DS.

Long-term effects of neonatal treatment with fluoxetine on cognitive performance in Ts65Dn mice.

Individuals with Down syndrome (DS), a genetic condition caused by triplication of chromosome 21, are characterized by intellectual disability and are prone to develop Alzheimer's disease (AD), due to triplication of the amyloid precursor protein (APP) gene. Recent evidence in the Ts65Dn mouse model of DS shows that enhancement of serotonergic transmission with fluoxetine during the perinatal period rescues neurogenesis, dendritic pathology and behavior, indicating that cognitive impairment can be pharmacologically restored. A crucial question is whether the short-term effects of early treatments with fluoxetine disappear at adult life stages. In the current study we found that hippocampal neurogenesis, dendritic pathology and hippocampus/amygdala-dependent memory remained in their restored state when Ts65Dn mice, which had been neonatally treated with fluoxetine, reached adulthood. Additionally, we found that the increased levels of the APP-derived ßCTF peptide in adult Ts65Dn mice were normalized following neonatal treatment with fluoxetine. This effect was accompanied by restoration of endosomal abnormalities, a ßCTF-dependent feature of DS and AD. While untreated adult Ts65Dn mice had reduced hippocampal levels of the 5-HT1A receptor (5-HT1A-R) and methyl-CpG-binding protein (MeCP2), a protein that promotes 5-HT1A-R transcription, in neonatally-treated mice both 5-HT1A-R and MeCP2 were normalized. In view of the crucial role of serotonin in brain development, these findings suggest that the enduring outcome of neonatal treatment with fluoxetine may be due to MeCP2-dependent restoration of the 5-HT1A-R. Taken together, results provide new hope for the therapy of DS, showing that early treatment with fluoxetine enduringly restores cognitive impairment and prevents early signs of AD-like pathology.

Prenatal pharmacotherapy rescues brain development in a Down's syndrome mouse model.

Intellectual impairment is a strongly disabling feature of Down's syndrome, a genetic disorder of high prevalence (1 in 700-1000 live births) caused by trisomy of chromosome 21. Accumulating evidence shows that widespread neurogenesis impairment is a major determinant of abnormal brain development and, hence, of intellectual disability in Down's syndrome. This defect is worsened by dendritic hypotrophy and connectivity alterations. Most of the pharmacotherapies designed to improve cognitive performance in Down's syndrome have been attempted in Down's syndrome mouse models during adult life stages. Yet, as neurogenesis is mainly a prenatal event, treatments aimed at correcting neurogenesis failure in Down's syndrome should be administered during pregnancy. Correction of neurogenesis during the very first stages of brain formation may, in turn, rescue improper brain wiring. The aim of our study was to establish whether it is possible to rescue the neurodevelopmental alterations that characterize the trisomic brain with a prenatal pharmacotherapy with fluoxetine, a drug that is able to restore post-natal hippocampal neurogenesis in the Ts65Dn mouse model of Down's syndrome. Pregnant Ts65Dn females were treated with fluoxetine from embryonic Day 10 until delivery. On post-natal Day 2 the pups received an injection of 5-bromo-2-deoxyuridine and were sacrificed after either 2 h or after 43 days (at the age of 45 days). Untreated 2-day-old Ts65Dn mice exhibited a severe neurogenesis reduction and hypocellularity throughout the forebrain (subventricular zone, subgranular zone, neocortex, striatum, thalamus and hypothalamus), midbrain (mesencephalon) and hindbrain (cerebellum and pons). In embryonically treated 2-day-old Ts65Dn mice, precursor proliferation and cellularity were fully restored throughout all brain regions. The recovery of proliferation potency and cellularity was still present in treated Ts65Dn 45-day-old mice. Moreover, embryonic treatment restored dendritic development, cortical and hippocampal synapse development and brain volume. Importantly, these effects were accompanied by recovery of behavioural performance. The cognitive deficits caused by Down's syndrome have long been considered irreversible. The current study provides novel evidence that a pharmacotherapy with fluoxetine during embryonic development is able to fully rescue the abnormal brain development and behavioural deficits that are typical of Down's syndrome. If the positive effects of fluoxetine on the brain of a mouse model are replicated in foetuses with Down's syndrome, fluoxetine, a drug usable in humans, may represent a breakthrough for the therapy of intellectual disability in Down's syndrome.

Cellularity Defects Are Not Ubiquitous in the Brains of Fetuses With Down Syndrome.

Down syndrome (DS) is a genetic pathology characterized by various developmental defects. Unlike other clinical problems, intellectual disability is an invariant clinical trait of DS. Impairment of neurogenesis accompanied by brain hypotrophy is a typical neurodevelopmental phenotype of DS, suggesting that a reduction in the number of cells forming the brain may be a key determinant of intellectual disability. Previous evidence showed that fetuses with DS exhibit widespread hypocellularity in brain regions belonging to the temporal lobe memory systems, which may account for the typical explicit memory impairment that characterizes DS. In the current study, we have examined the basal ganglia, the insular cortex (INS), and the cingulate cortex (CCX) of fetuses with DS and age-matched controls (18-22 weeks of gestation), to establish whether cellularity defects involve regions that are not primarily involved in explicit memory. We found that fetuses with DS exhibit a notable hypocellularity in the putamen (-30%) and globus pallidus (-35%). In contrast, no cellularity differences were found in the INS and CCX, indicating that hypocellularity is not ubiquitous in the DS brain. The hypocellularity found in the basal ganglia, which are critically implicated in the control of movement, suggests that such alterations may contribute to the motor abnormalities of DS. The normal cytoarchitecture of the INS and CCX suggests that the alterations exhibited by people with DS in functions in which these regions are involved are not attributable to neuron paucity.

Reversal of neurodevelopmental impairment and cognitive enhancement by pharmacological intervention with the polyphenol polydatin in a Down syndrome model.

Intellectual disability (ID) is the unavoidable hallmark of Down syndrome (DS), a genetic condition due to triplication of chromosome 21. ID in DS is largely attributable to neurogenesis and dendritogenesis alterations taking place in the prenatal/neonatal period, the most critical time window for brain development. There are currently no treatments for ID in DS. Considering the timeline of brain development, treatment aimed at improving the neurological phenotypes of DS should be initiated as early as possible and use safe agents. The goal of this study was to establish whether it is possible to improve DS-linked neurodevelopmental defects through early treatment with polydatin, a natural polyphenol. We used the Ts65Dn mouse model of DS and focused on the hippocampus, a brain region fundamental for long-term memory. We found that in Ts65Dn mice of both sexes treated with polydatin from postnatal (P) day 3 to P15 there was full restoration of neurogenesis, neuron number, and dendritic development. These effects were accompanied by normalization of Cyclin D1 and DSCAM levels, which may account for the rescue of neurogenesis and dendritogenesis, respectively. Importantly, in Ts65Dn mice treated with polydatin from P3 to adolescence (∼P50) there was full restoration of hippocampus-dependent memory, indicating a pro-cognitive outcome of treatment. No adverse effects were observed on the body and brain weight. The efficacy and safety of polydatin in a model of DS prospect the possibility of its use during early life stages for amelioration of DS-linked neurodevelopmental alterations.

Increased hippocampal epigenetic age in the Ts65Dn mouse model of Down Syndrome.

Down syndrome (DS) is a segmental progeroid genetic disorder associated with multi-systemic precocious aging phenotypes, which are particularly evident in the immune and nervous systems. Accordingly, people with DS show an increased biological age as measured by epigenetic clocks. The Ts65Dn trisomic mouse, which harbors extra-numerary copies of chromosome 21 (Hsa21)-syntenic regions, was shown to recapitulate several progeroid features of DS, but no biomarkers of age have been applied to it so far. In this pilot study, we used a mouse-specific epigenetic clock to measure the epigenetic age of hippocampi from Ts65Dn and euploid mice at 20 weeks. Ts65Dn mice showed an increased epigenetic age in comparison with controls, and the observed changes in DNA methylation partially recapitulated those observed in hippocampi from people with DS. Collectively, our results support the use of the Ts65Dn model to decipher the molecular mechanisms underlying the progeroid DS phenotypes.

APP-dependent up-regulation of Ptch1 underlies proliferation impairment of neural precursors in Down syndrome.

Mental retardation in Down syndrome (DS) appears to be related to severe neurogenesis impairment during critical phases of brain development. Recent lines of evidence in the cerebellum of a mouse model for DS (the Ts65Dn mouse) have shown a defective responsiveness to Sonic Hedgehog (Shh), a potent mitogen that controls cell division during brain development, suggesting involvement of the Shh pathway in the neurogenesis defects of DS. Based on these premises, we sought to identify the molecular mechanisms underlying derangement of the Shh pathway in neural precursor cells (NPCs) from Ts65Dn mice. By using an in vitro model of NPCs obtained from the subventricular zone and hippocampus, we found that trisomic NPCs had an increased expression of the Shh receptor Patched1 (Ptch1), a membrane protein that suppresses the action of a second receptor, Smoothened (Smo), thereby maintaining the pathway in a repressed state. Partial silencing of Ptch1 expression in trisomic NPCs restored cell proliferation, indicating that proliferation impairment was due to Ptch1 overexpression. The overexpression of Ptch1 in trisomic NPCs resulted from increased levels of AICD [a transcription-promoting fragment of amyloid precursor protein (APP)] and increased AICD binding to the Ptch1 promoter. Our data provide novel evidence that Ptch1 overexpression underlies derangement of the Shh pathway in trisomic NPCs with consequent proliferation impairment. The demonstration that Ptch1 overexpression in trisomic NPCs is due to an APP fragment provides a link between this trisomic gene and the defective neuronal production that characterizes the DS brain.

Early-occurring proliferation defects in peripheral tissues of the Ts65Dn mouse model of Down syndrome are associated with patched1 over expression.

Down syndrome (DS) is a genetic pathology due to the triplication of human chromosome 21. In addition to mental retardation, individuals with DS exhibit a large range of variable traits, including co-occurring congenital malformations. It is now clear that neurogenesis impairment underlies the typically reduced brain size and, hence, mental retardation in individuals with DS. The small body size and the constellation of congenital malformations in children with DS suggest that proliferation defects may involve peripheral tissues, in addition to the brain. The goal of the current study was to establish whether a generalized impairment of cell proliferation is a key feature of the trisomic condition. We used the Ts65Dn mouse, a widely used DS model, and examined proliferation in tissues with different embryological origin by 5-bromo-2-deoxyuridine immunohistochemistry. We found that 2-day-old (P2) Ts65Dn mice had notably fewer proliferating cells in the heart and liver, and in all proliferating niches of the skin and intestine. A reduced proliferation rate was still present in the intestine at P15. In all tissues, Ts65Dn mice had a similar number of apoptotic cells as euploid mice, indicating no unbalance in cell death. In the skin, liver and intestine of trisomic mice, we found a higher expression of patched1 (Ptch1), a receptor that represses the mitogenic sonic hedgehog (Shh) pathway. This suggests that Ptch1-dependent inhibition of Shh signaling may underlie proliferation impairment in trisomic peripheral tissues. In agreement with the widespread reduction in proliferation, neonate trisomic mice had a reduced body weight and this defect was still present at 30 days of age. Our findings show that, in all examined peripheral tissues, Ts65Dn mice exhibit a notable reduction in proliferation rate, suggesting that proliferation impairment may be a generalized defect of trisomic precursor cells.

Early pharmacotherapy restores neurogenesis and cognitive performance in the Ts65Dn mouse model for Down syndrome.

Down syndrome (DS) is a genetic pathology characterized by intellectual disability and brain hypotrophy. Widespread neurogenesis impairment characterizes the fetal and neonatal DS brain, strongly suggesting that this defect may be a major determinant of mental retardation. Our goal was to establish, in a mouse model for DS, whether early pharmacotherapy improves neurogenesis and cognitive behavior. Neonate Ts65Dn mice were treated from postnatal day (P) 3 to P15 with fluoxetine, an antidepressant that inhibits serotonin (5-HT) reuptake and increases proliferation in the adult Ts65Dn mouse (Clark et al., 2006). On P15, they received a BrdU injection and were killed after either 2 h or 1 month. Results showed that P15 Ts65Dn mice had notably defective proliferation in the hippocampal dentate gyrus, subventricular zone, striatum, and neocortex and that proliferation was completely rescued by fluoxetine. In the hippocampus of untreated P15 Ts65Dn mice, we found normal 5-HT levels but a lower expression of 5-HT1A receptors and brain-derived neurotrophic factor (BDNF). In Ts65Dn mice, fluoxetine treatment restored the expression of 5-HT1A receptors and BDNF. One month after cessation of treatment, there were more surviving cells in the dentate gyrus of Ts65Dn mice, more cells with a neuronal phenotype, more proliferating precursors, and more granule cells. These animals were tested for contextual fear conditioning, a hippocampus-dependent memory task, and exhibited a complete recovery of memory performance. Results show that early pharmacotherapy with a drug usable by humans can correct neurogenesis and behavioral impairment in a model for DS.

Is it possible to improve neurodevelopmental abnormalities in Down syndrome?

Down syndrome (DS) is a genetic pathology caused by the triplication of human chromosome 21. Although individuals with DS have various medical problems, intellectual disability is the most invalidating aspect of the pathology. Despite numerous efforts, the mechanisms whereby gene triplication leads to the DS phenotype have not been elucidated and there are, at present, no therapies to rescue brain developmental alterations and mental disability in individuals with DS. In this review, we focused on the major defects of the DS brain, comparing data regarding humans with DS and mouse models for DS, and therapeutic interventions attempted on animal DS models. Based on the promising results of pharmacotherapies in these models, we believe that it is possible to conclude that tools to improve brain development in DS are now almost at hand. We now know that it is possible to rescue and/or improve neurogenesis, neuron maturation, connectivity, neurodegeneration and behavior. We believe that the knowledge gained in DS mouse models provides a rational basis to start new clinical trials in infants, children and adults with DS, exploiting drugs that have proved able to rescue various facets of the DS neurologic phenotype. It is not unreasonable to consider that the results of these trials may provide a positive answer to the question: 'Is it possible to improve brain development in DS?'.

The flavonoid 7,8-DHF fosters prenatal brain proliferation potency in a mouse model of Down syndrome.

Neurogenesis impairment is a key determinant of intellectual disability in Down syndrome (DS), a genetic pathology due to triplication of chromosome 21. Since neurogenesis ceases after birth, apart in the hippocampus and olfactory bulb, the only means to tackle the problem of neurogenesis impairment in DS at its root is to intervene during gestation. A few studies in DS mouse models show that this is possible, although the drugs used may raise caveats in terms of safety. We previously found that neonatal treatment with 7,8-dihydroxyflavone (7,8-DHF), a flavonoid present in plants, restores hippocampal neurogenesis in the Ts65Dn model of DS. The goal of the current study was to establish whether prenatal treatment with 7,8-DHF improves/restores overall brain proliferation potency. Pregnant Ts65Dn females received 7,8-DHF from embryonic day 10 until delivery. On postnatal day 2 (P2) the pups were injected with BrdU and were killed after either 2 h or 52-60 days (P52-60). Evaluation of the number of proliferating (BrdU+) cells in various forebrain neurogenic niches of P2 mice showed that in treated Ts65Dn mice proliferation potency was improved or even restored in most of the examined regions, including the hippocampus. Quantification of the surviving BrdU+ cells in the dentate gyrus of P52-60 mice showed no difference between treated and untreated Ts65Dn mice. At P52-60, however, treated Ts65Dn mice exhibited a larger number of granule cells in comparison with their untreated counterparts, although their number did not reach that of euploid mice. Results show that 7,8-DHF has a widespread impact on prenatal proliferation potency in Ts65Dn mice and exerts mild long-term effects. It remains to be established whether treatment extending into the neonatal period can lead to an improvement in brain development that is retained in adulthood.