Timothy H. Moran

A mouse model for Down syndrome exhibits learning and behaviour deficits

Trisomy 21 or Down syndrome (DS) is the most frequent genetic cause of mental retardation, affecting one in 800 live born human beings. Mice with segmental trisomy 16 (Ts65Dn mice) are at dosage imbalance for genes corresponding to those on human chromosome 21q21–22.3—which includes the so–called DS ‘critical region’. They do not show early–onset of Alzheimer disease pathology; however, Ts65Dn mice do demonstrate impaired performance in a complex learning task requiring the integration of visual and spatial information. The reproducibility of this phenotype among Ts65Dn mice indicates that dosage imbalance for a gene or genes in this region contributes to this impairment. The corresponding dosage imbalance for the human homologues of these genes may contribute to cognitive deficits in DS.

Two brain cholecystokinin receptors: implications for behavioral actions

The distribution and relative specificity of cholecystokinin (CCK) receptors in the rat brain was mapped by in vitro autoradiography with [125I]CCK-33. We identified two distinct binding patterns, suggesting two CCK receptor types. The first is widespread and relatively non-specific. The second, localized to a few subcortical nuclei, has the specificity demonstrated for pancreatic CCK receptors. Localization of this receptor type to the area postrema provides a possible entry site into brain for circulating CCK that would distinguish between CCK and gastrin and could mediate some of CCKs behavioral effects.

Neurobiology of Exercise

Voluntary physical activity and exercise training can favorably influence brain plasticity by facilitating neurogenerative, neuroadaptive, and neuroprotective processes. At least some of the processes are mediated by neurotrophic factors. Motor skill training and regular exercise enhance executive functions of cognition and some types of learning, including motor learning in the spinal cord. These adaptations in the central nervous system have implications for the prevention and treatment of obesity, cancer, depression, the decline in cognition associated with aging, and neurological disorders such as Parkinsons disease, Alzheimers dementia, ischemic stroke, and head and spinal cord injury. Chronic voluntary physical activity also attenuates neural responses to stress in brain circuits responsible for regulating peripheral sympathetic activity, suggesting constraint on sympathetic responses to stress that could plausibly contribute to reductions in clinical disorders such as hypertension, heart failure, oxidative stress, and suppression of immunity. Mechanisms explaining these adaptations are not as yet known, but metabolic and neurochemical pathways among skeletal muscle, the spinal cord, and the brain offer plausible, testable mechanisms that might help explain effects of physical activity and exercise on the central nervous system.

p53 Mediates Cellular Dysfunction and Behavioral Abnormalities in Huntington’s Disease

We present evidence for a specific role of p53 in the mitochondria-associated cellular dysfunction and behavioral abnormalities of Huntingtons disease (HD). Mutant huntingtin (mHtt) with expanded polyglutamine (polyQ) binds to p53 and upregulates levels of nuclear p53 as well as p53 transcriptional activity in neuronal cultures. The augmentation is specific, as it occurs with mHtt but not mutant ataxin-1 with expanded polyQ. p53 levels are also increased in the brains of mHtt transgenic (mHtt-Tg) mice and HD patients. Perturbation of p53 by pifithrin-alpha, RNA interference, or genetic deletion prevents mitochondrial membrane depolarization and cytotoxicity in HD cells, as well as the decreased respiratory complex IV activity of mHtt-Tg mice. Genetic deletion of p53 suppresses neurodegeneration in mHtt-Tg flies and neurobehavioral abnormalities of mHtt-Tg mice. Our findings suggest that p53 links nuclear and mitochondrial pathologies characteristic of HD.

Inducible expression of mutant human DISC1 in mice is associated with brain and behavioral abnormalities reminiscent of schizophrenia

A strong candidate gene for schizophrenia and major mental disorders, disrupted-in-schizophrenia 1 (DISC1) was first described in a large Scottish family in which a balanced chromosomal translocation segregates with schizophrenia and other psychiatric illnesses. The translocation mutation may result in loss of DISC1 function via haploinsufficiency or dominant-negative effects of a predicted mutant DISC1 truncated protein product. DISC1 has been implicated in neurodevelopment, including maturation of the cerebral cortex. To evaluate the neuronal and behavioral effects of mutant DISC1, the Tet-off system under the regulation of the CAMKII promoter was used to generate transgenic mice with inducible expression of mutant human DISC1 (hDISC1) limited to forebrain regions, including cerebral cortex, hippocampus and striatum. Expression of mutant hDISC1 was not associated with gross neurodevelopmental abnormalities, but led to a mild enlargement of the lateral ventricles and attenuation of neurite outgrowth in primary cortical neurons. These morphological changes were associated with decreased protein levels of endogenous mouse DISC1, LIS1 and SNAP-25. Compared to their sex-matched littermate controls, mutant hDISC1 transgenic male mice exhibited spontaneous hyperactivity in the open field and alterations in social interaction, and transgenic female mice showed deficient spatial memory. The results show that the neuronal and behavioral effects of mutant hDISC1 are consistent with a dominant-negative mechanism, and are similar to some features of schizophrenia. The present mouse model may facilitate the study of aspects of the pathogenesis of schizophrenia.

Disordered food intake and obesity in rats lacking cholecystokinin A receptors

Otsuka Long-Evans Tokushima Fatty (OLETF) rats develop obesity, hyperglycemia, and non-insulin-dependent diabetes mellitus and do not express cholecystokinin A (CCK-A) receptors, the receptor subtype mediating the satiety actions of CCK. In short-term feeding tests, male OLETF rats were completely resistant to exogenous CCK, and their response to bombesin was attenuated. Comparisons of liquid meal consumption in OLETF and control Long-Evans Tokushima (LETO) rats demonstrated that 1) OLETF rats had greater intakes during 30-min scheduled daytime meals and significantly larger and fewer spontaneous night-time meals and 2) although the initial rates of licking were the same, OLETF rats maintained the initial rate longer and the rate at which their licking declined was slower. In 24-h solid food access tests, OLETF rats consumed significantly more pellets than LETO controls, and this increase was attributable to significant increases in meal size. Together, these data are consistent with the interpretation that the lack of CCK-A receptors in OLETF rats results in a satiety deficit leading to increases in meal size, overall hyperphagia, and obesity.Otsuka Long-Evans Tokushima Fatty (OLETF) rats develop obesity, hyperglycemia, and non-insulin-dependent diabetes mellitus and do not express cholecystokinin A (CCK-A) receptors, the receptor subtype mediating the satiety actions of CCK. In short-term feeding tests, male OLETF rats were completely resistant to exogenous CCK, and their response to bombesin was attenuated. Comparisons of liquid meal consumption in OLETF and control Long-Evans Tokushima (LETO) rats demonstrated that 1) OLETF rats had greater intakes during 30-min scheduled daytime meals and significantly larger and fewer spontaneous nighttime meals and 2) although the initial rates of licking were the same, OLETF rats maintained the initial rate longer and the rate at which their licking declined was slower. In 24-h solid food access tests, OLETF rats consumed significantly more pellets than LETO controls, and this increase was attributable to significant increases in meal size. Together, these data are consistent with the interpretation that the lack of CCK-A receptors in OLETF rats results in a satiety deficit leading to increases in meal size, overall hyperphagia, and obesity.

Cholecystokinin and satiety: current perspectives ☆

In the almost 30 years since the ability of peripheral administration of the brain/gut peptide cholecystokinin (CCK) to inhibit food intake was first demonstrated, significant progress in our overall understanding of the role of CCK in ingestive behavior has been made. A physiologic role for endogenous CCK in the control of meal size has been demonstrated and sites and mechanisms of action for CCK in food intake have been investigated. Recent work has uncovered roles for the CCK satiety pathway in the mediation of the feeding modulatory actions of estradiol, insulin, and leptin. The availability of the Otsuka Long Evans Tokushima Fatty (OLETF) rat, a strain lacking CCK(A) receptors, provides a unique model for the study of how deficits in a within-meals satiety signaling pathway may result in long-term changes in food intake and body weight.

Prenatal Stress or High-Fat Diet Increases Susceptibility to Diet-Induced Obesity in Rat Offspring

OBJECTIVE Perturbations to the prenatal environment have been associated with the development of adult chronic disease, findings that gave rise to the “Barker Hypothesis” or the “developmental origins of adult disease” concept. In this study, we used an animal model to determine the metabolic consequences of maternal prenatal stress and high-fat feeding on the developing offspring. RESEARCH DESIGN AND METHODS Pregnant female Sprague-Dawley rats were maintained on standard chow or 60% high-fat diet throughout gestation and lactation. Half of each group were exposed to a novel variable stress paradigm during the 3rd week of gestation, whereas control dams were left undisturbed. Body weight, body composition, glucose tolerance, and endocrine parameters were measured in offspring through early adulthood. RESULTS Male and female pups from dams that experienced prenatal stress and/or were on a high-fat diet weighed more beginning on postnatal day 7 compared with standard chow–control pups. Access to high-fat diet at weaning increased the body weight effect through early adulthood and was attributable to greater adiposity. Pups weaned onto standard chow diet showed no significant difference in glucose clearance or insulin secretion. However, pups weaned onto high-fat diet had impaired glucose tolerance if their dams were on a high-fat diet, experienced prenatal stress, or both. CONCLUSIONS Our data demonstrate that prenatal stress and/or high-fat diet during the intrauterine or postnatal environment affects offspring in a manner that increases their susceptibility to diet-induced obesity and leads to secondary adverse metabolic consequences.

Neuroprotective role of Sirt1 in mammalian models of Huntington's disease through activation of multiple Sirt1 targets

Huntingtons disease is a fatal neurodegenerative disorder caused by an expanded polyglutamine repeat in huntingtin (HTT) protein. We previously showed that calorie restriction ameliorated Huntingtons disease pathogenesis and slowed disease progression in mice that model Huntingtons disease (Huntingtons disease mice). We now report that overexpression of sirtuin 1 (Sirt1), a mediator of the beneficial metabolic effects of calorie restriction, protects neurons against mutant HTT toxicity, whereas reduction of Sirt1 exacerbates mutant HTT toxicity. Overexpression of Sirt1 improves motor function, reduces brain atrophy and attenuates mutant-HTT–mediated metabolic abnormalities in Huntingtons disease mice. Further mechanistic studies suggested that Sirt1 prevents the mutant-HTT–induced decline in brain-derived neurotrophic factor (BDNF) concentrations and the signaling of its receptor, TrkB, and restores dopamine- and cAMP-regulated phosphoprotein, 32 kDa (DARPP32) concentrations in the striatum. Sirt1 deacetylase activity is required for Sirt1-mediated neuroprotection in Huntingtons disease cell models. Notably, we show that mutant HTT interacts with Sirt1 and inhibits Sirt1 deacetylase activity, which results in hyperacetylation of Sirt1 substrates such as forkhead box O3A (Foxo3a), thereby inhibiting its pro-survival function. Overexpression of Sirt1 counteracts the mutant-HTT–induced deacetylase deficit, enhances the deacetylation of Foxo3a and facilitates cell survival. These findings show a neuroprotective role for Sirt1 in mammalian Huntingtons disease models and open new avenues for the development of neuroprotective strategies in Huntingtons disease.

A Drosophila model for LRRK2-linked parkinsonism

Mutations in the leucine-rich repeat kinase (LRRK2) gene cause late-onset autosomal dominant Parkinsons disease (PD) with pleiomorphic pathology. Previously, we and others found that expression of mutant LRRK2 causes neuronal degeneration in cell culture. Here we used the GAL4/UAS system to generate transgenic Drosophila expressing either wild-type human LRRK2 or LRRK2-G2019S, the most common mutation associated with PD. Expression of either wild-type human LRRK2 or LRRK2-G2019S in the photoreceptor cells caused retinal degeneration. Expression of LRRK2 or LRRK2-G2019S in neurons produced adult-onset selective loss of dopaminergic neurons, locomotor dysfunction, and early mortality. Expression of mutant G2019S-LRRK2 caused a more severe parkinsonism-like phenotype than expression of equivalent levels of wild-type LRRK2. Treatment with l-DOPA improved mutant LRRK2-induced locomotor impairment but did not prevent the loss of tyrosine hydroxylase-positive neurons. To our knowledge, this is the first in vivo“gain-of-function” model which recapitulates several key features of LRRK2-linked human parkinsonism. These flies may provide a useful model for studying LRRK2-linked pathogenesis and for future therapeutic screens for PD intervention.