Disturbed neurotransmitter homeostasis in ether lipid deficiency.

Plasmalogens, the most prominent ether (phospho)lipids in mammals, are structural components of most cellular membranes. Due to their physicochemical properties and abundance in the central nervous system, a role of plasmalogens in neurotransmission has been proposed, but conclusive data are lacking. Here, we targeted this issue in the glyceronephosphate O-acyltransferase (Gnpat) KO mouse, a model of complete deficiency in ether lipid biosynthesis. Throughout the study, focusing on adult male animals, we found reduced brain levels of various neurotransmitters. In the dopaminergic nigrostriatal tract, synaptic endings but not neuronal cell bodies were affected. Neurotransmitter turnover was altered in ether lipid-deficient murine as well as human post-mortem brain tissue. A generalized loss of synapses did not account for the neurotransmitter deficits, since the levels of several presynaptic proteins appeared unchanged. However, reduced amounts of vesicular monoamine transporter indicate a compromised vesicular uptake of neurotransmitters. As exemplified by norepinephrine, the release of neurotransmitters from Gnpat KO brain slices was diminished in response to strong electrical and chemical stimuli. Finally, addressing potential phenotypic correlates of the disturbed neurotransmitter homeostasis, we show that ether lipid deficiency manifests as hyperactivity and impaired social interaction. We propose that the lack of ether lipids alters the properties of synaptic vesicles leading to reduced amounts and release of neurotransmitters. These features likely contribute to the behavioral phenotype of Gnpat KO mice, potentially modeling some human neurodevelopmental disorders like autism or attention deficit hyperactivity disorder.

Enhanced Dopamine Transmission and Hyperactivity in the Dopamine Transporter Heterozygous Mice Lacking the D3 Dopamine Receptor.

Dopamine transporter knockout (DATk) mice are known to demonstrate profound hyperactivity concurrent with elevated (5-fold) extracellular dopamine in the basal ganglia. At the same time, heterozygous DAT mice (DATh) demonstrate a 2-fold increase in dopamine levels yet only a marginal elevation in locomotor activity level. Another model of dopaminergic hyperactivity is the D3 dopamine receptor knockout (D3k) mice, which present only a modest hyperactivity phenotype, predominately manifested as stereotypical behaviors. In the D3k mice, the hyperactivity is also correlated with elevated extracellular dopamine levels (2-fold) in the basal ganglia. Cross-breeding was used to evaluate the functional consequences of the deletion of both genes. In the heterozygous DAT mice, inactivation of the D3R gene (DATh/D3k) resulted in significant hyperactivity and further elevation of striatal extracellular dopamine above levels observed in respective single mutant mice. The decreased weight of DATk mice was evident regardless of the D3 dopamine receptor genotype. In contrast, measures of thermoregulation revealed that the marked hypothermia of DATk mice (-2 °C) was reversed in double knockout mice. Thus, the extracellular dopamine levels elevated by prolonging uptake could be elevated even further by eliminating the D3 receptor. These data also suggest that the hypothermia observed in DATk mice may be mediated through D3 receptors.

Disrupting the hippocampal Piwi pathway enhances contextual fear memory in mice.

The Piwi pathway is a conserved gene regulatory mechanism comprised of Piwi-like proteins and Piwi-interacting RNAs, which modulates gene expression via RNA interference and through interaction with epigenetic mechanisms. The mammalian Piwi pathway has been defined by its role in transposon control during spermatogenesis; however, despite an increasing number of studies demonstrating its expression in the nervous system, relatively little is known about its function in neurons or potential contribution to behavioural regulation. We have discovered that all three Piwi-like genes are expressed in the adult mouse brain, and that viral-mediated knockdown of the Piwi-like genes Piwil1 and Piwil2 in the dorsal hippocampus leads to enhanced contextual fear memory without affecting generalised anxiety. These results implicate the Piwi pathway in behavioural regulation in the adult mammalian brain, likely through modulation of plasticity-related gene expression.

TRIO loss of function is associated with mild intellectual disability and affects dendritic branching and synapse function.

Recently, we marked TRIO for the first time as a candidate gene for intellectual disability (ID). Across diverse vertebrate species, TRIO is a well-conserved Rho GTPase regulator that is highly expressed in the developing brain. However, little is known about the specific events regulated by TRIO during brain development and its clinical impact in humans when mutated. Routine clinical diagnostic testing identified an intragenic de novo deletion of TRIO in a boy with ID. Targeted sequencing of this gene in over 2300 individuals with ID, identified three additional truncating mutations. All index cases had mild to borderline ID combined with behavioral problems consisting of autistic, hyperactive and/or aggressive behavior. Studies in dissociated rat hippocampal neurons demonstrated the enhancement of dendritic formation by suppressing endogenous TRIO, and similarly decreasing endogenous TRIO in organotypic hippocampal brain slices significantly increased synaptic strength by increasing functional synapses. Together, our findings provide new mechanistic insight into how genetic deficits in TRIO can lead to early neuronal network formation by directly affecting both neurite outgrowth and synapse development.

Autistic-like behaviours and hyperactivity in mice lacking ProSAP1/Shank2.

Autism spectrum disorders comprise a range of neurodevelopmental disorders characterized by deficits in social interaction and communication, and by repetitive behaviour. Mutations in synaptic proteins such as neuroligins, neurexins, GKAPs/SAPAPs and ProSAPs/Shanks were identified in patients with autism spectrum disorder, but the causative mechanisms remain largely unknown. ProSAPs/Shanks build large homo- and heteromeric protein complexes at excitatory synapses and organize the complex protein machinery of the postsynaptic density in a laminar fashion. Here we demonstrate that genetic deletion of ProSAP1/Shank2 results in an early, brain-region-specific upregulation of ionotropic glutamate receptors at the synapse and increased levels of ProSAP2/Shank3. Moreover, ProSAP1/Shank2(-/-) mutants exhibit fewer dendritic spines and show reduced basal synaptic transmission, a reduced frequency of miniature excitatory postsynaptic currents and enhanced N-methyl-d-aspartate receptor-mediated excitatory currents at the physiological level. Mutants are extremely hyperactive and display profound autistic-like behavioural alterations including repetitive grooming as well as abnormalities in vocal and social behaviours. By comparing the data on ProSAP1/Shank2(-/-) mutants with ProSAP2/Shank3αß(-/-) mice, we show that different abnormalities in synaptic glutamate receptor expression can cause alterations in social interactions and communication. Accordingly, we propose that appropriate therapies for autism spectrum disorders are to be carefully matched to the underlying synaptopathic phenotype.

Peer Problems and Hyperactivity-Impulsivity Among Norwegian and American Children: The Role of 5-HTTLPR.

Peer problems are linked to attention deficit hyperactivity disorder (ADHD) symptoms and the serotonin system is thought to be involved in ADHD-related behavior. Hence, from a Gene × Environment perspective, the serotonin transporter 5-HTTLPR may play a moderating role. In two large community samples, the moderating role of 5-HTTLPR was examined related to more hyperactivity-impulsivity symptoms (HI symptoms) predicted by more peer problems. In Study 1, involving 642 Norwegian children, results indicated that for s-allele carriers only, caregiver-reported peer problems at age 4 predicted more parent-reported HI symptoms at age 6. In Study 2, similar results emerged involving 482 American children. Discussion focuses on differential sensitivity to the adverse effects of poor peer relations.

Biomarkers of agitation and aggression in Alzheimer's disease: A systematic review.

INTRODUCTION: Agitation is one of the most challenging neuropsychiatric symptoms to treat in Alzheimer's disease and has significant implications for patient and caregiver. A major source of difficulty in identifying safe and effective treatments for agitation is the lack of validated biomarkers. As such, patients may not be appropriately targeted, and biological response to pharmacotherapy cannot be adequately monitored. METHODS: This systematic review aimed to summarize evidence on the association between biomarkers and agitation/aggression in patients with Alzheimer's disease, utilizing the National Institute on Aging-Alzheimer's Association Research Framework and the Biomarkers, EndpointS, and other Tools Resource of the Food and Drug Association-National Institutes of Health Biomarker Working Group. RESULTS: This review identified six classes of biomarkers (neuropathological, neurotransmitter, neuroimaging, apolipoprotein E (APOE) genotype, inflammatory, and clusterin) associated with agitation/aggression, which were mostly diagnostic in nature. DISCUSSION: Future studies should investigate the predictive, prognostic, and monitoring capacity of biomarkers to provide insight into the longitudinal course of agitation/aggression, as well as predict and monitor biological response to a pharmacological intervention.

Cytochrome P450 20A1 in zebrafish: Cloning, regulation and potential involvement in hyperactivity disorders.

Cytochrome P450 (CYP) enzymes for which there is no functional information are considered "orphan" CYPs. Previous studies showed that CYP20A1, an orphan, is expressed in human hippocampus and substantia nigra, and in zebrafish (Danio rerio) CYP20A1 maternal transcript occurs in eggs, suggesting involvement in brain and in early development. Moreover, hyperactivity is reported in humans with chromosome 2 microdeletions including CYP20A1. We examined CYP20A1 in zebrafish, including impacts of chemical exposure on expression. Zebrafish CYP20A1 cDNA was cloned, sequenced, and aligned with cloned human CYP20A1 and predicted vertebrate orthologs. CYP20A1s share a highly conserved N-terminal region and unusual sequences in the I-helix and the heme-binding CYP signature motifs. CYP20A1 mRNA expression was observed in adult zebrafish organs including the liver, heart, gonads, spleen and brain, as well as the eye and optic nerve. Putative binding sites in proximal promoter regions of CYP20A1s, and response of zebrafish CYP20A1 to selected nuclear and xenobiotic receptor agonists, point to up-regulation by agents involved in steroid hormone response, cholesterol and lipid metabolism. There also was a dose-dependent reduction of CYP20A1 expression in embryos exposed to environmentally relevant levels of methylmercury. Morpholino knockdown of CYP20A1 in developing zebrafish resulted in behavioral effects, including hyperactivity and a slowing of the optomotor response in larvae. The results suggest that altered expression of CYP20A1 might be part of a mechanism linking methylmercury exposure to neurobehavioral deficits. The expanded information on CYP20A1 brings us closer to "deorphanization", that is, identifying CYP20A1 functions and its roles in health and disease.

Association between Sirtuin 1 Gene rs10997870 Polymorphism and Suicide Behaviors in Bipolar Disorder.

BACKGROUND/AIMS: Suicidal behavior (SB) in bipolar disorder (BD) is a complex multifactorial event resulting from an interaction of genetic, neurobiological and psychosocial factors. Recent studies identified new possible mechanisms, suggesting a role for sirtuins (SIRTs 1-7), a family of nicotinamide adenine dinucleotide-dependent enzymes with a multifaceted role in the central nervous system. The aims of the present study were: (1) to investigate the effects of the rs10997870 SIRT1 gene variant on SB in BD; (2) to explore the effects of the same gene variant on specific depressive symptoms at the severest episode. METHODS: One hundred and eighty DSM-IV bipolar outpatients were enrolled in a naturalistic cohort study. The rs10997870 polymorphism within the SIRT1 gene was analyzed. RESULTS: An association between the GG genotype and SB was detected (lifetime: p = 0.015). Compared to other genotypes, GG carriers presented more frequently psychomotor agitation (p = 0.009) and a higher Hamilton Depression Rating Scale total score (p = 0.014) at the severest depressive episode. SB and psychomotor agitation were found to be associated with GG carriers and G allele in a multivariate analysis as well. CONCLUSION: Our findings suggest a role of the rs10997870 SIRT1 gene variant in SB amongst BD patients and its association with specific depressive symptoms. Despite a number of limitations of this exploratory study, our results may provide new insight into the mechanisms underlying SB in BD.

Citalopram for the Treatment of Agitation in Alzheimer Dementia: Genetic Influences.

OBJECTIVE: To assess potential genetic influences on citalopram treatment efficacy for agitation in individuals with Alzheimer dementia (AD). Six functional genetic variants were studied in the following genes: serotonin receptor 2A (HTR2A-T102C), serotonin receptor 2C (HTR2C-Cys23Ser), serotonin transporter (5HTT-LPR), brain-derived neurotropic factor (BDNF-Val66Met), apolipoprotein E (ε2, ε3, ε4 variants), and cytochrome P450 (CYP2C19). Treatment response by genotype was measured by (1) the agitation domain of the Neurobehavioral Rating Scale, (2) the modified Alzheimer Disease Cooperative Study-Clinical Global Impression of Change scale (mADCS-CGIC), (3) the agitation domain of the Neuropsychiatric Inventory (NPI), and (4) the Cohen-Mansfield Agitation Inventory. METHOD: We utilized data from the Citalopram for Agitation in Alzheimer's Disease (CitAD) database. CitAD was a 9-week randomized, double-blind, placebo-controlled multicenter clinical trial showing significant improvement in agitation and caregiver distress in patients treated with citalopram. Proportional odds logistic regression and mixed effects models were used to examine the above-mentioned outcome measures. RESULTS: Significant interactions were noted on the NPI agitation domain for HTR2A (likelihood ratio [LR] = 6.19, df = 2, P = .04) and the mADCS-CGIC for HTR2C (LR = 4.33, df = 2, P = .02) over 9 weeks. DISCUSSION: Treatment outcomes in CitAD showed modest, although statistically significant, influence of genetic variation at HTR2A and HTR2C loci. Future studies should continue to examine the interaction of known genetic variants with antidepressant treatment in patients with AD having agitation.

An affective disorder in zebrafish with mutation of the glucocorticoid receptor.

Upon binding of cortisol, the glucocorticoid receptor (GR) regulates the transcription of specific target genes, including those that encode the stress hormones corticotropin-releasing hormone (CRH) and adrenocorticotropic hormone. Dysregulation of the stress axis is a hallmark of major depression in human patients. However, it is still unclear how glucocorticoid signaling is linked to affective disorders. We identified an adult-viable zebrafish mutant in which the negative feedback on the stress response is disrupted, due to abolition of all transcriptional activity of GR. As a consequence, cortisol is elevated, but unable to signal through GR. When placed into an unfamiliar aquarium ('novel tank'), mutant fish become immobile ('freeze'), show reduced exploratory behavior and do not habituate to this stressor upon repeated exposure. Addition of the antidepressant fluoxetine to the holding water and social interactions restore normal behavior, followed by a delayed correction of cortisol levels. Fluoxetine does not affect the overall transcription of CRH, the mineralocorticoid receptor (MR), the serotonin transporter (Serta) or GR itself. Fluoxetine, however, suppresses the stress-induced upregulation of MR and Serta in both wild-type fish and mutants. Our studies show a conserved, protective function of glucocorticoid signaling in the regulation of emotional behavior and reveal novel molecular aspects of how chronic stress impacts vertebrate brain physiology and behavior. Importantly, the zebrafish model opens up the possibility of high-throughput drug screens in search of new classes of antidepressants.

Autism traits in children and adolescents with Cornelia de Lange syndrome.

Cornelia de Lange syndrome (CdLS) is a cohesinopathy causing delayed growth and limb deficits. Individuals with CdLS have mild to profound intellectual disability and autistic features. This study characterizes the behavioral phenotype of children with CdLS, focusing on autistic features, maladaptive behaviors, and impact of age. Children with CdLS (5-18 years) were administered normed instruments to characterize autism features (Childhood Autism Rating Scale, CARS), maladaptive behaviors (Aberrant Behavior Checklist), and adaptive skills (Vineland Adaptive Behaviors Scales). CdLS features and severity were rated with Diagnostic Criteria for CdLS. Forty-one children with CdLS (23 females, 18 males) were classified as having "no autism" (n = 7; 17.1%), "mild autism" (n = 17; 41.4%), and "severe autism" (n = 17; 41.4%), using CARS scores. Characteristic items were abnormal emotional response, stereotypies, odd object use, rigidity, lack of verbal communication, and low intellectual functioning. Verbal communication deficits and repetitive behaviors were higher compared to sensory, social cognition, and behavior abnormalities (P ≤ 0.0001). Maladaptive behaviors associated with autism traits were stereotypies (P = 0.003), hyperactivity (P = 0.01), and lethargy (P = 0.03). Activities of daily living were significantly affected; socialization adaptive skills were a relative strength. However, with advancing age, both socialization (P < 0.0001) and communication (P = 0.001) domains declined significantly. CdLS is characterized by autistic features, notably excessive repetitive behaviors and expressive language deficits. While other adaptive skills are impacted, socialization adaptive skills are less affected. Advancing age can worsen communication and socialization deficits relative to neurotypical peers.

GFAP expression and social deficits in transgenic mice overexpressing human sAPPα.

Autistic individuals display impaired social interactions and language, and restricted, stereotyped behaviors. Elevated levels of secreted amyloid precursor protein-alpha (sAPPα), the product of α-secretase cleavage of APP, are found in the plasma of some individuals with autism. The sAPPα protein is neurotrophic and neuroprotective and recently showed a correlation to glial differentiation in human neural stem cells (NSCs) via the IL-6 pathway. Considering evidence of gliosis in postmortem autistic brains, we hypothesized that subsets of patients with autism would exhibit elevations in CNS sAPPα and mice generated to mimic this observation would display markers suggestive of gliosis and autism-like behavior. Elevations in sAPPα levels were observed in brains of autistic patients compared to controls. Transgenic mice engineered to overexpress human sAPPα (TgsAPPα mice) displayed hypoactivity, impaired sociability, increased brain glial fibrillary acidic protein (GFAP) expression, and altered Notch1 and IL-6 levels. NSCs isolated from TgsAPPα mice, and those derived from wild-type mice treated with sAPPα, displayed suppressed ß-tubulin III and elevated GFAP expression. These results suggest that elevations in brain sAPPα levels are observed in subsets of individuals with autism and TgsAPPα mice display signs suggestive of gliosis and behavioral impairment.

Alterations in local thyroid hormone signaling in the hippocampus of the SAMP8 mouse at younger ages: association with delayed myelination and behavioral abnormalities.

The senescence-accelerated mouse (SAM) strains were established through selective inbreeding of the AKR/J strain based on phenotypic variations of aging and consist of senescence-prone (SAMP) and senescence-resistant (SAMR) strains. Among them, SAMP8 is considered as a model of neurodegeneration displaying age-associated learning and memory impairment and altered emotional status. Because adult hypothyroidism is one of the common causes of cognitive impairment and various psychiatric disorders, we examined the possible involvement of thyroid hormone (TH) signaling in the pathological aging of SAMP8 using the senescence-resistant SAMR1 as control. Although plasma TH levels were similar in both strains, a significant decrease in type 2 deiodinase (D2) gene expression was observed in the SAMP8 hippocampus from 1 to 8 months of age, which led to a 35-50% reductions at the protein level and 20% reduction of its enzyme activity at 1, 3, and 5 months. D2 is responsible for local conversion of thyroxine into transcriptionally active 3,5,3'-triiodothyronine (T3), so the results suggest a reduction in T3 level in the SAMP8 hippocampus. Attenuation of local TH signaling was confirmed by downregulation of TH-dependent genes and by immunohistochemical demonstration of delayed and reduced accumulation of myelin basic protein, the expression of which is highly dependent on TH. Furthermore, we found that hyperactivity and reduced anxiety were not age-associated but were characteristic of young SAMP8 before they start showing impairments in learning and memory. Early alterations in local TH signaling may thus underlie behavioral abnormalities as well as the pathological aging of SAMP8.

Analysis of CYP2D6 genotype and response to tetrabenazine.

Tetrabenazine is effective in the treatment of the chorea associated with Huntington disease and other hyperkinetic movement disorders. Following oral administration, tetrabenazine is hepatically transformed into 2 active metabolites that are CYP2D6 substrates. There are 4 CYP2D6 genotypes: poor metabolizers, intermediate metabolizers, extensive metabolizers, and ultrarapid metabolizers. CYP2D6 genotyping was performed on sequential subjects treated with tetrabenazine, but results were not known at the time of titration. Duration of titration to a stable dose, total daily dose, response rating scores, and adverse events were retrospectively collected and subsequently analyzed. Of 127 patients, the majority (n = 100) were categorized as extensive metabolizers, 14 as intermediate metabolizers, 11 as poor metabolizers, and 2 as ultrarapid metabolizers. Ultrarapid metabolizer patients needed a longer titration (8 vs 3.3, 4.4, and 3 weeks, respectively; P < .01) to achieve optimal benefit and required a higher average daily dose than the other patients, but this difference did not reach statistical significance. The treatment response was less robust in the intermediate metabolizer group when compared with the extensive metabolizer patients (P = .013), but there were no statistically significant differences between the various groups with regard to adverse effects. Our findings demonstrate that, aside from the need for a longer titration in the ultrarapid metabolizers, there are no distinguishing features of patients with various CYP2D6 genotypes, and therefore the current recommendation to systematically genotype all patients prescribed more than 50 mg/day of tetrabenazine should be reconsidered.

Genetic selection for temperament affects behaviour and the secretion of adrenal and reproductive hormones in sheep subjected to stress.

We investigated the effect of genetic selection for temperament on the way that stressors affect the behaviour and the adrenal and reproductive axes of sheep. We tested three hypotheses: (i) isolation would increase cortisol secretion and decrease luteinising hormone (LH) secretion more in nervous sheep than in calm sheep; (ii) isolation combined with simulated human presence would increase cortisol secretion and decrease LH secretion more in nervous sheep than in calm sheep and (iii) isolation combined with stressors that were not specific to the selection process (i.e. non-selection stressors) would increase cortisol secretion and decrease LH secretion equally in calm and nervous sheep. Isolation alone increased cortisol secretion and decreased LH secretion in nervous sheep but not in calm sheep. Compared to calm sheep, nervous sheep were more agitated during the first 2 h of isolation but not during the second 2 h of isolation. Exposure to non-selection stressors increased cortisol secretion, decreased LH pulse amplitude and the mean plasma concentrations of LH in both calm and nervous sheep. We conclude that genetic selection for temperament affects the behavioural expression of the stress response and the secretion of adrenal and reproductive hormones during isolation, but has less impact on their reactivity to non-selection stressors.

Blood DNA Methylation Levels in the WNT5A Gene Promoter Region: A Potential Biomarker for Agitation in Subjects with Dementia.

BACKGROUND: Behavioral and psychological symptoms of dementia (BPSD) cause a heavy burden for both patient and caregivers. These symptoms are diverse, and their mechanism is still unclear. Agitation is the most common and difficult to treat among BPSD. In recent years, while changes in DNA methylation levels have been receiving attention as a biomarker of aging and dementia, associations with BPSD have not been examined. OBJECTIVE: Focusing on agitation, the objective of the present study was to identify a region where changes in DNA methylation levels are associated with agitation. METHODS: Using genome-wide DNA methylation analysis data for 7 dementia subjects with agitation, 5 dementia subjects without agitation, and 4 normal elderly controls, we determined a signaling pathway in the WNT5A gene promoter region to be associated with agitation. Based on this result, we measured DNA methylation levels in this region for 26 dementia subjects with agitation and 82 dementia subjects without agitation by means of methylation-sensitive high-resolution melting (MS-HRM) analysis. RESULTS: The WNT5A DNA methylation level in dementia subjects with agitation was significantly lower than in those without agitation (p = 0.001). Changes in WNT5A DNA methylation levels were not influenced by age, sex, body mass index, APOE ɛ4, medication, or inflammatory cytokines. CONCLUSION: Our results suggested an association of agitation with Wnt signaling, in particular with changes in WNT5A DNA methylation levels, which could be a potentially useful biomarker for predicting the appearance of agitation. It may contribute to the elucidation of the mechanism of BPSD.

Pten and the brain: sizing up social interaction.

In a comprehensive set of experiments in this issue of Neuron, Kwon et al. demonstrate that conditional inactivation of Pten leads to behavioral abnormalities and neuropathological changes. Pten mutants displayed reduced social interaction and heightened responses to sensory stimuli. Additionally, neuronal hypertrophy, as well as macrocephaly was observed. Based on rare human mutations in PTEN and the PI3K pathway, the authors suggest they have produced a potential animal model of autism with macrocephaly.

"Skittish" Abca2 knockout mice display tremor, hyperactivity, and abnormal myelin ultrastructure in the central nervous system.

The ATP-binding cassette transporter 2 (ABCA2) is an endolysosomal protein most highly expressed in the central and peripheral nervous system tissues and macrophages. Previous studies indicated its role in cholesterol/steroid (estramustine, estradiol, and progesterone) trafficking/sequestration, oxidative stress response, and Alzheimer's disease. Developmental studies have shown its expression during macrophage and oligodendrocyte differentiation, processes requiring membrane growth. To determine the in vivo function(s) of this transporter, we generated a knockout mouse from a gene-targeted disruption of the murine ABCA2 gene. Knockout males and females are viable and fertile. However, a non-Mendelian inheritance pattern was shown among male progeny of heterozygous crosses. Compared to wild-type and heterozygous littermates, knockout mice displayed a tremor without ataxia, hyperactivity, and reduced body weight; the latter two phenotypes were more marked in females than in males. This sexual disparity suggests a role for ABCA2 in hormone-dependent neurological and/or developmental pathways. Myelin sheath thickness in the spinal cords of knockout mice was greatly increased compared to that in wild-type mice, while a significant reduction in myelin membrane periodicity (compaction) was observed in both spinal cords and cerebra of knockout mice. Loss of ABCA2 function in vivo resulted in abnormal myelin compaction in spinal cord and cerebrum, an ultrastructural defect that we propose to be the cause of the phenotypic tremor.