A disturbance of late myelination as a trigger for Alzheimer's disease

Apolipoprotein E ε4 Mediates Myelin Breakdown by Targeting Oligodendrocytes in Sporadic Alzheimer Disease

White matter degradation in the frontal lobe is one of the earliest detectable changes in aging and Alzheimer disease. The ε4 allele of apolipoprotein E (APOE4) is strongly associated with such myelin pathology but the underlying cellular mechanisms remain obscure. We hypothesized that, as a lipid transporter, APOE4 directly triggers pathology in the cholesterol-rich myelin sheath independent of AD pathology. To test this, we performed immunohistochemistry on brain tissues from healthy controls, sporadic, and familial Alzheimer disease subjects. While myelin basic protein expression was largely unchanged, in frontal cortex the number of oligodendrocytes (OLs) was significantly reduced in APOE4 brains independent of their Braak stage or NIA-RI criteria. This high vulnerability of OLs was confirmed in humanized APOE3 or APOE4 transgenic mice. A gradual decline of OL numbers was found in the aging brain without associated neuronal loss. Importantly, the application of lipidated human APOE4, but not APOE3, proteins significantly reduced the formation of myelinating OL in primary cell culture derived from Apoe-knockout mice, especially in cholesterol-depleted conditions. Our findings suggest that the disruption of myelination in APOE4 carriers may represent a direct OL pathology, rather than an indirect consequence of amyloid plaque formation or neuronal loss.

Neurosteroids and oxysterols as potential therapeutic agents for glaucoma and Alzheimer's disease

Glaucoma is one of the most frequent causes of visual impairment worldwide and involves selective damage to retinal ganglion cells (RGCs) resulting in degeneration of neural pathways connecting retina to visual cortex. It is of interest that similarities in pathological changes have been described in Alzheimer's disease (AD), the most common cause of progressive memory loss and dementia in older people. Accumulation of amyloid-beta (Abeta) and hyperphosphorylated tau is thought to contribute to apoptotic neuronal death in Alzheimer's disease, and similar changes have been linked to apoptotic RGC death in glaucoma. Both glaucoma and Alzheimer's disease also suffer from a lack of effective treatments prompting a search for novel therapeutic interventions. Neurosteroids (NSs) (including oxysterols) are endogenous molecules synthesized in the nervous system from cholesterol that can modulate glutamate and GABA receptors, the primary mediators of fast excitatory and inhibitory neurotransmission in the brain, respectively. Because changes in the glutamate and GABA neurotransmitter systems contribute to the pathogenesis of AD and glaucoma, NSs are possible therapeutic targets for these disorders. In this review, we present recent evidence supporting pathological links between Alzheimer's disease and glaucoma, and focus on the possible role of NSs in these diseases and how NSs might be developed for therapeutic purposes.

Iodine Deficiency and the Brain: Effects and Mechanisms

Iodine is an essential micronutrient needed in human diets. As iodine is an integral component of thyroid hormone, it mediates the effects of thyroid hormone on brain development. Iodine deficiency is the most prevalent and preventable cause of mental impairment in the world. The exact mechanism through which iodine influences the brain is unclear, but is generally thought to begin with genetic expression. Many brain structures and systems appear to be affected with iodine deficiency, including areas such as the hippocampus, microstructures such as myelin, and neurotransmitters. The clearest evidence comes from the studies examining cognition in the cases of iodine deprivation or interventions involving iodine supplementation. Nevertheless, there are many inconsistencies and gaps in the literature of iodine deficiency, especially over the lifespan. This paper summarizes the literature on this topic, suggests a causal mechanism for iodine's effect on the brain, and indicates areas for the future research (e.g., using magnetic resonance imaging (MRI) and functional MRI to examine how iodine supplementation facilitates cognitive functioning).

Midlife measurements of white matter microstructure predict subsequent regional white matter atrophy in healthy adults

OBJECTIVES

Although age-related brain changes are becoming better understood, midlife patterns of change are still in need of characterization, and longitudinal studies are lacking. The aim of this study was to determine if baseline fractional anisotropy (FA), obtained from diffusion tensor imaging (DTI) predicts volume change over a 4-year interval.

EXPERIMENTAL DESIGN

Forty-four cognitively healthy middle-age adults underwent baseline DTI and longitudinal T1-weighted magnetic resonance imaging. Tensor-based morphometry methods were used to evaluate volume change over time. FA values were extracted from regions of interest that included the cingulum, entorhinal white matter, and the genu and splenium of the corpus callosum. Baseline FA was used as a predictor variable, whereas gray and white matter atrophy rates as indexed by Tensor-based morphometry were the dependent variables.

PRINCIPAL OBSERVATIONS

Over a 4-year period, participants showed significant contraction of white matter, especially in frontal, temporal, and cerebellar regions (P < 0.05, corrected for multiple comparisons). Baseline FA in entorhinal white matter, genu, and splenium was associated with longitudinal rates of atrophy in regions that included the superior longitudinal fasciculus, anterior corona radiata, temporal stem, and white matter of the inferior temporal gyrus (P < 0.001, uncorrected for multiple comparisons).

CONCLUSIONS

Brain change with aging is characterized by extensive shrinkage of white matter. Baseline white matter microstructure as indexed by DTI was associated with some of the observed regional volume loss. The findings suggest that both white matter volume loss and microstructural alterations should be considered more prominently in models of aging and neurodegenerative diseases.

Allopregnanolone levels are reduced in temporal cortex in patients with Alzheimer's disease compared to cognitively intact control subjects

The neurosteroid allopregnanolone has pronounced neuroprotective actions, increases myelination, and enhances neurogenesis. Evidence suggests that allopregnanolone dysregulation may play a role in the pathophysiology of Alzheimer's disease (AD) and other neurodegenerative disorders. Our prior data demonstrate that allopregnanolone is reduced in prefrontal cortex in male patients with AD compared to male cognitively intact control subjects, and inversely correlated with neuropathological disease stage (Braak and Braak). We therefore determined if allopregnanolone levels are also reduced in AD patients compared to control subjects in temporal cortex, utilizing a larger set of samples from both male and female patients. In addition, we investigated if neurosteroids are altered in subjects who are APOE4 allele carriers. Allopregnanolone, dehydroepiandrosterone (DHEA), and pregnenolone levels were determined in temporal cortex postmortem samples by gas chromatography/mass spectrometry, preceded by high performance liquid chromatography (40 subjects with AD/41 cognitively intact control subjects). Allopregnanolone levels are reduced in temporal cortex in patients with AD (median 2.68 ng/g, n=40) compared to control subjects (median 5.64 ng/g, n=41), Mann-Whitney p=0.0002, and inversely correlated with Braak and Braak neuropathological disease stage (Spearman r=-0.38, p=0.0004). DHEA and pregnenolone are increased in patients with AD compared to control subjects. Patients carrying an APOE4 allele demonstrate reduced allopregnanolone levels in temporal cortex (Mann-Whitney p=0.04). In summary, our findings indicate that neurosteroids are altered in temporal cortex in patients with AD and related to neuropathological disease stage. In addition, the APOE4 allele is associated with reduced allopregnanolone levels. Neurosteroids may be relevant to the neurobiology and therapeutics of AD.

Age-related and cuprizone-induced changes in myelin and transcription factor gene expression and in oligodendrocyte cell densities in the rostral corpus callosum of mice

During aging, there is a decrease both in the stability of central nervous system (CNS) myelin once formed and in the efficiency of its repair by oligodendrocytes (OLs). To study CNS remyelination during aging, we used the cuprizone (a copper chelator) mouse model. Inclusion of cuprizone in the diet kills mature OLs and demyelinates axons in the rostral corpus callosum (CC) of mice, which enabled us to characterize age-related changes (i.e., 2-16 months of age) in glial cell response during the recruitment (i.e., demyelination) and differentiation (i.e., remyelination) phases of myelin repair. We have found that the time between 12 and 16 months of age is a critical period during which there is an age-related decrease in the number of OL lineage cells (Olig2(Nuc)+ve/GFAP-ve cells) in the rostral CC of both control mice and mice recovering from cuprizone-induced demyelination. Our results also show there was an age-related impaired recruitment of progenitor cells to replace lost OLs in spite of there being no major age-related decrease in the size of the progenitor cell pool (PDGFalphaR+ve/GFAP-ve, and Olig2(Nuc) +ve/PDGFalphaR+ve cells). However, there were cuprizone-induced increased numbers of astrocyte progenitor cells (Olig2(Cyto)+ve/PDGFalphaR+ve) in these same mice; thus PDGFalphaR+ve progenitor cells in mice as old as 16 months of age retain the ability to differentiate into astrocytes, with this fate choice occurring following cytoplasmic translocation of Olig2. These data reveal for the first time age-related differences in the differentiation of PDGFalphaR+ve progenitor cells into OLs and astrocytes and lead us to suggest that during aging there must be a transcriptional switch mechanism in the progenitor cell fate choice in favor of astrocytes. This may at least partially explain the age-related decrease in efficiency of OL myelination and remyelination.

Effects of mental illness and aging in two thalamic nuclei

We previously reported a schizophrenia associated reduction of neuronal and oligodendrocyte number in the anterior principal thalamic nucleus (APN) in a cohort of severely impaired elderly subjects with schizophrenia (SZ) relative to age matched nonpsychiatric controls (NCs). The present study was undertaken to determine 1) if those findings could be replicated in an independent sample of less chronically impaired subjects with SZ and NCs stratified across a broader age range; 2) if the findings are specific to SZ or are also seen in unipolar major depressive (MDD) or bipolar disorder (BPD); and 3) if the findings are specific to the APN or also seen in another thalamic nucleus. Computer assisted stereological methods were employed to determine the number of neurons and oligodendrocytes in the APN and centromedian nucleus (CMN) of the Nissl-stained thalamic sections maintained by the Stanley Foundation Brain Bank. This collection includes specimens from NCs and age matched subjects with diagnoses of SZ, MDD, or BPD who died between the ages of 25 and 68. Data were analyzed by mixed-effects linear regressions adjusting for demographic variables and known history of exposure to psychotropic medications. Oligodendrocyte number was decreased in both nuclei relative to NCs in subjects with SZ and in that subset of subjects with BPD who had experienced psychotic episodes. Compared to NCs both of these patient groups also exhibited an attenuation of an age-related increase in the number of oligodendrocytes. Contrary to our previous report, we did not detect a SZ-associated deficit in neuronal number in the APN. A history of exposure to neuroleptics, however, was associated with a decrease in neuronal number in both nuclei, but this decrease did not vary in relation to cumulative lifetime neuroleptic exposure in fluphenazine equivalents. Among subjects with psychiatric diagnoses, exposure to lithium was associated with an increase in the number of oligodendrocytes. No effects were detected for exposure to anticonvulsants or for abuse of alcohol or other substances.

Iron, copper, and iron regulatory protein 2 in Alzheimer's disease and related dementias

Accumulating evidence implicates a role for altered iron and copper metabolism in the pathogenesis of neurodegenerative disorders such as Alzheimer's disease (AD). However, imbalances in the levels of the various forms of iron at different stages of AD have not been examined. In this pilot study we extracted and measured the levels of loosely bound, non-heme and total iron and copper in the frontal cortex and hippocampus of patients with mild-moderate AD (n=3), severe AD (n=8) and dementia with Lewy bodies (DLB, n=6), using graphite furnace atomic absorption spectrometry (GFAAS). Additionally, the expression of iron regulatory protein 2 (IRP2) was examined in relation to the pathological hallmarks of AD and DLB, amyloid plaques, neurofibrillary tangles (NFT), and Lewy bodies, by immunohistochemistry. We found significantly decreased loosely bound iron in the hippocampal white matter of mild-moderate and severe AD patients and a trend towards increased non-heme iron in the hippocampal gray matter of severe AD patients. Furthermore, decreased levels of total copper were seen in severe AD and DLB frontal cortex compared to controls, suggesting an imbalance in brain metal levels in both AD and DLB. The decrease in loosely bound iron in mild-moderate AD patients may be associated with myelin breakdown seen in the beginning stages of AD and implicates that iron dysregulation is an early event in AD pathogenesis.

The neurosteroid allopregnanolone is reduced in prefrontal cortex in Alzheimer's disease

BACKGROUND

Few data are currently available investigating neurosteroids (NS) in Alzheimer's disease (AD). The NS allopregnanolone may be decreased in serum and plasma in patients with AD, but it is unclear if allopregnanolone is also reduced in brain. Because a number of NS exhibit neuroprotective effects and impact cognitive performance in rodent models, these molecules may be relevant to the pathophysiology of neurodegenerative disorders. We therefore investigated prefrontal cortex (PFC) NS levels in AD.

METHODS

Neurosteroid levels (allopregnanolone, pregnenolone, dehydroepiandrosterone [DHEA]) were determined in postmortem PFC in 14 male subjects with AD and 15 cognitively intact male control subjects by gas chromatography/mass spectrometry preceded by high-performance liquid chromatography purification.

RESULTS

Subjects with AD exhibit significant reductions in allopregnanolone compared with cognitively intact control subjects (median levels = 2.50 ng/g vs. 5.59 ng/g, respectively; p = .02). Allopregnanolone levels are inversely correlated with neuropathological disease stage (Braak), r = -.49, p = .007. Median DHEA levels are elevated in subjects with AD (p = .01).

CONCLUSIONS

Subjects with AD demonstrate significant reductions in PFC allopregnanolone levels, a finding that may be relevant to neuropathological disease stage severity. Neurosteroids may have utility as candidate biomarkers in AD.

Cognitive deficit associated with cholinergic and nerve growth factor down-regulation in experimental allergic encephalomyelitis in rats

Clinical symptoms in multiple sclerosis include cognitive dysfunction. Difficulties in learning and remembering new information represent the most common cognitive deficit and are associated with a general and progressive brain pathology. Possible pathogenetic mechanisms for neuronal damage such as neuroprotective strategies are under active investigation also in experimental allergic encephalomyelitis, the most widely used experimental model for multiple sclerosis. In this paper we demonstrate that a selective deficit in learning and memory performance, as investigated by the Morris water maze test, is a consistent feature in rat encephalomyelitis, which correlates with a decline in choline acetyltransferase activity and nerve growth factor mRNA level in cerebral cortex, hippocampus, and basal forebrain. Treatment aimed to restore acetylcholine content through chronic administration of selective acetylcholinesterase inhibitors (rivastigmine and donepezil) restores cognitive performance, choline acetyltransferase activity, and nerve growth factor mRNA expression.

Neurotropic action of androgens: principles, mechanisms and novel targets

The importance of androgen signaling is well recognized for numerous aspects of central nervous system (CNS) function, ranging from sex-specific organization of neuroendocrine and behavioral circuits to adaptive capacity, resistance and repair. Nonetheless, concepts for the therapeutic use of androgens in neurological and mental disorders are far from being established. This review outlines some critical issues which interfere with decisions on the suitability of androgens as therapeutic agents for CNS conditions. Among these, sex-specific organization of neural substrates and resulting differential responsiveness to endogenous gonadal steroids, convergence of steroid hormone actions on common molecular targets, co-presence of different sex steroid receptors in target neuronal populations, and in situ biotransformation of natural androgens apparently pose the principal obstacles for the characterization of specific neurotropic effects of androgens. Additional important, albeit less explored aspects consist in insufficient knowledge about molecular targets in the CNS which are under exclusive or predominant androgen control. Own experimental data illustrate the variability of pharmacological effects of natural and synthetic androgens on CNS functions of adaptive relevance, such as sexual behavior, anxiety and endocrine responsiveness to stress. Finally, we present results from an analysis of the consequences of aging for the rat brain transcriptome and examination of the influence of androgens on differentially expressed genes with presumable significance in neuropathology.