GFAP immunoreactivity reveals astrogliosis in females heterozygous for jimpy

Exercise-Induced Modulation of Neuroinflammation in Models of Alzheimer's Disease

 Alzheimer's disease (AD), a progressive, neurodegenerative condition characterised by accumulation of toxic βeta-amyloid (Aβ) plaques, is one of the leading causes of dementia globally. The cognitive impairment that is a hallmark of AD may be caused by inflammation in the brain triggered and maintained by the presence of Aβ protein, ultimately leading to neuronal dysfunction and loss. Since there is a significant inflammatory component to AD, it is postulated that anti-inflammatory strategies may be of prophylactic or therapeutic benefit in AD. One such strategy is that of regular physical activity, which has been shown in epidemiological studies to be protective against various forms of dementia including AD. Exercise induces an anti-inflammatory environment in peripheral organs and also increases expression of anti-inflammatory molecules within the brain. Here we review the evidence, mainly from animal models of AD, supporting the hypothesis that exercise can reduce or slow the cellular and cognitive impairments associated with AD by modulating neuroinflammation.

CCR5 knockout prevents neuronal injury and behavioral impairment induced in a transgenic mouse model by a CXCR4-using HIV-1 glycoprotein 120

The innate immune system has been implicated in several neurodegenerative diseases, including HIV-1-associated dementia. In this study, we show that genetic ablation of CCR5 prevents microglial activation and neuronal damage in a transgenic model of HIV-associated brain injury induced by a CXCR4-using viral envelope gp120. The CCR5 knockout (KO) also rescues spatial learning and memory in gp120-transgenic mice. However, the CCR5KO does not abrogate astrocytosis, indicating it can occur independently from neuronal injury and behavioral impairment. To characterize further the neuroprotective effect of CCR5 deficiency we performed a genome-wide gene expression analysis of brains from HIVgp120tg mice expressing or lacking CCR5 and nontransgenic controls. A comparison with a human brain microarray study reveals that brains of HIVgp120tg mice and HIV patients with neurocognitive impairment share numerous differentially regulated genes. Furthermore, brains of CCR5 wild-type and CCR5KO gp120tg mice express markers of an innate immune response. One of the most significantly upregulated factors is the acute phase protein lipocalin-2 (LCN2). Using cerebrocortical cell cultures, we find that LCN2 is neurotoxic in a CCR5-dependent fashion, whereas inhibition of CCR5 alone is not sufficient to abrogate neurotoxicity of a CXCR4-using gp120. However, the combination of pharmacologic CCR5 blockade and LCN2 protects neurons from toxicity of a CXCR4-using gp120, thus recapitulating the finding in CCR5-deficient gp120tg mouse brain. Our study provides evidence for an indirect pathologic role of CCR5 and a novel protective effect of LCN2 in combination with inhibition of CCR5 in HIV-associated brain injury.

Myelin expression is altered in the brains of neonatal rats reared in a fluctuating oxygen atmosphere

BACKGROUND

Preterm infants receiving supplemental oxygen therapy experience frequent fluctuations in their blood oxygen levels, the magnitude of which has been associated with the incidence and severity of retinopathy of prematurity in such infants.

OBJECTIVE

Our objective was to investigate in a relevant animal model whether the immature brain with its poorly vascularised white matter might also be susceptible to injury when exposed to such fluctuations in blood oxygen.

METHODS

Newborn rats were reared in an atmosphere in which a computer reproduced the oxygen fluctuations derived from the transcutaneous oxygen levels of a 24-week preterm infant who had developed severe retinopathy. Following 14 days of exposure, we measured the expression of active caspase-3, myelin basic protein (MBP) and glial fibrillary acidic protein (GFAP) in the brains comparing with rat pups raised in room air.

RESULTS

Compared to room air controls, at day 14, the expression of active caspase-3 was increased by up to 162% (significant increase in 7 of 9 regions), MBP decreased by up to 70% (significant in the hypothalamus only) and GFAP increased by up to 103% (significant in 6 of 7 regions. On day 21, following 7 days of reparative recovery, GFAP levels in most areas of oxygen-exposed brains had returned to near control levels. There were no longer significant differences in caspase-3 levels apart from the cerebral cortex, cerebellum and striatum. In contrast, MBP expression was now much higher in most regions of the treated brains compared to controls.

CONCLUSION

We conclude that fluctuations in blood oxygen, observed in preterm survivors, may constitute a source of injury to the white matter and corpus striatum of the developing brain and contribute to the neurological sequelae in extremely premature infants.

Recovery of myelin after induction of oligodendrocyte cell death in postnatal brain

A transgenic mouse line (Oligo-TTK) was established to monitor oligodendrocyte cell death and myelin formation in the CNS. The expression of a conditionally toxic gene, the herpes simplex virus-1 thymidine kinase (HSV1-TK), was made under control of the MBP (myelin basic protein) gene promoter. A truncated form of the HSV1-TK (TTK) gene was used to avoid both bystander effect resulting from leaking in thymidine kinase activity and sterility in transgenic males observed in previous transgenic mice. The transgene was expressed in the CNS with a restricted localization in oligodendrocytes. Oligodendrocyte proliferation and myelin formation are therefore tightly controlled experimentally by administration of ganciclovir (GCV) via the induction of oligodendrocyte cell death. The most severe and irreversible hypomyelination was obtained when GCV was given daily from postnatal day 1 (P1) to P30. Oligodendrocyte plasticity and myelin recovery were analyzed in another phenotype generated by GCV treatment from P1 to P15. In this model, after dysmyelination, an apparent normal behavior was restored with no visible pathological symptoms by P30. Proliferating cells, which may be implicated in myelin repair in this model, are detected primarily in myelin tracts expressing the oligodendrocyte phenotype. Therefore, the endogenous potential of oligodendrocytes to remyelinate was clearly demonstrated in the mice of this study.

A transgenic mouse model for inducible and reversible dysmyelination

Oligodendrocytes are glial cells devoted to the production of myelin sheaths. Myelination of the CNS occurs essentially after birth. To delineate both the times of oligodendrocyte proliferation and myelination, as well as to study the consequence of dysmyelination in vivo, a model of inducible dysmyelination was developed. To achieve oligodendrocyte ablation, transgenic animals were generated that express the herpes virus 1 thymidine kinase (HSV1-TK) gene under the control of the myelin basic protein (MBP) gene promoter. The expression of the MBP-TK transgene in oligodendrocytes is not toxic on its own; however, toxicity can be selectively induced by the systemic injection of animals with nucleoside analogs, such as FIAU [1-(2-deoxy-2-fluoro-beta-delta-arabinofuranosyl)-5-iodouracil]. This system allows us to control the precise duration of the toxic insult and the degree of ablation of oligodendrocytes in vivo. We show that chronic treatment of MBP-TK mice with FIAU during the first 3 postnatal weeks triggers almost a total depletion of oligodendrocytes in the CNS. These effects are accompanied by a behavioral phenotype characterized by tremors, seizures, retarded growth, and premature animal death. We identify the period of highest oligodendrocytes division in the first 9 postnatal days. Delaying the beginning of FIAU treatments results in different degrees of dysmyelination. Dysmyelination in MBP-TK mice is always accompanied by astrocytosis. Thus, this transgenic line provides a model to study the events occurring during dysmyelination of various intensities. It also represents an invaluable tool to investigate remyelination in vivo.

Understanding glial abnormalities associated with myelin deficiency in the jimpy mutant mouse

Jimpy is a shortened life-span murine mutant showing recessive sex-linked inheritance. The genetic defect consists of a point mutation in the PLP gene and produces a severe CNS myelin deficiency that is associated with a variety of complex abnormalities affecting all glial populations. The myelin deficiency is primarily due to a failure to produce the normal amount of myelin during development. However, myelin destruction and oligodendrocyte death also account for the drastic myelin deficit observed in jimpy. The oligodendroglial cell line shows complex abnormalities in its differentiation pattern, including the degeneration of oligodendrocytes through an apoptotic mechanism. Oligodendrocytes seem to be the most likely candidate to be primarily altered in a disorder affecting myelination, but disturbances affecting astrocytes and microglia are also remarkable and may have a crucial significance in the development of the jimpy disorder. In fact, the jimpy phenotype may not be attributed to a defect in a single cell but rather to a deficiency in the normal relations between glial cells. Evidences from a variety of sources indicate that the jimpy mutant could be a model for disturbed glial development in the CNS. The accurate knowledge of the significance of PLP and its regulation during development must be of vital importance in order to understand glial abnormalities in jimpy.

Oligodendrocyte survival and function in the long-lived strain of the myelin deficient rat

This study has examined cellular and molecular aspects of glial cell function in a newly described long-lived myelin deficient rat mutant. In contrast to the shorter-lived mutants which died at 25-30 days, the longer-lived mutant rats lived to 75-80 days of age. Despite living longer, these mutants had a similar frequency of seizures to their younger counterparts. In the spinal cord and optic nerves of the older mutants, myelinated fibres in similar numbers to those seen in the younger myelin deficient rats were present. However, the total glial cell numbers were markedly reduced with few remaining normal appearing oligodendrocytes, and very few microglia compared to the younger mutants. In addition, little or no cell death or division was seen in the longer-lived rats. However, there was some evidence of ongoing myelination and the persistence of immature oligodendrocytes or their progenitors in the older mutant. There was some continued myelin gene expression, although this was at much reduced levels compared to normal, with proteolipid protein and myelin basic protein being most affected. In situ hybridization analysis for proteolipid protein mRNA showed that few proteolipid protein expressing oligodendrocytes remained in the 70-80-day-old mutant. Polymerase chain reaction analysis of exon 3 of the long-lived mutant revealed the same point mutation as described in the younger myelin deficient rat.

Oligodendrocytes in female carriers of the jimpy gene make more myelin than normal oligodendrocytes

The female carrier of the jimpy (jp) gene is a model system to study the plasticity of neuroglial cells and the mechanisms they use to compensate for a temporary deficit in myelin. Myelin in the female carriers is reduced 30-40% during the first postnatal month but is normal in adults. We hypothesized that the number of oligodendrocytes (OLs) in the female carriers is increased, based upon previous data showing OL proliferation is increased but the number of dying OLs is only slightly elevated in development. To test this hypothesis, antibodies to carbonic anhydrase (CA)II, an OL-specific marker, were used to quantify the number of OLs in the spinal cords of 1-month-old and adult female carriers. Contrary to expectations, the number of OLs is significantly reduced in the dorsal funiculus and grey matter by 21% in adult female carriers compared to controls. A reduction of lesser magnitude is present in the 1-month-old animals. Electron microscopic montages prepared from normal and carrier dorsal funiculus were used to count total numbers of glia. Ultrastructural quantification shows a similar reduction in the number of OLs and confirms the validity of the CAII immunostaining as a means to quantify OLs. These data show that there are 21% fewer OLs in the central nervous system (CNS) of adult female carriers but normal amounts of myelin. Presumably, some OLs in the carrier CNS are maintaining more myelin than their counterparts in normal CNS would. These findings demonstrate that (1) a reduction in number of OLs does not necessarily involve a reduction in the amount of myelin, and (2) OLs have considerable flexibility in the amount of myelin they can make.

Developmental expression of glial fibrillary acidic protein and glutamine synthetase mRNAs in normal and jimpy mice

Astrogliosis is a prominent feature in the CNS of the dysmyelinating mutant, jimpy. In the following study the expression of the glial markers, glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS) mRNAs were examined the cerebra of normal and jimpy mice. The relative abundance of GFAP and GS mRNAs increased rapidly in the CNS of normal mice during the first two postnatal weeks. During the third week the content of GFAP and GS mRNA remained constant. The pattern of developmental accumulation of these transcripts in jimpy animals was distinctly different. Levels of GFAP transcripts in 6- and 10-day-old jimpy animals were essentially the same as controls. In 14-day-old animals, however, the content of GFAP mRNA in jimpy had increased dramatically, and was 3-fold greater than that found in normal animals. The levels of GFAP message remained significantly elevated above control values for the life of the animals, approximately 22-24 days. In contrast, no significant difference in GS mRNA content was detected between control and jimpy brain tissue. The results of this study indicated that increased accumulation of GFAP mRNA was significant component of reactive gliosis and that the mechanisms responsible for the induction of GFAP were dissociated from those that regulate GS expression.

Patchy myelination pattern in the jimpy mouse brain: immunohistochemical study

The jimpy (jp) mutation of the mouse leads to a dramatic decrease of myelination in the hemizygous mutant central nervous system (CNS). Several descriptions based on classical histology, immunohistochemistry, and electron microscopy (EM) have demonstrated the scarcity of myelin formation in the different parts of the CNS. The immunohistochemical study presented here showed a very singular patchy pattern of myelin distribution in the different areas of the whole mutant brain. The myelin patches are randomly dispersed without bilateral symmetry, and their density and location vary from one animal to another. No reproducible pattern of myelination could be found among the population observed. This distribution has been compared with observations on young heterozygotes and wild-type homozygotes from the same strain. A similar patchy and random distribution of myelin could be observed in heterozygotes, which present an intermediate level of myelination. This strongly suggests that a migration of precursors or immature oligodendrocytes (ODCs) from the periventricular zone followed by local multiplication of colonies of ODCs before myelination is a general feature in normal as well as pathological conditions.

Astroglial plasticity in hemizygous and heterozygous jimpy mice

Gliosis is a common phenomenon which occurs in many human diseases and in experimentally altered nervous tissue. The factors activating astrocytes to respond are still unclear but recent evidence suggests that diverse substances can provoke a gliotic response. This paper describes the nature of the gliosis in the myelin deficient jimpy and relates these findings to other recent studies of experimentally induced demyelination in which gliosis is a prominent feature of the disorder. In jimpy males, an astroglial hypertrophy which consists of an increase in the number of cell processes can be demonstrated by both electron microscopy and immunocytochemistry using antibodies to glial fibrillary acidic protein. Increased glial fibrillary acidic protein staining in the white matter of jimpy males correlates with the normal time of myelination in different tracts. The immunostaining is not, however, restricted to white matter. Increased staining can be demonstrated in spinal cord grey matter when hardly any myelinated fibers are present, it is especially prominent around blood vessels of both white and grey matter, and is found in the corpus callosum and in the underlying subventricular zone shortly before or at the time myelination begins in this tract. These observations suggest that the hypertrophy is not simply a response by the astrocyte to the absence of myelin sheaths. While an astroglial hypertrophy is dramatic in jimpy males, quantitative counts of astrocytes and electron microscopic autoradiograms do not reveal an increase in the total number of this cell type. These findings suggest that hyperplasia and hypertrophy of astrocytes may be under separate regulatory control with different factors involved in each phenomenon. In the female carriers of the jimpy gene, myelination is temporarily delayed during postnatal development but after several months, the amount of myelin, whether measured morphometrically or biochemically, reaches normal levels. In the white matter of the young female carrier, staining for glial fibrillary acidic protein is increased in terms of the number of processes and the total volume of neuropil but a normal pattern of staining is observed within a year. These and other observations suggest that the glial hypertrophy in the young mosaic is temporary and that regression and reorganization of glial processes takes place as myelination proceeds.

Myelin mosaicism in female heterozygotes of the canine shaking pup and myelin-deficient rat mutants

Female heterozygotes of the shaking pup and myelin-deficient rat sex-linked recessive traits, show myelin mosaicism of the optic nerve and spinal cord. This is most marked in the optic nerve especially in the rat where mosaic patches persist with aging. In both the rat and dog, abnormal oligodendrocytes with distended rough endoplasmic reticulum are found in the abnormal patches and are a marker of the trait. Female heterozygote dogs can develop a marked tremor which disappears with age.

Biochemical correlates of myelination in brain and spinal cord of mice heterozygous for the jimpy gene

Brain and spinal cord of female mice heterozygous for the jimpy gene were analyzed during development for activity of ceramide galactosyl transferase (CGT) and for levels of myelin basic protein (MBP). CGT activity was low at 13-14 days in brains of heterozygous jimpy females but showed normal levels by 31-36 days, in agreement with our earlier study of this enzyme. In cord, CGT activity was normal or slightly above normal at all ages studied, from 13-14 days into adulthood. In both brain and cord, decreased levels of MBP were observed at 13 days; by 100 days, amounts of MBP approached normal levels. Proven female carriers of the jimpy gene also showed normal levels of CGT activity, MBP, and isolated myelin at 200-250 days of age in both brain and cord. These biochemical findings agree with previous morphologic measurements in cord demonstrating deficits in myelin at early ages but compensation by 100 days. Our results show that compensation occurs earlier in cord than in brain and that levels of MBP show a closer correlation than CGT activity with amounts of myelin, as measured by either morphometric analysis or direct isolation.

Development of cerebellar astroglia: transitions in form and cytoskeletal content

The forms, disposition, and cytoskeletal contents of astroglia in immature mouse cerebellum were studied by immunocytochemical staining with antisera against two intermediate filament proteins, vimentin (Vim) (58,000 daltons) and glial filament protein (GF) (51,000 daltons). From embryonic (E) Day 15 to postnatal (P) Day 2, Vim is expressed in cells throughout the cerebellar anlage, including radial glia and Bergmann fibers, cells with amorphous shapes and 2-3 processes, and thick longitudinal elements oriented parallel to axons within axon tracts. GF is not expressed during the first few postnatal days, but by P7, there is a dramatic increase in GF-positive astrocyte-like cells in the putative white matter that are more densely stained and more crowded than at any other age. Between P7 and P14 all astrocytes throughout the cerebellum express both Vim and GF. From P21 on, Vim expression is progressively rarer in all astrocytes except for Bergmann fibers, and GF-positive astrocytes become less numerous. These findings raise two issues: (a) the lineage and relationships of cells expressing Vim and GF; (b) Since GF-positive cells appear as axon ingrowth ceases, axons must grow in a terrain comprised of glial cells that have a different cytoskeletal composition (vimentin), reflecting a less differentiated state, than mature astrocytes or than the GF-rich astrocytes that proliferate after injury in adult CNS.

Biochemical expression of mosaicism in female mice heterozygous for the jimpy gene

Expression of the jimpy gene in heterozygous females was analyzed by measuring galactolipid synthesis in brain during early myelination. Sulfatide labeling in brains of heterozygous females at 13-14 days is decreased to 40-80% that of female littermates who do not carry the jimpy gene. The activities of ceramide galactosyl transferase and cerebroside sulfotransferase and levels of myelin basic protein were similarly depressed. Since the jimpy gene was maintained with the Tabby gene in these studies, the effect of the Tabby gene on these parameters was examined and found to have no effect. These biochemical findings indicate that myelination is retarded in the brains of heterozygous jimpy females during the second week of development. However, at 18 days and thereafter, sulfatide labeling is less reduced, suggesting that the oligodendrocytes in brain attempt to compensate, in agreement with morphologic studies which show that myelin is decreased in brain during early development, but appears normal in adult animals.