Gender effect on neurodegeneration and myelin markers in an animal model for multiple sclerosis
© Massella et al; licensee BioMed Central Ltd. 2012
Received: 25 August 2011
Accepted: 24 January 2012
Published: 24 January 2012
Multiple sclerosis (MS) varies considerably in its incidence and progression in females and males. In spite of clinical evidence, relatively few studies have explored molecular mechanisms possibly involved in gender-related differences. The present study describes possible cellular- and molecular-involved markers which are differentially regulated in male and female rats and result in gender-dependent EAE evolution and progression. Attention was focused on markers of myelination (MBP and PDGFαR) and neuronal distress and/or damage (GABA synthesis enzymes, GAD65 and GAD67, NGF, BDNF and related receptors), in two CNS areas, i.e. spinal cord and cerebellum, which are respectively severely and mildly affected by inflammation and demyelination. Tissues were sampled during acute, relapse/remission and chronic phases and results were analysed by two-way ANOVA.
1. A strong gender-dependent difference in myelin (MBP) and myelin precursor (PDGFαR) marker mRNA expression levels is observed in control animals in the spinal cord, but not in the cerebellum. This is the only gender-dependent difference in the expression level of the indicated markers in healthy animals; 2. both PDGFαR and MBP mRNAs in the spinal cord and MBP in the cerebellum are down-regulated during EAE in gender-dependent manner; 3. in the cerebellum, the expression profile of neuron-associated markers (GAD65, GAD67) is characterized by a substantial down-regulation during the inflammatory phase of the disease, which does not differ between male and female rats (two-way ANOVA); 4. there is an up-regulation of NGF, trkA and p75 mRNA expression in the early phases of the disease (14 and 21 days post-immunization), which is not different between male and female.
It is reported herein that the regulation of markers involved in demyelination and neuroprotection processes occurring during EAE, a well-established MS animal model, is gender- and time-dependent. These findings might contribute to gender- and phase disease-based therapy strategies.
Keywordsexperimental allergic encephalomyelitis gender-related rat spinal cord cerebellum neurotrophins and related receptors
Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS), which can progress over decades. The progressive failure of remyelination leads to the cumulative loss of axons, grey matter atrophy and prevalent neurodegeneration responsible for chronic disability and cognitive decline . There is a considerable difference in the way MS affects females and males, as has been highlighted by epidemiology studies and MRI analyses [2–4]. The way of gender-influence in MS is complex and still obscure. From a pathogenic point of view, females tend to have stronger Th1-mediated immune responses and are more prone to develop autoimmune diseases, including MS . However, gender might influence white matter establishment and maintenance of the mature structure of white tracts, thus affecting their repair capability. A review of in vivo imaging studies suggests that gender-related differences in white matter in the human brain exist in healthy subjects  and affect in particular age-related changes in precentral, cingulate, and anterior temporal white matter areas . As well as in the normal brain, white matter pathology seems also to differ in males and females in several neurological and psychiatric conditions. For example, in schizophrenia there is a subtle and gender-dependent alteration in the forebrain commissures, and the neurotoxic effect of metamphetamine on frontal white matter seems to be less prominent in women than in men . Sex differences in lesion size, neuronal cell loss, and mortality rates have been observed after ischemia and trauma . Moreover, a gender effect in the progression of several neurodegenerative diseases has been noted. Epidemiological data point to women's proneness to Alzheimer's disease , and indicate that after traumatic brain injury (TBI) women show better recovery than men .
Apart from this clinical evidence, on which several gonadal steroids-oriented clinical trials have been based , relatively few studies have explored possible molecular mechanisms involved in gender-related differences. In this context, there has been much speculation concerning the gonadal hormone role in immune function and cytokines production during inflammation, myelination, and neurodegeneration/neuroprotection [4, 5, 12], but direct evidence regarding gender-related differences in these and other critical molecular and cellular hallmarks of the diseases is still very scanty.
We are exploring possible molecular determinants for gender-dependent differences in inflammatory demyelinating diseases using experimental allergic encephalomyelitis (EAE) in rats as a disease model for MS. EAE induced in Dark Agouti rats is Th1 mediated, having a relapsing-remitting course, and comprises also persistent demyelination, remyelination, neuronal distress and cognitive defects [13–18]. In this model, we have demonstrated by liquid chromatography-tandem mass spectrometry that the levels of neuroactive steroids display sex, regional and temporal differences in both control and EAE, and these changes did not correlate to the plasma levels of gonadal hormones [19, 20].
In this paper data are presented regarding molecular markers for oligodendrocyte precursor cells, myelin proteins, neurotransmitters and neurotrophins in the course of EAE. Attention was focused on the spinal cord as a white matter-rich area in which most of the tissue is occupied by heavily myelinated tracts that is severely affected by inflammation and demyelination, and on the cerebellum as a grey matter-rich area since ataxia is a common symptom in EAE rats, and a cerebellar cortical atrophy has been described late in the disease in spite of poor inflammation and demyelination .
Animals and disease progression
At this stage, and as already reported in previously published paper  very few cellular infiltrates were found in cerebellum and cerebral cortex (not shown).
GAD, neurotrophins and related receptors
Gender bias in autoimmune diseases is a well-known and hitherto unexplained fact. In particular, MS is more prevalent in females than in males, and this female predominance increases as time goes by. Gender appears to play a critical role also in the progression of MS, suggesting that not only immune reaction, but also remyelination, axonal pathology and neural damage might be gender-dependent . In all cases, the histopathological and molecular mechanisms underlying the inherent differences in male and female MS are still obscure .
In the present study molecular markers for myelin and grey matter that are differentially regulated in male and female rats in the experimental model of the disease have been described. Two areas were investigated: the spinal cord, which is the area with extensive inflammation and demyelination, and the cerebellum, in view of the ataxia symptom in EAE, where inflammatory cellular infiltrates are scare in this disease model . While the clinical profile of the disease differs between sexes being more severe in female than male rats, the inflammatory cellular infiltration in the spinal cord is lower in females, but produces a stronger astroglial reaction than in males. Moreover, disease-induced alteration of several markers is different between the two sexes. In particular, a strong gender-dependent difference in MBP and PDGFaR mRNA expression level in the spinal cord of healthy animals was found, which correspond to a different regulation during the disease.
EAE in male and female Dark Agouti rats
Here we confirm that both male and female Dark Agouti rats are highly susceptible to EAE . While the clinical score profile in the acute phase is similar in males and females, male rats show a more pronounced recovery than female rats. In this EAE model, a massive infiltration by inflammatory cells, and a massive demyelination were observed in the spinal cord, whereas small and localized lesions were spread over the main white tracts, including the cerebellar peduncoli, corpus callosum and optic nerve [15, 16, 26]. The remyelination process starts quite early also in Dark Agouti rats, but it is not yet complete at 40 DPI . In spite of the fact that severity of inflammatory infiltrate in the spinal cord is lower in female compared to male rats, astroglial reaction is more pronounced in female than in male rats. This could be related to humoral rather cellular immune reaction. Overall, the females of all the species used for MS models display stronger immune responses than males [26–28]. This is attributed to cytokines production [29, 30]. For example, cytokine IL-13 is implicated in gender differences in EAE severity in C57BL/6 mice, where the absence of the anti-inflammatory IL-13 entails lower susceptibility to EAE in females vs males or WT females with normal levels of IL-13 .
Gender-dependent differences in white matter during health and inflammatory-demyelinating disease
When comparing the expression level of the genes included in the study in male vs female healthy rats, the most significant result was the 5-fold higher MBP and the 2.5-fold higher PDGFαR expression in males than females in the spinal cord, but not in the cerebellum. This difference is not present at protein level, and, more generally, no significant differences in myelin sheaths in the spinal cord were found between male and female rats. This result might suggest that MBP synthesis could be regulated in a different way at post-transcriptional and translational levels in males and females. In fact, mRNA levels and protein levels do not always correlate [32, 33] and this might be due to the post-transcriptional mechanisms playing and/or to the different in vivo half life of proteins. Gonadal steroid and neurosteroid may elicit effects through non-genomic mechanisms via ERs localized on the plasma membrane, and ligand-independent pathways to activate ERs have been also described . Moreover, the mechanisms controlling the rates of degradation/synthesis for a given mRNA and protein are not homogeneous, even within proteins that have similar functions . Technical limits of the methods for quantifying mRNA and proteins should also be taken into account.
More generally, the issue of gender-related difference in the anatomy of white tracts is complex and controversial. The gender-dependent difference in PDGFαR and MBP mRNA in the spinal cord correlates with data from Cerghet et al. [35, 36] in mice. They found that the density of oligodendrocytes and the content of several myelin proteins in white tracts is higher in males than in females, whereas the lifespan of oligodendrocytes is shorter in females than in males, thus suggesting that myelin turnover is greater in females than in males. It may thus be argued that males have a greater functional reserve than females, whereas females have a greater vulnerability related to higher myelin protein turnover.
We and others already described the variation of MBP protein content, such as different markers for OPCs in male and female rats during EAE [13, 15, 16, 37]. In this study we report that there is a gender-dependent difference in the regulation of PDGFαR and MBP during the experimental disease, involving both genes in the spinal cord, and MBP, only in the cerebellum. This correlates with other reports describing differences in remyelination in old male and female rats  and in cuprizone intoxication model (reversible inflammatory demyelination) .
Neurotrophins and neuronal markers in inflammatory-demyelinating diseases
The EAE model in rodents (and primates) allows neuronal distress/damage to be investigated. We already described how the expression level of the acetylcholine synthesis enzyme choline acetyltransferase mRNA level was transiently reduced in motor neurons in the spinal cord  and in cholinergic neuron of the basal forebrain  during EAE. Here investigation was focused on cerebellum, which is involved in motor symptoms in EAE. In spite of scant inflammation and demyelination, a grey matter atrophy  and a Purkinje cell loss  has been described in the cerebellum. As index of neuronal injury, we investigated the expression level of the mRNAs encoding for the GABA synthesis enzyme γ-aminobutyric acid decarboxylase (GAD) . A transient down-regulation of both GAD65 and GAD67 was observed in male and female EAE rats, thus confirming that the acute phase of the disease is associated with reversible neuron distress.
Due to the neurotrophins role as endogenous neuroprotectors, their expression levels were investigated during EAE, focusing on NGF and BDNF. Previous results from our laboratory have described higher levels of NGF in certain brain areas, like the thalamus and cerebral cortex, but not the spinal cord, in EAE compared to healthy rats, associated to a strong up-regulation of p75- and trkA-like immunoreactivity . This up-regulation diminishes over time and a drop in NGF mRNA expression level was reported in the cerebral cortex at 104 DPI . Here we confirm the up-regulation of NGF and its high-affinity receptor trkA mRNA in the cerebellum in both male and female in the early, inflammatory phase of EAE, while BDNF is down-regulated at the same times. NGF, but also BDNF, modulate inflammation and immune cell function in many diseases [42–45]. Both the high and low affinity NGF receptors are widely expressed in the immune system, thus indicating a potential for responding to this neurotrophin through an autocrine mechanism [43, 45]. NGF increase during EAE may possibly also result in increased neuroprotection , working with the marmoset model, showed that NGF icv administration delayed the onset of clinical EAE, and also prevented the full development of EAE lesions. NGF administration also influences EAE development and progression in rats  by reducing the severity of the disease compared to that in saline treated EAE mice.
The low-affinity receptor p75 is also up-regulated. Notably, the up-regulation of p75 in Purkinje neurons has been described in EAE . The p75 up-regulation in EAE could be related to the Purkinje neurons death and cerebellar atrophy, since p75 can induce autophagy and death in these cells .
Since the original report describing the different content of NGF in the submaxillary gland in male and female mice , many other reports have illustrated sexually dimorphic distribution of NGF in tissues and plasma of different animal species [51–53]. The cerebellum and particularly the Purkinje cells have been recognized as a major source for neurosteroid production , and we reported that neurosteroids are differentially regulated in the cerebellum of control male and female rats, such as during EAE . Our mRNA data indicate that there are no sex differences in the NGF and BDNF levels in cerebellum either in healthy and EAE rats, thus confirming previous reports suggesting that neurotrophin levels do not correlate with estrogen levels in females or with estrogen or testosterone levels in males at this age [55, 56].
Differences in vulnerability and disease evolution have already given rise to clinical trials for MS based on gonadal steroids. With regard to animal models of inflammatory-demyelinating diseases, it has recently been reported that the combined administration of 17 beta-estradiol and progesterone prevents cuprizone-provoked demyelination of the corpus callosum in male mice . A similar effect of combined treatment was also found in MOG (40-45)-induced EAE mice . However, rational future prospects regarding the use of sex steroids as adjuvant therapy in MS should be based on the identification of the pathological process, cell type, and molecular pathways positively affected by steroids with regard to disease onset and progression. This study provides a description of the gender-dependent and disease-dependent regulation of markers for these different processes in a well-established animal model, suggesting that demyelination/remyelination might be a target for gender-dependent therapies.
Animals, EAE induction, groups size
Dark-Agouti (Harlan, Italy) male and female rats, 150-175 g body weight were used in this study. In both female and male rats, a group was sensitized with a medium containing 0.15 g/ml guinea pig spinal cord tissue in complete Freund's adjuvant (CFA, Sigma), 50% v/v, to which 5 mg/ml of heat-inactivated Mycobacterium (Difco H37Ra) was added. Un-injected rats were used as controls. Rats were regularly weighed and examined for clinical signs of EAE by a trained observer according to a semiquantitative score: 1 = loss of tail tone, 2 = mono or bilateral weakness of hind legs or middle ataxia, 3 = ataxia or paralysis, 4 = severe hind legs paralysis, 5 = severe hind leg paralysis and urinary incontinence. To determine the different phases of the estrous cycle, female rats were monitored by daily vaginal smears and only those demonstrating at least two consecutive 4-day cycles were used in the study and killed on the day of proestrus. Tissues were collected in two independent experiments, the first for molecular biology studies, the second for morphology and western blotting. Sixty-four animals (32 females and 32 males) were included in the first experiment; eight animals were included in each time-point groups (sacrificed 14, 21, 40 DPI) for both male and female, and for un-injected animals. Twenty-four animals (12 females and 12 males) were include in the second experiment, were tissue was collected at 14 DPI. All animal protocols described herein were carried out according to the European Community Council Directives (86/609/EEC) and approved by the intramural ethical committee for animal experimentation of Bologna University and Ministry of Health, comply with the guidelines published in the NIH Guide for the Care and Use of Laboratory Animals.
Histology, histochemistry and immunohistochemistry
Six animals/group were included in this part of the study. EAE animals were killed at 14 DPI, i.e. during the acute phase of the disease. The lumbar tract of the spinal cord was rapidly dissected out, briefly washed in PBS and then fixed using a paraformaldehyde 4% + saturated picric acid 14% solution in PBS 0.2 M, pH 6.9 for 12 hours (h). Cryostat sections (14 μm) were then collected from the spinal cord. The extent of inflammation, demyelination and astroglial rection was evaluated on tissue sections stained with hematoxylin and eosin to visualize infiltrate cells, with FluoroMyelin™ Fluorescence Myelin Staining (Molecular Probes, Eugene, OR) for the myelin sheaths. Indirect immunofluorescence (IF) procedures were used to visualize the anti-fibrillary acid protein (GFAP, Chemicon International Inc. Temecula, CA, USA). Briefly, sections were first incubated in 0.1 M phosphate buffered saline (PBS) at room temperature for 10-30 min, followed by incubation at 4°C for 24 h in a humid atmosphere with the primary antibodies diluted in PBS containing 0.3% Triton X-100, v/v. After rinsing in PBS for 20 min (2 × 10 min), sections were incubated at 37°C for 30 min in a humid atmosphere with the secondary antisera conjugated with different fluorochromes diluted in PBS/Triton X-100 0.3%. Sections were then rinsed in PBS (as above) and mounted in glycerol containing 1,4-phenylendiamine 0.1 g/l (Sigma). Images were taken by Olympus AX70-PROVIS microscope equipped with motorized z-stage control and F-VIEW II CCD Camera. The inflammatory infiltration was evaluated by two independent operators in blind on 5 replicate sections per animal, by counting the number and severity of cellular infiltrates over each, entire coronal section. Cellular infiltrates were scored as follows: 0, none; 1, a few inflammatory cells; 2, organization of perivascular infiltrates; 3, increasing severity of perivascular cuffing with extension into the adjacent tissue [59–61]. The inflammation score, expressed as score/section, derives from the sum of infiltration score in each cellular infiltrate. To minimize the bias due to random distribution of the demyelinating lesions, the FluoroMyelin and GFAP-staining were both measured in the fasciculus gracilis, and calculated as percentage positive areas using the Image ProPlus software (Imaging Research Inc, St. Catharines, Ont., Canada). Myelin sheath thickness was measured on confocal images (Olympus FluoView 500) using Image ProPlus software (MediaCybernetics, Bethesda, MD). The G-ratio (ratio of axon diameter to total fibre diameter) was calculated on confocal images by dividing the circumference of an axon without myelin by the circumference of the same axon including myelin. At least 250 fibers/group were included in the analysis.
RNA isolation, reverse transcription and Semiquantitative real-time PCR
Nucleotide sequences of primers used for gene expression study by real-time semiquantitative PCR assays.
The efficiency of each pair of used primers was calculated by amplifying cDNA serial dilutions in the conditions above described obtaining efficiency values in the range of 95-102%. The 2^(-ΔΔCT) method was used for the calculation of gene expression relative to a given reference group (CT, threshold cycle). When comparing gene expression between sexes,ΔΔCT was calculated as follow: ΔΔCT = (CT target gene- CT GAPDH) female - (CT target gene- CT GAPDH) male. When instead comparing gene expression between healthy (Control) and EAE animals at different phases of the disease, the ΔΔCT formula was: ΔΔCT = (CT target gene- CT GAPDH) Control - (CT target gene- CT GAPDH) EAE acute (14 DPI), ΔΔCT = (CT target gene- CT GAPDH) Control - (CT target gene- CT GAPDH) EAE remission (21 DPI), ΔΔCT = (CT target gene- CT GAPDH) Control - (CT target gene- CT GAPDH) EAE chronic (40 DPI).
The specificity of real-time PCR reactions was evidenced by the melting curve of the amplified products, obtaining a unique peak at the correspondent melting temperature (Tm) (Figure 3E). No CT was obtained after real-time PCR of no RT sample for any of the pair of primers used. Template controls gave always no CT as well. Random amplified products were resolved in a 2.5% agarose gel and TAE electrophoresis buffer, obtaining a unique band of the expected size. DNA marker of 100 bp ladder (Fermentas) was used.
Western blotting procedure
Tissue homogenates were prepared using a lysing buffer consisting on 10 mM Hepes, 1 mM DTT, pH 7.5 and protease inhibitor cocktail (Sigma). Equal amounts of protein, concentrations determined by Lowry method with the Protein assay kit (Bio-Rad, Hercules, California, USA), were separated in 15% SDS-polyacrylamide gels and electroblotted to nitrocellulose membranes. In order to block nonspecific protein binding sites, filters were incubated with blocking solution (Pierce, Rockford, IL, USA) for 2 h at room temperature and primary antibodies were then incubated overnight at 4°C. After washing for 1 hour with TTBS (TBS-0.05% Tween-20), filters were incubated with secondary antibodies for 30 min at room temperature and washed again for another hour. Rabbit polyclonal anti-MBP (DAKO), dilution 1:2000 and GAPDH (glyceraldehyde 3-phosphate dehydrogenase) (Millipore, Milan, Italy), dilution 1:150 were used as primary antibodies while anti-rabbit (1:30,000) and anti-mouse (1:2000), respectively, antiserum conjugated to horseradish peroxidase (DAKO) were used as secondary antibody. Finally, proteins were detected using an ECL chemioluminescent kit (Pierce) and exposition to radiographic film. Densitometric analysis was performed using the AIS Imaging System software (Ontario, Canada) and the data obtained statistically analysed and represented using PrismGraph software (GraphPad Software, San Diego, CA, USA). Seven animals/group were used to perform these studies.
Data presentation and statistical analysis
For mRNAs expression data, we first analyzed the difference between female and male control animals (data normalized vs female, because the disease is usually induced in female rats.). Student's t test was used for the statistical analysis. Since the aim of the study was to investigate the disease effect at different time point in male and female rats, we then analyzed the disease and gender influence by two-way ANOVA and Bonferroni post-test. Results for two-way ANOVA are reported in the figures and figure legends. Morphological data were analyzed by Student's t test, and clinical data by two-way ANOVA. Results were considered significant when the probability of their occurrence by chance alone was less than 5%. GraphPad Software (San Diego, CA, USA) was used for statistics and graph preparation.
Brain-derived neurotrophic factor
complete Freund's adjuvant
Central nervous system
experimental allergic encephalomyelitis
γ-aminobutyric acid decarboxylase
glyceraldehyde 3-phosphate dehydrogenase
anti-fibrillary acid protein
myelin basic protein
myelin oligodendrocyte glycoprotein
Nerve growth factor
oligodendrocyte precursor cells
platelet-derived growth factor.
This work was supported by AISM-FISM (LG), Health Sciences and Technology - Interdepartmental Center for Industrial Research (HST-ICIR), University of Bologna and Assessorato alle attività Produttive (LC); Fondazione IRET, Ozzano Emilia (LG). The financial support of Fondazione Italiana Sclerosi Multipla (2010/23) to RC Melcangi is gratefully acknowledged.
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