Morphological and functional alterations in the substantia nigra pars compacta of the Mecp2-null mouse.

Rett syndrome (RTT) is a severe neurological disorder caused by mutations in the MECP2 gene, in which older patients often develop parkinsonian features. Although Mecp2 has been shown to modulate the catecholaminergic metabolism of the RTT mouse model, little is known about the central dopaminergic neurons. Here we found that the progression of the motor dysfunction in the Mecp2-deficient mouse becomes more severe between 4 and 9 weeks of age. We then studied the phenotype of the dopaminergic neurons of the substantia nigra pars compacta (SNpc). We found a major reduction in the number of tyrosine hydroxylase (Th)-expressing neurons, as well as a reduction in their soma size, by 5 weeks of age. We showed that this deficit is not due to apoptosis and that the remaining neurons express a mature dopaminergic phenotype. A reduction in the Th-staining intensity was also found in the caudate-putamen (CPu), the main dopaminergic target for SNpc. We found that the amount of activated-Th (pSer40-Th) is slightly reduced at 5 weeks of age in the Mecp2-deficient mouse, but that this amount is affected more importantly by 9 weeks of age. Neurochemical measurements revealed a significant reduction of dopamine content at 5 and 9 weeks of age in the CPu whereas SNpc contents were preserved. Finally, we found that chronic L-Dopa treatment improved the motor deficits previously identified. Altogether, our findings demonstrate that Mecp2-deficiency induces nigrostriatal deficits, and they offer a new perspective to better understand the origin of motor dysfunction in RTT.

Progressive motor and respiratory metabolism deficits in post-weaning Mecp2-null male mice.

The methyl-CpG binding protein 2 (Mecp2) gene encodes a nuclear transcriptional modulator highly expressed in post-mitotic neurons. Mutations of this gene cause a large spectrum of neurological disorders in humans. Several lines of mice harboring a constitutional deletion of Mecp2 are available. The use of these models is crucial to understand the basis of Mecp2-related pathologies. However, most of the studies performed using these lines focused on different postnatal time points. The aim of the present study was to provide a more complete description of the behavioral phenotype of the Mecp2(tm1.1Bird) mice. To this aim, we used a modified version of the SHIRPA protocol and a set of sensorimotor tests and respiratory metabolism measurements, in a longitudinal study of the Mecp2-null male mice (Mecp2(-/y)) from three weeks (weaning) to eight weeks of age. Our data document, for the first time, the sequential appearance of the in vivo deficits in this mouse line. The observed deficits initially concern major parameters (such as body weight), and are followed by involuntary and sensitive defects (reflexes). Subsequently, motor functions and respiratory metabolism are severally impaired. A detailed description of these gradual defects may help to identify their neuronal origin and to elaborate novel therapeutic strategies.

Detection of social approach in inbred mice.

An experiment was designed to automatically assess the relative level of social interaction during encounters involving trios of inbred mice consisting of two familiar cage mate males plus an unfamiliar third male. The automation of the spatial positioning was obtained by using a video-tracking program. In addition social behaviours were manually scored. To evaluate the influence of basic motor properties on the evaluation of the level of social interaction, we analysed two strains (C57BL/6J and 129S2/Sv) that are frequently employed in transgenic research, and show very different levels of motor activity. Correlations between manual and automated parameters showed that spatial parameters correctly fitted the level of social interaction between mice. In both strains C57BL/6J and 129S2/Sv, a proximity parameter (duration of bouts during which two individuals were close to each other) defined the social approach and correctly assessed the discrimination of social novelty.

Impairment of novelty detection in mice targeted for the Chl1 gene.

A deficit in cell adhesion molecules including the human Chl1 (close homologue of the L1 cell adhesion molecule) gene may cause impairment of cognitive processes. Aberrant connectivity in the CA3 region of the hippocampus has been reported in mice lacking the CHL1 protein after Chl1 gene targeting. Previous studies have observed a deficit in the processing of novel information by CHL1-deficient mice. We investigated deficits in spatial discrimination and object discrimination in three groups of mice--Chl1(+/+), Chl1(+/-) and Chl1(-/-)--performing spatial and object novelty tasks. The results indicated that wild-type mice easily recognized objects that were either "displaced" or "substituted". Chl1(-/-) mice showed severe impairment of the capacity to react to both spatial and non-spatial novelty. Chl1(+/-) mice were severely restricted in their ability to detect spatial changes, but succeeded in novel object discrimination. A dose-dependent sensitivity of the organization of the CA3 layer to the CHL1 protein may explain this result. However, the observations suggest that a dysfunction of parts of the brain other than the hippocampus may be involved in the impairment.

Attack behaviors in mice: from factorial structure to quantitative trait loci mapping.

The emergence or non-emergence of attack behavior results from interaction between the genotype and the conditions under which the mice are tested. Inbred mice of the same strain reared or housed under conditions do not react the same way; reactions also vary according to the place selected for testing and the different opponents. A factor analysis showed that the attack behavior in non-isolated males, tested in neutral area covaried with high testosterone and steroid sulfatase and low brain 5-hydroxytriptamine (5-HT), beta-endorphin and Adrenocorticotropic Hormone (ACTH) concentration, whereas, for isolated males tested in their own housing cage, it covaried with high testosterone activity and low brain 5-HT concentration. A wide genome scan was performed with two independent populations derived from C57BL/6J and NZB/BlNJ, each being reared, housed and tested under highly contrasting conditions, as described above, and confronted with A/J standard males. Common Quantitative Trait Loci emerged for two rearing/testing conditions. For rattling latency we detected Quantitative Trait Loci on Mus musculus chromosome 8 (MMU8) (at 44, LOD score=3.51 and 47 cM, LOD score=6.22, for the first and the second conditions) and on MMU12 (at 39 cM, LOD score=3.69 and at 41 cM, LOD score=2.99, respectively). For the number of attacks, Quantitative Trait Loci were common: on MMU11 at 39 cM LOD score=4.51 and 45 cM, LOD score=3.05, respectively, and on MMU12 (17 cM, LOD score=2.71 and 24 cM, LOD score=3.10). The steroid sulfatase gene (Sts), located on the X-Y pairing region, was linked, but only in non-isolated males, tested in neutral area for rattling latency, first attack latency, and number of attacks (LOD scores=4.9, 4.79 and 3.57, respectively). We found also that the Quantitative Trait Locus encompassing Sts region interacted with other Quantitative Trait Loci. These results indicate that attack behavior measured in different rearing and testing conditions have different biological and genetic correlates. This suggests that further explorations should be done with standardized tests and, in addition, with a wide range of tests, so as to gain an understanding of the true impact of genes or pharmacological treatments on specific categories of aggressive behavior.