Noradrenergic deficits and respiratory dysfunction in the "Rett" mouse
The“Rett” mice have major respiratory problems.
We have studied the maturation of the respiratory function in the Mecp2 deficient mice. We have shown that the respiratory rhythm of the mice was normal until 30 days after birth. Subsequently, the respiratory rhythm becomes irregular and the animals show severe apneas a few days before their death. Light anesthesia allows to suppress these respiratory problems. Based on what is known in humans, we think that the respiratory rhythm generator is able to produce a normal rhythm but that its regulatory inputs are abnormal. This point is very important if we plan to design a pharmaceutical intervention to correct the respiratory deficits.
Recordings of the respiratory rhythm of the Mecp2-deficient mice between 30 days (P30) and 60 days (P60) of age. The frequency of duration of apneas increase with age. Several mice presented apneas lasting for 10 seconds, which is excessively long for an animal whose breathing frequency is 160 movements per minute.
The respiratory dysfunction is associated with cellular deficits in the brainstem of the mutant animals.
In a second step, we investigated whether this breathing disturbances could be due to neurochemical defects. Using high performance liquid chromatography, we measured the content of various metabolites in different regions of the brain of knock-out animals. Unexpectedly, we observed an important deficit in the content of noradrenaline (30%) in the brainstem of the mutant mice, before the first clinical manifestations. A specific labelling (see below) showed that two major neuronal groups (A1C1 and A2C2) are abnormal in the brainstem of these animals. These cell groups are involved in the modulation of the respiratory rhythm. This is the first demonstration of a major cellular deficit in this model (40% less neurones in the deficient animals in comparison to the controls). We have also shown that this cellular deficit was specific to these cell groups and is not a generalized neuronal loss. Indeed, we observed no defect in the other neuronal populations that we studied in the same region of the brain (serotoninergic neurones or neurons from the X and XII motor nuclei).
Immunodetection of tyrosine hydroxylase allowing the detection of noradrenergic neurones in the brainstem of the Mecp2 deficient mice. Arrows indicate the location of the A2C2 neurones. We have showed that the mutant mice have a much smaller number of labelled neurones in this structure (and in the A1C1 nuclei).
Our results indicate that Mecp2 deficiency causes alterations of the bioaminergic systems in the mouse. These alterations, in turn, could explain the respiratory rhythm disturbances. The important cellular deficit that we have identified could also potentially explain other defects of the autonomic functions. This work provides very interesting perspectives to further understand the phenotypic consequences of Mecp2 deficit. Many questions are left for investigation following these results. We need to know of the cellular defects evidences in the brainstem of the Mecp2 deficient mice are a cause or a consequence of respiratory dysfunction. To answer these questions, we need to study young animals at a time when the respiration is still normal. Based on the putative function of the Mecp2 protein in the regulation of gene expression, we have also started the transcriptional analysis of cathecholaminergic nuclei of these animals at different developmental ages.