MeCP2 deficiency is associated with reduced levels of tubulin acetylation and can be restored using HDAC6 inhibitors.

UNLABELLED: Rett syndrome (RTT) is a severe neurodevelopmental disorder, predominantly caused by loss of function mutations in the X-linked methyl-CpG-binding protein 2 (MECP2) gene. Despite the genetic cause being known in the majority of cases, the pathophysiology of the neurological phenotype of RTT is largely unknown. Tubulin and the microtubule network play an essential role in neuronal function whereby the acetylation state of microtubules dictates the efficiency of neuronal migration and differentiation, synaptic targeting and molecular motor trafficking of mRNA, high-energy mitochondria and brain-derived neurotrophic factor (BDNF)-containing vesicles. Recent reports have shown perturbations in tubulin and microtubule dynamics in MeCP2-deficient cells, suggesting a link between the aberrations of these cellular entities and the neurobiology of RTT. We have interrogated the functional state of the microtubule network in fibroblasts derived from two patients with RTT as well as cortical neurons from a RTT mouse model and observed a reduction in acetylated α-tubulin and an increase in the tubulin-specific deacetylase, histone deacetylase 6 (HDAC6). Furthermore, we show that inhibition of HDAC6 by Tubastatin A can restore tubulin acetylation levels. We also demonstrate microtubule instability in the RTT patient fibroblasts in response to nocodazole, which is progressively ameliorated in a mutation-dependent manner by Tubastatin A. We conclude that Tubastatin A is capable of counteracting the microtubule defects observed in MeCP2-deficient cells, which could in turn lead to the restoration of molecular trafficking along the microtubules and thus could be a potentially new therapeutic option for RTT. KEY MESSAGE: Cells from MeCP2-deficient cells show reduced levels of acetylated α-tubulin. Cells from two patients and a RTT mouse model have increased levels of HDAC6 but not sirtuin 2 (SIRT2). Inhibition of HDAC6 by Tubastatin A increases the in vitro acetylation of α-tubulin. Inhibition of HDAC6 by Tubastatin A does not increase MECP2 expression. Cells from two patients show microtubule instability, which is ameliorated by Tubastatin A.

Mitochondrial dysfunction in the skeletal muscle of a mouse model of Rett syndrome (RTT): implications for the disease phenotype.

Rett syndrome (RTT) is a severe neurodevelopmental disorder, predominantly caused by mutations in the X-linked Methyl-CpG-binding protein 2 (MECP2) gene. Patients present with numerous functional deficits including intellectual disability and abnormalities of movement. Clinical and biochemical features may overlap with those seen in patients with primary mitochondrial respiratory chain disorders. In the late stages of the disorder, patients suffer from motor deterioration and usually require assisted mobility. Using a mouse model of RTT (Mecp2(tm1Tam)), we studied the mitochondrial function in the hind-limb skeletal muscle of these mice. We identified a reduction in cytochrome c oxidase subunit I (MTCO1) at both the transcript and protein level, in accordance with our previous findings in RTT patient brain studies. Mitochondrial respiratory chain (MRC) enzyme activity of complexes II+III (COII+III) and complex IV (COIV), and glutathione (GSH) levels were significantly reduced in symptomatic mice, but not in the pre-symptomatic mice. Our findings suggest that mitochondrial abnormalities in the skeletal muscle may contribute to the progressive deterioration in mobility in RTT through the accumulation of free radicals, as evidenced by the decrease in reduced glutathione (GSH). We hypothesise that a diminution in GSH leads to an accumulation of free radicals and an increase in oxidative stress. This may impact on respiratory chain function and contribute in part to the progressive neurological and motor deterioration seen in the Mecp2-mutant mouse. Treatment strategies aimed at restoring cellular GSH levels may prove to be a novel target area to consider in future approaches to RTT therapies.