Early Treatment Reverses Cell Defects, Improves Survival in Mice

Early administration of a compound intended to enhance signals between brain cells lessened disease severity, and improved overall health and survival in a mouse model of Rett syndrome.

The improvements, which also included benefits in nerve cell activity, indicate that treatments enhancing neuronal activity early in brain development may have therapeutic potential, scientists said.

The study, “The enhancement of activity rescues the establishment of Mecp2 null neuronal phenotypes,” was published in the journal EMBO Molecular Medicine.  

Rett syndrome results mainly from mutations in the MECP2 gene, leading to developmental delays as well as cognitive and motor disturbances. During brain development, neural precursor cells (NPCs) develop into mature neurons in a highly organized process regulated by many different genes, including MECP2.

Loss of working MECP2 disrupts this process, resulting in functional and structural defects in mature neurons that contribute to Rett syndrome. Although Rett symptoms typically emerge from 6 to 18 months of age, developmental abnormalities in the brain are detected earlier. 

Researchers in Italy sought to investigate if reversing early alterations in neuronal development might reduce Rett symptoms. To do this, they used the ampakine CX-546 to enhance neuronal activity in NPCs and mature neurons grown in cell culture, as well as a mouse model of Rett. 

The scientists grew mature neurons from NPCs collected from the cerebral cortex of the brain, which is affected by Rett syndrome. MECP2 deficiency in these cells produces well-documented defects.

Compared to wild-type (control) cells, NPCs lacking MECP2 showed delayed maturation, diminished expression (activity) of genes involved in responsiveness to external stimuli, and structural defects such as reduced cell body size and growth of dendrites — the short projections that receive signals from other cells. 

CX-546 treatment reduced the structural abnormalities in MECP2-deficient neurons, including increased cell body size and dendrite growth. Notably, the rescue effect was stronger when CX-546 was administered earlier in nerve cell development.

Also, CX-546 boosted gene activity in MECP2-deficient neurons, with earlier treatment again having a more pronounced effect than later treatment. 

Treatment with CX-546 earlier in NPC development further increased response to external stimuli, bringing the MECP2-deficient neuron responsiveness to within the range of controls. By contrast, later treatment did not rescue the functional defects, again suggesting that early treatment is more effective.

In MECP2-deficient neurons, early CX-546 administration also induced the activity of several genes related to normal neuron function, which was reduced in untreated cells lacking MECP2. Among the increased genes were Kcc2, whose reduction has been linked to Rett developmental defects, and Grin1, whose changes were identified recently as the genetic cause of Rett in a young girl.

Significantly, CX-546 treatment reduced the activity of REST, which provides instructions for making a protein that cooperates with MECP2. Together, these results suggest that early CX-546 treatment enhances neuronal activity and rescues the structural and functional changes that result from MECP2 deficiency.

In a mouse model of Rett, daily injections of CX-546 early after birth significantly increased survival, so much so that 90% of treated Rett mice survived beyond 88 days with an average survival of 110 days. 

Treated animals showed a significant improvement in overall health, including a decrease in disease severity, with symptoms in 40% of CX-546-treated mice being classified in the highest severity score compared to 100% of untreated mice at 80 days of age. 

CX-546 also led to a significant reduction in hind limb clasping, a hallmark of Rett syndrome in mice, as well as improved coordination and ability to distinguish between different objects. 

Interestingly, CX-546 treatment also restored gene activity in Kcc2 and BDNF, which is regulated by MECP2 and is key to brain development.

“Our data show that Ampakine administration during early development produces beneficial effects that persist long after the end of the treatment,” the researchers wrote.

“In the future we will apply longitudinal studies and test different time windows of treatment to gain preclinical insights … Eventually, we will test the benefits of repeating the treatment in order to ensure the preservation, rather than the establishment alone, of functional neuronal networks,” they added.

The team further said that the observed alterations very early in development support “the importance of re‐evaluating the definition of the ‘pre‐symptomatic’ phase of Rett syndrome.”