UConn professor of physiology and neurobiology Daniel Mulkey has received a $2.1 million grant from the National Heart, Lung, and Blood Institute to investigate the cellular and molecular basis for disordered breathing in a rare genetic condition called Rett syndrome. This project is part of a collaboration with Michelle Olsen, associate professor of neurobiology at Virginia Tech, and it has been continuously funded since 2010, receiving a total of $5.4 million in federal support.
Rett syndrome is a rare neurodevelopmental disorder linked to mutations on the X chromosome, meaning it occurs almost exclusively in girls. Rett syndrome significantly impacts patients’ ability to walk, speak, eat, and breathe. Disordered breathing is associated with a high mortality rate for this population.
Rett syndrome is caused by loss of function mutations in methyl-CpG-binding protein 2 (MeCP2), a transcription factor that regulates gene expression in many cells. However, scientists do not understand how this disruption to MeCP2 affects breathing.
Evidence from patients and animal models of Rett syndrome suggests disordered breathing in this disease results from disruption in the brains ability to regulate breathing in response to changes in carbon dioxide (CO2)/hydrogen (H+) levels. This process is known as respiratory chemoreception.
The retrotrapezoid nucleus (RTN) is a tiny region of the brainstem that serves as a CO2/H+ sensor to regulate breathing. Neurons in this region are directly activated by CO2/H+ and regulate breathing but recent work suggests astrocytes – a cell type long-assumed to be a support cell with no specific function – also contribute to RTN chemoreception. Previous studies also suggest a type of potassium channel known as Kir4.1/5.1 channels, which is almost exclusively expressed on astrocytes, concurs CO2/H+ sensitivity to these cells. However, it remains unclear how astrocytes and Kir4.1/5.1 channels contribute to the drive to breathe under normal conditions, and whether disruption of these mechanisms contributes to breathing problems in disease.
A main objective of this work is to determine whether Kir4.1/5.1 function is disrupted by loss of MeCP2 and contributes to breathing problems in Rett syndrome. Kir4.1 potassium channels can function as homomeric channels composed of two Kir4.1 subunits, but they also be heteromerized with Kir5.1. The heteromeric channels gain the ability to sense changes in CO2/H+.
Mulkey and his longtime collaborator Michelle Olsen showed MeCP2 regulates expression of Kir4.1 and Kir5.1. The loss of this transcription factor resulted in diminished expression of Kir4.1/5.1 channels and diminished ventilatory response to CO2. These results suggest dysregulation of astrocyte Kir4.1/5.1 channels contributes to breathing problems in Rett syndrome.
The ability of astrocytes to regulate neural activity, and consequently respiratory drive, depends on their close proximity to neurons. Interestingly, previous work showed astrocytes in certain brain regions undergo dramatic changes in volume across sleep and wake states. Astrocytes are snuggled up close to neurons during wakefulness but retract during sleep in a way that will limit communication between these cell types. Based on this, Mulkey and Olsen predict that Rett syndrome-induced disruption of normal astrocyte functions due to loss of Kir4.1/5.1 will have a greater effect on respiratory neurons during wakefulness.
This new grant will use a combination of genetics, cellular electrophysiology, and advanced confocal microscopy to determine whether and how astrocytes and Kir4.1/5.1 channels contribute to sleep-wake state dependent control of breathing under normal conditions and in Rett syndrome.
“This work will provide novel insight regarding how astrocytes contribute to control of breathing, and in doing so potentially identify new avenues for treatment of disordered breathing in Rett syndrome,” Mulkey says.
The Mulkey lab studies electrophysiological characteristics of mammalian neurons in brainstem regions associated with respiratory control, specifically defining properties unique to respiratory chemoreceptors and cellular mechanisms by which chemoreceptors sense changes in CO2/pH.