The aberrant protein that causes juvenile Batten disease, a fatal neurodegenerative disorder of childhood, also plays a key role in normal cell function, Duke University Medical Center researchers have found.
The Duke study is the first to identify where the cln3 protein resides in human brain cells and to link the protein's location to its function. The researchers discovered that cln3 transports a vital lipid, or complex fat molecule, within a cell. The breakdown of this transportation system results in uncontrolled apoptosis, or cell death, and the neural degeneration that is a hallmark of Batten disease.
"Genetic diseases such as Batten disease provide a unique opportunity to understand the role of proteins, like cln3, that are vital for normal cell function," said Rose-Mary Boustany, M.D., a professor of pediatrics and neurobiology at Duke University Medical Center and senior author of the study.
The results appear in the September 2004 issue of Pediatric Research. The study was supported by the National Institute of Neurological Disorders and Stroke, a division of the National Institutes of Health.
Understanding the role of cln3 has implications beyond juvenile Batten disease – a rare, untreatable disorder -- because the protein is overproduced by cells in a number of cancers. And the lipid transported by cln3 plays an important role in infection by the AIDS virus and prion proteins, and in developing Alzheimer's disease. Prions are infectious proteins that can transmit diseases such as Creutzfeld-Jacob or mad cow disease.
Batten disease collectively refers to inherited disorders also called the neuronal ceroid lipofuscinoses, or NCL, which share similar clinical and pathological traits but are genetically distinct. All NCL disorders cause progressive loss of motor skills, mental retardation, loss of speech, blindness, uncontrolled seizures and, eventually, premature death.
The juvenile form of Batten disease, caused by mutations in the CLN3 gene, is the most common inherited neurodegenerative disease of childhood. Children who inherit a copy of the defective CLN3 gene from both parents produce a non-functional form of the protein. Those affected develop normally until about age five. Between the ages of five and eight years, their vision, motor and cognitive skills begin to deteriorate, with uncontrolled seizures and massive cell loss in the brain eventually resulting in death in the twenties.
Duke graduate student Dixie-Ann Persaud-Sawin, lead author of the study, focused her investigation of the protein primarily on human and rat brain cells because most of the disease's effects occur in the brain. She also studied cultured skin cells from patients, as these cells die more rapidly than their normal counterparts.
Persaud-Sawin found that the normal cln3 protein resides in the Golgi, a cellular body that packages proteins for use and transport within a cell. Cln3 also appears in complex fat platforms, or lipid rafts, within the cell surface and protective membrane coating. Time-lapse microscopy revealed that cln3 moves rapidly back and forth between the Golgi and the cell surface.
"It turns out that the cln3 protein may be a crucial protein involved in the transport of vital lipids from the Golgi, where they are made, to where they are needed -- for example, at the cell surface," Boustany said.
One of those vital lipids is galactosylceramide (GalCer). GalCer is a major constituent of lipid rafts. These lipid platforms are important for the initiation of signaling events in the cell, such as programmed cell death, or apoptosis.
The mutant cln3 protein lacks the amino acids that serve as a structural binding site for GalCer. In mutant patient cells, both cln3 and GalCer remain trapped in the Golgi and never move into the cell's plasma membrane, the Duke team discovered. With GalCer missing from the lipid platforms at the cell surface, apoptosis is inappropriately initiated and proceeds uncontrolled, resulting in death of brain cells and the clinical deterioration of juvenile Batten disease.
"We may be able to devise a way to treat juvenile neuronal ceroid lipofuscinosis [Batten disease] by finding an alternate route for delivery of GalCer to the cell surface," Boustany said.
GalCer also interacts with the AIDS virus and beta amyloid, the protein responsible for Alzheimer's disease, and prionic proteins that cause human Creutzfeldt-Jakob Disease and bovine spongiform encephalopathy (mad cow disease). A protein on HIV's surface binds with GalCer and may be important in the transmission of the disease. The beta amyloid and prionic proteins also enter cells by binding to GalCer.