Inherited genetic diseases such as retinitis pigmentosa and nephrogenic diabetes may become treatable if the initial results achieved by a team of researchers led by Prof. Arun Shukla from the Department of Biological Sciences and Bioengineering at the Indian Institute of Technology (IIT), Kanpur, are reproducible in animal models and humans.
Retinitis pigmentosa is an inherited, degenerative eye disease that leads to progressive loss of vision as one gets older, while genetic nephrogenic diabetes arises from kidney cells’ inability to retain water leading to extreme thirst and dehydration. In both these cases, the cause of disease is a mutation in the G-protein-coupled receptors (GPCR) which causes the receptors to be pulled from the plasma membrane to the inside of the cell just as the receptors reach the cell membrane to start signalling. In the absence of the receptors (rhodopsin GPCR in the case of retinitis pigmentosa and vasopressin GPCR for genetic nephrogenic diabetes) the cells fail to signal and do not function normally.
The pulling in or trafficking of the GPCR receptors from the membrane surface to inside the cells (which is called endocytosis) happens when a small family of proteins called beta-arrestins bind to GPCR receptors and to another class of proteins called clathrin.
“We have designed synthetic antibody fragments which specifically bind to beta-arrestins at the position where clathrin gets bound. So our antibodies prevent the clathrin protein from binding to beta-arrestins thus preventing endocytosis,” says Prof. Shukla. The results were published in the journal Nature Nanotechnology.
Theoretically, preventing the trafficking of the GPCR receptors from the membrane surface to inside the cells is like turning the clock back; rhodopsin and vasopressin receptors would stay intact on the cell membrane, start signalling and enable the otherwise mutant cells to function normally. “Our designer proteins provide unexplored territory for therapeutic applications for inherited diseases,” he says.
Limited lifespan
Proteins such as GPCR receptors, beta-arrestins and clathrin have limited life span of a few hours, while the antibodies introduced survive for a loner time. Once introduced, the antibodies can get bound to beta-arrestins that are freshly produced and prevent clathrin from binding to beta-arrestins thus preventing endocytosis.
“Even when beta-arrestins bound to antibodies decay, the antibodies with their longer life span can bind to newly formed beta-arrestins. The other possibility is increasing the concentration of the antibodies so that they are always present in the cells to bind to freshly produced beta-arrestins,” Prof. Shukla says.
Since the mechanism of the two diseases is the same, the antibodies can work equally well immaterial of the cell type involved. “This is a proof-of-concept study using modified kidney cell lines. The next step will be to develop new strategies to deliver the antibodies into human live cells and animal models,” he says. The team is considering starting these studies in near future.
The big advantage of using antibodies is that they selectively block receptor endocytosis but not signalling. “This provides a unique handle, currently not available anywhere in the world, for targeting a specific GPCR function. This makes our designer proteins superior to knock-out approaches,” he comments.
