Fluorescence microscopy image showing the dramatic alteration in brain morphology of SNX27-deficient mice, demonstrating the importance of this gene in maintaining normal brain architecture and function.
Within every cell, numerous lipid-coated bubbles shuttle to and fro, transporting cargoes between the outer plasma membrane and various cellular organelles. Proteins known as ‘sorting nexins’ help shepherd molecules into this system by delivering specific targets into the early endosome, a compartment from which subsequent cellular trafficking is coordinated. To date, however, only a fraction of the 47 known sorting nexins have been characterized in detail.
Li Shen Loo at the A*STAR Institute of Molecular and Cell Biology and co-workers have now shown that one of these sorting nexins, known as SNX27, has a vital role in normal physiology. Sorting nexin 27 is unique among the sorting nexins in that it contains a PDZ domain, which enables it to interact selectively with other proteins containing a PDZ-interacting domain, although little else was known about its function.
In an initial set of experiments, the researchers determined that mice express this protein in virtually every tissue of the body, and confirmed that SNX27 selectively localizes to the early endosome via interaction with phosphatidylinositol 3-phosphate, a lipid that is particularly enriched in this structure. They subsequently examined this protein’s function by generating mice lacking the SNX27 gene, as well as heterozygotes that possess only one of two gene copies.
“SNX27-deficient mice unexpectedly failed to thrive and all died within the first three weeks, while heterozygotes behaved just like wild-type mice and lived up to 24 months,” says Loo. “This indicates that SNX27 plays a pivotal role in normal physiology, growth and survival.”
To identify this protein’s trafficking targets, Loo and her co-workers performed a screen to uncover interacting partners, revealing more than a dozen candidates. One protein struck them as particularly interesting—N-methyl-D-aspartate receptor 2C (NR2C), a component of the NMDAR neurotransmitter receptor complex. “Increased NMDAR activity has been implicated in stroke, epilepsy and traumatic brain injury,” says Loo, “while decreased NMDAR activity has been implicated in schizophrenia.”
NR2C contains a PDZ-binding domain, and interacts directly with SNX27 in a PDZ-dependent fashion. Brain tissue from SNX27-deficient mice exhibited a 35–40% excess of NR2C relative to wild-type animals (see image), while levels of NR1, another NMDAR subunit, remained unaffected. In parallel, the absence of SNX27 led to an 85% reduction in the trafficking of NR2C away from the cell membrane. These data suggest that this interaction directly influences levels of active NR2C in the brain. The researchers are now investigating the neurological significance of the SNX27–NR2C association as part of ongoing studies.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Molecular and Cell Biology.
