X-linked Angelman-like syndrome caused by Slc9a6 knockout in mice exhibits evidence of endosomal–lysosomal dysfunction

Abstract

Mutations in solute carrier family 9 isoform 6 on chromosome Xq26.3 encoding sodium–hydrogen exchanger 6, a protein mainly expressed in early and recycling endosomes are known to cause a complex and slowly progressive degenerative human neurological disease. Three resulting phenotypes have so far been reported: an X-linked Angelman syndrome-like condition, Christianson syndrome and corticobasal degeneration with tau deposition, with each characterized by severe intellectual disability, epilepsy, autistic behaviour and ataxia. Hypothesizing that a sodium–hydrogen exchanger 6 deficiency would most likely disrupt the endosomal–lysosomal system of neurons, we examined Slc9a6 knockout mice with tissue staining and related techniques commonly used to study lysosomal storage disorders. As a result, we found that sodium–hydrogen exchanger 6 depletion leads to abnormal accumulation of GM2 ganglioside and unesterified cholesterol within late endosomes and lysosomes of neurons in selective brain regions, most notably the basolateral nuclei of the amygdala, the CA3 and CA4 regions and dentate gyrus of the hippocampus and some areas of cerebral cortex. In these select neuronal populations, histochemical staining for β-hexosaminidase activity, a lysosomal enzyme involved in the degradation of GM2 ganglioside, was undetectable. Neuroaxonal dystrophy similar to that observed in lysosomal disease was observed in the cerebellum and was accompanied by a marked and progressive loss of Purkinje cells, particularly in those lacking the expression of Zebrin II. On behavioural testing, Slc9a6 knockout mice displayed a discrete clinical phenotype attributable to motor hyperactivity and cerebellar dysfunction. Importantly, these findings show that sodium–hydrogen exchanger 6 loss of function in the Slc9a6-targeted mouse model leads to compromise of endosomal–lysosomal function similar to lysosomal disease and to conspicuous neuronal abnormalities in specific brain regions, which in concert could provide a unified explanation for the cellular and clinical phenotypes in humans with SLC9A6 mutations.

Subcellular organization of UBE3A in neurons

“…Whether UBE3A directly regulates NHE8 or NHE7 is unknown, but supporting a possible link between UBE3A and Na + /H + exchangers, we note that mutations of the related exchanger NHE6 result in endosomal-lysosomal dysfunction and Christianson syndrome, a disorder phenotypically similar to AS (e.g. (Stromme et al, 2011;Pescosolido et al, 2014).…”

Ganglioside accumulation in activated glia in the developing brain: comparison between WT and GalNAcT KO mice

“…While mechanisms of preferential elevation of GM2 in phagocytic microglia in WT mice remain to be elucidated, accumulation of simple gangliosides, such as GM3 and GM2, have often been reported in lysosomal storage diseases, neurodegenerative diseases, and neuroinjury models ( 23, 27-29, 34-36, 50, 51 ). Even when a primary defi ciency in a lysosomal glycohydrolase, activator protein, or saposin directly involved in the degradation of GM2 or GM3 ( 52 ) is not present, the accumulation of GM2/GM3 appears to occur, at least in part, in the endosomal/lysosomal system ( 26,29,30,34,53 ) and/or in lipid rafts ( 24,27,33 ). As well as for defi ciencies in catabolic proteins not directly involved in degradation of these gangliosides, defects in traffi cking of gangliosides from endosomes to the Golgi apparatus ( 53,54 ) or from late endosomes to lysosomes ( 55,56 ) also observed in some lysosomal storage diseases can produce similar accumulation.…”

“…In addition, accumulation of GM2 has been observed in neurodegenerative diseases such as Alzheimer disease (31)(32)(33) and Angelmanlike syndrome ( 34 ), and in rodent acute brain injury models such as blast-induced mild traumatic brain injury ( 35 ) and a transient focal cerebral ischemia model ( 36 ). However, the mechanisms behind and the roles of accumulated GM2 have not been well elucidated.…”

Traditional and emerging roles for the SLC9 Na+/H+ exchangers

“…The neurological phenotype of an NHE6 KO mouse was recently described by Walkley et al [158]. Mutant mice were born at the expected mendelian ratio and had no obvious phenotype, even at older age.…”