Diverse species-specific phenotypic consequences of loss of function sorting nexin 14 mutations

Abstract

Mutations in the SNX14 gene cause spinocerebellar ataxia, autosomal recessive 20 (SCAR20) in both humans and dogs. Studies implicating the phenotypic consequences of SNX14 mutations to be consequences of subcellular disruption to autophagy and lipid metabolism have been limited to in vitro investigation of patient-derived dermal fibroblasts, laboratory engineered cell lines and developmental analysis of zebrafish morphants. SNX14 homologues Snz (Drosophila) and Mdm1 (yeast) have also been conducted, demonstrated an important biochemical role during lipid biogenesis. In this study we report the effect of loss of SNX14 in mice, which resulted in embryonic lethality around mid-gestation due to placental pathology that involves severe disruption to syncytiotrophoblast cell differentiation. In contrast to other vertebrates, zebrafish carrying a homozygous, maternal zygotic snx14 genetic loss-of-function mutation were both viable and anatomically normal. Whilst no obvious behavioural effects were observed, elevated levels of neutral lipids and phospholipids resemble previously reported effects on lipid homeostasis in other species. The biochemical role of SNX14 therefore appears largely conserved through evolution while the consequences of loss of function varies between species. Mouse and zebrafish models therefore provide valuable insights into the functional importance of SNX14 with distinct opportunities for investigating its cellular and metabolic function in vivo. Mutations in the human Sorting Nexin 14 (SNX14) gene cause spinocerebellar ataxia, autosomal recessive 20 (SCAR20; OMIM 616354) 1. These mutations most often lead to complete loss or truncation of the SNX14 protein, resulting in early onset cerebellar atrophy, ataxia, developmental delay, intellectual disability and coarse facial features, with hearing loss, relative macrocephaly and seizures only reported in some patients 1-7. SNX14 is ubiquitously expressed among tissues, accounting for the clinically recognisable syndromic presentation characteristic of SCAR20 1,5,7. SNX14 belongs to the RGS-PX protein family, which includes SNX13, SNX19 and SNX25 8. No mutations in these other members have yet been identified as the cause of human diseases. Inside the cell, SNX14 mutations impact both autophagy and lipid metabolism 1,2,9. The most apparent subcellular phenotype is the accumulation of autolysosomes containing lipids 1,9. SNX14 is localised to the endoplasmic reticulum membrane via its N-terminal transmembrane domain where it is enriched in proximity to lipid

Zebrafish Models of Autosomal Recessive Ataxias

“…It was observed that total fatty acids from neutral lipids (both saturated and unsaturated) were elevated in snx14 knock-out embryos in comparison with wild type and heterozygous embryos. These results confirm that SNX14 has a conserved role in lipid biogenesis [ 61 ].…”

“…The discrepancies in these studies could be because Bryant et al [ 61 ] used sub toxic doses of morpholinos. Morpholinos are only considered reliable when a mutant model supports morpholino findings due to the previously observed off target effects of morpholinos [ 62 ].…”

“…In addition, the premature stop codon of mutants resulted in a nonsense mediated decay that upregulated homologous gene sequences that rescue the phenotype. The consequence of the mutation at the protein level could not be demonstrated in zebrafish because there was no SNX14 antibody available that could react with the corresponding zebrafish protein [ 61 ].…”

SNX-PXA-RGS-PXC Subfamily of SNXs in the Regulation of Receptor-Mediated Signaling and Membrane Trafficking

“…There is a dynamic coordinated interaction among the recycling, retrograde, and degradative pathways, which maintains normal cellular functions [ 2 , 3 ]. However, if the SNX-PXA-RGS-PXC subfamily, like all other SNX subfamilies, is dysfunctional and disabled to transport receptor cargoes to their appropriate cellular destinations, there will be the impairment of the above-mentioned pathways, which will negatively affect cellular functions, causing disorders, such as those listed in Table 4 [ 17 , 18 , 19 , 20 , 23 , 27 , 33 , 38 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 ].…”

“…SNX14, localized in the lysosome [ 16 ], functions as a negative regulator of the signaling and trafficking of 5-HT 6 R [ 13 ] and probably other receptor cargoes, as well. SNX14 is also localized at the membrane contact site of ER-lipid droplets in yeast, drosophila, and mammals [ 14 , 42 , 91 , 100 ], indicating important roles of SNX14 in lipid drop biogenesis and trafficking of lipid transfer proteins [ 101 ].…”