NIPA1(SPG6), the Basis for Autosomal Dominant Form of Hereditary Spastic Paraplegia, Encodes a Functional Mg2+ Transporter

Angela Goytain,Rochelle M Hines,Alaa El-Husseini,Gary A Quamme

doi:10.1074/jbc.m610314200

Angela Goytain, Rochelle M Hines + Show 2 more

Open Access

https://doi.org/10.1074/jbc.m610314200

Copy DOI

Abstract

Mutations in the NIPA1(SPG6) gene, named for "nonimprinted in Prader-Willi/Angelman" has been implicated in one form of autosomal dominant hereditary spastic paraplegia (HSP), a neurodegenerative disorder characterized by progressive lower limb spasticity and weakness. However, the function of NIPA1 is unknown. Here, we show that reduced magnesium concentration enhances expression of NIPA1 suggesting a role in cellular magnesium metabolism. Indeed NIPA1 mediates Mg2+ uptake that is electrogenic, voltage-dependent, and saturable with a Michaelis constant of 0.69+/-0.21 mM when expressed in Xenopus oocytes. Subcellular localization with immunofluorescence showed that endogenous NIPA1 protein associates with early endosomes and the cell surface in a variety of neuronal and epithelial cells. As expected of a magnesium-responsive gene, we find that altered magnesium concentration leads to a redistribution between the endosomal compartment and the plasma membrane; high magnesium results in diminished cell surface NIPA1 whereas low magnesium leads to accumulation in early endosomes and recruitment to the plasma membrane. The mouse NIPA1 mutants, T39R and G100R, corresponding to the respective human mutants showed a loss-of-function when expressed in oocytes and altered trafficking in transfected COS7 cells. We conclude that NIPA1 normally encodes a Mg2+ transporter and the loss-of function of NIPA1(SPG6) due to abnormal trafficking of the mutated protein provides the basis of the HSP phenotype.

Highlights

Drome [1,2,3,4,5]
The T39R and G100R mutations resulted in altered intracellular trafficking of NIPA1 protein and diminished Mg2ϩ transport suggesting a role in the hereditary spastic paraplegia (HSP) phenotype
NIPA1 cDNA Is a Magnesium-responsive Gene—With the knowledge that differential gene expression is involved with selective control of epithelial cell magnesium conservation, our strategy was to use microarray analysis to identify cDNAs that were up-regulated with low magnesium [15]

Summary

Introduction

Drome [1,2,3,4,5]. NIPA1 has been implicated in another distinct disorder termed autosomal dominant hereditary spastic paraplegia (HSP) (OMIM 608145 and 600363). Mutations in at least six genes have been associated with autosomal dominant HSP including NIPA1(SPG6) This heterogenous group presents with progressive lower limb spasticity and weakness. The data suggest that epithelial cells can sense the environmental magnesium and through transcription- and translation-dependent processes alter Mg2ϩ transport and maintain magnesium balance. In an attempt to identify genes underlying cellular changes resulting from adaptation to low extracellular magnesium, we used oligonucleotide microarray analysis to screen for magnesium-regulated transcripts in epithelial cells [16]. Our data indicate that NIPA1 protein mediates Mg2ϩ transport and is regulated by magnesium indicating that it may play a role in control of cellular magnesium homeostasis. The T39R and G100R mutations resulted in altered intracellular trafficking of NIPA1 protein and diminished Mg2ϩ transport suggesting a role in the HSP phenotype

Objectives

Methods

Results

Conclusion