A spastic paraplegia mouse model reveals REEP1-dependent ER shaping
Christian Beetz, … , Britta Qualmann, Christian A. Hübner
Christian Beetz, … , Britta Qualmann, Christian A. Hübner
Published September 24, 2013
Citation Information: J Clin Invest. 2013;123(10):4273-4282. https://doi.org/10.1172/JCI65665.
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Research Article

A spastic paraplegia mouse model reveals REEP1-dependent ER shaping

Abstract

Axonopathies are a group of clinically diverse disorders characterized by the progressive degeneration of the axons of specific neurons. In hereditary spastic paraplegia (HSP), the axons of cortical motor neurons degenerate and cause a spastic movement disorder. HSP is linked to mutations in several loci known collectively as the spastic paraplegia genes (SPGs). We identified a heterozygous receptor accessory protein 1 (REEP1) exon 2 deletion in a patient suffering from the autosomal dominantly inherited HSP variant SPG31. We generated the corresponding mouse model to study the underlying cellular pathology. Mice with heterozygous deletion of exon 2 in Reep1 displayed a gait disorder closely resembling SPG31 in humans. Homozygous exon 2 deletion resulted in the complete loss of REEP1 and a more severe phenotype with earlier onset. At the molecular level, we demonstrated that REEP1 is a neuron-specific, membrane-binding, and membrane curvature–inducing protein that resides in the ER. We further show that Reep1 expression was prominent in cortical motor neurons. In REEP1-deficient mice, these neurons showed reduced complexity of the peripheral ER upon ultrastructural analysis. Our study connects proper neuronal ER architecture to long-term axon survival.

Authors

Christian Beetz, Nicole Koch, Mukhran Khundadze, Geraldine Zimmer, Sandor Nietzsche, Nicole Hertel, Antje-Kathrin Huebner, Rizwan Mumtaz, Michaela Schweizer, Elisabeth Dirren, Kathrin N. Karle, Andrey Irintchev, Victoria Alvarez, Christoph Redies, Martin Westermann, Ingo Kurth, Thomas Deufel, Michael M. Kessels, Britta Qualmann, Christian A. Hübner

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Figure 1

Lack of REEP1 exon 2 is associated with spastic paraplegia in humans and causes a severe motor phenotype in mice.

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Lack of REEP1 exon 2 is associated with spastic paraplegia in humans and...
(A) MLPA-based screening of genomic DNA from an HSP index patient for copy number alterations in REEP1. The assay targets exons 3–6 with single probes, and exons 1, 2, and 7 with 2 or 3 probes each (ex1-1, ex1-2, etc.). Relative MLPA signals of approximately 1.0 indicate a normal diploid copy number (gray bars), whereas signals of approximately 0.5 (black bars) indicate a heterozygous genomic deletion. (B) Alignment of REEP1 intron 1 and intron 2 sequences with the sequence of the fusion allele. The box highlights the microhomology of 3 bp at the junction and the exclamation points denote nucleotide identity. (C) Strategy of Reep1 exon 2 deletion in mice. Wild-type locus (top schematic); the thick horizontal line marks the genomic region used for the targeting construct. The targeted locus is shown in the middle schematic, and the targeted locus after Cre-mediated exon 2 excision is shown in the bottom schematic. Numbered black squares: exons; triangles: loxP sites; NEO: neomycin selection cassette; BamHI, EcoRI, SmaI, SalI: restriction sites used for cloning. (D) Western blot analysis of brain lysates from Reep1+/+ and Reep1–/– mice using affinity-purified anti-REEP1 antibodies shows the absence of REEP1 protein in Reep1–/– mice. (E) Sixteen-month-old Reep1–/– mice suffer from gait abnormalities (see also Supplemental Videos 1 and 2). Note the broad-based positioning of the hind limbs, the kyphotic posture, and that neither the hind body nor the tail is properly lifted.