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 2

The REEP1 knockout phenotype is progressive, dose-dependent, and associated with a neurodegenerative rather than a neurodevelopmental pathology of upper motor neuron axons.

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The REEP1 knockout phenotype is progressive, dose-dependent, and associa...
(A) Measurement of the foot-base angle. Shown are images of 20-week-old mice immediately prior to hind paw lifting. (B) Genotype dependence of foot-base angle over time. Error bars represent the SEM. *P < 0.05 (two-way ANOVA). (C) Semithin sections of the corticospinal tract in the spinal cord of 30-week-old mice at the lumbar level. Arrow and asterisk denote pathological structures, which are interspersed between morphological intact axons of Reep1–/– but not Reep+/+ mice. Scale bars: 1 μm. (D) Ultrastructural characterization of pathologies as highlighted in C. Dark structures are packed with electron-dense material (left panel); vacuolar structures are regions devoid of cellular material (right panel). The presence of myelin sheaths (arrowheads) indicates an axonal origin. Scale bars: 400 nm. (E) Neuronal cell bodies in the motor cortices of 13-month-old Reep1+/+ and Reep1–/– mice were identified by NeuN immunoreactivity. Scale bars: 200 μm. (F) Layer-wise quantification revealed no neuron loss in Reep1–/– mice. Error bars represent the SEM. (G) Lower motor neurons are not involved as judged from CMAPs upon sciatic nerve stimulation recorded from the musculus triceps surae. Error bars represent the SEM. (H) Cortical neurons obtained from P1 pups and cultured for 3 days. Neurites were visualized with phalloidin (Phl). Axons were labeled by the pan-axonal neurofilament marker SMI312 (SMI). Scale bars: 50 μm. (I) Quantification of the length of the longest axonal projection of each neuron. Error bars represent the SEM.