Relevant to disease mechanism
Reduced Spastin levels in neurons are associated with disruption to calcium balance and relevant to the disease-causing mechanism in SPG4 HSP. Restoring Spastin levels by gene editing was found crucial to correcting calcium imbalance.
Prof Winner discusses this research in the video. For translation from German, turn on closed captions in the bottom toolbar, then choose preferred language in Settings.
Here is a translated summary: We are currently testing various substances in SPG4 cell cultures that change the shape of (impaired) microtubules and have an effect on (impaired) calcium regulation. We have achieved very positive results particularly with one substance, in that we can significantly improve the amount of incoming calcium (to near-normal levels) and also reduce swollen neurite processes.
These are new findings and need confirmation by further testing, including whether or not impaired axonal transport improves as well. If successful, future plans include preclinical investigations, including animal studies, and the prospect of clinical trials in people with SPG4 HSP.
Pathogenic variants in SPAST, the gene coding for spastin, are the single most common cause of hereditary spastic paraplegia, a progressive motor neuron disease. Spastin regulates key cellular functions, including microtubule-severing and endoplasmic reticulum-morphogenesis. However, it remains unclear how alterations in these cellular functions due to SPAST pathogenic variants result in motor neuron dysfunction. Since spastin influences both microtubule network and endoplasmic reticulum structure, we hypothesized that spastin is necessary for the regulation of Ca2+ homeostasis via store-operated calcium entry.
Here, we show that the lack of spastin enlarges the endoplasmic reticulum and reduces store-operated calcium entry. In addition, elevated levels of different spastin variants induced clustering of STIM1 within the endoplasmic reticulum, altered the transport of STIM1 to the plasma membrane and reduced store-operated calcium entry, which could be rescued by exogenous expression of STIM1. Importantly, store-operated calcium entry was strongly reduced in induced pluripotent stem cell-derived neurons from hereditary spastic paraplegia patients with pathogenic variants in SPAST resulting in spastin haploinsufficiency. These neurons developed axonal swellings in response to lack of spastin.
We were able to rescue both store-operated calcium entry and axonal swellings in SPAST patient neurons by restoring spastin levels, using CRISPR/Cas9 to correct the pathogenic variants in SPAST.
These findings demonstrate that proper amounts of spastin are a key regulatory component for store-operated calcium entry mediated Ca2+ homeostasis and suggest store-operated calcium entry as a disease relevant mechanism of spastin-linked motor neuron disease.
SOURCE: Brain. Volume 145 Issue 9, Sept 2022, 3131–3146 https://doi.org/10.1093/brain/awac122
Store-operated calcium entry is reduced in spastin-linked hereditary spastic paraplegia
Tania Rizo 1 , Lisa Gebhardt 2 , Julia Riedlberger 1 , Esther Eberhardt 3 , Lars Fester 4 , Dalia Alansary 5 , Jürgen Winkler 6 7 , Soeren Turan 8 , Philipp Arnold 9 , Barbara A Niemeyer 5 , Michael J M Fischer 2 10 , Beate Winner 1 7
1. Department of Stem Cell Biology, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany.
2. Institute of Physiology and Pathophysiology, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany.
3. Department of Anesthesiology, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany.
4. Institute of Anatomy and Cell Biology, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany.
5. Molecular Biophysics, University of Saarland, Center for Integrative Physiology and Molecular Medicine, 66421 Homburg/Saar, Germany.
6. Department of Molecular Neurology, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany.
7. Center of Rare Diseases Erlangen (ZSEER), Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany.
8. Institute of Biochemistry (Emil-Fischer-Center), Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany.
9. Institute of Anatomy, Functional and Clinical Anatomy, Friedrich-Alexander-University Erlangen-Nürnberg, 91054 Erlangen, Germany.
10. Center of Physiology and Pharmacology, Medical University of Vienna, 1090 Vienna, Austria.