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The supporting actor steals the show: Role of β‑spectrin N-terminus in SCA5

Written by Sophia Leung
Edited by Dr Hannah K Shorrock

In SCA5, the N-terminus of β‑spectrin induces toxicity and therefore a new target for therapeutics to treat SCA5.

The research group led by Dr. Adam Avery explored the roles a fragment of the β‑spectrin protein, the N-terminus, plays in SCA5. They discovered that the N-terminus is far from merely an extra in the show: it supports the normal function of β‑spectrin and plays the role of an accomplice in inducing toxicity in SCA5. Because of this, the researchers suggest that the N-terminus of β‑spectrin is a potential target for treating SCA5.

What is significant about this study?

SCA5 is caused by mutations in the gene that encodes a protein called β-spectrin. β-spectrin interacts with other proteins in the cell to create a scaffold under the cell membrane. This scaffold offers structural support to stabilize the shape of the cell, in addition to holding other important cellular machines in place. However, we still don’t fully understand the characteristics and normal function of the protein, or how mutations in different places of β-spectrin all cause SCA5.  A recent study has found that not only are the sites of mutations relevant for the dysfunction of the protein but so is the N-terminus. The N-terminus is the beginning part of a protein, in this case, the beginning of β-spectrin, which is not a place where SCA5 mutations are found. This makes understanding the role of the N-terminus important for understanding how mutations in β‑spectrin lead to SCA5 disease.

What does the N-terminus do?

To study the function of the β‑spectrin N-terminus, the researchers engineered versions of β‑spectrin without the N-terminus and called this βspecΔN-WT. The researchers used this construct as well as full-length, or wild-type, β‑spectrin (βspecWT) to generate genetically engineered fruit flies. When they compared flies that express βspecWT and flies without the N-terminus (βspecΔN-WT), they noticed that βspecΔN-WT flies had lower survival rates. This proved that the N-terminus is not redundant, but important to the normal function of β‑spectrin and the survival of fruit flies.

To understand the effect of SCA5 mutations on β‑spectrin function, the researchers generated flies that expressed full-length β‑spectrin containing SCA5 mutations (βspecSCA5) and flies that expressed β‑spectrin containing SCA5 mutations without the N-terminal (βspecΔN-SCA5). Since β‑spectrin is important for maintaining cell shape, the researchers next looked at the shapes of neurons for the different versions of β‑spectrin. In flies that had the SCA5 mutation (βspecSCA5), neurons had fewer branches, and these branches were shorter. This means that the neurons were not formed properly. This could indicate that they make fewer connections with other neurons and therefore have a lower ability to receive signals from others. However, when the N-terminus was removed (βspecΔN-SCA5), the neurons appeared normal despite having the SCA5 mutations. This showed that the N-terminus is required for the SCA5 mutation to cause alternations in neuron branching: and thus makes the N-terminus an accomplice.

Next, the researchers explored why removing the N-terminus restored normal cell shape. They first checked if neurons are producing different amounts of the β‑spectrin protein. Indeed, compared to normal flies (βspecWT), SCA5 flies (βspecSCA5) expressed three times more β‑spectrin protein. This indicates that in SCA5, an abnormal amount of β‑spectrin accumulates in neurons. However, if the N-terminus is removed (βspecΔN-SCA5), the β‑spectrin protein level is similar to wild-type flies (βspecWT). This means that the N-terminus is involved in increasing the levels of the mutated β‑spectrin protein: again, the N-terminus is an accomplice in SCA5 toxicity.

An important function of β‑spectrin is to bind to actin, another structural protein in cells. The researchers hypothesized that the N-terminus participates in this interaction. To test this, they compared wild-type flies (βspecWT) and SCA5 flies (βspecSCA5) and found that βspecSCA5 showed abnormally strong binding to actin. This was abolished in SCA5 flies without the N-terminus (βspecΔN-SCA5). This suggests that the N-terminus is involved in binding actin and is necessary for the abnormal actin binding in SCA5.

Putting it all together

The researchers have shown that the N-terminus of the β‑spectrin protein plays an active role in SCA5 toxicity: it causes alterations to neuron branching, accumulation of the mutated β‑spectrin protein in neurons, and excessively strong binding with actin. While current preclinical therapeutic strategies in SCA5 consider targeting the site of mutations in β‑spectrin, this study suggests that the N-terminus is a novel therapeutic target. Treatments that disarm N-terminus, the accomplice, will have the potential to be generalized for treating SCA5 caused by different mutations, making this a promising direction of research.

Key Words

β‑spectrin: The mutated protein in SCA5; a protein that forms a scaffold under the cell membrane to provide structural support to the cell.

N-terminus: a term biologists use to describe the position in a protein. N-terminus means the front or beginning part of a protein. As oppose to C-terminus, which is the end part of a protein. This originates from the chemical structure of the building blocks of proteins: they all start with the element nitrogen (N) and end with the element carbon (C).

Conflict of Interest Statement

The writer and editor declare no conflict of interest.

Citation of Article Reviewed

Denha, S. A., Atang, A. E., Hays, T. S. & Avery, A. W. beta-III-spectrin N-terminus is required for high-affinity actin binding and SCA5 neurotoxicity. Sci Rep 12, 1726, doi:10.1038/s41598-022-05762-2 (2022).

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