Investigation into the structure and function of a novel cellular structure
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Down’s syndrome (DS) is a congenital disorder caused by trisomy of chromosome 21, giving rise to symptoms including intellectual disability, poor muscle tone and characteristic facial features. Located on chromosome 21 is a gene encoding ubiquitin specific protease 25 (USP25), a member of the deubiquitinating family of enzymes. Studies of partial trisomies have revealed that although the USP25 gene is situated outside the critical region of chromosome 21 required to be triplicated to induce full DS symptoms, it is in a region linked to mild mental retardation and muscle hypotonia. Previous data has shown that, when overexpressed, USP25 forms novel rod shaped structures approximately 3-5 μm in length and 0.3-0.6 μm wide, and with a copy number, on average, of no more than 1-2 per cell. These structures do not associate with any known cellular organelle or with any component of the cytoskeleton. In this work, endogenous USP25 rods were detected in cultures of primary human foetal astrocytes, and a comparison of healthy and DS human primary foetal astrocytes revealed that in the DS culture a higher percentage of astrocytes contain rods. The domains of USP25 required for rod formation were identified using a series of GFP-tagged deletion constructs. USP25 rods could be purified from HEK293 cells using subcellular fractionation techniques and were assayed for deubiquitinating activity; however, none was detected suggesting that rods may be catalytically inactive. Interacting partners of USP25 were identified using mass spectrometry, but none were found to localise in rods. Additionally, a USP25 null human embryonic stem cell line was generated in order to interrogate the function of USP25 in astrocytes, and was found to be deficient in maintaining the integrity of an epithelial cell layer in an in vitro model of the blood brain barrier.
AuthorsAbrehart, Robert W
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