Voermans, N. | bioRxiv Science

bioRxiv2025

Muscle stem cells produce a protective Fibrillin-1 matrix to prevent precocious activation.

Multiple biological mechanisms have been uncovered to regulate muscle stem cell quiescence, including inhibition of differentiation, adhesion-dependent anchoring, and translational control, which can be broadly classified as intrinsic or extrinsic properties. Here, we identify the matrix glycoprotein Fibrillin-1 (FBN1) as a Notch-regulated, cell-autonomous effector, essential for maintaining quiescence in muscle stem cells. Known for its causal role in Marfan syndrome (MFS), a connective tissue disorder that also presents with skeletal muscle atrophy, our work positions FBN1 as a critical niche component that protects stem cells from aberrant growth factor signalling. We demonstrate that targeted deletion of Fbn1 in muscle stem cells leads to dose-dependent quiescence defects, characterized by loss of cellular projections, depletion of the stem cell pool, and progressive decline in muscle function. Consistently, human MFS muscle biopsies show abnormally activated satellite cells, implicating stem cell imbalance in the development of MFS-associated myopathy. Mechanistically, the loss of FBN1 upregulates TGF{beta} signalling, and pharmacological inhibition of this pathway using the Angiotensin Receptor blocker losartan restores the cellular and physiological defects of mutant muscles. These findings reveal a new quiescence-preserving mechanism through extracellular matrix-mediated shielding from mitogenic signals, and position stem cell dysfunction as a driver of MFS myopathy. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=159 SRC="FIGDIR/small/666604v1_ufig1.gif" ALT="Figure 1"> View larger version (64K): org.highwire.dtl.DTLVardef@1895e71org.highwire.dtl.DTLVardef@1974e87org.highwire.dtl.DTLVardef@a49979org.highwire.dtl.DTLVardef@5813c9_HPS_FORMAT_FIGEXP M_FIG C_FIG Model of Fibrillin-1 protective barrier in quiescent satellite cells.Satellite cells produce Fibrillin-1 (FBN1) to establish a localized extracellular matrix barrier that limits exposure to mitogenic signals. Mechanistically, FBN1 expression is induced by Notch signalling and serves to sequester latent TGF{beta} complexes in an inactive form, thereby preventing their activation within the immediate niche. This barrier is spatially restricted beneath the basal lamina and remains functionally independent from the abundant interstitial FBN1 produced by fibro-adipogenic progenitors (FAPs). The model describes a self-contained, satellite cell-derived ECM system that maintains quiescence by insulating the cells from activating cues.

cell biology↗

bioRxiv2024

Optical genome mapping enables accurate repeat expansion testing

Short tandem repeats (STRs) are amongst the most abundant class of variations in human genomes and are meiotically and mitotically unstable which leads to expansions and contractions. STR expansions are frequently associated with genetic disorders, with the size of expansions often correlating with the severity and age of onset. Therefore, being able to accurately detect the total repeat expansion length and to identify potential somatic repeat instability is important. Current standard of care (SOC) diagnostic assays include laborious repeat-primed PCR-based tests as well as Southern blotting, which are unable to precisely determine long repeat expansions and/or require a separate set-up for each locus. Sequencing-based assays have proven their potential for the genome-wide detection of repeat expansions but have not yet replaced these diagnostic assays due to their inaccuracy to detect long repeat expansions (short-read sequencing) and their costs (long-read sequencing). Here, we tested whether optical genome mapping (OGM) can efficiently and accurately identify the STR length and assess the stability of known repeat expansions. We performed OGM for 85 samples with known clinically relevant repeat expansions in DMPK, CNBP and RFC1, causing myotonic dystrophy type 1 and 2 and cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS), respectively. After performing OGM, we applied three different repeat expansion detection workflows, i.e. manual de novo assembly, local guided assembly (local-GA) and molecule distance script of which the latter two were developed as part of this study. The first two workflows estimated the repeat size for each of the two alleles, while the third workflow was used to detect potential somatic instability. The estimated repeat sizes were compared to the repeat sizes reported after the SOC and concordance between the results was determined. All except one known repeat expansions above the pathogenic repeat size threshold were detected by OGM, and allelic differences were distinguishable, either between wildtype and expanded alleles, or two expanded alleles for recessive cases. An apparent strength of OGM over current SOC methods was the more accurate length measurement, especially for very long repeat expansion alleles, with no upper size limit. In addition, OGM enabled the detection of somatic repeat instability, which was detected in 9/30 DMPK, 23/25 CNBP and 4/30 RFC1 samples, leveraging the analysis of intact, native DNA molecules. In conclusion, for tandem repeat expansions larger than [~]300 bp, OGM provides an efficient method to identify exact repeat lengths and somatic repeat instability with high confidence across multiple loci simultaneously, enabling the potential to provide a significantly improved and generic genome-wide assay for repeat expansion disorders.

genomics↗