Follistatin-like 1 in development and human diseases.

Follistatin-like 1 (FSTL1) is a secreted glycoprotein displaying expression changes during development and disease, among which cardiovascular disease, cancer, and arthritis. The cardioprotective role of FSTL1 has been intensively studied over the last years, though its mechanism of action remains elusive. FSTL1 is involved in multiple signaling pathways and biological processes, including vascularization and regulation of the immune response, a feature that complicates its study. Binding to the DIP2A, TLR4 and BMP receptors have been shown, but other molecular partners probably exist. During cancer progression and rheumatoid arthritis, controversial data have been reported with respect to the proliferative, apoptotic, migratory, and inflammatory effects of FSTL1. This controversy might reside in the extensive post-transcriptional regulation of FSTL1. The FSTL1 primary transcript also encodes for a microRNA (miR-198) in primates and multiple microRNA-binding sites are present in the 3'UTR. The switch between expression of the FSTL1 protein and miR-198 is an important regulator of tumour metastasis and wound healing. The glycosylation state of FSTL1 is a determinant of biological activity, in cardiomyocytes the glycosylated form promoting proliferation and the non-glycosylated working anti-apoptotic. Moreover, the glycosylation state shows differences between species and tissues which might underlie the differences observed in in vitro studies. Finally, regulation at the level of protein secretion has been described.

Deletion of Fstl1 (Follistatin-Like 1) From the Endocardial/Endothelial Lineage Causes Mitral Valve Disease.

OBJECTIVE: Fstl1 (Follistatin-like 1) is a secreted protein that is expressed in the atrioventricular valves throughout embryonic development, postnatal maturation, and adulthood. In this study, we investigated the loss of Fstl1 in the endocardium/endothelium and their derived cells. APPROACH AND RESULTS: We conditionally ablated Fstl1 from the endocardial lineage using a transgenic Tie2-Cre mouse model. These mice showed a sustained Bmp and Tgfß signaling after birth. This resulted in ongoing proliferation and endocardial-to-mesenchymal transition and ultimately in deformed nonfunctional mitral valves and a hypertrophic dilated heart. Echocardiographic and electrocardiographic analyses revealed that loss of Fstl1 leads to mitral regurgitation and left ventricular diastolic dysfunction. Cardiac function gradually deteriorated resulting in heart failure with preserved ejection fraction and death of the mice between 2 and 4 weeks after birth. CONCLUSIONS: We report on a mouse model in which deletion of Fstl1 from the endocardial/endothelial lineage results in deformed mitral valves, which cause regurgitation, heart failure, and early cardiac death. The findings provide a potential molecular target for the clinical research into myxomatous mitral valve disease.

Considerations for Measurement of Embryonic Organ Growth.

Organogenesis is a complex coordinated process of cell proliferation, growth, migration, and apoptosis. Differential growth rates, particularly during cardiogenesis, play a role in establishing morphology. Studies using stereological and cell sorting methods derive averages of morphogenetic parameters for an organ. To understand tissue composition and differential growth, the researcher must determine a number of morphogenetic parameters in the developing organ. Such measurements require sectioning to enable identification of organ borders, tissue components and cell types, three-dimensional (3D)-reconstruction of sections to visualize morphology and a 3D-measurement scheme to build local morphogenetic information. Although thick the section confocal microscopy partially solves these issues, information loss at the section surface hampers the reconstruction of 3D morphology. Episcopic imaging provides the correct morphology but lacks histological procedures to identify multiple cell types. The 3D-measurement scheme is based on systematic sampling, with overlapping sample volumes, of the entire organ in the aligned image stack. For each sample volume, morphogenetic variables are calculated and results projected back to the cube (boxel) at the sample volume center. Boxel size determines spatial resolution of the final quantitative 3D-reconstruction whereas size of the sample volume determines the precision of the morphogenetic information. The methods described here can be used to measure tissue volume, proliferation and cell size, to determine contribution and distribution of cell types in a tissue and to display this information in a quantitative 3D-reconstruction. Anat Rec, 302:49-57, 2019. © 2018 Wiley Periodicals, Inc.

Identifying pathogenic variants in the Follistatin-like 1 gene (FSTL1) in patients with skeletal and atrioventricular valve disorders.

BACKGROUND: Follistatin-like 1 (Fstl1) is a glycoprotein expressed throughout embryonic development. Homozygous loss of Fstl1 in mice results in skeletal and respiratory defects, leading to neonatal death due to a collapse of the trachea. Furthermore, Fstl1 conditional deletion from the endocardial/endothelial lineage results in postnatal death due to heart failure and profound atrioventricular valve defects. Here, we investigated patients with phenotypes similar to the phenotypes observed in the transgenic mice, for variants in FSTL1. METHODS: In total, 69 genetically unresolved patients were selected with the following phenotypes: campomelic dysplasia (12), small patella syndrome (2), BILU (1), and congenital heart disease patients (54), of which 16 also had kyphoscoliosis, and 38 had valve abnormalities as their main diagnosis. Using qPCR, none of 69 patients showed copy number variations in FSTL1. The entire gene body, including microRNA-198 and three validated microRNA-binding sites, were analyzed using Sanger sequencing. RESULTS: No variants were found in the coding region. However, 8 intronic variants were identified that differed significantly in their minor allele frequency compared to controls. Variant rs2272515 was found to significantly correlate (p < 0.05) with kyphoscoliosis. CONCLUSION: We conclude that pathogenic variants in FSTL1 are unlikely to be responsible for skeletal or atrioventricular valve anomalies in humans.

Endothelial follistatin-like-1 regulates the postnatal development of the pulmonary vasculature by modulating BMP/Smad signaling.

Bone morphogenetic protein (BMP) signaling regulates vascular smooth muscle maturation, endothelial cell proliferation, and tube formation. The endogenous BMP antagonist Follistatin-like 1 (Fstl1) is highly expressed in pulmonary vascular endothelium of the developing mouse lung, suggesting a role in pulmonary vascular formation and vascular homeostasis. The aim of this study was to investigate the role of Fstl1 in the pulmonary vascular endothelium. To this aim, Fstl1 was conditionally deleted from endothelial and endothelial-derived cells using Tie2-cre driven Fstl1-KO mice (Fstl1-eKO mice). Endothelial-specific Fstl1 deletion was postnatally lethal, as ∼70% of Fstl1-eKO mice died at three weeks after birth. Deletion of Fstl1 from endothelium resulted in a reduction of right ventricular output at three weeks after birth compared with controls. This was associated with pulmonary vascular remodeling, as the percentage of actin-positive small pulmonary vessels was increased at three weeks in Fstl1-eKO mice compared with controls. Endothelial deletion of Fstl1 resulted in activation of Smad1/5/8 signaling and increased BMP/Smad-regulated gene expression of Jagged1, Endoglin, and Gata2 at one week after birth compared with controls. In addition, potent vasoconstrictor Endothelin-1, the expression of which is driven by Gata2, was increased in expression, both on the mRNA and protein levels, at one week after birth compared with controls. At three weeks, Jagged1 was reduced in the Fstl1-eKO mice whereas Endoglin and Endothelin-1 were unchanged. In conclusion, loss of endothelial Fstl1 in the lung is associated with elevated BMP-regulated genes, impaired small pulmonary vascular remodeling, and decreased right ventricular output.