Thyroid follicle development requires Smad1/Smad5- and endothelial-dependent basement membrane assembly

Abstract

Thyroid follicles, the functional units of the thyroid gland, are delineated by a monolayer of thyrocytes resting on a continuous basement membrane. The developmental mechanisms of folliculogenesis, whereby follicles are formed by the reorganization of a non-structured mass of non-polarized epithelial cells, are largely unknown. Here we show that assembly of the epithelial basement membrane is crucial for folliculogenesis and is controlled by endothelial cell invasion and by BMP-Smad signaling in thyrocytes. Thyroid-specific Smad1 and Smad5 double-knockout (Smad1/5 dKO ) mice displayed growth retardation, hypothyroidism and defective follicular architecture. In Smad1/5 dKO embryonic thyroids, epithelial cells remained associated in large clusters and formed small follicles. Although similar follicular defects are found in Vegfa knockout (Vegfa KO ) thyroids, Smad1/5 dKO thyroids had normal endothelial cell density yet impaired endothelial differentiation. Interestingly, both Vegfa KO and Smad1/5 dKO thyroids displayed impaired basement membrane assembly. Furthermore, conditioned medium (CM) from embryonic endothelial progenitor cells (eEPCs) rescued the folliculogenesis defects of both Smad1/5 dKO and Vegfa KO thyroids. Laminin α1, β1 and γ1, abundantly released by eEPCs into CM, were crucial for folliculogenesis. Thus, epithelial Smad signaling and endothelial cell invasion promote folliculogenesis via assembly of the basement membrane.

TGF-β Family Signaling in Ductal Differentiation and Branching Morphogenesis

“…The final growth and completion of morphogenesis into organized follicles in the thyroid glands of these mice are defective, resembling the loss-of-function phenotype of the proangiogenic vascular endothelial growth factor-A (Villacorte et al 2016). Thyroid glands with defective BMP signaling show defective basement membrane, and extracellular matrix proteins like laminins can rescue the developmental defect (Villacorte et al 2016). Thus, BMP signaling assists in the architectural organization of the epitheliumto-basement membrane barrier (Villacorte et al 2016).…”

“…Conditional expression of TGF-b1 in thyrocytes of transgenic NOD mice promotes the development of thyroid hyperplasia, and inhibits autoimmune thyroiditis that is presumably because of enhanced recruitment of regulatory T cells into the thyroid glands (Yu et al 2010). BMP signaling promotes thyroid epithelial development, as evidenced from the effects of conditional inactivation of both Smad1 and Smad5 in thyrocytes (Villacorte et al 2016). The final growth and completion of morphogenesis into organized follicles in the thyroid glands of these mice are defective, resembling the loss-of-function phenotype of the proangiogenic vascular endothelial growth factor-A (Villacorte et al 2016).…”

“…BMP signaling promotes thyroid epithelial development, as evidenced from the effects of conditional inactivation of both Smad1 and Smad5 in thyrocytes (Villacorte et al 2016). The final growth and completion of morphogenesis into organized follicles in the thyroid glands of these mice are defective, resembling the loss-of-function phenotype of the proangiogenic vascular endothelial growth factor-A (Villacorte et al 2016). Thyroid glands with defective BMP signaling show defective basement membrane, and extracellular matrix proteins like laminins can rescue the developmental defect (Villacorte et al 2016).…”

“…Thyroid glands with defective BMP signaling show defective basement membrane, and extracellular matrix proteins like laminins can rescue the developmental defect (Villacorte et al 2016). Thus, BMP signaling assists in the architectural organization of the epitheliumto-basement membrane barrier (Villacorte et al 2016). The potential role of TGF-b family members in thyroid gland development remains to be better clarified in vivo.…”

Modelling of Epithelial Growth, Fission and Lumen Formation During Embryonic Thyroid Development: A Combination of Computational and Experimental Approaches

“…Moreover, this decrease in thyroid endothelial density negatively impacts on the organisation of epithelial cells into pre-follicular structures at E15.5 and eventually into large polarised epithelial monolayers (follicles) around birth ( 9 ). We further showed that the assembly of the basement membrane, partly controlled by endothelial cell invasion in the thyroid, was crucial for epithelial cell orientation, acquisition of polarity and in fine follicle formation ( 10 ).…”