TGFβ and BMP Dependent Cell Fate Changes Due to Loss of Filamin B Produces Disc Degeneration and Progressive Vertebral Fusions

Spondylocarpotarsal synostosis (SCT) is an autosomal recessive disorder characterized by progressive vertebral fusions and caused by loss of function mutations in Filamin B (FLNB). FLNB acts as a signaling scaffold by linking the actin cytoskleteon to signal transduction systems, yet the disease mechanisms for SCT remain unclear. Employing a Flnb knockout mouse, we found morphologic and molecular evidence that the intervertebral discs (IVDs) of Flnb–/–mice undergo rapid and progressive degeneration during postnatal development as a result of abnormal cell fate changes in the IVD, particularly the annulus fibrosus (AF). In Flnb–/–mice, the AF cells lose their typical fibroblast-like characteristics and acquire the molecular and phenotypic signature of hypertrophic chondrocytes. This change is characterized by hallmarks of endochondral-like ossification including alterations in collagen matrix, expression of Collagen X, increased apoptosis, and inappropriate ossification of the disc tissue. We show that conversion of the AF cells into chondrocytes is coincident with upregulated TGFβ signaling via Smad2/3 and BMP induced p38 signaling as well as sustained activation of canonical and noncanonical target genes p21 and Ctgf. These findings indicate that FLNB is involved in attenuation of TGFβ/BMP signaling and influences AF cell fate. Furthermore, we demonstrate that the IVD disruptions in Flnb–/–mice resemble aging degenerative discs and reveal new insights into the molecular causes of vertebral fusions and disc degeneration.

脊柱腕跗骨融合症（Spondylocarpotarsal synostosis，SCT）是一种常染色体隐性遗传病，以进行性脊柱融合为特征，由细丝蛋白B（Filamin B，FLNB）的功能丧失性突变引发。FLNB作为信号支架蛋白，可将肌动蛋白细胞骨架与信号转导系统相连，但目前SCT的致病机制仍不明确。本研究通过构建Flnb基因敲除小鼠，发现形态学与分子生物学证据表明，Flnb–/–小鼠的椎间盘（intervertebral discs，IVDs）在出生后发育过程中会发生快速且进行性的退变，这一过程源于椎间盘内细胞命运的异常改变，尤以纤维环（annulus fibrosus，AF）最为显著。在Flnb–/–小鼠中，纤维环细胞丧失其典型的成纤维细胞样特性，并获得肥大软骨细胞的分子与表型特征。该改变以软骨内样骨化的标志性特征为表现，包括胶原基质异常、X型胶原（Collagen X）表达上调、细胞凋亡增加，以及椎间盘组织的异常骨化。研究证实，纤维环细胞向软骨细胞的转化与转化生长因子β（TGFβ）/Smad2/3信号通路的激活、骨形态发生蛋白（BMP）介导的p38信号通路激活，以及经典与非经典靶基因p21和结缔组织生长因子（Ctgf）的持续上调同步发生。上述研究结果表明，FLNB参与调控TGFβ/BMP信号通路的衰减，并影响纤维环细胞的命运决定。此外，本研究证实Flnb–/–小鼠的椎间盘损伤与衰老相关性退变椎间盘的表型相似，为脊柱融合与椎间盘退变的分子致病机制提供了新的见解。