RESUMEN
Humans lacking sclerostin display progressive bone overgrowth due to increased bone formation. Although it is well established that sclerostin is an osteocyte-secreted bone formation inhibitor, the underlying molecular mechanisms are not fully elucidated. We identified in tandem affinity purification proteomics screens LRP4 (low density lipoprotein-related protein 4) as a sclerostin interaction partner. Biochemical assays with recombinant proteins confirmed that sclerostin LRP4 interaction is direct. Interestingly, in vitro overexpression and RNAi-mediated knockdown experiments revealed that LRP4 specifically facilitates the previously described inhibitory action of sclerostin on Wnt1/ß-catenin signaling. We found the extracellular ß-propeller structured domain of LRP4 to be required for this sclerostin facilitator activity. Immunohistochemistry demonstrated that LRP4 protein is present in human and rodent osteoblasts and osteocytes, both presumed target cells of sclerostin action. Silencing of LRP4 by lentivirus-mediated shRNA delivery blocked sclerostin inhibitory action on in vitro bone mineralization. Notably, we identified two mutations in LRP4 (R1170W and W1186S) in patients suffering from bone overgrowth. We found that these mutations impair LRP4 interaction with sclerostin and its concomitant sclerostin facilitator effect. Together these data indicate that the interaction of sclerostin with LRP4 is required to mediate the inhibitory function of sclerostin on bone formation, thus identifying a novel role for LRP4 in bone.
Asunto(s)
Proteínas Morfogenéticas Óseas/metabolismo , Proteínas Relacionadas con Receptor de LDL/metabolismo , Osteocitos/metabolismo , Osteogénesis , Proteínas Adaptadoras Transductoras de Señales , Sustitución de Aminoácidos , Animales , Proteínas Morfogenéticas Óseas/genética , Marcadores Genéticos/genética , Células HEK293 , Humanos , Proteínas Relacionadas con Receptor de LDL/genética , Ratones , Mutación Missense , Transducción de Señal/genética , Proteína Wnt1/genética , Proteína Wnt1/metabolismo , beta Catenina/genética , beta Catenina/metabolismoRESUMEN
The impact of transient gene expression approaches (TGE) on the rapid production of recombinant proteins is undisputed, despite that all efforts are currently relying on two host cell families only, namely HEK293 derivatives and CHO cell line(s). Yet, the increasing complexity of biological targets calls for more than two host cell types to meet the challenges of difficult-to-express proteins. For this reason, we evaluated the more recently established novel CAP-T® cell line derived from human amniocytes for its performance and potential in transient gene expression. Upon careful analyses and adaptation of all process parameters we show here that indeed the CAP-T® cells are extremely amenable to transient gene expression and recombinant protein production. Additionally, they possess inherent capabilities to express and secrete complex and difficult target molecules, thus adding an attractive alternative to the repertoire of existing host cell lines used in transient production processes.