Transforming growth factor-beta (TGF-beta) superfamily members are multifunctional cell-cell signaling proteins that play pivotal roles in tissue homeostasis and development of multicellular animals. They mediate their pleiotropic effects from membrane to nucleus through distinct combinations of type I and type II serine/threonine kinase receptors and their downstream effectors, known as Smad proteins. Certain Smads, termed receptor-regulated Smads, become phosphorylated by activated type I receptors and form heteromeric complexes with a common-partner Smad4, which translocates into the nucleus to control gene transcription. In addition to these signal transducing Smads, inhibitory Smads have been identified that inhibit the activation of receptor-regulated Smads. In contrast to the still growing TGF-beta superfamily (with approximately 30 members in mammals), relatively few type I and type II receptors as well as Smads have been identified. We will focus on recent insights into the molecular mechanisms by which signaling specificity between different TGF-beta superfamily members is achieved and regulated, and how a single family member can elicit a broad scala of biological responses.-Piek, E., Heldin, C.-H., ten Dijke, P. Specificity, diversity, and regulation in TGF-beta superfamily signaling.

Highlights d Spatial proteogenomic single-cell atlas of healthy and obese murine and human liver d Validated flow cytometry and microscopy panels for all hepatic cells d LAMs are differentially located in the lean and obese liver d Evolutionary conserved BMP9/10-ALK1 axis is essential for KC development

Bone morphogenetic proteins (BMPs) are multifunctional proteins structurally related to transforming growth factor-beta (TGF beta) and activin that can induce cartilage and bone growth in vivo. Members of the TGF beta superfamily exert their biological effects via heteromeric serine/threonine kinase complexes of type I and type II receptors. We previously obtained six different type I receptors, termed activin receptor-like kinase-1 (ALK-1) to -6. ALK-5 is a TGF beta type I receptor, ALK-2 and ALK-4 are activin type I receptors, and ALK-3 and ALK-6 are type I receptors for osteogenic protein-1 (OP-1)/bone morphogenetic protein-7 (BMP-7) and BMP-4. Here we report the complementary DNA cloning of the mouse homolog of ALK-3, which is highly conserved between mouse and man. ALK-3 messenger RNA (mRNA) is ubiquitously expressed in various adult mouse tissues, whereas ALK-6 mRNA is only found in brain and lung. The distribution of ALK-3 and ALK-6 mRNA in the postimplantation mouse embryo [6.5-15.5 days postcoitum (pc)] was studied by in situ hybridization. ALK-3 was nearly ubiquitously expressed throughout these stages of development, but was notably absent in the liver. In contrast, ALK-6 showed a more restricted expression pattern. ALK-6 mRNA was absent in early postimplantation embryos, was detected first in 9.5 days pc embryos, and persisted until 15.5 days pc. In midgestation embryos, ALK-6 transcripts were detected in mesenchymal precartilage condensations, premuscle masses, blood vessels, central nervous system, parts of the developing ear and eye, and epithelium. The expression in sites of developing cartilage and bone supports the idea that ALK-3 and -6 are receptors for BMPs in vivo. In addition, the expression of these genes in many soft tissues suggests broader functions for BMPs in embryogenesis.

Fibrodysplasia ossificans progressiva (FOP) is a rare disabling disease characterized by heterotopic ossification for which there is currently no treatment available. FOP has been linked recently to a heterozygous R206H mutation in the bone morphogenetic protein (BMP) type I receptor activin receptor-like kinase 2 (ALK2). Expression of the mutant ALK2-R206H receptor (FOP-ALK2) results in increased phosphorylation of the downstream Smad1 effector proteins and elevated basal BMP-dependent transcriptional reporter activity, indicating that FOP-ALK2 is constitutively active. FOP-ALK2-induced transcriptional activity could be blocked by overexpressing either of the inhibitory Smads, Smad6 or -7, or by treatment with the pharmacological BMP type I receptor inhibitor dorsomorphin. However, in contrast to wild-type ALK2, FOP-ALK2 is not inhibited by the negative regulator FKBP12. Mesenchymal cells expressing the FOP-ALK2 receptor are more sensitive to undergoing BMP-induced osteoblast differentiation and mineralization. In vivo bone formation was assessed by loading human mesenchymal stem cells (hMSCs) expressing the ALK2-R206H receptor onto calcium phosphate scaffolds and implantation in nude mice. Compared with control cells FOP-ALK2-expressing cells induced increased bone formation. Taken together, the R206H mutation in ALK2 confers constitutive activity to the mutant receptor, sensitizes mesenchymal cells to BMP-induced osteoblast differentiation, and stimulates new bone formation. We have generated an animal model that can be used as a stepping stone for preclinical studies aimed at inhibiting the heterotopic ossification characteristic of FOP.

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