Myostatin is a transforming growth factor ␤ family member that acts as a negative regulator of skeletal muscle growth. Myostatin circulates in the blood of adult mice in a noncovalently held complex with other proteins, including its propeptide, which maintain the C-terminal dimer in a latent, inactive state. This latent form of myostatin can be activated in vitro by treatment with acid; however, the mechanisms by which latent myostatin is activated in vivo are unknown. Here, we show that members of the bone morphogenetic protein-1͞tolloid (BMP-1͞TLD) family of metalloproteinases can cleave the myostatin propeptide in this complex and can thereby activate latent myostatin. Furthermore, we show that a mutant form of the propeptide resistant to cleavage by BMP-1͞TLD proteinases can cause significant increases in muscle mass when injected into adult mice. These findings raise the possibility that members of the BMP-1͞TLD family may be involved in activating latent myostatin in vivo and that molecules capable of inhibiting these proteinases may be effective agents for increasing muscle mass for both human therapeutic and agricultural applications.

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IL-17F and IL-17A are members of the IL-17 pro-inflammatory cytokine family. IL-17A has been implicated in the pathogenesis of autoimmune diseases. IL-17F is a disulfidelinked dimer that contains a cysteine-knot motif. We hypothesized that IL-17F and IL-17A could form a heterodimer due to their sequence homology and overlapping pattern of expression. We evaluated the structure of recombinant IL-17F and IL-17A proteins, as well as that of natural IL-17F and IL-17A derived from activated human CD4؉ T cells, by enzyme-linked immunosorbent assay, immunoprecipitation followed by Western blotting, and mass spectrometry. We find that both IL-17F and IL-17A can form both homodimeric and heterodimeric proteins when expressed in a recombinant system, and that all forms of the recombinant proteins have in vitro functional activity. Furthermore, we find that in addition to the homodimers of IL-17F and IL-17A, activated human CD4؉ T cells also produce the IL-17F/IL-17A heterodimer. These data suggest that the IL-17F/IL-17A heterodimer may contribute to the T cell-mediated immune responses.Interleukins 17F 3 and 17A (IL-17A) are closely related members of the IL-17 cytokine family, and share 50% amino acid identity. Studies in the mouse have identified Th17 cells as a distinct CD4ϩ T cell lineage that is defined by the production of IL-17F and IL-17A (1-7). IL-6 and transforming growth factor-␤ (TGF-␤) are required for the differentiation of naïve CD4ϩ T cells to Th17 cells (1,8), which are maintained in the presence of IL-23 and IL-1␤. Conversely, IL-4 and interferon-␥ can inhibit the development of Th17 cells (9, 10). Th17 cells have been implicated in the pathology of mouse autoimmune disease models (2).Expression of IL-17F and IL-17A has been detected in activated human peripheral blood lymphocytes. It has been shown by reverse transcriptase-PCR experiments that the cytokines are expressed in activated human CD4ϩ T cells (11,12). Expression of IL-17F and IL-17A has also been observed in tissue samples from various autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, psoriasis, inflammatory bowel disease, and asthma (2, 3, 13-22).The crystal structure of IL-17F has been solved and shows that the protein forms a disulfide-linked dimeric glycoprotein (23). IL-17A is also a disulfide-linked homodimeric glycoprotein (24), although crystal structure or data defining the precise subunit interactions are lacking. The IL-17F homodimer includes a classical cysteine knot motif, which is found in the TGF-␤, bone morphogenetic protein, and nerve growth factor superfamilies (25, 26). One difference in the cysteine knot motif of IL-17F compared with the other known cysteine knot protein families is that it only utilizes four cysteines instead of the classical six cysteines to form the knot.There have been reports that some of the cysteine knot family members can exist as heterodimers in vivo. TGF-␤1.2 and -␤2.3 were identified in bovine bone extracts, whereas inhibin and activin AB have been found in gonadal fluids (27...

IL-17A and IL-17F, produced by the Th17 CD4+ T cell lineage, have been linked to a variety of inflammatory and autoimmune conditions. We recently reported that activated human CD4+ T cells produce not only IL-17A and IL-17F homodimers but also an IL-17F/IL-17A heterodimeric cytokine. All three cytokines can induce chemokine secretion from bronchial epithelial cells, albeit with different potencies. In this study, we used small interfering RNA and Abs to IL-17RA and IL-17RC to demonstrate that heterodimeric IL-17F/IL-17A cytokine activity is dependent on the IL-17RA/IL-17RC receptor complex. Interestingly, surface plasmon resonance studies indicate that the three cytokines bind to IL-17RC with comparable affinities, whereas they bind to IL-17RA with different affinities. Thus, we evaluated the effect of the soluble receptors on cytokine activity and we find that soluble receptors exhibit preferential cytokine blockade. IL-17A activity is inhibited by IL-17RA, IL-17F is inhibited by IL-17RC, and a combination of soluble IL-17RA/IL-17RC receptors is required for inhibition of the IL-17F/IL-17A activity. Altogether, these results indicate that human IL-17F/IL-17A cytokine can bind and signal through the same receptor complex as human IL-17F and IL-17A. However, the distinct affinities of the receptor components for IL-17A, IL-17F, and IL-17F/IL-17A heterodimer can be exploited to differentially affect the activity of these cytokines.