Thomas B. Thompson

Structures of an ActRIIB:activin A complex reveal a novel binding mode for TGF‐β ligand:receptor interactions

The TGF‐β superfamily of ligands and receptors stimulate cellular events in diverse processes ranging from cell fate specification in development to immune suppression. Activins define a major subgroup of TGF‐β ligands that regulate cellular differentiation, proliferation, activation and apoptosis. Activins signal through complexes formed with type I and type II serine/threonine kinase receptors. We have solved the crystal structure of activin A bound to the extracellular domain of a type II receptor, ActRIIB, revealing the details of this interaction. ActRIIB binds to the outer edges of the activin finger regions, with the two receptors juxtaposed in close proximity, in a mode that differs from TGF‐β3 binding to type II receptors. The dimeric activin A structure differs from other known TGF‐β ligand structures, adopting a compact folded‐back conformation. The crystal structure of the complex is consistent with recruitment of two type I receptors into a close packed arrangement at the cell surface and suggests that diversity in the conformational arrangements of TGF‐β ligand dimers could influence cellular signaling processes.

The Structure of Apolipoprotein A-I in High Density Lipoproteins

Not long ago, high density lipoproteins (HDL)2 were second class citizens with regard to therapeutic strategies for lowering the risk of atherosclerosis and coronary artery disease (CAD). To date, most successful approaches have focused on the better understood pathways of cholesterol synthesis and low density lipoprotein (LDL) production, the “forward” cholesterol transport pathway. For example, the statin class of cholesterol synthesis inhibitors significantly reduces LDL levels resulting in a less atherogenic plasma lipoprotein profile. However, the relatively modest improvements in mortality conferred by these drugs suggest that other factors also play significant roles in defining CAD risk. The recent discoveries of HDL-interacting cell surface proteins such as scavenger receptor BI (SR-BI) and ATP-binding cassette transporters A1 (ABCA1) and G1 (for recent reviews see Refs. 1 and 2) have helped define the steps of reverse cholesterol transport (RCT), i.e. the movement of cholesterol from the periphery to the liver for catabolism (3, 4). Additionally, there is growing evidence that HDL anti-inflammatory properties may contribute significant protective effects (5), apparently via specific cell signaling pathways (6). These discoveries have fueled a new interest in HDL as a target for CAD treatment (7). Unfortunately, a complete understanding ofHDL function has been hampered by a lack of information on its structure and the molecular basis of its interactions with other proteins. This review summarizes the latest efforts in understanding the structure of the defining protein component of HDL, apoA-I, in the various stages of the RCT pathway.

Growth differentiation factor 9:bone morphogenetic protein 15 heterodimers are potent regulators of ovarian functions

Significance Although genetic studies have uncovered critical functions of GDF9 and BMP15 in female reproduction, many genetic and physiologic data for these ligands remain perplexing. Here we establish that mouse and human GDF9:BMP15 heterodimers are the most biopotent regulators of ovarian granulosa cell functions. Moreover, GDF9:BMP15 heterodimers require a unique signaling complex that includes the type 2 receptor BMPR2, an ALK4/5/7 type 1 kinase receptor, and an ALK6 type 1 co-receptor. GDF9:BMP15 binding to this complex stimulates phosphorylation of SMAD2/3. Our findings explain intraspecies and interspecies functions of these oocyte-synthesized proteins and have key implications for the regulation of female fertility. The TGF-β superfamily is the largest family of secreted proteins in mammals, and members of the TGF-β family are involved in most developmental and physiological processes. Growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP15), oocyte-secreted paralogs of the TGF-β superfamily, have been shown genetically to control ovarian physiology. Although previous studies found that GDF9 and BMP15 homodimers can modulate ovarian pathways in vitro, the functional species-specific significance of GDF9:BMP15 heterodimers remained unresolved. Therefore, we engineered and produced purified recombinant mouse and human GDF9 and BMP15 homodimers and GDF9:BMP15 heterodimers to compare their molecular characteristics and physiological functions. In mouse granulosa cell and cumulus cell expansion assays, mouse GDF9 and human BMP15 homodimers can up-regulate cumulus expansion-related genes (Ptx3, Has2, and Ptgs2) and promote cumulus expansion in vitro, whereas mouse BMP15 and human GDF9 homodimers are essentially inactive. However, we discovered that mouse GDF9:BMP15 heterodimer is ∼10- to 30-fold more biopotent than mouse GDF9 homodimer, and human GDF9:BMP15 heterodimer is ∼1,000- to 3,000-fold more bioactive than human BMP15 homodimer. We also demonstrate that the heterodimers require the kinase activities of ALK4/5/7 and BMPR2 to activate SMAD2/3 but unexpectedly need ALK6 as a coreceptor in the signaling complex in granulosa cells. Our findings that GDF9:BMP15 heterodimers are the most bioactive ligands in mice and humans compared with homodimers explain many puzzling genetic and physiological data generated during the last two decades and have important implications for improving female fertility in mammals.

The structure of myostatin:follistatin 288: insights into receptor utilization and heparin binding.

Myostatin is a member of the transforming growth factor‐β (TGF‐β) family and a strong negative regulator of muscle growth. Here, we present the crystal structure of myostatin in complex with the antagonist follistatin 288 (Fst288). We find that the prehelix region of myostatin very closely resembles that of TGF‐β class members and that this region alone can be swapped into activin A to confer signalling through the non‐canonical type I receptor Alk5. Furthermore, the N‐terminal domain of Fst288 undergoes conformational rearrangements to bind myostatin and likely acts as a site of specificity for the antagonist. In addition, a unique continuous electropositive surface is created when myostatin binds Fst288, which significantly increases the affinity for heparin. This translates into stronger interactions with the cell surface and enhanced myostatin degradation in the presence of either Fst288 or Fst315. Overall, we have identified several characteristics unique to myostatin that will be paramount to the rational design of myostatin inhibitors that could be used in the treatment of muscle‐wasting disorders.

Circulating Growth Differentiation Factor 11/8 Levels Decline With Age

RATIONALE Growth differentiation factor 11 (GDF11) and GDF8 are members of the transforming growth factor-β superfamily sharing 89% protein sequence homology. We have previously shown that circulating GDF11 levels decrease with age in mice. However, a recent study by Egerman et al reported that GDF11/8 levels increase with age in mouse serum. OBJECTIVE Here, we clarify the direction of change of circulating GDF11/8 levels with age and investigate the effects of GDF11 administration on the murine heart. METHODS AND RESULTS We validated our previous finding that circulating levels of GDF11/8 decline with age in mice, rats, horses, and sheep. Furthermore, we showed by Western analysis that the apparent age-dependent increase in GDF11 levels, as reported by Egerman et al, is attributable to cross-reactivity of the anti-GDF11 antibody with immunoglobulin, which is known to increase with age. GDF11 administration in mice rapidly activated SMAD2 and SMAD3 signaling in myocardium in vivo and decreased cardiac mass in both young (2-month-old) and old (22-month-old) mice in a dose-dependent manner after only 9 days. CONCLUSIONS Our study confirms an age-dependent decline in serum GDF11/8 levels in multiple mammalian species and that exogenous GDF11 rapidly activates SMAD signaling and reduces cardiomyocyte size. Unraveling the molecular basis for the age-dependent decline in GDF11/8 could yield insight into age-dependent cardiac pathologies.

Beta A versus beta B: is it merely a matter of expression?

Activins are members of the transforming growth factor (TGF) beta (beta) superfamily of proteins that function in a wide array of physiological processes. Like other TGFbeta ligands, activins are biologically active as dimers. An activin molecule is comprised of two beta-subunits, of which four isoforms have been identified: betaA, betaB, betaC, and betaE. The most widely studied activins to date are activin A (betaA/betaA), activin B (betaB/betaB), and activin AB (betaA/betaB). Inhibin is a naturally occurring activin antagonist that consists of an alpha-subunit disulfide-linked to one of the activin beta-subunits, producing inhibin A (alpha/betaA), or inhibin B (alpha/betaB). The development of assays distinguishing between different forms of activins and inhibins, along with knock-in and knock-out models, have provided evidence that the betaA- and betaB-subunits have independent and separate roles physiologically. Additionally, evaluation of ligand-receptor interactions indicates significant differences in receptor affinity between activin isoforms, as well as between inhibin isoforms. In this review we explore the differences between activin/inhibin betaA- and betaB-subunits, including expression patterns, binding properties, and the specific structural aspects of each. From the growing pool of knowledge regarding activins and inhibins, the emerging data support the hypothesis that betaA- and betaB-subunits are functionally differently.

The Structure of FSTL3·Activin A Complex DIFFERENTIAL BINDING OF N-TERMINAL DOMAINS INFLUENCES FOLLISTATIN-TYPE ANTAGONIST SPECIFICITY

Transforming growth factor β family ligands are neutralized by a number of structurally divergent antagonists. Follistatin-type antagonists, which include splice variants of follistatin (FS288 and FS315) and follistatin-like 3 (FSTL3), have high affinity for activin A but differ in their affinity for other ligands, particularly bone morphogenetic proteins. To understand the structural basis for ligand specificity within FS-type antagonists, we determined the x-ray structure of activin A in complex with FSTL3 to a resolution of 2.5 Å. Similar to the previously resolved FS·activin A structures, the ligand is encircled by two antagonist molecules blocking all ligand receptor-binding sites. Recently, the significance of the FS N-terminal domain interaction at the ligand type I receptor site has been questioned; however, our data show that for FSTL3, the N-terminal domain forms a more intimate contact with activin A, implying that this interaction is stronger than that for FS. Furthermore, binding studies revealed that replacing the FSTL3 N-terminal domain with the corresponding FS domain considerably lowers activin A affinity. Therefore, both structural and biochemical evidence support a significant interaction of the N-terminal domain of FSTL3 with activin A. In addition, structural comparisons with bone morphogenetic proteins suggest that the interface where the N-terminal domain binds may be the key site for determining FS-type antagonist specificity.

Structure of Myostatin·Follistatin-like 3 N-TERMINAL DOMAINS OF FOLLISTATIN-TYPE MOLECULES EXHIBIT ALTERNATE MODES OF BINDING

Background: Myostatin is a strong inhibitor of muscle growth and a therapeutic target for the treatment of muscle wasting. Results: Follistatin-like 3, a myostatin inhibitor, interacts uniquely with myostatin as compared with other ligands through its N-terminal domain. Conclusion: The N-terminal domains of follistatin-type molecules may be specificity determinants in ligand binding. Significance: Follistatin-type molecules form unique, specific interactions with different TGF-β family ligands. TGF-β family ligands are involved in a variety of critical physiological processes. For instance, the TGF-β ligand myostatin is a staunch negative regulator of muscle growth and a therapeutic target for muscle-wasting disorders. Therefore, it is important to understand the molecular mechanisms of TGF-β family regulation. One form of regulation is through inhibition by extracellular antagonists such as the follistatin (Fst)-type proteins. Myostatin is tightly controlled by Fst-like 3 (Fstl3), which is the only Fst-type molecule that has been identified in the serum bound to myostatin. Here, we present the crystal structure of myostatin in complex with Fstl3. The structure reveals that the N-terminal domain (ND) of Fstl3 interacts uniquely with myostatin as compared with activin A, because it utilizes different surfaces on the ligand. This results in conformational differences in the ND of Fstl3 that alter its position in the type I receptor-binding site of the ligand. We also show that single point mutations in the ND of Fstl3 are detrimental to ligand binding, whereas corresponding mutations in Fst have little effect. Overall, we have shown that the NDs of Fst-type molecules exhibit distinctive modes of ligand binding, which may affect overall affinity of ligand·Fst-type protein complexes.

Structural and Functional Analysis of Tetracenomycin F2 Cyclase from Streptomyces glaucescens A TYPE II POLYKETIDE CYCLASE

Tetracenomycin F2 cyclase (tcmI gene product), catalyzes an aromatic rearrangement in the biosynthetic pathway for tetracenomycin C in Streptomyces glaucescens. The x-ray structure of this small enzyme has been determined to 1.9-Å resolution together with an analysis of site-directed mutants of potential catalytic residues. The protein exhibits a dimeric βαβ ferredoxin-like fold that utilizes strand swapping between subunits in its assembly. The fold is dominated by four strands of antiparallel sheet and a layer of α-helices, which creates a cavity that is proposed to be the active site. This type of secondary structural arrangement has been previously observed in polyketide monooxygenases and suggests an evolutionary relationship between enzymes that catalyze adjacent steps in these biosynthetic pathways. Mutational analysis of all of the obvious catalytic bases within the active site suggests that the enzyme functions to steer the chemical outcome of the cyclization rather than providing a specific catalytic group. Together, the structure and functional analysis provide insight into the structural framework necessary to perform the complex rearrangements catalyzed by this class of polyketide cyclases.

Structural Basis for a Functional Antagonist in the Transforming Growth Factor β Superfamily

Within the transforming growth factor β superfamily, the agonist-antagonist relationship between activin and inhibin is unique and critical to integrated reproductive function. Activin acts in the pituitary to stimulate follicle-stimulating hormone, and is antagonized by endocrine acting, gonadally derived inhibin. We have undertaken a mutational analysis of the activin βA subunit to determine the precise structural aspects that contribute to inhibin antagonism of activin. By substituting specific amino acid residues in the activin βA subunit with similarly aligned amino acids from the α subunit, we have pinpointed the residues required for activin receptor binding and activity, as well as for inhibin antagonism of activin through its receptors. Additionally, we have identified an activin mutant with a higher affinity for the activin type I receptor that provides structural evidence for the evolution of ligand-receptor interactions within the transforming growth factor β superfamily.