Regulation of anterior/posterior patterning of the axial skeleton by growth/differentiation factor 11

The Proprotein Convertases in Hypercholesterolemia and Cardiovascular Diseases: Emphasis on Proprotein Convertase Subtilisin/Kexin 9

The secretory proprotein convertase (PC) family comprises nine members, as follows: PC1/3, PC2, furin, PC4, PC5/6, paired basic amino acid cleaving enzyme 4, PC7, subtilisin kexin isozyme 1/site 1 protease (SKI-1/S1P), and PC subtilisin/kexin type 9 (PCSK9). The first seven PCs cleave their substrates at single/paired basic residues and exhibit specific and often essential functions during development and/or in adulthood. The essential SKI-1/S1P cleaves membrane-bound transcription factors at nonbasic residues. In contrast, PCSK9 cleaves itself once, and the secreted inactive protease drags the low-density lipoprotein receptors (LDLR) and very LDLR (VLDLR) to endosomal/lysosomal degradation. Inhibitory PCSK9 monoclonal antibodies are now prescribed to treat hypercholesterolemia. This review focuses on the implication of PCs in cardiovascular functions and diseases, with a major emphasis on PCSK9. We present a phylogeny of the PCs and the analysis of PCSK9 haplotypes in modern and archaic human species. The absence of PCSK9 in mice led to the discovery of a sex- and tissue-specific subcellular distribution of the LDLR and VLDLR. PCSK9 inhibition may have other applications because it reduces inflammation and sepsis in a LDLR-dependent manner. Our present understanding of the cellular mechanism(s) that enables PCSK9 to induce the degradation of receptors is reviewed, as well as the consequences of its key natural mutations. The PCSK9 ongoing clinical trials are reviewed. Finally, how the other PCs may impact cardiovascular disease and the metabolic syndrome, and become relevant targets, is discussed.

GDF11 Treatment Attenuates the Recovery of Skeletal Muscle Function After Injury in Older Rats

Loss of skeletal muscle mass and function results in loss of mobility for elderly patients. Novel therapies that can protect and/or restore muscle function during aging would have profound effects on the quality of life for this population. Growth differentiation factor 11 (GDF11) has been proposed as a "youthful" circulating factor that can restore cardiac, neural, and skeletal muscle functions in aging animals. However, conflicting data has been recently published that casts doubt on these assertions. We used a complex rat model of skeletal muscle injury that physiologically mimics injuries seen in patients; to investigate the ability of GDF11 and to enhance skeletal muscle regeneration after injury in older rats. Our data showed that GDF11 treatment resulted in a significant increase in tissue fibrosis, accompanied by attenuated functional recovery, as compared to animals treated with vehicle alone. GDF11 impaired the recovery of skeletal muscle function in older rats after injury.

Supt20 is required for development of the axial skeleton

Somitogenesis and subsequent axial skeletal development is regulated by the interaction of pathways that determine the periodicity of somite formation, rostrocaudal somite polarity and segment identity. Here we use a hypomorphic mutant mouse line to demonstrate that Supt20 (Suppressor of Ty20) is required for development of the axial skeleton. Supt20 hypomorphs display fusions of the ribs and vertebrae at lower thoracic levels along with anterior homeotic transformation of L1 to T14. These defects are preceded by reduction of the rostral somite and posterior shifts in Hox gene expression. While cycling of Notch target genes in the posterior presomitic mesoderm (PSM) appeared normal, expression of Lfng was reduced. In the anterior PSM, Mesp2 expression levels and cycling were unaffected; yet, expression of downstream targets such as Lfng, Ripply2, Mesp1 and Dll3 in the prospective rostral somite was reduced accompanied by expansion of caudal somite markers such as EphrinB2 and Hes7. Supt20 interacts with the Gcn5-containing SAGA histone acetylation complex. Gcn5 hypomorphic mutant embryos show similar defects in axial skeletal development preceded by posterior shift of Hoxc8 and Hoxc9 gene expression. We demonstrate that Gcn5 and Supt20 hypomorphs show similar defects in rostral-caudal somite patterning potentially suggesting shared mechanisms.

GDF11 Inhibits Bone Formation by Activating Smad2/3 in Bone Marrow Mesenchymal Stem Cells

Growth differentiation factor 11 (GDF11) is a member of the transforming growth factor-β superfamily. Recent studies confirmed that GDF11 plays an important role in regulating the regeneration of brain, skeletal muscle, and heart during aging; however, its role in bone metabolism remains unclear. Thus, the aim of this study was to determine the effects of GDF11 on bone metabolism, including bone formation and bone resorption, both in vitro and in vivo. Our results showed that GDF11 inhibited osteoblastic differentiation of bone marrow mesenchymal stem cells in vitro. Mechanistically, GDF11 repressed Runx2 expression by inducing SMAD2/3 phosphorylation during osteoblast differentiation. Moreover, intraperitoneal injection of GDF11 inhibited bone formation and accelerated age-related bone loss in mice. Our results also showed that GDF11 had no effect on osteoclast differentiation or bone resorption both in vitro and in vivo. These results provide a further rationale for the therapeutic targeting of GDF11 for the treatment of age-related osteoporosis.

A Genome-Wide Association Study Identifies Multiple Regions Associated with Head Size in Catfish

Skull morphology is fundamental to evolution and the biological adaptation of species to their environments. With aquaculture fish species, head size is also important for economic reasons because it has a direct impact on fillet yield. However, little is known about the underlying genetic basis of head size. Catfish is the primary aquaculture species in the United States. In this study, we performed a genome-wide association study using the catfish 250K SNP array with backcross hybrid catfish to map the QTL for head size (head length, head width, and head depth). One significantly associated region on linkage group (LG) 7 was identified for head length. In addition, LGs 7, 9, and 16 contain suggestively associated regions for head length. For head width, significantly associated regions were found on LG9, and additional suggestively associated regions were identified on LGs 5 and 7. No region was found associated with head depth. Head size genetic loci were mapped in catfish to genomic regions with candidate genes involved in bone development. Comparative analysis indicated that homologs of several candidate genes are also involved in skull morphology in various other species ranging from amphibian to mammalian species, suggesting possible evolutionary conservation of those genes in the control of skull morphologies.

GDF11 decreases bone mass by stimulating osteoclastogenesis and inhibiting osteoblast differentiation

Osteoporosis is an age-related disease that affects millions of people. Growth differentiation factor 11 (GDF11) is a secreted member of the transforming growth factor beta (TGF-β) superfamily. Deletion of Gdf11 has been shown to result in a skeletal anterior-posterior patterning disorder. Here we show a role for GDF11 in bone remodelling. GDF11 treatment leads to bone loss in both young and aged mice. GDF11 inhibits osteoblast differentiation and also stimulates RANKL-induced osteoclastogenesis through Smad2/3 and c-Fos-dependent induction of Nfatc1. Injection of GDF11 impairs bone regeneration in mice and blocking GDF11 function prevents oestrogen-deficiency-induced bone loss and ameliorates age-related osteoporosis. Our data demonstrate that GDF11 is a previously unrecognized regulator of bone remodelling and suggest that GDF11 is a potential target for treatment of osteoporosis.

Biochemistry and Biology of GDF11 and Myostatin: Similarities, Differences, and Questions for Future Investigation

Growth differentiation factor 11 (GDF11) and myostatin (or GDF8) are closely related members of the transforming growth factor β superfamily and are often perceived to serve similar or overlapping roles. Yet, despite commonalities in protein sequence, receptor utilization and signaling, accumulating evidence suggests that these 2 ligands can have distinct functions in many situations. GDF11 is essential for mammalian development and has been suggested to regulate aging of multiple tissues, whereas myostatin is a well-described negative regulator of postnatal skeletal and cardiac muscle mass and modulates metabolic processes. In this review, we discuss the biochemical regulation of GDF11 and myostatin and their functions in the heart, skeletal muscle, and brain. We also highlight recent clinical findings with respect to a potential role for GDF11 and/or myostatin in humans with heart disease. Finally, we address key outstanding questions related to GDF11 and myostatin dynamics and signaling during development, growth, and aging.

Is Growth Differentiation Factor 11 a Realistic Therapeutic for Aging-Dependent Muscle Defects?

This "Controversies in Cardiovascular Research" article evaluates the evidence for and against the hypothesis that the circulating blood level of growth differentiation factor 11 (GDF11) decreases in old age and that restoring normal GDF11 levels in old animals rejuvenates their skeletal muscle and reverses pathological cardiac hypertrophy and cardiac dysfunction. Studies supporting the original GDF11 hypothesis in skeletal and cardiac muscle have not been validated by several independent groups. These new studies have either found no effects of restoring normal GDF11 levels on cardiac structure and function or have shown that increasing GDF11 or its closely related family member growth differentiation factor 8 actually impairs skeletal muscle repair in old animals. One possible explanation for what seems to be mutually exclusive findings is that the original reagent used to measure GDF11 levels also detected many other molecules so that age-dependent changes in GDF11 are still not well known. The more important issue is whether increasing blood [GDF11] repairs old skeletal muscle and reverses age-related cardiac pathologies. There are substantial new and existing data showing that GDF8/11 can exacerbate rather than rejuvenate skeletal muscle injury in old animals. There is also new evidence disputing the idea that there is pathological hypertrophy in old C57bl6 mice and that GDF11 therapy can reverse cardiac pathologies. Finally, high [GDF11] causes reductions in body and heart weight in both young and old animals, suggestive of a cachexia effect. Our conclusion is that elevating blood levels of GDF11 in the aged might cause more harm than good.

Of Mice and Snakes: A Tail of Oct4

The vertebrate axial skeleton comprises regions of specialized vertebrae, which vary in length between lineages. Aires et al. (2016) uncover a key role for Oct4 in determining trunk length in mice. Additionally, a heterochronic shift in Oct4 expression may underlie the extreme elongation of the trunk in snakes.

Relationship of serum GDF11 levels with bone mineral density and bone turnover markers in postmenopausal Chinese women

Growth differentiation factor 11 (GDF11) is an important circulating factor that regulates aging. However, the role of GDF11 in bone metabolism remains unclear. The present study was undertaken to investigate the relationship between serum GDF11 level, bone mass, and bone turnover markers in postmenopausal Chinese women. Serum GDF11 level, bone turnover biochemical markers, and bone mineral density (BMD) were determined in 169 postmenopausal Chinese women (47-78 years old). GDF11 serum levels increased with aging. There were negative correlations between GDF11 and BMD at the various skeletal sites. After adjusting for age and body mass index (BMI), the correlations remained statistically significant. In the multiple linear stepwise regression analysis, age or years since menopause, BMI, GDF11, and estradiol were independent predictors of BMD. A significant negative correlation between GDF11 and bone alkaline phosphatase (BAP) was identified and remained significant after adjusting for age and BMI. No significant correlation was noted between cross-linked N-telopeptides of type I collagen (NTX) and GDF11. In conclusion, GDF11 is an independent negative predictor of BMD and correlates with a biomarker of bone formation, BAP, in postmenopausal Chinese women. GDF11 potentially exerts a negative effect on bone mass by regulating bone formation.

Quantification of GDF11 and Myostatin in Human Aging and Cardiovascular Disease

Growth and differentiation factor 11 (GDF11) is a transforming growth factor β superfamily member with a controversial role in aging processes. We have developed a highly specific LC-MS/MS assay to quantify GDF11, resolved from its homolog, myostatin (MSTN), based on unique amino acid sequence features. Here, we demonstrate that MSTN, but not GDF11, declines in healthy men throughout aging. Neither GDF11 nor MSTN levels differ as a function of age in healthy women. In an independent cohort of older adults with severe aortic stenosis, we show that individuals with higher GDF11 were more likely to be frail and have diabetes or prior cardiac conditions. Following valve replacement surgery, higher GDF11 at surgical baseline was associated with rehospitalization and multiple adverse events. Cumulatively, our results show that GDF11 levels do not decline throughout aging but are associated with comorbidity, frailty, and greater operative risk in older adults with cardiovascular disease.

Serum Levels of Growth Differentiation Factor 11 Are Independently Associated with Low Hemoglobin Values in Hemodialysis Patients

Circulating levels of growth differentiation factor 11 (GDF11) have been shown to decrease with age in several mammalian species, and supplementation of GDF11 by heterochronic parabiosis or systemic administration reverses age-related organ damage. However, there is some controversy about the pathophysiological role of GDF11 in aging-associated organ damage. Since aging process is accelerated in uremia, we compared serum levels of GDF11 in hemodialysis (HD) patients with those in age-matched healthy controls, and then determined the independent clinical correlates of GDF11 in HD subjects. Sixty-two maintenance HD patients (34 male and 28 female; mean age, 52.6 years; mean duration of HD, 7.7 months) were enrolled in the present study. Twenty-nine age-matched subjects were used as a control. GDF11 was measured by a commercially available enzyme-linked immunosorbent assay kit. Serum GDF11 levels in HD patients were significantly higher than those in controls (9.4 ± 5.1 pg/mL vs. 7.3 ± 5.9 pg/mL). A statistical significance was demonstrated between GDF11 and hemoglobin (inversely). Multiple stepwise regression analysis revealed that hemoglobin (p < 0.001) was a sole independent correlate of GDF11 levels in HD patients (R² = 0.168). Our present study suggests that kinetics and regulation of circulating GDF11 may differ between normal physiological aging process and accelerated pathological aging conditions, such as uremia. Given that GDF11 has been shown to inhibit erythroid maturation in mice, elevation of GDF11 levels may be involved in erythropoietin-resistant anemia in HD patients.

Bone Morphogenetic Proteins

Bone morphogenetic proteins (BMPs), originally identified as osteoinductive components in extracts derived from bone, are now known to play important roles in a wide array of processes during formation and maintenance of various organs including bone, cartilage, muscle, kidney, and blood vessels. BMPs and the related "growth and differentiation factors" (GDFs) are members of the transforming growth factor β (TGF-β) family, and transduce their signals through type I and type II serine-threonine kinase receptors and their intracellular downstream effectors, including Smad proteins. Furthermore, BMP signals are finely tuned by various agonists and antagonists. Because deregulation of the BMP activity at multiple steps in signal transduction is linked to a wide variety of human diseases, therapeutic use of activators and inhibitors of BMP signaling will provide potential avenues for the treatment of the human disorders that are caused by hypo- and hyperactivation of BMP signals, respectively.

Developmental Mechanism of Limb Field Specification along the Anterior-Posterior Axis during Vertebrate Evolution

In gnathostomes, limb buds arise from the lateral plate mesoderm at discrete positions along the body axis. Specification of these limb-forming fields can be subdivided into several steps. The lateral plate mesoderm is regionalized into the anterior lateral plate mesoderm (ALPM; cardiac mesoderm) and the posterior lateral plate mesoderm (PLPM). Subsequently, Hox genes appear in a nested fashion in the PLPM and provide positional information along the body axis. The lateral plate mesoderm then splits into the somatic and splanchnic layers. In the somatic layer of the PLPM, the expression of limb initiation genes appears in the limb-forming region, leading to limb bud initiation. Furthermore, past and current work in limbless amphioxus and lampreys suggests that evolutionary changes in developmental programs occurred during the acquisition of paired fins during vertebrate evolution. This review presents these recent advances and discusses the mechanisms of limb field specification during development and evolution, with a focus on the role of Hox genes in this process.

New insights into iron regulation and erythropoiesis

PURPOSE OF REVIEW

Iron homeostasis and erythropoiesis regulate each other to ensure optimal delivery of oxygen and iron to cells and tissues. Defining the mechanisms of this crosstalk is important for understanding the pathogenesis of common conditions associated with disordered iron metabolism and erythropoiesis.

RECENT FINDINGS

Stress erythropoiesis causes suppression of hepcidin to increase iron availability for hemoglobin synthesis. The erythroid hormone erythroferrone (ERFE) was identified as the mediator of this process. ERFE and additional candidates (TWSG1 and GDF15) may also mediate hepcidin suppression in ineffective erythropoiesis. Several mechanisms by which iron regulates erythropoiesis were also recently identified. Iron deficiency suppresses erythropoietin production via the IRP1-HIF2α axis to prevent excessive iron usage by erythropoiesis during systemic iron restriction. Iron restriction also directly impairs erythroid maturation by inhibiting aconitase, and this can be reversed by the administration of the aconitase product isocitrate. Another novel target is GDF11, which is thought to autoinhibit erythroid maturation. GDF11 traps show promising pharmacologic activity in models of both ineffective erythropoiesis and iron-restricted anemia.

SUMMARY

This review summarizes exciting advances in understanding the mechanisms of iron and erythropoietic regulation, and development of novel therapeutic tools for disorders resulting from dysregulation of iron metabolism or erythropoiesis.

Position-dependent plasticity of distinct progenitor types in the primitive streak

The rostrocaudal (head-to-tail) axis is supplied by populations of progenitors at the caudal end of the embryo. Despite recent advances characterising one of these populations, the neuromesodermal progenitors, their nature and relationship to other populations remains unclear. Here we show that neuromesodermal progenitors are a single Sox2^lowT^low entity whose choice of neural or mesodermal fate is dictated by their position in the progenitor region. The choice of mesoderm fate is Wnt/β-catenin dependent. Wnt/β-catenin signalling is also required for a previously unrecognised phase of progenitor expansion during mid-trunk formation. Lateral/ventral mesoderm progenitors represent a distinct committed state that is unable to differentiate to neural fates, even upon overexpression of the neural transcription factor Sox2. They do not require Wnt/β-catenin signalling for mesoderm differentiation. This information aids the correct interpretation of in vivo genetic studies and the development of in vitro protocols for generating physiologically-relevant cell populations of clinical interest.

DOI:http://dx.doi.org/10.7554/eLife.10042.001

Our bodies, like those of all animals with a backbone, form during embryo development in a head-to-tail sequence. This process is fuelled by populations of proliferating cells called progenitor cells, which are found in an early embryonic structure called the primitive streak, and later at the tail-end of the embryo.

One of these populations – known as the neuromesodermal progenitors (or NMPs) – produces the animal’s spinal cord, muscle and bone tissue. However, it is not clear how this cell population is maintained or what triggers these cells to specialise into the correct cell type. It is even unclear whether NMPs are a single cell type or a collection of several types of progenitor, each with a slightly different propensity to make spinal cord or muscle and bone. Answering these questions could inform the future development of cell-replacement therapies for conditions such as spinal injuries.

Wymeersch et al. used a range of techniques to identify, map the fate, and assess the developmental potential of progenitors in the primitive streak. This revealed fine-grained differences in the fates adopted by cells in the progenitor region. However, these regional differences were found to result from the progenitor cells’ extensive ability to respond to signals they receive from their environment, rather than being hard-wired into the progenitor cells. In fact, Wymeersch et al. detected only two distinct cell types: the NMPs and a new cell population termed lateral/paraxial mesoderm progenitors (or LPMPs), which, unlike NMPs, do not form nerve cells.

Further experiments investigated the molecular signals present in the environment of these progenitors that help to decide their fate. NMPs respond to an important developmental signal, called Wnt, by adopting a so-called mesoderm fate. This signal also induces NMPs to undergo a previously unknown phase of proliferation during the formation of the animal’s body. LPMPs, on the other hand, do not require Wnt to form mesoderm.

These findings show that studies with embryos can identify new progenitor populations that might be clinically relevant, and reveal new ways in which a cell’s environment inside an embryo can determine its fate.

DOI:http://dx.doi.org/10.7554/eLife.10042.002

Generating Diverse Spinal Motor Neuron Subtypes from Human Pluripotent Stem Cells

Resolving the mechanisms underlying human neuronal diversification remains a major challenge in developmental and applied neurobiology. Motor neurons (MNs) represent a diverse pool of neuronal subtypes exhibiting differential vulnerability in different human neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). The ability to predictably manipulate MN subtype lineage restriction from human pluripotent stem cells (PSCs) will form the essential basis to establishing accurate, clinically relevant in vitro disease models. I first overview motor neuron developmental biology to provide some context for reviewing recent studies interrogating pathways that influence the generation of MN diversity. I conclude that motor neurogenesis from PSCs provides a powerful reductionist model system to gain insight into the developmental logic of MN subtype diversification and serves more broadly as a leading exemplar of potential strategies to resolve the molecular basis of neuronal subclass differentiation within the nervous system. These studies will in turn permit greater mechanistic understanding of differential MN subtype vulnerability using in vitro human disease models.

Common mechanisms in development and disease: BMP signaling in craniofacial development

BMP signaling is one of the key pathways regulating craniofacial development. It is involved in the early patterning of the head, the development of cranial neural crest cells, and facial patterning. It regulates development of its mineralized structures, such as cranial bones, maxilla, mandible, palate, and teeth. Targeted mutations in the mouse have been instrumental to delineate the functional involvement of this signaling network in different aspects of craniofacial development. Gene polymorphisms and mutations in BMP pathway genes have been associated with various non-syndromic and syndromic human craniofacial malformations. The identification of intricate cellular interactions and underlying molecular pathways illustrate the importance of local fine-regulation of Bmp signaling to control proliferation, apoptosis, epithelial-mesenchymal interactions, and stem/progenitor differentiation during craniofacial development. Thus, BMP signaling contributes both to shape and functionality of our facial features. BMP signaling also regulates postnatal craniofacial growth and is associated with dental structures life-long. A more detailed understanding of BMP function in growth, homeostasis, and repair of postnatal craniofacial tissues will contribute to our ability to rationally manipulate this signaling network in the context of tissue engineering.

Next generation limb development and evolution: old questions, new perspectives

The molecular analysis of limb bud development in vertebrates continues to fuel our understanding of the gene regulatory networks that orchestrate the patterning, proliferation and differentiation of embryonic progenitor cells. In recent years, systems biology approaches have moved our understanding of the molecular control of limb organogenesis to the next level by incorporating next generation ‘omics’ approaches, analyses of chromatin architecture, enhancer-promoter interactions and gene network simulations based on quantitative datasets into experimental analyses. This Review focuses on the insights these studies have given into the gene regulatory networks that govern limb development and into the fin-to-limb transition and digit reductions that occurred during the evolutionary diversification of tetrapod limbs.

The central role of muscle stem cells in regenerative failure with aging

Skeletal muscle mass, function, and repair capacity all progressively decline with aging, restricting mobility, voluntary function, and quality of life. Skeletal muscle repair is facilitated by a population of dedicated muscle stem cells (MuSCs), also known as satellite cells, that reside in anatomically defined niches within muscle tissues. In adult tissues, MuSCs are retained in a quiescent state until they are primed to regenerate damaged muscle through cycles of self-renewal divisions. With aging, muscle tissue homeostasis is progressively disrupted and the ability of MuSCs to repair injured muscle markedly declines. Until recently, this decline has been largely attributed to extrinsic age-related alterations in the microenvironment to which MuSCs are exposed. However, as highlighted in this Perspective, recent reports show that MuSCs also progressively undergo cell-intrinsic alterations that profoundly affect stem cell regenerative function with aging. A more comprehensive understanding of the interplay of stem cell-intrinsic and extrinsic factors will set the stage for improving cell therapies capable of restoring tissue homeostasis and enhancing muscle repair in the aged.

A study of vertebra number in pigs confirms the association of vertnin and reveals additional QTL

Background

Formation of the vertebral column is a critical developmental stage in mammals. The strict control of this process has resulted in little variation in number of vertebrae across mammalian species and no variation within most mammalian species. The pig is quite unique as considerable variation exists in number of thoracic vertebrae as well as number of lumbar vertebrae. At least two genes have been identified that affect number of vertebrae in pigs yet considerable genetic variation still exists. Therefore, a genome-wide association (GWA) analysis was conducted to identify additional genomic regions that affect this trait.

Results

A total of 1883 animals were phenotyped for the number of ribs and thoracolumbar vertebrae as well as successfully genotyped with the Illumina Porcine SNP60 BeadChip. After data editing, 41,148 SNP markers were included in the GWA analysis. These animals were also phenotyped for kyphosis. Fifty-three 1 Mb windows each explained at least 1.0 % of the genomic variation for vertebrae counts while 16 regions were significant for kyphosis. Vertnin genotype significantly affected vertebral counts as well. The region with the largest effect for number of lumbar vertebrae and thoracolumbar vertebrae were located over the Hox B gene cluster and the largest association for thoracic vertebrae number was over the Hox A gene cluster. Genetic markers in significant regions accounted for approximately 50 % of the genomic variation. Less genomic variation for kyphosis was described by QTL regions and no region was associated with kyphosis and vertebra counts.

Conclusions

The importance of the Hox gene families in vertebral development was highlighted as significant associations were detected over the A, B and C families. Further evaluation of these regions and characterization of variants within these genes will expand our knowledge on vertebral development using natural genetic variants segregating in commercial swine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-015-0286-9) contains supplementary material, which is available to authorized users.

Roles of GASP-1 and GDF-11 in Dental and Craniofacial Development

PURPOSE

Growth and differentiation factor-11 (GDF-11) is a TGF-β family member that plays important regulatory roles in development of multiple tissues which include axial skeletal patterning, palatal closure, and tooth formation. Proteins that have been identified as GDF-11 inhibitors include GDF-associated serum protein-1 (GASP-1) and GASP-2. Recently, we found that mice genetically engineered to lack both Gasp1 and Gdf11 have an increased frequency of cleft palate. The goal of this study was to investigate the roles of GDF-11 and its inhibitors, GASP-1 and GASP-2, during dental and craniofacial development and growth.

METHODS

Mouse genetic studies were used in this study. Homozygous knockout mice for Gasp1 (Gasp1^-/- ) and Gasp2 (Gasp2^-/- ) were viable and fertile, but Gdf11 homozygous knockout (Gdf11^-/- ) mice died within 24 hours after birth. The effect of either Gasp1 or Gasp2 deletion in Gdf11^-/- mice during embryogenesis was evaluated in Gasp1^-/-;Gdf11^-/- and Gasp2^-/-;Gdf11^-/- mouse embryos at 18.5 days post-coitum (E18.5). For the analysis of adult tissues, we used Gasp1^-/-;Gdf11^+/- and Gasp2^-/-;Gdf11^+/- mice to evaluate the potential haploinsufficiency of Gdf11 in Gasp1^-/- and Gasp2^-/- mice.

RESULTS

Although Gasp2 expression decreased after E10.5, Gasp1 expression was readily detected in various ectodermal tissues at E17.5, including hair follicles, epithelium in nasal cavity, retina, and developing tooth buds. Interestingly, Gasp1^-/- ;Gdf11^-/- mice had abnormal formation of lower incisors: tooth buds for lower incisors were under-developed or missing. Although Gdf11^+/- mice were viable and had mild transformations of the axial skeleton, no specific defects in the craniofacial development have been observed in Gdf11^+/- mice. However, loss of Gasp1 in Gdf11^+/- mice occasionally resulted in small and abnormally shaped auricles.

CONCLUSIONS

These findings suggest that both GASP-1 and GDF-11 play important roles in dental and craniofacial development both during embryogenesis and in adult tissues.

A Tale from TGF-β Superfamily for Thymus Ontogeny and Function

Multiple signaling pathways control every aspect of cell behavior, organ formation, and tissue homeostasis throughout the lifespan of any individual. This review takes an ontogenetic view focused on the large superfamily of TGF-β/bone morphogenetic protein ligands to address thymus morphogenesis and function in T cell differentiation. Recent findings on a role of GDF11 for reversing aging-related phenotypes are also discussed.

Anorectal malformation: the etiological factors

Anorectal malformation (ARM) is a congenital anomaly commonly encountered in pediatric surgery practice. Although surgical procedures correct the anatomical anomalies, the post-operative bowel function is not universally satisfactory. The etiology of ARM remains unclear. In this review, we summarize the current understanding of the genetic and epigenetic factors contributing to the pathogenesis of ARM, based on published animal models, human genetics and epidemiological researches. Appreciation of these factors may be helpful in the management of ARM in the future.

A review on the mechanisms of blood-flow restriction resistance training-induced muscle hypertrophy

It has traditionally been believed that resistance training can only induce muscle growth when the exercise intensity is greater than 65% of the 1-repetition maximum (RM). However, more recently, the use of low-intensity resistance exercise with blood-flow restriction (BFR) has challenged this theory and consistently shown that hypertrophic adaptations can be induced with much lower exercise intensities (<50% 1-RM). Despite the potent hypertrophic effects of BFR resistance training being demonstrated by numerous studies, the underlying mechanisms responsible for such effects are not well defined. Metabolic stress has been suggested to be a primary factor responsible, and this is theorised to activate numerous other mechanisms, all of which are thought to induce muscle growth via autocrine and/or paracrine actions. However, it is noteworthy that some of these mechanisms do not appear to be mediated to any great extent by metabolic stress but rather by mechanical tension (another primary factor of muscle hypertrophy). Given that the level of mechanical tension is typically low with BFR resistance exercise (<50% 1-RM), one may question the magnitude of involvement of these mechanisms aligned to the adaptations reported with BFR resistance training. However, despite the low level of mechanical tension, it is plausible that the effects induced by the primary factors (mechanical tension and metabolic stress) are, in fact, additive, which ultimately contributes to the adaptations seen with BFR resistance training. Exercise-induced mechanical tension and metabolic stress are theorised to signal a number of mechanisms for the induction of muscle growth, including increased fast-twitch fibre recruitment, mechanotransduction, muscle damage, systemic and localised hormone production, cell swelling, and the production of reactive oxygen species and its variants, including nitric oxide and heat shock proteins. However, the relative extent to which these specific mechanisms are induced by the primary factors with BFR resistance exercise, as well as their magnitude of involvement in BFR resistance training-induced muscle hypertrophy, requires further exploration.

Independent regulation of vertebral number and vertebral identity by microRNA-196 paralogs

The Hox genes play a central role in patterning the embryonic anterior-to-posterior axis. An important function of Hox activity in vertebrates is the specification of different vertebral morphologies, with an additional role in axis elongation emerging. The miR-196 family of microRNAs (miRNAs) are predicted to extensively target Hox 3' UTRs, although the full extent to which miR-196 regulates Hox expression dynamics and influences mammalian development remains to be elucidated. Here we used an extensive allelic series of mouse knockouts to show that the miR-196 family of miRNAs is essential both for properly patterning vertebral identity at different axial levels and for modulating the total number of vertebrae. All three miR-196 paralogs, 196a1, 196a2, and 196b, act redundantly to pattern the midthoracic region, whereas 196a2 and 196b have an additive role in controlling the number of rib-bearing vertebra and positioning of the sacrum. Independent of this, 196a1, 196a2, and 196b act redundantly to constrain total vertebral number. Loss of miR-196 leads to a collective up-regulation of numerous trunk Hox target genes with a concomitant delay in activation of caudal Hox genes, which are proposed to signal the end of axis extension. Additionally, we identified altered molecular signatures associated with the Wnt, Fgf, and Notch/segmentation pathways and demonstrate that miR-196 has the potential to regulate Wnt activity by multiple mechanisms. By feeding into, and thereby integrating, multiple genetic networks controlling vertebral number and identity, miR-196 is a critical player defining axial formulae.

Effect of variation in ovine WFIKKN2 on growth traits appears to be gender-dependent

WFIKKN2 may play a role in the regulation of muscle growth and development, but to date there have been no reports on the effect of variation in WFIKKN2 on growth and carcass traits in livestock. In this study, the effect of variation in ovine WFIKKN2 was investigated in 800 New Zealand Romney lambs (395 male and 405 female), with five previously described variants (A to E) being identified. Variation in ovine WFIKKN2 was not found to affect various growth traits in the female lambs, but the presence of variant B was associated (P < 0.05) with decreased birth weight, tailing weight, weaning weight and pre-weaning growth rate; and increased post-weaning growth rate in male lambs. In male lambs, the presence of variant B was associated (P < 0.05) with an increased shoulder yield and proportion shoulder yield. No associations with growth or carcass traits were detected for the presence (or absence) of the other variants. These results suggest that variation in ovine WFIKKN2 may have a differential effect on growth in male and female lambs, and hence that the gene may be expressed in, or act in, a gender-specific fashion.

Mechanisms of BMP-Receptor Interaction and Activation

Bone morphogenetic proteins (BMPs), together with the eponymous transforming growth factor (TGF) β and the Activins form the TGFβ superfamily of ligands. This protein family comprises more than 30 structurally highly related proteins, which determine formation, maintenance, and regeneration of tissues and organs. Their importance for the development of multicellular organisms is evident from their existence in all vertebrates as well as nonvertebrate animals. From their highly specific functions in vivo either a strict relation between a particular ligand and its cognate cellular receptor and/or a stringent regulation to define a distinct temperospatial expression pattern for the various ligands and receptor is expected. However, only a limited number of receptors are found to serve a large number of ligands thus implicating highly promiscuous ligand-receptor interactions instead. Since in tissues a multitude of ligands are often found, which signal via a highly overlapping set of receptors, this raises the question how such promiscuous interactions between different ligands and their receptors can generate concerted and highly specific cellular signals required during embryonic development and tissue homeostasis.

GDF11 Increases with Age and Inhibits Skeletal Muscle Regeneration

Age-related frailty may be due to decreased skeletal muscle regeneration. The role of TGF-β molecules myostatin and GDF11 in regeneration is unclear. Recent studies showed an age-related decrease in GDF11 and that GDF11 treatment improves muscle regeneration, which were contrary to prior studies. We now show that these recent claims are not reproducible and the reagents previously used to detect GDF11 are not GDF11 specific. We develop a GDF11-specific immunoassay and show a trend toward increased GDF11 levels in sera of aged rats and humans. GDF11 mRNA increases in rat muscle with age. Mechanistically, GDF11 and myostatin both induce SMAD2/3 phosphorylation, inhibit myoblast differentiation, and regulate identical downstream signaling. GDF11 significantly inhibited muscle regeneration and decreased satellite cell expansion in mice. Given early data in humans showing a trend for an age-related increase, GDF11 could be a target for pharmacologic blockade to treat age-related sarcopenia.

Strategies of Manipulating BMP Signaling in Microgravity to Prevent Bone Loss

Bone structure and function is shaped by gravity. Prolonged exposure to microgravity leads to 1-2% bone loss per month in crew members compared to 1% bone loss per year in postmenopausal women. Exercise countermeasures developed to date are ineffective in combating bone loss in microgravity. The search is on for alternate therapies to prevent bone loss in space. Microgravity is an ideal stimulus to understand bone interactions at different levels of organizations. Spaceflight experiments are limited by high costs and lack of opportunity. Ground-based microgravity analogs have proven to simulate biological responses in space. Mice experiments have given important signaling clues in microgravity-associated bone loss, but are restricted by numbers and human application. Cell-based systems provide initial clues to signaling changes; however, the information is simplistic and limited to the cell type. There is a need to integrate information at different levels and provide a complete picture which will help develop a unique strategy to prevent bone weakening. Limited exposure to simulated microgravity using random positioning machine induces proliferation and differentiation of bipotential murine oval liver stem cells. Bone morphogenetic proteins (BMPs) are the prototypal osteogenic signaling molecule with multitude of bone protective functions. In this chapter, we discuss the basic BMP structure, its significance in bone repair, and stem cell differentiation in microgravity. Based on the current information, we propose a model for BMP signaling in space. Development of new technologies may help osteoporosis patients, bedridden people, spinal injuries, or paralytic patients.

PCSK5 mutation in a patient with the VACTERL association

BACKGROUND

The VACTERL association is a typically sporadic, non-random collection of congenital anomalies that includes vertebral defects, anal atresia, cardiac defects, tracheoesophageal fistula with esophageal atresia, renal anomalies, and limb abnormalities. Although several chromosomal aberrations and gene mutations have been reported as disease-causative, these findings have been sparsely replicated to date.

CASE PRESENTATION

In the present study, whole exome sequencing of a case with the VACTERL association uncovered a novel frameshift mutation in the PCSK5 gene, which has been reported as one of the causative genes for the VACTERL association. Although this mutation appears potentially pathogenic in its functional aspects, it was also carried by the healthy father. Furthermore, a database survey revealed several other deleterious variants in the PCSK5 gene in the general population.

CONCLUSIONS

Further studies are necessary to clarify the etiological role of the PCSK5 mutation in the VACTERL association.

An allelic series of miR-17 ∼ 92-mutant mice uncovers functional specialization and cooperation among members of a microRNA polycistron

Polycistronic microRNA clusters are a common feature of vertebrate genomes. The coordinated expression of miRNAs belonging to different seed families from a single transcription unit suggests functional cooperation, but this hypothesis has not been experimentally tested. Here we report the characterization of an allelic series of genetically engineered mice harboring selective targeted deletions of individual components of miR-17~92. Our results demonstrate the co-existence of functional cooperation and specialization among members of this cluster, identify a novel function for the miR-17 seed family in controlling axial patterning in vertebrates, and show that loss of miR-19 selectively impairs Myc-driven tumorigenesis in two models of human cancer. By integrating phenotypic analysis and gene expression profiling, we provide a genome-wide view of how components of a polycistronic miRNA-cluster affect gene expression in vivo. The reagents and datasets reported here will accelerate exploration of the complex biological functions of this important miRNA cluster.

Alternative binding modes identified for growth and differentiation factor-associated serum protein (GASP) family antagonism of myostatin

Myostatin, a member of the TGF-β family of ligands, is a strong negative regulator of muscle growth. As such, it is a prime therapeutic target for muscle wasting disorders. Similar to other TGF-β family ligands, myostatin is neutralized by binding one of a number of structurally diverse antagonists. Included are the antagonists GASP-1 and GASP-2, which are unique in that they specifically antagonize myostatin. However, little is known from a structural standpoint describing the interactions of GASP antagonists with myostatin. Here, we present the First low resolution solution structure of myostatin-free and myostatin-bound states of GASP-1 and GASP-2. Our studies have revealed GASP-1, which is 100 times more potent than GASP-2, preferentially binds myostatin in an asymmetrical 1:1 complex, whereas GASP-2 binds in a symmetrical 2:1 complex. Additionally, C-terminal truncations of GASP-1 result in less potent myostatin inhibitors that form a 2:1 complex, suggesting that the C-terminal domains of GASP-1 are the primary mediators for asymmetric complex formation. Overall, this study provides a new perspective on TGF-β antagonism, where closely related antagonists can utilize different ligand-binding strategies.

Proteasome dysfunction induces muscle growth defects and protein aggregation

The ubiquitin–proteasome and autophagy–lysosome pathways are the two major routes of protein and organelle clearance. The role of the proteasome pathway in mammalian muscle has not been examined in vivo. In this study, we report that the muscle-specific deletion of a crucial proteasomal gene, Rpt3 (also known as Psmc4), resulted in profound muscle growth defects and a decrease in force production in mice. Specifically, developing muscles in conditional Rpt3-knockout animals showed dysregulated proteasomal activity. The autophagy pathway was upregulated, but the process of autophagosome formation was impaired. A microscopic analysis revealed the accumulation of basophilic inclusions and disorganization of the sarcomeres in young adult mice. Our results suggest that appropriate proteasomal activity is important for muscle growth and for maintaining myofiber integrity in collaboration with autophagy pathways. The deletion of a component of the proteasome complex contributed to myofiber degeneration and weakness in muscle disorders that are characterized by the accumulation of abnormal inclusions.

Growth differentiation factor 11 is an encephalic regionalizing factor in neural differentiated mouse embryonic stem cells

Background

The central nervous system has a complex structural organization and consists of different subdomains along the antero-posterior axis. However, questions remain about the molecular mechanisms leading to the regionalization of this organ. We used a previously developed methodology to identify the novel patterning role of GDF11, a TGF-β signaling factor.

Findings

Using an assay based on neural differentiated mouse embryonic stem cells, GDF11 is shown to induce diencephalic (posterior forebrain), mesencephalic (midbrain) and metencephalic (anterior hindbrain) fates at the expense of telencephalic (anterior forebrain) specification. GDF11 has not previously been implicated in the early patterning of the nervous system. In addition, inhibition of the TGF-β type I receptors Alk4, Alk5 and Alk7 by the pharmacological inhibitor SB431542 caused a strong anteriorization of the cells.

Conclusions

Our findings suggest that GDF11 is involved in the earliest steps of the brain patterning during neurogenesis in the vertebrate embryo and is shown to be a regionalizing factor of the regional fate in the developing brain. This regionalization is not a typical posteriorizing signal as seen with retinoic acid, FGF or BMP molecules. To our knowledge, this is the first time that GDF11 is implicated in the earliest steps of the patterning of the neural plate.

Bone Morphogenetic Protein (BMP) signaling in development and human diseases

Bone Morphogenetic Proteins (BMPs) are a group of signaling molecules that belongs to the Transforming Growth Factor-β (TGF-β) superfamily of proteins. Initially discovered for their ability to induce bone formation, BMPs are now known to play crucial roles in all organ systems. BMPs are important in embryogenesis and development, and also in maintenance of adult tissue homeostasis. Mouse knockout models of various components of the BMP signaling pathway result in embryonic lethality or marked defects, highlighting the essential functions of BMPs. In this review, we first outline the basic aspects of BMP signaling and then focus on genetically manipulated mouse knockout models that have helped elucidate the role of BMPs in development. A significant portion of this review is devoted to the prominent human pathologies associated with dysregulated BMP signaling.

A phase 1 study of ACE-536, a regulator of erythroid differentiation, in healthy volunteers

ACE-536, a recombinant protein containing a modified activin receptor type IIB, is being developed for the treatment of anemias caused by ineffective erythropoiesis, such as thalassemias and myelodysplastic syndromes. ACE-536 acts through a mechanism distinct from erythropoiesis-stimulating agents to promote late-stage erythroid differentiation by binding to transforming growth factor-β superfamily ligands and inhibiting signaling through transcription factors Smad 2/3. The goal of this Phase 1 study was to evaluate the safety, tolerability, pharmacokinetics, and pharmacodynamic effects of ascending dose levels of ACE-536 in healthy volunteers. Thirty-two postmenopausal women were randomized in sequential cohorts of eight subjects each to receive up to two doses of either ACE-536 (0.0625–0.25 mg/kg) or placebo (3:1 randomization) given subcutaneously every 2 weeks. Mean baseline age was 59.4 years, and hemoglobin was 13.2 g/dL. ACE-536 was well tolerated at dose levels up to 0.25 mg/kg over the 1-month treatment period. There were no serious or severe adverse events, nor clinically meaningful changes in safety laboratory measures or vital signs. Mean ACE-536 AUC_0–14d and C_max increased proportionally after first dose; mean t_½ was 15–16 days. Dose-dependent increases in hemoglobin concentration were observed, beginning 7 days after initiation of treatment and maintained for several weeks following treatment. The proportion of subjects with a hemoglobin increase ≥1.0 g/dL increased in a dose-dependent manner to 83.3% of subjects in the highest dose group, 0.25 mg/kg. ACE-536 was well tolerated and resulted in sustained increases in hemoglobin levels in healthy postmenopausal women. Am. J. Hematol. 89:766–770, 2014. © 2014 Wiley Periodicals, Inc.

Restoring systemic GDF11 levels reverses age-related dysfunction in mouse skeletal muscle

Parabiosis experiments indicate that impaired regeneration in aged mice is reversible by exposure to a young circulation, suggesting that young blood contains humoral "rejuvenating" factors that can restore regenerative function. Here, we demonstrate that the circulating protein growth differentiation factor 11 (GDF11) is a rejuvenating factor for skeletal muscle. Supplementation of systemic GDF11 levels, which normally decline with age, by heterochronic parabiosis or systemic delivery of recombinant protein, reversed functional impairments and restored genomic integrity in aged muscle stem cells (satellite cells). Increased GDF11 levels in aged mice also improved muscle structural and functional features and increased strength and endurance exercise capacity. These data indicate that GDF11 systemically regulates muscle aging and may be therapeutically useful for reversing age-related skeletal muscle and stem cell dysfunction.

Myostatin regulates tissue potency and cardiac calcium-handling proteins

Attenuating myostatin enhances striated muscle growth, reduces adiposity, and improves cardiac contractility. To determine whether myostatin influences tissue potency in a manner that could control such pleiotropic actions, we generated label-retaining mice with wild-type and mstn(-/-) (Jekyll) backgrounds in which slow-cycling stem, transit-amplifying, and progenitor cells are preferentially labeled by histone 2B/green fluorescent protein. Jekyll mice were born with fewer label-retaining cells (LRCs) in muscle and heart, consistent with increased stem/progenitor cell contributions to embryonic growth of both tissues. Cardiac LRC recruitment from noncardiac sources occurred in both groups, but lasted longer in Jekyll hearts, whereas heightened β-adrenergic sensitivity of mstn(-/-) hearts was explained by elevated SERCA2a, phospholamban, and β2-adrenergic receptor levels. Jekyll mice were also born with more adipose LRCs despite significantly smaller tissue weights. Reduced adiposity in mstn(-/-) animals is therefore due to reduced lipid deposition as adipoprogenitor pools appear to be enhanced. By contrast, increased bone densities of mstn(-/-) mice are likely compensatory to hypermuscularity because LRC counts were similar in Jekyll and wild-type tibia. Myostatin therefore significantly influences the potency of different tissues, not just muscle, as well as cardiac Ca²⁺-handling proteins. Thus, the pleiotropic phenotype of mstn(-/-) animals may not be due to enhanced muscle development per se, but also to altered stem/progenitor cell pools that ultimately influence tissue potency.

Variation in the ovine WFIKKN2 gene

WFIKKN2 may play a role in the regulation of muscle growth and development through its interaction with growth and differentiation factor 8 (GDF8) and growth and differentiation factor 11 (GDF11), but to date research into the function of the protein has been focused on mice, even though the WFIKKN2 gene (WFIKKN2) was first identified in humans in 2001. In this study two regions (intron 1 and the 3' UTR) of ovine WFIKKN2 were investigated, using Polymerase Chain Reaction-Single Stranded Conformational Polymorphism (PCR-SSCP). Two different PCR-SSCP patterns, representing two unique DNA sequences (designated a and b) were detected in a 399-bp amplicon derived from the 3' UTR, with sequence analysis revealing one single nucleotide polymorphism (SNP). In a 421-bp amplicon from intron 1, five different PCR-SSCP patterns (designated A-E) were observed and twelve SNPs were detected. Either one or two different sequences were detected in individual sheep and all the sequences identified shared homology with the WFIKKN2 sequences from cattle and other animal species, suggesting that these sequences represent variants of the ovine WFIKKN2 gene. In intron 1 of 487 sheep from eight breeds, variants B and C were the most common, followed by A, D and E. These results indicate that ovine WFIKKN2 is polymorphic and suggest that further analysis is required to see if variation in the gene is associated with variation in growth and muscle traits in sheep.