Formin1 disruption confers oligodactylism and alters Bmp signaling

A 12 Week Fetus with Anophthalmia, Limb Anomalies and Infratemporal Teratoma

Background

Microphthalmia is a rare autosomal recessive condition commonly known as Waardenburg anophthalmia syndrome (WAS) or oculo-acromal formation syndrome (MIM#206920).

Case Description

Here, we report the case of a woman whose fetal ultrasonography at 12 weeks of pregnancy revealed multiple fetal abnormalities. These included the absence of the left upper limb, an unclear display of the right orbit, a visible maxillary space, and a round, echoless appearance measuring 4 mm in diameter in the middle of the forehead. There was also a significant echo in the sac wall. The possibility of a frontal meningocele or a proboscis-like nose was considered. The fetus was delivered with absence of the left upper limb, absence of the right eye, a cleft lip on the right side, and a milky white sac with a diameter of 5 mm on the forehead after the pregnancy was terminated at the hospital. Pathological investigation revealed a mature cystic teratoma. The conclusion was microphthalmia with limb anomalies (MLA) after missing limbs, absence of eyes, and cleft lip were input into the Online Mendelian Inheritance in Man database. The case was diagnosed with fetal microphthalmia with limb anomalies and an interfrontal teratoma.

Conclusion

In this case, the entire exon analysis was not conducted, and as a result, the final diagnosis remains unclear. Based exclusively on the phenotype of the induced fetus, MLA was diagnosed. It is proposed that cases satisfying the requirements for a pathological diagnosis should undergo a pathological examination to establish a definitive diagnosis.

Probing the ligand binding specificity of FNBP4 WW domains and interaction with FH1 domain of FMN1

Formins are a group of actin-binding proteins that mediate nascent actin filament polymerization, filament elongation, and barbed end-capping function, thereby regulating different cellular and developmental processes. Developmental processes like vertebrate gastrulation, neural growth cone dynamics, and limb development require formins functioning in a regulated manner. Formin-binding proteins like Rho GTPase regulate the activation of auto-inhibited conformation of diaphanous formins. Unlike other diaphanous formins, Formin1 (FMN1) a non-diaphanous formin is not regulated by Rho GTPase. FMN1 acts as an antagonist of the Bone Morphogenetic Protein (BMP) signaling pathway during limb development. Several previous reports demonstrated that WW domain-containing proteins can interact with poly-proline-rich amino acid stretches of formins and play a crucial role in developmental processes. In contrast, WW domain-containing Formin-binding Protein 4 (FNBP4) protein plays an essential role in limb development. It has been hypothesized that the interaction between FNBP4 and FMN1 can further attribute to the role in limb development through the BMP signaling pathway. In this study, we have elucidated the binding kinetics of FNBP4 and FMN1 using surface plasmon resonance (SPR) and enzyme-linked immunosorbent assays (ELISA). Our findings confirm that the FNBP4 exhibits interaction with the poly-proline-rich formin homology 1 (FH1) domain of FMN1. Furthermore, only the first WW1 domains are involved in the interaction between the two domains. Thus, this study sheds light on the binding potentialities of WW domains of FNBP4 that might contribute to the regulation of FMN1 function.

Unique genetic signature and selection footprints in Dutch population of German Longhaired Pointer dogs

The German Longhaired Pointer (GLP) breed is a versatile pointer dog breed. In the current study, we investigated the genetic diversity of these dogs based on SNP array data and compared it to 11 other pointer setter breeds. The results show that GLPs have a relatively low level of inbreeding among these pointer breeds. In addition, with the availability of pedigree information of the GLPs, we demonstrate that the correlation between pedigree-based inbreeding and genotype-based inbreeding coefficients was high (R = 0.89 and 0.85). By investigating population structure between these 12 pointer setter breeds we showed that GLP is a breed distinct from other pointers and shares common ancestry with a few other pointing breeds. Finally, we identified selection signatures in GLPs using the runs of homozygosity islands method. The most significant runs of homozygosity island was detected on chromosome 30 harboring the genes RYR3, FMN1, and GREM1. The RYR3 gene plays a role in skeletal muscle contraction while the FMN1 and GREM1 genes are involved in limb development. The selection on these three genes could have contributed to the excellent athletic performance of GLPs, which is an extremely important characteristic for this hunting dog.

Adaptive mechanoproperties mediated by the formin FMN1 characterize glioblastoma fitness for invasion

Glioblastoma are heterogeneous tumors composed of highly invasive and highly proliferative clones. Heterogeneity in invasiveness could emerge from discrete biophysical properties linked to specific molecular expression. We identified clones of patient-derived glioma propagating cells that were either highly proliferative or highly invasive and compared their cellular architecture, migratory, and biophysical properties. We discovered that invasiveness was linked to cellular fitness. The most invasive cells were stiffer, developed higher mechanical forces on the substrate, and moved stochastically. The mechano-chemical-induced expression of the formin FMN1 conferred invasive strength that was confirmed in patient samples. Moreover, FMN1 expression was also linked to motility in other cancer and normal cell lines, and its ectopic expression increased fitness parameters. Mechanistically, FMN1 acts from the microtubule lattice and promotes a robust mechanical cohesion, leading to highly invasive motility.

Formins in Human Disease

Almost 25 years have passed since a mutation of a formin gene, DIAPH1, was identified as being responsible for a human inherited disorder: a form of sensorineural hearing loss. Since then, our knowledge of the links between formins and disease has deepened considerably. Mutations of DIAPH1 and six other formin genes (DAAM2, DIAPH2, DIAPH3, FMN2, INF2 and FHOD3) have been identified as the genetic cause of a variety of inherited human disorders, including intellectual disability, renal disease, peripheral neuropathy, thrombocytopenia, primary ovarian insufficiency, hearing loss and cardiomyopathy. In addition, alterations in formin genes have been associated with a variety of pathological conditions, including developmental defects affecting the heart, nervous system and kidney, aging-related diseases, and cancer. This review summarizes the most recent discoveries about the involvement of formin alterations in monogenic disorders and other human pathological conditions, especially cancer, with which they have been associated. In vitro results and experiments in modified animal models are discussed. Finally, we outline the directions for future research in this field.

Uncovering Modifier Genes of X-Linked Alport Syndrome Using a Novel Multiparent Mouse Model

BACKGROUND

Mutations in COL4A5 are responsible for 80% of cases of X-linked Alport Syndrome (XLAS). Although genes that cause AS are well characterized, people with AS who have similar genetic mutations present with a wide variation in the extent of kidney impairment and age of onset, suggesting the activities of modifier genes.

METHODS

We created a cohort of genetically diverse XLAS male and female mice using the Diversity Outbred mouse resource and measured albuminuria, GFR, and gene expression. Using a quantitative trait locus approach, we mapped modifier genes that can best explain the underlying phenotypic variation measured in our diverse population.

RESULTS

Genetic analysis identified several loci associated with the variation in albuminuria and GFR, including a locus on the X chromosome associated with X inactivation and a locus on chromosome 2 containing Fmn1. Subsequent analysis of genetically reduced Fmn1 expression in Col4a5 knockout mice showed a decrease in albuminuria, podocyte effacement, and podocyte protrusions in the glomerular basement membrane, which support the candidacy of Fmn1 as a modifier gene for AS.

CONCLUSION

With this novel approach, we emulated the variability in the severity of kidney phenotypes found in human patients with Alport Syndrome through albuminuria and GFR measurements. This approach can identify modifier genes in kidney disease that can be used as novel therapeutic targets.

Elevated diversity in loci linked to facial morphology is consistent with the hypothesis that individual facial recognition is important across hominoids

OBJECTIVES

The ability to use visual signals to identify individuals is an important feature of primate social groups, including humans. Sheehan and Nachman (2014) showed that loci linked to facial morphology had elevated levels of diversity and interpreted this as evidence that the human face is under frequency-dependent selection to enhance individual recognition (Nature Communications 5). In our study, we tested whether this pattern is found in non-human ape species, to help understand whether individual recognition might also play a role in species other than humans.

MATERIALS AND METHODS

We examined levels of genetic diversity in an available population genomic dataset of humans, chimpanzees, bonobos, gorillas, and orangutans for three sets of loci, (1) loci linked to facial morphology, (2) loci linked to height, and (3) neutrally evolving regions. We tested whether loci linked to facial morphology were more variable than loci linked to height or neutrally evolving loci in each of these species.

RESULTS

We found significantly elevated diversity in loci linked to facial morphology in chimpanzees, gorillas, and Sumatran and Bornean orangutans.

DISCUSSION

Our findings closely parallel those of Sheehan and Nachman and are consistent with the idea that selection for facial diversity and individual recognition has not only shaped the evolution of the human face, but it has similarly shaped the evolution of most of our closest primate relatives. We also discuss alternative hypotheses for this pattern.

Deletion of morpholino binding sites (DeMOBS) to assess specificity of morphant phenotypes

Two complimentary approaches are widely used to study gene function in zebrafish: induction of genetic mutations, usually using targeted nucleases such as CRISPR/Cas9, and suppression of gene expression, typically using Morpholino oligomers. Neither method is perfect. Morpholinos (MOs) sometimes produce off-target or toxicity-related effects that can be mistaken for true phenotypes. Conversely, genetic mutants can be subject to compensation, or may fail to yield a null phenotype due to leakiness (e.g. use of cryptic splice sites or downstream AUGs). When discrepancy between mutant and morpholino-induced (morphant) phenotypes is observed, experimental validation of such phenotypes becomes very labor intensive. We have developed a simple genetic method to differentiate between genuine morphant phenotypes and those produced due to off-target effects. We speculated that indels within 5' untranslated regions would be unlikely to have a significant negative effect on gene expression. Mutations induced within a MO target site would result in a Morpholino-refractive allele thus suppressing true MO phenotypes whilst non-specific phenotypes would remain. We tested this hypothesis on one gene with an exclusively zygotic function, tbx5a, and one gene with strong maternal effect, ctnnb2. We found that indels within the Morpholino binding site are indeed able to suppress both zygotic and maternal morphant phenotypes. We also observed that the ability of such indels to suppress morpholino phenotypes does depend on the size and the location of the deletion. Nonetheless, mutating the morpholino binding sites in both maternal and zygotic genes can ascertain the specificity of morphant phenotypes.

Rab27a co-ordinates actin-dependent transport by controlling organelle-associated motors and track assembly proteins

Cell biologists generally consider that microtubules and actin play complementary roles in long- and short-distance transport in animal cells. On the contrary, using melanosomes of melanocytes as a model, we recently discovered that the motor protein myosin-Va works with dynamic actin tracks to drive long-range organelle dispersion in opposition to microtubules. This suggests that in animals, as in yeast and plants, myosin/actin can drive long-range transport. Here, we show that the SPIRE-type actin nucleators (predominantly SPIRE1) are Rab27a effectors that co-operate with formin-1 to generate actin tracks required for myosin-Va-dependent transport in melanocytes. Thus, in addition to melanophilin/myosin-Va, Rab27a can recruit SPIREs to melanosomes, thereby integrating motor and track assembly activity at the organelle membrane. Based on this, we suggest a model in which organelles and force generators (motors and track assemblers) are linked, forming an organelle-based, cell-wide network that allows their collective activity to rapidly disperse the population of organelles long-distance throughout the cytoplasm.

Analysis of the genomic expression profile in trisomy 18: insight into possible genes involved in the associated phenotypes

Trisomy 18, sometimes called Edwards syndrome, occurs in about 1 in 6000 live births and causes multiple birth defects in affected infants. The extra copy of chromosome 18 causes the altered expression of many genes and leads to severe skeletal, cardiovascular and neurological systems malformations as well as other medical problems. Due to the low rate of survival and the massive genetic imbalance, little research has been aimed at understanding the molecular consequences of trisomy 18 or considering potential therapeutic approaches. Our research is the first study to characterize whole-genome expression in fibroblast cells obtained from two patients with trisomy 18 and two matched controls, with follow-up expression confirmation studies on six independent controls. We show a detailed analysis of the most highly dysregulated genes on chromosome 18 and those genome-wide. The identified effector genes and the dysregulated downstream pathways provide hints of possible genotype-phenotype relationships to some of the most common symptoms observed in trisomy 18. We also provide a possible explanation for the sex-specific differences in survival, a unique characteristic of trisomy 18. Our analysis of genome-wide expression data moves us closer to understanding the molecular consequences of the second most common human autosomal trisomy of infants who survive to term. These insights might also translate to the understanding of the etiology of associated birth defects and medical conditions among those with trisomy 18.

Phenotype loss is associated with widespread divergence of the gene regulatory landscape in evolution

Detecting the genomic changes underlying phenotypic changes between species is a main goal of evolutionary biology and genomics. Evolutionary theory predicts that changes in cis-regulatory elements are important for morphological changes. We combined genome sequencing, functional genomics and genome-wide comparative analyses to investigate regulatory elements in lineages that lost morphological traits. We first show that limb loss in snakes is associated with widespread divergence of limb regulatory elements. We next show that eye degeneration in subterranean mammals is associated with widespread divergence of eye regulatory elements. In both cases, sequence divergence results in an extensive loss of transcription factor binding sites. Importantly, diverged regulatory elements are associated with genes required for normal limb patterning or normal eye development and function, suggesting that regulatory divergence contributed to the loss of these phenotypes. Together, our results show that genome-wide decay of the phenotype-specific cis-regulatory landscape is a hallmark of lost morphological traits.

Cis-regulatory elements are important factors for morphological changes. Here, the authors show widespread divergence of limb and eye regulatory elements in limb loss in snakes and eye degeneration in subterranean mammals respectively.

Association between a single-nucleotide polymorphism in the GREM1 gene and non-syndromic orofacial cleft in the Chinese population

BACKGROUND

Non-syndromic orofacial cleft (NSOC) is a common craniofacial deformity among newborns. The GREM1 gene is correlated with orofacial development. The aim of our study was to investigate the association between a single-nucleotide polymorphism in the GREM1 gene and this malformation in the Chinese population.

METHODS

The SNaPshot mini-sequencing technique was used to genotype the locus rs1258763 of the GREM1 gene in 331 patients with NSOC and 271 individuals in a control group.

RESULTS

For GREM1 rs1258763, there was a significant difference between the NSOC case group and control group (P = .022). Children carrying GA and GA/AA genotypes had an increased risk of NSOC (OR=1.62, 95%CI: 1.15-2.30; OR=1.52, 95%CI: 1.09-2.12). In the cleft subgroup, we found that the GREM1 rs1258763 GA genotype might contribute to the elevated risk of the cleft lip with or without cleft palate (CL/P) (P = .029). Non-significant differences were found between the cleft palate only (CPO) and control groups (P = .077).

CONCLUSION

Our findings revealed that the GREM1 polymorphism was significantly associated with the risk of NSOC in the Chinese population.

Non-canonical activity of the podosomal formin FMNL1γ supports immune cell migration

Having previously located the formin FMNL1 in macrophage podosomes, we developed an in vivo model to assess the role of FMNL1 in the migration activities of primary macrophages. Deletion of FMNL1 in mice was genetically lethal; however, targeted deletion in macrophages was achieved by employing macrophage-specific Cre. Unchallenged FMNL1-deficient mice exhibited an unexpected reduction in tissue-resident macrophages despite normal blood monocyte numbers. Upon immune stimulus, the absence of FMNL1 resulted in reduced macrophage recruitment in vivo, decreased migration in two-dimensional in vitro culture and a decrease in the number of macrophages exhibiting podosomes. Of the three described isoforms of FMNL1 - α, β and γ - only FMNL1γ rescued macrophage migration when expressed exogenously in depleted macrophages. Surprisingly, mutation of residues in the FH2 domain of FMNL1γ that disrupt barbed-end actin binding did not limit rescue of macrophage migration and podosome numbers. These observations suggest that FMNL1 contributes to macrophage migration activity by stabilizing the lifespan of podosomes without interaction of fast-growing actin termini.

Meta-analysis Reveals Genome-Wide Significance at 15q13 for Nonsyndromic Clefting of Both the Lip and the Palate, and Functional Analyses Implicate GREM1 As a Plausible Causative Gene

Nonsyndromic orofacial clefts are common birth defects with multifactorial etiology. The most common type is cleft lip, which occurs with or without cleft palate (nsCLP and nsCLO, respectively). Although genetic components play an important role in nsCLP, the genetic factors that predispose to palate involvement are largely unknown. In this study, we carried out a meta-analysis on genetic and clinical data from three large cohorts and identified strong association between a region on chromosome 15q13 and nsCLP (P = 8.13×10⁻¹⁴ for rs1258763; relative risk (RR): 1.46, 95% confidence interval (CI): 1.32–1.61)) but not nsCLO (P = 0.27; RR: 1.09 (0.94–1.27)). The 5 kb region of strongest association maps downstream of Gremlin-1 (GREM1), which encodes a secreted antagonist of the BMP4 pathway. We show during mouse embryogenesis, Grem1 is expressed in the developing lip and soft palate but not in the hard palate. This is consistent with genotype-phenotype correlations between rs1258763 and a specific nsCLP subphenotype, since a more than two-fold increase in risk was observed in patients displaying clefts of both the lip and soft palate but who had an intact hard palate (RR: 3.76, CI: 1.47–9.61, P_diff<0.05). While we did not find lip or palate defects in Grem1-deficient mice, wild type embryonic palatal shelves developed divergent shapes when cultured in the presence of ectopic Grem1 protein (P = 0.0014). The present study identified a non-coding region at 15q13 as the second, genome-wide significant locus specific for nsCLP, after 13q31. Moreover, our data suggest that the closely located GREM1 gene contributes to a rare clinical nsCLP entity. This entity specifically involves abnormalities of the lip and soft palate, which develop at different time-points and in separate anatomical regions.

Clefts of the lip and palate are common birth defects, and require long-term multidisciplinary management. Their etiology involves genetic factors and environmental influences and/or a combination of both, however, these interactions are poorly defined. Moreover, although clefts of the lip may or may not involve the palate, the determinants predisposing to specific subphenotypes are largely unknown. Here we demonstrate that variations in the non-coding region near the GREM1 gene show a highly significant association with a particular phenotype in which cleft lip and cleft palate co-occur (nsCLP; P = 8.13×10⁻¹⁴). Our data suggest that the risk is even higher for patients who have a cleft lip and a cleft of the soft palate, but not of the hard palate. Interestingly, this subphenotype corresponds to the expression of the mouse Grem1 gene, which is found in the developing lip and soft palate but not in the hard palate. While Grem1-deficient mice display no lip or palate defects, we demonstrate that ectopic Grem1 protein alters palatal shelve morphogenesis. Together, our results identify a region near GREM1 as the second, genome-wide significant risk locus for nsCLP, and suggest that deregulated GREM1 expression during craniofacial development may contribute to this common birth defect.

Is toxicant-induced Sertoli cell injury in vitro a useful model to study molecular mechanisms in spermatogenesis?

Sertoli cells isolated from rodents or humans and cultured in vitro are known to establish a functional tight junction (TJ)-permeability barrier that mimics the blood-testis barrier (BTB) in vivo. This model has been widely used by investigators to study the biology of the TJ and the BTB. Studies have shown that environmental toxicants (e.g., perfluorooctanesulfonate (PFOS), bisphenol A (BPA) and cadmium) that exert their disruptive effects to induce Sertoli cell injury using this in vitro model are reproducible in studies in vivo. Thus, this in vitro system provides a convenient approach to probe the molecular mechanism(s) underlying toxicant-induced testis injury but also to provide new insights in understanding spermatogenesis, such as the biology of cell adhesion, BTB restructuring that supports preleptotene spermatocyte transport, and others. Herein, we provide a brief and critical review based on studies using this in vitro model of Sertoli cell cultures using primary cells isolated from rodent testes vs. humans to monitor environmental toxicant-mediated Sertoli cell injury. In short, recent findings have shown that environmental toxicants exert their effects on Sertoli cells to induce testis injury through their action on Sertoli cell actin- and/or microtubule-based cytoskeleton. These effects are mediated via their disruptive effects on actin- and/or microtubule-binding proteins. Sertoli cells also utilize differential spatiotemporal expression of these actin binding proteins to confer plasticity to the BTB to regulate germ cell transport across the BTB.

Regulation of blood-testis barrier by actin binding proteins and protein kinases

The blood-testis barrier (BTB) is an important ultrastructure in the testis, since the onset of meiosis and spermiogenesis coincides with the establishment of a functional barrier in rodents and humans. It is also noted that a delay in the assembly of a functional BTB following treatment of neonatal rats with drugs such as diethylstilbestrol or adjudin also delays the first wave of spermiation. While the BTB is one of the tightest blood-tissue barriers, it undergoes extensive remodeling, in particular, at stage VIII of the epithelial cycle to facilitate the transport of preleptotene spermatocytes connected in clones across the immunological barrier. Without this timely transport of preleptotene spermatocytes derived from type B spermatogonia, meiosis will be arrested, causing aspermatogenesis. Yet the biology and regulation of the BTB remains largely unexplored since the morphological studies in the 1970s. Recent studies, however, have shed new light on the biology of the BTB. Herein, we critically evaluate some of these findings, illustrating that the Sertoli cell BTB is regulated by actin-binding proteins (ABPs), likely supported by non-receptor protein kinases, to modulate the organization of actin microfilament bundles at the site. Furthermore, microtubule-based cytoskeleton is also working in concert with the actin-based cytoskeleton to confer BTB dynamics. This timely review provides an update on the unique biology and regulation of the BTB based on the latest findings in the field, focusing on the role of ABPs and non-receptor protein kinases.

Formins: Actin nucleators that regulate cytoskeletal dynamics during spermatogenesis

Formins are a growing class of actin nucleation proteins that promote the polymerization of actin microfilaments, forming long stretches of actin microfilaments to confer actin filament bundling in mammalian cells. As such, microfilament bundles can be formed in specific cellular domains, in particular in motile mammalian cells, such as filopodia. Since ectoplasmic specialization (ES), a testis-specific adherens junction (AJ), at the Sertoli cell-cell and Sertoli-spermatid interface is constituted by arrays of actin microfilament bundles, it is likely that formins are playing a significant physiological role on the homeostasis of ES during the epithelial cycle of spermatogenesis. In this Commentary, we provide a timely discussion on formin 1 which was recently shown to be a crucial regulator of actin microfilaments at the ES in the rat testis (Li N et al. Endocrinology, 2015, in press; DOI: 10.1210/en.2015-1161, PMID:25901598). We also highlight research that is needed to unravel the functional significance of formins in spermatogenesis.

Advances in the Molecular Genetics of Non-syndromic Syndactyly

Syndactyly, webbing of adjacent digits with or without bony fusion, is one of the most common hereditary limb malformations. It occurs either as an isolated abnormality or as a component of more than 300 syndromic anomalies. There are currently nine types of phenotypically diverse nonsyndromic syndactyly. Non-syndromic syndactyly is usually inherited as an autosomal dominant trait, although the more severe presenting types and subtypes may show autosomal recessive or X-linked pattern of inheritance. The phenotype appears to be not only caused by a main gene, but also dependant on genetic background and subsequent signaling pathways involved in limb formation. So far, the principal genes identified to be involved in congenital syndactyly are mainly involved in the zone of polarizing activity and sonic hedgehog pathway. This review summarizes the recent progress made in the molecular genetics, including known genes and loci responsible for non-syndromic syndactyly, and the signaling pathways those genetic factors involved in, as well as clinical features and animal models. We hope our review will contribute to the understanding of underlying pathogenesis of this complicated disorder and have implication on genetic counseling.

Cytoskeletal proteins in cortical development and disease: actin associated proteins in periventricular heterotopia

The actin cytoskeleton regulates many important cellular processes in the brain, including cell division and proliferation, migration, and cytokinesis and differentiation. These developmental processes can be regulated through actin dependent vesicle and organelle movement, cell signaling, and the establishment and maintenance of cell junctions and cell shape. Many of these processes are mediated by extensive and intimate interactions of actin with cellular membranes and proteins. Disruption in the actin cytoskeleton in the brain gives rise to periventricular heterotopia (PH), a malformation of cortical development, characterized by abnormal neurons clustered deep in the brain along the lateral ventricles. This disorder can give rise to seizures, dyslexia and psychiatric disturbances. Anatomically, PH is characterized by a smaller brain (impaired proliferation), heterotopia (impaired initial migration) and disruption along the neuroependymal lining (impaired cell-cell adhesion). Genes causal for PH have also been implicated in actin-dependent processes. The current review provides mechanistic insight into actin cytoskeletal regulation of cortical development in the context of this malformation of cortical development.

Biallelic truncating mutations in FMN2, encoding the actin-regulatory protein Formin 2, cause nonsyndromic autosomal-recessive intellectual disability

Dendritic spines represent the major site of neuronal activity in the brain; they serve as the receiving point for neurotransmitters and undergo rapid activity-dependent morphological changes that correlate with learning and memory. Using a combination of homozygosity mapping and next-generation sequencing in two consanguineous families affected by nonsyndromic autosomal-recessive intellectual disability, we identified truncating mutations in formin 2 (FMN2), encoding a protein that belongs to the formin family of actin cytoskeleton nucleation factors and is highly expressed in the maturing brain. We found that FMN2 localizes to punctae along dendrites and that germline inactivation of mouse Fmn2 resulted in animals with decreased spine density; such mice were previously demonstrated to have a conditioned fear-learning defect. Furthermore, patient neural cells derived from induced pluripotent stem cells showed correlated decreased synaptic density. Thus, FMN2 mutations link intellectual disability either directly or indirectly to the regulation of actin-mediated synaptic spine density.

A formin-g role during development and disease

Several different protein families were shown to be involved in the regulation of actin filament formation and have been studied extensively in processes such as cell migration. Among them are members of the formin family, which tend to promote the formation of linear actin filaments. Studies in recent years, often using loss of function animal models, have indicated that formin family members play roles beyond cell motility in vitro and are involved in processes ranging from tissue morphogenesis and cell differentiation to diseases such as cancer and cardiomyopathy. Therefore the aim of this review is to discuss these findings and to start putting them into a subcellular context.

Formin 1 and filamin B physically interact to coordinate chondrocyte proliferation and differentiation in the growth plate

Filamin B (FlnB) is an actin-binding protein thought to transduce signals from various membrane receptors and intracellular proteins onto the actin cytoskeleton. Formin1 (Fmn1) is an actin-nucleating protein, implicated in actin assembly and intracellular signaling. Human mutations in FLNB cause several skeletal disorders associated with dwarfism and early bone fusion. Mouse mutations in Fmn1 cause aberrant fusion of carpal digits. We report here that FlnB and Fmn1 physically interact, are co-expressed in chondrocytes in the growth plate and share overlapping expression in the cell cytoplasm and nucleus. Loss of FlnB leads to a dramatic decrease in Fmn1 expression at the hypertrophic-to-ossification border. Loss of Fmn1-FlnB in mice leads to a more severe reduction in body size, weight and growth plate length, than observed in mice following knockout of either gene alone. Shortening of the long bone is associated with a decrease in chondrocyte proliferation and an overall delay in ossification in the double-knockout mice. In contrast to FlnB null, Fmn1 loss results in a decrease in the width of the prehypertrophic zone. Loss of both proteins, however, causes an overall decrease in the width of the proliferation zone and an increase in the differentiated hypertrophic zone. The current findings suggest that Fmn1 and FlnB have shared and independent functions. FlnB loss promotes prehypertrophic differentiation whereas Fmn1 leads to a delay. Both proteins, however, regulate chondrocyte proliferation, and FlnB may regulate Fmn1 function at the hypertrophic-to-ossification border, thereby explaining the overall delay in ossification.

Interaction between microtubules and the Drosophila formin Cappuccino and its effect on actin assembly

Formin family actin nucleators are potential coordinators of the actin and microtubule cytoskeletons, as they can both nucleate actin filaments and bind microtubules in vitro. To gain a more detailed mechanistic understanding of formin-microtubule interactions and formin-mediated actin-microtubule cross-talk, we studied microtubule binding by Cappuccino (Capu), a formin involved in regulating actin and microtubule organization during Drosophila oogenesis. We found that two distinct domains within Capu, FH2 and tail, work together to promote high-affinity microtubule binding. The tail domain appears to bind microtubules through nonspecific charge-based interactions. In contrast, distinct residues within the FH2 domain are important for microtubule binding. We also report the first visualization of a formin polymerizing actin filaments in the presence of microtubules. Interestingly, microtubules are potent inhibitors of the actin nucleation activity of Capu but appear to have little effect on Capu once it is bound to the barbed end of an elongating filament. Because Capu does not simultaneously bind microtubules and assemble actin filaments in vitro, its actin assembly and microtubule binding activities likely require spatial and/or temporal regulation within the Drosophila oocyte.

Whole-exome sequencing identified a homozygous FNBP4 mutation in a family with a condition similar to microphthalmia with limb anomalies

Microphthalmia with limb anomalies (MLA), also known as Waardenburg anophthalmia syndrome or ophthalmoacromelic syndrome, is a rare autosomal recessive disorder. Recently, we and others successfully identified SMOC1 as the causative gene for MLA. However, there are several MLA families without SMOC1 abnormality, suggesting locus heterogeneity in MLA. We aimed to identify a pathogenic mutation in one Lebanese family having an MLA-like condition without SMOC1 mutation by whole-exome sequencing (WES) combined with homozygosity mapping. A c.683C>T (p.Thr228Met) in FNBP4 was found as a primary candidate, drawing the attention that FNBP4 and SMOC1 may potentially modulate BMP signaling.

Formins: emerging players in the dynamic plant cell cortex

Formins (FH2 proteins) are an evolutionarily conserved family of eukaryotic proteins, sharing the common FH2 domain. While they have been, until recently, understood mainly as actin nucleators, formins are also engaged in various additional aspects of cytoskeletal organization and signaling, including, but not limited to, the crosstalk between the actin and microtubule networks. A surprising diversity of domain organizations has been discovered among the FH2 proteins, and specific domain setups have been found in plants. Seed plants have two clades of formins, one of them (Class I) containing mostly transmembrane proteins, while members of the other one (Class II) may be anchored to membranes via a putative membrane-binding domain related to the PTEN antioncogene. Thus, plant formins present good candidates for possible mediators of coordination of the cortical actin and microtubule cytoskeletons, as well as their attachment to the plasma membrane, that is, aspects of cell cortex organization likely to be important for cell and tissue morphogenesis. Although experimental studies of plant formin function are hampered by the large number of formin genes and their functional redundancy, recent experimental work has already resulted in some remarkable insights into the function of FH2 proteins in plants.

Conserved cis-regulatory regions in a large genomic landscape control SHH and BMP-regulated Gremlin1 expression in mouse limb buds

Background

Mouse limb bud is a prime model to study the regulatory interactions that control vertebrate organogenesis. Major aspects of limb bud development are controlled by feedback loops that define a self-regulatory signalling system. The SHH/GREM1/AER-FGF feedback loop forms the core of this signalling system that operates between the posterior mesenchymal organiser and the ectodermal signalling centre. The BMP antagonist Gremlin1 (GREM1) is a critical node in this system, whose dynamic expression is controlled by BMP, SHH, and FGF signalling and key to normal progression of limb bud development. Previous analysis identified a distant cis-regulatory landscape within the neighbouring Formin1 (Fmn1) locus that is required for Grem1 expression, reminiscent of the genomic landscapes controlling HoxD and Shh expression in limb buds.

Results

Three highly conserved regions (HMCO1-3) were identified within the previously defined critical genomic region and tested for their ability to regulate Grem1 expression in mouse limb buds. Using a combination of BAC and conventional transgenic approaches, a 9 kb region located ~70 kb downstream of the Grem1 transcription unit was identified. This region, termed Grem1 Regulatory Sequence 1 (GRS1), is able to recapitulate major aspects of Grem1 expression, as it drives expression of a LacZ reporter into the posterior and, to a lesser extent, in the distal-anterior mesenchyme. Crossing the GRS1 transgene into embryos with alterations in the SHH and BMP pathways established that GRS1 depends on SHH and is modulated by BMP signalling, i.e. integrates inputs from these pathways. Chromatin immunoprecipitation revealed interaction of endogenous GLI3 proteins with the core cis-regulatory elements in the GRS1 region. As GLI3 is a mediator of SHH signal transduction, these results indicated that SHH directly controls Grem1 expression through the GRS1 region. Finally, all cis-regulatory regions within the Grem1 genomic landscape locate to the DNAse I hypersensitive sites identified in this genomic region by the ENCODE consortium.

Conclusions

This study establishes that distant cis-regulatory regions scattered through a larger genomic landscape control the highly dynamic expression of Grem1, which is key to normal progression of mouse limb bud development.

Dishevelled-associated activator of morphogenesis 1 (Daam1) is required for heart morphogenesis

Dishevelled-associated activator of morphogenesis 1 (Daam1), a member of the formin protein family, plays an important role in regulating the actin cytoskeleton via mediation of linear actin assembly. Previous functional studies of Daam1 in lower species suggest its essential role in Drosophila trachea formation and Xenopus gastrulation. However, its in vivo physiological function in mammalian systems is largely unknown. We have generated Daam1-deficient mice via gene-trap technology and found that Daam1 is highly expressed in developing murine organs, including the heart. Daam1-deficient mice exhibit embryonic and neonatal lethality and suffer multiple cardiac defects, including ventricular noncompaction, double outlet right ventricles and ventricular septal defects. In vivo genetic rescue experiments further confirm that the lethality of Daam1-deficient mice results from the inherent cardiac abnormalities. In-depth analyses have revealed that Daam1 is important for regulating filamentous actin assembly and organization, and consequently for cytoskeletal function in cardiomyocytes, which contributes to proper heart morphogenesis. Daam1 is also found to be important for proper cytoskeletal architecture and functionalities in embryonic fibroblasts. Biochemical analyses indicate that Daam1 does not regulate cytoskeletal organization through RhoA, Rac1 or Cdc42. Our study highlights a crucial role for Daam1 in regulating the actin cytoskeleton and tissue morphogenesis.

SMOC1 is essential for ocular and limb development in humans and mice

Microphthalmia with limb anomalies (MLA) is a rare autosomal-recessive disorder, presenting with anophthalmia or microphthalmia and hand and/or foot malformation. We mapped the MLA locus to 14q24 and successfully identified three homozygous (one nonsense and two splice site) mutations in the SPARC (secreted protein acidic and rich in cysteine)-related modular calcium binding 1 (SMOC1) in three families. Smoc1 is expressed in the developing optic stalk, ventral optic cup, and limbs of mouse embryos. Smoc1 null mice recapitulated MLA phenotypes, including aplasia or hypoplasia of optic nerves, hypoplastic fibula and bowed tibia, and syndactyly in limbs. A thinned and irregular ganglion cell layer and atrophy of the anteroventral part of the retina were also observed. Soft tissue syndactyly, resulting from inhibited apoptosis, was related to disturbed expression of genes involved in BMP signaling in the interdigital mesenchyme. Our findings indicate that SMOC1/Smoc1 is essential for ocular and limb development in both humans and mice.

Bare bones pattern formation: a core regulatory network in varying geometries reproduces major features of vertebrate limb development and evolution

BACKGROUND

Major unresolved questions regarding vertebrate limb development concern how the numbers of skeletal elements along the proximodistal (P-D) and anteroposterior (A-P) axes are determined and how the shape of a growing limb affects skeletal element formation. There is currently no generally accepted model for these patterning processes, but recent work on cartilage development (chondrogenesis) indicates that precartilage tissue self-organizes into nodular patterns by cell-molecular circuitry with local auto-activating and lateral inhibitory (LALI) properties. This process is played out in the developing limb in the context of a gradient of fibroblast growth factor (FGF) emanating from the apical ectodermal ridge (AER).

RESULTS

We have simulated the behavior of the core chondrogenic mechanism of the developing limb in the presence of an FGF gradient using a novel computational environment that permits simulation of LALI systems in domains of varying shape and size. The model predicts the normal proximodistal pattern of skeletogenesis as well as distal truncations resulting from AER removal. Modifications of the model's parameters corresponding to plausible effects of Hox proteins and formins, and of the reshaping of the model limb, bud yielded simulated phenotypes resembling mutational and experimental variants of the limb. Hypothetical developmental scenarios reproduce skeletal morphologies with features of fossil limbs.

CONCLUSIONS

The limb chondrogenic regulatory system operating in the presence of a gradient has an inherent, robust propensity to form limb-like skeletal structures. The bare bones framework can accommodate ancillary gene regulatory networks controlling limb bud shaping and establishment of Hox expression domains. This mechanism accounts for major features of the normal limb pattern and, under variant geometries and different parameter values, those of experimentally manipulated, genetically aberrant and evolutionary early forms, with no requirement for an independent system of positional information.

Unleashing formins to remodel the actin and microtubule cytoskeletons

Formins are highly conserved proteins that have essential roles in remodelling the actin and microtubule cytoskeletons to influence eukaryotic cell shape and behaviour. Recent work has identified numerous cellular factors that locally recruit, activate or inactivate formins to bridle and unleash their potent effects on actin nucleation and elongation. The effects of formins on microtubules have also begun to be described, which places formins in a prime position to coordinate actin and microtubule dynamics. The emerging complexity in the mechanisms governing formins mirrors the wide range of essential functions that they perform in cell motility, cell division and cell and tissue morphogenesis.

The role of formins in human disease

Formins are a conserved family of proteins that play key roles in cytoskeletal remodeling. They nucleate and processively elongate non-branched actin filaments and also modulate microtubule dynamics. Despite their significant contributions to cell biology and development, few studies have directly implicated formins in disease pathogenesis. This review highlights the roles of formins in cell division, migration, immunity, and microvesicle formation in the context of human disease. In addition, we discuss the importance of controlling formin activity and protein expression to maintain cell homeostasis.