Inhibition of Shh signalling in the chick wing gives insights into digit patterning and evolution

Sprouty2/4 deficiency disrupts early signaling centers impacting chondrogenesis in the mouse forelimb

The FGF signaling pathway plays an important role in the regulation of limb development, controlling cell migration, proliferation, differentiation, and apoptosis. Sprouty proteins act as antagonists of the FGF pathway and control the extent of FGF signaling as part of a negative feedback loop. Sprouty2/4 deficient mice evince defects in endochondral bone formation and digit patterning in their forelimbs, with pathogenesis recently related to ciliopathies. To understand the mechanisms behind these pathologies, the limb defects in Sprouty2+/-;Sprouty4-/- male and female mice were characterized and correlated to the dynamic expression patterns of Sprouty2 and Sprouty4, and the impact on the main signaling centers of the limb bud was assessed. Sprouty2 and Sprouty4 exhibited dynamic expressions during limb development. Interestingly, despite similar expression patterns in all limbs, the hindlimbs did not evince any obvious alterations in development, while the forelimbs showed consistent phenotypes of variable severity. Prenatally as well as postnatally, the left forelimb was significantly more severely affected than the right one. A broad variety of pathologies was present in the autopodium of the forelimb, including changes in digit number, size, shape, and number of bones, hand clefts, and digit fusions. Ectopic ossification of bones and abnormal bone fusions detected in micro-CT scans were frequently observed in the digital as well as in the carpal and metacarpal areas. Sprouty2+/-;Sprouty4-/- limb buds showed patchy loss of Fgf8 expression in the apical ectodermal ridge, and a loss of tissue underlying these regions. The zone of polarizing activity was also impacted, with lineage analysis highlighting a change in the contribution of Sonic hedgehog expressing cells. These findings support the link between Sproutys and Hedgehog signaling during limb development and highlight the importance of Sprouty2 and Sprouty4 in controlling early signaling centers in the limb.

From mesenchymal niches to engineered in vitro model systems: Exploring and exploiting biomechanical regulation of vertebrate hedgehog signalling

Tissue patterning is the result of complex interactions between transcriptional programs and various mechanical cues that modulate cell behaviour and drive morphogenesis. Vertebrate Hedgehog signalling plays key roles in embryogenesis and adult tissue homeostasis, and is central to skeletal development and the osteogenic differentiation of mesenchymal stem cells. The expression of several components of the Hedgehog signalling pathway have been reported to be mechanically regulated in mesodermal tissue patterning and osteogenic differentiation in response to external stimulation. Since a number of bone developmental defects and skeletal diseases, such as osteoporosis, are directly linked to aberrant Hedgehog signalling, a better knowledge of the regulation of Hedgehog signalling in the mechanosensitive bone marrow-residing mesenchymal stromal cells will present novel avenues for modelling these diseases and uncover novel opportunities for extracellular matrix-targeted therapies. In this review, we present a brief overview of the key molecular players involved in Hedgehog signalling and the basic concepts of mechanobiology, with a focus on bone development and regeneration. We also highlight the correlation between the activation of the Hedgehog signalling pathway in response to mechanical cues and osteogenesis in bone marrow-derived mesenchymal stromal cells. Finally, we propose different tissue engineering strategies to apply the expanding knowledge of 3D material-cell interactions in the modulation of Hedgehog signalling in vitro for fundamental and translational research applications.

Avian Coloration Genetics: Recent Advances and Emerging Questions

The colorful phenotypes of birds have long provided rich source material for evolutionary biologists. Avian plumage, beaks, skin, and eggs-which exhibit a stunning range of cryptic and conspicuous forms-inspired early work on adaptive coloration. More recently, avian color has fueled discoveries on the physiological, developmental, and-increasingly-genetic mechanisms responsible for phenotypic variation. The relative ease with which avian color traits can be quantified has made birds an attractive system for uncovering links between phenotype and genotype. Accordingly, the field of avian coloration genetics is burgeoning. In this review, we highlight recent advances and emerging questions associated with the genetic underpinnings of bird color. We start by describing breakthroughs related to 2 pigment classes: carotenoids that produce red, yellow, and orange in most birds and psittacofulvins that produce similar colors in parrots. We then discuss structural colors, which are produced by the interaction of light with nanoscale materials and greatly extend the plumage palette. Structural color genetics remain understudied-but this paradigm is changing. We next explore how colors that arise from interactions among pigmentary and structural mechanisms may be controlled by genes that are co-expressed or co-regulated. We also identify opportunities to investigate genes mediating within-feather micropatterning and the coloration of bare parts and eggs. We conclude by spotlighting 2 research areas-mechanistic links between color vision and color production, and speciation-that have been invigorated by genetic insights, a trend likely to continue as new genomic approaches are applied to non-model species.

Lewis Wolpert (1929-2021)

Lewis Wolpert was a brilliant and inspiring scientist who made hugely significant contributions which underpin and influence our understanding of developmental biology today. He spent his career interested in how the fertilised egg can give rise to the whole embryo (and ultimately the adult) with one head, two arms, two legs, all its organs and importantly how cells become different from each other and how they 'know' what to become. His ideas revolutionised the way developmental biology was perceived and also reinvigorated, in particular, the key question of how pattern formation in embryonic development is achieved. He published over 200 scientific articles and received many accolades over his career for his work and services to science in the UK. These included a CBE (Commander of the Order of the British Empire) from the Queen, being elected a Fellow of the Royal Society and a Fellow of the Royal Society of Literature. He was also a recipient of the Waddington Medal from the British Society for Developmental Biology and was awarded The Royal Society's top honour, the Royal Medal in 2018. Lewis was also a gifted teacher and communicator, including being the author of a textbook on developmental biology used around the world to train the next generation of developmental biologists. This contribution was recognised in 2003, by the award of the Viktor Hamburger Outstanding Educator Award from the Society of Developmental Biology in the USA. Lewis always enjoyed giving talks and lectures, having an infectious and persuasive enthusiasm coupled with a sharp sense of humour. He also published articles in popular science journals (aimed at the public) such as New Scientist, Scientific American and The Scientist. Lewis also wrote several popular science books. He was a passionate advocate for the public understanding of science and was the Chair of The Royal Society/Royal Institution/British Association for the Advancement of Science Committee for Public Understanding of Science (1994-1998). For this contribution he was awarded The Royal Society Michael Faraday Medal for "excellence in communicating science to UK audiences". He presented the prestigious Royal Institution Christmas Lectures in 1986 entitled 'Frankenstein's Quest: development of life'. These lectures, six in total, are presented by leading scientists and aimed at the general public and broadcast on national television. On a personal level, Lewis influenced all who came into contact with him, shaped his students and postdocs careers and instilled in them, and the community as whole, a life-long love of developmental biology.

Establishing the pattern of the vertebrate limb

The vertebrate limb continues to serve as an influential model of growth, morphogenesis and pattern formation. With this Review, we aim to give an up-to-date picture of how a population of undifferentiated cells develops into the complex pattern of the limb. Focussing largely on mouse and chick studies, we concentrate on the positioning of the limbs, the formation of the limb bud, the establishment of the principal limb axes, the specification of pattern, the integration of pattern formation with growth and the determination of digit number. We also discuss the important, but little understood, topic of how gene expression is interpreted into morphology.

Sonic hedgehog specifies flight feather positional information in avian wings

Classical tissue recombination experiments performed in the chick embryo provide evidence that signals operating during early limb development specify the position and identity of feathers. Here, we show that Sonic hedgehog (Shh) signalling in the embryonic chick wing bud specifies positional information required for the formation of adult flight feathers in a defined spatial and temporal sequence that reflects their different identities. We also reveal that Shh signalling is interpreted into specific patterns of Sim1 and Zic transcription factor expression, providing evidence of a putative gene regulatory network operating in flight feather patterning. Our data suggest that flight feather specification involved the co-option of the pre-existing digit patterning mechanism and therefore uncovers an embryonic process that played a fundamental step in the evolution of avian flight.

The benefits differential equations bring to limb development

Systems biology is a large field, offering a number of advantages to a variety of biological disciplines. In limb development, differential-equation based models can provide insightful hypotheses about the gene/protein interactions and tissue differentiation events that form the core of limb development research. Differential equations are like any other communicative tool, with misuse and limitations that can come along with their advantages. Every theory should be critically analyzed to best ascertain whether they reflect the reality in biology as well they claim. Differential equation-based models have consistent features which researchers have drawn upon to aid in more realistic descriptions and hypotheses. Nine features are described that highlight these trade-offs. The advantages range from more detailed descriptions of gene interactions and their consequence and the capacity to model robustness to the incorporation of tissue size and shape. The drawbacks come with the added complication that additional genes and signaling pathways that require additional terms within the mathematical model. They also come in the translation between the mathematical terms of the model, values and matrices, to the real world of genes, proteins, and tissues that constitute limb development. A critical analysis is necessary to ensure that these models effectively expand the understanding of the origins of a diversity of limb anatomy, from evolution to teratology. This article is categorized under: Vertebrate Organogenesis > Musculoskeletal and Vascular Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Establishment of Spatial and Temporal Patterns > Repeating Patterns and Lateral Inhibition.

An autoregulatory cell cycle timer integrates growth and specification in chick wing digit development

A fundamental question is how proliferation and growth are timed during embryogenesis. Although it has been suggested that the cell cycle could be a timer, the underlying mechanisms remain elusive. Here we describe a cell cycle timer that operates in Sonic hedgehog (Shh)-expressing polarising region cells of the chick wing bud. Our data are consistent with Shh signalling stimulating polarising region cell proliferation via Cyclin D2, and then inhibiting proliferation via a Bmp2-p27^kip1 pathway. When Shh signalling is blocked, polarising region cells over-proliferate and form an additional digit, which can be prevented by applying Bmp2 or by inhibiting D cyclin activity. In addition, Bmp2 also restores posterior digit identity in the absence of Shh signalling, thus indicating that it specifies antero-posterior (thumb to little finger) positional values. Our results reveal how an autoregulatory cell cycle timer integrates growth and specification and are widely applicable to many tissues.

Polarizing Region Tissue Grafting in the Chick Embryo Limb Bud

The polarizing region of the developing limb bud is an important organizing center that is involved in anteroposterior (thumb to little finger) patterning and has three main functions that are now considered to depend on the secreted protein Sonic hedgehog (Shh). These are (1) specifying anteroposterior positional values by autocrine and graded paracrine signaling; (2) promoting growth in adjacent mesenchyme; (3) maintaining the distal epithelium that is essential for limb outgrowth by induction of a factor in adjacent mesenchyme. The polarizing region was identified using classical tissue grafting techniques in chicken embryos. Here we describe this procedure using tissue from transgenic Green Fluorescent Protein-expressing chicken embryos that allows the long-term fate of the polarizing region to be determined. This technique provides a highly useful and effective method to understand how the polarizing region patterns the limb and has implications for other organizing centers.

Distal Dorsal Dimelia: A Disturbance of Dorsal-Ventral Digit Development

PURPOSE

Congenital palmar nail (distal dorsal dimelia [dDD]) of the hand is a rare malformation most commonly affecting the little finger. The purpose of this report was to review the features and associations of this rare disorder and discuss the suspected underlying etiology in light of our current understanding of developmental biology.

METHODS

In this retrospective cohort study from 3 practices, we describe our collective experience and review the reported literature on this disorder both as an isolated condition and in conjunction with other anomalies.

RESULTS

We examined 15 fingers with dDD, 5 of which involved little fingers. We also found dDD in 6 cases with radial polydactyly (preaxial polydactyl type II [PPD2]) and in 1 case of cleft hand involving digits adjacent to the clefted web space (the index and middle fingers). Cases of little finger dDD were also associated with prominent clefting of the adjacent web space in 4 of 5 cases. All cases had stiffness of the interphalangeal joints and loss of palmar creases consistent with dorsalization of the palmar aspect of the digit. When combined with 63 fingers reported in the literature with dDD, 3 patterns were evident. The most common form occurred in little fingers (n = 50; 64%; dDDu). The next most common form was reported in association with cleft hands (n = 16; 21%; dDDc). Radial digits in association with either radial polydactyly (PPD2) or radial longitudinal deficiency were also susceptible to dDD (n = 12; 15%; dDDr).

CONCLUSIONS

Congenital dDD is a disturbance of terminal dorsal-ventral digit patterning. The distribution of this condition with little fingers, clefting, and altered radial digit formation (PPD2 or radial longitudinal deficiency), as well as recent genetic and animal studies, suggests that dDD and altered dorsal-ventral patterning are linked to abnormal apical ectodermal ridge boundary formation.

TYPE OF STUDY/LEVEL OF EVIDENCE

Diagnostic IV.

Evolution of antero-posterior patterning of the limb: Insights from the chick

The developing limbs of chicken embryos have served as pioneering models for understanding pattern formation for over a century. The ease with which chick wing and leg buds can be experimentally manipulated, while the embryo is still in the egg, has resulted in the discovery of important developmental organisers, and subsequently, the signals that they produce. Sonic hedgehog (Shh) is produced by mesenchyme cells of the polarizing region at the posterior margin of the limb bud and specifies positional values across the antero-posterior axis (the axis running from the thumb to the little finger). Detailed experimental embryology has revealed the fundamental parameters required to specify antero-posterior positional values in response to Shh signaling in chick wing and leg buds. In this review, the evolution of the avian wing and leg will be discussed in the broad context of tetrapod paleontology, and more specifically, ancestral theropod dinosaur paleontology. How the parameters that dictate antero-posterior patterning could have been modulated to produce the avian wing and leg digit patterns will be considered. Finally, broader speculations will be made regarding what the antero-posterior patterning of chick limbs can tell us about the evolution of other digit patterns, including those that were found in the limbs of the earliest tetrapods.

John Saunders' ZPA, Sonic hedgehog and digit identity - How does it really all work?

Among John Saunders' many seminal contributions to developmental biology, his discovery of the limb 'zone of polarizing activity' (ZPA) is arguably one of the most memorable and ground-breaking. This discovery introduced the limb as a premier model for understanding developmental patterning and promoted the concept of patterning by a morphogen gradient. In the 50 years since the discovery of the ZPA, Sonic hedgehog (Shh) has been identified as the ZPA factor and the basic components of the signaling pathway and many aspects of its regulation have been elucidated. Although much has also been learned about how it regulates growth, the mechanism by which Shh patterns the limb, how it acts to instruct digit 'identity', nevertheless remains an enigma. This review focuses on what has been learned about Shh function in the limb and the outstanding puzzles that remain to be solved.

Genetics of clubfoot; recent progress and future perspectives

Clubfoot or talipes equinovarus (TEV) is an inborn three-dimensional deformity of leg, ankle and foot. It results from structural defects of several tissues of foot and lower leg leading to abnormal positioning of foot and ankle joints. TEV can lead to long-lasting functional disability, malformation and discomfort if left untreated. Substantial progress has been achieved in the management and diagnosis of limb defects; however, not much is known about the molecular players and signalling pathways underlying TEV disorder. The homeostasis and development of the limb depends on the complex interactions between the lateral plate mesoderm cells and outer ectoderm. These complex interactions include HOX signalling and PITX1-TBX4 pathways. The susceptibility to develop TEV is determined by a number of environmental and genetic factors, although the nature and level of interplay between them remains unclear. Familial occurrence and inter and intra phenotypic variability of TEV is well documented. Variants in genes that code for contractile proteins of skeletal myofibers might play a role in the aetiology of TEV but, to date, no strong candidate genes conferring increased risk have emerged, although variants in TBX4, PITX1, HOXA, HOXC and HOXD clusters genes, NAT2 and others have been shown to be associated with TEV. The mechanisms by which variants in these genes confer risk and the nature of the physical and genetic interaction between them remains to be determined. Elucidation of genetic players and cellular pathways underlying TEV will certainly increase our understanding of the pathophysiology of this deformity.

Transcriptional changes in chick wing bud polarization induced by retinoic acid

BACKGROUND

Retinoic acid is implicated in the induction of the gene encoding Sonic hedgehog (Shh) that specifies anteroposterior positional values and promotes growth of the developing limb bud. However, because retinoic acid is involved in limb initiation, it has been difficult to determine if it could have additional roles in anteroposterior patterning. To investigate this, we implanted retinoic acid-soaked beads to the anterior margin of the chick wing bud and performed microarray analyses prior to onset of Shh expression.

RESULTS

Retinoic acid up-regulates expression of Hoxd11-13 that encode transcription factors implicated in inducing Shh transcription and that are involved in digit development. In our assay, retinoic acid induces Shh transcription and, consequently, a new pattern of digits at a much later stage than anticipated. Retinoic acid represses many anteriorly expressed genes, including Bmp4, Lhx9, Msx2, and Alx4. We provide evidence that retinoic acid influences transcription via induction of dHAND and inhibition of Gli3 to establish a new anteroposterior pre-pattern. We show that transient exposure to retinoic acid can suppress distal development and expedite cells to transcriptionally respond to Shh.

CONCLUSIONS

Our findings reveal how retinoic acid and Shh signaling could cooperate in anteroposterior patterning of the limb. Developmental Dynamics 246:682-690, 2017. © 2017 Wiley Periodicals, Inc.

Sonic Hedgehog Signaling in Limb Development

The gene encoding the secreted protein Sonic hedgehog (Shh) is expressed in the polarizing region (or zone of polarizing activity), a small group of mesenchyme cells at the posterior margin of the vertebrate limb bud. Detailed analyses have revealed that Shh has the properties of the long sought after polarizing region morphogen that specifies positional values across the antero-posterior axis (e.g., thumb to little finger axis) of the limb. Shh has also been shown to control the width of the limb bud by stimulating mesenchyme cell proliferation and by regulating the antero-posterior length of the apical ectodermal ridge, the signaling region required for limb bud outgrowth and the laying down of structures along the proximo-distal axis (e.g., shoulder to digits axis) of the limb. It has been shown that Shh signaling can specify antero-posterior positional values in limb buds in both a concentration- (paracrine) and time-dependent (autocrine) fashion. Currently there are several models for how Shh specifies positional values over time in the limb buds of chick and mouse embryos and how this is integrated with growth. Extensive work has elucidated downstream transcriptional targets of Shh signaling. Nevertheless, it remains unclear how antero-posterior positional values are encoded and then interpreted to give the particular structure appropriate to that position, for example, the type of digit. A distant cis-regulatory enhancer controls limb-bud-specific expression of Shh and the discovery of increasing numbers of interacting transcription factors indicate complex spatiotemporal regulation. Altered Shh signaling is implicated in clinical conditions with congenital limb defects and in the evolution of the morphological diversity of vertebrate limbs.