Molecular mechanism of synovial joint site specification and induction in developing vertebrate limbs

The vertebrate appendage comprises three primary segments, the stylopod, zeugopod, and autopod, each separated by joints. The molecular mechanisms governing the specification of joint sites, which define segment lengths and thereby limb architecture, remain largely unknown. Existing literature suggests that reciprocal gradients of Retinoic Acid (RA) and Fibroblast Growth Factor (FGF) signaling define the expression domains of putative segment markers, Meis1, Hoxa11, and Hoxa13. Barx1 is expressed in the presumptive joint sites. Our data demonstrate that RA-FGF signaling gradients define expression domain of Barx1 in the first presumptive joint site. When misexpressed, Barx1 induces novel interzone-like structures and its loss-of-function partially blocks interzone development. Simultaneous perturbations of RA-FGF signaling gradients result in predictable shifts of Barx1 expression domains along the proximo-distal axis and consequently, formation of repositioned joints. Our data suggest that during early limb bud development, Meis1 and Hoxa11 expression domains are overlapping while Barx1 expression domain resides within the Hoxa11 expression domain. However, once the interzone is formed, the expression domains are refined and Barx1 expression domain becomes congruent with the border of these two putative segment markers.

Coxa Vara Deformity in Fibrous Dysplasia/McCune-Albright Syndrome: Prevalence, Natural History and Risk Factors, a two-center study

This study aimed to evaluate the prevalence of and risk factors for coxa vara deformity in patients with Fibrous Dysplasia/McCune-Albright Syndrome (FD/MAS). This study was conducted at the National Institutes of Health and Leiden University Medical Center. All patients with any subtype of FD/MAS, FD involving the proximal femur, ≥1 X-ray available and age <30 years were included. X-rays were scored for the neck-shaft angle (NSA). Varus deformity was defined as NSA <110^o or >10^o below age-specific values. Risk factors for deformity were assessed by nested case-control analysis, comparing patients and femurs with and without deformity, and by linear mixed effects model, modelling temporal NSA decrease (the natural course of the NSA) in non-operated femurs with ≥2 X-rays. Assessed variables included growth hormone excess, hyperthyroidism, hypophosphatemia, >25% of the femur affected, calcar destruction, radiolucency and bilateral involvement. In total 180 patients were studied, 57% female. Mean baseline age was 13.6 (±SD 7.5) years; median follow-up 5.4 (IQR 11.1) years. 63% were diagnosed with MAS. 94 patients were affected bilaterally; 274 FD-femurs were analyzed; 99 femurs had a varus deformity (36%). In the nested case-control analysis, risk factors were: presence of MAS (p<0.001), hyperthyroidism (p<0.001), hypophosphatemia (p<0.001), high percentage of femur affected (p<0.001), calcar destruction (p<0.001). The linear mixed effects model included 114 femurs, identified risk factors were: growth hormone excess (β=7.2, p=0.013), hyperthyroidism (β=11.3, p<0.001), >25% of the femur affected (β=13.2, p=0.046), calcar destruction (β=8.3, p=0.004), radiolucency (β=3.9, p=0.009), bilateral involvement (β=9.8, p=0.010). Visual inspection of the graph of the model demonstrated most progression of deformity if NSA <120^o with age <15. In conclusion, in tertiary care centers, the prevalence of FD/MAS coxa vara deformity was 36%. Risk factors included presence of MAS, high percentage of femur affected, calcar destruction, radiolucency, NSA <120^o and age <15. This article is protected by copyright. All rights reserved.

Co-option of the limb patterning program in cephalopod eye development

Background

Across the Metazoa, similar genetic programs are found in the development of analogous, independently evolved, morphological features. The functional significance of this reuse and the underlying mechanisms of co-option remain unclear. Cephalopods have evolved a highly acute visual system with a cup-shaped retina and a novel refractive lens in the anterior, important for a number of sophisticated behaviors including predation, mating, and camouflage. Almost nothing is known about the molecular-genetics of lens development in the cephalopod.

Results

Here we identify the co-option of the canonical bilaterian limb patterning program during cephalopod lens development, a functionally unrelated structure. We show radial expression of transcription factors SP6-9/sp1, Dlx/dll, Pbx/exd, Meis/hth, and a Prdl homolog in the squid Doryteuthis pealeii, similar to expression required in Drosophila limb development. We assess the role of Wnt signaling in the cephalopod lens, a positive regulator in the developing Drosophila limb, and find the regulatory relationship reversed, with ectopic Wnt signaling leading to lens loss.

Conclusion

This regulatory divergence suggests that duplication of SP6-9 in cephalopods may mediate the co-option of the limb patterning program. Thus, our study suggests that this program could perform a more universal developmental function in radial patterning and highlights how canonical genetic programs are repurposed in novel structures.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01182-2.

Brd4 is required for chondrocyte differentiation and endochondral ossification

Differentiation of multi-potent mesenchymal stromal cells (MSCs) is directed by the activities of lineage-specific transcription factors and co-factors. A subset of these proteins controls the accessibility of chromatin by recruiting histone acetyl transferases or deacetylases that regulate acetylation of the N-termini of H3 and H4 histone proteins. Bromodomain (BRD) proteins recognize these acetylation marks and recruit the RNA pol II containing transcriptional machinery. Our previous studies have shown that Brd4 is required for osteoblast differentiation in vitro. Here, we investigated the role of Brd4 on endochondral ossification in C57BL/6 mice and chondrogenic differentiation in cell culture models. Conditional loss of Brd4 in the mesenchyme (Brd4 cKO, Brd4^fl/fl: Prrx1-Cre) yields smaller mice that exhibit alteration in endochondral ossification. Importantly, abnormal growth plate morphology and delayed long bone formation is observed in juvenile Brd4 cKO mice. One week old Brd4 cKO mice have reduced proliferative and hypertrophic zones within the physis and exhibit a delay in the formation of the secondary ossification center. At the cellular level, Brd4 function is required for chondrogenic differentiation and maturation of both ATDC5 cells and immature mouse articular chondrocytes. Mechanistically, Brd4 loss suppresses Sox9 levels and reduces expression of Sox9 and Runx2 responsive endochondral genes (e.g., Col2a1, Acan, Mmp13 and Sp7/Osx). Collectively, our results indicate that Brd4 is a key epigenetic regulator required for normal chondrogenesis and endochondral ossification.

A dot-stripe Turing model of joint patterning in the tetrapod limb

Iterative joints are a hallmark of the tetrapod limb, and their positioning is a key step during limb development. Although the molecular regulation of joint formation is well studied, it remains unclear what controls the location, number and orientation (i.e. the pattern) of joints within each digit. Here, we propose the dot-stripe mechanism for joint patterning, comprising two coupled Turing systems inspired by published gene expression patterns. Our model can explain normal joint morphology in wild-type limbs, hyperphalangy in cetacean flippers, mutant phenotypes with misoriented joints and suggests a reinterpretation of the polydactylous Ichthyosaur fins as a polygonal joint lattice. By formulating a generic dot-stripe model, describing joint patterns rather than molecular joint markers, we demonstrate that the insights from the model should apply regardless of the biological specifics of the underlying mechanism, thus providing a unifying framework to interrogate joint patterning in the tetrapod limb.

Morphology and development of a novel murine skeletal dysplasia

Background

Limb bones develop and grow by endochondral ossification, which is regulated by specific cell and molecular pathways. Changes in one or more of these pathways can have severe effects on normal skeletal development, leading to skeletal dysplasias. Many skeletal dysplasias are known to result from mis-expression of major genes involved in skeletal development, but the etiology of many skeletal dysplasias remains unknown. We investigated the morphology and development of a mouse line with an uncharacterized mutation exhibiting a skeletal dysplasia-like phenotype (Nabo).

Methods

We used µCT scanning and histology to comprehensively characterize the phenotype and its development, and to determine the developmental stage when this phenotype first appears.

Results

Nabo mice have shorter limb elements compared to wildtype mice, while clavicles and dermal bones of the skull are not affected. Nabo embryos at embryonic stage E14 show shorter limb cartilage condensations. The tibial growth plate in Nabo mice is wider than in wildtype, particularly in the proliferative zone, however proliferative chondrocytes show less activity than wildtype mice. Cell proliferation assays and immunohistochemistry against the chondrogenic marker Sox9 suggest relatively lower, spatially-restricted, chondrocyte proliferation activity in Nabo. Bone volume and trabecular thickness in Nabo tibiae are also decreased compared to wildtype.

Discussion

Our data suggest that the Nabo mutation affects endochondral ossification only, with the strongest effects manifesting in more proximal limb structures. The phenotype appears before embryonic stage E14, suggesting that outgrowth and patterning processes may be affected. Nabo mice present a combination of skeletal dysplasia-like characteristics not present in any known skeletal dysplasia. Further genomic and molecular analysis will help to identify the genetic basis and precise developmental pathways involved in this unique skeletal dysplasia.

Reduced β-catenin expression affects patterning of bone primordia, but not bone maturation

Wnt/β-catenin signaling is involved in patterning of bone primordia, but also plays an important role in the differentiation of chondrocytes and osteoblasts. During these processes the level of β-catenin must be tightly regulated. Excess β-catenin leads to conditions with increased bone mass, whereas loss of β-catenin is associated with osteoporosis or, in extreme cases, the absence of limbs. In this study, we examined skeletogenesis in mice, which retain only 25% of β-catenin. These embryos showed severe morphological abnormalities of which the lack of hindlimbs and misshaped front paws were the most striking. Surprisingly however, calcification of bone primordia occurred normally. Moreover, the Wnt-dependent regulatory network of transcription factors driving the differentiation of cartilage and bone, as well as the expression of extracellular matrix components, were preserved. These findings show that 25% β-catenin is insufficient for the correct patterning of bone primordia, but sufficient for their mineralization. Our approach helps to identify bone morphogenetic processes that can proceed normally even at low β-catenin levels, in contrast to those that require high β-catenin dosages. This information could be exploited to improve the treatment of bone diseases by fine-tuning the individual β-catenin dosage requirements.

Summary: The correct allocation and patterning of skeletal elements during development requires a higher dosage of β-catenin than the maturation and mineralization of these primordia.

Limb patterning genes and heterochronic development of the emu wing bud

Background

The forelimb of the flightless emu is a vestigial structure, with greatly reduced wing elements and digit loss. To explore the molecular and cellular mechanisms associated with the evolution of vestigial wings and loss of flight in the emu, key limb patterning genes were examined in developing embryos.

Methods

Limb development was compared in emu versus chicken embryos. Immunostaining for cell proliferation markers was used to analyze growth of the emu forelimb and hindlimb buds. Expression patterns of limb patterning genes were studied, using whole-mount in situ hybridization (for mRNA localization) and RNA-seq (for mRNA expression levels).

Results

The forelimb of the emu embryo showed heterochronic development compared to that in the chicken, with the forelimb bud being retarded in its development. Early outgrowth of the emu forelimb bud is characterized by a lower level of cell proliferation compared the hindlimb bud, as assessed by PH3 immunostaining. In contrast, there were no obvious differences in apoptosis in forelimb versus hindlimb buds (cleaved caspase 3 staining). Most key patterning genes were expressed in emu forelimb buds similarly to that observed in the chicken, but with smaller expression domains. However, expression of Sonic Hedgehog (Shh) mRNA, which is central to anterior–posterior axis development, was delayed in the emu forelimb bud relative to other patterning genes. Regulators of Shh expression, Gli3 and HoxD13, also showed altered expression levels in the emu forelimb bud.

Conclusions

These data reveal heterochronic but otherwise normal expression of most patterning genes in the emu vestigial forelimb. Delayed Shh expression may be related to the small and vestigial structure of the emu forelimb bud. However, the genetic mechanism driving retarded emu wing development is likely to rest within the forelimb field of the lateral plate mesoderm, predating the expression of patterning genes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0063-5) contains supplementary material, which is available to authorized users.

The transformation suppressor gene Reck is required for postaxial patterning in mouse forelimbs

The membrane-anchored metalloproteinase-regulator RECK has been characterized as a tumor suppressor. Here we report that mice with reduced Reck-expression show limb abnormalities including right-dominant, forelimb-specific defects in postaxial skeletal elements. The forelimb buds of low-Reck mutants have an altered dorsal ectoderm with reduced Wnt7a and Igf2 expression, and hypotrophy in two signaling centers (i.e., ZPA and AER) that are essential for limb outgrowth and patterning. Reck is abundantly expressed in the anterior mesenchyme in normal limb buds; mesenchyme-specific Reck inactivation recapitulates the low-Reck phenotype; and some teratogens downregulate Reck in mesenchymal cells. Our findings illustrate a role for Reck in the mesenchymal-epithelial interactions essential for mammalian development.