SHH Protein Variance in the Limb Bud Is Constrained by Feedback Regulation and Correlates with Altered Digit Patterning

Foxa1 and Foxa2 orchestrate development of the urethral tube and division of the embryonic cloaca through an autoregulatory loop with Shh

Congenital anomalies of external genitalia affect approximately 1 in 125 live male births. Development of the genital tubercle, the precursor of the penis and clitoris, is regulated by the urethral plate epithelium, an endodermal signaling center. Signaling activity of the urethral plate is mediated by Sonic hedgehog (SHH), which coordinates outgrowth and patterning of the genital tubercle by controlling cell cycle kinetics and expression of downstream genes. The mechanisms that govern Shh transcription in urethral plate cells are largely unknown. Here we show that deletion of Foxa1 and Foxa2 results in persistent cloaca, an incomplete separation of urinary, genital, and anorectal tracts, and severe hypospadias, a failure of urethral tubulogenesis. Loss of Foxa2 and only one copy of Foxa1 results in urethral fistula, an additional opening of the penile urethra. Foxa1/a2 participate in an autoregulatory feedback loop with Shh, in which FOXA1 and FOXA2 positively regulate transcription of Shh in the urethra, and SHH feeds back to negatively regulate Foxa1 and Foxa2 expression. These findings reveal novel roles for Foxa genes in development of the urethral tube and in division of the embryonic cloaca.

L-type voltage-gated Ca2+ channel CaV1.2 regulates chondrogenesis during limb development

All cells, including nonexcitable cells, maintain a discrete transmembrane potential (Vmem), and have the capacity to modulate Vmem and respond to their own and neighbors' changes in Vmem Spatiotemporal variations have been described in developing embryonic tissues and in some cases have been implicated in influencing developmental processes. Yet, how such changes in Vmem are converted into intracellular inputs that in turn regulate developmental gene expression and coordinate patterned tissue formation, has remained elusive. Here we document that the Vmem of limb mesenchyme switches from a hyperpolarized to depolarized state during early chondrocyte differentiation. This change in Vmem increases intracellular Ca2+ signaling through Ca2+ influx, via CaV1.2, 1 of L-type voltage-gated Ca2+ channels (VGCCs). We find that CaV1.2 activity is essential for chondrogenesis in the developing limbs. Pharmacological inhibition by an L-type VGCC specific blocker, or limb-specific deletion of CaV1.2, down-regulates expression of genes essential for chondrocyte differentiation, including Sox9, Col2a1, and Agc1, and thus disturbs proper cartilage formation. The Ca2+-dependent transcription factor NFATc1, which is a known major transducer of intracellular Ca2+ signaling, partly rescues Sox9 expression. These data reveal instructive roles of CaV1.2 in limb development, and more generally expand our understanding of how modulation of membrane potential is used as a mechanism of developmental regulation.

Making and breaking symmetry in development, growth and disease

Consistent asymmetries between the left and right sides of animal bodies are common. For example, the internal organs of vertebrates are left-right (L-R) asymmetric in a stereotyped fashion. Other structures, such as the skeleton and muscles, are largely symmetric. This Review considers how symmetries and asymmetries form alongside each other within the embryo, and how they are then maintained during growth. I describe how asymmetric signals are generated in the embryo. Using the limbs and somites as major examples, I then address mechanisms for protecting symmetrically forming tissues from asymmetrically acting signals. These examples reveal that symmetry should not be considered as an inherent background state, but instead must be actively maintained throughout multiple phases of embryonic patterning and organismal growth.

Sonic Hedgehog Signaling Is Required for Cyp26 Expression during Embryonic Development

Deciphering how signaling pathways interact during development is necessary for understanding the etiopathogenesis of congenital malformations and disease. In several embryonic structures, components of the Hedgehog and retinoic acid pathways, two potent players in development and disease are expressed and operate in the same or adjacent tissues and cells. Yet whether and, if so, how these pathways interact during organogenesis is, to a large extent, unclear. Using genetic and experimental approaches in the mouse, we show that during development of ontogenetically different organs, including the tail, genital tubercle, and secondary palate, Sonic hedgehog (SHH) loss-of-function causes anomalies phenocopying those induced by enhanced retinoic acid signaling and that SHH is required to prevent supraphysiological activation of retinoic signaling through maintenance and reinforcement of expression of the Cyp26 genes. Furthermore, in other tissues and organs, disruptions of the Hedgehog or the retinoic acid pathways during development generate similar phenotypes. These findings reveal that rigidly calibrated Hedgehog and retinoic acid activities are required for normal organogenesis and tissue patterning.

Global optimization using Gaussian processes to estimate biological parameters from image data

Parameter estimation is a major challenge in computational modeling of biological processes. This is especially the case in image-based modeling where the inherently quantitative output of the model is measured against image data, which is typically noisy and non-quantitative. In addition, these models can have a high computational cost, limiting the number of feasible simulations, and therefore rendering most traditional parameter estimation methods unsuitable. In this paper, we present a pipeline that uses Gaussian process learning to estimate biological parameters from noisy, non-quantitative image data when the model has a high computational cost. This approach is first successfully tested on a parametric function with the goal of retrieving the original parameters. We then apply it to estimating parameters in a biological setting by fitting artificial in-situ hybridization (ISH) data of the developing murine limb bud. We expect that this method will be of use in a variety of modeling scenarios where quantitative data is missing and the use of standard parameter estimation approaches in biological modeling is prohibited by the computational cost of the model.

Association of SNP rs80659072 in the ZRS with polydactyly in Beijing You chickens

The Beijing You chicken is a Chinese native breed with superior meat quality and a unique appearance. The G/T mutation of SNP rs80659072 in the Shh long-range regulator of GGA2 is highly associated with the polydactyly phenotype in some chicken breeds. In the present study, this SNP was genotyped using the TaqMan detection method, and its association with the number of toes was analyzed in a flock of 158 birds of the Beijing You population maintained at the Beijing Academy of Agriculture and Forestry Sciences. Furthermore, the skeletal structure of the digits was dissected and assembled in 113 birds. The findings revealed that the toes of Beijing You chickens were rich and more complex than expected. The plausible mutation rs80659072 in the zone of polarizing activity regulatory sequence (ZRS) in chickens was an essential but not sufficient condition for polydactyly and polyphalangy in Beijing You chickens. Several individuals shared the T allele but showed normal four-digit conformations. However, breeding trials demonstrated that the T allele could serve as a strong genetic marker for five-toe selection in Beijing You chickens.