Limb specific Acvr1-knockout during embryogenesis in mice exhibits great toe malformation as seen in Fibrodysplasia Ossificans Progressiva (FOP).

PURPOSE: This study analyzes Prx1-specific conditional knockout of Acvr1 aiming to elucidate the endogenous role of Acvr1 during limb formation in early embryonic development. ACVR1 can exhibit activating and inhibiting function in BMP signaling. ACVR1 gain-of-function mutations can cause the rare disease fibrodysplasia ossificans progressiva (FOP), where patients develop ectopic bone replacing soft tissue, tendons and ligaments. METHODS: Whole-mount in situ hybridization and skeletal preparations revealed that following limb-specific conditional knockout of Acvr1, metacarpals and proximal phalanges were shortened and additional cartilage and bone elements were formed. RESULTS: The analysis of a set of marker genes including ligands and receptors of BMP signaling as well as genes involved in patterning and tendon and cartilage formation, revealed temporal disturbances with distinct spatial patterns. The most striking result was that in the absence of Acvr1 in mesoderm precursor cells, first digits were drastically malformed. CONCLUSION: In FOP, malformation of big toes can serve as a first soft marker in diagnostics. The surprising similarities in phenotype between the described conditional knockout of Acvr1 and the FOP mouse model, indicates a natural inhibitory function of ACVR1. This represents a further step towards better understanding the role of Acvr1 and developing treatment options for FOP.

A GDF5 point mutation strikes twice--causing BDA1 and SYNS2.

Growth and Differentiation Factor 5 (GDF5) is a secreted growth factor that belongs to the Bone Morphogenetic Protein (BMP) family and plays a pivotal role during limb development. GDF5 is a susceptibility gene for osteoarthritis (OA) and mutations in GDF5 are associated with a wide variety of skeletal malformations ranging from complex syndromes such as acromesomelic chondrodysplasias to isolated forms of brachydactylies or multiple synostoses syndrome 2 (SYNS2). Here, we report on a family with an autosomal dominant inherited combination of SYNS2 and additional brachydactyly type A1 (BDA1) caused by a single point mutation in GDF5 (p.W414R). Functional studies, including chondrogenesis assays with primary mesenchymal cells, luciferase reporter gene assays and Surface Plasmon Resonance analysis, of the GDF5(W414R) variant in comparison to other GDF5 mutations associated with isolated BDA1 (p.R399C) or SYNS2 (p.E491K) revealed a dual pathomechanism characterized by a gain- and loss-of-function at the same time. On the one hand insensitivity to the main GDF5 antagonist NOGGIN (NOG) leads to a GDF5 gain of function and subsequent SYNS2 phenotype. Whereas on the other hand, a reduced signaling activity, specifically via the BMP receptor type IA (BMPR1A), is likely responsible for the BDA1 phenotype. These results demonstrate that one mutation in the overlapping interface of antagonist and receptor binding site in GDF5 can lead to a GDF5 variant with pathophysiological relevance for both, BDA1 and SYNS2 development. Consequently, our study assembles another part of the molecular puzzle of how loss and gain of function mutations in GDF5 affect bone development in hands and feet resulting in specific types of brachydactyly and SYNS2. These novel insights into the biology of GDF5 might also provide further clues on the pathophysiology of OA.

Extraembryonic gut endoderm cells undergo programmed cell death during development.

Despite a distinct developmental origin, extraembryonic cells in mice contribute to gut endoderm and converge to transcriptionally resemble their embryonic counterparts. Notably, all extraembryonic progenitors share a non-canonical epigenome, raising several pertinent questions, including whether this landscape is reset to match the embryonic regulation and if extraembryonic cells persist into later development. Here we developed a two-colour lineage-tracing strategy to track and isolate extraembryonic cells over time. We find that extraembryonic gut cells display substantial memory of their developmental origin including retention of the original DNA methylation landscape and resulting transcriptional signatures. Furthermore, we show that extraembryonic gut cells undergo programmed cell death and neighbouring embryonic cells clear their remnants via non-professional phagocytosis. By midgestation, we no longer detect extraembryonic cells in the wild-type gut, whereas they persist and differentiate further in p53-mutant embryos. Our study provides key insights into the molecular and developmental fate of extraembryonic cells inside the embryo.

Reconstructing axial progenitor field dynamics in mouse stem cell-derived embryoids.

Embryogenesis requires substantial coordination to translate genetic programs to the collective behavior of differentiating cells, but understanding how cellular decisions control tissue morphology remains conceptually and technically challenging. Here, we combine continuous Cas9-based molecular recording with a mouse embryonic stem cell-based model of the embryonic trunk to build single-cell phylogenies that describe the behavior of transient, multipotent neuro-mesodermal progenitors (NMPs) as they commit into neural and somitic cell types. We find that NMPs show subtle transcriptional signatures related to their recent differentiation and contribute to downstream lineages through a surprisingly broad distribution of individual fate outcomes. Although decision-making can be heavily influenced by environmental cues to induce morphological phenotypes, axial progenitors intrinsically mature over developmental time to favor the neural lineage. Using these data, we present an experimental and analytical framework for exploring the non-homeostatic dynamics of transient progenitor populations as they shape complex tissues during critical developmental windows.

Spatiotemporal transcriptomic maps of whole mouse embryos at the onset of organogenesis.

Spatiotemporal orchestration of gene expression is required for proper embryonic development. The use of single-cell technologies has begun to provide improved resolution of early regulatory dynamics, including detailed molecular definitions of most cell states during mouse embryogenesis. Here we used Slide-seq to build spatial transcriptomic maps of complete embryonic day (E) 8.5 and E9.0, and partial E9.5 embryos. To support their utility, we developed sc3D, a tool for reconstructing and exploring three-dimensional 'virtual embryos', which enables the quantitative investigation of regionalized gene expression patterns. Our measurements along the main embryonic axes of the developing neural tube revealed several previously unannotated genes with distinct spatial patterns. We also characterized the conflicting transcriptional identity of 'ectopic' neural tubes that emerge in Tbx6 mutant embryos. Taken together, we present an experimental and computational framework for the spatiotemporal investigation of whole embryonic structures and mutant phenotypes.

Mutations in GDF5 reveal a key residue mediating BMP inhibition by NOGGIN.

Signaling output of bone morphogenetic proteins (BMPs) is determined by two sets of opposing interactions, one with heterotetrameric complexes of cell surface receptors, the other with secreted antagonists that act as ligand traps. We identified two mutations (N445K,T) in patients with multiple synostosis syndrome (SYM1) in the BMP-related ligand GDF5. Functional studies of both mutants in chicken micromass culture demonstrated a gain of function caused by a resistance to the BMP-inhibitor NOGGIN and an altered signaling effect. Residue N445, situated within overlapping receptor and antagonist interfaces, is highly conserved among the BMP family with the exception of BMP9 and BMP10, in which it is substituted with lysine. Like the mutant GDF5, both BMPs are insensitive to NOGGIN and show a high chondrogenic activity. Ectopic expression of BMP9 or the GDF5 mutants resulted in massive induction of cartilage in an in vivo chick model presumably by bypassing the feedback inhibition imposed by endogenous NOGGIN. Swapping residues at the mutation site alone was not sufficient to render Bmp9 NOG-sensitive; however, successive introduction of two additional substitutions imparted high to total sensitivity on customized variants of Bmp9. In conclusion, we show a new mechanism for abnormal joint development that interferes with a naturally occurring regulatory mechanism of BMP signaling.

Emergence and patterning dynamics of mouse-definitive endoderm.

The segregation of definitive endoderm (DE) from bipotent mesendoderm progenitors leads to the formation of two distinct germ layers. Dissecting DE commitment and onset has been challenging as it occurs within a narrow spatiotemporal window in the embryo. Here, we employ a dual Bra/Sox17 reporter cell line to study DE onset dynamics. We find Sox17 expression initiates in vivo in isolated cells within a temporally restricted window. In 2D and 3D in vitro models, DE cells emerge from mesendoderm progenitors at a temporally regular, but spatially stochastic pattern, which is subsequently arranged by self-sorting of Sox17 + cells. A subpopulation of Bra-high cells commits to a Sox17+ fate independent of external Wnt signal. Self-sorting coincides with upregulation of E-cadherin but is not necessary for DE differentiation or proliferation. Our in vivo and in vitro results highlight basic rules governing DE onset and patterning through the commonalities and differences between these systems.

Hijacking of transcriptional condensates by endogenous retroviruses.

Most endogenous retroviruses (ERVs) in mammals are incapable of retrotransposition; therefore, why ERV derepression is associated with lethality during early development has been a mystery. Here, we report that rapid and selective degradation of the heterochromatin adapter protein TRIM28 triggers dissociation of transcriptional condensates from loci encoding super-enhancer (SE)-driven pluripotency genes and their association with transcribed ERV loci in murine embryonic stem cells. Knockdown of ERV RNAs or forced expression of SE-enriched transcription factors rescued condensate localization at SEs in TRIM28-degraded cells. In a biochemical reconstitution system, ERV RNA facilitated partitioning of RNA polymerase II and the Mediator coactivator into phase-separated droplets. In TRIM28 knockout mouse embryos, single-cell RNA-seq analysis revealed specific depletion of pluripotent lineages. We propose that coding and noncoding nascent RNAs, including those produced by retrotransposons, may facilitate 'hijacking' of transcriptional condensates in various developmental and disease contexts.

Comparative topographical description of the central nervous system of the macaw (Ara ararauna) and the owl (Tyto furcata)Comparative Topographical Description of the Central Nervous System of the Macaw (Ara ararauna) and the Owl (Tyto furcata) / Descripción topográfica comparativa del sistema nervioso central del guacamayo (Ara ararauna) y el búho (Tyto furcata)

SUMMARY: The study of animal neurology has historically focused on the closest descendants of humans, such as monkeys and chimpanzees. Because of this, the neurology of birds remains poorly studied and understood by humans compared to other groups of animals. Thus, the objective was to describe the central nervous system to better understand its functioning, correlating the findings with the role it plays in the physiology and biology of birds, comparing species with different behaviors between herbivores and carnivores, filling gaps in the literature serving as subsidy for future research.

RESUMEN: El estudio de la neurología animal se ha centrado históricamente en los descendientes más cercanos de los humanos, como los monos y los chimpancés. Debido a esto, la neurología de las aves sigue siendo poco estudiada y comprendida, en comparación con la de otros grupos de animales. Así, el objetivo fue describir el sistema nervioso central para comprender mejor su funcionamiento, correlacionando los hallazgos con el rol que juega en la fisiología y biología de las aves, comparando especies con diferentes comportamientos entre herbívoros y carnívoros y llenando vacíos en la literatura que sirvan como base para futuras investigaciones.

Comparative Topographical Description of the Central Nervous System of the Macaw (Ara ararauna) and the Owl (Tyto furcata)

SUMMARY: The study of animal neurology has historically focused on the closest descendants of humans, such as monkeys and chimpanzees. Because of this, the neurology of birds remains poorly studied and understood by humans compared to other groups of animals. Thus, the objective was to describe the central nervous system to better understand its functioning, correlating the findings with the role it plays in the physiology and biology of birds, comparing species with different behaviors between herbivores and carnivores, filling gaps in the literature serving as subsidy for future research.

El estudio de la neurología animal se ha centrado históricamente en los descendientes más cercanos de los humanos, como los monos y los chimpancés. Debido a esto, la neurología de las aves sigue siendo poco estudiada y comprendida, en comparación con la de otros grupos de animales. Así, el objetivo fue describir el sistema nervioso central para comprender mejor su funcionamiento, correlacionando los hallazgos con el rol que juega en la fisiología y biología de las aves, comparando especies con diferentes comportamientos entre herbívoros y carnívoros y llenando vacíos en la literatura que sirvan como base para futuras investigaciones.

Alternative splicing and extensive RNA editing of human TPH2 transcripts.

Brain serotonin (5-HT) neurotransmission plays a key role in the regulation of mood and has been implicated in a variety of neuropsychiatric conditions. Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of 5-HT. Recently, we discovered a second TPH isoform (TPH2) in vertebrates, including man, which is predominantly expressed in brain, while the previously known TPH isoform (TPH1) is primarly a non-neuronal enzyme. Overwhelming evidence now points to TPH2 as a candidate gene for 5-HT-related psychiatric disorders. To assess the role of TPH2 gene variability in the etiology of psychiatric diseases we performed cDNA sequence analysis of TPH2 transcripts from human post mortem amygdala samples obtained from individuals with psychiatric disorders (drug abuse, schizophrenia, suicide) and controls. Here we show that TPH2 exists in two alternatively spliced variants in the coding region, denoted TPH2a and TPH2b. Moreover, we found evidence that the pre-mRNAs of both splice variants are dynamically RNA-edited in a mutually exclusive manner. Kinetic studies with cell lines expressing recombinant TPH2 variants revealed a higher activity of the novel TPH2B protein compared with the previously known TPH2A, whereas RNA editing was shown to inhibit the enzymatic activity of both TPH2 splice variants. Therefore, our results strongly suggest a complex fine-tuning of central nervous system 5-HT biosynthesis by TPH2 alternative splicing and RNA editing. Finally, we present molecular and large-scale linkage data evidencing that deregulated alternative splicing and RNA editing is involved in the etiology of psychiatric diseases, such as suicidal behaviour.