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Whole organism lineage tracing by combinatorial and cumulative genome editing

Multicellular systems develop from single cells through a lineage, but current lineage tracing approaches scale poorly to whole organisms. Here we use genome editing to progressively introduce and accumulate diverse mutations in a DNA barcode over multiple rounds of cell division. The barcode, an array of CRISPR/Cas9 target sites, records lineage relationships in the patterns of mutations shared between cells. In cell culture and zebrafish, we show that rates and patterns of editing are tunable, and that thousands of lineage-informative barcode alleles can be generated. We find that most cells in adult zebrafish organs derive from relatively few embryonic progenitors. Genome editing of synthetic target arrays for lineage tracing (GESTALT) will help generate large-scale maps of cell lineage in multicellular systems.

Developmental Biology

Splice Variants of the SWR1-type Nucleosome Remodeling Factor Domino Have Distinct Functions during Drosophila melanogaster Oogenesis

SWR1-type nucleosome remodeling factors replace histone H2A by variants to endow chromatin locally with specialized functionality. In Drosophila melanogaster a single H2A variant, H2A.V, combines functions of mammalian H2A.Z and H2A.X in transcription regulation and DNA damage response. A major role in H2A.V incorporation for the only SWR1-like enzyme in flies, Domino, is assumed, but not well documented in vivo. It is also unclear, whether the two alternatively spliced isoforms, dom-A and dom-B, have redundant or specialized functions. Loss of both DOM isoforms compromises oogenesis causing female sterility. Therefore, we systematically explored roles of the two DOM isoforms during oogenesis using a cell type-specific knockdown approach. Despite their ubiquitous expression, DOM-A and DOM-B have non-redundant functions in germline and soma for egg chamber formation. We show that chromatin incorporation of H2A.V in germline and somatic cells depends on DOM-B, while incorporation in endoreplicating germline nurse cells is independent of DOM. In contrast, DOM-A promotes the removal of H2A.V from stage 5 nurse cells. Remarkably, the two DOM isoforms have distinct functions in cell type-specific development and H2A.V exchange.\n\nSummary Statement\n\nIsoforms of nucleosome remodeling factor Domino change chromatin structure by histone variant exchange to direct essential cellular processes in oocyte development.

Developmental Biology

The Drosophila genes disconnected and disco-related together specify development of adult legs

In the fruit fly, Drosophila melanogaster, specification of the legs begins during embryogenesis when Wingless signaling induces small groups of cells to form the imaginal disc primordia in the thoracic segments. This signal initiates expression of transcription factors that will later be used to pattern the legs. The paralogous genes disconnected and disco-related encode transcription factors that are expressed in the disc primordia during early embryogenesis, and their expression continues in the leg discs during larval and pupal stages. The importance of these two genes in establishing the leg development trajectory was indicated by our previous observation that ectopic expression of either gene in the wing discs cells caused legs to develop in place of wings. However, because of their redundancy and requirement for survival during embryogenesis, we were unable to define their role in development of the adult legs. Here, we report loss-of-function analyses of the disco genes during development of the legs. We discovered that loss of both genes functions causes both truncation of the distal leg with apparent overgrowth of proximal regions and complete loss of legs and ventral thoracic body patterning. At the molecular level we noted reduction or loss of signaling and transcription factors that pattern the proximal-distal axis of the legs. We conclude from these studies that the disco genes promote leg development through regulation of signaling processes, but also by stabilizing expression of the leg determination gene network.

Developmental Biology

Multiscale quantification of tissue behavior during amniote embryo axis elongation

Embryonic axis extension is a complex multi-tissue morphogenetic process responsible for the formation of the posterior part of the amniote body. Cells located in the caudal part of the embryo divide and rearrange to participate in the elongation of the different embryonic tissues (e.g. neural tube, axial and paraxial mesoderm, lateral plate, ectoderm, endoderm). We previously identified the paraxial mesoderm as a crucial player of axis elongation, but how movements and growth are coordinated between the different posterior tissues to drive morphogenesis remain largely unknown. Here we use the quail embryo as a model system to quantify cell behavior and movements in the various tissues of the elongating embryo. We first quantify the tissue-specific contribution to axis elongation by using 3D volumetric techniques, then quantify tissue-specific parameters such as cell density and proliferation at different embryonic stages. To be able to study cell behavior at a multi-tissue scale we used high-resolution 4D imaging of transgenic quail embryos expressing constitutively expressed fluorescent proteins. We developed specific tracking and image analysis techniques to analyze cell motion and compute tissue deformations in 4D. This analysis reveals extensive sliding between tissues during axis extension. Further quantification of \"tissue tectonics\" showed patterns of rotations, contractions and expansions, which are coherent with the multi-tissue behavior observed previously. Our results confirm the central role of the PSM in axis extension; we propose that the PSM specific cell proliferation and migration programs control the coordination of elongation between tissues during axis extension.

Developmental Biology

Apontic regulates cell proliferation and development by activating the expression of hedgehog and cyclin E

Hedgehog (Hh) signaling pathway and Cyclin E are key players in cell proliferation and development. Hyperactivation of hh and cyclin E. has been linked to several types of cancer. However, transcriptional regulation of hh and cyclin E. are not well understood. Here we show that an evolutionarily conserved transcription factor Apontic (Apt) is an activator of hh and cyclin E. in Drosophila. Apt directly promotes the expression of hh and cyclin E. through its binding site in the promoter regions of hh and cyclin E. during wing development. This Apt-dependent proper expression of hh and cyclin E. is required for cell proliferation and development of the wing. Apt-mediated expression of hh and cyclin E. can direct proliferation of Hh-expressing cells and simultaneous growth, patterning and differentiation of Hh-recipient cells. The discovery of the coordinated expression of Hh and principal cell-cycle regulator Cyclin E. by Apt implicates insight into the mechanism by which deregulated hh and cyclin E promotes tumor formation.\n\nSummary statementWe identified a novel role for Apontic as an important common regulator of the transcription of hedgehog and cyclin E. Our study provides important insights into the mechanism of organ development.

Developmental Biology

The C. elegans NF2/Merlin Molecule NFM-1 Non-Autonomously Regulates Neuroblast Migration and Interacts Genetically with the Guidance Cue SLT-1/Slit

During nervous system development, neurons and their progenitors often migrate to their final destinations. In Caenorhabditis elegans, the bilateral Q neuroblasts and their descendants migrate long distances in opposite directions, despite being born in the same posterior region. QR on the right migrates anteriorly and generates the AQR neuron positioned near the head, and QL on the left migrates posteriorly, giving rise to the PQR neuron positioned near the tail. In a screen for genes required for AQR and PQR migration, we identified an allele of nfm-1, which encodes a molecule similar to vertebrate NF2/Merlin, an important tumor suppressor in humans. Mutations in NF2 lead to Neurofibromatosis Type II, characterized by benign tumors of glial tissues. These molecules contain Four-point-one Ezrin Radixin Moesin (FERM) domains characteristic of cytoskeletal-membrane linkers, and vertebrate NF2 is required for epidermal integrity. Vertebrate NF2 can also regulate several transcriptional pathways including the Hippo pathway. Here we demonstrate that in C. elegans, nfm-1 is required for complete migration of AQR and PQR, and that it likely acts outside of the Q cells themselves in a non-autonomous fashion. We also show a genetic interaction between nfm-1 and the C. elegans Slit homolog slt-1, which encodes a conserved secreted guidance cue. In vertebrates, NF2 can control Slit2 mRNA levels through the hippo pathway in axon pathfinding, suggesting a conserved interaction of NF2 and Slit2 in regulating migration.

Developmental Biology

A Key Cytoskeletal Regulator of Ubiquitination Amplifies TGFβ Signaling during Mouse Developmental Vascular Patterning

Vascular development involves de novo formation of a capillary plexus, which is then pruned and remodeled by angiogenic events. Cytoskeletal remodeling and directional endothelial migration are essential for developmental and pathological angiogenesis. Smad-dependent TGF{beta} signaling controls vascular patterning and is negatively regulated by microtubules. Here we show that a positive regulator of TGF{beta} signaling is essential for developmental vascular patterning and microtubule stability. Rudhira/BCAS3 is known to bind microtubules and to play a nodal role in cytoskeletal remodeling and directional endothelial cell (EC) migration in vitro. We demonstrate that the molecular and cellular function of Rudhira is deployed at critical steps in vascular patterning. We generated the first floxed mice for rudhira and find that global or endothelial knockout of rudhira results in mid-gestation lethality due to aberrant embryonic and extra-embryonic vessel patterning and defective cardiac morphogenesis. Rudhira null yolk sac ECs show random and retarded migration. Yolk sac transcriptome analysis revealed key mediators of angiogenic processes and TGF{beta} receptor signaling were perturbed in rudhira null mutants. Molecular and biochemical analyses showed that rudhira depletion reduced microtubule stability but increased expression of pathway inhibitors leading to high levels of SMAD2/3 ubiquitination and reduced activation. These effects were not rescued by exogenous TGF{beta}. However, TGF{beta} treatment of wild type ECs increased Rudhira expression. Further, exogenous Rudhira, which promotes directional cell migration, caused increased SMAD2/3 nuclear translocation and reduced inhibitor levels. Therefore, we propose that Rudhira and TGF{beta} signaling are mutually dependent. Rudhira has a dual function in promoting TGF{beta} signaling, possibly by sequestering microtubules and simultaneously preventing SMAD2/3 ubiquitination to permit EC migration and vascular patterning. TGF{beta} signaling and aberrant human Rudhira (Breast Cancer Amplified Sequence 3, BCAS3) expression are both associated with tumour metastasis. Our study identifies a cytoskeletal, cell type-specific modulator of TGF{beta} signaling important in development and cancer.

Developmental Biology

Antagonistic roles of Polycomb repression and Notch signaling in the maintenance of somatic cell fate in C. elegans.

Reprogramming of somatic cells in intact nematodes allows characterization of cell plasticity determinants, which knowledge is crucial for regenerative cell therapies. By inducing muscle or endoderm transdifferentiation by the ectopic expression of selector transcription factors, we show that cell fate is remarkably robust in fully differentiated larvae. This stability depends on the presence of the Polycomb-associated histone H3K27 methylation, but not H3K9 methylation: in the absence of this epigenetic mark, many cells can be transdifferentiated which correlates with definitive developmental arrest. A candidate RNAi screen unexpectedly uncovered that knock-down of somatic NotchLIN-12 signaling rescues this larval arrest. Similarly in a wild-type context, genetically increasing NotchLIN-12 signaling renders a fraction of the animals sensitive to induced transdifferentiation. This reveals an antagonistic role of the Polycomb repressive complex 2 stabilizing cell fate and Notch signaling enhancing cell plasticity.

Developmental Biology

Genomic dimensions of Su(H)-targeted regulatory belts in Drosophila

Asymmetric Notch signaling promotes divergent fates in select cells throughout metazoan development. In the receiving cell, signaling results in cleavage of the Notch intracellular domain and its import into the nucleus, where it binds Suppressor of Hairless [Su(H)] to promote gene expression in conjunction with contextual cues in the surrounding DNA sequence. To investigate the nature of this contextual logic, we identify 1344 Su(H)-site containing regulatory belts that are conserved across the Drosophila genus. Each Su(H)-type regulatory belt (SUH-RB) is a 0.6-1.0 kb chain of conservation peaks consistent with a transcriptional enhancer or core promoter. These regulatory belts contain one or more canonical binding sites for Su(H) along with ~15-30 other binding sites. SUH-RBs are densely clustered in certain chromosomal regions such as the E(spl)-complex, the Wnt gene complex, and genes encoding Notch receptor ligands (Delta and Serrate). SUH-RBs overlap most known Su(H)/Notch-target enhancers and others, including non-embryonic enhancers that are not identified by embryonic ChIP-seq peaks. Thus, SUH-RBs overcome the stage-specific nature of embryonic ChIP-seq peaks and suggest a pervasive role for contextual tissue-specific pioneer and/or enhancer-licensing factors. SUH-RBs also delineate false positive ChIP-seq peaks, which do not overlap SUH-RBs, are missing even the weakest Su(H)-binding sequences, and have the shortest ChIP peak widths. Last, we characterize several novel enhancers including Su(H)-dependent enhancers at Notch and Delta, intestinal enhancers at A2bp1 and hedgehog, and distinct enhancers at roughest, E2f1, and escargot.

Developmental Biology

A Facilitated Diffusion Mechanism Establishes the Drosophila Dorsal Gradient

The transcription factor NF-{kappa}B plays an important role in the immune system as an apoptotic and inflammatory factor. In the Drosophila melanogaster embryo, a homolog of NF-KB called Dorsal (dl) patterns the dorsal-ventral (DV) axis in a concentration-dependent manner. During early development, dl is sequestered outside the nucleus by Cactus (Cact), homologous to I{kappa}B. Toll signaling at the ventral midline breaks the dl/Cact complex, allowing dl to enter the nucleus where it transcribes target genes. Here we show that dl accumulates on the ventral side of the embryo over the last 5 cleavage cycles and that this accumulation is the result of facilitated diffusion of dl/Cact complex. We speculate that the predominant role for Cact in DV axis specification is to shuttle dl towards the ventral midline. Given that this mechanism has been found in other, independent systems, we suggest it may be more prevalent than previously thought.

Developmental Biology

Drosophila sensory cilia lacking MKS-proteins exhibit striking defects during development but only subtle defects in adults

Cilia are conserved organelles that have important motility, sensory and signalling roles. The transition zone (TZ) at the base of the cilium is critical for cilia function, and defects in several TZ proteins are associated with human congenital ciliopathies such as Nephronophthisis (NPHP) and Meckel Gruber syndrome (MKS). In several species, MKS and NPHP proteins form separate complexes that cooperate with Cep290 to assemble the TZ, but flies lack an obvious NPHP complex. We show that MKS proteins in flies are spatially separated from Cep290 at the TZ, and that flies mutant for individual MKS genes fail to recruit other MKS proteins to the TZ, while Cep290 appears to be recruited normally. Although there are abnormalities in microtubule and membrane organisation in developing MKS mutant cilia, these defects are less apparent in adults, where sensory cilia and sperm flagella function quite normally. Thus, surprisingly, MKS proteins are not essential for cilia or flagella function in flies.

Developmental Biology

Hipk is required for JAK/STAT activity and promotes hemocyte-derived tumorigenesis

Dysregulation of key signaling molecules and pathways are causative of many Human diseases and cancers. A point mutation in the Drosophila Janus kinase (called hop) causes constitutive activation of the JAK/STAT pathway and results in blood cell tumours. We provide robust genetic evidence that Hipk is required for endogenous JAK/STAT activity. Overexpression of Hipk can phenocopy the effects of overactive JAK/STAT mutations and lead to melanized tumors and loss of Hipk can suppress the effects of hyperactive JAK/STAT. Furthermore, Hipk expression in blood cell progenitors causes tumors. PLA experiments show that Hipk can interact with the pathway effector Stat92E. Together our results show that Hipk is a novel factor required for effective JAK/STAT signaling.\n\nSummary StatementLoss of hipk impairs JAK/STAT activity in multiple tissue types and elevated Hipk leads to the formation of blood cell tumors in Drosophila.

Developmental Biology

RNase reverses segment sequence in the anterior of a beetle egg (Callosobruchus maculatus, Coleoptera)

The genetic regulation of anterior-posterior segment pattern development has been elucidated in detail for Drosophila, but it is not canonical for insects. A surprising diversity of regulatory mechanisms is being uncovered not only between insect Orders, but also within the Order of the Diptera. This raises the question whether the same diversity of regulatory mechanisms exists within other insect Orders. This paper draws attention to the promise of the pea beetle Callosobruchus maculatus for elucidating the evolution of pattern regulation mechanisms in Coleoptera and other insect Orders. Introduction of RNase in eggs of Callosobruchus replaces anterior segments with posterior segments oriented in mirror image symmetry to the original posterior segments (double abdomens). Reversal is specific for RNase activity, for treatment of the anterior egg pole and for cytoplasmic RNA. Yield depends on developmental stage, enzyme concentration and temperature. A maximum of 30% of treated eggs reversed segment sequence after puncture in 10.0 g/ml RNase S reconstituted from S-protein and S-peptide at 30 {degrees}C. This result sets the stage for an analysis of the genetic regulation of segment pattern formation in the long germ embryo of the Coleopteran Callosobruchus and for comparison with the short germ embryo of the Coleopteran Tribolium.

Developmental Biology

Information Isometry Technique Reveals Organizational Features in Developmental Cell Lineages

Lineage trees of embryonic development contain much subtle information about the embryogenetic process. One type of information is contained in how the order of nodes are sorted at each level of the tree. Sorting of lineage trees is accomplished using a specific criterion for all levels (each representing a division event) of the tree, resulting in new types of trees (e.g. differentiation tree). Another type of information can be revealed from pairwise comparisons of each type of tree. By using a binary classifier to quantify the first type of information (ordering by level), we can obtain a quantitative measure for the second type of information by using the Hamming distance between equivalent positions in two trees. In this paper, we will introduce a method for calculating and visualizing the information content of embryogenesis called the information isometry technique. Information is extracted from developmental lineages using a binary classification system, and visualization is accomplished through the construction of isometric graphs, which re-represent a tree topology as a series of isometric lines. As the points representing each segment of an isometric line changes color, there is a shift in the underlying tree and its constituent cells. Isometric graphs reveal a number of patterns within cell lineages, including the relative information content of specific subtrees.

Developmental Biology

A correlation between Long noncoding RNA and unpaired DNA silencing in Drosophila

Hybrid transgenes are often recognized as foreign genetic material by cell surveillance mechanisms and are repressed in expression inversely to their copy numbers. Here, we compare the expression of multiple Adh-promoter-white reporter (Adh-w) inserts in paired and unpaired configurations in Drosophila somatic cells. The unpaired copies exhibit a clear repression at the transcriptional level relative to paired gene dosage effect, which is dependent upon long noncoding RNA, Polycomb and piwi. Deficiency mapping using Adh-w constructs showed that a minimal sequence of 532 bp of the Adh promoter is required for unpaired DNA silencing. Long noncoding RNA detected from this region of the Adh promoter is abundant in the unpaired condition. It serves as a docking site for at least two proteins POLYCOMB and Piwi that are essential for active transcriptional silencing. The lesser abundance of noncoding RNAs in the paired configuration only allows PC binding. An active RNA-Protein complex binds to unpaired copies. The loss-of-function piwi mutation relieves transcriptional silencing even in association with POLYCOMB. It suggests that functional RNA-Piwi complex might create a silencing driven chromatin configuration by accumulating histone modifying enzymes at the Adh-w promoter target. This distinct transcriptional silencing that is stronger for unpaired DNA represents a novel mechanism to repress new transposon and foreign DNA insertions for protection of genome integrity.

Developmental Biology

Precision of readout at the hunchback gene

The simultaneous expression of the hunchback gene in the multiple nuclei of the developing fly embryo gives us a unique opportunity to study how transcription is regulated in functional organisms. A recently developed MS2-MCP technique for imaging transcription in living Drosophila embryos allows us to quantify the dynamics of the developmental transcription process. The initial measurement of the morphogens by the hunchback promoter takes place during very short cell cycles, not only giving each nucleus little time for a precise readout, but also resulting in short time traces. Additionally, the relationship between the measured signal and the promoter state depends on the molecular design of the reporting probe. We develop an analysis approach based on tailor made autocorrelation functions that overcomes the short trace problems and quantifies the dynamics of transcription initiation. Based on life imaging data, we identify signatures of bursty transcription initiation from the hunchback promoter. We show that the precision of the expression of the hunchback gene to measure its position along the anterior-posterior axis is low both at the boundary and in the anterior even at cycle 13, suggesting additional post-translational averaging mechanisms to provide the precision observed in fixed material.

Developmental Biology

FORENSIC POST-MORTEM INTERVAL (PMI) ESTIMATES: VARIATION IN FLY DEVELOPMENTAL TIMES OF INDIVIDUALS

Immature stages of flies are paramount in establishing the post-mortem interval (PMI) in forensic practice. My focus is on differences in developmental time that can be influenced by genetic differences or individual life history traits, which latter may be interpreted as life history decisions.\n\nData of a calliphorid fly species (Lucilia ampullacea) are presented: one female produced 300 eggs within an hour and the individual developmental time varied subsequently to a great extent - when the first flies emerged from their puparia there were still first instar larvae in the food (pig liver) provided.\n\nIn conclusion the estimated PMI must be based on a wide range of collected flies (not simply the oldest or largest or widest individual), since a limited sample can be one extremity of a potentially bell shaped (Gaussian) frequency distribution of developmental times -- unrepresentative sampling will bias the PMI in an unpredictable way. One possible solution can be to use large, randomized samples and their body measurement means.

developmental biology

Independent modes of ganglion cell translocation ensure correct lamination of the zebrafish retina

The arrangement of neurons into distinct layers is critical for neuronal connectivity and function of the nervous system. During development, most neurons move from their birthplace to the appropriate layer, where they polarize. However, kinetics and modes of many neuronal translocation events still await exploration. Here, we investigate ganglion cell (RGC) translocation across the embryonic zebrafish retina. After completing their translocation, RGCs establish the most basal retinal layer where they form the optic nerve. Using in toto light sheet microscopy, we show that somal translocation of RGCs is a fast and directed event. It depends on basal process attachment and stabilized microtubules. Interestingly, interference with somal translocation induces a switch to multipolar migration. This multipolar mode is less efficient but still leads to successful RGC layer formation. When both modes are inhibited, RGCs that fail to translocate induce lamination defects, indicating that correct RGC translocation is crucial for subsequent retinal lamination.

Developmental Biology