Left-right development from embryos to brains

Cellular bases of olfactory circuit assembly revealed by systematic time-lapse imaging

Neural circuit assembly features simultaneous targeting of numerous neuronal processes from constituent neuron types, yet the dynamics is poorly understood. Here, we use the Drosophila olfactory circuit to investigate dynamic cellular processes by which olfactory receptor neurons (ORNs) target axons precisely to specific glomeruli in the ipsi- and contralateral antennal lobes. Time-lapse imaging of individual axons from 30 ORN types revealed a rich diversity in extension speed, innervation timing, and ipsilateral branch locations and identified that ipsilateral targeting occurs via stabilization of transient interstitial branches. Fast imaging using adaptive optics-corrected lattice light-sheet microscopy showed that upon approaching target, many ORN types exhibiting "exploring branches" consisted of parallel microtubule-based terminal branches emanating from an F-actin-rich hub. Antennal nerve ablations uncovered essential roles for bilateral axons in contralateral target selection and for ORN axons to facilitate dendritic refinement of postsynaptic partner neurons. Altogether, these observations provide cellular bases for wiring specificity establishment.

Laterality and Left-sidedness in the Nose, Face, and Body: A New Finding

Background:

Asymmetry is a common occurrence in bilaterian animals, particularly human beings. Through examination of patients and their photographs during rhinoplasty, we noted wider left-sided nasal and facial features in most patients. This observation led us to hypothesize that this might be consistent to the whole body.

Methods:

We conducted a study in 3 parts to test the question above. First, we analyzed operating notes of 50 rhinoplasty patients to determine the wider side of the upper, middle, and lower thirds of the nose. Second, we analyzed the width of the face and chest wall in 31 patients to discern any correlation between facial and bodily asymmetry. Third, computerized tomographic scans of the thorax and body of 48 patients were studied to measure the width of the hemithorax and hemipelvic bone.

Results:

(1) Upper vault width was wider on left side (78%). Left middle vault width was wider (88%). The lower lateral cartilage, lateral crura convexity was more prominent on left side (48%), and a wider scroll area was found and trimmed in 21 (left) and 0 (right) cases. The alar base was wider on left side (56%). (2) In the body and face analysis, 64.5% had a wider left-sided face and body. (3) In the computed tomographic scan analysis, same-sided thorax and pelvis asymmetry was seen (85.35%), 33 and 7 of which were left- and right-sided, respectively.

Conclusion:

We observed generalized asymmetry of the face and body with left-sided predominance.

Heterotaxy in Caenorhabditis: widespread natural variation in left-right arrangement of the major organs

Although the arrangement of internal organs in most metazoans is profoundly left-right (L/R) asymmetric with a predominant handedness, rare individuals show full (mirror-symmetric) or partial (heterotaxy) reversals. While the nematode Caenorhabditis elegans is known for its highly determinate development, including stereotyped L/R organ handedness, we found that L/R asymmetry of the major organs, the gut and gonad, varies among natural isolates of the species in both males and hermaphrodites. In hermaphrodites, heterotaxy can involve one or both bilaterally asymmetric gonad arms. Male heterotaxy is probably not attributable to relaxed selection in this hermaphroditic species, as it is also seen in gonochoristic Caenorhabditis species. Heterotaxy increases in many isolates at elevated temperature, with one showing a pregastrulation temperature-sensitive period, suggesting a very early embryonic or germline effect on this much later developmental outcome. A genome-wide association study of 100 isolates showed that male heterotaxy is associated with three genomic regions. Analysis of recombinant inbred lines suggests that a small number of loci are responsible for the observed variation. These findings reveal that heterotaxy is a widely varying quantitative trait in an animal with an otherwise highly stereotyped anatomy, demonstrating unexpected plasticity in an L/R arrangement of the major organs even in a simple animal.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Left-right asymmetry in gut development: what happens next?

The gastrointestinal tract is an asymmetrically patterned organ system. The signals which initiate left-right asymmetry in the developing embryo have been extensively studied, but the downstream steps required to confer asymmetric morphogenesis on the gut organ primordia are less well understood. In this paper we outline key findings on the tissue mechanics underlying gut asymmetry, across a range of species, and use these to synthesise a conserved model for asymmetric gut morphogenesis. We also discuss the importance of correct establishment of left-right asymmetry for gut development and the consequences of perturbations in this process.

Wnt genes define distinct boundaries in the developing human brain: implications for human forebrain patterning

Understanding the factors that govern human forebrain regionalization along the dorsal-ventral and left-right (L-R) axes is likely to be relevant to a wide variety of neurodevelopmental and neuropsychiatric conditions. Recent work in lower vertebrates has identified several critical signaling molecules involved in embryonic patterning along these axes. Among these are the Wingless-Int (WNT) proteins, involved in the formation of dorsal central nervous system (CNS) structures, as well as in visceral L-R asymmetry. We examined the expression of WNT2b and WNT7b in the human brain, because these genes have highly distinctive expression patterns in the embryonic mouse forebrain. In the human fetal telencephalon, WNT2b expression appears to define the cortical hem, a dorsal signaling center previously characterized in mouse, which is also confirmed by BMP7 expression. In diencephalon, WNT2b expression is restricted to medial dorsal structures, including the developing pineal gland and habenular nucleus, both implicated in CNS L-R asymmetry in lower organisms. At 5 weeks gestation, WNT7b is expressed in cerebral cortical and diencephalic progenitor cells. As the cortical plate develops, WNT7b expression shifts, demarcating deep layer neurons of the neocortex and the hippocampal formation. Spatial and temporal expression patterns show startling similarity between human and mouse, suggesting that the developmental roles of these WNT genes may be highly conserved, despite the far greater size and complexity of the human forebrain.

A microRNA controlling left/right neuronal asymmetry in Caenorhabditis elegans

How left/right functional asymmetry is layered on top of an anatomically symmetrical nervous system is poorly understood. In the nematode Caenorhabditis elegans, two morphologically bilateral taste receptor neurons, ASE left (ASEL) and ASE right (ASER), display a left/right asymmetrical expression pattern of putative chemoreceptor genes that correlates with a diversification of chemosensory specificities. Here we show that a previously undefined microRNA termed lsy-6 controls this neuronal left/right asymmetry of chemosensory receptor expression. lsy-6 mutants that we retrieved from a genetic screen for defects in neuronal left/right asymmetry display a loss of the ASEL-specific chemoreceptor expression profile with a concomitant gain of the ASER-specific profile. A lsy-6 reporter gene construct is expressed in less than ten neurons including ASEL, but not ASER. lsy-6 exerts its effects on ASEL through repression of cog-1, an Nkx-type homeobox gene, which contains a lsy-6 complementary site in its 3' untranslated region and that has been shown to control ASE-specific chemoreceptor expression profiles. lsy-6 is the first microRNA to our knowledge with a role in neuronal patterning, providing new insights into left/right axis formation.

Pitx2c attenuation results in cardiac defects and abnormalities of intestinal orientation in developing Xenopus laevis

The experimental manipulation of early embryologic events, resulting in the misexpression of the homeobox transcription factor pitx2, is associated with subsequent defects of laterality in a number of vertebrate systems. To clarify the role of one pitx2 isoform, pitx2c, in determining the left-right axis of amphibian embryos, we examined the heart and gut morphology of Xenopus laevis embryos after attenuating pitx2c mRNA levels using chemically modified antisense oligonucleotides. We demonstrate that the partial depletion of pitx2c mRNA in these embryos results in alteration of both cardiac morphology and intestinal coiling. The most common cardiac abnormality seen was a failure of rightward migration of the outflow tract, while the most common intestinal laterality phenotype seen was a full reversal in the direction of coiling, each present in 23% of embryos injected with the pitx2c antisense oligonucleotide. An abnormality in either the heart or gut further predisposed to a malformation in the other. In addition, a number of other cardiac anomalies were observed after pitx2c mRNA attenuation, including abnormalities of atrial septation, extracellular matrix restriction, relative atrial-ventricular chamber positioning, and restriction of ventricular development. Many of these findings correlate with cardiac defects previously reported in pitx2 null and hypomorphic mice, but can now be assigned specifically to attenuation of the pitx2c isoform in Xenopus.

Analysis of quantitative trait loci for behavioral laterality in mice

Laterality is believed to have genetic components, as has been deduced from family studies in humans and responses to artificial selection in mice, but these genetic components are unknown and the underlying physiological mechanisms are still a subject of dispute. We measured direction of laterality (preferential use of left or right paws) and degree of laterality (absolute difference between the use of left and right paws) in C57BL/6ByJ (B) and NZB/BlNJ (N) mice and in their F(1) and F(2) intercrosses. Measurements were taken of both forepaws and hind paws. Quantitative trait loci (QTL) did not emerge for direction but did for degree of laterality. One QTL for forepaw (LOD score = 5.6) and the second QTL for hind paw (LOD score = 7.2) were both located on chromosome 4 and their peaks were within the same confidence interval. A QTL for plasma luteinizing hormone concentration was also found in the confidence interval of these two QTL. These results suggest that the physiological mechanisms underlying degree of laterality react to gonadal steroids.

Embryonic and larval staging of summer flounder (Paralichthys dentatus)

Early development of flatfishes such as the summer flounder Paralichthys dentatus (Pleuronectiformes) has not been extensively documented, largely because of a dearth of material; however, the recent expansion of flatfish aquaculture has made embryos of P. dentatus readily available for developmental studies. We divide development of P. dentatus embryos and larvae into two main periods, pre- and posthatching, and assign stages within each of those primary divisions. Stages from fertilization to hatching loosely follow the general teleost staging scheme suggested by Shardo ([1995] J Morphol 225:125-167); stages from hatching through metamorphosis are aligned with the series used for Japanese flounder, P. olivaceus (Minami [1982] Nippon Suisan Gakkaishi 48:1581-1588; Fukuhara [1986] Nippon Suisan Gakkaishi 52:81-91). Although length, width, and age may serve as approximate indicators of developmental progression in summer flounder, these characteristics are too variable to form the sole basis of a staging table. Therefore, we define stages by morphological criteria drawn from the development of the jaw apparatus and digestive system, eye migration, and notochord tip flexion. Examination of these morphological features in hatched larvae allows accurate and consistent assessment of developmental stage despite variation in timing and size. The staging scheme for flounder embryonic and larval development presented here should facilitate both experimental and comparative research on summer flounder and other flatfish species.

Heritability of lobar brain volumes in twins supports genetic models of cerebral laterality and handedness

Although the left and right human cerebral hemispheres differ both functionally and anatomically, little is known about the environmental or genetic factors that govern central nervous system asymmetry. Nevertheless, cerebral asymmetry is strongly correlated with handedness, and handedness does have a significant genetic component. To explore the relative contribution of environmental and genetic influences on cerebral asymmetry, we examined the volumes of left and right cerebral cortex in a large cohort of aging identical and fraternal twins and explored their relationship to handedness. Cerebral lobar volumes had a major genetic component, indicating that genes play a large role in changes in brain volume that occur with aging. Shared environment, which likely represents in utero events, had about twice the effect on the left hemisphere as on the right, consistent with less genetic control over the left hemisphere. To test the major genetic models of handedness and cerebral asymmetry, twin pairs were divided into those with two right handers and those with at least one left hander (nonright handers). Genetic factors contributed twice the influence to left and right cerebral hemispheric volumes in right-handed twin pairs, suggesting a large decrement in genetic control of cerebral volumes in the nonright-handed twin pairs. This loss of genetic determination of the left and right cerebral hemispheres in the nonright-handed twin pairs is consistent with models postulating a right-hand/left-hemisphere-biasing genetic influence, a "right-shift" genotype that is lost in nonright handers, resulting in decreased cerebral asymmetry.

Asymmetry: molecular, biologic, embryopathic, and clinical perspectives

This overview of asymmetry addresses the following topics: chiral molecules; asymmetric signaling molecules, including N-cadherin, Shh, Fgf8, lefty1, lefty2, nodal, Pitx2, activin betaB, activin receptor IIA, and cSnR; situs abnormalities; asymmetric cell division; laterality in humans and animals; behavioral asymmetry in humans and animals; asymmetric embryopathies, including Tessier-type "clefts"; hemiasymmetries such as hemihyperplasia, hemihypoplasia, and hemiatrophy; asymmetric vascular syndromes, including Klippel-Trenaunay and Sturge-Weber syndromes; plagiocephaly of the synostotic and deformational types; somatic mosaicism, including a discussion of McCune-Albright syndrome, fibrous dysplasia, GNAS1 mutations, and Proteus syndrome.

Establishment of left-right asymmetry

The vertebrate body plan has bilateral symmetry and left-right asymmetries that are highly conserved. The molecular pathways for left-right development are beginning to be elucidated. Several distinct mechanisms to initiate the vertebrate left-right axis have been proposed. These mechanisms appear to converge on highly conserved expression patterns of genes in the transforming growth factor-beta (TGFbeta) family of cell-cell signaling factors, nodal and lefty-2, and subsequently the expression of the transcription regulator Pitx2, in left lateral plate mesoderm. It is possible that downstream signaling pathways diverge in distinct classes of vertebrates.

Multiple pathways in the midline regulate concordant brain, heart and gut left-right asymmetry

The embryonic midline in vertebrates has been implicated in left-right development, but the mechanisms by which it regulates left-right asymmetric gene expression and organ morphogenesis are unknown. Zebrafish embryos have three domains of left-right asymmetric gene expression that are useful predictors of organ situs. cyclops (nodal), lefty1 and pitx2 are expressed in the left diencephalon; cyclops, lefty2 and pitx2 are expressed in the left heart field; and cyclops and pitx2 are expressed in the left gut primordium. Distinct alterations of these expression patterns in zebrafish midline mutants identify four phenotypic classes that have different degrees of discordance among the brain, heart and gut. These classes help identify two midline domains and several genetic pathways that regulate left-right development. A cyclops-dependent midline domain, associated with the prechordal plate, regulates brain asymmetry but is dispensable for normal heart and gut left-right development. A second midline domain, associated with the anterior notochord, is dependent on no tail, floating head and momo function and is essential for restricting asymmetric gene expression to the left side. Mutants in spadetail or chordino give discordant gene expression among the brain, heart and gut. one-eyed pinhead and schmalspur are necessary for asymmetric gene expression and may mediate signaling from midline domains to lateral tissues. The different phenotypic classes help clarify the apparent disparity of mechanisms proposed to explain left-right development in different vertebrates.

Cardiac looping in the chick embryo: a morphological review with special reference to terminological and biomechanical aspects of the looping process

Understanding early cardiac morphogenesis, especially the process of cardiac looping, is of fundamental interest for diverse biomedical disciplines. During the past few years, remarkable progress has been made in identifying molecular signaling cascades involved in the control of cardiac looping. Given the rapid accumulation of new data on genetic, molecular, and cellular aspects of early cardiac morphogenesis, and given the widespread interest in cardiac looping, it seems worth reviewing those aspects of the looping process that have received less attention during the past few years. These are terminological problems, the "gross" morphological aspects, and the biomechanical concepts of cardiac looping. With respect to terminology, emphasis is given to the unperceived fact that different viewpoints exist as to which part of the normal sequence of morphogenetic events should be called cardiac looping. In a short-term version, which is preferred by developmental biologists, cardiac looping is also called dextral- or rightward-looping. Dextral-looping comprises only those morphogenetic events leading to the transformation of the originally straight heart tube into a c-shaped loop, whose convexity is normally directed toward the right of the body. Cardioembryologists, however, regard cardiac looping merely as a long-term process that may continue until the subdivisions of the heart tube and vessel primordia have approximately reached their definitive topographical relationship to each other. Among cardioembryologists, therefore, three other definitions are used. Taking into account the existence of four different definitions of the term cardiac looping will prevent some confusion in communications on early cardiac morphogenesis. With respect to the gross morphological aspects, emphasis is given to the following points. First, the straight heart tube does not consist of all future regions of the mature heart but only of the primordia of the apical trabeculated regions of the future right and left ventricles, and possibly a part of the primitive conus (outflow tract). The remaining part of the primitive conus and the primordia of the great arteries (truncus arteriosus), the inflow of both ventricles, the primitive atria, and the sinus venosus only appear during looping at the arterial (truncus arteriosus) and venous pole (other primordia). Second, dextral-looping is not simply a bending of the straight heart tube toward the right of the body, as it has frequently been misinterpreted. It results from three different morphogenetic events: (a) bending of the primitive ventricular region of the straight heart tube toward its original ventral side; (b) rotation or torsion of the bending ventricular region around a craniocaudal axis to the right of the body, so that the original ventral side of the heart tube finally forms the right convex curvature and the original dorsal side forms the left concave curvature of the c-shaped heart loop; (c) displacement of the primitive conus to the right of the body by kinking with respect to the arterial pole. Third, dextral-looping does not bring the subdivisions of the heart tube and vessel primordia approximately into their definitive topographical relationship to each other. This is achieved by the morphogenetic events following dextral-looping. This review seeks to bring together data from the diverse disciplines working on the developing heart.

Early cerebro-craniofacial dysmorphogenesis in schizophrenia: a lifetime trajectory model from neurodevelopmental basis to 'neuroprogressive' process

Understanding the temporal origin(s) of schizophrenia, through specifying the earliest identifiable pathology, might indicate when to look for etiological factor(s), what their nature might be, and how course of illness might evolve from these origins. From this premise, earlier formulations are elaborated to offer a rigorously data-driven model that roots schizophrenia in cerebro-craniofacial dysmorphogenesis, particularly along the mid-line but involving other structures, over weeks 9/10 through 14/15 of gestation. However, a brain that has been compromised very early in fetal life is still subject to the normal endogenous programme of developmental, maturational and involutional processes on which a variety of exogenous biological insults and psychosocial stressors can impact adversely over later pregnancy, through infancy and childhood, to maturation and into old age, to sculpt brain structure and function; it should be emphasised that the effects of such endogenous programmes and exogenous insults on such an already developmentally-compromised brain may be different from their effects on a brain whose early fetal origins were unremarkable. From these early origins, a lifetime trajectory model for schizophrenia from developmental basis to 'neuroprogressive' process is constructed. Thereafter, consideration is given to what the model can explain, including cerebral asymmetry and homogeneity, what it cannot explain, what empirical findings would challenge or disprove the model, what cellular and molecular mechanisms might underpin the model, and what are its implications.

Cerberus regulates left-right asymmetry of the embryonic head and heart

BACKGROUND

Most of the molecules known to regulate left-right asymmetry in vertebrate embryos are expressed on the left side of the future trunk region of the embryo. Members of the protein family comprising Cerberus and the putative tumour suppressor Dan have not before been implicated in left-right asymmetry. In Xenopus, these proteins have been shown to antagonise members of the transforming growth factor beta (TGF-beta) and Wnt families of signalling proteins.

RESULTS

Chick Cerberus (cCer) was found to be expressed in the left head mesenchyme and in the left flank of the embryo. Expression on the left side of the head was controlled by Sonic hedgehog (Shh) acting through the TGF-beta family member Nodal; in the flank, cCer was also regulated by Shh, but independently of Nodal. Surprisingly, although no known targets of Cerberus are expressed asymmetrically on the right side of the embryo at these stages, misexpression of cCer on this side of the embryo led to upregulation of the transcription factor Pitx2 and reversal of the direction of heart and head turning, apparently as independent events. Consistent with the possibility that cCer may be acting on bilaterally expressed TGF-beta family members such as the bone morphogenetic proteins (BMPs), this result was mimicked by right-sided misexpression of the BMP antagonist, Noggin.

CONCLUSIONS

Our findings suggest that cCer maintains a delicate balance of different TGF-beta family members involved in laterality decisions, and reveal the existence of partially overlapping molecular pathways regulating left-right asymmetry in the head and trunk of the embryo.