Lymph heart musculature is under distinct developmental control from lymphatic endothelium

Spinal neuron diversity scales exponentially with swim-to-limb transformation during frog metamorphosis

Vertebrates exhibit a wide range of motor behaviors, ranging from swimming to complex limb-based movements. Here we take advantage of frog metamorphosis, which captures a swim-to-limb-based movement transformation during the development of a single organism, to explore changes in the underlying spinal circuits. We find that the tadpole spinal cord contains small and largely homogeneous populations of motor neurons (MNs) and V1 interneurons (V1s) at early escape swimming stages. These neuronal populations only modestly increase in number and subtype heterogeneity with the emergence of free swimming. In contrast, during frog metamorphosis and the emergence of limb movement, there is a dramatic expansion of MN and V1 interneuron number and transcriptional heterogeneity, culminating in cohorts of neurons that exhibit striking molecular similarity to mammalian motor circuits. CRISPR/Cas9-mediated gene disruption of the limb MN and V1 determinants FoxP1 and Engrailed-1, respectively, results in severe but selective deficits in tail and limb function. Our work thus demonstrates that neural diversity scales exponentially with increasing behavioral complexity and illustrates striking evolutionary conservation in the molecular organization and function of motor circuits across species.

A study on the vertebral column of the dice snake Natrix tessellata (Serpentes, Natricidae) from Denizli (western Anatolia, Turkey)

The vertebral anatomy of snakes has attracted the attention of researchers for decades and numerous studies have been made for extinct and extant species. The present study investigated the morphological variations in vertebral structure among different vertebral regions in the dice snake Natrix tessellata, and provides a detailed anatomical and microstructural description of the vertebral column. Vertebrae were analyzed and compared using x-ray imaging, scanning electron microscopy, micro-computed tomography, and histological techniques. The vertebral column of N. tessellata is divided into three regions: precloacal, cloacal, and caudal. Unlike in many other tetrapods and snakes, the atlas of N. tessellata does not form a complete ring. It has a flat and roughly trilobate shape with a prominent middle lobe. The axis has two hypapophyses. The anterior precloacal region of the vertebral column has longer and more paddle-shaped hypapophyses, distinguishing it from the posterior and mid-trunk vertebrae. The anterior cloacal vertebrae have a short hypapophysis rather than a hemal keel, and the lymphapophysis extends outward, curving slightly. The cotyle and condyle of the caudal vertebrae exhibited a closer resemblance to a rounded shape, while the pleurapophysis extended ventrolaterally and curved ventrally near its distal end. Paired hemapophyses were present at the posterior-most point of the centrum instead of a hypapophysis. In light of previous fossil findings, our anatomical comparison of the vertebral and transverse processes indicates that the extant Natrix has a more flexible and less rigid spine than its ancestors. Overall, the vertebral differences among snake anatomical regions or taxa are a testament to the remarkable diversity and adaptability of these fascinating reptiles.

The Enigmas of Lymphatic Muscle Cells: Where Do They Come From, How Are They Maintained, and Can They Regenerate?

Lymphatic muscle cell (LMC) contractility and coverage of collecting lymphatic vessels (CLVs) are integral to effective lymphatic drainage and tissue homeostasis. In fact, defects in lymphatic contractility have been identified in various conditions, including rheumatoid arthritis, inflammatory bowel disease, and obesity. However, the fundamental role of LMCs in these pathologic processes is limited, primarily due to the difficulty in directly investigating the enigmatic nature of this poorly characterized cell type. LMCs are a unique cell type that exhibit dual tonic and phasic contractility with hybrid structural features of both vascular smooth muscle cells (VSMCs) and cardiac myocytes. While advances have been made in recent years to better understand the biochemistry and function of LMCs, central questions regarding their origins, investiture into CLVs, and homeostasis remain unanswered. To summarize these discoveries, unexplained experimental results, and critical future directions, here we provide a focused review of current knowledge and open questions related to LMC progenitor cells, recruitment, maintenance, and regeneration. We also highlight the high-priority research goal of identifying LMC-specific genes towards genetic conditional- inducible in vivo gain and loss of function studies. While our interest in LMCs has been focused on understanding lymphatic dysfunction in an arthritic flare, these concepts are integral to the broader field of lymphatic biology, and have important potential for clinical translation through targeted therapeutics to control lymphatic contractility and drainage.

Structural and functional analysis of the newt lymphatic system

Regeneration competent vertebrates such as newts and salamanders possess a weakened adaptive immune system characterized by multiple connections between the lymphatic system and the blood vascular system called lymphatic hearts. The role of lymphatic vasculature and these lymphaticovenous connections in regeneration is unknown. We used in-vivo near-infrared lymphangiography, ultra-high frequency ultrasonography, micro-CT lymphangiography, and histological serial section 3-dimentional computer reconstruction to evaluate the lymphatic territories of Cynops pyrrhogaster. We used our model and supermicrosurgery to show that lymphatic hearts are not essential for lymphatic circulation and limb regeneration. Instead, newts possess a novel intraosseous network of lymphatics inside the bone expressing VEGFR-3, LYVE-1 and CD-31. However, we were unable to show Prox-1 expression by these vessels. We demonstrate that adult newt bone marrow functions as both a lymphatic drainage organ and fat reservoir. This study reveals the fundamental anatomical differences between the immune system of urodeles and mammals and provides a model for investigating lymphatics and regeneration.

The Lymphatic System in Zebrafish Heart Development, Regeneration and Disease Modeling

Heart disease remains the single largest cause of death in developed countries, and novel therapeutic interventions are desperately needed to alleviate this growing burden. The cardiac lymphatic system is the long-overlooked counterpart of the coronary blood vasculature, but its important roles in homeostasis and disease are becoming increasingly apparent. Recently, the cardiac lymphatic vasculature in zebrafish has been described and its role in supporting the potent regenerative response of zebrafish heart tissue investigated. In this review, we discuss these findings in the wider context of lymphatic development, evolution and the promise of this system to open new therapeutic avenues to treat myocardial infarction and other cardiopathologies.

New reporter zebrafish line unveils heterogeneity among lymphatic endothelial cells during development

BACKGROUND

In zebrafish, lymphatic endothelial cells (LECs) originate from multiple/several distinct progenitor populations and generate organ-specific lymphatic vasculatures. Cell fate and tissue specificities were determined using a combination of genetically engineered transgenic lines in which the promoter of a LEC-specific gene drives expression of a fluorescent reporter protein.

RESULTS

We established a novel zebrafish transgenic line expressing eGFP under the control of part of the zebrafish batf3 promoter (Basic Leucine Zipper ATF-Like Transcription Factor 3). Spatiotemporal examination of Tg(batf3MIN:eGFP) transgenic fish revealed a typical lymphatic expression pattern, which does not perfectly recapitulate the expression pattern of existing LEC transgenic lines. eGFP⁺ cells constitute a heterogeneous endothelial cell population, which expressed LEC and/or blood endothelial cells (BEC) markers in different tissues. In addition, we characterize the renal eGFP⁺ cell as a population of interest to study kidney diseases and regeneration.

CONCLUSION

Our Tg(batf3MIN:eGFP) reporter zebrafish line provides a useful system to study LEC populations, of which heterogeneity depends on origin of progenitors, tissue environment and physiological conditions. We further developed a novel fish-adapted tissue clearing method, which allows deep imaging and 3D-visualization of vascular and lymphatic networks in the whole organism.

A novel perivascular cell population in the zebrafish brain

The blood-brain barrier is essential for the proper homeostasis and function of the CNS, but its mechanism of function is poorly understood. Perivascular cells surrounding brain blood vessels are thought to be important for blood-brain barrier establishment, but their roles are not well defined. Here, we describe a novel perivascular cell population closely associated with blood vessels on the zebrafish brain. Based on similarities in their morphology, location, and scavenger behavior, these cells appear to be the zebrafish equivalent of cells variably characterized as Fluorescent Granular Perithelial cells (FGPs), perivascular macrophages, or 'Mato Cells' in mammals. Despite their macrophage-like morphology and perivascular location, zebrafish FGPs appear molecularly most similar to lymphatic endothelium, and our imaging studies suggest that these cells emerge by differentiation from endothelium of the optic choroidal vascular plexus. Our findings provide the first report of a perivascular cell population in the brain derived from vascular endothelium.

Characterisation of Development and Electrophysiological Mechanisms Underlying Rhythmicity of the Avian Lymph Heart

Despite significant advances in tissue engineering such as the use of scaffolds, bioreactors and pluripotent stem cells, effective cardiac tissue engineering for therapeutic purposes has remained a largely intractable challenge. For this area to capitalise on such advances, a novel approach may be to unravel the physiological mechanisms underlying the development of tissues that exhibit rhythmic contraction yet do not originate from the cardiac lineage. Considerable attention has been focused on the physiology of the avian lymph heart, a discrete organ with skeletal muscle origins yet which displays pacemaker properties normally only found in the heart. A functional lymph heart is essential for avian survival and growth in ovo. The histological nature of the lymph heart is similar to skeletal muscle although molecular and bioelectrical characterisation during development to assess mechanisms that contribute towards lymph heart contractile rhythmicity have not been undertaken. A better understanding of these processes may provide exploitable insights for therapeutic rhythmically contractile tissue engineering approaches in this area of significant unmet clinical need. Here, using molecular and electrophysiological approaches, we describe the molecular development of the lymph heart to understand how this skeletal muscle becomes fully functional during discrete in ovo stages of development. Our results show that the lymph heart does not follow the normal transitional programme of myogenesis as documented in most skeletal muscle, but instead develops through a concurrent programme of precursor expansion, commitment to myogenesis and functional differentiation which offers a mechanistic explanation for its rapid development. Extracellular electrophysiological field potential recordings revealed that the peak-to-peak amplitude of electrically evoked local field potentials elicited from isolated lymph heart were significantly reduced by treatment with carbachol; an effect that could be fully reversed by atropine. Moreover, nifedipine and cyclopiazonic acid both significantly reduced peak-to-peak local field potential amplitude. Optical recordings of lymph heart showed that the organ’s rhythmicity can be blocked by the HCN channel blocker, ZD7288; an effect also associated with a significant reduction in peak-to-peak local field potential amplitude. Additionally, we also show that isoforms of HCN channels are expressed in avian lymph heart. These results demonstrate that cholinergic signalling and L-type Ca²⁺ channels are important in excitation and contraction coupling, while HCN channels contribute to maintenance of lymph heart rhythmicity.

An early post-traumatic reaction of lymph-heart striated muscle fibers in adult frog Rana temporaria during the first postoperative week: An electron microscopic and autoradiographic study

According to the current opinion, lymph-heart striated muscle represents a specialized type of skeletal muscles in frogs. Here, we studied muscle fibers in mechanically damaged lymph hearts during the first postoperative week using electron-microscopic autoradiography. We present evidence that both, the satellite cells and pre-existing muscle fibers bordering the site of injury, contribute directly to the lymph-heart muscle regeneration. Several muscle fibers located in the vicinity of the damaged area displayed features of nuclear and sarcoplasmic activation. We also observed ultrastructural changes indicating activation of a few satellite cells, namely decondensation of chromatin, enlargement of nuclei and nucleoli, appearance of free ribosomes and rough endoplasmic reticulum tubules in the cytoplasm. Electron-microscopic autoradiography showed that 4 h after single (3)H-thymidine administration on the seventh day after injury not only the activated satellite cells, but also some nuclei of myofibers bordering the injured zone are labeled. We showed that both, the myonuclei of fibers displaying the signs of degenerative/reparative processes in the sarcoplasm and the myonuclei of the fibers enriched with highly organized myofibrils, can re-enter into the S-phase. Our results indicate that the nuclei of lymph-heart myofibers can reactivate DNA synthesis during regenerative myogenesis, unlike the situation in regenerating frog skeletal muscle where myogenic cells do not synthesize DNA at the onset of myofibrillogenesis.

Differential regulation of CASZ1 protein expression during cardiac and skeletal muscle development

BACKGROUND

The zinc-finger transcription factor CASZ1 is required for differentiation of a distinct population of cardiomyocytes during development. However, expression of Casz1 mRNA is detected throughout the developing heart, suggesting the spatial regulation of CASZ1 occurs at the protein level. Relatively little is known about posttranscriptional regulation of Casz1 in the heart.

RESULTS

We generated antibodies that specifically recognize CASZ1 in developing Xenopus embryos, and performed immunofluorescence analysis of CASZ1 during cardiac development. CASZ1 was detected throughout the developing myocardium. CASZ1 was restricted to terminally differentiated cardiomyocytes, and was down-regulated in cells that re-enter the cell cycle. We determined that CASZ1 expression correlated with terminal differentiation in cardiac muscle cells, skeletal muscle cells, and lymph-heart musculature.

CONCLUSIONS

This study indicates that spatially distinct expression of CASZ1 protein may be due to posttranscriptional control of Casz1 mRNA during cardiac development. The results of this study provide insights into the role of Casz1 in cardiac function and in the differentiation of other cell types, including skeletal muscle and lymph heart.

Mechanical forces in lymphatic vascular development and disease

The lymphatic vasculature is essential for fluid homeostasis and transport of immune cells, inflammatory molecules, and dietary lipids. It is composed of a hierarchical network of blind-ended lymphatic capillaries and collecting lymphatic vessels, both lined by lymphatic endothelial cells (LECs). The low hydrostatic pressure in lymphatic capillaries, their loose intercellular junctions, and attachment to the surrounding extracellular matrix (ECM) permit passage of extravasated blood plasma from the interstitium into the lumen of the lymphatic capillaries. It is generally thought that interstitial fluid accumulation leads to a swelling of the ECM, to which the LECs of lymphatic capillaries adhere, for example via anchoring filaments. As a result, LECs are pulled away from the vascular lumen, the junctions open, and fluid enters the lymphatic vasculature. The collecting lymphatic vessels then gather the plasma fluid from the capillaries and carry it through the lymph nodes to the blood circulation. The collecting vessels contain intraluminal bicuspid valves that prevent fluid backflow, and are embraced by smooth muscle cells that contribute to fluid transport. Although the lymphatic vessels are regular subject to mechanical strain, our knowledge of its influence on lymphatic development and pathologies is scarce. Here, we discuss the mechanical forces and molecular mechanisms regulating lymphatic vascular growth and maturation in the developing mouse embryo. We also consider how the lymphatic vasculature might be affected by similar mechanomechanisms in two pathological processes, namely cancer cell dissemination and secondary lymphedema.

Lymphatic regulation in nonmammalian vertebrates

All vertebrate animals share in common the production of lymph through net capillary filtration from their closed circulatory system into their tissues. The balance of forces responsible for net capillary filtration and lymph formation is described by the Starling equation, but additional factors such as vascular and interstitial compliance, which vary markedly among vertebrates, also have a significant impact on rates of lymph formation. Why vertebrates show extreme variability in rates of lymph formation and how nonmammalian vertebrates maintain plasma volume homeostasis is unclear. This gap hampers our understanding of the evolution of the lymphatic system and its interaction with the cardiovascular system. The evolutionary origin of the vertebrate lymphatic system is not clear, but recent advances suggest common developmental factors for lymphangiogenesis in teleost fishes, amphibians, and mammals with some significant changes in the water-land transition. The lymphatic system of anuran amphibians is characterized by large lymphatic sacs and two pairs of lymph hearts that return lymph into the venous circulation but no lymph vessels per se. The lymphatic systems of reptiles and some birds have lymph hearts, and both groups have extensive lymph vessels, but their functional role in both lymph movement and plasma volume homeostasis is almost completely unknown. The purpose of this review is to present an evolutionary perspective in how different vertebrates have solved the common problem of the inevitable formation of lymph from their closed circulatory systems and to point out the many gaps in our knowledge of this evolutionary progression.

Getting out and about: the emergence and morphogenesis of the vertebrate lymphatic vasculature

The lymphatic vascular system develops from the pre-existing blood vasculature of the vertebrate embryo. New insights into lymphatic vascular development have recently been achieved with the use of alternative model systems, new molecular tools, novel imaging technologies and growing interest in the role of lymphatic vessels in human disorders. The signals and cellular mechanisms that facilitate the emergence of lymphatic endothelial cells from veins, guide migration through the embryonic environment, mediate interactions with neighbouring tissues and control vessel maturation are beginning to emerge. Here, we review the most recent advances in lymphatic vascular development, with a major focus on mouse and zebrafish model systems.

Exon capture and bulk segregant analysis: rapid discovery of causative mutations using high-throughput sequencing

Background

Exome sequencing has transformed human genetic analysis and may do the same for other vertebrate model systems. However, a major challenge is sifting through the large number of sequence variants to identify the causative mutation for a given phenotype. In models like Xenopus tropicalis, an incomplete and occasionally incorrect genome assembly compounds this problem. To facilitate cloning of X. tropicalis mutants identified in forward genetic screens, we sought to combine bulk segregant analysis and exome sequencing into a single step.

Results

Here we report the first use of exon capture sequencing to identify mutations in a non-mammalian, vertebrate model. We demonstrate that bulk segregant analysis coupled with exon capture sequencing is not only able to identify causative mutations but can also generate linkage information, facilitate the assembly of scaffolds, identify misassembles, and discover thousands of SNPs for fine mapping.

Conclusion

Exon capture sequencing and bulk segregant analysis is a rapid, inexpensive method to clone mutants identified in forward genetic screens. With sufficient meioses, this method can be generalized to any model system with a genome assembly, polished or unpolished, and in the latter case, it also provides many critical genomic resources.

A revised model of Xenopus dorsal midline development: differential and separable requirements for Notch and Shh signaling

The development of the vertebrate dorsal midline (floor plate, notochord, and hypochord) has been an area of classical research and debate. Previous studies in vertebrates have led to contrasting models for the roles of Shh and Notch signaling in specification of the floor plate, by late inductive or early allocation mechanisms, respectively. Here, we show that Notch signaling plays an integral role in cell fate decisions in the dorsal midline of Xenopus laevis, similar to that observed in zebrafish and chick. Notch signaling promotes floor plate and hypochord fates over notochord, but has variable effects on Shh expression in the midline. In contrast to previous reports in frog, we find that Shh signaling is not required for floor plate vs. notochord decisions and plays a minor role in floor plate specification, where it acts in parallel to Notch signaling. As in zebrafish, Shh signaling is required for specification of the lateral floor plate in the frog. We also find that the medial floor plate in Xenopus comprises two distinct populations of cells, each dependent upon different signals for its specification. Using expression analysis of several midline markers, and dissection of functional relationships, we propose a revised allocation mechanism of dorsal midline specification in Xenopus. Our model is distinct from those proposed to date, and may serve as a guide for future studies in frog and other vertebrate organisms.