Immunohistochemical demonstration of hyaluronan and its possible involvement in axolotl neural crest cell migration

The Role of Posterior Neural Plate-Derived Presomitic Mesoderm (PSM) in Trunk and Tail Muscle Formation and Axis Elongation

Elongation of the posterior body axis is distinct from that of the anterior trunk and head. Early drivers of posterior elongation are the neural plate/tube and notochord, later followed by the presomitic mesoderm (PSM), together with the neural tube and notochord. In axolotl, posterior neural plate-derived PSM is pushed posteriorly by convergence and extension of the neural plate. The PSM does not go through the blastopore but turns anteriorly to join the gastrulated paraxial mesoderm. To gain a deeper understanding of the process of axial elongation, a detailed characterization of PSM morphogenesis, which precedes somite formation, and of other tissues (such as the epidermis, lateral plate mesoderm and endoderm) is needed. We investigated these issues with specific tissue labelling techniques (DiI injections and GFP⁺ tissue grafting) in combination with optical tissue clearing and 3D reconstructions. We defined a spatiotemporal order of PSM morphogenesis that is characterized by changes in collective cell behaviour. The PSM forms a cohesive tissue strand and largely retains this cohesiveness even after epidermis removal. We show that during embryogenesis, the PSM, as well as the lateral plate and endoderm move anteriorly, while the net movement of the axis is posterior.

Axolotl: A resourceful vertebrate model for regeneration and beyond

The regenerative capacity varies significantly among the animal kingdom. Successful regeneration program in some animals results in the functional restoration of tissues and lost structures. Among the highly regenerative animals, axolotl provides multiple experimental advantages with its many extraordinary characteristics. It has been positioned as a regeneration model organism due to its exceptional renewal capacity, including the internal organs, central nervous system, and appendages, in a scar-free manner. In addition to this unique regeneration ability, the observed low cancer incidence, its resistance to carcinogens, and the reversing effect of its cell extract on neoplasms strongly suggest its usability in cancer research. Axolotl's longevity and efficient utilization of several anti-aging mechanisms underline its potential to be employed in aging studies.

Diverse Roles for Hyaluronan and Hyaluronan Receptors in the Developing and Adult Nervous System

Hyaluronic acid (HA) plays a vital role in the extracellular matrix of neural tissues. Originally thought to hydrate tissues and provide mechanical support, it is now clear that HA is also a complex signaling molecule that can regulate cell processes in the developing and adult nervous systems. Signaling properties are determined by molecular weight, bound proteins, and signal transduction through specific receptors. HA signaling regulates processes such as proliferation, differentiation, migration, and process extension in a variety of cell types including neural stem cells, neurons, astrocytes, microglia, and oligodendrocyte progenitors. The synthesis and catabolism of HA and the expression of HA receptors are altered in disease and influence neuroinflammation and disease pathogenesis. This review discusses the roles of HA, its synthesis and breakdown, as well as receptor expression in neurodevelopment, nervous system function and disease.

Ultrastructural immunolocalization of telomerase and hyaluronate in migrating keratinocytes in a case of oro-pharyngeal squamous cancer

The ultrastructural immunolocalization of telomerase and hyaluronate has been studied in a case of oropharyngeal squamous carcinoma. Immunofluorescence shows that telomerase immunolabeling is present in the cytoplasm and in nuclei of some keratinocytes during their migration into the underlying connective tissue. The electron microscope shows that the nuclear localization of telomerase mainly occurs in the large nucleoli and in likely Cajal bodies, the sites of assembling and maturation of proteins forming the telomerase complex. Aside ribosomes, the nucleolus has a role in the biosynthesis of this reverse transcriptase during cell proliferation in normal tissues and in tumors. The cytoplasmic labeling for telomerase is frequently associated with an irregular network of keratin bundles but the significance of this observation is unclear. Hyaluronate, detected through ultrastructural immunolocalization of a hyaluronate binding protein, is abundant mostly along the cell membrane of the detaching basal keratinocytes during epithelial mesenchymal transition. A coat of hyaluronate surrounds the free keratinocytes of the squamous epithelium and is present around the connective cells present underneath. The study supports the hypothesis that hyaluronate forms a pathway along which epithelial cells can migrate during epidermal mesenchymal transition and may also shield cancer cells from immune cells.

Ultrastructural immunolocalization of hyaluronate in regenerating tail of lizards and amphibians supports an immune-suppressive role to favor regeneration

Hyaluronate is produced in high amount during the initial stages of regeneration of the tail and limbs of lizards, newts, and frog tadpoles. The fine distribution of hyaluronate in the regenerating tail blastemas has been assessed by ultrastructural immunolocalization of the Hyaluronate Binding Protein (HABP), a protein that indirectly reveals the presence of hyaluronate in tissues. The present electron microscopic study shows that HABP is detected in the cytoplasm but this proteins is mainly localized on the surfaces of cells in the wound epidermis and mesenchymal cells of the blastema. HABP appears, therefore, accumulated along the cell surface, indicating that hyaluronate coats these embryonic-like cells and their antigens. The high level of hyaluronate in the blastema, aside favoring tissue hydration, cell movements, and remodeling for blastema formation and growth, likely elicits a protection from the possible immune-reaction of lymphocytes and macrophages to embryonic-fetal-like antigens present on the surface of blastema and epidermal cells. Their survival, therefore, allows the continuous multiplication of these cells in regions rich in hyaluronate, promoting the regeneration of a new tail or limbs. The study suggests that organ regeneration in vertebrates is only possible in the presence of high hyaluronate content and hydration. These two conditions facilitate cell movement, immune-protection, and activate the Wnt signaling pathway, like during development.

A transitional extracellular matrix instructs cell behavior during muscle regeneration

Urodele amphibians regenerate appendages through the recruitment of progenitor cells into a blastema that rebuilds the lost tissue. Blastemal formation is accompanied by extensive remodeling of the extracellular matrix. Although this remodeling process is important for appendage regeneration, it is not known whether the remodeled matrix directly influences the generation and behavior of blastemal progenitor cells. By integrating in vivo 3-dimensional spatiotemporal matrix maps with in vitro functional time-lapse imaging, we show that key components of this dynamic matrix, hyaluronic acid, tenascin-C and fibronectin, differentially direct cellular behaviors including DNA synthesis, migration, myotube fragmentation and myoblast fusion. These data indicate that both satellite cells and fragmenting myofibers contribute to the regeneration blastema and that the local extracellular environment provides instructive cues for the regenerative process. The fact that amphibian and mammalian myoblasts exhibit similar responses to various matrices suggests that the ability to sense and respond to regenerative signals is evolutionarily conserved.

Electron microscopy of high pressure frozen samples: bridging the gap between cellular ultrastructure and atomic resolution

Transmission electron microscopy has provided most of what is known about the ultrastructural organization of tissues, cells, and organelles. Due to tremendous advances in crystallography and magnetic resonance imaging, almost any protein can now be modeled at atomic resolution. To fully understand the workings of biological "nanomachines" it is necessary to obtain images of intact macromolecular assemblies in situ. Although the resolution power of electron microscopes is on the atomic scale, in biological samples artifacts introduced by aldehyde fixation, dehydration and staining, but also section thickness reduces it to some nanometers. Cryofixation by high pressure freezing circumvents many of the artifacts since it allows vitrifying biological samples of about 200 mum in thickness and immobilizes complex macromolecular assemblies in their native state in situ. To exploit the perfect structural preservation of frozen hydrated sections, sophisticated instruments are needed, e.g., high voltage electron microscopes equipped with precise goniometers that work at low temperature and digital cameras of high sensitivity and pixel number. With them, it is possible to generate high resolution tomograms, i.e., 3D views of subcellular structures. This review describes theory and applications of the high pressure cryofixation methodology and compares its results with those of conventional procedures. Moreover, recent findings will be discussed showing that molecular models of proteins can be fitted into depicted organellar ultrastructure of images of frozen hydrated sections. High pressure freezing of tissue is the base which may lead to precise models of macromolecular assemblies in situ, and thus to a better understanding of the function of complex cellular structures.

Close-to-native ultrastructural preservation by high pressure freezing

The objective of modern transmission electron microscopy (TEM) in life science is to observe biological structures in a state as close as possible to the living organism. TEM samples have to be thin and to be examined in vacuum; therefore only solid samples can be investigated. The most common and popular way to prepare samples for TEM is to subject them to chemical fixation, staining, dehydration, and embedding in a resin (all of these steps introduce considerable artifacts) before investigation. An alternative is to immobilize samples by cooling. High pressure freezing is so far the only approach to vitrify (water solidification without ice crystal formation) bulk biological samples of about 200 micrometer thick. This method leads to an improved ultrastructural preservation. After high pressure freezing, samples have to be subjected to follow-up procedure, such as freeze-substitution and embedding. The samples can also be sectioned into frozen hydrated sections and analyzed in a cryo-TEM. Also for immunocytochemistry, high pressure freezing is a good and practicable way.

The combination of chemical fixation procedures with high pressure freezing and freeze substitution preserves highly labile tissue ultrastructure for electron tomography applications

The emergence of electron tomography as a tool for three dimensional structure determination of cells and tissues has brought its own challenges for the preparation of thick sections. High pressure freezing in combination with freeze substitution provides the best method for obtaining the largest volume of well-preserved tissue. However, for deeply embedded, heterogeneous, labile tissues needing careful dissection, such as brain, the damage due to anoxia and excision before cryofixation is significant. We previously demonstrated that chemical fixation prior to high pressure freezing preserves fragile tissues and produces superior tomographic reconstructions compared to equivalent tissue preserved by chemical fixation alone. Here, we provide further characterization of the technique, comparing the ultrastructure of Flock House Virus infected DL1 insect cells that were (1) high pressure frozen without fixation, (2) high pressure frozen following fixation, and (3) conventionally prepared with aldehyde fixatives. Aldehyde fixation prior to freezing produces ultrastructural preservation superior to that obtained through chemical fixation alone that is close to that obtained when cells are fast frozen without fixation. We demonstrate using a variety of nervous system tissues, including neurons that were injected with a fluorescent dye and then photooxidized, that this technique provides excellent preservation compared to chemical fixation alone and can be extended to selectively stained material where cryofixation is impractical.

Hyaluronan is organized into fiber-like structures along migratory pathways in the developing mouse cerebellum

Hyaluronan is a free glycosaminoglycan which is abundant in the extracellular matrix of the developing brain. Although not covalently linked to any protein it can act as a backbone molecule forming aggregates with chondroitin sulfate proteoglycans of the lectican family and link proteins. Using neurocan-GFP as a direct histochemical probe we analyzed the distribution and organization of hyaluronan in the developing mouse cerebellum, and related its fine structure to cell types of specified developmental stages. We observed a high affinity of this probe to fiber-like structures in the prospective white matter which are preferentially oriented parallel to the cerebellar cortex during postnatal development suggesting a specially organized form of hyaluronan. In other layers of the cerebellar cortex, the hyaluronan organization seemed to be more diffuse. During the second postnatal week, the overall staining intensity of hyaluronan in the white matter declined but fiber-like structures were still present at the adult stage. This type of hyaluronan organization is different from perineuronal nets e.g. found in deep cerebellar nuclei. Double staining experiments with cell type specific markers indicated that these fiber-like structures are predominantly situated in regions where motile cells such as Pax2-positive inhibitory interneuron precursors and MBP-positive oligodendroglial cells are located. In contrast, more stationary cells such as mature granule cells and Purkinje cells are associated with lower levels of hyaluronan in their environment. Thus, hyaluronan-rich fibers are concentrated at sites where specific neural precursor cell types migrate, and the anisotropic orientation of these fibers suggests that they may support guided neural migration during brain development.

XHas2 activity is required during somitogenesis and precursor cell migration in Xenopus development

In vertebrates, hyaluronan biosynthesis is regulated by three transmembrane catalytic enzymes denoted Has1, Has2 and Has3. We have previously cloned the Xenopus orthologues of the corresponding genes and defined their spatiotemporal distribution during development. During mammalian embryogenesis, Has2 activity is known to be crucial, as its abrogation in mice leads to early embryonic lethality. Here, we show that, in Xenopus,morpholino-mediated loss-of-function of XHas2 alters somitogenesis by causing a disruption of the metameric somitic pattern and leads to a defective myogenesis. In the absence of XHas2, early myoblasts underwent apoptosis, failing to complete their muscle differentiation programme. XHas2 activity is also required for migration of hypaxial muscle cells and trunk neural crest cells (NCC). To approach the mechanism whereby loss of HA,following XHas2 knockdown, could influence somitogenesis and precursor cell migration, we cloned the orthologue of the primary HA signalling receptor CD44 and addressed its function through an analogous knockdown approach. Loss of XCD44 did not disturb somitogenesis, but strongly impaired hypaxial muscle precursor cell migration and the subsequent formation of the ventral body wall musculature. In contrast to XHas2,loss of function of XCD44 did not seem to be essential for trunk NCC migration, suggesting that the HA dependence of NCC movement was rather associated with an altered macromolecular composition of the ECM structuring the cells' migratory pathways. The presented results, extend our knowledge on Has2 function and, for the first time, demonstrate a developmental role for CD44 in vertebrates. On the whole, these data underlie and confirm the emerging importance of cell-ECM interactions and modulation during embryonic development.