Programmed cell death in Xenopus laevis spinal cord, tail and other tissues, prior to, and during, metamorphosis

Protein S-nitrosylation: Nitric oxide signalling during anuran tail regression

Tail regression is a remarkable process where a complex organ like the tail is completely resorbed by cell death during anuran metamorphosis. Nitric oxide is a signalling molecule involved in various physiological processes and along with reactive nitrogen species induces apoptosis. The present study describes the contribution of nitric oxide and reactive nitrogen species (nitrosative stress) during tail regression in the tadpoles of Indian tree frog, Polypedates maculatus. Spectrophotometric estimation revealed significantly higher levels of nitrite, nitrate and peroxynitrite in the regressing tails of the late climactic stages as compared to the early climactic stages and pre-regressing tails. S-nitrosylated proteins were detected in the apoptotic cells of epidermis and muscle, denervated and fragmented myofibres, outer notochordal sheath of the degenerating notochord, endothelium of blood vessels, blood cells and spinal cord of the regressing tail of the late climactic stages using fluorescent detection methods. Thus, a higher level of nitrosative stress in the late climactic stages is suggested to cause S-nitrosylation of proteins and subsequent apoptosis in the tail tissues. Macrophages were found engulfing the apoptotic cells and cell debris at the distal end of the regressing tail. Interestingly, macrophages were always found to be associated with melanocytes suggesting a close association for clearing cell debris by phagocytosis.

Comparisons of cell proliferation and cell death from tornaria larva to juvenile worm in the hemichordate Schizocardium californicum

Background

There are a wide range of developmental strategies in animal phyla, but most insights into adult body plan formation come from direct-developing species. For indirect-developing species, there are distinct larval and adult body plans that are linked together by metamorphosis. Some outstanding questions in the development of indirect-developing organisms include the extent to which larval tissue undergoes cell death during the process of metamorphosis and when and where the tissue that will give rise to the adult originates. How do the processes of cell division and cell death redesign the body plans of indirect developers? In this study, we present patterns of cell proliferation and cell death during larval body plan development, metamorphosis, and adult body plan formation, in the hemichordate Schizocardium californium (Cameron and Perez in Zootaxa 3569:79–88, 2012) to answer these questions.

Results

We identified distinct patterns of cell proliferation between larval and adult body plan formation of S. californicum. We found that some adult tissues proliferate during the late larval phase prior to the start of overt metamorphosis. In addition, using an irradiation and transcriptomic approach, we describe a genetic signature of proliferative cells that is shared across the life history states, as well as markers that are unique to larval or juvenile states. Finally, we observed that cell death is minimal in larval stages but begins with the onset of metamorphosis.

Conclusions

Cell proliferation during the development of S. californicum has distinct patterns in the formation of larval and adult body plans. However, cell death is very limited in larvae and begins during the onset of metamorphosis and into early juvenile development in specific domains. The populations of cells that proliferated and gave rise to the larvae and juveniles have a genetic signature that suggested a heterogeneous pool of proliferative progenitors, rather than a set-aside population of pluripotent cells. Taken together, we propose that the gradual morphological transformation of S. californicum is mirrored at the cellular level and may be more representative of the development strategies that characterize metamorphosis in many metazoan animals.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13227-022-00198-1.

Growth at Cold Temperature Increases the Number of Motor Neurons to Optimize Locomotor Function

During vertebrate development, spinal neurons differentiate and connect to generate a system that performs sensorimotor functions critical for survival. Spontaneous Ca²⁺ activity regulates different aspects of spinal neuron differentiation. It is unclear whether environmental factors can modulate this Ca²⁺ activity in developing spinal neurons to alter their specialization and ultimately adjust sensorimotor behavior to fit the environment. Here, we show that growing Xenopus laevis embryos at cold temperatures results in an increase in the number of spinal motor neurons in larvae. This change in spinal cord development optimizes the escape response to gentle touch of animals raised in and tested at cold temperatures. The cold-sensitive channel TRPM8 increases Ca²⁺ spike frequency of developing ventral spinal neurons, which in turn regulates expression of the motor neuron master transcription factor HB9. TRPM8 is necessary for the increase in motor neuron number of animals raised in cold temperatures and for their enhanced sensorimotor behavior when tested at cold temperatures. These findings suggest the environment modulates neuronal differentiation to optimize the behavior of the developing organism.

Spencer et al. discover that Xenopus larvae reared in cold temperature are better equipped to escape upon touch at cold temperature relative to warm-grown siblings. This advantage is dependent on the cold-sensitive channel TRPM8, which is necessary for increased Ca²⁺ spike frequency in embryonic spinal neurons, their differentiation, and survival.

Immunohistochemical Localization of Calpains in the Amphibian Xenopus laevis

Though histochemical techniques have been used for decades, they are still very important in basic research. They make it possible to work on fixed tissues and provide a large amount of information in a relatively short time and at a low cost. Here we describe methods for indirect immunohistochemistry and immunofluorescence on sections of tadpoles and tissues of adult amphibians belonging to the species Xenopus laevis. The objective is to localize calpains within tissues in order to understand their involvement in cellular processes.

Apoptosis in larval and frog skin of Rana pipiens, R. catesbeiana, and Ceratophrys ornata

Apoptosis (programmed cell death) occurs during normal development of anurans in organs such as gills, gut, and tail. For example, apoptotic cells have been reported in the luminal epithelium along the length of the digestive tract of both larvae and frogs; however, timing of the peak number of such cells varies in different species. The purpose of the present study was to ascertain whether apoptosis also varies by species during metamorphic restructuring of the skin (as larval epithelium is replaced by adult epidermis). To determine this, cross-sections of dorsal skin from representative larval stages and frogs of Rana pipiens, R. catesbeiana, and Ceratophrys ornata were incubated with monoclonal antibody against active caspase-3, one of the main enzymes in the apoptotic cascade. We observed apoptotic cells in the epidermis of the skin of the three species and found that such cells were more numerous in larval stages than in frogs and more abundant in the two ranid species than in C. ornata. These results contribute to our understanding of metamorphic changes in anuran skin.

Immunohistochemical localization of cathepsin D and a possible role of melanocytes during tail resorption in tadpoles of a tropical toad

Programmed cell death during anuran tail resorption is primarily brought about by apoptosis. Cathepsin D, a lysosomal aspartyl protease, is involved in the death of tail tissues. Thus, anuran tail resorption presents an ideal model to study cathepsin-mediated cell death during vertebrate development. Present study describes the trend of specific activity of cathepsin D in the tail of different developmental stages and immunohistochemical localization of cathepsin D in the tail tissues of the common Asian toad, Duttaphrynus melanostictus. Cathepsin D was involved in programmed cell death in epidermis, muscle, spinal cord, and blood cells in the resorbing tail. Interestingly, it was also involved in the pre-resorbing tail before visible tail resorption which indicates initiation of cell death even before actually the tail resorbs. Melanocytes were found to be one of the causative agents in degrading tail tissues and were associated with the degradation of muscle, epidermis and spinal cord of the resorbing tail.

Silver nanoparticle exposure causes apoptotic response in the earthworm Lumbricus terrestris (Oligochaeta)

In terrestrial ecotoxicology there is a serious lack of data for potential hazards posed by engineered nanoparticles (ENPs). This is partly due to complex interactions between ENPs and the soil matrix, but also to the lack of suitable toxicological end points in organisms that are exposed to ENPs in a relevant manner. Earthworms are key organisms in terrestrial ecosystems, but so far only physiological end points of low sensitivity have been used in ecotoxicity studies with ENPs. We exposed the earthworm Lumbricus terrestris to silver nanoparticles and measured their impact on apoptosis in different tissues. Increased apoptotic activity was detected in a range of tissues both at acute and sublethal concentrations (down to 4 mg/kg soil). Comparing exposure in water and soil showed reduced bioavailability in soil reflected in the apoptotic response. Apoptosis appears to be a sensitive end point and potentially a powerful tool for quantifying environmental hazards of ENPs.

The anatomy and development of the claws of Xenopus laevis (Lissamphibia: Anura) reveal alternate pathways of structural evolution in the integument of tetrapods

Digital end organs composed of hard, modified epidermis, generally referred to as claws, are present in mammals and reptiles as well as in several non-amniote taxa such as clawed salamanders and frogs, including Xenopus laevis. So far, only the claws and nails of mammals have been characterized extensively and the question of whether claws were present in the common ancestor of all extant tetrapods is as yet unresolved. To provide a basis for comparisons between amniote and non-amniote claws, we investigated the development, growth and ultrastructure of the epidermal component of the claws of X. laevis. Histological examination of developing claws of X. laevis shows that claw formation is initiated at the tip of the toe by the appearance of superficial cornified cells that are dark brown. Subsequent accumulation of new, proximally extended claw sheath corneocyte layers increases the length of the claw. Histological studies of adult claws show that proliferation of cornifying claw sheath cells occurs along the entire length of the claw-forming epidermis. Living epidermal cells that are converting into the cornified claw sheath corneocytes undergo a form of programmed cell death that is accompanied by degradation of nuclear DNA. Subsequently, the cytoplasm and the nuclear remnants acquire a brown colour by an as-yet unknown mechanism that is likely homologous to the colouration mechanism that occurs in other hard, cornified structures of amphibians such as nuptial pads and tadpole beaks. Transmission electron microscopy revealed that the cornified claw sheath consists of parallel layers of corneocytes with interdigitations being confined to intra-layer contacts and a cementing substance filling the intercorneocyte spaces. Together with recent reports that showed the main molecular components of amniote claws are absent in Xenopus, our data support the hypothesis that claws of amphibians likely represent clade-specific innovations, non-homologous to amniote claws.

Apoptosis in amphibian organs during metamorphosis

During amphibian metamorphosis, the larval tissues/organs rapidly degenerate to adapt from the aquatic to the terrestrial life. At the cellular level, a large quantity of apoptosis occurs in a spatiotemporally-regulated fashion in different organs to ensure timely removal of larval organs/tissues and the development of adult ones for the survival of the individuals. Thus, amphibian metamorphosis provides us a good opportunity to understand the mechanisms regulating apoptosis. To investigate this process at the molecular level, a number of thyroid hormone (TH) response genes have been isolated from several organs of Xenopus laevis tadpoles and their expression and functional analyses are now in progress using modern molecular and genetic technologies. In this review, we will first summarize when and where apoptosis occurs in typical larva-specific and larval-to-adult remodeling amphibian organs to highlight that the timing of apoptosis is different in different tissues/organs, even though all are induced by the same circulating TH. Next, to discuss how TH spatiotemporally regulates the apoptosis, we will focus on apoptosis of the X. laevis small intestine, one of the best characterized remodeling organs. Functional studies of TH response genes using transgenic frogs and culture techniques have shown that apoptosis of larval epithelial cells can be induced by TH either cell-autonomously or indirectly through interactions with extracellular matrix (ECM) components of the underlying basal lamina. Here, we propose that multiple intra- and extracellular apoptotic pathways are coordinately controlled by TH to ensure massive but well-organized apoptosis, which is essential for the proper progression of amphibian metamorphosis.

Neuromodulation and developmental plasticity in the locomotor system of anuran amphibians during metamorphosis

Metamorphosis in frogs has long fascinated laymen and scientists alike. This remarkable developmental transformation involves the simultaneous remodelling of almost every organ in the body, including the gut, associated with a switch in diet from filter feeder to predator, and the visual system, from laterally-directed monocular to forward-directed binocular vision. In the context of locomotion there is the complete loss of the tail, the main structure involved in generating thrust during swimming in larvae, and the gain of the limbs which produce rhythmic extension-flexion kicks during swimming and jumping. Here we review recent evidence from experiments utilizing novel in vitro isolated preparations of the Xenopus laevis spinal cord and brainstem which remain viable for several days and can generate motor rhythms similar to those that would normally drive locomotion in vivo. The results indicate that the developing limb circuitry is born from within the existing axial-based network, which acts like a functional scaffold. Initially the limb activity shares the same left-right alternation coordination and relatively high frequency as the tail swimming network. Only later, once the limbs are fully functional, does the limb network break free to produce left-right synchrony of limb motoneuron bursting and with a different, slower cadence than the tail-based system. During the initial formation of the limb networks nitric oxide-producing neurons appear in the spinal cord, but occupy regions other than those in which the new limb circuitry is developing. Now exogenous nitric oxide facilitates locomotor activity, in contrast to its inhibitory effects on swimming at earlier larval stages of development.

Developmental and regional expression of NADPH-diaphorase/nitric oxide synthase in spinal cord neurons correlates with the emergence of limb motor networks in metamorphosing Xenopus laevis

Metamorphosis in anuran amphibians requires a complete transformation in locomotor strategy from undulatory tadpole swimming to adult quadrupedal propulsion. The underlying reconfiguration of spinal networks may be influenced by various neuromodulators including nitric oxide, which is known to play an important role in CNS development and plasticity in diverse species, including metamorphosis of amphibians. Using NADPH-diaphorase (NADPH-d) staining and neuronal nitric oxide synthase (nNOS) immunofluorescence labelling, the expression and developmental distribution of NOS-containing neurons in the spinal cord and brainstem were analysed in all metamorphic stages of Xenopus laevis. Wholemount preparations of the spinal cord from early stages of metamorphosis (coincident with emergence of the fore- and hindlimb buds) revealed two clusters of NOS-positive neurons interspersed with areas devoid of stained somata. These cells were distributed in three topographic subgroups, the most ventral of which had axonal projections that crossed the ventral commissure. Motoneurons innervating the fore- and hindlimb buds were retrogradely labelled with horseradish peroxidase (HRP) to determine their position in relation to the two NOS-expressing cord regions. Limb motoneurons and NOS-positive cells did not overlap, indicating that during early stages of metamorphosis nitrergic neurons are excluded from regions where spinal limb circuits are forming. As metamorphosis progresses, NOS expression became distributed along the length of the spinal cord together with an increase in the number and intensity of labelled cells and fibers. NOS expression reached a peak as the forelimbs emerge then declined. These findings are consistent with a role for nitric oxide (NO) in the developmental transition from undulatory swimming to quadrupedal locomotion.

Larval arm resorption proceeds concomitantly with programmed cell death during metamorphosis of the sea urchin Hemicentrotus pulcherrimus

Sea urchins are excellent models to elucidate metamorphic phenomena of echinoderms. However, little attention has been paid to the way that their organ resorption is accomplished by programmed cell death (PCD) and related cellular processes. We have used cytohistochemistry and transmission electron microscopy to study arm resorption in competent larvae of metamorphosing sea urchins, Hemicentrotus pulcherrimus, induced to metamorphose by L-glutamine treatment. The results show that: (1) columnar epithelial cells, which are constituents of the ciliary band, undergo PCD in an overlapping fashion with apoptosis and autophagic cell death; (2) squamous epithelial cells, which are distributed between the two arrays of the ciliary band, display a type of PCD distinct from that of columnar epithelial cells, i.e., a cytoplasmic type of non-lysosomal vacuolated cell death; (3) epithelial integrity is preserved even when PCD occurs in constituent cells of the epithelium; (4) secondary mesenchyme cells, probably blastocoelar cells, contribute to the elimination of dying epithelial cells; (5) nerve cells have a delayed initiation of PCD. Taken together, our data indicate that arm resorption in sea urchins proceeds concomitantly with various types of PCD followed by heterophagic elimination, but that epithelial organization is preserved during metamorphosis.

NGF and IL-1beta are co-localized in the developing nervous system of the frog, Xenopus laevis

NGF, a neurotrophic factor best known for its role in promoting cell survival, regulates many neurodevelopmental processes, including synaptic plasticity, neurite outgrowth and programmed cell death. Although there is a large amount of data regarding NGF in the developing nervous system of many species, there is little known about its regulation and role in the frog, Xenopus laevis. In this report, immunocytochemistry was used to characterize NGF protein expression in developing tadpoles. Protein expression was analyzed in tadpoles from stage 44/45 through stage 50, a period of development characterized by extensive neurite outgrowth, neuronal differentiation and an initial period of programmed cell death. Similar to other species, NGF was expressed in sensory cells and tissues, including the inner ear, eye, olfactory system, lateral line organs, papillae in the oral cavity, and gills tufts. In addition, NGF was expressed in specific cells in the central nervous system, cranial and dorsal root ganglia, spinal sensory and motoneurons, and muscle tissues in the tail and body cavity. In the mammalian nervous system, the cytokine, interleukin-1beta (IL-1beta) induces expression of NGF. In this report, double-label immunocytochemistry was used to determine the relationship between NGF and IL-1beta. Results showed most cell types and/or tissues that expressed NGF also expressed IL-1beta. However, NGF was typically associated with cellular and nuclear membranes, whereas IL-1beta appeared in the cytoplasm and nucleolus. The nuclear localization of IL-1beta supports the idea that it regulates gene transcription in the frog. The appearance of NGF and IL-1beta in the same cells suggests they may interact to influence neural development.

Apoptotic wing degeneration and formation of an altruism-regulating glandular appendage (gemma) in the ponerine ant Diacamma sp. from Japan (Hymenoptera, Formicidae, Ponerinae)

We here show an example of morphological novelties, which have evolved from insect wings into the specific structures controlling social behaviour in an ant species. Most ant colonies consist of winged queen(s) and wingless workers. In the queenless ponerine ant Diacamma sp. from Japan, however, all female workers have a pair of small thoracic appendages, called "gemmae", which are homologous to the forewings and acts as an organ regulating altruism expression. Most workers, whose gemmae are clipped off by other colony members, become nonreproductive helpers, while only a single individual with complete gemmae becomes functionally reproductive. We examined histologically the development of gemmae, and compared it with that of functional wings in males. Female larvae had well-developed wing discs for both fore- and hindwings. At pupation, however, the wing discs started to evaginate and later degenerate. The hindwing discs completely degenerated, while the degeneration of forewing discs was incomplete, leading to the formation of gemmae. The degeneration process involved apoptotic cell death as confirmed by TUNEL assay. In addition, glandular cells differentiated from the epithelial cells of the forewing buds after completion of pupation. The mechanism of developmental transition from wing to gemma can be regarded as an evolutionary gain of new function, which can be seen in insect appendages and vertebrate limbs.