Expression and function of an even-skipped homolog in the leech Helobdella robusta

Glossiphoniid leeches as a touchstone for studies of development in clitellate annelids

My goals in this chapter are to share my enthusiasm for studying the biology of leeches, to place this work in context by presenting my rationale for studying non-traditional biological models in general, and to sample just three of the questions that intrigue me in leech biology, namely segmentation, genome evolution and neuronal fate specification. I first became excited about the idea of using leeches as a subject of investigation as an undergraduate in 1970 and have been engaged in this work since I arrived at Berkeley as a postdoc in 1976, intending to study leech neurobiology. Both my research interests and the rationale for the work have expanded greatly since then. What follows is a fragmentary personal and historical account-the interested reader may find more comprehensive treatments elsewhere (Kuo et al., 2020; Shankland & Savage, 1997; Shain, 2009; Weisblat & Huang, 2001; Weisblat & Kuo, 2009, 2014; Weisblat & Winchell, 2020).

Unravelling spiral cleavage

Snails, earthworms and flatworms are remarkably different animals, but they all exhibit a very similar mode of early embryogenesis: spiral cleavage. This is one of the most widespread developmental programs in animals, probably ancestral to almost half of the animal phyla, and therefore its study is essential for understanding animal development and evolution. However, our knowledge of spiral cleavage is still in its infancy. Recent technical and conceptual advances, such as the establishment of genome editing and improved phylogenetic resolution, are paving the way for a fresher and deeper look into this fascinating early cleavage mode.

Functional role of pax6 during eye and nervous system development in the annelid Capitella teleta

The transcription factor Pax6 is an important regulator of early animal development. Loss of function mutations of pax6 in a range of animals result in a reduction or complete loss of the eye, a reduction of a subset of neurons, and defects in axon growth. There are no studies focusing on the role of pax6 during development of any lophotrochozoan representative, however, expression of pax6 in the developing eye and nervous system in a number of species suggest that pax6 plays a highly conserved role in eye and nervous system formation. We investigated the functional role of pax6 during development of the marine annelid Capitella teleta. Expression of pax6 transcripts in C. teleta larvae is similar to patterns found in other animals, with distinct subdomains in the brain and ventral nerve cord as well as in the larval and juvenile eye. To perturb pax6 function, two different splice-blocking morpholinos and a translation-blocking morpholino were used. Larvae resulting from microinjections with either splice-blocking morpholino show a reduction of the pax6 transcript. Development of both the larval eyes and the central nervous system architecture are highly disrupted following microinjection of each of the three morpholinos. The less severe phenotype observed when only the homeodomain is disrupted suggests that presence of the paired domain is sufficient for partial function of the Pax6 protein. Preliminary downstream target analysis confirms disruption in expression of some components of the retinal gene regulatory network, as well as disruption of genes involved in nervous system development. Results from this study, taken together with studies from other species, reveal an evolutionarily conserved role for pax6 in eye and neural specification and development.

On the origin of leeches by evolution of development

Leeches are a unique group of annelids arising from an ancestor that would be characterized as a freshwater oligochaete worm. Comparative biology of the oligochaetes and the leeches reveals that body plan changes in the oligochaete-to-leech transition probably occurred by addition or modification of the terminal steps in embryonic development and that they were likely driven by a change in the feeding behavior in the ancestor of leeches. In this review article, developmental changes that are associated with the evolution of several leech-specific traits are discussed. These include (1) the evolution of suckers, (2) the loss of chaetae, (3) the loss of septa, and (4) a fixed number of segments. An altered developmental fate of the teloblast is further proposed to be a key factor contributing to the fixation of the segment number, and the evolutionary change in teloblast development may also account for the loss of the ability to regenerate the lost body segments in the leech.

Cleavage modification did not alter blastomere fates during bryozoan evolution

BACKGROUND

Stereotypic cleavage patterns play a crucial role in cell fate determination by precisely positioning early embryonic blastomeres. Although misplaced cell divisions can alter blastomere fates and cause embryonic defects, cleavage patterns have been modified several times during animal evolution. However, it remains unclear how evolutionary changes in cleavage impact the specification of blastomere fates. Here, we analyze the transition from spiral cleavage - a stereotypic pattern remarkably conserved in many protostomes - to a biradial cleavage pattern, which occurred during the evolution of bryozoans.

RESULTS

Using 3D-live imaging time-lapse microscopy (4D-microscopy), we characterize the cell lineage, MAPK signaling, and the expression of 16 developmental genes in the bryozoan Membranipora membranacea. We found that the molecular identity and the fates of early bryozoan blastomeres are similar to the putative homologous blastomeres in spiral-cleaving embryos.

CONCLUSIONS

Our work suggests that bryozoans have retained traits of spiral development, such as the early embryonic fate map, despite the evolution of a novel cleavage geometry. These findings provide additional support that stereotypic cleavage patterns can be modified during evolution without major changes to the molecular identity and fate of embryonic blastomeres.

Evolutionary crossroads in developmental biology: annelids

Annelids (the segmented worms) have a long history in studies of animal developmental biology, particularly with regards to their cleavage patterns during early development and their neurobiology. With the relatively recent reorganisation of the phylogeny of the animal kingdom, and the distinction of the super-phyla Ecdysozoa and Lophotrochozoa, an extra stimulus for studying this phylum has arisen. As one of the major phyla within Lophotrochozoa, Annelida are playing an important role in deducing the developmental biology of the last common ancestor of the protostomes and deuterostomes, an animal from which >98% of all described animal species evolved.

Lineage analysis of micromere 4d, a super-phylotypic cell for Lophotrochozoa, in the leech Helobdella and the sludgeworm Tubifex

The super-phylum Lophotrochozoa contains the plurality of extant animal phyla and exhibits a corresponding diversity of adult body plans. Moreover, in contrast to Ecdysozoa and Deuterostomia, most lophotrochozoans exhibit a conserved pattern of stereotyped early divisions called spiral cleavage. In particular, bilateral mesoderm in most lophotrochozoan species arises from the progeny of micromere 4d, which is assumed to be homologous with a similar cell in the embryo of the ancestral lophotrochozoan, more than 650 million years ago. Thus, distinguishing the conserved and diversified features of cell fates in the 4d lineage among modern spiralians is required to understand how lophotrochozoan diversity has evolved by changes in developmental processes. Here we analyze cell fates for the early progeny of the bilateral daughters (M teloblasts) of micromere 4d in the leech Helobdella sp. Austin, a clitellate annelid. We show that the first six progeny of the M teloblasts (em1-em6) contribute five different sets of progeny to non-segmental mesoderm, mainly in the head and in the lining of the digestive tract. The latter feature, associated with cells em1 and em2 in Helobdella, is seen with the M teloblast lineage in a second clitellate species, the sludgeworm Tubifex tubifex and, on the basis of previously published work, in the initial progeny of the M teloblast homologs in molluscan species, suggesting that it may be an ancestral feature of lophotrochozoan development.

High resolution cell lineage tracing reveals developmental variability in leech

Knowing the normal patterns of embryonic cell proliferation, migration, and differentiation is a cornerstone for understanding development. Yet for most species, the precision with which embryonic cell lineages can be determined is limited by technical considerations (the large numbers of cells, extended developmental times, opacity of the embryos), and these are exacerbated by the inherent variability of the lineages themselves. Here, we present an improved method of cell lineage tracing in the leech Helobdella, driving the expression of a nuclearly localized histone H2B:GFP (green fluorescent protein) fusion protein in selected lineages by microinjection of a plasmid vector. This construct generates a long lasting and minimally mosaic signal with single cell resolution, and does not disrupt the development of most lineages tested. We have validated this technique by elucidating details of cell lineages contributing to segmental and prostomial tissues that could not be observed with standard dextran lineage tracers.

Grandparental stem cells in leech segmentation: differences in CDC42 expression are correlated with an alternating pattern of blast cell fates

Embryonic segmentation in clitellate annelids (oligochaetes and leeches) is a cell lineage-driven process. Embryos of these worms generate a posterior growth zone consisting of 5 bilateral pairs of identified segmentation stem cells (teloblasts), each of which produces a column of segmental founder cells (blast cells). Each blast cell generates a lineage-specific clone via a stereotyped sequence of cell divisions, which are typically unequal both in terms of the relative size of the sister cells and in the progeny to which they give rise. In two of the five teloblast lineages, including the ventralmost, primary neurogenic (N) lineage, the blast cells adopt two different fates, designated nf and ns, in exact alternation within the blast cell column; this is termed a grandparental stem cell lineage. To lay groundwork for investigating unequal divisions in the leech Helobdella, we have surveyed the Helobdella robusta genome for genes encoding orthologs of the Rho family GTPases, including the rho, rac and cdc42 sub-families, which are known to be involved in multiple processes involving cell polarization in other systems. We find that, in contrast to most other known systems the Helobdella genome contains two cdc42 orthologs, one of which is expressed at higher levels in the ns blast cells than in nf blast cells. We also demonstrate that the asymmetric divisions of the primary nf and ns blast cells are regulated by the polarized distribution of the activated form of the Cdc42 protein, rather than by the overall level of expression. Our results provide the first molecular insights into the mechanisms of the grandparental stem cell lineages, a novel, yet evolutionarily ancient stem cell division pattern. Our results also provide an example in which asymmetries in the distribution of Cdc42 activity, rather than in the overall levels of Cdc42 protein, are important regulating unequal divisions in animal cells.

And Lophotrochozoa makes three: Notch/Hes signaling in annelid segmentation

Segmentation is unquestionably a major factor in the evolution of complex body plans, but how this trait itself evolved is unknown. Approaching this problem requires comparing the molecular mechanisms of segmentation in diverse segmented and unsegmented taxa. Notch/Hes signaling is involved in segmentation in sequentially segmenting vertebrates and arthropods, as judged by patterns of expression of one or more genes in this network and by the disruption of segmental patterning when Notch/Hes signaling is disrupted. We have previously shown that Notch and Hes homologs are expressed in the posterior progress zone (PPZ), from which segments arise, in the leech Helobdella robusta, a sequentially segmenting lophotrochozoan (phylum Annelida). Here, we show that disrupting Notch/Hes signaling disrupts segmentation in this species as well. Thus, Notch/Hes functions in either the maintenance of the PPZ and/or the patterning processes of segmentation in representatives of all three superphyla of bilaterally symmetric animals. These results are consistent with two evolutionary scenarios. In one, segmentation was already present in the ancestor of all three superphyla. In the other, Notch/Hes signaling functioned in axial growth by terminal addition in an unsegmented bilaterian ancestor, and was subsequently exapted to function in segmentation as that process evolved independently in two or more taxa.

Notch signaling during larval and juvenile development in the polychaete annelid Capitella sp. I

Notch signaling is involved in a large range of developmental processes, and has been functionally implicated in body plan segmentation in two of the three diverse segmented taxa, the vertebrates and arthropods. Here we investigate expression of Notch, Delta, and hes gene homologues during larval and juvenile development in the polychaete annelid Capitella sp. I., a member of the third group of segmented animals. During larval stages, CapI-Notch, CapI-Delta, CapI-hes2, and CapI-hes3 transcripts are initially detected in broad ectodermal domains in future segments as well as in the brain and foregut; later, CapI-Notch, CapI-Delta, and CapI-hes2 transcripts are detected in the presumptive chaetal sacs. In contrast, CapI-hes1 has a segmentally reiterated pattern in a restricted region of the mesoderm in each presumptive segment. CapI-Notch, CapI-Delta, CapI-hes2, and CapI-hes3 and CapI-hes1 are all expressed in the terminal growth zone that generates post-metamorphic segments, however, CapI-hes1 has a non-overlapping complementary expression pattern to that of CapI-Notch and CapI-Delta. CapI-Delta and CapI-Notch transcripts are localized to already formed segments, with posterior boundaries that correlate with the posterior boundary of the nascent segment, while CapI-hes1 lies posterior to CapI-Notch and CapI-Delta. The localization of CapI-Notch, CapI-Delta, and CapI-hes transcripts correlate with areas of rapid cell proliferation in Capitella, which include the brain, foregut, and terminal growth zone.

Mesoderm in spiralians: the organizer and the 4d cell

The Spiralia is a clade of protostome invertebrate phyla that share a highly conserved mode of early development. Spiralian development is characterized by regularities in the arrangement of early cleavages, the fates of the cells that are produced by these divisions, and the development of the distinctive trochophore larva. Because of the strong conservation in early development, homologies can be identified between cells in divergent taxa. Some of the most striking examples of conservation in the spiralian embryo are in the cells that generate the mesoderm. The specification of the mesodermal precursors has been well characterized by embryological approaches, and recently the molecular mechanisms of mesoderm specification are starting to be elucidated. This review examines the development of mesoderm in spiralians in a comparative context, with particular focus on the relationship between the mesendodermal cell 4d and the embryonic organizer.

Hau-Pax3/7A is an early marker of leech mesoderm involved in segmental morphogenesis, nephridial development, and body cavity formation

Two genes of the Pax III subfamily, Hau-Pax3/7A and -Pax3/7B, were identified from the leech Helobdella, and the expression and function of Hau-Pax3/7A in development are described. Leech embryos undergo spiral cleavage, then produce a set of teloblastic stem cells that generate segmented mesoderm and ectoderm. Hau-Pax3/7A is present as a maternal transcript in both ectodermal and mesodermal progenitors, but this pool of early RNA disappears and is replaced by a pattern of zygotic transcription restricted to the blast cell progeny of the mesodermal M teloblasts. Each mesodermal blast cell clone goes through multiple phases of Hau-Pax3/7A expression, the last of which is associated with the organogenesis of the nephridia and other segment-specific structures. Morpholino-mediated knockdown of Hau-Pax3/7A expression causes the mesodermal blast cell clones to undergo irregular patterns of morphogenesis that disrupt the segmental organization of the germinal plate, and interferes with both the specification and morphological differentiation of the mesodermal nephridia. Knockdown of Hau-Pax3/7A in the mesoderm can also lead to abnormalities in the formation of the dorsal cavities, possibly through indirect effects of this germ layer on neighboring tissues. This is the first report of broad mesodermal Pax III expression outside of chordates, and raises the possibility that such expression may be a primitive trait inherited from the last common ancestor of the bilaterian superphyla.

Characterization of Notch-class gene expression in segmentation stem cells and segment founder cells in Helobdella robusta (Lophotrochozoa; Annelida; Clitellata; Hirudinida; Glossiphoniidae)

To understand the evolution of segmentation, we must compare segmentation in all three major groups of eusegmented animals: vertebrates, arthropods, and annelids. The leech Helobdella robusta is an experimentally tractable annelid representative, which makes segments in anteroposterior progression from a posterior growth zone consisting of 10 identified stem cells. In vertebrates and some arthropods, Notch signaling is required for normal segmentation and functions via regulation of hes-class genes. We have previously characterized the expression of an hes-class gene (Hro-hes) during segmentation in Helobdella, and here, we characterize the expression of an H. robusta notch homolog (Hro-notch) during this process. We find that Hro-notch is transcribed in the segmental founder cells (blast cells) and their stem-cell precursors (teloblasts), as well as in other nonsegmental tissues. The mesodermal and ectodermal lineages show clear differences in the levels of Hro-notch expression. Finally, Hro-notch is shown to be inherited by newly born segmental founder cells as well as transcribed by them before their first cell division.

Terminal addition, the Cambrian radiation and the Phanerozoic evolution of bilaterian form

We examine terminal addition, the process of addition of serial elements in a posterior subterminal growth zone during animal development, across modern taxa and fossil material. We argue that terminal addition was the basal condition in Bilateria, and that modification of terminal addition was an important component of the rapid Cambrian evolution of novel bilaterian morphology. We categorize the often-convergent modifications of terminal addition from the presumed ancestral condition. Our focus on terminal addition and its modification highlights trends in the history of animal evolution evident in the fossil record. These trends appear to be the product of departure from the initial terminal addition state, as is evident in evolutionary patterns within-fossil groups such as trilobites, but is also more generally related to shifts in types of morphologic change through the early Phanerozoic. Our argument is contingent on dates of metazoan divergence that are roughly convergent with the first appearance of metazoan fossils in the latest Proterozoic and Cambrian, as well as on an inference of homology of terminal addition across bilaterian Metazoa.

caudal and even-skipped in the annelid Platynereis dumerilii and the ancestry of posterior growth

In order to address the question of the conservation of posterior growth mechanisms in bilaterians, we have studied the expression patterns of the orthologues of the genes caudal, even-skipped, and brachyury in the annelid Platynereis dumerilii. Annelids belong to the still poorly studied third large branch of the bilaterians, the lophotrochozoans, and have anatomic and developmental characteristics, such as a segmented body plan, indirect development through a microscopic ciliated larva, and building of the trunk through posterior addition, which are all hypothesized by some authors (including us) to be present already in Urbilateria, the last common ancestor of bilaterians. All three genes are shown to be likely involved in the building of the anteroposterior axis around the slit-like amphistomous blastopore as well as in the patterning of the terminal anus-bearing piece of the body (the pygidium). In addition, caudal and even-skipped are likely involved in the posterior addition of segments. Together with the emerging results on the conservation of segmentation genes, these results reinforce the hypothesis that Urbilateria had a segmented trunk developing through posterior addition.

Hirudo medicinalis: a platform for investigating genes in neural repair

We have used the nervous system of the medicinal leech as a preparation to study the molecular basis of neural repair. The leech central nervous system, unlike mammalian CNS, can regenerate to restore function, and contains identified nerve cells of known function and connectivity. We have constructed subtractive cDNA probes from whole and regenerating ganglia of the ventral nerve cord and have used these to screen a serotonergic Retzius neuron library. This identifies genes that are regulated as a result of axotomy, and are expressed by the Retzius cell. This approach identifies many genes, both novel and known. Many of the known genes identified have homologues in vertebrates, including man. For example, genes encoding thioredoxin (TRX), Rough Endoplasmic Reticulum Protein 1 (RER-1) and ATP synthase are upregulated at 24 h postinjury in leech nerve cord. To investigate the functional role of regulated genes in neuron regrowth we are using microinjection of antisense oligonucleotides in combination with horseradish peroxidase to knock down expression of a chosen gene and to assess regeneration in single neurons in 3-D ganglion culture. As an example of this approach we describe experiments to microinject antisense oligonucleotide to a leech isoform of the structural protein, Protein 4.1. Our approach thus identifies genes regulated at different times after injury that may underpin the intrinsic ability of leech neurons to survive damage, to initiate regrowth programs and to remake functional connections. It enables us to determine the time course of gene expression in the regenerating nerve cord, and to study the effects of gene knockdown in identified neurons regenerating in defined conditions in culture.

Segmentation

The three major taxa with metameric segmentation (annelids, arthropods, and chordates) appear to use three very different molecular strategies to generate segments. However, unexpected similarities are starting to emerge from characterization of pair-rule patterning and segmental border formation. Moreover, the existence of an ancestral segmentation clock based on Notch signaling has become likely. An old concept of comparative anatomy, the enterocoele theory, is compatible with a single origin of segmentation mechanisms and could therefore provide a conceptual framework for assessing these molecular similarities.

Cell cycle-dependent expression of a hairy and Enhancer of split (hes) homolog during cleavage and segmentation in leech embryos

We have cloned genes related to hairy and Enhancer of split (hes) from glossiphoniid leeches, Helobdella robusta and Theromyzon rude. In leech, segments arise sequentially in anteroposterior progression from a posterior growth zone that consists of five bilaterally paired embryonic stem cells called teloblasts. Each teloblast gives rise to segmental founder cells (primary blast cells) that contribute iterated sets of definitive progeny in each segment. Thus, in leech, the "segmentation clock," is closely identified with the cell cycle clock of the teloblasts. We have characterized normal expression patterns of mRNA and protein for the H. robusta hes-class gene (Hro-hes). Semiquantitative RT-PCR revealed that Hro-hes mRNA levels peak while the teloblasts are actively producing primary blast cells. RT-PCR, in situ hybridization and immunostaining revealed that Hro-hes is expressed as early as the first zygotic mitosis and throughout early development. Hro-hes is expressed in macromeres, pro-teloblasts, teloblasts and primary blast cells. HRO-HES protein is localized in the nuclei of cells expressing HRO-HES during interphase; nuclear HRO-HES is reduced during mitosis. In contrast, Hro-hes is transcribed during mitosis and its transcripts are associated with mitotic apparatus (MA). Thus, Hro-hes transcription cycles in antiphase to the nuclear localization of HRO-HES protein. These results indicate that Hro-hes expression, and thus possibly its biological activity, is linked to the cell cycle.