As Viewed Through the Eyes of Hydra, Regeneration Cannot Occur Without the Presence of a Properly Formed Extracellular Matrix
Published Date: July 12, 2017
As Viewed Through the Eyes of Hydra, Regeneration Cannot Occur Without the Presence of a Properly Formed Extracellular Matrix
Michael P. Sarras Jr
Department of Cell Biology and Anatomy, Rosalind Franklin University of Medicine and Science, 3333 Green Bay Road, North Chicago, IL 60064, USA
*Corresponding author: Michael P. Sarras Jr, Department of Cell Biology and Anatomy, Rosalind Franklin University of Medicine and Science, 3333 Green Bay Road, North Chicago, IL 60064, USA, E-mail: firstname.lastname@example.org.
Citation: Sarras Jr MP (2017) As Viewed Through the Eyes of Hydra, Regeneration Cannot Occur Without the Presence of a Properly Formed Extracellular Matrix. J Dev Biol Reg Med 1(1): 101.
Extensive studies on cell/extracellular matrix (cell/ECM) interactions in Hydra (an early divergent metazoan, dating back some 600 million years) indicate that in this animal model of cell-matrix interactions, an ECM is a critical and an absolute requirement for regeneration and morphogenesis to occur. This short review will highlight the data behind this statement and bring the reader up to date on our most current understanding of the molecular components and function of Hydra's ECM and its associated regulatory molecules.
Keywords: Extracellular matrix; Cnidarians; Hydra; Development; Regeneration
ECM: Extracellular Matrix; HCol-I: Hydra Collagen I; HCol-IV: Hydra Collagen IV; MMPs: Matrix Metalloproteinases; HMMP: Hydra Matrix Metalloproteinase.
For those who object to title's reference to "the eyes of Hydra", it should be noted that while as a Cnidarian of the Hydrozoan class, Hydra does not have anatomical eyes composed of vertebrate-like photoreceptors; it does have light sensitive systems based on such transduction pathways as opsin-coupled cyclic nucleotide gated ion channels coupled to the molecule arrestin .
Hydra's simplified structure makes it an excellent model for the study of cell/Extracellular Matrix (cell/ECM) interactions. As shown in Hydra's body structure is organized as an epithelial bilayer with an intervening ECM, called the mesoglea by early invertebrate biologists. The epithelial bilayer has an outer ectoderm and an inner endoderm. This is similar to the mammalian glomerular filtration system with its epithelial bilayer composed of podocytes and endothelial cells with an intervening basement membrane. A broad array of interstitial cells (neurons, nematocysts, gametes, etc) resides within the bilayer; however, chemically-induced epithelial hydra that lack any interstitial cells, are fully capable of complete regeneration and morphogenesis.
The highly regenerative capability of Hydra has been known since Trembley's studies beginning in about 1740 . Besides surgical ablation of the head or foot that induces subsequent regeneration of these structures , Hydra has the remarkable ability to regenerate its entire body structure from pellets of its cells formed from isolated cell preparations obtained by non-enzymatic dissociation of adult Hydra . This entire regenerative process (termed Hydra cell aggregation) occurs within 72-96 hours and results in a fully functional adult Hydra at the end of this period. These attributes allow the investigator to utilize Hydra for a wide variety of cell/ECM interactions to include: 1) de novo biogenesis of ECM using Hydra cell aggregates, 2) introduction of blocking Abs, peptides, etc to Hydra cell aggregates or adult polyps for analysis of cell migration, morphogenesis, or cell differentiation, and 3) use of electroporation-introduced antisense molecules into Hydra undergoing head or foot regeneration for knockdown experiments.
This short review will 1) describe the overall structure of Hydra in relation to its ECM, 2) describe the overall structure of Hydra ECM and cite recent studies reviewing current Hydra NCBI data bases, and 3) describe functional studies that elucidated the relationship of Hydra ECM formation to its critical function in Hydra's regenerative and morphogenetic processes.
Figure 1: Hydra’s Body Organization. Overall body structure depicted in the left panel and the cellular bilayer with its intervening ECM (termed mesoglea by early invertebrate biologists) shown in the right panel. The red box to the right side of the Hydra body diagram shows the position of the expanded bilayer diagram in the right panel. Figure modified from Google freeware.
Hydra ECM Structure and Molecular Composition
Hydra's simple structure and high regenerative capabilities make it an ideal animal model to study the dynamics of ECM formation and its role in morphogenesis. Taking advantage of the Hydra model, experiments beginning in the mid 1980s focused on biochemically defining Hydra's ECM and determining its role in the organism's regenerative and morphogenetic abilities. Using a convergent cellular, biochemical, and molecular approach; methods were first developed for the isolation of large quantities of Hydra ECM . Subsequent biochemical analysis of these ECM preparations indicated that Hydra mesoglea had the properties of a true collagen-based ECM . These purified ECM preparations were then utilized for the development of both polyclonal and monoclonal antibodies that could be used as biochemical, molecular, and immunological probes [10–12]. Screening Hydra expression cDNA libraries with these antibodies yielded a broad array of cDNA clones that matched vertebrate class ECM components [10–12]. To summarize years of work, figure 2 shows the overall structure and molecular composition of Hydra ECM. As indicated in figure 2, Hydra ECM is in fact, a combination of two epithelial-associated basal laminas (type IV collagen, laminin, etc) [10,11] with a central interstitial-like matrix composed of fibrillar collagens . The results of whole mount and ultrastructural TEM immunocytochemical localization studies are shown in figure 2 to give the reader examples of the types of distribution patterns observed for ECM components.
Figure 2: Hydra’s ECM Structure. Hydra ECM is organized as two subepithelial basal lamina (containing type- IV collagen and laminin) with a central interstitial fibrous matrix (containing Hcol-I, etc.) as shown in the upper panel based on initial published studies. In the lower panel immuno-images are shown using mAb to Hydra laminin (β1 chain). Image A is a whole mount with the signal observed throughout the polyp’s mesoglea (arrows) with the apical border of the ectoderm indicated by the arrowhead. Image B is a frozen section of the bilayer showing Laminin localized to the two sub-epithelial zones of the ECM (arrowheads). The inset in B shows TEM immuno-gold localization to the basal lamina region of Hydra’s ECM (arrow). Images C and D show whole mounts with laminin along the entire longitudinal axis of Hydra (arrowheads). Tentacles and body column are shown while the basal disc region is not; however, the basal disc does have ECM with all its components. The apical border of the ectoderm is indicated by arrows in D while the underlying β1 laminin chain in the ECM is indicated by the arrow heads.
Most recently, a 2017 study reviewed NCBI Hydra databases  and found that the 1) original single interstitial Hydra collagen (Hcol-I) and the 2) two basal lamina components (Hydra Type IV collagen) and Hydra laminin (a5 and b1 chains) represent only a small percentage of the actual number of Hydra ECM molecules that exist based on recent genomic sequence analysis . In fact, based on current databases, we can identify thirty one Hydra ECM components classified in the following matrix molecule classes: proteoglycans, collagens, and glycoproteins . These thirty one Hydra ECM Components are shown in table 1. In addition, eight Integrin chains (both a and b class chains) exist  [from the original single Integrin-like molecule that was described ] and sixteen matrix metalloproteinases are now known to exist  [from the original single Hydra MMP molecule, HMMP ]. Biochemical analysis of these additional ECM and ECM-associated molecules awaits protein-expression type studies.
Table 1: Hydra Extracellular Matrix (ECM) Proteins.
The Relationship of Hydra ECM Formation to its Function in Regeneration and Morphogenesis
The dynamics of Hydra ECM formation was elucidated by a combination of immunological and molecular studies as originally reviewed by Sarras in 2012 . Following establishing an understanding of how Hydra forms its ECM, studies were then performed to determine the role of individual ECM components in regulating regenerative and morphogenetic processes. The clearest data came from studies with the Hydra cell pellet system described above (Hydra cell aggregate studies) . With the aid of specific ECM probes (both antibody and RNA-in situ based) it was clear that the stages in Hydra regeneration from the original cellular pellets involves the following steps: 1) cell pellets sorted with ectoderm cells moving to the periphery of the pellet and endoderm cells moving to the internal region of the pellet, 2) a fluid-filled cyst-like structure then formed whose outer cell layer was composed of ectodermal cells while just internal to the ectoderm cells were the endodermal cells, 3) ECM components were then produced and secreted by the bilayer into the space between the ectoderm and endoderm cell layers and these matrix components organized into a defined ECM with the basal lamina components localized sub-epithelial while the interstitial matrix components were localized to the central region of the ECM between the two basal lamina regions. Interestingly, ECM components were made in either the ectoderm or endoderm (not both) in a particular temporal sequence; indicating cross-talk of the bilayer for coordinated secretion and polymerization of Hydra's ECM . Utilizing blocking antibodies to ECM components, fragments of ECM components, or antisense oligos; functional studies found that inhibition in the secretion of individual ECM components or any disruption of ECM polymerization [17,18] resulted in complete blockage of Hydra morphogenesis in the cyst stage [8,18]. Later studies applied to head or foot regeneration showed the same sequence in ECM formation and the same functional results . In addition to these ECM formation studies, parallel studies showed that blockage of the migration of interstitial cells along the body column required interstitial cell/ECM interactions ; indicating a broader role for the ECM in Hydra's cell-physiological processes beyond that of just the epithelium. For the above mentioned functional studies, antibodies and/or antisense molecules were developed against Hydra laminin (b1 chain), Hydra Hcol-I, Hcol-IV, and Hydra matrix metalloproteinase (HMMP). Table 2 provides a summary of the various developmental processes shown to involve cell/ECM interactions in Hydra as well as the ECM molecules identified in those processes [8–12,15,18–27]. In total, these studies clearly indicate that in Hydra, ECM formation is essential for the organism to regenerate and for morphogenesis to occur; however, the precise underlying mechanisms for these cell/ECM interactions remain to be determined.
Table 2: Developmental Processes in Hydra that have been Reported to Involve Cell–ECMinteractions. (*HCol-lV: Hydra Collagen lV; HMMP: Hydra Matrix Metalloproteinase; HCol-l: Hydra Collagen l.)
Given the early divergence of Hydra during formation of metazoans (dating back some 600 million years), the studies establishing the role of ECM in hydra regeneration and morphogenesis reflect the fundamental and critical function of ECM in these processes for metazoans. This relationship continues into the chordates and forms the basis for normal develops as well as disease processes. The inter-relationship of signals between the ECM, cells, and internal signaling pathways is clearly complex [28,29] but provides an essential approach to understanding basic developmental and regenerative processes for application to the reversal of human disease and degenerative processes.
The author wishes to express his appreciation to the National Institutes of Health, USA (DK092721) for funds that supported preparation and writing of this review. The author also wishes to thank those who have developed the genomic and EST databases that have allowed a more in depth study of Hydra ECM and its associated regulatory molecules.
The authors have no conflict of interests or commercial interests as related to the information provided in this review.
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Copyright: © 2017 Sarras Jr MP. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.