Abstract
Cell–cell communication, facilitating the exchange of small metabolites, ions and second messengers, takes place via aqueous proteinaceous channels called gap junctions. Connexins (cx) are the subunits of a gap junction channel. Mutations in zebrafish cx43 produces the short fin (sof b123) phenotype and is characterized by short fins due to reduced segment length of the bony fin rays and reduced cell proliferation. Previously established results from our lab demonstrate that Cx43 plays a dual role regulating both cell proliferation (growth) and joint formation (patterning) during the process of skeletal morphogenesis. In this study, we show that Hapln1a (Hyaluronan and Proteoglycan Link Protein 1a) functions downstream of cx43. Hapln1a belongs to the family of link proteins that play an important role in stabilizing the ECM by linking the aggregates of hyaluronan and proteoglycans. We validated that hapln1a is expressed downstream of cx43 by in situ hybridization and quantitative RT-PCR methods. Moreover, in situ hybridization at different time points revealed that hapln1a expression peaks at 3 days post amputation. Expression of hapln1a is located in the medial mesenchyme and the in the lateral skeletal precursor cells. Furthermore, morpholino mediated knock-down of hapln1a resulted in reduced fin regenerate length, reduced bony segment length and reduced cell proliferation, recapitulating all the phenotypes of cx43 knock-down. Moreover, Hyaluronic Acid (HA) levels are dramatically reduced in hapln1a knock-down fins, attesting the importance of Hapln1a in stabilizing the ECM. Attempts to place hapln1a in our previously defined cx43–sema3d pathway suggest that hapln1a functions in a parallel genetic pathway. Collectively, our data suggest that Cx43 mediates independent Sema3d and Hapln1a pathways in order to coordinate skeletal growth and patterning.
Highlights
Gap junctions play a critical role in coupling tissue function and they have long been hypothesized to play a role in the maintenance of homeostasis, morphogenesis, cell differentiation, growth control and the process of skeletogenesis in multicellular organisms [1]
We find that hapln1a functions downstream of cx43, providing novel insights into how skeletal morphogenesis could be influenced by Cx43
We did not observe an upregulation of hapln1a in alf dty86, suggesting that among a population of alf dty86 regenerating fins, there is little difference in the expression level of hapln1a between WT and alf dty86.To determine the tissue-specific expression of hapln1a, we performed in situ hybridization on cryosections
Summary
Gap junctions play a critical role in coupling tissue function and they have long been hypothesized to play a role in the maintenance of homeostasis, morphogenesis, cell differentiation, growth control and the process of skeletogenesis in multicellular organisms [1]. The syndrome oculodentodigital dysplasia (ODDD), characterized by abnormalities in craniofacial elements, limbs and dentition, has been linked to missense mutations in the GJA1 gene locus in humans [3]. Targeted gene knock-down of cx results in embryonic heart defects in zebrafish, signifying the essential role of cx during development [8]. The skeletal defects seen in the CX432/2 KO mouse model exhibited hypomineralization of craniofacial bones and severely delayed ossification of the appendicular skeleton [9]. The ODDD phenotype is similar to a set of craniofacial abnormalities observed in the targeted CX43 knockdown chick model [10,11]. A homozygous mutation in cx causes the short fin phenotype, characterized by shorter tail fins due to defects in the fin skeleton.
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