Combining physiology and genetics in the zebrafish retina

Tensile Loaded Tissue-Engineered Human Tendon Constructs Stimulate Myotube Formation

Skeletal muscle possesses adaptability to mechanical loading and regenerative potential following muscle injury due to muscle stem cell activity. So far, it is known that muscle stem cell activity is supported by the roles of several interstitial cells within skeletal muscle in response to muscle damage. The adjacent tendon is also exposed to repetitive mechanical loading and possesses plasticity like skeletal muscle. However, the interplay between the skeletal muscle and adjacent tendon tissue has not been fully investigated. In this study, we tested whether factors released by three-dimensional engineered human tendon constructs in response to uniaxial tensile loading can stimulate the proliferation and differentiation of human-derived myogenic cells (myoblasts). Tendon constructs were subjected to repetitive mechanical loading (4% strain at 0.5 Hz for 4 h) and nonrepetitive loading (0% strain at 0 Hz for 4 h), and the conditioned media from mechanically loaded and nonmechanically loaded control constructs were applied to myoblasts. Immunofluorescence analysis revealed both an increase of myotube fusion index (≥5 nuclei within one desmin+ myotube) and the myotube diameter when conditioned medium from mechanically loaded tendon constructs was applied. Myostatin, myosin heavy chain 7, and AXIN2 gene expressions were downregulated in myotubes treated with conditioned medium from mechanically loaded tendon constructs. However, proliferative potential (number of Ki67+ and bromodeoxyuridine+ myoblasts) did not differ between the two groups. These results indicate that tendon fibroblasts enhance myotube formation by mechanical loading-induced factors. Our finding suggests that mechanical loading affects the signaling interplay between skeletal muscle and tendon tissue and is thus important for musculoskeletal tissue development and regeneration in humans. Impact statement The interplay between satellite cells and various types of resident cells within the skeletal muscle for muscle regeneration has been extensively studied. However, even though tendon tissue is located adjacent to skeletal muscle tissue and cells in these tissues are exposed to repetitive mechanical loading together, the interaction between muscle and tendon tissues for muscle regeneration remains to be elucidated. In this study, we report that the conditioned media from engineered human tendon tissues undergoing repetitive tensile mechanical loading enhanced myotube formation. Our in vitro findings extend the fundamental understanding of the crosstalk between adjacent tissues of the muscle-tendon unit.

Formation of stromal collagen fibrils and proteoglycans in the developing zebrafish cornea

PURPOSE

Collagen fibrils and proteoglycans are the main components of the corneal extracellular matrix and corneal transparency depends crucially on their proper arrangement. In the present study, we investigated the formation of collagen fibrils and proteoglycans in the developing cornea of the zebrafish, a model organism used to study vertebrate embryonic development and genetic disease.

METHODS

We employed thin-section electron microscopy to investigate the ultrastructure of the zebrafish cornea at different developmental stages.

RESULTS

The layering of the zebrafish cornea into an epithelium, a Bowman's layer, stroma and endothelium was observed starting at 72 hr post-fertilization. At this stage, the stroma contained orthogonally arranged collagen fibrils and small proteoglycans. The density of proteoglycans increased gradually throughout subsequent development of the cornea. In the stroma of 2-week-old larvae, the collagen fibrils were organized into thin lamellae and were separated by very large, randomly distributed proteoglycans. At 4 weeks, a regular arrangement of proteoglycans in relation to the collagen fibrils was observed for the first time and the lamellae were also thickened.

CONCLUSION

The present study, for the first time, provides ultrastructural details of collagen fibril and proteoglycan development in the zebrafish cornea. Furthermore, it directly correlates the collagen fibril and proteoglycan composition of the zebrafish cornea with that of the human cornea. The similarities between the two species suggest that the zebrafish could serve as a model for investigating the genetics of human corneal development and diseases.

The Mechanism of Activity-Dependent Sodium Channel Inhibition by the Antidepressants Fluoxetine and Desipramine

The effect of monoamine uptake inhibitor-type antidepressants on sodium channels of hippocampal neurons was investigated. Members of the tricyclic group of antidepressants are known to modify multiple targets, including sodium channels, whereas selective serotonin-reuptake inhibitors (SSRIs) are regarded as highly selective compounds, and their effect on sodium channels was not investigated in detail. In this study, a representative member of each group was chosen: the tricyclic antidepressant desipramine and the SSRI fluoxetine. The drugs were roughly equipotent use-dependent inhibitors of sodium channels, with IC(50) values approximately 100 microM at -150 mV holding potential, and approximately 1 microM at -60 mV. We suggest that therapeutic concentrations of antidepressants affect neuronal information processing partly by direct, activity-dependent inhibition of sodium channels. As for the mechanism of inhibition, use-dependent inhibition by antidepressants was believed to be due to a preferential affinity to the fast-inactivated state. Using a voltage and perfusion protocol by which relative affinities to fast-versus slow-inactivated states could be assessed, we challenged this view and found that the affinity of both drugs to slowinactivated state(s) was higher. We propose a different mechanism of action for these antidepressants, in which slow rather than fast inactivation plays the dominant role. This mechanism is similar but not equivalent with the novel mechanism of usedependent sodium channel inhibition previously described by our group (Neuroscience 125:1019-1028, 2004; Neuroreport 14:1945-1949, 2003). Our results suggest that different drugs can produce use-dependent sodium channel inhibition by different mechanisms.

Fluorescence visualization of ultraviolet-sensitive cone photoreceptor development in living zebrafish

Cone photoreceptor cells of fish retinae are arranged in a highly organized fashion. However, the molecular mechanisms underlying photoreceptor development and retinal pattern formation are largely unknown. Here we established transgenic lines of zebrafish carrying green fluorescent protein (GFP) cDNA with the 5.5-kb upstream region of the ultraviolet-sensitive cone opsin gene (SWS1). In the transgenic fish, GFP gene expression proceeded in the same spatiotemporal pattern as SWS1 in the retinae of embryos. In the adult retina, GFP expression was observed throughout the short single cone (SSC) layer where SWS1 is specifically expressed. Therefore, the transgenic fish provides an excellent genetic background to study retinal pattern formation, photoreceptor determination and differentiation, and factors regulating these processes and SSC-specific expression of SWS1.

Visualization of rod photoreceptor development using GFP-transgenic zebrafish

Zebrafish retina contains five morphologically distinct classes of photoreceptors, each expressing a distinct type of opsin gene. Molecular mechanisms underlying specification of opsin expression and differentiation among the cell types are largely unknown. This is partly because mutants affected with expression of a particular class of opsin gene are difficult to find. In this study we established the transgenic lines of zebrafish carrying green fluorescent protein (GFP) gene under the 1.1-kb and 3.7-kb upstream regions of the rod-opsin gene. In transgenic fish, GFP expression initiated and proceeded in the same spatiotemporal pattern with rod-opsin gene. The retinal section from adult transgenic fish showed GFP expression throughout the rod cell layer. These results indicate that the proximal 1.1-kb region is sufficient to drive gene expression in all rod photoreceptor cells. These transgenic fish should facilitate screening of mutants affected specifically with rod-opsin expression or rod cell development by visualization of rod cells by GFP.