We have previously reported that the retinoic acid (RA) catabolizing enzyme CYP26A1 plays an important role in protecting tail bud tissues from inappropriate exposure to RA generated in the adjacent trunk tissues by RALDH2, and that Cyp26a1-null animals exhibit spina bifida and caudal agenesis. We now show that, in the absence of Cyp26a1, retinoic acid receptor gamma (RARγ) mediates ectopic RA-signaling in the tail bud. We also show that activated RARγ results in downregulation of Wnt3a and Fgf8, which integrate highly conserved signaling pathways known for their role in specifying caudal morphogenesis.Ablation of the gene for RARγ (Rarg) rescues Cyp26a1-null mutant animals from caudal regression and embryonic lethality, thus demonstrating that CYP26A1 suppresses the RA-mediated downregulation of WNT3A and FGF8 signaling pathways by eliminating ectopic RA in gastrulating tail bud mesoderm. 1450 assays involving a RA-responsive lacZ reporter transgene reveal that tail bud tissues normally devoid of RA stain positively for RA activity (Sakai et al., 2001). Loss of CYP26A1 function is embryonic lethal, with mutant embryos dying from caudal regression associated with spina bifida, imperforate anus, agenesis of the caudal portions of the digestive and urogenital tracts, malformed lumbosacral skeletal elements, and lack of caudal tail vertebrae Sakai et al., 2001). This phenotype indicates that CYP26A1 is also essential for maintaining morphogenetic events during late gastrulation, probably by protecting tail bud tissues from exposure to RA.The caudal regression phenotype of Cyp26a1 mutants closely resembles that of wild-type embryos treated with teratogenic doses of RA at E8.5-E9.5 (Alles and Sulik, 1990;Kessel, 1992;Padmanabhan, 1998). Such RA-treated wildtype embryos show elevated levels of apoptosis, develop ectopic neural tubes and exhibit severe downregulation of Wnt3a expression in tail bud tissues (Alles and Sulik, 1990;Iulianella et al., 1999;Shum et al., 1999;Yasuda et al., 1990). Interestingly, loss of WNT3A function leads to a caudal regression syndrome similar to that observed in Cyp26a1 mutants and RA-treated wild-type embryos (Takada et al., 1994). Finally, resistance to the teratogenic effects of RA has been observed in mice that lack Rarg, which develop caudal tissues normally and show no change in Wnt3a expression after RA treatment during late gastrulation (Lohnes et al., 1993;Iulianella et al., 1999). Based on these observations, we were interested in testing whether we could rescue Cyp26a1 mutants from endogenous RA teratogenicity by disrupting the Rarg gene.We report that loss of Rarg rescues Cyp26a1 mutants from embryonic lethality and suppresses their caudal regression phenotype. Furthermore, ablation of Rarg restores normal gene expression patterns in the tail bud of Cyp26a1 -/-Rarg -/-double mutants. Our results suggest that RA signaling mediated by RARγ downregulates WNT3A and FGF8 signaling activities, leading to defects in caudal mesoderm and definitive endoderm formation. Using this doubl...

Large volume deficiencies in skeletal muscle tissue fail to heal with conservative treatments, and improved treatment methods are needed. Tissue engineered scaffolds for skeletal muscle need to mimic the optimal environment for muscle development by providing the proper electric, mechanical, and chemical cues. Electroactive polymers, polymers that change in size or shape in response to an electric field, may be able to provide the optimal environment for muscle growth. In this study, an electroactive polymer made from poly(ethylene glycol) diacrylate (PEGDA) and acrylic acid (AA) is characterized and optimized for movement and biocompatibility. Hydrogel sample thickness, overall polymer concentration, and the ratio of PEGDA to AA were found to significantly impact the actuation response. C2C12 mouse myoblast cells attached and proliferated on hydrogel samples with various ratios of PEGDA to AA. Future experiments will produce hydrogel samples combined with aligned guidance cues in the form of electrospun fibers to provide a favorable environment for muscle development.

Skeletal muscle is made up of hundreds of multinucleated, aligned fibers that work together during contraction. While smaller injuries are typically able to be repaired by the body, large volumetric muscle loss (VML) typically results in loss of function. Tissue engineering (TE) applications that use cells seeded onto hydrogels are one potential option for regenerating the lost tissue. Hydrogels are described as soft crosslinked polymeric networks with high water content that simulates the body's natural aqueous environment. They can be formulated from many different starting materials into biocompatible, biodegradable systems. Fabrication methods such as electrospinning, freeze-drying, molding, and 3D printing can be used with the hydrogel solution to form 3D structures. In this review, natural, semi-synthetic, synthetic, and composite hydrogels for skeletal muscle regeneration are discussed. It was ascertained that the majority of the current research focused on natural polymeric hydrogels including collagen, gelatin, agarose, alginate, fibrin, chitosan, keratin, and combinations of the aforementioned. This category was followed by a discussion of composite hydrogels, defined in this review as at least one synthetic and one natural polymer combined to form a hydrogel, and these are the next most favored materials. Synthetic polymer hydrogels came in third with semi-synthetic polymers, chemically modified natural polymers, being the least common. While many of the hydrogels show promise for skeletal muscle regeneration, continued investigation is needed in order to regenerate a functional muscle tissue replacement. Lay SummarySkeletal muscle tissue engineering focuses on regenerating large amounts of skeletal muscle tissue lost due to tumor removal, traumatic injuries, and/or disease. Neither natural repair processes by the body nor current medical interventions are able to completely restore function after volumetric muscle loss. Thus, scientists are investigating alternative approaches to regenerate the lost muscle, restore function, and increase patient quality of life. This review paper summarizes the research from 2013 to early 2018 using hydrogels, a soft material with a high water content, as a tool to regenerate muscle. The review is categorized into hydrogels made from natural materials, semi-synthetic materials, synthetic materials, and composite materials (at least one natural and one synthetic material combined).

No abstract