br Collagens as new drivers of zebrafish
Collagens as new drivers of zebrafish tissue regeneration Contrary to mammals, zebrafish possess very high regeneration capacities and thereby represent a versatile model to study tissue and organ regeneration. Adult fish can fully reconstruct their brain, spinal cord, retina, kidney, liver, pancreas and all its appendages . For evident reasons related to human regenerative medicine, the heart reconstruction was highly studied as well as the caudal fin regeneration for practical reasons. Collagen IX was involved in the formation of the vascular plexus during adult zebrafish caudal fin regeneration, a structure that is abnormally persistent at 10 days post-amputation (dpa) in persistent plexus mutants (prp). This mutant carries a point mutation causing a single leucine (176)-to-histidine change in the collagen IX TSPN subdomain. This mutation in the collagen IX gene resulted in smaller fins with rounded edges and osteoblast mispatterning. Both prp embryos and adult animals exhibited defects in fin skeleton regeneration, a collagen II-rich structure to which collagen IX is associated . The possible role of collagen IX in angiogenesis is intriguing and has not been described in mammals. Several fibrillar collagens are expressed in fin skeleton, the collagen types I, II, V, XI and XXIV and XXVII . As mentioned above, fish bones also express collagen type X. Collagen I is encoded by four genes in zebrafish: col1a1a, col1a1b, col1a2a and col1a2b. Analysis of collagen I gene hmg-coa reductase inhibitor during fin skeleton regeneration suggested that, while col1a2a was not detected, col1a2b is highly expressed and could assemble with one of the two other chains to form collagen I heterotrimers . In situ hybridization at 4 dpa showed that col1a1b, col5a1, col10a1, col24a1, col27a1 are restricted to the proximal regenerate that will form the collagenous lepidotrichia part of the fin skeleton while col1a1a and col2a1b are also present in the distal outgrowth corresponding to unmineralized actinotrichia bundles . The fin skeleton regeneration is also regulated by the Hsp47 procollagen chaperone. Its knockdown reduced cell proliferation and the length of the different segments that compose the fin skeleton, resulting in smaller fins . As illustrated by collagen II staining, the regeneration of fin skeleton is compromised in hsp47 MO-injected fins. This suggested that Hsp47 is required for proper fin growth and patterning probably through its collagen chaperone activity. Finally, the collagen-based ECM was shown to be critical for proper wounding and its formation and reorganization at the leading edge of the wound was dependent on reactive oxygen species and vimentin . Collagens are also critical for spinal cord regeneration. A conditional heat-inducible mutant for lysyl hydroxylase 3 (lh3), encoding a glycosyltransferase required for collagen secretion and deposition has been generated. Lh3 invalidation caused defects in axons target-selective regeneration through its main substrate in Schwann cells, collagen IV α5 chain . Interestingly, the collagen IV α5 chain was shown to destabilize mistargeted axons during regeneration in order to ensure target-selective regeneration through interaction with the axonal repellent slit1a. Collagen IV α5 could sequester Slit1a to form a repulsive barrier that directs axons onto their original path. The FACIT collagen XII is also part of the zebrafish embryo axonal regenerative program. Its expression was induced in fibroblast-like cells by Wnt/β-catenin signalling after spinal lesion in zebrafish larvae . The collagen XII-containing ECM was necessary for axons navigation within the damaged site to promote axonal bridging through the lesion and functional recovery. Strikingly, col12a1a overexpression was sufficient to rescue the effects of Wnt/β-catenin inhibition showing that collagen XII actively promotes axonal regeneration. Similarly, a collagen XII-rich ECM was critical for adult zebrafish heart regeneration. After cryoinjury, collagen types XII and I were highly upregulated at the wound site upon TGF-β signalling . In the regenerating edge of the myocardium, collagen XII partially co-localized with tenascin C and fibronectin that composed the transitional regenerative ECM. Together, these studies positioned collagen XII as a new actor of the provisional ECM. Overall, ECM components actively contribute to regeneration. This has been illustrated by a recent study showing that decellularized ECM from zebrafish heart promotes cardiac functional recovery in mammals  opening a new area of future regenerative medicine.