The matrilins are non-collagenous extracellular matrix proteins that form a subbranch of the superfamily of proteins containing VWA domains. Four matrilins are present in mammals, matrilin-1, -2, -3 and �4. The matrilins contain one or two VWA domains which are connected by a varying number of EGF-like domains, followed by a C-terminal a-helical coiled-coil domain. Matrilins serve as adaptors in the assembly of supramolecular structures in the extracellular matrix, but it is not known if this role is static or dynamic in nature. The in vivo functions of matrilins remain unclear and need to be elucidated in detail, in particular to understand the role of matrilins in inherited disease. Mutations in the gene encoding human matrilin-3 lead to autosomal dominant skeletal disorders, such as multiple epiphyseal dysplasia (MED), which is characterized by short stature and early onset osteoarthritis, and bilateral hereditary microepiphyseal dysplasia, a variant form of MED characterized by pain in the hip and knee joints. In addition, a mutation in the first EGF-like domain of matrilin-3 has been linked to hand osteoarthritis in the Icelandic population. Matrilin-3 null mice and matrilin-1/-3 double deficient mice were characterized. Homozygous matrilin-3 mutant mice appear normal, are fertile, and show no obvious skeletal malformations. Histological and ultrastructural analyses reveal an endochondral bone formation indistinguishable from that of wildtype animals. Northern blot, immunohistochemical, and biochemical analyses showed no compensatory upregulation of any other member of the matrilin family. In matrilin-1/-3 double null mice, biochemical analyses revealed a molecular phenotype in which the amount of matrilin-4 protein is increased and the band patterns of matrilin-3 and -4 are altered. The upregulation of matrilin-4 is likely to represent a compensatory mechanism. Altogether, the findings suggest functional redundancy among matrilins in mammals and demonstrate that the phenotypes of MED-like disorders are not caused by the absence of matrilin-3, but are likely to be due to dominant negative effects of the mutant proteins. The zebrafish is a well established model organism for the study of vertebrate development. The matrilins are present in neither Drosophila nor in C. elegans and the zebrafish is therefore among the simplest organisms which express matrilins. Highly conserved orthologues, matrilin-1, -3a, -3b and �4, are present in zebrafish, while the matrilin-2 gene is missing. The temporal and spatial expression of zebrafish matrilins was characterized. Zebrafish matrilin-1 was found not only in skeletal tissue but also in notochord and intestine. Matrilin-3a expression is restricted to skeletal tissues, while the expression pattern of matrilin-3b has not yet been elucidated due to the lack of a specific antibody. Nevertheless, RT-PCR analysis reveals that matrilin-3b is expressed at 24 hpf and, interestingly, splice variants of matrilin-3b containing a proline- and serine/threonine-rich domain are found only in embryos but not in adult fish, indicating that this new domain probably has an important function during zebrafish development. Similar to in mammals, matrilin-4 is the earliest and most widely expressed matrilin in zebrafish. Matrilin-4 is strongly expressed already at 24 hpf and is present in the skeletal tissues, soft connective tissues and nervous tissues. Morpholino antisense oligonucleotides were used to knockdown matrilins expressed in zebrafish. Malformations were seen at all the doses used and the phenotypes matched to the tissue distribution of the respective matrilin. Injection of matrilin-1 or matrilin-4 morpholinos give curled body shape, smaller eyes or a truncated body axis depending on dosage. The matrilin-3a knockdown embryos showed a serious skeletal phenotype
