Primary structure of the long and short splice variants of mouse collagen XII and their tissue‐specific expression during embryonic development

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Abstract
Type XII collagen, a member of the FACIT group of extracellular matrix proteins, consists of molecules that are trimers of α1(XII) chains. The three chains in each molecule form a cross-shaped structure with a central globule from which a triple-helical tail and three finger-like regions (containing von Willebrand factor A-like domains and fibronectin type III repeats) extend. cDNA cloning/sequencing of chicken α1(XII) collagen and protein studies with mouse, bovine, and human material suggest that the α1(XII) collagen gene gives rise to two molecular variants, differing in the length of the finger-like regions, by alternative splicing of the primary transcript. To provide a basis for studies of the function of the two variants in an organism that can be genetically manipulated, we have isolated and sequenced mouse cDNAs encoding both splice variants. The sequence provides the first complete nucleotide and amino acid sequence of mammalian type XII collagen. From these cDNAs we have generated digoxigenin-labeled RNA probes for in situ hybridization of developing mouse embryos to find out whether the splicing mechanism responsible for generation of the two forms is developmentally regulated. The results, combined with Northern blot and RT-PCR analysis of RNA from embryos at various developmental stages, demonstrate that the long form of collagen XII, XIIA, is the predominant form at early stages (ED7 and 11); at later stages of development (ED15 and 17) the short form, XIIB, becomes the major form. As the short form becomes the major product, the long splice variant continues to be expressed in several tissues, even after birth. An exception is dermis, which is positive for the long form up to embryonic day 15, but negative at day 18, when only the short form RNA can be detected.