Until now, no biological tools have been available to determine if a cross-linked collagen fibrillar network derived entirely from type IIA procollagen isoforms, can form in the extracellular matrix (ECM) of cartilage. Recently, homozygous knock-in transgenic mice (Col2a1+ex2, ki/ki) were generated that exclusively express the IIA procollagen isoform during post-natal development while type IIB procollagen, normally present in the ECM of wild type mice, is absent. The difference between these Col2a1 isoforms is the inclusion (IIA) or exclusion (IIB) of exon 2 that is alternatively spliced in a developmentally regulated manner. Specifically, chondroprogenitor cells synthesize predominantly IIA mRNA isoforms while differentiated chondrocytes produce mainly IIB mRNA isoforms. Recent characterization of the Col2a1+ex2 mice has surprisingly shown that disruption of alternative splicing does not affect overt cartilage formation. In the present study, biochemical analyses showed that type IIA collagen extracted from ki/ki mouse rib cartilage can form homopolymers that are stabilized predominantly by hydroxylysyl pyridinoline (HP) cross-links at levels that differed from wild type rib cartilage. The findings indicate that mature type II collagen derived exclusively from type IIA procollagen molecules can form hetero-fibrils with type XI collagen and contribute to cartilage structure and function. Heteropolymers with type XI collagen also formed. Electron microscopy revealed mainly thin type IIA collagen fibrils in ki/ki mouse rib cartilage. Immunoprecipitation and mass spectrometry of purified type XI collagen revealed a heterotrimeric molecular composition of α1(XI)α2(XI)α1(IIA) chains where the α1(IIA) chain is the IIA form of the α3(XI) chain. Since the N-propeptide of type XI collagen regulates type II collagen fibril diameter in cartilage, the retention of the exon 2-encoded IIA globular domain would structurally alter the N-propeptide of type XI collagen. This structural change may subsequently affect the regulatory function of type XI collagen resulting in the collagen fibril and cross-linking differences observed in this study.

The present study describes the generation of a knock-in mouse model to address the role of type II procollagen (Col2a1) alternative splicing in skeletal development and maintenance. Alternative splicing of Col2a1 precursor mRNA is a developmentally-regulated event that only occurs in chondrogenic tissue. Normally, chondroprogenitor cells synthesize predominantly exon 2-containing mRNA isoforms (type IIA and IID) while Col2a1 mRNA devoid of exon 2 (type IIB) is the major isoform produced by differentiated chondrocytes. Another isoform, IIC, has also been identified that contains a truncated exon 2 and is not translated into protein. The biological significance of this IIA/IID to IIB splicing switch is not known. Utilizing a splice site targeting knock-in approach, a 4 nucleotide mutation was created to convert the 5 splice site of Col2a1 exon 2 from a weak, non-consensus sequence to a strong, consensus splice site. This resulted in apparent expression of only the IIA mRNA isoform, as confirmed in vitro by splicing of a type II procollagen mini-gene containing the 5′ splice site mutation. To test the splice site targeting approach in vivo, homozygote mice engineered to retain IIA exon 2 (Col2a1+ex2) were generated. Chondrocytes from hindlimb epiphyseal cartilage of homozygote mice were shown to express only IIA mRNA and protein at all pre- and post-natal developmental stages analyzed (E12.5, E16.5, P0, P3, P7, P14, P28 and P70). As expected, type IIB procollagen was the major isoform produced in wild type cartilage at all post-natal time points. Col2a1+ex2 homozygote mice are viable, appear healthy and display no overt phenotype to date. However, research is currently underway to investigate the biological consequence of persistent expression of the exon 2-encoded conserved cysteine-rich domain in post-natal skeletal tissues.

The fibrillar collagen types I, II, and V/XI have recently been shown to have partially 3-hydroxylated proline (3Hyp) residues at sites other than the established primary Pro-986 site in the collagen triple helical domain. These sites showed tissue specificity in degree of hydroxylation and a pattern of D-periodic spacing. This suggested a contributory role in fibril supramolecular assembly. The sites in clade A fibrillar ␣1(II), ␣2(V), and ␣1(I) collagen chains share common features with known prolyl 3-hydroxylase 2 (P3H2) substrate sites in ␣1(IV) chains implying a role for this enzyme. We pursued this possibility using the Swarm rat chondrosarcoma cell line (RCS-LTC) found to express high levels of P3H2 mRNA. Mass spectrometry determined that all the additional candidate 3Hyp substrate sites in the pN type II collagen made by these cells were highly hydroxylated. In RNA interference experiments, P3H2 protein synthesis was suppressed coordinately with prolyl 3-hydroxylation at Pro-944, Pro-707, and the C-terminal GPP repeat of the pN␣1(II) chain, but Pro-986 remained fully hydroxylated. Furthermore, when P3H2 expression was turned off, as seen naturally in cultured SAOS-2 osteosarcoma cells, full 3Hyp occupancy at Pro-986 in ␣1(I) chains was unaffected, whereas 3-hydroxylation of residue Pro-944 in the ␣2(V) chain was largely lost, and 3-hydroxylation of Pro-707 in ␣2(V) and ␣2(I) chains were sharply reduced. The results imply that P3H2 has preferred substrate sequences among the classes of 3Hyp sites in clade A collagen chains.Collagen, the major structural protein of vertebrates has evolved a range of post-translational modifications that are essential for triple helix assembly and stability, intermolecular cross-linking, and strength of fibrils and tissue function. Enzymes from the family of 2-oxo-glutarate-dependent dioxygenases are responsible for most of these modifications. For example, prolyl 4-hydroxylase catalyzes proline 4-hydroxylation, a modification necessary for the secondary and tertiary structure of collagen (1, 2), and the lysyl hydroxylase isoenzymes catalyze lysine hydroxylation, a modification essential for the formation of intermolecular cross-links in collagen (2-4).Prolyl 3-hydroxlase 1 (P3H1), 2 another member of the 2-oxo-glutarate-dependent dioxygenase family, catalyzes the post-translational 3-hydroxylation of certain proline residues in fibril-forming collagens. This enzyme was only recently cloned from chicken tissues (5) after being partially purified Ͼ25 years ago (6). P3H1 was identified as the chick homologue of rat leprecan, a glycoprotein, which was first isolated from a rat parietal yolk sac tumor cell line (7). Within the past few years, novel mutations in the human P3H1 gene and genes encoding P3H1-associated proteins were shown to cause recessive forms of osteogenesis imperfecta (8 -11). By mass spectrometry, we and other investigators have shown that Pro-986 in the ␣1(I) collagen chain of bone and skin from such patients can be severely under-3-hydroxyated. The P3H1 enz...