Takashi Muramatsu

Midkine is a heparin‐binding growth factor with survival‐promoting and migration‐enhancing activities. In order to understand the regulation of midkine signaling, we isolated midkine‐binding proteoglycans from day 13 mouse embryos, when midkine is intensely expressed. Deglycosylation followed by SDS/PAGE revealed various protein bands; one of these was identified as PG‐M/versican by in gel trypsin digestion and sequencing the resulting peptides. PG‐M/versican isolated from day 13 mouse embryos bound midkine with aK_dof 1.0 nm. Pleiotrophin/heparin‐binding growth‐associated molecule, which has a structure related to midkine, was also bound similarly. Digestion with chondroitinase ABC, AC‐I or B abolished the binding to midkine. Heparin as well as chondroitin sulfate D and E inhibited the binding. After chondroitinase ABC digestion, the midkine‐binding PG‐M/versican released 4‐sulfated, 6‐sulfated, 2,6‐disulfated and 4,6‐disulfated unsaturated disaccharides. These results suggest that midkine binds to a polysulfated domain in the chondroitin sulfate chain with a region of dermatan sulfate structure. This proteoglycan may modulate the midkine activity, as binding to midkine can enhance midkine action by concentrating it to the cell periphery or inhibit the action by competing with the binding to a signaling receptor.

Background: Midkine (MK) is a growth factor implicated in the development and repair of various tissues, especially neural tissues. However, its in vivo function has not been clarified.Results: Knockout mice lacking the MK gene (Mdk) showed no gross abnormalities. We closely analysed postnatal brain development in Mdk(–/–) mice using calcium binding proteins as markers to distinguish neuronal subpopulations. Intense and prolonged calretinin expression was found in the dentate gyrus granule cell layer of the hippocampus of infant Mdk(–/–) mice. In infant Mdk(+/+) mice, calretinin expression in the granule cell layer was weaker, and had disappeared by 4 weeks after birth, when calretinin expression still persisted in Mdk(–/–) mice. Furthermore, 4 weeks after birth, Mdk(–/–) mice showed a deficit in their working memory, as revealed by a Y‐maze test, and had an increased anxiety, as demonstrated by the elevated plus‐maze test.Conclusion: Midkine plays an important role in the regulation of postnatal development of the hippocampus.