Secreted protein that functions as cytokine and growth factor and mediates its signal through cell-surface proteoglycan and non-proteoglycan receptors (PubMed:16901907). Binds cell-surface proteoglycan receptors via their chondroitin sulfate (CS) groups (PubMed:17230638). Thereby regulates many processes like inflammatory response, cell proliferation, cell adhesion, cell growth, cell survival, tissue regeneration, cell differentiation and cell migration (PubMed:17230638, PubMed:19060126, PubMed:17015789, PubMed:28183532, PubMed:10683378, PubMed:15482347, PubMed:15509530, PubMed:24458438, PubMed:25551381, PubMed:29233575). Participates in inflammatory processes by exerting two different activities. Firstly, mediates neutrophils and macrophages recruitment to the sites of inflammation both by direct action by cooperating namely with ITGB2 via LRP1 and by inducing chemokine expression (PubMed:10683378, PubMed:15509530, PubMed:24458438, PubMed:28183532). This inflammation can be accompanied by epithelial cell survival and smooth muscle cell migration after renal and vessel damage, respectively (PubMed:15509530, PubMed:10683378). Secondly, suppresses the development of tolerogenic dendric cells thereby inhibiting the differentiation of regulatory T cells and also promote T cell expansion through NFAT signaling and Th1 cell differentiation (PubMed:28183532). Promotes tissue regeneration after injury or trauma. After heart damage negatively regulates the recruitment of inflammatory cells and mediates cell survival through activation of anti-apoptotic signaling pathways via MAPKs and AKT pathways through the activation of angiogenesis (PubMed:17015789, PubMed:19060126). Also facilitates liver regeneration as well as bone repair by recruiting macrophage at trauma site and by promoting cartilage development by facilitating chondrocyte differentiation (PubMed:15482347, PubMed:25551381). Plays a role in brain by promoting neural precursor cells survival and growth through interaction with heparan sulfate proteoglycans (PubMed:17230638). Binds PTPRZ1 and promotes neuronal migration and embryonic neurons survival (By similarity). Binds SDC3 or GPC2 and mediates neurite outgrowth and cell adhesion (By similarity). Binds chondroitin sulfate E and heparin leading to inhibition of neuronal cell adhesion induced by binding with GPC2 (PubMed:10978312). Binds CSPG5 and promotes elongation of oligodendroglial precursor-like cells (PubMed:16901907). Also binds ITGA6:ITGB1 complex; this interaction mediates MDK-induced neurite outgrowth (By similarity). Binds LRP1; promotes neuronal survival (By similarity). Binds ITGA4:ITGB1 complex; this interaction mediates MDK-induced osteoblast cells migration through PXN phosphorylation (By similarity). Binds anaplastic lymphoma kinase (ALK) which induces ALK activation and subsequent phosphorylation of the insulin receptor substrate (IRS1), followed by the activation of mitogen-activated protein kinase (MAPK) and PI3-kinase, and the induction of cell proliferation (By similarity). Promotes epithelial to mesenchymal transition through interaction with NOTCH2 (By similarity). During arteriogenesis, plays a role in vascular endothelial cell proliferation by inducing VEGFA expression and release which in turn induces nitric oxide synthase expression. Moreover activates vasodilation through nitric oxide synthase activation (PubMed:29233575). Negatively regulates bone formation in response to mechanical load by inhibiting Wnt/beta-catenin signaling in osteoblasts (PubMed:21185956, PubMed:20200993). In addition plays a role in hippocampal development, working memory, auditory response, early fetal adrenal gland development and the female reproductive system (PubMed:16619002, PubMed:17121547, PubMed:10096022). Homodimer. Interacts with ALK. Interacts with LRP1; promotes neuronal survival. Interacts with LRP2. Interacts with NCAM1 (By similarity). Interacts (via C-terminal) with PTPRZ1 (via chondroitin sulfate chains); this interaction is inhibited by PTN; this interaction promotes neuronal migration (PubMed:10212223). Interacts with NCL; this interaction promotes NCL clustering and lateral movements of this complex into lipid rafts leading to MDK internalization. Interacts with LRP6 and LRP8: this interaction is calcium dependent. Interacts with ITGA4. Interacts with ITGA6. Interacts with ITGB1. Interacts with ITGA4:ITGB1 complex; this interaction mediates MDK-induced osteoblast cells migration through PXN phosphorylation. Interacts with ITGA6:ITGB1 complex; this interaction mediates MDK-induced neurite outgrowth. Interacts with NOTCH2; this interactio mediates a nuclear accumulation of NOTCH2 and therefore activation of NOTCH2 signaling leading to interaction between HES1 and STAT3. Interacts with GPC2 (via heparan sulfate chain); this interaction is inhibited by heparin followed by chondroitin sulfate E; this interaction induces GPC2 clustering through heparan sulfate chain; this interaction induces neuronal cell adhesion and neurite outgrowth (By similarity). Interacts with SDC3; this interaction induces SDC3 clustering; this interaction induces neuronal cell adhesion and neurite outgrowth (PubMed:9089390, PubMed:12084985). Interacts with SDC1 (PubMed:9089390). Interacts with CSPG5; this interaction promotes elongation of oligodendroglial precursor-like cells (PubMed:16901907). Expressed in the follicular epithelium and granulosa cells of the ovary. Is expressed temporarily during the early stages of retinoic acid-induced differentiation of embryonal carcinoma cells and during the mid-gestation period of mouse embryogenesis. In late embryos and in adults expression is restricted to the kidney. By retinoic acid (PubMed:2345177). Induced after tissue damage (PubMed:17015789, PubMed:19060126). Induced by inflammatory cells, in particular, CD4(+) T cells under inflammatory conditions (PubMed:22323540). Induced during the early and intermediate phase of fracture repair (PubMed:25551381). Homozygous knockout MDK mice are viable and reproduce normally. Mice have no apparent abnormalities except that postnatal development of the hippocampus is delayed. However 4 weeks after birth, mice have a deficit in their working memory and have an increased anxiety (PubMed:10096022). Knockout MDK mice exhibit low to moderate levels of auditory deficits and generally respond at around 50 dB. PTN and MDK double knockoutmice have a deficit of auditory response (PubMed:16619002). PTN and MDK double knockout mice are born in only one third the number expected by Mendelian segregation and 4 weeks after birth weigh about half as much as wild-type mice. Most of the female are infertile. Both male and female one-month-old mice show a defect in spontaneous locomotive activity of 50-60% of that of wild-type mice. Although the difference in activity decrease with age, the activity of 3-month-old male double knockout mice is still about 80% of that of the wild-type mice. The diestrus and proestrus periods are long and the estrus period is short. Furthermore, vaginal abnormality is found in about half of the double deficient mice (PubMed:17121547). Homozygous knockout MDK mice display not significant difference from wild-type until the age of 6 month. Mice at 12 and 18 months of age show an increased trabecular bone volume, accompanied by cortical porosity (PubMed:20200993). Belongs to the pleiotrophin family.