Myelination is essential for proper functioning of the central nervous system (CNS). In this study, we have identified a mouse mutation, designated furue, which causes tremors and hypomyelination in the CNS, particularly in the spinal cord, but not in the sciatic nerve of the peripheral nervous system (PNS). In the spinal cord of the furue mice, myelination of small diameter axons was dramatically reduced and differentiation of oligodendrocytes, the myelin-forming cells in the CNS, was inhibited. We subsequently found that the furue mutation was associated with a transgene insertion into the teneurin-4 (Ten-4/Odz4) gene, encoding a transmembrane protein of unknown function. Ten-4 was strongly expressed in the spinal cord of wild-type mice and was induced during normal oligodendrocyte differentiation. In contrast, in the furue mice, the expression of Ten-4 was absent. Differentiation and cellular process formation of oligodendrocytes were inhibited in primary cell culture from the furue mice. Cell differentiation and process formation were also inhibited in the oligodendrocyte progenitor cell line CG-4 following suppression of Ten-4 expression by shRNA. Further, Ten-4 positively regulated focal adhesion kinase (FAK), an essential signaling molecule for oligodendrocyte process formation and myelination of small diameter axons. These findings suggest that Ten-4 is a novel regulator of oligodendrocyte differentiation and that it plays a critical role in the myelination of small diameter axons in the CNS.

Perlecan (Hspg2) is a heparan sulfate proteoglycan expressed in basement membranes and cartilage. Perlecan deficiency (Hspg2−/−) in mice and humans causes lethal chondrodysplasia, which indicates that perlecan is essential for cartilage development. However, the function of perlecan in endochondral ossification is not clear. Here, we report the critical role of perlecan in VEGF signaling and angiogenesis in growth plate formation. The Hspg2−/− growth plate was significantly wider but shorter due to severely impaired endochondral bone formation. Hypertrophic chondrocytes were differentiated in Hspg2−/− growth plates; however, removal of the hypertrophic matrix and calcified cartilage was inhibited. Although the expression of MMP-13, CTGF, and VEGFA was significantly upregulated in Hspg2−/− growth plates, vascular invasion into the hypertrophic zone was impaired, which resulted in an almost complete lack of bone marrow and trabecular bone. We demonstrated that cartilage perlecan promoted activation of VEGF/VEGFR by binding to the VEGFR of endothelial cells. Expression of the perlecan transgene specific to the cartilage of Hspg2−/− mice rescued their perinatal lethality and growth plate abnormalities, and vascularization into the growth plate was restored, indicating that perlecan in the growth plate, not in endothelial cells, is critical in this process. These results suggest that perlecan in cartilage is required for activating VEGFR signaling of endothelial cells for vascular invasion and for osteoblast migration into the growth plate. Thus, perlecan in cartilage plays a critical role in endochondral bone formation by promoting angiogenesis essential for cartilage matrix remodeling and subsequent endochondral bone formation.

Perlecan is a component of the basement membrane that surrounds skeletal muscle. The aim of the present study is to identify the role of perlecan in skeletal muscle hypertrophy and myostatin signaling, with and without mechanical stress, using a mouse model (Hspg2 −/− -Tg) deficient in skeletal muscle perlecan. We found that myosin heavy chain (MHC) type IIb fibers in the tibialis anterior (TA) muscle of Hspg2 −/− -Tg mice had a significantly increased fiber cross-sectional area (CSA) compared to control (WT-Tg) mice. Hspg2 −/− -Tg mice also had an increased number of type IIx fibers in the TA muscle. Myostatin and its type I receptor (ALK4) expression was substantially decreased in the Hspg2 −/− -Tg TA muscle. Myostatin-induced Smad activation was also reduced in a culture of myotubes from the Hspg2 −/− -Tg muscle, suggesting that myostatin expression and its signaling were decreased in the Hspg2 −/− -Tg muscle. To examine the effects of mechanical overload or unload on fast and slow muscles in Hspg2 −/− -Tg mice, we performed tenotomy of the plantaris (fast) muscle and the soleus (slow) muscle. Mechanical overload on the plantaris muscle of Hspg2 −/− -Tg mice significantly increased wet weights compared to those of control mice, and unloaded plantaris muscles of Hspg2 −/− -Tg mice caused less decrease in wet weights compared to those of control mice. The decrease in myostatin expression was significantly profound in the overloaded plantaris muscle of Hspg2 −/− -Tg mice, compared with that of control mice. In contrast, overloading the soleus muscle caused no changes in either type of muscle. These results suggest that perlecan is critical for maintaining fast muscle mass and fiber composition, and for regulating myostatin signaling.