Demyelination is the hallmark of numerous neurodegenerative conditions, including multiple sclerosis. Oligodendrocyte progenitors (OPCs), which normally mature into myelin-forming oligodendrocytes, are typically present around demyelinated lesions but do not remyelinate affected axons. Here, we find that the glycosaminoglycan hyaluronan accumulates in demyelinated lesions from individuals with multiple sclerosis and in mice with experimental autoimmune encephalomyelitis. A high molecular weight (HMW) form of hyaluronan synthesized by astrocytes accumulates in chronic demyelinated lesions. This form of hyaluronan inhibits remyelination after lysolecithin-induced white matter demyelination. OPCs accrue and do not mature into myelin-forming cells in demyelinating lesions where HMW hyaluronan is present. Furthermore, the addition of HMW hyaluronan to OPC cultures reversibly inhibits progenitor-cell maturation, whereas degrading hyaluronan in astrocyte-OPC cocultures promotes oligodendrocyte maturation. HMW hyaluronan may therefore contribute substantially to remyelination failure by preventing the maturation of OPCs that are recruited to demyelinating lesions.

Multipotent neural stem cells (NSCs) are competent for commitment to the oligodendrocyte (OL) lineage both in vitro and in vivo. We exploited this property to develop a rat neurospheres (NS)/oligospheres (OS)-based culture system to generate large numbers of highly enriched late OL progenitors (preOLs) and mature OLs (MatOLs). CNS neuroblastoma cell line B104-derived conditioned medium promoted the generation of nearly pure populations of preOLs from dissociated OS. The subsequent culture of preOLs with ciliary neurotrophic factor (CNTF) and 3,3¢,5¢-triiodo-L-thyronine (T 3 ) generated nearly pure populations of MatOLs. OL lineage specificity was confirmed by immunocytochemistry, quantitative RT-PCR and gene expression profiling, which demonstrated large differences between preOLs and MatOLs. The insulin-like growth factors (IGFs) are potent neuro-protective agents required for OL survival. We used this system to systematically define maturation-dependent changes in IGF signaling during the course of OL differentiation. The IGF-I and insulin receptors, insulin receptor substrate-1 (IRS-1) and IRS-2, protein kinase B (PKB)/Akt and Janus kinase (JNK) were expressed at higher levels in NS and preOLs compared with OS and MatOLs. Erk expression increased markedly from NS to OS, decreased only partially upon commitment to preOLs, and, in MatOLs, returned to a low level similar to NS. IGF activation of the generally proliferative Erk pathway was gradually acquired during NSC differentiation, whereas IGF activation of the generally pro-survival, anti-apoptotic PI3K/PKB pathway was consistently robust at each developmental stage.