Targeting Insulin-Like Growth Factor-1 Signaling into the Central Nervous System for Promoting Myelin Repair

Multiple sclerosis (MS) is the most common demyelinating disease of the central nervous system (CNS). Without myelin, nerve impulses in the CNS are slowed or stopped, leading to a constellation of neurological symptoms. Demyelination also provides a permitting condition for irreversible axonal damage. Remyelination of MS lesions largely fails, although oligodendrocyte precursors and premyelinating oligodendrocytes (myelin forming cells) are present in many demyelinated plaques. Insulin-like growth factor (IGF)-1 is a growth factor that should provide the appropriate signals to promote repair of MS lesions, because it acts as a survival factor for cells of the oligodendrocyte lineage and stimulates myelin synthesis. In a pilot study on MS patients, no detectable remyelinating effects in the CNS were observed following subcutaneous administration of IGF-1. A number of reasons might explain a lack of beneficial effects: a) it is unlikely that subcutaneous administration of IGF-1 provides sufficient passage across the blood-brain-barrier and into the CNS, b) the biological actions of IGF-1 are tightly regulated by several insulin-like growth factor binding proteins (IGFBPs), which become upregulated in the demyelinated lesions and may prevent access of IGF-1 to its receptor, c) IGF-1 not only acts on oligodendrocytes, but also stimulates the proliferation of astrocytes, which form the glial scar that impedes repair processes. In this review, we will discuss strategies to enhance IGF-1 signaling in the CNS utilizing a) alternative routes of administration, b) IGF analogues that displace IGF-1 from regulatory IGFBPs and c) strategies to selectively target IGF-1 to oligodendrocytes.