Towards a comprehensive understanding of the trophic support of motoneurons.

Motoneurons played an essential role in establishing the concept of target-mediated support of innervating neurons. However, it took several decades until molecules were identified which trophically support motoneurons in vitro and in vivo. The most potent molecule identified so far is ciliary neurotrophic factor (CNTF). It is expressed as a cytosolic molecule in myelinating Schwann cells rather than in skeletal muscle in the postnatal period and therefore does not qualify as a target-derived neurotrophic factor regulating motoneuron survival during embryonic development. However, the inactivation of CNTF by gene targeting experiments results in progressive atrophy and degeneration of motoneurons, demonstrating that CNTF plays an essential role as a maintenance factor for motoneurons postnatally. Secretory molecules which are expressed in skeletal muscle during embryonic development and which support motoneurons in culture and partially also in vivo include members of the NGF gene family (BDNF, NT-3, NT-4/5), FGF-5, IGF-I, and LIF. The evaluation of the physiological importance of these molecules is under investigation.