The molecular mechanisms controlling the process of myelination by Schwann cells remain elusive, despite recent progress in the identification and characterization of genes encoding myelin components (reviewed in ref. 1). We have created a null allele in the mouse Krox-20 gene, which encodes a zinc-finger transcription factor, by in-frame insertion of the Escherichia coli lacZ gene, and have shown that hindbrain segmentation is affected in Krox-20-/- embryos. We demonstrate here that Krox-20 is also activated in Schwann cells before the onset of myelination and that its disruption blocks Schwann cells at an early stage in their differentiation, thus preventing myelination in the peripheral nervous system. In Krox-20-/- mice, Schwann cells wrap their cytoplasmic processes only one and a half turns around the axon, and although they express the early myelin marker, myelin-associated glycoprotein, late myelin gene products are absent, including those for protein zero and myelin basic protein. Therefore Krox-20 is likely to control a set of genes required for completion of myelination in the peripheral nervous system.

The effect of mouse nerve growth factor (NGF) on cultured human fetal sensory neurons was assayed by measuring neurite length, density and rate of growth. Addition of NGF increased adhesion of dissociated sensory neurons cultured on collagen coated surfaces. Almost all neurons of 9 to 10 week old fetuses are postmitotic, contain neuron-specific enolase, (an enzyme linked to differentiation), and require NGF for optimal neurite growth. Sensory ganglia re-explanted on collagen showed maximal neurite length and density when treated with 1 ng/ml of NGF. Neurite density was reduced considerably in the absence of mouse NGF and was almost abolished by addition of antimouse NGF antibodies. Surfaces coated with the matrix glycoproteins laminin or fibronectin further stimulated neurite growth of ganglia in the presence of NGF. Increasing amounts of matrix proteins could partly compensate for the absence of mouse NGF or the inhibition of NGF activity by antibodies. Stimulation of neurite growth by matrix proteins was time-dependent, and neurites showed maximum length at 10 days (2 to 3 mm). Neurite growth was more pronounced with laminin than with fibronectin and collagen, and antibodies to laminin suppressed all neurite growth. In the presence of a constant amount of NGF, mean neurite growth reached 26 microns/hr (at 1 day), and was 2.1 and 1.7 times faster on laminin and fibronectin (respectively) than on collagen. Thus, laminin, and to a lesser degree fibronectin, may enhance neurite growth of human sensory neurons in synergy with NGF.

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