Induction of neurite outgrowth by a conditioned-medium factor bound to the culture substratum

Heart-cell conditioned medium (HCM) induces rapid neurite outgrowth from isolated neurons in culture. The following evidence indicates that this action of HCM is due to a trypsin-sensitive factor which attaches to the polyornithinecoated culture substratum: (i) Pretreatment of the culture substratum with HCM allows rapid neurite outgrowth to occur even in unconditioned media. The active factor remains bound to the substratum during the period of neurite outgrowth. (ii) The substratum-bound activity is destroyed by trypsin treatment, but is insensitive to collagenase, RNase, and DNase. (iii) The factor that binds to the substratum is essential for neurite outgrowth, because HCM is no longer active when the material that binds to the polyornithine substratum has been removed by passage of the HCM over a series of culture dishes. However, this "depleted" HCM is still able to support the growth of nonneuronal cells. (iv) Most significantly, when neurons are cultured in whole HCM, the extent of neurite outgrowth is proportional to the amount of substratum-bound activity and not to the amount in solution, indicating that the substratum-bound form of the factor is more active. Previous observations [Collins, F. (1978) Dev. Biol. 65, 50-57] suggest that HCM promotes neurite outgrowth by increasing the adhesion between nerve cell surface extensions and the polyornithine-coated culture substratum. It is possible, therefore, that the factor in HCM that binds to the substratum possesses sites to which nerve cell surface components adhere.