Programmed cell death without DNA fragmentation in the jimpy mouse: secreted factors can enhance survival

Jimpy is one of many related mutations affecting the myelin proteolipid protein gene that causes severe hypomyelination in the central nervous system (CNS). Underlying the hypomyelination is a failure of oligodendrocytes (OLs) to differentiate, and the premature death of large numbers of OLs during the developmental period. Previous light and electron microscopic evidence suggested that jimpy OLs die in a manner consistent with programmed cell death. We have used TUNEL staining as a biochemical marker for apoptosis in conjunction with immunostaining for OL and myelin markers. At 13 - 14 days postnatal, a time when the number of dying OLs in jimpy CNS is increased more than five times normal, there are only modest increases (70% in spinal cord; 20% in cerebral cortex) in TUNEL labeled cells in mutant CNS tissues. The results in vitro are similar, and only a small per cent of TUNEL labeled cells have the antigenic phenotype of OLs. The discrepancy between numbers of dying and TUNEL labeled cells suggests either that most jimpy OLs do not undergo programmed cell death or that the biochemical pathways leading to their death do not involve DNA fragmentation which is detected by the TUNEL method. We also present evidence that jimpy OLs show increased survival and enhanced differentiation when they are grown in vitro in medium conditioned by cells lines which express products of the proteolipid protein gene. Cell lines expressing proteolipid protein and the alternatively spliced DM20 protein have differential effects on cell numbers and production of myelin-like membranes.