TEMPORAL CHANGES IN BETA-TUBULIN AND NEUROFILAMENT MESSENGER-RNA LEVELS AFTER TRANSECTION OF ADULT-RAT RETINAL GANGLION-CELL AXONS IN THE OPTIC-NERVE

Axons of adult mammalian retinal ganglion cells (RGCs) do not regenerate spontaneously after injury in the optic nerve and show a persistent decrease in the rate of transport of tubulin and neurofilament proteins. To investigate further the expression of cytoskeletal proteins in these axotomized CNS neurons, mRNA levels of beta-tubulin and the 150 kDa neurofilament subunit (NF-M) were measured after interrupting the optic nerve 9 mm from the eye. Northern blots of RNA extracted from whole retinas after optic nerve transection showed that the total level of both of these mRNAs fell after injury. To determine if this decrease was a result of the death of axotomized RGCs or reflected changes in individual neurons, RNA probes were hybridized to radial cryostat sections of normal and axotomized retinas from 1 d to 6 months after injury. Grain counts revealed two trends of tubulin expression in RGCs. An early increase in tubulin mRNAs in the axotomized RGCs was followed by a later decrease. Such an increase in tubulin mRNA levels has been correlated with regenerative growth in other neurons. By 1 week after injury, the beta-tubulin mRNA levels decreased to 70% of the control value. Moreover, the time of this fall coincided with the onset of a marked slowing of cytoskeletal transport that follows injury in the optic nerve. In contrast, NF-M mRNA levels dropped immediately after axotomy, and remained at 80% of the control level. It is suggested that the transient increase in tubulin mRNAs may reflect an early regenerative response whose persistence depends on further growth cone interactions with the substrate.