Differential expression of membrane conductances underlies spontaneous event initiation by rostral midline neurons in the embryonic mouse hindbrain.

Spontaneous activity is expressed in many developing CNS structures and is crucial in correct network development. Previous work using [Ca(2+)](i) imaging showed that in the embryonic mouse hindbrain spontaneous activity is initiated by a driver population, the serotonergic neurons of the nascent raphe. Serotonergic neurons derived from former rhombomere 2 drive 90% of all hindbrain events at E11.5. We now demonstrate that the electrical correlate of individual events is a spontaneous depolarization, which originates at the rostral midline and drives events laterally. Midline events have both a rapid spike and a large plateau component, while events in lateral tissue comprise only a smaller amplitude plateau. Lateral cells have a large resting conductance and are highly coupled via neurobiotin-permeant gap junctions, while midline cells are significantly less gap junction-coupled and uniquely express a T-type Ca(2+) channel. We propose that the combination of low resting conductance and expression of T-type Ca(2+) current is permissive for midline neurons to acquire the initiator or driver phenotype, while cells without these features cannot drive activity. This demonstrates that expression of specific conductances contributes to the ability to drive spontaneous activity in a developing network.

Induction and loss of a TP53-dependent radioadaptive response in the human lymphoblastoid cell model TK6 and its abrogation by BCL2 over-expression.

PURPOSE: To characterize the radioadaptive response in the human lymphoblastoid cell model TK6, and determine: (i) Whether repeated low dose exposures are more effective than single acute exposures in inducing resistance, (ii) the time-course for induction and loss of resistance following chronic exposures, and (iii) the effect of TP53 deletion or BCL2 over-expression on the induction of an adaptive response. MATERIALS AND METHODS: TK6, a human B-lymphoblastoid cell line, TK6-BCL2, a TK6 line that over-expresses BCL2 and is resistant to radiation-induced apoptosis, and NH32, a TP53 knockout of TK6 that is also resistant to apoptosis were studied. Cells were exposed to chronic, daily doses of 10 cGy given over 1 -21 days before being challenged with 1 -5 Gy exposures. Cell survival and chromatid break induction following high dose challenge were used to evaluate adaptive radiation responses. RESULTS: Exposure to 10 cGy gamma rays induced resistance to killing and chromosome break induction in TK6 cells, but not in either TK6-BCL2 or NH32 cells. Resistance in TK6 was observed 4 h after exposure, and cells remained resistant for about 48 h. Maximal resistance was induced by a single 10 cGy dose. Repeated 10 cGy exposures had no additional effect on radiation sensitivity, except to maintain the induced radioresistance. CONCLUSION: An adaptive response is maximally and rapidly induced by a single low dose exposure in TK6 cells, and it has a limited lifespan. Induction of an adaptive response in TK6 cells can be abrogated by either TP53 loss or BCL2 over-expression. The characteristics of induced resistance in TK6 cells suggest that alterations in TP53-dependent apoptotic responses may be one mechanism for resistance.