update 2013-07-31: here’s a quick summary table of what’s available in the published literature:

Half lives of PrP mRNA, PrPC and PrPSc

molecule N2a cells in vivo
PrP mRNA 7h [Pfeifer 1993]
PrPC 5.2h [Borcheldt 1990 (ft)]
3-6h [Caughey 1989 (ft)]		 18h [Safar 2005 (ft)]
sPrPSc 36h [Safar 2005 (ft)]
rPrPSc 30h [Peretz 2001 (ft)] 36h [Safar 2005 (ft)]

rPrPSc = proteinase K-resistant PrPSc, aka PrP-res.
sPrPSc = proteinase K-sensitive PrPSc.

end update. original post follows.

A few days ago I commented that it was unclear what timeline of drug treatment would be needed to observe an effect of compounds that inhibit PrP transcription.  I did some more digging and found that there is a bit of literature on this.

First, to clear up an earlier confusion, Safar 2005 (ft) states that PrP expression in Tet-off mice declined to ~40% of initial levels after 1 day of doxycycline and to ~5% after one week.  But note that ‘initial levels’ in this mouse model are about double the normal expression level for wild-type mice due to the strong Tet promoter [Tremblay 1998], so this 5% corresponds to something more like 10% of wild-type levels.  To be quite precise, Safar’s homozygous Tg(tTA : PrP+/+)3 mice expressed PrP at 4 times wild-type levels, but these experiments were done on heterozygous Tg(tTA : PrP+/0)3 mice, and Safar states that:

When Tg(tTA : PrP+/0)3 mice were given 2 mg doxycycline ml−1 in the drinking water, PrPC expression decreased to 0·1-fold

Thus confirming the figure of ~10% of wild-type levels.  See also Table 1.

Even at this level of expression, the mice did eventually succumb to prion disease, suggesting that the ‘critical threshold’ of tolerable PrPSc production, if there is one, is yet lower than 10% – a challenge for the therapeutic strategy of depleting PrP.

Safar 2005 (ft) also quantifies the half-life of PrP and its mRNA.  PrPC is said to have a half-life of 18h in the mouse brain.  The longer half-life of PrPSc (measured at 36h) means that once PrP production begins to decline following doxycycline administration, PrPSc begins to account for a proportionally larger fraction of total PrP in the brain.

However, Safar notes that the half-life of PrPC in ScN2a cells – the RML prion-infected mouse neuroblastoma cell line most widely used in drug screens – is no more than 7h [Table 2], citing data from [Borcheldt & Prusiner 1988, Caughey 1989, Borcheldt 1990 (ft), Peretz 2001 (ft)].  PrPSc takes longer, though, with a half-life of perhaps 30h in ScN2a cells [Peretz 2001 (ft)], suggesting a longer timeline  may be needed to observe therapeutic effects in infected cultures.

As for PrP mRNA, Safar states that “PrP mRNA and PrPC have consistently shortt1/2 values in ScN2a cells” and appears to cite [Borcheldt 1990 (ft), Muller 1997].  But in fact Borcheldt does not quantify mRNA, instead citing Chesebro 1985, which was the original landmark study that used complementary RNA probes to discover PrP-encoding DNA in both infected and uninfected mice, an important piece of evidence for the prion hypothesis.  But Chesebro did not quantify mRNA half-life.  That leaves Muller 1997, an old study in an obscure journal for which I cannot find full text – but the abstract certainly doesn’t say anything about mRNA half-life.  The Safar 2005 (ft) paper itself does not explicitly quantify PrP mRNA at any point, though since PrP expression is being shut off via a transcriptional mechanism, some information can be inferred from the data.  Looking at Safar’s data, on one hand, PrPC declined to 40% of original levels in just a day, suggesting a fast degradation of the mRNA (and the protein itself), but then it is hard to explain exactly why the peak inhibition was not attained until after 7 days.  And these data concern the mouse brain, so they can’t tell us much about the half life of PrP mRNA in N2a cells.

To summarize:

  • PrP Tet-off mice with PrP expression reduced to 10% of wild-type levels still succumb to scrapie, suggesting that PrP depletion strategies will need to do better than this in order to reverse symptomatic disease.
  • Rapid reduction in PrP levels in Tet-off mice following doxycycline administration may suggest a short half-life of PrP mRNA in the mouse brain.
  • I cannot find any studies which have explicitly quantified the half-life of PrP mRNA in live animals or in cell cultures, including the N2a cells widely used in drug discovery screens. see below

update 2013-04-09: @g dug up this old paper: [Pfeifer 1993] which quantified PrP mRNA half life at 7h in both N2a and ScN2a cells.