Knockout of the prion protein (PrP)-like Sprn gene does not produce embryonic lethality in combination with PrP ^C -deficiency

Abstract

The Sprn gene encodes Shadoo (Sho), a glycoprotein with biochemical properties similar to the unstructured region of cellular prion protein (PrP C ). Sho has been considered a candidate for the hypothetical π protein that supplies a PrP C -like function to maintain the viability of Prnp 0/0 mice lacking the PrP C protein. To understand these relationships more clearly we probed the cell biology of Sho and created knockout mice. Besides full-length and a “C1” C-terminal fragment, we describe a 6-kDa N-terminal Sho neuropeptide, “N1,” which is present in membrane-enriched subcellular fractions of wild-type mice. Sprn null alleles were produced that delete all protein coding sequences yet spare the Mtg 1 gene transcription unit that overlaps the Sprn 3′ UTR; the resulting mice bred to homozygosity were viable and fertile, although Sprn 0/0 mice maintained in two genetic backgrounds weighed less than wild-type mice. Lack of Sho protein did not affect prion incubation time. Contrasting with lethality reported for knockdown of expression in Prnp 0/0 embryos using lentiviruses targeted against the Sprn 3′ UTR, we established that double-knockout mice deficient in both Sho and PrP C are fertile and viable up to 690 d of age. Our data reduce the impetus for equating Sho with the notional π protein and are not readily reconciled with hypotheses wherein expression of PrP C and Sho are both required for completion of embryogenesis. Alternatively, and in accord with some reports for PrP C , we infer that Sho’s activity will prove germane to the maintenance of neuronal viability in postnatal life.

Prion Infectivity Plateaus and Conversion to Symptomatic Disease Originate from Falling Precursor Levels and Increased Levels of Oligomeric PrP Sc Species

“…Although cell-based mechanisms to attenuate the accumulation of rPrP Sc or infectivity may exist (for example, clearance [29] and phagocytosis [30]), it is unlikely they could have the same magnitude of impact as the downregulation of PrP C , which is closely linked in time to the drop in replication rate for infectivity and PrP Sc isoforms. This is because (i) PrP C is an obligatory precursor for the formation of PrP Sc and infectivity (shown by the use of PrP transgenic mice [31] and Prnp 0/0 knockout mice [32]) and, conversely, (ii) there is no alternative pathway of cross-seeding to create mouse-adapted prions by misfolding of Sho protein (33)(34)(35). Although it remains possible that a particular subpool of PrP C is eligible for conversion to PrP Sc (for example, partially unfolded or particularly glycosylated molecules), the size of this hypothetical pool would have to expand and contract in direct proportion with the net starting concentration of PrP C because constitutive and inducible transgenes that alter PrP C levels have a profound effect upon the generation of prion infectivity and disease incubation times (31,36,37).…”

The role of prion protein in stem cell regulation

“…In fact, controversies about the particular impact of these proteins on embryo development have arisen. In the case of Shadoo, a recent study failed to observe a clear phenotype in the mouse Shadoo-PrP C double-KO embryos (Daude et al 2012) while a previous report described a lethal embryonic phenotype (Young et al 2009). However, the inactivation of PrP C in zebrafish induced embryonic lethality (Malaga-Trillo & Sempou 2009), highlighting the lack of a clear model that could connect the observations.…”

Prion disease tempo determined by host-dependent substrate reduction

“…Declining levels of PrP C in the course of protracted prion infections were in most instances shadowed by a drop in endogenous mouse Sho (refs. 10, 11; Figures 1, 4, and 5; and Supplemental Figures 1 and 9), a cousin of PrP C that does not modulate creation of PrP Sc and is not converted to a protease-resistant isoform (41). Depletion of PrP C as a simple consequence of synaptic damage or neuronal cell death is unlikely, because PrP C downregulation occurred nearly 300 days before clinical disease in hemizygous Prnp 0/+ mice and was absent in certain rapid models with an abundance of vacuolation at end stage disease (i.e., TgPrnp a mice and bank voles; Figure 5D and Supplemental Figure 9, B and E).…”