Myenteric neurons of the ileum that express somatostatin are a target of prion neuroinvasion in an alimentary model of sheep scrapie

Abstract

Neuroinvasion of the enteric nervous system by prions is an important step in dissemination to the brain, yet very little is known about the basic process of enteric neuroinvasion. Using an alimentary model of neonatal disease transmission, neuroinvasion by scrapie prions in the ileum of lambs was detected by immunohistochemical staining for the disease-associated form of the prion protein, PrP Sc . Odds ratios (OR) were determined for the frequency of PrP Sc staining within enteric somata categorized by plexus location (myenteric, submucosal) and neurochemical staining (PGP 9.5, neural nitric oxide synthase, somatostatin, substance P, and vasoactive intestinal polypeptide). PrP Sc was observed in 4.48 § 4.26% of myenteric neurons and 2.57 § 1.82% of submucosal neurons in Wve lambs aged 208-226 days but not in a lamb aged 138 days. The relative frequency of PrP Sc within enteric somata was interdependent on plexus location and neurochemical type. Interestingly, PrP Sc was observed more frequently within myenteric neurons than in submucosal neurons (PGP 9.5; OR = 1.72, 95% conWdence interval = 1.21-2.44), and was observed within the myenteric plexus approximately 4£ (2.16-6.94) more frequently in somatostatin neurons than in the general neural population stained by PGP 9.5. Nerve Wbers stained for somatostatin were present in the mucosa and near PrP Sc staining within Peyer's patches. The results suggest that somatostatin-expressing enteric neurons, with Wber projections near Peyer's patches, but with somata present in greatest proportion within the myenteric plexus, are an early target for neuroinvasion by scrapie prions and could serve an important role in neural dissemination.

Prions and neuronal death

“…1 In this respect, calbindin (CALB)-immunoreactive (IR), neuronal nitric oxide synthase (nNOS)-IR (Figure 1), and somatostatin-IR neurons have been recently identified as three distinct cell populations residing within ileal ENS plexuses that are selectively targeted during natural and oral experimental sheep scrapie infection. 4, 5 However, differently from that reported at the CNS level, 1 we did not observe significant differences in total (HuC/D-IR) as well as in CALB-IR and nNOS-IR neuron cell counts between the ileal ENS plexuses from scrapie-affected sheep and those from uninfected control sheep carrying an identical ‘susceptible' (ARQ/ARQ) PrP genotype. 4 Furthermore, no evidence of gut dysfunction and/or functional loss was found in these scrapie-affected sheep, which is also in open contrast with that constantly observed in the brain of any PD-affected individual.…”

The brain to gut pathway: a possible route of prion transmission

“…Other subclasses of neurons could also be affected by peripheral spread of prions. In orally infected sheep, PrP Sc –IR was prominent in somatostatin (SOM)-IR MP neurons and, to a smaller extent, in nNOS-IR neurons 41. In the mouse ileum, SOM-IR MP neurons represent a small subpopulation (about 4%), which also express CALR-IR,24 which we did not specifically investigate.…”

“…In general, during natural prion disease, the SMP seems less involved in prion neuroinvasion compared to the MP41; this is intriguing because the processes of SMP neurons that project to the mucosa and Peyer's patches in grazing mammals, such as cattle43 and sheep,42 44 as well as in mouse,45 46 would be expected to be exposed to prions.…”

Ovine progressive pneumonia provirus levels are unaffected by the prion 171R allele in an Idaho sheep flock

“…DNA amplification and sequencing of the ovine PRNP ORF was performed similarly to previous experiments using forward primer 5'-GGCATTTGATGCTGACACC-3' and reverse primer 5'-TACAGGGCTGCAGGTAGAC-3' [33]. Reverse primer 5'-GGTGGTGACTGTGTGTTGCTGA-3' was used for standard dideoxynucleotide sequencing.…”