Several lines of evidence have recently suggested that skin is the place where misfolded proteins often stay, which may play a role in the pathogenesis and early detection of neurodegenerative diseases25. While it has been known for a long time that sheep and goats with scrapie often have skin lesions26, prions had not been detected in skin until prion infectivity was first found in skin of prion-infected greater kudu using an animal-based bioassay27. Subsequently, skin PrPSc was detected directly by western blotting after the enrichment of PrPSc in experimentally or naturally scrapie-infected hamsters and sheep, as well as in a single cadaver with vCJD28,29. We recently observed both prion-seeding activity and prion infectivity in the skin of patients with sCJD and vCJD at the terminal stage of the diseases using RT-QuIC assay and a bioassay humanized Tg mouse-based, respectively13. In the current study, we further demonstrated that skin PrPSc is preclinically detectable not only by RT-QuIC, but also by sPMCA before brain damage occurs in two animal models of prion diseases, 263K-inoculated hamsters and sCJDMM1-inoculated humanized Tg40h mice, with parallel RT-QuIC findings in an independent set of scrapie-infected hamsters done in an independent laboratory.

In terms of the earliest time point at which skin PrPSc becomes detectable in animals infected by the intracerebral inoculation of prions, sPMCA showed detection at 2 wpi for hamsters and 4 wpi for Tg40h mice, while RT-QuIC detection was at 3 wpi for hamsters and 20 wpi for Tg40h (Fig. 8a). These findings indicate that skin PrPSc is detectable at least 5 weeks earlier in scrapie-infected animals before brain pathology is observed. Of the five body areas examined, the ear pinna and back skin were the areas that showed earliest prion-seeding activity (3 wpi), while the thigh skin was the latest (9 wpi). The latter was also confirmed by two sets of hamsters examined in two-independent laboratories in this study. In contrast to the prion-seeding activity found in the skin of infected hamsters at the early stage of infection, the earliest time point showing skin prion-seeding activity was at 20 wpi in humanized Tg mice by RT-QuIC (Fig. 8b). This time point was similar whether recombinant hamster or human PrP substrate was used despite our expectation that human PrP might provide a more sensitive RT-QuIC for human PrPSc based on better sequence homology between the seeds and substrate.

Fig. 8 Schematic diagram of time-points with detectable PrPSc, brain pathology, or signs. a 263K-inoculated hamsters were assessed at different weeks post inoculation (wpi) from day 0 to endpoint at 11 wpi. b sCJDMM1-inoculated Tg40h mice were assessed at different wpi from day 0 to endpoint at 24 wpi Full size image

Although the reasons for early and widespread presence of PrPSc in the skin remain unclear, possibilities include the spread of the prion inoculum itself, or endogenously replicating prions, from the brain through the peripheral nerves to the skin within the 2–3 weeks required for the first detection by our ultrasensitive sPMCA and RT-QuIC assays. PrP seeding activity has been detected in the blood in the prion-infected hamsters and deer immediately after peripheral inoculation including oral, nasal, or blood route30. However, no reports have shown that PrPSc is consistently detectable in the blood of prion-infected hamsters within 2 weeks post intracerebral inoculation. Thus, the early spread of PrPSc from the brain-to-the skin in the intracerebrally 263K-inoculated hamsters is likely either not through the blood or, if initially from the blood, requires time-dependent concentration or replication in the skin to become detectable.

It is unclear why, according to RT-QuIC, the back skin more consistently accumulates PrPSc than the other skin areas tested. It may depend on the dermatomes of nerves and their distance from the CNS. Between the back and thigh areas examined, the back dermatome is more proximate to the CNS. Similarly, we found prion-seeding activity much earlier in the ear area than the thigh (3 wpi vs 9 wpi). Analogously, misfolded α-synuclein deposition in Parkinson’s disease patients is more frequently detected in proximate (100% in the cervical C7 site) compared to distal (35% in the thoracic T12 region) skin areas by immunofluorescence microscopy31,32,33. In future studies, it would be interesting to determine whether PrPSc in the skin of sCJD has a similar distribution, and whether factors besides dermatome distance from the brain are involved.

Both sPMCA and RT-QuIC assays detected skin PrPSc early in scrapie-infected hamsters. However, sPMCA amplified PrPSc in skin samples from CJD-infected Tg40 mice at 4 wpi, while RT-QuIC assay detected prion-seeding activity only at 20 wpi (Fig. 8). The reason for the difference in Tg40 mice is not clear, but may be due in part to differences between the assays and the prion strains involved. sPMCA is performed in brain homogenates, which provide naturally post-translationally modified (glycosylated and GPI-anchored) PrPC as the substrate, and other potential brain-derived co-factors. RT-QuIC reactions include only unmodified recombinant PrPC as substrate, and no natural co-factors. sPMCA reactions are accelerated by sonication, whereas RT-QuIC reactions are shaken. Also, in successive rounds of sPMCA, the substrate and other brain components are refreshed, but our RT-QuIC reactions were performed in one round, with no refreshment. To exclude the effect of mismatch between seeds and substrates on the sensitivity of RT-QuIC reactions, we tested two recombinant PrP molecules as substrates from two different species including hamster and human and they all showed the similar sensitivity with the same earliest time point at 20 wpi. Finally, 263K scrapie and MM1 sCJD prions undoubtedly differ in conformation, and therefore, perhaps, their interactions with co-factors, various PrPC substrates, and/or skin-derived inhibitors of RT-QuIC reactions. These factors might differentially affect the sensitivity of detection of MM1 sCJD in the skin of Tg40 mice by sPMCA and RT-QuIC. It is also possible that the RT-QuIC assay may become as sensitive as sPMCA for skin prion detection in the Tg40h mice after further optimization of RT-QuIC’s experimental conditions.

Our early detection of PrPSc in the skin of sCJD- and scrapie-infected rodents suggests that it may be possible to do the same with the skin of humans who carry PrP mutations associated with genetic prion diseases such as familial CJD, Gerstmann–Sträussler–Scheinker syndrome, or fatal familial insomnia because it is expected that their mutant PrPC spontaneously converts into PrPSc and accumulates later in life. Skin-based RT-QuIC may reveal early prion-seeding activity in PrP mutation-carriers, or people with suspected exposures to prion infections, while they are still asymptomatic. Even for suspected sCJD cases, who are only identified in the symptomatic phase, skin-based RT-QuIC might be useful for monitoring disease progression, defining severity and diversity, and evaluating the treatment efficacy when potential drugs become available.

Although neither clinical signs nor brain PrPSc were observed in control animals cohabitating with 263K-inoculated hamsters within 12 weeks, the mock-inoculated animals that were housed with scrapie-infected animals had amplifiable PrPSc in the brain and skin via amplification techniques. Moreover, in contrast to the skin PrPSc amplified from the 263K-inoculated hamsters as early as 2 wpi, the control animals that co-habitated with infected hamsters were found to have amplified skin PrPSc after cohabitation for 11 weeks. This finding implies that the skin PrPSc detected early in the scrapie-infected hamsters is not the result of environmental contamination; otherwise, the control animals would exhibit skin PrPSc at 2 wpi as well. The finding of skin PrPSc in the cohabitating control animals may be relevant to the environmental transmission of prions observed in natural animal prion diseases, such as scrapie and CWD. Interestingly, prion transmission has been observed in hamsters by contact with prion-contaminated surfaces through rubbing and bedding34, in which cases skin is expected to be involved. The role that skin may plays in the environmental transmission of prions warrants future investigation.

Skin PrPSc may derive from urine or fecal prion contamination in addition to possible skin shedding due to scratching or biting each other. Indeed, scrapie infectivity was reported in the urine of prion-infected mice coincident with lymphocytic nephritis during their preclinical and clinical stages of prion infection35,36. It was also observed in their urine in intracerebrally inoculated hamsters even without any apparent inflammation21. In addition, deer with clinical CWD and mild to moderate nephritis were found to have sPMCA-detectable PrPSc and CWD-infectivity in urine22. Using sPMCA, PrPSc was detected in urine of ~80% of the hamsters intraperitoneally inoculated with 263K prions at the symptomatic stage23. Notably, PrPSc was detected in urine, but only at the terminal stage of disease in intracerebrally inoculated hamsters, except for a few days immediately after oral administration24. Similar to the observations by Gonzalez-Romero et al.23, Murayama et al. also found that not all infected hamsters had detectable urine PrPSc even at the terminal stage24. The skin PrPSc detected early in the intracerebrally infected hamsters, but not in the co-habitated-negative controls, at 2 wpi suggests that skin prions may not result from urine at the early stage of infection.

Unlike the situation with urine, it has not been very clear whether PrPSc is present in feces of intracerebrally inoculated hamsters at the early stage of prion infection. High titers of prion infectivity were detected in feces throughout the disease incubation in orally inoculated hamsters while low levels of infectivity were occasionally observed in intracerebrally- or intraperitoneally-inoculated animals18. For instance, no prion infectivity was detected in feces of hamsters within 3 wpi, including at 1, 2, and 22 days post inoculation (dpi), except for 8 dpi when 17% transmission rate was detected in feces18. However, fecal PrPSc was only detected during the clinical stage of disease by sPMCA in hamsters with lower doses of oral inoculum19. Western blotting of fecal extracts showed shedding of PrPSc in the excrement at 24–72 h post inoculation, but not at 0–24 h post inoculation, or at later preclinical or clinical time points19. Consistent with this observation, prion infectivity was not detected in feces of mule deer after oral challenge with CWD prions within the first 12–16 wpi, but feces contained infectivity after 36 wpi through to clinical disease stages at 64–80 wpi20. It is likely that PrPSc is present in feces of infected animals at the late stage of prion infection, which may contaminate the skin of cohabitating control animals.

Although prion contamination of skin by excrement may not be a major concern in human prion diseases, it is an important issue for prion transmission in animals, such as cattle, sheep, goats, and cervids. It is worth noting that the high incidence of scrapie in sheep and goats as well as CWD in cervids is believed to be attributable to contamination of the environment due to high prion shedding. The detection of PrPSc in excretions including saliva, urine, and feces clearly indicates this shedding. Oral ingestion due to the coprophagic behavior of animals has been believed to cause wide horizontal transmission of scrapie and CWD. However, our current finding of skin PrPSc in cohabitating prion-inoculated and PBS-inoculated control animals, as well as the occurrence of brain PrPSc in the PBS-inoculated animals at the late stage suggests that prion contamination of skin may be a potential route of transmission of prion diseases.

In conclusion, our study indicates that skin PrPSc may be a useful biomarker not only for the preclinical diagnosis of prion diseases, but also for monitoring disease progression following infection and treatment. Since the chance that PrPSc can be consistently detected in blood and urine of sCJD patients by sPMCA and RT-QuIC assays has been virtually very low10,11,37, it is possible that detection of PrPSc in the skin, a highly accessible tissue, could be developed for evaluating therapeutic efficiency and drug screening. As mentioned earlier, RT-QuIC analysis of CSF and nasal brushing specimens to date has been used for diagnosis of human prion diseases only at the clinical stage. Moreover, it is much less practical in live animals to collect CSF and nasal brushing specimens. In cervids, at least, there has been more focus on RT-QuIC analyses of RAMALT biopsies and various excreta38. Although these analyses are currently the most accurate tests available for chronic wasting disease in live cervids, they do not yet provide 100% diagnostic sensitivity and specificity38. Thus, it may be helpful to have additional or alternative diagnostic specimens, such as skin or ear pinna punches, for RT-QuIC and sPMCA testing.