Age-related alterations affect the susceptibility of mice to prion infection
Introduction
Creutzfeldt-Jakob disease (CJD), a fatal neurodegenerative disorder, is the most common form of human prion diseases also denominated transmissible spongiform encephalopathies (TSEs). The ultimate marker of prion infection is the accumulation of PrPSc, an aberrant conformer of the membrane glycoprotein PrPC (Prusiner, 1998). During the disease incubation period, PrPSc accumulates primarily in lymphoreticular organs such as the spleen (Fraser and Dickinson, 1978, Hilton et al., 1998, Kimberlin and Walker, 1979) and thereafter reaches the central nervous system (CNS) through peripheral nerves (Beekes et al., 1998, Bencsik et al., 2001, Glatzel et al., 2001).
CJD can manifest at three different etiologies—sporadic, familial and infectious. Both the sporadic CJD (sCJD) and the familial form of CJD (fCJD; linked to mutations in the PrP gene) are considered as late onset neurodegenerative diseases with an average appearance age of 60–70 for sCJD, and age 50 for fCJD (Brown et al., 1994). In contrast, the transmissible prion diseases, such as Kuru and most iatrogenic CJD, often appear at early age (Gajdusek and Zigas, 1957). In addition, variant CJD (vCJD), which is related to the consumption of meat contaminated with bovine spongiform encephalopathy (BSE), also show an early age of onset of 26 on average (Spencer et al., 2002, Will et al., 1996). The early age of onset typical to acquired prion diseases could be attributed to the characteristics of each specific prion strain, as defined by parameters such as incubation time and vacuolar pathology following transmission to wild-type mice (Collinge et al., 1996, McLean et al., 1998, Ritchie et al., 2009). As for Kuru and iatrogenic CJD, additional epidemiological factors were found that affect the age at onset (Gibbs et al., 1985, Koch et al., 1985, Will, 2003). However, regarding vCJD and despite many efforts, no age-related epidemiological factor was found that could explain this bias (Boelle et al., 2004, Ghani et al., 2003). Thus, young age may constitute a risk factor for infectious prion diseases following exposure to the prion agent.
Normal aging is associated with alterations in various organs and functional pathways. In the brain, such variations include changes in neuron morphology, genomes instability (Bohr et al., 2007), production of reactive oxygen species (Sohal and Weindruch, 1996), glial cells activation, inflammation and neuronal damage (Block et al., 2007, Blumenthal, 1997, Nakanishi, 2003). Several genes whose expression levels change during aging were identified in several tissues including brain (Lee et al., 1999, Lee et al., 2000, Lu et al., 2004). Among those, genes involved in stress and inflammatory responses were shown to be upregulated. Interestingly, several of these genes were also shown to be upregulated in the brains of prion-infected animals and of human sporadic CJD patients (Booth et al., 2004, Riemer et al., 2004, Xiang et al., 2005, Xiang et al., 2004).
In this study, we used a mouse model for infectious prion disease to examine the effects of age on the susceptibility to prion infection and the pathology of the disease. To this end, we infected young (1 month) and aged (16 months) C57BL/6 mice intraperitoneally with Rocky Mountain Laboratory (RML) strain mouse prions. As controls we used mock-inoculated age-match mice (referred here as ‘naïve’). We followed the mice for clinical signs of prion disease and measured disease-specific pathological markers such as gliosis, vacuolation and accumulation of PrPSc in the spleen and brain, and the expression profile of age and disease-related genes. Our results demonstrate that younger mice are more susceptible to prion infection, and that this differential susceptibility correlates with the age-related response of pro-inflammatory and stress-response genes to prion infection. Whether the expression of these genes plays a causal role in prion infection remains to be established.
Section snippets
Animal study
All groups of mice comprised of 5–10 animals. 1- and 16-month-old C57BL/6 female mice (Harlan, Israel) were inoculated intraperitoneally (IP) with 85–100 μl of 1% Rocky Mountain Laboratory (RML) strain of prion-infected brain homogenate. The inoculum volume was adjusted according to the mice weight, which was 15% lower in the young mice. However, we found that such changes in the amount of inoculum were insignificant, as previously shown (Prusiner et al., 1982). Age-matched control mice (naïve)
Extended incubation time of prion disease in mice infected at old age
To elucidate the effect of normal aging on subjects infected with prions, we inoculated intraperitoneally (I.P.) C57BL/6 mice, at the ages of 1 or 16 months with 1% RML strain of murine prions. Age-matched control mice (naïve) were inoculated with sterile PBS not containing infectious material. The mice were then followed twice a week for progressive clinical signs of prion disease, including truncal ataxia, hind-limb paresis, loss of extensor reflex, and tail stiffening. The incubation time
Discussion
We have shown in this work that older mice present a lower susceptibility to prion infection as compared to young mice. This is reflected in the longer incubation time required to reach the terminal stage of the disease (Fig. 1) and the milder neuropathological features such as spongiform changed and gliosis (Fig. 2). Importantly, prion disease in older mice was associated with a markedly reduced accumulation of PrPSc (Fig. 3), in spite of the higher levels of PrPC (Fig. 4a) and the extended
Conflict of statement
We state that we have no actual or potential conflicts of interest including any financial, personal or other relationships with other people or organizations within 3 years of beginning the work submitted that could inappropriately influence (bias) their work.
References (73)
- et al.
A longitudinal study of age-related loss of noradrenergic nerves and lymphoid cells in the rat spleen
Exp. Neurol.
(1992) - et al.
Evidence for an early inflammatory response in the central nervous system of mice with scrapie
Neuroscience
(1996) - et al.
Genome instability and DNA repair in brain, ageing and neurological disease
Neuroscience
(2007) - et al.
Inflammaging and anti-inflammaging: a systemic perspective on aging and longevity emerged from studies in humans
Mech. Ageing Dev.
(2007) - et al.
Studies of the lymphoreticular system in the pathogenesis of scrapie: the role of spleen and thymus
J. Comp. Pathol.
(1978) - et al.
Sympathetic innervation of lymphoreticular organs is rate limiting for prion neuroinvasion
Neuron
(2001) - et al.
The cellular isoform of the prion protein, PrPc, is associated with caveolae in mouse neuroblastoma (N2a) cells
Biochem. Biophys. Res. Commun.
(1995) - et al.
Prion immunoreactivity in appendix before clinical onset of variant Creutzfeldt-Jakob disease
Lancet
(1998) - et al.
Onset of accumulation of PrPres in murine ME7 scrapie in relation to pathological and PrP immunohistochemical changes
J. Comp. Pathol.
(2001) - et al.
Pathogenesis of mouse scrapie: dynamics of agent replication in spleen, spinal cord and brain after infection by different routes
J. Comp. Pathol.
(1979)