Multiple Factors Influence the Incubation Period of ALS Prion-like Transmission in SOD1 Transgenic Mice

Mutations in superoxide dismutase 1 (SOD1) that are associated with amyotrophic lateral sclerosis (ALS) cause its misfolding and aggregation. Prior studies have demonstrated that the misfolded conformation of ALS-SOD1 can template with naïve SOD1 "host proteins" to propagate, spread, and induce paralysis in SOD1 transgenic mice. These observations have advanced the argument that SOD1 is a host protein for an ALS conformer that is prion-like and experimentally transmissible. Here, we investigated the propagation of different isolates of G93A-SOD1 ALS conformers using a paradigm involving transmission to mice expressing human G85R-SOD1 fused to yellow fluorescent protein (G85R-SOD1:YFP). In these studies, we also utilized a newly developed line of mice in which the G85R-SOD1:YFP construct was flanked by loxp sites, allowing its temporal and spatial regulation. We used methods in which the G93A ALS conformers were injected into the sciatic nerve or hindlimb muscle of adult transgenic mice. We observed that the incubation period to paralysis varied significantly depending upon the source of inoculum containing misfolded G93A SOD1. Serial passage and selection produced stable isolates of G93A ALS conformers that exhibited a defined minimum incubation period of ~2.5 months when injected into the sciatic nerve of young adult mice. As expected, neuronal excision of the transgene in loxpG85R-SOD1:YFP mice blocked induction of paralysis by transmission of G93A ALS conformers. Our findings indicate that G93A ALS conformers capable of inducing disease require neuronal expression of a receptive host SOD1 protein for propagation, with a defined incubation period to paralysis.

Guam ALS-PDC is a distinct double-prion disorder featuring both tau and Aβ prions

The amyotrophic lateral sclerosis-parkinsonism dementia complex (ALS-PDC) of Guam is an endemic neurodegenerative disease that features widespread tau tangles, occasional α-synuclein Lewy bodies, and sparse β-amyloid (Aβ) plaques distributed in the central nervous system. Extensive studies of genetic or environmental factors have failed to identify a cause of ALS-PDC. Building on prior work describing the detection of tau and Aβ prions in Alzheimer's disease (AD) and Down syndrome brains, we investigated ALS-PDC brain samples for the presence of prions. We obtained postmortem frozen brain tissue from 26 donors from Guam with ALS-PDC or no neurological impairment and 71 non-Guamanian donors with AD or no neurological impairment. We employed cellular bioassays to detect the prion conformers of tau, α-synuclein, and Aβ proteins in brain extracts. In ALS-PDC brain samples, we detected high titers of tau and Aβ prions, but we did not detect α-synuclein prions in either cohort. The specific activity of tau and Aβ prions was increased in Guam ALS-PDC compared with sporadic AD. Applying partial least squares regression to all biochemical and prion infectivity measurements, we demonstrated that the ALS-PDC cohort has a unique molecular signature distinguishable from AD. Our findings argue that Guam ALS-PDC is a distinct double-prion disorder featuring both tau and Aβ prions.

Variation in the vulnerability of mice expressing human superoxide dismutase 1 to prion-like seeding: a study of the influence of primary amino acid sequence

Misfolded forms of superoxide dismutase 1 (SOD1) with mutations associated with familial amyotrophic lateral sclerosis (fALS) exhibit prion characteristics, including the ability to act as seeds to accelerate motor neuron disease in mouse models. A key feature of infectious prion seeding is that the efficiency of transmission is governed by the primary sequence of prion protein (PrP). Isologous seeding, where the sequence of the PrP in the seed matches that of the host, is generally much more efficient than when there is a sequence mis-match. Here, we used paradigms in which mutant SOD1 seeding homogenates were injected intraspinally in newborn mice or into the sciatic nerve of adult mice, to assess the influence of SOD1 primary sequence on seeding efficiency. We observed a spectrum of seeding efficiencies depending upon both the SOD1 expressed by mice injected with seeds and the origin of the seed preparations. Mice expressing WT human SOD1 or the disease variant G37R were resistant to isologous seeding. Mice expressing G93A SOD1 were also largely resistant to isologous seeding, with limited success in one line of mice that express at low levels. By contrast, mice expressing human G85R-SOD1 were highly susceptible to isologous seeding but resistant to heterologous seeding by homogenates from paralyzed mice over-expressing mouse SOD1-G86R. In other seeding experiments with G85R SOD1:YFP mice, we observed that homogenates from paralyzed animals expressing the H46R or G37R variants of human SOD1 were less effective than seeds prepared from mice expressing the human G93A variant. These sequence mis-match effects were less pronounced when we used purified recombinant SOD1 that had been fibrilized in vitro as the seeding preparation. Collectively, our findings demonstrate diversity in the abilities of ALS variants of SOD1 to initiate or sustain prion-like propagation of misfolded conformations that produce motor neuron disease.

Novel SOD1 monoclonal antibodies against the electrostatic loop preferentially detect misfolded SOD1 aggregates

Amyotrophic lateral sclerosis (ALS) is a progressive neurological disease that leads to motor neuron degeneration and paralysis. Superoxide dismutase (SOD1) mutations are the second most common cause of familial ALS and are responsible for up to 20 % of familial ALS cases. In ALS patients, SOD1 can form toxic misfolded aggregates that deposit in the brain and spinal cord. To better detect SOD1 aggregates and expand the repertoire of conformational SOD1 antibodies, SOD1 monoclonal antibodies were generated by immunizing SOD1 knockout mice with an SOD1 fragment consisting of amino acids 129-146, which make up part of the electrostatic loop. A series of hybridomas secreting antibodies were screened and five different SOD1 monoclonal antibodies (2C10, 2F8, 4B11, 5H5, and 5A10) were found to preferentially detect denatured or aggregated SOD1 by enzyme-linked immunosorbent assay (ELISA), filter trap assay, and immunohistochemical analysis in SOD1 mouse models. The staining with these antibodies was compared to Campbell-Switzer argyrophilic reactivity of pathological inclusions. These new conformational selective SOD1 antibodies will be useful for clinical diagnosis of SOD1 ALS and potentially for passive immunotherapy.

Tryptophan residue 32 in human Cu-Zn superoxide dismutase modulates prion-like propagation and strain selection

Mutations in Cu/Zn superoxide dismutase 1 (SOD1) associated with familial amyotrophic lateral sclerosis cause the protein to aggregate via a prion-like process in which soluble molecules are recruited to aggregates by conformational templating. These misfolded SOD1 proteins can propagate aggregation-inducing conformations across cellular membranes. Prior studies demonstrated that mutation of a Trp (W) residue at position 32 to Ser (S) suppresses the propagation of misfolded conformations between cells, whereas other studies have shown that mutation of Trp 32 to Phe (F), or Cys 111 to Ser, can act in cis to attenuate aggregation of mutant SOD1. By expressing mutant SOD1 fused with yellow fluorescent protein (YFP), we compared the relative ability of these mutations to modulate the formation of inclusions by ALS-mutant SOD1 (G93A and G85R). Only mutation of Trp 32 to Ser persistently reduced the formation of the amorphous inclusions that form in these cells, consistent with the idea that a Ser at position 32 inhibits templated propagation of aggregation prone conformations. To further test this idea, we produced aggregated fibrils of recombinant SOD1-W32S in vitro and injected them into the spinal cords of newborn mice expressing G85R-SOD1: YFP. The injected mice developed an earlier onset paralysis with a frequency similar to mice injected with WT SOD1 fibrils, generating a strain of misfolded SOD1 that produced highly fibrillar inclusion pathology. These findings suggest that the effect of Trp 32 in modulating the propagation of misfolded SOD1 conformations may be dependent upon the “strain” of the conformer that is propagating.

Loss of charge mutations in solvent exposed Lys residues of superoxide dismutase 1 do not induce inclusion formation in cultured cell models

Mutations in superoxide dismutase 1 (SOD1) associated with familial amyotrophic lateral sclerosis (fALS) induce the protein to misfold and aggregate. Missense mutations at more than 80 different amino acid positions have been associated with disease. How these mutations heighten the propensity of SOD1 to misfold and aggregate is unclear. With so many mutations, it is possible that more than one mechanism of aggregation may be involved. Of many possible mechanisms to explain heightened aggregation, one that has been suggested is that mutations that eliminate charged amino acids could diminish repulsive forces that would inhibit aberrant protein:protein interactions. Mutations at twenty-one charged residues in SOD1 have been associated with fALS, but of the 11 Lys residues in the protein, only 1 has been identified as mutated in ALS patients. Here, we examined whether loss of positively charged surface Lys residues in SOD1 would induce misfolding and formation of intracellular inclusions. We mutated four different Lys residues (K30, K36, K75, K91) in SOD1 that are not particularly well conserved, and expressed these variants as fusion proteins with yellow fluorescent protein (YFP) to assess inclusion formation. We also assessed whether these mutations induced binding to a conformation-restricted SOD1 antibody, designated C4F6, which recognizes non-natively folded protein. Although we observed some mutations to cause enhanced C4F6 binding, we did not observe that mutations that reduce charge at these positions caused the protein to form intracellular inclusions. Our findings may have implications for the low frequency of mutations at Lys residues SOD1 in ALS patients.

Prion-like Spreading in Tauopathies

Tau is a microtubule-associated protein that functions in regulating cytoskeleton dynamics, especially in neurons. Misfolded and aggregated forms of tau produce pathological structures in a number of neurodegenerative diseases, including Alzheimer's disease (AD) and tauopathy dementias. These disorders can present with a sporadic etiology, such as in AD, or a familial etiology, such as in some cases of frontotemporal dementia with parkinsonism. Notably, the pathological features of tau pathology in these diseases can be very distinct. For example, the tau pathology in corticobasal degeneration is distinct from that of an AD patient. A wealth of evidence has emerged within the last decade to suggest that the misfolded tau in tauopathies possesses prion-like features and that such features may explain the diverse characteristics of tauopathies. The prion-like concept for tauopathies arose initially from the observation that the progressive accumulation of tau pathology as the symptoms of AD progress seemed to follow anatomically linked pathways. Subsequent studies in cell and animal models revealed that misfolded tau can propagate from cell to cell and from region to region in the brain through direct neuroanatomical connections. Studies in these cell and mouse models have demonstrated that experimentally propagated forms of misfolded tau can exist as conformationally distinct "strains" with unique biochemical, morphological, and neuropathological characteristics. This review discusses the clinical, pathological, and genetic diversity of tauopathies and the discoveries underlying the emerging view that the unique features of clinically distinct tauopathies may be a reflection of the strain of misfolded tau that propagates in each disease.

Prion-like mechanisms in amyotrophic lateral sclerosis

The prion hypothesis - a protein conformation capable of replicating without a nucleic acid genome - was heretical at the time of its discovery. However, the characteristics of the disease-misfolded prion protein and its ability to transmit disease, replicate, and spread are now widely accepted throughout the scientific community. In fact, in the last decade a wealth of evidence has emerged supporting similar properties observed for many of the misfolded proteins implicated in other neurodegenerative diseases, such as Alzheimer disease, Parkinson disease, tauopathies, and as described in this chapter, amyotrophic lateral sclerosis (ALS). Multiple studies have now demonstrated the ability for superoxide dismutase-1, 43-kDa transactive response (TAR) DNA-binding protein, fused-in sarcoma, and most recently, C9orf72-encoded polypeptides to display properties similar to those of prions. The majority of these are cell-free and in vitro assays, while superoxide dismutase-1 remains the only ALS-linked protein to demonstrate several prion-like properties in vivo. In this chapter, we provide an introduction to ALS and review the recent literature linking several proteins implicated in the familial forms of the disease to properties of the prion protein.

Lysine acylation in superoxide dismutase-1 electrostatically inhibits formation of fibrils with prion-like seeding

The acylation of lysine residues in superoxide dismutase-1 (SOD1) has been previously shown to decrease its rate of nucleation and elongation into amyloid-like fibrils linked to amyotrophic lateral sclerosis. The chemical mechanism underlying this effect is unclear, i.e. hydrophobic/steric effects versus electrostatic effects. Moreover, the degree to which the acylation might alter the prion-like seeding of SOD1 in vivo has not been addressed. Here, we acylated a fraction of lysine residues in SOD1 with groups of variable hydrophobicity, charge, and conformational entropy. The effect of each acyl group on the rate of SOD1 fibril nucleation and elongation were quantified in vitro with thioflavin-T (ThT) fluorescence, and we performed 594 iterate aggregation assays to obtain statistically significant rates. The effect of the lysine acylation on the prion-like seeding of SOD1 was assayed in spinal cord extracts of transgenic mice expressing a G85R SOD1-yellow fluorescent protein construct. Acyl groups with >2 carboxylic acids diminished self-assembly into ThT-positive fibrils and instead promoted the self-assembly of ThT-negative fibrils and amorphous complexes. The addition of ThT-negative, acylated SOD1 fibrils to organotypic spinal cord failed to produce the SOD1 inclusion pathology that typically results from the addition of ThT-positive SOD1 fibrils. These results suggest that chemically increasing the net negative surface charge of SOD1 via acylation can block the prion-like propagation of oligomeric SOD1 in spinal cord.

Relationship between mutant Cu/Zn superoxide dismutase 1 maturation and inclusion formation in cell models

A common property of Cu/Zn superoxide dismutase 1 (SOD1), harboring mutations associated with amyotrophic lateral sclerosis, is a high propensity to misfold and form abnormal aggregates. The aggregation of mutant SOD1 has been demonstrated in vitro, with purified proteins, in mouse models, in human tissues, and in cultured cell models. In vitro translation studies have determined that SOD1 with amyotrophic lateral sclerosis mutations is slower to mature, and thus perhaps vulnerable to off-pathway folding that could generate aggregates. The aggregation of mutant SOD1 in living cells can be monitored by tagging the protein with fluorescent fluorophores. In this study, we have taken advantage of the Dendra2 fluorophore technology in which excitation can be used to switch the output color from green to red, thereby clearly creating a time stamp that distinguishes pre-existing and newly made proteins. In cells that transiently over-express the Ala 4 to Val variant of SOD1-Dendra2, we observed that newly made mutant SOD1 was rapidly captured by pathologic intracellular inclusions. In cell models of mutant SOD1 aggregation over-expressing untagged A4V-SOD1, we observed that immature forms of the protein, lacking a Cu co-factor and a normal intramolecular disulfide, persist for extended periods. Our findings fit with a model in which immature forms of mutant A4V-SOD1, including newly made protein, are prone to misfolding and aggregation.

Substantially elevating the levels of αB-crystallin in spinal motor neurons of mutant SOD1 mice does not significantly delay paralysis or attenuate mutant protein aggregation

There has been great interest in enhancing endogenous protein maintenance pathways such as the heat-shock chaperone response, as it is postulated that enhancing clearance of misfolded proteins could have beneficial disease modifying effects in amyotrophic lateral sclerosis and other neurodegenerative disorders. In cultured cell models of mutant SOD1 aggregation, co-expression of αB-crystallin (αB-crys) has been shown to inhibit the formation of detergent-insoluble forms of mutant protein. Here, we describe the generation of a new line of transgenic mice that express αB-crys at > 6-fold the normal level in spinal cord, with robust increases in immunoreactivity throughout the spinal cord grey matter and, specifically, in spinal motor neurons. Surprisingly, spinal cords of mice expressing αB-crys alone contained 20% more motor neurons per section than littermate controls. Raising αB-crys by these levels in mice transgenic for either G93A or L126Z mutant SOD1 had no effect on the age at which paralysis developed. In the G93A mice, which showed the most robust degree of motor neuron loss, the number of these cells declined by the same proportion as in mice expressing the mutant SOD1 alone. In paralyzed bigenic mice, the levels of detergent-insoluble, misfolded, mutant SOD1 were similar to those of mice expressing mutant SOD1 alone. These findings indicate that raising the levels of αB-crys in spinal motor neurons by 6-fold does not produce the therapeutic effects predicted by cell culture models of mutant SOD1 aggregation. Enhancing the protein chaperone function may present a therapeutic approach to amyotrophic lateral sclerosis caused by mutations in SOD1, and other neurodegenerative disorders characterized by cytosolic protein aggregation. Previous studies in cell models suggested that the chaperone known as αB-crystallin (αB-crys) can prevent mutant SOD1 aggregation. We report that transgenic expression of αB-crys at > 6-fold the normal level in spinal cords of mice expressing mutant SOD1 produces no therapeutic benefit.

Experimental transmissibility of mutant SOD1 motor neuron disease

By unknown mechanisms, the symptoms of amyotrophic lateral sclerosis (ALS) seem to spread along neuroanatomical pathways to engulf the motor nervous system. The rate at which symptoms spread is one of the primary drivers of disease progression. One mechanism by which ALS symptoms could spread is by a prion-like propagation of a toxic misfolded protein from cell to cell along neuroanatomic pathways. Proteins that can transmit toxic conformations between cells often can also experimentally transmit disease between individual organisms. To survey the ease with which motor neuron disease (MND) can be transmitted, we injected spinal cord homogenates prepared from paralyzed mice expressing mutant superoxide dismutase 1 (SOD1-G93A and G37R) into the spinal cords of genetically vulnerable SOD1 transgenic mice. From the various models we tested, one emerged as showing high vulnerability. Tissue homogenates from paralyzed G93A mice induced MND in 6 of 10 mice expressing low levels of G85R-SOD1 fused to yellow fluorescent protein (G85R-YFP mice) by 3-11 months, and produced widespread spinal inclusion pathology. Importantly, second passage of homogenates from G93A → G85R-YFP mice back into newborn G85R-YFP mice induced disease in 4 of 4 mice by 3 months of age. Homogenates from paralyzed mice expressing the G37R variant were among those that transmitted poorly regardless of the strain of recipient transgenic animal injected, a finding suggestive of strain-like properties that manifest as differing abilities to transmit MND. Together, our data provide a working model for MND transmission to study the pathogenesis of ALS.

Direct and indirect mechanisms for wild-type SOD1 to enhance the toxicity of mutant SOD1 in bigenic transgenic mice

Co-expression of wild-type human superoxide dismutase 1 (WT-hSOD1) with ALS mutant hSOD1 accelerates disease onset relative to mice expressing only mutant protein. Here, we analyzed the effect of co-expressed WT-hSOD1 in two established mutant mouse models (L126Z and G37R), and a new model that expresses the first 102 amino acids of SOD1 with mutations at histidines 46, 48 and 63 to eliminate Cu binding (Cu-V103Z). A subset of Cu-V103Z mice developed paralysis between 500 and 730 days. Similar to mice expressing L126Z-SOD1, the spinal cords of this new model showed SOD1 immunoreactive fibrillar inclusions. Co-expression of WT-hSOD1 with Cu-V103Z SOD1 moderately accelerated the age to paralysis, similar in magnitude to WT/L126Z mice. In either combination of these bigenic mice, the severity of fibrillar inclusion pathology was diminished and unreactive to antibodies specific for the C terminus of WT protein. Co-expression of WT-hSOD1 fused to yellow fluorescent protein (WT-hSOD1:YFP) with G37R-hSOD1 produced earlier disease, and spinal cords of paralyzed bigenic mice showed YFP fluorescent inclusion-like structures. In bigenic L126Z/WT-hSOD1:YFP mice, disease was not accelerated and WT-hSOD1:YFP remained diffusely distributed. A combination of split luciferase complementation assays and affinity capture-binding assays demonstrated that soluble G37R-hSOD1 efficiently and tightly bound soluble WT-hSOD1, whereas soluble forms of the Cu-V103Z and L126Z variants demonstrated low affinity. These data indicate that WT-hSOD1 may indirectly augment the toxicity of mutant protein by competing for protective factors, but disease onset seems to be most accelerated when WT-hSOD1 interacts with mutant SOD1 and becomes misfolded.

Widespread and efficient transduction of spinal cord and brain following neonatal AAV injection and potential disease modifying effect in ALS mice

The architecture of the spinal cord makes efficient delivery of recombinant adeno-associated virus (rAAV) vectors throughout the neuraxis challenging. We describe a paradigm in which small amounts of virus delivered intraspinally to newborn mice result in robust rAAV-mediated transgene expression in the spinal cord. We compared the efficacy of rAAV2/1, 2/5, 2/8, and 2/9 encoding EGFP delivered to the hindlimb muscle (IM), cisterna magna (ICM), or lumbar spinal cord (IS) of neonatal pups. IS injection of all four capsids resulted in robust transduction of the spinal cord with rAAV2/5, 2/8, and 2/9 vectors appearing to be transported to brain. ICM injection resulted in widespread expression of EGFP in the brain, and upper spinal cord. IM injection resulted in robust muscle expression, with only rAAV2/8 and 2/9 transducing spinal motor and sensory neurons. As proof of concept, we use the IS paradigm to express murine Interleukin (IL)-10 in the spinal cord of the SOD1-G93A transgenic mouse model of amyotrophic lateral sclerosis. We show that expression of IL-10 in the spinal axis of SOD1-G93A mice altered the immune milieu and significantly prolonged survival. These data establish an efficient paradigm for somatic transgene delivery of therapeutic biologics to the spinal cord of mice.

Conformational specificity of the C4F6 SOD1 antibody; low frequency of reactivity in sporadic ALS cases

Greater than 160 missense mutations in copper-zinc superoxide dismutase-1 (SOD1) can cause amyotrophic lateral sclerosis (ALS). These mutations produce conformational changes that reveal novel antibody binding epitopes. A monoclonal antibody, clone C4F6 - raised against the ALS variant G93A of SOD1, has been identified as specifically recognizing a conformation shared by many ALS mutants of SOD1. Attempts to determine whether non-mutant SOD1 adopts a C4F6-reactive conformation in spinal tissues of sporadic ALS (sALS) patients has produced inconsistent results. To define the epitope recognized by C4F6, we tested its binding to a panel of recombinant ALS-SOD1 proteins expressed in cultured cells, producing data to suggest that the C4F6 epitope minimally contains amino acids 90-93, which are normally folded into a tight hairpin loop. Multiple van der Waals interactions between the 90-93 loop and a loop formed by amino acids 37-42, particularly a leucine at position 38, form a stable structure termed the β-plug. Based on published modeling predictions, we suggest that the binding of C4F6 to multiple ALS mutants of SOD1 occurs when the local structure within the β-plug, including the loop at 90-93, is destabilized. In using the antibody to stain tissues from transgenic mice or humans, the specificity of the antibody for ALS mutant SOD1 was influenced by antigen retrieval protocols. Using conditions that showed the best discrimination between normal and misfolded mutant SOD1 in cell and mouse models, we could find no obvious difference in C4F6 reactivity to spinal motor neurons between sALS and controls tissues.

The strain-encoded relationship between PrP replication, stability and processing in neurons is predictive of the incubation period of disease

Prion strains are characterized by differences in the outcome of disease, most notably incubation period and neuropathological features. While it is established that the disease specific isoform of the prion protein, PrP^Sc, is an essential component of the infectious agent, the strain-specific relationship between PrP^Sc properties and the biological features of the resulting disease is not clear. To investigate this relationship, we examined the amplification efficiency and conformational stability of PrP^Sc from eight hamster-adapted prion strains and compared it to the resulting incubation period of disease and processing of PrP^Sc in neurons and glia. We found that short incubation period strains were characterized by more efficient PrP^Sc amplification and higher PrP^Sc conformational stabilities compared to long incubation period strains. In the CNS, the short incubation period strains were characterized by the accumulation of N-terminally truncated PrP^Sc in the soma of neurons, astrocytes and microglia in contrast to long incubation period strains where PrP^Sc did not accumulate to detectable levels in the soma of neurons but was detected in glia similar to short incubation period strains. These results are inconsistent with the hypothesis that a decrease in conformational stability results in a corresponding increase in replication efficiency and suggest that glia mediated neurodegeneration results in longer survival times compared to direct replication of PrP^Sc in neurons.

Prion diseases are a group of infectious fatal neurodegenerative diseases that affect animals including humans. This unique infectious agent is the result of a post-translational conformational change of the normal form of the prion protein, PrP^C, to an infectious form of the prion protein, PrP^Sc. Different strains of the infectious agent result in characteristic incubation periods and neuropathological features within a single host species. These strain-specific differences in disease outcome are likely due to strain-specific conformations of PrP^Sc, though the mechanisms by which different conformation can affect prion strain properties are not understood. The aim of this study was to investigate the relationship between the biochemical properties of PrP^Sc to the corresponding neuropathological characteristics of eight hamster-adapted prion strains. Our findings indicate that PrP^Sc from short incubation period strains were more efficiently replicated, had a more stable conformation, and were observed to be more resistant to clearance from the soma of neurons compared to prion strains with a relatively long incubation period. These results suggest the progression of prion disease is influenced by the balance between replication and clearance of PrP^Sc in neurons.

Prion interference is due to a reduction in strain-specific PrPSc levels

When two prion strains infect a single host, one strain can interfere with the ability of the other to cause disease but it is not known whether prion replication of the second strain is also diminished. To further investigate strain interference, we infected hamsters in the sciatic nerve with the long-incubation-period transmissible mink encephalopathy (TME) agent DY TME prior to superinfection of hamsters with the short-incubation-period HY TME agent. Increases in the interval between TME agent inoculations resulted in an extension of the incubation period of HY TME or a complete block of the ability of the HY TME agent to cause disease. The sciatic nerve route of inoculation gave the two TME strains access to the same population of neurons, allowing for the potential of prion interference in the lumbar spinal cord. The ability of the DY TME agent to extend the incubation period of HY TME corresponds with detection of DY TME PrP(Sc), the abnormal isoform of the prion protein, in the lumbar spinal cord. The increased incubation period of HY TME or the inability of the HY TME agent to cause disease in the coinfected animals corresponds with a reduction in the abundance of HY TME PrP(Sc) in the lumbar spinal cord. When the two strains were not directed to the same populations of neurons within the lumbar spinal cord, interference between HY TME and DY TME did not occur. This suggests that DY TME agent replication interferes with HY TME agent replication when the two strains infect a common population of neurons.