Could Alzheimer’s disease begin in the womb? One theory for the origin of some neurological diseases, such as schizophrenia (see Brown and Derkits, 2010) and autism (see Atladóttir et al., 2010), is that an infection in the mother alters fetal neurodevelopment, setting up offspring for disease brought on by another trigger later in life. Could the same be true for Alzheimer's disease? Irene Knuesel from the University of Zurich in Switzerland reported at the Society for Neuroscience annual meeting held 12-16 November 2011 in Washington, DC, that a two-hit immune challenge, one in late gestation and another one later in adult life, leads to amyloid plaques and hyperphosphorylated tau accumulation in wild-type mouse brains. The pathology follows a pattern that "strikingly" resembles that observed in humans with Alzheimer's disease, wrote the authors in their abstract. This new mouse model could allow study of late-onset sporadic Alzheimer's disease—as opposed to the familial form modeled by transgenic mice—and may implicate infection and inflammation as a driving force in AD, according to Knuesel.

"We have not studied the sporadic form of Alzheimer's disease sufficiently enough, and yet this makes up the major population of the patients," said Knuesel. "Now we have a model that would allow us to study, in a morphological context, the processing changes and upstream factors involved in initiation of plaque and tangle pathology—almost impossible to do in a transgenic mouse."

Knuesel and colleagues' work suggests that inflammation plays a prominent role in the sporadic form of the disease, perhaps even initiating it. They first found that in four-month-old triple-transgenic mice, a single systemic immune reaction—induced by injecting the viral mimic polyinosinic:polycytidylic acid (poly[I:C])—caused a dramatic boost in the plaque and tau pathology observed 11 months later. "I've never seen that much plaque deposition in triple-transgenic mice at that age," said Cindy Lemere, Brigham and Women's Hospital, Boston. "Giving that injection early in life led to very strong acceleration of AD-like pathology, showing that early exposure to an immune stimulus can fast-forward pathology."

Could an early immune challenge elicit AD-like pathology in wild-type mice as well? Knowing that some neurodevelopmental disorders might have their roots in gestation, the researchers wondered if the same might be true of Alzheimer's. The team found a window in late gestation when injecting mothers with poly(I:C), permanently altered their pups' immune systems. Circulating levels of several inflammatory cytokines remained elevated throughout the offspring's lives. The animals had more APP, and produced more Aβ42 and somatodendritic, hyperphosphorylated tau than controls, in which neuronal tau was predominantly axonal. A Y-maze test also demonstrated profound working memory deficits in these offspring at 22 months of age.

Knuesel and colleagues gave those same mice a second injection of poly(I:C) at 12 months of age, and looked at their brains three months later. Amyloid plaques—detected by an antibody against rodent Aβ40/42—were pronounced in the entorhinal (ERC) and piriform cortices. Relative to controls, there was also more Aβ immunoreactivity within the hippocampus, particularly in regions receiving projections from the ERC. The pattern suggests that the deposits started in the cortices and expanded to the hippocampus, much like they do in the early human form of Alzheimer's disease. The plaques resembled the diffuse ones found in humans, Knuesel said. Hyperphosphorylated tau also accumulated in the mouse neurons. While these aggregates were not similar to human neurofibrillary tangles (they were Gallyas silver stain-negative), Knuesel thought NFTs might form as the mice age. The mice also showed elevated microglial activation and some evidence of microglia degeneration. The researchers are conducting behavioral tests on the 12- to 15-month-old mice to test if they have cognitive deficits. The results suggest that an immune challenge early in development, followed by a second in adulthood, puts the brain at risk for AD-like pathology.

"The very early immune challenge seems to have long-term consequences for aging in the brain," said Lemere. "It appears that the brain is then set up so another immune challenge later in life makes the brain more susceptible to neurodegeneration."

Knuesel believes that the cytokines and chemokines produced by the mother as a result of infection cross the placental barrier, enter the fetus, and make their way across the underdeveloped blood-brain barrier to the central nervous system of the offspring. In late gestation, the fetus also contributes to the cytokine production. Knuesel thinks the elevated cytokines alter microglia and genes involved in early brain development and immune functions. Cytokines may also hamper division of microglial precursors so that there are fewer microglia in old age, making them less able to phagocytose protein deposits.

"I think it's a moderately low elevation of inflammatory cytokines that may damage the brain chronically," said Knuesel. Genetic factors and repeated infections in old age may also infer risk. "Then a systemic infection can be the last little kick that sends the system all the way downhill," Knuesel added.

Inflammation has long been thought to be involved in AD pathology, but it is not clear whether immune responses are a cause or a result of the disease (see ARF related news story). Microglia have been found to phagocytose plaques and clear them from the brain (see ARF related news story on Simard et al., 2006 and ARF related news story on El Khoury et al., 2007). But evidence that systemic inflammation in people with Alzheimer's disease speeds up cognitive decline suggests that some immune responses exacerbate AD (see Holmes et al., 2009). In addition, genomewide association studies reported that genes related to the innate immune system confer risk for AD (see ARF related news story on Harold et al., 2009 and Lambert et al., 2009).

There have been mixed reactions to Knuesel’s findings. At several conferences where she presented this work, scientists pointed out that rodent Aβ is not known to aggregate, leading them to question the nature of the amyloid deposits in these wild-type mice. "It would be helpful to clarify that they are extracellular versus intracellular, as well as the exact composition of β amyloid in these deposits, " said Lemere. Knuesel says she has done the biochemistry but is keeping the results under wraps until they are published, which she expects to be in the coming months.

This isn't the first time that Alzheimer's-like plaques have been induced in wild-type mice by infection. Chlamydia pneumoniae has been reported to induce amyloid plaques, for instance (see Little et al., 2004). Herpes simplex virus infection also causes Aβ42 to deposit in wild-type mice (see Wozniak et al., 2007 and recent ARF Webinar on herpes simplex virus as a possible trigger of AD). There is still much work to do before a link can be made between infection and the human form of Alzheimer's disease, including testing AD patients for immune markers and closely reviewing epidemiological data, Knuesel said.

"I don't think there's any epidemiology out there that has looked at that carefully," said Bruce Lamb of the Cleveland Clinic in Ohio. "But I think maybe it is time to do that kind of study." He added that the mechanism of how poly(I:C) works in these mice also needs further exploration.

"Knuesel’s two-hit strategy to develop a wild-type AD model has a sound scientific background because there is now genetic evidence for the role of innate immunity [in AD], and there is epidemiological and clinical evidence that systemic inflammatory mediators could contribute to the development of clinical Alzheimer’s," said Piet Eikelenboom of the Vrije Universiteit in Amsterdam, The Netherlands. But he is not yet convinced that the plaques seen in the rodents are comparable to the ones found in humans. He said he will reserve judgment until the new data are published.

Michael Chumley of the Texas Christian University in Fort Worth and his team are also looking into the effects of simulated bacterial infections on adult wild-type mice. In a poster presentation, graduate student Marielle Kahn reported that peripheral injections of lipopolysaccharide (LPS)—a bacterial coat component—over seven days in four- to six-month-old C57BL/6J mice (a common wild-type lab strain) led to immediate cognitive deficits. Animals had trouble with contextual fear learning and spent less time in the platform zone in the Morris water maze test. The mice were no longer sick at the time of testing, nor did they have elevated levels of some common pro-inflammatory cytokines left in their systems. However, their Aβ42 levels in the hippocampus were significantly elevated.

Taken together, these findings and Knuesel's support the idea that infection may trigger Alzheimer’s. "The implication could be that systemic inflammation is an instigating factor for Alzheimer's disease," said Chumley.

Chumley and colleagues do not yet know if mice fully recover cognitive function after the LPS injections, or if they continue to decline. The group will check for cognitive deficits a few weeks after injection, and will test the effects of repeated simulated infections on the mice. Recurring infections often occur in the older population, Chumley said, and the team wants to know if chronic infections and inflammation could drive Alzheimer's-like deficits and pathology.—Gwyneth Dickey Zakaib.

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  1. Comment by Debomoy Lahiri and Bryan Maloney
    The previously proposed LEARn (Latent Early-life Associated Regulation) pathway may explain the recently featured results from Chumley and Kahn and from Knuessel et al. regarding infection and AD. Work such as theirs that draws pathways between environmentally induced stress, such as infection, and AD (or AD-like results in model animals) significantly adds to our understanding of the etiology and prevention of AD. However, we would like to point out that the concept of AD as a “two-hit” (actually “n-hit”) disorder is not quite novel. We have previously proposed that a significant proportion of sporadic AD likely arises through latent influences of environment (Lahiri et al., 2009a). We have previously reported latent induction of expression of AD-related genes from early life environmental stress in mice (Basha et al., 2005 and ARF related news story) and in monkeys, and presented this as the “latent early-life associated regulation” (LEARn) model of sporadic/idiopathic neuropsychiatric disease etiology (Lahiri and Maloney, 2010). Briefly, our LEARn model proposes that AD is the result of multiple “hits” against a patient. Such “hits” may include a genetic risk factor (such as ApoE4), but would also include exposure to environmental toxins, heavy metals, variations in life events such as disease exposure, levels of dietary cholesterol, or even interpersonal and sociocultural factors such as emotional stress during childhood, educational attainment, or economic status.

    One of the hits is likely to be early in life (Basha et al., 2005). If such a “first hit” is lacking, further hits would usually be insufficient to cause AD. However, even a first hit would be insufficient, in and of itself, to result in AD. It would be the combined hits that lead to AD. LEARn further proposes a testable mechanism. The effects of the first hit and later hits would be latently preserved as epigenetic modifications to DNA and chromatin. Eventually, if hits have accumulated in critical regions of disease-associated genes, AD will result (Lahiri et al., 2009a; Lahiri and Maloney, 2010). Infection has already been shown to result in functional epigenetic modifications, such as changes in DNA methylation and oxidation (Katoh, 2007; Paschos and Allday, 2010; Cabrera et al., 2011; Chakraborty et al., 2011). Immune stimulation/inflammation from multiple causes is a known mechanism for alterations in DNA methylation and oxidation (Risom et al., 2005; Jawad et al., 2011; Saito et al., 2011; Weill et al., 2011). It is, therefore, reasonable to suggest that the results of Chumley and Kahn and of Kneussel et al. may likely reflect such an effect tied to AD-like conditions in model animals.

    However, multiple potential causes exist for epigenetic modification. This suggests that infection may act as one likely but not necessary only possible “hit” in AD etiology, if AD is determined to be a disease related to epigenetic regulation. It should be noted that we have already explicitly drawn parallels between a possible “n-hit” model of AD (LEARn) and currently accepted oncological models (Lahiri et al., 2009a). In some ways, the results of Kneussel’s and Chumley’s groups are reminiscent of early results in the “war on cancer” that promised to attribute all or most cancer to infections. While this hypothesis did not directly pan out, it did lead to a broader oncological model based on induced changes in the epigenome, of which infection is one potential cause. Significant differences in DNA methylation have been found in several disorders, including schizophrenia, bipolar disorder, suicide following abuse during childhood, and AD, among others (Heindel et al., 2006; Jirtle and Skinner, 2007; Dosunmu et al., 2009). More specifically, comparison of DNA methylation for selected gene sequences between AD and non-AD brain samples determined disease-associated differences in DNA methylation levels (Wang et al., 2008; Mastroeni et al., 2010). Likewise, a pair of monozygotic twins had been raised together but was discordant for AD. They had differential DNA methylation in temporal neocortical neurons (Mastroeni et al., 2009a; Mastroeni et al., 2009b).

    It may be tempting to dismiss these as prenatal “imprintation” effects, but it has been established that genomic DNA methylation can and does change across a large proportion of the human population (Bjornsson et al., 2008). The results reported by Alzforum certainly agree with the LEARn model, which has also proposed specific hypotheses to test environmental influences, consisting of 1) target sites within gene regulatory regions sensitive to epigenetic modification (e.g., unusually high or low concentrations of “CpG” and “GG” dimers); 2) potential critical time points in lifespan for environmental exposures (e.g., early development, midlife, etc.); and 3) environmental factors that tend to alter epigenetic markers. These elements would be combined with knowledge of candidate genes gained by linkage association studies to produce more specific, broadly applicable pathways for AD etiology.

    This is, of course, not to denigrate the work recently reported in Alzforum. Confirmation of a potentially important model for AD etiology is always welcome. However, AD as a “multiple-hit” disorder with significant environmental input is an idea that has been explicitly proposed before and overlooked by the Alzforum.

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References

News Citations

  1. Barcelona: Inflammation—That Two-Faced Beast
  2. Calling for Backup: Microglia from Bone Marrow Fight Plaques in AD Mice
  3. Microglia—Medics or Meddlers in Dementia
  4. Paper Alert: GWAS Hits Clusterin, CR1, PICALM Formally Published

Webinar Citations

  1. Herpes Simplex and Alzheimer’s—Time to Think Again?

Paper Citations

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  2. . Maternal infection requiring hospitalization during pregnancy and autism spectrum disorders. J Autism Dev Disord. 2010 Dec;40(12):1423-30. PubMed.
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  4. . Ccr2 deficiency impairs microglial accumulation and accelerates progression of Alzheimer-like disease. Nat Med. 2007 Apr;13(4):432-8. PubMed.
  5. . Systemic inflammation and disease progression in Alzheimer disease. Neurology. 2009 Sep 8;73(10):768-74. PubMed.
  6. . Genome-wide association study identifies variants at CLU and PICALM associated with Alzheimer's disease. Nat Genet. 2009 Oct;41(10):1088-93. PubMed.
  7. . Genome-wide association study identifies variants at CLU and CR1 associated with Alzheimer's disease. Nat Genet. 2009 Oct;41(10):1094-9. PubMed.
  8. . Chlamydia pneumoniae induces Alzheimer-like amyloid plaques in brains of BALB/c mice. Neurobiol Aging. 2004 Apr;25(4):419-29. PubMed.
  9. . Herpes simplex virus infection causes cellular beta-amyloid accumulation and secretase upregulation. Neurosci Lett. 2007 Dec 18;429(2-3):95-100. PubMed.

Other Citations

  1. triple-transgenic mice

Further Reading

Papers

  1. . Microglia and memory: modulation by early-life infection. J Neurosci. 2011 Oct 26;31(43):15511-21. PubMed.