Some scientists believe that brain inflammation can trigger sporadic Alzheimer’s disease, but the precise pathways remain unclear. In the May 30 Journal of Cell Biology, researchers led by Aldo Pagano at the University of Genoa, Italy, present evidence for an RNA-mediated mechanism. Using neuroblastoma cells, the scientists found that inflammation triggers transcription of a non-coding RNA that alters splicing of a potassium channel-interacting protein. The alternately spliced protein interferes with nerve conductance and promotes amyloid-β production in these cultures. The researchers did not look at primary neuronal cultures. However, the scientists report that the non-coding RNA is enriched in the brains of some AD patients, suggesting a connection to the disease.

“[This paper] offers an alternative narrative to the neurodegenerative events of AD, which stitches the story together in a brand-new way,” said Karl Herrup at Rutgers University, Piscataway, New Jersey. “I recommend this paper.” Herrup was not involved in the research.

Growing evidence indicates that non-coding RNAs play a role in neurodegenerative disease (see, e.g., ARF related conference story Part 1 and Part 2). For example, microRNAs have been shown to regulate AD-related genes (see ARF related news story), and larger non-coding RNAs may also affect gene expression (see ARF related news story). To find more such RNA regulators, Pagano’s group previously searched for promoter elements recognized by RNA polymerase III, an enzyme that synthesizes various non-coding, housekeeping RNAs. They turned up about 30 novel non-coding RNA transcripts (see Pagano et al., 2007).

In the current paper, joint first authors Sara Massone, Irene Vassallo, and Manuele Castelnuovo characterized one of these non-coding RNAs, 38A. This transcript maps to an intron of the potassium channel-interacting protein KCNIP4, and has an anti-sense configuration to KCNIP4 RNA. This suggests that 38A could bind to the KCNIP4 transcript, covering up a splice site and altering the way the RNA is processed. In support of this, the authors found that overexpression of 38A in neuroblastoma cells doubled the ratio of alternatively spliced variant IV protein over the more common variant I. KCNIP4 variant IV alters the kinetics of A-type voltage-dependent potassium channels, causing these channels to lose their fast inactivation (see Holmqvist et al., 2002), a finding Massone and colleagues confirmed in their cell cultures. Since A-type current has an important role in long-term potentiation, this change might perturb memory and brain plasticity, the authors note. In normal brains, variant IV is expressed in globus pallidus and basal forebrain, which contain slowly inactivating potassium channels, but not in striatum or hippocampus, where fast inactivation is the rule (see Baranauskas, 2004; Trimmer and Rhodes, 2004).

Since KCNIP4 variant I has also been shown to bind presenilin-2 (see Morohashi et al., 2002; Parks and Curtis, 2007), Massone and colleagues examined variant IV’s interactions with the secretase in their neuroblastoma cells. Co-immunoprecipitations showed that variant IV does not bind PS2. Moreover, in cells containing triple the ratio of variant IV to variant I protein, γ-secretase processing favored Aβ42 production. Cells with threefold more variant IV produced twice as much Aβ42 and 1.4-fold more Aβ40 than normal. RNA silencing of variant IV reversed this effect, demonstrating a tight link between the KCNIP4 isoform and altered APP processing. In contrast to PS2, Massone and colleagues found that variant IV had no effect on PS1 levels or processing. Mutations in both PS1 and PS2 can cause familial AD.

To see if 38A could be a factor in human AD, Massone and colleagues examined 17 postmortem AD and 10 control brains. They found that, on average, 38A RNA was increased 10-fold in AD cerebral cortex over control, although they saw wide variability from brain to brain. They also confirmed that variant IV protein was enriched in cortical extracts from AD brains. The authors looked for a genetic connection, and found that variations in the 38A promoter occurred more often in AD brains than in controls. This suggests that inherited factors might increase a person’s risk for 38A overexpression.

What else might lead to an imbalance in variant IV production? Increasing evidence suggests that inflammation promotes the development of AD and other neurodegenerative diseases (see, e.g., ARF related conference story; Griffin et al., 1989; McGeer et al., 2006; Wyss-Coray, 2006). Massone and colleagues found that adding the pro-inflammatory cytokine IL1-α to their neuroblastoma cells increased 38A transcription threefold. When they pretreated the cultures with a non-steroidal anti-inflammatory drug, this effect disappeared.

“If this study is confirmed…then these results would extend the rationale for using an anti-inflammatory approach very early in the disease to reduce the buildup of Aβ pathology,” Greg Cole at the University of California, Los Angeles, wrote to ARF (see full comment below).

Christian Haass, at the German Center for Neurodegenerative Diseases (DZNE) and Ludwig-Maximilians-University, both in Munich, Germany, suggested caution in connecting this 38A-driven mechanism to AD. “I am not sure if the observed modest increase of Aβ42 is sufficient to drive disease pathology in vivo,” he wrote to ARF (see full comment below).

To extend this work, the next logical step would be to see how this pathway behaves in primary neuronal cultures, Herrup said. For example, he speculated that the involvement of PS1 might be different in another cellular context. Another promising route would be to follow up on the human data and nail down the sources of individual variation, Herrup said, noting there are multiple possible explanations for this variation, including the age of the samples and the different brain regions examined. “To me, this is part of a growing body of evidence that simple linear explanations for AD are increasingly untenable,” he said. Herrup recently led an Alzforum Webinar on the potential etiologies of AD.—Madolyn Bowman Rogers

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  1. This paper by Massone and colleagues is intriguing in that they report that IL1-α elevates 38A RNA in a cyclo-oxygenase (COX) inhibitor-sensitive manner, and that AD brain has markedly elevated 38A RNA consistent with an inflammation-driven increase. They find that 38A modulates KCNIP4 splicing to yield more variant IV and less variant I, which the authors also observe in AD brain. Given functional differences in the variants, the alternative splicing reported in AD brain is a novel and interesting result. In-vitro data in neuronal cell lines support a possible impact on A-type potassium currents and perturbed LTP, but this does not seem to have been demonstrated in vivo or in real neurons. Previous work on overexpression of variant 1 KCNIP4 did not show an impact on γ-secretase, so these data are novel. The in-vitro data argue for a KCNIP4 regulation of PS2, but not PS1, leading to a γ-secretase modulation with a shift toward increased Aβ42. I am not a presenilin expert, so it is difficult for me to assess the import of a selective effect on PS2. PS1 has appeared the more critical modulator of Aβ42 production, but given that PS2 mutations can cause AD, I think the new work in this paper will be important to repeat and perhaps build on.

    If this study is confirmed, and it is true that this provides a new mechanism for inflammation to increase Aβ42 production and for COX inhibitors to modulate γ-secretase, then these results would extend the rationale for using an anti-inflammatory approach very early in the disease to reduce the buildup of Aβ pathology.

    It has been clear for many years that COX inhibitors are not very helpful with established AD and show a lagging protective effect in the epidemiology requiring intervention two or more years prior to AD to show protection. Recent trial results suggest that γ-secretase inhibition in mild to moderate AD may also be unable to provide protection. This raises the question of whether other anti-Aβ strategies will show similar lagging effects, as the non-steroidal anti-inflammatory drug epidemiology suggests for COX inhibitors.

    View all comments by Gregory Cole
  2. PS2 complexes do not represent the main γ-secretase activity in the brain; rather, this is exerted by PS1-containing γ-secretase complexes. Moreover, I am not sure if the observed modest increase of Aβ42 is sufficient to drive disease pathology in vivo. It should be kept in mind that all FAD-associated PS2 mutations cause a huge increase of Aβ42 generation. This makes sense, considering that PS2 is not contributing very much to the γ-secretase activity in the brain.

    In addition, the γ-secretase interactions are monitored by co-immunoprecipitations of full-length PS1 or PS2. However, it has been known for a long time that full-length PS is not part of the γ-secretase complex.

    View all comments by Christian Haass
  3. In agreement with Christian Haass, I recommend prudence in linking, unequivocally, the forced expression of 38A ncRNA to the pathological manifestations of AD. We must be cautious in ascribing to its over-synthesis a causative mechanism of AD onset. In this regard, our work reports that “at present it is not clear if 38A is essential for AD onset or rather it is part of a pathologic condition of the brain that occurs in numerous neurodegenerative disorders,” and that our results “suggest its contribution to the disease in association with other possible elements.”

    Nonetheless, the fact that “this model provides a novel way to investigate neurodegeneration and brain function based on an alternative cascade of reactions unexpectedly controlled by an ncRNA transcribed by pol III” is relevant, and might open the way to novel explorations of brain pathology. In this context, as suggested by Greg Cole, the best way to know the involvement of 38A ncRNA in AD is to evaluate the consequences of its synthesis in mouse hippocampal and cortical neurons, a work currently in progress. Different lines of transgenic mice stably overexpressing 38A ncRNA have been generated and, hopefully, will provide novel information about 38A's biological role. If a causative action of 38A ncRNA overexpression on AD onset can be clearly demonstrated, this mouse model will also provide a suitable way to test the role of inflammation (and of anti-inflammatory drugs) on this small ncRNA, and on the possible pathological consequences of its synthesis.

    View all comments by Aldo Pagano
  4. Although inflammation appears to be a contributor to the pathology of AD, it is unlikely to be a causal event of this dementia, as I have recently reviewed in a paper that analyzes current AD hypotheses. A considerable number of hypothetical proposals suggesting the cause of AD have been published in the last decades, and none so far has reached a consensus of agreement by experts in the field. Hence, it is fundamental that any likely proposed cause of AD needs to satisfy strict criteria derived by evidence-based medicine from randomized controlled trials.

    AD progress will not move forward until we set in motion efficient therapeutic initiatives designed to prevent, reverse, or slow down the likely cause of this disorder, not the imagined or convenient cause that is good for grant-getting but is unsupported by the available clinical evidence.

    References:

    . Three postulates to help identify the cause of Alzheimer’s disease. J Alzheimers Dis. 2011;24(4):657-68. PubMed.

    View all comments by Jack de la Torre

References

News Citations

  1. Keystone: More Than Mere Nucleotides—miRNAs as Master Regulators, Part 1
  2. Keystone: More Than Mere Nucleotides—miRNAs as Master Regulators, Part 2
  3. BACE in Alzheimer’s—Does MicroRNA Control Translation?
  4. Research Brief: The RNA World Expands, Again
  5. Barcelona: Inflammation—That Two-Faced Beast

Webinar Citations

  1. Reimagining Alzheimer's Disease—Time for Bright New Ideas?

Paper Citations

  1. . New small nuclear RNA gene-like transcriptional units as sources of regulatory transcripts. PLoS Genet. 2007 Feb 2;3(2):e1. PubMed.
  2. . Elimination of fast inactivation in Kv4 A-type potassium channels by an auxiliary subunit domain. Proc Natl Acad Sci U S A. 2002 Jan 22;99(2):1035-40. PubMed.
  3. . Cell-type-specific splicing of KChIP4 mRNA correlates with slower kinetics of A-type current. Eur J Neurosci. 2004 Jul;20(2):385-91. PubMed.
  4. . Localization of voltage-gated ion channels in mammalian brain. Annu Rev Physiol. 2004;66:477-519. PubMed.
  5. . Molecular cloning and characterization of CALP/KChIP4, a novel EF-hand protein interacting with presenilin 2 and voltage-gated potassium channel subunit Kv4. J Biol Chem. 2002 Apr 26;277(17):14965-75. PubMed.
  6. . Presenilin diversifies its portfolio. Trends Genet. 2007 Mar;23(3):140-50. PubMed.
  7. . Brain interleukin 1 and S-100 immunoreactivity are elevated in Down syndrome and Alzheimer disease. Proc Natl Acad Sci U S A. 1989 Oct;86(19):7611-5. PubMed.
  8. . Inflammation, anti-inflammatory agents and Alzheimer disease: the last 12 years. J Alzheimers Dis. 2006;9(3 Suppl):271-6. PubMed.
  9. . Inflammation in Alzheimer disease: driving force, bystander or beneficial response?. Nat Med. 2006 Sep;12(9):1005-15. PubMed.

Further Reading

Primary Papers

  1. . RNA polymerase III drives alternative splicing of the potassium channel-interacting protein contributing to brain complexity and neurodegeneration. J Cell Biol. 2011 May 30;193(5):851-66. PubMed.