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