Researchers are beginning to uncover genetic factors that modify the ability of amyloid plaques to spur tau pathology. Now, scientists led by Michael Ewers at Ludwig-Maximilians University, Munich, add a longevity gene into the mix. In a preprint posted July 31 to medRxiv, they propose that for a given amount of amyloid, people who carry one copy of the protective VS klotho variant accumulate fewer neurofibrillary tangles than noncarriers. This lighter tangle load correlated with better episodic memory. The finding sheds light on how klotho protects the brain, and could point to therapeutic strategies, the authors suggest. “This study adds evidence that the association between amyloid and tau pathologies depends on genotype,” Bernard Hanseeuw at Massachusetts General Hospital, Boston, wrote to Alzforum.
- People with one copy of the klotho VS variant have fewer tangles for a given amyloid load.
- Their memories are better, too.
- These protective effects are stronger in APOE4 carriers.
Others found the study a significant advance. “[The authors] are the first to publish a beneficial klotho-tau-cognition relationship,” Carmela Abraham at Boston University School of Medicine wrote to Alzforum. Abraham, who first implicated klotho in brain aging, called the work elegant and said it strengthened the rationale for boosting klotho to treat AD (full comment below). Michael Belloy at Stanford University noted that klotho initially was believed to be a resilience factor that helped the brain cope with plaques and tangles, but recent evidence suggests it directly protects against their accumulation. “It would be promising to figure out the molecular machinery behind this,” Belloy said.
High klotho levels associate with longer life and sharper cognition (Dec 2007 conference news; May 2013 conference news; May 2014 news). The protective VS variant occurs in about 25 percent of the population. It comprises two SNPs, F352V and C370S, that are inherited together. One copy of VS boosts klotho expression, leading to bigger brains and better memory (Yokoyama et al., 2015; de Vries et al., 2017). Curiously, two copies of the VS variant lower klotho production and confer no benefit.
Other groups previously found that the VS variant best protects APOE4 carriers, who are more at risk for amyloid accumulation. The variant reduces their plaque burden and cuts Alzheimer’s risk by a third (Erickson et al., 2019; Apr 2020 news). However, no one had examined klotho’s effect on tangles.
To investigate, first author Julia Neitzel analyzed data from 354 participants in ADNI3, all of whom had amyloid and tau PET scans. Of these, 213 were cognitively healthy, 111 had mild cognitive impairment, and 30 had AD. Their average age was 71. About a quarter of the cohort, 92 people, were heterozygous VS carriers.
At a given amyloid burden, the VS carriers bound less flortaucipir tau tracer in their brains than did noncarriers. This held true in both the inferior temporal cortex, where tau pathology usually starts, and in the whole brain. The attenuation remained significant after adjusting for age, sex, education, disease status, and APOE genotype.
Within this overall signal, the researchers found hints of stronger protection in APOE4 carriers. When they stratified the cohort by APOE genotype, having the VS variant correlated with less brain-wide flortaucipir uptake, indicating fewer tangles, in the 130 APOE4 carriers, but not in the 224 people with other APOE genotypes. The authors noted that the protective effect of the VS variant on tangles seems to be limited to people who either have a high amyloid burden or who carry APOE4. Both these groups are at higher risk of tangles.
A regional analysis of the brain hinted that klotho may act locally. The gene is most strongly expressed in mesotemporal, inferior temporal, and posterior cingulum cortices, according to the Allen Brain Atlas. In the ADNI3 cohort, the VS variant conferred a stronger protective effect against tangles in those brain regions than in others.
Does this matter for cognition? To address this question, the researchers limited their analysis to the 258 participants who were amyloid-positive, since they are most likely to have cognitive decline. In this subgroup, the 67 VS carriers outperformed the 191 noncarriers on an episodic memory measure in the ADNI-MEM composite. This remained significant even after adjusting for APOE4 genotype and other confounders. Statistical mediation analysis, which can help tease out cause and effect, showed the effect was mediated by the lower tau tracer uptake.
Previous studies had reported that VS correlated with less amyloid accumulation. Neitzel and colleagues did not see this in the 354 ADNI3 participants who had a tau PET scan; however, in a larger ADNI3 cohort of all 959 participants who had had amyloid scans and genotyping, heterozygous VS carriers did have less amyloid in the brain than did VS noncarriers, at least among those younger than 80. In agreement with prior studies, the amyloid burden difference was driven by APOE4 carriers. Because the authors saw a reduction in tangles in a small subset of the cohort, they concluded that the klotho variant more strongly affects tau than amyloid. They calculated a Cohen’s effect size of 0.069 for amyloid, and 0.118 for tangles.
In ongoing research, Neitzel and colleagues are testing serum and cerebrospinal fluid samples from the same study participants to see if the protective effects in VS carriers are mediated by klotho protein levels. They will also try to decipher the mechanism of how klotho prevents tangles. The protein takes part in numerous cellular processes, including redox reactions and autophagy, and it regulates calcium, insulin, and growth factor signaling. In a mouse model of amyloidosis, klotho overexpression reduced plaque buildup by activating autophagy (Zeng et al., 2019). Ewers and colleagues will analyze proteomes from volunteers to evaluate whether autophagic proteins are upregulated in VS carriers.
The findings imply that somehow raising klotho levels might stave off AD. This is supported by work in mouse models of amyloidosis, where injecting different forms of klotho protein into blood or brain sharpens memory (Aug 2017 news; Nov 2017 news). Ligustilide, a small molecule found in essential oils extracted from plants, promotes klotho processing, increasing the secreted form of the protein, and could have therapeutic potential, Ewers said. In mouse models of amyloidosis, ligustilide treatment lowered amyloid load and improved memory (Kuang et al., 2017). As the primary active component in the Chinese herbal extract Rhizoma Chuanxiong, the compound is used to treat cardiovascular ailments, but the liver rapidly breaks it down, reducing its bioavailability (Yan et al., 2008).
Beyond therapeutic considerations, the finding elucidates the relationship between plaques and tangles. Other genetic variants have been found to weaken that association. In a woman with the presenilin 1 Paisa mutation that causes familial AD, the Christchurch mutation in APOE3 was reported to protect from cognitive decline. Now in her 70s, her brain is full of plaques but contains few tangles (Nov 2019 news). The Christchurch mutation is rare, however, while klotho-VS is common. “Our data show the potential for tangles to be modulated by common genetic variants,” Ewers told Alzforum.
Recently, researchers led by Alfredo Ramirez at the University of Cologne, Germany, and Agustín Ruiz at the International University of Catalonia in Barcelona, Spain, identified another gene that intervenes in the pathogenic cascade at the step between plaques and tangles. They reported that a rare coding variant in the microglial phospholipase-C-γ2 (PLCG2) gene protected against AD. In 1,261 MCI patients, those with the variant had less CSF p-tau181 and slower cognitive decline than noncarriers. This marker rises early in AD and thus far appears specific for amyloid-driven tau pathology (Mar 2020 news). The drop in p-tau181 predominantly occurred in MCI patients who did have amyloid buildup, as determined by low CSF Aβ42. The effect size of this protective association was similar to that of APOE4, although in the opposite direction.
This suggests that, like klotho, PLCG2 may intervene between amyloid and tau, the authors noted. In protein network analysis, PLCG2 interacts with other known AD risk factors such as TREM2, APOE, and complement (Kleineidam et al., 2020). Ewers noted that PLCG2 acts downstream of fibroblast growth factor. FGF interacts with klotho, hinting that klotho and PLCG2 could be part of the same mechanistic pathway (Mar 2019 news).
“Future studies should investigate how the combination of distinct genetic protective and risk factors impacts the interaction between amyloid and tau, and whether these effects are additive or multiplicative,” Hanseeuw wrote.—Madolyn Bowman Rogers
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