Linda Younkin

Mayo Clinic, Jacksonville, Florida, United States

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Publications (115)890.48 Total impact

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    ABSTRACT: Alzheimer's disease is a neurodegenerative disorder in which extracellular deposition of β-amyloid (Aβ) oligomers causes synaptic injury resulting in early memory loss, altered homeostasis, accumulation of hyperphosphorylated tau and cell death. Since proteins in the SNAP (Soluble N-ethylmaleimide-sensitive factor Attachment Protein) REceptors (SNARE) complex are essential for neuronal Aβ release at pre-synaptic terminals, we hypothesized that genetically controlled SNARE expression could alter neuronal Aß release at the synapse and hence play an early role in Alzheimer's pathophysiology. Here we report 5 polymorphisms in Vesicle-Associated Membrane Protein 1 (VAMP1), a gene encoding a member of the SNARE complex, associated with bidirectionally altered cerebellar VAMP1 transcript levels (all p < 0.05). At the functional level, we demonstrated that control of VAMP1 expression by heterogeneous knockdown in mice resulted in up to 74% reduction in neuronal Aβ exocytosis (p < 0.001). We performed a case-control association study of the 5 VAMP1 expression regulating polymorphisms in 4,667 Alzheimer's disease patients and 6,175 controls to determine their contribution to Alzheimer's disease risk. We found that polymorphisms associated with increased brain VAMP1 transcript levels conferred higher risk for Alzheimer's disease than those associated with lower VAMP1 transcript levels (p = 0.03). Moreover, we also report a modest protective association for a common VAMP1 polymorphism with Alzheimer's disease risk (OR = 0.88, p = 0.03). This polymorphism was associated with decreased VAMP1 transcript levels (p = 0.02) and was functionally active in a dual luciferase reporter gene assay (p < 0.01). Genetically regulated VAMP1 expression in the brain may modify both Alzheimer's disease risk and may contribute to Alzheimer's pathophysiology.
    Molecular Neurodegeneration 04/2015; 10(1):18. DOI:10.1186/s13024-015-0015-x · 6.56 Impact Factor
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    ABSTRACT: Background Plasma amyloid-β (Aβ) peptide levels have been examined as a low-cost accessible marker for risk of incident Alzheimer's disease (AD) and dementia, but results have varied between studies. We reassessed these associations in one of the largest, prospective, community-based studies to date. Methods A total of 2189 dementia-free, Framingham Study participants aged >60 years (mean age, 72 ± 8 years; 56% women) had plasma Aβ1-42 and Aβ1-40 measured and were followed prospectively (mean, 7.6 ± 3.0 years) for dementia/AD. Results Increased plasma Aβ1-42 levels were associated with lower risk of dementia (Aβ1-42: hazard ratio [HR] = 0.80 [0.71‒0.90], P < .001; Aβ1-42-to-Aβ1-40 ratio: HR = 0.86 [0.76‒0.98], P = .027) and AD (Aβ1-42: HR = 0.79 [0.69‒0.90], P < .001; Aβ1-42-to-Aβ1-40 ratio: HR = 0.83 [0.72‒0.96], P = .012). Conclusion Our results suggest that lower plasma Aβ levels are associated with risk of incident AD and dementia. They encourage further evaluation of plasma Aβ levels as a biomarker for risk of developing clinical AD and dementia.
    Alzheimer's & dementia: the journal of the Alzheimer's Association 09/2014; 11(3). DOI:10.1016/j.jalz.2014.07.001 · 12.41 Impact Factor
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    Alzheimer's and Dementia 07/2014; 10(4):P270-P271. DOI:10.1016/j.jalz.2014.04.439 · 12.41 Impact Factor
  • Alzheimer's and Dementia 07/2013; 9(4):P238-P239. DOI:10.1016/j.jalz.2013.05.459 · 12.41 Impact Factor
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    ABSTRACT: Leucine rich repeat transmembrane protein 3 (LRRTM3) is member of a synaptic protein family. LRRTM3 is a nested gene within α-T catenin (CTNNA3) and resides at the linkage peak for late-onset Alzheimer's disease (LOAD) risk and plasma amyloid β (Aβ) levels. In-vitro knock-down of LRRTM3 was previously shown to decrease secreted Aβ, although the mechanism of this is unclear. In SH-SY5Y cells overexpressing APP and transiently transfected with LRRTM3 alone or with BACE1, we showed that LRRTM3 co-localizes with both APP and BACE1 in early endosomes, where BACE1 processing of APP occurs. Additionally, LRRTM3 co-localizes with APP in primary neuronal cultures from Tg2576 mice transduced with LRRTM3-expressing adeno-associated virus. Moreover, LRRTM3 co-immunoprecipitates with both endogenous APP and overexpressed BACE1, in HEK293T cells transfected with LRRTM3. SH-SY5Y cells with knock-down of LRRTM3 had lower BACE1 and higher CTNNA3 mRNA levels, but no change in APP. Brain mRNA levels of LRRTM3 showed significant correlations with BACE1, CTNNA3 and APP in ∼400 humans, but not in LRRTM3 knock-out mice. Finally, we assessed 69 single nucleotide polymorphisms (SNPs) within and flanking LRRTM3 in 1,567 LOADs and 2,082 controls and identified 8 SNPs within a linkage disequilibrium block encompassing 5'UTR-Intron 1 of LRRTM3 that formed multilocus genotypes (MLG) with suggestive global association with LOAD risk (p = 0.06), and significant individual MLGs. These 8 SNPs were genotyped in an independent series (1,258 LOADs and 718 controls) and had significant global and individual MLG associations in the combined dataset (p = 0.02-0.05). Collectively, these results suggest that protein interactions between LRRTM3, APP and BACE1, as well as complex associations between mRNA levels of LRRTM3, CTNNA3, APP and BACE1 in humans might influence APP metabolism and ultimately risk of AD.
    PLoS ONE 06/2013; 8(6):e64164. DOI:10.1371/journal.pone.0064164 · 3.23 Impact Factor
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    ABSTRACT: Deposition of amyloid-β (Aβ) in cerebral arteries, known as cerebral amyloid angiopathy (CAA), occurs both in the setting of Alzheimer's disease and independent of it, and can cause cerebrovascular insufficiency and cognitive deficits. The mechanisms leading to CAA have not been established, and no therapeutic targets have been identified. We investigated the role of CD36, an innate immunity receptor involved in Aβ trafficking, in the neurovascular dysfunction, cognitive deficits, and amyloid accumulation that occurs in mice expressing the Swedish mutation of the amyloid precursor protein (Tg2576). We found that Tg2576 mice lacking CD36 have a selective reduction in Aβ1-40 and CAA. This reduced vascular amyloid deposition was associated with preservation of the Aβ vascular clearance receptor LRP-1, and protection from the deleterious effects of Aβ on cerebral arterioles. These beneficial vascular effects were reflected by marked improvements in neurovascular regulation and cognitive performance. Our data suggest that CD36 promotes vascular amyloid deposition and the resulting cerebrovascular damage, leading to neurovascular dysfunction and cognitive deficits. These findings identify a previously unrecognized role of CD36 in the mechanisms of vascular amyloid deposition, and suggest that this scavenger receptor is a putative therapeutic target for CAA and related conditions.
    Proceedings of the National Academy of Sciences 02/2013; 110(8). DOI:10.1073/pnas.1300021110 · 9.67 Impact Factor
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    ABSTRACT: Neuronal network hyperexcitability underlies the pathogenesis of seizures and is a component of some degenerative neurological disorders such as Alzheimer's disease (AD). Recently, the microtubule-binding protein tau has been implicated in the regulation of network synchronization. Genetic removal of Mapt, the gene encoding tau, in AD models overexpressing amyloid-β (Aβ) decreases hyperexcitability and normalizes the excitation/inhibition imbalance. Whether this effect of tau removal is specific to Aβ mouse models remains to be determined. Here, we examined tau as an excitability modifier in the non-AD nervous system using genetic deletion of tau in mouse and Drosophila models of hyperexcitability. Kcna1(-/-) mice lack Kv1.1-delayed rectifier currents and exhibit severe spontaneous seizures, early lethality, and megencephaly. Young Kcna1(-/-) mice retained wild-type levels of Aβ, tau, and tau phospho-Thr(231). Decreasing tau in Kcna1(-/-) mice reduced hyperexcitability and alleviated seizure-related comorbidities. Tau reduction decreased Kcna1(-/-) video-EEG recorded seizure frequency and duration as well as normalized Kcna1(-/-) hippocampal network hyperexcitability in vitro. Additionally, tau reduction increased Kcna1(-/-) survival and prevented megencephaly and hippocampal hypertrophy, as determined by MRI. Bang-sensitive Drosophila mutants display paralysis and seizures in response to mechanical stimulation, providing a complementary excitability assay for epistatic interactions. We found that tau reduction significantly decreased seizure sensitivity in two independent bang-sensitive mutant models, kcc and eas. Our results indicate that tau plays a general role in regulating intrinsic neuronal network hyperexcitability independently of Aβ overexpression and suggest that reducing tau function could be a viable target for therapeutic intervention in seizure disorders and antiepileptogenesis.
    The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 01/2013; 33(4):1651-9. DOI:10.1523/JNEUROSCI.3191-12.2013 · 6.34 Impact Factor
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    ABSTRACT: To investigate whether prostate cancer patients receiving leuprolide demonstrated objective cognitive decline accompanied by a change in plasma levels of amyloid-β. Between November 19, 2003, and July 21, 2008, we prospectively enrolled 50 patients with biochemical recurrence of prostate cancer and measured plasma amyloid-β peptide 40 and amyloid-β peptide 42 levels with sandwich enzyme-linked immunosorbent assay at baseline before the first leuprolide injection and at 2, 4, and 12 months. The Mini-Mental State Examination was used to assess 49 patients at baseline and at subsequent visits, and 24 were also assessed by the California Verbal Learning Test-Short Form. Patients were a median age of 71 years (range, 59-89 years). Compared with baseline levels, plasma amyloid-β peptide 40 levels were increased at 2 months (P = .04) and 4 months (P = .02). Age was correlated with plasma amyloid-β peptide 40 levels (P = .003) and likely accounted for this relationship. Plasma amyloid-β peptide 42 and performance on cognitive tasks did not differ from baseline, but memory measures improved slightly after baseline, most likely due to a practice effect. Leuprolide therapy was not associated with a decline in cognition or memory function or with elevated plasma amyloid short-term. Larger studies are needed to confirm these findings.
    Urology 01/2013; 81(1):150-4. DOI:10.1016/j.urology.2012.08.066 · 2.19 Impact Factor
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    ABSTRACT: Increasing evidence indicates that cerebrovascular dysfunction plays a pathogenic role in Alzheimer's dementia (AD). Amyloid-β (Aβ), a peptide central to the pathogenesis of AD, has profound vascular effects mediated, for the most part, by reactive oxygen species produced by the enzyme NADPH oxidase. The mechanisms linking Aβ to NADPH oxidase-dependent vascular oxidative stress have not been identified, however. We report that the scavenger receptor CD36, a membrane glycoprotein that binds Aβ, is essential for the vascular oxidative stress and neurovascular dysfunction induced by Aβ1-40. Thus, topical application of Aβ1-40 onto the somatosensory cortex attenuates the increase in cerebral blood flow elicited by neural activity or by endothelium-dependent vasodilators in WT mice but not in CD36-null mice (CD36(0/0)). The cerebrovascular effects of infusion of Aβ1-40 into cerebral arteries are not observed in mice pretreated with CD36 blocking antibodies or in CD36(0/0) mice. Furthermore, CD36 deficiency prevents the neurovascular dysfunction observed in transgenic mice overexpressing the Swedish mutation of the amyloid precursor protein Tg2576 despite elevated levels of brain Aβ1-40. CD36 is also required for the vascular oxidative stress induced by exogenous Aβ1-40 or observed in Tg2576 mice. These observations establish CD36 as a key link between Aβ1-40 and the NADPH oxidase-dependent vascular oxidative stress underlying the neurovascular dysfunction and suggest that CD36 is a potential therapeutical target to counteract the cerebrovascular dysfunction associated with Aβ.
    Proceedings of the National Academy of Sciences 03/2011; 108(12):5063-8. DOI:10.1073/pnas.1015413108 · 9.67 Impact Factor
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    ABSTRACT: Lower plasma β-amyloid 42 and 42/40 levels have been associated with incident dementia, but results are conflicting and few have investigated cognitive decline among elders without dementia. To determine if plasma β-amyloid is associated with cognitive decline and if this association is modified by measures of cognitive reserve. We studied 997 black and white community-dwelling older adults from Memphis, Tennessee, and Pittsburgh, Pennsylvania, who were enrolled in the Health ABC Study, a prospective observational study begun in 1997-1998 with 10-year follow-up in 2006-2007. Participant mean age was 74.0 (SD, 3.0) years; 55.2% (n = 550) were female; and 54.0% (n = 538) were black. Association of near-baseline plasma β-amyloid levels (42 and 42/40 measured in 2010) and repeatedly measured Modified Mini-Mental State Examination (3MS) results. Low β-amyloid 42/40 level was associated with greater 9-year 3MS cognitive decline (lowest β-amyloid tertile: mean change in 3MS score, -6.59 [95% confidence interval [CI], -5.21 to -7.67] points; middle tertile: -6.16 [95% CI, -4.92 to -7.32] points; and highest tertile: -3.60 [95% CI, -2.27 to -4.73] points; P < .001). Results were similar after multivariate adjustment for age, race, education, diabetes, smoking, and apolipoprotein E [APOE ] e4 status and after excluding the 72 participants with incident dementia. Measures of cognitive reserve modified this association whereby among those with high reserve (at least a high school diploma, higher than sixth-grade literacy, or no APOE e4 allele), β-amyloid 42/40 was less associated with multivariate adjusted 9-year decline. For example, among participants with less than a high school diploma, the 3MS score decline was -8.94 (95% CI, -6.94 to -10.94) for the lowest tertile compared with -4.45 (95% CI, -2.31 to -6.59) for the highest tertile, but for those with at least a high school diploma, 3MS score decline was -4.60 (95% CI,-3.07 to -6.13) for the lowest tertile and -2.88 (95% CI,-1.41 to -4.35) for the highest tertile (P = .004 for interaction). Interactions were also observed for literacy (P = .005) and for APOE e4 allele (P = .02). Lower plasma β-amyloid 42/40 is associated with greater cognitive decline among elderly persons without dementia over 9 years, and this association is stronger among those with low measures of cognitive reserve.
    JAMA The Journal of the American Medical Association 01/2011; 305(3):261-6. DOI:10.1001/jama.2010.1995 · 35.29 Impact Factor
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    ABSTRACT: Behavioural and psychological signs and symptoms of dementia encompass a wide range of neuropsychiatric disturbances which coincide with progressing cognitive decline in Alzheimer's disease (AD). Physical aggression and agitation, which occurs in 20-65% of AD patients, is physically and emotionally stressful, not only to patients but also to immediate family and caregivers. The exact mechanisms underlying the increased aggressive behaviour in AD has yet to be elucidated. We used a transgenic mouse model, denoted Tg2576, which over-expresses a mutated human amyloid precursor protein (APP) gene implicated in familial AD, to investigate aggressive behaviour of males at the stage of amyloid beta pathology preceding overt amyloid plaque deposition in the brain. The aggressive behaviour of transgenic and non-transgenic littermate males was evaluated in a standard resident-intruder test in which an isolated resident male responded aggressively toward an experimentally naïve intruder male of A/J strain. We showed that 7-month-old Tg2576 resident males demonstrated significantly higher and unchanged level of aggression towards intruder males during 3 consecutive encounters as compared to their non-transgenic littermate counterparts. These results validate further the Tg2576 mouse model of AD underscoring its usefulness in studying non-mnemonic changes in behaviour related to the disease.
    Behavioural brain research 01/2011; 216(1):77-83. DOI:10.1016/j.bbr.2010.07.016 · 3.03 Impact Factor
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    ABSTRACT: By analyzing late onset Alzheimer’s disease (LOAD) in a genome wide association study of 3 American Caucasian series and evaluating the 25 SNPs with most significant allelic association in 4 additional series, we identifi ed a SNP (rs5984894) on Xq21.3 in PCDH11X that is strongly associated with LOAD in American Caucasians (total n=4,855; AD:2,391; control:2,464). Analysis of rs5984894 by logistic regression using sex as a covariate gave a global p value of 3.9 x 10-12 in the combined series. Odds ratios were 1.75( 95% CI 1.42-2.16) for female homozygotes (P=2.0x10-7) and 1.26 (95% CI 1.05-1.51) for female heterozygotes (P=0.01) compared to female non-carriers. For male hemizygotes (P=0.07) compared to male non-carriers the odds ratio was 1.18 (95% CI 0.99-1.41). The eff ect of this variant was dose dependent, as females homozygotes for the minor allele were at signifi cantly greater risk than heterozygous females and hemizygous males. We tested additional variants in PCDH11X for association with LOAD. One of these variants, rs2573905, showed association with LOAD similar to that for rs5984894, albeit with a slightly more significant p-value (5.4x10-13). This is not surprising since these two variants are in near perfect linkage disequilibrium (r2 = 0.98, D’ = 0.99), and the minor allele of these two SNPs occur on the same haplotype. However, rs2573905 is in a sequence that has been evolutionarily conserved between human and mice, with 70% sequence identity over 100bp, suggesting a possible functional role for this SNP. Joint analysis of APOE and rs2573905 genotypes showed that 75 women with LOAD (4.9%) were homozygous both for APOE e4 and for rs2573905 whereas only 2 unaff ected women (0.15%), both age 73, were double homozygotes. These fi nding suggest that the double homozygote may be fully penetrant in women over the age of 73 and that this combination may account for ~5% of the AD that occurs in women.
    Hirosaki Medical Journal 07/2010; 61.
  • Alzheimer's and Dementia 07/2010; 6(4). DOI:10.1016/j.jalz.2010.05.500 · 12.41 Impact Factor
  • Alzheimer's and Dementia 07/2010; 6(4). DOI:10.1016/j.jalz.2010.05.1691 · 12.41 Impact Factor
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    ABSTRACT: Late-onset Alzheimer disease (LOAD) is a common disorder with a substantial genetic component. We postulate that many disease susceptibility variants act by altering gene expression levels. We measured messenger RNA (mRNA) expression levels of 12 LOAD candidate genes in the cerebella of 200 subjects with LOAD. Using the genotypes from our LOAD genome-wide association study for the cis-single nucleotide polymorphisms (SNPs) (n = 619) of these 12 LOAD candidate genes, we tested for associations with expression levels as endophenotypes. The strongest expression cis-SNP was tested for AD association in 7 independent case-control series (2,280 AD and 2,396 controls). We identified 3 SNPs that associated significantly with IDE (insulin degrading enzyme) expression levels. A single copy of the minor allele for each significant SNP was associated with approximately twofold higher IDE expression levels. The most significant SNP, rs7910977, is 4.2 kb beyond the 3' end of IDE. The association observed with this SNP was significant even at the genome-wide level (p = 2.7 x 10(-8)). Furthermore, the minor allele of rs7910977 associated significantly (p = 0.0046) with reduced LOAD risk (OR = 0.81 with a 95% CI of 0.70-0.94), as expected biologically from its association with elevated IDE expression. These results provide strong evidence that IDE is a late-onset Alzheimer disease (LOAD) gene with variants that modify risk of LOAD by influencing IDE expression. They also suggest that the use of expression levels as endophenotypes in genome-wide association studies may provide a powerful approach for the identification of disease susceptibility alleles.
    Neurology 02/2010; 74(6):480-6. DOI:10.1212/WNL.0b013e3181d07654 · 8.29 Impact Factor
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    ABSTRACT: The insulin-degrading enzyme gene (IDE) is a strong functional and positional candidate for late onset Alzheimer's disease (LOAD). We examined conserved regions of IDE and its 10 kb flanks in 269 AD cases and 252 controls thereby identifying 17 putative functional polymorphisms. These variants formed eleven haplotypes that were tagged with ten variants. Four of these showed significant association with IDE transcript levels in samples from 194 LOAD cerebella. The strongest, rs6583817, which has not previously been reported, showed unequivocal association (p = 1.5x10(-8), fold-increase = 2.12,); the eleven haplotypes were also significantly associated with transcript levels (global p = 0.003). Using an in vitro dual luciferase reporter assay, we found that rs6583817 increases reporter gene expression in Be(2)-C (p = 0.006) and HepG2 (p = 0.02) cell lines. Furthermore, using data from a recent genome-wide association study of two Croatian isolated populations (n = 1,879), we identified a proxy for rs6583817 that associated significantly with decreased plasma Abeta40 levels (ss = -0.124, p = 0.011) and total measured plasma Abeta levels (b = -0.130, p = 0.009). Finally, rs6583817 was associated with decreased risk of LOAD in 3,891 AD cases and 3,605 controls. (OR = 0.87, p = 0.03), and the eleven IDE haplotypes (global p = 0.02) also showed significant association. Thus, a previously unreported variant unequivocally associated with increased IDE expression was also associated with reduced plasma Abeta40 and decreased LOAD susceptibility. Genetic association between LOAD and IDE has been difficult to replicate. Our findings suggest that targeted testing of expression SNPs (eSNPs) strongly associated with altered transcript levels in autopsy brain samples may be a powerful way to identify genetic associations with LOAD that would otherwise be difficult to detect.
    PLoS ONE 01/2010; 5(1):e8764. DOI:10.1371/journal.pone.0008764 · 3.23 Impact Factor
  • The Clinical Neuropsychologist 01/2010; 24(4):601-601. · 1.72 Impact Factor
  • Alzheimer's and Dementia 07/2009; 5(4). DOI:10.1016/j.jalz.2009.04.1097 · 12.41 Impact Factor
  • Alzheimer's and Dementia 07/2009; 5(4). DOI:10.1016/j.jalz.2009.04.796 · 12.41 Impact Factor
  • Alzheimers & Dementia - ALZHEIMERS DEMENT; 07/2009

Publication Stats

9k Citations
890.48 Total Impact Points


  • 2006–2015
    • Mayo Clinic
      Jacksonville, Florida, United States
  • 1999–2014
    • Mayo Foundation for Medical Education and Research
      • Department of Neuroscience
      Rochester, Michigan, United States
    • Cardiff University
      • School of Psychology
      Cardiff, Wales, United Kingdom
  • 2013
    • The Rockefeller University
      New York, New York, United States
  • 2007
    • Johns Hopkins University
      Baltimore, Maryland, United States
  • 2004
    • Mayo Clinic - Rochester
      Рочестер, Minnesota, United States
  • 2002
    • University of Minnesota Duluth
      Duluth, Minnesota, United States
  • 1985–1995
    • Case Western Reserve University
      • Institute of Pathology
      Cleveland, Ohio, United States
  • 1982–1994
    • Case Western Reserve University School of Medicine
      • Department of Pharmacology
      Cleveland, Ohio, United States
  • 1989
    • Brigham and Women's Hospital
      • Center for Neurologic Diseases
      Boston, Massachusetts, United States
  • 1978–1979
    • University of Cincinnati
      • Department of Pharmacology and Cell Biophysics
      Cincinnati, Ohio, United States