First authored by postgraduate student Yuek Ling, we report here our latest findings in dementia neurochemistry. Alzheimer’s disease (AD) is characterized by accumulation of β-amyloid plaques (AP) and neurofibrillary tangles (NFT) in the cortex, together with synaptic loss and amyloid angiopathy. Perturbations in the brain lysosomal system, including the cathepsin family of proteases, have been implicated in AD where they may be involved in proteolytic clearance of misfolded and abnormally aggregated peptides. However, the status of cathepsin D (catD) is unclear in Lewy body dementia, the second most common form of neurodegenerative dementia after AD, and characterized by Lewy bodies (LB) containing aggregated α-synuclein. Furthermore, earlier reports of catD changes in AD have not been entirely consistent. We measured CatD immunoreactivities in the temporal (Brodmann area BA21) and parietal (BA40) cortices of well characterized AD brains as well as two clinical subtypes of Lewy body dementia, namely Parkinson disease dementia (PDD) and dementia with Lewy bodies (DLB), known to show varying degrees of concomitant AD pathology. Increased catD immunoreactivities in AD were found for both neocortical regions measured, where they also correlated with neuropathological NFT scores and phosphorylated pSer396 tau burden, and appeared to co-localize at least partly to NFT-containing neurons. In contrast, catD was increased only in BA40 in DLB and not at all in PDD, did not correlate with LB scores, and did not appreciably co-localize with α-synuclein inclusions. Our study suggests that catD upregulation may be an adaptive response to AD-related processes leading to neurofibrillary degeneration, but may not be directly associated with formation of α-synuclein inclusions in Lewy body dementia.
Chai YL, Chong JR, Weng J, et al. (This paper).
While hepatocyte growth factor (HGF) is known to exert cell growth, migration and morphogenic effects in various organs, recent studies suggest that HGF may also play a role in synaptic maintenance and cerebrovascular integrity. Although increased levels of HGF have been reported in brain and cerebrospinal fluid (CSF) samples of patients with Alzheimer’s disease (AD), it is unclear whether peripheral HGF may be associated with cerebrovascular disease (CeVD) and dementia. In this study, we examined the association of baseline serum HGF with neuroimaging markers of CeVD in a cohort of pre-dementia (cognitive impaired no dementia, CIND) and AD patients. Serum samples from aged, Non-cognitively impaired (NCI) controls, CIND and AD subjects were measured for HGF levels. CeVD (cortical infarcts, microinfarcts, lacunes, white matter hyperintensities (WMH) and microbleeds) were assessed by magnetic resonance imaging (MRI). After controlling for covariates, higher levels of HGF were associated with both CIND and AD. Among the different CeVD MRI markers in CIND and AD, only small vessel disease, but not large vessel disease markers were associated with higher HGF levels. We conclude that serum HGF may be a useful peripheral biomarker for small vessel disease in subjects with cognitive impairment and AD.
Zhu Y, Hilal S, Chai YL et al. (This paper).
The translocase of the outer membrane (TOM) is a vital mitochondrial transport system facilitating the importation of nuclear encoded proteins into the organelle. While mitochondrial dysfunction, including perturbation of oxidative phosphorylation (OXPHOS) complex, is evident in Alzheimer’s disease (AD), it remains unclear whether the observed OXPHOS deficits may be associated with TOM alterations. In this study, the objective was to correlate TOM subunits with OXPHOS complex proteins in AD. We used postmortem neocortex (BA40) from AD and age-matched controls were processed to obtain mitochondrial enriched homogenates for the measurement of Tom20, Tom22, Tom40, Tom70 as well as components of OXPHOS complex I – V by immunoblotting. We found that Tom20 and Tom70 immunoreactivities were significantly reduced in AD, as were components of OXPHOS complex I and III. Both Tom20 and Tom70 positively correlated with complex III and V, while Tom20 also correlated with complex IV. In conclusion, reductions in certain TOM subunits and their correlations with specific OXPHOS complex proteins suggest that an impaired mitochondrial transportation system may contribute to previously observed oxidative phosphorylation deficits in AD. Follow-up studies are needed to corroborate the present correlative study.
Atamna H, Frey WH, 2nd (2007) Mechanisms of mitochondrial dysfunction and energy deficiency in Alzheimer’s disease. Mitochondrion 7:297-310.
Chai YL, Xing H, Chong JR et al. (This paper).
In collaboration with the proteomics team led by Newman Sze, we reported an iTRAQ-based study on Lewy body dementias, the second most common cause of neurodegenerative dementia in the elderly after Alzheimer’s disease (AD). The two clinical subgroups of Lewy body dementias, namely, dementia with Lewy bodies (DLB) and Parkinson’s disease dementia (PDD), are differentiated by the chronology of cognitive symptoms relative to parkinsonism. At present, there remains a debate on whether DLB and PDD are separate disease entities, or fall within the same spectrum of Lewy body dementias. In this study, we compared the detergent-soluble proteome via an 8-plex isobaric tag for relative and absolute quantitation (iTRAQ) analysis of pooled lysates from the prefrontal cortex (BA9) of DLB (n = 19) and PDD (n = 21) patients matched a priori for amyloid (total Aβ42) burden, semi-quantitative scores for Lewy bodies and neurofibrillary tangles together with age-matched control (n = 21) subjects. A total of 1914 proteins were confidently identified by iTRAQ (false discovery rate = 0%). None of the proteins showed a significant yet opposite regulation in between DLB and PDD when compared to aged controls in the proteomic data set as well as following immunoblot analysis of the pooled and individual lysates involving all 61 subjects. The postsynaptic protein, synaptopodin (SYNPO) was significantly down-regulated in both DLB and PDD subgroups, suggesting a defective synaptic transmission in the demented patients. In conclusion, the largely similar proteome of DLB and PDD matched for amyloid burden suggests that variations in concomitant AD-related pathology, abnormal post-translational modifications or protein-protein interactions, defective intracellular trafficking or misfolding of proteins could play a part in driving the clinically observed differences between these two subgroups of Lewy body dementias. This further indicates that amyloid-targeting therapeutic strategies may show different efficacies in DLB versus PDD.
Datta A, Chai YL, Tan JM, et al. (This paper).
In this study led by Michelle Tan, and following hot on the heels of our previous paper showing association of the FynT isoform with astrocyte activation in AD (Lee et al, 2016), we demonstrated selective FynT induction in murine cortex and primary astrocyte culture after prolonged exposure to inflammatory stimulants, suggesting that FynT may mediate persistent neuroinflammation. To delineate the functional role of astrocytic FynT in association with TNF-mediated inflammatory responses, immortalized normal human astrocytes (iNHA) stably expressing FynT kinase constitutively active (FynT-CA) or kinase dead (FynT-KD) mutants were treated with TNF and compared for inflammatory responses using high-throughput real-time RT-PCR and Luminex multi-analyte immunoassays. FynT-CA but not FynT-KD mutant exhibited drastic induction of proinflammatory cytokines and chemokines after prolonged exposure to TNF, which could be attenuated by treating with Fyn kinase inhibitor PP2 or silencing via FynT-specific DsiRNA. FynT kinase activity-dependent induction of PKCδ expression, PKCδ phosphorylation, as well as NFκB activation was detected at the late phase but not the early phase of TNF signaling. In conclusion, selective FynT induction by TNF may facilitate persistent inflammatory responses in astrocytes, which is highly relevant to chronic neuroinflammation in neurodegenerative diseases including but not limited to AD.
Lee C, Low CY, Wong SY, et al. (This paper).
Lee C, Low CY, Francis PT, Attems J, Wong PT, Lai MK, Tan MG (2016) An isoform-specific role of FynT tyrosine kinase in Alzheimer’s disease. J Neurochem 136:637-50.
First authored by post-graduate student Julie Zhu, we report here that Serum IL-8 may have clinical utility as a biomarker for white matter hyperintensities (WMH) in AD. This is the latest work in our ongoing efforts to examine serum inflammatory markers in preclinical stages of dementia and in AD, as well as to investigate their associations with concomitant cerebrovascular disease (CeVD), given that CeVD have been implicated in cognitive impairment and may worsen AD severity (Zekry et al., 2002). We performed a cross-sectional case-control study including 96 AD, 140 cognitively impaired no dementia (CIND), and 79 noncognitively impaired participants. All subjects underwent neuropsychological and neuroimaging assessments, as well as collection of blood samples for measurements of serum samples interleukin (IL)-6, IL-8, and tumor necrosis factor α levels. Subjects were classified as CIND or dementia based on clinical criteria. Significant CeVD, including white-matter hyperintensities (WMHs), lacunes, and cortical infarcts, was assessed by magnetic resonance imaging. After controlling for covariates, higher concentrations of IL-8, but not the other measured cytokines, were associated with both CIND and AD only in the presence of significant CeVD (CIND with CeVD: odds ratios [ORs] 4.53; 95% confidence interval [CI] 1.5-13.4 and AD with CeVD: OR 7.26; 95% CI 1.2-43.3). Subsequent multivariate analyses showed that among the types of CeVD assessed, only WMH was associated with higher IL-8 levels in CIND and AD (WMH: OR 2.81; 95% CI 1.4-5.6). Longitudinal follow-up studies would help validate these findings.
Zhu Y, Chai YL, Hilal S, et al. (This paper).
Zekry D, Duyckaerts C, Moulias R, et al. Degenerative and vascular lesions of the brain have synergistic effects in dementia of the elderly. Acta Neuropathol 103:481-7.
Collapsin response mediator protein-2 (CRMP2) regulates axonal growth cone extension, and increased CRMP2 phosphorylation may lead to axonal degeneration. Axonal and synaptic pathology is an important feature of Lewy body dementias (LBD), but the state of CRMP2 phosphorylation (pCRMP2) as well as its correlations with markers of neurodegeneration have not been studied in these dementias. Hence, we measured CRMP2 phosphorylation at Thr509, Thr514 and Ser522, as well as markers of β-amyloid (Aβ), tau-phosphorylation, α-synuclein and synaptic function in the postmortem neocortex of a longitudinally assessed cohort of LBD patients characterized by low (Parkinson’s disease dementia, PDD) and high (dementia with Lewy bodies, DLB) burden of Alzheimer type pathology. We found specific increases of pCRMP2 at Thr514 in DLB, but not PDD. The increased CRMP2 phosphorylation correlated with fibrillogenic Aβ as well as with losses of markers for axon regeneration (β-III-tubulin) and synaptic integrity (synaptophysin) in LBD. In contrast, pCRMP2 alterations did not correlate with tau-phosphorylation or α-synuclein, and also appear unrelated to immunoreactivities of putative upstream kinases glycogen synthase kinase 3β and cyclin-dependent kinase 5, as well as to protein phosphatase 2A. In conclusion, increased pCRMP2 may underlie the axonal pathology of DLB, and may be a novel therapeutic target. However, antecedent signaling events as well as the nature of pCRMP2 association with Aβ and other neuropathologic markers require further study.
Xing H, Lim YA, Chong JR, Lee JH, Aarsland D, Ballard CG, Francis PT, Chen CP, Lai MK (This paper).
Vascular pathology plays an important role in the development of cognitive decline and dementia. In this context, growth differentiation factor-15 (GDF-15) has been suggested to be a biomarker due to its regulatory roles in inflammatory and trophic responses during tissue injury. However, limited data exist on the associations of GDF-15 with either cerebrovascular disease (CeVD) burden or the spectrum of cognitive impairment. Therefore, we aimed to study peripheral levels of GDF-15 incognitive impairment no dementia (CIND) or Alzheimer disease (AD) subjects assessed for CeVD using a case–control cohort design, with cases recruited from memory clinics and controls from memory clinics and the community. All subjects underwent detailed neuropsychological assessment, 3-Tesla magnetic resonance imaging, and venous blood draw. Subjects were classified as CIND or AD based on clinical criteria, while significant CeVD was defined as the presence of cortical infarcts and/or 2 lacunes or more, and/or confluent white matter hyperintensities (WMHs) in 2 or more brain regions. A total of 324 subjects were included in the study, of whom 80 had no cognitive impairment, 144 CIND and 100with AD. Higher GDF-15 levels were significantly associated with disease groups, especially in the presence of CeVD, namely, CIND with CeVD (odds ratios [OR]: 7.21; 95% confidence interval [CI]: 2.14–24.27) and AD with CeVD (OR: 21.87; 95% CI: 2.01–237.43). Among the different CeVD markers, only WMH was associated with higher GDF-15 levels (OR: 3.97; 95% CI: 1.79–8.83). The associations between GDF-15 and cognitive impairment as well as with WMH remained significant after excluding subjects with cardiovascular diseases. In conclusion, we showed that increased GDF-15 may be a biomarker for CIND and AD in subjects with WMH.
Chai YL, Hilal S, Chong JP, Ng YX, Liew OW, Xu X, Ikram MK, Venketasubramanian N, Richards AM, Lai MK, Chen CP (This paper).
White matter lesions (WML) are thought to contribute to vascular cognitive impairment in elderly patients. Growing evidence show that failure of myelin formation arising from the disruption of oligodendrocyte progenitor cell (OPC) differentiation is a cause of chronic vascular white matter damage. The sphingosine kinase (SphK)/sphingosine-1-phosphate (S1P) signaling pathway regulates oligodendroglia differentiation and function, and is known to be altered in hypoxia. In this study, we measured SphK, S1P as well as markers of WML, hypoxia and OPC (NG2) in a mouse bilateral carotid artery stenosis (BCAS) model of chronic cerebral hypoperfusion. Our results indicated that BCAS induced hypoxia inducible factor (HIF)-1α, Sphk2, S1P, and NG2 up-regulation together with accumulation of WML. In contrast, BCAS mice treated with the SphK inhibitor, SKI-II, showed partial reversal of SphK2, S1P and NG2 elevation and amelioration of WML. In an in vitro model of hypoxia, SKI-II reversed the suppression of OPC differentiation. Our study suggests a mechanism for hypoperfusion-associated WML involving HIF-1α-SphK2-S1P-mediated disruption of OPC differentiation, and proposes the SphK signaling pathway as a potential therapeutic target for white matter disease.
Yang Y, Torta F, Arai K, Wenk MR, Herr DR, Wong PT, Lai MK (This paper).