We investigated whether plasma clusterin concentration predicts subsequent rates of change in brain volumes in MCI and control individuals. To test whether peripheral clusterin concentration was related to progression of early pathological changes in incipient stages of the disease, we first determined whether associations between plasma clusterin concentration at baseline and longitudinal changes in brain volumes differed between MCI and cognitively normal participants from the neuroimaging substudy of the BLSA. We then further defined these group differences in associations by investigating associations between plasma clusterin concentration and rates of change in brain volumes within each group. Finally, we examined whether there was an association between peripheral concentration of clusterin and that in brain tissue samples in regions vulnerable to AD pathology.
We observed widespread and significant differences in the association of plasma clusterin concentration with rates of brain atrophy between cognitively normal and MCI groups for the same cohort of BLSA participants in whom we recently reported accelerated tissue loss in MCI compared to normal individuals (Driscoll et al., 2009
Significant differences in associations occurred for several global, lobar and regional brain volumes. In most of these regions, associations between baseline clusterin concentration and subsequent rates of brain atrophy reached significance within the MCI (but not control) group. The direction of this effect suggests that higher concentrations of plasma clusterin are associated with lower rates of brain atrophy in the MCI individuals. In addition to the associations with global rates of atrophy (whole brain and vCSF), higher plasma clusterin concentration was related to lower rate of atrophy in temporal gray matter in the MCI group. Other regional brain volumes showing similar associations in MCI included those vulnerable to pathological changes in AD, including superior temporal, parahippocampal and cingulate gyri ().
By showing that peripheral concentration of clusterin is associated with changes in brain volumes over time in individuals with MCI, our current observations provide further evidence linking clusterin with pathological processes intrinsic to AD. While we have not addressed the precise mechanisms underlying this association, several lines of evidence suggest biological roles for clusterin in pathways relevant to AD pathogenesis, including amyloid clearance, complement modulation and apoptosis (DeMattos et al., 2004
; DeMattos et al., 2002
; French et al., 1994
; Nuutinen et al., 2009
; Tschopp and French, 1994
). Our current results also suggest that clusterin is elevated in the process of AD, perhaps as a protective mechanism. The fact that higher clusterin levels predict slower rates of atrophy in MCI may reflect its role in enhancing amyloid clearance and/or modulating neuroinflammation during disease progression, such as in individuals with MCI. Consistent with this hypothesis, a previous post-mortem study suggested a neuroprotective role for clusterin in brain regions vulnerable to AD pathology (Giannakopoulos et al., 1998
). Previous reports on differences in clusterin concentration in the CSF between AD and controls have been inconclusive (Lidstrom et al., 2001
; Sihlbom et al., 2008
). More recently, Schrijvers and colleagues reported a significant association between higher plasma clusterin concentration and disease severity in AD as well as an increase relative to healthy controls and suggest that increased expression of clusterin in AD reflects a neuroprotective response.
These findings extend our recent observations on the association between plasma clusterin concentration and endophenotypes of disease pathology in patients with established AD (Thambisetty et al., 2010
). In this previous cross-sectional investigation, we found that higher concentrations of plasma clusterin were associated with greater atrophy of the entorhinal cortex in AD. In the light of these previous results, we predicted that we would observe a similar direction of effect in this longitudinal analysis of MCI and control subjects. However, contrary to our expectation, we found a significant association between increased plasma clusterin concentration and decreased rate of brain atrophy in MCI subjects. One plausible explanation might be that in individuals with MCI, elevated plasma clusterin is associated with decreased rate of atrophy as would be expected for a protective mechanism. However, in individuals with established AD, the association of plasma clusterin with pathology may reflect the eventual failure of such protective mechanisms in the setting of genetic risk factors and/or adverse environmental influences. This model would predict that in individuals with established AD, increased plasma clusterin concentration and its association with greater extent of brain atrophy might be indicative of a more ‘aggressive’ disease course. Indeed, in our previous cross-sectional study, we observed that higher baseline plasma clusterin concentration in patients with established AD was associated with a faster rate of clinical progression (Thambisetty et al., 2010
). While this hypothesis is in line with experimental evidence suggesting a protective effect of clusterin in pathological processes in AD (Calero et al., 2000
), we acknowledge that alternative models to explain our observations are possible. The potential role of clusterin in pathological processes distinct from brain atrophy was highlighted by a recent study where the Alzheimer risk variant of the clusterin gene (CLU) was associated with lower white matter integrity in young healthy adults (Braskie et al., 2011
). Future studies combining analyses of genetic variation in the CLU gene with clusterin protein concentration in relation to pathological processes in at-risk individuals are likely to be revealing of the precise role of clusterin in Alzheimer’s biology.
Finally, in order to test whether plasma clusterin concentration reflects its role in the brain in response to AD pathology, we also asked whether peripheral concentration of clusterin was associated with its concentration in brain regions vulnerable to such pathological changes. By demonstrating a significant association between plasma concentration of clusterin and its concentration within the superior temporal gyrus, a region especially vulnerable to AD pathology (Whitwell et al.
), we suggest that peripheral clusterin levels do reflect its biological role in brain regions undergoing pathological changes in AD. The precise mechanisms underlying this observation however remain to be identified as it is unclear whether changes in plasma clusterin levels are a consequence of early brain pathology in incipient AD or precede these changes and are a systemic peripheral signature in response to physiological stress such as oxidative damage. It is worth noting in this context that a variety of perturbations including changes in temperature, pH, oxidative stress and protease degradation can provoke extracellular chaperone responses from proteins like clusterin (Wyatt et al., 2011
) (Naiki and Nagai, 2009
The strengths of this study include the longitudinal design incorporating data points from a large number (>900 MRI observations) of serial assessments, relatively long follow-up interval and extensive characterization of the study sample. However, some caveats must be considered in the interpretation of our results. Since our principal aim was to undertake an exploratory analysis of the association between plasma clusterin concentration and longitudinal changes in brain volumes in MCI and control individuals, we have chosen to present results from all brain regions examined without correction for multiple comparisons. Furthermore, as the outcome variables in these analyses were measures of changes in brain volumes that are themselves highly correlated with each other, correcting for multiple comparisons in this scenario is especially problematic. While our approach might increase the risk of type-I error, we believe that it now allows for further directed analyses of the relationship between clusterin and AD-related changes in specific brain regions as well as testing a priori hypotheses on the putative role of clusterin as a biological modifier of such changes.
In summary, our results substantially extend recent studies that have found an association between plasma clusterin concentration and measures of disease severity and progression in patients with AD. By demonstrating an association between plasma clusterin concentration and longitudinal changes in regional brain volumes in at-risk older individuals, they add to the mounting evidence linking this multi-functional lipoprotein with pathological mechanisms in Alzheimer’s disease.