Accessible biomarkers monitoring synaptic dysfunction and loss would be of great clinical use in AD for early diagnosis, prediction and monitoring of cognitive decline, and for drug evaluation. In this study, we report that plasma synaptic marker NRG1 i/ was increased in AD patients already at MCI stage ii/ had a promising AUC to discriminate AD patients both at MCI and dementia stage, from NC, iii/was associated with CSF AD biomarkers in Aβ-positive individuals and iv/ correlated with CSF synaptic markers and v/ was inversely correlated with cognition.
NRG1 is expressed at the synapse in multiple brain regions, including those preferentially affected in AD, as hippocampus and entorhinal cortex(32–34).Post-mortem studies have reported NRG1 accumulation in neuritic plaques in association with dystrophic neurites, activated astrocytes, and microglia in human AD brains(21,22). NRG1 and ErbB4 directed immunoreactivity was observed in the hippocampus located in neuronal cell bodies and dendrites (22). Interestingly, NRG1 can be measured in human fluids(17,23,24,30,31,35,36). Increased levels of CSF NRG1 in AD compared with controls and with non-AD related cognitive decline, have been reported in the literature, including our prior work (23,24). In Pankonin et al., CSF NRG1 was increased in AD patients from an early stage of the disease. More recently, in a larger cohort using the most recent AD diagnosis criteria including CSF biomarkers, we have confirmed those results(24). CSF NRG1 was significantly associated with CSF AD core biomarkers, suggesting a possible implicationin AD pathophysiological processes. Moreover,CSF NRG1 levels correlated with other CSF synaptic markers, also suggesting that NRG1 was mainly originating from the synapse.
A previous study has already reported increased levels of plasma NRG1 in AD patients, with higher levels in advanced disease(35). However, in this work, AD was clinically diagnosedwith no biomarker to confirm underlying AD pathophysiological process andcorrelation with CSF NRG1 levels was not studied.Our study brings evidence that plasma NRG1 is increased in patients with confirmed underlying amyloid pathology, already at MCI stage.It is interesting to noteas APP, at the origin of Aβ, and NRG1 are both cleaved by BACE1 in the brain(18,20).
Plasma NRG1 levels were significantly correlated with CSF levels in the whole cohort and this association was sustained in the Aβ-positive patients group.The existence of extra cerebral expression of NRG1 is known but the significant correlation between plasma and CSF levels indicates that plasma level modifications substantially arise from the central nervous system (37). Thus, this flags plasma NRG1 levels as a potential surrogate for brain NRG1 modifications in AD.Correlation to CSF synaptic markers GAP-43, neurogranin and SNAP-25 in the Aβ-positive patients tends to indicate that detected NRG1 changes are related to synaptic modifications.
There was a significant association between plasma NRG1 levelsand MMSE in our whole cohort as well as in the Aβ-positive patients.This finding is in agreement with the previous studies in plasma and CSF again showing that NRG1 levels associate with cognition already at early stages of the disease (23,24,35).
Plasma NRG1 also displayed increased levels in non-AD dementia compared to NC and its accuracy in identifying AD at dementia stage was moderate. In a study on vascular dementia, plasma NRG1 levels were found to be increased and inversely correlated to cognitive severity(38).Neuropathological studies and synaptic CSF biomarkers results have highlighted the fact that synapse dysfunction is a prominent feature in AD but that it is not entirely specific to it(39,40). It can also be observed in non-AD dementia, although to a much lesser extentthan in AD, a finding in line with our results (41).
An underlying mechanistic question to this marker iswhether alterations in NRG1 levels are related to a general process of synaptic degeneration and clearance or whether these changes occur as a response, positive or negative, to the development of AD pathology, or to an increase in synaptic synthesis and release.
NRG1-ErbB4 signaling is important in regulating synaptic function at both excitatory and inhibitory synapses (42). While loss of NRG1 signaling has been shown to be pejorative to synaptic transmission, excessive NRG1 activity is also associated with synaptic dysfunction resulting from alterationof LTP at glutamatergic synapses(34,43). NRG1 has also been identified as a major susceptibility gene in schizophrenia(Stefansson et al. 2002; Mei and Xiong 2008). Mutant NRG1 mice display both excitatory and inhibitory synaptic impairment and schizophrenia-like behavioral disorder(46).NRG1 function must be precisely balanced to maintain normal glutamatergic receptor functions at synapses and excitatory-inhibitory neurotransmission.In line with those findings, evidence suggests that NRG1 increase may specifically influence cognitive function and neuropathology in AD(47–49). Although not formally established,the mechanism of the increase of NRG1 could beexplained by the increased levels and activity of BACE1 observed in AD (47). Yet, its beneficial or detrimental effect is not solved. In experimental works, NRG1 overexpression could rescue APP induced toxicity in primary cortical neurons(48). In an AD mouse model, NRG1 treatment prevented Aβ-induced impairment of long-term potentiation in hippocampal slices via its receptor ErbB4(50).Conversely, other experimental works have suggested a negative effect of the NRG1-ErbB4 signaling in AD. Perfusion of NRG1 in hippocampus decreased LTP in AD mice model as well as in control mice (51). Further understandings of NRG1 responseupon amyloid pathology will allow to specify the exact synaptic eventsassociated with CSF and plasma NRG1 modifications observed in AD patients.
In addition to contributing to better understanding of AD mechanisms, our finding that plasma NRG1 levels could reflect synaptic impairment in AD may have major practical utility. Development of blood biomarkers measuring Aβ, tau and neurodegeneration processes has known great advancement recently, but, to date, there is no validated blood biomarkers reflecting synaptic pathology (14). Recent studies have reported that the measure of markers of AD hallmarksin plasma such as Aβ42, p-tau 181, p-tau217 and p-231can identify and monitor AD brain pathology with high accuracy, demonstratingthat they can be used as non-invasive toolsin AD diagnosis(52–54).As synaptic impairment is one of the earliestabnormal features in AD,already present at the preclinical phase, an accessible non-invasive synaptic marker would be of high added value for early diagnosis(12).
Moreover, synaptic markers hold important promise for monitoringthe effects of disease-modifying treatments on synaptic degeneration. Compared with CSF markers, validated blood-based synaptic AD biomarkers would provide a fast, acceptable and cost-effective method of early detection, diagnosis and follow-up as well as a screening and follow-up tool in therapeutic trials.Our work shows that plasma NRG1 levels could be one of these potential biomarkers.
This study has several limitations. The correlation between plasma and CSF NRG1 remained moderate. Further studies will be needed to understand if this variability is related to the blood-brain barrier’s passage, NRG1 metabolism in plasma, matrix effects or interaction with peripheral NRG1. In our cohort, cognition was evaluated using MMSE, a general test. Study of plasma NRG1 relation to cognition using neuropsychological assessment with testsevaluating specifically episodic memory should give more robust evidence. We could not include measurements of AD‐specific blood biomarkers such as p-tau or Aβ. Finally,confirmation of our results in larger cohorts is needed, including larger samples of non-AD dementia patients. Study of plasma NRG1 at preclinical phase will also be needed to better characterize its kinetic on the whole AD spectrum.