Background and objectives

Synaptic dysfunction and degeneration is a predominant feature of brain aging and synaptic preservation buffers against Alzheimer's disease (AD) protein-related brain atrophy. We tested whether cerebrospinal fluid (CSF) synaptic protein concentrations similarly moderate the effects of axonal injury, indexed via CSF neurofilament light [NfL], on brain atrophy in clinically normal adults.

Methods

Clinically normal older adults enrolled in the observational Hillblom Aging Network study at the UCSF Memory and Aging Center completed baseline lumbar puncture and longitudinal brain MRI (Mean scan [follow-up]=2.6 [3.7 years]). CSF was assayed for synaptic proteins (synaptotagmin-1, synaptosomal-associated protein 2 [SNAP-25], neurogranin, growth associated protein 43 [GAP-43]), axonal injury (NfL), and core AD biomarkers (ptau₁₈₁/Aβ₄₂ ratio_; reflecting AD proteinopathy). Ten bilateral temporo-parietal gray matter ROIs shown to be sensitive to clinical AD were summed to generate a composite temporo-parietal ROI. Linear mixed-effects models tested statistical moderation of baseline synaptic proteins on baseline NfL-related temporo-parietal trajectories, controlling for ptau181/Aβ42 ratios.

Results

Forty-six clinically normal older adults (Mean age=70; 43% female) were included. Synaptic proteins exhibited small to medium correlations with NfL (r range: .10 to .36). Higher baseline NfL, but not ptau181/Aβ42 ratios, predicted steeper temporo-parietal atrophy (NfL x time: β=-0.08, p<.001; ptau181/Aβ42 x time: β=-0.02, p=.31). SNAP-25, neurogranin, and GAP-43 significantly moderated NfL-related atrophy trajectories (-0.07≤βs≥-0.06, ps<.05) such that NfL was associated with temporo-parietal atrophy at high (more abnormal) but not low (more normal) synaptic protein concentrations. At high NfL concentrations, atrophy trajectories were 1.5 to 4.5 times weaker when synaptic protein concentrations were low (β range: -0.21 to -0.07) than high (β range: -0.33 to -0.30).

Conclusions

The association between baseline CSF NfL and longitudinal temporo-parietal atrophy is accelerated by synaptic dysfunction and buffered by synaptic integrity. Beyond AD proteins, concurrent examination of in vivo axonal and synaptic biomarkers may improve detection of neural alterations that precede overt structural changes in AD-sensitive brain regions.