TUBB4B mRNA in CSF and plasma exosomes as a biomarker for tracking Alzheimer's disease progression

Background TUBB4B, critical for neuronal microtubule stability, has been indicated by bioinformatics analyses to exhibit reduced expression in Alzheimer's disease (AD) brain tissue. However, TUBB4B expression level in biofluid exosomes and its diagnostic utility remain unexplored. Methods Bioinformatics analysis, animal models, and clinical validation were employed. Total exosomes were isolated from plasma (n = 138: 52 NC, 36 MCI, 50 AD), with paired CSF samples collected from a subset of 70 participants (30 NC, 20 MCI, 20 AD). TUBB4B mRNA levels were quantified by qPCR. Tubb4b mRNA and protein levels were measured in hippocampal samples from APP/PS1, 5×FAD, and wild-type C57BL/6 mice aged 8 months. Results Analysis of the AlzData database confirmed significant downregulation of TUBB4B expression in brain tissue from AD patients. In CSF exosomes, TUBB4B mRNA was decreased from the MCI stage ( p < 0.001), correlated positively with MMSE scores (r = 0.6, p < 0.001), and distinguished MCI from NC (AUC = 0.877, 95% CI: 0.767– 0.9 86) and AD from NC (AUC = 0.927, 95% CI: 0.858–0.995). In plasma exosomes, TUBB4B mRNA levels also exhibited decreases in MCI and AD ( p < 0.05), and correlated with paired CSF levels (r = 0.61, p < 0.001). In APP/PS1 and 5×FAD mice, hippocampal Tubb4b mRNA and protein were reduced at 8 months, with an age-dependent mRNA decline observed in APP/PS1 and 5×FAD mice aged 2, 5, and 8 months. Conclusion TUBB4B mRNA is consistently downregulated in CSF and plasma exosomes of AD patients from the MCI stage. CSF exosomal TUBB4B mRNA may serve as a candidate biomarker reflecting neuronal microtubule dysfunction across the AD continuum, from MCI to advanced stages, and its concordant changes in plasma exosomes support further development of non-invasive blood-based tests for AD.