Cyclin-Dependent Kinase 5–Dependent BAG3 Degradation Modulates Synaptic Protein Turnover

Abstract

BACKGROUND: Synaptic protein dyshomeostasis and functional loss is an early invariant feature of Alzheimer's disease (AD), yet the unifying etiological pathway remains largely unknown. Knowing that cyclin-dependent kinase 5 (CDK5) plays critical roles in synaptic formation and degeneration, its phosphorylation targets were reexamined in search of candidates with direct global impacts on synaptic protein dynamics, and the associated regulatory network was also analyzed. METHODS: Quantitative phosphoproteomics and bioinformatics analyses were performed to identify top-ranked candidates. A series of biochemical assays was used to investigate the associated regulatory signaling networks. Histological, electrochemical, and behavioral assays were performed in conditional knockout, small hairpin RNAmediated knockdown, and AD-related mice models to evaluate the relevance of CDK5 to synaptic homeostasis and functions. RESULTS: Among candidates with known implications in synaptic modulations, BAG3 ranked the highest. CDK5mediated phosphorylation on S297/S291 (mouse/human) destabilized BAG3. Loss of BAG3 unleashed the selective protein degradative function of the HSP70 machinery. In neurons, this resulted in enhanced degradation of a number of glutamatergic synaptic proteins. Conditional neuronal knockout of Bag3 in vivo led to impairment of learning and memory functions. In human AD and related mouse models, aberrant CDK5-mediated loss of BAG3 yielded similar effects on synaptic homeostasis. Detrimental effects of BAG3 loss on learning and memory functions were confirmed in these mice, and such effects were reversed by ectopic BAG3 reexpression. CONCLUSIONS: Our results highlight that the neuronal CDK5-BAG3-HSP70 signaling axis plays a critical role in modulating synaptic homeostasis. Dysregulation of the signaling pathway directly contributes to synaptic dysfunction and AD pathogenesis.

BAG3 regulation of Rab35 mediates the Endosomal Sorting Complexes Required for Transport /endolysosome pathway and tau clearance

“…We noticed that the overexpression of BAG3 also increased the density of dendrites and postsynaptic compartments in the P301S mice. This finding echoes the results from a recent study indicating that exogenous expression of BAG3 in the 5XFAD mouse model increased the expression of synaptic proteins [82].…”

“…BAG3 is increased with aging [71]. However, recent data suggest that BAG3 expression is decreased in human AD brains compared with the aged control [82]. A previous study in our lab showed that depletion of BAG3 in rat cortical neurons increased the levels of endogenous phosphorylated tau [51].…”

“…BAG3 is essential for maintaining proteostasis and increases in neurons during the aging process [71], and a recent report indicated that BAG3 levels are likely decreased in AD brains compared to aged matched controls [82]. Further, previous studies suggest that overexpression of BAG3 in rat cortical neuron promotes tau clearance [51].…”

“…Bcl-2-associated anthogene 3 (BAG3) is a stress-induced, multi-domain protein that plays a critical role in maintaining proteostasis, and thus neuronal health [5,69,82,12]. As a stress-induced protein, BAG3 protein levels increase during ageing [71].…”

“…As a stress-induced protein, BAG3 protein levels increase during ageing [71]. Interestingly, BAG3 is expressed at higher levels in neurons that are resistant to tau pathology compared to those that are susceptible [23], and recent data suggest that BAG3 levels are lower in AD brain compared to aged-matched controls [82]. BAG3 has been shown to promote the clearance of tau [51] and other diseaserelevant aggregation prone proteins such as α-synuclein [8], mutant SOD1, and mutant huntingtin [69].…”