From neurodegeneration to neurohomeostasis: The role of ubiquitin

The ability of synapses to undergo changes in structure and function in response to alterations of neuronal activity is an essential property of neural circuits. One way that this is achieved is through global changes in the molecular composition of the synapse; however, it is not clear how these changes are coupled to the dynamics of neuronal activity. Here we found that, in cultured rat cortical neurons, bidirectional changes of neuronal activity led to corresponding alterations in the expression of brain-derived neurotrophic factor (BDNF) and phosphorylation of its receptor tropomyosin-related kinase B (TrkB), as well as in the level of synaptic proteins. Exogenous BDNF reversed changes in synaptic proteins induced by chronic activity blockade, while inhibiting Trk kinase activity or depleting endogenous BDNF abolished the concentration changes induced by chronic activity elevation. Both tetrodotoxin and bicuculline had significant, but opposite, effects on synaptic protein ubiquitination in a time-dependent manner. Furthermore, exogenous BDNF was sufficient to increase ubiquitination of synaptic proteins, whereas scavenging endogenous BDNF or inhibiting Trk kinase activity prevented the ubiquitination of synaptic proteins induced by chronic elevation of neuronal activity. Inhibiting the proteasome or blocking protein polyubiquitination mimicked the effect of tetrodotoxin on the levels of synaptic proteins and canceled the effects of BDNF. Our study indicates that BDNF-TrkB signaling acts upstream of the ubiquitin proteasome system, linking neuronal activity to protein turnover at the synapse.Synaptic remodeling is critical for brain development and neural plasticity (1-3). During neural circuit maturation and in response to alterations in neuronal activity, synapses undergo remarkable cytoarchitectural and functional changes (1, 4 -6). Such plasticity is associated with changes to the molecular composition of synapses (1). In dissociated neuron cultures, for example, chronic increases or reductions in neuronal activity cause dramatic changes in the expression of a large number of proteins at the synapse. These changes are at least in part determined by the rate of protein degradation by the ubiquitin proteasome system (UPS) 2 (4). It remains to be determined, however, how modulation of neural activity translates into changes in the rate of protein degradation.Previous studies have shown that the expression of brainderived neurotrophic factor (BDNF) in cortical neurons and its subsequent secretion are regulated by neuronal activity in vivo and in vitro (7-9). As BDNF is known to exert many modulatory actions on neuronal and synaptic functions (10 -13), we hypothesized that BDNF signaling serves to link neuronal activity to proteasome-dependent synapse remodeling. In this study, we show that BDNF expression and TrkB activation in cultures of cortical neurons are modulated by chronic alteration of activity with tetrodotoxin (TTX) or bicuculline. Application of exogenous BDNF prevents TTX-induced changes in syna...

Section: Discussionmentioning

confidence: 76%

Section: Discussionmentioning

confidence: 99%

Brain-derived Neurotrophic Factor-Tropomyosin-related Kinase B Signaling Contributes to Activity-dependent Changes in Synaptic Proteins

Jia

Chen

Zhou

et al. 2008

“…Ub conjugates are found associated with intracellular deposits of aggregated protein in neurons in several amyloidoses and other aggregation-associated neurodegenerative diseases such as Alzheimer's, Parkinson's, and prion diseases (2,3). This, as well as a growing body of new evidence, suggests a linkage between Ub-proteasome system malfunction and amyloid-associated pathogenesis (4).…”

mentioning

confidence: 99%

Pleiotropic Effects of Ubp6 Loss on Drug Sensitivities and Yeast Prion Are Due to Depletion of the Free Ubiquitin Pool

Chernova

Allen

Wesoloski

et al. 2003

106

151

Mutation of the mouseSecond, fluorescence microscopy analysis shows that Ubp6-GFP fusion protein is localized to the nucleus of yeast cell, as are most proteasomes. Third, the Nterminal Ub-like domain, although it is not required for nuclear localization of Ubp6, targets Ubp6 to the proteasome and cannot be functionally replaced by Ub. The human ortholog of Ubp6, USP14, probably plays a similar role in higher eukaryotes, since it fully compensates for ubp6⌬ defects and binds to the yeast proteasome. These data link the Ub system to prion expression and propagation and have broad implications for other neuronal inclusion body diseases.

“…Despite their heterogeneity, they have similar features, including abnormal deposition of ubiquitinated protein aggregates in inclusion bodies within neurons in the respective affected areas of the CNS (reviewed in Ref. 1). The ubiquitinated protein aggregates are thought to result from dysfunction of the ubiquitin/proteasome pathway (UPP) 2 or from structural changes in the protein substrates that prevent their degradation by the UPP (reviewed in Ref.…”

mentioning

confidence: 99%

Dynamics of the Degradation of Ubiquitinated Proteins by Proteasomes and Autophagy

Myeku

Figueiredo‐Pereira

2011

184

128

Proteotoxicity resulting from accumulation of damaged/unwanted proteins contributes prominently to cellular aging and neurodegeneration. Proteasomal removal of these proteins upon covalent polyubiquitination is highly regulated. Recent reports proposed a role for autophagy in clearance of diffuse ubiquitinated proteins delivered by p62/SQSTM1. Here, we compared the turnover dynamics of endogenous ubiquitinated proteins by proteasomes and autophagy by assessing the effect of their inhibitors. Autophagy inhibitors bafilomycin A1, ammonium chloride, and 3-methyladenine failed to increase ubiquitinated protein levels. The proteasome inhibitor epoxomicin raised ubiquitinated protein levels at least 3-fold higher than the lysosomotropic agent chloroquine. These trends were observed in SK-N-SH cells under serum or serum-free conditions and in WT or Atg5 ؊/؊ mouse embryonic fibroblasts (MEFs). Notably, chloroquine considerably inhibited proteasomes in SK-N-SH cells and MEFs. In these cells, elevation of p62/SQSTM1 was greater upon proteasome inhibition than with all autophagy inhibitors tested and was reduced in Atg5 ؊/؊ MEFs. With epoxomicin, soluble p62/SQSTM1 associated with proteasomes and p62/SQSTM1 aggregates contained inactive proteasomes, ubiquitinated proteins, and autophagosomes. Prolonged autophagy inhibition (96 h) failed to elevate ubiquitinated proteins in rat cortical neurons, although epoxomicin did. Moreover, prolonged autophagy inhibition in cortical neurons markedly increased p62/SQSTM1, supporting its degradation mainly by autophagy and not by proteasomes. In conclusion, we clearly demonstrate that pharmacologic or genetic inhibition of autophagy fails to elevate ubiquitinated proteins unless the proteasome is affected. We also provide strong evidence that p62/SQSTM1 associates with proteasomes and that autophagy degrades p62/SQSTM1. Overall, the function of p62/ SQSTM1 in the proteasomal pathway and autophagy requires further elucidation.