Phosphorylation of Rpt6 regulates synaptic strength in hippocampal neurons.

It has become increasingly evident that protein degradation via the ubiquitin proteasome system plays a fundamental role in the development, maintenance and remodeling of synaptic connections in the CNS. We and others have recently described the activity-dependent regulation of proteasome activity and recruitment of proteasomes into spine compartments involving the phosphorylation of the 19S ATPase subunit, Rpt6, by the plasticity kinase Ca(2+)/calmodulin-dependent protein kinase II α (CaMKIIα) (Bingol and Schuman, 2006; Djakovic et al., 2009; Bingol et al, 2010). Here, we investigated the role of Rpt6 phosphorylation on proteasome function and synaptic strength. Utilizing a phospho-specific antibody we verified that Rpt6 is phosphorylated at Serine 120 (S120) by CaMKIIα. In addition, we found that Rpt6 is phosphorylated by CaMKIIα in an activity-dependent manner. Furthermore, we showed that a serine 120 to aspartic acid phospho-mimetic mutant of Rpt6 (S120D) increases its resistance to detergent extraction in rat hippocampal dendrites, indicating phosphorylated Rpt6 may promote the tethering of proteasomes to scaffolds and cytoskeletal components. Expression of Rpt6 S120D decreased miniature EPSC (mEPSC) amplitude, while expression of a phospho-dead mutant (S120A) increased mEPSC amplitude. Surprisingly, homeostatic scaling of mEPSC amplitude produced by chronic application of bicuculline or tetrodotoxin is both mimicked and occluded by altered Rpt6 phosphorylation. Together, these data suggest that CaMKII-dependent phosphorylation of Rpt6 at S120 may be an important regulatory mechanism for proteasome-dependent control of synaptic remodeling in slow homeostatic plasticity.

Mice lacking doublecortin and doublecortin-like kinase 2 display altered hippocampal neuronal maturation and spontaneous seizures.

Mutations in doublecortin (DCX) are associated with intractable epilepsy in humans, due to a severe disorganization of the neocortex and hippocampus known as classical lissencephaly. However, the basis of the epilepsy in lissencephaly remains unclear. To address potential functional redundancy with murin Dcx, we targeted one of the closest homologues, doublecortin-like kinase 2 (Dclk2). Here, we report that Dcx; Dclk2-null mice display frequent spontaneous seizures that originate in the hippocampus, with most animals dying in the first few months of life. Elevated hippocampal expression of c-fos and loss of somatostatin-positive interneurons were identified, both known to correlate with epilepsy. Dcx and Dclk2 are coexpressed in developing hippocampus, and, in their absence, there is dosage-dependent disrupted hippocampal lamination associated with a cell-autonomous simplification of pyramidal dendritic arborizations leading to reduced inhibitory synaptic tone. These data suggest that hippocampal dysmaturation and insufficient receptive field for inhibitory input may underlie the epilepsy in lissencephaly, and suggest potential therapeutic strategies for controlling epilepsy in these patients.

Regulation of synaptic structure by ubiquitin C-terminal hydrolase L1.

Ubiquitin C-terminal hydrolase L1 (UCH-L1) is a deubiquitinating enzyme that is selectively and abundantly expressed in the brain, and its activity is required for normal synaptic function. Here, we show that UCH-L1 functions in maintaining normal synaptic structure in hippocampal neurons. We found that UCH-L1 activity is rapidly upregulated by NMDA receptor activation, which leads to an increase in the levels of free monomeric ubiquitin. Conversely, pharmacological inhibition of UCH-L1 significantly reduces monomeric ubiquitin levels and causes dramatic alterations in synaptic protein distribution and spine morphology. Inhibition of UCH-L1 activity increases spine size while decreasing spine density. Furthermore, there is a concomitant increase in the size of presynaptic and postsynaptic protein clusters. Interestingly, however, ectopic expression of ubiquitin restores normal synaptic structure in UCH-L1-inhibited neurons. These findings point to a significant role of UCH-L1 in synaptic remodeling, most likely by modulating free monomeric ubiquitin levels in an activity-dependent manner.

Regulation of the proteasome by neuronal activity and calcium/calmodulin-dependent protein kinase II.

Protein degradation via the ubiquitin proteasome system has been shown to regulate changes in synaptic strength that underlie multiple forms of synaptic plasticity. It is plausible, therefore, that the ubiquitin proteasome system is itself regulated by synaptic activity. By utilizing live-cell imaging strategies we report the rapid and dynamic regulation of the proteasome in hippocampal neurons by synaptic activity. We find that the blockade of action potentials (APs) with tetrodotoxin inhibited the activity of the proteasome, whereas the up-regulation of APs with bicuculline dramatically increased the activity of the proteasome. In addition, the regulation of the proteasome is dependent upon external calcium entry in part through N-methyl-D-aspartate receptors and L-type voltage-gated calcium channels and requires the activity of calcium/calmodulin-dependent protein kinase II (CaMKII). Using in vitro and in vivo assays we find that CaMKII stimulates proteasome activity and directly phosphorylates Rpt6, a subunit of the 19 S (PA700) subcomplex of the 26 S proteasome. Our data provide a novel mechanism whereby CaMKII may regulate the proteasome in neurons to facilitate remodeling of synaptic connections through protein degradation.

Let-7 coordinately suppresses components of the amino acid sensing pathway to repress mTORC1 and induce autophagy.

Macroautophagy (hereafter autophagy) is the major pathway by which macromolecules and organelles are degraded. Autophagy is regulated by the mTOR signaling pathway-the focal point for integration of metabolic information, with mTORC1 playing a central role in balancing biosynthesis and catabolism. Of the various inputs to mTORC1, the amino acid sensing pathway is among the most potent. Based upon transcriptome analysis of neurons subjected to nutrient deprivation, we identified let-7 microRNA as capable of promoting neuronal autophagy. We found that let-7 activates autophagy by coordinately downregulating the amino acid sensing pathway to prevent mTORC1 activation. Let-7 induced autophagy in the brain to eliminate protein aggregates, establishing its physiological relevance for in vivo autophagy modulation. Moreover, peripheral delivery of let-7 anti-miR repressed autophagy in muscle and white fat, suggesting that let-7 autophagy regulation extends beyond CNS. Hence, let-7 plays a central role in nutrient homeostasis and proteostasis regulation in higher organisms.