Sudarshan C. Upadhya

Proteasome inhibition enhances the induction and impairs the maintenance of late-phase long-term potentiation

Protein degradation by the ubiquitin-proteasome pathway plays important roles in synaptic plasticity, but the molecular mechanisms by which proteolysis regulates synaptic strength are not well understood. We investigated the role of the proteasome in hippocampal late-phase long-term potentiation (L-LTP), a model for enduring synaptic plasticity. We show here that inhibition of the proteasome enhances the induction of L-LTP, but inhibits its maintenance. Proteasome inhibitor-mediated enhancement of the early part of L-LTP requires activation of NMDA receptors and the cAMP-dependent protein kinase. Augmentation of L-LTP induction by proteasome inhibition is blocked by a protein synthesis inhibitor anisomycin and is sensitive to the drug rapamycin. Our findings indicate that proteasome inhibition increases the induction of L-LTP by stabilizing locally translated proteins in dendrites. In addition, our data show that inhibition of the proteasome blocks transcription of brain-derived neurotrophic factor (BDNF), which is a cAMP-responsive element-binding protein (CREB)-inducible gene. Furthermore, our results demonstrate that the proteasome inhibitors block degradation of ATF4, a CREB repressor. Thus, proteasome inhibition appears to hinder CREB-mediated transcription. Our results indicate that blockade of proteasome activity obstructs the maintenance of L-LTP by interfering with transcription as well as translation required to sustain L-LTP. Thus, proteasome-mediated proteolysis has different roles during the induction and the maintenance of L-LTP.

Role of ubiquitin-proteasome-mediated proteolysis in nervous system disease

Proteolysis by the ubiquitin-proteasome pathway (UPP) is now widely recognized as a molecular mechanism controlling myriad normal functions in the nervous system. Also, this pathway is intimately linked to many diseases and disorders of the brain. Among the diseases connected to the UPP are neurodegenerative disorders such as Alzheimers, Parkinsons and Huntingtons diseases. Perturbation in the UPP is also believed to play a causative role in mental disorders such as Angelman syndrome. The pathology of neurodegenerative diseases is characterized by abnormal deposition of insoluble protein aggregates or inclusion bodies within neurons. The ubiquitinated protein aggregates are believed to result from dysfunction of the UPP or from structural changes in the protein substrates which prevent their recognition and degradation by the UPP. An early effect of abnormal UPP in diseases of the nervous system is likely to be impairment of synaptic function. Here we discuss the UPP and its physiological roles in the nervous system and how alterations in the UPP relate to development of nervous system diseases. This article is part of a Special Issue entitled The 26S Proteasome: When degradation is just not enough!

Role of the ubiquitin proteasome system in Alzheimer's disease

Though Alzheimers disease (AD) is a syndrome with well-defined clinical and neuropathological manifestations, an array of molecular defects underlies its pathology. A role for the ubiquitin proteasome system (UPS) was suspected in the pathogenesis of AD since the presence of ubiquitin immunoreactivity in AD-related neuronal inclusions, such as neurofibrillary tangles, is seen in all AD cases. Recent studies have indicated that components of the UPS could be linked to the early phase of AD, which is marked by synaptic dysfunction, as well as to the late stages of the disease, characterized by neurodegeneration. Insoluble protein aggregates in the brain of AD patients could result from malfunction or overload of the UPS, or from structural changes in the protein substrates, which prevent their recognition and degradation by the UPS. Defective proteolysis could cause the synaptic dysfunction observed early in AD since the UPS is known to play a role in the normal functioning of synapses. In this review, we discuss recent observations on possible links between the UPS and AD, and the potential for utilizing UPS components as targets for treatment of this disease.Publication history: Republished from Current BioDatas Targeted Proteins database (TPdb; http://www.targetedproteinsdb.com).

The ubiquitin-proteasome pathway in health and disease of the nervous system

In recent years, proteolysis by the ubiquitin-proteasome pathway has attained prominence as a new molecular mechanism that regulates many vital functions of the nervous system, including development of synaptic connections and synaptic plasticity. Here, we review the latest findings on the role of proteolysis in sculpting the nervous system through control of axonal growth, axonal and dendritic pruning, and regulation of synaptic size and number. We also discuss how protein degradation functions in synaptic plasticity and the roles of local proteolysis in neuronal compartments. In addition, we describe how proteolysis is associated with Alzheimers disease and ataxia. Furthermore, we highlight the recent approaches that exploit components of the ubiquitin-proteasome pathway for amelioration of these diseases in animal models.

A potential proteasome-interacting motif within the ubiquitin-like domain of parkin and other proteins

Parkin and other unrelated proteins contain a ubiquitin-like domain (UbLD). This article describes a motif that might be important in the interaction of UbLD-containing proteins (UbLPs) with the proteasome. The proteasome-interacting motif, which is conserved in a subset of UbLPs, such as parkin, Rad23 and several transcription factors, is likely to enable the UbLPs to form a complex with the proteasome for proteolysis or the recently discovered non-proteolytic functions of the proteasome.

Ubiquitin‐proteasome‐mediated CREB repressor degradation during induction of long‐term facilitation

Long‐term facilitation in Aplysia and other forms of long‐term memory in invertebrates and vertebrates require the gene expression cascade induced by cAMP‐responsive element binding protein (CREB). Normally, gene expression by CREB is inhibited by repressors. The molecular mechanisms by which the repression is relieved are not understood. Our results show that Aplysia CREB repressor is a substrate for degradation by the ubiquitin‐proteasome pathway. Treatment with the facilitatory neurotransmitter 5‐hydroxy tryptamine (5‐HT) leads to CREB repressor degradation in vivo and the degradation can be blocked by a specific proteasome inhibitor. Our biochemical studies show that attachment of ubiquitin molecules marks the CREB repressor for degradation by the proteasome. Protein kinase C (PKC) stimulates ubiquitination and degradation of the CREB repressor. Our results suggest that proteolytic removal of the CREB repressor is a potential mechanism for controlling gene expression by CREB. Without stimulation, gene expression is suppressed by the CREB repressor. Upon stimulation with 5‐HT, PKC is activated, causing enhancement in ubiquitination and degradation of the CREB repressor. Thus, regulation of proteolysis of the CREB repressor by PKC might be critical in determining whether or not CREB‐mediated gene expression goes forward during induction of long‐term facilitation.

Differential regulation of proteasome activity in the nucleus and the synaptic terminals

Proteasome is a multi-subunit proteolytic complex that degrades proteins covalently linked to multiple molecules of ubiquitin. Earlier studies showed a role for the ubiquitin-proteasome pathway in several models of long-term memory and other forms of synaptic plasticity. In Aplysia, the ubiquitin-proteasome pathway has been shown to contribute to the induction of long-term facilitation. In other model systems, ubiquitin-proteasome-mediated proteolysis has also been shown to play a role in synapse development. Previous studies of synaptic plasticity focused on changes in components or the substrates of the ubiquitin-proteasome pathway in whole neurons. Modification of specific synapses would require precise spatial and temporal regulation of the components of the ubiquitin-proteasome pathway within the subcellular compartments of neurons during learning. As a first step towards testing the idea of local regulation of the ubiquitin-proteasome pathway in neurons, we investigated proteasome activity in nuclear and synaptosomal fractions. Here we show that proteasome activity in the synaptic terminals is higher compared to the activity in the nucleus in the Aplysia nervous system as well as in the mouse brain. Furthermore, the proteasome activity in the two neuronal compartments is differentially modulated by protein kinases. Differential regulation of proteasome activity in neuronal compartments such as the synaptic terminals is likely to be a key mechanism underlying synapse-specific plasticity.

Ubiquitin-proteasome pathway components as therapeutic targets for CNS maladies.

In the central nervous system (CNS), abnormal deposition of insoluble protein aggregates or inclusion bodies within nerve cells is commonly observed in association with several neurodegenerative diseases. The ubiquitinated protein aggregates are believed to result from malfunction or overload of the ubiquitin-proteasome pathway or from structural changes in the protein substrates which prevent their recognition and degradation by the ubiquitin-proteasome pathway. Impaired proteolysis might also contribute to the synaptic dysfunction seen early in neurodegenerative diseases because the ubiquitin-proteasome pathway is known to play a role in normal functioning of synapses. Because specificity of the ubiquitin proteasome mediated proteolysis is determined by specific ubiquitin ligases (E3s), identification of specific E3s and their allosteric modulators are likely to provide effective therapeutic targets for the treatment of several CNS disorders. Another unexplored area for the discovery of drug targets is the proteasome. Although many inhibitors of the proteasome are available, no effective drugs exist that can stimulate the proteasome. Since abnormal protein aggregation is a common feature of different neurodegenerative diseases, enhancement of proteasome activity might be an efficient way to remove the aggregates that accumulate in the brain. In this review, we discuss how the components of the ubiquitin-proteasome pathway could be potential targets for therapy of CNS diseases and disorders.

Signal transduction and gene expression in cultured accessory olfactory bulb neurons

Glutamate and norepinephrine (NE) are believed to mediate the long-lasting synaptic plasticity in the accessory olfactory bulb (AOB) that underlies pheromone recognition memory. The mechanisms by which these neurotransmitters bring about the synaptic changes are not clearly understood. In order to study signals that mediate synaptic plasticity in the AOB, we used AOB neurons in primary culture as a model system. Because induction of pheromone memory requires coincident glutamatergic and noradrenergic input to the AOB, and requires new protein synthesis, we reasoned that glutamate and NE must induce gene expression in the AOB. We used a combination of agonists that stimulate alpha1 and alpha2 adrenergic receptors in combination with N-methyl-d-aspartic acid and tested expression of the immediate-early gene (IEG) c-Fos. We found that the glutamatergic and noradrenergic stimulation caused significant induction of c-Fos mRNA and protein. Induction of c-Fos was significantly reduced in the presence of inhibitors of protein kinase C, mitogen-activated protein kinase (MAPK) and phospholipase C. These results suggest that glutamate and NE induce gene expression in the AOB through a signaling pathway mediated by protein kinase C and MAPK.

Expression Profiling Reveals Differential Gene Induction Underlying Specific and Non-Specific Memory for Pheromones in Mice

Memory for the mating males pheromones in female mice is thought to require synaptic changes in the accessory olfactory bulb (AOB). Induction of this memory depends on release of glutamate in response to pheromonal exposure coincident with release of norepinephrine (NE) in the AOB following mating. A similar memory for pheromones can also be induced artificially by local infusion of the GABA(A) receptor antagonist bicuculline into the AOB. The natural memory formed by exposure to pheromones during mating is specific to the pheromones sensed by the female during mating. In contrast, the artificial memory induced by bicuculline is non-specific and results in the female mice recognizing all pheromones as if they were from the mating male. Although protein synthesis has been shown to be essential for development of pheromone memory, the gene expression cascades critical for memory formation are not known. We investigated changes in gene expression in the AOB using oligonucleotide microarrays during mating-induced pheromone memory (MIPM) as well as bicuculline-induced pheromone memory (BIPM). We found the set of genes induced during MIPM and BIPM are largely non-overlapping and Ingenuity Pathway Analysis revealed that the signaling pathways in MIPM and BIPM also differ. The products of genes induced during MIPM are associated with synaptic function, indicating the possibility of modification at specific synapses, while those induced during BIPM appear to possess neuron-wide functions, which would be consistent with global cellular changes. Thus, these results begin to provide a mechanistic explanation for specific and non-specific memories induced by pheromones and bicuculline infusion respectively.