Constitutively expressed catalytic proteasomal subunits are up-regulated during neuronal differentiation and required for axon initiation, elongation and maintenance

Directed mechanisms for apical dendrite development during neuronal polarization

The development of the dendrite and the axon during neuronal polarization underlies the directed flow of information in the brain. Seminal studies on axon development have dominated the mechanistic analysis of neuronal polarization. These studies, many originating from examinations in cultured hippocampal and cortical neurons in vitro, have established a prevalent view that axon formation precedes and is necessary for neuronal polarization. There is also in vivo evidence supporting this view. Nevertheless, the establishment of bipolar polarity and the leading edge, and apical dendrite development in pyramidal neurons in vivo occur when axon formation is prevented. Furthermore, recent mounting evidence suggest that directed mechanisms might mediate bipolar polarity/leading process and subsequent apical dendrite development. In the presence of spatially directed extracellular cues in the developing brain, these events may operate independently of axon forming events. In this perspective we summarize evidence in support of these evolving views in neuronal polarization and highlight recent findings on dedicated mechanisms acting in apical dendrite development.

Axonal Organelles as Molecular Platforms for Axon Growth and Regeneration after Injury

Investigating the molecular mechanisms governing developmental axon growth has been a useful approach for identifying new strategies for boosting axon regeneration after injury, with the goal of treating debilitating conditions such as spinal cord injury and vision loss. The picture emerging is that various axonal organelles are important centers for organizing the molecular mechanisms and machinery required for growth cone development and axon extension, and these have recently been targeted to stimulate robust regeneration in the injured adult central nervous system (CNS). This review summarizes recent literature highlighting a central role for organelles such as recycling endosomes, the endoplasmic reticulum, mitochondria, lysosomes, autophagosomes and the proteasome in developmental axon growth, and describes how these organelles can be targeted to promote axon regeneration after injury to the adult CNS. This review also examines the connections between these organelles in developing and regenerating axons, and finally discusses the molecular mechanisms within the axon that are required for successful axon growth.

Non-Hematologic Toxicity of Bortezomib in Multiple Myeloma: The Neuromuscular and Cardiovascular Adverse Effects

Simple Summary

Multiple myeloma (MM) is a still uncurable tumor of mainly elderly patients originating from the terminally differentiated B cells. Introduction to the treatment of MM patients of a new class of drugs called proteasome inhibitors (bortezomib followed by carfilzomib and ixazomib) significantly improved disease control. Proteasome inhibitors interfere with the major mechanism of protein degradation in a cell leading to the severe imbalance in the protein turnover that is deadly to MM cells. Currently, these drugs are the mainstream of MM therapy but are also associated with an increased rate of the injuries to multiple organs and tissues. In this review, we summarize the current knowledge on the molecular mechanisms of the first-in-class proteasome inhibitor bortezomib-induced disturbances in the function of peripheral nerves and cardiac and skeletal muscle.

Abstract

The overall approach to the treatment of multiple myeloma (MM) has undergone several changes during the past decade. and proteasome inhibitors (PIs) including bortezomib, carfilzomib, and ixazomib have considerably improved the outcomes in affected patients. The first-in-class selective PI bortezomib has been initially approved for the refractory forms of the disease but has now become, in combination with other drugs, the backbone of the frontline therapy for newly diagnosed MM patients, as well as in the maintenance therapy and relapsed/refractory setting. Despite being among the most widely used and highly effective agents for MM, bortezomib can induce adverse events that potentially lead to early discontinuation of the therapy with negative effects on the quality of life and outcome of the patients. Although peripheral neuropathy and myelosuppression have been recognized as the most relevant bortezomib-related adverse effects, cardiac and skeletal muscle toxicities are relatively common in MM treated patients, but they have received much less attention. Here we review the neuromuscular and cardiovascular side effects of bortezomib. focusing on the molecular mechanisms underlying its toxicity. We also discuss our preliminary data on the effects of bortezomib on skeletal muscle tissue in mice receiving the drug.

Bortezomib- and thalidomide-induced peripheral neuropathy in multiple myeloma: clinical and molecular analyses of a phase 3 study

A subanalysis of the GIMEMA-MMY-3006 trial was performed to characterize treatment-emergent peripheral neuropathy (PN) in patients randomized to thalidomide-dexamethasone (TD) or bortezomib-TD (VTD) before and after double autologous transplantation (ASCT) for multiple myeloma (MM). A total of 236 patients randomized to VTD and 238 to TD were stratified according to the emergence of grade ≥2 PN. Gene expression profiles (GEP) of CD138+ plasma cells were analyzed in 120 VTD-treated patients. The incidence of grade ≥2 PN was 35% in the VTD arm and 10% in the TD arm (P < 0.001). PN resolved in 88 and 95% of patients in VTD and TD groups, respectively. Rates of complete/near complete response, progression-free and overall survival were not adversely affected by emergence of grade ≥2 PN. Baseline characteristics were not risk factors for PN, while GEP analysis revealed the deregulated expression of genes implicated in cytoskeleton rearrangement, neurogenesis, and axonal guidance. In conclusion, in comparison with TD, incorporation of VTD into ASCT was associated with a higher incidence of PN which, however, was reversible in most of the patients and did not adversely affect their outcomes nor their ability to subsequently receive ASCT. GEP analysis suggests an interaction between myeloma genetic profiles and development of VTD-induced PN.

Autophagy negatively regulates early axon growth in cortical neurons

Neurite growth requires neurite extension and retraction, which are associated with protein degradation. Autophagy is a conserved bulk degradation pathway that regulates several cellular processes. However, little is known about autophagic regulation during early neurite growth. In this study, we investigated whether autophagy was involved in early neurite growth and how it regulated neurite growth in primary cortical neurons. Components of autophagy were expressed and autophagy was activated during early neurite growth. Interestingly, inhibition of autophagy by atg7 small interfering RNA (siRNA) caused elongation of axons, while activation of autophagy by rapamycin suppressed axon growth. Surprisingly, inhibition of autophagy reduced the protein level of RhoA. Moreover, expression of RhoA suppressed axon overelongation mediated by autophagy inhibition, whereas inhibition of the RhoA signaling pathway by Y-27632 recovered rapamycin-mediated suppression of axon growth. Interestingly, hnRNP-Q1, which negatively regulates RhoA, accumulated in autophagy-deficient neurons, while its protein level was reduced by autophagy activation. Overall, our study suggests that autophagy negatively regulates axon extension via the RhoA-ROCK pathway by regulating hnRNP-Q1 in primary cortical neurons. Therefore, autophagy might serve as a fine-tuning mechanism to regulate early axon extension.

Disruption of E3 ligase NEDD4 in peripheral neurons interrupts axon outgrowth: Linkage to PTEN

Exploiting molecules and pathways important in developmental axon behaviour may offer new insights into regenerative behaviour of adult peripheral neurons after injury. In previous work, we have provided evidence that inhibition or knockdown of PTEN (phosphatase and tensin homolog deleted on chromosome ten) dramatically increases adult peripheral axon outgrowth, especially in preconditioned neurons (Christie et al., 2010). PTEN appears to operate as an endogenous brake to regeneration. Recent reports from Drinjakovic et al. (2010) have highlighted a role for the ubiquitin proteasome system (UPS) during neurite outgrowth in developing Xenopus retinal ganglion cells. Specifically, disruption of the UPS E3 ligase Nedd4 (neural precursor cell-expressed developmentally down-regulated protein 4) inhibited neurite branching through up-regulation of PTEN. We explored the potential role of Nedd4 in the peripheral neurons of adult rat dorsal root ganglia (DRG), particularly its impact on regenerative behaviour. Global inhibition of the UPS in vitro was associated with a severe decrease in neurite branching, both in preconditioned (injured) and control DRG sensory neurons. These involved neurons however maintained or qualitatively increased their PTEN expression, suggesting ongoing PTEN activity during UPS inhibition. Considering component's of UPS more specifically, Nedd4 co-localized with PTEN within sensory neurons in vivo and in vitro. Nedd4 also co-localized with PTEN and NF200 labelled regenerating axons at the injury site in the periphery following a 3 day sciatic nerve cut. A significant role for this unique co-expression was observed with fluorescently tagged siRNA inhibition of Nedd4, which decreased neurite outgrowth, an impact associated with greater expression of PTEN and that was completely reversed with application of a PTEN inhibitor. Overall, our results suggest an important role for Nedd4 regulation of PTEN in the response of peripheral neurons to injury. By degrading PTEN among other potential actions, Nedd4 supports axonal outgrowth whereas its inhibition facilitates PTEN inhibition of regeneration.

Proteasome inhibition causes epithelial-mesenchymal transition upon TM4SF5 expression

Transmembrane 4 L six family member 5 (TM4SF5) is highly expressed in hepatocarcinoma and causes epithelial-mesenchymal transition (EMT) of hepatocytes. We found that TM4SF5-expressing cells showed lower mRNA levels but maintained normal protein levels in certain gene cases, indicating that TM4SF5 mediates stabilization of proteins. In this study, we explored whether regulation of proteasome activity and TM4SF5 expression led to EMT. We observed that TM4SF5 expression caused inhibition of proteasome activity and proteasome subunit expression, causing morphological changes and loss of cell-cell contacts. shRNA against TM4SF5 recovered proteasome expression, with leading to blockade of proteasome inactivation and EMT. Altogether, TM4SF5 expression appeared to cause loss of cell-cell adhesions via proteasome suppression and thereby proteasome inhibition, leading to repression of cell-cell adhesion molecules, such as E-cadherin.

Initiating and growing an axon

The ability of neurons to form a single axon and multiple dendrites underlies the directional flow of information transfer in the central nervous system. Dendrites and axons are molecularly and functionally distinct domains. Dendrites integrate synaptic inputs, triggering the generation of action potentials at the level of the soma. Action potentials then propagate along the axon, which makes presynaptic contacts onto target cells. This article reviews what is known about the cellular and molecular mechanisms underlying the ability of neurons to initiate and extend a single axon during development. Remarkably, neurons can polarize to form a single axon, multiple dendrites, and later establish functional synaptic contacts in reductionist in vitro conditions. This approach became, and remains, the dominant model to study axon initiation and growth and has yielded the identification of many molecules that regulate axon formation in vitro (Dotti et al. 1988). At present, only a few of the genes identified using in vitro approaches have been shown to be required for axon initiation and outgrowth in vivo. In vitro, axon initiation and elongation are largely intrinsic properties of neurons that are established in the absence of relevant extracellular cues. However, the importance of extracellular cues to axon initiation and outgrowth in vivo is emerging as a major theme in neural development (Barnes and Polleux 2009). In this article, we focus our attention on the extracellular cues and signaling pathways required in vivo for axon initiation and axon extension.

Cell-specific targeting in the mouse inner ear using nanoparticles conjugated with a neurotrophin-derived peptide ligand: potential tool for drug delivery

Cell specific targeting is an emerging field in nanomedicine. Homing of the multifunctional nanoparticles (MFNPs) is achieved by the conjugation of targeting moieties on the nanoparticle surface. The inner ear is an attractive target for new drug delivery strategies as it is hard to access and hearing loss is a significant worldwide problem. In this work we investigated the utility of a Nerve Growth Factor-derived peptide (hNgf_EE) functionalized nanoparticles (NPs) to target cells of the inner ear. These functionalized NPs were introduced to organotypic explant cultures of the mouse inner ear and to PC-12 rat pheochromocytoma cells. The NPs did not show any signs of toxicity. Specific targeting and higher binding affinity to spiral ganglion neurons, Schwann cells and nerve fibers of the explant cultures were achieved through ligand mediated multivalent binding to tyrosine kinase receptors and to p75 neurotrophin receptors. Unspecific uptake of NPs was investigated using NPs conjugated with scrambled hNgf_EE peptide. Our results indicate a selective cochlear cell targeting by MFNPs, which may be a potential tool for cell specific drug and gene delivery to the inner ear.

Establishment of axon-dendrite polarity in developing neurons

Neurons are among the most highly polarized cell types in the body, and the polarization of axon and dendrites underlies the ability of neurons to integrate and transmit information in the brain. Significant progress has been made in the identification of the cellular and molecular mechanisms underlying the establishment of neuronal polarity using primarily in vitro approaches such as dissociated culture of rodent hippocampal and cortical neurons. This model has led to the predominant view suggesting that neuronal polarization is specified largely by stochastic, asymmetric activation of intracellular signaling pathways. Recent evidence shows that extracellular cues can play an instructive role during neuronal polarization in vitro and in vivo. In this review, we synthesize the recent data supporting an integrative model whereby extracellular cues orchestrate the intracellular signaling underlying the initial break of neuronal symmetry leading to axon-dendrite polarization.

The role of local protein synthesis and degradation in axon regeneration

In axotomised regenerating axons, the first step toward successful regeneration is the formation of a growth cone. This requires a variety of dynamic morphological and biochemical changes in the axon, including the appearance of many new cytoskeletal, cell surface and signalling molecules. These changes suggest the activation of coordinated complex cellular processes. A recent development has been the demonstration that the regenerative ability of some axons depends on their capacity to locally synthesise new proteins and degrade others at the injury site autonomously from the cell body. There are also events involving the degradation of cytoskeletal and other molecules, and activation of signalling pathways, with axotomy-induced calcium changes probably being an initiating event. A future challenge will be to understand how this complex network of processes interacts in order to find therapeutic ways of promoting the regeneration of CNS axons.

The flavonoid fisetin promotes nerve cell survival from trophic factor withdrawal by enhancement of proteasome activity

To explore the possibility that specific flavonoids can substitute for neurotrophic factors, we examined the ability of the flavonol fisetin and several related flavonoids to support the survival of low density, serum-free cultures of rat cortical neurons. Normally these cells die within 24h in the absence of trophic factors but in the presence of fisetin and several related flavonoids the cells survive and produce long neurites. While the survival-promoting effect of several of the fisetin-related flavonoids was partially dependent on ERK activation, the effect of fisetin was not. Fisetin can enhance glutathione synthesis but the survival-promoting effect of fisetin was also not dependent on glutathione. However, proteasome inhibitors almost completely blocked the ability of fisetin to promote survival. Consistent with this observation, fisetin increased proteasome activity. Together these results demonstrate a new activity for fisetin and tie this activity to its neurotrophic effects.

In vitro, lidocaine-induced axonal injury is prevented by peripheral inhibition of the p38 mitogen-activated protein kinase, but not by inhibiting caspase activity

BACKGROUND

All local anesthetics (LAs) are, to some extent, neurotoxic. Toxicity studies have been performed in dissociated neuron cultures, immersing both axon and soma in LA. This approach, however, does not accurately reflect the in vivo situation for peripheral nerve blockade, where LA is applied to the axon alone.

METHODS

We investigated lidocaine neurotoxicity in compartmental sensory neuron cultures, which are composed of one central compartment containing neuronal cell bodies and a peripheral compartment containing their axons, allowing for selective incubation. We applied lidocaine +/- neuroprotective drugs to neuronal somata or axons, and assessed neuron survival and axonal outgrowth.

RESULTS

Lidocaine applied to the peripheral compartment led to a decreased number of axons (to 59% +/- 9%), without affecting survival of cell bodies. During axonal incubation with lidocaine, the p38 mitogen-activated protein kinase inhibitor SB203580 (10 microM) attenuated axonal injury when applied to the axon (insignificant reduction of maximal axonal distance to 93% +/- 9%), but not when applied to the cell body (deterioration of maximal axonal length to 48% +/- 6%). Axonal co-incubation of lidocaine with the caspase inhibitor z-vad-fmk (20 microM) was not protective.

CONCLUSIONS

Whereas inhibition of either p38 mitogen-activated protein kinase or caspase activity promote neuronal survival after LA treatment of dissociated neuronal cultures, axonal degeneration induced by lidocain (40 mM) is prevented by p38 MAP kinase but not by caspase inhibition. We conclude that processes leading to LA-induced neurotoxicity in dissociated neuronal culture may be different from those observed after purely axonal application.

Association of Rpn10 with high molecular weight complex is enhanced during retinoic acid-induced differentiation of neuroblastoma cells

The ubiquitin-binding Rpn10 protein serves as an ubiquitin receptor that delivers client proteins to the 26S proteasome, the protein degradation complex. It has been suggested that the ubiquitin-dependent protein degradation is critical for neuronal differentiation and for preventing neurodegenerative diseases. Our previous study indicated the importance of Rpn10 in control of cellular differentiation (Shimada et al., Mol Biol Cell 17:5356-5371, 2006), though the functional relevance of Rpn10 in neuronal cell differentiation remains a mystery to be uncovered. In the present study, we have examined the level of Rpn10 in a proteasome-containing high molecular weight (HMW) protein fraction prepared from the mouse neuroblastoma cell line Neuro2a. We here report that the protein level of Rpn10 in HMW fraction from un-differentiated Neuro2a cells was significantly lower than that of other cultured cell lines. We have found that retinoic acid-induced neural differentiation of Neuro2a cells significantly stimulates the incorporation of Rpn10 into HMW fractions, although the amounts of 26S proteasome subunits were not changed. Our findings provide the first evidence that the modulation of Rpn10 is linked to the control of retinoic acid-induced differentiation of neuroblastoma cells.