Ubiquitin-directed antibodies inhibit neuronal transporters in rat brain synaptosomes

Almost 50 Years of Monomeric Extracellular Ubiquitin (eUb)

Monomeric ubiquitin (Ub) is a 76-amino-acid highly conserved protein found in eukaryotes. The biological activity of Ub first described in the 1970s was extracellular, but it quickly gained relevance due to its intracellular role, i.e., post-translational modification of intracellular proteins (ubiquitination) that regulate numerous eukaryotic cellular processes. In the following years, the extracellular role of Ub was relegated to the background, until a correlation between higher survival rate and increased serum Ub concentrations in patients with sepsis and burns was observed. Although the mechanism of action (MoA) of extracellular ubiquitin (eUb) is not yet well understood, further studies have shown that it may ameliorate the inflammatory response in tissue injury and multiple sclerosis diseases. These observations, compounded with the high stability and low immunogenicity of eUb due to its high conservation in eukaryotes, have made this small protein a relevant candidate for biotherapeutic development. Here, we review the in vitro and in vivo effects of eUb on immunologic, cardiovascular, and nervous systems, and discuss the potential MoAs of eUb as an anti-inflammatory, antimicrobial, and cardio- and brain-protective agent.

Ubiquitin-proteasome system and proteasome inhibition: new strategies in stroke therapy

BACKGROUND AND PURPOSE

Proteasomes are large multicatalytic proteinase complexes that are found in the cytosol and in the nucleus of eukaryotic cells with a central role in cellular protein turnover. The ubiquitin-proteasome system (UPS) has a central role in the selective degradation of intracellular proteins. Among the key proteins whose levels are modulated by the proteasome are those involved in the control of inflammatory processes, cell cycle regulation, and gene expression. There are now overwhelming data suggesting that the UPS contributes to cerebral ischemic injury.

SUMMARY OF REVIEW

Proteasome inhibition is a potential treatment option for stroke. Thus far, proof of principle has been obtained from studies in several animal models of cerebral ischemia. Treatment with proteasome inhibitors reduces effectively neuronal and astrocytic degeneration, cortical infarct volume, infarct neutrophil infiltration, and NF-kappaB immunoreactivity with an extension of the neuroprotective effect at least 6 hours after ischemic insult. However, it is clear that the UPS represents a central pathway for the processing and metabolism of multiple proteins with critical roles in cellular function. To avoid eliciting significant side effects associated with complete inhibition of the proteasome and the possible immunosuppressive effects from persistent suppression of NF-kappaB activation, it is critical that we understand how to partially and temporally attenuate proteasome function to elicit the desired therapeutic effect before any large-scale use in humans.

CONCLUSIONS

This review highlights the most recent advances in our knowledge on UPS, as well as the early experience of using proteasome inhibition strategies to treat acute stroke.

Multiple ubiquitin conjugates are present in rat brain synaptic membranes and postsynaptic densities

The pattern of ubiquitin-protein conjugates present in a range of adult rat forebrain subcellular fractions has been investigated by immunoblotting with a monoclonal antibody specific for ubiquitin and its conjugates. Each fraction contains a complex and characteristic pattern of ubiquitin conjugates. Many integral synaptic membrane proteins are ubiquitinated, including a subset of high M(r) (> 120 kD) concanavalin A-binding glycoproteins. Postsynaptic densities are also enriched in ubiquitin conjugates, the profile being distinct from that of synaptic membranes. These results suggest that many plasma membrane and synaptic proteins are ubiquitinated.

Effects of chronic nicotine and pilocarpine administration on neocortical neuronal density and [3H]GABA uptake in nucleus basalis lesioned rats

We investigated the possible long-term neuroprotective roles of (-)nicotine and muscarinic agonist, pilocarpine, in the neocortices of rats receiving bilateral nucleus basalis lesions. Ibotenic acid-lesioned animals eventually displayed a 15-20% reduction in the density of neocortical Nissl staining neurons in layers II, III and VI, as well as a 27% loss in high-affinity GABA uptake 8 months post-lesioning. Deficits were not observed at earlier intervals. (-)Nicotine (0.2 mg/kg, i.p.) or (-)nicotine plus pilocarpine (1 mg/kg, i.p.) attenuated these losses when administered once daily to rats from 5-8 months post-lesioning. Pilocarpine alone had no protective effect on neuronal density or GABA uptake. These results suggest that nicotine receptor activation may counteract neocortical neuronal loss/atrophy following loss of ascending basal forebrain neurons.

Actions of a monoclonal antibody Tor 23 on rat brain presynaptic cholinergic processes

Tor 23 is a monoclonal antibody, generated against cholinergic terminals of the Torpedo californica, that has been found to bind to the extracellular surface of cholinergic neurons in a variety of tissues. This study shows that Tor 23 inhibits: 1) high affinity [3H]hemicholinium-3 binding to detergent-solubilized membranes prepared from rat neocortices; 2) high affinity [3H]choline uptake in rat neocortical and striatal P2 preparations; and 3) [3H]acetylcholine synthesis in isolated nerve terminals. Tor 23 does not appear to affect low affinity [3H]choline uptake or [3H]acetylcholine release. These results are consistent with the hypothesis that Tor 23 may bind to nerve terminal high affinity choline transporters in the rat brain.

Axonal transport of two major components of the ubiquitin system: free ubiquitin and ubiquitin carboxyl-terminal hydrolase PGP 9.5

Ubiquitin (Ub), a stress protein thought to target abnormal proteins for degradation, is present in abnormal structures that occur in neuronal perikarya and axons of degenerative diseases including Alzheimer disease. To begin to assess the role of the Ub system in the axon, we studied expression and axonal transport of Ub and other stress proteins, as well as of Ub carboxyl-terminal hydrolase PGP 9.5, in the rat visual system in normal conditions and following heat-shock (HS). In the retina, both the constitutive and inducible forms of HSPs 70 were expressed under normal conditions, while in the superior colliculus the inducible form was detected only following HS. Ub, PGP 9.5 and HSPs 70 were transported in the axon exclusively with the slow component b (SCb), known to carry cytoskeletal and cytoplasmic proteins. The exceedingly long time needed for stress proteins to reach distant axonal locales at the rate of SCb (approximately 3 mm/day) makes it unlikely that they could contribute significantly to the stress response at those sites.

Impairment of protein ubiquitination may cause delayed neuronal death

The hippocampus is a brain structure specifically vulnerable to short periods of transient cerebral ischemia, and which displays delayed neuronal necrosis. Protein ubiquitination is a posttranslational modification of proteins and an important factor in heat shock response and a regulator of ATP-dependent protein degradation. Using affinity purified antibodies against ubiquitin and ubiquitin-protein conjugates we have found that the ubiquitin immunoreactivity (UIR), normally present in all neurons of the hippocampus, disappears in the early recirculation period following cerebral ischemia from all hippocampal cells except the interneurons. Later UIR reappears in the different hippocampal regions over a 72 h period in the following order: granule cells-CA3 pyramidal cells-CA2 pyramidal cells. This is the inverse order of sensitivity of these cells to ischemia. The UIR never recovers in the CA1 pyramidal neurons where a 95% neuronal necrosis is seen following three days of recovery. We propose that the loss of UIR in the pyramidal neurons in the CA1 region signifies a persistent impairment of protein ubiquitination, and thus a change in the turnover of structural and regulatory proteins, which could be an essential part of the mechanism of slow neuronal death following cerebral ischemia.

Ubiquitin is a component of neurofibrillary tangles in a variety of neurodegenerative diseases

Ubiquitin has been shown to be a component of neurofibrillary tangles in Alzheimer's disease. We now show immunocytochemically that it is also a component of neurofibrillary tangles in several other neurodegenerative diseases of diverse aetiology, including Down's syndrome, dementia pugilistica and postencephalitic parkinsonism, and in normal ageing. Ubiquitin immunoreactivity is not, however, generally found in the neurofibrillary tangles of progressive supranuclear palsy. These findings show that while associated ubiquitin is not a feature unique to the tangles of Alzheimer's disease, it is not simply a non-specific response to the presence of an inclusion body within the cell. The observations suggest that ubiquitin may have an important role in the formation of neurofibrillary tangles in a variety of neurodegenerative diseases.

Antigen recognized by monoclonal antibodies to mesencephalic neural crest and to ciliary ganglion neurons is involved in the high affinity choline uptake mechanism in these cells

High-affinity choline uptake mechanisms are among the characteristics of cholinergic neurons such as the ciliary and choroid subpopulations in the ciliary ganglion (Barald and Berg, 1979). We have produced three monoclonal antibodies (Mabs), two of which were made to 8-day embryonic chick ciliary ganglion (CG) neurons (CG-1, CG-4) (Barald, 1982) and one of which was made to cultured mesencephalic neural crest (NC) cells (CG-14) removed from the embryo 31 hr after incubation. We have shown that all three Mabs label a common 75 kD antigen present on the cell surface of both CG neurons and NC cells (Barald, 1988). Here we report that the CG-1 and CG-4 antibodies, used in the same ratios in which they are synergistically cytotoxic for both the CG and NC cells (Barald, 1988), and Mab CG-14 alone, have specific effects on the high-affinity choline uptake mechanism (HACU) of CG neurons and isolated antigen-positive NC cells in the absence of complement. CG-1 and CG-4 in ratios of 8/1 (the same ratios that are used to kill the CG and the NC subpopulation), but neither singly, inhibit the HACU of CG neurons by 40% and that of isolated antigen-positive NC cells by 75%. However, CG-14 alone, at 1 microgram/ml, inhibits the HACU of both CG neurons and isolated NC cells by 95%. None of the antibodies had an effect on numbers of ouabain binding sites (a measure of the Na+/K+ ATPase) or cell surface acetylcholinesterase (AChE) of CG neurons or NC cells isolated by "no-flow" fluorescence cytometry with a Meridian Instruments ACAS470 cytometer. CG or NC cells grown in the presence of the antibodies without complement grow and remain healthy for many weeks. They exhibit no difference in morphology, protein content, lactate dehydrogenase activity (LDH), or division time from untreated sister cultures. Therefore, the antigen recognized by all three Mabs may be involved in a high-affinity choline uptake mechanism, a common characteristic of cholinergic neurons. The Mabs themselves may possibly label some element of the high-affinity transporter or a proximal membrane component. This implies that such a high-affinity uptake mechanism is present in the subpopulation of NC cells at early times in development. If these cells in fact are destined to contribute to the avian CG, these characteristics are present in the subpopulation before the NC cells take on a neuronal morphology.