A review of non-prostanoid, eicosanoid receptors: expression, characterization, regulation, and mechanism of action

Eicosanoid signaling controls a wide range of biological processes from blood pressure homeostasis to inflammation and resolution thereof to the perception of pain and to cell survival itself. Disruption of normal eicosanoid signaling is implicated in numerous disease states. Eicosanoid signaling is facilitated by G-protein-coupled, eicosanoid-specific receptors and the array of associated G-proteins. This review focuses on the expression, characterization, regulation, and mechanism of action of non-prostanoid, eicosanoid receptors.

A Review of Prostanoid Receptors: Expression, Characterization, Regulation, and Mechanism of Action

Prostaglandin signaling controls a wide range of biological processes from blood pressure homeostasis to inflammation and resolution thereof to the perception of pain to cell survival. Disruption of normal prostanoid signaling is implicated in numerous disease states. Prostaglandin signaling is facilitated by G-protein-coupled, prostanoid-specific receptors and the array of associated G-proteins. This review focuses on the expression, characterization, regulation, and mechanism of action of prostanoid receptors with particular emphasis on human isoforms.

The Role of Eicosanoids in Alzheimer's Disease

Alzheimer's disease (AD) is one of the most common neurodegenerative disorders known. Estimates from the Alzheimer's Association suggest that there are currently 5.8 million Americans living with the disease and that this will rise to 14 million by 2050. Research over the decades has revealed that AD pathology is complex and involves a number of cellular processes. In addition to the well-studied amyloid-β and tau pathology, oxidative damage to lipids and inflammation are also intimately involved. One aspect all these processes share is eicosanoid signaling. Eicosanoids are derived from polyunsaturated fatty acids by enzymatic or non-enzymatic means and serve as short-lived autocrine or paracrine agents. Some of these eicosanoids serve to exacerbate AD pathology while others serve to remediate AD pathology. A thorough understanding of eicosanoid signaling is paramount for understanding the underlying mechanisms and developing potential treatments for AD. In this review, eicosanoid metabolism is examined in terms of in vivo production, sites of production, receptor signaling, non-AD biological functions, and known participation in AD pathology.

Capillary endothelial Na(+), K(+), ATPase transporter homeostasis and a new theory for migraine pathophysiology

BACKGROUND

Cerebrospinal fluid sodium concentration ([Na(+)](csf)) increases during migraine, but the cause of the increase is not known.

OBJECTIVE

Analyze biochemical pathways that influence [Na(+)](csf) to identify mechanisms that are consistent with migraine.

METHOD

We reviewed sodium physiology and biochemistry publications for links to migraine and pain.

RESULTS

Increased capillary endothelial cell (CEC) Na(+), K(+), -ATPase transporter (NKAT) activity is probably the primary cause of increased [Na(+)](csf). Physiological fluctuations of all NKAT regulators in blood, many known to be involved in migraine, are monitored by receptors on the luminal wall of brain CECs; signals are then transduced to their abluminal NKATs that alter brain extracellular sodium ([Na(+)](e)) and potassium ([K(+)](e)).

CONCLUSIONS

We propose a theoretical mechanism for aura and migraine when NKAT activity shifts outside normal limits: (1) CEC NKAT activity below a lower limit increases [K(+)](e), facilitates cortical spreading depression, and causes aura; (2) CEC NKAT activity above an upper limit elevates [Na(+)](e), increases neuronal excitability, and causes migraine; (3) migraine-without-aura may arise from CEC NKAT over-activity without requiring a prior decrease in activity and its consequent spreading depression; (4) migraine triggers disturb, and treatments improve, CEC NKAT homeostasis; (5) CEC NKAT-induced regulation of neural and vasomotor excitability coordinates vascular and neuronal activities, and includes occasional pathology from CEC NKAT-induced apoptosis or cerebral infarction.

The morphology and biochemistry of nanostructures provide evidence for synthesis and signaling functions in human cerebrospinal fluid

Background

Cerebrospinal fluid (CSF) contacts many brain regions and may mediate humoral signaling distinct from synaptic neurotransmission. However, synthesis and transport mechanisms for such signaling are not defined. The purpose of this study was to investigate whether human CSF contains discrete structures that may enable the regulation of humoral transmission.

Methods

Lumbar CSF was collected prospectively from 17 participants: with no neurological or psychiatric disease, with Alzheimer's disease, multiple sclerosis, or migraine; and ventricular CSF from two cognitively healthy participants with long-standing shunts for congenital hydrocephalus. Cell-free CSF was subjected to ultracentrifugation to yield supernatants and pellets that were examined by transmission electron microscopy, shotgun protein sequencing, electrophoresis, western blotting, lipid analysis, enzymatic activity assay, and immuno-electron microscopy.

Results

Over 3,600 CSF proteins were identified from repeated shotgun sequencing of cell-free CSF from two individuals with Alzheimer's disease: 25% of these proteins are normally present in membranes. Abundant nanometer-scaled structures were observed in ultracentrifuged pellets of CSF from all 16 participants examined. The most common structures included synaptic vesicle and exosome components in 30-200 nm spheres and irregular blobs. Much less abundant nanostructures were present that derived from cellular debris. Nanostructure fractions had a unique composition compared to CSF supernatant, richer in omega-3 and phosphoinositide lipids, active prostanoid enzymes, and fibronectin.

Conclusion

Unique morphology and biochemistry features of abundant and discrete membrane-bound CSF nanostructures are described. Prostaglandin H synthase activity, essential for prostanoid production and previously unknown in CSF, is localized to nanospheres. Considering CSF bulk flow and its circulatory dynamics, we propose that these nanostructures provide signaling mechanisms via volume transmission within the nervous system that are for slower, more diffuse, and of longer duration than synaptic transmission.

Cerebrospinal fluid sodium increases in migraine

BACKGROUND

Pharmaceuticals with calcium- or sodium-channel-blocking activity have proven useful for migraine prophylaxis, and calcium channel, sodium transporter, and sodium channel gene mutations have been found in familial hemiplegic migraine. However, it is not known whether calcium or sodium homeostasis is altered in migraine.

OBJECTIVE

To compare levels of sodium, calcium, potassium, and magnesium in cerebrospinal fluid (CSF) and blood plasma between migraineurs and controls.

METHODS

We recruited 20 migraineurs without aura and 11 controls prospectively, and studied migraineurs in sick (MH(+)) and well (MH(-)) states. We collected lumbar CSF and venous blood plasma, quantified elements with ion-selective electrodes or colorimetry, and determined osmolality by depression of freezing point. We compared levels of Na(+), Ca(2+), K(+), and Mg among and also within subjects who were studied in both MH(+) and MH(-) states.

RESULTS

Mean CSF Na(+) levels were increased by 3 mmol/L in MH(+) compared with MH(-) and by 4 mmol/L compared to controls (P < 0.005). In 4 subjects who were sampled in both MH(+) and MH(-) states, mean CSF Na(+) concentration increased by 2 mmol/L in the MH(+) state compared with the MH(-) state (P < 0.05). Simultaneous plasma Na(+) levels did not differ among the 3 clinical groups, nor did osmolality, total Ca and Ca(2+), K(+), and total Mg levels in CSF.

CONCLUSIONS

Compared to both controls and the MH(-) state, CSF Na(+) concentration increased in MH(+) independently from other clinical or pharmacological fluctuations, CSF concentrations of Ca(2+), Mg, and K(+), and blood plasma Na(+) levels. These results implicate a deviation of Na(+) homeostasis in migraine. The modestly elevated extracellular Na(+) in MH(+) may cause the neural changes that underlie clinical features of migraine.

Enhanced sequence coverage of proteins in human cerebrospinal fluid using multiple enzymatic digestion and linear ion trap LC-MS/MS

The cerebrospinal fluid (CSF) provides a ready access into the health state of the central nervous system, and alterations in some CSF proteins have been documented in brain disease. However, the complete variety of proteins is not known and methods to identify protein components are still being developed. The goal of this study was to examine the sequence coverage obtained from human CSF digests produced with different proteases. Enzymatic digests of CSF proteins were obtained with arginine-C endopeptidase (ArgC), glutamic acid endopeptidase (GluC), chymotrypsin, trypsin and their combinations, and then examined using reverse phase chromatography and a Finnigan LTQ linear ion trap mass spectrometer. Peptide sequences were identified with BioWorks 3.1 and sequence coverage calculated for the 38 most confidently identified proteins. Trypsin and GluC yielded greater coverage than chymotrypsin, while ArgC had the least sequence coverage. Protein sequence coverage was affected only slightly over four orders of magnitude dynamic range of abundance. Combining the peptides derived from different proteases further increased the coverage. Maximal sequence coverage was achieved by combining digest results from both GluC and trypsin. These results have implications for future studies to identify CSF proteins and their post-translational modifications.

Prostaglandin D synthase isoforms from cerebrospinal fluid vary with brain pathology

Glutathione independent prostaglandin D synthase (Swissprot P41222, PTGDS) has been identified in human cerebrospinal fluid and some changes in PTGDS in relation to disease have been reported. However, little is known of the extent that PTGDS isoforms fluctuate across a large range of congenital and acquired diseases. The purpose of this study was to examine changes in PTGDS isoforms in such a population. Spinal fluid from 22 healthy study participants (normal controls) with no classifiable neurological or psychiatric diagnosis was obtained and PTGDS isoforms were identified by specific immunostaining and mass spectrometry after denaturing 2D gel electrophoresis. The PTGDS isoforms in controls consisted of five charge isoforms that were always present and a small number of occasional, low abundance isoforms. A qualitative survey of 98 different people with a wide range of congenital and acquired diseases revealed striking changes. Loss of the control isoforms occurred in congenital malformations of the nervous system. Gain of additional isoforms occurred in some degenerative, most demyelinating and vasculitic diseases, as well as in Creutzfeldt-Jakob disease. A retrospective analysis of published data that quantified relative amounts of PTGDS in multiple sclerosis, schizophrenia and Parkinson's disease compared to controls revealed significant dysregulation. It is concluded that qualitative and quantitative fluctuations of cerebrospinal fluid PTGDS isoforms reflect both major and subtle brain pathophysiology.

Identification of disease markers in human cerebrospinal fluid using lipidomic and proteomic methods

Lipids comprise the bulk of the dry mass of the brain. In addition to providing structural integrity to membranes, insulation to cells and acting as a source of energy, lipids can be rapidly converted to mediators of inflammation or to signaling molecules that control molecular and cellular events in the brain. The advent of soft ionization procedures such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) have made it possible for compositional studies of the diverse lipid structures that are present in brain. These include phospholipids, ceramides, sphingomyelin, cerebrosides, cholesterol and their oxidized derivatives. Lipid analyses have delineated metabolic defects in disease conditions including mental retardation, Parkinson's Disease (PD), schizophrenia, Alzheimer's Disease (AD), depression, brain development, and ischemic stroke. In this review, we examine the structure of the major lipid classes in the brain, describe methods used for their characterization, and evaluate their role in neurological diseases. The potential utility of characterizing lipid markers in the brain, with specific emphasis on disease mechanisms, will be discussed. Additionally, we describe several proteomic strategies for characterizing lipid-metabolizing proteins in human cerebrospinal fluid (CSF). These proteins may be potential therapeutic targets since they transport lipids required for neuronal growth or convert lipids into molecules that control brain physiology. Combining lipidomics and proteomics will enhance existing knowledge of disease pathology and increase the likelihood of discovering specific markers and biochemical mechanisms of brain diseases.

Protein analysis in human cerebrospinal fluid: Physiological aspects, current progress and future challenges

The introduction of lumbar puncture into clinical medicine over 100 years ago marks the beginning of the study of central nervous system diseases using the human cerebrospinal fluid (CSF). Ever since, CSF has been analyzed extensively to elucidate the physiological and biochemical bases of neurological disease. The proximity of CSF to the brain makes it a good target for studying the pathophysiology of brain functions, but the barrier function of the CSF also impedes its diagnostic value. Today, measurements to determine alterations in the composition of CSF are central in the differential diagnosis of specific diseases of the central nervous system (CNS). In particular, the analysis of the CSF protein composition provides crucial information in the diagnosis of CNS diseases. This enables the assessment of the physiology of the blood-CSF barrier and of the immunology of intrathecial responses. Besides those routine measurements, protein compositional studies of CSF have been extended recently to many other proteins in the expectation that comprehensive analysis of lower abundance CSF proteins will lead to the discovery of new disease markers. Disease marker discovery by molecular profiling of the CSF tissue has the enormous potential of providing many new disease relevant molecules. New developments in protein profiling techniques hold promise for the discovery and validation of relevant disease markers. In this review, we summarize the current efforts and progress in CSF protein profiling measurements using conventional and current protein analysis tools. We also discuss necessary development in methodology in order to have the highest impact on the study of the molecular composition of CSF proteins.

Intermediates in the refolding of ribonuclease at subzero temperatures. 2. Monitoring by inhibitor binding and catalytic activity

The kinetics of refolding of ribonuclease A were monitored by the return of catalytic activity and inhibitor binding at -15 degrees C in 35% methanol cryosolvent at pH* 3.0 and 6.0. Catalytic activity was measured with cytidine 2',3'-cyclic monophosphate as substrate; inhibitor binding was determined with the competitive inhibitor cytidine 2'-monophosphate. Biphasic kinetics were observed at pH* 3.0 for both return of catalytic activity and inhibitor binding. At pH* 6.0 the rate of return of catalytic activity was monophasic, whereas that of inhibitor binding was biphasic. For both inhibitor binding and catalytic activity one of the observed rates was pH-dependent. Full return of catalytic activity was obtained at the completion of the refolding process. The observations are interpreted in terms of two parallel pathways of refolding for slow-refolding ribonuclease, with several native-like, partially folded intermediate states on the minor slow-refolding pathway. Of particular note is the presence of at least one such species that has inhibitor-binding capacity but not catalytic activity. This may be rationalized in terms of the known native structure. In addition, an intermediate is postulated which has the incorrect Pro-93 conformation and only partial catalytic activity (42% of the native). The slowest observed transient is attributed to the isomerization of this proline residue and return of full catalytic activity.

Intermediates in the refolding of ribonuclease at subzero temperatures. 1. Monitoring by nitrotyrosine absorbance

Derivatives of ribonuclease A in which tyrosines-73, -76, and -115 were nitrated have been synthesized, purified to homogeneity, and characterized by NMR, isoelectric points, absorbance spectra, and catalytic activity. The positions of their reversible thermal unfolding transitions were determined in 35% methanol at pH* 3.0 and 6.0. In the present study the kinetics of the refolding of these nitrotyrosine derivatives were measured at -15 degrees C at pH* 3.0 and 6.0 by using a cryosolvent composed of 35% aqueous methanol. The rates of folding of different regions of the molecule were determined by using the nitrotyrosines as environmentally sensitive probes. Multiphasic kinetics were observed for the refolding of the nitro-Tyr-115, -73, and -76 derivatives. The native environment about Tyr-115 was formed more rapidly than that about Tyr-73 and -76, and the native environment about both these tyrosines was attained much sooner than the native state itself, as judged by other probes. The results indicate that different regions of the molecule attain their native environments at different rates. This observation shows that the folding pathway must involve partially folded intermediate states.

Intermediates in the refolding of ribonuclease at subzero temperatures. 3. Multiple folding pathways

The kinetics of refolding of ribonuclease A have been measured at -15 degrees C by monitoring the intrinsic fluorescence and absorbance signals from the six tyrosine residues. For each probe multiphasic kinetics were observed. The burial of tyrosine residues, as determined by the change in absorbance at 286 nm, revealed four phases, whereas the kinetics of refolding monitored by fluorescence revealed only two phases. The rates of the transients detected by fluorescence were independent of pH. One of the faster transients detected by delta A286 involved a decrease in absorbance, which is consistent with solvent exposure, rather than burial, and suggests the possibility of an abortive partially folded intermediate in the earlier stages of folding. Double-jump unfolding assays were used to follow the buildup and decay of an intermediate in the refolding reaction at -15 degrees C. At both pH* 3.0 and pH* 6.0 the maximum concentration of the intermediate was 25-30% of the total protein. The existence of a second pathway of slow folding was inferred from the difference in rate of formation of native enzyme and breakdown of the observed intermediate, and by computer simulations. In addition, the unfolding assay demonstrated that 20% of the unfolded protein was converted to native at a much faster rate, consistent with observations in aqueous solution that 80% of unfolded ribonuclease A consists of slow-folding species. Kinetics and amplitude data from these and other refolding experiments with different probes were used to develop possible models for the pathway of refolding. The simplest system consistent with the results for the slow-refolding species involves two parallel pathways with multiple intermediates on each of them. Several independent lines of evidence indicate that about 30% of the unfolded state refolds by the minor pathway, in which the slowest observed phase is attributed to the isomerization of Pro-93. The major pathway involves 50% of the unfolded state; the reason why it refolds slowly is not apparent. A native-like intermediate is formed considerably more rapidly in the major slow-refolding pathway, compared to the minor pathway.

Observation of intermediates in the folding of ribonuclease A at low temperature using proton nuclear magnetic resonance

The refolding of ribonuclease A (RNase A) has been investigated in aqueous methanol cryosolvents in the 0 to -20 degrees C range. When a thermally unfolded sample was brought under renaturing conditions (e.g., -16 degrees C, 35% methanol, pH 2.8), the refolding, as monitored by the absorbance change at 286 nm (which reflects the degree of solvent exposure of Tyr), was triphasic and took approximately 1 h for completion. The 360-MHz proton nuclear magnetic resonance (NMR) spectrum of the native enzyme in either 35% or 50% aqueous methanol is very similar to that in aqueous solution. When the refolding of RNase A was monitored in the subzero temperature range with the signals of the His C2 protons, new resonances rapidly appeared, in addition to those from native protein. The new resonances are attributed to a partially folded intermediate state that has a relatively compact structure. Time-dependent changes were observed in the areas of the resonances from both native and partially folded species. The rates of peak area reduction for the intermediate state were the same as those for the increase in area of the native resonances, and similar to those for the second phase observed in the absorbance experiments. The results are consistent with the slow-refolding form of RNase A consisting of a least two distinct populations. A model for the folding of RNase A is proposed.