The aluminum-induced increase in blood-brain barrier permeability to delta-sleep-inducing peptide occurs throughout the brain and is independent of phosphorus and acetylcholinesterase levels

Viktor Mutt lecture: Peptides can cross the blood-brain barrier

The field of peptides exploded in the 1970's and has continued to be a major area of discovery. Among the early discoveries was that peptides administered peripherally could affect brain functions. This led Kastin to propose that peptides could cross the blood-brain barrier (BBB). Although initially very controversial, Kastin, I, and others demonstrated not only that peptides can cross the BBB, but elucidated many fundamental characteristics of that passage. That work was in large part the basis of the 2022 Viktor Mutt Lectureship. Here, we review some of the early work with current updates on topics related to the penetration of peptides across the BBB. We briefly review mechanisms by which peripherally administered peptides can affect brain function without crossing the BBB, and then review the major mechanisms by which peptides and their analogs have been show to cross the BBB: transmembrane diffusion, saturable transport, and adsorptive transcytosis. Saturable transport systems are adaptable to physiologic changes and can be altered by disease states. In particular, the transport across the BBB of insulin and of pituitary adenylate cyclase activating polypeptide (PACAP) illustrate many of the concepts regarding peptide transport across the BBB.

Metal Imbalance in Neurodegenerative Diseases with a Specific Concern to the Brain of Multiple Sclerosis Patients

There is increasing evidence that deregulation of metals contributes to a vast range of neurodegenerative diseases including multiple sclerosis (MS). MS is a chronic inflammatory disease of the central nervous system (CNS) manifesting disability and neurological symptoms. The precise origin of MS is unknown, but the disease is characterized by focal inflammatory lesions in the CNS associated with an autoimmune reaction against myelin. The treatment of this disease has mainly been based on the prescription of immunosuppressive and immune-modulating agents. However, the rate of progressive disability and early mortality is still worrisome. Metals may represent new diagnostic and predictive markers of severity and disability as well as innovative candidate drug targets for future therapies. In this review, we describe the recent advances in our understanding on the role of metals in brain disorders of neurodegenerative diseases and MS patients.

Olfactory Function in Chronic Renal Failure

Background

Patients with chronic renal failure (CRF) show a high prevalence of poor nutritional state so that dietary treatment becomes a significant part of the therapeutic regimen. Because smell plays an important role in nutrition, this study aimed to investigate olfactory function in CRF patients.

Methods

A total of 64 CRF patients were investigated. Forty-nine of them were treated with hemodialysis, 15 CRF patients were not dialysis dependent. For comparison we examined 15 healthy subjects. Olfactory function was assessed for odor discrimination, odor identification, and butanol odor thresholds.

Results

Olfactory loss was found in 56% of the patients, with 3 functional anosmics and 33 hyposmics. CRF had specific effects on individual tests of olfactory function. Elevated odor thresholds were found in 11% of patients, 38% of patients had reduced odor discrimination, and 48% of patients exhibited deficits in odor identification. Results of psychological tests (Mini–Mental State Examination and Trail-Making Test) correlated with results from odor identification (p < 0.01) and discrimination (p < 0.01) but not with odor thresholds.

Conclusions

The ability to discriminate and identify odors was found severely impaired whereas odor thresholds were similar to what is seen in the general population. Consequently, CRF patients should be counseled with regard to the possibility of reduced chemosensory functions.

Aluminium sulfate exposure: A set of effects on hydrolases from brain, muscle and digestive tract of juvenile Nile tilapia (Oreochromis niloticus)

Aluminium is a major pollutant due to its constant disposal in aquatic environments through anthropogenic activities. The physiological effects of this metal in fish are still scarce in the literature. This study investigated the in vivo and in vitro effects of aluminium sulfate on the activity of enzymes from Nile tilapia (Oreochromis niloticus): brain acetylcholinesterase (AChE), muscle cholinesterases (AChE-like and BChE-like activities), pepsin, trypsin, chymotrypsin and amylase. Fish were in vivo exposed during 14days when the following experimental groups were assayed: control group (CG), exposure to Al₂(SO₄)₃ at 1μg·mL^-1 (G1) and 3μg·mL^-1 (G3) (concentrations compatible with the use of aluminium sulfate as coagulant in water treatment). In vitro exposure was performed using animals of CG treatment. Both in vivo and in vitro exposure increased cholinesterase activity in relation to controls. The highest cholinesterase activity was observed for muscle BChE-like enzyme in G3. In contrast, the digestive enzymes showed decreased activity in both in vivo and in vitro exposures. The highest inhibitory effect was observed for pepsin activity. The inhibition of serine proteases was also quantitatively analyzed in zymograms using pixel optical densitometry as area under the peaks (AUP) and integrated density (ID). These results suggest that the inhibition of digestive enzymes in combination with activation of cholinesterases in O. niloticus is a set of biochemical effects that evidence the presence of aluminium in the aquatic environment. Moreover, these enzymatic alterations may support further studies on physiological changes in this species with implications for its neurological and digestive metabolisms.

Peptides at the blood brain barrier: Knowing me knowing you

When the Davis Lab was first asked to contribute to this special edition of Peptides to celebrate the career and influence of Abba Kastin on peptide research, it felt like a daunting task. It is difficult to really understand and appreciate the influence that Abba has had, not only on a generation of peptide researchers, but also on the field of blood brain barrier (BBB) research, unless you lived it as we did. When we look back at our careers and those of our former students, one can truly see that several of Abba's papers played an influential role in the development of our personal research programs.

In vivo and in vitro effects of aluminum on the activity of mouse brain acetylcholinesterase

Cholinesterases are a large family of enzymatic proteins widely distributed throughout both neuronal and non-neuronal tissues. In Alzheimer's disease (AD), analytical as well as epidemiological studies suggest an implication of an abnormal focal accumulation of aluminum in the brain. In this devastating disease, aluminum may interfere with various biochemical processes including acetylcholine metabolism, and can thus act as a possible etiopathogenic cofactor. Acetylcholinesterase (AChE) exists in several molecular forms that differ in solubility and mode of membrane attachment rather than in catalytic activity. Mice were treated orally with aluminum chloride or aluminum lactate (Al(lac)(3)), and AChE activity in their brain homogenates was then assayed. Results showed that this in vivo treatment augmented the activity of the enzyme. An activating effect was also observed in vitro, when the aluminum compounds were added directly to mouse brain homogenates. However, the activating effect observed in vivo was much more marked than that observed in vitro. In addition, the activation produced by Al(lac)(3) was higher than that obtained after aluminum chloride treatment. Kinetics measurements of AChE activity in the absence and presence of treatment with aluminum both in vivo and in vitro are reported. The influence of the metal speciation on enzymatic activity is discussed in relation to a possible implication of aluminum in some neurodegenerative diseases.

Profiling of endogenous peptides as a tool for studying development and neurological disease

The use of a method to follow changes in endogenous peptide production, as they occur in biological studies, is an excellent complement to other molecular techniques. It has the unique ability to characterize peptides that have been produced from protein precursors, and instrumentation is available that provides high resolution peptide separations that are quantitative, sensitive, and amenable to automation. All tissues express a large number of peptide species that can be visualized, or profiled, on chromatographic separations using reverse-phase high-performance liquid chromatography. This large number of peptides offers many potential molecules that can be used to identify biological mechanisms associated with experimental paradigms. Peptide analysis has been used successfully in many types of studies. In this review, we outline our experience in using peptides as biological markers and provide a description of the evolution of peptide profiling in our laboratories. Peptide expression has been used in studies ranging from how brain regions develop to identifying changes in disease processes including Alzheimer's disease and models of stroke. Some of the findings provided by these studies have been new pathways of peptide processing and the identification of accelerated proteolysis on proteins such as hemoglobin as a function of Alzheimer's disease and brain insult. Peptide profiling has also proven to be an excellent technique for studying many well-known nervous system proteins including calmodulin, PEP-19, myelin basic protein, cytoskeletal proteins, and others. It is the purpose of this review to describe our experience using the technique and to highlight improvements that have added to the power of the approach. Peptide analysis and the expansion in the instrumentation that can detect peptides will no doubt make these types of studies a powerful addition to our molecular armamentarium for conducting biological studies.

Selective, physiological transport of insulin across the blood-brain barrier: novel demonstration by species-specific radioimmunoassays

Insulin in blood is thought to cross the blood-brain barrier (BBB) to act within the brain to help control appetite. We examined the ability of blood-borne insulin to cross the BBB. Human insulin was infused for 48 h subcutaneously at several doses into mice and the amount of human and murine insulin in serum and brain measured with species-specific radioimmunoassays. For the exogenous human insulin, both brain and blood concentrations increased with increasing doses of infused insulin, whereas for the endogenous murine insulin, brain and blood concentrations decreased. Since the mouse cannot make human insulin, blood was the only source for the human insulin in brain, demonstrating that insulin does indeed cross the BBB. The relationship between the concentrations of human insulin in brain and blood was nonlinear, showing that passage is by a saturable mechanism. Partial saturation of the transporter occurred at euglycemic concentrations of serum insulin. Thus, insulin enters the brain by a saturable transport system that is operational primarily at physiological levels of serum insulin.

Passage of peptides across the blood-brain barrier: pathophysiological perspectives

Blood-borne peptides are capable of affecting the central nervous system (CNS) despite being separated from the CNS by the blood-brain barrier (BBB), a monolayer comprised of brain endothelial and ependymal cells. Blood-borne peptides can directly affect the CNS after they cross the BBB by nonsaturable and saturable transport mechanisms. The ability of peptides to cross the BBB to a meaningful degree suggests that the BBB may act as a modulatory pathway in the exchange of informational molecules between the brain and the peripheral circulation. The permeability of the BBB to peptides is a regulatory process affected by developmental, physiological, and pathological events. This regulation sets the stage for the relation between peptides and the BBB to be involved in pathophysiological events. For example, some of the classic actions of melanocortins on the CNS are explained by their abilities to cross the BBB, whereas aspects of feeding and alcohol-related behaviors are associated with the passage of other specific peptides across the BBB. The BBB should no longer be considered a static barrier but should be recognized as a regulatory interface controlling the exchange of informational molecules, such as peptides, between the blood and CNS.

Aluminum, altered transcription, and the pathogenesis of Alzheimer's disease

The etiology of some, if not all, cases of Alzheimer's disease is linked to a mutation in the proximal portion of the long arm of chromosome 21∶21q11.2 → 21q22.2. While the functional consequences of the mutation are unknown, we speculate that one consequence of the mutation is loss of the natural barriers and intracellular ligands for aluminum. As a result, aluminum gains access to several brain sites including the nuclear compartment in certain neurons of the central nervous system.Both sporadic and familial Alzheimer's disease are associated with an increased compaction of DNA within chromatin as measured by physical shearing and resistance to digestion by micrococcal nuclease and DNase I. There is also an increase in linker histone Hl(o) content on dinucleosomes released by light (3-5% ASN) micrococcal nuclease digestion, and an increase in the affinity of histone Hl(o) for DNA as measured by a salt elution technique. The change in enzyme accessibility to chromatin also involves the 5' promoter region of at least one physiologically important gene: the gene which codes for the low molecular weight moiety of neurofilament (NF-L). The conformation change involving the 5' regulator region probably reduces transcription because the pool size of the mRNA coding for NF-L is reduced to 14% of age matched control in cerebral grey matter. Reduced transcription may account for many disorders in cellular metabolic processes including the regulation of phosphorylation, calcium homeostasis, free radical metabolism, proteolysis and neurotransmitter metabolism.The experimental evidence indicates that one important toxic action of aluminum in Alzheimer's disease neocortex is to increase the binding of histones, particularly Hl(o), to DNA which results in increased compaction of chromatin and reduced transcription. The supporting evidence includes: (1) A statistically reliable correlation between the aluminum to DNA ratio on intermediate euchromatin and the amount of highly condensed heterochromatin found in a given preparation from Alzheimer affected neocortex (Crapperet al., 1980). (2) A nine-fold increase in aluminum content in Alzheimer's disease in the di- and tri- nucleosome fraction released by light micrococcal nuclease digestion of nuclei from cerebral grey matter compared to age matched controls. Compared to age matched control dinucleosomes, the Alzheimer affected dinucleosomes contain an increased abundance of the linker histone Hl(o) and an increased proportion of DNA containing the promoter region of the gene coding for NF-L. (3) A reduction in abundance to 14% of control mRNA coding for NF-L in Alzheimer affected neocortex (Crapper McLachlanet al., 1988). (4) In vitro evidence that Alzheimer linker histones bind more tightly to DNA than control and that aluminum added to nuclei,in vitro, extracted from normal control brain, enhances DNA-protein binding of Hl and Hl(o) at concentrations found in the Alzheimer affected chromatin (Lukiwet al., 1987). (5) Application of a band retardation assay indicates that aluminum,in vitro, selectively binds human Hl(o) to a 300 bp human ALU DNA fragment from a crude extract of 5% per chloric acid soluble proteins. (6) Aluminum experimentally applied to rabbit CNS induces a marked reduction in NF-L mRNA in anterior horn cells (Mumaet al., 1988). We therefore conclude that aluminum plays a major role in the pathogenesis of Alzheimer's disease. Further understanding of the role of aluminum in Alzheimer's disease requires a detailed investigation of the precise sites of co-ordination of this trivalent metal within chromatin.

Aluminum-induced neurotoxicity: alterations in membrane function at the blood-brain barrier

Aluminum is established as a neurotoxin, although the basis for its toxicity is unknown. It recently has been shown to alter the function of the blood-brain barrier (BBB), which regulates exchanges between the central nervous system (CNS) and peripheral circulation. The BBB owes its unique properties to the integrity of the cell membranes that comprise it. Aluminum affects some of the membrane-like functions of the BBB. It increases the rate of transmembrane diffusion and selectively changes saturable transport systems without disrupting the integrity of the membranes or altering CNS hemodynamics. Such alterations in the access to the brain of nutrients, hormones, toxins, and drugs could be the basis of CNS dysfunction. Aluminum is capable of altering membrane function at the BBB; many of its effects on the CNS as well as peripheral tissues can be explained by its actions as a membrane toxin.

Interactions between the blood-brain barrier and endogenous peptides: emerging clinical implications

The effects of peptides on brain function suggest therapeutic and pathologic roles for these substances. Many peptides cross the blood-brain barrier (BBB) by transmembrane diffusion as a function of their lipid solubilities. Other peptides, such as the enkephalins, Tyr-MIF-1, vasopressin-related peptides, and peptide T-like peptides, are transported by carrier-mediated systems. Passage is influenced by aging, stress, lighting, drugs, amino acids, and neurotoxins. Disruption of the BBB results in complex changes in the blood and CSF levels of peptides. Peptides influence the passage of glucose, amino acids, and inorganic acids and may affect the integrity of the BBB. Peptide-BBB interactions have been suggested to play direct roles in dialysis dementia and maple syrup urine disease; they may be expected to be involved in other disorders of the CNS.

D-[Ala1]-peptide T-amide is transported from blood to brain by a saturable system

It is becoming increasingly evident that peptides can cross the blood-brain barrier. The entry into the central nervous system of a commercially available analog of Peptide T, an octapeptide derived from the human immunodeficiency virus envelope glycoprotein 120, was studied in several experiments. It was found that 125I-Peptide T analog given intravenously in the periphery entered the brain in an intact form, as confirmed by HPLC, to a greater extent than did the labeled albumin control. This entry occurred despite the very low lipid solubility, measured by the octanol/buffer partition coefficient, for the iodinated analog. The rate of entry was decreased by unlabeled Peptide T analog, but not by iodo-tyrosine. Saturable transport out of the brain was not observed after intraventricular administration. Thus, results with 125I-Peptide T analog indicate that saturable systems can transport peptides from the blood into the central nervous system.

Delta-sleep-inducing peptide (DSIP): an update

The isolation and characterization of delta-sleep-inducing peptide (DSIP) achieved from 1963 to 1977 were reviewed in 1984. The first reports describing sleep as well as extra-sleep effects of DSIP also were included in that work. Only two years later, much additional literature concerning DSIP has accumulated. Besides further sleep-inducing and/or -supporting effects of DSIP in animals, considerable work has been carried out to evaluate the potential use of the peptide for therapeutic purposes such as treatment of insomnia, pain, and withdrawal. Immunohistochemical as well as radioimmunochemical studies provided further insights into the natural occurrence of the nonpeptide and the distribution of DSIP-like material in the body, suggesting possible relations of the peptide to certain diseases. Various physiological functions of DSIP and a possible mechanism of action involving the modulation of adrenergic transmission remain to be established.

Carrier-mediated transport of enkephalins and N-Tyr-MIF-1 across blood-brain barrier

The saturable, carrier-mediated system capable of the brain-to-blood transport of small peptides with an N-terminal tyrosine was characterized. The rate of disappearance of intraventricularly injected iodinated peptide in the presence or absence of the inhibitor being tested was determined from formulas based on the residual radioactivity in the brains of mice after decapitation. The injection of 100 nmol/mouse of unlabeled N-Tyr-MIF-1 (TMIF) increased the half-time disappearance of 125I-TMIF (ITMIF) in the central nervous system (CNS) from 14.1 to 88.7 min (P less than 0.00005). Technetium, a substance transported out of the brain by the same system that transports iodine, was used as a control; the half-time disappearance of technetium pertechnetate was unaffected by unlabeled TMIF. With two related but distinct techniques, the maximum transport rate out of the CNS (Vmax) for TMIF was 0.266 nmol X g of brain per min (method 1) and 0.297 nmol X g-1 X min-1 (method 2), while the amount of unlabeled material needed to achieve 50% of Vmax (Km) was 15.2 nmol/g (method 1) and 15.1 nmol/g (method 2). The lack of effect of the tyrosinated fragments of TMIF as inhibitors indicates that TMIF is being transported in intact form. The Vmax for methionine enkephalin determined with labeled and unlabeled methionine enkephalin was 0.630 nmol X g-1 X min-1 and the Km was 24.95 nmol/g. Studies with the metabolic modulators furosemide, acetozolamide, reserpine, ouabain, and theophylline suggest that the system is sodium dependent and probably independent of ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptides and the blood-brain barrier: lipophilicity as a predictor of permeability

Eighteen peptides were examined for penetration across the blood-brain barrier (BBB) in rats. Iodinated peptides were injected via the carotid artery and 5 sec later the rats were decapitated. The results were expressed as a brain to blood ratio. The results showed that small amounts of most peptides enter the brain. The octanol coefficient, a measurement of lipophilicity, was a good predictor of penetration for most peptides. The low molecular weight, N-tyrosinated peptides (leucine-enkephalin, methionine enkephalin, N-Tyr-MIF-1, and N-Tyr-FMRF), that have been shown to be transported out of the brain by a saturable, carrier-mediated system, had much lower penetration rates than those predicted by their octanol coefficients. Molecular weight, percent of unbound peptide, total charge, net charge, and absolute charge were not good predictors of peptide penetration. Pretreatment of the rats with IP aluminum, which has been suggested to increase lipophilic permeability of the BBB, enhanced passage of 15 of the 18 peptides but not radioiodinated serum albumin or radioactive red blood cells. Thus, of the various physicochemical characteristics of peptides tested here, lipophilicity was most important in determining penetration of peptides across the BBB. However, the existence of one class of peptides that deviates from this trend (those of low molecular weight with an N-tyrosine) and the variability among the remaining peptides suggests that other, unidentified factors may be important in the prediction of peptide penetration across the BBB.

Permeability of the blood-brain barrier to neuropeptides: the case for penetration

Evidence that peptides can cross the blood-brain barrier (BBB) is reviewed. Penetration is suggested by the observations that blood levels correlate with cerebrospinal fluid levels for many peptides and that peripheral administration of peptides results in effects on the CNS. Passage is confirmed by experiments involving administration of a peptide (immunoactive or radioactive) in one compartment and identification of its appearance in the other, supported by such methods as selective labeling, cross-reactivity with highly specific antibodies, and chromatography. The degree of passage varies among peptides and their analogs. The major route of passage is probably by a non-competitive, non-saturable mechanism, wih the physicochemical characteristics of the peptide (e.g. lipophilicity, charge, molecular weight, and protein binding) determining the degree of passage. A competitive transport mechanism also exists for some peptides. Penetration of the BBB via large pores or by pinocytosis does not appear to be of major importance for peptides. Permeability of the BBB to peptides, but not to the larger iodinated albumin, is affected by intraperitoneal administration of aluminum, apparently by an increase in the permeability of the membrane to lipophilic materials.