Treatment effects in a transgenic mouse model of Alzheimer's disease: a magnetic resonance spectroscopy study after passive immunization

Despite the enormous public health impact of Alzheimer's disease (AD), no disease-modifying treatment has yet been proven to be efficacious in humans. A rate-limiting step in the discovery of potential therapies for humans is the absence of efficient non-invasive methods of evaluating drugs in animal models of disease. Magnetic resonance spectroscopy (MRS) provides a non-invasive way to evaluate the animals at baseline, at the end of treatment, and serially to better understand treatment effects. In this study, MRS was assessed as potential outcome measure for detecting disease modification in a transgenic mouse model of AD. Passive immunization with two different antibodies, which have been previously shown to reduce plaque accumulation in transgenic AD mice, was used as intervention. Treatment effects were detected by MRS, and the most striking finding was attenuation of myo-inositol (mIns) increases in APP-PS1 mice with both treatments. Additionally, a dose-dependent effect was observed with one of the treatments for mIns. MRS appears to be a valid in vivo measure of anti-Aβ therapeutic efficacy in pre-clinical studies. Because it is noninvasive, and can detect treatment effects, use of MRS-based endpoints could substantially accelerate drug discovery.

Regional differences in MRI detection of amyloid plaques in AD transgenic mouse brain

Our laboratory and others have reported the ability to detect individual Alzheimer's disease (AD) amyloid plaques in transgenic mouse brain in vivo by magnetic resonance imaging (MRI). Since amyloid plaques contain iron, most MRI studies attempting to detect plaques in AD transgenic mouse brain have employed techniques that exploit the paramagnetic effect of iron and have had mixed results. In the present study, using five-way anatomic spatial coregistration of MR images with three different histological techniques, properties of amyloid plaques in AD transgenic mouse brain were revealed that may explain their variable visibility in gradient- and spin-echo MR images. The results demonstrate differences in the visibility of plaques in the cortex and hippocampus, compared to plaques in the thalamus, by the different MRI sequences. All plaques were equally detectable by T(2)SE, while only thalamic plaques were reliably detectable by T(2)*GE pulse sequences. Histology revealed that cortical/hippocampal plaques have low levels of iron while thalamic plaques have very high levels. However, the paramagnetic effect of iron does not appear to be the sole factor leading to the rapid decay of transverse magnetization (short T(2)) in cortical/hippocampal plaques. Accordingly, MRI methods that rely less on iron magnetic susceptibility effect may be more successful for eventual human AD plaque MR imaging, particularly since human AD plaques more closely resemble the cortical and hippocampal plaques of AD transgenic mice than thalamic plaques.

Amyloid beta peptide as a vaccine for Alzheimer's disease involves receptor-mediated transport at the blood-brain barrier

Much research is now focused on a potential vaccine for Alzheimer's disease (AD). Current studies involve administering the amyloid beta peptide (Abeta) in Freund's complete adjuvant, which cannot be used in humans. Our studies show that the immune complex of Abeta is taken up by a receptor-mediated process at the blood-brain barrier (BBB). The success of immunization for AD, therefore, may be critically dependent on circulating Abeta levels which are lower in AD patients compared to AD transgenic mice. Moreover, we have found that modifying the antibody with polyamine increases its BBB permeability and may provide a better approach to passive immunization for Alzheimer's disease.

Permeability of proteins at the blood-brain barrier in the normal adult mouse and double transgenic mouse model of Alzheimer's disease

The permeability of albumin, insulin, and human A beta 1--40 at the blood-brain barrier (BBB) was determined in the normal adult mouse (B6/SJL) and in the double transgenic Alzheimer mouse (APP, PS1) by using an I.V. bolus injection technique to quantify the permeability coefficient-surface area (PS) product for each protein after correction for the residual plasma volume (V(p)) occupied by the protein in the blood vessels of different brain regions using a second aliquot of the same protein radiolabeled with a different isotope of iodine ((125)I vs (131)I) as a vascular space marker. This technology for quantifying BBB permeability of proteins was adapted from the rat to the mouse and involved catheterizing the femoral artery and vein of the mouse instead of the brachial artery and vein as for the rat. Because of the smaller blood volume in the mouse, serial sampling (20 microl) of blood from the femoral artery of the mouse was performed and directly TCA precipitated to generate a whole blood washout curve for the intact protein. When similar blood sampling techniques were used in the rat, the PS values for albumin and insulin at the BBB were similar in these two species. In the double transgenic mouse, the V(p) values for albumin were significantly increased 1.4- to 1.6-fold in five of six brain regions compared to the normal adult mouse, which indicated increased adherence of albumin to vessel walls. As a result, the PS values were significantly decreased, from 1.4- to 3.2-fold, which likely reflected decreased transport of albumin by passive diffusion. In contrast, insulin, which is taken up into the brain by a receptor-mediated transport mechanism at the BBB, showed no significant difference in the V(p) values but a significant increase in the PS values in four of six brain regions. This suggests a compensatory mechanism in the Alzheimer's transgenic brain whereby there is an increased permeability to insulin at the BBB. Surprisingly, there was no significant difference in the V(p) or PS values for human A beta 1--40 at the BBB in the double transgenic Alzheimer mouse at 24, 32, or 52 weeks of age, when there is both significant A beta levels in the plasma and amyloid burden in the brains of these animals. These data suggest that there is not an alteration in permeability to human A beta 1--40 at the BBB with increasing amyloid burden in the double transgenic Alzheimer mouse. Although these observations suggest structural alterations at the BBB, they do not support the concept of extensive BBB damage with substantial increases in BBB permeability in Alzheimer's disease.

Quantitative histological analysis of amyloid deposition in Alzheimer's double transgenic mouse brain

The development of transgenic mice has created new opportunities for the generation of animal models of human neurodegenerative diseases where previously there was no animal counterpart. The first successful transgenic mouse model of Alzheimer's disease expressed increased levels of mutant human amyloid precursor protein, exhibiting neuritic-type amyloid deposits and behavioral deficits at six to nine months of age. More recently, it was shown that transgenic mice expressing both mutant human amyloid precursor protein and presenilin 1 exhibit neuritic-type amyloid deposits and behavioral deficits in as little as 12 weeks. This accelerated Alzheimer phenotype greatly reduces the time necessary to conduct preclinical drug trials, as well as animal housing costs. The purpose of this study was to quantify the deposition of amyloid in five regions of the cortex and two regions of the hippocampus of transgenic mice expressing amyloid precursor protein (K670N, M671L) and presenilin 1 (M146L) mutations at various ages, using quantitative methods of confocal laser scanning microscopy and image analysis. Amyloid burden, expressed as the percentage area occupied by thioflavin S-positive amyloid deposits, increased an average of 179-fold from 12 to 54 weeks of age (0.02+/-0.01% to 3.57+/-0.29%, mean+/-S.E.M., respectively) in five regions of the cortex and two of the hippocampus. This was a function of increases in both deposit number and size. This transgenic mouse provides an ideal animal model for evaluating the efficacy of potential therapeutic agents aimed at reducing amyloid deposition, such as inhibitors of amyloid fibril formation or secretase inhibitors.

Therapeutic benefit of polyamine-modified catalase as a scavenger of hydrogen peroxide and nitric oxide in familial amyotrophic lateral sclerosis transgenics

Continuous subcutaneous administration of polyamine-modified catalase that has increased permeability at the blood-brain barrier showed both a highly significant delay in onset and an increase in survival in a transgenic mouse model of familial amyotrophic lateral sclerosis having a point mutation in the gene encoding copper/zinc superoxide dismutase. These results suggest that hydrogen peroxide-mediated oxidative stress with subsequent free radical damage involving nitric oxide and possibly hydroxyl radicals in motor neurons may be the culprit in familial amyotrophic lateral sclerosis.

Targeting alzheimer amyloid plaques in vivo

The only definitive diagnosis for Alzheimer disease (AD) at present is postmortem observation of neuritic plaques and neurofibrillary tangles in brain sections. Radiolabeled amyloid-beta peptide (Abeta), which has been shown to label neuritic plaques in vitro, therefore could provide a diagnostic tool if it also labels neuritic plaques in vivo following intravenous injection. In this study, we show that the permeability of Abeta at the blood-brain barrier can be increased by at least twofold through covalent modification with the naturally occurring polyamine, putrescine. We also show that, following intravenous injection, radiolabeled, putrescine-modified Abeta labels amyloid deposits in vivo in a transgenic mouse model of AD, as well as in vitro in human AD brain sections. This technology, when applied to humans, may be used to detect plaques in vivo, allowing early diagnosis of the disease and therapeutic intervention before cognitive decline occurs.

Obesity is associated with a decreased leptin transport across the blood-brain barrier in rats

Leptin exerts important effects on the regulation of food intake and energy expenditure by acting in the brain. Leptin is secreted by adipocytes into the bloodstream and must gain access to specific regions in the brain involved in regulating energy balance. Its action is mediated by interaction with a receptor that is mainly expressed in the hypothalamus but is also present in other cerebral areas. To reach these target areas, leptin most likely needs to cross the blood-brain barrier (BBB). In this study, we compared the permeability of leptin at the BBB in homozygous lean (FA/FA), high-fat diet-induced (HFD) obese rats (FA/FA rats on a highfat diet), and genetically obese fa/fa Zucker rats by quantifying the permeability coefficient surface area (PS) product after correction for the residual plasma volume (Vp) occupied by leptin in the vessel bed of different brain regions. The intravenous bolus injection technique was used in the cannulated brachial vein and artery using leptin radioiodinated with 2 isotopes of iodine (125I and 131I) to separately determine the PS and Vp values. The PS for leptin at the BBB in lean FA/FA rats ranged from 11.0 +/- 1.6 at the cortex to 14.8 +/- 1.4 x 10(-6) ml x g(-1) x ml(-1) at the posterior hypothalamus. The PS for leptin in HFD obese FA/FA and obese fa/fa rats ranged from 3.0- to 4.0-fold lower than in lean FA/FA rats. The Vp values were not significantly different among the 3 groups studied. SDS-PAGE analysis of the radioiodinated leptin after 60 min of uptake revealed intact protein in the 8 different brain regions. Plasma leptin levels were significantly higher in both obese rat groups compared with those in lean FA/FA rats. Leptin levels in cerebrospinal fluid were not significantly different among the 3 groups of rats. These findings strongly suggest that the leptin receptor (OB-R) in the BBB can be easily saturated. Saturation of the BBB OB-R in obese individuals would explain the defect in leptin transport into the brain described in this study.

Therapeutic benefits of putrescine-modified catalase in a transgenic mouse model of familial amyotrophic lateral sclerosis

Dominant mutations in the copper/zinc superoxide dismutase (SOD1) gene have been observed in 15-20% of familial amyotrophic lateral sclerosis (FALS) cases. The mechanism by which SOD1 mutations result in motor neuron degeneration in FALS mice partly involves oxidative damage and an increased peroxidase activity of the mutant SOD1. A new therapeutic approach designed to eliminate the substrate of this peroxidase activity was examined in two lines of transgenic mice expressing the FALS-linked mutation glycine to alanine (G93A). We investigated the ability of putrescine-modified catalase (PUT-CAT), an antioxidant enzyme that removes hydrogen peroxide and has increased permeability at the blood-brain barrier, to modify the time course of the SOD1 mutation-induced motor neuron disease in these FALS mice. Continuous, subcutaneous administration of PUT-CAT significantly delayed the age at which onset of clinical disease occurred (indicated by loss of splay and/or tremors of hindlimbs) in a high-expressor line of FALS transgenic mice. Intraperitoneal injection of PUT-CAT given two times per week also significantly delayed the onset of clinical disease in a low-expressor line of FALS mice. PUT-CAT also significantly delayed the age at which clinical weakness developed (quantified by measuring the shortening of stride length) in both lines of FALS animals. No significant changes were observed in the survival times of the high-expressor FALS mice in any of the treatment groups. However, a trend toward a prolongation of survival was observed in the PUT-CAT-treated low-expressor FALS mice. These results support the role of free radical-mediated damage in the cascade of events leading to motor neurodegeneration in FALS and indicate that PUT-CAT interacts with a critical step in this cascade to delay the onset of clinical disease as well as the development of clinical weakness in FALS transgenic mice.

Plasma pharmacokinetics, nervous system biodistribution and biostability, and spinal cord permeability at the blood-brain barrier of putrescine-modified catalase in the adult rat

Free radical-mediated oxidative damage has been proposed to be an underlying mechanism in several neurodegenerative disorders. Previous investigations in our laboratory have shown that putrescine-modified catalase (PUT-CAT) has increased permeability at the blood-brain (BBB) and blood-nerve barriers with retained enzymatic activity after parenteral administration when compared to native catalase (CAT). The goals of the present study were to examine the plasma stability, spinal cord BBB permeability, nervous system biodistribution, and spinal cord enzyme activity of CAT and PUT-CAT after parenteral administration in the adult rat. TCA precipitation and chromatographic analyses revealed that CAT and PUT-CAT were found intact in the plasma and in the central nervous system (CNS) after iv, ip, or sc bolus injections. The highest percentages of intact CAT or PUT-CAT proteins were found in the plasma after iv administration, and similar percentages of intact CAT or PUT-CAT were found in the CNS following all three types of administration. Increases of 2.4- to 4.7-fold in permeability at the BBB and similar increases in the levels of intact PUT-CAT were found in different brain regions compared to the levels of CAT. A 2.4-fold higher level of intact PUT-CAT compared to that of CAT (P < 0.05) was found in the spinal cord 60 min after a sc bolus injection. CAT enzyme activity in the spinal cord was 50% higher (P < 0.05) in rats treated with PUT-CAT continuously for 1 week by subcutaneously implanted, osmotic pumps than the activity found in rats treated with PBS. These results provide evidence that intact, enzymatically active PUT-CAT is efficiently delivered to the nervous system following iv, ip, and sc administration and suggest that sc administration of PUT-CAT may be effective in treating neurodegenerative disorders in which the underlying mechanisms involve the action of free radicals and oxidative damage.

Beta-sheet breaker peptide inhibitor of Alzheimer's amyloidogenesis with increased blood-brain barrier permeability and resistance to proteolytic degradation in plasma

Short synthetic peptides homologous to the central region of Abeta but bearing proline residues as beta-sheet blockers have been shown in vitro to bind to Abeta with high affinity, partially inhibit Abeta fibrillogenesis, and redissolve preformed fibrils. While short peptides have been used extensively as therapeutic drugs in medicine, two important problems associated with their use in central nervous system diseases have to be addressed: (a) rapid proteolytic degradation in plasma, and (b) poor blood-brain barrier (BBB) permeability. Recently, we have demonstrated that the covalent modification of proteins with the naturally occurring polyamines significantly increases their permeability at the BBB. We have extended this technology to iAbeta11, an 11-residue beta-sheet breaker peptide that inhibits Abeta fibrillogenesis, by covalently modifying this peptide with the polyamine, putrescine (PUT), and evaluating its plasma pharmacokinetics and BBB permeability. After a single intravenous bolus injection in rats, both 125I-YiAbeta11 and 125I-PUT-YiAbeta11 showed rapid degradation in plasma as determined by trichloroacetic acid (TCA) precipitation and paper chromatography. By switching to the all D-enantiomers of YiAbeta11 and PUT-YiAbeta11, significant protection from degradation by proteases in rat plasma was obtained with only 1.9% and 5.7% degradation at 15 min after intravenous bolus injection, respectively. The permeability coefficient x surface area product at the BBB was five- sevenfold higher in the cortex and hippocampus for the 125I-PUT-D-YiAbeta11 compared to the 125I-D-YiAbeta11, with no significant difference in the residual plasma volume. In vitro assays showed that PUT-D-YiAbeta11 retains its ability to partially inhibit Abeta fibrillogenesis and dissolve preformed amyloid fibrils. Because of its five- to sevenfold increase in permeability at the BBB and its resistance to proteolysis in the plasma, this polyamine-modified beta-sheet breaker peptide may prove to be an effective inhibitor of amyloidogenesis in vivo and, hence, an important therapy for Alzheimer's disease.

Receptor-mediated transport of human amyloid beta-protein 1-40 and 1-42 at the blood-brain barrier

Since amyloid beta-protein (A beta) is the primary component of both vascular and parenchymal amyloid deposits in Alzheimer's disease, information regarding its permeability at the blood-brain barrier (BBB) will help elucidate the contribution of circulating A beta to vascular and parenchymal A beta deposition in this disease and in brain aging. The permeability of the D- and L-enantiomers of A beta 1-40 and L-A beta 1-42 at the BBB was determined in the normal adult rat by quantifying the permeability coefficient-surface area product (PS) for each protein after correction for the residual plasma volume (Vp) occupied by the protein [labeled with a different isotope of iodine (125I vs 131I)] in blood vessels of different brain regions. After a single i.v. bolus injection, the plasma pharmacokinetics determined by TCA precipitation, paper chromatography, and SDS-PAGE were similar for both 125I-L-A beta 1-40 and 125I-L-A beta 1-42. The PS at the BBB for L-A beta 1-42 was significantly (1.4- to 1.8-fold) higher than for L-A beta 1-40 and ranged from 17.7 to 26.4 x 10(-6) ml/g/s for different brain regions. A comparison of the PS values at the BBB for L-A beta 1-40 showed no significant difference when determined at 15 or 30 min after i.v. bolus injection, times that reflect different levels of degradation in plasma (37.9% at 15 min and 65.5% at 30 min). The PS values obtained, therefore, were representative of the intact protein rather than degradation products. The PS values obtained for the all-D-enantiomer of A beta 1-40 were very low and comparable to that of albumin and IgG, whose mechanism of transport is by passive diffusion. Taken together, these data imply a stereoisomer-specific, ligand-receptor interaction at the BBB for the L-A beta proteins. The high PS values observed for L-A beta 1-40 and 1-42 compare to insulin, whose uptake is decidedly by a receptor-mediated transport process, and suggest a similar mechanism for L-A beta entry into the brain.

Putrescine-modified nerve growth factor: bioactivity, plasma pharmacokinetics, blood-brain/nerve barrier permeability, and nervous system biodistribution

Previous investigations from our laboratory have demonstrated that the covalent modification of a variety of proteins, including antioxidant enzymes, with the naturally occurring polyamines--putrescine (PUT), spermidine, and spermine--dramatically increases their permeability coefficient-surface area product (PS) at the blood-brain and blood-nerve barriers after parenteral administration. In the present study, we have covalently modified nerve growth factor (NGF) with PUT by targeting carboxylic groups for their graded modification by controlling the ionization of these groups with pH. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis, western, and isoelectric focusing analyses demonstrated conversion of NGF to its polyamine-modified derivatives at different pH values. Although the immunoreactivity of PUT-NGF determined by ELISA and western analysis decreased with decreasing pH, the biological activity of PUT-NGF was not affected at any pH as determined by survival and neurite extension of dorsal root ganglia and PC12 cultures. Plasma pharmacokinetics after a single intravenous bolus administration revealed intact PUT-NGF through 10 min and 73-82% intact protein at 15 min. The PS value for PUT-NGF was maximized and the residual plasma volume (Vp) of the protein in the blood vessels minimized when the pH of the modification reaction was >6.4. The biodistribution of PUT-NGF at 15 min showed 22-33% intact protein in different brain regions, which represented 0.4-5.9 ng of PUT-NGF in different brain regions, a physiological dose that is capable of eliciting a bioresponse. The design of this polyamine-modified NGF derivative that has enhanced permeability at the blood-brain and blood-nerve barriers with retained bioactivity may obviate the necessity to create small-molecule mimics of NGF and may be applicable to neurotrophins, engineered multifunctional chimeric neurotrophins, antioxidant enzymes, and other therapeutic proteins with specific clinical application to neurological diseases.

Activity of cyclic AMP phosphodiesterases and adenylyl cyclase in peripheral nerve after crush and permanent transection injuries

Recent studies demonstrate that cAMP levels are tightly controlled during demyelination and remyelination in Schwann cells as cAMP decreases to 8-10% of normal following both sciatic nerve crush or permanent transection injury and only begins to increase in the crushed nerve after remyelination (Poduslo, J. F., Walikonis, R. S., Domec, M., Berg, C. T., and Holtz-Heppelmann, C. J. (1995) J. Neurochem. 65, 149-159). To investigate the mechanisms responsible for this change in cAMP levels, cAMP phosphodiesterase (PDE) and adenylyl cyclase activities were determined before and after sciatic nerve injury. Basal cAMP PDE activity in soluble endoneurial homogenates of normal nerve was 34.9 +/- 1.9 pmol/mg of protein/min (chi +/- S.E.; n = 10). This activity increased about 3-fold within 6 days following both injuries. Basal PDE activity remained elevated in the transected nerve, but declined to 70 pmol/mg of protein/min in the crushed nerve at 21 and 35 days following injury. Isozyme-specific inhibitors and stimulators were used to identify the PDE families in the sciatic nerve. The low Km cAMP-specific (PDE4) and the Ca2+/calmodulin-stimulated (PDE1) families were found to predominate in assays using endoneurial homogenates. The PDE4 inhibitor rolipram also increased cAMP levels significantly after incubation of endoneurial tissue with various isozyme-specific inhibitors, indicating that PDE4 plays a major role in determining cAMP levels. PDE4 mRNA was localized by in situ hybridization to cells identified as Schwann cells by colabeling of S100, a Schwann cell specific protein. Adenylyl cyclase activity declined following injury, from 3.7 pmol/mg of protein/min in normal nerve to 0.70 pmol/mg/min by 7 days following injury. Both decreased synthesis and increased degradation contribute, therefore, to the reduced levels of cAMP following peripheral nerve injury and are likely critical to the process of Wallerian degeneration.

Putrescine-modified catalase with preserved enzymatic activity exhibits increased permeability at the blood-nerve and blood-brain barriers

Much evidence exists in support of the hypothesis that free radicals contribute to the pathogenesis of several neurodegenerative disorders and that mechanisms of free radical generation occur both intracellularly and extracellularly. Previous studies in this laboratory have shown that covalent modification of growth factors and antioxidant enzymes with the naturally occurring polyamine, putrescine, increases their permeability at the blood-nerve and blood-brain barriers (BNB and BBB), but does not significantly inhibit bioactivity. Furthermore, putrescine-modified superoxide dismutase (SOD) was shown to reduce neurodegeneration in a rat model of global cerebral ischemia. The purpose of the present study was to modify the antioxidant enzyme, catalase (CAT), with putrescine (PUT) at carboxylic acid groups whose ionization, and hence reactivity, was controlled with pH and investigate the effects on permeability and enzymatic activity. Modification of CAT with PUT increased its permeability 2-3-fold and preserved 67% of its enzymatic activity compared to native CAT and 137% compared to lyophilized CAT. The results of this study indicate that modification of CAT with putrescine increases its permeability while preserving enzymatic activity. PUT-SOD administered in combination with PUT-CAT may eliminate both the superoxide radical and the H2O2 produced from the dismutation of superoxide, respectively, and thus prevent the formation of hydroxyl radicals. This combination may exhibit increased neuroprotective effects, compared to native enzymes, following systemic administration for the treatment of free radical associated neurodegenerative disorders.

Postischemic, systemic administration of polyamine-modified superoxide dismutase reduces hippocampal CA1 neurodegeneration in rat global cerebral ischemia

Antioxidant enzymes such as superoxide dismutase (SOD) have shown neuroprotective effects in animal models of cerebral ischemia, but only at very high doses. Modifications to increase the plasma half-life or blood-brain barrier (BBB) permeability of SOD have resulted in limited neuroprotective effects. No one has demonstrated neuroprotection with postischemic administration. The specific aim of the present study was to administer systemically a polyamine-modified SOD, having increased BBB permeability and preserved enzymatic activity, following global cerebral ischemia in rats and analyze the effects on the selective vulnerability of CA1 hippocampal neurons. Following 12 min of four-vessel occlusion, global cerebral ischemia, male Wistar rats were dosed (i.v.) with either saline, native SOD (5000 U/kg), polyamine-modified SOD (5000 U/kg), or enzymatically inactive, polyamine-modified SOD (2.1 mg/kg) twice daily for 3 days. Neuroprotective effects on hippocampal CA1 neurons were assessed using standard histological methods. Saline-treated animals had very few remaining CA1 neurons (1.44 +/- 0.60 neurons/reticle; x +/- S.E.M.) compared to sham rats (58.57 +/- 0.69). Native (10.38 +/- 2.96) or inactive, polyamine-modified SOD (7.32 +/- 2.68) did not show significant neuroprotective effects. Polyamine-modified SOD, however, resulted in the survival of significantly more CA1 neurons (24.61 +/- 5.90; P < 0.01). Postischemic, systemic administration of polyamine-modified SOD, having increased BBB permeability and preserved enzymatic activity, significantly reduced hippocampal CA1 neuron loss following global cerebral ischemia. Similar modification of other antioxidant enzymes and neurotrophic factors with polyamines may provide a useful technique for the systemic delivery of therapeutic proteins across the BBB for the treatment of stroke and other neurodegenerative disorders.

Permeability and residual plasma volume of human, Dutch variant, and rat amyloid beta-protein 1-40 at the blood-brain barrier

The permeability of normal human, the human Dutch variant, and the rat A beta 1-40 proteins at the blood-brain barrier (BBB) was determined in the normal adult rat by quantifying the permeability coefficient-surface area (PS) product for each protein after correction for the residual plasma volume (Vp) occupied by the protein in the blood vessels of different brain regions. The PS for normal and Dutch A beta ranged from 13 x 10(-6) to 22 x 10(-6) ml/g/s in different brain regions, which is 130 to 220 times greater than albumin. These high PS values compare to that of insulin, whose uptake is decidedly by a receptor-mediated transport process, and suggest a similar mechanism for A beta. Remarkably, the PS for rat A beta was 4 times higher and ranged from 54 x 10(-6) to 82 x 10(-6) ml/g/s for different brain regions, suggesting a distinctive species specificity. While the Vp values of human and rat A beta were comparable, the Dutch variant was 2 to 3 times higher, indicating adherence to the vessel walls in different brain regions, consistent with the heavy A beta deposition that has been described in intracerebral vessel walls with this variant. The high PS values observed for A beta at the BBB suggest that sources outside the nervous system could contribute, at least in part, to the cerebral A beta deposits seen in Alzheimer's disease. SDS-PAGE of 125I-labeled human A beta after 60 min of uptake revealed intact protein in plasma and in different brain regions. In addition, 125I-labeled human A beta binding to a protein of 67,000 in both plasma and brain tissue regions was observed with SDS-PAGE. This protein was tentatively identified as albumin, and it was not detectable in the brain regions of animals that had undergone intracardiac perfusion; hence, a portion of A beta binds tightly to and is likely transported by albumin in plasma. The absence of this A beta-albumin complex in brain regions after perfusion and the low permeability of albumin at the BBB imply that A beta itself is efficiently transported at the BBB to account for the high PS values, although presentation of A beta to the capillary endothelial cell by albumin or other plasma proteins cannot be excluded.

Increased permeability of superoxide dismutase at the blood-nerve and blood-brain barriers with retained enzymatic activity after covalent modification with the naturally occurring polyamine, putrescine

Our previous studies have demonstrated that modification of superoxide dismutase (SOD) with the naturally occurring polyamines--putrescine (PUT), spermidine, and spermine--dramatically increases the permeability-coefficient surface area (PS) product at the blood-brain barrier and blood-nerve barrier after parenteral administration. Because of this increased permeability, the efficient delivery of polyamine-modified SOD (pSOD) across these barriers may enhance its therapeutic usefulness in treating ischemic neuronal degeneration, neurodegenerative disease, or even aging as an important antioxidant therapeutic strategy. Because PUT-SOD had the highest PS values, SOD was modified in the present experiments by activating carboxylic acid groups to the reactive ester with water-soluble carbodiimide and then reacted with PUT as the nucleophilic reagent. Preservation of SOD enzyme activity while maximizing the permeability was accomplished by adjusting the ionization of the protein carboxylic acid with pH. Both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing analyses demonstrated graded conversion of SOD to its polyamine-modified derivative when performed at different pH. Although modification at pH 4.7 resulted in only 6.6% retained SOD activity and the highest PS value (43.35 +/- 3.81 x 10(-6) ml/g/s for the hippocampus), modification at pH 5.7 resulted in 50.1 % retained activity with a PS value of 24.48 1.30 x 10(-6) ml/g/s for nerve endoneurium and 21.95 +/- 1.62 x 10(-6) ml/g/s for hippocampus. This contrasts with a PS of 1.8-3.2 x 10(-6) ml/g/s for native SOD in nerve and various brain regions. Reaction conditions are therefore defined that titrate enzyme activity of PUT-SOD with PS changes in the intact animal after intravenous administration. These studies will allow an evaluation of the therapeutic usefulness of pSOD in animal models of neuronal degeneration.

Polyamine modification increases the permeability of proteins at the blood-nerve and blood-brain barriers

The permeability of the blood-nerve barrier (BNB) and the blood-brain barrier (BBB) to superoxide dismutase (SOD), insulin, albumin, and IgG in normal adult rats was quantified by measuring the permeability coefficient-surface area product (PS) with the intravenous bolus injection technique before and after covalent protein modification with naturally occurring polyamines-putrescine (PUT), spermidine (SPD), and spermine (SPM). The PS value of the BNB for PUT-SOD was 21.1-fold greater than the native SOD, and the PS values of the BBB for PUT-SOD ranged from 17.6-fold greater for the thalamus to 23.6-fold greater for the caudate-putamen compared with native SOD. In a similar manner, polyamine-modified insulin showed a 1.7-2.0-fold increase in PS of the BNB and BBB compared with the high values of native insulin. Polyamine-modified albumin showed a remarkable 54-165-fold increase in PS of the BNB and BBB compared with native albumin, whereas PUT-IgG resulted in an even higher increase in the PS that ranged from 111- to 349-fold for nerve and different brain regions compared with native IgG. Polyamine modification of proteins, therefore, can dramatically increase the permeability at the BNB and BBB of a variety of proteins with widely differing M(r) and function. It is surprising that the PS values of the BNB and BBB decreased with the increasing number of positive charges of the protonated amino groups on the polyamines (PUT>SPD>SPM). Although cationic proteins are known to interact with fixed anionic charges on the lumen of the microvascular endothelium, this observation of decreased permeability with increased positive charge distribution along the aliphatic carbon chain of the polyamines implies mechanisms other than simple electrostatic interaction involving charge density. It is suggested that the polyamine transporter may be responsible for the transport of these polyamine-modified proteins. Systemic administration of polyamine-modified peptides and proteins might prove to be an efficient approach to deliver therapeutic agents into the CNS and PNS for the treatment of a variety of neurological diseases.

Permeability at the blood-brain and blood-nerve barriers of the neurotrophic factors: NGF, CNTF, NT-3, BDNF

A comparison was made of the permeabilities of different neurotrophic factors at the blood-brain barrier (BBB) and blood-nerve barrier (BNB) in normal adult rats by quantifying the permeability coefficient-surface area (PS) product after correction for the residual plasma volume (Vp) occupied by the protein in the capillary bed of the nerve endoneurium or different brain regions. The i.v. bolus injection technique was used in the cannulated brachial vein and artery using the same protein radioiodinated with a second isotope of iodine (125I vs. 131I) to separately determine the PS and Vp values. The plasma washout showed a decreasing plasma half-life in the order of brain-derived neurotrophic factor (BDNF) < neurotrophin-3 (NT-3) < ciliary neurotrophic factor (CNTF) < nerve growth factor (NGF). The PS at the BNB for NGF was 1.40 +/- 0.15 x 10(-6) ml/g/s (mean +/- SEM). The other neurotrophic proteins were all significantly higher than NGF (CNTF: 9.5 x ; NT-3: 20.8 x ; BDNF: 18.9 x ). The Vp for NGF at the BNB was 1.92 +/- 0.12 microliters/g and was not significantly different from the other proteins except for NGF vs. BDNF (P < 0.05). The PS for NGF at the BBB ranged from 1.5 to 2.7 x 10(-6) ml/g/s for six different brain regions. The PS for CNTF ranged from 6.0 to 8.0-fold higher than NGF; NT-3: 10.6 to 15.2-fold higher; and BDNF: 11.3 to 16.4-fold higher. The Vp values were not significantly different except for CNTF in the hippocampus and cortex (P < 0.05). SDS-PAGE analyses of all the radioiodinated neurotrophic proteins after 60 min of uptake revealed intact protein in the endoneurium and in the six different brain regions with exposure times of 2-42 days. The quantification of the permeability of these neurotrophic proteins provides baseline values for comparison of different protein modifications that enhance the PS while still preserving the neurotrophic activity (e.g., protein glycation; Poduslo and Curran, Mol. Brain Res., 23 (1994) 157). Enhanced permeability following modification might allow the use of systematic delivery of these proteins for practical therapeutic treatment of various neurodegenerative disorders.

The second messenger, cyclic AMP, is not sufficient for myelin gene induction in the peripheral nervous system

The adenylyl cyclase-cyclic AMP (cAMP) second messenger pathway has been proposed to regulate myelin gene expression; however, a clear correlation between endogenous cAMP levels and myelin-specific mRNA levels has never been demonstrated during the induction or maintenance of differentiation by the myelinating Schwann cell. Endogenous cAMP levels decreased to 8-10% of normal nerve by 3 days after crush or permanent transection injury of adult rat sciatic nerve. Whereas levels remained low after transection injury, cAMP levels reached only 27% of the normal values by 35 days after crush injury. Because P0 mRNA levels were 60% of normal levels by 14 days and 100% by 21 days after crush injury, cAMP increased only well after P0 gene induction. cAMP, therefore, does not appear to trigger myelin gene induction but may be involved in myelin assembly or maintenance. Forskolin, an activator of adenylyl cyclase, increased endoneurial cAMP levels only in the normal nerve, and in the crushed nerve beginning at 16 days after injury, but at no time in the transected nerve. Only by treating transected nerve with 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of cAMP phosphodiesterases, in combination with forskolin was it possible to increase cAMP levels. No induction of myelin genes, however, was observed with short- or long-term treatment with IBMX and forskolin in the transected nerve. A three-fold increase in phosphodiesterase activity was observed at 35 days after both injuries, and a nonmyelinated nerve was shown to have even higher activity. These experiments, therefore, suggest an important role for phosphodiesterase in the inactivation of this second messenger-dependent stimuli when Schwann cells are non-myelinating, such as after sciatic nerve injury or in the nonmyelinated nerve, which again implies that cAMP may be required for the maintenance of the myelin sheath.

Macromolecular permeability across the blood-nerve and blood-brain barriers

The permeability of insulin (Ins), nerve growth factor (NGF), albumin (Alb), transferrin (Trf), and IgG across the blood-nerve barrier (BNB) and blood-brain barrier (BBB) in normal adult rats was quantified by measuring the (permeability coefficient x surface area) product (PS) with the i.v. bolus-injection technique in the cannulated brachial vein and artery using radioiodinated proteins. The PS values of the BNB for IgG and Alb were low: 0.079 +/- 0.029 x 10(-6) and 0.101 +/- 0.088 x 10(-6) ml.g-1.s-1, (mean +/- SD, respectively). The PS values for NGF and Trf were 16.1-fold and 25.5-fold higher than for Alb. The PS for Ins across the BNB was 33.190 +/- 2.053 x 10(-6) ml.g-1.s-1--a remarkable 329-fold increase compared with Alb. The PS values of the BBB for IgG and Alb in different brain regions were all low, from 0.028 +/- 0.017 to 0.151 +/- 0.035 x 10(-6) ml.g-1.s-1 (mean +/- SD). NGF and Trf had comparable PS values from 13- to 32-fold higher than for Alb, except for the brain stem, where the PS for Trf was 66-fold higher than for Alb. The mean PS for Ins across the BBB ranged from 15.78 +/- 5.45 x 10(-6) ml.g-1.s-1 for the cortex to 22.62 +/- 7.50 x 10(-6) ml.g-1.s-1 for the brain stem--again a remarkable 105- to 390-fold increase relative to Alb. Because reliable PS measurements were obtained for all proteins tested, the BBB and BNB cannot be considered impermeable to proteins--a concept that has plagued brain- and nerve-barrier research. The low PS values for IgG and Alb indicate low rates of transfer; however, Alb, in particular, is the major protein of endoneurial and ventricular fluid, which suggests that these PS values may be significant. Ins had the highest PS values, which likely reflect the mechanism of transport across the barriers--that is, receptor-mediated transport. Because NGF and Trf had PS values 13- to 66-fold higher than for Alb, whether this reflects receptor-mediated uptake, adsorptive-mediated transcytosis, or some other mechanism is unclear. That the PS values for NGF and Trf differ from Alb and IgG clearly suggests, however, a different uptake mechanism. Finally, the remarkably high PS values for Ins across the BBB and BNB identify this protein and its putative receptor on capillary endothelial cells as a potential target for drug delivery into the central and peripheral nervous systems.

Glycation increases the permeability of proteins across the blood-nerve and blood-brain barriers

Our previous investigations have demonstrated increased permeability across the blood-nerve barrier of human plasma albumin after glycation with D-glucose [J.F. Poduslo and G.L. Curran, Proc. Natl. Acad. Sci. USA, 89 (1992) 2218-2222]. In the present investigation, the generality of this observation was evaluated by measuring the permeability coefficient-surface area product (PS) after correction for the residual plasma volume (Vp) across the blood-nerve barrier (BNB), as well as the blood-brain barrier (BBB), for nerve growth factor (NGF) and human IgG after in vitro glycation with D-glucose using an i.v. bolus injection technique in the cannulated brachial vein and artery of normal adult rats. Glycated proteins (gNGF and gIgG) had significantly decreased circulating plasma half-lives compared to the non-glycated proteins. The PS across the BNB obtained for gNGF was significantly increased compared to NGF with a 2.0-fold increase observed after 8 weeks of glycation and a 5.1-fold increase at 21 weeks of glycation. The Vp measurement for NGF and gNGF across the BNB was not significantly different at 8 weeks of glycation but was 1.3-fold greater at 21 weeks of glycation. The PS across the BBB for gNGF was about 2-fold greater than NGF with a glycation time of 8 weeks and 3.2-3.6-fold greater with a glycation time of 21 weeks for six different brain regions. No changes were observed in the Vp for any of the brain regions for gNGF compared to NGF. The PS across the BNB for gIgG compared to IgG was significantly greater with a 4.1-fold relative increase and no significant difference in the Vp values.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of myelin genes during peripheral nerve remyelination requires a continuous signal from the ingrowing axon

The effect of a permanent transection on myelin gene expression in a regenerating sciatic nerve and in an adult sciatic nerve was compared to establish the degree of axonal control exerted upon Schwann cells in each population. First, the adult sciatic nerve was crushed, and the distal segment allowed to regenerate. At 12 days post-crush, the sciatic nerve was transected distal to the site of crush to disrupt the Schwann cell-axonal contacts that had reformed. Messenger RNA (mRNA) levels coding for five myelin proteins were assayed in the distal segment of the crush-transected nerve after 9 days and were compared to corresponding levels in the distal segments of sciatic nerves at 21 days post-crush and 21 days post-transection using Northern blot and slot-blot analysis. Levels of mRNAs found in the distal segment of the transected and crush-transected nerve suggested that Schwann cells in the regenerating nerve and in the mature adult nerve are equally responsive to axonal influences. The crush-transected model allowed the genes that were studied to be classified according to their response to Schwann cell-axonal contact. The levels of mRNAs were 1) down-regulated to basal levels (P0 and MBP mRNAs), 2) down-regulated to undetectable levels (myelin-associated glycoprotein mRNAs), 3) upregulated (mRNAs encoding 2'3'-cyclic nucleotide phosphodiesterase and beta-actin), or 4) not stringently controlled by the removal of Schwann cell-axonal contact (proteolipid protein mRNAs). This novel experimental model has thus provided evidence that the expression of some of the important myelin genes during peripheral nerve regeneration is dependent on continuous signals from the ingrowing axons.

Regulation of 2',3'-cyclic nucleotide phosphodiesterase gene expression in experimental peripheral neuropathies

2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNPase) is an enzyme associated with central nervous system myelination. Although present in the mammalian peripheral nerve, it is not clear what its role is during myelination nor how the expression of this gene is regulated in the PNS. In this study, CNPase gene expression was studied in the crushed and permanently transected rat sciatic nerve, two models of peripheral nerve neuropathy. The Schwann cells of the crushed nerve initially demyelinate, remain in a non-myelinating condition until active regeneration induces remyelination (10-21 days after injury), whereas those of the permanently transected nerve remain in a quiescent, non-myelinating state after the initial demyelination. An increase of CNPase mRNA levels is observed during degeneration and remains high whether the peripheral nerve is regenerating or not, suggesting transcriptional activation of CNPase mRNA and/or increased CNPase mRNA stability as a response to nerve injury. In contrast, the steady state level of CNPase protein did not increase during degeneration or regeneration suggesting either negative translational regulation of CNPase gene expression or a higher turnover of this protein in the injured peripheral nerve. Furthermore, CNPase activity dropped sharply during early degeneration and remained low in the quiescent cells of the permanently transected nerve while it increased in the regenerating nerve. The results suggest that although transcriptional or post-transcriptional regulation of CNPase gene expression is not dependent on Schwann cell-axonal contact, the activity of CNPase appears to be dependent on myelination and indirectly dependent on the presence of axons in the peripheral nerve.

Increased permeability across the blood-nerve barrier of albumin glycated in vitro and in vivo from patients with diabetic polyneuropathy

The blood-nerve transfer of human plasma albumin glycated with D-glucose was investigated by measuring the permeability coefficient-surface area product (PS) of the blood-nerve barrier to radioiodinated albumin in normal adult rat sciatic nerve. Human albumin (ALB) from normal individuals, freshly isolated by CM-Affi-Gel Blue affinity chromatography, was glycated in vitro for 1, 3, 10, 19, and 30 weeks. Glycated ALB (gALB) was separated from the nonglycated form by boronate-affinity chromatography. The efficiency of this separation was assessed by chromatography of ALB glycated with [14C]glucose and by rechromatography of isolated ALB and gALB after radioiodination. The gALB was also shown to have a higher molecular weight and be completely separated from ALB after SDS/pore gradient electrophoresis in a Tris borate/EDTA buffer. After 1 week of glycation, the gALB PS was 2.2-fold greater than the ALB PS (0.724 +/- 0.063 x 10(-6) vs. 0.328 +/- 0.053 x 10(-6) ml.g-1.s-1; mean +/- SD; P less than 0.0001) and it increased with the time of glycation reaching a maximum value of 16.2-fold greater at 30 weeks (4.656 +/- 1.117 x 10(-6) vs. 0.288 +/- 0.042 x 10(-6) ml.g-1.s-1; mean +/- SD; P less than 0.0001). No change was observed in the residual endoneurial plasma volume. In addition, the PS of gALB isolated from patients with diabetic polyneuropathy was significantly increased (P less than 0.0001) compared to the PS for ALB isolated from the same patients. It is hypothesized that the increased permeability of gALB and presumably other glycated serum components across the blood-nerve barrier, as well as the observed quantitative increase in ALB, IgG, and IgM in sural nerve biopsies from patients with diabetic polyneuropathy contribute to the development of diabetic polyneuropathy over a prolonged period of time by mechanisms that might involve osmotic changes in the nerve microenvironment, direct toxic effects of glycated macromolecules on cells within the endoneurium, or nerve damage by classical immunological mechanisms due to trapping of glycated immunoglobulins within nerve.

P0 gene expression in Schwann cells is modulated by an increase of cAMP which is dependent on the presence of axons

The role of cAMP in the regulation of P0 gene expression was investigated in Schwann cells of normal, regenerated, and permanently transected rat sciatic nerve. Forskolin treatment of endoneurial segments of rat sciatic nerve resulted in increased cAMP and P0 mRNA levels in normal and regenerated nerves but not in permanently transected nerves, where axonal regeneration is prevented. This increase of cAMP and P0 mRNA occurred within 30 and 90 min, respectively. P0 mRNA levels in the endoneurial segment of the permanently transected nerve were not increased with dibutyryl cAMP. The Schwann cells of the permanently transected nerve, however, retained the ability to myelinate 15 embryonic day (E15) dorsal root ganglia (DRG) neuron and neurite networks cultured in vitro. P0 mRNA levels increased within 4 days in transected endoneurium segments cocultured with E15 DRG neurons and neurites and further increased in 21 day myelinating cocultures. Although cAMP was not detectable in 4 day cocultures, it increased to detectable levels in 21 day cultures, suggesting that cAMP is involved in the myelinating process. These results indicate that the presence of the axon is required for the observed increase of cAMP and P0 mRNA levels and suggest that the increase of cAMP occurs within the axon which then presumably activates a different Schwann cell second messenger pathway to induce P0 gene expression.

Aminoguanidine effects on nerve blood flow, vascular permeability, electrophysiology, and oxygen free radicals

Since advanced glycosylation end products have been suggested to mediate hyperglycemia-induced microvascular atherogenesis and because aminoguanidine (AG) prevents their generation, we examined whether AG could prevent or ameliorate the physiologic and biochemical indices of streptozotocin (STZ)-induced experimental diabetic neuropathy. Four groups of adult Sprague-Dawley rats were studied: group I received STZ plus AG (25 mg.kg-1.day-1), group II received STZ plus AG (50 mg.kg-1.day-1), group III received STZ alone, and group IV was a control. We monitored conduction and action potential amplitudes serially in sciatic-tibial and caudal nerves, nerve blood flow, oxygen free radical activity (conjugated dienes and hydroperoxides), and the product of the permeability coefficient and surface area to 125I-labeled albumin. STZ-induced diabetes (group III) caused a 57% reduction in nerve blood flow and in abnormal nerve conduction and amplitudes and a 60% increase in conjugated dienes. Nerve blood flow was normalized by 8 weeks with AG (groups I and II) and conduction was significantly improved, in a dose-dependent manner, by 16 and 24 weeks in sciatic-tibial and caudal nerves, respectively. The permeability coefficient was not impaired, suggesting a normal blood-nerve barrier function for albumin, and the oxygen free-radical indices were not ameliorated by AG. We suggest that AG reverses nerve ischemia and more gradually improves their electrophysiology by an action on nerve microvessels. AG may have potential in the treatment of diabetic neuropathy.

Levels of proteolipid protein mRNAs in peripheral nerve are not under stringent axonal control

The proteolipid protein (PLP) is the major protein in the myelin sheath of the CNS. It was recently reported that PLP coding transcripts are also found in the PNS, although the protein was not detectable in peripheral nerve myelin. In the present investigation, levels of mRNA for PLP in sciatic nerve were studied during development and following transection and crush injury. Results were compared to those for P0, the major PNS myelin protein, and the myelin-associated glycoprotein (MAG). PLP transcript levels were very low at 21 days in sciatic nerve and remained unchanged in the adult sciatic nerve. This contrasts markedly with P0 and MAG mRNAs, which are expressed at high levels during development and decrease in content significantly by adulthood. The level of PLP messages was reduced approximately 40% in the quiescent Schwann cells in the distal segment of the sciatic nerve at 21 days after permanent transection, yet P0 mRNA levels were very low, and MAG mRNAs were undetectable in this tissue. The distal segment of the crush-injured sciatic nerve is characterized by transient demyelination followed by rapid myelination. PLP mRNA levels remained comparatively unaffected in the 3-week period following crush injury. RNase protection experiments using two antisense riboprobes confirmed that levels of PLP-derived protected fragments, corresponding to PLP and DM-20 messages, remained unchanged in the developing and adult sciatic nerve. These results indicate that myelin-specific P0 and MAG genes are tightly controlled at the level of transcription through Schwann cell-axonal interactions, whereas PLP transcription in the peripheral nerve remains nearly dissociated from axonal influences.

Polyglucosan body axonal enlargement increases myelin spiral length but not lamellar number

The area of the unrolled myelin sheet of internodes of myelinated fibers (MF) of peripheral nerve is thought to be determined by axonal caliber and internodal length. We studied the effect of a focal increase of axonal caliber due to the deposition of polyglucosan bodies (PGB), amylopectin-like glucose polymers, on number of myelin lamellae (NL), interlamellar distance (periodicity), and myelin spiral length (MSL) from a sural nerve biopsy specimen of a patient with chronic inflammatory demyelinating polyneuropathy. Axonal area, NL, periodicity, and MSL were estimated within internodes of MF above, at, and below PGB. The axon caliber at the level of the PGB was significantly (P less than 0.002) increased when the PGB was included. At the PGB, NL and their periodicity were not significantly different from those above or below the PGB. The MSL was significantly longer overlying the PGB than it was in the same internode above or below the PGB. Because slippage or stretching of the myelin sheath as well as movement of molecular constituents of myelin is not likely over large distances, localized biosynthesis and assembly of new myelin may explain this increase of MSL.

Axonal modulation of myelin gene expression in the peripheral nerve

Myelin gene expression (P0, MBP, P2, and MAG) was investigated during Wallerian degeneration and in the presence or absence of subsequent axonal regeneration and remyelination. The steady state levels of mRNA and protein were assessed in the crushed or permanently transected rat sciatic nerve at 0, 1, 4, 7, 10, 12, 14, 21, and 35 days after injury. The mRNA and protein steady state levels of the myelin specific genes, P0 and the MBPs, decreased to low yet detectable levels during Wallerian degeneration and returned to normal levels with subsequent axonal regeneration. The steady state level of P2 protein also followed a similar pattern of expression. The steady state level of MAG mRNA decreased to undetectable levels by 4 days of injury in the permanently transected nerve. After crush injury, re-expression of MAG to levels comparable to those of normal nerves preceded that of P2 by 2 days and that of P0 and the MBPs by 3 weeks during axonal regeneration and remyelination. These results support the proposed roles for MAG in the formation of initial Schwann cell-axonal contact required for myelin assembly, for P2 in fatty acid transport during myelination, and for P0 and the MBPs in the maintenance of the integrity and compactness of the myelin sheath. In addition, these results indicate that the expression of the myelin specific genes, P0 and MBP, is constitutive and that the level of myelin specific mRNAs is modulated by axonal contact and myelin assembly.

Developmental changes in blood-nerve transfer of albumin and endoneurial albumin content in rat sciatic nerve

The rate of entry of albumin into the endoneurial space and its content within that compartment during development were investigated by measuring the permeability coefficient-surface area product to 125I-albumin (PSA) of the blood-nerve interface (BNI), endoneurial residual plasma volume (Vp), blood-nerve interface index to albumin, and endoneurial water content in sciatic nerves of rats ranging in age from 1 to 24 weeks. There was a 30-fold reduction in PSA and a 4-fold decrease in Vp from 2 to 24 weeks, indicating that the endoneurial capillaries and perineurium become less permeable during development. On the other hand, the Alb-BNI index was relatively small at 1 week, increased to a peak value around 6-8 weeks, and then decreased to adult values by 13 weeks. The smaller Alb-BNI index in the neonatal period is consistent with endoneurial albumin being cleared across a permeable perineurium by epineurial lymphatics. Subsequently, as the perineurium becomes less permeable, endoneurial albumin content increases. It then decreases as the endoneurial capillary permeability also decreases. Additionally, metabolic clearance of albumin, especially during the first 2-3 weeks, by axons and glia to meet the nutritive requirements of rapid axonal growth and myelination could be partly responsible for a lower Alb-BNI index. It is suggested that in the developing nerve, the combination of epineurial lymphatics and a relatively permeable perineurium, together with axonal and glial uptake and protein catabolism aid in the clearance of plasma-derived osmolytes from the endoneurial space, and thus prevent the elevation of endoneurial hydrostatic pressure and onset of oedema that would have been seen in an adult nerve with a comparably permeable BNI.

Sulfate incorporation into peripheral nerve endoneurial glycolipids after crush and permanent transection injury

The sulfation of peripheral nerve glycolipids was examined at 35 days after both crush injury or permanent transection of the adult rat sciatic nerve by in vitro incorporation of [35S]sulfate into endoneurial slices. These experimental models of neuropathy are characterized by the presence and absence of both axonal regeneration and subsequent myelin assembly. Although the sulfo-glucuronosyl glycosphingolipids (SGGLs) were not detected by alpha-napthol reagent after HPTLC separation of the total acidic lipid extract, fluorographic analysis after sulfate incorporation revealed a 4.7-fold increase in [35S]sulfate in the sulfo-glucuronosyl paragloboside (SGPG) and a 3.5-fold increase in the sulfo-glucuronosyl-lactosaminosyl paragloboside (SGLPG) after the crush injury compared to permanent transection. These [35S]sulfate-labeled lipids were identified by comigration after HPTLC separation by immunostaining with specific IgM monoclonal antibodies from a patient with demyelinating neuropathy and plasma cell dyscrasia. Enhanced incorporation of sulfate in the crushed nerves was also observed in the sulfatides and in several unknown lipids migrating between GM2 and GM3, between GM1, and GM2, slightly above the origin, and at the origin. Since previous studies (Yao and Poduslo: J Neurochem 50:630-638, 1988) have shown [35S]sulfate incorporation, but not [3H]Gal or [3H]Glc, into sulfatides at 35 days after transection, it is possible that the sulfation observed in the present studies does not represent de novo biosynthesis but rather sulfation of an endogenous pool of glycolipids that results from the nerve injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of axons in the regulation of P0 biosynthesis by Schwann cells

The role of axons in the expression of the major myelin glycoprotein, P0, has been investigated using neuron/Schwann cell cultures. These cultures were either nonmyelinating or myelinating due to growth in defined medium or in medium containing serum and chick embryo extract, respectively. The neurons and Schwann cells used in the studies were derived from embryonic day 15 rat dorsal root ganglia (DRG), and the Schwann cells from these ganglia are shown not to synthesize appreciable levels of P0 prior to growth in culture. Myelinating cultures of Schwann cells and neurons grown together for 18-21 days synthesize P0 that is readily identified by immunoblotting. The nonmyelinating cultures, which do not assemble basal lamina, also synthesize P0 that is detectable by either [3H]mannose precursor incorporation or by immunoblotting. The steady-state level of P0 in the nonmyelinating cultures is less than that of the myelinating cultures, and the P0 that is synthesized by the former appears to be catabolized shortly after its biosynthesis. Since nonmyelinating Schwann cells synthesize P0 when in contact with neurites in vitro, we have examined the ability of such nonmyelinating cells to express the glycoprotein in vivo. Very little steady-state P0 is detected in immunoblots of the adult rat cervical sympathetic trunk (CST), a nerve in which approximately 99% of the axons are nonmyelinated. Similarly, the amounts of [3H]mannose and [3H]amino acids that are incorporated into newly synthesized P0 are much lower in the CST than in the adult sciatic nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological changes in the sciatic nerve endoneurium of lead-intoxicated rats: a model of endoneurial homeostasis

The effects of inorganic lead (Pb) on peripheral nerves were studied by measuring the permeability coefficient-surface area product to albumin (PSA) of the blood-nerve interface (BNI), water and Pb content of endoneurium, and residual endoneurial plasma volume (Vp) of sciatic nerves of adult rats maintained on a 4% Pb diet for 2-12 weeks. Additionally, body weight, haematocrit, and blood Pb levels were also monitored. Within 1 week, both blood and endoneurial Pb levels had increased significantly above background levels and continued to increase up to the 6th week. Thereafter, over the next 6 weeks, the blood Pb levels were relatively stable, while the endoneurial Pb levels decreased sharply to a quarter of their peak values. The wet/dry weight ratio of the endoneurium, an indicator of nerve oedema, increased by about 30% from the 4th to 12th week, reaching a water content of 75% corresponding to a 24% increase of the total fascicular area. PSA increased only by the 8th week, without an accompanying increase in Vp. This suggests that the increase in PSA reflects an increase in the permeability of the BNI. Furthermore, the moderate 3-fold increase in PSA is more consistent with an adaptive response of the BNI to changes in the endoneurial microenvironment rather than a massive breakdown or disruption of the BNI. This is supported by the inconsistent or mild leakage of horseradish peroxidase from endoneurial capillaries observed in Pb-intoxicated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Golgi sulfation of the oligosaccharide chain of P0 occurs in the presence of myelin assembly but not in its absence

To decipher the intracellular targeting mechanism by which the major glycoprotein of peripheral nerve myelin, P0, is delivered to myelin after crush injury, as well as to the lysosome after permanent transection injury of the sciatic nerve--experimental paradigms characterized by the presence and absence of axonal regeneration and subsequent myelin assembly, respectively--the role of sulfation of P0 was investigated. P0 sulfation is shown to occur within the Golgi apparatus as a post-translational modification of the oligosaccharide chain which is dependent on processing beyond the action of mannosidase I. It is associated with myelination as observed during development and after crush injury, but does not occur after transection injury, even in the presence of the mannosidase II inhibitor, swainsonine, or the lysosomotrophic agent, L-methionine methyl ester. Although P0 accumulation can be demonstrated with both agents when other precursors are used (e.g. fucose, mannose, amino acids) and indicates lysosomal targeting and delivery of P0 after the action of GlcNAc transferase I, the absence of P0 sulfation after transection suggests that the lack of this modification may result in a default mechanism for lysosomal targeting after nerve transection. Lysosomal degradation of P0 was evaluated after crush injury by pulse-chase analyses with 35SO4 and [3H] mannose in the presence and absence of chlorate, an inhibitor of ATP-sulfarylase. Although P0 sulfation of the oligosaccharide chain is a stable modification whose labeling is dramatically inhibited by chlorate, no decrease in mannose-labeled P0 was seen with chlorate even with prolonged chase times. Because of this lack of degradation of mannose-labeled P0 in the presence of chlorate in the crushed nerve, it is concluded that the absence of P0 sulfation does not result in a default mechanism for lysosomal delivery.

Axonal regulation of Schwann cell glycolipid biosynthesis

Schwann cell biosynthesis of glycolipids was studied by in vitro incorporation of [3H]galactose into neonatal rat sciatic nerves before and after endoneurial explant culture and in culture of purified Schwann cells. In neonatal nerves prior to culture, [3H]galactose was actively incorporated into galactocerebrosides (GalCe), monogalactosyl diacylglycerol (MGDG), and the sulfatides (Su). In contrast, the incorporation of [3H]galactose into MGDG, GalCe, and Su was nearly undetected in endoneurial explants after 4 days in vitro (div). Instead, there was increased 3H-labeling of glucocerebrosides (GlcCe) and its homologues, with tetrahexosylceramides (GL-4) being a major product, which continued through 8 div. This shift in glycolipid biosynthesis was further demonstrated in the purified Schwann cell cultures. These observations, together with our early findings in the permanent transection paradigm support a direct role of axons in specifying Schwann cell biosynthesis of the GalCe, MGDG, and Su and that the absence of this Schwann cell-axon interaction results in the phenotypic expression of glucocerebroside homologues by the Schwann cell.

Regulation of apolipoprotein E gene expression after injury of the rat sciatic nerve

The expression of apolipoprotein E (apo E) is dramatically increased following peripheral nerve injury. This increased expression has been postulated to be negatively influenced by unknown mechanisms during subsequent axonal regeneration (Muller et al.: Science 228:499-501, 1985). The present study investigates the role of the regenerating axon in regulating apo E gene expression in two experimental paradigms which permit or prevent axonal regeneration in the adult rat sciatic nerve--the crush or permanent transection injuries. The nerves in these two models undergo axonal degeneration, demyelination, and Schwann cell proliferation; however, subsequent axonal regeneration and remyelination occur only in the distal segment of the crush-injured and not in the permanently transected nerve. The steady-state levels of apo E mRNA in both models increase sharply between 1 and 4 days and reach a maximum level at 12-14 days, which did not change significantly between 14 and 35 days after injury. No significant difference is observed in the steady-state levels of apo E mRNA between the crushed and permanently transected nerves as a function of time after injury. The steady-state protein level of apo E in the endoneurial segments initially increases, peaks at 14-21 days, and then decreases between 35 and 60 days after injury in both models. In contrast, the rate of newly translated and secreted apo E significantly increases by fourfold (P less than 0.005) between 35 and 60 days after permanent transection whereas it does not significantly differ at these times after crush injury. The increased rate of translation and secretion of apo E after transection compared to the constant rate observed after crush injury, together with the comparable steady-state levels of apo E mRNA and protein in both models, suggests translational or post-translational control, but not transcriptional and/or posttranscriptional control, by the regenerating axons. Furthermore, the increasing rate of biosynthesis and secretion of apo E after permanent transection concomitant with the decreasing steady-state levels of the protein suggests that apo E is either removed from the endoneurium or subsequently utilized or degraded by mechanisms that are independent from nerve regeneration.

Catabolic regulation of the expression of the major myelin glycoprotein by Schwann cells in culture

Previous studies have suggested that neonatal Schwann cell cultures deprived of axonal contact do not express components of the myelin membrane, including the major myelin glycoprotein, P0. In contrast, Schwann cells from permanently transected, adult nerve exhibit continued biosynthesis of P0 after culture, suggesting that the ability to express the myelin glycoprotein may depend on the degree of cellular differentiation. To examine further the ability of Schwann cell cultures to express P0 as a function of age, we have performed precursor incorporation studies on endoneurial explants from 4- to 12-day-old rat sciatic nerves after 5 days in culture. The data reveal that explants from 12-day-old animals synthesize detectable levels of this integral myelin protein when assayed by [3H]mannose incorporation, even though there is no apparent myelin assembly in the cultures. Pulse-chase analysis of cultures from 12-day-old rats demonstrates that [3H]mannose-labeled P0 is substantially degraded within 3 h. This catabolism largely can be prevented by the addition of swainsonine, ammonium chloride, or L-methionine methyl ester to the pulse-chase media. The former agent alters oligosaccharide processing whereas the latter two compounds inhibit lysosomal function. The P0 synthesized by the 12-day explant cultures following the addition of swainsonine is readily fucosylated, implying that the protein has progressed at least as far as the medial Golgi before its exit and subsequent catabolism. If cultures from 4-, 6-, and 8-day-old animals are analyzed for P0 biosynthesis by [3H]mannose incorporation in the presence of swainsonine, detectable levels of the glycoprotein are seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Myelin-associated glycoprotein gene expression in the presence and absence of Schwann cell-axonal contact

The distal segments of the crush-injured and permanently transected sciatic nerve provide models to study Schwann cell activity in the presence and absence of Schwann cell-axonal contact, respectively. We examined the quantity and quality of transcript coding for the myelin-associated glycoprotein (MAG) over a 3-week period following crush injury and at 35 days after transection to investigate possible regulation of this gene during nerve injury and subsequent repair. Northern blot and slot blot analysis indicated a sharp decrease in levels of MAG mRNA 2 days after crush injury which was followed by a progressive increase in levels of message between 7 and 21 days after injury. Western blot analysis showed that levels of MAG protein decreased substantially 7 days after crush injury, which returned to 70% of the adult value by 21 days after injury. MAG mRNA and protein were undetectable by Northern and Western analysis, respectively, in the distal segment of the sciatic nerve 35 days after permanent transection. This infers distinct down-regulation of MAG gene expression after permanent transection of a peripheral nerve. These comparative studies of MAG transcripts and encoded protein may indicate regulation of MAG gene expression at the level of transcription, and possibly at the level of post-transcription in these experimental models of peripheral neuropathies.

Blood-nerve transfer of albumin and its implications for the endoneurial microenvironment

Blood-nerve transfer of plasma albumin was studied by measuring the permeability coefficient-surface area (PS) product of the blood-nerve barrier (BNB) to 125I-albumin in rat sciatic nerve using the i.v. bolus injection method. The calculated PS was 6.3 +/- 0.5 (S.E.M.) x 10(-7) ml.g-1.s-1. This value is smaller by more than an order of magnitude of that measured for sucrose and confirms the relative impermeance of the BNB to blood-borne solutes. From a review of the available evidence, it is concluded that normal blood-nerve exchange occurs predominantly across the endoneurial microvasculature, and the PS of the BNB reflects the permeability of capillaries to a greater extent than that of the perineurium. The only capillaries found to be less permeable than these are the cerebral capillaries. Proximo-distal differences (sciatic vs tibial) of the PS could not be detected. Blood-nerve albumin transfer was calculated at 1.2 mg.g-1.day-1, and the daily turnover of endoneurial albumin to be about 30%. It is postulated that small increases in PS of BNB to albumin lead to an elevation of endoneurial albumin concentration and, through the operation of Starling forces, subsequently produce endoneurial oedema. A major question posed by the results of this study is the identity of pathways for clearance of albumin and other macromolecules from the endoneurium.

Regulation of UDP-galactose:ceramide galactosyltransferase and UDP-glucose:ceramide glucosyltransferase after crush and transection nerve injury

The enzyme activities of ceramide galactosyltransferase and ceramide glucosyltransferase were assayed as a function of time (0, 1, 2, 4, 7, 14, 21, 28, and 35 days) after crush injury or permanent transection of the adult rat sciatic nerve. These experimental models of neuropathy are characterized by the presence and absence of axonal regeneration and subsequent myelin assembly. Within the first 4 days after both injuries, a 50% reduction of ceramide galactosyltransferase-specific activity was observed compared to values found in the normal adult nerve. This activity remained unchanged at 7 days after injury; however, by 14 days the ceramide galactosyltransferase activity diverged in the two models. The activity increased in the crushed nerve and reached control values by 21 days, whereas a further decrease was observed in the transected nerve such that the activity was nearly immeasurable by 35 days. In contrast, the ceramide glucosyltransferase activity showed a rapid increase between 1 and 4 days, followed by a plateau that was 3.4-fold greater than that in the normal adult nerve, which persisted throughout the observation period in both the crush and transection models. [3H]Galactose precursor incorporation studies at 7, 14, 21, and 35 days after injury confirmed the previously observed shift in biosynthesis from the galactocerebrosides during myelin assembly in the crush model to the glucocerebrosides and oligohexosylceramide homologues in the absence of myelin assembly in the transection model. The transected nerves were characterized by a peak of biosynthesis of the glucocerebrosides at 14 days. Of particular interest is the biosynthesis of the glucocerebrosides and the oligohexosylceramides at 7 and 14 days after crush injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve glucose, fructose, sorbitol, myo-inositol, and fiber degeneration and regeneration in diabetic neuropathy

We measured the alcohol sugars in sural nerves from 11 controls, 21 conventionally treated patients with diabetes and neuropathy, and 4 diabetics without neuropathy. The results were related to metabolic control and to clinical, neuropathological, and morphometric abnormalities in the nerves. The mean endoneurial glucose, fructose, and sorbitol values were higher in diabetic patients than in controls. Linear regression analysis revealed that nerve sorbitol content in the diabetics was inversely related to the number of myelinated fibers (P = 0.003). Mean nerve levels of myo-inositol were not decreased in the diabetic patients, with or without neuropathy, and were not associated with any of the neuropathological end points of diabetes. Our results indicate that myo-inositol deficiency is not part of the pathogenesis of human diabetic neuropathy, as had been hypothesized. Other accumulated alcohol sugars, however, were increased in diabetes and were associated with the severity of neuropathy. On repeat biopsy, six diabetics, treated for a year with the aldose reductase inhibitor sorbinil, had decreased endoneurial levels of sorbitol (P less than 0.01) and fructose (0.05 less than P less than 0.1), but unchanged levels of myo-inositol.

Temporal changes in PO and MBP gene expression after crush-injury of the adult peripheral nerve

The crush-injured sciatic nerve provides a model to study Schwann cell regulation of myelin gene expression during the process of demyelination and remyelination. In order to investigate the possible transcriptional regulation of myelin gene expression, the quantity, quality and translational efficiency of PO (the major myelin glycoprotein) and MBP (the myelin basic proteins) coding messages were investigated as a function of time following crush-injury of the adult rat sciatic nerve. Northern blot analysis indicated that the size of the PO and MBP transcripts remain unchanged in the distal segments of crushed sciatic nerves at 1, 2, 4, 7, 10, 14 and 21 days after crush-injury. Dot-blot analysis showed a sharp drop in levels of PO and MBP coding transcripts 1 day after crush-injury with the lowest steady-state levels at 4-7 days. Message levels were found to increase after 7 days, the highest increase in levels of message was found to be between 10 and 14 days. The highest steady-state level of both transcripts was observed at 21 days. In vitro translation and immunoprecipitation of PO-translated products from various stages of crush-injury also indicated this trend. The pattern of gene expression of PO- and MBP-coding transcripts parallel each other and follow the pattern of demyelination and remyelination. The results are also consistent with our previous interpretation which suggests that PO and MBP gene expression is regulated at the level of transcription and that these two genes might be coordinately expressed. Western blot analysis of PO protein from these stages revealed a similar decrease and then increase in the levels of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Increase in albumin, IgG, and IgM blood-nerve barrier indices in human diabetic neuropathy

The albumin (Alb), IgG, and IgM concentrations in the endoneurium of fascicular sural nerve biopsy samples were evaluated in controls (n = 9 or 10), diabetic patients without neuropathy (n = 6), and diabetic patients with polyneuropathy (n = 17 or 18). These values were significantly increased in diabetic patients with and without neuropathy when expressed both per endoneurial dry weight or endoneurial total protein compared to biopsy samples from healthy controls. When these concentrations, expressed per endoneurial total protein, were related to plasma concentrations similarly expressed, the resulting blood-nerve barrier (BNB) indices were significantly increased for Alb (6.1 times; P less than 0.00001), IgG (4.9 times; P = 0.00037), and IgM (2.7 times; P = 0.015). The diabetic patients without neuropathy (defined as having an index of pathology of greater than 0.65; a measure of the severity of the pathological abnormality based on morphological criteria) also had significant increases in two of these BNB indices that were intermediate between the diabetic neuropathy patients and controls (Alb, 3.9 times controls; P = 0.00002: IgG, 4.6 times controls; P = 0.00016: IgM, 1.8 times controls; not significant). No correlations were observed between the endoneurial concentrations of these plasma proteins or the BNB indices and the index of pathology, suggesting that these increases in endoneurial plasma proteins precede the pathologic alterations. The increased values for the diabetics in the absence of pathological abnormalities may prove useful in predicting neuropathic complications. The ratio of the IgG-BNB index to the Alb-BNB index was decreased 19%, and the ratio of the IgM-BNB index to the Alb-BNB index was decreased 56% in diabetic neuropathy patients compared to controls. Although the IgG and IgM concentrations are increased in the diabetic endoneurium, the Alb increase is greater and a mechanism other than size indiscriminate extravasation of plasma proteins, therefore, is suggested. Morphometric assessment of the endoneurial compartments, which would be expected to contain these plasma proteins, suggests that they are not altered in diabetic neuropathy; hence, it is hypothesized that the observed increase in endoneurial concentration of these plasma proteins results from altered transport through the endothelial or perineurial barrier, which supports an underlying vascular mechanism in the development of diabetic polyneuropathy.

IgM/IgG solid-phase antibody-capture assay with biotin/125I-streptavidin amplification: application to normal human sural nerve biopsies

A highly sensitive and specific solid-phase antibody-capture assay was developed to measure IgM and IgG in endoneurial preparations of human sural nerve biopsies. Assay amplification was obtained by utilizing biotin-labeled anti-IgM or anti-IgG antibody and 125I-streptavidin, resulting in multiple streptavidin molecules binding per biotinylated antibody molecule. A minimal detectable dose of 0.16 +/- 0.08 ng (mean +/- SD; n = 7) for IgM and 0.03 +/- 0.02 ng (mean +/- SD; n = 5) for IgG was obtained in a 100 microliters sample. When this assay was applied to normal fascicular biopsies from human sural nerve, the percent of IgM and IgG, respectively, of the total endoneurial protein was 0.026 +/- 0.015% (n = 9) and 0.27 +/- 0.15% (n = 10; mean +/- SD). When these endoneurial concentrations of IgM and IgG were related to the plasma concentrations (mg IgM or IgG/mg total plasma protein), an IgM-blood-nerve barrier (BNB) index of 4.09 +/- 1.95 and an IgG-BNB index of 2.07 +/- 1.10 were obtained (X10(2); mean +/- SD). These values were also related to the albumin (Alb) concentration in the biopsies as a percent of total endoneurial protein (2.48 +/- 1.07%; mean +/- SD) and with the Alb-BNB index (5.40 +/- 2.53; X10(2); mean +/- SD; n = 10). Although these normal values will be expected to change with age, sex, nerve, and proximal-distal distance from nerve root, they should provide a basis for the comparison of BNB indices from patients with peripheral neuropathy.