myelin-basic-protein and Phenylketonurias

Untreated phenylketonuria (PKU) patients and PKU animal models show hypomyelination in the central nervous system and white matter damages, which are accompanied by myelin basic protein (MBP) impairment. Despite many assumptions, the primary explanation of the mentioned cerebral outcomes remains elusive. In this study, MBP protein and mRNA expression on brains of wild type (WT) and phenylketonuric (ENU2) mice were analyzed throughout mice lifespan (14-60-180-270-360-540 post-natal days, PND). The results confirmed the low MBP expression at first PND times, while revealed an unprecedented progressive MBP protein expression recovery in aged ENU2 mice. Unexpectedly, unaltered MBP mRNA expression between WT and ENU2 was always observed. Additionally, for the same time intervals, a significant decrease of the phenylalanine concentration in the peripheral blood and brain of ENU2 mice was detected, to date, for the first time. In this scenario, a translational hindrance of MBP during initial and late cerebral development in ENU2 mice was hypothesized, leading to the execution of a microRNA microarray analysis on 60 PND brains, which was followed by a proteomic assay on 60 and 360 PND brains in order to validate in silico miRNA-target predictions. Taken together, miR-218-1-3p, miR-1231-3p and miR-217-5p were considered as the most impactful microRNAs, since a downregulation of their potential targets (MAG, CNTNAP2 and ANLN, respectively) can indirectly lead to a low MBP protein expression. These miRNAs, in addition, follow an opposite expression trend compared to MBP during adulthood, and their target proteins revealed a complete normalization in aged ENU2 mice. In conclusion, these results provide a new perspective on the PKU pathophysiology understanding and on a possible treatment, emphasizing the potential modulating role of differentially expressed microRNAs in MBP expression on PKU brains during PKU mouse lifespan.

Topics: Animals; Longevity; Membrane Proteins; Mice; MicroRNAs; Myelin Basic Protein; Nerve Tissue Proteins; Phenylketonurias; Proteomics; RNA, Messenger

Phenylketonuria (PKU) is a metabolic genetic disease characterized by deficient phenylalanine hydroxylase (PAH) enzymatic activity. Brain hypomyelination has been reported in untreated patients, but its mechanism remains unclear. We therefore investigated the influence of phenylalanine (Phe), phenylpyruvate (PP), and phenylacetate (PA) on oligodendrocytes. We first showed in a mouse model of PKU that the number of oligodendrocytes is not different in corpus callosum sections from adult mutants or from control brains. Then, using enriched oligodendroglial cultures, we detected no cytotoxic effect of high concentrations of Phe, PP, or PA. Finally, we analyzed the impact of Phe, PP, and PA on the myelination process in myelinating cocultures using both an in vitro index of myelination, based on activation of the myelin basic protein (MBP) promoter, and the direct quantification of myelin sheaths by both optical measurement and a bioinformatics method. None of these parameters was affected by the increased levels of Phe or its derivatives. Taken together, our data demonstrate that high levels of Phe, such as in PKU, are unlikely to directly induce brain hypomyelination, suggesting involvement of alternative mechanisms in this myelination defect.

Topics: Animals; Cells, Cultured; Corpus Callosum; Disease Models, Animal; Lac Operon; Mice; Mice, Mutant Strains; Mice, Transgenic; Myelin Basic Protein; Myelin Sheath; Oligodendroglia; Phenylalanine; Phenylalanine Hydroxylase; Phenylketonurias; Promoter Regions, Genetic; Rats; Rats, Wistar

Phenylketonuria is caused by specific mutations in the phenylalanine hydroxylase gene and is characterized by elevated blood phenylalanine levels, hypomyelination in forebrain structures, reduced dopamine levels, and cognitive difficulties. To determine whether brain tyrosine levels and/or myelination play a role in the up-regulation of dopamine, phenylketonuric mice were placed on a low phenylalanine diet for 4 weeks and as blood phenylalanine levels dropped to normal, the relationships between phenylalanine, tyrosine, dopamine, myelin proteins, and axonal proteins in frontal cortex and striatum were determined using gas chromatography mass spectrometry, histology, and western blotting techniques. Blood phenylalanine rapidly decreased from an eight-fold elevation to near control levels, and blood tyrosine gradually rose from about 50% to near normal values. In frontal cortex and striatum, phenylalanine levels dropped to 2- and 1.5-fold elevations above control, respectively, and tyrosine levels increased but remained less than 70% of control in both structures. In frontal cortex, increases in dopamine and myelin basic protein occurred in a similar biphasic pattern, reaching near normal levels by week 4. In striatum, dopamine and MBP dramatically increased to near normal levels in the first week. Myelination was confirmed histologically and by western blot quantification of phosphorylated neurofilaments. In summary, our results showed: (i) an increase in dopamine despite low brain tyrosine levels and (ii) similar recovery patterns for myelination and dopamine. Since myelin/axonal interactions trigger signaling pathways that result in axonal maturation, we speculate that this interaction also may trigger signals that up-regulate neurotransmitter synthesis.

Topics: Animals; Blotting, Western; Brain; Brain Chemistry; Corpus Striatum; Disease Models, Animal; Dopamine; Food, Formulated; Frontal Lobe; Male; Mice; Mice, Mutant Strains; Myelin Basic Protein; Myelin Sheath; Neurofilament Proteins; Phenylalanine; Phenylketonurias; Phosphorylation; Tyrosine; Tyrosine 3-Monooxygenase

Previous reports have suggested that elevated levels of phenylalanine inhibit cholesterol synthesis. The goals of this study were to investigate if perturbations in cholesterol synthesis exist in the PAH(enu2) genetic mouse model for phenylketonuria (PKU), and if so, initiate studies determining if they might underlie the white matter pathology that exists in PKU forebrain. Gross sections and electron microscopy showed that select tracts were hypomyelinated in adult PKU mouse forebrain but not hindbrain. The activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), the rate controlling enzyme in the cholesterol biosynthetic pathway, was examined in isolated microsomes from forebrain, hindbrain, and liver to assess if perturbations in cholesterol biosynthesis were occurring. HMGR activity was normal in unaffected PKU hindbrain and was increased 2-4-fold in PKU liver compared to control. HMGR activity in the forebrain, however, was decreased by 30%. Because normal numbers of MBP-expressing glia (oligodendrocytes) were present, but the number of glia expressing HMGR was reduced by 40% in the hypomyelinated tracts, the decreased HMGR activity seemed to result from a down-regulation of HMGR expression in affected oligodendrocytes. Exposure of an oligodendrocyte-like glioma cell line to physiologically relevant elevated levels of Phe resulted in a 30% decrease in cholesterol synthesis, a 28% decrease in microsomal HMGR activity, and a 28% decrease in HMGR protein levels. Measurement of HMGR activity after addition of exogenous Phe to control brain microsomes revealed that Phe is a noncompetitive inhibitor of HMGR; physiologically relevant elevated levels of exogenous Phe inhibited HMGR activity by 30%. Taken together, these data suggest that HMGR is moderately inhibited in the PKU mouse. Unlike other cell types in the body, a subset of oligodendrocytes in the forebrain seems to be unable to overcome this inhibition. We speculate that this may be the cause of the observed pathology in PKU brain.

Topics: Alkyl and Aryl Transferases; Animals; Brain Chemistry; Cell Count; Cell Line; Cholesterol; Demyelinating Diseases; Disease Models, Animal; Farnesyltranstransferase; Hydroxymethylglutaryl CoA Reductases; Hydroxymethylglutaryl-CoA Reductase Inhibitors; Liver; Mice; Mice, Neurologic Mutants; Mice, Transgenic; Microsomes; Myelin Basic Protein; Oligodendroglia; Phenylalanine; Phenylalanine Hydroxylase; Phenylketonurias; Prosencephalon; Rhombencephalon

Phenylketonuria (PKU) is caused by mutation(s) in the phenylalanine hydroxylase (PAH) gene which lead to deficient PAH activity and an accumulation of phenylalanine in the blood. The primary pathologic finding is hypomyelination and gliosis of central nervous system white matter. Similar white matter pathology is observed in the Pahenu2 mouse, a genetic model for PKU. We studied this mouse to examine the basis for these neuropathologic changes in PKU and to determine if hypomyelination and gliosis occur independently or are interrelated. Although white matter tracts within PKU brains are hypomyelinated, immunostaining and Western blot analyses revealed that these tracts contain abundant amounts of myelin markers, i.e. myelin basic protein (MBP), 2',3'-cyclic nucleotide 3'phosphohydrolase, and myelin/oligodendrocyte-specific protein (MOSP). However, Western blot analyses also showed that MBP isoform expression was aberrant. Investigation of individual cells was performed by extraction of tissue sections with Triton X-100. Most of the MOSP was extracted, with the remaining MOSP clearly visible in dual labeled cells, i.e. MOSP was colocalized along glial fibrillary acidic protein (GFAP) filaments. Cells expressing both MBP and GFAP were also identified in optic tract. Double labeling with a riboprobe for MBP and antibodies specific for GFAP revealed that the majority of GFAP-positive cells expressed MBP mRNA. Our in vitro studies examined the response of cultured wild type oligodendrocytes to elevated phenylalanine for 4 weeks (wk). Under these conditions, about 50 these conditions, about 50% of the oligodendrocytes expressed GFAP filaments and failed to elaborate membrane sheets. Proliferation of astrocytes appears not to be the source of gliosis, since the nuclei of GFAP-positive cells in the PKU brains did not immunostain for proliferating cell nuclear antigen. Dual-labeled cells were detected in normal mouse brain sections; however, PKU mouse white matter tracts were found to contain about twice the number of dual-labeled cells compared to normal tissue. Taken together, these data suggest that both myelinating and nonmyelinating oligodendrocytes are present in the normal adult brain, and that in response to a toxic factor such as elevated phenylalanine, myelinating oligodendrocytes adopt a nonmyelinating phenotype that expresses GFAP. Since myelinating Schwann cells and GFAP-positive nonmyelinating Schwann cells are normally present in adult peripheral n

Topics: Adult; Animals; Biomarkers; Brain; Glial Fibrillary Acidic Protein; Gliosis; Humans; Mice; Mice, Mutant Strains; Mutation; Myelin Basic Protein; Myelin Sheath; Neuroglia; Neuronal Plasticity; Oligodendroglia; Phenylalanine; Phenylalanine Hydroxylase; Phenylketonurias; RNA, Messenger

The severe hypomyelination seen in the CNS of untreated phenylketonuria (PKU) patients has been suggested to be the result of a defect in the process of myelination itself. Using chronic hyperphenylalaninemia (HPA) in rats as a model of PKU we have previously shown, by immunohistochemistry, that axonal maturation as well as myelination was severely retarded. In the present study we have used image analysis techniques to quantitate changes in myelin basic protein (MBP) and 200-kDa neurofilament protein (NF-H) immunostaining in the corpus callosum and cerebral cortical grey matter of HPA rats. No difference in the density of MBP+ myelin was observed in the corpus callosum after 24 days HPA treatment although the width of the tract was much reduced. In contrast there was a deficit in NF-H immunostaining. Large deficits in both myelin and axonal maturity were seen in the cortical grey matter. Following a 6-week recovery period, despite recovery in the corpus callosum, large deficits in both MBP and NF-H were still seen in all cortical layers. Deficits in NF-H immunostaining were two to three times greater than those for MBP. On increasing the recovery period to 18 weeks significant deficits in myelin remained in layers I-III of the cortical grey matter whereas NF-H immunostaining had returned to normal levels in all layers. Our data suggest a primary effect of HPA on neuronal development, in particular axonal maturation, with a secondary hypomyelination and show that permanent deficits in myelinated axons in outer cortical layers can result when myelination is severely inhibited during a critical developmental period.

Topics: Animals; Animals, Newborn; Axons; Corpus Callosum; Disease Models, Animal; Female; Male; Molecular Weight; Myelin Basic Protein; Myelin Sheath; Neurofilament Proteins; Neurons; Oligodendroglia; Phenylalanine; Phenylketonurias; Prosencephalon; Rats; Rats, Sprague-Dawley

We examined developmental changes of myelin-associated glycoprotein (MAG), basic protein (BP), and proteolipid protein (PLP) in central nervous system myelin isolated from experimental hyperphenylalaninemic rats (PKU rats) and controls. Higher amounts of MAG, including high-molecular-weight MAG in myelin, were found in 12- to 21-day-old control rats than in adult rats. MAG in developing myelin was at a maximum in 18-day-old rats and began to decrease in 21-day-old rats, while PLP and BP in developing myelin increased at these developmental stages. The level of high-molecular-weight MAG decreased in myelin prepared from 21-day-old rats. These results suggest that the decreasing high-molecular-weight MAG is important for compaction of myelin in the early stage of myelination. In myelin from 12- to 18-day-old PKU rats, the ratio of each protein such as MAG, PLP, or BP to that of control was about 0.5 at 12 days, and increased to almost 1.0 at 18 days. The myelination seems to be initially delayed but to be close to that of controls in PKU rats about 18 days old.

Topics: Animals; Brain; Brain Chemistry; Molecular Weight; Myelin Basic Protein; Myelin Proteins; Myelin Proteolipid Protein; Myelin-Associated Glycoprotein; Phenylalanine; Phenylketonurias; Rats; Rats, Inbred Lew