Cobalamin (vitamin B12) regulation of PrPC, PrPC-mRNA and copper levels in rat central nervous system

Prion protein with a mutant N-terminal octarepeat region undergoes cobalamin-dependent assembly into high-molecular weight complexes

The cellular prion protein (PrP^C) has a C-terminal globular domain and a disordered N-terminal region encompassing five octarepeats (ORs). Encounters between Cu(II) ions and four OR sites produce interchangeable binding geometries; however, the significance of Cu(II) binding to ORs in different combinations is unclear. To understand the impact of specific binding geometries, OR variants were designed that interact with multiple or single Cu(II) ions in specific locked coordinations. Unexpectedly, we found that one mutant produced detergent-insoluble, protease-resistant species in cells in the absence of exposure to the infectious prion protein isoform, scrapie-associated prion protein (PrP^Sc). Formation of these assemblies, visible as puncta, was reversible and dependent upon medium formulation. Cobalamin (Cbl), a dietary cofactor containing a corrin ring that coordinates a Co³⁺ ion, was identified as a key medium component, and its effect was validated by reconstitution experiments. Although we failed to find evidence that Cbl interacts with Cu-binding OR regions, we instead noted interactions of Cbl with the PrP^C C-terminal domain. We found that some interactions occurred at a binding site of planar tetrapyrrole compounds on the isolated globular domain, but others did not, and N-terminal sequences additionally had a marked effect on their presence and position. Our studies define a conditional effect of Cbl wherein a mutant OR region can act in cis to destabilize a globular domain with a wild type sequence. The unexpected intersection between the properties of PrP^Sc's disordered region, Cbl, and conformational remodeling events may have implications for understanding sporadic prion disease that does not involve exposure to PrP^Sc.

Epidermal Growth Factor in the CNS: A Beguiling Journey from Integrated Cell Biology to Multiple Sclerosis. An Extensive Translational Overview

This article reviews the wealth of papers dealing with the different effects of epidermal growth factor (EGF) on oligodendrocytes, astrocytes, neurons, and neural stem cells (NSCs). EGF induces the in vitro and in vivo proliferation of NSCs, their migration, and their differentiation towards the neuroglial cell line. It interacts with extracellular matrix components. NSCs are distributed in different CNS areas, serve as a reservoir of multipotent cells, and may be increased during CNS demyelinating diseases. EGF has pleiotropic differentiative and proliferative effects on the main CNS cell types, particularly oligodendrocytes and their precursors, and astrocytes. EGF mediates the in vivo myelinotrophic effect of cobalamin on the CNS, and modulates the synthesis and levels of CNS normal prions (PrP^Cs), both of which are indispensable for myelinogenesis and myelin maintenance. EGF levels are significantly lower in the cerebrospinal fluid and spinal cord of patients with multiple sclerosis (MS), which probably explains remyelination failure, also because of the EGF marginal role in immunology. When repeatedly administered, EGF protects mouse spinal cord from demyelination in various experimental models of autoimmune encephalomyelitis. It would be worth further investigating the role of EGF in the pathogenesis of MS because of its multifarious effects.

Prevention of clinical and histological signs of MOG-induced experimental allergic encephalomyelitis by prolonged treatment with recombinant human EGF

Epidermal growth factor (EGF) represents the prototype of the group I EGF family. The pleiotropic effects of the EGF have attracted attention to the possibility that it could be implicated in autoimmune diseases, such as Multiple Sclerosis (MS). We show here that treatment with EGF, as a late prophylactic regime, improved the clinical and histological features of EAE, a preclinical model of MS. In silico analysis further corroborated these findings by demonstrating that EGF receptors are less expressed in CNS from patients with MS as compared to controls. Taken together these data provide clear-cut in vivo proof of concept for a beneficial role of exogenously administered EGF in MS, that may, therefore, represent a novel therapeutic approach.

Investigation of Receptor-Mediated Cyanocobalamin (Vitamin B12) Transport across the Inner Blood-Retinal Barrier Using Fluorescence-Labeled Cyanocobalamin

The blood-to-retina supply of cyanocobalamin (vitamin B₁₂) across the blood-retinal barrier (BRB) was investigated by synthesizing a fluorescence-labeled cyanocobalamin (Cy5-cyanocobalamin). In the in vivo analysis following internal jugular injection of Cy5-cyanocobalamin, confocal microscopy showed the distribution of Cy5-cyanocobalamin in the inner plexiform layer (IPL), the outer plexiform layer (OPL), and the retinal pigment epithelium (RPE). In the in vitro analysis with TR-iBRB2 cells, an in vitro model cell line of the inner BRB, Cy5-cyanocobalamin uptake by TR-iBRB2 cells exhibited a time-dependent increase after preincubation with transcobalamin II (TCII) protein, during its residual uptake without preincubation with TCII protein. The Cy5-cyanocobalamin uptake by TR-iBRB2 cells was significantly reduced in the presence of unlabeled cyanocobalamin, chlorpromazine, and chloroquine and was also significantly reduced under Ca²⁺-free conditions. Confocal microscopy of the TR-iBRB2 cells showed fluorescence signals of Cy5-cyanocobalamin and GFP-TCII protein, and these signals merged with each other. The RT-PCR, Western blot, and immunohistochemistry clearly suggested the expression of TCII receptor (TCII-R) in the inner and outer BRB. These results suggested the involvement of receptor-mediated endocytosis in the blood-to-retina transport of cyanocobalamin at the inner BRB with implying its possible involvement at the outer BRB.

Low levels of cobalamin, epidermal growth factor, and normal prions in multiple sclerosis spinal cord

We have previously demonstrated that multiple sclerosis (MS) patients have abnormal cerebrospinal fluid (CSF) levels of the key myelin-related molecules cobalamin (Cbl), epidermal growth factor (EGF), and normal cellular prions (PrP(C)s), thus confirming that some CSF abnormalities may be co-responsible for remyelination failure. We determined the levels of these three molecules in post-mortem spinal cord (SC) samples taken from MS patients and control patients. The control SC samples, almost all of which came from non-neurological patients, did not show any microscopic lesions of any type. All of the samples were supplied by the U.K. MS Tissue Bank. The Cbl, EGF, and PrP(C) levels were determined using enzyme-linked immunosorbent assays. The SC total homocysteine level was determined using a competitive immunoenzymatic assay. CSF samples, taken from a further group of MS patients, were used for the assay of holo-transcobalamin (holo-TC) levels. The Cbl, EGF, and PrP(C) levels were significantly decreased in MS SCs in comparison with controls and, paradoxically, the decreased Cbl levels were associated with decreased SC levels of homocysteine, a biochemical marker of Cbl deficiency. The trends of EGF and PrP(C) levels paralleled those previously found in CSF, whereas that of Cbl was the opposite. There was no significant difference in CSF holo-TC levels between the MS patients and the controls. Given that we have previously demonstrated that Cbl positively regulates central nervous system EGF levels, it is conceivable that the low EGF levels in the MS SC may be causally related to a local decrease in Cbl levels. Only PrP(C) levels were invariably decreased in both the SC and CSF regardless of the clinical course of the disease. These findings suggest that the simultaneous lack of Cbl, EGF, and PrP(C)s may greatly hamper the remyelination process in MS patients, because they are key molecules of the machinery for remyelination.

Relationships between cobalamin, epidermal growth factor, and normal prions in the myelin maintenance of central nervous system

Cobalamin (Cbl), epidermal growth factor (EGF), and prions (PrPs) are key molecules for myelin maintenance in the central and peripheral nervous systems. Cbl and EGF increase normal prion (PrP(C)) synthesis and PrP(C) levels in rat spinal cord (SC) and elsewhere. Cbl deficiency increases PrP(C) levels in rat SC and cerebrospinal fluid (CSF), and decreases PrP(C)-mRNA levels in rat SC. The administration of anti-octapeptide repeat PrP(C) region antibodies (Abs) to Cbl-deficient (Cbl-D) rats prevents SC myelin lesions and a local increase in tumor necrosis factor (TNF)-α levels, whereas anti-TNF-α Abs prevent SC myelin lesions and the increase in SC and CSF PrP(C) levels. As it is known that both Cbl and EGF regulate SC PrP(C) synthesis independently, and that Cbl regulates SC EGF synthesis, EGF may play both Cbl-independent and Cbl-dependent roles. When Cbl-D rats undergo Cbl replacement therapy, SC PrP(C) levels are similar to those observed in Cbl-D rats. In rat frontal cortex (which is marginally affected by Cbl deficiency in histological terms), Cbl deficiency decreases PrP(C) levels and the increase induced by Cbl replacement leads to their normalization. Increased nerve PrP(C) levels are detected in the myelin lesions of the peripheral neuropathy of Cbl-D rats, and CSF PrP(C) levels are also increased in Cbl-D patients (but not in patients with Cbl-unrelated neurological diseases). Various common steps in the downstream signaling pathway of Cbl, EGF, and PrP(C) underlines the close relationship between the three molecules in keeping myelin normal.

Cobalamin and normal prions: a new horizon for cobalamin neurotrophism

It is known that cobalamin (Cbl) deficiency damages myelin by increasing tumor necrosis factor (TNF)-α and decreasing epidermal growth factor (EGF) levels in rat central nervous system (CNS), and affects the peripheral nervous system (PNS) morphologically and functionally. It is also known that some polyneuropathies not due to Cbl deficiency are connected with increased TNF-α levels, and that various cytokines (including TNF-α) and growth factors regulate the in vitro synthesis of normal prions (PrP(C)s). Given that there is extensive evidence that PrP(C)s play a key role in the maintenance of CNS and PNS myelin, we investigated whether the PrP(C) octapeptide repeat (OR) region is involved in the pathogenesis of rat Cbl-deficient (Cbl-D) polyneuropathy. After intracerebroventricularly administering antibodies (Abs) against the OR region (OR-Abs) to Cbl-D rats to prevent myelin damage and maximum nerve conduction velocity (MNCV) abnormalities, and PrP(C)s to otherwise normal rats to reproduce PNS Cbl-D-like lesions, we measured PrP(C) levels and MNCV of the sciatic and tibial nerves. PrP(C) and TNF-α levels were increased in sciatic and tibial nerves of Cbl-D and saline-treated rats, and the OR-Abs normalized the myelin ultrastructure, TNF-α levels, and MNCV values of the sciatic and tibial nerves of Cbl-D rats. The same peripheral nerves of the otherwise normal PrP(C)-treated rats showed typical Cbl-D myelin lesions, significantly increased TNF-α levels, and significantly decreased MNCV values. These findings demonstrate that Cbl deficiency induces excess PrP(C)s and thereby excess OR regions, which seem to be responsible for the PNS myelin damage, as has recently been found in the case of CNS myelin damage [66]. Furthermore, excess TNF-α is also involved in the pathogenesis of Cbl-D polyneuropathy. In conclusion, we have extended the list of prion diseases by adding one caused by excess PrP(C)s and the polyneuropathies related to excess TNF-α.