Maximal load of the vitamin B12 transport system: a study on mice treated for four weeks with high-dose vitamin B12 or cobinamide

Vitamin B12 Deficiency and the Nervous System: Beyond Metabolic Decompensation-Comparing Biological Models and Gaining New Insights into Molecular and Cellular Mechanisms

Vitamin B12 (VitB12) is a micronutrient and acts as a cofactor for fundamental biochemical reactions: the synthesis of succinyl-CoA from methylmalonyl-CoA and biotin, and the synthesis of methionine from folic acid and homocysteine. VitB12 deficiency can determine a wide range of diseases, including nervous system impairments. Although clinical evidence shows a direct role of VitB12 in neuronal homeostasis, the molecular mechanisms are yet to be characterized in depth. Earlier investigations focused on exploring the biochemical shifts resulting from a deficiency in the function of VitB12 as a coenzyme, while more recent studies propose a broader mechanism, encompassing changes at the molecular/cellular levels. Here, we explore existing study models employed to investigate the role of VitB12 in the nervous system, including the challenges inherent in replicating deficiency/supplementation in experimental settings. Moreover, we discuss the potential biochemical alterations and ensuing mechanisms that might be modified at the molecular/cellular level (such as epigenetic modifications or changes in lysosomal activity). We also address the role of VitB12 deficiency in initiating processes that contribute to nervous system deterioration, including ROS accumulation, inflammation, and demyelination. Consequently, a complex biological landscape emerges, requiring further investigative efforts to grasp the intricacies involved and identify potential therapeutic targets.

Vitamin B12 is a limiting factor for induced cellular plasticity and tissue repair

Transient reprogramming by the expression of OCT4, SOX2, KLF4 and MYC (OSKM) is a therapeutic strategy for tissue regeneration and rejuvenation, but little is known about its metabolic requirements. Here we show that OSKM reprogramming in mice causes a global depletion of vitamin B12 and molecular hallmarks of methionine starvation. Supplementation with vitamin B12 increases the efficiency of reprogramming both in mice and in cultured cells, the latter indicating a cell-intrinsic effect. We show that the epigenetic mark H3K36me3, which prevents illegitimate initiation of transcription outside promoters (cryptic transcription), is sensitive to vitamin B12 levels, providing evidence for a link between B12 levels, H3K36 methylation, transcriptional fidelity and efficient reprogramming. Vitamin B12 supplementation also accelerates tissue repair in a model of ulcerative colitis. We conclude that vitamin B12, through its key role in one-carbon metabolism and epigenetic dynamics, improves the efficiency of in vivo reprogramming and tissue repair.

The beneficial role of vitamin B12 in injury induced by ischemia/reperfusion: Beyond scavenging superoxide?

Vitamin B12 (B12) is required for cellular metabolism and DNA synthesis as a co-enzyme; it also possesses anti-reactive oxygen species (ROS) property as a superoxide scavenger. B12 deficiency has been implicated in multiple diseases such as megaloblastic anemia, and this disease can be effectively cured by supplementation of B12. Multiple studies suggest that B12 also benefits the conditions associated with excess ROS. Recently, we have reported that oral high dose B12 decreases superoxide level and renal injury induced by ischemia/reperfusion in mice. Here, we discuss potential mechanism(s) other than decreasing superoxide by which B12 executes its beneficial effects.

A second-generation glucagon-like peptide-1 receptor agonist mitigates vomiting and anorexia while retaining glucoregulatory potency in lean diabetic and emetic mammalian models

AIM

To develop a conjugate of vitamin B12 bound to the glucagon-like peptide-1 receptor (GLP-1R) agonist exendin-4 (Ex4) that shows reduced penetrance into the central nervous system while maintaining peripheral glucoregulatory function.

METHODS

We evaluated whether a vitamin B12 conjugate of Ex4 (B12-Ex4) improves glucose tolerance without inducing anorexia in Goto-Kakizaki (GK) rats, a lean type 2 diabetes model of an understudied but medically compromised population of patients requiring the glucoregulatory effects of GLP-1R agonists without anorexia. We also utilized the musk shrew (Suncus murinus), a mammalian model capable of emesis, to test B12-Ex4 on glycaemic profile, feeding and emesis.

RESULTS

In both models, native Ex4 and B12-Ex4 equivalently blunted the rise in blood glucose levels during a glucose tolerance test. In both GK rats and shrews, acute Ex4 administration decreased food intake, leading to weight loss; by contrast, equimolar administration of B12-Ex4 had no effect on feeding and body weight. There was a near absence of emesis in shrews given systemic B12-Ex4, in contrast to reliable emesis produced by Ex4. When administered centrally, both B12-Ex4 and Ex4 induced similar potency of emesis, suggesting that brain penetrance of B12-Ex4 is required for induction of emesis.

CONCLUSIONS

These findings highlight the potential therapeutic value of B12-Ex4 as a novel treatment for type 2 diabetes devoid of weight loss and with reduced adverse effects and better tolerance, but similar glucoregulation to current GLP-1R agonists.

Oral high dose vitamin B12 decreases renal superoxide and post-ischemia/reperfusion injury in mice

Renal ischemia/reperfusion injury (IRI) is a leading cause of acute kidney injury (AKI), a potentially fatal syndrome characterized by a rapid decline in kidney function. Excess production of superoxide contributes to the injury. We hypothesized that oral administration of a high dose of vitamin B12 (B12 - cyanocobalamin), which possesses a superoxide scavenging function, would protect kidneys against IRI and provide a safe means of treatment. Following unilateral renal IR surgery, C57BL/6J wild type (WT) mice were administered B12 via drinking water at a dose of 50 mg/L. After 5 days of the treatment, plasma B12 levels increased by 1.2-1.5x, and kidney B12 levels increased by 7-8x. IRI mice treated with B12 showed near normal renal function and morphology. Further, IRI-induced changes in RNA and protein markers of inflammation, fibrosis, apoptosis, and DNA damage response (DDR) were significantly attenuated by at least 50% compared to those in untreated mice. Moreover, the presence of B12 at 0.3 μM in the culture medium of mouse proximal tubular cells subjected to 3 hr of hypoxia followed by 1 hr of reperfusion in vitro showed similar protective effects, including increased cell viability and decreased reactive oxygen species (ROS) level. We conclude that a high dose of B12 protects against perfusion injury both in vivo and in vitro without observable adverse effects in mice and suggest that B12 merits evaluation as a treatment for I/R-mediated AKI in humans.

Systemically Administered Plant Recombinant Holo-Intrinsic Factor Targets the Liver and is not Affected by Endogenous B12 levels

Precision targeting imaging agents and/or treatment agents to select cells or organs in the body remains a significant need and is an area of intense research. It has been hypothesized that the vitamin B12 (B12) dietary pathway, or components thereof, may be exploitable in this area. The question of whether gastric Intrinsic factor (IF), critical for B12 absorption in the GI tract via the cubilin receptor, could be used as a targeting moiety for the cubilin receptor systemically, has not been investigated. Cubilin is the only known receptor for holo-IF and is found primarily in the kidney and ear (outside of the ileum of the GI) offering significant scope for specific targeting. We utilized plant derived human gastric IF in fluorescent cell and PET based in vivo imaging and biodistribution studies and demonstrated that plant derived IF primarily targets the liver, likely a consequence of the unique glycosylation profile of the IF, and is not affected by endogenous B12 levels.

Facilitating the Cellular Accumulation of Pt-Based Chemotherapeutic Drugs

Cisplatin, carboplatin, and oxaliplatin are Pt-based drugs used in the chemotherapeutic eradication of cancer cells. Although most cancer patient cells initially respond well to the treatment, the clinical effectiveness declines over time as the cancer cells develop resistance to the drugs. The Pt-based drugs are accumulated via membrane-bound transporters, translocated to the nucleus, where they trigger various intracellular cell death programs through DNA interaction. Here we illustrate how resistance to Pt-based drugs, acquired through limitation in the activity/subcellular localization of canonical drug transporters, might be circumvented by the facilitated uptake of Pt-based drug complexes via nanocarriers/endocytosis or lipophilic drugs by diffusion.

Imaging Cobalamin Uptake within Malignant Breast Tumors In Vivo

PURPOSE

To image the uptake of cobalamin (Cbl) within malignant breast tumors in vivo.

PROCEDURES

Prior to surgery 20 female patients with clinically suspected breast tumors were intravenously administered 0.25 μg of an In-111 labeled 5-deoxyadenosylcobalamin (AC) analog ([¹¹¹In]AC) and sequentially imaged with whole-body planar (WBP) and single-photon emission computed tomography (SPECT) between 2-5 h and 20-24 h post-injection (P.I.). The tumor to background (T/B) ratio for [¹¹¹In]AC in breast tumors at 2-5 h was correlated to its expression of estrogen (ER), progesterone (PR), and human epidermal growth factor 2 (HER2) receptors. Subsequent pulse chase (PC) experiments in nude mice burdened with the MDA-MB-231 triple-negative (TN) breast tumor xenograft measured the effect that pulses of AC or dexamethasone (DEX) had on [¹¹¹In]AC uptake in both normal murine tissue and the TN breast tumor.

RESULTS

The mean [¹¹¹In]AC T/B ratio of the patients' 18 resected tumors was 5.8. Comparing ER- and PR-positive tumors (n = 11) to TN and HER2-positive tumors (n = 7), the mean [¹¹¹In]AC T/B ratios at 2-5 h P.I. were 3.2 (range 1.8-5.6) and 10.4 (range 3.3-22.5), respectively. Pulses of 2.0 μg of AC at 2, 8, or 24 h; or 40.0 μg of DEX at 24 h prior to injecting 0.5 μg of [¹¹¹In]AC, increased mean tracer uptake in the MDA-MB-231 tumors by 26.4, 71.5, 92.6, and 49.1 %, respectively. Only the 2- and 24-h PC intervals concomitantly suppressed [¹¹¹In]AC uptake in normal murine tissue while enhancing [¹¹¹In]AC uptake in MDA-MB-231 tumors.

CONCLUSION

The uptake of Cbl within malignant breast tumors can be imaged clinically. Cbl uptake is greatest in TN and HER2-positive breast tumors. A solitary bolus of AC or DEX increases the [¹¹¹In]AC uptake within a breast tumor in vivo. Investigating the cytogenetic mechanisms controlling the endocytosis of Cbl in malignant breast tumors is warranted.

Organometallic DNA-B12 Conjugates as Potential Oligonucleotide Vectors: Synthesis and Structural and Binding Studies with Human Cobalamin-Transport Proteins

The synthesis and structural characterization of Co-(dN)₂₅ -Cbl (Cbl: cobalamin; dN: deoxynucleotide) and Co-(dN)₃₉ -Cbl, which are organometallic DNA-B₁₂ conjugates with single DNA strands consisting of 25 and 39 deoxynucleotides, respectively, and binding studies of these two DNA-Cbl conjugates to three homologous human Cbl transporting proteins, transcobalamin (TC), intrinsic factor (IF), and haptocorrin (HC), are reported. This investigation tests the suitability of such DNA-Cbls for the task of eventual in vivo oligonucleotide delivery. The binding of DNA-Cbl to TC, IF, and HC was investigated in competition with either a fluorescent Cbl derivative and Co-(dN)₂₅ -Cbl, or radiolabeled vitamin B₁₂ (⁵⁷ Co-CNCbl) and Co-(dN)₂₅ -Cbl or Co-(dN)₃₉ -Cbl. Binding of the new DNA-Cbl conjugates was fast and tight with TC, but poorer with HC and IF, which extends a similar original finding with the simpler DNA-Cbl, Co-(dN)₁₈ -Cbl. The contrasting affinities of TC versus IF and HC for the DNA-Cbl conjugates are rationalized herein by a stepwise mechanism of Cbl binding. Critical contributions to overall affinity result from gradual conformational adaptations of the Cbl-binding proteins to the DNA-Cbl, which is first bound to the respective β domains. This transition is fast with TC, but slow with IF and HC, with which weaker binding results. The invariably tight interaction of the DNA-Cbl conjugates with TC makes the Cbl moiety a potential natural vector for the specific delivery of oligonucleotide loads from the blood into cells.

Structure of the human transcobalamin beta domain in four distinct states

Vitamin B12 (cyanocobalamin, CNCbl) is an essential cofactor-precursor for two biochemical reactions in humans. When ingested, cobalamins (Cbl) are transported via a multistep transport system into the bloodstream, where the soluble protein transcobalamin (TC) binds Cbl and the complex is taken up into the cells via receptor mediated endocytosis. Crystal structures of TC in complex with CNCbl have been solved previously. However, the initial steps of holo-TC assembly have remained elusive. Here, we present four crystal structures of the beta domain of human TC (TC-beta) in different substrate-bound states. These include the apo and CNCbl-bound states, providing insight into the early steps of holo-TC assembly. We found that in vitro assembly of TC-alpha and TC-beta to a complex was Cbl-dependent. We also determined the structure of TC-beta in complex with cobinamide (Cbi), an alternative substrate, shedding light on the specificity of TC. We finally determined the structure of TC-beta in complex with an inhibitory antivitamin B12 (anti-B12). We used this structure to model the binding of anti-B12 into full-length holo-TC and could rule out that the inhibitory function of anti-B12 was based on an inability to form a functional complex with TC.

Cellular uptake of metallated cobalamins

Cellular uptake of vitamin B12-cisplatin conjugates was estimated via detection of their metal constituents (Co, Pt, and Re) by inductively coupled plasma mass spectrometry (ICP-MS). Vitamin B12 (cyano-cob(iii)alamin) and aquo-cob(iii)alamin [Cbl-OH2](+), which differ in the β-axial ligands (CN(-) and H2O, respectively), were included as control samples. The results indicated that B12 derivatives delivered cisplatin to both cellular cytosol and nuclei with an efficiency of one third compared to the uptake of free cisplatin cis-[Pt(II)Cl2(NH3)2]. In addition, uptake of charged B12 derivatives including [Cbl-OH2](+), [{Co}-CN-{cis-PtCl(NH3)2}](+), [{Re}-{Co}-CN-{cis-PtCl(NH3)2}](+), and [{Co}-CN-{trans-Pt(Cyt)(NH3)2}](2+) (Cyt = cytarabin) was high compared to neutral B12, which implied the existence of an additional internalization pathway for charged B12 vitamin analogs. The affinities of the charged B12 derivatives to the B12 transporters HC, IF and TC were similar to that of native vitamin B12.

Elevation of urinary methylmolonic acid induces the suppression of megalin-mediated endocytotic cycles during vitamin B12 deficiency

Megalin is a scavenger receptor that serves in the endocytosis of a highly diverse group of ligands that includes Vitamin B12. We found an accumulation of megalin closed to apical region in renal proximal tubule cells of Vitamin B12-deficient rats. Interestingly, Vitamin B12 levels also controlled resorption of renal retinol binding protein. Using L2 yolk sac cells, megalin localized to the submembrane compartment by methylmalonic acid (MMA), which accumulates during vitamin B12 deficiency. In addition, MMA inhibited megalin-mediated endocytosis via YWTD repeats motif in an ectodomain of megalin. Therefore, megalin endocytosis may be regulated by MMA.

Organometallic B12-DNA conjugate: synthesis, structure analysis, and studies of binding to human B12-transporter proteins

Design, synthesis, and structural characterization of a B12-octadecanucleotide are presented herein, a new organometallic B12-DNA conjugate. In such covalent conjugates, the natural B12 moiety may be a versatile vector for controlled in vivo delivery of oligonucleotides to cellular targets in humans and animals, through the endogenous B12 transport systems. Binding of the organometallic B12 octadecanucleotide to the three important human proteins of B12 transport was studied, to examine its structural suitability for the task of eventual in vivo oligonucleotide delivery. Binding was efficient with transcobalamin (TC), but not so efficient with the homologous glycoproteins intrinsic factor and haptocorrin. Binding of the B12 octadecanucleotide to TC suggests the capacity of the B12 moiety to serve as a natural vector for specific transport of single stranded, organometallic oligonucleotide loads from the blood stream into cells.

Investigation of a vitamin B12 conjugate as a PET imaging probe

Nutrient demand is a fundamental characteristic of rapidly proliferating cells. Vitamin B12 is vital for cell proliferation; thus neoplastic cells have an increased demand for this essential nutrient. In this study we exploited the vitamin B12 uptake pathway to probe the nutritional demand of proliferating cells with a radiolabeled B12 derivative in various preclinical tumor models. We describe the synthesis and biological evaluations of copper-64-labeled B12 -ethylenediamine-benzyl-1,4,7-triazacyclononane-N,N',N''-triacetic acid (B12 -en-Bn-NOTA-(64) Cu), the first example of a B12 derivative for positron emission tomography (PET) imaging. Small-animal imaging and pharmacological evaluation show high tumor uptake ranging from 2.20 to 4.84% ID g(-1) at 6 h post-administration. Competition studies with excess native B12 resulted in a 95% decrease in tumor accumulation, indicating the specificity of this radiopharmaceutical for B12 endocytotic transport proteins. These results show that a vitamin B12 PET radiopharmaceutical has potential utility for non-invasive imaging of enhanced nutrient demand in proliferating cells.

4-ethylphenyl-cobalamin impairs tissue uptake of vitamin B12 and causes vitamin B12 deficiency in mice

Co_β-4-ethylphenyl-cob(III) alamin (EtPhCbl) is an organometallic analogue of vitamin B₁₂ (CNCbl) which binds to transcobalamin (TC), a plasma protein that facilitates the cellular uptake of cobalamin (Cbl). In vitro assays with key enzymes do not convert EtPhCbl to the active coenzyme forms of Cbl suggesting that administration of EtPhCbl may cause cellular Cbl deficiency. Here, we investigate the in vivo effect of EtPhCbl in mice and its ability, if any, to induce Cbl deficiency. We show that EtPhCbl binds to mouse TC and we examined mice that received 3.5 nmol/24h EtPhCbl (n=6), 3.5 nmol/24h CNCbl (n=7) or NaCl (control group) (n=5) through osmotic mini-pumps for four weeks. We analyzed plasma, urine, liver, spleen, submaxillary glands and spinal cord for Cbl and markers of Cbl deficiency including methylmalonic acid (MMA) and homocysteine (tHcy). Plasma MMA (mean±SEM) was elevated in animals treated with EtPhCbl (1.01±0.12 µmol/L) compared to controls (0.30±0.02 µmol/L) and CNCbl (0.29±0.01 µmol/L) treated animals. The same pattern was observed for tHcy. Plasma total Cbl concentration was higher in animals treated with EtPhCbl (128.82±1.87 nmol/L) than in CNCbl treated animals (87.64±0.93 nmol/L). However, the organ levels of total Cbl were significantly lower in animals treated with EtPhCbl compared to CNCbl treated animals or controls, notably in the liver (157.07±8.56 pmol/g vs. 603.85±20.02 pmol/g, and 443.09±12.32 pmol/g, respectively). Differences between the three groups was analysed using one-way ANOVA and, Bonferroni post-hoc test. EtPhCbl was present in all tissues, except the spinal cord, accounting for 35-90% of total Cbl. In conclusion, treatment with EtPhCbl induces biochemical evidence of Cbl deficiency. This may in part be caused by a compromised tissue accumulation of Cbl.

Vitamin B₁₂ dependent changes in mouse spinal cord expression of vitamin B₁₂ related proteins and the epidermal growth factor system

Chronic vitamin B12 (cobalamin) deficiency in the mammalian central nervous system causes degenerative damage, especially in the spinal cord. Previous studies have shown that cobalamin status alters spinal cord expression of epidermal growth factor (EGF) and its receptor in rats. Employing a mouse model of cobalamin-depletion and loading, we have explored the influence of Cbl status on spinal cord expression of cobalamin related proteins, as well as all four known EGF receptors and their activating ligands. Following four weeks of osmotic minipump infusion (n=7 in each group) with cobinamide (4.25nmol/h), saline or cobalamin (1.75nmol/h) the spinal cords were analyzed for cobalamin and for the mRNA levels of cobalamin related proteins and members of the EGF system using quantitative reverse transcription PCR. The median spinal cord cobalamin content was 17, 32, and 52pmol/gr of tissues in cobinamide, saline, and cobalamin treated animals, respectively. Both cobinamide and cobalamin induced a significant decrease in the expression of the lysosomal membrane cobalamin transporter. All four EGF receptors and their activating ligands, except for EGF, were expressed in the spinal cord. Notably, the expression of one of the EGF receptors, HER3, and the ligands heparin-binding EGF-like growth factor, transforming growth factor-α, and neuregulins 1α was increased in cobalamin treated mice. Our studies show that four weeks treatment of mice with cobinamide induces spinal cord cobalamin depletion and that cobalamin loading induces an altered expression pattern of the EGF system thus confirming a spinal cord cross talk between Cbl and the EGF system.

Synthesis, characterization and pharmacodynamics of vitamin-B(12)-conjugated glucagon-like peptide-1

Clearing the way: Glucagon-like peptide-1 (GLP-1) receptor agonists are proving a potent weapon in the treatment of type II diabetes. A new vitamin B(12)-GLP-1 conjugate is investigated and shown to have insulinotropic properties similar to the unmodified peptide. These results are critical to the exploitation of the vitamin B(12) oral uptake pathway for peptide delivery.