Megalin is essential for renal proximal tubule reabsorption and accumulation of transcobalamin-B(12)

Salvage pathway of vitamin B12 absorption in chickens with mutant tumor virus a receptor

The tumor virus A receptor (TVA), a member of the low-density lipoprotein receptor (LDLR) family, serves as an entry receptor for Avian Leukosis Virus (ALV) subgroups A and K, as well as a receptor for vitamin B12 bound to transcobalamin. Naturally occurring genetic variants in the TVA gene determine susceptibility or resistance to ALV-A and -K, but the effects of these mutated TVA on vitamin B12 uptake have not been investigated systemically. We found four TVA variants comprising the wild type (TVAWT), a single nucleotide polymorphism variant (TVASNP), and two partial deletions in the splicing branch point region (TVAR). This study investigates the relationship between the various genotypes of TVA alleles and uptake of vitamin B12 in chickens. A protein interaction model suggested that mutant TVAs (i.e., TVASNP, TVAR) may have reduced ability to take up vitamin B12 due to a disrupted LDL-A domain, a pivotal region involved in vitamin B12 uptake; however, we found no significant difference in absorption of vitamin B12 in TVAWT and TVASNP chickens, or levels of its metabolite in serum. Notably, TVAR chickens had significantly higher levels of vitamin B12 than TVAWT chickens, a finding contrary to the predicted lower uptake. Expression of vitamin B12 carrier related genes (i.e., AMN, GIF, and TCN2) in chickens showed a stepwise increase: TVAWT > TVASNP > TVAR. These results suggest a mechanism by which mutant TVA chickens with a disrupted TVA protein acquire natural resistance to ALV-A -K, with no impairment of vitamin B12 metabolism.

Megalin: A Sidekick or Nemesis of the Kidney?

Megalin is an endocytic receptor in the proximal tubules that reabsorbs filtered proteins in the kidneys. Recycling of megalin after endocytosis and its expression on the apical plasma membrane of the proximal tubule are critical for its function. The expression of megalin in the kidney undergoes dynamic changes under physiologic and pathophysiologic conditions. Receptors and various effector signaling components regulate megalin expression and, potentially, function. Genetic manipulation and rare mutations in megalin suggest that a lack of or deficiency in megalin expression/function promotes tubular proteinuria and albuminuria. However, the role of megalin in kidney diseases associated with obesity, diabetes, hypertension, and nephrotoxicity remains unclear. To address these questions, animal and human studies have indicated megalin as a protective, injurious, and potentially urinary marker of nephropathy. This article reviews the literature on the regulation of megalin expression and the role of megalin in the pathophysiology of the kidney under experimental and clinical conditions. Moreover, this review articulates the need for studies that can clarify whether megalin can serve as a therapeutic target, in one way or the other, to treat kidney disease.

Proteomic and transcriptomic analyses identify apo-transcobalamin-II as a biomarker of overall survival in osteosarcoma

Background

The large-scale proteomic platform known as the SomaScan® assay is capable of simultaneously measuring thousands of proteins in patient specimens through next-generation aptamer-based multiplexed technology. While previous studies have utilized patient peripheral blood to suggest serum biomarkers of prognostic or diagnostic value in osteosarcoma (OSA), the most common primary pediatric bone cancer, they have ultimately been limited in the robustness of their analyses. We propose utilizing this aptamer-based technology to describe the systemic proteomic milieu in patients diagnosed with this disease.

Methods

To determine novel biomarkers associated with overall survival in OSA, we deployed the SomaLogic SomaScan® 7k assay to investigate the plasma proteomic profile of naive primary, recurrent, and metastatic OSA patients. Following identification of differentially expressed proteins (DEPs) between 2-year deceased and survivor cohorts, publicly available databases including Survival Genie, TIGER, and KM Plotter Immunotherapy, among others, were utilized to investigate the significance of our proteomic findings.

Results

Apo-transcobalamin-II (APO-TCN2) was identified as the most DEP between 2-year deceased and survivor cohorts (Log2 fold change = 6.8, P-value = 0.0017). Survival analysis using the Survival Genie web-based platform indicated that increased intratumoral TCN2 expression was associated with better overall survival in both OSA (TARGET-OS) and sarcoma (TCGA-SARC) datasets. Cell-cell communication analysis using the TIGER database suggested that TCN2+ Myeloid cells likely interact with marginal zone and immunoglobin-producing B lymphocytes expressing the TCN2 receptor (CD320) to promote their proliferation and survival in both non-small cell lung cancer and melanoma tumors. Analysis of publicly available OSA scRNA-sequencing datasets identified similar populations in naive primary tumors. Furthermore, circulating APO-TCN2 levels in OSA were then associated with a plasma proteomic profile likely necessary for robust B lymphocyte proliferation, infiltration, and formation of intratumoral tertiary lymphoid structures for improved anti-tumor immunity.

Conclusions

Overall, APO-TCN2, a circulatory protein previously described in various lymphoproliferative disorders, was associated with 2-year survival status in patients diagnosed with OSA. The relevance of this protein and apparent immunological function (anti-tumor B lymphocyte responses) was suggested using publicly available solid tumor RNA-sequencing datasets. Further studies characterizing the biological function of APO-TCN2 and its relevance in these diseases is warranted.

Emerging Roles of Vitamin B12 in Aging and Inflammation

Vitamin B12 (cobalamin) is an essential nutrient for humans and animals. Metabolically active forms of B12-methylcobalamin and 5-deoxyadenosylcobalamin are cofactors for the enzymes methionine synthase and mitochondrial methylmalonyl-CoA mutase. Malfunction of these enzymes due to a scarcity of vitamin B12 leads to disturbance of one-carbon metabolism and impaired mitochondrial function. A significant fraction of the population (up to 20%) is deficient in vitamin B12, with a higher rate of deficiency among elderly people. B12 deficiency is associated with numerous hallmarks of aging at the cellular and organismal levels. Cellular senescence is characterized by high levels of DNA damage by metabolic abnormalities, increased mitochondrial dysfunction, and disturbance of epigenetic regulation. B12 deficiency could be responsible for or play a crucial part in these disorders. In this review, we focus on a comprehensive analysis of molecular mechanisms through which vitamin B12 influences aging. We review new data about how deficiency in vitamin B12 may accelerate cellular aging. Despite indications that vitamin B12 has an important role in health and healthy aging, knowledge of the influence of vitamin B12 on aging is still limited and requires further research.

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.

β2-microglobulin and colorectal cancer among inpatients: a case-control study

Β2-microglobulin (β2-M) is associated with various malignancies. However, the relationship between β2-M and colorectal cancer (CRC) remains unclear. We explored the association between β2-M and CRC among inpatients who underwent colonoscopy and explored factors that may modify the association. All consecutive inpatients who underwent colonoscopy were enrolled in a tertiary hospital between April 2015 and June 2022. Inpatients with initial CRC or normal colonoscopies were considered eligible as cases or controls, respectively. Baseline characteristics and laboratory indicators of the participants were collected from electronic medical records. Logistic regression analysis, smooth curve fitting, sensitivity analysis, and subgroup analysis were conducted in the present study. After adjusting for baseline clinical characteristics and laboratory parameters, β2-M was positively associated with CRC (odds ratio [OR] 1.32; 95% confidence interval [CI] 1.11-1.58) among inpatients. When the β2-M level was assigned as tertiles, participants in the highest tertile presented with a higher risk of CRC (OR 2.33; 95% CI 1.57-3.48). A positive linear association was observed between β2-M and CRC with smooth curve fitting. In particular, it may be of great importance to monitor β2-M levels for predicting CRC patients.

Vitamin B12 in Cats: Nutrition, Metabolism, and Disease

Cobalamin is a water-soluble molecule that has an important role in cellular metabolism, especially in DNA synthesis, methylation, and mitochondrial metabolism. Cobalamin is bound by intrinsic factor (IF) and absorbed in the ileal tract. The IF in cats is synthesized exclusively by pancreatic tissue. About 75% of the total plasma cobalamin in cats is associated with transcobalamin II, while in this species, transcobalamin I is not present. In cats, the half-life of cobalamin is 11-14 days. Diagnostic biomarkers for B12 status in cats include decreased levels of circulating total cobalamin and increased levels of methylmalonic acid. The reference interval for serum cobalamin concentrations in cats is 290-1500 ng/L, and for the serum methylmalonic acid concentration, it is 139-897 nmol/L. Therapy for hypocobalaminemia mainly depends on the underlying disease. In some cases, subcutaneous or intramuscular injection of 250 μg/cat is empirically administered. In recent years, it has been demonstrated that oral cobalamin supplementation can also be used successfully in dogs and cats as a less invasive alternative to parental administration. This review describes the current knowledge regarding B12 requirements and highlights improvements in diagnostic methods as well as the role of hypocobalaminemia in its associated diseases.

The complex machinery of human cobalamin metabolism

Vitamin B₁₂ (cobalamin, Cbl) is required as a cofactor by two human enzymes, 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR) and methylmalonyl-CoA mutase (MMUT). Within the body, a vast array of transporters, enzymes and chaperones are required for the generation and delivery of these cofactor forms. How they perform these functions is dictated by the structure and interactions of the proteins involved, the molecular bases of which are only now being elucidated. In this review, we highlight recent insights into human Cbl metabolism and address open questions in the field by employing a protein structure and interactome based perspective. We discuss how three very similar proteins-haptocorrin, intrinsic factor and transcobalamin-exploit slight structural differences and unique ligand receptor interactions to effect selective Cbl absorption and internalisation. We describe recent advances in the understanding of how endocytosed Cbl is transported across the lysosomal membrane and the implications of the recently solved ABCD4 structure. We detail how MMACHC and MMADHC cooperate to modify and target cytosolic Cbl to the client enzymes MTR and MMUT using ingenious modifications to an ancient nitroreductase fold, and how MTR and MMUT link with their accessory enzymes to sustainably harness the supernucleophilic potential of Cbl. Finally, we provide an outlook on how future studies may combine structural and interactome based approaches and incorporate knowledge of post-translational modifications to bring further insights.

Vitamin B12 (Cobalamin): Its Fate from Ingestion to Metabolism with Particular Emphasis on Diagnostic Approaches of Acquired Neonatal/Infantile Deficiency Detected by Newborn Screening

Acquired vitamin B12 (vB12) deficiency (vB12D) of newborns is relatively frequent as compared with the incidence of inherited diseases included in newborn screening (NBS) of different countries across the globe. Infants may present signs of vB12D before 6 months of age with anemia and/or neurologic symptoms when not diagnosed in asymptomatic state. The possibility of identifying vitamin deficient mothers after their pregnancy during the breastfeeding period could be an additional benefit of the newborn screening. Vitamin supplementation is widely available and easy to administer. However, in many laboratories, vB12D is not included in the national screening program. Optimized screening requires either second-tier testing or analysis of new urine and blood samples combined with multiple clinical and laboratory follow ups. Our scope was to review the physiologic fate of vB12 and the pathobiochemical consequences of vB12D in the human body. Particular emphasis was put on the latest approaches for diagnosis and treatment of vB12D in NBS.

Newly Identified Deficiencies in the Multiple Sclerosis Central Nervous System and Their Impact on the Remyelination Failure

The pathogenesis of multiple sclerosis (MS) remains enigmatic and controversial. Myelin sheaths in the central nervous system (CNS) insulate axons and allow saltatory nerve conduction. MS brings about the destruction of myelin sheaths and the myelin-producing oligodendrocytes (ODCs). The conundrum of remyelination failure is, therefore, crucial in MS. In this review, the roles of epidermal growth factor (EGF), normal prions, and cobalamin in CNS myelinogenesis are briefly summarized. Thereafter, some findings of other authors and ourselves on MS and MS-like models are recapitulated, because they have shown that: (a) EGF is significantly decreased in the CNS of living or deceased MS patients; (b) its repeated administration to mice in various MS-models prevents demyelination and inflammatory reaction; (c) as was the case for EGF, normal prion levels are decreased in the MS CNS, with a strong correspondence between liquid and tissue levels; and (d) MS cobalamin levels are increased in the cerebrospinal fluid, but decreased in the spinal cord. In fact, no remyelination can occur in MS if these molecules (essential for any form of CNS myelination) are lacking. Lastly, other non-immunological MS abnormalities are reviewed. Together, these results have led to a critical reassessment of MS pathogenesis, partly because EGF has little or no role in immunology.

CD320 expression and apical membrane targeting in renal and intestinal epithelial cells

Vitamin B12 is an essential nutrient acquired via dietary intake. Receptor-mediated endocytosis is a key mechanism in vitamin B12 absorption, cellular uptake, and reabsorption. CD320 is a type I transmembrane protein responsible for cellular uptake of vitamin B12 in peripheral tissues. In this study, we examined segmental distribution and cellular expression of CD320 in mouse kidneys and intestines. We show that CD320 is expressed on the luminal surface in the small intestine and in proximal tubules in the kidney, suggesting that, in addition to its role in vitamin B12 uptake in peripheral tissues, CD320 may participate in vitamin B12 absorption in the small intestine and reabsorption in the kidney. Moreover, we show that an amino acid motif, DSSDE, in the second low-density lipoprotein receptor class A domain of CD320 is a key apical membrane targeting signal in both renal and intestinal epithelial cells. Mutations or deletion of this motif abolish the specific apical membrane expression of CD320 in polarized Madin-Darby canine kidney cells and human colon cancer-derived Caco-2 cells. In short-hairpin RNA-based gene knockdown experiments, we show that the apical membrane targeting of CD320 is mediated by a Rab11a-dependent mechanism. These results extend our knowledge regarding the cell biology of CD320 and its role in vitamin B12 metabolism.

Perspective: The High-Folate-Low-Vitamin B-12 Interaction Is a Novel Cause of Vitamin B-12 Depletion with a Specific Etiology-A Hypothesis

Vitamin B-12 is a water-soluble vitamin that plays important roles in intermediary metabolism. Vitamin B-12 deficiency has many identifiable causes, including autoimmune and other gastrointestinal malabsorption disorders, dietary deficiency, and congenital defects in genes that are involved in vitamin B-12 trafficking and functions. Another putative cause of vitamin B-12 deficiency is the high-folate-low vitamin B-12 interaction, first suspected as the cause for observed relapse and exacerbation of the neurological symptoms in patients with pernicious anemia who were prescribed high oral doses of folic acid. We propose that this interaction is real and represents a novel cause of vitamin B-12 depletion with specific etiology. We hypothesize that excessive intake of folic acid depletes serum holotranscobalamin (holoTC), thereby decreasing active vitamin B-12 in the circulation and limiting its availability for tissues. This effect is specific for holoTC and does not affect holohaptocorrin, the inert form of serum vitamin B-12. Depletion of holoTC by folic acid in individuals with already low vitamin B-12 status further compromises the availability of vitamin B-12 coenzymes to their respective enzymes, and consequently a more pronounced state of biochemical deficiency. This hypothesis is drawn from evidence of observational and intervention studies of vitamin B-12-deficient patients and epidemiological cohorts. The evidence also suggests that, in a depleted state, vitamin B-12 is diverted to the hematopoietic system or the kidney. This most likely reflects a selective response of tissues expressing folate receptors with high affinity for unmetabolized folic acid (UMFA; e.g., hematopoietic progenitors and renal tubules) compared with those tissues (e.g., liver) that only express the reduced folate carrier, which is universally expressed but has poor affinity for UMFA. The biochemical and physiological mechanisms underlying this interaction require elucidation to clarify its potential public health significance.

Kidney disease genetic risk variants alter lysosomal beta-mannosidase (MANBA) expression and disease severity

More than 800 million people in the world suffer from chronic kidney disease (CKD). Genome-wide association studies (GWAS) have identified hundreds of loci where genetic variants are associated with kidney function; however, causal genes and pathways for CKD remain unknown. Here, we performed integration of kidney function GWAS and human kidney-specific expression quantitative trait analysis and identified that the expression of beta-mannosidase (MANBA) was lower in kidneys of subjects with CKD risk genotype. We also show an increased incidence of renal failure in subjects with rare heterozygous loss-of-function coding variants in MANBA using phenome-wide association analysis of 40,963 subjects with exome sequencing data. MANBA is a lysosomal gene highly expressed in kidney tubule cells. Deep phenotyping revealed structural and functional lysosomal alterations in human kidneys from subjects with CKD risk alleles and mice with genetic deletion of Manba Manba heterozygous and knockout mice developed more severe kidney fibrosis when subjected to toxic injury induced by cisplatin or folic acid. Manba loss altered multiple pathways, including endocytosis and autophagy. In the absence of Manba, toxic acute tubule injury induced inflammasome activation and fibrosis. Together, these results illustrate the convergence of common noncoding and rare coding variants in MANBA in kidney disease development and demonstrate the role of the endolysosomal system in kidney disease development.

COX-2-derived prostaglandins as mediators of the deleterious effects of nicotine in chronic kidney disease

Tobacco smoking has been identified as a risk factor in the progression of chronic kidney disease (CKD). In previous studies, we showed that nicotine induces cyclooxygenase (COX)-2 expression in vivo and in vitro and that the administration of nicotine in vivo worsens the severity of renal injury in a model of subtotal renal ablation. In the present study, we tested the role of COX-2-derived prostaglandins on the deleterious effects of nicotine in CKD. Sham and 5/6 nephrectomy (5/6Nx) rats received tap water or nicotine (100 μg/mL) in the drinking water for 12 wk. Additional groups also systemically received the COX-2 inhibitor NS-398 (1.5 mg·kg^-1·day^-1 via osmotic minipump). The administration of nicotine worsened renal injury and proteinuria in 5/6Nx rats and increased proteinuria in sham rats. 5/6Nx rats had increased cortical production of the prostaglandins PGE₂, PGI₂, PGD₂, and PGF_2α and of thromboxane A₂. In these rats, nicotine reduced the production of all prostaglandins examined except thromboxane A₂. Treatment with the COX-2 inhibitor NS-398 resulted in complete inhibition of all prostaglandins studied and ameliorated renal injury and proteinuria in 5/6Nx rats on nicotine but not in 5/6 Nx rats on tap water. Nicotine also reduced the expression of megalin in all groups examined, and this was partially prevented by COX-2 inhibition. In the present study, we showed that in CKD, nicotine worsens renal injury at least in part by producing an imbalance in the production of prostaglandins. This imbalance in the production of prostaglandins likely plays a role in the deleterious effects of smoking on the progression of CKD.

Review of cobalamin status and disorders of cobalamin metabolism in dogs

Disorders of cobalamin (vitamin B₁₂) metabolism are increasingly recognized in small animal medicine and have a variety of causes ranging from chronic gastrointestinal disease to hereditary defects in cobalamin metabolism. Measurement of serum cobalamin concentration, often in combination with serum folate concentration, is routinely performed as a diagnostic test in clinical practice. While the detection of hypocobalaminemia has therapeutic implications, interpretation of cobalamin status in dogs can be challenging. The aim of this review is to define hypocobalaminemia and cobalamin deficiency, normocobalaminemia, and hypercobalaminemia in dogs, describe known cobalamin deficiency states, breed predispositions in dogs, discuss the different biomarkers of importance for evaluating cobalamin status in dogs, and discuss the management of dogs with hypocobalaminemia.

Vitamin B12 , folate, and the methionine remethylation cycle-biochemistry, pathways, and regulation

Vitamin B₁₂ (cobalamin, Cbl) is a nutrient essential to human health. Due to its complex structure and dual cofactor forms, Cbl undergoes a complicated series of absorptive and processing steps before serving as cofactor for the enzymes methylmalonyl-CoA mutase and methionine synthase. Methylmalonyl-CoA mutase is required for the catabolism of certain (branched-chain) amino acids into an anaplerotic substrate in the mitochondrion, and dysfunction of the enzyme itself or in production of its cofactor adenosyl-Cbl result in an inability to successfully undergo protein catabolism with concomitant mitochondrial energy disruption. Methionine synthase catalyzes the methyl-Cbl dependent (re)methylation of homocysteine to methionine within the methionine cycle; a reaction required to produce this essential amino acid and generate S-adenosylmethionine, the most important cellular methyl-donor. Disruption of methionine synthase has wide-ranging implications for all methylation-dependent reactions, including epigenetic modification, but also for the intracellular folate pathway, since methionine synthase uses 5-methyltetrahydrofolate as a one-carbon donor. Folate-bound one-carbon units are also required for deoxythymidine monophosphate and de novo purine synthesis; therefore, the flow of single carbon units to each of these pathways must be regulated based on cellular needs. This review provides an overview on Cbl metabolism with a brief description of absorption and intracellular metabolic pathways. It also provides a description of folate-mediated one-carbon metabolism and its intersection with Cbl at the methionine cycle. Finally, a summary of recent advances in understanding of how both pathways are regulated is presented.

Neuropathology of vitamin B12 deficiency in the Cd320-/- mouse

In humans, vitamin B12 deficiency causes peripheral and CNS manifestations. Loss of myelin in the peripheral nerves and the spinal cord (SC) contributes to peripheral neuropathy and motor deficits. The metabolic basis for the demyelination and brain disorder is unknown. The transcobalamin receptor-knockout mouse ( Cd320-/-) develops cobalamin (Cbl) deficiency in the nervous system, with mild anemia. A decreased S-adenosylmethionine: S-adenosylhomocysteine ratio and increased methionine were seen in the brain with no significant changes in neurotransmitter metabolites. The structural pathology in the SC presented as loss of myelin in the axonal tracts with inflammation. The sciatic nerve (SN) showed increased nonuniform, internodal segments suggesting demyelination, and remyelination in progress. Consistent with these changes, the Cd320-/- mouse showed an increased latency to thermal nociception. Further, lower amplitude of compound action potential in the SN suggested that the functional capacity of the heavily myelinated axons were preferentially compromised, leading to loss of peripheral sensation. Although the metabolic basis for the demyelination and the structural and functional alterations of the nervous system in Cbl deficiency remain unresolved, the Cd320-/- mouse provides a unique model to investigate the pathologic consequences of vitamin B12 deficiency. -Arora, K., Sequeira, J. M., Alarcon, J. M., Wasek, B., Arning, E., Bottiglieri, T., Quadros, E. V. Neuropathology of vitamin B12 deficiency in the Cd320-/- mouse.

Kinetic analysis of transcellular passage of the cobalamin-transcobalamin complex in Caco-2 monolayers

We suggest a novel kinetic approach to quantifying receptor–ligand interactions via the cellular transport and/or accumulation of the ligand. The system of cobalamin (Cbl, vitamin B12) transport was used as a model, because Cbl is an obligatory cofactor, taken up by animal cells with the help of a transport protein and a membrane receptor. Bovine transcobalamin (bTC) stimulated the cellular accumulation and transcytosis of radioactive [⁵⁷Co]Cbl in polarized monolayers of Caco-2 cells. The bovine protein was much more efficient than human TC. The transport was inhibited in a dose-dependent manner by the unlabeled bTC-Cbl complex, the ligand-free bTC, and the receptor-associated protein (RAP). This inhibition pattern implied the presence of a megalin-like receptor. Quantitative assessment of kinetic records by the suggested method revealed the apparent concentration of receptors in vitro (≈15 nM), as well as the dissociation constants of bTC–Cbl (K_d = 13 nM) and RAP (K_d = 1.3 nM). The data were used to estimate the effective luminal concentrations of TC-specific receptors in kidneys (3.8 µM) and intestine (50 nM), the tissues resembling polarized Caco-2 cells.

Mice lacking the transcobalamin-vitamin B12 receptor, CD320, suffer from anemia and reproductive deficits when fed vitamin B12-deficient diet

In humans, poor nutrition, malabsorption and variation in cobalamin (vitamin B12) metabolic genes are associated with hematological, neurological and developmental pathologies. Cobalamin is transported from blood into tissues via the transcobalamin (TC) receptor encoded by the CD320 gene. We created mice carrying a targeted deletion of the mouse ortholog, Cd320. Knockout (KO) mice lacking this TC receptor have elevated levels of plasma methylmalonic acid and homocysteine but are otherwise healthy, viable, fertile and not anemic. To challenge the Cd320 KO mice we maintained them on a vitamin B12-deficient diet. After 5 weeks on this diet, reproductive failure develops in Cd320 KO females but not males. In vitro, homozygous Cd320 KO embryos from cobalamin-deficient Cd320 KO dams develop normally to embryonic day (E) 3.5, while in vivo, few uterine decidual implantation sites are observed at E7.5, suggesting that embryos perish around the time of implantation. Dietary restriction of vitamin B12 induces a severe macrocytic anemia in Cd320 KO mice after 10-12 months while control mice on this diet are anemia-free up to 2 years. Despite the severe anemia, cobalamin-deficient KO mice do not exhibit obvious neurological symptoms. Our results with Cd320 KO mice suggest that an alternative mechanism exists for mice to transport cobalamin independent of the Cd320 encoded receptor. Our findings with deficient diet are consistent with historical and epidemiological data suggesting that low vitamin B12 levels in humans are associated with infertility and developmental abnormalities. Our Cd320 KO mouse model is an ideal model system for studying vitamin B12 deficiency.

Vitamin B12 deficiency and impaired expression of amnionless during aging

BACKGROUND

Physical frailty and loss of mobility in elderly individuals lead to reduced independence, quality of life, and increased mortality. Vitamin B12 deficiency has been linked to several age-related chronic diseases, including in the musculo-skeletal system, where vitamin B12 deficiency is generally believed to be linked to poor nutritional intake. In the present study, we asked whether aging and frailty associate with altered vitamin B12 homeostasis in humans and investigated the underlying molecular mechanisms using preclinical models.

METHODS

We analysed a subset of the Singapore Longitudinal Aging Study and stratified 238 participants based on age and Fried frailty criteria. Levels of methyl-malonic acid (MMA), a marker for vitamin B12 deficiency, and amnionless, the vitamin B12 co-receptor that anchors the vitamin B12 transport complex to the membrane of epithelial cells, were measured in plasma. In addition, vitamin B12 levels and the molecular mechanisms of vitamin B12 uptake and excretion were analysed in ileum, kidney, liver, and blood using a rat model of natural aging where nutritional intake is fully controlled.

RESULTS

We demonstrate that aging and frailty are associated with a higher prevalence of functional vitamin B12 deficiency that can be detected by increased levels of MMA in blood (ρ = 0.25; P = 0.00013). The decline in circulating vitamin B12 levels is recapitulated in a rat model of natural aging where food composition and intake are stable. At the molecular level, these perturbations involve altered expression of amnionless in the ileum and kidney. Interestingly, we demonstrate that amnionless can be detected in serum where its levels increase during aging in both rodents and human (P = 3.3e-07 and 9.2e-07, respectively). Blood amnionless levels negatively correlate with vitamin B12 in rats (r²  = 0.305; P = 0.0042) and positively correlate with the vitamin B12 deficiency marker MMA in humans (ρ = 0.22; P = 0.00068).

CONCLUSIONS

Our results demonstrate that aging and frailty cause intrinsic vitamin B12 deficiencies, which can occur independently of nutritional intake. Mechanistically, vitamin B12 deficiency involves the physio-pathological decline of both the intestinal uptake and the renal reabsorption system for vitamin B12. Finally, amnionless is a novel biomarker which can detect perturbed vitamin B12 bioavailability during aging and physical frailty.

Novel differences in renal gene expression in a diet-induced obesity model

Obesity is a significant risk factor for both chronic kidney disease and end-stage renal disease. To better understand disease development, we sought to identify novel genes differentially expressed early in disease progression. We first confirmed that mice fed a high-fat (HF) diet exhibit early signs of renal injury including hyperfiltration. We then performed RNA-Seq using renal cortex RNA from C57BL6/J male mice fed either HF or control (Ctrl) diet. We identified 1,134 genes differentially expressed in the cortex on HF vs. Ctrl, of which 31 genes were selected for follow-up analysis. This included the 9 most upregulated, the 11 most downregulated, and 11 genes of interest (primarily sensory receptors and G proteins). Quantitative (q)RT-PCR for these 31 genes was performed on additional male renal cortex and medulla samples, and 11 genes (including all 9 upregulated genes) were selected for further study based on qRT-PCR. We then examined expression of these 11 genes in Ctrl and HF male heart and liver samples, which demonstrated that these changes are relatively specific to the renal cortex. These 11 genes were also examined in female renal cortex, where we found that the expression changes seen in males on a HF diet are not replicated in females, even when the females are started on the diet sooner to match weight gain of the males. In sum, these data demonstrate that in a HF-diet model of early disease, novel transcriptional changes occur that are both sex specific and specific to the renal cortex.

A missense mutation in TCN2 is associated with decreased risk for congenital heart defects and may increase cellular uptake of vitamin B12 via Megalin

Deregulation of folate and vitamin B12 (VB12) metabolism contributes to the risk of congenital heart defects (CHDs). Transcobalamin (TCN2) is essential for transporting VB12 from blood to cells as TCN2-bound VB12 (holo-TC) is the only form for somatic cellular uptake. In this study, we performed an association study between common polymorphisms in 46 one carbon metabolism genes and CHD in 412 CHDs and 213 controls. Only two significant association signals in coding regions were identified: FTCD c.1470C>T & TCN2 c.230A>T. The only missense mutation, TCN2 c.230A>T, was further validated in 412 CHDs and 1177 controls. TCN2 c.230T is significantly associated with reduced CHD risk in North Chinese (odds ratio = 0.67, P = 4.62e-05), compared with the 230A allele. Interestingly, the mean level of plasma holo-TC in women with the TA genotype was 1.77-fold higher than that in women with the AA genotype. Further analysis suggested that c.230A>T enhanced the cellular uptake of holo-TC via the LRP2 receptor. Our results determined that a functional polymorphism in TCN2 contributes to the prevalence of CHDs. TCN2 c.230A>T is significantly associated with a reduced CHD risk, likely due to TCN2 c.230T improving the interaction between holo-TC and its LRP2 receptor.

Characteristics of Vitamin B12 Deficiency in Patients With Plasma Cell Disorders

BACKGROUND

Although increased rates of vitamin B12 deficiency have been reported in patients with plasma cell dyscrasias (PCDs), no mechanism has been identified. Excess free light chains (FLCs) could disrupt the renal proximal tubule receptors where B12 is reabsorbed. We sought to characterize the relationship between B12 deficiency and PCDs. We hypothesized that rates of B12 deficiency would be highest in patients with PCDs with high FLC burdens.

METHODS

We reviewed the electronic medical records of 501 patients who met inclusion criteria (diagnosed PCD with documented serum B12 and FLC levels) to obtain clinical data recorded prior to patients' lowest B12 levels.

RESULTS

Overall, 20.0% of patients had low vitamin B12. There was an expected negative correlation between estimated glomular filtration rate and FLC (r_s = -0.317; P < .001). However, low B12 levels were more prevalent in patients with preserved renal function (P = .047). Low B12 was associated with lower mean corpuscular volume (P = .037).

CONCLUSION

Higher FLC burden was associated with poor kidney function but not with low B12. Low B12 was seen more commonly in patients with preserved kidney function. Mean corpuscular volume was statistically but not clinically different between patients with low and normal B12 and, therefore, may not be a reliable indicator of B12 deficiency in PCDs. Prospective studies should compare B12 metabolites with FLC levels. Detection of B12 deficiency among patients with PCDs remains important to reduce neurologic dysfunction and cytopenias, sequelae common to B12 deficiency and PCDs.

Maternofetal transport of vitamin B12: role of TCblR/CD320 and megalin

Vitamin B₁₂ deficiency causes megaloblastic anemia and neurologic disorder in humans. Gene defects of transcobalamin (TC) and the transcobalamin receptor (TCblR), needed for cellular uptake of the TC-bound B₁₂, do not confer embryonic lethality. TC deficiency can produce the hematologic and neurologic complications after birth, whereas TCblR/CD320 gene defects appear to produce mild metabolic changes. Alternate maternofetal transport mechanisms appear to provide adequate B₁₂ to the fetus. To understand this mechanism, we evaluated the role of TC, TCblR/CD320, and megalin in maternofetal transport of B₁₂ in a TCblR/CD320-knockout (KO) mouse. Our results showed high expression of TCblR/CD320 in the labyrinth of the placenta, embryonic brain, and spinal column in wild-type (WT) mice. Megalin expression was about the same in both WT and KO mouse visceral yolk sac, brain, and spinal column. Megalin mRNA was down-regulated in the KO embryonic spinal cord (SC) and kidneys. Megalin expression remained unaltered in adult WT and KO mouse brain, SC, and kidneys. Injected dsRed-TC-B₁₂ and TC-⁵⁷CoB₁₂ accumulated in the visceral yolk sac of KO mice where megalin is expressed and provides an alternate mechanism for the maternofetal transport of Cbl during fetal development.-Arora, K., Sequeira, J. M., Quadros, E. V. Maternofetal transport of vitamin B₁₂: role of TCblR/CD320 and megalin.

Megalin and cubilin in proximal tubule protein reabsorption: from experimental models to human disease

Proximal tubule protein uptake is mediated by 2 receptors, megalin and cubilin. These receptors rescue a variety of filtered ligands, including biomarkers, essential vitamins, and hormones. Receptor gene knockout animal models have identified important functions of the receptors and have established their essential role in modulating urinary protein excretion. Rare genetic syndromes associated with dysfunction of these receptors have been identified and characterized, providing additional information on the importance of these receptors in humans. Using various disease models in combination with receptor gene knockout, the implications of receptor dysfunction in acute and chronic kidney injury have been explored and have pointed to potential new roles of these receptors. Based on data from animal models, this paper will review current knowledge on proximal tubule endocytic receptor function and regulation, and their role in renal development, protein reabsorption, albumin uptake, and normal renal physiology. These findings have implications for the pathophysiology and diagnosis of proteinuric renal diseases. We will examine the limitations of the different models and compare the findings to phenotypic observations in inherited human disorders associated with receptor dysfunction. Furthermore, evidence from receptor knockout mouse models as well as human observations suggesting a role of protein receptors for renal disease will be discussed in light of conditions such as chronic kidney disease, diabetes, and hypertension.

Absorption and blood/cellular transport of folate and cobalamin: Pharmacokinetic and physiological considerations

The systems involving folate and cobalamin have several features in common: 1) their dietary forms require luminal digestion for absorption; 2) intestinal bacteria in the upper intestine synthesize and utilize both vitamins, creating possible competition for the nutrients; 3) there is one major intestinal brush border protein essential for absorption; 4) both are subject to extensive entero-hepatic circulation. Finally, human mutations have confirmed the role of specific transporters and receptors in these processes. There are other features, however, that distinguish the metabolism of these vitamins: 1) upper intestinal bacteria tend to produce folate, while cobalamin (cbl) utilization is more common; 2) cbl absorption requires a luminal binding protein, but folate does not; 3) folate absorption can occur throughout the small bowel, but the cbl receptor, cubilin, is restricted to the distal half of the small bowel; 4) movement into cells uses transporters, exchangers, and symporters, whereas cbl is transferred by receptor-mediated endocytosis; 5) folate is carried in the blood mostly in red blood cells, whereas cbl is carried on specific binding-proteins; 6) folate can enter cells via multiple systems, but cbl uptake into all tissues use the transcobalamin receptor (TC-R), with the asialoglycoprotein receptor (ASGP-R) present in hepatocytes for uptake of haptocorrin-cbl (HC-cbl) complexes. In summary, the systems for absorption and distribution of folate and cobalamin are complex. These complexities help to explain the variable clinical responses after oral administration of the vitamins, especially when provided as supplements.

A population of mitochondrion-rich cells in the pars recta of mouse kidney

Following perfusion of adult mouse kidney with a solution of nitroblue tetrazolium (NBT), certain epithelial cells in the pars recta (S3) segments of proximal tubules react to form cytoplasmic deposits of blue diformazan particles. Such cells are characterized by dark cytoplasm, small and often elliptical nuclei, elaborate, process-bearing profiles, and abundant mitochondria. The atypical epithelial cells display the additional characteristic of immunoreactivity for a wide spectrum of antigens, including mesenchymal proteins such as vimentin. Though present in kidneys of untreated or sham-operated animals, they are particularly evident under experimental conditions such as unilateral ureteral obstruction (UUO), appearing in both contralateral and obstructed kidneys over the course of a week's duration, but disappearing from the obstructed kidney as it undergoes the profound atrophy attributable to deterioration of the population of its proximal tubules. The cells do not appear in neonatal kidneys, even those undergoing UUO, but begin to be recognizable soon after weaning (28 days). It is possible that diformazan-positive cells in the mouse S3 tubular segment constitute a resident population of cells that can replenish or augment the tubule. Although somewhat similar cells, with dark cytoplasm and vimentin expression, have been described in human, rat, and transgenic mouse kidney (Smeets et al. in J Pathol 229: 645-659, 2013; Berger et al. in Proc Natl Acad Sci U S A 111: 1533-1538, 2014), those cells-known as "scattered tubule cells" or "proximal tubule rare cells"- differ from the S3-specific cells in that they are present throughout the entire proximal tubule, often lack a brush border, and have only a few mitochondria.

β2-Microglobulin-mediated signaling as a target for cancer therapy

β2-microglobulin (β2-m) has become the focus of intense scrutiny since the discovery of its undesirable roles promoting osteomimicry and cancer progression. β2-m is a well-known housekeeping protein that forms complexes with the heavy chain of major histocompatibility complex class I molecules, which are heterodimeric cell surface proteins that present antigenic peptides to cytotoxic T cells. On recognition of foreign peptide antigens on cell surfaces, T cells actively bind and lyse antigen-presenting cancer cells. In addition to its roles in tumor immunity, β2-m has two different functions in cancer cells, either tumor promoting or tumor suppressing, in cancer cell context-dependent manner. Our studies have demonstrated that β2-m is involved extensively in the functional regulation of growth, survival, apoptosis, and even metastasis of cancer cells. We found that β2-m is a soluble growth factor and a pleiotropic signaling molecule which interacts with its receptor, hemochromatosis protein, to modulate epithelial-to-mesenchymal transition (EMT) through iron-responsive pathways. Specific antibodies against β2-m have remarkable tumoricidal activity in cancer, through β2-m action on iron flux, alterations of intracellular reactive oxygen species, DNA damage and repair enzyme activities, β-catenin activation and cadherin switching, and tumor responsiveness to hypoxia. These novel functions of β2-m and β2-m signaling may be common to several solid tumors including human lung, breast, renal, and prostate cancers. Our experimental results could lead to the development of a novel class of antibody-based pharmaceutical agents for cancer growth control. In this review, we briefly summarize the recent data regarding β2-m as a promising new cancer therapeutic target and discuss antagonizing this therapeutic target with antibody therapy for the treatment of localized and disseminated cancers.

Low-density lipoprotein receptors in liver: old acquaintances and a newcomer

The lipoprotein receptors low-density lipoprotein receptor (LDLR), the low-density lipoprotein receptor-related protein 1 (LRP1) and megalin/LRP2 share characteristic structural elements. In addition to their well-known roles in endocytosis of lipoproteins and systemic lipid homeostasis, it has been established that LRP1 mediates the endocytotic clearance of a multitude of extracellular ligands and regulates diverse signaling processes such as growth factor signaling, inflammatory signaling pathways, apoptosis, and phagocytosis in liver. Here, possible functions of LRP1 expression in hepatocytes and non-parenchymal cells in healthy and injured liver are discussed. Recent studies indicate the expression of megalin (LRP2) by hepatic stellate cells, myofibroblasts and Kupffer cells and hypothesize that LRP2 might represent another potential regulator of hepatic inflammatory processes. These observations provide the experimental framework for the systematic and dynamic analysis of the LDLR family during chronic liver injury and fibrogenesis.

The soluble receptor for vitamin B12 uptake (sCD320) increases during pregnancy and occurs in higher concentration in urine than in serum

Background

Cellular uptake of vitamin B₁₂ (B12) demands binding of the vitamin to transcobalamin (TC) and recognition of TC-B12 (holoTC) by the receptor CD320, a receptor expressed in high quantities on human placenta. We have identified a soluble form of CD320 (sCD320) in serum and here we present data on the occurrence of this soluble receptor in both serum and urine during pregnancy.

Methods

We examined serum from twenty-seven pregnant women (cohort 1) at gestational weeks 13, 24 and 36 and serum and urine samples from forty pregnant women (cohort 2) tested up to 8 times during gestational weeks 17-41. sCD320, holoTC, total TC and complex formation between holoTC and sCD320 were measured by in-house ELISA methods, while creatinine was measured on the automatic platform Cobas 6000. Size exclusion chromatography was performed on a Superdex 200 column.

Results

Median (range) of serum sCD320 increased from 125 (87-839) pmol/L (week 15) to reach a peak value of 199 (72-672) pmol/L (week 35) then dropped back to its baseline level just before birth (week 40). Around one third of sCD320 was precipitated with holoTC at all-time points studied. The urinary concentration of sCD320 was around two fold higher than in serum. Urinary sCD320/creatinine ratio correlated with serum sCD320 and reached a peak median level of 53 (30–101) pmol/mmol creatinine (week 35). sCD320 present in serum and urine showed the same elution pattern upon size exclusion chromatography.

Conclusion

We report for the first time that sCD320 is present in urine and in a higher concentration than in serum and that serum and urine sCD320 increase during pregnancy. The high urinary concentration and the strong correlation between urinary and serum sCD320 suggests that sCD320 is filtered in the kidney.

Endocytic Receptors in the Renal Proximal Tubule

Protein reabsorption is a predominant feature of the renal proximal tubule. Animal studies show that the ability to rescue plasma proteins relies on the endocytic receptors megalin and cubilin. Recently, studies of patients with syndromes caused by dysfunctional receptors have supported the importance of these for protein clearance of human ultrafiltrate. This review focuses on the molecular biology and physiology of the receptors and their involvement in renal pathological conditions.

Vitamin B12 transport from food to the body's cells--a sophisticated, multistep pathway

Vitamin B(12) (B(12); also known as cobalamin) is a cofactor in many metabolic processes; deficiency of this vitamin is associated with megaloblastic anaemia and various neurological disorders. In contrast to many prokaryotes, humans and other mammals are unable to synthesize B(12). Instead, a sophisticated pathway for specific uptake and transport of this molecule has evolved. Failure in the gastrointestinal part of this pathway is the most common cause of nondietary-induced B(12) deficiency disease. However, although less frequent, defects in cellular processing and further downstream steps in the transport pathway are also known culprits of functional B(12) deficiency. Biochemical and genetic approaches have identified novel proteins in the B(12) transport pathway--now known to involve more than 15 gene products--delineating a coherent pathway for B(12) trafficking from food to the body's cells. Some of these gene products are specifically dedicated to B(12) transport, whereas others embrace additional roles, which explains the heterogeneity in the clinical picture of the many genetic disorders causing B(12) deficiency. This Review describes basic and clinical features of this multistep pathway with emphasis on gastrointestinal transport of B(12) and its importance in clinical medicine.

Internalization of odorant-binding proteins into the mouse olfactory epithelium

The detection of odorants in vertebrates is mediated by chemosensory neurons that reside in the olfactory epithelium of the nose. In land-living species, the hydrophobic odorous compounds inhaled by the airstream are dissolved in the nasal mucus by means of specialized globular proteins, the odorant-binding proteins (OBPs). To assure the responsiveness to odors of each inhalation, a rapid removal of odorants from the microenvironment of the receptor is essential. In order to follow the fate of OBP/odorant complexes, a recombinant OBP was fluorescently labeled, loaded with odorous compounds, and applied to the nose of a mouse. Very quickly, labeled OBP appeared inside the sustentacular cells of the epithelium. This uptake occurred only when the OBP was loaded with appropriate odorant compounds. A search for candidate transporters that could mediate such an uptake process led to the identification of the low density lipoprotein receptor Lrp2/Megalin. In the olfactory epithelium, megalin was found to be specifically expressed in sustentacular cells and the Megalin protein was located in their microvilli. In vitro studies using a cell line that expresses megalin revealed a rapid internalization of OBP/odorant complexes into lysosomes. The uptake was blocked by a Megalin inhibitor, as was the internalization of OBPs into the sustentacular cells of the olfactory epithelium. The results suggest that a Megalin-mediated internalization of OBP/odorant complexes into the sustentacular cells may represent an important mechanism for a rapid and local clearance of odorants.

Proximal tubule cell hypothesis for cardiorenal syndrome in diabetes

Incidence of cardiovascular disease (CVD) is remarkably high among patients with chronic kidney disease (CKD), even in the early microalbuminuric stages with normal glomerular filtration rates. Proximal tubule cells (PTCs) mediate metabolism and urinary excretion of vasculotoxic substances via apical and basolateral receptors and transporters. These cells also retrieve vasculoprotective substances from circulation or synthesize them for release into the circulation. PTCs are also involved in the uptake of sodium and phosphate, which are critical for hemodynamic regulation and maintaining the mineral balance, respectively. Dysregulation of PTC functions in CKD is likely to be associated with the development of CVD and is linked to the progression to end-stage renal disease. In particular, PTC dysfunction occurs early in diabetic nephropathy, a leading cause of CKD. It is therefore important to elucidate the mechanisms of PTC dysfunction to develop therapeutic strategies for treating cardiorenal syndrome in diabetes.

Preliminary studies on the selective accumulation of vitamin-targeted polymers within tumors

Many different cancer types have previously been found to show increased uptake of the vitamins folate, vitamin B12, and biotin; however, it is not known whether these tumor lines show increased uptake of one or more of the vitamins. The current study was designed to examine the relative uptake of the three vitamins in 10 different types of cell lines. Rhodamine-labeled hydroxypropyl-methacrylamide (HPMA) was targeted with vitamin B(12), folate, or biotin, and the uptake of the labeled polymer was compared both in in vitro cell cultures and in mice-bearing tumors from a variety of tumor cell lines. Fluorescent microscopy of cell cultures and histological examination of tumor sections showed greatly increased uptake of the fluorescently labeled polymer in many tumors when the polymer was targeted with folate, biotin, or vitamin B(12). Tumors with enhanced uptake of vitamin B(12)- or folate-targeted rhodamine-HPMA also showed increased uptake of biotin-Rho-HPMA. In contrast, tumors with increased uptake of folate-Rho-HPMA did not show increased uptake of vitamin B12 (VB(12))-HPMA and vice versa. These findings suggest that vitamin-targeted polymers may greatly increase the uptake of drug-polymer complexes in certain tumors, which may result in an increased efficacy of antitumor agents, and which may allow for easier imaging of both the primary and metastatic tumors.

Cellular uptake of steroid carrier proteins--mechanisms and implications

Steroid hormones are believed to enter cells solely by free diffusion through the plasma membrane. However, recent studies suggest the existence of cellular uptake pathways for carrier-bound steroids. Similar to the clearance of cholesterol via lipoproteins, these pathways involve the recognition of carrier proteins by endocytic receptors on the surface of target cells, followed by internalization and cellular delivery of the bound sterols. Here, we discuss the emerging concept that steroid hormones can selectively enter steroidogenic tissues by receptor-mediated endocytosis, and we discuss the implications of these uptake pathways for steroid hormone metabolism and action in vivo.

Identification of multidrug resistance protein 1 (MRP1/ABCC1) as a molecular gate for cellular export of cobalamin

Cobalamin (Cbl, vitamin B(12)) deficiency in humans is a cause of hematologic and neurologic disorders. We show here that the cellular export of Cbl, in contrast to the carrier- and receptor-dependent cellular import of Cbl, occurs by transmembrane transport of "free" Cbl. Screening of candidate transporters by cellular gene silencing showed a role in cellular Cbl efflux of the ATP-binding cassette (ABC)-drug transporter, ABCC1, alias multidrug resistance protein 1 (MRP1), which is present in the basolateral membrane of intestinal epithelium and in other cells. The ability of MRP1 to mediate ATP-dependent Cbl transport was confirmed by vesicular transport experiments, and a physiologic role of MRP1 in mammalian Cbl homeostasis is indicated by the phenotype of knockout mice with targeted disruption of MRP1. These animals have a reduced concentration of Cbl in plasma and in the storage organs liver and kidney. In contrast, Cbl accumulates in the terminal part of the intestine of these mice, suggesting a functional malabsorption because of a lower epithelial basolateral Cbl efflux. The identification of this Cbl export mechanism now allows the delineation of a coherent pathway for Cbl trafficking from food to the body cells.

Age-related alterations of B-group vitamin contents in urine, blood and liver from rats

To investigate how aging alters B-group vitamin metabolism, rats were fed with niacin-free 20% casein diet from 3 to 80 wk old, and the urinary excretions of the B group vitamins were periodically measured. The blood and liver B-group vitamin levels in 80-wk-old rats were also compared with those in 8-wk-old rats. The urinary excretion of thiamin, riboflavin, vitamin B6 metabolite 4-pyridoxic acid, pantothenic acid, folic acid and biotin were not altered during 540 d. The urinary vitamin B12 increased by 8-fold at 29 wk old, and further increased at 80 wk old. Conversion of nicotinamide from tryptophan gradually decreased to 60% from 29 to 48 wk old. Plasma PLP, vitamin B12 and folate levels in 80-wk-old rats were lower than those in 8-wk-old rats, consistent with lower liver vitamin B6 and folate levels in aged rats. Plasma and liver biotin levels in aged rats were higher than those in young rats. Other B-group vitamins such as vitamin B1, vitamin B2, niacin and pantothenic acid levels in blood and liver from aged rats were same as those from young rats. Alteration of vitamin B6 metabolism in particular is similar to the observations in elderly humans reported previously. Our findings suggest that aged rats can be useful models to investigate aging-related B-group vitamin metabolism.

Sensing, signaling and sorting events in kidney epithelial cell physiology

The kidney regulates body fluid, ion and acid/base homeostasis through the interaction of a host of channels, transporters and pumps within specific tubule segments, specific cell types and specific plasma membrane domains. Furthermore, renal epithelial cells have adapted to function in an often unique and challenging environment that includes high medullary osmolality, acidic pHs, variable blood flow and constantly changing apical and basolateral 'bathing' solutions. In this review, we focus on selected protein trafficking events by which kidney epithelial cells regulate body fluid, ion and acid-base homeostasis in response to changes in physiological conditions. We discuss aquaporin 2 and G-protein-coupled receptors in fluid and ion balance, the vacuolar H(+)-adenosine triphosphatase (V-ATPase) and intercalated cells in acid/base regulation and acidification events in the proximal tubule degradation pathway. Finally, in view of its direct role in vesicle trafficking that we outline in this study, we propose that the V-ATPase itself should, under some circumstances, be considered a fourth category of vesicle 'coat' protein (COP), alongside clathrin, caveolin and COPs.

Embryonic development in the reduced folate carrier knockout mouse is modulated by maternal folate supplementation

BACKGROUND

The reduced folate carrier (RFC1) is a ubiquitously expressed integral membrane protein that mediates delivery of 5-methyltetrahydrofolate into mammalian cells. In this study, embryonic/fetal development is characterized in an RFC1 knockout mouse model in which pregnant dams receive different levels of folate supplementation.

METHODS

RFC1(+/-) males were mated to RFC1(+/-) females, and pregnant dams were treated with vehicle (control) or folic acid (25 or 50 mg/kg) by daily subcutaneous injection (0.1 mL/10 g bwt), beginning on E0.5 and continuing throughout gestation until the time of sacrifice.

RESULTS

Without maternal folate supplementation, RFC1 nullizygous embryos die shortly postimplantation. Supplementation of pregnant dams with 25 mg/kg/day folic acid prolongs survival of mutant embryos until E9.5-E10.5, but they are developmentally delayed relative to wild-type littermates, display a marked absence of erythropoiesis, severe neural tube and limb bud defects, and failure of chorioallantoic fusion. Fgfr2 protein levels are significantly reduced or absent in the extraembryonic membranes of RFC1 nullizygous embryos. Maternal folate supplementation with 50 mg/kg/day results in survival of 22% of RFC1 mutants to E18.5, but they develop with multiple malformations of the eyelids, lungs, heart, and skin.

CONCLUSIONS

High doses of daily maternal folate supplementation during embryonic/fetal development are necessary for early postimplantation embryonic viability of RFC1 nullizygous embryos, and play a critical role in chorioallantoic fusion, erythropoiesis, and proper development of the neural tube, limbs, lungs, heart, and skin.

New Derivatives of Vitamin B12 Show Preferential Targeting of Tumors

Rapidly growing cells show an increased demand for nutrients and vitamins. The objective of our work is to exploit the supply route of vitamin B12 to deliver new derivatives of this vital vitamin to hyperproliferative cells. To date, radiolabeled (57Co and 111In) vitamin B12 derivatives showed labeling of tumor tissue but also undesired high accumulation of radioactivity in normal tissue. By abolishing the interaction of a tailored vitamin B12 derivative to its transport protein transcobalamin II and therefore interrupting transcobalamin II receptor and megalin mediated uptake in normal tissue, preferential accumulation of a radiolabeled vitamin in cancer tissue could be accomplished. We identified transcobalamin I on tumors as a possible new receptor for this preferential accumulation of vitamin-mediated targeting. The low systemic distribution of radioactivity and the high tumor to blood ratio opens the possibility of a more successful clinical application of vitamin B12 for imaging or therapy. [Cancer Res 2008;68(8):2904–11]

Microarray analysis of E9.5 reduced folate carrier (RFC1; Slc19a1) knockout embryos reveals altered expression of genes in the cubilin-megalin multiligand endocytic receptor complex

BACKGROUND

The reduced folate carrier (RFC1) is an integral membrane protein and facilitative anion exchanger that mediates delivery of 5-methyltetrahydrofolate into mammalian cells. Adequate maternal-fetal transport of folate is necessary for normal embryogenesis. Targeted inactivation of the murine RFC1 gene results in post-implantation embryolethality, but daily folic acid supplementation of pregnant dams prolongs survival of homozygous embryos until mid-gestation. At E10.5 RFC1-/- embryos are developmentally delayed relative to wildtype littermates, have multiple malformations, including neural tube defects, and die due to failure of chorioallantoic fusion. The mesoderm is sparse and disorganized, and there is a marked absence of erythrocytes in yolk sac blood islands. The identification of alterations in gene expression and signaling pathways involved in the observed dysmorphology following inactivation of RFC1-mediated folate transport are the focus of this investigation.

RESULTS

Affymetrix microarray analysis of the relative gene expression profiles in whole E9.5 RFC1-/- vs. RFC1+/+ embryos identified 200 known genes that were differentially expressed. Major ontology groups included transcription factors (13.04%), and genes involved in transport functions (ion, lipid, carbohydrate) (11.37%). Genes that code for receptors, ligands and interacting proteins in the cubilin-megalin multiligand endocytic receptor complex accounted for 9.36% of the total, followed closely by several genes involved in hematopoiesis (8.03%). The most highly significant gene network identified by Ingenuitytrade mark Pathway analysis included 12 genes in the cubilin-megalin multiligand endocytic receptor complex. Altered expression of these genes was validated by quantitative RT-PCR, and immunohistochemical analysis demonstrated that megalin protein expression disappeared from the visceral yolk sac of RFC1-/- embryos, while cubilin protein was widely misexpressed.

CONCLUSION

Inactivation of RFC1 impacts the expression of several ligands and interacting proteins in the cubilin-amnionless-megalin complex that are involved in the maternal-fetal transport of folate and other nutrients, lipids and morphogens such as sonic hedgehog (Shh) and retinoids that play critical roles in normal embryogenesis.

Properties of the Glomerular Barrier and Mechanisms of Proteinuria

This review focuses on the intricate properties of the glomerular barrier. Other reviews have focused on podocyte biology, mesangial cells, and the glomerular basement membrane (GBM). However, since all components of the glomerular membrane are important for its function, proteinuria will occur regardless of which layer is affected by disease. We review the properties of endothelial cells and their surface layer, the GBM, and podocytes, discuss various methods of studying glomerular permeability, and analyze data concerning the restriction of solutes by size, charge, and shape. We also review the physical principles of transport across biological or artificial membranes and various theoretical models used to predict the fluxes of solutes and water. The glomerular barrier is highly size and charge selective, in qualitative agreement with the classical studies performed 30 years ago. The small amounts of albumin filtered will be reabsorbed by the megalin-cubulin complex and degraded by the proximal tubular cells. At present, there is no unequivocal evidence for reuptake of intact albumin from urine. The cellular components are the key players in restricting solute transport, while the GBM is responsible for most of the resistance to water flow across the glomerular barrier.

The role of megalin (LRP-2/Gp330) during development

Megalin (LRP-2/GP330), a member of the LDL receptor family, is an endocytic receptor expressed mainly in polarised epithelial cells. Identified as the pathogenic autoantigen of Heymann nephritis in rats, its functions have been studied in greatest detail in adult mammalian kidney, but there is increasing recognition of its involvement in embryonic development. The megalin homologue LRP-1 is essential for growth and development in Caenorhabditis elegans and megalin plays a role in CNS development in zebrafish. There is now also evidence for a homologue in Drosophila. However, most research concerns mammalian embryogenesis; it is widely accepted to be important during forebrain development and the developing renal proximal tubule. Megalin is also expressed in lung, eye, intestine, uterus, oviduct, and male reproductive tract. It is found in yolk sacs and the outer cells of pre-implantation mouse embryos, where interactions with cubilin result in nutrient endocytosis, and it may be important during implantation. Models for megalin interaction(s) with Sonic Hedgehog (Shh) have been proposed. The importance of Shh signalling during embryogenesis is well established; how and when megalin interacts with Shh is becoming a pertinent question in developmental biology.

The kidney in vitamin B12and folate homeostasis: characterization of receptors for tubular uptake of vitamins and carrier proteins

Over the past 10 years, animal studies have uncovered the molecular mechanisms for the renal tubular recovery of filtered vitamin and vitamin carrier proteins. Relatively few endocytic receptors are responsible for the proximal tubule uptake of a number of different vitamins, preventing urinary losses. In addition to vitamin conservation, tubular uptake by endocytosis is important to vitamin metabolism and homeostasis. The present review focuses on the receptors involved in renal tubular recovery of folate, vitamin B12, and their carrier proteins. The multiligand receptor megalin is important for the uptake and tubular accumulation of vitamin B12. During vitamin load, the kidney accumulates large amounts of free vitamin B12, suggesting a possible storage function. In addition, vitamin B12is metabolized in the kidney, suggesting a role in vitamin homeostasis. The folate receptor is important for the conservation of folate, mediating endocytosis of the vitamin. Interaction between the structurally closely related, soluble folate-binding protein and megalin suggests that megalin plays an additional role in the uptake of folate bound to filtered folate-binding protein. A third endocytic receptor, the intrinsic factor-B12receptor cubilin-amnionless complex, is essential to the renal tubular uptake of albumin, a carrier of folate. In conclusion, uptake is mediated by interaction with specific endocytic receptors also involved in the renal uptake of other vitamins and vitamin carriers. Little is known about the mechanisms regulating intracellular transport and release of vitamins, and whereas tubular uptake is a constitutive process, this may be regulated, e.g., by vitamin status.

Megalin-mediated reuptake of retinol in the kidneys of mice is essential for vitamin A homeostasis

The reuptake of retinol (ROH) and retinol-binding protein (RBP) in the kidneys is mediated by the endocytic receptor megalin, suggesting an important role for this receptor in vitamin A (VA) metabolism. We examined the extent to which megalin deficiency may affect urinary ROH excretion, levels of ROH and RBP in plasma, as well as storage of VA in liver and kidney. For this purpose, mice with a kidney-specific megalin gene defect (megalin(lox/lox); apoE(Cre)) and control mice (megalin(lox/lox)) were fed either a basal diet containing 4500 retinol equivalents (RE)/kg diet or a diet without VA during experimental periods of 42 and 84 d. Urinary ROH excretion was observed only in megalin(lox/lox); apoE(Cre) mice (P < 0.0001, 2-way ANOVA) and not in the controls. Plasma ROH and RBP differed only by diet (P < 0.05), but not genotype (P = 0.615). A major effect of megalin deficiency, however, was evident in retinyl ester levels in the liver (P < 0.05), which were approximately 37% lower than those in megalin(lox/lox) controls (P < 0.05, Student's t test) during the 84-d period of dietary VA deprivation. Kidney levels of VA were not affected by the receptor gene defect. The findings demonstrate that urinary ROH excretion caused by megalin deficiency requires accelerated mobilization of hepatic VA stores to maintain normal plasma ROH levels, which suggests that megalin plays an essential role in systemic VA homeostasis.

Protein reabsorption in renal proximal tubule-function and dysfunction in kidney pathophysiology

The endocytic receptors megalin and cubilin are highly expressed in the early parts of the endocytic apparatus of the renal proximal tubule. The two receptors appear to be responsible for the tubular clearance of most proteins filtered in the glomeruli. Since cubilin is a peripheral membrane protein it has no endocytosis signaling sequence. Cubilin binds to megalin and it appears that megalin is responsible for internalization of cubilin and its ligands, in addition to internalizing its own ligands. The importance of the receptors is underscored by the proteinuria observed in megalin-deficient mice, in dogs lacking functional cubilin, and in patients with distinct mutations of the cubilin gene. In this review we focus on the role of megalin- and cubilin-mediated endocytosis in renal pathophysiology. Association between disorders characterized by tubular proteinuria, such as megaloblastic anemia type-1, Dent disease, cystinosis, and Fabry disease and the dysfunction of proximal tubular endocytosis is discussed. The correlation between the high capacity of endocytosis in the proximal tubule and progressive renal disease in overload proteinuria is considered.