The Hereditary Hemochromatosis protein HFE and its chaperone β2-microglobulin localise predominantly to the endosomal-recycling compartment

The roles of iron and HFE genotype in neurological diseases

Iron accumulation is a recurring pathological phenomenon in many neurological diseases including Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, and others. Iron is essential for normal development and functions of the brain; however, excess redox-active iron can also lead to oxidative damage and cell death. Especially for terminally differentiated cells like neurons, regulation of reactive oxygen species is critical for cell viability. As a result, cellular iron level is tightly regulated. Although iron accumulation related to neurological diseases has been well documented, the pathoetiological contributions of the homeostatic iron regulator (HFE), which controls cellular iron uptake, is less understood. Furthermore, a common HFE variant, H63D HFE, has been identified as a modifier of multiple neurological diseases. This review will discuss the roles of iron and HFE in the brain as well as their impact on various disease processes.

Novel role for cardiac myocyte-derived β-2 microglobulin in mediating cardiac fibrosis

Hypertension is a significant risk factor for the development of cardiovascular ailments, including ischemic heart disease and diastolic dysfunction. In a recent issue of Clinical Science, Li et al. [Clin. Sci. (2018) 132, 1855-1874] report that β-2 microglobulin (β2M) is a novel secreted soluble factor released by cardiac myocytes during pressure overload that promotes profibrotic gene expression in cardiac fibroblasts both in vitro and in vivo Their study further identifies elevated β2M levels as a possible biomarker for hypertensive patients with cardiac complications. The authors propose a mechanism that mechanically stretched cardiomyocytes release soluble β2M which, through paracrine communication with cardiac fibroblasts, transactivates epidermal growth factor receptor (EGFR) to initiate acute signal transduction and up-regulate profibrotic genes, thereby promoting fibrosis. Here, we will discuss the background, significance of the study, alternative mechanisms, and future directions.

Rediscovering Beta-2 Microglobulin As a Biomarker across the Spectrum of Kidney Diseases

There is currently an unmet need for better biomarkers across the spectrum of renal diseases. In this paper, we revisit the role of beta-2 microglobulin (β₂M) as a biomarker in patients with chronic kidney disease and end-stage renal disease. Prior to reviewing the numerous clinical studies in the area, we describe the basic biology of β₂M, focusing in particular on its role in maintaining the serum albumin levels and reclaiming the albumin in tubular fluid through the actions of the neonatal Fc receptor. Disorders of abnormal β₂M function arise as a result of altered binding of β₂M to its protein cofactors and the clinical manifestations are exemplified by rare human genetic conditions and mice knockouts. We highlight the utility of β₂M as a predictor of renal function and clinical outcomes in recent large database studies against predictions made by recently developed whole body population kinetic models. Furthermore, we discuss recent animal data suggesting that contrary to textbook dogma urinary β₂M may be a marker for glomerular rather than tubular pathology. We review the existing literature about β₂M as a biomarker in patients receiving renal replacement therapy, with particular emphasis on large outcome trials. We note emerging proteomic data suggesting that β₂M is a promising marker of chronic allograft nephropathy. Finally, we present data about the role of β₂M as a biomarker in a number of non-renal diseases. The goal of this comprehensive review is to direct attention to the multifaceted role of β₂M as a biomarker, and its exciting biology in order to propose the next steps required to bring this recently rediscovered biomarker into the twenty-first century.

Expression and alternative splicing of classical and nonclassical MHCI genes in the hippocampus and neuromuscular junction

The major histocompatibility complex class I (MHCI) is a large gene family, with over 20 members in mouse. Some MHCIs are well-known for their critical roles in the immune response. Studies in mice which lack stable cell-surface expression of many MHCI proteins suggest that one or more MHCIs also play unexpected, essential roles in the establishment, function, and modification of neuronal synapses. However, there is little information about which genes mediate MHCI's effects in neurons. In this study, RT-PCR was used to simultaneously assess transcription of many MHCI genes in regions of the central and peripheral nervous system where MHCI has a known or suspected role. In the hippocampus, a part of the CNS where MHCI regulates synapse density, synaptic transmission, and plasticity, we found that more than a dozen MHCI genes are transcribed. Single-cell RT-PCR revealed that individual hippocampal neurons can express more than one MHCI gene, and that the MHCI gene expression profile of CA1 pyramidal neurons differs significantly from that of CA3 pyramidal neurons or granule cells of the dentate gyrus. MHCI gene expression was also assessed at the neuromuscular junction (NMJ), a part of the peripheral nervous system (PNS) where MHCI plays a role in developmental synapse elimination, aging-related synapse loss, and neuronal regeneration. Four MHCI genes are expressed at the NMJ at an age when synapse elimination is occurring in three different muscles. Several MHCI mRNA splice variants were detected in hippocampus, but not at the NMJ. Together, these results establish the first profile of MHCI gene expression at the developing NMJ, and demonstrate that MHCI gene expression is under tight spatial and temporal regulation in the nervous system. They also identify more than a dozen MHCIs that could play important roles in regulating synaptic transmission and plasticity in the central and peripheral nervous systems.

Inhibition of protein-tyrosine phosphatase 1B (PTP1B) mediates ubiquitination and degradation of Bcr-Abl protein

Chronic myelogenous leukemia (CML) is a myeloproliferative disorder characterized at the molecular level by the expression of Bcr-Abl, a chimeric protein with deregulated tyrosine kinase activity. The protein-tyrosine phosphatase 1B (PTP1B) is up-regulated in Bcr-Abl-expressing cells, suggesting a regulatory link between the two proteins. To investigate the interplay between these two proteins, we inhibited the activity of PTP1B in Bcr-Abl-expressing TonB.210 cells by either pharmacological or siRNA means and examined the effects of such inhibition on Bcr-Abl expression and function. Herein we describe a novel mechanism by which the phosphatase activity of PTP1B is required for Bcr-Abl protein stability. Inhibition of PTP1B elicits tyrosine phosphorylation of Bcr-Abl that triggers the degradation of Bcr-Abl through ubiquitination via the lysosomal pathway. The degradation of Bcr-Abl consequently inhibits tyrosine phosphorylation of Bcr-Abl substrates and the downstream production of intracellular reactive oxygen species. Furthermore, PTP1B inhibition reduces cell viability and the IC(50) of the Bcr-Abl inhibitor imatinib mesylate. Degradation of Bcr-Abl via PTP1B inhibition is also observed in human CML cell lines K562 and LAMA-84. These results suggest that inhibition of PTP1B may be a useful strategy to explore in the development of novel therapeutic agents for the treatment of CML, particularly because host drugs currently used in CML such as imatinib focus on inhibiting the kinase activity of Bcr-Abl.

N-glycosylation is important for the correct intracellular localization of HFE and its ability to decrease cell surface transferrin binding

HFE is a type 1 transmembrane protein that becomes N-glycosylated during transport to the cell membrane. It influences cellular iron concentrations through multiple mechanisms, including regulation of transferrin binding to transferrin receptors. The importance of glycosylation in HFE localization and function has not yet been studied. Here we employed bioinformatics to identify putative N-glycosylation sites at residues N110, N130 and N234 of the human HFE protein, and used site-directed mutagenesis to create combinations of single, double or triple mutants. Compared with the wild-type protein, which co-localizes with the type 1 transferrin receptor in the endosomal recycling compartment and on distributed punctae, the triple mutant co-localized with BiP in the endoplasmic reticulum. This was similar to the localization pattern described previously for the misfolding HFE-C282Y mutant that causes type 1 hereditary haemachromatosis. We also observed that the triple mutant was functionally deficient in beta2-microglobulin interactions and incapable of regulating transferrin binding, once again, reminiscent of the HFE-C282Y variant. Single and double mutants that undergo limited glycosylation appeared to have a mixed phenotype, with characteristics primarily of the wild-type, but also some from the glycosylation-deficient protein. Therefore, although they displayed an endosomal recycling compartment/punctate localization like the wild-type protein, many cells simultaneously displayed additional reticular localization. Furthermore, although the majority of cells expressing these single and double mutants showed decreased surface binding of transferrin, a number appeared to have lost this ability. We conclude that glycosylation is important for the normal intracellular trafficking and functional activity of HFE.

Knockdown of beta2-microglobulin perturbs the subcellular distribution of HFE and hepcidin

Hereditary Haemochromatosis is an iron overload disorder associated with mutations in the HFE gene, and to a lesser degree, the gene encoding its chaperone protein beta-2 microglobulin (beta2M). Here, we report that knockdown of beta2M by RNAi restricts HFE distribution to the endoplasmic reticulum (ER). Additionally, we demonstrate that hepcidin, an iron homeostasis-associated protein, localises predominantly to LBPA-positive late endosomes. Interestingly, we show that knockdown of beta2M by RNAi perturbs hepcidin localisation to late endosomes. In summary, our data suggest that beta2M is essential for the correct subcellular distribution of both HFE and hepcidin, two proteins, which are critical for iron homeostasis.