Transferrin receptor functions as a signal-transduction molecule for its own recycling via increases in the internal Ca2+ concentration

CAMKK2-CAMK4 signaling regulates transferrin trafficking, turnover, and iron homeostasis

Background

Circulatory iron is a hazardous biometal. Therefore, iron is transported in a redox-safe state by a serum glycoprotein - transferrin (TF). Different organs acquire iron from the systemic circulation through a tightly regulated mechanism at the blood-tissue interface which involves receptor-mediated internalization of TF. Thus, abnormal TF trafficking may lead to iron dyshomeostasis associated with several diseases including neurodegeneration. Iron -induced toxicity can cause neuronal damage to iron-sensitive brain regions. Recently, it was discovered that CAMKK2, a calcium (Ca²⁺)/calmodulin-activated kinase, controls receptor-mediated TF trafficking in mouse tissues, specifically in the brain. The biological function of CAMKK2 is mediated through multiple downstream effectors. Both CAMKK2 and one of its downstream kinase, CAMK4, exhibit overlapping expression in mouse brain. The role of CAMK4 in vesicular transport has been reported and loss of CAMKK2 or CAMK4 leads to cognitive defects in mouse. Therefore, it was hypothesized that CAMKK2-CAMK4 signaling regulates receptor-mediated TF trafficking and iron homeostasis which may be responsible for the neuronal malfunction observed in CAMKK2- or CAMK4-deficient mice.

Methods

CAMK4^−/− mouse was used to study tissue-specific turnover of TF, TF-receptor (TFRC) and iron. CRISPR/Cas9-based CAMKK2 and/or CAMK4 deleted human embryonic kidney-derived HEK293 cell clones were used to study the molecular defects in receptor-mediated TF trafficking. Further, a “zero functional G protein” condition in HEK293 cell was exploited to study CAMKK2-CAMK4 signaling-mediated regulation of intracellular Ca²⁺ homeostasis which was linked to calcium signaling during TF trafficking.

Results

Loss of CAMK4 leads to abnormal post-translational modifications (PTMs) and turnover of TF in mouse cerebellum and liver which was associated with iron dyshomeostasis in these tissues. The HEK293 cell-based study revealed that the absence of CAMKK2-CAMK4 signaling altered intracellular Ca²⁺ homeostasis and lead to abnormal calcium signaling during TF trafficking. Also, CAMKK2-CAMK4 signaling deficiency affected the molecular interaction of TF and TF-receptor-associated protein complexes which indicated a potential failure in the recruitment of interacting proteins due to differential PTMs in TF.

Conclusion

Overall, this study established a novel mechanistic link between intracellular Ca²⁺ level, receptor-mediated TF trafficking, and iron homeostasis, all regulated by CAMKK2-CAMK4 signaling.

Heterologous downregulation of vasopressin type 2 receptor is induced by transferrin

Vasopressin (VP) binds to the vasopressin type 2 receptor (V2R) to trigger physiological effects including body fluid homeostasis and blood pressure regulation. Signaling is terminated by receptor downregulation involving clathrin-mediated endocytosis and V2R degradation. We report here that both native and epitope-tagged V2R are internalized from the plasma membrane of LLC-PK1 kidney epithelial cells in the presence of another ligand, transferrin (Tf). The presence of iron-saturated Tf (holo-Tf; 4 h) reduced V2R binding sites at the cell surface by up to 33% while iron-free (apo-Tf) had no effect. However, no change in green fluorescent protein-tagged V2R distribution was observed in the presence of bovine serum albumin, atrial natriuretic peptide, or ANG II. Conversely, holo-Tf did not induce the internalization of another G protein-coupled receptor, the parathyroid hormone receptor. In contrast to the effect of VP, Tf did not increase intracellular cAMP or modify aquaporin-2 distribution in these cells, although addition of VP and Tf together augmented VP-induced V2R internalization. Tf receptor coimmunoprecipitated with V2R, suggesting that they interact closely, which may explain the additive effect of VP and Tf on V2R endocytosis. Furthermore, Tf-induced V2R internalization was abolished in cells expressing a dominant negative dynamin (K44A) mutant, indicating the involvement of clathrin-coated pits. We conclude that Tf can induce heterologous downregulation of the V2R and this might desensitize VP target cells without activating downstream V2R signaling events. It also provides new insights into urine-concentrating defects observed in rat models of hemochromatosis.

SRC-mediated phosphorylation of dynamin and cortactin regulates the "constitutive" endocytosis of transferrin

The mechanisms by which epithelial cells regulate clathrin-mediated endocytosis (CME) of transferrin are poorly defined and generally viewed as a constitutive process that occurs continuously without regulatory constraints. In this study, we demonstrate for the first time that endocytosis of the transferrin receptor is a regulated process that requires activated Src kinase and, subsequently, phosphorylation of two important components of the endocytic machinery, namely, the large GTPase dynamin 2 (Dyn2) and its associated actin-binding protein, cortactin (Cort). To our knowledge these findings are among the first to implicate an Src-mediated endocytic cascade in what was previously presumed to be a nonregulated internalization process.

TRPV5 is internalized via clathrin-dependent endocytosis to enter a Ca2+-controlled recycling pathway

The epithelial Ca(2+) channel TRPV5 plays an essential role in transcellular Ca(2+) transport and is one of the most Ca(2+)-selective members of the transient receptor potential superfamily. Regulation of the abundance of TRPV5 at the cell surface is critical in body Ca(2+) homeostasis. However, little is known about the mechanisms underlying TRPV5 endo- and exocytosis. Here, we show that TRPV5 is constitutively internalized in a dynamin- and clathrin-dependent manner. Internalized TRPV5 first appears in small vesicular structures and then localizes to perinuclear structures positive for Rab11a. TRPV5 has a half-life of more than 8 h and is stable even after internalization from the cell surface for more than 3 h. Disruption of cell surface delivery of newly synthesized TRPV5 by brefeldin A does not reduce TRPV5-mediated Ca(2+) influx in cells, suggesting the presence of a stable intracellular pool of the channel capable of recycling back to the surface. Furthermore, the endocytic recycling kinetics is decreased upon treatment with Ca(2+) chelator BAPTA-AM, indicating that the channel's trafficking pathways are dynamically controlled by Ca(2+).

Ca2+-dependent calmodulin binding to FcRn affects immunoglobulin G transport in the transcytotic pathway

The Fcgamma receptor FcRn transports immunoglobulin G (IgG) so as to avoid lysosomal degradation and to carry it bidirectionally across epithelial barriers to affect mucosal immunity. Here, we identify a calmodulin-binding site within the FcRn cytoplasmic tail that affects FcRn trafficking. Calmodulin binding to the FcRn tail is direct, calcium-dependent, reversible, and specific to residues comprising a putative short amphipathic alpha-helix immediately adjacent to the membrane. FcRn mutants with single residue substitutions in this motif, or FcRn mutants lacking the cytoplasmic tail completely, exhibit a shorter half-life and attenuated transcytosis. Chemical inhibitors of calmodulin phenocopy the mutant FcRn defect in transcytosis. These results suggest a novel mechanism for regulation of IgG transport by calmodulin-dependent sorting of FcRn and its cargo away from a degradative pathway and into a bidirectional transcytotic route.

Effects of endurance training on intracellular calcium concentration in T lymphocytes

The purpose of the present study was to determine whether 12 months of endurance training reduced [Ca2+]i in T helper (CD4+) lymphocytes in trained (TR) men compared to untrained (UT). Fourteen trained (Ironman triathletes) and nine untrained (sedentary) men volunteered for the study. The TR group averaged 12 km of swimming, 300 km of cycling and 60 km of running per week during the year. Resting blood samples were taken from TR (VO(2peak) 64 +/- 2 ml kg(-1) min(-1)) and UT (VO(2peak) 42 +/- 2 ml kg(-1) min(-1)) subjects every 4 weeks for 52 weeks (October 1, 1999-October 1, 2000). Leukocyte concentration was measured using a full blood count. Unstimulated CD4+ lymphocytes were separated and analysed for changes in free ([Ca2+]i) and total ([Ca2+]t) calcium using flow cytometry. There were no significant differences in leukocyte concentration between UT and TR groups. There were significant differences between TR and UT in [Ca2+]i (October B and November), and [Ca2+]t (January and March). There were also significant sequential monthly changes in both [Ca2+]i and [Ca2+]t for TR and UT groups during the study. Significant increases in [Ca2+]i and [Ca2+]t during summer (January and March) for both TR and UT groups suggest an increase in intracellular signalling during hot weather. [Ca2+]i and [Ca2+]t were significantly lower in TR lymphocytes during November and March, suggesting that endurance training during warmer months may decrease [Ca2+]i through altered intracellular signalling, possibly to maintain lymphocyte function during heat stress.

Heat shock protein 27 downregulates the transferrin receptor 1-mediated iron uptake

It has been reported that over-expression of human heat shock protein 27 (hsp27) in murine cells decreased the intracellular iron level [Arrigo, A. P., Virot, S., Chaufour, S., Firdaus, W., Kretz-Remy, C., & Diaz-Latoud, C. (2005). Hsp27 consolidates intracellular redox homeostasis by upholding glutathione in its reduced form and by decreasing iron intracellular levels. Antioxidants & Redox Signalling, 7, 412-422]. However, the mechanism involved is unknown. In this study, the regulation of transferrin receptor 1 (TfR1)-mediated iron uptake by human hsp27 was investigated in CCL39 cells by overexpression of human hsp27 and its dominant-negative mutant (hsp27-3G). The results showed that overexpression of hsp27 diminished intracellular labile iron pool, increased the binding activity of iron regulatory protein (IRP) to iron responsive element (IRE) and the cell surface-expressed TfR1s. However, the increased surface-expressed TfR1s resulted in decrease rather than increase of iron uptake. Further study revealed that overexpression of hsp27 decelerated transferrin endocytosis and recycling, while overexpressed hsp27-3G had a reversal effect. Moreover, flowcytometric analysis showed an enhanced actin polymerization in the cells overexpressing hsp27. In particular, fluorescence imaging of cytoskeleton displayed highly stabilized microfilaments and preferential localization of hsp27 in cortical area of the actin cytoskeleton. In contrast, disruption of actin cytoskeleton by cytochalasin B resulted in acceleration of the endocytosis and recycling of Tf, as well as increase of iron uptake. Meanwhile, the possible involvement of ferroportin 1 in down-regulation of intracellular iron level by overexpression of hsp27 was checked. However, the outcome was negative. Our findings indicate that hsp27 down-regulates TfR1-mediated iron uptake via stabilization of the cortical actin cytoskeleton rather than the classical IRP/IRE mode. The study may also imply that hsp27 protects cells from oxidative stress by reducing cellular iron uptake.

Parvovirus infection of cells by using variants of the feline transferrin receptor altering clathrin-mediated endocytosis, membrane domain localization, and capsid-binding domains

The feline and canine transferrin receptors (TfRs) bind canine parvovirus to host cells and mediate rapid capsid uptake and infection. The TfR and its ligand transferrin have well-described pathways of endocytosis and recycling. Here we tested several receptor-dependent steps in infection for their role in virus infection of cells. Deletions of cytoplasmic sequences or mutations of the Tyr-Thr-Arg-Phe internalization motif reduced the rate of receptor uptake from the cell surface, while polar residues introduced into the transmembrane sequence resulted in increased degradation of transferrin. However, the mutant receptors still mediated efficient virus infection. In contrast, replacing the cytoplasmic and transmembrane sequences of the feline TfR with those of the influenza virus neuraminidase (NA) resulted in a receptor that bound and endocytosed the capsid but did not mediate viral infection. This chimeric receptor became localized to detergent-insoluble membrane domains. To test the effect of structural virus receptor interaction on infection, two chimeric receptors were prepared which contained antibody-variable domains that bound the capsid in place of the TfR ectodomain. These chimeric receptors bound CPV capsids and mediated uptake but did not result in cell infection. Adding soluble feline TfR ectodomain to the virus during that uptake did not allow infection.

Exosome release is regulated by a calcium-dependent mechanism in K562 cells

Multivesicular bodies (MVBs) are endocytic structures that contain small vesicles formed by the budding of an endosomal membrane into the lumen of the compartment. Fusion of MVBs with the plasma membrane results in secretion of the small internal vesicles termed exosomes. K562 cells are a hematopoietic cell line that releases exosomes. The application of monensin (MON) generated large MVBs that were labeled with a fluorescent lipid. Exosome release was markedly enhanced by MON treatment, a Na+/H+ exchanger that induces changes in intracellular calcium (Ca2+). To explore the possibility that the effect of MON on exosome release was caused via an increase in Ca2+, we have used a calcium ionophore and a chelator of intracellular Ca2+. Our results indicate that increasing intracellular Ca2+ stimulates exosome secretion. Furthermore, MON-stimulated exosome release was completely eliminated by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM), implying a requirement for Ca2+ in this process. We have observed that the large MVBs generated in the presence of MON accumulated Ca2+ as determined by labeling with Fluo3-AM, suggesting that intralumenal Ca2+ might play a critical role in the secretory process. Interestingly, our results indicate that transferrin (Tf) stimulated exosome release in a Ca2+-dependent manner, suggesting that Tf might be a physiological stimulus for exosome release in K562 cells.

Intracellular Ca(2+) regulates the cellular iron uptake in K562 cells

Fluorescence quenching was used to study the kinetics of the transferrin receptor (TfR)-mediated iron uptake in the calcein-loaded K562 cells. It was found that elevation of intracellular free Ca(2+) ([Ca(2+)](i)) by thapsigargin (TG) speeds up the initial rate of iron uptake and increases the overall capacity of the cells in taking up iron. Depletion of intracellular Ca(2+) or complete chelation of extracellular Ca(2+) results in complete inhibition of the iron uptake in cells. To gain insight into molecular mechanism, IANBD-labeled transferrin (Tf) and microscopic fluorescence imaging were used to observe the endocytosis and recycling of the Tf-TfR complex in single live cells. The study showed that the preincubation of cells with TG or phorbol myristate acetate (PMA), the direct activator of protein kinase C (PKC), accelerated the endocytosis and recycling of the complex in a dose-dependent manner. W-7, the calmodulin antagonist, and GF109203X, a selected cell-permeant inhibitor of PKC, can reverse the acceleration. Analysis of actin polymerization in controlled, [Ca(2+)](i)-elevated and W-7-treated cells revealed that the actin polymerization is enhanced as [Ca(2+)](i) is raised, but reduced by W-7. The results suggest that the regulation of actin polymerization by intracellular Ca(2+) may play a central role in Ca(2+)-dependent iron uptake.

Tumour necrosis factor-alpha transcription in transferrin-stimulated human blood mononuclear cells: is transferrin receptor involved in the signalling mechanism?

Transferrin (Tf) and tumour necrosis factor-alpha (TNF-alpha) participate in immune response regulation. We studied the capacity of Tf to modulate 'in vitro' TNF-alpha secretion, membrane expression and transcription by human blood mononuclear cells (BMNC). Women 25-45 years of age with normal iron status (n = 20) or with iron deficiency (ID, n = 20) due to gynaecological bleeding were studied. BMNC were incubated with different proportions of Fe-exempt and Fe-saturated Tf (apo-Tf:holo-Tf). Apo-Tf or holo-Tf uniformly induced TNF-alpha secretion in the cell supernatants from both groups. Nevertheless, cytokine levels were significantly lower in ID subjects. For all Tf-Fe saturations assayed, mean TNF-alpha levels varied between 1.4-1.6 ng/ml and 0.4-0.7 ng/ml for normal and ID women respectively (P < 0.001). The addition of apo-Tf enhanced TNF-alpha secretion in a dose-dependent manner, but the cytokine levels were lower in ID group. Tf did not induce pro-TNF-alpha expression in monocytes and lymphocytes from either group. Tf-treated cells from normal individuals expressed approximately two to three times more TNF-alpha mRNA than cells from ID subjects. Mean values ranged 96-110 atmol/ml in normal women and 24-31 atmol/ml in ID women for all Tf-Fe saturation levels tested (P < 0.001). These results show that Tf-induced TNF-alpha secretion is transcriptionally regulated. The impaired TNF-alpha transcription in cells from ID subjects indicates that the quality of the immune response is linked to the Fe status of mononuclear cells.

Extra-renal erythropoietin secretion after bilateral nephrectomy

Serum concentrations of erythropoietin and soluble transferrin receptor were examined immediately after removal of the remaining kidney in a patient with a history of unilateral radical nephrectomy. Unexpectedly, the erythropoietin concentration increased from 18.2 U/L to 42.5 within 6 h after the operation. Soluble transferrin receptor (TfR) concentration rapidly decreased from 2,725 microgram/L to 1,548 during the 12-h postoperative period. Extra-renal erythropoietin secretion could increase to more than twice as much as the renal secretion of erythropoietin immediately after the loss of renal tissue, and the rise in erythropoietin could accelerate the recycling of TfR, thus causing a decrease in its serum concentration.

Calcium-dependent transferrin receptor recycling in bovine chromaffin cells

The release of regulated secretory granules is known to be calcium dependent. To examine the Ca2+-dependence of other exocytic fusion events, transferrin recycling in bovine chromaffin cells was examined. Internalised 125I-transferrin was released constitutively from cells with a half-time of about 7 min. Secretagogues that triggered catecholamine secretion doubled the rate of 125I-transferrin release, the time courses of the two triggered secretory responses being similar. The triggered 125I-transferrin release came from recycling endosomes rather than from sorting endosomes or a triggered secretory vesicle pool. Triggered 125I-transferrin release, like catecholamine secretion from the same cells, was calcium dependent but the affinities for calcium were very different. The extracellular calcium concentrations that gave rise to half-maximal evoked secretion were 0.1 mm for 125I-transferrin and 1.0 mm for catecholamine, and the intracellular concentrations were 0.1 microm and 1 microm, respectively. There was significant 125I-transferrin recycling in the virtual absence of intracellular Ca2+, but the rate increased when Ca2+ was raised above 1 nm, and peaked at 1 microm when the rate had doubled. Botulinum toxin type D blocked both transferrin recycling and catecholamine secretion. These results indicate that a major component of the vesicular transport required for the constitutive recycling of transferrin in quiescent cells is calcium dependent and thus under physiological control, and also that some of the molecular machinery involved in transferrin recycling/fusion processes is shared with that for triggered neurosecretion.

Molecular keys to the problems of cerebral vasospasm

The mechanisms responsible for subarachnoid hemorrhage (SAH)-induced vasospasm are under intense investigation but remain incompletely understood. A consequence of SAH-induced vasospasm, cerebral infarction, produces a nonrecoverable ischemic tissue core surrounded by a potentially amenable penumbra. However, successful treatment has been inconsistent. In this review, we summarize the basic molecular biology of cerebrovascular regulation, describe recent developments in molecular biology to elucidate the mechanisms of SAH-induced vasospasm, and discuss the potential contribution of cerebral microcirculation regulation to the control of ischemia. Our understanding of the pathogenesis of SAH-induced vasospasm remains a major scientific challenge; however, molecular biological techniques are beginning to uncover the intracellular mechanisms involved in vascular regulation and its failure. Recent findings of microvascular regulatory mechanisms and their failure after SAH suggest a role in the development and size of the ischemia. Progress is being made in identifying the various components in the blood that cause SAH-induced vasospasm. Thus, our evolving understanding of the underlying molecular mechanism may provide the basis for improved treatment after SAH-induced vasospasm, especially at the level of the microcirculation.

Transferrin stimulates iron absorption, exocytosis, and secretion in cultured intestinal cells

The cellular mechanism by which basolateral transferrin (Tf) produces an increase in apical-to-basolateral Fe flux in Caco-2 cells was analyzed. After a pulse of 59Fe from the apical medium, three types of basolateral 59Fe efflux were found: a 59Fe efflux that was independent of the presence of Tf in the basolateral medium, a 59Fe efflux in which 59Fe left the cell bound to Tf, and a Tf-dependent 59Fe efflux in which 59Fe came off the cell not bound to Tf. Furthermore, addition of Tf to the basolateral medium doubled the exocytosis rate of Tf and increased the secretion of apolipoprotein A, a basolateral secretion marker. Both apotransferrin and Fe-containing Tf produced similar increases in 59Fe efflux, Tf exocytosis, and apolipoprotein A secretion. The Ca2+ channel inhibitor SKF-96365 inhibited both the Tf-mediated increase in transepithelial Fe transport and the secretion of apolipoprotein A. Thus the activation of transepithelial Fe transport by Tf seems to be mediated by Ca2+ entry into the cells.