Calcium-dependent transferrin receptor recycling in bovine chromaffin cells

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.

δ-Tocopherol Effect on Endocytosis and Its Combination with Enzyme Replacement Therapy for Lysosomal Disorders: A New Type of Drug Interaction?

Induction of lysosomal exocytosis alleviates lysosomal storage of undigested metabolites in cell models of lysosomal disorders (LDs). However, whether this strategy affects other vesicular compartments, e.g., those involved in endocytosis, is unknown. This is important both to predict side effects and to use this strategy in combination with therapies that require endocytosis for intracellular delivery, such as lysosomal enzyme replacement therapy (ERT). We investigated this using δ-tocopherol as a model previously shown to induce lysosomal exocytosis and cell models of type A Niemann-Pick disease, a LD characterized by acid sphingomyelinase (ASM) deficiency and sphingomyelin storage. δ-Tocopherol and derivative CF3-T reduced net accumulation of fluid phase, ligands, and polymer particles via phagocytic, caveolae-, clathrin-, and cell adhesion molecule (CAM)-mediated pathways, yet the latter route was less affected due to receptor overexpression. In agreement, δ-tocopherol lowered uptake of recombinant ASM by deficient cells (known to occur via the clathrin pathway) and via targeting intercellular adhesion molecule-1 (associated to the CAM pathway). However, the net enzyme activity delivered and lysosomal storage attenuation were greater via the latter route. Data suggest stimulation of exocytosis by tocopherols is not specific of lysosomes and affects endocytic cargo. However, this effect was transient and became unnoticeable several hours after tocopherol removal. Therefore, induction of exocytosis in combination with therapies requiring endocytic uptake, such as ERT, may represent a new type of drug interaction, yet this strategy could be valuable if properly timed for minimal interference.

Single particle tracking of internalized metallic nanoparticles reveals heterogeneous directed motion after clathrin dependent endocytosis in mouse chromaffin cells

Most accepted single particle tracking methods are able to obtain high-resolution trajectories for relatively short periods of time. In this work we apply a straightforward combination of single-particle tracking microscopy and metallic nanoparticles internalization on mouse chromaffin cells to unveil the intracellular trafficking mechanism of metallic-nanoparticle-loaded vesicles (MNP-V) complexes after clathrin dependent endocytosis. We found that directed transport is the major route of MNP-Vs intracellular trafficking after stimulation (92.6% of the trajectories measured). We then studied the MNP-V speed at each point along the trajectory, and found that the application of a second depolarization stimulus during the tracking provokes an increase in the percentage of low-speed trajectory points in parallel with a decrease in the number of high-speed trajectory points. This result suggests that stimulation may facilitate the compartmentalization of internalized MNPs in a more restricted location such as was already demonstrated in neuronal and neuroendocrine cells (Bronfman et al 2003 J. Neurosci. 23 3209-20). Although further experiments will be required to address the mechanisms underlying this transport dynamics, our studies provide quantitative evidence of the heterogeneous behavior of vesicles mobility after endocytosis in chromaffin cells highlighting the potential of MNPs as alternative labels in optical microscopy to provide new insights into the vesicles dynamics in a wide variety of cellular environments.

Membrane cycling after the excess retrieval mode of rapid endocytosis in mouse chromaffin cells

AIM

After exocytosis, neuroendocrine cells and neurones keep constant the plasma membrane and the releasable vesicle pools by performing endocytosis and vesicular cycling. Patch-clamp capacitance measurements on chromaffin cells showed that strong Ca(+2) entry activates excess retrieval: a rapid endocytosis process that retrieves more membrane than the one fused by preceding exocytosis. The main purpose of the present experiments was to study the recycling pathway that follows excess retrieval, which is unknown.

METHODS

Membrane recycling after exocytosis-endocytosis can be studied by fluorescence imaging assays with FM1-43 (Perez Bay et al. Am J Physiol Cell Physiol 2007; 293, C1509). In this work, we used this assay in combination with fluorescent dextrans and specific organelle-targeted antibodies to study the membrane recycling after excess retrieval in mouse chromaffin cells.

RESULTS

Excess retrieval was observed after the application of high-K(+) or cholinergic agonists during 15 or 30 s in the presence of FM1-43. We found that the excess retrieval membrane pool (defined as endocytosis-exocytosis) was associated with the generation of a non-releasable fraction of membrane (up to 30% of plasma membrane surface) colocalizing with the lysosomal compartment. The excess retrieval membrane pool followed a saturable cytosolic Ca(2+) dependency, and it was suppressed by inhibitors of L-type Ca(2+) channels, endoplasmic reticulum Ca(2+) release and PKC.

CONCLUSION

Excess retrieval is not associated with the cycling of releasable vesicles, but it is related to the formation of non-releasable endosomes. This process is activated by a concerted contribution of Ca(2+) entry through L-channels and Ca(2+) release from endoplasmic reticulum.

Regulation of amino acid/carnitine transporter B 0,+ (ATB 0,+) in astrocytes by protein kinase C: independent effects on raft and non-raft transporter subpopulations

Neutral and basic amino acid transporter B(0,+) belongs to a Na,Cl-dependent superfamily of proteins transporting neurotransmitters, amino acids and osmolytes, known to be regulated by protein kinase C (PKC). The present study demonstrates an increased phosphorylation of B(0,+) on serine moiety after treatment of rat astrocytes with phorbol 12-myristate 13-acetate, a process correlated with an augmented activity of l-leucine transport and an enhanced presence of the transporter at the cell surface. After solubilization with Triton X-100 and sucrose gradient centrifugation, B(0,+) was detected in non-raft as well as in detergent-resistant raft fractions under control conditions, while phorbol 12-myristate 13-acetate treatment resulted in a complete disappearance of the transporter from the raft fraction. B(0,+) was observed to interact with caveolin-1 and flotillin-1 (reggie-2) proteins, the markers of detergent-resistant microdomains of plasma membrane. As verified in immunocytochemistry and immunoprecipitation experiments, modification of PKC activity did not affect these interactions. It is proposed that PKC reveals different effects on raft and non-raft subpopulations of B(0,+). Phorbol ester treatment results in trafficking of the transporter from the intracellular pool to non-raft microdomains and increased activity, while B(0,+) present in raft microdomains undergoes either internalization or is transferred laterally to non-raft domains.

Do astrocytes really exocytose neurotransmitters?

In the past 20 years, an extra layer of information processing, in addition to that provided by neurons, has been proposed for the CNS. Neuronally evoked increases of the intracellular calcium concentration in astrocytes have been suggested to trigger exocytotic release of the 'gliotransmitters' glutamate, ATP and D-serine. These are proposed to modulate neuronal excitability and transmitter release, and to have a role in diseases as diverse as stroke, epilepsy, schizophrenia, Alzheimer's disease and HIV infection. However, there is intense controversy about whether astrocytes can exocytose transmitters in vivo. Resolving this issue would considerably advance our understanding of brain function.

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+).

Macropinocytosis: regulated coordination of endocytic and exocytic membrane traffic events

Macropinocytosis, a form of bulk uptake of fluid and solid cargo into cytoplasmic vacuoles, called macropinosomes, has been studied mostly in relation to antigen presentation. Early membrane traffic events occurring in this process are, however, largely unknown. Using human dendritic cells we show that a marked increase in the rate of macropinocytosis occurs a few minutes after application of two markers (small latex beads or dextran), depends on a slow intracellular Ca2+ concentration ([Ca2+](i)) rise that precedes the PI3K-dependent step, and is preceded and accompanied by exocytosis of enlargeosomes compensating in part for the macropinocytic plasma membrane internalization. Unexpectedly, macropinosomes themselves, which share markers with endosomes, undergo Ca2+ -dependent exocytosis so that, after approximately 20 minutes of continuous bead or dextran uptake, an equilibrium is reached preventing cells from overloading themselves with the organelles. Large [Ca2+](i) increases induced by ionomycin trigger rapid (<1 minute) exocytic regurgitation of all macropinosomes, whereas endosomes remain apparently unaffected. We conclude that, in dendritic cells, the rate of macropinocytosis is not constant but increases in a regulated fashion, as previously shown in other cell types. Moreover, macropinosomes are not simple containers that funnel cargo to an endocytic pathway, but unique organelles, distinct from endosomes by their competence for regulated exocytosis and other membrane properties.

Triggering receptor expressed in myeloid cells 2 (TREM2) trafficking in microglial cells: continuous shuttling to and from the plasma membrane regulated by cell stimulation

Cell biology of triggering receptor expressed in myeloid cells 2, a receptor expressed in brain cells (microglia and possibly neurons and oligodendrocytes) which is responsible for a neurological and psychiatric genetic disease, polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy otherwise called the Nasu-Hakola disease, is still largely unknown. Using immortalized mouse N9 microglial cells we demonstrate that triggering receptor expressed in myeloid cells 2 is mostly distributed intracellularly in two pools: a deposit in the Golgi complex and a population of exocytic vesicles, distinct from endosomes and lysosomes, which is continuously translocated to, and recycled from the cell surface. Results with ionomycin and gamma-interferon, showing rapid and slow increases, respectively, of triggering receptor expressed in myeloid cells 2 surface density, documented that the exocytosis of the receptor-rich vesicles is regulated. Pulse labeling in the cold of surface triggering receptor expressed in myeloid cells 2 with its antibody (or Fab fragment) followed by chase at 37 degrees C showed internalization, with recovery of the antibody in endosomes and lysosomes. However, part of the receptor/antibody complex, internalized for up to 30 min chase, was recycled to the cell surface within 2 min of ionomycin stimulation, together with a fraction of the total biotinylated surface protein chased in parallel. The internalized receptor appears therefore to get access to exocytic organelles distinct from lysosomes which may resemble the exocytic vesicles of resting cells. These results document that, in microglial cells, the surface density of the triggering receptor expressed in myeloid cells 2 and thus, presumably, the response to its activation, is continuously adapted and can be greatly increased, even at rapid rate, as a function of cell activation.

Neurotransmitter transporter trafficking: endocytosis, recycling, and regulation

Sodium- and chloride-dependent transporters in the SLC6 gene family are key regulators of extracellular neurotransmitter levels and are required for normal neurotransmission. Copious evidence supports the premise that membrane trafficking dynamically modulates transporter surface expression in response to psychostimulant exposure, receptor activation, and neuronal activity. Recent work from our group and others demonstrates that many SLC6 transporters not only traffic in response to exogenous stimuli, but also constitutively traffic, with exogenous signaling modulating intrinsic transporter trafficking kinetics. This review focuses on what is currently understood about constitutive and regulated transporter trafficking, and poses a model wherein endocytic trafficking dynamically primes transporters for multi-faceted regulatory events.

Recycling endosomes supply AMPA receptors for LTP

Long-term potentiation (LTP) of synaptic strength, the most established cellular model of information storage in the brain, is expressed by an increase in the number of postsynaptic AMPA receptors. However, the source of AMPA receptors mobilized during LTP is unknown. We report that AMPA receptors are transported from recycling endosomes to the plasma membrane for LTP. Stimuli that triggered LTP promoted not only AMPA receptor insertion but also generalized recycling of cargo and membrane from endocytic compartments. Thus, recycling endosomes supply AMPA receptors for LTP and provide a mechanistic link between synaptic potentiation and membrane remodeling during synapse modification.

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.