Possible involvement of zinc transporter ZIP13 in myogenic differentiation.

Ehlers-Danlos syndrome spondylodysplastic type 3 (EDSSPD3, OMIM 612350) is an inherited recessive connective tissue disorder that is caused by loss of function of SLC39A13/ZIP13, a zinc transporter belonging to the Slc39a/ZIP family. We previously reported that patients with EDSSPD3 harboring a homozygous loss of function mutation (c.221G > A, p.G64D) in ZIP13 exon 2 (ZIP13G64D) suffer from impaired development of bone and connective tissues, and muscular hypotonia. However, whether ZIP13 participates in the early differentiation of these cell types remains unclear. In the present study, we investigated the role of ZIP13 in myogenic differentiation using a murine myoblast cell line (C2C12) as well as patient-derived induced pluripotent stem cells (iPSCs). We found that ZIP13 gene expression was upregulated by myogenic stimulation in C2C12 cells, and its knockdown disrupted myotubular differentiation. Myocytes differentiated from iPSCs derived from patients with EDSSPD3 (EDSSPD3-iPSCs) also exhibited incomplete myogenic differentiation. Such phenotypic abnormalities of EDSSPD3-iPSC-derived myocytes were corrected by genomic editing of the pathogenic ZIP13G64D mutation. Collectively, our findings suggest the possible involvement of ZIP13 in myogenic differentiation, and that EDSSPD3-iPSCs established herein may be a promising tool to study the molecular basis underlying the clinical features caused by loss of ZIP13 function.

Protective Effects of Imeglimin and Metformin Combination Therapy on ß-Cells in db/db Male Mice.

Imeglimin and metformin act in metabolic organs, including ß-cells, via different mechanisms. In the present study, we investigated the impacts of imeglimin, metformin, or their combination (Imeg + Met) on ß-cells, the liver, and adipose tissues in db/db mice. Imeglimin, metformin, or Imeg + Met treatment had no significant effects on glucose tolerance, insulin sensitivity, respiratory exchange ratio, or locomotor activity in db/db mice. The responsiveness of insulin secretion to glucose was recovered by Imeg + Met treatment. Furthermore, Imeg + Met treatment increased ß-cell mass by enhancing ß-cell proliferation and ameliorating ß-cell apoptosis in db/db mice. Hepatic steatosis, the morphology of adipocytes, adiposity assessed by computed tomography, and the expression of genes related to glucose or lipid metabolism and inflammation in the liver and fat tissues showed no notable differences in db/db mice. Global gene expression analysis of isolated islets indicated that the genes related to regulation of cell population proliferation and negative regulation of cell death were enriched by Imeg + Met treatment in db/db islets. In vitro culture experiments confirmed the protective effects of Imeg + Met against ß-cell apoptosis. The expression of Snai1, Tnfrsf18, Pdcd1, Mmp9, Ccr7, Egr3, and Cxcl12, some of which have been linked to apoptosis, in db/db islets was attenuated by Imeg + Met. Treatment of a ß-cell line with Imeg + Met prevented apoptosis induced by hydrogen peroxide or palmitate. Thus, the combination of imeglimin and metformin is beneficial for the maintenance of ß-cell mass in db/db mice, probably through direct action on ß-cells, suggesting a potential strategy for protecting ß-cells in the treatment of type 2 diabetes.

Zinc and iron dynamics in human islet amyloid polypeptide-induced diabetes mouse model.

Metal homeostasis is tightly regulated in cells and organisms, and its disturbance is frequently observed in some diseases such as neurodegenerative diseases and metabolic disorders. Previous studies suggest that zinc and iron are necessary for the normal functions of pancreatic ß cells. However, the distribution of elements in normal conditions and the pathophysiological significance of dysregulated elements in the islet in diabetic conditions have remained unclear. In this study, to investigate the dynamics of elements in the pancreatic islets of a diabetic mouse model expressing human islet amyloid polypeptide (hIAPP): hIAPP transgenic (hIAPP-Tg) mice, we performed imaging analysis of elements using synchrotron scanning X-ray fluorescence microscopy and quantitative analysis of elements using inductively coupled plasma mass spectrometry. We found that in the islets, zinc significantly decreased in the early stage of diabetes, while iron gradually decreased concurrently with the increase in blood glucose levels of hIAPP-Tg mice. Notably, when zinc and/or iron were decreased in the islets of hIAPP-Tg mice, dysregulation of glucose-stimulated mitochondrial respiration was observed. Our findings may contribute to clarifying the roles of zinc and iron in islet functions under pathophysiological diabetic conditions.

Establishment of an enzyme-linked immunosorbent assay for mouse pancreatic polypeptide clarifies the regulatory mechanism of its secretion from pancreatic γ cells.

Pancreatic polypeptide (PP), secreted from γ cells of the islets of Langerhans, is a 36 amino-acid peptide encoded by the Ppy gene. Although previous studies have reported that PP causes a decrease in appetite, the molecular mechanism that regulates PP secretion has not been fully elucidated. Lack of understanding of the regulatory mechanism of PP secretion may be partially owing to the lack of assay systems that can specifically detect PP. We recently developed the mouse monoclonal antibody 23-2D3 that specifically recognizes PP. In the present study, we developed a sandwich enzyme-linked immunosorbent assay for the measurement of mouse PP, and directly monitored intracellular Ca2+ concentrations in Ppy-expressing cells from a newly developed reporter mouse. Using these systems, we identified agonists, such as carbachol and glucose-dependent insulinotropic polypeptide (GIP), which stimulate PP secretion. We further demonstrated that, unlike the case of GIP-induced insulin secretion from ß cells, there is a unique mechanism by which PP secretion is triggered by an increase in intracellular Ca2+ concentrations via voltage-dependent calcium channels even in low-glucose conditions.

Characterisation of Ppy-lineage cells clarifies the functional heterogeneity of pancreatic beta cells in mice.

AIMS/HYPOTHESIS: Pancreatic polypeptide (PP) cells, which secrete PP (encoded by the Ppy gene), are a minor population of pancreatic endocrine cells. Although it has been reported that the loss of beta cell identity might be associated with beta-to-PP cell-fate conversion, at present, little is known regarding the characteristics of Ppy-lineage cells. METHODS: We used Ppy-Cre driver mice and a PP-specific monoclonal antibody to investigate the association between Ppy-lineage cells and beta cells. The molecular profiles of endocrine cells were investigated by single-cell transcriptome analysis and the glucose responsiveness of beta cells was assessed by Ca2+ imaging. Diabetic conditions were experimentally induced in mice by either streptozotocin or diphtheria toxin. RESULTS: Ppy-lineage cells were found to contribute to the four major types of endocrine cells, including beta cells. Ppy-lineage beta cells are a minor subpopulation, accounting for 12-15% of total beta cells, and are mostly (81.2%) localised at the islet periphery. Unbiased single-cell analysis with a Ppy-lineage tracer demonstrated that beta cells are composed of seven clusters, which are categorised into two groups (i.e. Ppy-lineage and non-Ppy-lineage beta cells). These subpopulations of beta cells demonstrated distinct characteristics regarding their functionality and gene expression profiles. Ppy-lineage beta cells had a reduced glucose-stimulated Ca2+ signalling response and were increased in number in experimental diabetes models. CONCLUSIONS/INTERPRETATION: Our results indicate that an unexpected degree of beta cell heterogeneity is defined by Ppy gene activation, providing valuable insight into the homeostatic regulation of pancreatic islets and future therapeutic strategies against diabetes. DATA AVAILABILITY: The single-cell RNA sequence (scRNA-seq) analysis datasets generated in this study have been deposited in the Gene Expression Omnibus (GEO) under the accession number GSE166164 ( www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE166164 ).

Leukotriene A4 hydrolase deficiency protects mice from diet-induced obesity by increasing energy expenditure through neuroendocrine axis.

Obesity is a health problem worldwide, and brown adipose tissue (BAT) is important for energy expenditure. Here, we explored the role of leukotriene A4 hydrolase (LTA4 H), a key enzyme in the synthesis of the lipid mediator leukotriene B4 (LTB4 ), in diet-induced obesity. LTA4 H-deficient (LTA4 H-KO) mice fed a high-fat diet (HFD) showed a lean phenotype, and bone-marrow transplantation studies revealed that LTA4 H-deficiency in non-hematopoietic cells was responsible for this lean phenotype. LTA4 H-KO mice exhibited greater energy expenditure, but similar food intake and fecal energy loss. LTA4 H-KO BAT showed higher expression of thermogenesis-related genes. In addition, the plasma thyroid-stimulating hormone and thyroid hormone concentrations, as well as HFD-induced catecholamine secretion, were higher in LTA4 H-KO mice. In contrast, LTB4 receptor (BLT1)-deficient mice did not show a lean phenotype, implying that the phenotype of LTA4 H-KO mice is independent of the LTB4 /BLT1 axis. These results indicate that LTA4 H mediates the diet-induced obesity by reducing catecholamine and thyroid hormone secretion.

Rubicon in pancreatic beta cells plays a limited role in maintaining glucose homeostasis following increased insulin resistance.

Autophagy has been reported to play a crucial role in the maintenance of intracellular homeostasis, including in pancreatic beta cells. Rubicon, which interacts with the phosphoinositide 3-kinase (PI3K) complex, through autophagy-related 14 (ATG14), is among the few autophagy regulators that have been reported to inhibit autophagic flux to date and the deletion of Rubicon has been shown to increase autophagic flux. Based on previous results showing a causal relationship between autophagic dysfunction and pancreatic beta-cell impairment, we hypothesized that the deletion of Rubicon in pancreatic beta cells would improve cell integrity and confer protective effects. To test this hypothesis, we first confirmed that Rubicon knockdown (KD) promoted autophagic flux in ßTC3 pancreatic beta-cell line. Next, we generated pancreatic beta-cell-specific Rubicon knockout (ßKO) mice, by administering tamoxifen to Rubiconflox/flox:MIP-Cre-ERT mice, which showed normal glucose tolerance and insulin secretion under a normal chow diet, despite successful gene recombination. We also attempted to increase insulin resistance by feeding the mice with a high-fat diet for an additional 2 months to find little differences among the parameters evaluated for glucose metabolism. Finally, severe insulin resistance was induced with insulin receptor antagonist treatment, which resulted in comparable glucose homeostasis measurements between Rubicon ßKO and control mice. In summary, these results suggest that in pancreatic beta cells, Rubicon plays a limited role in the maintenance of systemic glucose homeostasis.

Detailed analyses of the crucial functions of Zn transporter proteins in alkaline phosphatase activation.

Numerous zinc ectoenzymes are metalated by zinc and activated in the compartments of the early secretory pathway before reaching their destination. Zn transporter (ZNT) proteins located in these compartments are essential for ectoenzyme activation. We have previously reported that ZNT proteins, specifically ZNT5-ZNT6 heterodimers and ZNT7 homodimers, play critical roles in the activation of zinc ectoenzymes, such as alkaline phosphatases (ALPs), by mobilizing cytosolic zinc into these compartments. However, this process remains incompletely understood. Here, using genetically-engineered chicken DT40 cells, we first determined that Zrt/Irt-like protein (ZIP) transporters that are localized to the compartments of the early secretory pathway play only a minor role in the ALP activation process. These transporters included ZIP7, ZIP9, and ZIP13, performing pivotal functions in maintaining cellular homeostasis by effluxing zinc out of the compartments. Next, using purified ALP proteins, we showed that zinc metalation on ALP produced in DT40 cells lacking ZNT5-ZNT6 heterodimers and ZNT7 homodimers is impaired. Finally, by genetically disrupting both ZNT5 and ZNT7 in human HAP1 cells, we directly demonstrated that the tissue-nonspecific ALP-activating functions of both ZNT complexes are conserved in human cells. Furthermore, using mutant HAP1 cells, we uncovered a previously-unrecognized and unique spatial regulation of ZNT5-ZNT6 heterodimer formation, wherein ZNT5 recruits ZNT6 to the Golgi apparatus to form the heterodimeric complex. These findings fill in major gaps in our understanding of the molecular mechanisms underlying zinc ectoenzyme activation in the compartments of the early secretory pathway.

Biphasic changes in ß-cell mass around parturition are accompanied by increased serotonin production.

Pancreatic ß-cell mass is known to be considerably altered during pregnancy and after parturition in rodents and humans. While ß-cell mass increases during pregnancy and starts to return toward its original level after parturition, the cellular mechanisms by which ß-cell mass during this period is regulated remains unclear. To address this issue in mice, we quantified ß-cell mass and investigated the mechanisms underlying its regulation throughout the perinatal and postpartum period. The increased ß-cell size and proliferation during pregnancy were significantly reduced shortly after parturition, whereas there was no evidence of ß-cell reprogramming or increased apoptosis. Direct RNA sequencing of islets from pregnant and postpartum mice demonstrated dynamic changes in gene expression patterns, showing robust downregulation of cell cycle-related genes 1 day after parturition, and the reupregulation of serotonin metabolism-related genes at postpartum day 7. Serotonin synthesis was activated only in lactating females, accompanied by increased ß-cell mass. Taken together, these findings demonstrate that ß-cell mass is decreased shortly after parturition owing to reduced ß-cell size and proliferation, and is subsequently increased, in association with lactation and serotonin biosynthesis.

Role of Zinc Homeostasis in the Pathogenesis of Diabetes and Obesity.

Zinc deficiency is a risk factor for obesity and diabetes. However, until recently, the underlying molecular mechanisms remained unclear. The breakthrough discovery that the common polymorphism in zinc transporter SLC30A8/ZnT8 may increase susceptibility to type 2 diabetes provided novel insights into the role of zinc in diabetes. Our group and others showed that altered ZnT8 function may be involved in the pathogenesis of type 2 diabetes, indicating that the precise control of zinc homeostasis is crucial for maintaining health and preventing various diseases, including lifestyle-associated diseases. Recently, the role of the zinc transporter ZIP13 in the regulation of beige adipocyte biogenesis was clarified, which indicated zinc homeostasis regulation as a possible therapeutic target for obesity and metabolic syndrome. Here we review advances in the role of zinc homeostasis in the pathophysiology of diabetes, and propose that inadequate zinc distribution may affect the onset of diabetes and metabolic diseases by regulating various critical biological events.

Normal pancreatic ß-cell function in mice with RIP-Cre-mediated inactivation of p62/SQSTM1.

Recent studies have suggested that decreased pancreatic ß-cell function and mass are common features of patients with type 2 diabetes mellitus. Pancreatic ß-cell homeostasis is regulated by various types of signaling molecules and stress responses. Sequestosome 1/p62 (SQSTM1, hereafter referred to as p62) is a ubiquitin-binding adaptor protein involved in cell signaling, oxidative stress, and autophagy. Because p62 appears to play an important role in maintaining mitochondrial quality control, it is possible that the loss of p62 in pancreatic ß cells contributes to mitochondrial dysfunction, and thus leading to impaired glucose tolerance. In this study we investigated the physiological roles of p62 by inactivating p62 in a ß-cell specific manner. We found that firstly, rat insulin-2 promoter-Cre (RIP-Cre)-mediated p62 inactivation did not cause body weight gain, although ubiquitous inactivation of p62 was previously shown to result in severe obesity. Secondly, we found no gross structural disorganization of the islets of p62-deficient mice. Consistent with normal islet morphology, no impairment in glucose tolerance was observed in mice with RIP-Cre-mediated p62 deletion. These results suggest that p62 is dispensable for normal islet organization and ß-cell function.

Zinc transporter ZIP13 suppresses beige adipocyte biogenesis and energy expenditure by regulating C/EBP-ß expression.

Given the relevance of beige adipocytes in adult humans, a better understanding of the molecular circuits involved in beige adipocyte biogenesis has provided new insight into human brown adipocyte biology. Genetic mutations in SLC39A13/ZIP13, a member of zinc transporter family, are known to reduce adipose tissue mass in humans; however, the underlying mechanisms remains unknown. Here, we demonstrate that the Zip13-deficient mouse shows enhanced beige adipocyte biogenesis and energy expenditure, and shows ameliorated diet-induced obesity and insulin resistance. Both gain- and loss-of-function studies showed that an accumulation of the CCAAT/enhancer binding protein-ß (C/EBP-ß) protein, which cooperates with dominant transcriptional co-regulator PR domain containing 16 (PRDM16) to determine brown/beige adipocyte lineage, is essential for the enhanced adipocyte browning caused by the loss of ZIP13. Furthermore, ZIP13-mediated zinc transport is a prerequisite for degrading the C/EBP-ß protein to inhibit adipocyte browning. Thus, our data reveal an unexpected association between zinc homeostasis and beige adipocyte biogenesis, which may contribute significantly to the development of new therapies for obesity and metabolic syndrome.

[Zinc and type 2 diabetes].

Pancreatic ß cells contain the highest amount of zinc among cells within the human body, and hence, the relationship between zinc and diabetes has been a topic of great interest. While many studies demonstrating possible involvement of zinc deficiency in diabetes have been reported, precise mechanisms how zinc regulates glucose metabolism are still far from understood. Recent studies revealed that zinc can transmit signals that are driven by a variety of zinc transporters in a tissue and cell-type specific manner and deficiency in some zinc transporters may cause human diseases. Here, we review the role of zinc in metabolism particularly focusing on the emerging role of zinc transporters in diabetes.

The PP-motif in luminal loop 2 of ZnT transporters plays a pivotal role in TNAP activation.

Secretory and membrane-bound zinc-requiring enzymes are thought to be activated by binding zinc in the early secretory pathway. One such enzyme, tissue-non-specific alkaline phosphatase (TNAP), is activated through a two-step mechanism, via protein stabilization and subsequent enzyme activation through metalation, by ZnT5-ZnT6 heterodimers or ZnT7 homodimers. However, little is known about the molecular basis underlying the activation process. In the present study, we found that the di-proline motif (PP-motif) in luminal loop 2 of ZnT5 and ZnT7 is important for TNAP activation. TNAP activity was significantly reduced in cells lacking ZnT5-ZnT6 heterodimers and ZnT7 homodimers [triple knockout (TKO) cells]. The decreased TNAP activity was restored by expressing hZnT5 with hZnT6 or hZnT7, but significantly less so (almost 90% less) by expressing mutants thereof in which the PP-motif was mutated to alanine (PP-AA). In TKO cells, overexpressed hTNAP was not completely activated, and it was converted less efficiently into the holo form by expressing a PP-AA mutant of hZnT5 with hZnT6, whose defects were not restored by zinc supplementation. The zinc transport activity of hZnT7 was not significantly impaired by the PP-AA mutation, indicating that the PP-motif is involved in the TNAP maturation process, although it does not control zinc transport activity. The PP-motif is highly conserved in ZnT5 and ZnT7 orthologues, and its importance for TNAP activation is conserved in the Caenorhabditis elegans hZnT5 orthologue CDF5. These results provide novel molecular insights into the TNAP activation process in the early secretory pathway.

Molecular pathogenesis of spondylocheirodysplastic Ehlers-Danlos syndrome caused by mutant ZIP13 proteins.

The zinc transporter protein ZIP13 plays critical roles in bone, tooth, and connective tissue development, and its dysfunction is responsible for the spondylocheirodysplastic form of Ehlers-Danlos syndrome (SCD-EDS, OMIM 612350). Here, we report the molecular pathogenic mechanism of SCD-EDS caused by two different mutant ZIP13 proteins found in human patients: ZIP13(G64D), in which Gly at amino acid position 64 is replaced by Asp, and ZIP13(ΔFLA), which contains a deletion of Phe-Leu-Ala. We demonstrated that both the ZIP13(G64D) and ZIP13(ΔFLA) protein levels are decreased by degradation via the valosin-containing protein (VCP)-linked ubiquitin proteasome pathway. The inhibition of degradation pathways rescued the protein expression levels, resulting in improved intracellular Zn homeostasis. Our findings uncover the pathogenic mechanisms elicited by mutant ZIP13 proteins. Further elucidation of these degradation processes may lead to novel therapeutic targets for SCD-EDS.

Human IAPP-induced pancreatic ß cell toxicity and its regulation by autophagy.

Pancreatic islets in patients with type 2 diabetes mellitus (T2DM) are characterized by loss of ß cells and formation of amyloid deposits derived from islet amyloid polypeptide (IAPP). Here we demonstrated that treatment of INS-1 cells with human IAPP (hIAPP) enhances cell death, inhibits cytoproliferation, and increases autophagosome formation. Furthermore, inhibition of autophagy increased the vulnerability of ß cells to the cytotoxic effects of hIAPP. Based on these in vitro findings, we examined the pathogenic role of hIAPP and its relation to autophagy in hIAPP-knockin mice. In animals fed a standard diet, hIAPP had no toxic effects on ß cell function; however, hIAPP-knockin mice did not exhibit a high-fat-diet-induced compensatory increase in ß cell mass, which was due to limited ß cell proliferation and enhanced ß cell apoptosis. Importantly, expression of hIAPP in mice with a ß cell-specific autophagy defect resulted in substantial deterioration of glucose tolerance and dispersed cytoplasmic expression of p62-associated toxic oligomers, which were otherwise sequestrated within p62-positive inclusions. Together, our results indicate that increased insulin resistance in combination with reduced autophagy may enhance the toxic potential of hIAPP and enhance ß cell dysfunction and progression of T2DM.

The diabetes-susceptible gene SLC30A8/ZnT8 regulates hepatic insulin clearance.

Recent genome-wide association studies demonstrated that common variants of solute carrier family 30 member 8 gene (SLC30A8) increase susceptibility to type 2 diabetes. SLC30A8 encodes zinc transporter-8 (ZnT8), which delivers zinc ion from the cytoplasm into insulin granules. Although it is well known that insulin granules contain high amounts of zinc, the physiological role of secreted zinc remains elusive. In this study, we generated mice with ß cell-specific Slc30a8 deficiency (ZnT8KO mice) and demonstrated an unexpected functional linkage between Slc30a8 deletion and hepatic insulin clearance. The ZnT8KO mice had low peripheral blood insulin levels, despite insulin hypersecretion from pancreatic ß cells. We also demonstrated that a substantial amount of the hypersecreted insulin was degraded during its first passage through the liver. Consistent with these findings, ZnT8KO mice and human individuals carrying rs13266634, a major risk allele of SLC30A8, exhibited increased insulin clearance, as assessed by c-peptide/insulin ratio. Furthermore, we demonstrated that zinc secreted in concert with insulin suppressed hepatic insulin clearance by inhibiting clathrin-dependent insulin endocytosis. Our results indicate that SLC30A8 regulates hepatic insulin clearance and that genetic dysregulation of this system may play a role in the pathogenesis of type 2 diabetes.

Essential role of the zinc transporter ZIP9/SLC39A9 in regulating the activations of Akt and Erk in B-cell receptor signaling pathway in DT40 cells.

The essential trace element zinc is important for all living organisms. Zinc functions not only as a nutritional factor, but also as a second messenger. However, the effects of intracellular zinc on the B cell-receptor (BCR) signaling pathway remain poorly understood. Here, we present data indicating that the increase in intracellular zinc level induced by ZIP9/SLC39A9 (a ZIP Zrt-/Irt-like protein) plays an important role in the activation of Akt and Erk in response to BCR activation. In DT40 cells, the enhancement of Akt and Erk phosphorylation following BCR activation requires intracellular zinc. To clarify this event, we used chicken ZnT5/6/7-gene-triple-knockout DT40 (TKO) cells and chicken Zip9-knockout DT40 (cZip9KO) cells. The levels of Akt and ERK phosphorylation significantly decreased in cZip9KO cells. In addition, the enzymatic activity of protein tyrosine phosphatase (PTPase) increased in cZip9KO cells. These biochemical events were restored by overexpressing the human Zip9 (hZip9) gene. Moreover, we found that the increase in intracellular zinc level depends on the expression of ZIP9. This observation is in agreement with the increased levels of Akt and Erk phosphorylation and the inhibition of total PTPase activity. We concluded that ZIP9 regulates cytosolic zinc level, resulting in the enhancement of Akt and Erk phosphorylation. Our observations provide new mechanistic insights into the BCR signaling pathway underlying the regulation of intracellular zinc level by ZIP9 in response to the BCR activation.

Tissue nonspecific alkaline phosphatase is activated via a two-step mechanism by zinc transport complexes in the early secretory pathway.

A number of enzymes become functional by binding to zinc during their journey through the early secretory pathway. The zinc transporters (ZnTs) located there play important roles in this step. We have previously shown that two zinc transport complexes, ZnT5/ZnT6 heterodimers and ZnT7 homo-oligomers, are required for the activation of alkaline phosphatases, by converting them from the apo- to the holo-form. Here, we investigated the molecular mechanisms of this activation. ZnT1 and ZnT4 expressed in chicken DT40 cells did not contribute to the activation of tissue nonspecific alkaline phosphatase (TNAP). The reduced activity of TNAP in DT40 cells deficient in both ZnT complexes was not restored by zinc supplementation nor by exogenous expression of other ZnTs that increase the zinc content in the secretory pathway. Moreover, we showed that expression of ZnT5/ZnT6 heterodimers reconstituted with zinc transport-incompetent ZnT5 mutant failed to restore TNAP activity but could stabilize the TNAP protein as the apo-form, regardless of zinc status. These findings demonstrate that TNAP is activated not simply by passive zinc binding but by an elaborate two-step mechanism via protein stabilization followed by enzyme conversion from the apo- to the holo-form with zinc loaded by ZnT complexes in the early secretory pathway.