1.Identification and activity analysis of ubiquitin ligase TP12446 gene from Trichinella spiralis
Shuyan ZHANG ; Zijian DONG ; Jianda PANG ; Saining WANG ; Qianqian DANG ; Fengying YIN ; Xiaolei LIU ; Xuelin WANG
Chinese Journal of Veterinary Science 2025;45(5):1017-1025
Based on the previous transcriptomic experimental data of Trichinella spiralis(T.spira-lis)in this study,the larval stage specific gene TP12446 was screened and its identity in the ubiq-uitin ligase RNF family was predicted.In the study,bioinformatics methods were used to analyze its physicochemical properties and its activity to lay the foundation for further exploring the func-tion of TP12446 gene.The physicochemical properties and protein structure of TP12446 protein were predicted by bioinformatics.Its ubiquitin ligase activity was also verified by ubiquitination re-actions in vitro.The expression characteristics of TP12446 protein in different stage of T.spiralis infection were analyzed by qPCR and Western blot.Bioinformatics analysis showed that TP12446 protein was composed of 453 amino acids and its molecular weight was 51.48 kDa.The protein had a transmembrane structure and contained signal peptides.The results indicated that it was a secre-tory protein and mainly located in the cytoplasmic membrane.The protein structure analysis re-vealed that the protein contained RING and PA domain,its secondary structure was mainly com-posed of α-helix and irregular crimp and there were 10 B cell epitopes on TP12446 protein.The prediction of glycosylation and phosphorylation sites indicated that TP12446 protein contained 38 potential phosphorylation sites.Results of PPI interaction protein prediction showed that TP12446 protein had strong interaction with Usp8,Tmem37,Otub1,Otub2,Ubox5 and CD151.The results of qPCR and Western blot showed that TP12446 gene expression was the highest in the larva stage of T.spiralis,the activity of ubiquitin ligase was verified by ubiquitination reaction in vitro.TP12446 protein was a secretory hydrophobic protein with E3 ubiquitin ligase activity,which was involved in regulating cell cycle and apoptosis.
2.Neural physiological basis and exercise-induced mechanism of central fatigue
Qifei WANG ; Xingbin DU ; Jianda KONG
Chinese Journal of Tissue Engineering Research 2025;29(32):6979-6988
BACKGROUND:Central fatigue refers to the phenomenon where changes in the functions of the central nervous system lead to a decline in an individual's exercise capacity and an increase in the sense of fatigue.With the continuous development of sports science,the mechanisms of exercise-induced central fatigue have gradually become a research hotspot.Although a large number of studies have explored the roles of biological mechanisms such as neurotransmitters,energy metabolism,and neuroinflammation in central fatigue,the interactions of these mechanisms and their manifestations in different exercise scenarios are still not fully understood.OBJECTIVE:To explore various biological mechanisms of exercise-induced central fatigue,clarify the interactions of neurotransmitters,energy metabolism,neural network activities,neuroinflammation,and oxidative stress in the formation of fatigue,and thereby provide a new perspective for understanding central fatigue.METHODS:Web of Science,PubMed,CNKI,WanFang,and VIP databases were searched with search terms"central fatigue,neurophysiological basis,exercise,physical activity,neurotransmitters,serotonin,dopamine,glutamate,GABA,energy metabolism,astrocytes,neuroinflammation,oxidative stress,cortical activity,neural network activity,sport"in Chinese and English.Relevant literature published from the establishment of each database to August 2024 was retrieved,and finally 74 articles were included for review.RESULTS AND CONCLUSION:Exercise-induced central fatigue involves multiple complex biological mechanisms.The imbalance of neurotransmitters(such as serotonin,dopamine,glutamate,and γ-aminobutyric acid)is a key factor in central fatigue.In particular,the increase in serotonin is closely related to the enhancement of the sense of fatigue.The disorder of energy metabolism,especially the glycogen metabolism of astrocytes and the supply of lactic acid,plays an important role in the formation of fatigue during long-term exercise.Neuroinflammation and oxidative stress exacerbate the manifestations of central fatigue by affecting the release of neurotransmitters and the functions of neural networks.In addition,changes in the activities of neural networks,such as the decrease in the excitability of the cerebral cortex,directly affect exercise performance.Future studies should strengthen the comprehensive exploration of neurotransmitter,metabolic,and inflammatory pathways,conduct more human studies to verify the findings of animal experiments,explore the differences in the mechanisms of central fatigue under different exercise conditions(such as intensity,time,and environment),and study the potential of drug interventions or nutritional supplements based on neurotransmitter regulation in alleviating central fatigue.
3.Single-cell transcriptome sequencing and clinical significance analysis of cellular heterogeneity in chronic skin ulcers
Chuwang WANG ; Jianda ZHOU ; Yanlian XIANG ; Peiting LI ; Shaohua WANG ; Jia CHEN ; Shuyue CHEN ; Wu XIONG ; Yu LIU ; Xiao FU
Chinese Journal of General Surgery 2025;34(2):327-337
Background and Aims:Chronic skin ulcers are a significant disease affecting patients'daily lives and psychological well-being.Abnormalities in the cells and extracellular matrix within the tissue may disrupt the balance of the microenvironment,hindering the normal skin repair process and leading to delayed healing of the ulcer.There is currently a lack of research on the mechanisms underlying the development of chronic ulcers and their diagnostic biomarkers.Single-cell sequencing,a newly developed high-throughput sequencing method in recent years,uses gene sequencing at the single-cell resolution to precisely reveal disease mechanisms and has been applied in various diseases.This study used single-cell transcriptome sequencing(scRNA-Seq)to investigate the cellular heterogeneity in chronic skin ulcer tissue to elucidate the potential molecular mechanisms behind delayed healing and provide new insights for clinical treatment.Methods:The scRNA-Seq technology was used to compare the differences in cell subpopulations and gene expression between chronic ulcer tissue and normal skin tissue.Single cells were sorted using a microfluidic platform,and cDNA libraries were constructed for subsequent differential gene analysis and functional enrichment analysis.Results:scRNA-Seq analysis revealed significant immune-metabolic remodeling features in chronic ulcer tissue:the number of B cells,monocytes,and macrophages in ulcer tissue increased by 2.1 to 3.5 times compared to the normal tissue control.This was accompanied by widespread activation of collagen synthesis genes(COL1A1/COL3A1)and synergistic suppression of immune regulators(e.g.,granzyme family GZMA/GZMB/H).Cross-cell subpopulation functional network analysis showed that hypoxia response mediated by the HIF-1 signaling pathway and PI3K/Akt pathway abnormalities formed a positive feedback loop,exacerbating the imbalance in the secretion of inflammatory factors(CXCL3/8,TGFBI)and compensatory upregulation of mitochondrial oxidative phosphorylation.Conclusion:Chronic skin ulcers exhibit significant differences in cellular heterogeneity and gene expression,suggesting that chronic ulcers are not simply tissue defects but a complex pathological process dominated by chronic inflammation and immune dysregulation.The coordinated dysregulation of multiple cell subpopulations in the ulcer microenvironment,along with persistent inflammatory responses and metabolic abnormalities,is interconnected through the HIF-1/TNF/MAPK pathway network.Downregulation of granzyme gene family members and abnormal histone modifications may contribute to immune clearance defects,providing a theoretical basis for developing novel therapies targeting epigenetic regulation or mitochondrial function.
4.Chitosan hydrogel loaded with human umbilical cord mesenchymal stem cell-derived exosomes promotes healing of chronic diabetic wounds in rats.
Xiaohui QIU ; Meng WANG ; Jiangjie TANG ; Jianda ZHOU ; Chen JIN
Journal of Southern Medical University 2025;45(10):2082-2091
OBJECTIVES:
To investigate the mechanism by which chitosan (CS) hydrogel loaded with human umbilical cord mesenchymal stem cell (HUVECs)-derived exosomes (hUCMSC-exos) (Exos@CS-Gel) improves diabetic wound healing.
METHODS:
hUCMSC-exos were extracted and Exos@CS-Gel was prepared. The effect of Exos@CS-Gel on proliferation and migration of HUVECs were evaluated using scratch wound assay and CCK-8 assay. Diabetic rat models with full-thickness skin wounds established by streptozotocin induction were randomized divided into 4 groups for treatment with Exos@CS-Gel (100 µg hUCMSC-exos dissolved in 100 µL 24% CS hydrogel), hUCMSC-exos (100 µg hUCMSC-exos dissolved in 100 µL PBS), CS hydrogel (100 µL 24% CS hydrogel), or PBS (control group). Wound healing and the therapeutic mechanisms were assessed using immunohistochemistry, HE staining, immunofluorescence, and qRT-PCR.
RESULTS:
In cultured HUVECs, Exos@CS-Gel treatment significantly promoted cell proliferation and migration. In the rat models of chronic diabetic wounds, the wound healing rate in Exos@CS-Gel group reached 92.7% on day 14, significantly higher than those in hUCMSC-exos group (9.12%), CS hydrogel group (16.28%), and control group (25.98%). Microvessel density and the expression levels of vascular endothelial growth factor and transforming growth factor β-1 were significantly increased in the Exos@CS-Gel group.
CONCLUSIONS
Exos@CS-Gel promotes survival capacity of hUCMSC-exos in vitro and accelerates diabetic wound healing in rats by promoting angiogenesis and cell proliferation.
Animals
;
Wound Healing
;
Humans
;
Chitosan
;
Exosomes
;
Mesenchymal Stem Cells/cytology*
;
Diabetes Mellitus, Experimental
;
Rats
;
Umbilical Cord/cytology*
;
Hydrogels
;
Human Umbilical Vein Endothelial Cells
;
Cell Proliferation
;
Rats, Sprague-Dawley
;
Male
5.Identification and activity analysis of ubiquitin ligase TP12446 gene from Trichinella spiralis
Shuyan ZHANG ; Zijian DONG ; Jianda PANG ; Saining WANG ; Qianqian DANG ; Fengying YIN ; Xiaolei LIU ; Xuelin WANG
Chinese Journal of Veterinary Science 2025;45(5):1017-1025
Based on the previous transcriptomic experimental data of Trichinella spiralis(T.spira-lis)in this study,the larval stage specific gene TP12446 was screened and its identity in the ubiq-uitin ligase RNF family was predicted.In the study,bioinformatics methods were used to analyze its physicochemical properties and its activity to lay the foundation for further exploring the func-tion of TP12446 gene.The physicochemical properties and protein structure of TP12446 protein were predicted by bioinformatics.Its ubiquitin ligase activity was also verified by ubiquitination re-actions in vitro.The expression characteristics of TP12446 protein in different stage of T.spiralis infection were analyzed by qPCR and Western blot.Bioinformatics analysis showed that TP12446 protein was composed of 453 amino acids and its molecular weight was 51.48 kDa.The protein had a transmembrane structure and contained signal peptides.The results indicated that it was a secre-tory protein and mainly located in the cytoplasmic membrane.The protein structure analysis re-vealed that the protein contained RING and PA domain,its secondary structure was mainly com-posed of α-helix and irregular crimp and there were 10 B cell epitopes on TP12446 protein.The prediction of glycosylation and phosphorylation sites indicated that TP12446 protein contained 38 potential phosphorylation sites.Results of PPI interaction protein prediction showed that TP12446 protein had strong interaction with Usp8,Tmem37,Otub1,Otub2,Ubox5 and CD151.The results of qPCR and Western blot showed that TP12446 gene expression was the highest in the larva stage of T.spiralis,the activity of ubiquitin ligase was verified by ubiquitination reaction in vitro.TP12446 protein was a secretory hydrophobic protein with E3 ubiquitin ligase activity,which was involved in regulating cell cycle and apoptosis.
6.Single-cell transcriptome sequencing and clinical significance analysis of cellular heterogeneity in chronic skin ulcers
Chuwang WANG ; Jianda ZHOU ; Yanlian XIANG ; Peiting LI ; Shaohua WANG ; Jia CHEN ; Shuyue CHEN ; Wu XIONG ; Yu LIU ; Xiao FU
Chinese Journal of General Surgery 2025;34(2):327-337
Background and Aims:Chronic skin ulcers are a significant disease affecting patients'daily lives and psychological well-being.Abnormalities in the cells and extracellular matrix within the tissue may disrupt the balance of the microenvironment,hindering the normal skin repair process and leading to delayed healing of the ulcer.There is currently a lack of research on the mechanisms underlying the development of chronic ulcers and their diagnostic biomarkers.Single-cell sequencing,a newly developed high-throughput sequencing method in recent years,uses gene sequencing at the single-cell resolution to precisely reveal disease mechanisms and has been applied in various diseases.This study used single-cell transcriptome sequencing(scRNA-Seq)to investigate the cellular heterogeneity in chronic skin ulcer tissue to elucidate the potential molecular mechanisms behind delayed healing and provide new insights for clinical treatment.Methods:The scRNA-Seq technology was used to compare the differences in cell subpopulations and gene expression between chronic ulcer tissue and normal skin tissue.Single cells were sorted using a microfluidic platform,and cDNA libraries were constructed for subsequent differential gene analysis and functional enrichment analysis.Results:scRNA-Seq analysis revealed significant immune-metabolic remodeling features in chronic ulcer tissue:the number of B cells,monocytes,and macrophages in ulcer tissue increased by 2.1 to 3.5 times compared to the normal tissue control.This was accompanied by widespread activation of collagen synthesis genes(COL1A1/COL3A1)and synergistic suppression of immune regulators(e.g.,granzyme family GZMA/GZMB/H).Cross-cell subpopulation functional network analysis showed that hypoxia response mediated by the HIF-1 signaling pathway and PI3K/Akt pathway abnormalities formed a positive feedback loop,exacerbating the imbalance in the secretion of inflammatory factors(CXCL3/8,TGFBI)and compensatory upregulation of mitochondrial oxidative phosphorylation.Conclusion:Chronic skin ulcers exhibit significant differences in cellular heterogeneity and gene expression,suggesting that chronic ulcers are not simply tissue defects but a complex pathological process dominated by chronic inflammation and immune dysregulation.The coordinated dysregulation of multiple cell subpopulations in the ulcer microenvironment,along with persistent inflammatory responses and metabolic abnormalities,is interconnected through the HIF-1/TNF/MAPK pathway network.Downregulation of granzyme gene family members and abnormal histone modifications may contribute to immune clearance defects,providing a theoretical basis for developing novel therapies targeting epigenetic regulation or mitochondrial function.
7.Neural physiological basis and exercise-induced mechanism of central fatigue
Qifei WANG ; Xingbin DU ; Jianda KONG
Chinese Journal of Tissue Engineering Research 2025;29(32):6979-6988
BACKGROUND:Central fatigue refers to the phenomenon where changes in the functions of the central nervous system lead to a decline in an individual's exercise capacity and an increase in the sense of fatigue.With the continuous development of sports science,the mechanisms of exercise-induced central fatigue have gradually become a research hotspot.Although a large number of studies have explored the roles of biological mechanisms such as neurotransmitters,energy metabolism,and neuroinflammation in central fatigue,the interactions of these mechanisms and their manifestations in different exercise scenarios are still not fully understood.OBJECTIVE:To explore various biological mechanisms of exercise-induced central fatigue,clarify the interactions of neurotransmitters,energy metabolism,neural network activities,neuroinflammation,and oxidative stress in the formation of fatigue,and thereby provide a new perspective for understanding central fatigue.METHODS:Web of Science,PubMed,CNKI,WanFang,and VIP databases were searched with search terms"central fatigue,neurophysiological basis,exercise,physical activity,neurotransmitters,serotonin,dopamine,glutamate,GABA,energy metabolism,astrocytes,neuroinflammation,oxidative stress,cortical activity,neural network activity,sport"in Chinese and English.Relevant literature published from the establishment of each database to August 2024 was retrieved,and finally 74 articles were included for review.RESULTS AND CONCLUSION:Exercise-induced central fatigue involves multiple complex biological mechanisms.The imbalance of neurotransmitters(such as serotonin,dopamine,glutamate,and γ-aminobutyric acid)is a key factor in central fatigue.In particular,the increase in serotonin is closely related to the enhancement of the sense of fatigue.The disorder of energy metabolism,especially the glycogen metabolism of astrocytes and the supply of lactic acid,plays an important role in the formation of fatigue during long-term exercise.Neuroinflammation and oxidative stress exacerbate the manifestations of central fatigue by affecting the release of neurotransmitters and the functions of neural networks.In addition,changes in the activities of neural networks,such as the decrease in the excitability of the cerebral cortex,directly affect exercise performance.Future studies should strengthen the comprehensive exploration of neurotransmitter,metabolic,and inflammatory pathways,conduct more human studies to verify the findings of animal experiments,explore the differences in the mechanisms of central fatigue under different exercise conditions(such as intensity,time,and environment),and study the potential of drug interventions or nutritional supplements based on neurotransmitter regulation in alleviating central fatigue.
8.Strengthening the integration of traditional Chinese and Western medicine to promote the construction of a Chinese characteristic wound repair and regeneration system
Yu LIU ; Xiaohui QIU ; Na YANG ; Hong YANG ; Lixin XU ; Jian WANG ; Lu ZHANG ; Ke CAO ; Ke TAO ; Wu XIONG ; Jianda ZHOU
Journal of Chinese Physician 2024;26(3):321-325
To promote the construction of a wound repair and regeneration system with Chinese characteristics, it is necessary to follow the principle of combining traditional Chinese and Western medicine, and integrate theory, clinical practice, and teaching. Traditional Chinese medicine emphasizes a holistic concept and the principle of dialectical treatment, while Western medicine focuses on etiological analysis and local treatment. The combination of Chinese and Western medicine can complement each other's advantages and improve treatment effectiveness. The key technological innovations in repairing and regenerating systems cover areas such as drug therapy, physical therapy, and the application of biomaterials. This article discusses the development potential and challenges of combining traditional Chinese and Western medicine in the field of wound repair and regeneration, providing new ideas and methods for the development of wound repair and regeneration. It is expected to bring better medical services and treatment effects to patients undergoing repair and regeneration.
9.Mitochondrial dysfunction in Parkinson's disease and the potential ameliorative effects of exercise
Jianda KONG ; Yingao XIE ; Wen MA ; Youhan LIU ; Qinglu WANG
Chinese Journal of Tissue Engineering Research 2024;28(27):4413-4420
BACKGROUND:Parkinson's disease is a neurodegenerative disease,and its pathogenesis involves mitochondrial dysfunction.Exercise has a potential ameliorative effect on mitochondrial dysfunction related to Parkinson's disease,but there is no comprehensive review and in-depth analysis in this field. OBJECTIVE:To comprehensively review and analyze mitochondrial dysfunction related to Parkinson's disease and the potential ameliorative effect of exercise,thereby providing new ideas and methods for the treatment and prevention of Parkinson's disease. METHODS:We searched the Web of Science,PubMed,CNKI,WanFang,and VIP databases with the keywords of"mitochondria,mitochondrial function,mitochondrial disease,mitochondrial dysfunction,Parkinson's disease,Parkinson,exercise,physical activity,exercise training,exercise therapy,mitochondrial impairment,mitochondrial damage,mitochondrial defects"in Chinese and"mitochondria,Parkinson's disease,Parkinson disease,physical exercise,exercise,physical activity,mitochondrial dysfunction,mitochondrial damage,mitochondrial impairment,athletic training,exercise training,rehabilitation"in English.A total of 89 articles were included for review and analysis. RESLUTS AND CONCLUSION:Parkinson's disease is closely related to mitochondrial dysfunction,including mitochondrial biogenesis inhibition,reduced autophagy,increased apoptosis,abnormal elevation of Ca2+ concentration,and increased oxidative stress in Parkinson's disease patients.Exercise has a positive effect on mitochondrial dysfunction related to Parkinson's disease,by promoting mitochondrial biogenesisand autophagy,regulating mitochondrial morphology,altering the plasticity of the mitochondrial respiratory chain,and reducing oxidative stress,thus helping to improve the development and progression of Parkinson's disease.However,the detailed mechanism between mitochondrial dysfunction and the ameliorative effect of exercise is still not fully understood,and future clinical studies can be conducted to validate the results of animal models and gain insights into the benefits and mechanisms of exercise in patients with Parkinson's disease.
10.Effects and mechanism of metformin on the wound healing of full-thickness skin defects in diabetic rats
Baohong WANG ; Yanbing ZHANG ; Xianping ZHANG ; Yuting LI ; Zhihui WU ; Rongying HU ; Shiyue ZHAO ; Hongna JIANG ; Yuwei YAO ; Jianda DONG
Chinese Journal of Burns 2024;40(6):579-588
Objective:To investigate the effects and mechanism of metformin on the wound healing of full-thickness skin defects in diabetic rats.Methods:This study was an experimental study. Eighteen 8-week-old male Sprague Dawley rats were divided into control group, diabetes group, and diabetes+metformin group according to complete random grouping method, with 6 rats in each group. The latter two groups of rats were used to create diabetic models, and then four circular full-thickness skin defect wounds with a diameter of 5 mm were made on the back of 18 rats. Metformin F-127 hydrogel was applied only to the wounds of rats in diabetes+metformin group. The wound healing status on post injury day (POD) 7 and 13 was observed and the wound healing rate was calculated. The wound tissue on POD 7 and 13 was collected for hematoxylin-eosin staining to measure the length of re-epithelialized epidermis and calculate the change rates in diameters of epidermal and dermal wounds, for immunohistochemical staining to detect the relative expressions of keratin 10 and proliferating cell nuclear antigen (PCNA), and for Western blotting to detect the protein expressions of keratin 10 and PCNA. The sample size in all the above experiments was 8 except that in the last experiment was 3. The correlations between the relative expressions of keratin 10 and PCNA in wound tissue in three groups of rats and their wound healing rates, and the correlation between the relative expressions of keratin 10 and PCNA in wound tissue were analyzed.Results:On POD 7, the wound healing rates of rats in diabetes group and diabetes+metformin group were 81.48% (77.89%, 85.53%) and 93.04% (92.51%, 94.24%), which were significantly lower than 100% (97.17%, 100%) in control group (with Z values of 2.37 and -3.36, respectively, P<0.05); the wound healing rate of rats in diabetes+metformin group was significantly higher than that in diabetes group ( Z=3.45, P<0.05). On POD 13, the wound healing rates of rats in control group and diabetes+metformin group were both 100% (100%, 100%), which were significantly higher than 94.47% (90.68%, 99.82%) in diabetes group (with Z values of 2.90 and -2.90, respectively, P<0.05). On POD 7, the change rates in epidermal wound diameter of rats in control group and diabetes+metformin group were significantly higher than that in diabetes group (with Z values of 3.36 and -2.74, respectively, P<0.05). The change rates in dermal wound diameter of rats in the three groups were similar on POD 7 and 13 ( P>0.05). The lengths of re-epithelialized epidermis of rats in control group and diabetes+metformin group on POD 13 were significantly longer than that in diabetes group (with Z values of 3.34 and -2.64, respectively, P<0.05). The relative expressions of keratin 10 in wound tissue of rats in diabetes group on POD 7 and 13 were significantly higher than those in control group (with Z values of -3.36 and -3.26, respectively, P<0.05) and diabetes+metformin group (with Z values of 3.36 and 3.15, respectively, P<0.05), and the relative expression of keratin 10 in wound tissue of rats in diabetes+metformin group on POD 7 was significantly lower than that in control group ( Z=3.05, P<0.05); the relative expressions of PCNA in wound tissue of rats in diabetes group on POD 7 and 13 were significantly lower than those in control group (with both Z values of 3.36, P<0.05) and diabetes+metformin group (with both Z values of -3.36, P<0.05). The protein expressions of keratin 10 in wound tissue of rats in control group and diabetes+metformin group on POD 7 as well as that in diabetes+metformin group on POD 13 were significantly lower than those in diabetes group ( P<0.05), and the protein expressions of PCNA in wound tissue of rats in control group and diabetes+metformin group on POD 7 were significantly higher than that in diabetes group ( P<0.05). There was a significant positive correlation between the relative expression of keratin 10 in wound tissue and the wound healing rate in control group and diabetes+metformin group of rats (with r values of 0.78 and 0.71, respectively, P<0.05), there was a significant negative correlation between the relative expression of PCNA in wound tissue and the wound healing rate in diabetes+metformin group of rats ( r=-0.60, P<0.05), and there was a significant negative correlation between the relative expressions of PCNA and keratin 10 in wound tissue of rats in diabetes group and diabetes+metformin group (with r values of -0.41 and -0.49, respectively, P<0.05). Conclusions:The diabetic rats with full-thickness skin defect wound exhibit delayed healing, accompanied by up-regulation of keratin 10 and down-regulation of PCNA in keratinocytes in the wound tissue. Metformin can promote wound healing in diabetic rats with full-thickness skin defects by down-regulating keratin 10 expression and up-regulating PCNA expression in keratinocytes in the wound tissue, and the wound healing rate was positively correlated with the expression of keratin 10 and negatively correlated with the expression of PCNA.

Result Analysis
Print
Save
E-mail