Effects of apocynin and hyperbaric oxygenation on ischemia/reperfusion acute kidney injury induced in spontaneously hypertensive rats

Ishemijsko/reperfuzijsko oštećenje je čest uzrok akutnog bubrežnog oštećenja (ABO), a hipertenzija može doprineti povećanoj učestalosti i lošijem ishodu ove bolesti. Cilj ove studije je da ispita efekte preventivnog tretmana hiperbaričnom oksigenacijom (HBO) i tretmana apokininom, samostalno ili u kombinaciji na: sistemske i bubrežne hemodinamske parametre, parametre bubrežne funkcije, oksidativni stres, lipidni profil i strukturu bubrežnog tkiva kod spontano hipertenzivnih pacova nakon ishemijsko/reperfuzijskog oštećenja bubrega. Studija je sprovedena na mužjacima spontano hipertenzivnih pacova, koji su nasumično podeljeni u sledeće grupe: lažno operisane pacove (SHAM), pacove sa indukovanim ABO (ABO), grupu sa indukovanim ABO, a kojoj je aplikovan apokinin (ABO+APO), grupu sa HBO tretmanom pre indukcije ABO (ABO+HBO) i grupu sa kombinacijom pretretmana HBO i tretmana apokininom (ABO+APO+HBO). HBO tretman sprovođen je izlaganjem čistom kiseoniku (2.026 bar) dva puta dnevno, dva uzastopna dana, u trajanju od 60 minuta, a 24 časa pre izvođenja ABO. ABO je izvođeno uklanjanjem desnog bubrega i klemovanjem leve bubrežne arterije u trajanju od 45 minuta. Apokinin je aplikovan (40 mg/kg t.m., intravenski) 5 minuta pre reperfuzije. Izazivanje ABO je dovodilo do značajnog pada srednjeg arterijskog pritiska i srčane frekvence, dok je u tretiranim grupama uočen statistički značajan pad ukupnog perifernog vaskularnog otpora. Značajn pad u protoku krvi kroz renalnu arteriju, kao i povećanje vaskularnog otpora u istom krvnom sudu nakon indukcije ABO, poboljšali su se nakon tretmana HBO. Takođe, do jednakog poboljšanja bubrežne hemodinamike dovodio je i tretman apokininom, sa ili bez HBO tretmana. Posmatrajući parametre bubrežne funkcije, ABO je dovodilo do statistički značajnog povećanja koncentracija kreatinina, uree i fosfata u plazmi, kao i smanjenja njihovih urinarnih klirensa. Značajno poboljšanje navedenih parametara, a samim tim i funkcije bubrega uočeno je u svim tretiranim grupama. ABO je dovodilo do promena u lipidnom profilu, sa statistički značajnim povećanjem koncentracija ukupnog i LDL holesterola. Sa druge strane, u poređenju sa ABO grupom, u svim tretiranim grupama uočen je porast HDL holesterola. HBO tretman, primanjen samostalno ili u kombinaciji sa apokininom, dovodio je do smanjenja intenzivne lipidne peroksidacije u plazmi, nastale usled ABO. Takođe, isti tretman, primenjen samostalno, povećavao je aktivnost antioksidativnog enzima glutation – reduktaze, dok je u kombinaciji sa apokininom dovodio do povećane aktivnosti katalaze u eritrocitima. HBO tretman, samostalno ili u kombinaciji sa apokininom, dovodio je do statistički značajnog pada vrednosti KIM – 1 u plazmi. Imunohistohemijska analiza ekspresije 4-HNE, NGAL, i HO-1 u bubrežnom tkivu ukazala je na značajno poboljšanje navedenih parametara u svim tretiranim grupama u odnosu na ABO grupu.Ischemia/reperfusion injury is common cause of acute kidney injury (AKI), while hypertension might contribute to the increased incidence and worse outcome of AKI. The purpose of this study was to investigate the effects of single and combined hyperbaric oxygen (HBO) preconditioning and apocynin treatment on systemic and renal haemodinamics, kidney function, oxidative stress levels, lipid profile and kidney structure in spontaneously hypertensive rats after renal ischemia/reperfusion injury. The study was conducted on male spontaneously hypertensive rats, randomly divided into following groups: sham-operated group (SHAM) AKI control group (AKI), AKI group with apocynin treatment (AKI+APO), group with HBO preconditioning before AKI induction (AKI+HBO) and group with HBO preconditioning before and apocynin treatment after AKI induction (AKI+APO+HBO). HBO preconditioning was performed by exposing to pure oxygen (2.026 bar), twice a day for two consecutive days for 60 minutes, and 24h before AKI induction. For AKI induction, the right kidney was removed and ischemia was performed by clamping the left renal artery for 45 minutes. Apocynin was applied (40 mg/kg b.m., intravenously) 5 minutes before reperfusion. After AKI induction, mean arterial pressure and hart rate were significantly decreased, while total peripheral vascular resistance was significantly increased in all treated groups. AKI significantly increased renal vascular resistance and reduced renal blood flow, which were significantly improved after HBO preconditioning. Also, apocynin treatment, with or without HBO preconditioning showed improvement on renal hemodynamics. Considering kidney function, AKI significantly increased plasma creatinine, urea and phosphate levels and decreased their urinary clearances. Remarkable improvement, both in plasma concentrations and urinary clarences, in comparison to AKI group was observed in all treated groups. AKI induction caused significant increase in total and LDL cholesterol concentrations in plasma. On the other hand, in all treated group significant increase in HDL cholesterol plasma concentrations was noticed in comparison to AKI group. HBO preconditioning, applied both, solitary or with apocynin decreased intense lipid peroxidation in plasma caused by AKI induction. Also, applied solitary, HBO preconditioning increased glutathione reductase enzyme activity while combined with apocynin, it increased catalase activity in erythrocytes. Increased KIM – 1 plasma levels after AKI induction were significantly decreased after HBO preconditioning, solitary or combined with apocynin. Immunohistochemical analysis of 4-HNE, NGAL, and HO-1 tissue expression in all treated groups showed also significant improvment

Potassium channels in renal proximal tubule

Potassium channels are a diverse family of membrane proteins which are present within all cells of the body. They contain two subunits α, which determine the structure of the channel, and β, which can modify the properties of the channel. Those transmembrane proteins take part in K+ movement across cell membranes, via a highly selective pore. The kidneys have crucial role in maintaining total body potassium content, by matching its intake and excretion. K+ absorption in the proximal tubule is primarily passive and proportional to Na+ and water, so that changes in fluid and potassium transport are closely coupled. Proximal tubular K+ channels are crucial for the maintenance of a hyperpolarized membrane voltage. In leaky epithelia, such as the proximal tubule, the hyperpolarization of the basolateral membrane also results in the hyperpolarization of the apical membrane, due to increase in the K+ conductance of that barrier. They are also involved in regulating cell volume and in recycling potassium across the basolateral membrane. K+ channels of the KCNK and KCNJ gene families have been discovered in the basolateral membrane cell of various species. One of the primary functions of basolateral K+ channels is to recycle K+ across the basolateral membrane for proper function of the Na+-K+-ATPase. Activation by extracellular alkalinization has been associated with a role of TASK-2 in kidney proximal tubule bicarbonate reabsorption. In renal proximal tubules, luminal K+ channels play an important role for restoring the driving force of Na+-coupled transport systems (amino acids, glucose), which depolarize the luminal membrane. Some of these luminal K+ channels are activated directly by the transport-associated depolarization; others are regulated by mediators, second messenger pathways and cell volume

Clinicopathological Relevance of PAX8 Expression Patterns in Acute Kidney Injury and Chronic Kidney Diseases

Transcription factor PAX8, expressed during embryonic kidney development, has been previously detected in various kidney tumors. In order to investigate expression of PAX8 transcription factor in acute kidney injury (AKI) and chronic kidney diseases (CKD), immunohistochemical analysis was performed. Presence, location and extent of PAX8 expression were analyzed among 31 human kidney samples of AKI (25 autopsy cases, 5 kidney biopsies with unknown etiology and 1 AKI with confirmed myoglobin cast nephropathy), as well as in animals with induced postischemic AKI. Additionally, expression pattern was analyzed in 20 kidney biopsy samples of CKD. Our study demonstrates that various kidney diseases with chronic disease course that results in the formation of tubular atrophy and interstitial fibrosis, lead to PAX8 expression in the nuclei of proximal tubules. Furthermore, patients with PAX8 detected within the damaged proximal tubuli would be carefully monitored, since deterioration in kidney function was observed during follow-up. We also showed that myoglobin provoked acute kidney injury followed with large extent of renal damage, was associated with strong nuclear expression of PAX8 in proximal tubular cells. These results were supported and followed by data obtained in experimental model of induced postischemic acute kidney injury. Considering these findings, we can assume that PAX8 protein might be involved in regeneration process and recovery after acute kidney injury. Thus, accordingly, all investigation concerning PAX8 immunolabeling should be performed on biopsy samples of the living individuals

Immunohistochemical Analysis of 4-HNE, NGAL, and HO-1 Tissue Expression after Apocynin Treatment and HBO Preconditioning in Postischemic Acute Kidney Injury Induced in Spontaneously Hypertensive Rats

Oxidative stress has been considered as a central aggravating factor in the development of postischemic acute kidney injury (AKI). The aim of this study was to perform the immunohistochemical analysis of 4-hydroxynonenal (4-HNE), neutrophil gelatinase-associated lipocalin (NGAL), and heme oxygenase-1 (HO-1) tissue expression after apocynin (APO) treatment and hyperbaric oxygenation (HBO) preconditioning, applied as single or combined protocol, in postischemic acute kidney injury induced in spontaneously hypertensive rats (SHR). Twenty-four hours before AKI induction, HBO preconditioning was carried out by exposing to pure oxygen (2.026 bar) twice a day, for 60 min in two consecutive days. Acute kidney injury was induced by removal of the right kidney while the left renal artery was occluded for 45 min by atraumatic clamp. Apocynin was applied in a dose of 40 mg/kg body weight, intravenously, 5 min before reperfusion. We showed increased 4-HNE renal expression in postischemic AKI compared to Sham-operated (SHAM) group. Apocynin treatment, with or without HBO preconditioning, improved creatinine and phosphate clearances, in postischemic AKI. This improvement in renal function was accompanied with decreased 4-HNE, while HO-1 kidney expression restored close to the control group level. NGAL renal expression was also decreased after apocynin treatment, and HBO preconditioning, with or without APO treatment. Considering our results, we can say that 4-HNE tissue expression can be used as a marker of oxidative stress in postischemic AKI. On the other hand, apocynin treatment and HBO preconditioning reduced oxidative damage, and this protective effect might be expected even in experimental hypertensive condition

Efekat deprivacije REM faze spavanja na aktivnost acetilholinesteraze i NA+/K+-ATPaze sinaptozoma mozga pacova u hipotiroidizmu

Hipotireoidizam i deprivacija REM spavanja prepoznati su kao mogući faktori rizika za nastanak neurodegenerativnih oboljenja, pre svega Alchajmerove bolesti (AB). Enzimi acetilholinesteraza (AchE) i Na+ /K+ -ATPaza imaju ulogu u taloženju amiloidnih plakova, tako da je njihova ukupna aktivnost u mozgu obolelog od AB snižena. Cilj istraživanja bio je utvrđivanje mogućeg efekta hipotireoidizma i deprivacije REM faze spavanja na aktivnost AchE i Na+ /K+ -ATPaze. Koristili smo eksperimentalni model hipotireoidizma izazvan propiltiouracilom. Nakon uvođenja u hipotiroidizam, eksperimentalna grupa životinja (n = 12) podeljena je u dve podrupe. Životinje iz prve podgrupe (n = 6) boravile su 72 h na maloj platformi, koja ne dozvoljava REM spavanje, jer usled atonije životinja upada u vodu i budi se. Drugoj podgrupi životinja (n = 6) omogućeno je spavanje. Nakon 72 h, životinje su žrtvovane, i u sinaptozomalnoj frakciji mozga pacova određivana je aktivnost AchE i Na+ /K+ -ATPaze spektrofotometrijski. Rezultati pokazuju da hipotireoidizam dovodi do snižene aktivnosti AchE i Na+ /K+ -ATPaze u odnosu na kontrolu (p ≤ 0,01), dok zajednički efekat hipotireoidizma i deprivacije REM spavanja dovodi do snižene aktivnosti AchE i Na+ /K+ -ATPaze u odnosu na kontrolu (n = 6, p ≤ 0,01), ali ne u odnosu na hipotiroidizam (p > 0,05). Na osnovu dobijenih rezultata možemo zaključiti da hipotireoidizam smanjuje aktivnost enzima AchE i Na+ /K+ -ATPaze, a da deprivacija REM faze spavanja dodatno ne smanjuje aktivnost ovih enzima u hipotireoidizmu, tako je mogući način delovanja hipotireoidizma kao faktora rizika za nastanak AB upravo sniženje aktivnosti ovih enzima.Četvrti srpski kongres o štitastoj žlezdi : Zbornik sažetaka : Septembar, 2017, Zlatibor

Effects of NADPH oxidase blockade and hyperbaric oxygen preconditioning on 4-HNE, NGAL, and HO-1 tissue expression in postischemic acute kidney injury induced in spontaneously hypertensive rat

Kidney disease represents a serious global health problem. Free radicals and prooxidants produced during ischemic acute kidney injury (AKI) may further aggravate the course of the disease and play a role in the pathogenesis of subsequent complications. The aims of our study were to examine the importance of NADPH oxidase blockade and to determine the effect of hyperbaric oxygen preconditioning on the immunohistochemical analysis of 4-hydroxynonenal (4-HNE), neutrophil gelatinase-associated lipocalin (NGAL), and heme oxygenase-1 (HO-1) tissue expression in postischemic acute kidney injury induced in spontaneously hypertensive rats (SHR). Twenty-four hours before AKI induction, HBO preconditioning was carried out by exposing to pure oxygen (2.026bar) twice a day, for 60 min in two consecutive days. Acute kidney injury was induced by removal of the right kidney while the left renal artery was occluded for 45 min by atraumatic clamp. NADPH oxidase blockade was performed by Apocynin (40 mg/kg body weigh)t, intravenously, 5 min before reperfusion. We showed increased 4-HNE renal expression in postischemic AKI compared to Sham operated (SHAM) group. Apocynin treatment, with or without HBO preconditioning, improved creatinine and phosphate clearances, in postischemic AKI. This improvement in renal function was accompanied with decreased 4-HNE, while HO-1 kidney expression restored close to the control group level. NGAL renal expression was also decreased after apocynin treatment, and HBO preconditioning,with or without APO treatment. Considering our results, we can say that 4-HNE tissue expression can be used as a marker of oxidative stress in postischemic AKI. On the other hand, NADPH oxidase blockade and HBO preconditioning reduced oxidative damage, and this protective effect might be expected even in experimental hypertensive condition

USING SOCIAL NETWORK IN MEDIA INDUSTRY

Završni rad s temom Primjena društvenih mreža u medijskoj industriji započeo sam analizom i definiranjem postojećih najvažnijih i najpopularnijih društvenih mreža poput Facebooka, Twittera, Instagrama, LinkedIna ali i recimo to tako zastarijelih tipova društvenih mreža poput foruma i blogova itd. Opisao sam njihovu povijest i razvoj od samih početaka sve do danas. U trećem poglavlju počeo sam pisati o medijskoj industriji tj. o njezinoj povijesti i karakteristikama. Četvrto poglavlje bazirao sam na statističkim podacima o korištenju interneta u svijetu i o korištenju najpopularnijih društvenih mreža. Peto poglavlje opisuje rezultate provedene ankete u kojoj je sudjelovalo 223 osobe koje su većinom studenti jer smo anketu provodili većinom na Facebook stranicama gdje se okupljaju bivši i današnji studenti. Rezultati su pokazali da društvene mreže najčešće koristimo za zabavu, dijljenje sadržaja sa prijateljima i komunikaciju. Zanimljivi su podaci koji govore kako je internet mladima danas primaran izvor svih informacija. Šesto poglavlje je dio gdje smo na temelju rezultata dobivenih iz ankete dali preporuku za korištenje društvenih mreža u medijskoj industriji. Društvene mreže danas su glavni izvor informacija i svi moderni mediji jednostavno su primorani na korištenje društvenih mreža

USING SOCIAL NETWORK IN MEDIA INDUSTRY

Završni rad s temom Primjena društvenih mreža u medijskoj industriji započeo sam analizom i definiranjem postojećih najvažnijih i najpopularnijih društvenih mreža poput Facebooka, Twittera, Instagrama, LinkedIna ali i recimo to tako zastarijelih tipova društvenih mreža poput foruma i blogova itd. Opisao sam njihovu povijest i razvoj od samih početaka sve do danas. U trećem poglavlju počeo sam pisati o medijskoj industriji tj. o njezinoj povijesti i karakteristikama. Četvrto poglavlje bazirao sam na statističkim podacima o korištenju interneta u svijetu i o korištenju najpopularnijih društvenih mreža. Peto poglavlje opisuje rezultate provedene ankete u kojoj je sudjelovalo 223 osobe koje su većinom studenti jer smo anketu provodili većinom na Facebook stranicama gdje se okupljaju bivši i današnji studenti. Rezultati su pokazali da društvene mreže najčešće koristimo za zabavu, dijljenje sadržaja sa prijateljima i komunikaciju. Zanimljivi su podaci koji govore kako je internet mladima danas primaran izvor svih informacija. Šesto poglavlje je dio gdje smo na temelju rezultata dobivenih iz ankete dali preporuku za korištenje društvenih mreža u medijskoj industriji. Društvene mreže danas su glavni izvor informacija i svi moderni mediji jednostavno su primorani na korištenje društvenih mreža

Effects of apocynin and hyperbaric oxygenation on ischemia/reperfusion acute kidney injury induced in spontaneously hypertensive rats

Ishemijsko/reperfuzijsko oštećenje je čest uzrok akutnog bubrežnog oštećenja (ABO), a hipertenzija može doprineti povećanoj učestalosti i lošijem ishodu ove bolesti. Cilj ove studije je da ispita efekte preventivnog tretmana hiperbaričnom oksigenacijom (HBO) i tretmana apokininom, samostalno ili u kombinaciji na: sistemske i bubrežne hemodinamske parametre, parametre bubrežne funkcije, oksidativni stres, lipidni profil i strukturu bubrežnog tkiva kod spontano hipertenzivnih pacova nakon ishemijsko/reperfuzijskog oštećenja bubrega. Studija je sprovedena na mužjacima spontano hipertenzivnih pacova, koji su nasumično podeljeni u sledeće grupe: lažno operisane pacove (SHAM), pacove sa indukovanim ABO (ABO), grupu sa indukovanim ABO, a kojoj je aplikovan apokinin (ABO+APO), grupu sa HBO tretmanom pre indukcije ABO (ABO+HBO) i grupu sa kombinacijom pretretmana HBO i tretmana apokininom (ABO+APO+HBO). HBO tretman sprovođen je izlaganjem čistom kiseoniku (2.026 bar) dva puta dnevno, dva uzastopna dana, u trajanju od 60 minuta, a 24 časa pre izvođenja ABO. ABO je izvođeno uklanjanjem desnog bubrega i klemovanjem leve bubrežne arterije u trajanju od 45 minuta. Apokinin je aplikovan (40 mg/kg t.m., intravenski) 5 minuta pre reperfuzije. Izazivanje ABO je dovodilo do značajnog pada srednjeg arterijskog pritiska i srčane frekvence, dok je u tretiranim grupama uočen statistički značajan pad ukupnog perifernog vaskularnog otpora. Značajn pad u protoku krvi kroz renalnu arteriju, kao i povećanje vaskularnog otpora u istom krvnom sudu nakon indukcije ABO, poboljšali su se nakon tretmana HBO. Takođe, do jednakog poboljšanja bubrežne hemodinamike dovodio je i tretman apokininom, sa ili bez HBO tretmana. Posmatrajući parametre bubrežne funkcije, ABO je dovodilo do statistički značajnog povećanja koncentracija kreatinina, uree i fosfata u plazmi, kao i smanjenja njihovih urinarnih klirensa. Značajno poboljšanje navedenih parametara, a samim tim i funkcije bubrega uočeno je u svim tretiranim grupama. ABO je dovodilo do promena u lipidnom profilu, sa statistički značajnim povećanjem koncentracija ukupnog i LDL holesterola. Sa druge strane, u poređenju sa ABO grupom, u svim tretiranim grupama uočen je porast HDL holesterola. HBO tretman, primanjen samostalno ili u kombinaciji sa apokininom, dovodio je do smanjenja intenzivne lipidne peroksidacije u plazmi, nastale usled ABO. Takođe, isti tretman, primenjen samostalno, povećavao je aktivnost antioksidativnog enzima glutation – reduktaze, dok je u kombinaciji sa apokininom dovodio do povećane aktivnosti katalaze u eritrocitima. HBO tretman, samostalno ili u kombinaciji sa apokininom, dovodio je do statistički značajnog pada vrednosti KIM – 1 u plazmi. Imunohistohemijska analiza ekspresije 4-HNE, NGAL, i HO-1 u bubrežnom tkivu ukazala je na značajno poboljšanje navedenih parametara u svim tretiranim grupama u odnosu na ABO grupu.Ischemia/reperfusion injury is common cause of acute kidney injury (AKI), while hypertension might contribute to the increased incidence and worse outcome of AKI. The purpose of this study was to investigate the effects of single and combined hyperbaric oxygen (HBO) preconditioning and apocynin treatment on systemic and renal haemodinamics, kidney function, oxidative stress levels, lipid profile and kidney structure in spontaneously hypertensive rats after renal ischemia/reperfusion injury. The study was conducted on male spontaneously hypertensive rats, randomly divided into following groups: sham-operated group (SHAM) AKI control group (AKI), AKI group with apocynin treatment (AKI+APO), group with HBO preconditioning before AKI induction (AKI+HBO) and group with HBO preconditioning before and apocynin treatment after AKI induction (AKI+APO+HBO). HBO preconditioning was performed by exposing to pure oxygen (2.026 bar), twice a day for two consecutive days for 60 minutes, and 24h before AKI induction. For AKI induction, the right kidney was removed and ischemia was performed by clamping the left renal artery for 45 minutes. Apocynin was applied (40 mg/kg b.m., intravenously) 5 minutes before reperfusion. After AKI induction, mean arterial pressure and hart rate were significantly decreased, while total peripheral vascular resistance was significantly increased in all treated groups. AKI significantly increased renal vascular resistance and reduced renal blood flow, which were significantly improved after HBO preconditioning. Also, apocynin treatment, with or without HBO preconditioning showed improvement on renal hemodynamics. Considering kidney function, AKI significantly increased plasma creatinine, urea and phosphate levels and decreased their urinary clearances. Remarkable improvement, both in plasma concentrations and urinary clarences, in comparison to AKI group was observed in all treated groups. AKI induction caused significant increase in total and LDL cholesterol concentrations in plasma. On the other hand, in all treated group significant increase in HDL cholesterol plasma concentrations was noticed in comparison to AKI group. HBO preconditioning, applied both, solitary or with apocynin decreased intense lipid peroxidation in plasma caused by AKI induction. Also, applied solitary, HBO preconditioning increased glutathione reductase enzyme activity while combined with apocynin, it increased catalase activity in erythrocytes. Increased KIM – 1 plasma levels after AKI induction were significantly decreased after HBO preconditioning, solitary or combined with apocynin. Immunohistochemical analysis of 4-HNE, NGAL, and HO-1 tissue expression in all treated groups showed also significant improvment

USING SOCIAL NETWORK IN MEDIA INDUSTRY

Završni rad s temom Primjena društvenih mreža u medijskoj industriji započeo sam analizom i definiranjem postojećih najvažnijih i najpopularnijih društvenih mreža poput Facebooka, Twittera, Instagrama, LinkedIna ali i recimo to tako zastarijelih tipova društvenih mreža poput foruma i blogova itd. Opisao sam njihovu povijest i razvoj od samih početaka sve do danas. U trećem poglavlju počeo sam pisati o medijskoj industriji tj. o njezinoj povijesti i karakteristikama. Četvrto poglavlje bazirao sam na statističkim podacima o korištenju interneta u svijetu i o korištenju najpopularnijih društvenih mreža. Peto poglavlje opisuje rezultate provedene ankete u kojoj je sudjelovalo 223 osobe koje su većinom studenti jer smo anketu provodili većinom na Facebook stranicama gdje se okupljaju bivši i današnji studenti. Rezultati su pokazali da društvene mreže najčešće koristimo za zabavu, dijljenje sadržaja sa prijateljima i komunikaciju. Zanimljivi su podaci koji govore kako je internet mladima danas primaran izvor svih informacija. Šesto poglavlje je dio gdje smo na temelju rezultata dobivenih iz ankete dali preporuku za korištenje društvenih mreža u medijskoj industriji. Društvene mreže danas su glavni izvor informacija i svi moderni mediji jednostavno su primorani na korištenje društvenih mreža