Mechanisms Underlying Exosome-Derived Bmal1 In Enhancing Cardiac Function

Objective:In China,heart diseases rank No.1 among all diseases affecting people’s health.Bmall(Brain and Muscle Arnt-like Protein-1)plays an important role in regulating heart function.We previously found that cardiac-specific knockout of Bmal1 gene in mice led to impaired cardiac function in mice.However,the mechanisms underlying exosomesderived Bmal1 in enhancing cardiac function remain unknown.Methods:Cardiomyocyte-specific Bmall knockout mice(Bmall-cKO)were constructed by injecting tamoxifen on the first and third days.Cardiac function was measured by echocardiography.The primary cardiomyocytes were isolated to detect cardiomyocyte binucleation rates.The mRNA and protein expression of RNA polymerase I(RNA Pol I)and Nucleolin were detected in different treatment groups of the wild type and Bmall-cKO mice,Finally,the role of Bmall in the heart was assessed by injecting exosomes containing different levels of Bmall through the tail vein.Results:Compared with normal mice,the cardiac function of Bmall-cKO mice was significantly decreased.The binucleation rate of cardiomyocytes increased in primary cardiomyocytes transfected with Bmall siRNA.The data showed that Bmall protein could directly interact with ribosome small subunit protein 10(RPS10)protein.When the Bmall gene is knocked out,it reduces ribosome biogenesis by decreasing the expression of RPS10 mRNA and protein.RT-PCR and Western Blot and immunofluorescence showed that RNA Pol I and Nucleolin expression were reduced in the absence of Bmall.This affected the binding of RPS10 protein to RhoA(Ras Homolog family member A)mRNA.RhoA is a key factor in cytokinesis.When the expression of Bmall or RPS10 is inhibited,the expression levels of RhoA mRNA and protein are decreased.Finally,mice were injected with tail vein or cardiomyocytes added with PBS,Exo,si Exo(si Bmall)to detect ribosome biogenesis level,and the binucleation rate of cardiomyocytes.The results showed that the expression of ribosome biogenesis was decreased and binucleation rate of cardiomyocytes was increased by si Exo.These results indicated that exosomes derived from human umbilical cord mesenchymal stem cells(UMSC)could enhance ribosome biogenesis expression by delivering Bmall.Bmal1 acts on RPS10 to increase RhoA expression,leading to a reduced binucleation rate of cardiomyocytes,thereby enhancing cardiac function.Conclusion:Loss of Bmal1 gene results in decreased expression of RPS10 mRNA and protein,which leads to decreased ribosome biogenesis level and RhoA expression,thereby increasing the binucleation rate of cardiomyocytes,resulting in decreased cardiac function in mice.This phenomenon can be reversed when exosomes are added.It was shown that human mesenchymal stem cell-derived exosomes could promote the recovery of cardiac function by delivering Bmal1 in mice.