SUMOylation pathway is disturbed, the cross-talk among SUMOylation and other post translational modifications, the role of miRNAs in controlling the function of transcripts, loading of RNA species into exosomes and the possible SUMOylation

SUMOylation, much of a similar process like ubiquitination catches attention across various research groups as a potential therapeutic target to fight various infectious and cancerous diseases. This idea take its strength from recent reports which unearth the molecular mechanisms of SUMOylation and its involvement in important diseases distributed across various kingdoms. At the beginning SUMOylation was considered a process affected only by viral diseases but subsequent reports enlighten its role in diseases caused by bacteria as well. This enhances the SUMOylation canvas and demanded more in-depth study of the process. The present review is an attempt to study the regulatory mechanism of genes when the natural SUMOylation pathway is disturbed, the cross-talk among SUMOylation and other post translational modifications, the role of miRNAs in controlling the function of transcripts, loading of RNA species into exosomes and the possible SUMOylation related therapeutic targets. Introduction The innate immune responses across kingdoms are tightly regulated to fight attacking pathogens. Post translational modification of proteins like acetylation, methylation, ubiquitination and SUMOylation have predominant role in activation/deactivation of immune responses by regulating various cellular processes including transcription, DNA repair, proliferation and apoptosis as these have the potential to relocate the protein to different organelles and modify its biological function. The deep understanding of those post translational mechanisms could provide insights in controlling several disease conditions and develop therapeutics based on those post translational modification markers. The small ubiquitin like modifiers (SUMO) proteins discovered in 1997 (Mahajan et al., 1997) has since been recognized as family of important modification proteins unlike ubiquitination which is involved in degradation of proteins, instead it modulates the function of target proteins. The covalent conjugation of SUMO molecules to protein residue lysine on a typical consensus sequence ψKxD/E (where ψ is large hydrophobic residue, K is the target lysine, D/E is acidic residue and x represent any amino acid) (Rodriguez et al., 2001; Sampson et al., 2001). The process of SUMOylation is carried out by a cascade of three enzymes (E1, E2 and E3). E1 is a SUMO activating enzyme SAE1, while E2 is conjugating enzyme e.g. Ubc9 and E3 is SUMO ligase. The SUMOylation reaction is reversed by SENPs termed as deSUMOylation having distinct regulatory roles. Till date four SUMO molecules have been reported in mammals viz., SUMO1, SUMO2, SUMO3 and SUMO4. The sequence similarity of SUMO2 and SUMO3 are almost 97% hence are often reported collectively as SUMO2/3. Various kinds of stimuli l ike oxidative stress often increase the rate of SUMOylation. A recent report shows the exposure to cigarette smoke alters the microRNA expression by SUMOylation of DICER (Gross et al., 2014). Viruses and chemical toxins also causes an increase in SUMOylation, a well known example is of proteins like p53. The role of SUMOylation in various disease pathways and its consequence is depicted in Figure 1. Most of the biological fluids if not all have special vesicles ca l led exosomes which serves as a mode of communication between cells, become an active area of research for immune related responses. These have the specific repertoires of mRNA, miRNAs, and other non coding RNA. All these RNA species can be transferred functionally to recipient cells. A recent report indicated the mechanism by which miRNA is localized into exosomes by recognizing its specific motif and loaded through a protein heterogeneous nuclear r ibonucleoprotein A2B1 (hnRNPA2B1) (Richard et al., 2013). Interestingly hnRNPA2B1 binding to miRNA is controlled by SUMOylation. This finding suggests that loading of miRNA in exosomes can be altered by identification of specific motifs or changes in expression levels of hnRNPA2B1 hence making exosomes important in drugs discovery. The discovery of PML/SUMO pathway that hinder viral replication by SUMO conjugation and might be increasing the number of SUMO targets, indicates a possible role in senescence or stem cell self renewal. (Sahin et al., 2014). Ubiquitination maintains the balance in cyclin-dependent kinase (CDK) activity by degradation of cyclins to maintain proper cell cycle progression. The glioblastoma, a deadly brain cancer have been linked to SUMOylation of CDK6 by SUMO1 which stabilizes the protein that inturn leads the cell cycle towards cancer progression (Bellail et al., 2014). The patients of another important cancerous disease the granulosa cell tumors reported to have mutation in FOXL2 at C134W. This mutation is recently linked to sequential post translational modifications where it undergo hyperphosphorylation at serine 33 by GSK3β which causes Curr. Issues Mol. Biol. (2016) 18: 49-56. horizonpress.com/cimb #49 SUMOylation: A link to future therapeutics SUMOylation Khan et al. MDM2 related ubiquitination results in proteasomal degradation (Kim et al., 2014). The FOXL2 in normal patients however is phosphorylated hence allows its SUMOylation leading to stabilized protein. These recent developments on regulatory role of post translational modifications especially SUMOylation and their cross-talks further open a window to develop epigenetic drugs that may have an answer to many of the untreated infectious and cancerous diseases. In present attempt, we also have summarized the recent advances regarding SUMOylation that might be useful in drug development. SUMOylation and Inflammatory Pathway The families of receptors including Toll Like Receptors, NLR and RIG receptors initiates NF-kb against attacking pathogens by recognizing the pathogen associated molecular patterns (PAMPS) and danger associated molecular patterns (DAMPS). In mammals an operative immune response is achieved by the functional activation of key inflammatory responses like the production of cytokines characterized by proliferation, differentiation and recruitment of blood cells to site of injury which plays a critical role in immune responses. The TLRs are broadly differentiated into two groups based on its location. Several TLRs including (TLR1, TLR2, TLR4, TLR6 and TLR10) are located on membrane while (TLR3, TLR7, TLR8 and TLR9) are located in endosomes (Zohaib et al., 2015). Innate immunity is bestowed by highly conserved toll signalling pathway which is regulated by several factors, one among them is β-arrestin Kurtz (Krz) which is reported as inhibitor of toll signalling in embryo of Drosophila (Tipping et al., 2010). It is recently proven experimentally that a conserved sequence of Krz interacts with SUMO protease Ulp1 and affects SUMOylation. The loss of Krz or Ulp1 causes same kind of inflammatory phenotypes. Furthermore a mutation in Krz and Ulp1 causes similar dose dependent synergistic effects providing grounds for the two proteins involvement in same pathway. The altered levels of Krz can affect the deconjugation ability of Ulp1 so is involved in controlling systemic inflammation and toll signalling at SUMOylation level (Anjum et al., 2013). As the interaction between βarrestin and SENP1 is conserved it can be suspected that the control mechanism might be similar in other organisms. This will be of interest to see whether mammalian β arrestins have role in Toll/NFkB signalling regulation (Anjum et al., 2013). NFkB activation is controlled by various post translational mechanisms including phosphorylation, acetylation and ubiquitination. A transcription factor p65 also known as Curr. Issues Mol. Biol. (2016) 18: 49-56. horizonpress.com/cimb #50 Infammation Heart physiology Autophagy TLRs SERCA2A LC3/ GABARAP Loading of miRNAs Exosomes Deadly brain cancer hnRNPB2