View source for GlycoRNA

{{Short description|Small non-coding RNA}}
'''GlycoRNAs''' are small non-coding RNAs with sialylated [[glycan]]s.
Glycans mediate inter- and intramolecular interactions by adding [[polysaccharide]] chains onto [[lipid]]s and [[protein]]s. Similar to these other [[macromolecules]], [[RNA]]s can undergo [[sialylation]] and bear glycan structures. Some examples include [[Small nuclear RNA|small nuclear (sn) RNAs]], [[Ribosomal RNA|ribosomal (r) RNAs]], [[Small nucleolar RNA SNORD116|small nucleolar (sno) RNAs]], [[Transfer RNA|transfer (t) RNAs]], and [[Y RNA]]s - the latter of which comprise the greatest percentage of glycosylated RNA species.<ref name=":2">{{cite journal | vauthors = Saygin D, Tabib T, Bittar HE, Valenzi E, Sembrat J, Chan SY, Rojas M, Lafyatis R | display-authors = 6 | title = Transcriptional profiling of lung cell populations in idiopathic pulmonary arterial hypertension | journal = Pulmonary Circulation | volume = 10 | issue = 1 | pages = 124–131 | date = 2022-09-09 | pmid = 32166015 | doi = 10.15212/bioi-2021-0032 | pmc = 7052475 }}</ref>
Found primarily on the cell surface, these glycoRNAs can participate in the [[immune system]] and cell-to-cell communication.<ref name=":0">{{cite journal | vauthors = Flynn RA, Pedram K, Malaker SA, Batista PJ, Smith BA, Johnson AG, George BM, Majzoub K, Villalta PW, Carette JE, Bertozzi CR | display-authors = 6 | title = Small RNAs are modified with N-glycans and displayed on the surface of living cells | language = English | journal = Cell | volume = 184 | issue = 12 | pages = 3109–3124.e22 | date = June 2021 | pmid = 34004145 | pmc = 9097497 | doi = 10.1016/j.cell.2021.04.023 }}</ref>
== History of discovery ==
GlycoRNAs were discovered in May 2021 by a [[Stanford University]] research team led by Chemical Biologist [[Carolyn Bertozzi|Dr. Carolyn Bertozzi]]. The discovery came from an experiment where Post-doc Ryan Flynn used a metabolic tagging technique to label precursor sugars of glycan. What he discovered in the process was glycosylated, cell membrane-bound RNA.
Until now, [[lipid]]s and [[protein]]s were the only kinds of similarly sugar-decorated [[macromolecule]]s previously understood by science. Although glycoRNAs are not rare, it was understood that their discovery happened only relatively recently, as their presence defied accepted principles of well-established cellular biology.<ref name=":0" />
== Biology of RNA glycolysation ==
The exact mechanism of GlycoRNA [[glycosylation]] is still unclear. It remains ambiguous whether it follows a similar process performed by [[glycolipid]]s and glycoproteins in the [[Golgi apparatus|ER-Golgi complex]] or through different machinery. Flynn and [[Carolyn Bertozzi|Bertozzi]] have offered the first description of glycoRNA, and suggest that RNAs are glycosylated using the same machinery and mechanism that produces [[N-linked glycosylation|N-]] and [[O-linked glycosylation|O-linked glycans]] in glycosylated proteins.
Previous experiments have not addressed where and how RNA is tagged with sugars in cells. Proteins and lipids acquire [[carbohydrate]] tags primarily in the [[endoplasmic reticulum]] and [[Golgi apparatus]]. Principles of cellular biology say the initial transfer of sugars to macromolecules occurs in the endoplasmic reticulum. Then, the subsequent addition of sugars in the Golgi results in the formation of mature [[glycan]] structures. In the Golgi, these newly made glycans are packaged into vesicles for transportation into organelles or secreted across the plasma membrane. However, RNA is not known to exist in these compartments.<ref name=":0" />
[[File:Glycans and RNA.png|thumb|440x440px|'''Proposed biosynthesis pathway of N-linked glycoRNA:''' The synthesis of glycans starts in the endoplasmic reticulum, continues in the Golgi before being transported to the plasma membrane, where the glycoRNA is either secreted or becomes embedded in the plasma membrane. This is very similar to the biosynthesis pathway of N-linked glycoproteins.]]
Using pharmacological and genetic inhibition approaches, Flynn and [[Carolyn Bertozzi|Bertozzi]] explored the effects of inhibiting key glycan biosynthetic machinery on the production of glycoRNA. They found that the production of glycoRNA is impaired in cells where the glycan biosynthetic machinery has been genetically altered, although adding exogenous [[glycan]] reverses the inhibition.<ref name=":2" /> In another study, inhibition of [[oligosaccharyltransferase]], a membrane protein that catalyzes the addition of glycans to [[asparagine]] residues, was also seen to diminish the production of glycoRNA. Flynn and Bertozzi's studies demonstrated that cellular glycoRNA is also produced by the glycan biosynthetic machinery.<ref name=":0" />
Some direct measurement of the glycan on RNA association would help disclose both the mechanism and its function. There are a few possible explanations for how the attachment proceeds. [[Carolyn Bertozzi|Bertozzi's]] lab conducted several experiments to understand what holds glycan RNA together. Running glycoRNAs through a [[Differential centrifugation|sucrose gradient]] suggested that the molecule connecting the two entities was no larger than the small RNA itself. Researchers first thought that RNA could bind [[Non-covalent interaction|non-covalently]] to each other - a bond strong enough to avoid degradation unless digested by an [[Ribonuclease|RNase]]. However, this was very unlikely, as all evidence points to the RNA-glycan linkage via a [[Covalent bond|covalent]] network.<ref name=":0" />
Another possibility is that glycans can bind proteins, which in turn bind RNA. Researchers have speculated that the RNA-glycan linkage is likely not due to the direct glycosylation of RNA bases. The sensitivity of glycoRNA moieties to [[PNGase F]], which cleaves the glycosidic linkage between asparagine and the proximal [[GlcNAc]] of [[N-linked glycosylation|N-glycans]], implies a non-nucleobase interaction.<ref name=":0" /> However, a precursor nucleobase modification may be necessary for the [[Glycosidic bond|glycosidic linkage]] to occur. The chemistry behind RNA glycosylation remains unknown and necessitates further research to define the chemical and structural features of the linkage.
== Relations to disease ==
[[File:PDB 1pgs EBI.jpg|thumb|'''PYMOL:''' Three-dimensional structure of PNGase F, a glycosylasparaginase from Flavobacterium meningosepticum. (PDB: 1PGS)]]
The implication of glycoRNAs in RNA biology research is immense, especially when relating to disease progression and drug therapy. Aberrant glycosylation is a well-recognized hallmark of various human diseases. It has been a popular belief that diseases caused by abnormal glycosylation of proteins and lipids contribute to the pathology of those diseases. The discovery of glycoRNAs points researchers in the direction of an alternative molecule that may be responsible for the disrupted glycosylation network in human diseases.
[[Fractionation]] and [[Immunohistochemistry|immunohistochemical]] imaging studies revealed that glycoRNAs are primarily associated with the cell surface, as evidenced by their loss from the cell surface after treatment with a [[sialic acid]]-cleaving enzyme.<ref>{{cite journal | vauthors = Disney MD | title = A glimpse at the glycoRNA world | language = English | journal = Cell | volume = 184 | issue = 12 | pages = 3080–3081 | date = June 2021 | pmid = 34115968 | doi = 10.1016/j.cell.2021.05.025 | s2cid = 235385901 | doi-access = free }}</ref> This suggests that they may be involved in extracellular molecular interactions. Cell surface glycoRNAs were found to interact with members of the sialic acid-binding immunoglobulin-like lectin ([[Siglec]]) receptor family in addition to binding an anti-RNA antibody used to detect RNA virus-infected cells. These receptors are known to play a role in immune response modulation, specifically [[Host–pathogen interaction|host-pathogen interactions]], [[immune evasion]] in [[cancer]], and genetic links to [[Autoimmunity|autoimmune]] illnesses. The Siglec receptor family ligand partners are largely unknown, at least in part because previous research assumed they bind glycoproteins or glycolipids. As a result, glycoRNAs may be potential ligands for these and other orphan glycan-binding receptors.<ref name=":1">{{cite journal | vauthors = Clyde D | title = Sugar-coated RNAs | journal = Nature Reviews. Genetics | volume = 22 | issue = 8 | pages = 480 | date = August 2021 | pmid = 34168329 | doi = 10.1038/s41576-021-00388-y | s2cid = 235635316 }}</ref>
[[Autoimmune disease|Autoimmune disorders]] are linked to a myriad of RNA and RNA-associated [[autoantigens]]. Flynn and Bertozzi employed soluble Siglec-Fc reagents and [[flow cytometry]] to examine the binding of human Siglec receptors to cell surface glycoRNA. They found that 9 of the 12 marketed Siglec-Fc reagents bind to [[HeLa|HeLa cells]], and two of which, Siglec-11 and Siglec-14, had binding that was vulnerable to RNase A treatment.<ref>{{cite journal | vauthors = Smith BA, Bertozzi CR | title = The clinical impact of glycobiology: targeting selectins, Siglecs and mammalian glycans | journal = Nature Reviews. Drug Discovery | volume = 20 | issue = 3 | pages = 217–243 | date = March 2021 | pmid = 33462432 | pmc = 7812346 | doi = 10.1038/s41573-020-00093-1 }}</ref> These data support the hypothesis that cell surface glycoRNAs might act as direct ligands for Siglec receptors.
== Biomedical applications ==
GlycoRNAs have the potential to revolutionize the field of [[RNA-Seq|RNA sequencing]] (RNA-Seq) and improve our understanding of the [[human genome]]. By performing ([[Real-time polymerase chain reaction|qRT-PCR]]) arrays and RNA-Seq, it is possible to identify glycosylated RNA biomarkers that can be used to develop targeted therapies. By detecting altered expression of [[glycosyltransferase]]s, it is possible to discover precisely which RNA is glycosylated and what proteins carry it out.<ref>{{cite journal|vauthors=Angata K, Sawaki H, Tsujikawa S, Ocho M, Togayachi A, Narimatsu H |title=Glycogene Expression Profiling of Hepatic Cells by RNA-Seq Analysis for Glyco-Biomarker Identification |journal=Frontiers in Oncology |volume=10 |pages=1224 |date=2020-07-28 |pmid=32850363 |pmc=7402167 |doi=10.3389/fonc.2020.01224 |doi-access=free}}</ref> In diseases such as [[cancer]] and [[cystic fibrosis]], driver mutations can cause variable RNA glycosylation.<ref>{{cite journal|vauthors=Nishimura S |title=Toward automated glycan analysis |journal=Advances in Carbohydrate Chemistry and Biochemistry |volume=65 |pages=219–271 |date=2011 |pmid=21763513 |doi=10.1016/B978-0-12-385520-6.00005-4 |publisher=Elsevier |isbn=9780123855206}}</ref> To fully comprehend the genetic landscapes and heterogeneity of these disorders, more study in this field of [[glycoproteomics]] is required.
GlycoRNAs may also have a role in immune signal transduction. With their ability to bind to [[Siglec]] receptors and [[Anti-dsDNA antibodies|anti-dsRNA antibodies]], glycoRNAs have the potential to be used as biomarkers, enabling rapid diagnosis and prognosis for specific diseases. Furthermore, modifying RNA with glycans may be sensitive to [[immunotherapy]] medicines and thus could serve as drug targets. In addition to immunotherapy, several other therapies, including [[Radiation therapy|radiotherapy]] and [[chemotherapy]], have the potential to alter the glycan structure of glycoRNAs on the surface of the cell. Thus, these are expected to be used as a type of immune response signal receiver in many drug therapy.<ref name=":1" />
== References ==
{{Reflist}}
[[Category:Non-coding RNA]]