Reticulon

'''Reticulons''' ('''RTN'''s in [[vertebrate]]s and '''reticulon-like proteins''' or '''RNTl'''s in other [[eukaryote]]s) are a group of [[conserved sequence|evolutionary conservative]] [[protein]]s residing predominantly in [[endoplasmic reticulum]], primarily playing a role in promoting [[lipid bilayer|membrane]] curvature.<ref name="pmid16469703">{{cite journal|last=Voeltz|first=GK|author2=Prinz WA |author3=Shibata Y |author4=Rist JM |author5=Rapoport TA |title=A class of membrane proteins shaping the tubular endoplasmic reticulum|journal=[[Cell (journal)|Cell]]|volume=124|issue=3|pages=573–586|year=2006|pmid= 16469703|doi=10.1016/j.cell.2005.11.047|s2cid=12760025|doi-access=free}}</ref> In addition, reticulons may play a role in [[nuclear pore complex]] formation, [[Vesicle (biology)|vesicle]] formation, and other processes yet to be defined.<ref name="pmid18177508">{{cite journal |vauthors=Yang YS, Strittmatter SM |title=The reticulons: a family of proteins with diverse functions |journal=[[Genome Biol.]] |volume=8 |issue=12 |pages=234 |year=2007 |pmid=18177508 |pmc=2246256 |doi=10.1186/gb-2007-8-12-234 |doi-access=free }}</ref> They have also been linked to [[oligodendrocyte]] roles in inhibition of neurite outgrowth. Some studies link RTNs with [[Alzheimer's disease]] and [[amyotrophic lateral sclerosis]].<ref name="pmid18177508" />

All [[eukaryote]]s studied so far carry RTN genes in their [[genome]]s. The reticulons are absent only in [[archaea]] and [[bacteria]]. [[Mammal]]s have four reticulon genes, [[RTN1]], [[RTN2]], [[RTN3]], [[RTN4]]. Plants possess a greater number of '''reticulon''' [[isoform]]s, with 21 having been identified in the commonly used [[model organism]] [[Arabidopsis thaliana]].<ref>{{cite journal |vauthors=Hawes C, Kiviniemi P, Keichbaumer, V |title=The endoplasmic reticulum: A dynamic and well connected organelle |journal=[[Journal of Integrative Plant Biology]] |volume=57 |issue=1 |pages=50–62 |date= 2015 |pmid=25319240 |doi=10.1111/jipb.12297 |bibcode=2015JIPB...57...50H |url=https://radar.brookes.ac.uk/radar/items/3a7e5bcb-c558-4f89-b2db-7fa5b8ac5726/1 }}</ref>

The genes possess a number of [[exon]]s and [[intron]]s and are accordingly spliced into many [[isoform]]s. [[C-terminal]] region of RTNs contains a highly conservative reticulon [[homology domain]] (RHD) while other parts of the protein may vary even within a single organism.<ref name="pmid18177508"/>

A peculiar feature of RTN4's isoform RTN4A (Nogo-A) is its ability to inhibit [[axon]]al growth. This reticulon subform is absent in [[fish]],<ref name="pmid15858203">{{cite journal |vauthors=Diekmann H, Klinger M, Oertle T, Heinz D, Pogoda HM, Schwab ME, Stuermer CA |title=Analysis of the reticulon gene family demonstrates the absence of the neurite growth inhibitor Nogo-A in fish |journal=[[Mol. Biol. Evol.]] |volume=22 |issue=8 |pages=1635–48 |date=August 2005 |pmid=15858203 |doi=10.1093/molbev/msi158 |doi-access=free }}</ref> a [[taxon]] known for the heightened ability of its [[Central nervous system|CNS]] to regenerate after injury.

Transmembrane 33 ([[TMEM33]]) exogenously suppresses reticulon 4C function and therefore may play a role in dictating membrane curvature through inhibition of reticulon function.<ref name=" Urade et al 2014 ">{{cite web | title = Identification and Characterization of TMEM33 as a Reticulon-binding Protein | url = http://www.med.kobe-u.ac.jp/journal/contents/60/E57.pdf | accessdate = 2015-02-22  }}</ref>

== Structure ==

=== Evolutionary history === Reticulon [[protein]]s, which range from 200-1,200 [[amino acid]]s, have been tracked in all eukaryotic organisms that have been examined. The family of vertebrate proteins are called reticulons, and all other located [[eukaryote]]s are called reticulon-like proteins. Some examples of explored reticulon genomes of eukaryotes are in ''[[Homo sapiens]], [[House mouse|Mus musculus]], [[Zebrafish|Danio rerio]], [[Drosophila melanogaster]], [[Arabidopsis thaliana]], and [[Saccharomyces cerevisiae]].''<ref name="pmid18177508" /> These [[genome]]s are not found in either archaea or bacteria. Because of their absence from prokaryotes and close association with the [[endoplasmic reticulum]] (ER), it is proposed that reticulons have evolved with the eukaryotic endomembrane system. In mammals, there are four reticulon genes: RTN1-4. RTN 3 and 4 have sequence identity more closely related at 73% than between 2 and 4 with only a 52% sequence identity.<ref name="pmid18177508" /> There is a divergence in sequence between reticulons as their splice isoforms can be variable, even in the same organism. This is consistent with the evolution of species and cell-specific roles for reticulons. The longest [[Protein isoform|isoform]], Nogo-A, has shown through studies that it can inhibit neurite outgrowth and regeneration.<ref name="pmid18177508" /> However, this [[Protein isoform|isoform]] is absent in fish where regeneration of central nervous system is extensive. Reticulons can vary in function between species.<ref name="pmid18177508" />

=== Reticulon protein structure === The reticulon family contain a carboxy-terminal reticulon homology domain (RHD) that has two hydrophobic regions of 28-36 amino acids. Those regions are supposedly embedded in the membrane. Those regions are separated by the 60-70 amino acids of the hydrophilic loop.<ref name="pmid18177508" /> Following the loop is a carboxy-terminal tail which has a length of about 50 [[amino acid]]s. The amino-terminal domains are not similar to reticulons within the [[Protein family|family]].<ref name="pmid18177508" /> However, the three-dimensional structure has been preserved from [[yeast]]s to [[plant]]s to humans. The hydrophobic region of the structure is abnormally long compared to other transmembrane domains. The structure of the reticulon may be related to the function of this [[protein]].<ref name="pmid18177508" />

=== Localization in ER, N- and C-terminal ends === Reticulons are typically in the ER of [[Cell (biology)|cells]]; however, they have additionally been found on cell surfaces in mammals and on the surface of [[oligodendrocyte]]s where they inhibit axon growth.<ref name="pmid18177508" /> The [[N-terminus|N-terminal]], loop region and [[C-terminus|C-terminal]] are all on the [[cytosol]] side of the ER membrane and they are able to interact with other cytosolic proteins.<ref name=":1">{{Cite journal|last1=Sparkes|first1=Imogen|last2=Tolley|first2=Nicholas|last3=Aller|first3=Isabel|last4=Svozil|first4=Julia|last5=Osterrieder|first5=Anne|last6=Botchway|first6=Stanley|last7=Mueller|first7=Christopher|last8=Frigerio|first8=Lorenzo|last9=Hawes|first9=Chris|date=2010-04-01|title=Five Arabidopsis Reticulon Isoforms Share Endoplasmic Reticulum Location, Topology, and Membrane-Shaping Properties|journal=The Plant Cell Online|language=en|pages=1333–43|doi=10.1105/tpc.110.074385|issn=1040-4651|pmid=20424177|volume=22|issue=4|pmc=2879755|bibcode=2010PlanC..22.1333S }}</ref> N-terminal regions in reticulon proteins are diversified in interacting with other substrates.<ref name=":2">{{Cite journal|last1=Lee|first1=Hyoung Yool|last2=Bowen|first2=Christopher Hyde|last3=Popescu|first3=George Viorel|last4=Kang|first4=Hong-Gu|last5=Kato|first5=Naohiro|last6=Ma|first6=Shisong|last7=Dinesh-Kumar|first7=Savithramma|last8=Snyder|first8=Michael|last9=Popescu|first9=Sorina Claudia|date=2011-09-01|title=Arabidopsis RTNLB1 and RTNLB2 Reticulon-Like Proteins Regulate Intracellular Trafficking and Activity of the FLS2 Immune Receptor|journal=The Plant Cell Online|language=en|volume=23|issue=9|pages=3374–3391|doi=10.1105/tpc.111.089656|issn=1040-4651|pmid=21949153|pmc=3203430|bibcode=2011PlanC..23.3374L }}</ref>

Overall, three models have been identified of RHD topology.<ref name="pmid18177508" /> One finding suggests that the amino-acid terminus and the 66-loop extend into the extracellular space. This would indicate that the hydrophobic region double-backs on itself in the membrane. Other data suggests that the amino-terminal is intracellular. Lastly, a third model explains the 66-loop and amino-terminal domain are [[cytoplasm]]ic. All of these models suggest that reticulons may have different topologies at different regions such as the [[Endoplasmic reticulum|ER]] and [[plasma membrane]].<ref name="pmid18177508" /> This would allow them to not only look different at each location, but be able to carry different roles in the cell and in different cell types.<ref name="pmid18177508" />

The first reticulon protein [[RTN1|RTN1]] was characterized as an [[antigen]] for [[neuroendocrine cell]]s from a [[Complementary DNA|cDNA]] in neural tissue.<ref name="pmid18177508" /> It was later renamed when it was proved to be associated with the [[Endoplasmic reticulum|ER]] from several different methods. Reticulons do not have an [[Endoplasmic reticulum|ER]] localization sequence, but the RHD hydrophobic region is able to target protein-RTN to the ER by green fluorescence. Without the RHD, there is no association with the [[Endoplasmic reticulum|ER]]. Reticulons have localized to the [[Endoplasmic reticulum|ER]] in the following organisms: yeast, ''[[Arabidopsis]], [[Xenopus]], [[Drosophila]]'' and mammals.<ref name="pmid18177508" />

== Function ==

=== Mechanisms === Evidence shows that reticulons influence [[Endoplasmic reticulum|ER]] and [[Golgi apparatus|Golgi-body]] trafficking in and out of the cell through plasma membrane-associated proteins.<ref name=":2" /> Reticulons additionally aid in the formation of [[Vesicle (biology and chemistry)|vesicles]] and membrane morphogenesis.<ref name="pmid18177508" /> When inhibiting [[Reticulon 4|RTN4A]] in mammalian cells, it does not allow for proper formation of membrane tubules. In ''[[Caenorhabditis elegans|C. elegans]]'', removing RTNL RET−1 and associated proteins interferes with the formation of the [[Endoplasmic reticulum|ER]] during [[mitosis]]. It also disrupts [[nuclear envelope]] reassembly. Reticulons have been found to interact with [[protein]]s that are involved with vesicular formation and morphogenesis of the [[Endoplasmic reticulum|ER]]. They are additionally involved in intracellular trafficking. In one example, it was shown that increasing expression of [[RTN3|RTN3]] keeps transport of proteins from retrograding from the [[Golgi apparatus|Golgi bodies]] to the [[Endoplasmic reticulum|ER]].<ref name="pmid18177508" /> Additionally, reticulons may be used to shape coated protein vesicles by interacting with a component of the adaptor protein complex (which maintains the coat on the [[Vesicle (biology and chemistry)|vesicle]]).<ref name=":3">{{Cite journal|last1=Nziengui|first1=H.|last2=Schoefs|first2=B.|date=2009-02-01|title=Functions of reticulons in plants: What we can learn from animals and yeasts|journal=Cellular and Molecular Life Sciences|language=en|volume=66|issue=4|pages=584–95|doi=10.1007/s00018-008-8373-y|pmid=18989623|s2cid=30563495|issn=1420-682X|pmc=11131481}}</ref> Reticulons may also be involved with apoptosis. The RTN1C inhibits [[Bcl-xL|Bcl-XL]], which is an inhibitor of [[apoptosis]]. RTN1C has also been shown in the [[Endoplasmic reticulum|ER]] to upregulate its sensitivity to stressors, which is able to modulate [[apoptosis]].<ref name="pmid18177508" />

Reticulons have also been linked with [[oligodendrocyte]] roles in inhibition of neurite outgrowth.<ref name="pmid18177508" /> The longest isoform of [[Reticulon 4|RTN4]] has been studied extensively to show that this protein (Nogo-A) was identified as an inhibitor of neurite outgrowth. More specifically, the 66-loop region (Nogo66) is a potent inhibitor of neurite growth.<ref name="pmid18177508" /> Many studies in animals have found that inhibition of a NogoA interaction has promoted [[axon]] growth and recovery after a [[spinal cord injury]].<ref name="pmid18177508" /> Subsequently, clinical trials of anti-Nogo [[Antibody|antibodies]] have begun to see if we can use this phenomenon in [[human]]s.<ref name="pmid18177508" />

There is increasing evidence that reticulons are involved with several different types of neurodegenerative diseases such as [[Alzheimer's disease]] and amyotrophic lateral sclerosis.<ref name="pmid18177508" /> In [[Alzheimer's disease|Alzheimer’s disease]], a specific [[enzyme]] produces a pathological agent. Reticulons can interfere with those enzymes by decreasing [[Amyloid beta|Aβ]] levels. It was found in the [[temporal lobe]]s of humans that [[RTN3|RTN3]] was depleted in Alzheimer’s patients.<ref name="pmid18177508" /> Nonetheless, the exact relationship between [[Alzheimer's disease|Alzheimer’s disease]] and reticulons is unknown. There may also be a link of reticulons to [[multiple sclerosis]] and [[hereditary spastic paraplegia]]. The serum of patients with [[multiple sclerosis]] contains [[Autoantibody|autoantibodies]] against the isoform A-specific region of [[Reticulon 4|RTN4]]. In the most common mutated protein in [[hereditary spastic paraplegia]], [[spastin]], there was an interaction with both [[RTN1]] and [[RTN3]] through two-hybrid screening.<ref name="pmid18177508" /> Lastly, reticulons can be associated with [[amyotrophic lateral sclerosis]] (ALS). In a mouse model, varied regulation of [[Reticulon 4|RTN4A]] was found. In muscle biopsies of rats, the levels of [[RTN4]] were related to the severity of the disease.<ref name="pmid18177508" /> Additionally, [[Amyotrophic lateral sclerosis|ALS]] could be predicted with increased expression of RTN4A in lower motor neuron syndromes.<ref name="pmid18177508" />

=== Reticulons in plants === Knowledge of the reticulon is more advanced in [[yeast]]s and [[animal]]s than [[plant]]s. Most of what we know for plants can be derived from research from the latter, with little research on plants alone. The localization of some RTNs has been recognized in the tubules of [[plant cell]]s forming the [[Endoplasmic reticulum|ER]].<ref name=":3" /> However, research shows that the reticulons are restricted to the edges of the [[Endoplasmic reticulum|ER]] cisternae.<ref name=":1" /> Scientists have inferred that reticulons have a role in assembling the nuclear envelope during [[cell division]]. Current research includes the search for Nogo-66 protein homologs in plants. There is also hope to determine the RHD domain receptor in plants.<ref name=":3" />

==== Reticulon-like proteins: ''Arabidopsis'' ==== Due to the lack of information of reticulons, scientists often study reticulon-like proteins. The genome ''[[Arabidopsis thaliana]]'' has at least 19 reticulon like proteins, and 15 of them have been explicitly identified.<ref name=":4">{{Cite journal|last1=Tolley|first1=Nicholas|last2=Sparkes|first2=Imogen A.|last3=Hunter|first3=Paul R.|last4=Craddock|first4=Christian P.|last5=Nuttall|first5=James|last6=Roberts|first6=Lynne M.|last7=Hawes|first7=Chris|last8=Pedrazzini|first8=Emanuela|last9=Frigerio|first9=Lorenzo|date=2008-01-01|title=Overexpression of a Plant Reticulon Remodels the Lumen of the Cortical Endoplasmic Reticulum but Does not Perturb Protein Transport|journal=Traffic|language=en|volume=9|issue=1|pages=94–102|doi=10.1111/j.1600-0854.2007.00670.x|pmid=17980018|issn=1600-0854|doi-access=free}}</ref> One study on ''[[Arabidopsis|Arabadopsis]]'' looks at transport between [[organelle]]s and specific receptors. The regulation of receptor transport to the [[plasma membrane]] is important for the recognition of [[pathogen]]s.<ref name=":2" /> Membrane associated proteins travel from the [[Endoplasmic reticulum|ER]] to the [[Golgi apparatus|Golgi bodies]], and eventually the plasma membrane. Immune receptors that are related to the plasma membrane are called [[pattern recognition receptor]]s (PRRs).<ref name=":2" /> Through ''[[Arabidopsis]]'' [[protein microarray]]s the FLAGELIN-SENSITIVE2 (FLS2) receptor, a [[Pattern recognition receptor|PRR]], was tagged to identify reticulon-like protein RTNLB1 and its homolog RTNLB2. When manipulating the expression levels of RTNLB1 and RTNLB2, signaling of the FLS2 receptor was interrupted. A serine cluster at the [[N-terminus|N-terminal]] of the protein is important for the FLS2 interaction.<ref name=":2" /> Although there is not a direct interference, RTNLB1 and RTNLB2 interact with newly created FLS2 to facilitate transport to the [[Cell membrane|plasma membrane.]] Through the RTNLB1 and RTNLB2 reticulon domain, their function is part of a larger protein system that moderates FLS2 secretion. Receptor trafficking is looked at through plant studies as an important process of receptor activity. The role of human reticulons which are involved in intracellular protein trafficking indicate the relationship between reticulons and plant RTNLBs.<ref name=":2" />

One way those [[protein]]s can be compared to reticulons is in looking at reticulon-depleted [[Yeast|yeast cells]]. [[Fluorescence spectroscopy|Fluorescence]] was detected in modified [[Endoplasmic reticulum|ER]] structures of those [[yeast cell]]s and the localization of the FLS2 was defected.<ref name=":2" />

In another study, members of the RTN family of the ''[[Arabidopsis thaliana]]'' (RTNLB13) were cloned. Those members were expressed in [[Tobacco|tobacco leaf]] epidermal cells with an attached [[yellow fluorescent protein]] (YFP).<ref name=":4" /> The RTNLB13 was localized in the [[Endoplasmic reticulum|ER]] of those cells. Additionally, an ER luminal marker was tagged to further show that when RTNLB13 was added, morphological changes existed in the lumen of the [[Endoplasmic reticulum|ER]]. [[Fluorescence recovery after photobleaching|Fluorescence recovery after photobleaching (FRAP)]] analysis has shown that increased expression of RTNLB13 decreases the likeliness of proteins to be soluble in the ER lumen.<ref name=":4" /> To further examine that location of RTNS are the [[Endoplasmic reticulum|ER]], the increased expression of RTNLB13 did not have an effect on the Golgi shape and secretion of a reporter protein.<ref name=":4" />

==References== {{reflist}}

==External links== * [http://genomebiology.com/2007/8/12/234/figure/F2 The structure and membrane topology of reticulons]

[[Category:Proteins]]