Sandbox Reserved 1174: Difference between revisions

<StructureSection load='4z34' size='340' side='right' caption=' LPA Receptor 1 ' scene='72/721545/Overall/2'>

The LPA₁ receptor consists of seven transmembrane alpha helices. It lies in the membrane as shown in Figure 2, and as shown by the <scene name='72/721545/Membrane/6'>fatty acid</scene> bound in the crystallization of LPA₁ in orange. Most <scene name='72/721545/Polarity/4'>polar amino acids</scene> (red) reside on the intracellular and extracellular areas of the receptor, while most residues positioned on the trans membrane helices inside the membrane are hydrophobic (blue). A cytochrome b (b₅₆₂RIL) protein was inserted into the third intracellular loop to facilitate crystallization (Figure 2).<ref name= "Chrencik"/> The intracellular region of this membrane protein is coupled to a [https://www.ebi.ac.uk/interpro/potm/2004_10/Page2.htm heterotrimeric G protein].

[[Image:LPA_in_membrane4.fw.png|200px|center|thumb|'''Figure 2:''' LPA receptor (blue) bound to the cell membrane. The binding pocket is highlighted in red. The added bRIL protein is highlighted in orange.]]

Three native <scene name='72/721545/Disulfides/5'>disulfide bonds</scene> in the extracellular region of this receptor provide fold stability.<ref name= "Chrencik"/> The first disulfide bond constrains the N terminal helix to extracellular loop(ECL) 2. The second disulfide bond shapes ECL2, and the third binds ECL3 to one of the transmembrane alpha helices. These disulfide bonds provide intramolecular stabilization along the extracellular region of the LPA₁ receptor, where the substrate enters into the binding pocket. The <scene name='72/721545/N-terminus/3'>N-terminus</scene> is a six turn alpha helix. It functions like a cap on the extracellular side of the protein, packing tightly against ECL1 and ECL2.<ref name= "Chrencik"/> The N-terminus helix also provides <scene name='72/721545/34_39_40/4'>polar amino acids</scene> that interact with the ligand when bound. The extracellular region of this receptor plays a role in substrate specificity.  

The ligand shown in this structure, ONO-9780307, has a similar structure to LPA, and was bound to LPA₁ for crystallization to visualize the binding pocket. <ref name= "Moolenaar" /> The <scene name='72/721545/Ligand/4'>binding pocket</scene> for LPA consists of both polar and nonpolar residues. <scene name='72/721545/All_polar_interactions/7'>Polar</scene> residues are located on the N terminus and within the binding pocket. A <scene name='72/721545/Hydrophobic_pocket/4'>hydrophobic pocket</scene> also interacts with the long acyl chain of LPA. The shape and polarity of the binding pocket makes it specific for molecules with a polar head and long hydrophobic tail shaped like LPA.

 

LPA is synthesized extracellularly and enters the binding pocket from the extracellular space near the N terminus, the exact location is not known.<ref name= "Chrencik"/> When LPA binds, the G protein bound to the intracellular region of LPA₁ is activated. This G protein then signals the cell, mainly for survival and proliferation.  

Lysophosphatidic Acid Receptors (LPA) are part of a larger family known as lysophospholipid receptor family ([http://jb.oxfordjournals.org/content/131/6/767.long EDG family]), including the archetype sphingosine-1-phosphate receptors (S1P₁). The only structure previously reported in this GPCR family was of S1P₁, and it provides a comparison for differential structure and function to LPA₁. <ref name= "Chrencik"/> A major difference was observed in ligand access between these two receptors.  The binding path in LPA₁ is located in the extracellular milieu, while in S1P₁ the ligand accesses the binding pocket through the membrane (Figure 3). The overall shape of each binding pocket is also different, as the S1P₁ binding pocket has more of an oval shape, whereas [[Image:LPA S1P.png|300px|left|thumb|'''Figure 3:''' Comparison of the binding pockets of LPA₁ and S1P₁ receptors.  The electron density (tan) of the binding pocket is shown around the ligand (purple). The limited binding sites of the receptors are shown in tan.]] the LPA₁ binding pocket has a more spherical shape (Figure 3). This is due to a change in three of the amino acids present for each receptor.  At position 129 LPA₁ has an aspartate and S1P₁ has a phenylalanine.  The second change is at position 210, LPA₁ has a tryptophan while S1P₁ has a cysteine.  The third change occurs at position 274, for LPA₁ there is a glycine and S1P₁ has a leucine <ref name= "Chrencik"/>.  The more spherical binding pocket for LPA₁ gives it the ability to recognize a larger group of chemical species.  In particular, LPA₁ has the ability to bind with ligands that have acyl chains of varying lengths <ref name= "Chrencik"/>.  Since LPA₁ binds with a variety of acyl chains, it can be used in multiple pathways.  

Many of the functions of LPA₁, i.e. cell proliferation, survival, and morphology, are implicated in cancers. LPA acts as a tumor mitogen and an inducer of tumor-derived cytokine to support the metastasis (spreading) of breast and ovarian cancer to bones. Inhibition of LPA₁ can significantly reduce this progression, and therefore may be a promising treatment for patients with bone metastasis. <ref name= "Boucharaba"> DOI: 10.1073/pnas.0600979103 </ref> LPA does not have an effect on primary tumor size. <ref name= "Jean > DOI: 10.1093/jnci/djs319 </ref>

When an injury occurs LPA is released in the body.  LPA will then activate G-protein-coupled receptors.  Within the nervous system, LPA plays a role in the nociceptive process (nociceptive pain is a sharp pain that can come from a mild burn or twisted ankle).  The LPA signaling will activate GTPase RhoA <ref name= "Inoue"> DOI:10.1038/nm1060 </ref>.  Once activated Rho translocates to the plasma membrane.  Rho will activate Rho kinase (ROCK) <ref name= "Inoue"/>.  The actiavtion of ROCK is a required step in the pathway in the stimulation of neurotic pain.  When ROCK was inhibited the remaining pathway no longer functioned normally.  Mice with the deletion of LPA₁ receptors had lower levels of pain <ref name= "Inoue"/>.   

Idiopathic pulmonary fibrosis (IPF) has high rates of mortality <ref name= "Tager"> PMID:18066075 </ref>.  Understanding how LPA can effect fibrosis, is an important factor to finding medication and a cure for this disease.  The pathway of LPA-LPA₁ is important in mediating fibroblast migration and [https://en.wikipedia.org/wiki/Wound_healing Wound Healing].  Once fibrosis has been contracted LPA levels increase in the bronchoalveolar lavage (BAL) fluid.  The study showed that mice lacking LPA₁ had protection from mortality and were able to survive fibrosis.  LPA₁ plays an active role between lung injury and contracting pulmonary fibrosis.  The absence of LPA results in a vascular leak after an initial injury, leading to fibrosis.  LPA₁ is a link between lung injury and [http://www.nature.com/nm/journal/v14/n1/fig_tab/nm1685_F4.html pulmonary fibrosis] <ref name= "Tager"/>.   

The endocannabinoid system regulates a variety of physiological processes including appetite, pain sensation, mood, and memory.<ref name= "Chrencik"/> Endocannabinoids, the natural ligands for cannabinoid receptors, are similar in structure to lysophosphatidic acid.<ref name= "Chrencik"/> Both the cannabinoid receptors and the LPA receptors have a preference for long unsaturated acyl chains.<ref name= "Chrencik"/> The polar amino acid <scene name='72/721545/210/1'>Trp210</scene> in the binding pocket of LPA₁ is unique to the lysophospholipid and cannabinoid receptors, suggesting that they are related.  

[[Image:2-AG.png|220px|right|thumb|'''Figure 4:''' 2-arachidonylglycerol (2-AG)]]