The relationship between autism spectrum disorder and calcium signaling

Leena Nezamuldeen (Mentor: Dr. Karen Ross, Department of Biochemistry and Molecular & Cellular Biology, Georgetown University)

December 14th, 2016, 2:00pm, Room 341, Basic Science

Background:
Autism Spectrum Disorder ’ASD’ is a neurodevelopmental disorder characterized by impaired in social interaction, and communication, and repetitive behaviors. Syndromic autism is when autism is associated with well-known disorders such as Fragile X syndrome (caused by mutations in FMR1) or tuberous sclerosis (caused by mutations in TSC1 and TSC2). Recently, it has been shown that calcium signaling is impaired in several syndromic autism disorders due to depressed activity of ITPRs, which are a family of calcium channel receptors localized in the endoplasmic reticulum, suggesting that a post-translational modification is affecting the activity. Because ITPRs phosphorylation by several kinases is known to affect the calcium channel activity, we investigated, how mutations in TSC1, TSC2, and FMR could result in ITPR phosphorylation that could, in turn, lead to reduce the receptor activity.

Methods:
Information about ITPR phosphorylation sites and kinases, mentioned in published literatures, were gathered using the text mining tool RLIMS-P and the phosphorylation database, iPTMnet. If the phosphorylation sites for a kinase were not reported, we used a kinase-specific phosphorylation site prediction tool (GPS) to identify the predicted sites that these kinases will modify in ITPRs. We manually reviewed articles about ITPR phosphorylation to determine the effects of the phosphorylation on the channel activity. This information was combined to create an ITPR phosphorylation network using Cytoscape. Finally, we reviewed the literatures to evaluate whether a TSC2 mutation would affect any of these phosphorylation events, leading to decreased the calcium channel activity.

Results:
We found that many sites in ITPR are phosphorylated by PKA, PKG, PKC, AKT/PKB, FYN, LYN, PTK, cdc2/CyB “CDK”, PIK1, MAPK1/ERK2, mTORC1, CAMK2, ROCK. Phosphorylation of S118, S1577, S1767, S1774, S1799, S1846, S1912, S421, S704, S826, T1058, T1766, T1845, T1937, T2665, T276, T420, T795, T800, T955, Y2664, Y353, Y482 increases the calcium channel activity, phosphorylation of S150, S1568, S1709, S1782, S1838, S1839, S1840, S1843, S1847, S2090, S2633, S2670, S436, S930, S940, T150, T2306, T2380, T2434, T2675 decreases the activity, and S1133, S1160, S120, S1567, S1581, S1598, S1687, S1764, S1832, S1834, S1855, S2609, S2690, S916, S934, S937, T1572, T1856, T1882 were inconclusive. Two of the ITPR’s kinases, mTOR and Akt, are regulated by TSC2, but a loss of TSC2 would lead to changes in ITPR phosphorylation that would increase ITPR activity, not decrease it as was observed. We investigated other kinases, including PKA, PKC, and ERK2 are upstream of TSC2 in their respective signaling pathways, so the loss of TSC2 would not be expected to affect phosphorylation of ITPR by these kinases.

Conclusion:
None of the kinases in the phosphorylation network that we investigated are likely to be affected at the level of their activity by loss of TSC2 in a way that would lead to decreased ITPR activity. TSC1, TSC2, and FMR1 mutations are known to affect protein translation. A future work will be to investigate whether translation of any of the kinases that phosphorylate ITPR might be affected in syndromic autism disorders, leading to depressive ITPR activity.