Evidence for the involvement of SARA in neuronal polarization and cortical layer lamination

MESTRES, IVAN; ARIAS, CRISTINA; SUNG, CHING-HWA; CACERES, ALFREDO; CONDE, CECILIA

Congreso; 24th Biennial Meeting of the International Society for Neurochemistry and the American Society for Neurochemistry; 2013

SARA (Smad anchor for receptor activation) localizes to early endo­somes (EE) where it regu­lates their morphology and func­tion. We now show that in cultured rat hippo­campal pyramidal neu­rons SARA over-ex­pression generated unusual large EE and re­duced neurite outgrowth. By contrast, SARA suppre­ssion with short hairpin (sh) RNAi gave rise to a large rab11 positive recycling endo­some, as well as long neu­ronal pro­cesses and large growth cones. In situ electro­poration of mouse embryonic brains also sug­gests a pivotal role for SARA in neu­ronal de­velop­ment. Three days after electro­poration, control Hc-Red neu­rons started migration through­out the cortical plate showing pro­per pia-directed orientation. By contrast, neu­rons ex­pressing Hc-Red-sh-SARA failed to migrate as expected, acquiring a horizontal orientation. Quanti­tative analysis poin­ts that 90% of these neu­rons lack vertical orientation, being tilted with angles ranging from 0 to 45 degrees. A similar analysis at a later time poin­t after electro­poration (5 days) reveals that SARA-suppre­ssed neu­rons remained in deep cortical layers, and failed to reach super­ficial ones. 3D cell recon­structions show that while control neu­rons ex­tended appro­priately polarized leading and trailing edges, Hc-Red-sh-SARA ex­pressing cells exhibited a longer and curved leading pro­cess and lacked a typical trailing ex­tension; besides, many of them were completely inverted. Length mea­sure­ments of the longest neurite indicate that SARA-silenced neu­rons have a 1.9 fold longer leading pro­cess than those of control neu­rons. To­gether, our results sug­gest a major con­tri­bution of SARA to key aspects of neu­ronal de­velop­ment including migration, polarized growth and neurite formation. It is likely that some of these func­tions could be related with a role of SARA as a regu­lator of endo­somal trafficking. Supported by ANPCyT and CONICET to AC and CC.