Sortilin-mediated endocytosis determines levels of the frontotemporal dementia protein, progranulin

VIDEO ABSTRACT

The most common inherited form of Frontotemporal Lobar Degeneration (FTLD) known stems from Progranulin (GRN) mutation and exhibits TDP-43 plus ubiquitin aggregates. Despite the causative role of GRN haploinsufficiency in FTLD-TDP, the neurobiology of this secreted glycoprotein is unclear. Here, we examined PGRN binding to the cell surface. PGRN binds to cortical neurons via its C terminus, and unbiased expression cloning identifies Sortilin (Sort1) as a binding site. Sort1⁻/⁻ neurons exhibit reduced PGRN binding. In the CNS, Sortilin is expressed by neurons and PGRN is most strongly expressed by activated microglial cells after injury. Sortilin rapidly endocytoses and delivers PGRN to lysosomes. Mice lacking Sortilin have elevations in brain and serum PGRN levels of 2.5- to 5-fold. The 50% PGRN decrease causative in FTLD-TDP cases is mimicked in GRN+/⁻ mice, and is fully normalized by Sort1 ablation. Sortilin-mediated PGRN endocytosis is likely to play a central role in FTLD-TDP pathophysiology.

Identification of likely orthologs of tobacco salicylic acid-binding protein 2 and their role in systemic acquired resistance in Arabidopsis thaliana

Salicylic acid-binding protein 2 (SABP2) is essential for the establishment of systemic acquired resistance (SAR) in tobacco; SABP2's methyl salicylate (MeSA) esterase activity is required in healthy systemic tissues of infected plants to release the active defense phytohormone SA from MeSA, which serves as a long-distance signal for SAR. In the current study, we characterize a new gene family from Arabidopsis thaliana encoding 18 potentially active alpha/beta fold hydrolases that share 32-57% identity with SABP2. Of 14 recombinant AtMES (MES for methyl esterase) proteins tested, five showed preference for MeSA as a substrate and displayed SA inhibition of MeSA esterase activity in vitro (AtMES1, -2, -4, -7, and -9). The two genes encoding MeSA esterases with the greatest activity, AtMES1 and -9, as well as AtMES7 were transcriptionally upregulated during infection of Arabidopsis with avirulent Pseudomonas syringae. In addition, conditional expression of AtMES1, -7, or -9 complemented SAR deficiency in SABP2-silenced tobacco, suggesting that these three members of the AtMES family are SABP2 functional homologs (orthologs). Underexpression by knockout mutation and/or RNAi-mediated silencing of multiple AtMES genes, including AtMES1, -2, -7, and -9, compromised SAR in Arabidopsis and correlated with enhanced accumulation of MeSA in the systemic tissue of SAR-induced plants. Together, the data show that several members of the AtMES gene family are functionally homologous to SABP2 and redundant for MeSA hydrolysis and probably SAR. These data suggest that MeSA is a conserved SAR signal in Arabidopsis and tobacco.