Regulation of autophagy by phosphatidylinositol 3-phosphate

Presenilin is necessary for efficient proteolysis through the autophagy-lysosome system in a γ-secretase-independent manner

Presenilins are ubiquitous, intramembrane proteins that function in Alzheimer's disease (AD) as the catalytic component of the γ-secretase complex. Familial AD mutations in presenilin are known to exacerbate lysosomal pathology. Hence, we sought to elucidate the function endogenous, wild-type presenilins play in autophagy-mediated protein degradation. We report the finding that genetic deletion or knockdown of presenilins alters many autophagy-related proteins demonstrating a buildup of autophagosomes, indicative of dysfunction in the system. Presenilin-deficient cells inefficiently clear long-lived proteins and fail to build up autophagosomes when challenged with lysosomal inhibitors. Our studies further show that γ-secretase inhibitors do not adversely impact autophagy, indicating that the role of presenilins in autophagy is independent of γ-secretase activity. Based on our findings, we conclude that endogenous, wild-type presenilins are necessary for proper protein degradation through the autophagosome-lysosome system by functioning at the lysosomal level. The role of presenilins in autophagy has many implications for its function in neurological diseases such as AD.

The myotubularin family of lipid phosphatases in disease and in spermatogenesis

The MTM (myotubularin)/MTMR (myotubularin-related) protein family is comprised of 15 lipid phosphatases, of which nine members are catalytically active. MTMs are known to play a fundamental role in human physiology as gene mutations can give rise to X-linked myotubular myopathy or Charcot-Marie-Tooth disease, which manifest in skeletal muscle or in peripheral neurons respectively. Interestingly, studies have shown MTMR2 and MTMR5, two MTM family members, to be highly expressed in the testis, particularly in Sertoli and germ cells, and knockout of either gene resulted in spermatogenic defects. Other studies have shown that MTMR2 functions in endocytosis and membrane trafficking. In the testis, MTMR2 interacts and co-localizes with c-Src/phospho-Src-(Tyr⁴¹⁶), a non-receptor protein tyrosine kinase that regulates the phosphorylation state of proteins at the apical ES (ectoplasmic specialization), a unique type of cell junction found between Sertoli cells and elongating/elongated spermatids. In the present review, we highlight recent findings that have made a significant impact on our understanding of this protein family in normal cell function and in disease, with the emphasis on the role of MTMs and MTMRs in spermatogenesis. We also describe a working model to explain how MTMR2 interacts with other proteins such as c-Src, dynamin 2, EPS8 (growth factor receptor pathway substrate 8) and ARP2/3 (actin-related protein 2/3) at the apical ES and the apical TBC (tubulobulbar complex; tubular-like invaginations that function in the disassembly of the apical ES and in the recycling of its components) to regulate spermiation at late stage VIII of the seminiferous epithelial cycle.

Biogenesis and cargo selectivity of autophagosomes

Autophagy is a major catabolic pathway in eukaryotes, which is required for the lysosomal/vacuolar degradation of cytoplasmic proteins and organelles. Interest in the autophagy pathway has recently gained momentum largely owing to identification of multiple autophagy-related genes and recognition of its involvement in various physiological conditions. Here we review current knowledge of the molecular mechanisms regulating autophagy in mammals and yeast, specifically the biogenesis of autophagosomes and the selectivity of their cargo recruitment. We discuss the different steps of autophagy, from the signal transduction events that regulate it to the completion of this pathway by fusion with the lysosome/vacuole. We also review research on the origin of the autophagic membrane, the molecular mechanism of autophagosome formation, and the roles of two ubiquitin-like protein families and other structural elements that are essential for this process. Finally, we discuss the various modes of autophagy and highlight their functional relevance for selective degradation of specific cargos.

The class IA phosphatidylinositol 3-kinase p110-beta subunit is a positive regulator of autophagy

p110-β associates with the Vps34–Vps15–Beclin 1–Atg14L complex and facilitates generation of PtdIns(3)P to promote autophagy.

Autophagy is an evolutionarily conserved cell renewal process that depends on phosphatidylinositol 3-phosphate (PtdIns(3)P). In metazoans, autophagy is inhibited by PtdIns(3,4,5)P₃, the product of class IA PI3Ks, which mediates the activation of the Akt–TOR kinase cascade. However, the precise function of class IA PI3Ks in autophagy remains undetermined. Class IA PI3Ks are heterodimeric proteins consisting of an 85-kD regulatory subunit and a 110-kD catalytic subunit. Here we show that the class IA p110-β catalytic subunit is a positive regulator of autophagy. Genetic deletion of p110-β results in impaired autophagy in mouse embryonic fibroblasts, liver, and heart. p110-β does not promote autophagy by affecting the Akt–TOR pathway. Rather, it associates with the autophagy-promoting Vps34–Vps15–Beclin 1–Atg14L complex and facilitates the generation of cellular PtdIns(3)P. Our results unveil a previously unknown function for p110-β as a positive regulator of autophagy in multicellular organisms.