Eaten alive: a history of macroautophagy

Mechanisms of mitophagy

Autophagy not only recycles intracellular components to compensate for nutrient deprivation but also selectively eliminates organelles to regulate their number and maintain quality control. Mitophagy, the specific autophagic elimination of mitochondria, has been identified in yeast, mediated by autophagy-related 32 (Atg32), and in mammals during red blood cell differentiation, mediated by NIP3-like protein X (NIX; also known as BNIP3L). Moreover, mitophagy is regulated in many metazoan cell types by parkin and PTEN-induced putative kinase protein 1 (PINK1), and mutations in the genes encoding these proteins have been linked to forms of Parkinson's disease.

Gain and loss of T cell subsets in old age--age-related reshaping of the T cell repertoire

The immune system is affected by the aging process and undergoes significant age-related changes, termed immunosenescence. Different T cell subsets are affected by this process. Alterations within the bone marrow and thymus lead to a shift in the composition of the T cell repertoire from naïve to antigen-experienced T cells, thereby compromising the diversity of the T cell pool. Additional infection with latent pathogens such as cytomegalovirus aggravates this process. In this review, we focus on the major age-related changes that occur in the naïve and the antigen-experienced T cell population. We discuss the mechanisms responsible for the generation and maintenance of these subsets and how age-related changes can be delayed or prevented by clinical interventions.

Autophagy as a therapeutic target in cancer

Autophagy is a self-catabolic process that maintains intracellular homeostasis and prolongs cell survival under stress via lysosomal degradation of cytoplasmic constituents and recycling of amino acids and energy. Autophagy is intricately involved in many aspects of human health and disease, including cancer. Autophagy is a double-edged sword in tumorigenesis, acting both as a tumor suppressor and a protector of cancer cell survival, and elucidation of its exact role at different stages of cancer progression and in treatment responsiveness is a complex and challenging task. Better understanding of autophagy regulation and its impact on treatment outcome will potentially allow us to identify novel therapeutic targets in cancer. In this review, we summarize current knowledge on the regulation and dual function of autophagy in tumorigenesis, as well as ongoing efforts in modulating autophagy for cancer treatment and prevention. This is a very exciting and highly promising area of cancer research, as pharmacologic modulation of autophagy appears to augment the efficacy of currently available anticancer regimens and opens the way to the development of new combinatorial therapeutic strategies that will hopefully contribute to cancer eradication.

Leukocyte-cancer cell fusion: initiator of the warburg effect in malignancy?

The causes of metastasis remain unknown, however it has been proposed for nearly a century that metastatic cells are generated by fusion of tumor cells with tumor-associated leukocytes such as macrophages. Indeed, regardless of cell or tissue origin, when cancer cells in the original in situ tumor transform to malignant, invasive cells, they generally become aneuploid and begin to express molecules and traits characteristic of activated macrophages. This includes two key features of malignancy: chemotactic motility and the use of aerobic glycolysis as a metabolic energy source (the Warburg effect). Here we review evidence that these phenomena can be well-explained by macrophage-cancer cell fusion, as evidenced by studies of experimental macrophage-melanoma hybrids generated in vitro and spontaneous host-tumor hybrids in animals and more recently humans. A key finding to emerge is that experimental and spontaneous cancer cell hybrids alike displayed a high degree of constitutive autophagy, a macrophage trait that is expressed under hypoxia and nutrient deprivation as part of the Warburg effect. Subsequent surveys of 21 different human cancers from nearly 2,000 cases recently revealed that the vast majority (~85%) exhibited autophagy and that this was associated with tumor proliferation and metastasis. While much work needs to be done, we posit that these findings with human cancers could be a reflection of widespread leukocyte-cancer cell fusion as an initiator of metastasis. Such fusions would generate hybrids that express the macrophage capabilities for motility and survival under adverse conditions of hypoxia and nutrient deprivation, while at the same time maintaining the deregulated mitotic cycle of the cancer cell fusion partner.

What can plant autophagy do for an innate immune response?

Autophagy plays an established role in the execution of senescence, starvation, and stress responses in plants. More recently, an emerging role for autophagy has been discovered during the plant innate immune response. Recent papers have shown autophagy to restrict, and conversely, to also promote programmed cell death (PCD) at the site of pathogen infection. These initial studies have piqued our excitement, but they have also revealed gaps in our understanding of plant autophagy regulation, in our ability to monitor autophagy in plant cells, and in our ability to manipulate autophagic activity. In this review, we present the most pressing questions now facing the field of plant autophagy in general, with specific focus on autophagy as it occurs during a plant-pathogen interaction. To begin to answer these questions, we place recent findings in the context of studies of autophagy and immunity in other systems, and in the context of the mammalian immune response in particular.

Culling sick mitochondria from the herd

The PINK1-Parkin pathway plays a critical role in mitochondrial quality control by selectively targeting damaged mitochondria for autophagy. In this issue, Tanaka et al. (2010. J. Cell Biol. doi: 10.1083/jcb.201007013) demonstrate that the AAA-type ATPase p97 acts downstream of PINK1 and Parkin to segregate fusion-incompetent mitochondria for turnover. p97 acts by targeting the mitochondrial fusion-promoting factor mitofusin for degradation through an endoplasmic reticulum-associated degradation (ERAD)-like mechanism.

Mitophagy: the latest problem for Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder of unknown cause. Some familial forms of PD are provoked by mutations in the genes encoding for the PTEN (phosphatase and tensin homolog)-induced putative kinase-1 (PINK1) and Parkin. Mounting evidence indicates that PINK1 and Parkin might function in concert to modulate mitochondrial degradation, termed mitophagy. However, the molecular mechanisms by which PINK1/Parkin affect mitophagy are just beginning to be elucidated. Herein, we review the main advances in our understanding of the PINK1/Parkin pathway. Because of the phenotypic similarities among the different forms of PD, a better understanding of PINK1/Parkin biology might have far-reaching pathogenic and therapeutic implications for both the inherited and the sporadic forms of PD.