Eukaryotic DNA damage checkpoint activation in response to double-strand breaks

Double-strand breaks (DSBs) are the most detrimental form of DNA damage. Failure to repair these cytotoxic lesions can result in genome rearrangements conducive to the development of many diseases, including cancer. The DNA damage response (DDR) ensures the rapid detection and repair of DSBs in order to maintain genome integrity. Central to the DDR are the DNA damage checkpoints. When activated by DNA damage, these sophisticated surveillance mechanisms induce transient cell cycle arrests, allowing sufficient time for DNA repair. Since the term “checkpoint” was coined over 20 years ago, our understanding of the molecular mechanisms governing the DNA damage checkpoint has advanced significantly. These pathways are highly conserved from yeast to humans. Thus, significant findings in yeast may be extrapolated to vertebrates, greatly facilitating the molecular dissection of these complex regulatory networks. This review focuses on the cellular response to DSBs in Saccharomyces cerevisiae, providing a comprehensive overview of how these signalling pathways function to orchestrate the cellular response to DNA damage and preserve genome stability in eukaryotic cells.     

TCR signaling requirements for activating T cells and for generating memory

Over the last two decades the molecular and cellular mechanisms underlying T cell activation, expansion, differentiation, and memory formation have been intensively investigated. These studies revealed that the generation of memory T cells is critically impacted by a number of factors, including the magnitude of the inflammatory response and cytokine production, the type of dendritic cell [DC] that presents the pathogen derived antigen, their maturation status, and the concomitant provision of costimulation. Nevertheless, the primary stimulus leading to T cell activation is generated through the T cell receptor [TCR] following its engagement with a peptide MHC ligand [pMHC]. The purpose of this review is to highlight classical and recent findings on how antigen recognition, the degree of TCR stimulation, and intracellular signal transduction pathways impact the formation of effector and memory T cells.     

Light-dependent phosphorylation of the carboxy tail of mouse melanopsin

Melanopsin-based phototransduction is involved in non-image forming light responses including circadian entrainment, pupil constriction, suppression of pineal melatonin synthesis, and direct photic regulation of sleep in vertebrates. Given that the functions of melanopsin involve the measurement and summation of total environmental luminance, there would appear to be no need for the rapid deactivation typical of other G-protein coupled receptors. In this study, however, we demonstrate that heterologously expressed mouse melanopsin is phosphorylated in a light-dependent manner, and that this phosphorylation is involved in regulating the rate of G-protein activation and the lifetime of melanopsin’s active state. Furthermore, we provide evidence for light-dependent phosphorylation of melanopsin in the mouse retina using an in situ proximity ligation assay. Finally, we demonstrate that melanopsin preferentially interacts with the GRK2/3 family of G-protein coupled receptor kinases through co-immunoprecipitation assays. Based on the complement of G-protein receptor kinases present in the melanopsin-expressing retinal ganglion cells, GRK2 emerges as the best candidate for melanopsin’s cognate GRK.     

Microautophagy: lesser-known self-eating

Microautophagy, the non-selective lysosomal degradative process, involves direct engulfment of cytoplasmic cargo at a boundary membrane by autophagic tubes, which mediate both invagination and vesicle scission into the lumen. With its constitutive characteristics, microautophagy of soluble substrates can be induced by nitrogen starvation or rapamycin via regulatory signaling complex pathways. The maintenance of organellar size, membrane homeostasis, and cell survival under nitrogen restriction are the main functions of microautophagy. In addition, microautophagy is coordinated with and complements macroautophagy, chaperone-mediated autophagy, and other self-eating pathways. Three forms of selective microautophagy, including micropexophagy, piecemeal microautophagy of the nucleus, and micromitophagy, share common ground with microautophagy to some degree. As the accumulation of experimental data, the precise mechanisms that govern microautophagy are becoming more appreciated. Here, we review the microautophagic molecular machinery, its physiological functions, and relevance to human diseases, especially in diseases involving multivesicular bodies and multivesicular lysosomes.     

Inflammation and repeated infections in CGD: two sides of a coin

Chronic granulomatous disease (CGD) is an uncommon congenital immunodeficiency seen approximately in 1 of 250,000 individuals. It is caused by a profound defect in a burst of oxygen consumption that normally accompanies phagocytosis in all myeloid cells (neutrophils, eosinophils, monocytes, and macrophages). This “respiratory burst” involves the catalytic conversion of molecular oxygen to the oxygen free-radical superoxide, which in turn gives rise to hydrogen peroxide, hypochlorous acid, and hydroxyl radicals. These oxygen derivatives play a critical role in the killing of pathogenic bacteria and fungi. As a result of the failure to activate the respiratory burst in their phagocytes, the majority of CGD patients suffer from severe recurrent infections and rather unexplained prolonged inflammatory reactions that may result in granulomatous lesions. Both may cause severe organ dysfunction depending on the tissues involved. Preventive measures as well as rapid (invasive) diagnostic procedures are required to successfully treat CGD. Hematopoietic stem cell transplantation may be a serious option in some of the patients.     

Familial hemophagocytic lymphohistiocytosis: a model for understanding the human machinery of cellular cytotoxicity

Cytotoxic T lymphocytes, natural killer cells, and NKT cells are effector cells able to kill infected cells. In some inherited human disorders, a defect in selected proteins involved in the cellular cytotoxicity mechanism results in specific clinical syndromes, grouped under the name of familial hemophagocytic lymphohistiocytosis. Recent advances in genetic studies of these patients has allowed the identification of different genetic subsets. Additional genetic immune deficiencies may also induce a similar clinical picture. International cooperation and prospective trials resulted in refining the diagnostic and therapeutic approach to these rare diseases with improved outcome but also with improved knowledge of the mechanisms underlying granule-mediated cellular cytotoxicity in humans.     

Metabolic syndrome as a risk factor for neurological disorders

The metabolic syndrome is a cluster of common pathologies: abdominal obesity linked to an excess of visceral fat, insulin resistance, dyslipidemia and hypertension. At the molecular level, metabolic syndrome is accompanied not only by dysregulation in the expression of adipokines (cytokines and chemokines), but also by alterations in levels of leptin, a peptide hormone released by white adipose tissue. These changes modulate immune response and inflammation that lead to alterations in the hypothalamic ‘bodyweight/appetite/satiety set point,’ resulting in the initiation and development of metabolic syndrome. Metabolic syndrome is a risk factor for neurological disorders such as stroke, depression and Alzheimer’s disease. The molecular mechanism underlying the mirror relationship between metabolic syndrome and neurological disorders is not fully understood. However, it is becoming increasingly evident that all cellular and biochemical alterations observed in metabolic syndrome like impairment of endothelial cell function, abnormality in essential fatty acid metabolism and alterations in lipid mediators along with abnormal insulin/leptin signaling may represent a pathological bridge between metabolic syndrome and neurological disorders such as stroke, Alzheimer’s disease and depression. The purpose of this review is not only to describe the involvement of brain in the pathogenesis of metabolic syndrome, but also to link the pathogenesis of metabolic syndrome with neurochemical changes in stroke, Alzheimer’s disease and depression to a wider audience of neuroscientists with the hope that this discussion will initiate more studies on the relationship between metabolic syndrome and neurological disorders.