ER and aging-Protein folding and the ER stress response

Susceptibility to Calcium Dysregulation during Brain Aging

Calcium (Ca(2+)) is a highly versatile intracellular signaling molecule that is essential for regulating a variety of cellular and physiological processes ranging from fertilization to programmed cell death. Research has provided ample evidence that brain aging is associated with altered Ca(2+) homeostasis. Much of the work has focused on the hippocampus, a brain region critically involved in learning and memory, which is particularly susceptible to dysfunction during senescence. The current review takes a broader perspective, assessing age-related changes in Ca(2+) sources, Ca(2+) sequestration, and Ca(2+) binding proteins throughout the nervous system. The nature of altered Ca(2+) homeostasis is cell specific and may represent a deficit or a compensatory mechanism, producing complex patterns of impaired cellular function. Incorporating the knowledge of the complexity of age-related alterations in Ca(2+) homeostasis will positively shape the development of highly effective therapeutics to treat brain disorders.

Accelerated senescence in kidneys of low-birth-weight rats after catch-up growth

Epidemiological studies show a strong association between low birth weight and hypertension, renal, and cardiovascular disease, especially after catch-up growth. Senescence is an important contributor to the progression of chronic disease. Developmentally programmed premature senescence may be a link among low birth weight, catch-up growth, and adult disease. Low birth weight was induced by feeding pregnant rats a low-protein diet from day 12 of gestation to 10 days postdelivery. Low- and normal-birth-weight male offspring were weaned onto regular or high-calorie diets to enhance catch-up growth. Kidneys and hearts of offspring were analyzed for RNA and protein markers of stress-induced senescence (p16, p21, p53, Rb). Markers of mitochondrial stress (p66Shc) and activation of endoplasmic reticulum protein secretion (Ero1alpha) were analyzed as regulators of reactive oxygen species generation. Reactive oxygen species are known to be associated with premature aging. Senescence markers were not different in low- or normal-birth-weight kidneys at birth. During rapid catch-up growth, p16 and p21 increased significantly in low-birth-weight kidneys and hearts (P < 0.01). Renal p16 levels increased progressively and were significantly higher in low-birth-weight kidneys at 3 and 6 mo (P < or = 0.02). Renal p66Shc and Ero1alpha were significantly higher in low- compared with normal- birth-weight kidneys at 6 mo, suggesting reactive oxygen species generation (P < or = 0.03). Low-birth-weight rats exhibit accelerated senescence in kidneys and hearts after rapid catch-up growth, a likely important link between early growth and subsequent hypertension, renal, and cardiovascular disease.

Essential role of the unfolded protein response regulator GRP78/BiP in protection from neuronal apoptosis

Neurodegenerative diseases are often associated with dysfunction in protein quality control. The endoplasmic reticulum (ER), a key site for protein synthesis, senses stressful conditions by activating the unfolded protein response (UPR). Here we report the creation of a novel mouse model where GRP78/BiP, a major ER chaperone and master regulator of UPR, is specifically eliminated in the Purkinje cells (PCs). GRP78 depleted PCs activate UPR including induction of GRP94, PDI, CHOP and GADD34, feedback suppression of eIF2α phosphorylation and apoptotic cell death. In contrast to current models of protein misfolding where abnormal accumulation of ubiquitinated protein is prominent, cytosolic ubiquitin staining is dramatically reduced in GRP78 null PCs. Ultrastructural evaluation reveals that the ER shows prominent dilatation with focal accumulation of electron-dense material within the ER. The mice show retarded growth and severe motor coordination defect by week 5 and cerebellar atrophy by week 13. Our studies uncover a novel link between GRP78 depletion and reduction in cytosolic ubiquitination and establish a novel mouse model of accelerated cerebellar degeneration with basic and clinical applications.