No abstract

No abstract

The ability to withstand mitochondrial damage is especially critical for cells such as neurons that survive long-term. We report that cytochrome c (cyt c), a key trigger of apoptosis that is released upon mitochondrial permeabilization, is targeted for proteasome-mediated degradation in postmitotic neurons but not in normal proliferating cells. Importantly, an unbiased siRNA screen identifiedp53 associated Parkin-like cytoplasmic protein (PARC/CUL9) as an E3 ligase that targets cyt c for degradation. PARC/CUL9 levels were markedly elevated with neuronal differentiation and over expression of PARC/CUL9 was sufficient to promote cyt c degradation. Conversely, PARC/CUL9 deficiency made neurons more vulnerable to mitochondrial damage, compromising their ability to survive long-term. Degradation of cyt c by an identical mechanism was also seen in brain tumor cells, highlighting this as an important strategy engaged by neurons and cancer cells to ensure optimal long-term survival.

Vaso-occlusion is a hallmark of sickle cell disease. Agonist-induced activation of sickle red blood cells (SS RBCs) promotes their adhesion to vascular proteins, potentially contributing to vasoocclusion. Previously, we described a cyclic adenosine monophosphate (cAMP)-dependent increase in SS RBC adhesion to laminin. Here, we investigated whether Rap1, a small guanosine triphosphatase (GTPase) known to promote integrinmediated adhesion in other cells, was involved in this signaling pathway. We found that agonists known to induce cAMP signaling promoted the GTPbound, active state of Rap1 in SS RBCs.The cAMP-dependent exchange factor Epac (exchange protein directly activated by cAMP) is a likely upstream activator of Rap1, since Epac is present in these cells and the Epac-specific cAMP analog 8CPT-2-Me (8-(4-cholorophenylthio)-2-O-methyl-cAMP) activated Rap1 and promoted SS RBC adhesion to laminin. This 8CPT-2-Me-stimulated adhesion was integrin independent, since it was insensitive to RGD peptide or antibodies against the only known integrin on SS RBCs, ␣4␤1. IntroductionRecurrent, painful vaso-occlusive crises are a hallmark of sickle cell disease. A likely contributor to vasoocclusion is the propensity of sickle red blood cells (SS RBCs) to adhere to proteins in the vasculature. We recently determined that SS RBC adhesiveness is promoted by intracellular signaling events leading to receptormediated adhesion via either the integrin ␣4␤1 or the immunoglobulin (Ig) superfamily adhesion receptor basal cell adhesion molecule/ Lutheran (BCAM/LU). 1,2 However, the signaling pathways leading to adhesion of SS RBCs are not well understood. One signaling molecule that may be responsible for promoting adhesion in SS RBCs is the small guanosine triphosphatase (GTPase) receptorassociated protein 1 (Rap1).Rap1 is a close relative of Ras. There are 2 known isoforms of Rap1, Rap1a and Rap1b, which are 95% identical in amino acid sequence. 3 Like other small G proteins, Rap1 is active when GTPbound and inactive when guanosine diphosphate (GDP)-bound. Guanine nucleotide exchange factors (GEFs) promote the exchange of GDP for GTP, thus activating Rap1. GTPase-activating proteins (GAPs) activate the intrinsic GTPase activity of Rap1, resulting in the hydrolysis of bound GTP to GDP, thus inactivating Rap1. Several GEFs can activate Rap1 and are themselves activated by a diverse array of signaling pathways. One class of GEFs is exchange proteins activated by cAMP (Epac's), which activate Rap1 upon binding cyclic adenosine monophosphate (cAMP).Epac's are widely expressed and are emerging as an important class of cAMP effectors. 4 The downstream effectors of Rap1 are largely unknown. However, it has been demonstrated that Rap1 promotes the activation of integrin adhesion receptors, leading to cellular adhesion. Integrins known to be activated by Rap1 include ␣IIb␤3, ␣5␤1, ␣L␤2, ␣M␤2, and ␣4␤1. 3 A role for Rap1 in activating other classes of adhesion receptors is not well characterized.Rap1 is present in a number of cell types, incl...