Upregulation of select rab GTPases in cholinergic basal forebrain neurons in mild cognitive impairment and Alzheimer's disease.

Endocytic system dysfunction is one of the earliest disturbances that occur in Alzheimer's disease (AD), and may underlie the selective vulnerability of cholinergic basal forebrain (CBF) neurons during the progression of dementia. Herein we report that genes regulating early and late endosomes are selectively upregulated within CBF neurons in mild cognitive impairment (MCI) and AD. Specifically, upregulation of rab4, rab5, rab7, and rab27 was observed in CBF neurons microdissected from postmortem brains of individuals with MCI and AD compared to age-matched control subjects with no cognitive impairment (NCI). Upregulated expression of rab4, rab5, rab7, and rab27 correlated with antemortem measures of cognitive decline in individuals with MCI and AD. qPCR validated upregulation of these select rab GTPases within microdissected samples of the basal forebrain. Moreover, quantitative immunoblot analysis demonstrated upregulation of rab5 protein expression in the basal forebrain of subjects with MCI and AD. The elevation of rab4, rab5, and rab7 expression is consistent with our recent observations in CA1 pyramidal neurons in MCI and AD. These findings provide further support that endosomal pathology accelerates endocytosis and endosome recycling, which may promote aberrant endosomal signaling and neurodegeneration throughout the progression of AD.

Akt2 deficiency promotes cardiac induction of Rab4a and myocardial ß-adrenergic hypersensitivity.

Patients with diabetes mellitus can develop cardiac dysfunction in the absence of underlying coronary artery disease or hypertension; a condition defined as diabetic cardiomyopathy. Mice lacking the intracellular protein kinase Akt2 develop a syndrome that is similar to diabetes mellitus type 2. Expression profiling of akt2(-/-) myocardium revealed that Rab4a, a GTPase involved in glucose transporter 4 translocation and ß-adrenergic receptor (ßAR) recycling to the plasma membrane, was significantly induced. We therefore hypothesized that Akt2 deficiency increases myocardial ß-adrenergic sensitivity. Confirmatory analysis revealed up-regulation of Rab4a mRNA and protein in akt2(-/-) myocardium. In cultured cardiomyocyte experiments, Rab4a was induced by pharmacological inhibition of Akt as well as by specific knockdown of Akt2 with siRNA. Isolated akt2(-/-) hearts were hypersensitive to isoproterenol (ISO) but exhibited normal sensitivity to forskolin. Prolonged ISO treatment led to increased cardiac hypertrophy in akt2(-/-) mice compared to wild type mice. In addition, spontaneous hypertrophy was noted in aged akt2(-/-) hearts that was inhibited by treatment with the ßAR blocker propranolol. In agreement with previous results demonstrating increased fatty acid oxidation rates in akt2(-/-) myocardium, we found increased peroxisome proliferator-activated receptor α (PPARα) activity in the hearts of these animals. Interestingly, increased myocardial Rab4a expression was present in mice with cardiac-specific overexpression of PPARα and was also observed upon stimulation of PPARα activity in cultured cardiomyocytes. Accordingly, propranolol attenuated the development of cardiac hypertrophy in the PPARα transgenic mice as well. Our results indicate that reduced Akt2 leads to up-regulation of Rab4a expression in cardiomyocytes in a cell-autonomous fashion that may involve activation of PPARα. This maladaptive response is associated with hypersensitivity of akt2(-/-) myocardium to ß-adrenergic stimulation.

Intracellular localization and constitutive endocytosis of CXCR4 in human CD34+ hematopoietic progenitor cells.

CXCR4, the stromal cell-derived factor-1 receptor, plays an important role in the migration of hematopoietic progenitor/stem cells. The surface and cytoplasmic expression of CXCR4 on human hematopoietic CD34(+) cells was investigated. We show that its surface expression is low, whereas a large part of CXCR4 protein is sequestered intracellularly. Using confocal microscopy, we demonstrated that CXCR4 is colocalized with EEA-1, Rab5, Rab4, and Rab11, which are localized in early and recycling endosomes. No significant colocalization of CXCR4 with lysosomal markers CD63 and Lamp-1 was detected. Using antibody feeding experiments, we report a role for CXCR4 constitutive endocytosis in subcellular localization in stably transduced UT7-CXCR4-GFP and CD34(+) cells. Agonist-independent endocytosis of CXCR4 occurs through clathrin-coated vesicles. These data implicate a constitutive endocytosis in the regulation of CXCR4 membrane expression and suggest that constitutive endocytosis may be involved in the regulation of trafficking the human hematopoietic progenitor/stem cells to and in the bone marrow microenvironment.

Expression of a prenylation-deficient Rab4 interferes with propagation of insulin signaling through insulin receptor substrate-1.

Rab proteins are small GTP-binding proteins of the Ras superfamily that function in the regulation of vesicle transport processes. The Rab4 isoform has been implicated in insulin action. For instance, overexpression of a prenylation-deficient form of Rab4 has been shown to inhibit insulin-dependent GLUT4 translocation. Other steps affected by Rab4 in the cascade of events resulting from insulin receptor activation have not been elucidated. In the present studies, we measured effects on insulin-signaling proteins in 3T3-L1 adipocytes transiently expressing cytoplasmic forms of Rab4 and Rab5. Expression of a mutant Rab4 lacking a prenylation site resulted in reduced insulin-dependent phosphorylation ofcytoplasmic and internal membrane-associated insulin receptor substrate-1, leading to decreased insulin receptor substrate-1-associated phosphatidylinositol 3'-OH kinase activation and decreased Akt activation. These effects were not observed upon introduction of a similar mutant form of Rab5. These data indicate that Rab4 or a Rab4-associated protein is involved at one or more steps in propagating the insulin signal, in addition to any role it may play in the regulation of GLUT4 vesicle translocation. Our results support models of insulin signaling in which regulation of internal membrane trafficking plays a role in transduction of the insulin signal.