No abstract

Memory consolidation is defined by the stabilization of a memory trace after acquisition, and consists of numerous molecular cascades that mediate synaptic plasticity. Commonly, a distinction is made between an early and a late consolidation phase, in which early refers to the first hours in which labile synaptic changes occur, whereas late consolidation relates to stable and long-lasting synaptic changes induced by de novo protein synthesis. How these phases are linked at a molecular level is not yet clear. Here we studied the interaction of the cyclic nucleotide-mediated pathways during the different phases of memory consolidation in rodents. In addition, the same pathways were studied in a model of neuronal plasticity, long-term potentiation (LTP). We demonstrated that cGMP/protein kinase G (PKG) signaling mediates early memory consolidation as well as early-phase LTP, whereas cAMP/protein kinase A (PKA) signaling mediates late consolidation and late-phase-like LTP. In addition, we show for the first time that early-phase cGMP/PKG signaling requires late-phase cAMP/PKA-signaling in both LTP and long-term memory formation.

Enhancement of central availability of the second messenger cAMP is a promising approach to improve cognitive function. Pharmacological inhibition of phosphodiesterase type 4 (PDE4), a group of cAMP hydrolyzing enzymes in the brain, has been shown to improve cognitive performances in rodents and monkeys. However, inhibition of PDE4 is generally associated with severe emetic side-effects. Roflumilast, an FDA-approved PDE4 inhibitor for treatment of chronic obstructive pulmonary disease (COPD), is yielding only mild emetic side effects. In the present study we investigate the potential of roflumilast as a cognition enhancer and to determine the potential coinciding emetic response in comparison to rolipram, a classic PDE4 inhibitor with pronounced emetic effects. Cognition enhancement was evaluated in mice and it was found that both roflumilast and rolipram enhanced memory in an object location task (0.03mg/kg), whereas only roflumilast was effective in a spatial Y-maze (0.1mg/kg). Emetic potential was measured using competition of PDE4 inhibition for α2-adrenergic receptor antagonism in which recovery from xylazine/ketamine-mediated anesthesia is used as a surrogate marker. While rolipram displayed emetic properties at a dose 10 times the memory-enhancing dose, roflumilast only showed increased emetic-like properties at a dose 100 times the memory-enhancing dose. Moreover, combining sub-efficacious doses of the approved cognition-enhancer donepezil and roflumilast, which did not improve memory when given alone, fully restored object recognition memory deficit in rats induced by the muscarinic receptor antagonist scopolamine. These findings suggest that roflumilast offers a more favorable window for treatment of cognitive deficits compared to rolipram.

The Goodpasture antigen-binding protein (GPBP) and its splice variant the ceramide transporter (CERT) are multifunctional proteins that have been found to play important roles in brain development and biology. However, the function of GPBP and CERT is controversial because of their involvement in two apparently unrelated research fields: GPBP was initially isolated as a protein associated with collagen IV in patients with the autoimmune disease Goodpasture syndrome. Subsequently, a splice variant lacking a serine-rich domain of 26 amino acids (GPBPD26) was found to mediate the cytosolic transport of ceramide and was therefore (re)named CERT. The two splice forms likely carry out different functions in specific sub-cellular localizations. Selective GPBP knockdown induces extensive apoptosis and tissue loss in the brain of zebrafish. GPBP/GPBPD26 knockout mice die as a result of structural and functional defects in endoplasmic reticulum and mitochondria. Because both mitochondria and ceramide play an important role in many biological events that regulate neuronal differentiation, cellular senescence, proliferation and cell death, we propose that GPBP and CERT are pivotal in neurodegenerative processes. In this review, we discuss the current state of knowledge on GPBP and CERT, including the molecular and biochemical characterization of GPBP in the field of autoimmunity as well as the fundamental research on CERT in ceramide transport, biosynthesis, localization, metabolism and cell homeostasis. Keywords: autoimmunity, ceramide, GPBP/CERT, neurodegenerative diseases. J. Neurochem. (2010) 113, 1369-1386. | 2010 | 113 | 1369-1386 doi: 10.1111/j.1471-4159.2010 (Raya et al. 1999). Goodpasture's syndrome is a strictly human disorder caused by antibodies directed against the non-collagenous domain of the a3-chain of collagen type IV [a3 (IV) NCI]. Collagen is a major component of the extracellular matrix. The autoantibodies cause rapid functional disruption of the basement membrane of lungs, kidneys and the choroid plexus (Salama et al. 2001). GPBP is a soluble extracellular protein which binds and phosphorylates the antigen in this syndrome. Its expression is increased in other spontaneous autoimmune disorders including Goodpasture's syndrome, lupus erythematosus, pemphigoid and lichen planus (Raya et al. 2000), suggesting that GPBP might be an important protein in autoimmune disorders, where autoantigens arise from abnormal protein domain organization. JOURNAL OF NEUROCHEMISTRYSubsequently, in 2003, Hanada and colleagues showed that a spliced variant of GPBP which lacks a serine-rich domain composed of 26 aa residues was responsible for the cytosolic trafficking of ceramide from the endoplasmic reticulum (ER), to the Golgi apparatus. Hence, this isoform was termed as CERT (Hanada et al. 2003). Since the longer isoform also has ceramide transport properties in vitro, GPBP was renamed as CERT L (Hanada et al. 2003).Thus far it is not clear how the extracellular function of GPBP/CERT L , which is related to immu...