No abstract

Acute and chronic stress differentially regulate immediate-early gene (IEG) expression in the brain. Although acute stress induces c-Fos and FosB, repeated exposure to stress desensitizes the c-Fos response, but FosB-like immunoreactivity remains high. Several other treatments differentially regulate IEG expression in a similar manner after acute versus chronic exposure. The form of FosB that persists after these chronic treatments has been identified as ⌬FosB, a splice variant of the fosB gene. This study was designed to determine whether the FosB form induced after chronic stress is also ⌬FosB and to map the brain regions and identify the cell populations that exhibit this effect. Western blotting, using an antibody that recognizes all Fos family members, revealed that acute restraint stress caused robust induction of c-Fos and full-length FosB, as well as a small induction of ⌬FosB, in the frontal cortex (fCTX) and nucleus accumbens (NAc). The induction of c-Fos (and to some extent full-length FosB) was desensitized after 10 d of restraint stress, at which point levels of ⌬FosB were high. A similar pattern was observed after chronic unpredictable stress. By use of immunohistochemistry, we found that chronic restraint stress induced ⌬FosB expression predominantly in the fCTX, NAc, and basolateral amygdala, with lower levels of induction seen elsewhere. These findings establish that chronic stress induces ⌬FosB in several discrete regions of the brain. Such induction could contribute to the long-term effects of stress on the brain.

␤-Amyloid peptide (A␤)1 is the essential component of senile plaques (1, 2), which are the major pathological hallmark of Alzheimer's disease (AD) (3). A␤ is derived from proteolytic processing of a ubiquitous transmembrane protein termed ␤-amyloid precursor protein (APP) (4). Although the factors governing production and deposition of A␤ are not fully understood, it has been shown that APP can undergo at least two post-translational processing pathways (3). In a nonamyloidogenic pathway APP is cleaved within the A␤ region by a proteinase activity known as ␣-secretase. This prevents generation of A␤ and gives rise to a soluble form of APP (sAPP␣), which is found in the extracellular milieu and constitutes a marker for nonamyloidogenic processing. Alternatively, APP can undergo proteolytic cleavage by ␤-and ␥-secretases to generate A␤. This is referred to as the amyloidogenic pathway of APP processing, which can take place while internalized cell surface APP trafficks through the endocytic pathway (5).Previous studies have demonstrated that the low density lipoprotein receptor-related protein (LRP), a member of the low density lipoprotein (LDL) receptor family, binds and mediates the endocytosis of soluble (6) as well as cell surface APP (7) isoforms containing a Kunitz proteinase inhibitor (KPI) domain. LRP is a large, multifunctional endocytic receptor abundantly expressed in liver and brain that mediates the hepatic uptake of circulating chylomicron remnants (8), serpin-enzyme complexes (9), and proteinases of the fibrinolytic pathway (10, 11). Furthermore, LRP has been shown to regulate the cell surface levels of two receptors, the urokinase receptor (12) and tissue factor (13). LRP is also expressed in fibroblasts, macrophages, smooth muscle cells, neurons, and activated but not resting glial cells (14), suggesting that this receptor is involved in the binding and removal of interstitial ligands (e.g. proteinases and lipoproteins) produced by these cells. Targeted deletion of the LRP gene in mice leads to death of the embryo at day 13.5 (15), demonstrating that LRP plays a critical role during development. A 39-kDa receptor-associated protein (RAP) (16) binds reversibly to LRP and other members of the LDL receptor family such as gp330/megalin (17) and the very low density lipoprotein receptor (18) and inhibits ligand binding (19,20). RAP is found primarily in the endoplasmic reticulum, where it is thought to function as a molecular chaperone by assisting in receptor folding and processing and by preventing the association of newly synthesized receptors with endogenous ligands (21,22). Because of its high affinity for LRP and its ability to antagonize ligand binding, RAP constitutes a powerful tool to study LRP-mediated mechanisms.The objective of the present investigation was to test the hypothesis that LRP plays a role in the pathobiology of AD by facilitating delivery of cell surface APP to endosomal compartments, where the A␤ peptide can be generated. To this end, we employed two main experimental strateg...

The low density lipoprotein receptor-related protein (LRP) 1 is a large endocytic receptor containing a 515-kDa heavy chain to which ligands bind and a non-covalently associated 85-kDa light chain containing a transmembrane and cytoplasmic domain (for review see Ref. 1). LRP is one of 12 or more receptors that make up the LDL receptor superfamily and is essential for embryonic development in mice (2). A remarkable feature of LRP is its ability to bind and mediate the internalization of a diverse array of ligands, including proteinases (3, 4), proteinase-inhibitor complexes (5, 6), and lipoproteins (7). After binding to the LRP, the ligands are transported into endosomes where they uncouple in the reduced pH environment and are sorted to lysosomes for degradation. LRP recycles back to the cell surface where it is once again available to bind ligands.Recent studies indicate that in addition to their cargo transport function, certain LDL receptor family members also participate in signaling pathways. For example, the very low density lipoprotein receptor and apoE receptor 2 both participate in a signal transduction pathways mediated by reelin (8 -10). Reelin is secreted by Cajal-Retzius cell in the outermost layer of the cerebral cortex and controls the final position of neurons that migrate from the ventricular zone. Binding of reelin to either the very low density lipoprotein receptor or apoE receptor 2 induces tyrosine phosphorylation of disabled-1 (Dab1) (9, 10), an adaptor protein that interacts with the cytoplasmic domains of LDL receptor family members (11, 12) and functions in tyrosine kinase signaling pathways.In the case of LRP, accumulating evidence suggests a prominent but undefined role for this receptor in regulating cell physiology by facilitating signal transduction pathways. For example, LRP has been implicated as a component of the receptor complex for midkine (13), a heparin binding growth factor with migration-promoting and survival-promoting activities. Another LRP ligand, tissue type plasminogen activator, promotes late phase long term potentiation (14), and this activity appears to require its association with LRP (15). Finally, the binding of activated ␣ 2 M (␣ 2 M*) to LRP mediates calcium influx in neurons in a process that also involves N-methyl-D-

The transcription factor deltaFosB is induced in the nucleus accumbens and dorsal striatum by chronic exposure to several drugs of abuse, and increasing evidence supports the possibility that this induction is involved in the addiction process. However, to date there has been no report of deltaFosB induction by drugs of abuse in the ventral tegmental area (VTA), which is also a critical brain reward region. In the present study, we used immunohistochemistry to demonstrate that chronic forced administration of cocaine induces deltaFosB in the rat VTA. This induction occurs selectively in a gamma-aminobutyric acid (GABA) cell population within the posterior tail of the VTA. A similar effect is seen after chronic cocaine self-administration. Induction of deltaFosB in the VTA occurs after psychostimulant treatment only: it is seen with both chronic cocaine and amphetamine, but not with chronic opiates or stress. The expression of deltaFosB appears to be mediated by dopamine systems, as repeated administration of a dopamine uptake inhibitor induced deltaFosB in the VTA, while administration of serotonin or norepinephrine uptake inhibitors failed to produce this effect. Time course analysis showed that, following 14 days of cocaine administration, deltaFosB persists in the VTA for almost 2 weeks after cocaine withdrawal. This accumulation and persistence may account for some of the long-lasting changes in the brain associated with chronic drug use. These results provide the first evidence of deltaFosB induction in a discrete population of GABA cells in the VTA, which may regulate the functioning of the brain's reward mechanisms.