Many forms of long-lasting behavioral and synaptic plasticity require the synthesis of new proteins. For example, long-term potentiation (LTP) that endures for more than an hour requires both transcription and translation. The signal-transduction mechanisms that couple synaptic events to protein translational machinery during long-lasting synaptic plasticity, however, are not well understood. One signaling pathway that is stimulated by growth factors and results in the translation of specific mRNAs includes the rapamycin-sensitive kinase mammalian target of rapamycin (mTOR, also known as FRAP and RAFT-1). Several components of this translational signaling pathway, including mTOR, eukaryotic initiation factor-4E-binding proteins 1 and 2, and eukaryotic initiation factor-4E, are present in the rat hippocampus as shown by Western blot analysis, and these proteins are detected in the cell bodies and dendrites in the hippocampal slices by immunostaining studies. In cultured hippocampal neurons, these proteins are present in dendrites and are often found near the presynaptic protein, synapsin I. At synaptic sites, their distribution completely overlaps with a postsynaptic protein, PSD-95. These observations suggest the postsynaptic localization of these proteins. Disruption of mTOR signaling by rapamycin results in a reduction of late-phase LTP expression induced by high-frequency stimulation; the early phase of LTP is unaffected. Rapamycin also blocks the synaptic potentiation induced by brain-derived neurotrophic factor in hippocampal slices. These results demonstrate an essential role for rapamycin-sensitive signaling in the expression of two forms of synaptic plasticity that require new protein synthesis. The localization of this translational signaling pathway at postsynaptic sites may provide a mechanism that controls local protein synthesis at potentiated synapses.

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BMP7 and activin are members of the transforming growth factor ␤ superfamily. Here we characterize endogenous activin and BMP7 signaling pathways in P19 embryonic carcinoma cells. We show that BMP7 and activin bind to the same type II receptors, ActRII and IIB, but recruit distinct type I receptors into heteromeric receptor complexes. The major BMP7 type I receptor observed was ALK2, while activin bound exclusively to ALK4 (ActRIB). BMP7 and activin elicited distinct biological responses and activated different Smad pathways. BMP7 stimulated phosphorylation of endogenous Smad1 and 5, formation of complexes with Smad4 and induced the promoter for the homeobox gene, Tlx2. In contrast, activin induced phosphorylation of Smad2, association with Smad4, and induction of the activin response element from the Xenopus Mix.2 gene. Biochemical analysis revealed that constitutively active ALK2 associated with and phosphorylated Smad1 on the COOH-terminal SSXS motif, and also regulated Smad5 and Smad8 phosphorylation. Activated ALK2 also induced the Tlx2 promoter in the absence of BMP7. Furthermore, we show that ALK1 (TSRI), an orphan receptor that is closely related to ALK2 also mediates Smad1 signaling. Thus, ALK1 and ALK2 induce Smad1-dependent pathways and ALK2 functions to mediate BMP7 but not activin signaling.Recent studies have advanced significantly our understanding of how TGF␤ 1 superfamily members mediate their biological effects. The discovery of TGF␤ receptors and Smad proteins along with recent insights into the mechanism of their activation have allowed us to trace a TGF␤ signal transduction pathway from the cell membrane to the nucleus (reviewed in Refs. 1 and 2). TGF␤ family members initiate signaling at the cell surface by binding and bringing together two different but related serine/threonine kinase receptors, type I and type II.First, the ligand binds to the type II receptor, which recruits and transphosphorylates the type I receptor on the GS domain, a region in the juxtamembrane domain that is rich in serine and glycine residues (3). A mutation in the GS domain can lead to constitutive activation of the receptor (4) and such an activated receptor mimics the effects of the entire receptor-ligand complex in the absence of growth factor and the type II receptor. Thus, the type I receptor is considered the primary transducer of signals to downstream components of the pathway. So far, seven type I receptors or activin receptor-like kinases (ALK1-7) have been identified in vertebrates (reviewed in Refs. 2 and 5).Once the type I receptor is activated, it associates with specific receptor-regulated Smad proteins and phosphorylates them on the last two serine residues on the carboxyl-terminal domain (6 -9). Smad proteins are essential components of TGF␤ signaling that link ligand/receptor signals to transcriptional control (10 -16). All members possess two highly conserved MAD homology domains in the amino (MH1) and carboxyl (MH2) terminus that are connected by a proline-rich nonconserved region (reviewed in R...

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RNAs are present in dendrites and may be used for local protein synthesis in response to synaptic activity. To begin to understand dendritic RNA targeting, we cloned a rat homolog of staufen, a Drosophila gene that participates in mRNA targeting during development. In hippocampal neurons, rat staufen protein displays a microtubule-dependent somatodendritic distribution pattern that overlaps with dendritic RNAs. To determine whether r-staufen is required for dendritic RNA targeting, we constructed a mutant version containing the RNA binding domains (stau-RBD) but lacking the C-terminal portion potentially involved in dendritic targeting. Stau-RBD expression was restricted to the cell bodies and proximal dendrites. Expression of stau-RBD significantly decreased, while overexpression of wild-type r-staufen increased, the amount of dendritic mRNA. Taken together, these results suggest that the rat staufen protein plays an important role in the delivery of RNA to dendrites.

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Studies with animal models have suggested that reactivation of glia, including microglia and astrocytes, critically contributes to the development and maintenance of chronic pain. However, the involvement of glial reactivation in human chronic pain is unclear. We performed analyses to compare the glial reactivation profiles in the spinal dorsal horn (SDH) from three cohorts of sex- and age-matched human postmortem tissues: (i) HIV-negative patients, (ii) HIV-positive patients without chronic pain, and (iii) HIV patients with chronic pain. Our results indicate that the expression levels of CD11b and Iba1, commonly used for labeling microglial cells, did not differ in the three patient groups. On the other hand, GFAP and S100β, often used for labeling astrocytes, were specifically up-regulated in the spinal dorsal horn (SDH) of the ‘pain-positive’ HIV patients but not in the ‘pain-negative’ HIV patients. In addition, pro-inflammatory cytokines, TNFα and IL-1β, were specifically increased in the SDH of ‘pain-positive’ HIV patients. Our findings suggest that reactivation of astrocytes in the SDH may play a role during the maintenance phase of HIV-associated chronic pain.

Objective Chronic pain is a common neurological comorbidity of HIV-1 infection, but the etiological cause remains elusive. The objective of this study was to identify the HIV-1 causal factor that critically contributes to the pathogenesis of HIV-associated pain. Methods We first compared the levels of HIV-1 proteins in postmortem tissues of the spinal cord dorsal horn (SDH) from HIV-1/AIDS patients who developed chronic pain (‘pain-positive’ HIV-1 patients) and HIV-1 patients who did not develop chronic pain (‘pain-negative’ HIV-1 patients). Then, we used the HIV-1 protein that was specifically increased in the ‘pain-positive’ patients to generate mouse models. Finally, we performed comparative analyses on the pathological changes in the models and the HIV-1 patients. Results We found that HIV-1 gp120 was significantly higher in ‘pain-positive’ HIV-1 patients (vs. ‘pain-negative’ HIV-1 patients). This finding suggested that gp120 was a potential causal factor of the HIV-associated pain. To test this hypothesis, we used a mouse model generated by intrathecal injection (i.t.) of gp120 and compared the pathologies of the model and the ‘pain-positive’ human HIV-1 patients. The results showed that the mouse model and ‘pain-positive’ human HIV-1 patients developed extensive similarities in their pathological phenotypes, including pain behaviors, peripheral neuropathy, glial reactivation, synapse degeneration and aberrant activation of pain-related signaling pathways in the SDH. Interpretation Our findings suggest that gp120 may critically contribute to the pathogenesis of HIV-associated pain.