Synaptic plasticity is the experience-dependent change in connectivity between neurons that is believed to underlie learning and memory. Here, we discuss the cellular and molecular processes that are altered when a neuron responds to external stimuli, and how these alterations lead to an increase or decrease in synaptic connectivity. Modification of synaptic components and changes in gene expression are necessary for many forms of plasticity. We focus on excitatory neurons in the mammalian hippocampus, one of the best-studied model systems of learning-related plasticity.

Mutations of the retinoblastoma tumor suppressor gene RB are frequently observed in human cancers, but rarely in non-small cell lung carcinomas (NSCLCs). Emerging evidence also suggests that the RB-related gene p130 is inactivated in a subset of human NSCLCs. To directly test the specific tumor suppressor roles of RB and p130 in NSCLC, we crossed Rb and p130 conditional mutant mice to mice carrying a conditional oncogenic K-Ras allele. In this model, controlled oncogenic K-Ras activation leads to the development of adenocarcinoma, a major subtype of NSCLC. We found that loss of p130 accelerated the death of mice, providing direct evidence in vivo that p130 is a tumor suppressor gene, albeit a weak one in this context. Loss of Rb increased the efficiency of lung cancer initiation and resulted in the development of high-grade adenocarcinomas and rapid death. Thus, despite the low frequency of RB mutations in human NSCLCs and reports that K-Ras activation and loss of RB function are rarely found in the same human tumors, loss of Rb clearly cooperates with activation of oncogenic K-Ras in lung adenocarcinoma development in mice.

AMPA-type glutamate receptors mediate fast, excitatory neurotransmission in the brain, and their concentrations at synapses are important determinants of synaptic strength. We investigated the post-transcriptional regulation of GluA2, the calcium-impermeable AMPA receptor subunit, by examining the subcellular distribution of its mRNA and evaluating its translational regulation by microRNA in cultured mouse hippocampal neurons. Using computational approaches, we identified a conserved microRNA-124 (miR-124) binding site in the 3'UTR of GluA2 and demonstrated that miR-124 regulated the translation of GluA2 mRNA reporters in a sequence-specific manner in luciferase assays. While we hypothesized that this regulation might occur in dendrites, our biochemical and fluorescent in situ hybridization (FISH) data indicate that GluA2 mRNA does not localize to dendrites or synapses of mouse hippocampal neurons. In contrast, we detected significant concentrations of miR-124 in dendrites. Overexpression of miR-124 in dissociated neurons results in a 30% knockdown of GluA2 protein, as measured by immunoblot and quantitative immunocytochemistry, without producing any changes in GluA2 mRNA concentrations. While total GluA2 concentrations are reduced, we did not detect any changes in the concentration of synaptic GluA2. We conclude from these results that miR-124 interacts with GluA2 mRNA in the cell body to downregulate translation. Our data support a model in which GluA2 is translated in the cell body and subsequently transported to neuronal dendrites and synapses, and suggest that synaptic GluA2 concentrations are modified primarily by regulated protein trafficking rather than by regulated local translation.

Long-lasting forms of synaptic plasticity that underlie learning and memory require new transcription and translation for their persistence. The remarkable polarity and compartmentalization of neurons raises questions about the spatial and temporal regulation of gene expression within neurons. Alternative cleavage and polyadenylation (APA) generates mRNA isoforms with different 3′ untranslated regions (3′UTRs) and/or coding sequences. Changes in the 3′UTR composition of mRNAs can alter gene expression by regulating transcript localization, stability and/or translation, while changes in the coding sequences lead to mRNAs encoding distinct proteins. Using specialized 3′ end deep sequencing methods, we undertook a comprehensive analysis of APA following induction of long-term potentiation (LTP) of mouse hippocampal CA3-CA1 synapses. We identified extensive LTP-induced APA changes, including a general trend of 3′UTR shortening and activation of intronic APA isoforms. Comparison with transcriptome profiling indicated that most APA regulatory events were uncoupled from changes in transcript abundance. We further show that specific APA regulatory events can impact expression of two molecules with known functions during LTP, including 3′UTR APA of Notch1 and intronic APA of Creb1. Together, our results reveal that activity-dependent APA provides an important layer of gene regulation during learning and memory.

In cancer cells, the retinoblastoma tumor suppressor RB is directly inactivated by mutation in the RB gene or functionally inhibited by abnormal activation of cyclin-dependent kinase activity. While variations in RB levels may also provide an important means of controlling RB function in both normal and cancer cells, little is known about the mechanisms regulating RB transcription. Here we show that members of the RB and E2F families bind directly to the RB promoter. To investigate how the RB/E2F pathway may regulate Rb transcription, we generated reporter mice carrying an eGFP transgene inserted into a bacterial artificial chromosome containing most of the Rb gene. Expression of eGFP largely parallels that of Rb in transgenic embryos and adult mice. Using these reporter mice and mutant alleles for Rb, p107, and p130, we found that RB family members modulate Rb transcription in specific cell populations in vivo and in culture. Interestingly, while Rb is a target of the RB/E2F pathway in mouse and human cells, Rb expression does not strictly correlate with the cell cycle status of these cells. These experiments identify novel regulatory feedback mechanisms within the RB pathway in mammalian cells.The RB tumor suppressor gene was first identified through its direct mutation or deletion in human retinoblastoma. The RB protein is thought to function largely as a transcriptional cofactor that can repress or potentiate the functions of numerous transcription factors, affecting the expression of a broad number of target genes. Since its initial discovery, RB function has been demonstrated to be inactivated in virtually all human cancers through a variety of mechanisms. In particular, in addition to direct mutation events in the RB gene, the RB protein is often functionally inactivated by phosphorylation in tumor cells with constitutive activation of cyclin/Cdk complexes (44,55). RB inactivation through reduced transcription may also participate in the development of cancer (4, 41); surprisingly, however, little is known about the mechanisms regulating RB transcription in normal and tumor cells.Interestingly, the RB promoter contains a conserved binding site for the E2F transcription factors, some of which are direct partners of RB and key downstream mediators of RB (9). Increasing evidence suggests that this site may contribute to the regulation of RB transcription. For instance, overexpression of E2F1 can activate RB transcription, which may contribute to the variations in RB mRNA levels during cell cycle progression that are observed in some contexts (30, 43) but not in others (5,13,20). In addition, methylation of the E2F site in the RB promoter is sufficient to recruit repressor complexes to inhibit RB transcription (15). Accordingly, reporter assays show that mutation of the E2F site in the RB promoter leads to an absence of RB repression (33). Furthermore, mutation of the E2F (and Sp1) sites in the mouse Rb promoter in transgenic mice has revealed the importance of these sites for Rb expression in vivo (1).Togeth...

Bone morphogenetic proteins (BMPs) play diverse roles in many aspects of skeletal development and bone homeostasis. During endochondral ossification, tight regulation of BMP activity is required to assure proper survival, proliferation and differentiation of skeletal progenitor cells into chondrocytes and osteoblasts. BMP3, a structurally divergent member of the BMP family, acts as a negative regulator of bone formation by limiting BMP signal transduction. In this study, we focus on the chick limb where we find BMP3 has a unique localization pattern with strong expression in the developing perichondrium. Overexpression of BMP3 in chick wing bud at the onset of chondrogenesis, using replication competent retrovirus, reduces BMP signaling leading to increased cell proliferation and delayed cell differentiation, resulting in expanded skeletal elements and joint fusions. Our results suggest that BMP3 expression in the perichondrium may serve to regulate cartilage cell proliferation by modulating the levels of BMP signaling, thus ensuring proper endochondral ossification.

No abstract

The RB tumor suppressor gene is mutated in a broad range of human cancers, including pediatric retinoblastoma. Strikingly, however, Rb mutant mice develop tumors of the pituitary and thyroid glands, but not retinoblastoma. Mouse genetics experiments have demonstrated that p107, a protein related to pRB, is capable of preventing retinoblastoma, but not pituitary tumors, in Rb-deficient mice. Evidence suggests that the basis for this compensatory function of p107 is increased transcription of the p107 gene in response to Rb inactivation. To begin to address the context-dependency of this compensatory role of p107 and to follow p107 expression in vivo, we have generated transgenic mice carrying an enhanced GFP (eGFP) reporter inserted into a bacterial artificial chromosome (BAC) containing the mouse p107 gene. Expression of the eGFP transgene parallels that of p107 in these transgenic mice and identifies cells with a broad range of expression level for p107, even within particular organs or tissues. We also show that loss of Rb results in the upregulation of p107 transcription in specific cell populations in vivo, including subpopulations of hematopoietic cells. Thus, p107 BAC-eGFP transgenic mice serve as a useful tool to identify distinct cell types in which p107 is expressed and may have key functions in vivo, and to characterize changes in cellular networks accompanying Rb deficiency.