Differential regulation of the regulatory subunits for phosphatidylinositol 3-kinase in response to motor nerve injury

PI3K couples long-term synaptic potentiation with cofilin recruitment and actin polymerization in dendritic spines via its regulatory subunit p85α

Long-term synaptic plasticity is typically associated with morphological changes in synaptic connections. However, the molecular mechanisms coupling functional and structural aspects of synaptic plasticity are still poorly defined. The catalytic activity of type I phosphoinositide-3-kinase (PI3K) is required for specific forms of synaptic plasticity, such as NMDA receptor-dependent long-term potentiation (LTP) and mGluR-dependent long-term depression (LTD). On the other hand, PI3K signaling has been linked to neuronal growth and synapse formation. Consequently, PI3Ks are promising candidates to coordinate changes in synaptic strength with structural remodeling of synapses. To investigate this issue, we targeted individual regulatory subunits of type I PI3Ks in hippocampal neurons and employed a combination of electrophysiological, biochemical and imaging techniques to assess their role in synaptic plasticity. We found that a particular regulatory isoform, p85α, is selectively required for LTP. This specificity is based on its BH domain, which engages the small GTPases Rac1 and Cdc42, critical regulators of the actin cytoskeleton. Moreover, cofilin, a key regulator of actin dynamics that accumulates in dendritic spines after LTP induction, failed to do so in the absence of p85α or when its BH domain was overexpressed as a dominant negative construct. Finally, in agreement with this convergence on actin regulatory mechanisms, the presence of p85α in the PI3K complex determined the extent of actin polymerization in dendritic spines during LTP. Therefore, this study reveals a molecular mechanism linking structural and functional synaptic plasticity through the coordinate action of PI3K catalytic activity and a specific isoform of the regulatory subunits.

Transcribing the path to neurological recovery-From early signals through transcription factors to downstream effectors of successful regeneration

The peripheral nervous system is known to regenerate comparatively well and this ability is mirrored in the de novo expression or upregulation of a wide variety of molecules involved in axonal outgrowth starting with transcription factors, but also including growth-stimulating substances, guidance and cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions. Recent studies using pharmacological agents, and global as well as neuron-selective gene inactivation techniques have shed light on those endogenous molecules that play a non-redundant role in mediating regenerative axonal outgrowth in vivo. The aim of the current review is to sketch the sequence of molecular events from early sensors of injury to transcription factors to downstream effectors that cooperate in successful regeneration and functional recovery.

Induction of pancreatitis-associated protein (PAP) family members in neurons after traumatic brain injury

The pancreatitis-associated protein (PAP) family is a group of 16-kDa secretory proteins initially identified in the pancreas in rats with acute pancreatitis. Although induction of PAP family genes was reported in peripheral nerve injury models, the expression in the central nervous system after traumatic injury has not been examined. In the present study, we examined the expression of PAP family members (PAP-I, PAP-II, and PAP-III) in the rat brain following traumatic brain injury (TBI) induced by weight drop. There was a significant upregulation of PAP-I and PAP-III mRNA in the injured cortex beginning at 1 day after TBI. Immunohistochemical double-staining indicated that PAP-I and PAP-III staining was localized in a subpopulation of neurons in the peri-injured region. Expression of both PAP-I and PAP-III mRNA was observed following a transient increase in inflammatory cytokines, including TNF-alpha, IL-6, and IL-1beta mRNA. The results of the present study suggest that expression of PAP family members in response to traumatic and inflammatory stimuli are not restricted to the pancreas, intestine, and peripheral nervous system, and are likely a more general cellular response, including the central nervous system in the rat. Thus, PAP family members may have an anti-inflammatory role, and this may contribute to the protection of injured neurons.

Regulation of p110delta PI 3-kinase gene expression

BACKGROUND

Despite an intense interest in the biological functions of the phosphoinositide 3-kinase (PI3K) signalling enzymes, little is known about the regulation of PI3K gene expression. This also applies to the leukocyte-enriched p110delta catalytic subunit of PI3K, an enzyme that has attracted widespread interest because of its role in immunity and allergy.

PRINCIPAL FINDINGS

We show that p110delta expression is mainly regulated at the transcriptional level. In fibroblasts, lymphocytes and myeloid cells, p110delta gene transcription appears to be constitutive and not subject to acute stimulation. 5'RACE experiments revealed that p110delta mRNA transcripts contain distinct upstream untranslated exons (named exon -1, -2a, -2b, -2c and -2d), which are located up to 81 kb upstream of the translational start codon in exon 1. The levels of all the different p110delta transcripts are higher in leukocytes compared to non-leukocytes, with the p110delta transcript containing exon -2a most abundantly expressed. We have identified a highly conserved transcription factor (TF) binding cluster in the p110delta gene which has enhanced promoter activity in leukocytes compared to non-leukocytes. In human, this TF cluster is located immediately upstream of exon -2a whilst in mouse, it is located within exon -2a.

CONCLUSION

This study identifies a conserved PIK3CD promoter region that may account for the predominant leukocyte expression of p110delta.

The making of successful axonal regeneration: Genes, molecules and signal transduction pathways

Unlike its central counterpart, the peripheral nervous system is well known for its comparatively good potential for regeneration following nerve fiber injury. This ability is mirrored by the de novo expression or upregulation of a wide variety of molecules including transcription factors, growth-stimulating substances, cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions, that promote neurite outgrowth in cultured neurons. However, their role in vivo is less known. Recent studies using neutralizing antibodies, gene inactivation and overexpression techniques have started to shed light on those endogenous molecules that play a key role in axonal outgrowth and the process of successful functional repair in the injured nervous system. The aim of the current review is to provide a summary on this rapidly growing field and the experimental techniques used to define the specific effects of candidate signaling molecules on axonal regeneration in vivo.

Cell target genes of Epstein-Barr virus transcription factor EBNA-2: induction of the p55alpha regulatory subunit of PI3-kinase and its role in survival of EREB2.5 cells

Microarray analysis covering most of the annotated RNAs in the human genome identified a panel of genes induced by the Epstein-Barr virus (EBV) EBNA-2 transcription factor in the EREB2.5 human B-lymphoblastoid cell line without the need for any intermediate protein synthesis. Previous data indicating that PIK3R1 RNA (the alpha regulatory subunit of PI3-kinase) was induced were confirmed, but it is now shown that it is the p55alpha regulatory subunit that is induced. Several EBV-immortalized lymphoblastoid cell lines were shown to express p55alpha. Expression of PI3-kinase p85 regulatory and p110 catalytic subunits was not regulated by EBNA-2. Proliferation of EREB2.5 lymphoblastoid cells was inhibited by RNAi knock-down of p55alpha protein expression, loss of p55alpha being accompanied by an increase in apoptosis. p55alpha is thus a functional target of EBNA2 in EREB2.5 cells and the specific regulation of p55alpha by EBV will provide an opportunity to investigate the physiological function of p55alpha in this human cell line.

Annexin III implicated in the microglial response to motor nerve injury

To identify proteins implicated in peripheral nerve regeneration, we performed two-dimensional polyacrylamide gel electrophoresis and subsequent mass spectrometry analysis using nerve-injured hypoglossal nuclei of rat. We have identified annexin III (ANX III/ANX A3) as an induced protein after rat hypoglossal nerve injury. ANX III is known as a Ca2+-dependent phospholipid-binding protein, but its physiological function is mostly unknown. By in situ hybridization and immunohistochemistry, we demonstrated that ANX III expression was induced specifically in activated (axotomy-stimulated) microglia after nerve injury. ANX III was the most prominent ANX expressed in microglia of the major ANX family members (ANX I-VI). Hybridization signals for other ANX mRNAs (II, IV, V, and VI) were mainly observed in neuronal cells, and no significant hybridization signal for ANX I mRNA was detected in hypoglossal nuclei. In cultured microglia, ATP treatment induced ANX III translocation to the ruffling membrane where F-actin was accumulated. Further in vitro studies revealed that ANX III was not secreted and had F-actin binding activity in a Ca2+-dependent manner. These results suggest that ANX III may be a Ca2+-dependent mediator between phospholipids and F-actin in microglia stimulated by peripheral nerve injury.