Pituitary adenylate cyclase-activating polypeptide 38-mediated Rin activation requires Src and contributes to the regulation of HSP27 signaling during neuronal differentiation

A Biomarker for Predicting Responsiveness to Stem Cell Therapy Based on Mechanism-of-Action: Evidence from Cerebral Injury

To date, no stem cell therapy has been directed to specific recipients-and, conversely, withheld from others-based on a clinical or molecular profile congruent with that cell's therapeutic mechanism-of-action (MOA) for that condition. We address this challenge preclinically with a prototypical scenario: human neural stem cells (hNSCs) against perinatal/neonatal cerebral hypoxic-ischemic injury (HII). We demonstrate that a clinically translatable magnetic resonance imaging (MRI) algorithm, hierarchical region splitting, provides a rigorous, expeditious, prospective, noninvasive "biomarker" for identifying subjects with lesions bearing a molecular profile indicative of responsiveness to hNSCs' neuroprotective MOA. Implanted hNSCs improve lesional, motor, and/or cognitive outcomes only when there is an MRI-measurable penumbra that can be forestalled from evolving into necrotic core; the core never improves. Unlike the core, a penumbra is characterized by a molecular profile associated with salvageability. Hence, only lesions characterized by penumbral > core volumes should be treated with cells, making such measurements arguably a regenerative medicine selection biomarker.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

Dp71Δ78-79 dystrophin mutant stimulates neurite outgrowth in PC12 cells via upregulation and phosphorylation of HspB1

PC12 cells acquire a neuronal phenotype in response to nerve growth factor (NGF). However, this phenotype is more efficiently achieved when the Dp71Δ78-79 dystrophin mutant is stably expressed in PC12-C11 cells. To investigate the effect of Dp71Δ78-79 overexpression on the protein profile of PC12-C11 cells, we compared the expression profiles of undifferentiated and NGF-differentiated PC12-C11 and PC12 cells by 2DE. In undifferentiated cultures, one protein was downregulated, and five were upregulated. Dp71Δ78-79 overexpression had a greater effect on differentiated cultures, with ten proteins downregulated and seven upregulated. The protein with the highest upregulation was HspB1. Changes in HspB1 expression were validated by Western blot and immunofluorescence analyses. Interestingly, the neurite outgrowth in PC12-C11 cells was affected by a polyclonal antibody against HspB1, and the level of HspB1 and HspB1Ser86 decreased, suggesting an important role for this protein in this cellular process. Our results show that Dp71Δ78-79 affects the expression level of some proteins and that the stimulated neurite outgrowth produced by this mutant is mainly through upregulation and phosphorylation of HspB1.

PACAP enhances axon outgrowth in cultured hippocampal neurons to a comparable extent as BDNF

Pituitary adenylate cyclase-activating polypeptide (PACAP) exerts neurotrophic activities including modulation of synaptic plasticity and memory, hippocampal neurogenesis, and neuroprotection, most of which are shared with brain-derived neurotrophic factor (BDNF). Therefore, the aim of this study was to compare morphological effects of PACAP and BDNF on primary cultured hippocampal neurons. At days in vitro (DIV) 3, PACAP increased neurite length and number to similar levels by BDNF, but vasoactive intestinal polypeptide showed much lower effects. In addition, PACAP increased axon, but not dendrite, length, and soma size at DIV 3 similarly to BDNF. The PACAP antagonist PACAP6–38 completely blocked the PACAP-induced increase in axon, but not dendrite, length. Interestingly, the BDNF-induced increase in axon length was also inhibited by PACAP6–38, suggesting a mechanism involving PACAP signaling. K252a, a TrkB receptor inhibitor, inhibited axon outgrowth induced by PACAP and BDNF without affecting dendrite length. These results indicate that in primary cultured hippocampal neurons, PACAP shows morphological actions via its cognate receptor PAC₁, stimulating neurite length and number, and soma size to a comparable extent as BDNF, and that the increase in total neurite length is ascribed to axon outgrowth.

Analysis of the Rit subfamily GTPase-mediated signaling and neuronal differentiation and survival

The Rit subfamily of GTPases is a founding branch within the Ras family of small G-proteins and preserves unique sequences in the G2 effector loop domain and the C-terminus. Rit proteins regulate a diversity of signal transduction pathways, some of which are similar to and others of which differ from the pathways that are regulated by other Ras family GTPases. Rit proteins have been demonstrated to be essential regulators in neuronal differentiation and survival. Here, we describe the materials and methods utilized to characterize cellular signaling for the Rit subfamily of G-proteins in neuronal differentiation and survival.

RIT2, a neuron-specific small guanosine triphosphatase, is expressed in retinal neuronal cells and its promoter is modulated by the POU4 transcription factors

PURPOSE

Ras-like without CAAX 2 (RIT2), a member of the Ras superfamily of small guanosine triphosphatases, is involved in regulating neuronal function. RIT2 is a unique member of the Ras family in that RIT2 is preferentially expressed in various neurons, including retinal neurons. The mechanisms that regulate RIT2 expression in neurons were studied.

METHODS

Reverse transcription-quantitative PCR (RT-qPCR), immunohistochemistry, western blotting, bioinformatic prediction, electrophoretic mobility shift assay (EMSA), and cell transfection methods were used.

RESULTS

With immunohistochemistry of the mouse retina, RIT2 protein was detected in the ganglion cell layer (GCL), inner plexiform layer, inner nuclear layer, and outer plexiform layer, with the strongest staining in the GCL and the inner plexiform layer. RT-qPCR combined with laser capture microdissection detected Rit2 messenger RNA in the GCL and the inner nuclear layer. Western blot analysis showed a large increase in the RIT2 protein in the retina during maturation from newborn to adult. Transient transfection identified the 1.3 kb upstream region of human RIT2 as capable of driving expression in neuronal cell lines. Based on the known expression pattern and biological activity, we hypothesized that POU4 family factors might modulate RIT2 expression in retinal ganglion cells (RGCs). Bioinformatic analyses predicted six POU4 factor-binding sites within the 1.3 kb human RIT2 promoter region. EMSA analyses showed binding of POU4 proteins to three of the six predicted sites. Cotransfection with expression vectors demonstrated that POU4 proteins can indeed modulate the human RIT2 promoter, and that ISL1, a LIM homeodomain factor, can further modulate the activity of the POU4 factors.

CONCLUSIONS

These studies confirm the expression of RIT2 in retinal neuronal cells, including RGCs, begin to reveal the mechanisms responsible for neuronal expression of RIT2, and suggest a role for the POU4 family factors in modulating RIT2 expression in RGCs.

Rit subfamily small GTPases: regulators in neuronal differentiation and survival

Ras family small GTPases serve as binary molecular switches to regulate a broad array of cellular signaling cascades, playing essential roles in a vast range of normal physiological processes, with dysregulation of numerous Ras-superfamily G-protein-dependent regulatory cascades underlying the development of human disease. However, the physiological function for many "orphan" Ras-related GTPases remain poorly characterized, including members of the Rit subfamily GTPases. Rit is the founding member of a novel branch of the Ras subfamily, sharing close homology with the neuronally expressed Rin and Drosophila Ric GTPases. Here, we highlight recent studies using transgenic and knockout animal models which have begun to elucidate the physiological roles for the Rit subfamily, including emerging roles in the regulation of neuronal morphology and cellular survival signaling, and discuss new genetic data implicating Rit and Rin signaling in disorders such as cancer, Parkinson's disease, autism, and schizophrenia.

RACK1 is a ribosome scaffold protein for β-actin mRNA/ZBP1 complex

In neurons, specific mRNAs are transported in a translationally repressed manner along dendrites or axons by transport ribonucleic-protein complexes called RNA granules. ZBP1 is one RNA binding protein present in transport RNPs, where it transports and represses the translation of cotransported mRNAs, including β-actin mRNA. The release of β-actin mRNA from ZBP1 and its subsequent translation depends on the phosphorylation of ZBP1 by Src kinase, but little is known about how this process is regulated. Here we demonstrate that the ribosomal-associated protein RACK1, another substrate of Src, binds the β-actin mRNA/ZBP1 complex on ribosomes and contributes to the release of β-actin mRNA from ZBP1 and to its translation. We identify the Src binding and phosphorylation site Y246 on RACK1 as the critical site for the binding to the β-actin mRNA/ZBP1 complex. Based on these results we propose RACK1 as a ribosomal scaffold protein for specific mRNA-RBP complexes to tightly regulate the translation of specific mRNAs.

Ras family small GTPase-mediated neuroprotective signaling in stroke

Selective neuronal cell death is one of the major causes of neuronal damage following stroke, and cerebral cells naturally mobilize diverse survival signaling pathways to protect against ischemia. Importantly, therapeutic strategies designed to improve endogenous anti-apoptotic signaling appear to hold great promise in stroke treatment. While a variety of complex mechanisms have been implicated in the pathogenesis of stroke, the overall mechanisms governing the balance between cell survival and death are not well-defined. Ras family small GTPases are activated following ischemic insults, and in turn, serve as intrinsic switches to regulate neuronal survival and regeneration. Their ability to integrate diverse intracellular signal transduction pathways makes them critical regulators and potential therapeutic targets for neuronal recovery after stroke. This article highlights the contribution of Ras family GTPases to neuroprotective signaling cascades, including mitogen-activated protein kinase (MAPK) family protein kinase- and AKT/PKB-dependent signaling pathways as well as the regulation of cAMP response element binding (CREB), Forkhead box O (FoxO) and hypoxiainducible factor 1(HIF1) transcription factors, in stroke.

Generation of immortal cell lines from the adult pituitary: role of cAMP on differentiation of SOX2-expressing progenitor cells to mature gonadotropes

The pituitary is a complex endocrine tissue composed of a number of unique cell types distinguished by the expression and secretion of specific hormones, which in turn control critical components of overall physiology. The basic function of these cells is understood; however, the molecular events involved in their hormonal regulation are not yet fully defined. While previously established cell lines have provided much insight into these regulatory mechanisms, the availability of representative cell lines from each cell lineage is limited, and currently none are derived from adult pituitary. We have therefore used retroviral transfer of SV40 T-antigen to mass immortalize primary pituitary cell culture from an adult mouse. We have generated 19 mixed cell cultures that contain cells from pituitary cell lineages, as determined by RT-PCR analysis and immunocytochemistry for specific hormones. Some lines expressed markers associated with multipotent adult progenitor cells or transit-amplifying cells, including SOX2, nestin, S100, and SOX9. The progenitor lines were exposed to an adenylate cyclase activator, forskolin, over 7 days and were induced to differentiate to a more mature gonadotrope cell, expressing significant levels of α-subunit, LHβ, and FSHβ mRNAs. Additionally, clonal populations of differentiated gonadotropes were exposed to 30 nM gonadotropin-releasing hormone and responded appropriately with a significant increase in α-subunit and LHβ transcription. Further, exposure of the lines to a pulse paradigm of GnRH, in combination with 17β-estradiol and dexamethasone, significantly increased GnRH receptor mRNA levels. This array of adult-derived pituitary cell models will be valuable for both studies of progenitor cell characteristics and modulation, and the molecular analysis of individual pituitary cell lineages.

An evolutionarily conserved Rit GTPase-p38 MAPK signaling pathway mediates oxidative stress resistance

Rit knockout mice and D-Ric null Drosophila were used to identify the Rit/RIC subfamily of Ras-related GTPases as regulators of an evolutionarily conserved, p38-dependent signaling cascade that functions as a survival mechanism for cells in response to reactive oxygen species exposure.

Ras-related small GTP-binding proteins control a wide range of cellular processes by regulating a variety of effector pathways, including prominent roles in the control of mitogen-activated protein kinase (MAPK) cascades. Although the regulatory role(s) for many Ras family GTPases are well established, the physiological function for the Rit/Rin subfamily has been lacking. Here, using both knockout mice and Drosophila models, we demonstrate an evolutionarily conserved role for Rit subfamily GTPases (mammalian Rit and Rin, and the Drosophila RIC homologue) in governing survival in response to oxidative stress. Primary embryonic fibroblasts derived from Rit knockout mice display increased apoptosis and selective disruption of MAPK signaling following reactive oxygen species (ROS) exposure but not in response to endoplasmic reticulum stress or DNA damage. These deficits include a reduction in ROS-mediated stimulation of a p38-MK2-HSP27 signaling cascade that controls Akt activation, directing Bad phosphorylation to promote cell survival. Furthermore, D-RIC null flies display increased susceptibility to environmental stresses and reduced stress-dependent p38 signaling, extending the Rit-p38 survival pathway to Drosophila. Together, our studies establish the Rit GTPases as critical regulators of an evolutionarily conserved, p38 MAPK–dependent signaling cascade that functions as an important survival mechanism for cells in response to oxidative stress.

A rit GTPase-p38 mitogen-activated protein kinase survival pathway confers resistance to cellular stress

Cells mobilize diverse signaling cascades to protect against stress-mediated injury. Ras family GTPases play a pivotal role in cell fate determination, serving as molecular switches to control the integration of multiple signaling pathways. p38 mitogen-activated protein kinase (MAPK) signaling serves as a critical fulcrum in this process, regulating networks that stimulate cellular apoptosis but also have the capacity to promote cell survival. However, relatively little is known concerning this functional dichotomy, particularly the regulation of p38-dependent survival pathways. Here, we demonstrate that the Rit GTPase promotes cell survival by directing an unexpected p38 MAPK-dependent AKT survival pathway. Following stress exposure, Rit small hairpin RNA interference (shRNAi)-treated cells display increased apoptosis and selective disruption of p38 MAPK signaling, while expression of constitutively activated Rit promotes p38-AKT-dependent cell survival. Rit, but not Ras or Rap GTPases, can associate with, and is critical for, stress-mediated activation of the scaffolded p38-MK2-HSP27-AKT prosurvival signaling complex. Together, our studies establish Rit as a central regulator of a p38 MAPK-dependent signaling cascade that functions as a critical cellular survival mechanism in response to stress.

PACAP up-regulates the expression of apolipoprotein D in 3T3-L1 adipocytes. DRG/3T3-L1 co-cultures study

The existence of a cross-talk between nerves and fatty tissue is increasingly recognized. Using co-cultures of dorsal root ganglion (DRG)-derived cells and 3T3-L1 adipocytes, we have previously shown that the presence of fat cells enhances neurite outgrowth and number of synapses. Vice versa, neural cells induced expression of neurotrophic adipokines apolipoprotein D and E (ApoD, ApoE) and angiopoietin-1 (Ang-1) by adipocytes. Here, we tested whether pituitary adenylate cyclase-activating peptide (PACAP), which is released by sensory fibres and causes Ca(2+) influx into fat cells, is involved in ApoD induction. Using 3T3-L1 cell cultures, we found that PACAP at a dose of 1 nM up-regulated the expression of ApoD protein and mRNA approx. 2.5 fold. This effect was driven by ERK1/2 acting upon PAC1/VPAC2 receptors. In turn, PACAP-treated 3T3-L1 adipocytes in co-cultures with DRG cells enhanced neurite ramification of neurofilament 200 (NF200)-positive neurons (measured using fluorescence microscopy) and neurofilament 68 protein levels (measured using Western blot analysis). This effect could be blocked using the PAC1/VPAC2 antagonist PACAP(6-38). Scanning cytometry revealed PACAP/ApoD induced low density lipoprotein receptors (LDLR) and ApoE receptor 2 (apoER2) in NF200-positive cells. Thus, a bidirectional loop seems to exist regulating the innervation of fatty tissues: PACAP released from sensory fibres might stimulate fat cells to synthesize neurotrophic adipokines, which, in turn, support peripheral innervation.

Src-dependent TrkA transactivation is required for pituitary adenylate cyclase-activating polypeptide 38-mediated Rit activation and neuronal differentiation

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a potent neuropeptide that possesses both neurotrophic and neurodevelopmental effects. Recently, the Rit GTPase was found to be activated by a novel Galpha/cAMP/exchange protein activated by cyclic AMP (Epac)-dependent signaling pathway and required for PACAP-dependent cAMP response element-binding protein activation and neuronal differentiation. However, Epac did not function as a Rit guanine nucleotide exchange factor (GEF), and the nature of the PACAP regulatory cascade remained unclear. Here, we show that PACAP-mediated Rit activation involves Src family kinase-dependent TrkA receptor transactivation. PACAP receptor (PACR1) stimulation triggered both G(i)alpha and G(s)alpha/cAMP/Epac regulatory cascades resulting in Src kinase activity, which in turn induced TrkA kinase tyrosine phosphorylation. Importantly, Src inhibition, or the lack of functional Trk receptors, was found to inhibit PACAP-mediated Rit activation, whereas constitutively active Src alone was sufficient to stimulate Rit-guanosine triphosphate levels. A single tyrosine (Y(499)) phosphorylation event was identified as critical to both PACAP-mediated transactivation and TrkA-dependent Rit activation. Accordingly, PACAP stimulation resulted in TrkA-dependent phosphorylation of both the Shc adaptor and son of sevenless (SOS)1/2 GEFs, and Rit activation was inhibited by RNA interference silencing of SOS1/2, implicating a TrkA/Shc/SOS signaling complex in Rit regulation. Together, these observations expand upon the nature of PACR1-mediated transactivation and identify TrkA-Rit signaling as a key contributor to PACAP-dependent neuronal differentiation.

Heat shock protein 27 in neuronal survival and neurite outgrowth

The small heat shock protein 27 (Hsp27) is well documented to promote neuronal survival in neurodegenerative diseases and following nerve injury. It can directly inhibit apoptotic pathways, and as a chaperone it can ameliorate the toxic effects of misfolded proteins. More recently, Hsp27 has been implicated to also play a role in neurite outgrowth. Thus, Hsp27 is situated at the intersection of neuronal survival and differentiation and, as such, it has dual potential as a key therapeutic target for neuroregeneration.