Soluble adenylyl cyclase is not required for axon guidance to netrin-1

David Garbers' Contributions to Chemotaxis Signaling in Sperm

This review focuses on the contribution of the late David Garbers to chemotaxis of sperm, in particular from sea urchin. We will describe his discovery of chemotactic peptides and their cognate receptors, his discovery of a sperm-specific, unique Na+/H+ exchanger that represents a chimera between a solute carrier (SLC) and an ion channel. Finally, we will discuss his contributions to the understanding of cAMP signaling in sperm via soluble adenylyl cyclase (sAC) and its control by Ca2+ ions.

Insights from the neural guidance factor Netrin-1 into neurodegeneration and other diseases

Netrin-1 was initially discovered as a neuronal growth cue for axonal guidance, and its functions have later been identified in inflammation, tumorigenesis, neurodegeneration, and other disorders. We have recently found its alterations in the brains with Alzheimer's disease, which might provide important clues to the mechanisms of some unique pathologies. To provide better understanding of this promising molecule, we here summarize research progresses in genetics, pathology, biochemistry, cell biology and other studies of Netrin-1 about its mechanistic roles and biomarker potentials with an emphasis on clinical neurodegenerative disorders in order to expand understanding of this promising molecular player in human diseases.

Mitochondrial Functionality in Male Fertility: From Spermatogenesis to Fertilization

Mitochondria are structurally and functionally distinct organelles that produce adenosine triphosphate (ATP) through oxidative phosphorylation (OXPHOS), to provide energy to spermatozoa. They can also produce reactive oxidation species (ROS). While a moderate concentration of ROS is critical for tyrosine phosphorylation in cholesterol efflux, sperm–egg interaction, and fertilization, excessive ROS generation is associated with male infertility. Moreover, mitochondria participate in diverse processes ranging from spermatogenesis to fertilization to regulate male fertility. This review aimed to summarize the roles of mitochondria in male fertility depending on the sperm developmental stage (from male reproductive tract to female reproductive tract). Moreover, mitochondria are also involved in testosterone production, regulation of proton secretion into the lumen to maintain an acidic condition in the epididymis, and sperm DNA condensation during epididymal maturation. We also established the new signaling pathway using previous proteomic data associated with male fertility, to understand the overall role of mitochondria in male fertility. The pathway revealed that male infertility is associated with a loss of mitochondrial proteins in spermatozoa, which induces low sperm motility, reduces OXPHOS activity, and results in male infertility.

cAMP-dependent cell differentiation triggered by activated CRHR1 in hippocampal neuronal cells

Corticotropin-releasing hormone receptor 1 (CRHR1) activates the atypical soluble adenylyl cyclase (sAC) in addition to transmembrane adenylyl cyclases (tmACs). Both cAMP sources were shown to be required for the phosphorylation of ERK1/2 triggered by activated G protein coupled receptor (GPCR) CRHR1 in neuronal and neuroendocrine contexts. Here, we show that activated CRHR1 promotes growth arrest and neurite elongation in neuronal hippocampal cells (HT22-CRHR1 cells). By characterising CRHR1 signalling mechanisms involved in the neuritogenic effect, we demonstrate that neurite outgrowth in HT22-CRHR1 cells takes place by a sAC-dependent, ERK1/2-independent signalling cascade. Both tmACs and sAC are involved in corticotropin-releasing hormone (CRH)-mediated CREB phosphorylation and c-fos induction, but only sAC-generated cAMP pools are critical for the neuritogenic effect of CRH, further highlighting the engagement of two sources of cAMP downstream of the activation of a GPCR, and reinforcing the notion that restricted cAMP microdomains may regulate independent cellular processes.

DSCAM promotes axon fasciculation and growth in the developing optic pathway

Although many aspects of optic pathway development are beginning to be understood, the mechanisms promoting the growth of retinal ganglion cell (RGC) axons toward visual targets remain largely unknown. Down syndrome cell adhesion molecule (Dscam) is expressed by mouse RGCs shortly after they differentiate at embryonic day 12 and is essential for multiple aspects of postnatal visual system development. Here we show that Dscam is also required during embryonic development for the fasciculation and growth of RGC axons. Dscam is expressed along the developing optic pathway in a pattern consistent with a role in regulating RGC axon outgrowth. In mice carrying spontaneous mutations in Dscam (Dscam^del17 ; Dscam^2J), RGC axons pathfind normally, but growth from the chiasm toward their targets is impaired, resulting in a delay in RGC axons reaching the dorsal thalamus compared with that seen in wild-type littermates. Conversely, Dscam gain of function results in exuberant growth into the dorsal thalamus. The growth of ipsilaterally projecting axons is particularly affected. Axon organization in the optic chiasm and tract and RGC growth cone morphologies are also altered in Dscam mutants. In vitro DSCAM promotes RGC axon growth and fasciculation, and can act independently of cell contact. In vitro and in situ DSCAM is required both in the RGC axons and in their environment for the promotion of axon outgrowth, consistent with a homotypic mode of action. These findings identify DSCAM as a permissive signal that promotes the growth and fasciculation of RGC axons, controlling the timing of when RGC axons reach their targets.

A plasma membrane microdomain compartmentalizes ephrin-generated cAMP signals to prune developing retinal axon arbors

The development of neuronal circuits is controlled by guidance molecules that are hypothesized to interact with the cholesterol-enriched domains of the plasma membrane termed lipid rafts. Whether such domains enable local intracellular signalling at the submicrometre scale in developing neurons and are required for shaping the nervous system connectivity in vivo remains controversial. Here, we report a role for lipid rafts in generating domains of local cAMP signalling in axonal growth cones downstream of ephrin-A repulsive guidance cues. Ephrin-A-dependent retraction of retinal ganglion cell axons involves cAMP signalling restricted to the vicinity of lipid rafts and is independent of cAMP modulation outside of this microdomain. cAMP modulation near lipid rafts controls the pruning of ectopic axonal branches of retinal ganglion cells in vivo, a process requiring intact ephrin-A signalling. Together, our findings indicate that lipid rafts structure the subcellular organization of intracellular cAMP signalling shaping axonal arbors during the nervous system development.

Visualizing directional Rab7 and TrkA cotrafficking in axons by pTIRF microscopy

Rab7 GTPase is known to regulate protein degradation and intracellular signaling via endocytic sorting and is also known to be involved in peripheral neurodegeneration. Mutations in the GTP-binding pocket of Rab7 cause Charcot-Marie-Tooth type 2B (CMT-2B) neuropathy. It has been suggested that the CMT-2B-associated Rab7 mutants may disrupt retrograde survival signaling by degrading the signaling endosomes carrying the nerve growth factor (NGF) and its TrkA receptor. Studying the cotrafficking of Rab7 and retrograde-TrkA endosomes in axons is therefore important to understand how Rab7 mutants affect the NGF signaling in neurons. However, tracking the axonal transport of Rab7 and TrkA with conventional microscopy and assigning the transport directionality in mass neuronal cultures pose some practical challenges. In this chapter, we describe the combination of a single-molecule imaging technique, pseudo-total internal reflection fluorescence (pTIRF) microscopy, with microfluidic neuron cultures that enables the simultaneous tracking of fluorescently labeled Rab7- and TrkA-containing endosomes in axons.

The role of soluble adenylyl cyclase in neurite outgrowth

Axon regeneration in the mature central nervous system is limited by extrinsic inhibitory signals and a postnatal decline in neurons' intrinsic growth capacity. Neuronal levels of the second messenger cAMP are important in regulating both intrinsic growth capacity and neurons' responses to extrinsic factors. Approaches which increase intracellular cAMP in neurons enhance neurite outgrowth and facilitate regeneration after injury. Thus, understanding the factors which affect cAMP in neurons is of potential therapeutic importance. Recently, soluble adenylyl cyclase (sAC, ADCY10), the ubiquitous, non-transmembrane adenylyl cyclase, was found to play a key role in neuronal survival and axon growth. sAC is activated by bicarbonate and cations and may translate physiologic signals from metabolism and electrical activity into a neuron's decision to survive or regenerate. Here we critically review the literature surrounding sAC and cAMP signaling in neurons to further elucidate the potential role of sAC signaling in neurite outgrowth and regeneration. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Intermingled cAMP, cGMP and calcium spatiotemporal dynamics in developing neuronal circuits

cAMP critically modulates the development of neuronal connectivity. It is involved in a wide range of cellular processes that require independent regulation. However, our understanding of how this single second messenger achieves specific modulation of the signaling pathways involved remains incomplete. The subcellular compartmentalization and temporal regulation of cAMP signals have recently been identified as important coding strategies leading to specificity. Dynamic interactions of this cyclic nucleotide with other second messenger including calcium and cGMP are critically involved in the regulation of spatiotemporal control of cAMP. Recent technical improvements of fluorescent sensors facilitate cAMP monitoring, whereas optogenetic tools permit spatial and temporal control of cAMP manipulations, all of which enabled the direct investigation of spatiotemporal characteristics of cAMP modulation in developing neurons. Focusing on neuronal polarization, neurotransmitter specification, axon guidance, and refinement of neuronal connectivity, we summarize herein the recent advances in understanding the features of cAMP signals and their dynamic interactions with calcium and cGMP involved in shaping the nervous system.

Established and potential physiological roles of bicarbonate-sensing soluble adenylyl cyclase (sAC) in aquatic animals

Soluble adenylyl cyclase (sAC) is a recently recognized source of the signaling molecule cyclic AMP (cAMP) that is genetically and biochemically distinct from the classic G-protein-regulated transmembrane adenylyl cyclases (tmACs). Mammalian sAC is distributed throughout the cytoplasm and it may be present in the nucleus and inside mitochondria. sAC activity is directly stimulated by HCO3(-), and sAC has been confirmed to be a HCO3(-) sensor in a variety of mammalian cell types. In addition, sAC can functionally associate with carbonic anhydrases to act as a de facto sensor of pH and CO2. The two catalytic domains of sAC are related to HCO3(-)-regulated adenylyl cyclases from cyanobacteria, suggesting the cAMP pathway is an evolutionarily conserved mechanism for sensing CO2 levels and/or acid/base conditions. Reports of sAC in aquatic animals are still limited but are rapidly accumulating. In shark gills, sAC senses blood alkalosis and triggers compensatory H(+) absorption. In the intestine of bony fishes, sAC modulates NaCl and water absorption. And in sea urchin sperm, sAC may participate in the initiation of flagellar movement and in the acrosome reaction. Bioinformatics and RT-PCR results reveal that sAC orthologs are present in most animal phyla. This review summarizes the current knowledge on the physiological roles of sAC in aquatic animals and suggests additional functions in which sAC may be involved.

Soluble adenylyl cyclase is necessary and sufficient to overcome the block of axonal growth by myelin-associated factors

Neurons in the CNS do not regenerate following injury; regeneration is blocked by inhibitory proteins in myelin, such as myelin-associated glycoprotein (MAG). Elevating neuronal levels of the second messenger cAMP overcomes this blocked axonal outgrowth. One way to elevate cAMP is pretreating neurons with neurotrophins, such as brain-derived neurotrophic factor (BDNF). However, pleiotropic effects and poor bioavailability make exogenous administration of neurotrophins in vivo problematic; therefore, alternative targets must be considered. In neurons, two families of adenylyl cyclases synthesize cAMP, transmembrane adenylyl cyclases (tmACs), and soluble adenylyl cyclase (sAC). Here, we demonstrate that sAC is the essential source of cAMP for BDNF to overcome MAG-dependent inhibition of neurite outgrowth. Elevating sAC in rat and mouse neurons is sufficient to induce neurite outgrowth on myelin in vitro and promotes regeneration in vivo. These results suggest that stimulators of sAC might represent a novel therapeutic strategy to promote axonal growth and regeneration.

Role of soluble adenylyl cyclase in cell death and growth

cAMP signaling is an evolutionarily conserved intracellular communication system controlling numerous cellular functions. Until recently, transmembrane adenylyl cyclase (tmAC) was considered the major source for cAMP in the cell, and the role of cAMP signaling was therefore attributed exclusively to the activity of this family of enzymes. However, increasing evidence demonstrates the role of an alternative, intracellular source of cAMP produced by type 10 soluble adenylyl cyclase (sAC). In contrast to tmAC, sAC produces cAMP in various intracellular microdomains close to specific cAMP targets, e.g., in nucleus and mitochondria. Ongoing research demonstrates involvement of sAC in diverse physiological and pathological processes. The present review is focused on the role of cAMP signaling, particularly that of sAC, in cell death and growth. Although the contributions of sAC to the regulation of these cellular functions have only recently been discovered, current data suggest that sAC plays key roles in mitochondrial bioenergetics and the mitochondrial apoptosis pathway, as well as cell proliferation and development. Furthermore, recent reports suggest the importance of sAC in several pathologies associated with apoptosis as well as in oncogenesis. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Guidance cue netrin-1 and the regulation of inflammation in acute and chronic kidney disease

Acute kidney injury (AKI) is a common problem in the hospital setting and intensive care unit. Despite improved understanding, there are no effective therapies available to treat AKI. A large body of evidence strongly suggests that ischemia reperfusion injury is an inflammatory disease mediated by both adaptive and innate immune systems. Cell migration also plays an important role in embryonic development and inflammation, and this process is highly regulated to ensure tissue homeostasis. One such paradigm exists in the developing nervous system, where neuronal migration is mediated by a balance between chemoattractive and chemorepulsive signals. The ability of the guidance molecule netrin-1 to repulse or abolish attraction of neuronal cells expressing the UNC5B receptor makes it an attractive candidate for the regulation of inflammatory cell migration. Recent identification of netrin-1 as regulators of immune cell migration has led to a large number of studies looking into how netrin-1 controls inflammation and inflammatory cell migration. This review will focus on recent advances in understanding netrin-1 mediated regulation of inflammation during acute and chronic kidney disease and whether netrin-1 and its receptor activation can be used to treat acute and chronic kidney disease.

Neuronal expression of soluble adenylyl cyclase in the mammalian brain

Cyclic 3',5'-adenosine monophosphate (cAMP) is a critical and ubiquitous second messenger involved in a multitude of signaling pathways. Soluble adenylyl cyclase (sAC) is a novel source of cAMP subject to unique localization and regulation. It was originally discovered in mammalian testis and found to be activated by bicarbonate and calcium. sAC has been implicated in diverse processes, including astrocyte-neuron metabolic coupling and axonal outgrowth of neurons. However, despite these functional studies, demonstration of sAC protein expression outside of testis has been controversial. Recently, we showed sAC immunoreactivity in astrocytes, but the question of neuronal expression of sAC remained. We now describe the generation of a second sAC knockout mouse model (C2KO) designed to more definitively address questions of sAC expression, and we demonstrate conclusively using immune-electron microscopy that sAC is expressed in neuronal profiles in the central nervous system.

Genetic analysis of DSCAM's role as a Netrin-1 receptor in vertebrates

Down syndrome cell adhesion molecule (DSCAM) has mainly been characterized for its function as an adhesion molecule in axon growth and in self-recognition between dendrites of the same neuron. Recently, it has been shown that DSCAM can bind to Netrin-1 and that downregulation of DSCAM expression by siRNAs in chick and rodent spinal cords leads to impaired growth and turning response of commissural axons to Netrin-1. To investigate the effect of complete genetic ablation of DSCAM on Netrin-1-induced axon guidance, we analyzed spinal commissural neurons in DSCAM-null mice and found that they extend axons that reach and cross the floor plate and express apparently normal levels of the Netrin receptors DCC (deleted in colorectal carcinoma) and Neogenin. In vitro, commissural neurons in dorsal spinal cord explants of DSCAM-null embryos show normal outgrowth in response to Netrin-1. We therefore conclude that DSCAM is not required for Netrin-induced commissural axon outgrowth and guidance in mice.

The netrin receptor DCC is required in the pubertal organization of mesocortical dopamine circuitry

Netrins are guidance cues involved in neural connectivity. We have shown that the netrin-1 receptor DCC (deleted in colorectal cancer) is involved in the functional organization of the mesocorticolimbic dopamine (DA) system. Adult mice with a heterozygous loss-of-function mutation in dcc exhibit changes in indexes of DA function, including DA-related behaviors. These phenotypes are only observed after puberty, a critical period in the maturation of the mesocortical DA projection. Here, we examined whether dcc heterozygous mice exhibit structural changes in medial prefrontal cortex (mPFC) DA synaptic connectivity, before and after puberty. Stereological counts of tyrosine-hydroxylase (TH)-positive varicosities were increased in the cingulate 1 and prelimbic regions of the pregenual mPFC. dcc heterozygous mice also exhibited alterations in the size, complexity, and dendritic spine density of mPFC layer V pyramidal neuron basilar dendritic arbors. Remarkably, these presynaptic and postsynaptic partner phenotypes were not observed in juvenile mice, suggesting that DCC selectively influences the extensive branching and synaptic differentiation that occurs in the maturing mPFC DA circuit at puberty. Immunolabeling experiments in wild-type mice demonstrated that DCC is segregated to TH-positive fibers innervating the nucleus accumbens, with only scarce DCC labeling in mPFC TH-positive fibers. Netrin had an inverted target expression pattern. Thus, DCC-mediated netrin-1 signaling may influence the formation/maintenance of mesocorticolimbic DA topography. In support of this, we report that dcc heterozygous mice exhibit a twofold increase in the density of mPFC DCC/TH-positive varicosities. Our results implicate DCC-mediated netrin-1 signaling in the establishment of mPFC DA circuitry during puberty.

Spatial and temporal second messenger codes for growth cone turning

Cyclic AMP (cAMP) and calcium are ubiquitous, interdependent second messengers that regulate a wide range of cellular processes. During development of neuronal networks they are critical for the first step of circuit formation, transducing signals required for axon pathfinding. Surprisingly, the spatial and temporal cAMP and calcium codes used by axon guidance molecules are unknown. Here, we identify characteristics of cAMP and calcium transients generated in growth cones during Netrin-1-dependent axon guidance. In filopodia, Netrin-1-dependent Deleted in Colorectal Cancer (DCC) receptor activation induces a transient increase in cAMP that causes a brief increase in calcium transient frequency. In contrast, activation of DCC in growth cone centers leads to a transient calcium-dependent cAMP increase and a sustained increase in frequency of calcium transients. We show that filopodial cAMP transients regulate spinal axon guidance in vitro and commissural axon pathfinding in vivo. These growth cone codes provide a basis for selective activation of specific downstream effectors.

Intracellular cAMP signaling by soluble adenylyl cyclase

Soluble adenylyl cyclase (sAC) is a recently identified source of the ubiquitous second messenger cyclic adenosine 3',5' monophosphate (cAMP). sAC is distinct from the more widely studied source of cAMP, the transmembrane adenylyl cyclases (tmACs); its activity is uniquely regulated by bicarbonate anions, and it is distributed throughout the cytoplasm and in cellular organelles. Due to its unique localization and regulation, sAC has various functions in a variety of physiological systems that are distinct from tmACs. In this review, we detail the known functions of sAC, and we reassess commonly held views of cAMP signaling inside cells.

Second messengers and membrane trafficking direct and organize growth cone steering

Graded distributions of extracellular cues guide developing axons toward their targets. A network of second messengers - Ca(2+) and cyclic nucleotides - shapes cue-derived information into either attractive or repulsive signals that steer growth cones bidirectionally. Emerging evidence suggests that such guidance signals create a localized imbalance between exocytosis and endocytosis, which in turn redirects membrane, adhesion and cytoskeletal components asymmetrically across the growth cone to bias the direction of axon extension. These recent advances allow us to propose a unifying model of how the growth cone translates shallow gradients of environmental information into polarized activity of the steering machinery for axon guidance.

Signaling from axon guidance receptors

Determining how axon guidance receptors transmit signals to allow precise pathfinding decisions is fundamental to our understanding of nervous system development and may suggest new strategies to promote axon regeneration after injury or disease. Signaling mechanisms that act downstream of four prominent families of axon guidance cues--netrins, semaphorins, ephrins, and slits--have been extensively studied in both invertebrate and vertebrate model systems. Although details of these signaling mechanisms are still fragmentary and there appears to be considerable diversity in how different guidance receptors regulate the motility of the axonal growth cone, a number of common themes have emerged. Here, we review recent insights into how specific receptors for each of these guidance cues engage downstream regulators of the growth cone cytoskeleton to control axon guidance.

Calcium-dependent increases in protein kinase-A activity in mouse retinal ganglion cells are mediated by multiple adenylate cyclases

Neurons undergo long term, activity dependent changes that are mediated by activation of second messenger cascades. In particular, calcium-dependent activation of the cyclic-AMP/Protein kinase A signaling cascade has been implicated in several developmental processes including cell survival, axonal outgrowth, and axonal refinement. The biochemical link between calcium influx and the activation of the cAMP/PKA pathway is primarily mediated through adenylate cyclases. Here, dual imaging of intracellular calcium concentration and PKA activity was used to assay the role of different classes of calcium-dependent adenylate cyclases (ACs) in the activation of the cAMP/PKA pathway in retinal ganglion cells (RGCs). Surprisingly, depolarization-induced calcium-dependent PKA transients persist in barrelless mice lacking AC1, the predominant calcium-dependent adenylate cyclase in RGCs, as well as in double knockout mice lacking both AC1 and AC8. Furthermore, in a subset of RGCs, depolarization-induced PKA transients persist during the inhibition of all transmembrane adenylate cyclases. These results are consistent with the existence of a soluble adenylate cyclase that plays a role in calcium-dependent activation of the cAMP/PKA cascade in neurons.

The ratio of 'deleted in colorectal cancer' to 'uncoordinated-5A' netrin-1 receptors on the growth cone regulates mossy fibre directionality

Proper axonal targeting is fundamental to the establishment of functional neural circuits. The hippocampal mossy fibres normally project towards the CA3 region. In the hippocampi of patients with temporal lobe epilepsy and related animal models, however, mossy fibres project towards the molecular layer and produce the hyperexcitable recurrent networks. The cellular and molecular mechanisms underlying this aberrant axonal targeting, known as mossy fibre sprouting, remain unclear. Netrin-1 attracts or repels axons depending on the composition of its attraction-mediating receptor, deleted in colorectal cancer, and its repulsion-mediating receptor, uncoordinated-5, on the growth cone; but the roles of netrin-1-dependent guidance in pathological conditions are largely unknown. In this study, we examined the role of netrin-1 and its receptors in mossy fibre guidance and report that enhanced neuronal activity changes netrin-1-mediated cell targeting by the axons under hyperexcitable conditions. Netrin-1 antibody or Dcc ribonucleic acid interference attenuated mossy fibre growth towards CA3 in slice overlay assays. The axons were repelled from CA3 and ultimately innervated the molecular layer when hyperactivity was pharmacologically introduced. We first hypothesized that a reduction in netrin-1 expression in CA3 underlies the phenomenon, but found that its expression was increased. We then examined two possible activity-dependent changes in netrin-1 receptor expression: a reduction in the deleted in colorectal cancer receptor and induction of uncoordinated-5 receptor. Hyperactivity did not affect the surface expression of the deleted in colorectal cancer receptor on the growth cone, but it increased that of uncoordinated-5A, which was suppressed by blocking cyclic adenosine monophosphate signalling. In addition, Dcc knockdown did not affect hyperactivity-induced mossy fibre sprouting in the slice cultures, whereas Unc5a knockdown rescued the mistargeting. Thus, netrin-1 appears to attract mossy fibres via the deleted in colorectal cancer receptor, while it repels them via cyclic adenosine monophosphate-induced uncoordinated-5A under hyperexcitable conditions, resulting in mossy fibre sprouting.

Axon growth and guidance: receptor regulation and signal transduction

The development of precise connectivity patterns during the establishment of the nervous system depends on the regulated action of diverse, conserved families of guidance cues and their neuronal receptors. Determining how these signaling pathways function to regulate axon growth and guidance is fundamentally important to understanding wiring specificity in the nervous system and will undoubtedly shed light on many neural developmental disorders. Considerable progress has been made in defining the mechanisms that regulate the correct spatial and temporal distribution of guidance receptors and how these receptors in turn signal to the growth cone cytoskeleton to control steering decisions. This review focuses on recent advances in our understanding of the mechanisms mediating growth cone guidance with a particular emphasis on the control of guidance receptor regulation and signaling.

From Guidance Signals to Movement: Signaling Molecules Governing Growth Cone Turning

Directed growth cone movements in response to external guidance signals are required for the establishment of functional neuronal connections during development, adult nerve regeneration, and adult neurogenesis. Growth cone intrinsic properties permit different growth cone responses (e.g., attraction or repulsion) to a guidance signal, and alterations to these intrinsic properties often result in opposite growth cone responses. This article reviews the current knowledge of growth cone signaling, emphasizing the dependency of Ca2+ signaling on membrane potential shifts, and cyclic nucleotide and phosphoinositide signaling pathways during growth cone turning in response to guidance signals. We also discuss how asymmetrical growth cone signaling is achieved for the fine-tuned growth cone movement.

Somatic 'soluble' adenylyl cyclase isoforms are unaffected in Sacy tm1Lex/Sacy tm1Lex 'knockout' mice

BACKGROUND

Mammalian Soluble adenylyl cyclase (sAC, Adcy10, or Sacy) represents a source of the second messenger cAMP distinct from the widely studied, G protein-regulated transmembrane adenylyl cyclases. Genetic deletion of the second through fourth coding exons in Sacy(tm1Lex)/Sacy(tm1Lex) knockout mice results in a male sterile phenotype. The absence of any major somatic phenotype is inconsistent with the variety of somatic functions identified for sAC using pharmacological inhibitors and RNA interference.

PRINCIPAL FINDINGS

We now use immunological and molecular biological methods to demonstrate that somatic tissues express a previously unknown isoform of sAC, which utilizes a unique start site, and which 'escapes' the design of the Sacy(tm1Lex) knockout allele.

CONCLUSIONS/SIGNIFICANCE

These studies reveal increased complexity at the sAC locus, and they suggest that the known isoforms of sAC play a unique function in male germ cells.

Ena/VASP: proteins at the tip of the nervous system

The emergence of neurites from a symmetrical cell body is an essential feature of nervous system development. Neurites are the precursors of axons and dendrites and are tipped by growth cones, motile structures that guide elongating axons in the developing nervous system. Growth cones steer the axon along a defined path to its appropriate target in response to guidance cues. This navigation involves the dynamic extension and withdrawal of actin-filled finger-like protrusions called filopodia that continuously sample their environment. Ena/VASP proteins, a conserved family of actin-regulatory proteins, are crucial for filopodia formation and function downstream of several guidance cues. Here we review recent findings into Ena/VASP function in neurite initiation, axon outgrowth and guidance.