Rapid arrest of axon elongation by brefeldin A: a role for the small GTP-binding protein ARF in neuronal growth cones

Physiological and Pathological Roles of the Cytohesin Family in Neurons

The cytohesin proteins, consisting of four closely related members (cytohesins-1, -2, -3, and -4), are a subfamily of the Sec7 domain-containing guanine nucleotide exchange factors for ADP ribosylation factors (Arfs), which are critical regulators of membrane trafficking and actin cytoskeleton remodeling. Recent advances in molecular biological techniques and the development of a specific pharmacological inhibitor for cytohesins, SecinH3, have revealed the functional involvement of the cytohesin-Arf pathway in diverse neuronal functions from the formation of axons and dendrites, axonal pathfinding, and synaptic vesicle recycling, to pathophysiological processes including chronic pain and neurotoxicity induced by proteins related to neurodegenerative disorders, such as amyotrophic lateral sclerosis and Alzheimer's disease. Here, we review the physiological and pathological roles of the cytohesin-Arf pathway in neurons and discuss the future directions of this research field.

Missense-depleted regions in population exomes implicate ras superfamily nucleotide-binding protein alteration in patients with brain malformation

Genomic sequence interpretation can miss clinically relevant missense variants for several reasons. Rare missense variants are numerous in the exome and difficult to prioritise. Affected genes may also not have existing disease association. To improve variant prioritisation, we leverage population exome data to identify intragenic missense-depleted regions (MDRs) genome-wide that may be important in disease. We then use missense depletion analyses to help prioritise undiagnosed disease exome variants. We demonstrate application of this strategy to identify a novel gene association for human brain malformation. We identified de novo missense variants that affect the GDP/GTP-binding site of ARF1 in three unrelated patients. Corresponding functional analysis suggests ARF1 GDP/GTP-activation is affected by the specific missense mutations associated with heterotopia. These findings expand the genetic pathway underpinning neurologic disease that classically includes FLNA. ARF1 along with ARFGEF2 add further evidence implicating ARF/GEFs in the brain. Using functional ontology, top MDR-containing genes were highly enriched for nucleotide-binding function, suggesting these may be candidates for human disease. Routine consideration of MDR in the interpretation of exome data for rare diseases may help identify strong genetic factors for many severe conditions, infertility/reduction in reproductive capability, and embryonic conditions contributing to preterm loss.

The COPI vesicle complex binds and moves with survival motor neuron within axons

Spinal muscular atrophy (SMA), an inherited disease of motor neuron dysfunction, results from insufficient levels of the survival motor neuron (SMN) protein. Movement of the SMN protein as granules within cultured axons suggests that the pathogenesis of SMA may involve defects in neuronal transport, yet the nature of axon transport vesicles remains enigmatic. Here we show that SMN directly binds to the α-subunit of the coat protein I (COPI) vesicle coat protein. The α-COP protein co-immunoprecipitates with SMN, small nuclear ribonucleoprotein-associated assembly factors and β-actin mRNA. Although typically Golgi associated, in neuronal cells α-COP localizes to lamellipodia and growth cones and moves within the axon, with a subset of these granules traveling together with SMN. Depletion of α-COP resulted in mislocalization of SMN and actin at the leading edge at the lamellipodia. We propose that neurons utilize the Golgi-associated COPI vesicle to deliver cargoes necessary for motor neuron integrity and function.

Caenorhabditits elegans LRK-1 and PINK-1 act antagonistically in stress response and neurite outgrowth

Mutations in two genes encoding the putative kinases LRRK2 and PINK1 have been associated with inherited variants of Parkinson disease. The physiological role of both proteins is not known at present, but studies in model organisms have linked their mutants to distinct aspects of mitochondrial dysfunction, increased vulnerability to oxidative and endoplasmic reticulum stress, and intracellular protein sorting. Here, we show that a mutation in the Caenorhabditits elegans homologue of the PTEN-induced kinase pink-1 gene resulted in reduced mitochondrial cristae length and increased paraquat sensitivity of the nematode. Moreover, the mutants also displayed defects in axonal outgrowth of a pair of canal-associated neurons. We demonstrate that in the absence of lrk-1, the C. elegans homologue of human LRRK2, all phenotypic aspects of pink-1 loss-of-function mutants were suppressed. Conversely, the hypersensitivity of lrk-1 mutant animals to the endoplasmic reticulum stressor tunicamycin was reduced in a pink-1 mutant background. These results provide the first evidence of an antagonistic role of PINK-1 and LRK-1. Due to the similarity of the C. elegans proteins to human LRRK2 and PINK1, we suggest a common role of both factors in cellular functions including stress response and regulation of neurite outgrowth. This study might help to link pink-1/PINK1 and lrk-1/LRRK2 function to the pathological processes resulting from Parkinson disease-related mutants in both genes, the first manifestations of which are cytoskeletal defects in affected neurons.

A functional equivalent of endoplasmic reticulum and Golgi in axons for secretion of locally synthesized proteins

Subcellular localization of protein synthesis provides a means to regulate the protein composition in far reaches of a cell. This localized protein synthesis gives neuronal processes autonomy to rapidly respond to extracellular stimuli. Locally synthesized axonal proteins enable neurons to respond to guidance cues and can help to initiate regeneration after injury. Most studies of axonal mRNA translation have concentrated on cytoplasmic proteins. While ultrastructural studies suggest that axons do not have rough endoplasmic reticulum or Golgi apparatus, mRNAs for transmembrane and secreted proteins localize to axons. Here, we show that growing axons with protein synthetic activity contain ER and Golgi components needed for classical protein synthesis and secretion. Isolated axons have the capacity to traffic locally synthesized proteins into secretory pathways and inhibition of Golgi function attenuates translation-dependent axonal growth responses. Finally, the capacity for secreting locally synthesized proteins in axons appears to be increased by injury.

Differential transport and local translation of cytoskeletal, injury-response, and neurodegeneration protein mRNAs in axons

Recent studies have begun to focus on the signals that regulate axonal protein synthesis and the functional significance of localized protein synthesis. However, identification of proteins that are synthesized in mammalian axons has been mainly based on predictions. Here, we used axons purified from cultures of injury-conditioned adult dorsal root ganglion (DRG) neurons and proteomics methodology to identify axonally synthesized proteins. Reverse transcription (RT)-PCR from axonal preparations was used to confirm that the mRNA for each identified protein extended into the DRG axons. Proteins and the encoding mRNAs for the cytoskeletal proteins beta-actin, peripherin, vimentin, gamma-tropomyosin 3, and cofilin 1 were present in the axonal preparations. In addition to the cytoskeletal elements, several heat shock proteins (HSP27, HSP60, HSP70, grp75, alphaB crystallin), resident endoplasmic reticulum (ER) proteins (calreticulin, grp78/BiP, ERp29), proteins associated with neurodegenerative diseases (ubiquitin C-terminal hydrolase L1, rat ortholog of human DJ-1/Park7, gamma-synuclein, superoxide dismutase 1), anti-oxidant proteins (peroxiredoxins 1 and 6), and metabolic proteins (e.g., phosphoglycerate kinase 1 (PGK 1), alpha enolase, aldolase C/Zebrin II) were included among the axonally synthesized proteins. Detection of the mRNAs encoding each of the axonally synthesized proteins identified by mass spectrometry in the axonal compartment indicates that the DRG axons have the potential to synthesize a complex population of proteins. Local treatment of the DRG axons with NGF or BDNF increased levels of cytoskeletal mRNAs into the axonal compartment by twofold to fivefold but had no effect on levels of the other axonal mRNAs studied. Neurotrophins selectively increased transport of beta-actin, peripherin, and vimentin mRNAs from the cell body into the axons rather than changing transcription or mRNA survival in the axonal compartment.

ARNO and ARF6 regulate axonal elongation and branching through downstream activation of phosphatidylinositol 4-phosphate 5-kinase alpha

In the developing nervous system, controlled neurite extension and branching are critical for the establishment of connections between neurons and their targets. Although much is known about the regulation of axonal development, many of the molecular events that regulate axonal extension remain unknown. ADP-ribosylation factor nucleotide-binding site opener (ARNO) and ADP-ribosylation factor (ARF)6 have important roles in the regulation of the cytoskeleton as well as membrane trafficking. To investigate the role of these molecules in axonogenesis, we expressed ARNO and ARF6 in cultured rat hippocampal neurons. Expression of catalytically inactive ARNO or dominant negative ARF6 resulted in enhanced axonal extension and branching and this effect was abrogated by coexpression of constitutively active ARF6. We sought to identify the downstream effectors of ARF6 during neurite extension by coexpressing phosphatidyl-inositol-4-phosphate 5-Kinase alpha [PI(4)P 5-Kinase alpha] with catalytically inactive ARNO and dominant negative ARF6. We found that PI(4)P 5-Kinase alpha plays a role in neurite extension and branching downstream of ARF6. Also, expression of inactive ARNO/ARF6 depleted the actin binding protein mammalian ena (Mena) from the growth cone leading edge, indicating that these effects on axonogenesis may be mediated by changes in cytoskeletal dynamics. These results suggest that ARNO and ARF6, through PI(4)P 5-Kinase alpha, regulate axonal elongation and branching during neuronal development.

Block of slow axonal transport and axonal growth by brefeldin A in compartmented cultures of rat sympathetic neurons

Disruption of the Golgi by brefeldin A (BFA) has been reported to block fast axonal transport and axonal growth. We used compartmented cultures of rat sympathetic neurons to investigate its effects on slow axonal transport. BFA (1 micro g/ml) applied to cell bodies/proximal axons for 6-20 h disrupted the Golgi, reversibly blocked axonal growth, and reversibly blocked anterograde transport of all proteins, including tubulin. The retrograde transport of nerve growth factor (NGF) was also blocked. The phosphorylation of Erk1 and Erk2 in response to NGF was unaffected after 6 h of treatment with BFA, suggesting that the block of axonal transport was specific and direct. Consistent with its principal site of action at the Golgi, no effects were observed when BFA was applied only to the distal axons. Block of fast anterograde and retrograde axonal transport is consistent with the role of the Golgi in supplying transport vesicles. Block of slow axonal transport was surprising, and further results indicated that transport of tubulin en route along the axon was arrested by application of BFA to the cell bodies, suggesting that a continuous supply of anterograde transport vesicles from the Golgi is required to maintain slow axonal transport of cytoskeletal proteins.

Identification of reciprocally regulated gene modules in regenerating dorsal root ganglion neurons and activated peripheral or central nervous system glia

Differential gene expression in the rat after injury of dorsal root ganglion neurons in vivo, and simulation injury of Schwann cells and oligodendrocytes in vitro was analyzed using high-density cDNA microarrays. The analyses were carried out to study the genetic basis of peripheral nerve regeneration, and to compare gene regulation in glia of the central (oligodendrocyte) and peripheral (Schwann cell) nervous systems. The genes showing significant differential regulation in the three study groups represented all aspects of cellular metabolism. However, two unexpected observations were made. Firstly, a number of identical genes were differentially regulated in activated Schwann cells, activated oligodendrocytes and regenerating DRG neurons. Specifically, a group of 113 out of 210 genes that were down-regulated in Schwann cells upon lipopolysaccharide (LPS) treatment, were identical to genes up-regulated in the injured, regenerating DRG. Furthermore, a group of 53 out of 71 genes that were down-regulated in interferon gamma (IFN-gamma)/LPS-activated oligodendrocytes, were identical to genes up-regulated in the DRG neurons. Finally, 22 genes were common to these three groups, i.e., down-regulated in activated oligodendrocytes, down-regulated in activated Schwann cells, and up-regulated in regenerating DRG neurons. Secondly, a group of 16 cell-cycle and proliferation-related genes were up-regulated in the DRG following sciatic nerve crush, despite the absence of cells undergoing mitosis in the DRG, or any significant presence of apoptosis-related gene expression. Therefore, it appears that in these three cell types, large sets of genes are reciprocally regulated upon injury and/or activation. This suggests that the activation of the injury-related gene expression program in cell derivatives of the neuroectoderm involves, in part, highly conserved genetic elements.

Brefeldin A-induced neurotoxicity in cultured spinal cord neurons

Brefeldin A (BFA) is a fungus metabolite that is known to cause the disassembly of the Golgi complex and apoptosis in exposed cells, both of which have been suggested as playing roles in the pathogenesis of neurodegenerative diseases, particularly amyotrophic lateral sclerosis (ALS). This study showed that BFA caused neurotoxicity and apoptotic nuclear changes in cultured spinal neurons of rat spinal cord in a dose- and time-dependent manner. The spinal motor neurons were more vulnerable to this neurotoxicity. The cultured spinal neurons showed irreversible disassembly of the Golgi apparatus as early as 1 hr after exposure to BFA. BFA induced the expression and activation of caspase-12 beginning 8 hr after exposure. The level of the cleaved form of caspase-3 had increased 12 hr after the addition of BFA. Free radical generation and loss of mitochondrial membrane potential were observed in the later stages of neurotoxicity caused by BFA. Collectively, our data suggests that BFA is an excellent agent for reproducing the pathophysiological features of ALS. This in vitro model may be useful in attempts to study the mechanisms of this neurodegenerative disease and to examine therapeutic potentials.

Localization of mRNAs for six ARFs (ADP-ribosylation factors) in the brain of developing and adult rats and changes in the expression in the hypoglossal nucleus after its axotomy

ADP-ribosylation factors (ARFs) play crucial roles in the intracellular vesicular transport and in regulation of phospholipid-modifying enzyme activities and cytoskeletons. Using in situ hybridization histochemistry, the gene expression for six isoforms of ARF was examined during normal development of rats and in the hypoglossal nucleus following its axotomy. In the embryonic brain, the expression for ARF-1, -4, -5, -6 mRNAs was distinct in the ventricular germinal zone while that for ARF-3, -4, -5 in the mantle zone. In early postnatal brain, the expression for six ARFs was seen widely in various loci of the gray matter with different intensity, and the expression of ARF-4, -5, -6 mRNAs was evident in the cerebellar external granule cell layer. In the adult brain, the gene expression for all ARF isoforms decreased more or less in most gray matters and the distinct expression was maintained mainly in the hippocampal and dentate neuronal layers and cerebellar cortex. The expression levels of ARF-2 and -4 mRNAs in affected hypoglossal nucleus increased after 24 h up to 7 days following axotomy of the hypoglossal nerve, while no such changes were seen in the expression levels for the other ARFs. The present findings suggest that ARFs are differentially involved in some processes essential to nerve regeneration as well as neuronal differentiation and maturation.

Expression of nitric oxide synthase and soluble guanylyl cyclase in the developing olfactory system of Manduca sexta

The gaseous messenger nitric oxide (NO), with its ability to mediate both intercellular and intracellular communication, can play important roles in mediating cellular communication in both the development and the function of the nervous system. The authors investigated the possible role of NO signaling in the developing olfactory system (antennal lobe) of the moth Manduca sexta. NO synthase (NOS), the enzyme that generates NO, was localized by using immunocytochemistry, in situ hybridization, and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry. Although NADPH-d staining appears to be a poor indicator of the presence of NOS in this system, immunocytochemistry and in situ hybridization reveal that NOS is expressed in the axons of olfactory receptor neurons throughout development and in the perineurial sheath that covers the brain early in development. NOS is present in axon terminals as they form protoglomeruli, raising the possibility that NO mediates cell-cell interactions during antennal lobe development. NO-sensitive soluble guanylyl cyclase (sGC), one of the best characterized targets of NO, was localized in the developing olfactory system by using in situ hybridization and immunocytochemistry for the Manduca sexta sGCalpha1 subunit. The ability of the developing olfactory system to respond to exogenous NO also was examined by using cyclic guanosine monophosphate immunocytochemistry. sGC is expressed in mechanosensory neurons in the developing antenna and in many antennal lobe neurons in both the medial and lateral cell body clusters. Thus, NOS and sGC are expressed in a pattern that suggests that this signaling pathway may mediate intercellular communication during development of the olfactory system in Manduca sexta.

A shift in protein S-palmitoylation, with persistence of growth-associated substrates, marks a critical period for synaptic plasticity in developing brain

In the mammalian cortex, the initial formation of synaptic connections is followed by a prolonged period during which synaptic circuits are functional, but retain an elevated capacity for activity-dependent remodeling and functional plasticity. During this period, synaptic terminals appear fully mature, morphologically and physiologically. We show here, however, that synaptic terminals during this period are distinguished by their simultaneous accumulation of multiple growth-associated proteins at levels characteristic of axonal growth cones, and proteins involved in synaptic transmitter release at levels characteristic of adult synapses. We show further that newly formed synapses undergo a switch in the dynamic S-palmitoylation of proteins early in the critical period, which includes a large and specific decrease in the palmitoylation of GAP-43 and other major substrates characteristic of growth cones. Previous studies have shown that a similar reduction in ongoing palmitoylation of growth cone proteins is sufficient to stop advancing axons in vitro, suggesting that a developmental switch in protein S-palmitoylation serves to disengage the molecular machinery for axon extension in the absence of local triggers for remodeling during the critical period. Only much later does a decline in the availability of major growth cone components mark the molecular maturation of cortical synapses at the close of the critical period.