Selective blockade of axonogenesis in cultured hippocampal neurons by the tyrosine phosphatase inhibitor orthovanadate

Reference compounds for alternative test methods to indicate developmental neurotoxicity (DNT) potential of chemicals: example lists and criteria for their selection and use

There is a paucity of information concerning the developmental neurotoxicity (DNT) hazard posed by industrial and environmental chemicals. New testing approaches will most likely be based on batteries of alternative and complementary (non-animal) tests. As DNT is assumed to result from the modulation of fundamental neurodevelopmental processes (such as neuronal differentiation, precursor cell migration or neuronal network formation) by chemicals, the first generation of alternative DNT tests target these processes. The advantage of such types of assays is that they capture toxicants with multiple targets and modes-of-action. Moreover, the processes modelled by the assays can be linked to toxicity endophenotypes, i.e. alterations in neural connectivity that form the basis for neurofunctional deficits in man. The authors of this review convened in a workshop to define criteria for the selection of positive/negative controls, to prepare recommendations on their use, and to initiate the setup of a directory of reference chemicals. For initial technical optimization of tests, a set of >50 endpoint-specific control compounds was identified. For further test development, an additional “test” set of 33 chemicals considered to act directly as bona fide DNT toxicants is proposed, and each chemical is annotated to the extent it fulfills these criteria. A tabular compilation of the original literature used to select the test set chemicals provides information on statistical procedures, and toxic/non-toxic doses (both for pups and dams). Suggestions are provided on how to use the >100 compounds (including negative controls) compiled here to address specificity, adversity and use of alternative test systems.

Preparation of a tissue-like cortical primary culture from embryonic rats using Matrigel and serum free Start V Medium

To address the scientific quest for unravelling signalling pathways crucial in CNS development and function, cell culture systems have to be developed that are mimicking the physiological state of brain cells more efficiently. Here, we describe a method for cultivation of a virtual three-dimensional structure consisting of neural stem cell-derived cell types by using Matrigel as surface substrate and Start V as a serum free medium. We demonstrate that free floating dissociated cells form attached neurospheres from which cells start migration to surrounding areas and develop a virtual three-dimensional cell structure composed of neurons, glia and neural stem cells. Neuronal precursor cells differentiate into cholinergic and GABAergic cells and express vesicle proteins. Further, neuronal cells are interwoven with Nestin positive stem cells and GFAP positive astrocytes. Additionally, oligodendrocytes and microglia can also be detected in this neural tissue-like structure. As an example for studying cell migration we added externally microglial cells (BV2) and performed a confocal time lapse study. It revealed, that co-cultivated microglial cells migrated towards neurospheres within 14 h. Thus, the described method provides a serum free, tissue-like primary cell culture system of neural cells useful for the investigations of basic cell-cell interactions under in vitro conditions.

Doublecortin microtubule affinity is regulated by a balance of kinase and phosphatase activity at the leading edge of migrating neurons

Doublecortin (Dcx) is a microtubule-associated protein that is mutated in X-linked lissencephaly (X-LIS), a neuronal migration disorder associated with epilepsy and mental retardation. Although Dcx can bind ubiquitously to microtubules in nonneuronal cells, Dcx is highly enriched in the leading processes of migrating neurons and the growth cone region of differentiating neurons. We present evidence that Dcx/microtubule interactions are negatively controlled by Protein Kinase A (PKA) and the MARK/PAR-1 family of protein kinases. In addition to a consensus MARK site, we identified a serine within a novel sequence that is crucial for the PKA- and MARK-dependent regulation of Dcx's microtubule binding activity in vitro. This serine is mutated in two families affected by X-LIS. Immunostaining neurons with an antibody that recognizes phosphorylated substrates of MARK supports the conclusion that Dcx localization and function are regulated at the leading edge of migrating cells by a balance of kinase and phosphatase activity.

Keratan sulfate proteoglycan phosphacan regulates mossy fiber outgrowth and regeneration

We have examined the role of chondroitin sulfate proteoglycans (CSPGs) and keratan sulfate proteoglycans (KSPGs) in directing mossy fiber (MF) outgrowth and regeneration in rat hippocampal slice cultures. MFs normally exhibit a very specific innervation pattern that is restricted to the stratum lucidum (SL). In addition, MFs in hippocampal slice cultures will regenerate this specific innervation pattern after transection. CSPGs are one of the best characterized inhibitory axon guidance molecules in the CNS and are widely expressed in all areas of the hippocampus except SL. KSPGs are also widely expressed in the hippocampus, but their role in axon outgrowth has not been extensively studied in the CNS where phosphacan is the only protein that appears to contain KS-GAGs. Cultured hippocampal slices were treated with either chondroitin ABC lyase or keratanases to reduce the inhibitory axon guidance properties of CS and KS proteoglycans, respectively. The ability of transected MFs to regenerate their normal innervation pattern after digestion of CS and KS-GAGS sugars with these enzymes was examined. Only keratanase treatment resulted in misrouting of MFs. Identifying the mechanism by which keratanase produced MF misrouting is complicated by the presence of splice variants of the phosphacan gene that include the extracellular form of phosphacan and the transmembrane receptor protein tyrosine phosphatase beta/zeta (RPTPbeta/zeta). Both forms of phosphacan are made by astrocytes, suggesting that keratanase alters MF outgrowth by modifying astrocyte function.

Activation of JNK by vanadate induces a Fas-associated death domain (FADD)-dependent death of cerebellar granule progenitors in vitro

Apoptosis is a highly regulated process that plays a critical role in neuronal development as well as the homeostasis of the adult nervous system. Vanadate, an environmental toxicant, causes developmental defects in the central nervous system. Here, we demonstrated that vanadate induced apoptosis in cultured cerebellar granule progenitors (CGPs). Treatment of cultured CGPs with vanadate activated ERKs and JNKs but not p38 MAPK and also induced c-Jun phosphorylation. In addition, vanadate induced FasL production, Fas (CD95) aggregation, and its association with the Fas-associated death domain (FADD), as well as the activation of caspase-8. Furthermore, vanadate generated reactive oxygen species (ROS) in CGPs; however, ROS was not involved in vanadate-mediated MAPK activation. Vanadate-induced FasL expression was ROS-dependent but JNK-independent. In contrast, vanadate-elicited Fas aggregation and Fas-FADD association, as well as caspase-8 activation, were dependent on JNK activation but were minimally regulated by ROS generation. The hydrogen peroxide scavenger, catalase, blocked vanadate-induced FasL expression and partially mitigated vanadate-induced cell death. On the other hand, dominant negative FADD and caspase-8 inhibitor completely eliminated vanadate-induced apoptosis. Thus, JNK signaling plays a major role in vanadate-mediated activation of the Fas-FADD-caspase-8 pathway that accounts for vanadate-induced apoptosis of CGPs.

Association of LAR-like receptor protein tyrosine phosphatases with an enabled homolog in Hirudo medicinalis

Receptor protein tyrosine phosphatases (RPTPs) are thought to play important functions in pathfinding and target recognition by growing neuronal processes. The leech RPTPs HmLAR1 and HmLAR2 are expressed selectively by central neurons, Comb cells, and peripheral muscle tissues in the Hirudo medicinalis embryo. To explore the functions of HmLARs, we have sought to determine their physiological substrates. We report here the cloning and embryonic expression of Lena, the leech homolog of Enabled, a cytosolic protein implicated in actin-based cell motility. Lena is expressed in embryonic central neurons and in the Comb cell. We present experimental evidences indicating that Lena associates selectively with the intracellular domain of HmLAR1 and HmLAR2. Additionally, RNA interference (RNAi) of HmLAR1 in intact leech embryos leads to the hyperphosphorylation of Lena. We propose, therefore, that Lena is an in vivo substrate of HmLAR1 in neurons and perhaps of HmLAR2 in the Comb cells.

Abnormal polarization and axon outgrowth in retinal ganglion cells lacking the POU-domain transcription factor Brn-3b

The POU domain transcription factor Brn-3b (also called Brn-3.2) is essential for the normal development of retinal ganglion cells (RGCs) in the mouse. Without Brn-3b, RGCs commit to their fate and migrate to the ganglion cell layer, but most cells die during fetal development. An earlier report (L. Gan et al., 1999, Dev. Biol. 210, 469-480) suggested that cell death was caused by abnormal axon formation. Here, we use retinal explants from wild-type and mutant embryos to show that brn-3b-deficient RGCs are not properly polarized and tend to form dendrites rather than axons. Compared with wild-type explants, neurites of RGCs from brn-3b-deficient retinal explants grew slower, were shorter, and did not fasciculate properly. Mutant neurites had more microtubules than wild-type controls, and the arrangement of microtubules and neurofilaments was characteristic of dendrites rather than axons. Neurites from individual mutant RGCs displayed abnormal polarity and had dendrite-like branches extending outward from their main axis. Most mutant RGCs exhibited abnormal migratory behavior, and their neurites labeled intensely with the dendrite marker MAP-2. A small number of mutant RGCs were not migratory, and their neurites were longer and labeled positively for the axon marker tau-1, suggesting that some RGCs were not as severely affected by the absence of Brn-3b as others. Although tau-1 was not observed in most mutant neurites, it did accumulate in mutant cell bodies, implying that the absence of Brn-3b caused a defect in axon transport. Thus, Brn-3b appears to control the activity of genes that function in establishing RGC polarity, and without Brn-3b, RGCs cannot extend normal axons.

Growth cones are not required for initial establishment of polarity or differential axon branch growth in cultured hippocampal neurons

Hippocampal neurons developing in culture exhibit two types of differential, seemingly competitive, process outgrowth in the absence of external cues. During the initial acquisition of polarity, one of several equivalent undifferentiated minor neurites preferentially grows to become the axon. Once the axon has formed, it typically branches, and the branches grow differentially rather than concurrently. In axons with only two branches, growth alternates between branches. In both axon establishment and branch growth alternation, growth among sibling processes or branches must be differentially regulated. We found that elaborate and dynamic growth cones were associated with growth, whereas diminished growth cones were associated with nongrowing processes or branches. To test whether growth cones were necessary for differential growth, growth cone motility was eliminated by application of cytochalasin E. Although cytochalasin treatment before axon formation yielded longer processes overall, a similar percentage of both treated and untreated neurons had one process that grew more rapidly and became much longer than its sibling processes. Immunostaining to visualize dephospho-tau, an axonal marker, demonstrated that these single dominant processes were axons. Axons that formed in cytochalasin were thicker and showed more intense anti-tubulin staining than their sibling processes. Branched axons deprived of growth cones retained a pattern of differential growth and often included alternation. These results indicate that neither formation of a single axon nor differential growth of branches are dependent on growth cone motility and suggest that the neuron can regulate neurite elongation at sites other than at the growth cone.

Regulation of signaling by protein-tyrosine phosphatases: potential roles in the nervous system

During neuronal development, cells respond to a variety of environmental cues through cell surface receptors that are coupled to a signaling transduction machinery based on protein tyrosine phosphorylation and dephosphorylation. Receptor and non-receptor tyrosine kinases have received a great deal of attention; however, in the last few years, receptor (plasma membrane associated) and non-receptor protein-tyrosine phosphatases (PTPs) have also been shown to play important roles in development of the nervous system. In many cases PTPs have provocative distribution patterns or have been shown to be associated with specific cell adhesion and growth factor receptors. Additionally, altering PTP expression levels or activity impairs neuronal behavior. In this review we outline what is currently known about the role of PTPs in development, differentiation and neuronal physiology.

Laminin stimulates protein tyrosine dephosphorylation in PC12 cells

Laminin stimulates neurite outgrowth in rat pheochromocytoma cells (PC12 cells). Here, we investigated laminin signal transduction mechanisms by adding the tyrosine kinase/phosphatase modulators, genistein, quercetin, aurin tricarboxylic acid (ATA), and vanadate to PC12 cells. At 10 microM both genistein and quercetin enhanced laminin-mediated neurite outgrowth by 1.7- and 2.3-fold, respectively, while at 10 microM, ATA inhibited laminin-mediated neurite outgrowth by 92%. Vanadate inhibited neurite outgrowth by 63% at 10 microM. Immunoblot analysis revealed four proteins of approximately 240, 22, 110, and 35 kDa, which were dephosphorylated on tyrosine residues in laminin-treated PC12 cells, but not in NIH 3T3 cells. These results demonstrate that laminin-mediated neurite outgrowth involves protein tyrosine dephosphorylation and suggests that this mechanism may have specificity to neuronal cells.

Evidence for mitochondrial control of neuronal polarity

An early and essential step in the formation of functional neuronal circuits is the establishment of cell polarity, a process involving the morphological and functional differentiation of the axon (axonogenesis). We now report that treatment of cultured embryonic hippocampal neurons with ethidium bromide (EtBr; an agent that depletes mitochondrial DNA), prior to the establishment of cell polarity, prevents axon formation while permitting outgrowth of minor processes. The polarity-suppressing action of EtBr occurs under conditions of maintained cellular ATP levels, and is not mimicked by ATP-depleting agents (iodoacetate, p-(trifluoromethyoxy)phenylhydrazone [FCCP], and cyanide). Levels of tau, a microtubule-associated protein involved in axonogenesis, were not decreased in neurons treated with EtBr. Electron and confocal microscope analyses showed that EtBr treatment altered mitochondrial ultrastructure and subcellular localization. Basal levels of intracellular calcium were elevated 2- to 3-fold, intramitochondrial calcium levels were greatly increased, and mitochondrial transmembrane potential was decreased in EtBr-treated neurons. Exposure of neurons to a calcium ionophore prevented axonogenesis. Confocal images of intracellular calcium levels and mitochondrial localization in the same cells revealed congregations of mitochondria at the base of the axon associated with local reductions of intracellular calcium levels. When taken together with previous data indicating important roles for calcium in regulating neurite outgrowth, the present findings suggest critical roles for mitochondrial function and modulation of calcium homeostasis in the establishment of neuronal polarity.