Analysis of slow-onset neurite formation in NG108-15 cells: implications for a unified model of neurite elongation

Neuronal intermediate filament protein alpha-internexin facilitates axonal neurite elongation in neuroblastoma cells

We examined the localization and role of alpha-IN vs. other neuronal intermediate filaments before and during differentiation. Vimentin but not alpha-IN localized within filopodia-like neurites of undifferentiated cells. During differentiation, alpha-IN immunoreactivity accumulated within axonal neurites following vimentin but, as previously describe in neurons in situ, before the appearance of NF-L. We therefore manipulated alpha-IN synthesis, accumulation, and function in attempts to determine whether or not this intermediate filament species played a role in axonal development. Intracellular delivery of anti-alpha-IN antisense oligonucleotides and antibodies was permissive for neuritogenesis, yet compromised neurite elongation; this effect was further reflected in diminished levels of stabilized axonal microtubules. These data suggest that alpha-IN plays a role in the development of neuronal polarity. Relatively more alpha-IN than NF-L accumulated within the plastic axonal neurites induced following serum-deprivation, while stable, dbcAMP-induced neurites treatment contained equivalent levels of each. Protease inhibition increased NF-L and NF-H but not alpha-IN immunoreactivity within serum-deprived neurites, suggesting that proteolysis restricts NF-L accumulation pending neurite stabilization. To test the possibility that NF-H accumulation is dependent upon NF-L and cannot be mediated by alpha-IN, we examined levels of NF-H co-precipitated from cells with alpha-IN and NF-L. Virtually all newly synthesized NF-H co-precipitated with NF-L, while only a small percentage co-precipitated with alpha-IN. Finally, NF-H or NF-M were absent from the axon hillock or perikaryal area at the base of neurites, where alpha-IN immunoreactivity is prominent. These data extend earlier cell-free demonstrations that NF-H preferentially associates with NF-L rather than alpha-IN.

A transcription-dependent switch controls competence of adult neurons for distinct modes of axon growth

Although maturing neurons undergo a precipitous decline in the expression of genes associated with developmental axon growth, structural changes in axon arbors occur in the adult nervous system under both normal and pathological conditions. Furthermore, some neurons support extensive regrowth of long axons after nerve injury. Analysis of adult dorsal root ganglion (DRG) neurons in culture now shows that competence for distinct types of axon growth depends on different patterns of gene expression. In the absence of ongoing transcription, newly isolated neurons can extend compact, highly branched arbors during the first day in culture. Neurons subjected to peripheral axon injury 2-7 d before plating support a distinct mode of growth characterized by rapid extension of long, sparsely branched axons. A transition from "arborizing" to "elongating" growth occurs in naive adult neurons after approximately 24 hr in culture but requires a discrete period of new transcription after removal of the ganglia from the intact animal. Thus, peripheral axotomy-by nerve crush or during removal of DRGs--induces a transcription-dependent change that alters the type of axon growth that can be executed by these adult neurons. This transition appears to be triggered, in large part, by interruption of retrogradely transported signals, because blocking axonal transport in vivo can elicit competence for elongating growth in many DRG neurons. In contrast to peripheral axotomy, interruption of the centrally projecting axons of DRG neurons in vivo leads to subsequent growth in vitro that is intermediate between "arborizing" and "elongating" growth. This suggests that the transition between these two modes of growth is a multistep process and that individual steps may be regulated separately. These observations together suggest that structural remodeling in the adult nervous system need not involve the same molecular apparatus as long axon growth during development and regeneration.

Acute neurite retraction triggered by lysophosphatidic acid: timing of the inhibitory effects of genistein

Acute neurite retraction, elicited by diverse agents in several neuronal cell types, has been reported to be inhibited by genistein, a kinase antagonist that is relatively (though not absolutely) selective for tyrosine kinases. It was hypothesized that genistein acts upon some final common pathway that integrates multiple extrinsic and intrinsic signals to regulate whether neurites will execute a retraction response (J. Neurochem., 61 (1993) 340-343). To define this pathway in more detail, a quantitative study of NG108-15 cell rapid-onset neurites was carried out as they retract in response to lysophosphatidic acid (LPA, 10 microM). Following the application of LPA, most neurites exhibited early morphologic changes between 0.5 and 1.5 min, followed by progressive shortening and eventual retraction, with 50% of neurites completely retracted by 5 min and 80% gone by 10 min. Genistein did not inhibit the formation of subcortical F-actin, nor its functional competence in several assays. Genistein protected neurites when added at any time prior to the onset of the earliest morphologic changes, but failed to block progression when added to neurites that were already undergoing retraction. These findings imply that the final common pathway (i.e. the critical target(s) for genistein) must be activated late, after the increase in F-actin levels has peaked and just before retraction is initiated.

Acute neurite retraction elicited by diverse agents is prevented by genistein, a tyrosine kinase inhibitor

The retraction of axonal branches is a prominent feature of nervous system development and function. Although various biological and pathological signals can elicit retraction, little is known regarding their underlying mode of action. An in vitro assay using NG108-15 cells was used to demonstrate that rapid-onset neurites exposed acutely to trypsin, serum, lysophosphatidic acid, extracellular ATP, the phorbol ester phorbol 12-myristate 13-acetate, and nocodazole were all protected from retraction by the tyrosine kinase inhibitor genistein. This finding indicates that a common (genistein-sensitive) cellular event is involved in integrating the influence of multiple extrinsic and intrinsic signals and in regulating whether or not neurites will execute a retraction response.

Regulation of neuronal migration and neuritogenesis by distinct surface proteases. Relative contribution of plasmin and a thrombin-like protease

The relative contribution of two neuronal surface proteases, plasmin and a protease with thrombin-like specificity, on NB2a/dl neuroblastoma migration and neuritogenesis were examined. Exogenous plasmin induced cell body rounding and increased cell migration, but did not prevent or reverse neurite outgrowth. Inhibition of endogenous plasmin by its specific inhibitor, aprotinin, suppressed migration but did not induce neuritogenesis. Removal or inhibition of the thrombin-like protease by serum deprivation or hirudin addition, respectively, induced neurite outgrowth, as shown in our previous studies, but did not suppress migration. By contrast, trypsin induced simultaneous cell rounding and neurite retraction. These findings indicated that plasmin may regulate cell migration, while the thrombin-like protease may regulate facets of neurite outgrowth. Although unable to induce de novo neuritogenesis, plasmin inhibition potentiated the otherwise transient neurites induced by simultaneous inhibition of the thrombin-like protease. Since cultured neuronal cells migrate primarily in the direction of newly elaborated neurites, this finding is interpreted to indicate that cessation of neuronal migration by plasmin inhibition enhances net neurite outgrowth by inhibition of the putative thrombin-like protease.

Sequential effects of astroglial-derived factors on neurite outgrowth: initiation by protease inhibitors and potentiation by extracellular matrix components

Astroglial-conditioned medium (GCM) induced two distinct, but intimately related, phases of neuritogenesis in NB2a/d1 neuroblastoma cells--a "rapid-outgrowth," unstable phase, and a delayed, relatively stable phase, which are apparently regulated by glial-derived protease inhibitors and laminin, respectively. The initial rapid outgrowth (less than 4 hr) may be mediated by inhibition of a thrombin-like protease, present as a serum component and/or adsorbed to the outer neuronal surface, since (1) a similar effect was obtained by serum removal or by adding the specific thrombin inhibitor, hirudin; (2) exogenous thrombin inhibited the rapid outgrowth of neurites by GCM; and (3) cell-free enzyme assays confirmed the presence of thrombin-inhibitory activity in GCM. Although neurites induced by removal of serum removal or hirudin addition are rapidly resorbed following serum replenishment or hirudin depletion, GCM-induced neurites continued to elongate after GCM removal, indicating that GCM contained additional neurite-promoting factors. Anti-laminin antiserum did not inhibit the initial elaboration of neurites by GCM but prevented their continued elongation. Anti-laminin antiserum had no affect on neurite outgrowth induced by serum deprivation. The more protracted, second phase of neurite outgrowth could also be achieved by the addition of soluble purified laminin to undifferentiated cells. Unlike neurites at 4 hr, neurites at 24 hr were no longer dependent on the protease inhibitors in GCM, since exogenous thrombin no longer caused them to retract. Simultaneous addition of thrombin and anti-laminin antiserum with GCM had identical inhibitory effects on continued neurite elaboration at 24 hr as did anti-laminin antiserum without thrombin.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple proteases regulate neurite outgrowth in NB2a/dl neuroblastoma cells

Mouse NB2a/dl neuroblastoma cells elaborate axonal neurites in response to various chemical treatments including dibutyryl cyclic AMP and serum deprivation. Hirudin, a specific inhibitor of thrombin, initiated neurite outgrowth in NB2a/dl cells cultured in the presence of serum; however, these neurites typically retracted within 24 h. The cysteine protease inhibitors leupeptin and N-acetyl-leucyl-leucyl-norleucinal (CI; preferential inhibitor of micromolar calpain but also inhibits millimolar calpain) at 10(-6) M considerably enhanced neurite outgrowth induced by serum deprivation, but could not induce neuritogenesis in the presence of serum. A third cysteine protease inhibitor, N-acetyl-leucyl-leucyl-methional (CII; preferential inhibitor of millimolar calpain but also inhibits micromolar calpain), had no detectable effects by itself. Cells treated simultaneously with hirudin and either leupeptin, CI, or CII elaborated stable neurites in the presence of serum. Cell-free enzyme assays demonstrated that hirudin inhibited thrombin but not calpain, CI and CII inhibited calpain but not thrombin, and leupeptin inhibited both proteases. These results imply that distinct proteolytic events, possibly involving more than one protease, regulate the initiation and subsequent elongation and stabilization of axonal neurites. Since the addition of exogenous thrombin or calpain to serum-free medium did not modify neurite outgrowth, the proteolytic events affected by these inhibitors may be intracellular or involve proteases distinct from thrombin or calpain.

Cell attachment and neurite stability in NG108-15 cells: what is the role of microtubules?

Undifferentiated NG108-15 cells forming rapid-onset neurites were acutely exposed to nocodazole or trypsin. Resorption, cell rounding and detachment were delayed or prevented by 5'-deoxy,5'-methylthioadenosine (MTA), which selectively enhanced the strength of attachment responses. However, taxol, which stabilized microtubules, did not protect cell shape appreciably when trypsin or mechanical stimuli were used to decrease the strength of attachment. Together with numerous control experiments, this evidence suggests that the mechanical properties of microtubules do not contribute acutely to maintaining cell shape, though microtubules may play an indirect regulatory role (e.g. through their interactions with actin and substratum attachment sites). Patterns of trypsin-induced resorption resembled those seen 'spontaneously' in NG108-15 cells growing on laminin, and in fibroblastic CHO cells, suggesting that these results may be both physiologically relevant and applicable widely to many cell types.

Cell attachment and neurite stability in NG108-15 cells: effects of 5'-deoxy, 5'-methyl thioadenosine (MTA) compared with laminin, kinase inhibitor H-7, and Mn2+ ions

Methylation inhibitors such as 5'-deoxy, 5'-methyl thioadenosine (MTA) have been shown to exert an intriguing spectrum of effects upon neural cells: inhibiting responses to nerve growth factor in PC12 cells; stimulating outgrowth of rapid-onset neurites in NG108-15 cells; inhibiting the resorption and remodelling of these rapid-onset neurites; and inducing fibroblasts to extend long, multipolar, branched processes. To learn whether the apparently diverse effects of these agents might reflect some common underlying cellular sites of action, we examined simpler, short-term effects of MTA upon cell attachment and stability of rapid-onset neurites in NG108-15 cells, and compared the effects with those produced by the kinase inhibitor H-7, substratum-bound laminin, or Mn2+ ions. MTA was shown to selectively enhance the response of cells and their neurites in attaching to their substrata, without inducing other 'adhesive' responses such as cell spreading or motility. The data suggest that MTA activates or increases the effectiveness of 'attachment receptors', and that this at least partially explains its neurite-promoting effects. While it is still premature to propose a common mechanism of action for MTA and related agents, all of their known effects thus appear to involve modulating responses generated at cell-surface receptors. A further clue is that kinase C activity appears to be critical, since cells pretreated with phorbol esters for 24 h (which down-regulated levels of kinase C) selectively failed to extend neurites in response to MTA, but responded normally to laminin and Mn2+ ions.

Altered cell shapes in fibroblasts treated with 5'-deoxy-5'-methylthioadenosine: relation to morphogenesis of neural cells

In the present study, mouse 3T3 fibroblasts and primary human foreskin fibroblasts exposed to MTA (5'-deoxy-5'-methylthioadenosine) were found to achieve neural-like cell shapes and to extend long, multipolar processes rapidly and reversibly. Time lapse recordings and pharmacologic studies revealed that process formation in MTA-treated fibroblasts was mechanistically related to the rapid-onset mode of neurite formation previously characterized in neural hybrid NG108-15 cells. These data, together with evidence presented elsewhere, indicate that MTA selectively reorganizes the mode of expression of a specific cytoplasmic machinery that is active in many types of cells, and which is involved in regulating cell shape and neurite formation in developing neurons.

Morphologic plasticity of rapid-onset neurites in NG108-15 cells stimulated by substratum-bound laminin

Undifferentiated NG108-15 cells, when replated onto laminin-coated substrata, extend multipolar, highly branched neurite-like extensions up to 200 microns in length within 4 h; morphologic and pharmacologic properties of these 'rapid-onset neurites' have been described recently. The present study has extended these observations, using time lapse video recordings of their dynamic behavior and additional pharmacologic studies. Rapid-onset neurites and neuronal growth cones were shown to be regulated in an identical manner in all respects examined, including inhibition of outgrowth by cytochalasin B. Of particular interest was the observation that rapid-onset neurites in contact with laminin exhibited an extremely high rate of turnover, which was inhibited by 5'-deoxy-5'-methylthioadenosine (MTA). This system provides a uniquely favorable in vitro preparation in which neuritic plasticity can be elicited, directly observed and experimentally modulated under controlled conditions.