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Growth cone motility and navigation in response to extracellular signals are regulated by actin dynamics. To better understand actin involvement in these processes we determined how and in what form actin reaches growth cones, and once there, how actin assembly is regulated. A continuous supply of actin is maintained at the axon tip by slow transport, the mobile component consisting of an unassembled form of actin. Actin is co-transported with actin-binding proteins, including ADF and cofilin, structurally related proteins essential for rapid turnover of actin filaments in vivo. ADF and cofilin activity is regulated through phosphorylation by LIM kinases, downstream effectors of the Rho family of GTPases, Cdc42, Rac and Rho. Attractive and repulsive extracellular guidance cues might locally alter actin dynamics by binding specific GTPase-linked receptors, activating LIM kinases, and subsequently modulating the activity of ADF/cofilin. ADF is enriched in growth cones and is required for neurite outgrowth. In addition, signals that influence growth cone behavior alter ADF/cofilin phosphorylation, and overexpression of ADF enhances neurite outgrowth. Growth promoting effects of laminin are mimicked by expression of constitutively active Cdc42 and blocked by expression of the dominant negative Cdc42. Repulsive effects of myelin and sema3D on growth cones are blocked by expression of constitutively active Rac1 and dominant negative Rac1, respectively. Thus a series of complex pathways must exist for regulating effectors of actin dynamics. The bifurcating nature of the ADF/cofilin phosphorylation pathway may provide the integration necessary for this complex regulation.

Protein F1/GAP-43 is differentially expressed in brain with high levels present in regions associated with memory functions. However, in hippocampus the granule cells lack F1/GAP-43 expression. To determine if this lack of expression is due to inhibitory signals from the target cells, we selectively destroyed CA3 pyramidal cells unilaterally using microinjections of excitotoxins. Kainate lesions induced F1/GAP-43 mRNA expression bilaterally in granule cells at 24 h post-injection. Since the induction contralateral to the lesion was not due to loss of target cells, that induction may be ascribed to consequences of seizure activity. However, F1/GAP-43 mRNA hybridization decreased by 3 d post-lesion and was at background levels by 6 d, indicating that the lack of F1/GAP-43 expression in granule cells is restored despite a lack of target neurons. Unilateral lesions of CA3 cells using ibotenate, which are not as complete as kainate but do not cause seizures, did not induce F1/GAP-43 mRNA in granule cells on either the contralateral or, in 4 of 5 cases, the ipsilateral side. Taken together, these data suggest that the CA3 target is not essential for the absence of F1/GAP-43 expression in granule cells. To compare the extent of damage caused by the lesions, we investigated the location of astrocytes undergoing reactive gliosis, employing as a reporter glial fibrillary acidic protein (GFAP) gene expression. After both kainate and ibotenate injections GFAP hybridization increased in the lesioned area as well as in the contralateral hippocampus. These results indicate that injections of kainate, and possibly ibotenate to a lesser extent, may affect behavior not only by damaging cells at the injection site, but also by altering gene expression in cells at distant sites.

The phosphorylation of MARCKS, but not protein F1/GAP-43, is increased in the intermediate and medial portion of the hyperstriatum ventrale (IMHV) after chick imprinting. Here we investigated if MARCKS, but not F1/GAP-43, gene expression would also be altered after imprinting. We first investigated the constitutive mRNA distribution of MARCKS and F1/GAP-43 in chick brain. MARCKS mRNA was expressed in most cells and exhibited a relatively homogeneous distribution. In contrast, F1/GAP-43 mRNA levels were elevated in discrete brain regions, as we had observed in mammals. The highest F1/GAP-43 mRNA levels in the chick brain were in sensory and associational structures such as the hyperstriatal complex and neostriatum, and lower levels were in structures involved in motor control, such as paleostriatum. These results in chick are consistent with the previously drawn generalization that F1/GAP-43 mRNA is expressed in those brain regions which exhibit synaptic plasticity. After imprinting, MARCKS mRNA levels in IMHV were higher in good learners than poor learners. In contrast, analysis of F1/GAP-43 mRNA levels revealed no differences related to training in any brain region sampled. These selective results for MARCKS but not F1/GAP-43 parallel the prior findings on their phosphorylation, and are consistent with our hypothesis that the very same proteins that are post-translationally modified in association with learning and memory also undergo alterations in their gene expression.

In the adult rat hippocampus mRNA of F1/GAP-43, an axonal growth-associated protein, is highly expressed in pyramidal cells, but is absent in granule cells. To determine whether granule cells can be induced to express mRNA of F1/GAP-43, transcript levels were studied after limbic seizures, which can induce sprouting of granule cell mossy fibers. Seizure-inducing electrolytic lesions were made in the dentate gyrus hilus with stainless-steel electrodes and mRNA levels were measured in contralateral hippocampus by quantitative in situ hybridization. Induction of F1/GAP-43 mRNA expression was observed in granule cells at 24 h, but not at 6 or 12 h, after the hilar lesion. When equivalent sized hilar lesions were made with platinum electrodes, which do not induce seizures, no hybridization was apparent over the granule cells. Hybridization over granule cells had declined by 48 h post-lesion, but even at 10 days it was still slightly higher than in control rats. F1/GAP-43 mRNA expression was also increased 2-fold in CA1 pyramidal cells with peak expression at 48 h post-lesion. These are the first data to our knowledge that demonstrate that F1/GAP-43 gene expression can be altered in neurons located within the adult brain. Induction of F1/GAP-43 mRNA expression in the granule cells may be important for the sprouting of mossy fibers and could be triggered by the elevated levels of brain-derived neurotrophic factor in CA3 cells which precede the increased F1/GAP-3 gene expression in granule cells.

It is becoming increasingly evident that proteins of the actin depolymerizing factor (ADF)/cofilin family are essential regulators of actin turnover required for many actin-based cellular processes, including motility. ADF can increase actin turnover by either increasing the rate of actin filament treadmilling or by severing actin filaments. In neurons ADF is highly expressed in neuronal growth cones and its activity is regulated by many signals that affect growth cone motility. In addition, increased activity of ADF causes an increase in neurite extension. ADF activity is inhibited upon phosphorylation by LIM kinases (LIMK), kinases activated by members of the Rho family of small GTPases. ADF become dephosphorylated downstream of signal pathways that activate PI-3 kinase or increase levels of intracellular calcium. The growth-regulating effects of ADF together with its ability to be regulated by a wide variety of guidance cues, suggest that ADF may regulate growth cone advance and navigation.

Three days after long-term potentiation (LTP) there is a decrease in the gene expression of protein F1 (GAP-43) and gamma-PKC in CA3 pyramidal cells that is correlated with the magnitude of LTP. We predicted these decreases would be preceded by an increment in gene expression. At 1 h, but not at 2 h after LTP, F1/GAP-43 and gamma-PKC mRNA hybridization were increased, but increases were also observed after control stimulation. At both 1 and 2 h after LTP, changes in F1/GAP-43 hybridization were positively correlated with gamma-PKC hybridization and negatively correlated with LTP magnitude. These data indicate that correlated alterations in F1/GAP-43 gene expression and synaptic efficacy can occur as early as 1 h after LTP and persist for days.

The mRNA levels of protein F1 (also known as GAP-43), and protein kinase C (PKC) subtypes were measured 3 days after the induction of long-term enhancement (also known as long-term potentiation) in the hippocampus of chronically prepared conscious rats by quantitative in situ hybridization. Altered mRNA levels correlated significantly with alternations in synaptic efficacy; such correlations have not been reported previously. Rats with greater synaptic enhancement had lower gene expression in the CA3 subfield of F1/GAP-43 and both beta-PKC and gamma-PKC, but not alpha-PKC. For microtubule-associated protein 2 (MAP-2), neurogranin, and the glutamate receptor subtype B-flip, no correlation was observed in any cell field between synaptic enhancement and hybridization to the mRNA. To our surprise, alterations in mRNA levels of F1/GAP-43 and gamma-PKC were highly correlated (r = +0.928, P < 0.001), suggesting coordinate regulation. Since F1/GAP-43 is associated with neurite growth, its lowered expression at 3 days would reduce potential growth, leading to synaptic stabilization. We propose that long-term synaptic change is mediated by gene expression of the very same proteins initially modified posttranslationally.