Abnormal retinal projections alter GAP-43 patterns in the diencephalon

Target- as well as source-derived factors direct the morphogenesis of anomalous retino-thalamic projections

Neonatal tectal lesions in hamsters result in the elimination of a major central target of retinal axons, massively denervate the lateral posterior nucleus of the thalamus (LP), and lead to a marked increase of the retino-LP projection. In such animals, retino-LP axons show all of the normally-occurring terminal types. In addition, large clusters of varicosities, whose tubular configuration resembles the major type of tecto-LP terminals observed in normal animals, are also noted if the tectal lesion is made on the day after birth (P1). If, however, the neonatal lesion occurs on P5 rather than on P1, terminals resembling normal tecto-LP endings are rarely observed; rather, the distribution and morphology of retino-LP terminals bear a greater resemblance to those seen in normal hamsters, but the size and complexity of the terminals, particularly those that form string-like arrangements, is significantly increased. Our findings suggest that the altered morphology of some abnormally induced retino-LP terminals may be orchestrated by target-associated signals. However, there are age-related limitations on the degree to which afferent systems can vary their terminal morphology; these restrictions may derive from the target, or may be a function of intrinsic changes within the cells of origin of the afferent fibers.

The expression of rat GAP-43 cDNA in transgenic carp

To understand more about growth-associated protein-43 (GAP-43), we produced transgenic carp by introduction of a rat GAP-43 cDNA linked to the Rous sarcoma virus-long terminal repeat into fertilized eggs. Of 180 eggs microinjected with exogenous gene, 59 embryos hatched and 4 fish were found to contain the exogenous gene sequences in the genomic DNA. From a mature female transgenic carp, parthenogenetically, 126 progeny were derived and 52 of them survived for more than 90 days. The exogenous gene sequences were detected in 22 F1 progeny, and its messenger RNA was detected in all of 10 transgenic F1 carp examined. In serum-free medium, cultured retinal ganglion cells isolated from transgenic carp elongated their axons, while non-transgenic cells did not elongate axons.

Changes in rapidly transported proteins associated with development of abnormal projections in the diencephalon

The development of the hamster visual system is accompanied by striking changes in the pattern of proteins that are synthesized in retinal ganglion cells and rapidly transported to their nerve terminals. To determine whether any of these protein changes are regulated by interactions between the developing nerve endings and the cells with which they form synapses, we induced retinofugal axons to form abnormal projections in the lateral posterior (LP) nucleus of the thalamus and dense patches of hyperinnervation in the lateral geniculate nucleus (LGN) by removing their principal target, the superior colliculus (SC), the day after birth. Under these experimental conditions, two rapidly transported proteins, including the neural cell adhesion molecule, NCAM, showed significant changes in their time course of expression. NCAM, identified here using a monospecific antibody, is normally synthesized and transported at high levels at early stages of development and then declines during the second and third postnatal weeks. However, this decline was delayed when optic fibers were re-routed. A second rapidly transported protein, M(r) = 67 kDa, pI = 4.7, normally shows a rise in its synthesis and transport during terminal arbor formation and a subsequent decline, but it also remained elevated for a prolonged period when the SC was absent. These findings cannot be accounted for by a simple delay in the retinal ganglion cells' program of axonal growth, since other rapidly transported proteins, including the growth-associated protein GAP-43, showed a normal developmental time-course when the SC was removed. Target interactions therefore appear to influence the retinal ganglion cells' expression of different proteins in a specific fashion.

Vibrissectomy induced changes in GAP-43 immunoreactivity in the adult rat barrel cortex

Within the rat primary somatosensory cortex, neurons responding principally to movement of each individual mystacial vibrissa are grouped together in structures termed barrels. Previous studies have examined changes in the area of cortex showing increased 2-deoxyglucose uptake in response to vibrissal stimulation. These studies have shown that chronic removal of all but the central (C3) vibrissa in adult rats induces an enlarged representation of the remaining C3 barrel in the contralateral cortex. This increase is prevented by cortical norepinephrine depletion. The major question raised by such studies is whether such plasticity is due to structural rearrangement or unmasking of otherwise silent synapses. In this study, antibodies to GAP-43, a presynaptic protein whose synthesis is related to neuronal development and regeneration, were used to investigate this issue. In adult rat brain, tangential sections through layer IV of the barrel receptor field normally show moderate levels of GAP-43 immunoreactivity (GAP-IR) in the inter-barrel septa and low levels within the barrels themselves. The present study examined changes in the pattern of GAP-IR from 1 to 8 weeks after vibrissectomy with sparing of C3 as an index of possible physical reorganization of cortical circuits. Quantitative analysis of the cortices of animals with unilateral vibrissectomy with sparing of C3 showed that the area of low GAP-IR within the barrels surrounding C3 was decreased at 1 week (8.4% shrinkage; P less than 0.01) and 8 weeks (12.0% shrinkage; P less than 0.015), relative to the cortex ipsilateral to the surgery. Both bilateral vibrissectomy with sparing of C3 and ibotenic acid lesions of the ventrobasal thalamus produced similar results. Some evidence was also seen that the area of low GAP-IR in the C3 barrel shrank to a similar degree after such manipulations. Cortical norepinephrine depletion had no apparent effect on vibrissectomy-induced GAP-IR changes. These results suggest that removal of vibrissal input to the adult rat barrel cortex produces transynaptic induction of axonal sprouting within the barrel cortex.

Role of the growth-associated protein B-50/GAP-43 in neuronal plasticity

The neuronal phosphoprotein B-50/GAP-43 has been implicated in neuritogenesis during developmental stages of the nervous system and in regenerative processes and neuronal plasticity in the adult. The protein appears to be a member of a family of acidic substrates of protein kinase C (PKC) that bind calmodulin at low calcium concentrations. Two of these substrates, B-50 and neurogranin, share the primary sequence coding for the phospho- and calmodulin-binding sites and might exert similar functions in axonal and dendritic processes, respectively. In the adult brain, B-50 is exclusively located at the presynaptic membrane. During neuritogenesis in cell culture, the protein is translocated to the growth cones, i.e., into the filopodia. In view of many positive correlations between B-50 expression and neurite outgrowth and the specific localization of B-50, a role in growth cone function has been proposed. Its phosphorylation state may regulate the local intracellular free calmodulin and calcium concentrations or vice versa. Both views link the B-50 protein to processes of signal transduction and transmitter release.