Overexpression of GAP-43 reveals unexpected properties of hippocampal mossy fibers

A Shift from a Pivotal to Supporting Role for the Growth-Associated Protein (GAP-43) in the Coordination of Axonal Structural and Functional Plasticity

In a number of animal species, the growth-associated protein (GAP), GAP-43 (aka: F1, neuromodulin, B-50, G50, pp46), has been implicated in the regulation of presynaptic vesicular function and axonal growth and plasticity via its own biochemical properties and interactions with a number of other presynaptic proteins. Changes in the expression of GAP-43 mRNA or distribution of the protein coincide with axonal outgrowth as a consequence of neuronal damage and presynaptic rearrangement that would occur following instances of elevated patterned neural activity including memory formation and development. While functional enhancement in GAP-43 mRNA and/or protein activity has historically been hypothesized as a central mediator of axonal neuroplastic and regenerative responses in the central nervous system, it does not appear to be the crucial substrate sufficient for driving these responses. This review explores the historical discovery of GAP-43 (and associated monikers), its transcriptional, post-transcriptional and post-translational regulation and current understanding of protein interactions and regulation with respect to its role in axonal function. While GAP-43 itself appears to have moved from a pivotal to a supporting factor, there is no doubt that investigations into its functions have provided a clearer understanding of the biochemical underpinnings of axonal plasticity.

New Insights on Temporal Lobe Epilepsy Based on Plasticity-Related Network Changes and High-Order Statistics

Epilepsy is a disorder of the brain characterized by the predisposition to generate recurrent unprovoked seizures, which involves reshaping of neuronal circuitries based on intense neuronal activity. In this review, we first detailed the regulation of plasticity-associated genes, such as ARC, GAP-43, PSD-95, synapsin, and synaptophysin. Indeed, reshaping of neuronal connectivity after the primary, acute epileptogenesis event increases the excitability of the temporal lobe. Herein, we also discussed the heterogeneity of neuronal populations regarding the number of synaptic connections, which in the theoretical field is commonly referred as degree. Employing integrate-and-fire neuronal model, we determined that in addition to increased synaptic strength, degree correlations might play essential and unsuspected roles in the control of network activity. Indeed, assortativity, which can be described as a condition where high-degree correlations are observed, increases the excitability of neural networks. In this review, we summarized recent topics in the field, and data were discussed according to newly developed or unusual tools, as provided by mathematical graph analysis and high-order statistics. With this, we were able to present new foundations for the pathological activity observed in temporal lobe epilepsy.

Septo-temporal distribution and lineage progression of hippocampal neurogenesis in a primate (Callithrix jacchus) in comparison to mice

Adult born neurons in the hippocampus show species-specific differences in their numbers, the pace of their maturation and their spatial distribution. Here, we present quantitative data on adult hippocampal neurogenesis in a New World primate, the common marmoset (Callithrix jacchus) that demonstrate parts of the lineage progression and age-related changes. Proliferation was largely (∼70%) restricted to stem cells or early progenitor cells, whilst the remainder of the cycling pool could be assigned almost exclusively to Tbr2+ intermediate precursor cells in both neonate and adult animals (20–122 months). Proliferating DCX+ neuroblasts were virtually absent in adults, although rare MCM2+/DCX+ co-expression revealed a small, persisting proliferative potential. Co-expression of DCX with calretinin was very limited in marmosets, suggesting that these markers label distinct maturational stages. In adult marmosets, numbers of MCM2+, Ki67+, and significantly Tbr2+, DCX+, and CR+ cells declined with age. The distributions of granule cells, proliferating cells and DCX+ young neurons along the hippocampal longitudinal axis were equal in marmosets and mice. In both species, a gradient along the hippocampal septo-temporal axis was apparent for DCX+ and resident granule cells. Both cell numbers are higher septally than temporally, whilst proliferating cells were evenly distributed along this axis. Relative to resident granule cells, however, the ratio of proliferating cells and DCX+ neurons remained constant in the septal, middle, and temporal hippocampus. In marmosets, the extended phase of the maturation of young neurons that characterizes primate hippocampal neurogenesis was due to the extension in a large CR+/DCX- cell population. This clear dissociation between DCX+ and CR+ young neurons has not been reported for other species and may therefore represent a key primate-specific feature of adult hippocampal neurogenesis.

Impaired sprouting and axonal atrophy in cerebellar climbing fibres following in vivo silencing of the growth-associated protein GAP-43

The adult mammalian central nervous system has a limited ability to establish new connections and to recover from traumatic or degenerative events. The olivo-cerebellar network represents an excellent model to investigate neuroprotection and repair in the brain during adulthood, due to its high plasticity and ordered synaptic organization. To shed light on the molecular mechanisms involved in these events, we focused on the growth-associated protein GAP-43 (also known as B-50 or neuromodulin). During development, this protein plays a crucial role in growth and in branch formation of neurites, while in the adult it is only expressed in a few brain regions, including the inferior olive (IO) where climbing fibres (CFs) originate. Following axotomy GAP-43 is usually up-regulated in association with regeneration. Here we describe an in vivo lentiviral-mediated gene silencing approach, used for the first time in the olivo-cerebellar system, to efficiently and specifically downregulate GAP-43 in rodents CFs. We show that lack of GAP-43 causes an atrophy of the CF in non-traumatic conditions, consisting in a decrease of its length, branching and number of synaptic boutons. We also investigated CF regenerative ability by inducing a subtotal lesion of the IO. Noteworthy, surviving CFs lacking GAP-43 were largely unable to sprout on surrounding Purkinje cells. Collectively, our results demonstrate that GAP-43 is essential both to maintain CFs structure in non-traumatic condition and to promote sprouting after partial lesion of the IO.

Adult learning and remodeling of hippocampal mossy fibers: unheralded participant in circuitry for long-lasting spatial memory

Two articles in this issue concerning the overexpression of GAP-43 on mossy fiber growth are related to the plasticity of these axons in relation to learning and memory.