Neurotrophic factors

Neurotrophic factors are a heterogeneous group of molecules that have, as a common feature, the ability to promote neuronal survival and/or differentiation. The prototypic neurotrophic factor, nerve growth factor (NGF), was first isolated by Levi-Montalcini. At first, NGF was thought to play a role only in the development of the peripheral nervous system, responsible for the maintenance of sympathetic and dorsal root ganglion neurons (29, 37). Initial investigations of a possible role for NGF in brain development failed to demonstrate its expression in brain. With the advent of modern molecular techniques, however, NGF was found to be expressed in the developing and adult rodent brain (48). Additionally, the low-affinity NGF receptor (a.k.a. P75) was shown to be produced by CNS cholinergic neurons (41, 79). Culture and in vivo studies established that NGF was capable of promoting the survival and process outgrowth of basal forebrain and striatal cholinergic neurons (11, 21, 32). Given the wide variety of CNS neuronal phenotypes and the restricted spectrum of NGF activity in the CNS, it was considered likely that other neurotrophic factors must exist that affected noncholinergic neurons. In the mid-1980s, small quantities of such a factor were isolated from brain. This factor, which had neurotrophic activity in vitro and was clearly not NGF, was called brain-derived neurotrophic factor (BDNF) (6). When BDNF was cloned and sequenced, there were clear homologies to NGF, making it part of the same molecular family, the neurotrophins. Subsequently other neurotrophins have been identified, including neurotrophin 3 (NT-3) (56), neurotrophin 4/5 (NT-4/5) (20) and neurotrophin 6 (NT-6) (30). There is further complexity to the neurotrophin story. NGF, BDNF, and NT-3 all have multiple splice variants that arise, at least in part, through differential regulation of multiple promoter regions (45, 52, 76). The functional significance of these variants is not known, but it appears that the multiple promoters provide a basis for region- and cell-specific differences in transcriptional regulation (50, 51).

The primary functional receptors for the neurotrophins have been identified as members of the trk family of tyrosine kinases. There is a selectivity in the response characteristics of the trks for the different neurotrophins (for a review, see 57). Trk (or trkA) responds preferentially to NGF, trkB responds to BDNF and NT-4/5, and trkC responds to NT-3. These response characteristics are not absolute, and there is some overlap in receptor responsiveness (e.g., NT-3 has some activity at the trkA receptor). All three trks are expressed in mammalian CNS. Expression of the appropriate trk receptor is required for most of the neurotrophic functions of neurotrophins, although each can bind the low-affinity P75 receptor and may thereby influence apoptotic activity. Like the neurotrophins, the trks also have numerous splice variants, including forms with deletions, or severe truncation, of the cytoplasmic domain (3, 16, 61, 84). The function of these splice variants is unknown, but the predominant expression of truncated forms by glial and ventricle epithelial cells has suggested that these receptors, which lack full signal transduction capacity, may serve to limit diffusion of active factor through the neuropil (87).

The interaction of neurotrophins with the trk receptors induces a biochemical cascade of events, many steps of which are now known (for review, see 40). Neurotrophin binding to the homodimeric trk receptor induces tyrosine autophosphorylation and an activation of the kinase activities of the receptor. This leads to the phosphorylation and activation of latent second messengers and transcription factors that modulate the transcription of a variety of immediate early genes (IEGs). The IEG products include other transcription factors that regulate the expression of a range of genes that in turn regulate biochemical and morphologic differentiation, such as the production of neurotransmitters or elaboration of neurites. Several of these gene products are of particular interest in regard to epilepsy, although those playing the most important role have yet to be identified. For example, the neurotrophins have been demonstrated to regulate the expression of calcium-binding proteins, long suspected to buffer intracellular calcium levels and to protect cells from excitotoxic insult (2). Moreover, BDNF can induce the expression of neuropeptide Y (NPY) and may indeed be responsible for the induction of NPY synthesis in response to seizures (63). NPY, in turn, has a strong antiepileptic effect (42, 71, 78).

A less well understood but equally important function of neurotrophins is the prevention of cell death. It has long been known that peripheral sympathetic neurons cultured in the absence of NGF undergo programmed, or apoptotic, cell death (58)—a cell death program that depends on both RNA and protein synthesis leading to DNA fragmentation prior to neuronal lysis. Neurotrophins signaling through the trk family of receptors may prevent apoptosis by enhancing transcription of protective factors (e.g., b-cl2) that inhibit apoptosis or by inhibiting the transcription of genes within the cell death program.

An exciting discovery in the field of neurotrophin signaling is that the P75 receptor, which binds all of the neurotrophins but was for a time thought to play little role in signal transduction, actually does play a role in the regulation of cell death (70). Neurotrophins enhance cell death when they interact with the P75 receptor in the absence of trk expression, but in other cases neurotrophins can prevent cell death by interacting with the P75 receptors. Additionally, under hypoxic-ischemic or excitotoxic conditions, neurotrophins may enhance the necrotic death of neurons that normally respond to neurotrophins in a trophic manner. It is not known if these effects are mediated by trk or P75 receptors (43).

Several recent experiments have documented novel, rather surprising neuromodulatory functions of neurotrophins. As was first shown in electrophysiologic studies of acute hippocampal slices in culture, BDNF and NT-3 potentiate glutamatergic transmission within hippocampus (38, 39). This effect builds over a period of about 30 minutes, can be longlasting, and is reported to depend on new protein synthesis. Subsequent studies demonstrated the potentiating effects of neurotrophins within the neocortex, as well (1). Together with evidence that seizures stimulate neurotrophin synthesis and increases in neurotrophin content within axons and terminal boutons (14), these findings further suggest that the contribution of the neurotrophins to synaptic physiology may be potentiated in the wake of seizures; as a consequence, these factors may make a particularly large contribution to postictal events.

Beyond the neurotrophins, the expression and activities of several additional families of central neurotrophic factors are of interest in relation to the effects of seizures. The cytokine superfamily includes glial-cell-line-derived neurotrophic factor (GDNF), ciliary neurotrophic factor (CNTF), leukemia inhibitory factor, and the bone morphogenetic proteins (BMPs). GDNF and CNTF have both been studied in clinical trials to test their therapeutic value in the treatment of neurodegenerative diseases. These factors interact with the serine-threonine protein kinase receptors (53). Another large family of fibroblast growth factors (FGFs) includes several members that are expressed in the CNS and, like the neurotrophins, are thought to play roles in neuronal and glial differentiation, protein synthesis, and survival. The FGFs interact with at least three tyrosine kinase receptors (7). The epidermal growth factor (EGF) family contains several members, the prototype of which is EGF, that interact with the 170-kd tyrosine kinase EGF receptor (EGF-R). These peptides also have numerous actions on CNS cells. Interestingly, members of both the EGF and FGF family can induce the proliferation of pluripotent CNS stem cells (73, 86). These cells are capable of producing neurons, astrocytes, and oligodendrocytes. Members of another family of peptide growth factors, the heregulins, interact with a group of receptors that share sequence homology with EGF-R but do not bind the EGF family of peptides (69). The functions of these peptides are not yet well understood, but they appear to be important for some aspects of brain development (15, 27, 47). Additionally, numerous other compounds have been reported to have neurotrophic activities.