Peptides derived from the solvent-exposed loops 3 and 4 of BDNF bind TrkB and p75(NTR) receptors and stimulate neurite outgrowth and survival.

Brain-derived neurotrophic factor (BDNF) is critically involved in modeling the developing nervous system and is an important regulator of a variety of crucial functions in the mature CNS. BDNF exerts its action through interactions with two transmembrane receptors, either separately or in concert. BDNF has been implicated in several neurological disorders, and irregularities in BDNF function may have severe consequences. Administration of BDNF as a drug has thus far yielded few practicable results, and the potential side effects when using a multifunctional protein are substantial. In an effort to produce more specific compounds without side effects, small peptides mimicking protein function have been developed. The present study characterized two mimetic peptides, Betrofin 3 and Betrofin 4, derived from the BDNF sequence. Both Betrofins bound the cognate BDNF receptors, TrkB and p75(NTR), and induced neurite outgrowth and enhanced neuronal survival, probably by inducing signaling through tha Akt and MAPK pathways. Distinct, charged residues within the Betrofin sequences were identified as important for generating the neuritogenic response, which was also inhibited when BDNF was added together with either Betrofin, indicating partial agonistic effects of the peptides. Thus, two peptides derived from BDNF induced neurite outgrowth and enhanced neuronal survival, probably through binding to BDNF receptors.

Evidence for a release of brain-derived neurotrophic factor from the brain during exercise.

Brain-derived neurotrophic factor (BDNF) has an important role in regulating maintenance, growth and survival of neurons. However, the main source of circulating BDNF in response to exercise is unknown. To identify whether the brain is a source of BDNF during exercise, eight volunteers rowed for 4 h while simultaneous blood samples were obtained from the radial artery and the internal jugular vein. To further identify putative cerebral region(s) responsible for BDNF release, mouse brains were dissected and analysed for BDNF mRNA expression following treadmill exercise. In humans, a BDNF release from the brain was observed at rest (P < 0.05), and increased two- to threefold during exercise (P < 0.05). Both at rest and during exercise, the brain contributed 70-80% of circulating BDNF, while that contribution decreased following 1 h of recovery. In mice, exercise induced a three- to fivefold increase in BDNF mRNA expression in the hippocampus and cortex, peaking 2 h after the termination of exercise. These results suggest that the brain is a major but not the sole contributor to circulating BDNF. Moreover, the importance of the cortex and hippocampus as a source for plasma BDNF becomes even more prominent in response to exercise.

The Mts1/S100A4 protein is a neuroprotectant.

Mts1 (S100A4) is a calcium-binding protein of the EF-hand type, belonging to the S100 family of proteins. The mts1/S100A4 gene was originally isolated from tumor cell lines, and the protein is believed to play an important role in tumor progression. More recently, oligomeric, but not dimeric, forms of Mts1 have been shown to have a neuritogenic effect when added extracellularly to hippocampal neurons. Here we show increased neurite outgrowth in two other cell types, dopaminergic and cerebellar neurons, in response to treatment with Mts1 oligomers. Moreover, we demonstrate that Mts1 acts as a neuroprotectant in primary cerebellar, dopaminergic, and hippocampal neurons induced to undergo cell death. Interestingly, the survival of the cerebellar and hippocampal neurons increased as a result of treatment with Mts1 not only in oligomeric form but also--although to a lesser extent--in dimeric form. The inhibition of death in cerebellar neurons by Mts1 was accompanied by an inhibition of DNA fragmentation, but Mts1 did not affect the activity of caspases-3 and -6. In hippocampal neurons, cell death induced by the amyloid-beta peptide (Abeta(25-35)) was characterized by an increase in caspase-3 and -6 activity, but no DNA fragmentation was observed. As in cerebellar neurons, the induced increase in caspase activity in hippocampal neurons was not affected by Mts1.

Neuritogenic and survival-promoting effects of the P2 peptide derived from a homophilic binding site in the neural cell adhesion molecule.

The neural cell adhesion molecule (NCAM) plays a pivotal role in neural development, regeneration, and plasticity. NCAM mediates adhesion and subsequent signal transduction through NCAM-NCAM binding. Recently, a peptide ligand termed P2 corresponding to a 12-amino-acid sequence in the FG loop of the second Ig domain of NCAM was shown to mimic NCAM homophilic binding as reflected by induction of neurite outgrowth in hippocampal neurons. We demonstrate here that in concentrations between 0.1 and 10 microM, P2 also induced neuritogenesis in primary dopaminergic and cerebellar neurons. Furthermore, it enhanced the survival rate of cerebellar neurons although not of mesencephalic dopaminergic neurons. Moreover, our data indicate that the protective effect of P2 in cerebellar neurons was due to an inhibition of the apoptotic process, in that caspase-3 activity and the level of DNA fragmentation were lowered by P2. Finally, treatment of neurons with P2 resulted in phosphorylation of the ser/thr kinase Akt. Thus, a small peptide mimicking homophilic NCAM interaction is capable of inducing differentiation as reflected by neurite outgrowth in several neuronal cell types and inhibiting apoptosis in cerebellar granule neurons.

The role of phosphatidylinositol 3-kinase in neural cell adhesion molecule-mediated neuronal differentiation and survival.

The neural cell adhesion molecule, NCAM, is known to stimulate neurite outgrowth from primary neurones and PC12 cells presumably through signalling pathways involving the fibroblast growth factor receptor (FGFR), protein kinase A (PKA), protein kinase C (PKC), the Ras-mitogen activated protein kinase (MAPK) pathway and an increase in intracellular Ca2+ levels. Stimulation of neurones with the synthetic NCAM-ligand, C3, induces neurite outgrowth through signalling pathways similar to the pathways activated through physiological, homophilic NCAM-stimulation. We present here data indicating that phosphatidylinositol 3-kinase (PI3K) is required for NCAM-mediated neurite outgrowth from PC12-E2 cells and from cerebellar and dopaminergic neurones in primary culture, and that the thr/ser kinase Akt/protein kinase B (PKB) is phosphorylated downstream of PI3K after stimulation with C3. Moreover, we present data indicating a survival-promoting effect of NCAM-stimulation by C3 on cerebellar and dopaminergic neurones induced to undergo apoptosis. This protective effect of C3 included an inhibition of both DNA-fragmentation and caspase-3 activation. The survival-promoting effect of NCAM-stimulation was also shown to be dependent on PI3K.