Reversible sedimentation and masking of nerve growth factor (NGF) antigen by high molecular weight fractions from rat brain

Locomotor exercise alters expression of pro-brain-derived neurotrophic factor, brain-derived neurotrophic factor and its receptor TrkB in the spinal cord of adult rats

Previous evidence indicates that locomotor exercise is a powerful means of increasing brain-derived neurotrophic factor (BDNF) and its signal transduction receptor TrkB mRNA levels, immunolabeling intensity and number of BDNF- and TrkB-immunopositive cells in the spinal cord of adult rats but the contribution of specific cell types to changes resulting from long-term activity is unknown. As changes in BDNF protein distribution due to systemic stimuli may reflect either its in-situ synthesis or its translocation from other sources, we investigated where BDNF and TrkB mRNA are expressed in the spinal lumbar segments. We report on the cell types defined by size, BDNF mRNA levels and number of cells with TrkB transcripts in sedentary and exercised animals following 28 days of treadmill walking. In the majority of cells, exercise increased perikaryonal levels of BDNF mRNA but did not affect TrkB transcript levels. Bidirectional changes in a number of TrkB mRNA-expressing cells occurred in small groups of ventral horn neurons. An increase in BDNF transcripts was translated into changes in pro-BDNF and BDNF levels. A 7-day walking regimen increased BDNF protein levels similarly to 28-day treadmill walking. Our observations indicate that long- and short-term locomotor activity of moderate intensity produce stimuli sufficient to recruit a majority of spinal cells to increased BDNF synthesis, suggesting that continuous tuning of pro-BDNF and BDNF levels permits spinal networks to undergo trophic modulation not requiring changes in TrkB mRNA supply.

Neurotrophin-like immunoreactivity in the human pre-term newborn, infant, and adult cerebellum

The immunohistochemical occurrence of the neurotrophin (NT) proteins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT-4), and neurotrophin-3 (NT-3) is shown in the pre-term newborn, infant, and adult human post-mortem cerebellum. The NT-like immunoreactive structures were unevenly distributed and showed regional differences among cerebellar lobules and folia. NGF-, NT-4-, and NT-3-positive neuronal perikarya were observed in all specimens examined. At variance with the other neurotrophins, the BDNF antiserum labelled neuronal cell bodies only in newborn life and infancy, as well as extensive nerve fibre systems, whose density increased with age. The NT-antibodies, tested by Western blot on human cerebellum homogenates, revealed immunoreactive bands corresponding to proteins of heterogenous molecular weight. The results obtained provide a first demonstration of the tissue localization of the NTs in the human cerebellum from perinatal to adult age, thus suggesting their involvement in the development, differentiation and maintenance of the cerebellar connectivity. Codistribution of the four NTs or sets of them was observed in cortical and deep nuclei neurons. Multiple trophic roles for NTs, encompassing the classic target-derived and local mechanisms of support, are envisaged as significant in development, differentiation, and maintenance of the human cerebellar connectivity.

Delivery of neurotrophic factors to the central nervous system: pharmacokinetic considerations

Neurotrophic factors are proteins with considerable potential in the treatment of central nervous system (CNS) diseases and traumatic injuries. However, a significant challenge to their clinical use is the difficulty associated with delivering these proteins to the CNS. Neurotrophic factors are hydrophilic, typically basic, monomeric or dimeric proteins, mostly in the size range of 5 to 30 kDa. Neurotrophic factors potently support the development, growth and survival of neurons, eliciting biological effects at concentrations in the nanomolar to femtomolar range. They are not orally bioavailable and the blood-brain and blood-cerebrospinal fluid barriers severely limit their ability to enter into and act on sites in the CNS following parenteral systemic routes of administration. Most neurotrophic factors have short in vivo half-lives and poor pharmacokinetic profiles. Their access to the CNS is restricted by rapid enzymatic inactivation, multiple clearance processes, potential immunogenicity and sequestration by binding proteins and other components of the blood and peripheral tissues. The development of targeted drug delivery strategies for neurotrophic factors will probably determine their clinical effectiveness for CNS conditions. Achieving significant CNS target site concentrations while limiting systemic exposure and distribution to peripheral sites of action will lessen unwanted pleiotropic effects and toxicity. Local introduction of neurotrophic factors into the CNS intraparenchymally by direct injection/infusion or by implantation of delivery vectors such as polymer matrices or genetically modified cells yields the highest degree of targeting, but is limited by diffusion restrictions and invasiveness. Delivery of neurotrophic factors into the cerebrospinal fluid (CSF) following intracerebroventricular or intrathecal administration is less invasive and allows access to a much wider area of the CNS through CSF circulation pathways. However, diffusional and cellular barriers to penetration into surrounding CNS tissue and significant clearance of CSF into the venous and lymphatic circulation are also limiting. Unconventional delivery strategies such as intranasal administration may offer some degree of CNS targeting with minimal invasiveness. This review presents a summary of the neurotrophic factors and their indications for CNS disorders, their physicochemical characteristics and the different approaches that have been attempted or suggested for their delivery to the CNS. Future directions for further research such as the potential for CNS disease treatment utilising combinations of neurotrophic factors, displacement strategies, small molecule mimetics, chimaeric molecules and gene therapy are also discussed.

Differential patterns of nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 mRNA and protein levels in developing regions of rat brain

The present studies were undertaken to characterize the regional and temporal patterns of neurotrophin messenger RNA and protein levels for beta-nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 in the developing CNS. We have examined the levels of these neurotrophin messenger RNAs with ribonuclease protection assays and corresponding protein levels with enzyme-linked immunosorbent assays in the developing Long-Evans rat hippocampus, neocortex and cerebellum on postnatal days 1, 7, 14, 21, and 92. In addition, immunohistochemistry was used to localize the neurotrophins in these developing brain regions. Results indicated that in neocortex and hippocampus, messenger RNA for both nerve growth factor and brain-derived neurotrophic factor increased in an age-dependent manner, reaching a plateau by postnatal day 14. In the neocortex, nerve growth factor and brain-derived neurotrophic factor protein levels both peaked at postnatal day 14. In hippocampus, nerve growth factor protein peaked at postnatal day 7 while brain-derived neurotrophic factor peaked at postnatal day 14. In cerebellum, nerve growth factor messenger RNA levels were flat, while nerve growth factor protein peaked at postnatal day 7. Brain-derived neurotrophic factor messenger RNA increased in an age-dependent manner while the pattern for its protein levels was mixed. Neurotrophin-3 messeger RNA levels increased in an age-dependent manner in hippocampus, peaked at postnatal day14 in cerebellum, and no changes occurred in neocortex. Neurotrophin-3 protein was at its peak at postnatal day 1 and thereafter decreased at other postnatal days in all three brain regions. Results of neurotrophin immunohistochemistry often paralleled and complemented enzyme-linked immunosorbent assay data, demonstrating specific cell groups containing neurotrophin proteins in these regions. Within each region, patterns with regard to messenger RNA and respective protein levels for each neurotrophin were unique. No consistent relationship between patterns of neurotrophin messenger RNAs and their cognate proteins was observed between regions. The different regional patterns for neurotrophin messengerRNA and protein levels in each brain region indicate that messenger RNA studies of neurotrophin messenger RNA must be augmented by protein determination to fully characterize spatial and temporal neurotrophin distribution.

Brain-derived neurotrophic factor levels in the nervous system of wild-type and neurotrophin gene mutant mice

Although brain-derived neurotrophic factor is the most abundant and widely distributed neurotrophin in the nervous system, reproducible determinations of its levels have been hampered by difficulties in raising suitable monoclonal antibodies. Following immunization of mice with recombinant fish and mammalian brain-derived neurotrophic factor, monoclonal antibodies were generated and used in an immunoassay based on the recognition of two different epitopes. Neither antibody crossreacts with neurotrophin homodimers other than brain-derived neurotrophic factor, although reactivity was detected with brain-derived neurotrophic factor/neurotrophin-3 heterodimers. As both nerve growth factor and neurotrophin-3 are known to affect the development of a variety of neurons expressing the brain-derived neurotrophic factor (bdnf) gene, this assay was used to determine levels in tissues isolated from newborn mice carrying a null mutation in the nerve growth factor (ngf) or the neurotrophin-3 (nt3) gene. Marked differences were observed between mutants and wild-type littermates in the PNS, but not in the CNS, suggesting that neither nerve growth factor nor neurotrophin-3 is a unique regulator of brain-derived neurotrophic factor levels in the newborn mouse CNS.