Neurotrophins and basic fibroblast growth factor induce the differentiation of calbindin-containing neurons in the cerebral cortex

GDNF increases the density of cells containing calbindin but not of cells containing calretinin in cultured rat and human fetal nigral tissue

Among the dopaminergic neurons in substantia nigra pars compacta and in the ventral tegmental area, subpopulations express the calcium-binding proteins calbindin (CB) and calretinin (CR), and the CB-containing neurons are supposed to be less prone to degeneration in Parkinson's disease. Glial cell line-derived neurotrophic factor (GDNF) is a potent survival factor for nigrostriatal dopaminergic neurons. Using free-floating roller-tube (FFRT) cultures derived from fetal rat (E14) ventral mesencephalon we found that GDNF (10 ng/ml) significantly increased the number of surviving tyrosine hydroxylase (TH)-immunoreactive neurons. The possible effects of GDNF treatment on CB-immunoreactive (CB-ir) and CR-ir neurons in such cultures were examined in the present study. The neuronal cell densities were measured by quantifying the numbers of CB-ir and CR-ir neurons in areas of sections through the most extensive parts of the spherical cultures. In 4-day-old and 8-day-old cultures GDNF treatment increased the density of CB-ir neurons by 50% and 59%, respectively. Partial co-existence of TH and CB was shown using the method of double immunolabeling. The density of CR-containing neurons was unaffected by GDNF treatment as confirmed by Western blotting for CR. Parallel effects of GDNF treatment were obtained for cultures of human fetal ventral mesencephalon (8 weeks postconception). In conclusion, our findings identify GDNF as a potent factor for fetal rat and human nigral CB-ir neurons able to promote their survival in culture. Referring to a suggested neuroprotective role of CB, the results may be of relevance in the context of neuronal transplantation of patients suffering from severe Parkinson's disease.

Basic FGF increases communication between cells of the developing neocortex

We have found that basic fibroblast growth factor (bFGF), applied to cortical progenitor cells in vitro, produces an increase in the expression of the gap junction protein connexin (Cx) 43 and in the mRNA encoding Cx 43. This effect was evident in both proliferating and nonproliferating cells. The elevated levels of mRNA suggest that bFGF is likely to exert its effect by upregulating the rate of transcription of the Cx 43 gene. We have further shown that the increase in Cx 43 expression is mediated through the receptor tyrosine kinase pathway and is associated with enhanced intercellular dye-coupling mediated by gap junctions. These results suggest that gap junction channels provide a direct conduit for mitogens released in response to bFGF to effectively regulate proliferation during corticogenesis.

Collagen type IV promotes the differentiation of neuronal progenitors and inhibits astroglial differentiation in cortical cell cultures

In an effort to elucidate the interactions between cells in the developing cortex and their microenvironment, we have employed dissociated cell cultures and immunocytochemistry to analyze the effect of collagen type IV (COL) on the proliferation and differentiation of rat cortical progenitor cells during the period of corticogenesis. COL, present in the proliferative zones throughout the period of neurogenesis, belongs to a group of macromolecular proteins that make up a considerable portion of the extracellular matrix (ECM). We have shown that this ECM molecule inhibits cell proliferation and glial cell differentiation while promoting neuronal differentiation. We have also demonstrated that COL, when applied to the cultures with basic fibroblast growth factor (bFGF), induces glial cell differentiation while continuing to promote neuronal differentiation. These results indicate that cortical progenitor cells respond differentially to local environmental signals, and that components of the ECM are involved in the regulation of corticogenesis.

BDNF regulates reelin expression and Cajal-Retzius cell development in the cerebral cortex

Cajal-Retzius (CR) cells of the cerebral cortex express receptors for the neurotrophin brain-derived neurotrophic factor (BDNF) and downregulate expression of the extracellular matrix protein Reelin during early postnatal development, coincident with the onset of cortical BDNF expression. During this period, mice lacking BDNF have elevated levels of Reelin in CR cells. Acute BDNF stimulation of cortical neuron cultures and overexpression of BDNF in the developing brain of transgenic mice prior to the onset of endogenous production causes a profound, dose-dependent reduction of Reelin expression in CR cells. In addition, overexpression of BDNF produces gaps and heterotopias in the marginal zone and disorganization and aggregation of cortical CR cells and induces several other malformations, including aberrant cortical lamination, similar to the phenotype of reeler mutant mice, which lack Reelin. These results demonstrate a role for BDNF on cortical CR cells and identify Reelin as a direct effector of this neurotrophin during brain development.

Basic fibroblast growth factor promotes the generation and differentiation of calretinin neurons in the rat cerebral cortex in vitro

Calretinin-expressing neurons are some of the earliest postmitotic cells to appear in the developing cerebral cortex. Lineage studies have shown that the expression of this calcium-binding protein in cortical neurons is not genetically programmed and is likely to be induced by external factors. A number of studies have clearly shown that basic fibroblast growth factor (bFGF) and a number of neurotrophins promote the proliferation and differentiation of cortical progenitor cells to a particular lineage. Here, using a culture system of dissociated rat cortical cells, we found that brain-derived neurotrophic factor and neurotrophin-3 promoted the morphological differentiation of one of the calretinin-containing neuronal subpopulations, the Cajal-Retzius cells. Another subpopulation of calretinin-expressing cells of smaller size and bipolar form was generated when cultures were treated with bFGF. The progenitors of these neurons were stimulated by bFGF to divide a number of times before initiating their differentiation programme. The number of calretinin-expressing neurons increased further when cultures were treated with a combination of bFGF and retinoic acid.

Lysosomal dysfunction reduces brain-derived neurotrophic factor expression

Brain-derived neurotrophic factor (BDNF) expression in hippocampus and cortex is considerably reduced in Alzheimer's disease. The present study tested if lysosomal disturbances, a concomitant of brain aging, impair basal and/or induced expression of BDNF. Cultured hippocampal slices were incubated with N- CBZ-L-phenylalanyl-L-alanine-diazomethylketone (ZPAD), an inhibitor of cathepsins B and L, for 6 days and processed for in situ hybridization using radiolabeled cRNA probes against BDNF mRNA. Multiple densitometric readings were collected from each of the three principal hippocampal subdivisions. Within-slice averages were substantially lower in the ZPAD-treated group compared to controls. Treatment with the inhibitor did not change average neuron diameter or packing density. Intense stimulation of glutamate receptors with kainate for 30 min (followed by a 90-min recovery period) caused a nearly threefold increase in BDNF mRNA concentrations in the dentate gyrus while having only marginal effects in the other subdivisions. Slice averages of ZPAD-exposed cultures treated with kainate were lower than those of controls exposed to the excitotoxin; however, on a percentage basis, the kainate-induced increase in the dentate gyrus was comparable for the two groups (175 +/- 31 vs 179 +/- 39%). Kainate for 1 h (with a 5-h recovery) affected BDNF mRNA in a manner similar to that found with shorter infusions, i.e., induction in stratum granulosum but not elsewhere, lower overall slice averages with ZPAD treatment, and no evidence that ZPAD blocked the percentage increase in the dentate gyrus. These results provide evidence that lysosomal dysfunction occurring during brain aging could disrupt ongoing BDNF production without substantially impairing the neurotrophin response to intense physiological activity. The first observation suggests a plausible aging sequence leading to pathology while the second may be of interest with regard to possible therapeutics.