Fgf-10 is required for both limb and lung development and exhibits striking functional similarity to Drosophila branchless

Fgf-10-deficient mice (Fgf-10(-/-)) were generated to determine the role(s) of Fgf-10 in vertebrate development. Limb bud initiation was abolished in Fgf-10(-/-) mice. Strikingly, Fgf-10(-/-) fetuses continued to develop until birth, despite the complete absence of both fore- and hindlimbs. Fgf-10 is necessary for apical ectodermal ridge (AER) formation and acts epistatically upstream of Fgf-8, the earliest known AER marker in mice. Fgf-10(-/-) mice exhibited perinatal lethality associated with complete absence of lungs. Although tracheal development was normal, main-stem bronchial formation, as well as all subsequent pulmonary branching morphogenesis, was completely disrupted. The pulmonary phenotype of Fgf-10(-/-) mice is strikingly similar to that of the Drosophila mutant branchless, an Fgf homolog.

Evidence for neuronal regulation of oligodendrocyte development: cellular localization of platelet-derived growth factor alpha receptor and A-chain mRNA during cerebral cortex development in the rat

Oligodendrocyte responses in vitro to platelet-derived growth factor (PDGF) include proliferation, survival, migration, and changes in cell morphology and molecular expression. Studies of mixed glial cultures established that astrocytes secrete PDGF; thus astrocytes are considered to be key regulators of oligodendrocyte development in vitro. We previously demonstrated PDGF alpha receptor mRNA expression by oligodendrocyte progenitors and preoligodendrocytes during postnatal development of rat cerebral cortex. In the present study, we have mapped the spatial and temporal expression of PDGF A-chain ligand mRNA and alpha receptor mRNA to determine if the cell-cell interactions that form the basis for PDGF regulation of oligodendrocyte development in vitro are also present in vivo. By in situ hybridization (ISH) we demonstrate that at embryonic day 17 (E17) cells expressing receptor mRNA (PDGFR alpha +) are initially in the subventricular zone, at a distance from cells expressing ligand mRNA (PDGF+) in the cortical plate. By E20 PDGFR alpha + cells are found throughout the corpus callosum and cortical gray matter. PDGF+ cells are restricted to the cortical plate prenatally and only appeared in the corpus callosum postnatally. Combined immunocytochemistry and ISH demonstrated the PDGF+ cells colocalized with neurofilament, but not with GFAP. These data establish that PDGF is expressed by neurons during PDGFR alpha + oligodendrocyte progenitor migration from the subventricular zone to the corpus callosum and gray matter. Furthermore, neurons continue to express PDGF during the generation and differentiation of appropriate numbers of oligodendrocytes needed to myelinate axons as the nervous system matures.

The myelin-deficient rat mutant: partial recovery of oligodendrocyte maturation in vitro

The morphological and immunocytochemical identification and characterization of the myelin-forming cell, the oligodendrocyte, have defined a model system for developmental studies. The myelin-deficient (md) rat mutant lacks myelin in the central nervous system and fails to express the normal developmental increase in oligodendroglial and myelin markers, apparently as a consequence of a point mutation in the proteolipid protein gene. In the present work, we compared the developmental pattern of primary glial cultures derived from newborn md rat brains to those derived from wild-type animals. Brain cell suspensions were prepared from each rat pup and cultured separately. We found by immunocytochemical and enzymatic analyses for the various markers that the developmental cascade of oligodendroglial marker expression is delayed, oligodendrocytes failing to mature compared to normal cultures. However, a partial recovery of marker expression was observed in md-derived cultures as compared to development previously reported in the intact md animals. We suggest that the partial recovery of the sequential expression of oligodendroglial markers may be due to a supportive environment provided to the oligodendrocyte progenitor cell (1) by tissue culture conditions or (2) by the absence of the blood-brain barrier in contrast to its presence in the intact animal.

Transcriptional regulation studies of myelin-associated genes in myelin-deficient mutant rats

To identify and assess the consequences of the mutation in myelin-deficient (md) rats, the myelin proteolipid protein (PLP) gene and its expression were studied in md rats. Southern blots of the PLP gene demonstrated that no major deletions or insertions have occurred in this gene. In addition, the mutation in this gene does not result in a splicing defect in the RNAs, since all exons are represented in md PLP RNAs. These data are consistent with results in another laboratory indicating that a point mutation in the PLP gene in md rats results in a single amino acid alteration in the protein. To elucidate the molecular mechanisms producing reduced levels of PLP, myelin basic protein (MBP) and glycerol phosphate dehydrogenase (GPDH) mRNAs, and their corresponding proteins in md rats, in vitro transcription assays were performed. Transcription of the PLP gene in nuclei isolated from 23-day-old md rat brains was dramatically reduced relative to normal tissue. Thus, the single amino acid alteration in this protein alters the regulation of transcription of this gene. In contrast, the transcriptional activities of the MBP and GPDH genes in md rats were indistinguishable from normal animals. Thus, the lower level of MBP and GPDH mRNA and protein in md rats relative to normal results from a posttranscriptional event.

Stability of neuronal and glial marker enzymes in post-mortem rat brain

The enzymatic activities in post-mortem rat brain kept at 4 degrees C and at 25 degrees C were determined for a number of enzymes localized in specific cell types in the central nervous system. Choline acetyltransferase (CAT), glycerol-3-phosphate dehydrogenase (GPDH), glutamine synthetase (GS), lactate dehydrogenase (LDH) and 2',3'-cyclic nucleotide phosphohydrolase (CNPase) were found to be very stable at both 4 degrees C and 25 degrees C with only slight, if any, losses of activity being seen even at periods as long as 72 hr. Glutamic acid decarboxylase (GAD) activity was less stable than that of the other enzymes. In brains kept at 4 degrees C GAD activity was stable out to 24 hr after which it began to decline rapidly to 65% of control at 72 hr. In brains kept at 25 degrees C, GAD activity was stable for 6-8 hr and then began to steadily decline to 58% of control at 24 hr and 29% of control at 72 hr. Assuming that these enzymes have similar stabilities in post-mortem human brain, the effect of post-mortem delay in processing tissues may be of lesser significance than other factors with regard to the measured enzyme activities in human brain samples.