Differences in transcription of unique DNA sequences between neuronal and glial cells

Gene expression in cells of the central nervous system

This review summarized a part of our studies over a long period of time, relating them to the literature on the same topics. We aimed our research toward an understanding of the genetic origin of brain specific proteins, identified by B. W. Moore and of the high complexity of the nucleotide sequence of brain mRNA, originally investigated by W. E. Hahn, but have not completely achieved the projected goal. According to our studies, the reason for the high complexity in the RNA of brain nuclei might be the high complexity in neuronal nuclear RNA as described in the Introduction. Although one possible explanation is that it results from the summation of RNA complexities of several neuronal types, our saturation hybridization study with RNA from the isolated nuclei of granule cells showed an equally high sequence complexity as that of brain. It is likely that this type of neuron also contains numerous rare proteins and peptides, perhaps as many as 20,000 species which were not detectable even by two-dimensional PAGE. I was possible to gain insight into the reasons for the high sequence complexity of brain RNA by cloning the cDNA and genomic DNA of the brain-specific proteins as described in the previous sections. These data provided evidence for the long 3'-noncoding regions in the cDNA of the brain-specific proteins which caused the mRNA of brain to be larger than that from other tissues. During isolation of such large mRNAs, a molecule might be split into a 3'-poly(A)+RNA and 5'-poly(A)-RNA. In the studies on genomic DNA, genes with multiple transcription initiation sites were found in brain, such as CCK, CNP and MAG, in addition to NSE which was a housekeeping gene, and this may contribute to the high sequence complexity of brain RNA. Our studies also indicated the presence of genes with alternative splicing in brain, such as those for CNP, MAG and NGF, suggesting a further basis for greater RNA nucleotide sequence complexity. It is noteworthy that alternative splicing of the genes for MBP and PLP also produced multiple mRNAs. Such a mechanism may be a general characteristic of the genes for the myelin-specific proteins produced by oligodendrocytes. In considering the high nucleotide sequence complexity, it is interesting that MAG and S-100 beta genes etc. possess two additional sites for poly(A).(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of brain and pituitary RNA metabolism: a review of recent methodologies

Recent characterization of brain and pituitary RNA metabolism is reviewed. Relative to other tissues, the brain transcribes more of the unique, single-copy DNA. This transcriptional diversity reflects the inherent heterogeneity in organization and development of the brain. The end product of transcriptional regulation in the brain is a population of functional cytoplasmic mRNAs with multiple components, differing in complexity and abundance. Analysis of nuclear and cytoplasmic RNA provides evidence that both brain-specific synthesis and processing may determine the final mRNA population. Both polyadenylated and non-polyadenylated RNA classes contribute significantly to the total brain polysomal mRNA fraction. Characterizations of individual species of mRNA from both brain and pituitary are described. One possible transcriptional modulator in both the pituitary and brain is the presence of steroid hormone at responsive sites. Functional consequences of steroid accumulation within the brain may be (1) interactions with neurotransmitter, especially catecholamine, metabolism and function, (2) developmental interactions with neuronal systems, and (3) differentiation of glial cell function. The pleiotropic nature of steroid hormone effects (both transcriptional and non-transcriptional) within one brain region is considered by examining the biochemical effects of glucocorticoids in the hippocampus.

Complexity of RNA from the neuronal and glial nuclei

The unique DNA expression value in the brain nuclear RNA was much higher than those from other tissues. The origin of this high complexity of brain nuclear RNA was examined using the [3H]DNA-RNA saturation hybridization technique. RNA was extracted from the isolated neuronal and glial nuclei. The experimental results may indicate that the high complexity in the brain nuclei does not result from the summation of many cell types but only from the neuronal nuclei.