Sequence complexity and frequency distribution of poly(A)-containing messenger RNA sequences from the glioma cell line (C6

Brain non-adenylated mRNAs

Most eukaryotic messenger RNA (mRNA) species contain a 3'-poly(A) tract. The histone mRNAs are a notable exception although a subclass of histone-encoding mRNAs is polyadenylated. A class of mRNAs lacking a poly(A) tail would be expected to be less stable than poly(A)+ mRNAs and might, like the histones, have a half-life that varied in response to changes in the intracellular milieu. Brain mRNA exhibits an unusually high degree of sequence complexity; studies published ten years ago suggested that a large component of this complexity might be present in a poly(A)- mRNA population that was expressed postnatally. The question of the existence of a complex class of poly(A)- brain mRNAs is particularly tantalizing in light of the heterogeneity of brain cells and the possibility that the stability of these poly(A)- mRNAs might vary with changes in synaptic function, changing hormonal stimulation or with other modulations of neuronal function. The mRNA complexity analyses, although intriguing, did not prove the existence of the complex class of poly(A)- brain mRNAs. The observed mRNA complexity could have resulted from a variety of artifacts, discussed in more detail below. Several attempts have been made to clone members of this class of mRNA. This search for specific poly(A)- brain mRNAs has met with only limited success. Changes in mRNA polyadenylation state do occur in brain in response to specific physiologic stimuli; however, both the role of polyadenylation and de-adenylation in specific neuronal activities and the existence and significance of poly(A)- mRNAs in brain remain unclear.

Patterns of gene expression in the murine brain revealed by in situ hybridization of brain-specific mRNAs

Biochemical differences between neuronal cell populations of the mammalian brain, including selection of neurotransmitters and distinct neural antigens, suggest that the regulation of gene expression plays an important role in defining brain function. Here we describe the use of in situ hybridization to identify cDNA clones of highly regulated mRNA species and to define directly their pattern of gene expression in brain at both gross morphological and cellular levels. One of the selected cDNA clones, pMuBr2, detected a single 3.0 kb mRNA species, which from in situ hybridization appears specific to oligodendroglia cells. Three other cDNA clones, pMuBr3, 8 and 85, identified polyadenylated mRNA transcripts expressed by neuronal cells of the murine brain. Viewed at the gross morphological level, the mRNAs hybridizing to these cDNA sequences exhibit different patterns of abundance distinguishing such brain structures as pons, anterior thalamus, hippocampus, basal ganglia and anterior lobe of the neuroendocrine pituitary gland. At the cellular level, in situ hybridization revealed that these mRNAs are differentially expressed by morphologically and functionally distinct neurons of the cerebellum and hippocampal formation. When examined in the context of known brain function, however, the regulated expression of the neuron-specific mRNAs does not correlate simply with known cellular morphology or previously demonstrated neuronal relationships suggesting novel patterns of gene expression which may contribute to brain function.

Biphasic regulation by dibutyryl cyclic AMP of tubulin and actin mRNA levels in neuroblastoma cells

Blot hybridization analysis that used labeled tubulin cDNA probes revealed that N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate [dibutyryl cyclic AMP (Bt2cAMP)] initially increases and later decreases the level of tubulin mRNA in a neuroblastoma-glioma hybrid cell line as well as in the parent cells. A significant increase in tubulin mRNA sequences is already evident 1 hr after the addition of Bt2cAMP to the neuroblastoma cells, and a maximal induction of 2-fold is seen after 12 hr. Continued treatment with Bt2cAMP for 4 days results in a down-regulation of the initial tubulin mRNA level independently of cell density. In the glioma cells Bt2cAMP also rapidly increases the level of tubulin mRNA sequences, reaching a maximum within 6 hr. However, in these cells the subsequent decrease in tubulin mRNA content depends on the culture's phase of growth: cells at the logarithmic growth phase do not down-regulate the tubulin mRNA content even after prolonged treatment with Bt2cAMP, whereas confluent cells do. The hybrid cell line manifests intermediate characteristics in Bt2cAMP regulation of tubulin mRNA level. The time course of induction and down-regulation of tubulin mRNA content observed in the hybrid cells is similar to that of the parent neuroblastoma, whereas the sensitivity to induction is glioma-like and is 8-fold over the initial level. Blot hybridization with labeled actin cDNA probes showed a similar but not identical induction of actin mRNA synthesis with the hybrid and glioma cells, whereas no significant change was observed with the neuroblastoma cells. Moreover, prolonged treatment with Bt2cAMP of all these cell lines did not result in down-regulation of actin mRNA sequences below the initial control value. It was also observed that the level of tubulin sequences in mRNA isolated from 12-day-old rat brain was higher than that in the newborn brain. However, at an age of 30 days, the level of tubulin sequences decreases to about 75% of the newborn level.