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 deadenylation in specific neuronal activities and the existence and significance of poly(A)^- mRNAs in brain remain unclear.