Heterogeneous ectopic RNP-derived structures (HERDS) are markers of transcriptional arrest

We have reviewed the reports in the literature concerning the segregation and clustering of ribonucleoprotein (RNP) -containing nuclear structures in either physiological or experimentally induced conditions (i.e., spermiogenesis and erythropoiesis in mammals, early embryonic development, hibernation, spontaneous and drug-induced apoptosis, treatment with different drugs). Irrespective of the biological system or the experimental conditions, heterogeneous ectopic RNP-derived structures (HERDS) are always found. We hypothesize that the common event at the basis of this phenomenon might be the block in transcription; this is also consistent with our own and others' results on cultured cells after actinomycin D treatment. HERDS may therefore be considered as the morphological sign of transcriptional arrest. Based on the evidence that the restructuring/relocation of RNPs may be reversible, we also hypothesize that HERDS may serve as storage sites from which RNPs may be eventually retrieved, as soon as transcription restarts. Under acute stress conditions or during apoptotic cell death, the same reorganization of RNP-containing structures would be an adaptively suitable mechanism to induce an irreversible arrest in RNA processing, thus effectively blocking protein synthesis.