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

We have reviewed the reports in the literature concerning the segregation a nd clustering of ribonucleoprotein (RNP) -containing nuclear structures in either physiological or experimentally induced conditions (i.e., spermiogen esis and erythropoiesis in mammals, early embryonic development, hibernatio n, spontaneous and drug-induced apoptosis, treatment with different drugs). Irrespective of the biological system or the experimental conditions, hete rogeneous ectopic RNP-derived structures (HERDS) are always found. We hypot hesize that the common event at the basis of this phenomenon might be the b lock in transcription; this is also consistent with our own and others' res ults on cultured cells after actinomycin D treatment. HERDS may therefore b e considered as the morphological sign of transcriptional arrest. Based on the evidence that the restructuring/relocation of RNPs may be reversible, w e also hypothesize that HERDS may serve as storage sites from which RNPs ma y be eventually retrieved, as soon as transcription restarts. Under acute s tress conditions or during apoptotic cell death, the same reorganization of RNP-containing structures would be an adaptively suitable mechanism to ind uce an irreversible arrest in RNA processing, thus effectively blocking pro tein synthesis.