Segregated regulatory elements direct beta-myosin heavy chain expression in response to altered muscle activity.

Our previous transgenic analyses revealed that a 600-base pair beta-myosin heavy chain (betaMyHC) promoter conferred mechanical overload (MOV) and non-weight-bearing (NWB) responsiveness to a chloramphenicol acetyltransferase reporter gene. Whether the same DNA regulatory element(s) direct betaMyHC expression following MOV or NWB activity in vivo remains unknown. We now show that a 293-base pair betaMyHC promoter fused to chloramphenicol acetyltransferase (beta293) responds to MOV, but not NWB activity, indicating a segregation of these two diverse elements. Inclusion of the betaMyHC negative regulatory element (-332 to -300; betaNRE) within transgene beta350 repressed expression in all transgenic lines. Electrophoretic mobility shift assays showed highly enriched binding activity only in NWB soleus nuclear extracts that was specific to the distal region of the betaNRE sense strand (dbetaNRE-S; -332 to -311). Supershift electrophoretic mobility shift assay revealed that the binding at the distal region of the betaNRE sense strand was antigenically distinct from cellular nucleic acid-binding protein and Y-box-binding factor 1, two proteins shown to bind this element. Two-dimensional UV cross-linking and shift Southwestern blotting analyses detected two proteins (50 and 52 kDa) that bind to this element. These in vivo results demonstrate that segregated betaMyHC promoter elements transcriptionally regulate betaMyHC transgene expression in response to two diverse modes of neuromuscular activity.