In cardiac myocytes
Calmodulin (CaM) bound to the
ryanodine receptor (
RyR2) constitutes a large pool of total myocyte CaM, but the CaM-RyR2 affinity is reduced in pathological conditions. Knock-in mice expressing
RyR2 unable to bind CaM also developed
hypertrophy and early death. However, it is unknown whether CaM released from this RyR2-bound pool participates in pathological
cardiac hypertrophy. We found that
angiotensin II (AngII) or
phenylephrine (PE) both cause CaM to dissociate from the
RyR2 and translocate to the nucleus. To test whether this nuclear CaM accumulation depends on CaM released from
RyR2, we enhanced CaM-RyR2 binding affinity (with
dantrolene), or caused CaM dissociation from
RyR2 (using
suramin).
Dantrolene dramatically reduced AngII- and PE-induced nuclear CaM accumulation. Conversely,
suramin enhanced nuclear CaM accumulation. This is consistent with nuclear CaM accumulation coming largely from the CaM-RyR2 pool. CaM lacks a
nuclear localization signal (NLS), but
G-protein coupled receptor kinase 5 (GRK5) binds CaM, has a NLS and translocates like CaM in response to AngII or PE.
Suramin also promoted GRK5 nuclear import, and caused nuclear export of
histone deacetylase 5 (HDAC5).
Dantrolene prevented these effects. After 2-8 weeks of pressure overload (TAC) CaM binding to
RyR2 was reduced, nuclear CaM and GRK5 were both elevated and there was enhanced nuclear export of HDAC5. Stress (acute AngII or TAC) causes CaM dissociation from
RyR2 and translocation to the nucleus with GRK5 with parallel HDAC5 nuclear export. Thus CaM dissociation from
RyR2 may be an important step in driving pathological hypertrophic gene transcription.