Background: Capacitively coupled electric stimulation has been
successfully used in the treatment of bone nonunions and to effect spinal
fusions. However, the pathway of biologic events whereby this is accomplished
has not been fully elucidated. To determine whether bone morphogenetic
proteins (BMPs) could be involved, the effect of electrical stimulation on BMP
gene expression was investigated.
Methods: Postconfluent cultures of MC3T3-E1 bone cells were exposed
to a series of capacitively coupled signals in which the duration, amplitude,
frequency, and duty cycle were sequentially and systematically varied. The
cellular response was measured by quantifying the mRNA levels of BMP-2 through
BMP-8, as well as the BMP antagonists gremlin and noggin, with use of reverse
transcription followed by real-time quantitative polymerase chain reaction.
BMP-2 protein was measured by enzyme-linked immunosorbent assay, and alkaline
phosphatase activity was measured by a specific colorimetric assay.
Results: The results showed that BMP-2 through BMP-8, gremlin, and
noggin were all normally expressed by MC3T3-E1 cells, and could be
significantly up-regulated by specific and selective capacitively coupled
electric fields (p < 0.05). However, mRNA expression for BMP-2, 4, 5, 6,
and 7 was consistently up-regulated several times higher than that for BMP-3
and BMP-8, gremlin, and noggin under identical conditions. Concomitantly,
BMP-2 protein production and alkaline phosphatase activity were both
significantly increased in the same electrically stimulated cultures (p =
0.001 and p < 0.01, respectively).
Conclusions: These data clearly show that our optimal capacitively
coupled signal (60 kHz, 20 mV/cm at a 50% duty cycle for twenty-four hours)
can specifically, selectively, and simultaneously up-regulate the expression
of a number of osteoinductive BMPs; other BMPs and antagonists are only
Clinical Relevance: Electrical stimulation may be a useful treatment
modality for in vivo situations where bone induction is required, since it is
noninvasive, safe, effective, can be easily targeted to a variety of anatomic
sites, can provide a controlled production of BMPs, and can be used
repeatedly. The optimal duration (continuous stimulation at 100% duty cycle)
and frequency (60 kHz) determined in this in vitro study are the
same—and the amplitude (20 mV/cm) is in the same range (12
mV/cm)—as are used clinically.