Biomedical Engineering Reference
In-Depth Information
This learning process extends far beyond biological pacemaking per se; it can be
applied to many areas of gene and cell therapy. In other words, rather than the
lamentable race ''to be first'' that too often complicates medical therapies these days,
the intent should be to provide the right therapy at the earliest possible date.
There are many approaches to preparing biological pacemakers, all of which are
reviewed in this volume. One or more should find its way to the clinic. In considering
all approaches, we should remember that morphologically, the sinus node is a
complex structure in which different types of nodal cells are present, all of them
embedded in collagen. In addition, sinoatrial nodal cells are heterogeneous in terms of
connexin expression, and there is a clear cell size-dependence in the pattern of
connexin expression. With regard to this complexity, one approach that might be
taken would be to engineer a morphological and functional replica of the sinus node.
Rather than assume this daunting task, we have taken a lesson from our engineering
colleagues who designed the electronic pacemaker; that is, we are working to finetune
a structure that mimics the sinus node functionally without recapitulating it
morphologically.
This approach to biological pacemaking revolves around both gene and cell
therapies [5,8,10], with the focus on one particular target, the HCN (hyperpolarization
activated, cyclic nucleotide gated) gene isoforms responsible for the
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pacemaker
current [1]. We have chosen the HCN isoforms for two reasons: first, because together
they constitute the family that initiates pacemaker activity in the mammalian heart
(with HCN4 and HCN1 predominating in sinoatrial node and HCN2 in ventricular
specialized conducting system); and second, because they not only initiate pacemaker
activity, but their activation is sped by catecholamines and slowed by acetylcholine,
making them autonomically responsive. And autonomic responsiveness is and should
be a cornerstone of pacemaker activity in heart: lack of this is a key shortcoming of
electronic pacemakers.
To date, we have used HCN2 as our primary research tool in a two-pronged
approach. A gene therapy limb [5, 10] utilizes adenoviral vectors to test mutant as
well as chimeric genes in an effort to optimize pacemaker activity and to test
interactions between biological and electronic pacemaker therapies (so called tandem
pacemaking). Importantly, the replication-deficient adenovirus we have used results in
only transient (about 2 weeks) expression of pacemaker function. While not
promising for any long-term therapeutic modality, this approach does provide a
convenient means for proof-of-concept experiments. We are working as well with
adeno-associated virus to enable more durable expression: these experiments are still
in their early phases.
The second limb of our research, that of cell therapy, uses adult human
mesenchymal stem cells (hMSCs) as a platform for delivery of HCN constructs to the
myocardium [8]. Both the gene and cell therapy approaches will be reviewed here.
2 Gene Therapy Using HCN
Figure 1 provides a starting point for understanding the role of HCN genes and the
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current they carry in initiating the pacemaker potential. It also explains why we have
focused on the HCN family. In brief, phase 4 depolarization is initiated by inward
