Big, bad hearts: From flies to man

Genetic screens have revealed essential genes that guide metazoan complexity, from fates of individual cells, to patterning of cell arrays, to their assembly into organs (1–3).

Now the Ur-genetic organism Drosophila is making a play to provide genes for a next wave of biology: integrative physiology. How do organs move, beat, digest, secrete, behave, and interact in manners adjusted to changing needs of an organism? How can such features be monitored without perturbing the very processes under study?

In a recent issue of PNAS, Wolf et al. (4) chose heart contractility as the physiological target. They monitored and quantitated how well the beating heart of an adult fly empties on each beat and did so noninvasively, using optical coherence tomography (OCT), a technique that creates an image of the sample by analyzing its interference pattern of a broadband light source emission. They showed that contractility declines and that the heart enlarges when the cardiac myocytes bear mutations in the sarcomeric or cytoskeletal proteins troponin I, tropomyosin 2, or δ-sarcoglycan. They proposed that this is a useful model to discover genes responsible for the human disorder cardiomyopathy.

Cardiomyopathy

Cardiomyopathies are primary disorders of heart muscle [by convention not including those due to high blood pressure, due to congenital disease, or after heart attacks (5)]. The most prevalent form in the U.S. is dilated cardiomyopathy, when the heart gets bigger because of chamber enlargement, most especially the left ventricle. The essential physiological problem is diminished contractility, which, as it progresses, generally leads to symptoms of heart failure and death, with >10% dying annually (5). Not all big hearts are bad hearts. Athletes often have enlarged ventricles that contract perfectly normally.

Alcohol and other toxins can cause cardiomyopathy, but, in the majority of cases, the etiology is unknown. Interestingly, in more than a quarter of these “idiopathic” cases, the cardiomyopathy is familial, suggesting genetic predisposition. Mutations leading to dilated cardiomyopathy have been identified in proteins of the sarcomere, cytoskeleton, basement membrane, and calcium regulatory systems. In many cases, the responsible gene is unknown, so it would be very useful to have a compendium of candidate genes. That is what a model system might offer.

Genetic Screens for Physiology?

One vertebrate species, the zebrafish, has already been subject to screens for heart mutations, including those that interfere with contractility (3). One advantage of the zebrafish embryo is its transparency, so contractility can be monitored visually. In that species, mutations in sarcomeric proteins and novel signaling pathways have been discovered to perturb contractility (6, 7). Although not as readily subject to large-scale genetic screens, mouse heart function can be assessed noninvasively as well, and contractility can be demonstrably affected, for example, by mutation in laminin-α4 (8).

Much of the cellular machinery of heart cells is the same between Drosophila and humans.

The big advantage of Drosophila is the ease of large-scale screens combined with the ready recovery of causal mutations, using inserted transposons as tags. In addition, genetic pathways can be assembled in Drosophila by using sensitization screens, where the tuning down of one gene is used to reveal interacting roles of a second.

Heart Evolution

So how much resemblance does the heart of Drosophila bear to that of the human, and how reasonable a model can it provide of the dilated cardiomyopathies? Much of the cellular machinery of heart cells is the same between these species, even though they are presumed to have diverged at the invertebrate–vertebrate junction >500 million years ago (Mya) and may have arrived at their hearts by convergent evolution not from a shared ancestral heart. Orthologous genes direct cardiac cell fate decisions (e.g., nkx 2.5) and the generation of the contractile sarcomeres (9, 10). It is reasonable to presume that important molecular pathways related to contractility are shared and, as Wolf et al. (4) demonstrate, that mutations of sarcomeric proteins do lead to diminished contractile function and cardiac enlargement.

Of course, the physiology of the human heart differs in many regards from that of Drosophila. The Drosophila heart is a tube, lacking endothelium, composed of two thin layers of muscle oriented in the circumferential and longitudinal directions (11). Contractions squeeze along the tube and drive perilymph in alternating directions, as pacemakers alternate between anterior and posterior heart (12). The low pressure generated suffices for an open circulation, in which the vascular fluid (perilymph) percolates around the tissues before returning to the heart. In contrast, the thick-walled muscular human heart must perfuse a much larger body at high pressure and does so through a closed circulation. The human heart has four distinctive chambers, separated from each other by valves to ensure unidirectional flow. A single pacemaker node sets the heart rate, and ramifications of specialized conduction tissue throughout the heart ensure that chambers beat in a synchronous and coordinated fashion. The heart and vessels are lined by endothelium, which is a key element during embryological development and subsequently helps to ensure fluidity of the blood. In addition, the physiology of the human heart is embedded in an integrated mammalian physiological system designed to maintain tissue oxygenation through all types of weather. Contractility increases to maintain cardiac output in the face of threat or hemorrhage, a homeostatic response coordinated by complex interplay of hormones released from the kidney, the adrenal, the heart, and the brain.

Further studies will be needed to see to what degree the failing fly heart resembles the human heart. For example, it is not known whether the fly heart contractility normally increases with dilation, an important compensatory element for the human, or how it responds pathologically to injury. Is there, as in the human with these diseases, fibrosis accompanied by cellular hypertrophy and atrophy, processes believed to represent activation of pathological signaling path- ways, themselves contributing to the dysfunction (5)?

The obvious discrepancies between hearts separated at the late Precambrian do not detract from the importance of doing a screen in Drosophila for contractility mutations, using the elegant system established by Wolf et al. (4). Cellular defects in contractile and cytoskeletal machinery certainly form the bulk of defined cardiomyopathy mutations. New mutations may point directly to genes that are novel candidates for human cardiomyopathy or, through sensitization screens, identify otherwise elusive components of pathways, perhaps with nodes amenable to therapeutic pharmaceutical intervention.

These studies are yet more proof of the power of genetics to reveal the function of key genes in the intact animal and to help to identify key functional elements, in this case at the level of organ physiology, that most integrative and clinically relevant of sciences.