Bill Pak's Vision: The Neurogenetics of Phototransduction

We have created a special issue of the Journal of Neurogenetics to honor Bill Pak in the year of his 80th birthday. His research program has been in place at Purdue University since 1965. The research efforts from his laboratory tell a remarkable story of focus and steady progress towards the understanding of the process of phototransduction: How does a photoreceptor cell convert a photon of light into a physiological response that is communicated to the nervous system?

There is no question that there has been great progress in the understanding of phototransduction. Bill Pak's research efforts span a time frame in which little was known about the molecular workings of the light response. The story now is much different. We now have a very solid understanding of the molecular machinery and biochemical processes responsible for the phototransduction response. We also know that there are two major classes of photoreceptors, microvillar and ciliary photoreceptors, and two different signaling pathways in each of these classes. Although historically these two classes were viewed as distinct invertebrate and vertebrate forms of a photoreceptor, we now know that both types of photoreceptors can be found in a single species, and that both are represented in most major divisions of the animal kingdom.

Bill's PhD training was in physics, initially specializing in characterizing the properties of high-energy cosmic rays. He transitioned to biology research while working as a postdoctoral at University of Chicago. In 1965, Bill started as an assistant professor at Purdue University. He brought with him research expertise in electrophysiology on the vertebrate retina, in particular the very rapid light-evoked response called the early receptor potential. His initial research at Purdue continued to use electrophysiology to study vertebrate photoreceptors. It was during this time that he made a profound and influential decision to bring a genetic approach to the study of vision. The approach has provided a wealth of valuable experimental tools and created a field that both dominated his research career and provided inspiration, insights, and opportunities for his trainees and other scientists to develop their own scientific careers.

His decision was to use Drosophila as a model organism, and approach the problem of phototransduction using the genetic analysis of vision-defective mutants (Alawi et al., 1972; Pak et al., 1969, 1970; Pak & Pinto, 1976). Of course, in hindsight, this looks to be an easy decision, given the success of the endeavor, and of course the myriad profound advances in developmental biology, cell biology, and physiology made possible by this experimental system. However, Bill will be among the first to tell you of the skepticism he heard in the 1960s, such as might be phrased as “Phototransduction is too fast a process to involve biochemical steps controlled by the enzymes directly under genetic control,” “The process of vision in Drosophila seems to be so specialized that it won't apply to other animals,” and “Vision and other nervous system functions are so closely connected that most visual deficits having a genetic basis will also cause lethality.”

For these and other reasons, a commitment to using a genetic approach in Drosophila required some resolve. To be fair, there was also a sense that a genetic approach was needed to understand processes such as vision. About this same time, others would also successfully use systematic searches for Drosophila mutations affecting complex processes in higher organisms. Closest to Bill's efforts were those of Seymour Benzer and Martin Heisenberg, as these two researchers capitalized on the phototactic (Benzer, 1967; Hotta & Benzer, 1969) and optomotor (Heisenberg, 1971) visual behaviors of Drosophila. All three scientists have played major roles in generating the momentum for using Drosophila as a model organism for neurobiological studies. Bill's efforts are distinguished by a focus and long-term commitment to the study of process of phototransduction.

Bill's major contributions are well documented within the scientific literature. His group has isolated vision-impaired Drosophila mutants that identify more than 60 different complementation groups (Pak, 1995). Ching Kung gave a humorous perspective on the effort in a presentation in 2002 (see Figure 1). At least 20 of these genes have been subjected to extensive molecular characterization (Hardie, 2012; Montell, 2012; Pak, 2010). One result is that Drosophila is now the premier research organism for studies on invertebrate visual function. It is a system in which the initial genetic approach has been significantly enhanced by the opportune development of electrophysiological and molecular biological techniques. His research group rapidly assimilated new techniques as required, always framing the questions to keep the research focus on the process of phototransduction.

Figure 1. Bill Pak with Ching Kung at an event celebrating Bill's 70th birthday. Kung delivered a casual after-dinner talk reviewing Bill's research accomplishments. He created the cartoon and edited the poem for this presentation.

There has been a steady stream of key insights coming from Bill's research efforts. The ninaE (neither inactivation nor afterpotential) gene encodes for the major rhodopsin of Drosophila, the first invertebrate visual pigment subjected to genetic and molecular characterization (O’Tousa et al., 1985). NinaE mutants show age-dependent photoreceptor degeneration, a finding that anticipated results showing a common cause of the human retinal disease retinitis pigmentosa is mutations in human rhodopsin. In another example, a highly significant insight into the process of phototransduction was the molecular characterization of the norpA (no receptor potential) gene. norpA was one of the first vision-defective mutants picked up in the Drosophila genetic screens, and shown by Bill's lab in 1988 to encode an eye-specific phospholipase C enzyme (Bloomquist et al., 1988). The discovery and cloning of norpA clearly established the role of phosphoinositide signaling in rhabdomeric photoreceptors. As a result, the Drosophila visual system provides an excellent model for the most ubiquitous type of signal transduction used within nervous systems. Another early gene identified was one for which Bill's lab carried out the initial characterization and named trp, for transient receptor potential (Minke et al., 1975). TRP is the major light-activated membrane channel of the photoreceptor. TRP is the founding member of a large superfamily of cation channels (Hardie, 2007; Minke, 2006; Montell, 2005). In both vertebrates and invertebrates, TRP channels play essential roles in a large number of physiological processes, including most sensory responses, cellular Ca²⁺ homeostasis, blood pressure regulation, inflammation, and axon guidance.

There are many other examples as well. It is not a stretch to say that Bill's contributions are evident within every topic in every scientific review detailing our current understanding of invertebrate phototransduction. The work serves as a premier testament to the concept that extensive genetic analysis will provide profound insights into the physiology of any specialized cell type, whether in Drosophila, in mice, or in any other genetically tractable organisms.

In this special issue, we have solicited new scientific articles from Bill's past students, associates, and colleagues. The impetus was to let these scientists tell their stories. The first group consists of reviews describing research areas influenced by Bill Pak's work. Fittingly, Baruch Minke, whose research on the prolonged depolarizing afterpotential influenced Bill to modify his genetic screening protocols in ways that enabled identification of both ina and nina mutants, has contributed a review on this topic. Hiro Matsumoto has contributed a historical review describing the knowledge gained from studying light-regulated phosphoproteins using approaches he developed while in Bill's laboratory. Randall Shortridge, who first worked on the topic while a postdoctoral with Bill, reviews the advances provided by the study of the norpA mutation. Paulo Ferreira, with Andrew Orry, has contributed a perspective describing insights into the cellular role of the cyclophilin family of molecular chaperones, a topic he first encountered working on the ninaA mutant as a graduate student in Bill's laboratory. Chenjian Li, along with coworkers Yong Huang, Sushila Shenoy, Bingwei Lu, and Wencheng Liu, presents a combined review and research report describing how studies in Drosophila are contributing to an understanding of the pathogenesis of Parkinson's disease. Jay Hirsh has added a perspective of the time he worked with Bill to identify the gene for Drosophila rhodopsin.

A second set of papers presents original research results, all emphasizing the application of genetic approaches to investigate vision and neurobiological research themes. Martin Burg and Chun-Fang Wu describe characterization of the seizures and paralytic behaviors associated with Drosophila “bang-sensitive” mutants. Joe O’Tousa, along with Karen Hibbard, presents the identification and study of an essential gene required for rhodopsin maturation. Hung-Tat Leung, Shikoh Shino, and Eunju Kim describe experimental approaches they used to reveal functional and physical interactions among molecular components of the phototransduction process. These three authors were joined by Jaesung Yoon to contribute a second report that describes mutations affecting the photoreceptor's ability to carry out synaptic transmission. Kunio Isono, with colleagues Ryota Adachi, Yuko Sasaki, Hiromi Morita, Michio Komai, Hitoshi Shirakawa, Tomoko Goto and Akira Furuyama, reports on his experimental approach aimed at using Drosophila gustatory receptors to express and study vertebrate taste receptors. Mamiko Ozaki, with coauthors Azusa Nishimura, Yuko Ishida, Aya Takahashi, Haruka Okamoto, Marina Sakabe, Masanobu Itoh, and Toshiyuki Takano-Shimizu, presents results showing strain differences in the modulation in both feeding behavior and the expression of a sugar taste receptor gene during periods of starvation.

It is a fortunate coincidence that this volume also contains a review article by Bill Pak describing the ina and nina classes of phototransduction mutants (Pak et al., 2012). Recent Journal of Neurogenetics volumes have included relevant review articles by Bill Pak (2010) and by Baruch Minke (2010).

We conclude this introductory statement by expressing a sincere “thank you” to Bill Pak. We thank him for providing us a supportive environment that allowed us to develop our research capabilities. We thank him for the genuine generosity in supplying research reagents, research advice, and more. All is greatly appreciated and was greatly needed as we embarked on our own independent careers. Finally, we thank him for being a true role model, a creative, dedicated, and productive research scientist. His efforts always focused on research objectives, a great perspective for navigating through the intensely competitive Drosophila neuroscience landscape. It has been a privilege to work for and with Bill and hope that these opportunities will continue to present themselves in the future.