It was with great sadness that the cell biology community learned of the death of Peter Satir in July 2022. Peter was a beloved family member, mentor to many, and Distinguished Professor Emeritus and former Chair of Anatomy and Structural Biology at Albert Einstein College of Medicine, NY. In his field of cell biology, Peter was an honored pioneer in understanding the structural basis of ciliary motility and in the role of the primary cilium required for development and the health of humans. His sliding microtubule model for ciliary motility was ground breaking and provided the foundation for much of the current work on ciliary motility. For his work, Peter received in 2002 the Henry Grey Laureate, the highest honor given by the American Association of Anatomists, and he was awarded with the E.B. Wilson Award in 2014, the highest honor of the American Society of Cell Biology (ASCB). Peter was a mentor to many and supporter of junior and underrepresented minority scientists, many of whom are now successful independent researchers around the world. We all extend our deepest sympathy to Peter's family – his wife Birgit and his sons, Jacob and Adam and their families.Peter Satir grew up in New York city, attended Columbia University for his undergraduate studies and was admitted in the 1956 graduate class at Rockefeller University, then the Rockefeller Institute for Medical Sciences. This was one of the first graduate classes at Rockefeller University, and Peter worked with Keith Porter using electron microscopy to reveal the structure of cilia (Satir, 1997, 1998). This was a special time at Rockefeller University when the discipline of cell biology began and the ASCB was established in 1960. In his excellent autobiography essay published by the ASCB, Peter refers to Rockefeller as the ‘cradle’ of cell biology, listing the Rockefeller pioneers in cell biology (Satir, 2014). Featured in this early work was the electron microscopic description of cell and organelle structure, including the structure of cilia.Between WW1 and WW2, the Rockefeller Foundation set up an important collaboration with the Carlsberg Foundation in Denmark, centered on several Danish Nobel Prize winners, including, but not limited to, Niels Bohr and August Krogh. In the 1950s, the Rockefeller Institute graduate program encouraged students to spend a year abroad. Peter decided to go to the Carlsberg laboratory in Copenhagen to work with Kaj Linderstrøm-Lang, who defined the structure of proteins by their different levels of complexity. Owing to illness, however, Linderstrøm-Lang had to cancel the visit. Instead, Keith R. Porter allowed Peter to visit one of the first cell biologists in Denmark, Erik Zeuthen, who had been appointed as Head of The Carlsberg Foundation Biological Institute (CBI) in Copenhagen, and whom Peter had met at an MBL course in Woods Hole, MA. At this time, the CBI had become an international research center with a stream of foreign visitors. Peter studied with Zeuthen from 1958 to 1959 where he deciphered the rate of growth of the giant amoeba Chaos chaos (Satir and Zeuthen, 1961). Peter also introduced Danish scientists to electron microscopy, as practiced by Porter and Palade, leading to the eventual purchase of a transmission electron microscope by the CBI.As Peter gracefully describes in his autobiography essay (Satir, 2014), his visit to Copenhagen influenced his life profoundly, since this is where he met Birgit, who was a graduate student in Zeuthen's lab working on the cell cycle in synchronized cultures of Tetrahymena. Peter and Birgit continued as husband and wife and close scientific colleagues for over 60 years, visited Denmark every year and contributed immensely to cell biology in Denmark. During their years at Albert Einstein College of Medicine (AECOM), NY, they almost continuously had a Danish scientist or student working in their labs. It therefore came as no surprise when they both received an honorary doctorate from the University of Copenhagen, Peter in 2005, and Birgit in 2021.After finishing his graduate studies, Peter accepted a position as an Assistant Professor at the University of Chicago, Department of Zoology (1966), where Birgit was to have her own lab space in the Whitman Labs as a research associate. It was here that paths of Joel Rosenbaum and the Satir's crossed. Joel Rosenbaum met Peter and Birgit for the first time at Chicago when he gave a talk, in the Whitman lab, on fractionation of Tetrahymena cells. Earlier, Joel Rosenbaum had read with interest Peter's abstract on ciliary beating in clam gill cilia. Birgit, at that time, was interested in the isolation and biochemistry of ciliary basal bodies (kinetosomes), and she and Rosenbaum collaborated on a paper for the Journal of Protozoology (Satir and Rosenbaum, 1965), showing the first isolation and electron microscopy documentation of basal body isolation from Tetrahymena.Chicago was an outstanding place to be at this time if one was interested in cilia, the axoneme and microtubules. Edwin Taylor had earlier come to Chicago as an Assistant Professor to work on the cell cycle, and he immediately attracted some excellent graduate students: Gary Borisy, Michael Shelanski, Richard Weisenberg and Mark Adelman. Another graduate student, David Derosier, was working in the new electron microscopy facility in the basement of the biophysics building. Peter indicates in his autobiographical essay that he chose that position, in part, “because Frank Child, one of the first people to seriously work on the molecular biology of cilia, was also a young faculty member in the department. In the following years, the University of Chicago did indeed become, if not the cradle, certainly the ‘romper room’ of cilia and microtubule (MT) biology. In addition to Frank, Birgit, and Peter, the following years saw Sid Tamm, Gary Borisy, Joel Rosenbaum, David Phillips, and eventually Fred Warner studying cilia, while across the street in biophysics, our colleague Ed Taylor and his group began working on the structure of microtubules.”In Chicago, Peter began his studies of the ciliary metachronal wave in the epithelia cells of the clam gill (Satir, 1963, 1965). By 1967, taking advantage of the fixed metachronal wave (Satir, 1965), Peter showed cilia whose tip patterns varied with beat stage, and using serial section electron microscopy began testing a sliding MT hypothesis of ciliary motion. As a result of his work in Chicago, Peter was recruited to the Department of Physiology–Anatomy at the University of California–Berkeley, where he hired a talented team of investigators and began pioneering studies of the ciliary membrane, cell junctions, chloroplasts and a range of entirely new topics. Students and postdocs included Bernie Gilula (graduate student, later Professor Rockefeller, Baylor College of Medicine and Scripts Research Institute). At this time, Peter and Birgit hosted a number of visiting professors including Michael Holwill (Professor at King's College, London) and Shoji Baba (Professor at Ochanomizu University, Tokyo). Each trainee and visiting scholar went on to highly successful careers with outstanding publications.The focus on the structural basis of cell function was an ongoing theme (Sale and Barkalow, 2001). Notably, Bernie Gilula became a renowned professor and investigator with a focus on studying the gap junction (Gilula and Satir, 1971) until his untimely death in 2000. Their studies included discovery of the ciliary necklace (Gilula and Satir, 1972), a series of well-defined strings of scalloped membrane particles that encircle the ciliary shaft defining a junction between the ciliary membrane and the cytoplasmic membrane. The ciliary necklace contributes to a barrier between membrane domains (Nachury and Mick, 2019; Verhey and Yang, 2016) and is part of the ciliary transition zone, which controls the entrance and exit of ciliary proteins (Garcia-Gonzalo and Reiter, 2017; Greenan et al., 2020; Nachury and Mick, 2019; Van Den Hoek et al., 2022; Yang et al., 2015; Garcia et al., 2018; Nachury and Mick, 2019; Reiter and Leroux, 2017; Takao and Verhey, 2016; Wang et al., 2022).Among Peter's most impactful contributions while in Berkeley were his evidence for and definition of a ‘sliding microtubule model’ of ciliary bending (Satir, 1967, 1968) and the discovery that dynein is a minus-end motor (Sale and Satir, 1977). Following the identification of ciliary structure and its microtubules, the question was how they contribute to the bending motion. Peter was able to perform serial section electron microscopy and examine the tips of cilia fixed in known bend positions, and known axial orientation, to determine that microtubules slide relative to one another during bending (Satir, 1965, 1967, 1968). Peter formally proposed a ‘sliding filament’ model for ciliary bending, postulating that the dynein ATPases, which had been discovered by Ian Gibbons (Gibbons and Rowe, 1965), power microtubule sliding by a ‘moving cross bridge’ mechanism. Gibbons later provided strong support of the sliding microtubule model by direct observation of ATP-induced axonemal microtubule sliding (Summers and Gibbons, 1971).An important question resulting from microtubule sliding was the polarity of the force generated by dynein. It was possible that dynein generates force along microtubules in both directions (for example, by acting as a plus- and minus-end directed motor) or in only one direction. Peter proposed to perform Summers’ and Gibbons’ ATP-induced microtubule sliding experiment on an electron microscope grid coated with polylysine and examine the direction of microtubule sliding. The results revealed that dynein only moves toward the minus-end of the microtubule (Sale and Satir, 1977). Based on these data, Peter subsequently developed a switching model for oscillatory ciliary bending – the ‘switch point hypothesis’ (Satir and Matsuoka, 1989). Peter was quite excited to see the recent cryoelectron tomography results of Daniela Nicastro and colleagues demonstrating changes in dynein structure on opposite sides of the axoneme, which provided direct evidence supporting a switching model for ciliary bending (Lin and Nicastro, 2018).In 1977, Peter was invited to become the Chair of the Department of Anatomy (later the Department of Anatomy and Structural Biology, ASB) at AECOM in the Bronx, NY. At the same time, AECOM offered Birgit a tenure track faculty position, which broke with the tradition from many other universities where it was difficult for women to obtain regular faculty positions that could lead to tenure. The tradition of recognizing and promoting women scientists, and of offering individual faculty positions to spouses at AECOM, had a major impact on Peter's and Birgit's decision to accept the invitation from AECOM.Peter recruited outstanding faculty and transformed the department into an international hub for cutting-edge research in both basic and translational science that addressed fundamental questions in cell and medical biology. Peter took great interest in mentoring and creating an exciting and supportive work environment for students, research fellows and new faculty members. Gary Bassell, Charles Howard Candler Professor of Cell Biology and Chair, Department of Cell Biology, Emory University School of Medicine says: “Dr. Satir was my first chair from 1995–1998. I remember meeting with Peter to discuss what first lecture I would give in the medical student course ‘Cell Design Principles’. He asked if I might be interested to take over his lecture on intermediate filaments (IFs), which he had given for many years. I was really honored. I attended his last IF lecture, he shared several of his slides, we met to discuss recent papers and discoveries, and how my lecture might evolve, which I gave for several subsequent years. Meeting in his office, he shared a wonderful history of science and debate. This was the style in which he led the department – engaged in robust and collegial discussions. Seminar speakers were welcomed but also challenged (in good spirit). He had suggestions and critical input on my science as a new faculty member setting up my new lab, which I really appreciated. Peter and Birgit Satir were gracious hosts, inviting faculty to their home for an annual holiday party.”Peter's work continued to flourish with students and colleagues visiting from around the world. Several tracks of his research defined novel mechanisms in microtubule-based motor protein trafficking and novel regulatory mechanisms that controlled the speed of ciliary beating (Hamasaki et al., 1991, 1995; Satir et al., 1995). In 1998, 3 years before his stepping down from the Chair in 2001, Peter offered one of us, Søren T. Christensen, a previous student with Leif Rasmussen, who worked in Zeuthen's lab at the time of Peter's visit in Copenhagen, a postdoc position to work on the regulation of microtubule sliding velocity in cilia. Thus, Peter's and CBI's paths crossed again, now leading to work on primary cilia, which around this time became the focus of many labs based on the seminal discovery of intraflagellar transport (IFT) by Joel L. Rosenbaum and co-workers (Kozminski et al., 1995, 1993; Pazour and Rosenbaum, 2002; Rosenbaum and Witman, 2002; Satir, 2017). As Peter noted in his essay ‘Cilia: before and after’ (Satir, 2017), the discovery of IFT was the turning point for renaissance of the primary cilium and discovery of and the link of this trafficking system to human health and disease (Anvarian et al., 2019; Christensen et al., 2007; Pazour et al., 2020; Pazour and Rosenbaum, 2002; Reiter and Leroux, 2017; Satir et al., 2010).Although the initial focus of Søren's postdoc project was aimed at understanding how outer arm dynein light chains regulate microtubule sliding velocity in Tetrahymena cilia (Christensen et al., 2001), they showed that these cilia also provide a sensory hub for detecting signaling molecules during cell survival and proliferation (Christensen et al., 2003). This sparked their interest in understanding the sensory capacity of primary cilia in mammalian cells. After Søren's return to Copenhagen in 2000 to build his own lab, their collaborative efforts expanded tremendously. This led to the discoveries that primary cilia orchestrate RTK and PDGFRα signaling in cell cycle control and directional cell migration (Christensen et al., 2013; Clement et al., 2013; Schneider et al., 2010, 2005, 2009), and that human embryonic stem cells form cilia with a functional Hedgehog signaling machinery (Kiprilov et al., 2008). Their partnership also led to an exchange of students and visits between their labs, culminating in the Satir's sabbaticals stays in 2012–2013 and 2019–2020 in Copenhagen, supported by the Lundbeck Foundation. In collaboration with Ana Maria Cuervo, Olatz Pampliega and others at AECOM, Birgit and Peter further discovered how primary cilia converge on autophagy and vice versa (Pampliega et al., 2013), opening an entirely new field within cilia science with important implications for ciliopathies and autophagy-associated disorders (Morleo et al., 2022).From 1999–2001, Peter was University Chairman and Professor of ASB and, until joining the Department of Developmental and Molecular Biology at AECOM in 2021, Distinguished University Professor of ASB. After their formal retirements, Peter as University Professor Emeritus in 2017, and Birgit as University Professor Emerita in 2019, they continued to thrive in the scientific community at national and international meetings and conferences. In addition, although their own lab space gradually became very limited, they never stopped supporting students and young investigators and promoting basic research, and they also continued to foster new ideas and concepts within the cilia field. During his hospitalization in June 2022, Peter's spirits remained intact, and he continued to talk about cilia until the end. Peter was especially interested to discuss with his oncologists how defects in primary cilia might contribute to tumorigenesis, and he explained to them how primary cilia could be targeted in the treatment of cancer.Peter Satir will always be remembered as a pioneer in cell biology testing new ideas with ingenious approaches toward understanding of cilia and cell function. He will also be remembered for his leadership in cell biology, the ASCB and at AECOM. He helped many new faculty and trainees with his care and mentorship, and his warmth and collegiality. He enjoyed seeing the success of multiple new generations of scientists. He will be remembered as a devoted father and life-long, beloved partner of Birgit Satir. Peter Satir will be deeply missed by many.Peter was Professor at UC Berkeley and my (Winfield Sale) mentor and Professor starting in 1970 when I took his undergraduate course in cell biology. I completed an undergraduate project in Peter's lab learning electron microscopy, and I had planned to go to medical school, but I was hooked on the research and cilia. I was admitted to graduate school at UC Berkeley in 1972 and began to study cilia from Tetrahymena. We had great success studying dynein-driven microtubule sliding in cilia. But most important, almost every day, I met and talked with Peter about ideas and our results. I had the opportunity to travel to national and international meetings, and meet the leaders in cilia and dynein. I am now retired from Emory University where I had a terrific 42 year research and teaching career and family life. I am very grateful to Peter and Birgit for getting me started, showing me the way and meeting with me in Berkeley, Copenhagen and New York from 1972–2022, 50 years.Ever since my first meeting with the Satir's in 1994 at a lunch party in the house of my graduate supervisor, Leif Rasmussen, I (Søren T. Christensen) was amazed by their genuine interest in my studies, their kindness and wonderful spirit. My time in Peter's lab from 1998–2000 was the beginning of an exciting journey together in science, and along the way, we developed a warm friendship with memorable get-togethers and celebrations with our families in Denmark and abroad. I still recall Peter's love for Birgit as expressed through his singing of Danish love songs at parties they held for family and friends. Peter was fluent in Danish, the language he also chose to converse in when meeting Her Majesty the Queen of Denmark, Margrethe II, at an reception after having received his honorary doctorate from the University of Copenhagen in 2005; the only place and time when an American introduced the cilia field to the Royal Danish Family. Peter has had a tremendous impact on my career, from the first day I entered his lab to the many conversations we have had over the years. He showed me the importance of socializing with students and PIs from across the world and between disciplines to share research and facilitate our understanding of fundamental mechanisms in cell biology. Indeed, Peter and Birgit furthered this by throwing their annual ASCB wine and beer get-togethers in their hotel suite for old and new students, colleagues and friends – excellently supported by their bathtub filled up to the top with bottles of beer and white wine, with everything well embedded in ice cubes. Remember to have fun while you are around, Peter often said. And so we did.We are grateful to Joel Rosenbaum, Yale University, Gary Bassell, Emory University, and Thoru Pederson, UMass Chan Medical School, for discussion of cell biology history and memories of Peter Satir.
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