THE 2008 recipient of the Genetics Society of America Medal is Susan Lindquist. Lindquist has completely transformed our understanding of the role of protein folding in biological systems. Her work has employed, and to great effect, a zoo of powerful genetic systems, including yeast, fruit flies, Arabidopsis, and mice. She is also a fearless biochemist, employing state of the art technologies and inventing new ones. Again and again, she has shown the power of biochemistry to expand and explicate fundamental insights gained from genetic analysis and the power of genetics to disentangle intractable problems in biochemistry. Her work has provided paradigm-shifting insights into the most basic aspects of cell biology, genetics, and evolution.
HEAT-SHOCK RESPONSE, HEAT-SHOCK PROTEINS, AND STRESS TOLERANCE
Lindquist's work began with studies of the heat-shock response when she was a graduate student at Harvard in Matthew Meselson's lab. (At that time she published under the name Susan McKenzie.) As a second-year student, she was casting about for a new project when she bumped into a new assistant professor in the hallway of the biology labs. Sally Elgin told her of some exciting new work by Tissiers and Mitchell: that proteins were made in response to heat shock in the salivary glands of fruit flies. Lindquist decided to see if tissue culture cells had the same response, which would make it tractable to molecular analysis and provide a powerful wedge to explore the then murky waters of eukaryotic gene regulation (McKenzie et al. 1975; McKenzie and Meselson 1977). She discovered that these cells did have the response and that it was governed by both translational and transcriptional mechanisms, making it the strongest most global change in eukaryotic gene expression known (McKenzie et al. 1975; McKenzie and Meselson 1977; Lindquist 1980, 1981). She also discovered that eukaryotic cells have the unexpected capacity to discriminate between coexisting mRNAs and independently regulate their translation. During heat shock they translate heat-shock mRNAs with high efficiency and block normal mRNAs from translation, yet hold them ready for reactivation after heat shock (McKenzie et al. 1975; Lindquist 1980, 1981; DiDomenico et al. 1982).
In a seminal series of experiments Lindquist continued to exploit the heat-shock response to establish how intricate and highly orchestrated the regulation of eukaryotic gene expression can be. Her work revealed regulation operating at the level of RNA splicing (Yost and Lindquist 1986, 1988, 1991), selective RNA and protein transport in and out of the nucleus (Velazquez et al. 1980; Wang and Lindquist 1998), selective RNA degradation (Petersen and Lindquist 1988, 1989), and selective deadenylation (Dellavalle et al. 1994).
Her group also established that it is the heat-shock proteins themselves that turn these mechanisms off, at every level (Yost and Lindquist 1986, 1991; Dellavalle et al. 1994; Vogel et al. 1995). As it became clear that heat-shock proteins help to prevent and repair the damage caused by stress (a story to which Lindquist also made important contributions), the extremely elegant logic of the regulatory circuitry was revealed: as heat-shock proteins (Hsps) restore normal protein homeostasis they reset the damaged regulatory systems that prevent the transcription, translation, splicing, degradation, and deadenylation of normal mRNAs. By restoring regulatory systems to their normal state, Hsps remove their own advantage, turning off the response. Due in large measure to her work (complimented by the seminal work of Spradling, Pelham, Lis, and Wu on transcription), the heat-shock response provides perhaps the most beautiful and complete example of eukaryotic gene regulation documented.
Turning her attention to the function of the induced proteins, in collaboration with Didier Picard in Keith Yamamoto's group, Lindquist established that Hsp90 is required for the maturation of steroid hormone receptors and oncogenic tyrosine kinases (Picard et al. 1990; Xu and Lindquist 1993; Xu et al. 1999). Since Hsp90 is complexed with the inactive forms of these proteins, previous assumptions had been that Hsp90 acted to repress their function. This paradigm shift, established through genetic analysis, was complemented by contemporaneous work on the biochemistry of the protein by Pratt and Toft and has held true for countless other metastable proteins that regulate key processes in growth, differentiation, development, and cancer.
Next, Lindquist's group discovered the tolerance factor that defends cells against extreme stresses. A single protein, Hsp104, increases survival up to 10,000-fold (Sanchez and Lindquist 1990; Sanchez et al. 1992). Using genetics and biochemistry to decipher its mechanism, her group turned another long-held assumption on its head. Proteins that are denatured and even massively aggregated do not need to be degraded after stress: using the energy of ATP, and with the aid of two chaperones, Hsp70 and hsp40, Hsp104 restores these proteins to function (Parsell et al. 1994; Glover and Lindquist 1998).
PRIONS AND INHERITANCE
In 1994 Reed Wickner made the remarkable suggestion that a mysterious, cytoplasmically inherited factor known as [PSI+] might be based upon some sort of self-perpetuating protein state and named it a prion. In collaboration with Chernoff and Leibman, Lindquist established that Hsp104 regulates the propagation of [PSI+]. Having found that Hsp104 is a protein-remodeling factor, this provided a strong genetic argument in support of inheritance based on protein conformation (Chernoff et al. 1995).
Lindquist's group went on to provide much of the key genetic, cell biological, and biochemical evidence that established that proteins can serve as elements of genetic inheritance. They showed that the protein Sup35 transitions from a soluble to an aggregated state in forming the prion and that this state is self propagating by the transmission of protein aggregates from mother cells to their daughters (Patino et al. 1996).
Next, they determined the nature of the conformational switch—from a natively disordered state to an amyloid filament—and, using purified protein, established that it was an inherent property of Sup35 itself. Remarkably, once a small fraction switches, it rapidly templates the conversion of other proteins to the same state. Prion+ cell lysates can template this conversion, but not prion− lysates. This work established the fundamental biochemical mechanism for protein-based inheritance (Glover et al. 1997; Serio et al. 2000). More recently, Lindquist has provided stunning insight into the previously mysterious complexities of prion strains (Krishnan and Lindquist 2005; Mukhopadhyay et al. 2007; Tessier and Lindquist 2007).
Sup35 is an essential translation termination factor. Its prion domain has conserved the ability to switch into an inactive prion state for 800 million years of evolution. Lindquist argues that this switch is not just a disease of yeast, but serves as a completely novel mechanism for creating phenotypic diversity. When cells switch to the prion state the loss of termination activity leads to the read-through of stop codons, creating a host of new phenotypes, many of which are advantageous. By revealing previously hidden genetic variation on a global, genomewide scale the prion allows survival in fluctuating environments and can thereby provide a route to the rapid evolution of complex traits (True and Lindquist 2000; True et al. 2004).
Collaborating with the laboratory of Eric Kandel, Lindquist was also instrumental in establishing that another prion might serve a beneficial purpose (Si et al. 2003). CPEB, which plays a key role in the maintenance of synapses in metazoan brains, has a prion-like ability to sustain itself in an altered self-perpetuating conformation much like the yeast prion. Since the prion is the active form of the protein, their results suggest that this self-sustaining conformation constitutes a “molecular memory” for maintaining synapses. This paradigm-shifting work has greatly expanded our view of the importance of self-sustaining changes in protein conformation in biological systems.
HSP90 AND EVOLUTION
Hsp90's role in the maturation of steroid receptors and oncogenic tryosine kinases (discovered by Lindquist's group in collaboration with Yamamoto's, see above) has now been confirmed for a wide variety of metastable signal transducers. This places the protein in a unique position to couple environmental contingency with evolutionary change. In the mid-1990s, Lindquist's lab made the stunning discovery that Hsp90 buffers vast amounts of naturally occurring genetic variation in fruit flies (Rutherford and Lindquist 1998). By robustly maintaining diverse signaling pathways Hsp90 allows a multitude of mutations to accumulate in a silent state. When the organism experiences protein homeostatic stress (e.g., growth at high temperature), the variants are exposed, creating new traits. The variation can then be enriched by selective breeding and the phenotypes retained, even when the environment has returned to normal. Recently Lindquist's lab extended this work to Arabidopsis, demonstrating that Hsp90's capacity to buffer and release genetic variation is conserved across enormous evolutionary distances (Queitsch et al. 2002, 2008; Sangster et al. 2008a,b).
The Lindquist lab's studies of Hsp90 provided the first molecular foundation for the decades-old theory of canalization: that development can be made insensitive to genetic variation. In one fell swoop, it also explained how that variation could be exposed by environmental stress (which overwhelms protein homeostasis). The mechanism can produce an extraordinary variety of traits, reveals variation on a global genomewide scale, and allows that variation to work in a combinatorial fashion. This stunning new concept can also be applied to other biological problems, such as the evolution of tumors within a host (Rutherford and Lindquist 1998; Queitsch et al. 2002; Sangster et al. 2004). Lindquist pointed the way to that extension herself, when she reported that the mutations that activate an oncogenic protein also makes it more dependent upon Hsp90 for folding (Xu and Lindquist 1993; Xu et al. 1999).
PROTEIN FOLDING AND HUMAN DISEASE
Susan Lindquist has also provided new insights on the multifaceted roles that protein homeostasis plays in human diseases.
Reasoning that many neurodegenerative diseases are due to problems in protein folding and trafficking, and moreover that these problems and the mechanisms for coping with them are universal, Lindquist's group is employing yeast cells as “living test tubes” to study the cellular basis of complex diseases, such as Huntington's chorea (huntingtin) and Parkinson's disease (α-synuclein) (Krobitsch and Lindquist 2000; Outeiro and Lindquist 2003). They have used these cells to discover several unexpected properties of the implicated proteins. Importantly, although these diseases are characterized by protein aggregation, the factors that govern their toxicity show virtually no overlap. Remarkably, many of the factors discovered in yeast have now been validated in neurons. They have also discovered potential new therapeutic strategies that are currently being tested (Cashikar et al. 2005; Cooper et al. 2006; Duennwald et al. 2006a,b).
Once again demonstrating her potential to blaze new trails, Lindquist's group has recently revealed another pivotal role that protein homeostasis plays in balancing health and disease. HSF, the main regulator of the heat-shock response, controls a host of survival mechanisms, including protein homeostasis, maintenance of signaling pathways, prevention of apoptosis, responses to growth factors, and flux through respiratory and glycolytic pathways. Lindquist's group recently found that these same mechanisms are subverted by cancer cells to promote their survival in the face of the multifarious stresses of deranged signaling, genetic mutations, anoxia, nutrient deprivation, and the stress of new environments. Working with mice and with human cancer cells driven by diverse oncogenic lesions, their work provides fundamental new insight on the “nononcogene” addiction of cancer cells. It also suggests that HSF may be a powerful therapeutic target, aimed at the unique biology of cancer cells rather than at any particular oncogenic lesion (Dai et al. 2007).
GENETIC TRICKS TO REMODEL GENOMES
Lindquist's laboratory has also had an important impact on biological research in devising several powerful new technologies. From the standpoint of genetics, one of the most important has been the development of heterologous, site-specific recombinases to precisely remodel genomes in living organisms. Kent Golic, a postdoc in the Lindquist laboratory, imported a yeast site-specific DNA recombinase, FLP, into flies and embedded genes containing FLP target sites in the fly genome, providing the first mechanism for precisely popping genes in and out of the genome in a higher organism. The method also provided a mechanism for inducing sister chromatid and homologous chromosome exchange at specific sites to generate an allelic series of insertions (Golic and Lindquist 1989; Welte et al. 1993; Golic et al. 1997). This work transformed the practice of Drosophila genetics and has been used by countless labs to develop new methods for screening and for cell fate and lineage analysis. It also strongly influenced the development of site-specific recombination systems in other organisms, such as the Cre/Lox system in mice.
In reviewing Susan Lindquist's numerous scientific contributions, one is overwhelmed by the breadth, diversity, and endless creativity of this scientist. Her research group opened up the molecular analysis of the heat-shock response, provided definitive evidence that heat-shock proteins are key to tolerance to stress, established a new model for the general mechanism of amyloid formation, provided evidence for mechanisms by which complex traits could evolve rapidly, created model systems for studying deadly neurodegenerative diseases in yeast, and provided the first plausible molecular explanation for self-perpetuating protein conformational changes that might also be responsible for prion diseases in man. Lindquist is not limited to being an exceptional, groundbreaking scientist. In addition to her spectacular research, she has demonstrated a deep commitment to young scientists' careers and to advancing women in science by serving both as a role model and an advocate, speaking frequently on this topic. For all these reasons the Genetics Society is honored to salute Susan Lindquist for her impact on her field and our profession.
- Copyright © 2008 by the Genetics Society of America