Annemarie Weber was a major contributor to the renaissance of muscle biology research in the 1950s to 1970s, when the components of the contractile machinery were identified; novel views of muscle contraction and regulation were elucidated; and principles of energy transduction, motility and intracellular signaling common to all cells were revealed. While Annemarie was introduced to research by her father, Hans H. Weber, a muscle physiologist, she strode off on her own, establishing her credentials in publications that were innovative, thorough and meticolus. She quickly achieved international scientific recognition and became a dominating figure in the field, a significant feat at a time when developing an independent scientific career was not easy for women.
Annemarie provided direct evidence for the role of calcium ions as intracellular messengers. Some hints already existed: Small amounts of calcium-contaning solution introduced into muscle fibers resulted in localized contractions (1, 2), an indication that calcium could be a physiological activator. However, in vitro experiments with isolated proteins and myofibrils were baffling and gave uncertain results. Before the availability of calcium-specific chelators, calcium was a contaminator of glassware and chemicals. Magnesium added further complication and, at low concentrations of ATP contraction, was calcium-independent, so major skepticism remained on the activator role of calcium.
Annemarie (3) calculated the effect of free calcium ion concentration on various ligands and established that very low concentrations of ionized calcium are uniquely necessary and sufficient to activate the contractile machinery of muscle in the presence of physiological (mM) concentrations of MgATP. She further interacted with the Japanese scientist Setsuro Ebashi, also a supporter of the calcium hypothesis, who had identified the calcium binding and ATPase activity of a vesicular fraction derived from sarcoplasmic reticulum, or SR (4).
Wilhelm Hasselbach (5) demonstrated the ATP-dependent calcium-pumping action of isolated SR, and Annemarie finally demonstrated that SR vesicles could account fully for muscle relaxation via their calcium-sequestering ability (6). Her experiments and calculations were so compelling that they ended years of dispute, eliminating the hypothesis of a soluble relaxing factor. Her insights paved the way for the discovery by Ebashi of a myofibrillar protein with high affinity for calcium, troponin C (7), and led to the subsequent realization that most cells use calcium as an intracellular messenger.
The basic mechanisms Annemarie discovered in muscle apply very broadly: The free calcium concentration in the cytoplasm is kept low mostly due to the sequestering action of the endoplasmic reticulum and SR, while cytoplasmic proteins, such as troponin (in muscle) and calmodulin (in other cells), act as second messengers for cellular processes via their high calcium affinity.
Annemarie next directed her attention to the mechanism by which calcium activates contraction. The steric hindrance model (8, 9, 10) suggested that tropomyosin and the troponins, components of the actin-containing thin filaments, act as a complex calcium-sensitive switch for contraction. Annemarie was able to clarify the mystery of the calcium-independent activation of actomyosin interaction at low MgATP concentration by showing that the myosin crossbridges still attached to actin and waiting to bind ATP effectively act as a foot in the door, producing a cooperative activation of seven actin subunits along the thin filament (11). Her work provided an additional graphic picture of the way in which tropomyosin, which covers and inhibits actin subunits at rest, is forced out of the way after calcium binding to troponin C.
The last scientific challenge for Annemarie was to understand the formation and disassembly of actin filaments, which are present in a dynamic state in all eukariotic cells. In collaborative work, Weber used her understanding of kinetic processes to define the the regulation of actin-subunit association and dissociation at the slow-growing (pointed) filament end. She demonstrated that the actions of DNaseI and tropomyosin on that end of the filament are due to their specific effects on the actin subunit off-rate. Subsequently, in nice symmetry to her earlier work, she collaborated with Velia M. Fowler to show that tropomodulin is the long-sought-after capper for the thin filament pointed ends in striated muscle (12, 13). Her original molecular and kinetic insights still form the basis for studies of tropomodulin action.
Annemarie was an incredibly successful teacher, because she was totally dedicated, she had an inexhaustible enthusiasm for the material, and she liked challenging students, colleagues and herself while striving for perfection. Advanced scientists still remember being inspired by attending her famous physiology course at the Marine Biology Laboratory in Woods Hole, Mass. At the University of Pennsylvania, which she joined in 1972, she completely revised the medical biochemistry course with a novel approach, earning the students’ enthusiasm and the university’s Provost Award for Distinguished Teaching.
To have known Annemarie Weber was an unforgettable experience. My own appreciation started with the initial admiration at her 1964 Federation of American Societies for Experimental Biology meeting presentation, when she shredded the last lingering concepts of a soluble relaxing factor. I read with a sense of discovery her 1972 definition of the cooperative activation of actin filaments: what wonderful intuition! I then shifted into definite panic when I had to talk in her presence at the Pennsylvania Muscle Insitute in Philadelphia. I feared to hear the famous “Look here, sweetie” that preceeded one of her incisive comments on some inconsistency she had noticed in the presentation. I had seen famous scientists shake in their boots when she asked one of her pointed questions.
|A.V. Hill and Annemarie Weber photographed by D.R. Wilkie in 1951 at University College London.
Later, when I knew her better, I discovered the enormous generosity of her friendship. She was a challenging and stimulating scientifc adviser. She gave me thoughtful insights into my children (who confided in her) and stimulating books covering all sorts of subjects. She took me mountain hiking. She challenged my husband, Clay, in friendly debates. She similarly took care of and stimulated a large network of friends, including her German nephews and nieces and their children, for whom she planned interesting scientific experiments during their visits to Woods Hole. Her collaborator, Velia Fowler, remembers fondly her rigorous explanations of actin polymerization kinetics in conversations prefaced by the words, “But this is kinetics 101!” while being graciously hosted at her house. Trainees both at Penn and outside were daunted by the challenge of creating experimental designs and generating data that met her approval. Most touching was the fact that she maintained a courageous and cheerful demeanor up to the very last stages of lung cancer to maintain her independence and to hide her suffering from friends and relatives.
Annemarie had a strong personality, an ever-active intellect and boundless generosity. A unique scientist, teacher, mentor and friend, she will be missed.
I thank Velia M. Fowler and Yale E. Goldman for help with this article.
- 1. Heillbrunn, L. V. and Wiercinski, F. J. J. Cellular Comparative Physiol. 29, 15 – 32 (1947).
- 2. Niedergerke, R. J. Physiol. 128, 12P – 13P (1955).
- 3. Weber, A. J. Biol. Chem. 234, 2764 – 2769 (1959).
- 4. Ebashi, S. and Lipmann F. J. Cell Biol. 14, 389 – 400 (1962).
- 5. Hasslebach, W. and Makinose, M. Biochem. Zeitschr. 33, 528 – 528 (1961).
- 6. Weber, A. et al. J. Gen. Physiol. 46, 679 – 702 (1963).
- 7. Ebashi, S. and Ebashi, F. J. Biochem. 55, 604 – 613 (1964).
- 8. Haselgrove, J. C. Cold Spring Harbor Symp. Quant. Biol. 37, 341 – 352 (1972).
- 9. Huxley, H. E. Cold Spring Harbor Symp. Quant. Biol. 37, 361 – 378 (1972).
- 10. Parry, D. A. D. and Squire, J. M. J. Mol. Biol. 75, 35 – 55 (1973).
- 11. Bremel, R. D. et al. Cold Spring Harbor Symp. Quant. Biol. 37, 267 – 275 (1972).
- 12. Weber, A. et al. J. Cell Biol. 127, 1627 – 1635 (1994).
- 13. Gregorio, C. C. et al. Nature. 377, 83 – 86 (1995).
Clara Franzini-Armstrong (firstname.lastname@example.org) is professor emerita at the University of Pennsylvania School of Medicine.