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Cell Fate Determination: When is a determinant a determinant?
STEPHEN COHEN AND ANTHONY A. HYMAN STEPHEN COHEN AND ANTHONY A. HYMAN
CELL FATE DETERMINATION CELL FATE DETERMINATION
When isa determinant a determinant? The Numb protein, which is asymmetrically distributed during certain cell divisions in the nervous system of Drosophila, may have a role in determining the developmental fates of the daughter cells. During the development of an organism, a distinction can be made between those cell divisions that serve only to increase cell number and those that are involved in determining the cells' future fate. The two daughters of a purely proliferative cell division have equal developmental potentials - if their positions are interchanged, their developmental fates will correspond to their new positions, independent of lineage. Determinative cell divisions are more interesting from a developmental perspective: they produce nonequivalent daughter cells with different developmental fates (reviewed in [1]). In the early days of developmental biology, it was noticed that the cytoplasm of invertebrate eggs is often non-homogeneous [2]. The formation of particular cell types during determinative cell divisions could be traced back to the inclusion in one daughter cell of visibly different cytoplasm, suggesting a correlation between the inclusion of special 'cytoplasmic determinants' and cell fate specification [3,4]. Inheritance of cytoplasmic determinants provides a simple way for daughters of a single cell to acquire distinct fates. But although simple in concept, definitive evidence for the importance of cytoplasmic determinants has been relatively slow in coming. Cell fate decisions in complex systems seem more often to depend on cell-cell interactions, so that the daughters of a division have equivalent developmental potentials and their fates are subsequently determined by interactions with their neighbours (for example, see [5]).
wild-type animals, a progenitor cell known as a senseorgan precursor undergoes two rounds of cell division to produce four distinct daughter cells that comprise the sense organ (Fig. 1); the divisions of both the senseorgan precursor and its daughter cells seem to be unequal. One daughter (IIa in Fig. 1) divides once more to produce the external hair and socket cells, while division of the other (IIb in Fig. 1) produces the neuron and its supporting sheath cell. Thus, four distinct types of cell are produced. The product of the numb gene seems to play a role in distinguishing between the first two daughter cells, IIa and IIb. Using antibody staining, Rhyu et al. [141 showed that the Numb protein is localized to one end of the sense-organ precursor cell prior to its first division. As a consequence of this localization, Numb is inherited by only one of the two daughters, namely IIb, the progenitor of the neuron and sheath cell. In the absence of numb, the inequality of this first division is lost, so both daughters, IIa and IIb, become progenitors of hair and socket cells. Conversely, if numb is expressed in both daughter cells, both are directed into
Until recently, the best evidence for a cytoplasmic determinant came from studying the formation of the cells of the germ line. Cytoplasmic inclusions, known as polar granules or P-granules, segregate with precursors of the germ line in worms, flies and some vertebrates [6,7]. In Drosophila, granule components are thought to include a number of RNAs and proteins [8-12]. The formation of these ribonucleoprotein particles has been shown to be required for specifying the fate of germline precursor cells [131, but it has not yet been possible to identify a specific determinant of germ-line fate among the known components of P-granules. A recent study of cell fate specification in the peripheral nervous system of Drosophila [14] provides the first direct evidence that a segregated cytoplasmic factor is necessary for cell fate determination. Flies mutant for the numb gene lack the neural component of the sense organs of their peripheral nervous system [15]. In 420
Fig. 1. The cell lineages that generate the hair, socket, neuron and sheath cells in a wild-type embryo. In numb mutants, two sets of hairs and sockets are formed.
© Current Biology 1994, Vol 4 No 5
DISPATCH
the lib lineage, resulting in duplicated pairs of neurons and sheath cells. Thus, differential distribution of Numb protein correlates with the specification of distinct cell fates after division of the sense-organ precursor cell. At this level, Numb protein seems to act as a simple determinant, specifying the lineage leading to the formation of neurons and sheath cells. So, Numb protein behaves like a determinant during the first division of the sense-organ precursor cell. But Rhyu et al. [14] also present evidence that Numb may be involved in cell fate specification during the next round of cell division, when IIa and Ilb divide to form the hair and socket cell or neuron and sheath cell, respectively. In numb mutants, division of the Ila cell does not yield distinct hair and socket cells after the second division; rather, two daughters with the same fate are produced, with each going on to become a socket cell. Ectopic expression of numb can cause the opposite transformation, leading both daughters of IIa to form hairs. Furthermore, asymmetric distribution of Numb protein may be involved in the development of the central nervous system. Divisions of neuroblasts are asymmetric, each producing a ganglion mother cell and another neuroblast, and Numb protein is found to be asymmetrically localized during these divisions. Thus, asymmetric distribution of Numb seems to be implicated in a number of different determinative cell divisions in the developing nervous system of Drosophila. Traditionally, a determinant has been thought to be a component that will induce a specific developmental fate when inherited by a cell. The multiplicity of cell fate decisions in which Numb is involved suggests that it is not a determinant in this traditional sense. So how does Numb affect multiple cell fates? The segregation of a component into only one of a pair of daughter cells is a complicated event. First, some intracellular machinery must exist that allows the Numb protein to accumulate at one end of the sense-organ precursor cell before division. Second, the division axis must be such that all of the Numb ends up in one daughter cell. If the division axis is not specified properly, then the determinant will end up in both daughters (Fig. 2), and many experiments have shown that altering the cleavage plane can affect development [2,16,17]. In other words, the mechanisms that direct segregation of the determinant must be linked to a machinery that defines the plan of cell division. The relationship between division axis and cell polarity can be seen in more detail in early embryos of the nematode Caenorhabditiselegans [7,18]. In these embryos, the P-granules segregate to one end of the zygote before division. Subsequently, the mitotic spindle takes up a position which ensures that the Pgranules are all in the germ-line precursor cell. The position of the mitotic spindle seems to be determined by cortical sites [191, but P-granules will segregate to one end of the cell even in the absence of a spindle [7]. The cell must, therefore, have an intrinsic polarity that defines both the orientation of the spindle and the segregation of the determinant.
Fig. 2. The plane of cleavage will determine the distribution of asymmetrically distributed cytoplasmic determinants to daughter cells.
It is possible, therefore, that Numb is involved in generating cell polarity during division, perhaps to ensure the segregation of other, as yet unidentified, components. But the fact that ectopic expression of Numb in other cells can induce cell fate changes without localization suggests that this possibility is unlikely. Posokony [20] has suggested that differential distribution of Numb might be involved in cell fate specification based on cell-cell interactions. The genes Notch and Delta encode transmembrane proteins implicated in cell interactions which are important for cell fate decisions in the nervous system. Delta protein serves as the ligand and Notch the receptor for a signal by which a cell of the neuronal lineage inhibits from neuronal development its neighbour cells within a neuronal precluster and its sister cells in the sense-organ lineage pathway [21,22]. In this system, differential localization of Numb might be involved in setting up the direction of the Notch-Delta interaction, which would then determine which of the sister cells is to become the neuron. This more passive role for Numb is consistent with the reciprocal nature of the phenotypes caused by lack or gain of numb function. The proposed mechanism would provide an elegant way of combining segregation of determinants with cell interactions to make some cell divisions determinative. We wonder whether this type of combination of inheriting cytoplasmic components and learning information from its neighbours will prove to be a general mechanism in the specification of cell fate. References 1. HORVITZ HR, HERSKOWITZ I: Mechanisms of asymmetric cell
division: two Bs or not two Bs, that is the question. Cell 1992, 68:237-255.
2. WILSON EB: The cell in development and heredity. New York: 3.
Macmillan; 1925. FERNANDEZ J, STENT GS: Embryonic development of the glossiphonid leach Theronyzon rude: structure and development of the germinal bands. Dev Biol 1980, 78:407-434.
421
422
Current Biology 1994, Vol 4 No 5 4. JEFFERY WR, MEIER S: A yellow crescent cytoskeletal domain in ascidian eggs and its role in early development. Dev Biol 1983, 96:128-143.
14. RHYU MS, JAN LY, JAN YN: Asymmetric distribution of numb protein during division of the sensory organ precursor cell confers distinct fates to daughter cells. Cell 1994, 76:477-492.
BOWERMAN B, TAX FE, THOMAS JH, PRIESS JR: Cell interactions
15. UEMURA T, SHEPHERD S, ACKERMAN L, JAN LY, JAN YN: numb: a
involved in development of the bilaterally symmetrical intestinal valve cells during embryogenesis in Caenorhabdltls elegans. Development 1992, 116:1113-1122. BEAMS HW, KESSEL RG: The problem of germ cell determinants. Int Rev Cytol 1974, 39:413-479. STROME S, WOOD WB: Generation of asymmetry and segregation of germline granules in Caenorhabditlselegans embryos. Cell 1983, 35:15-25. HAY B, JAN LY, JAN YN: Localization of vasa, a component of Drosophila polar granules, in maternal-effect mutants that alter embryonic anteroposterior polarity. Development 1990, 109:425-433.
gene required in determination of cell fate during sense organ formation in Drosophlla. Cell 1994, 58:349-360. 16. WHITAKER JR: Acetylcholinesterase development in extra cells caused by changing the distribution of myoplasm in ascidian embryos. JEmbryolExp Morphol 1980, 55:343-354.
5.
6. 7.
8.
9.
EPHRUSSI A, DICKINSON LK, LEHMANN R: oskar organizes the
germ plasm and directs localization of the posterior determinant nanos. Cell 1991, 66:37-50. 10.
KIM-HA, SMITH JL, MACDONALD PM: oskar messenger RNA is
localized to the posterior pole of the Drosophla oocyte. Cell 1991, 66:23-34. 11.
JONGENS T, HAY B, JAN LY, JAN YN: The germ cell-less gene
product: A posteriorly localized component necessary for germ cell development in Drosophlla. Cell 1992, 70:539-584. 12.
BARDSLEY A, MCDONALD K, BOSWELL RE: Distribution of tudor
protein in the Drosophlla embryo suggests separation of functions based on site of localization. Development 1993, 119:207-219. 13.
EPHRUSSI A, LEHMANN R: Induction of germ cell formation by
oskar. Nature 1992, 358:387-392.
17. BERLETH T, JRGENS G: The role of the monopteros gene in
18.
19.
20. 21. 22.
organising the basal body region of the Arabldopsls embryo. Development 1993, 118:575-587. HYMAN AA, WHITE J: Determination of cell division axes in the early embryogenesis of Caenorhabdltlselegans. J Cell Biol 1987, 105:2123-2135. HYMAN AA: Centrosome movement in the early divisions of Caenorhabdltlselegans: a cortical site determining centrosome position. J Cell Biol 1989, 109:1185-1193. POSOKONY JW: Nature versus nurture: asymmetric cell divisions in Drosophilabristle development. Cell 1994, 76:415-418. HEITZLER P, SIMPSON P: The choice of cell fate in the epidermis of Drosophlla. Cell 1991, 64:1083-1092. HEITZLER P, SIMPSON P: Altered epidermal growth factor-like
sequences provide evidence for a role of Notch as a receptor in cell fate decisions. Development 1993, 117:1113-1123.
Stephen Cohen and Anthony A. Hyman, European Molecular Biology Laboratory, Meyerhofstrasse 1, 69012 Heidelberg, Germany.
CELL FATE DETERMINATION CELL FATE DETERMINATION
When isa determinant a determinant? The Numb protein, which is asymmetrically distributed during certain cell divisions in the nervous system of Drosophila, may have a role in determining the developmental fates of the daughter cells. During the development of an organism, a distinction can be made between those cell divisions that serve only to increase cell number and those that are involved in determining the cells' future fate. The two daughters of a purely proliferative cell division have equal developmental potentials - if their positions are interchanged, their developmental fates will correspond to their new positions, independent of lineage. Determinative cell divisions are more interesting from a developmental perspective: they produce nonequivalent daughter cells with different developmental fates (reviewed in [1]). In the early days of developmental biology, it was noticed that the cytoplasm of invertebrate eggs is often non-homogeneous [2]. The formation of particular cell types during determinative cell divisions could be traced back to the inclusion in one daughter cell of visibly different cytoplasm, suggesting a correlation between the inclusion of special 'cytoplasmic determinants' and cell fate specification [3,4]. Inheritance of cytoplasmic determinants provides a simple way for daughters of a single cell to acquire distinct fates. But although simple in concept, definitive evidence for the importance of cytoplasmic determinants has been relatively slow in coming. Cell fate decisions in complex systems seem more often to depend on cell-cell interactions, so that the daughters of a division have equivalent developmental potentials and their fates are subsequently determined by interactions with their neighbours (for example, see [5]).
wild-type animals, a progenitor cell known as a senseorgan precursor undergoes two rounds of cell division to produce four distinct daughter cells that comprise the sense organ (Fig. 1); the divisions of both the senseorgan precursor and its daughter cells seem to be unequal. One daughter (IIa in Fig. 1) divides once more to produce the external hair and socket cells, while division of the other (IIb in Fig. 1) produces the neuron and its supporting sheath cell. Thus, four distinct types of cell are produced. The product of the numb gene seems to play a role in distinguishing between the first two daughter cells, IIa and IIb. Using antibody staining, Rhyu et al. [141 showed that the Numb protein is localized to one end of the sense-organ precursor cell prior to its first division. As a consequence of this localization, Numb is inherited by only one of the two daughters, namely IIb, the progenitor of the neuron and sheath cell. In the absence of numb, the inequality of this first division is lost, so both daughters, IIa and IIb, become progenitors of hair and socket cells. Conversely, if numb is expressed in both daughter cells, both are directed into
Until recently, the best evidence for a cytoplasmic determinant came from studying the formation of the cells of the germ line. Cytoplasmic inclusions, known as polar granules or P-granules, segregate with precursors of the germ line in worms, flies and some vertebrates [6,7]. In Drosophila, granule components are thought to include a number of RNAs and proteins [8-12]. The formation of these ribonucleoprotein particles has been shown to be required for specifying the fate of germline precursor cells [131, but it has not yet been possible to identify a specific determinant of germ-line fate among the known components of P-granules. A recent study of cell fate specification in the peripheral nervous system of Drosophila [14] provides the first direct evidence that a segregated cytoplasmic factor is necessary for cell fate determination. Flies mutant for the numb gene lack the neural component of the sense organs of their peripheral nervous system [15]. In 420
Fig. 1. The cell lineages that generate the hair, socket, neuron and sheath cells in a wild-type embryo. In numb mutants, two sets of hairs and sockets are formed.
© Current Biology 1994, Vol 4 No 5
DISPATCH
the lib lineage, resulting in duplicated pairs of neurons and sheath cells. Thus, differential distribution of Numb protein correlates with the specification of distinct cell fates after division of the sense-organ precursor cell. At this level, Numb protein seems to act as a simple determinant, specifying the lineage leading to the formation of neurons and sheath cells. So, Numb protein behaves like a determinant during the first division of the sense-organ precursor cell. But Rhyu et al. [14] also present evidence that Numb may be involved in cell fate specification during the next round of cell division, when IIa and Ilb divide to form the hair and socket cell or neuron and sheath cell, respectively. In numb mutants, division of the Ila cell does not yield distinct hair and socket cells after the second division; rather, two daughters with the same fate are produced, with each going on to become a socket cell. Ectopic expression of numb can cause the opposite transformation, leading both daughters of IIa to form hairs. Furthermore, asymmetric distribution of Numb protein may be involved in the development of the central nervous system. Divisions of neuroblasts are asymmetric, each producing a ganglion mother cell and another neuroblast, and Numb protein is found to be asymmetrically localized during these divisions. Thus, asymmetric distribution of Numb seems to be implicated in a number of different determinative cell divisions in the developing nervous system of Drosophila. Traditionally, a determinant has been thought to be a component that will induce a specific developmental fate when inherited by a cell. The multiplicity of cell fate decisions in which Numb is involved suggests that it is not a determinant in this traditional sense. So how does Numb affect multiple cell fates? The segregation of a component into only one of a pair of daughter cells is a complicated event. First, some intracellular machinery must exist that allows the Numb protein to accumulate at one end of the sense-organ precursor cell before division. Second, the division axis must be such that all of the Numb ends up in one daughter cell. If the division axis is not specified properly, then the determinant will end up in both daughters (Fig. 2), and many experiments have shown that altering the cleavage plane can affect development [2,16,17]. In other words, the mechanisms that direct segregation of the determinant must be linked to a machinery that defines the plan of cell division. The relationship between division axis and cell polarity can be seen in more detail in early embryos of the nematode Caenorhabditiselegans [7,18]. In these embryos, the P-granules segregate to one end of the zygote before division. Subsequently, the mitotic spindle takes up a position which ensures that the Pgranules are all in the germ-line precursor cell. The position of the mitotic spindle seems to be determined by cortical sites [191, but P-granules will segregate to one end of the cell even in the absence of a spindle [7]. The cell must, therefore, have an intrinsic polarity that defines both the orientation of the spindle and the segregation of the determinant.
Fig. 2. The plane of cleavage will determine the distribution of asymmetrically distributed cytoplasmic determinants to daughter cells.
It is possible, therefore, that Numb is involved in generating cell polarity during division, perhaps to ensure the segregation of other, as yet unidentified, components. But the fact that ectopic expression of Numb in other cells can induce cell fate changes without localization suggests that this possibility is unlikely. Posokony [20] has suggested that differential distribution of Numb might be involved in cell fate specification based on cell-cell interactions. The genes Notch and Delta encode transmembrane proteins implicated in cell interactions which are important for cell fate decisions in the nervous system. Delta protein serves as the ligand and Notch the receptor for a signal by which a cell of the neuronal lineage inhibits from neuronal development its neighbour cells within a neuronal precluster and its sister cells in the sense-organ lineage pathway [21,22]. In this system, differential localization of Numb might be involved in setting up the direction of the Notch-Delta interaction, which would then determine which of the sister cells is to become the neuron. This more passive role for Numb is consistent with the reciprocal nature of the phenotypes caused by lack or gain of numb function. The proposed mechanism would provide an elegant way of combining segregation of determinants with cell interactions to make some cell divisions determinative. We wonder whether this type of combination of inheriting cytoplasmic components and learning information from its neighbours will prove to be a general mechanism in the specification of cell fate. References 1. HORVITZ HR, HERSKOWITZ I: Mechanisms of asymmetric cell
division: two Bs or not two Bs, that is the question. Cell 1992, 68:237-255.
2. WILSON EB: The cell in development and heredity. New York: 3.
Macmillan; 1925. FERNANDEZ J, STENT GS: Embryonic development of the glossiphonid leach Theronyzon rude: structure and development of the germinal bands. Dev Biol 1980, 78:407-434.
421
422
Current Biology 1994, Vol 4 No 5 4. JEFFERY WR, MEIER S: A yellow crescent cytoskeletal domain in ascidian eggs and its role in early development. Dev Biol 1983, 96:128-143.
14. RHYU MS, JAN LY, JAN YN: Asymmetric distribution of numb protein during division of the sensory organ precursor cell confers distinct fates to daughter cells. Cell 1994, 76:477-492.
BOWERMAN B, TAX FE, THOMAS JH, PRIESS JR: Cell interactions
15. UEMURA T, SHEPHERD S, ACKERMAN L, JAN LY, JAN YN: numb: a
involved in development of the bilaterally symmetrical intestinal valve cells during embryogenesis in Caenorhabdltls elegans. Development 1992, 116:1113-1122. BEAMS HW, KESSEL RG: The problem of germ cell determinants. Int Rev Cytol 1974, 39:413-479. STROME S, WOOD WB: Generation of asymmetry and segregation of germline granules in Caenorhabditlselegans embryos. Cell 1983, 35:15-25. HAY B, JAN LY, JAN YN: Localization of vasa, a component of Drosophila polar granules, in maternal-effect mutants that alter embryonic anteroposterior polarity. Development 1990, 109:425-433.
gene required in determination of cell fate during sense organ formation in Drosophlla. Cell 1994, 58:349-360. 16. WHITAKER JR: Acetylcholinesterase development in extra cells caused by changing the distribution of myoplasm in ascidian embryos. JEmbryolExp Morphol 1980, 55:343-354.
5.
6. 7.
8.
9.
EPHRUSSI A, DICKINSON LK, LEHMANN R: oskar organizes the
germ plasm and directs localization of the posterior determinant nanos. Cell 1991, 66:37-50. 10.
KIM-HA, SMITH JL, MACDONALD PM: oskar messenger RNA is
localized to the posterior pole of the Drosophla oocyte. Cell 1991, 66:23-34. 11.
JONGENS T, HAY B, JAN LY, JAN YN: The germ cell-less gene
product: A posteriorly localized component necessary for germ cell development in Drosophlla. Cell 1992, 70:539-584. 12.
BARDSLEY A, MCDONALD K, BOSWELL RE: Distribution of tudor
protein in the Drosophlla embryo suggests separation of functions based on site of localization. Development 1993, 119:207-219. 13.
EPHRUSSI A, LEHMANN R: Induction of germ cell formation by
oskar. Nature 1992, 358:387-392.
17. BERLETH T, JRGENS G: The role of the monopteros gene in
18.
19.
20. 21. 22.
organising the basal body region of the Arabldopsls embryo. Development 1993, 118:575-587. HYMAN AA, WHITE J: Determination of cell division axes in the early embryogenesis of Caenorhabdltlselegans. J Cell Biol 1987, 105:2123-2135. HYMAN AA: Centrosome movement in the early divisions of Caenorhabdltlselegans: a cortical site determining centrosome position. J Cell Biol 1989, 109:1185-1193. POSOKONY JW: Nature versus nurture: asymmetric cell divisions in Drosophilabristle development. Cell 1994, 76:415-418. HEITZLER P, SIMPSON P: The choice of cell fate in the epidermis of Drosophlla. Cell 1991, 64:1083-1092. HEITZLER P, SIMPSON P: Altered epidermal growth factor-like
sequences provide evidence for a role of Notch as a receptor in cell fate decisions. Development 1993, 117:1113-1123.
Stephen Cohen and Anthony A. Hyman, European Molecular Biology Laboratory, Meyerhofstrasse 1, 69012 Heidelberg, Germany.