Asymmetric cell division: lessons from flies and worms

Insights into the mechanisms of asymmetric cell division have recently been obtained from studies in genetically amenable systems such as Drosophila and Caenorhabditis elegans. These studies have emphasized the importance of cortically localized polarity organizing molecules, adapter molecules, and the actin cytoskeleton in controlling unequal segregation of cell-fate determinants and spindle orientation. The control of asymmetric cell divisions by Wnt signaling in C. elegans and Frizzled signaling in Drosophila reveals additional mechanisms for modulating cellular polarity and suggests that there are some similarities between the two systems.

Miranda as a multidomain adapter linking apically localized Inscuteable and basally localized Staufen and Prospero during asymmetric cell division in Drosophila

Neuroblasts in the developing Drosophila CNS asymmetrically localize the cell fate determinants Numb and Prospero as well as prospero RNA to the basal cortex during mitosis. The localization of Prospero requires the function of inscuteable and miranda, whereas prospero RNA localization requires inscuteable and staufen function. We demonstrate that Miranda contains multiple functional domains: an amino-terminal asymmetric localization domain, which interacts with Inscuteable, a central Numb interaction domain, and a more carboxy-terminal Prospero interaction domain. We also show that Miranda and Staufen have similar subcellular localization patterns and interact in vitro. Furthermore, miranda function is required for the asymmetric localization of Staufen. Miranda localization is disrupted by the microfilament disrupting agent latrunculin A. Our results suggest that Miranda directs the basal cortical localization of multiple molecules, including Staufen and prospero RNA, in mitotic neuroblasts in an actin-dependent manner.

Molecular basis for interactions of G protein betagamma subunits with effectors

Both the alpha and betagamma subunits of heterotrimeric guanine nucleotide-binding proteins (G proteins) communicate signals from receptors to effectors. Gbetagamma subunits can regulate a diverse array of effectors, including ion channels and enzymes. Galpha subunits bound to guanine diphosphate (Galpha-GDP) inhibit signal transduction through Gbetagamma subunits, suggesting a common interface on Gbetagamma subunits for Galpha binding and effector interaction. The molecular basis for interaction of Gbetagamma with effectors was characterized by mutational analysis of Gbeta residues that make contact with Galpha-GDP. Analysis of the ability of these mutants to regulate the activity of calcium and potassium channels, adenylyl cyclase 2, phospholipase C-beta2, and beta-adrenergic receptor kinase revealed the Gbeta residues required for activation of each effector and provides evidence for partially overlapping domains on Gbeta for regulation of these effectors. This organization of interaction regions on Gbeta for different effectors and Galpha explains why subunit dissociation is crucial for signal transmission through Gbetagamma subunits.

G-protein signaling: fine-tuning signaling kinetics

Mammalian 'regulators of G protein signaling' (RGS proteins) help shut off G-protein-mediated signaling by GTPase activation. But new evidence shows that RGS proteins can also speed up the activation of signaling. The combined effect is a change in signaling kinetics without a decrease in signaling intensity.

Asymmetric cell division

With the recent identification of intrinsic cell-fate determinants for asymmetric cell division in several systems, biologists have begun to gain insight into the cellular mechanisms by which these determinants are preferentially segregated into one of the two daughter cells during mitosis so that the daughter cells acquire different fates.

Delta and Serrate are redundant Notch ligands required for asymmetric cell divisions within the Drosophila sensory organ lineage

Asymmetric divisions allow a precursor to produce four distinct cells of a Drosophila sensory organ lineage (SOL). Whereas this process requires cell-cell communication via Notch (N) receptor, mitotic recombination that removes the N ligand Delta (Dl) or Serrate (Ser) in the SOL had mild or no effect. Removal of both Dl and Ser, however, led to cell fate transformations similar to the N phenotype. Cell fate transformation occurred even when a single SOL cell lost both Dl and Ser. Thus, Dl and Ser are redundant in mediating signaling between daughter cells to specify their distinct cell fates.

Presynaptic localization of Kv1.4-containing A-type potassium channels near excitatory synapses in the hippocampus

Mammalian Shaker voltage-gated potassium channels that contain the Kv1.4 subunit exhibit rapid activation and prominent inactivation processes, which enable these channels to integrate brief (approximiately milliseconds) depolarizations over time intervals of up to tens of seconds. In the hippocampus, Kv1.4 immunoreactivity is detected at greatest density in two regions: (1) the middle molecular layer (MML), where perforant path axons synapse with dentate granule cells, and (2) the stratum lucidum (SL) of CA3, where the mossy fibers travel in tight fasciculi and form en passante synapses onto CA3 pyramidal cells. We have studied the localization of Kv1.4 within these regions in detail. First, we compared the distribution of Kv1.4 and synaptophysin (a synaptic vesicle protein primarily localized near termini) under confocal immunofluorescence microscopy. In the MML, Kv1.4 and synaptophysin immunofluorescence appeared to overlap. In the SL, however, Kv1.4 and synaptophysin staining was detected in nonoverlapping, irregular patches ( approximately 5-10 micro m in diameter). Ultrastructural studies of these two regions revealed that Kv1.4 immunoreactivity was absent from the surface membranes of cell bodies and dendrites and occurred prominently on axons, including axonal "necks" near termini. Small excitatory synaptic boutons also were labeled in the MML; by contrast, the mossy fiber synaptic expansions in the SL were not stained. These localizations may enable Kv1.4-containing channels to regulate the process of neurotransmitter release at these excitatory synapses.

Numb-associated kinase interacts with the phosphotyrosine binding domain of Numb and antagonizes the function of Numb in vivo

During asymmetric cell division, the membrane-associated Numb protein localizes to a crescent in the mitotic progenitor and is segregated predominantly to one of the two daughter cells. We have identified a putative serine/threonine kinase, Numb-associated kinase (Nak), which interacts physically with the phosphotyrosine binding (PTB) domain of Numb. The PTB domains of Shc and insulin receptor substrate bind to an NPXY motif which is not present in the region of Nak that interacts with Numb PTB domain. We found that the Numb PTB domain but not the Shc PTB domain interacts with Nak through a peptide of 11 amino acids, implicating a novel and specific protein-protein interaction. Overexpression of Nak in the sensory organs causes both daughters of a normally asymmetric cell division to adopt the same cell fate, a transformation similar to the loss of numb function phenotype and opposite the cell fate transformation caused by overexpression of Numb. The frequency of cell fate transformation is sensitive to the numb gene dosage, as expected from the physical interaction between Nak and Numb. These findings indicate that Nak may play a role in cell fate determination during asymmetric cell divisions.

Probing the G-protein regulation of GIRK1 and GIRK4, the two subunits of the KACh channel, using functional homomeric mutants

In heart, G-protein-activated channels are complexes of two homologous proteins, GIRK1 and GIRK4. Expression of either protein alone results in barely active or non-active channels, making it difficult to assess the individual contribution of each subunit to the channel complex. The residue Phe137, located within the H5 region of GIRK1, is critical to the synergy between GIRK1 and GIRK4 (Chan, K. W., Sui, J. L., Vivaudou, M., and Logothetis, D. E. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 14193-14198). By modifying this residue or the matching residue of GIRK4, Ser143, we have been able to generate mutant proteins that produced large inwardly rectifying, G-protein-modulated currents when expressed alone in Xenopus oocytes. The enhanced activity of the heterologous expression of each of two active mutants, GIRK1(F137S) and GIRK4(S143T), was not caused by association with an endogenous oocyte channel subunit, and these mutants did not display apparent differences in the ability to localize to the cell surface compared with their wild-type counterparts. When these functional mutant channels were compared individually with wild-type heteromeric channels, they responded with only small differences to a number of maneuvers involving coexpression with muscarinic receptors, G-protein betagamma subunits, wild-type or mutated G-protein alpha subunits, and active protomers of pertussis toxin. These experiments, which confirmed the crucial, though not exclusive, role of Gbetagamma in regulating channel activity, demonstrated that GIRK1(F137S) and GIRK4(S143T), and by extrapolation their wild-type counterparts, interact in a qualitatively similar way with G-protein subunits. These findings suggest that functionally important sites of interaction with G-proteins are likely to be located within the homologous regions of GIRK1 and GIRK4 rather than within the divergent terminal regions. They also raise the question of the functional advantage of a heteromeric over homomeric design for G-protein-gated channels.

Voltage-gated and inwardly rectifying potassium channels

This lecture is dedicated to Max Delbrück and Seymour Benzer. Max Delbrück was our graduate advisor. He introduced us to a variety of biophysical problems, and taught us ways of thinking about these problems by example. Potassium channels was one of the topics included in his journal club in the early seventies; Max also carefully considered the feasibility of purifying potassium channels then. It was in Seymour Benzer's laboratory that we began to look for Drosophila mutants that affect synaptic transmission at the larval neuromuscular junction. Shaker was the first behavioural mutant we tested that gave a robust phenotype, a phenotype that could be mimicked by treating wild-type preparations with a potassium channel blocker. This mutant fly has led us to our subsequent molecular studies of potassium channels. Since we settled in the University of California, San Francisco, and began to study neural development as well as potassium channels, we have settled into the pattern of each attending meetings and presenting our studies on one of these two areas so as to avoid both being away from home and our children at the same time. In following this pattern, I will be presenting the studies of potassium channels as part of our long-term collaboration. In this talk I will first briefly take you through the path that led us to the molecular studies of potassium channels and then discuss the diversity and modulation of these potassium channels at the molecular and physiological level.

Genghis Khan (Gek) as a putative effector for Drosophila Cdc42 and regulator of actin polymerization

The small GTPases Cdc42 and Rac regulate a variety of biological processes, including actin polymerization, cell proliferation, and JNK/mitogen-activated protein kinase activation, conceivably via distinct effectors. Whereas the effector for mitogen-activated protein kinase activation appears to be p65PAK, the identity of effector(s) for actin polymerization remains unclear. We have found a putative effector for Drosophila Cdc42, Genghis Khan (Gek), which binds to Dcdc42 in a GTP-dependent and effector domain-dependent manner. Gek contains a predicted serine/threonine kinase catalytic domain that is 63% identical to human myotonic dystrophy protein kinase and has protein kinase activities. It also possesses a large coiled-coil domain, a putative phorbol ester binding domain, a pleckstrin homology domain, and a Cdc42 binding consensus sequence that is required for its binding to Dcdc42. To study the in vivo function of gek, we generated mutations in the Drosophila gek locus. Egg chambers homozygous for gek mutations exhibit abnormal accumulation of F-actin and are defective in producing fertilized eggs. These phenotypes can be rescued by a wild-type gek transgene. Our results suggest that this multidomain protein kinase is an effector for the regulation of actin polymerization by Cdc42.

The N terminus of the Drosophila Numb protein directs membrane association and actin-dependent asymmetric localization

Drosophila Numb is a membrane associated protein of 557 amino acids (aa) that localizes asymmetrically into a cortical crescent in mitotic neural precursor cells and segregates into one of the daughter cells, where it is required for correct cell fate specification. We demonstrate here that asymmetric localization but not membrane localization of Numb in Drosophila embryos is inhibited by latrunculin A, an inhibitor of actin assembly. We also show that deletion of either the first 41 aa or aa 41-118 of Numb eliminates both localization to the cell membrane and asymmetric localization during mitosis, whereas C-terminal deletions or deletions of central portions of Numb do not affect its subcellular localization. Fusion of the first 76 or the first 119 aa of Numb to beta-galactosidase results in a fusion protein that localizes to the cell membrane, but fails to localize asymmetrically during mitosis. In contrast, a fusion protein containing the first 227 aa of Numb and beta-galactosidase localizes asymmetrically during mitosis and segregates into the same daughter cell as the endogenous Numb protein, demonstrating that the first 227 aa of the Numb protein are sufficient for asymmetric localization.

Only a subset of the binary cell fate decisions mediated by Numb/Notch signaling in Drosophila sensory organ lineage requires Suppressor of Hairless

In Drosophila, an adult external sensory organ (bristle) consists of four distinct cells which arise from a sensory organ precursor cell via two rounds of asymmetric divisions. The sensory organ precursor cell first divides to generate two secondary precursor cells, IIa and IIb. The IIa cell then divides to produce the hair cell and the socket cell. Shortly after, the IIb cell divides to generate the neuron and the sheath cell. The membrane-associated protein Numb has been shown to be required for the first two asymmetric divisions. We now report that a new hypomorphic numb mutant not only displays a double-socket phenotype, due to a hair cell to socket cell transformation, but also a double-sheath phenotype, due to a neuron to sheath cell transformation. This provides direct evidence that numb functions in the neuron/sheath cell lineage as well. Those results, together with our observation from immunofluorescence analysis that Numb forms a crescent in the dividing IIa and IIb cells suggest that asymmetric localization of Numb is important for the cell fate determination in all three asymmetric cell divisions in the sensory organ lineage. Interestingly, we found that in the hair/socket cell lineage but not the neuron/sheath cell lineage, a Suppressor of Hairless mutation acts as a dominant suppressor of numb mutations whereas Hairless mutations act as enhancers of numb. Moreover, epistasis analysis indicates that Suppressor of Hairless acts downstream of numb, and results from in vitro binding analysis suggest that the genetic interaction between numb and Hairless may occur through direct protein-protein interaction. These studies reveal that Suppressor of Hairless is required for only a subset of the asymmetric divisions that depend on the function of numb and Notch.

Xath5 participates in a network of bHLH genes in the developing Xenopus retina

We examined the function of basic-helix-loop-helix (bHLH) transcription factors during retinal neurogenesis. We identified Xath5, a Xenopus bHLH gene related to Drosophila atonal, which is expressed in the developing Xenopus retina. Targeted expression of Xath5 in retinal progenitor cells biased the differentiation of these cells toward a ganglion cell fate, suggesting that Xath5 can regulate the differentiation of retinal neurons. We examined the relationship between the three bHLH genes Xash3, NeuroD, and Xath5 during retinal neurogenesis and found that Xash3 is expressed in early retinoblasts, followed by coexpression of Xath5 and NeuroD in differentiating cells. We provide evidence that the expression of Xash3, NeuroD, and Xath5 is coupled and propose that these bHLH genes regulate successive stages of neuronal differentiation in the developing retina.

Defective gamma-aminobutyric acid type B receptor-activated inwardly rectifying K+ currents in cerebellar granule cells isolated from weaver and Girk2 null mutant mice

Stimulation of inhibitory neurotransmitter receptors, such as gamma-aminobutyric acid type B (GABAB) receptors, activates G protein-gated inwardly rectifying K+ channels (GIRK) which, in turn, influence membrane excitability. Seizure activity has been reported in a Girk2 null mutant mouse lacking GIRK2 channels but showing normal cerebellar development as well as in the weaver mouse, which has mutated GIRK2 channels and shows abnormal development. To understand how the function of GIRK2 channels differs in these two mutant mice, we compared the G protein-activated inwardly rectifying K+ currents in cerebellar granule cells isolated from Girk2 null mutant and weaver mutant mice with those from wild-type mice. Activation of GABAB receptors in wild-type granule cells induced an inwardly rectifying K+ current, which was sensitive to pertussis toxin and inhibited by external Ba2+ ions. The amplitude of the GABAB receptor-activated current was severely attenuated in granule cells isolated from both weaver and Girk2 null mutant mice. By contrast, the G protein-gated inwardly rectifying current and possibly the agonist-independent basal current appeared to be less selective for K+ ions in weaver but not Girk2 null mutant granule cells. Our results support the hypothesis that a nonselective current leads to the weaver phenotype. The loss of GABAB receptor-activated GIRK current appears coincident with the absence of GIRK2 channel protein and the reduction of GIRK1 channel protein in the Girk2 null mutant mouse, suggesting that GABAB receptors couple to heteromultimers composed of GIRK1 and GIRK2 channel subunits.

The Drosophila neurogenic gene big brain, which encodes a membrane-associated protein, acts cell autonomously and can act synergistically with Notch and Delta

In the developing nervous system of Drosophila, cells in each proneural cluster choose between neural and epidermal cell fates. The neurogenic genes mediate the cell-cell communication process whereby one cell adopts the neural cell fate and prevents other cells in the cluster from becoming neural. In the absence of neurogenic gene function, most, if not all of the cells become neural. big brain is a neurogenic gene that encodes a protein with sequence similarity to known channel proteins. It is unique among the neurogenic genes in that previous genetic studies have not revealed any interaction between big brain and the other neurogenic genes. Furthermore, the neural hypertrophy in big brain mutant embryos is less severe than that in embryos mutant for other neurogenic genes. In this paper, we show by antibody staining that bib is expressed in tissues that give rise to neural precursors and in other tissues that are affected by loss of neurogenic gene function. By immunoelectron microscopy, we found that bib is associated with the plasma membrane and concentrated in apical adherens junctions as well as in small cytoplasmic vesicles. Using mosaic analysis in the adult, we demonstrate that big brain activity is required autonomously in epidermal precursors to prevent neural development. Finally, we demonstrate that ectopically expressed big brain acts synergistically with ectopically expressed Delta and Notch, providing the first evidence that big brain may function by augmenting the activity of the Delta-Notch pathway. These results are consistent with bib acting as a channel protein in proneural cluster cells that adopt the epidermal cell fate, and serving a necessary function in the response of these cells to the lateral inhibition signal.

G protein-coupled inwardly rectifying K+ channels (GIRKs) mediate postsynaptic but not presynaptic transmitter actions in hippocampal neurons

To study the role of G protein-coupled, inwardly rectifying K+ (GIRK) channels in mediating neurotransmitter actions in hippocampal neurons, we have examined slices from transgenic mice lacking the GIRK2 gene. The outward currents evoked by agonists for GABA(B) receptors, 5HT1A receptors, and adenosine A1 receptors were essentially absent in mutant mice, while the inward current evoked by muscarinic receptor activation was unaltered. In contrast, the presynaptic inhibitory action of a number of presynaptic receptors on excitatory and inhibitory terminals was unaltered in mutant mice. These included GABA(B), adenosine, muscarinic, metabotropic glutamate, and NPY receptors on excitatory synapses and GABA(B) and opioid receptors on inhibitory synapses. These findings suggest that a number of G protein-coupled receptors activate the same class of postsynaptic K+ channel, which contains GIRK2. In addition, the GIRK2 channels play no role in the inhibition mediated by presynaptic G protein-coupled receptors, suggesting that the same receptor can couple to different effector systems according to its subcellular location in the neuron.

Miranda is required for the asymmetric localization of Prospero during mitosis in Drosophila

Asymmetric division of Drosophila neuroblasts, sensory organ precursor cells, and cells in the procephalic neurogenic region involves the segregation of Numb and Prospero proteins into one of the two daughter cells. We have isolated a novel gene, miranda, based on the ability of its gene product to interact with the Prospero asymmetric localization domain. miranda expression coincides spatially and temporally with asymmetric cell divisions and asymmetric localization of Prospero. Miranda protein is localized asymmetrically, along with Prospero, to the basal cell membrane during mitosis. Loss of miranda gene function abolishes asymmetric Prospero localization during mitosis. The asymmetric localization of Miranda protein requires inscuteable. Our results suggest that miranda functions downstream of inscuteable and works as an adapter that connects Prospero to the basal cell membrane during asymmetric cell division.

Regulation of IRK3 inward rectifier K+ channel by m1 acetylcholine receptor and intracellular magnesium

Inward rectifier K+ channels control the cell's membrane potential and neuronal excitability. We report that the IRK3 but not the IRK1 inward rectifier K+ channel activity is inhibited by m1 muscarinic acetylcholine receptor. This m1 modulation cannot be accounted for by protein kinase C, Ca2+, or channel phosphorylation, but can be mimicked by Mg2+. Based on quantitative analyses of IRK3 and two different IRK1 mutant channels bestowed with sensitivity to m1 modulation, we suggest that the resting Mg2+ level causes chronic inhibition of IRK3 channels, and m1 receptor stimulation may lead to an increase of cytoplasmic Mg2+ concentration and further channel inhibition, due to the ability of Mg2+ to lead these channels into a prolonged inactivated state.

Differential expression of mammalian Numb, Numblike and Notch1 suggests distinct roles during mouse cortical neurogenesis

During Drosophila neurogenesis, asymmetric cell divisions are achieved by differential segregation of Numb (d-Numb) into one of the daughter cells to cause a bias in the Notch mediated cell-cell interaction. We have isolated a second mammalian gene with significant sequence similarity to d-numb, mouse numblike. When expressed in dividing neural precursors in Drosophila, Numblike is symmetrically distributed in the cytoplasm, unlike endogenous d-Numb or expressed mouse Numb (m-Numb), both of which are asymmetrically localized to one half of the cell membrane. In d-numb loss-of-function mutant embryos, expression of Numblike allows both daughter cells of a neural precursor to adopt the fate of the cell that normally inherits d-Numb. In mice, numblike mRNA is preferentially expressed in adult and embryonic nervous system. In the developing neocortex, Numblike is expressed in postmitotic neurons in the cortical plate, but not in progenitors within the ventricular zone where m-Numb and Notch1 are expressed. We have also found that, in dividing cortical progenitors, Notch1 is distributed around the entire membrane, unlike m-Numb which is asymmetrically localized to the apical membrane. We propose that an interplay between cell-intrinsic mechanisms (executed by m-numb and numblike) and cell-extrinsic mechanisms (mediated by Notch1) may be involved in both progenitor cell proliferation and neuronal differentiation during mammalian cortical neurogenesis.

Scanning mutagenesis of the putative transmembrane segments of Kir2.1, an inward rectifier potassium channel

Structural models of inward rectifier K+ channels incorporate four identical or homologous subunits, each of which has two hydrophobic segments (M1 and M2) which are predicted to span the membrane as alpha helices. Since hydrophobic interactions between proteins and membrane lipids are thought to be generally of a nonspecific nature, we attempted to identify lipid-contacting residues in Kir2.1 as those which tolerate mutation to tryptophan, which has a large hydrophobic side chain. Tolerated mutations were defined as those which produced measurable inwardly rectifying currents in Xenopus oocytes. To distinguish between water-accessible positions and positions adjacent to membrane lipids or within the protein interior we also mutated residues in M1 and M2 individually to aspartate, since an amino acid with a charged side chain should not be tolerated at lipid-facing or interior positions, due to the energy cost of burying a charge in a hydrophobic environment. Surprisingly, 17 out of 20 and 17 out of 22 non-tryptophan residues in M1 and M2, respectively, tolerated being mutated to tryptophan. Moreover, aspartate was tolerated at 15 out of 22 and 15 out of 21 non-aspartate M1 and M2 positions respectively. Periodicity in the pattern of tolerated vs. nontolerated mutations consistent with alpha helices or beta strands did not emerge convincingly from these data. We consider the possibility that parts of M1 and M2 may be in contact with water.