SNSS 2007 Systems Neurobiology Spring School

A fundamental aspect of neuronal development is the formation of neurites from a spherical cell body. Yet, there is relatively little research on the mechanisms by which neurites are formed. Recently, analysis of mice lacking all three Ena/VASP proteins revealed an important role for this family of proteins in axon tract formation in cortex. Ena/VASP proteins are known to regulate actin filament dynamics and are important for filopodia formation in neurons. By using live cell microscopy and platinum replica electron microscopy we found that Ena/VASP dependent actin bundles that comprise the core of the filopodia were essential for neuritogenesis. These results reveal an unexpected and intriguing role for filopodia in early cortical development.

Recommended Reading:

1. Dehmelt, L., and Halpain, S. (2004). Actin and microtubules in neurite initiation: are MAPs the missing link? J Neurobiol 58, 18-33.
2. Krause, M., E.W. Dent, J.E. Bear, J.J. Loureiro and F.B. Gertler. (2003) Ena/VASP proteins: regulators of the actin cytoskeleton and cell migration. Annual Review of Cell and Developmental Biology, 19: 541-64.
3. Lebrand, C., Dent, E. W., Strasser, G. A., Lanier, L. M., Krause, M., Svitkina, T. M., Borisy, G. G., and Gertler, F. B. (2004). Critical role of Ena/VASP proteins for filopodia formation in neurons and in function downstream of netrin-1. Neuron 42, 37-49.
4. da Silva, J. S., and Dotti, C. G. (2002). Breaking the neuronal sphere: regulation of the actin cytoskeleton in neuritogenesis. Nat Rev Neurosci 3, 694-704.
5. Dehmelt, L., Smart, F. M., Ozer, R. S., and Halpain, S. (2003). The role of microtubule-associated protein 2c in the reorganization of microtubules and lamellipodia during neurite initiation. J Neurosci 23, 9479-9490.

Optional Reading:

1. Dent, E. W., and Gertler, F. B. (2003). Cytoskeletal dynamics and transport in growth cone motility and axon guidance. Neuron 40, 209-227.

The olfactory system provides several operations that are essential for the survival of most animal species. These include the ability to detect and discriminate a considerable range of signal molecules and mediate instinctive behaviours including food selection, mating and reproduction and social organisation. The olfactory system must do this not only for only familiar but also unfamiliar odors that may enter the environment unpredictably. To achieve this the olfactory system has evolved several fundamental features that highlight the importance of the functional organisation of the early olfactory system. We will discuss how the olfactory bulb might makes use of its functional architecture to represent odor stimuli.

1. Mombaerts P. Molecular biology of odorant receptors in vertebrates. Annu Rev Neurosci. 1999;22:487-509. Review.
2. Buck L, Axel R. A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell. 1991 Apr 5;65(1):175-87.
3. Schaefer, A.T. And Margrie, T.W. Spatiotemporal representations in the olfactory system. TINS 2007 (in press)
4. Rinberg D, Koulakov A, Gelperin A. Speed-accuracy tradeoff in olfaction. Neuron. 2006 Aug 3;51(3):351-8.
5. Mori K, Takahashi YK, Igarashi KM, Yamaguchi M. Maps of odorant molecular features in the Mammalian olfactory bulb. Physiol Rev. 2006 Apr;86(2):409-33. Review.

Recommended:

1. Lledo PM, Saghatelyan A. Integrating new neurons into the adult olfactory bulb: joining the network, life-death decisions, and the effects of sensory experience. Trends Neurosci. 2005 May;28(5):248-54. Review.
2. Friedrich RW, Laurent G. Dynamic optimization of odor representations by slow temporal patterning of mitral cell activity. Science. 2001 Feb 2;291(5505):889-94.
3. Kaba H, Hayashi Y, Higuchi T, Nakanishi S. Induction of an olfactory memory by the activation of a metabotropic glutamate receptor. Science. 1994 Jul 8;265(5169):262-4.

In part I of this lecture, we will begin with the basic concepts underlying cell polarity and cell locomotion and an introduction to the basic molecular components that are important for these processes. This is then followed by a more detailed discussion of two cell polarity model systems that have brought important insights in recent years, including: 1) budding yeast undergoing polarized growth; 2) embryonic polarity in C. elegans zygotes. In part II of the lecture, we will begin with an in-depth discussion on the molecular machinery that provides the force for amoeboid cell movement. Then we will discuss how this machinery is controlled by conserved signaling pathways to ensure cell movement in physiologically relevant orientations. We will conclude the lecture with a discussion on future challenges in the field of cell polarity and cell motility.

Required reading:

Cell Polarity

1. Munro, E., J. Nance, et al. (2004). "Cortical flows powered by asymmetrical contraction transport PAR proteins to establish and maintain anterior-posterior polarity in the early C. elegans embryo." Dev Cell 7: 413-24.
2. Wedlich-Soldner, R., S. Altschuler, et al. (2003). "Spontaneous cell polarization through actomyosin-based delivery of the Cdc42 GTPase." Science 299(5610): 1231-5.

Cell Motility

1. Gupton, S. L. and C. M. Waterman-Storer (2006). "Spatiotemporal feedback between actomyosin and focal-adhesion systems optimizes rapid cell migration." Cell 125(7): 1361-74.
2. Pollard, T. D. and G. G. Borisy (2003). "Cellular motility driven by assembly and disassembly of actin filaments." Cell 112(4): 453-65.

Recommended reading:

1. Brandman, O., J. E. Ferrell, Jr., et al. (2005). "Interlinked fast and slow positive feedback loops drive reliable cell decisions." Science 310: 496-8.
2. Etienne-Manneville, S. and A. Hall (2003). "Cell polarity: Par6, aPKC and cytoskeletal crosstalk." Curr Opin Cell Biol 15(1): 67-72.
3. Medeiros, N. A., D. T. Burnette, et al. (2006). "Myosin II functions in actin-bundle turnover in neuronal growth cones." Nat Cell Biol 8(3): 215-26.
4. Svitkina, T. M. and G. G. Borisy (1999). "Arp2/3 complex and actin depolymerizing factor/cofilin in dendritic organization and treadmilling of actin filament array in lamellipodia." J Cell Biol 145(5): 1009-26.
5. Wedlich-Soldner, R., S. C. Wai, et al. (2004). "Robust cell polarity is a dynamic state established by coupling transport and GTPase signaling." J Cell Biol 166: 889-900.
6. Xu, J., F. Wang, et al. (2003). "Divergent signals and cytoskeletal assemblies regulate self-organizing polarity in neutrophils." Cell 114(2): 201-14.

Symmetry breaking into a polarized structure is a fundamental step in morphogenesis of various biological systems. Accumulating observations show that cell polarization patterns can be generated from symmetry in the absence or presence of asymmetric cues. Neurons also polarize by forming a single axon and multiple dendrites. Using highly sensitive 2-DE-based proteomics, we recently identified novel brain specific proteins, shootin1 and singar1. Shootin1 appears to be involved in axon outgrowth and neuronal polarity formation. On the other hand, singar1 may ensure the robustness of neuronal polarity by suppressing formation of surplus axons.

1. Craig, A. M., and Banker, G. (1994). Neuronal polarity. Annu Rev Neurosci 17, 267-310.
2. Mitchison, T., and Kirschner, M. (1988). Cytoskeletal dynamics and nerve growth. Neuron 1, 761-772.
3. Toriyama, M., Shimada, T., Kim, K. B., Mitsuba, M., Nomura, E., Katsuta, K., Sakumura, Y., Roepstorff, P., and Inagaki, N. (2006). Shootin1: a protein involved in organization of an asymmetric signal for neuronal polarization. J Cell Biol 175, 147-157.

Neuronal circuits in the brain are shaped by experience during 'critical periods' in early postnatal life. In the primary visual cortex, this activity-dependent development is triggered by the functional maturation of local inhibitory connections and driven by a specific, late-developing subset of interneurons. Ultimately, the structural consolidation of competing sensory inputs is mediated by a proteolytic reorganization of the extracellular matrix that occurs only during the critical period. The reactivation of this process, and subsequent recovery of function in conditions such as amblyopia, can now be studied with realistic circuit models that might generalize across systems.

Essential:

1. Hensch, T.K. (2005) Critical period plasticity in local cortical circuits. Nat. Rev. Neurosci. 6: 877-888.
2. Fagiolini M, Fritschy JM, Low K, Mohler H, Rudolph U, Hensch TK. (2004) Specific GABAA circuits for visual cortical plasticity. Science 303:1681-1683.
3. Mataga N, Mizuguchi Y, Hensch TK. (2004) Experience-dependent pruning of dendritic spines in visual cortex by tissue plasminogen activator. Neuron 44:1031-1041.

Recommended:

1. Maffei A, Nataraj K, Nelson SB, Turrigiano GG. (2006) Potentiation of cortical inhibition by visual deprivation. Nature. 443:81-84.
2. Feller MB, Scanziani M. (2005) A precritical period for plasticity in visual cortex. Curr Opin Neurobiol. 15:94-100.
3. Prusky GT, Douglas RM. (2003) Developmental plasticity of mouse visual acuity. Eur J Neurosci. 17:167-173.

Optional:

1. Kanold PO, Shatz CJ. (2006) Subplate neurons regulate maturation of cortical inhibition and outcome of ocular dominance plasticity. Neuron 51:627-638.

Neurons have a wide range of dendritic morphologies whose functions are largely unknown. We used an optimization procedure to find neuronal morphological structures for two computational tasks: First, neuronal morphologies were selected for the task of summing excitatory synaptic potentials (EPSPs) linearly. Second, structures were selected that distinguished the temporal order of EPSPs. The solutions resembled the morphology of real neurons. In particular the neurons optimized for linear summation electrotonically separated their synapses, as found in avian nucleus laminaris neurons. Neurons optimized for spike order detection had primary dendrites of significantly different diameter, as found in the basal and apical dendrites of cortical pyramidal neurons. The automated mapping between neuronal function and structure introduced allows a large catalog of computational functions to be built indexed by morphological structure.

Neurons display a wide variety of different non-linerar dynamics, such as action potential firing, sub-threshold oscillations and the non-linear integration of synaptic inputs. We have used in-vitro patch clamp recordings in slices of the mouse visual cortex and biophysical simulations to investigate some of these phenomena. In particular, we investigated the irregularity of spike trains in pyramidal neurons and interneurons, the effect of the neuromodulator acetyl-choline on the spike phase-reset curves of neurons and the interaction of sub-threshold oscillations with inhibitory synaptic potentials. A framework derived from dynamical-systems theory unifying of these phenomena is presented.

1. The effects of spike frequency adaptation and negative feedback on the synchronization of neural oscillators.
2. Interneurons of the neocortical inhibitory system.

Techniques that allow researchers to track and manipulate single molecules in living cells at high frame rates (1-250 kHz) are becoming important tools for investigating nanosystems working in living cells. In this lecture, I will talk about the following three recent developments. (1) The plasma membranes of practically all of the mammalian cells are parceled up into apposed domains of 30-200 nm. (2) Many signaling processes in the cell membrane take place in transient signaling complexes that interact with actin membrane skeleton and the plasma membrane. (3) Long analogue signals in the cell may occcur as the the sum of many pulse like single-molecule signaling events.

Essential:

1. N. Morone, T. K. Fujiwara, K. Murase, R. S. Kasai, H. Ike, S. Yuasa, J. Usukura, and A. Kusumi. Three-dimensional reconstruction of the membrane skeleton at the cell membrane interface by electron tomography. J. Cell Biol. 174, 851-862 (2006).
2. A. Kusumi, C. Nakada, K. Ritchie, K. Murase, K. Suzuki, H. Murakoshi, R. S. Kasai, J. Kondo, and T. Fujiwara. Paradigm shift of the plasma membrane concept from the two-dimensional continuum fluid to the partitioned fluid: high-speed single-molecule tracking of membrane molecules. Annu. Rev. Biophys. Biomol. Struct. 34, 351-378 (2005).
3. H. Murakoshi, R. Iino, T. Kobayashi, T. Fujiwara, C. Ohshima, A. Yoshimura, and A. Kusumi. Single-molecule imaging analysis of Ras activation in living cells. Proc. Natl. Acad. Sci. U.S. A. 101, 7317-7322 (2004).