Paul Letourneau | University of Minnesota - Twin Cities (original) (raw)

Papers by Paul Letourneau

Journal of Cell Science, 1996

The extracellular matrix through which growth cones navigate contains molecules, such as chondroi... more The extracellular matrix through which growth cones navigate contains molecules, such as chondroitin sulfate proteoglycan, that can inhibit growth cone advance and induce branching and turning. Growth cone turning is accompanied by rearrangement of the cytoskeleton. To identify changes in the organization of actin filaments and microtubules that occur as growth cones turn, we used time-lapse phase contrast videomicroscopy to observe embryonic chick dorsal root ganglion neuronal growth cones at a substratum border between fibronectin and chondroitin sulfate proteoglycan, in the presence and absence of cytochalasin B. Growth cones were fixed and immunocytochemically labeled to identify actin filaments and dynamic and stable microtubules. Our results suggest that microtubules are rearranged within growth cones to accomplish turning to avoid chondroitin sulfate proteoglycan. Compared to growth cones migrating on fibronectin, turning growth cones were more narrow, and they contained dyna...

Intrinsic mechanisms of growth cone motility influence of extrinsic factors growth cones in intac... more Intrinsic mechanisms of growth cone motility influence of extrinsic factors growth cones in intact developing systems the growth cone in regeneration.

The Journal of Neuroscience, 1994

The differentiation and morphogenesis of neural tissues involves a diversity of interactions betw... more The differentiation and morphogenesis of neural tissues involves a diversity of interactions between neural cells and their environment. Many potentially important interactions occur with the extracellular matrix (ECM), a complex association of extracellular glycoproteins organized into aggregates and polymers. In this article, we discuss recent findings on neuronal interactions with the ECM and their roles in neural cell migration and neurite growth. First, we examine the expression and putative functions of the molecules of the neural ECM. Second, we discuss cell surface molecules that mediate neural interactions with ECM components. Last, we address proteoglycans (PGs), a diverse class of glycoproteins, present both as ECM components and as cell surface molecules, which may mediate neural interactions with their environment. The best-understood cellular interactions with the ECM are adhesive, mediated by binding between specific cell surface molecules and cell binding domains of ECM components (Strittmater and Fishman, 199 1; Damsky and Werb, 1992). Cellsubstratum adhesion is necessary for major cell movements of neuron morphogenesis, that is, the migrations of neural cells and their precursors and the migratory behavior ofgrowth cones at the extending tips of axons and dendrites. As cells move, adhesive molecules at the surface of the leading edge of a migrating cell or growth cone bind to ligands on other cell surfaces or ECM components. These bonds stabilize filopodia and lamellipodia, and, in some cases, provide anchorage against which cytoskeletal filaments, associated with the plasma membrane, exert forces to pull the cell or growth cone forward. Thus, ECM has been primarily viewed as an adhesive substratum to provide traction for migrating cells and to stabilize the position and, perhaps, the state of differentiation of nonmotile cells. However, the interactions between neural cells and the ECM are not longer regarded as only adhesive or mechanical. Two points are now clear. First, some of these interactions are definitely not adhesive, but, rather, they may even be antiadhesive (Chiquet-Ehrismann, 199 1). Second, evidence has accumulated to indicate that the cell surface molecules that mediate cell-cell and cell-ECM interactions (immunoglobulin superfamily, cad-Preparation of this review was supported by NIH Grants HDI 9950 and NS28807

The Journal of Neuroscience, Jul 1, 1992

We thank Judith Kahm and David Gremmels for assistance in antibody production, Vi&i VanDisse for ... more We thank Judith Kahm and David Gremmels for assistance in antibody production, Vi&i VanDisse for purification of the synthetic peptides, Anne Fassen and Joanne Giusetmetti for helnful suaaestions. and Gerald Sedaewick for excellent photographic assi&nce. We are also;ndebted to Lia Abrahzrms, Sheryl Stucky, and Ann Parsons for culturing rat neurons, and to Drs. Steve McLoon and Diane Snow for providing antibodies. This research was supported by March of Dimes

The Journal of Neuroscience, Nov 24, 2004

The molecular mechanisms by which neurotrophins regulate growth cone motility are not well unders... more The molecular mechanisms by which neurotrophins regulate growth cone motility are not well understood. This study investigated the signaling involved in transducing BDNF-induced increases of filopodial dynamics. Our results indicate that BDNF regulates filopodial length and number through a Rho kinase-dependent mechanism. Additionally, actin depolymerizing factor (ADF)/cofilin activity is necessary and sufficient to transduce the effects of BDNF. Our data indicate that activation of ADF/cofilin mimics the effects of BDNF on filopodial dynamics, whereas ADF/cofilin inactivity blocks the effects of BDNF. Furthermore, BDNF promotes the activation of ADF/ cofilin by reducing the phosphorylation of ADF/cofilin. Although inhibition of myosin II also enhances filopodial length, our results indicate that BDNF signaling is independent of myosin II activity and that the two pathways result in additive effects on filopodial length. Thus, filopodial extension is regulated by at least two independent mechanisms. The BDNF-dependent pathway works via regulation of ADF/cofilin, independently of myosin II activity.

PubMed, May 1, 1996

The surface of a normal ovary is covered by a monolayer of epithelial cells that rest on a baseme... more The surface of a normal ovary is covered by a monolayer of epithelial cells that rest on a basement membrane. The glycoprotein laminin is the major noncollagenous protein present in the basement membrane. The integrins alpha 1 beta 1, alpha 2 beta 1, alpha 3 beta 1, alpha 6 beta 1, and alpha 6 beta 4 serve as cell surface receptors for laminin. During the progression of serous ovarian carcinoma, tumor cells are frequently exfoliated from the surface of the ovary, thereby losing contact with the basement membrane. This study was designed to determine whether alterations in integrin expression may be associated with the malignant phenotype of the primary ovarian tumor and exfoliated ovarian carcinoma cells in the ascites fluid. By immunohistochemical staining, the entire surface of epithelial cells of normal ovaries stained positively for beta 1, alpha 2, and alpha 3 integrins, whereas only the basal surface of the epithelial cells, where they are in contact with laminin, stained positively for alpha 6 and beta 4. The entire surface of epithelial cells of solid tumors from patients with serous ovarian carcinoma stained positively for beta 1, alpha 2, and alpha 3 integrins. In most cases, no intact basement membrane surrounded the tumor nests, and staining for alpha 6 and beta 4 was irregular. When present, the basement membrane stained positively for laminin, and the basal surface of the epithelial cells stained positively for alpha 6 and beta 4. Ovarian carcinoma ascites cells exhibited a distinct phenotype, with a significant decrease in expression of the alpha 6 and beta 4 integrin subunits. As alpha 6 and beta 4 integrin subunits are present at the basal surface of many epithelial cells and serve as receptors for laminin, it is possible that ovarian carcinoma epithelial cells may be released from the basement membrane of the ovary due to their deficit of alpha 6 and beta 4 integrin subunits.

Journal of Cell Science, Aug 1, 1996

The regulation of filopodial dynamics by neurotrophins and other guidance cues plays an integral ... more The regulation of filopodial dynamics by neurotrophins and other guidance cues plays an integral role in growth cone pathfinding. Filopodia are F-actin-based structures that explore the local environment, generate forces and play a role in growth cone translocation. Here, we review recent research showing that the actin-depolymerizing factor (ADF)/cofilin family of proteins mediates changes in the length and number of growth cone filopodia in response to brain-derived neurotrophic factor (BDNF). Although inhibition of myosin contractility also causes filopodial elongation, the elongation in response to BDNF does not occur through a myosin-dependent pathway. Active ADF/cofilin increases the rate of cycling between the monomer and polymer pools and is critical for the BDNF-induced changes. Thus, we discuss potential mechanisms by which ADF/cofilin may affect filopodial initiation and length change via its effects on F-actin dynamics in light of past research on actin and myosin function in growth cones.

Journal of Cell Biology, Apr 1, 1984

We cultured sensory neurons from chick embryos in media containing the alkaloid taxol at concentr... more We cultured sensory neurons from chick embryos in media containing the alkaloid taxol at concentrations from 7 X 10-9 to 3.5 X 10-6 M. When plated at taxol concentrations above 7 X 10-8 M for 24 h, neurons have short broad extensions that do not elongate on the culture substratum. Whèn actively growing neurites are exposed to these levels of taxol, neurite growth stops immediately and does not recommence. The broad processes of neurons cultured 24 h with taxol contain densely packed arrays of microtubules that loop back at the ends of the process. Neurofilaments are segregated from microtubules into bundles and tangled masses in these taxol-treated neurons. At the ends of neurites treated for 5 min with taxol, microtubules also turn and loop back abnormally toward the perikaryon. In the presence of 7 X 10-9 M taxol neurites do grow, although they are broader and less branched than normally. The neurites of these cells appear to have normal structure except for a large number of microtubules. Taxol probably stimulates microtubule polymerization in these cultured neurons. At high levels of the drug, this action inhibits neurite initiation and outgrowth by removing free tubulin from the cytoplasm and destroying the normal control of microtubule assembly in growing neurites. The rapid inhibition suggests that microtubule assembly may occur at neurite tips. At lower concentrations, taxol may slightly enhance the mechanisms of microtubule assembly in neurons, and this alteration of normal processes changes the morphogenetic properties of the growing neurites .

Acta Histochemica Et Cytochemica, 2002

Journal of Biological Chemistry, 1993

FN-C/H II (KNNQKSEPLIGRKKT), a heparin-binding peptide derived from the COOH-terminal heparin-bin... more FN-C/H II (KNNQKSEPLIGRKKT), a heparin-binding peptide derived from the COOH-terminal heparin-binding domain of fibronectin, mediates cell adhesion for a variety of cell types and promotes neurite outgrowth. By systematic amino acid substitution of synthetic peptide analogues of FN-C/H II, the basic structural features necessary for activity have been identified in the COOH-terminal residues LIGRKK. This biologically &amp;quot;active&amp;quot; sequence has been located in several other heparin/heparan sulfate-binding proteins and may represent a potential binding motif for sulfated polyanions. NMR structural studies indicate that the COOH-terminal segment of FN-C/H II displays significant multiple-turn or helix-like character suggesting that the RKK sequence may lie on the same surface of the protein, as opposed to alternating in an extended chain motif.

Trends in Neurosciences, 1986

Growth cones are the highly motile tips of growing axons and dendrites. They have held our attent... more Growth cones are the highly motile tips of growing axons and dendrites. They have held our attention ever since Ram6n y Cajal first saw one in the central nervous system of the developing chick and guessed, with characteristic accuracy, at their purpose. That was nearly one hundred years ago and we still have no definitive description of how growth cones move about or how axons and dendrites elongate. Perhaps even further from our grasp is a molecular description of the recognition events involved in synapse formation. Because the growth cone is responsible not only for navigating the axon or dendrite to its destination bet also for recognizing the correct synaptic partner, growth cones are crucial in the development and regeneration of the nervous system. For those interested in these areas this book is essential reading. As the editors point out in their preface, which serves as a potted history of the field, there has been a resurgence of late in growth cone research after a lull of 20 years or so since the heydays of Harrison (who invented tissue culture to study them) and Speidel. They attribute this renascence to the introduction of new and powerful techniques such as time lapse cinematography coupled with high contrast optics and dissociated cell culture. Examples of these techniques as applied to growth cone research and more recently introduced ones such as patch clamping and scanning electron microscopy are represented in the book. The book starts off with an introduction by Trinkaus which cogently argues a place for growth cone motility in the more general context of directional cell movement. This implies that those studying growth cone behaviour would benefit from an awareness of the state of knowledge of other examples of cell movement (and vice versa). It is a pity then that a few papers in this book were not devoted to such pertinent topics as fibroblast movement, chemotaxis in neutrophils or even to microvillus structure. The book is divided up into three sections dealing with in-vivo, in-vitro and electrophysiological aspects. The individual contributions collectively represent the most comprehensive assembly of papers on growth cones so far published. The book is also something of an imaginative publishing venture because it started life as an issue of the Journal of Neuroscience Research (Vol. 13, number 1 /2 ) entirely devoted to invited research papers on growth cones. The consequences of this unusual genesis are that it is very up-todate and the papers have a uniformity of style, in marked contrast to many books of symposia or meetings which it most closely resembles. Several of the papers are mainly descriptive, dealing with such things as the appearance of HRP-filled growth cones /n v./vo .(Mason, Reh and Constantine-Pa~om; Harris et ~/.) ,and, beautifally revealed, in the scanning electron microscope (Roberts and Patton), but these do not bring us noticeably closer to understanding growth cone motility or neurite extension. Studying channels in the growth cone plasma membrane and associated electrical fields or the responses of the growth cone to directly applied substances such as serotonin (Haydon et al.), NGF (Connolly et al., and Gundersen) or substrate bound molecules such as laminin (Hammarback et al.) are already beginning to do so. Two papers are technically ingenious, the first, by Freeman et al., reports the measurement of currents generated by growth cones in culture using a circularly vibrating probe. Ion substitution experiments seem to indicate that a Ca 2+ flux is responsible and the authors postulate that this may be involved in several processes including neurotransmitter release a property now known of growth cones. Importantly, Freeman et al. also show that the threshold for applied current densities necessary to re-direct growth cone movement is several orders of magnitude higher than the endogenous currents. The second paper, by O'Lagne et aL, reports on the morphological and electrophysiological properties of giant growth cones produced by fusion of the neuroae-like clone PC12. These monsters look set to provide us with much useful information. What is conspicuously lacking here, because of past technical difficulties, is a biochemical analysis of growth cones, in particular the contractile apparatus through which, presumably, all factors affecting motility operate. Perhaps the recent development of techniques to isolate growth cones from developing brain will help close this gap. In the meantime this book is highly recommended.

The Journal of Neuroscience, Mar 1, 1994

Fresenius' Zeitschrift f�r Analytische Chemie, 1986

1. Gross J, Lun A (1985) Z Kiln Med 40:1531-1534 2. Jolles J, Schrama LH, Gispen WH (1981) Biochi... more 1. Gross J, Lun A (1985) Z Kiln Med 40:1531-1534 2. Jolles J, Schrama LH, Gispen WH (1981) Biochim Biophys Acta 666 : 9 0 98 3. Majerus PW, Wilson DB, Connoly TM, Bross TE, Neufeld EJ (1985) TIBS 10 :168-17 t 4. Michell RH (1975) Biochim Biophys Acta 415:81 1 4 7 5. Odarjuk J (1984) Dissertation, Humboldt-Universit/it 6. Odarjuk J, Maretzki D, Gross J, Gerber G (1986) Biomed Biochim Acta (in press) 7. Wustmann Ch, Schmidt J, Ihle W, Gross J, Fischer HD (1983) Biomed Biochim Acta 42:265 273

Journal of neurobiology, 2004

Encyclopedia of Neuroscience, 2009

This article was originally published in the Encyclopedia of Neuroscience published by Elsevier, ... more This article was originally published in the Encyclopedia of Neuroscience published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for noncommercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution's administrator.

eLS, 2016

Axons and dendrites are the neuronal processes that transmit nerve signals throughout the body. ... more Axons and dendrites are the neuronal processes
that transmit nerve signals throughout the body.
These processes are formed and maintained by the
neuronal cytoskeleton, particularly microtubules
and actin filaments. Microtubules are the supportive
framework of neurites, axons and dendrites,
as well as the rails for transporting organelles
and cytoplasmic components. Actin filaments support
the neuron’s outer cortex, and the dynamic
protrusion of actin-filled filopodia and lamellipodia
drives neuronal morphogenesis, including axon
guidance, branching and regeneration. The initiation
of axon formation requires advance of
microtubules into filopodial protrusions adherent
to the substratum. Polymerisation of microtubule
plus ends and dynein-driven transport of short
microtubules provides ‘push’ for continued axonal
growth. Actin-filled protrusions at the axonal terminal,
the growth cone, explore the environment
for cues that bind receptors and trigger either protrusion
and microtubule advance or cessation of
protrusion and myosin II contractility that retracts
protrusions and limits microtubule advance.

The Journal of neuroscience : the official journal of the Society for Neuroscience, Jan 15, 1998

The sprouting of axon collateral branches is important in the establishment and refinement of neu... more The sprouting of axon collateral branches is important in the establishment and refinement of neuronal connections during both development and regeneration. Collateral branches are initiated by the appearance of localized filopodial activity along quiescent axonal shafts. We report here that sensory neuron axonal shafts rapidly sprout filopodia at sites of contact with nerve growth factor-coated polystyrene beads. Some sprouts can extend up to at least 60 micro(m) through multiple bead contacts. Axonal filopodial sprouts often contained microtubules and exhibited a debundling of axonal microtubules at the site of bead-axon contact. Cytochalasin treatment abolished the filopodial sprouting, but not the accumulation of actin filaments at sites of bead-axon contact. The axonal sprouting response is mediated by the trkA receptor and likely acts through a phosphoinositide-3 kinase-dependent pathway, in a manner independent of intracellular Ca2+ fluctuations. These findings implicate neur...

Current Biology, 2002

Accurate navigation by a neuronal growth cone requires the modulation of the growth cone's respon... more Accurate navigation by a neuronal growth cone requires the modulation of the growth cone's responsiveness to spatial and temporal changes in expression of guidance cues. These adaptations involve local protein synthesis and turnover in growth cones and distal axons. Local Protein Synthesis at an Intermediate Target Spinal cord commissural axons are attracted to the ventral midline, a source of the attractant netrin. Once