European Journal of Neuroscience European Journal of Neuroscience, Vol. 28, pp. 268–278, 2008 doi:10.1111/j.1460-9568.2008.06327.x Growth cone responses to growth and chemotropic factors Staci D. Sanford,1 Jesse C. Gatlin,1,* Tomas Ho¨kfelt2 and Karl H. Pfenninger1 1Departments of Pediatrics, and of Cell and Developmental Biology, and Neuroscience Program, University of Colorado School of Medicine, and University of Colorado Cancer Center, Aurora, CO 80045, USA 2Department of Neuroscience, Karolinska Institutet, Stockholm, Sweden Keywords: attractants, axonal pathfinding, growth cone, growth promotion, neuropeptides, repellents Abstract During nervous system development axons reach their target areas under the influence of numerous guidance cues that affect rate and direction of growth. This report addresses the unsettled question of whether and to what extent growth velocity and turning responses (attraction, repulsion) are interdependent. We exposed individual growth cones of fetal rat dorsal root ganglion neurons in culture asymmetrically to gradients of seven different factors and recorded their growth rates and turning angles. Growth cones exhibited divergent patterns of turning and growth responses. For example, hepatocyte growth factor (HGF), insulin-like growth factor-1 (IGF-1) and thrombin all promoted growth, but HGF was a powerful attractant, thrombin a potent repellent and IGF-1 did not elicit turning. Galanin and neuropeptide Y also affected growth and ⁄ or turning differentially. Finally, nerve growth factor in the culture medium not only inhibited the turning responses to HGF, but also converted growth promotion of HGF and IGF-1 into inhibition. Overall, our studies indicate that: (i) turning and advance are regulated independently, except that strong attractive or repulsive responses generally are accompanied by growth promotion; (ii) asymmetric growth factor application per se does not elicit attraction; (iii) regulation of the two parameters may occur through a single receptor; and (iv) the effects of combined growth factors may not be additive and can be inhibitory. Introduction Nerve growth cones respond to a variety of microenvironmental cues Slit (Stein & Tessier-Lavigne, 2001). Such observations add to the that control their advancement and growth direction (see, e.g. Tessier- complexity of the issue. Lavigne & Goodman, 1996). These cues may be soluble or substrate- The data just reviewed come from a variety of experimental systems bound and may elicit simple growth responses (increased or decreased studied with diverse approaches. In combination, they generate an growth rate), chemoattractant responses (turning toward source of incoherent picture. In other words, the question of the degree to which factor ⁄ positive chemotropism) or chemorepellent responses (turning attraction and growth promotion, on the one hand, and repulsion and away from source of factor ⁄ negative chemotropism). However, growth inhibition, on the other hand, are correlated has not been neuron and growth cone reactions to a single factor can be complex. investigated systematically, especially not in mammalian neurons. For example, nerve growth factor (NGF) can operate not only as a Therefore, we decided to address this issue by exposing individual, differentiation and growth factor, but also as an attractant (Kater et al., identified mammalian growth cones, in a chemically defined medium 1988; Henley & Poo, 2004). This has been shown for other factors and under identical conditions, to gradients of different factors using since [e.g. brain-derived neurotrophic factor (BDNF) and netrin-1; the approach developed by Poo and collaborators (Lohof et al., 1992). Stein & Tessier-Lavigne, 2001; ]. Yet, other communications reported These assays generated, over relatively short observation times, that growth rate and direction might not be linked. For example, in quantitative data for growth cone turning angles as well as for the rate Xenopus spinal neurons BDNF and acetylcholine (ACh) trigger an of growth in response to specific factors. Using NGF-responsive attractive response without enhancing outgrowth (Zheng et al., 1994; neurons of rat dorsal root ganglia (DRG), we compared the reactions Song et al., 1997), and Slit promotes outgrowth without eliciting of growth cones to seven different factors that can be grouped as turning (Stein & Tessier-Lavigne, 2001). This raises the question of follows. (i) Classic growth factors: the neurotrophin, NGF; a broadly the degree to which growth and turning responses are interdependent. acting growth factor, IGF-1 (insulin-like growth factor-1); and the Other studies revealed cross-talk between different cues, such as the growth and chemotactic factor, HGF (hepatocyte growth factor). inhibition of semaphorin 3A (Sema3A)-mediated chemorepulsion by (ii) Neuropeptides: galanin and NPY (neuropeptide Y). (iii) Repellents: NGF (Tuttle & O’Leary, 1998; Dontchev & Letourneau, 2002; Sema3A and thrombin, which is known primarily as a platelet- and Chalasani et al., 2003) or the inhibition of netrin-induced turning by fibroblast-activating protease factor (Fenton, 1988). These compounds are highly diverse and have been reported to influence axonal growth (for review, see Table 1). By quantifying growth cone turning and 1 Correspondence: Dr K. H. Pfenninger, * present address below. advancement under identical conditions we were able to assemble a E-mail: [email protected] sizeable catalog that describes a single mammalian growth cone’s *Present address: Department of Biology, University of North Carolina, Chapel Hill, NC response patterns to various factors. This allowed us to begin to 27599, USA answer the question of the degree to which changes in growth rates are Received 21 August 2007, revised 7 May 2008, accepted 13 May 2008 linked to turning responses. We also show effects of combining ª The Authors (2008). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd Growth cone responses to growth and chemotropic factors 269 Table 1. Effects of factors on neurite growth through Dr M. Nordlander, Gothenburg, Sweden); HGF (EMD Biosciences ⁄ Calbiochem; San Diego, CA, USA); IGF-1 and thrombin Factor Effect Reference (Sigma, St Louis, MO, USA); NGF (Alomone Labs, Jerusalem, Israel). NGF Growth promotion Cohen et al. (1954); Thoenen & Barde (1980) Chemoattraction Letourneau (1978); DRG neuron culture Gundersen & Barrett (1980) This study was approved by the Animal Care and Use Committee, IGF-1 Growth promotion, Caroni & Grandes (1990); UCHSC. Time pregnant females were deeply anesthetized with membrane expansion Pfenninger et al. (2003) Halothane or Isoflurane. After removal of the uterus by caesarean at growth cone section, the rats were killed while under anesthesia by cutting the heart. HGF Growth promotion Maina et al. (1997); The removed fetus was decapitated prior to dissection. For explant Korhonen et al. (2000) cultures, DRG were dissected from 15-day gestation Sprague–Dawley Chemoattraction Caton et al. (2000) rat fetus and cultured on laminin-coated coverslips (Assistent brand) in Neurobasal medium supplemented with B27, 10% v v fetal bovine Galanin Growth promotion Mahoney et al. (2003); ⁄ Hobson et al. (2006) serum and 100 ng ⁄ mL NGF (3.8 nm). After 24 h incubation (37°C, 5% CO2 in air) this medium was replaced with fresh Neurobasal NPY Growth promotion White (1998); Hansel et al. (2001) medium + B27, without other supplementation. After a second day in Sema3A Chemorepulsion Fan & Raper (1995); Kolodkin (1996) culture, neurites with spread growth cones were used for assays. Thrombin Growth inhibition Hawkins & Seeds (1986); Suidan et al. (1992) Growth and turning assays Chemorepulsion de La Houssaye et al. (1999) Gradients of the factors were generated in the proximity of cultured HGF, hepatocyte growth factor; IGF-1, insulin-like growth factor-1; NGF, nerve growth cones by repetitive-pulse application (Lohof et al., nerve growth factor; NPY, neuropeptide Y; Sema3A, semaphorin 3A. 1992). Micropipettes (inner diameter of the tip consistently 1–2 lm) were connected to a Picospritzer (set at 6 p.s.i.; General Valve, different growth factors and approach the question of whether Fairfield, NJ, USA) controlled by a square wave generator (2 Hz, multiple receptors are needed for the growth and the turning duration 10 ms; Astro-Med, West Warwick, RI, USA). The system responses. was calibrated by generating a model gradient of fluorescein- conjugated dextran. By tightly controlling factor concentration in the micropipette, frequency of ejections, volume of ejected factor solution Materials and methods and position of the pipette tip relative to the growing neurite (see below), we generated such gradients highly reproducibly, and they Materials were stable over time. Culture coverslips were placed in an Attofluor Reagents and their sources were: culture media, media supplements cell chamber (Molecular Probes ⁄ Invitrogen, Carlsbad, CA, USA) with and laminin (Invitrogen, Carlsbad, CA, USA). Sema3A: the culture medium, and layered-over with inert mineral oil (embryo-tested, supernatant of stably transfected HEK 293 cells secreting Sema3A sterile-filtered; Sigma) to maintain pH and avoid evaporation. (generous gift from Dr M. Tessier-Lavigne) was concentrated by Media and factor concentrations used are listed in Table 2. The ultrafiltration (Centriplus membrane, 50 000 MW cut-off; Millipore, effective concentrations of the factors at the growth cone were two- to Bedford,