We are interested in understanding the cellular and molecular mechanisms responsible for the formation of spinal motor circuits in vertebrates; the phenomena studied include mechanisms of motor axon pathfinding, the formation of functionally effective synapses, the assembly of motor- and interneurons into locomotor circuits, and the role of spontaneous rhythmic electrical activity in these processes. We believe that the mechanisms used during development to assemble the nervous system will be relevant to strategies for restoring neural circuits that have been damaged by injury or disease. Using embryonic mouse and avian spinal cords as models, the laboratory applies a wide range of techniques including electrophysiology and Ca²⁺ imaging of isolated spinal cord preparations or cord slices, molecular approaches to alter the expression of molecules in-vivo or in motoneuron-myotube cell cultures, specific tract tracing, dynamic imaging of fluorescently labeled cells, and biochemistry.

By combining electrophysiology and imaging of synaptic vesicle cycling at mouse neuromuscular junctions we recently discovered that different isoforms of NCAM play distinct and essential roles in both synaptic maturation and in maintaining effective transmission when adult junctions are activated repetitively. A specific domain on NCAM 180 is needed to activate an activity dependent signaling pathway, involving MLCK and myosin II, which is required to rapidly replenish synaptic vesicles during high frequency stimulation. We are now using bioinformatic, molecular, and physiological approaches to further decipher this pathway as well as a frequency dependent switch in the mode of synaptic vesicle cycling. The role of NCAM isoforms (and other molecules such as CD24) in the selective targeting of transmitter release machinery to synapses is being studied in motoneuron-myotube cultures in which one or more isoforms is selectively expressed pre- or post-synaptically. We believe that these observations are relevant to other parts of the nervous system as NCAM null mice also exhibit impaired secretion of adrenalin from chromaffin cells, and defects in hippocampal LTP, learning, and memory.

A second area of interest is to understand the role that rhythmic spontaneous activity, which is widespread throughout the developing nervous system, plays in the development of spinal motor circuits. By elucidating the transmitters and circuit that drives this activity we were able to alter it during precise periods of chick embryo development via in-ovo drug application. We made the surprising discovery that modest alterations in the frequency of this activity, as axons were growing to their targets, caused either dorsal-ventral or motoneuron pool-specific axon pathfinding errors depending on the sign of the alteration. We are now using channels that can be activated by light to drive activity in intact developing embryos with different stimulus patterns to elucidate downstream signaling pathways responsible for these pathfinding errors. By altering activity even earlier and by using 2-photon Ca²⁺ imaging to asses the alterations, we will test whether the generation and phenotypic differentiation of specific subtypes of motor- and intern-neurons and their assembly into circuits is sensitive to activity. Our observations show that any maternally taken drugs, which affect such early activity, have the potential to cause defects in the formation of spinal cord circuits.

1. Maeno-Hikichi, Y., Polo-Parada, L., Kastanenka, K., Landmesser, L.T. (2011)
   Frequency dependent modes of synaptic vesicle exocytosis and endocytosis at adult mouse neuromuscular junctions. J. Neurosci. (in press).
   
2. Park G-H, Maeno-Hikichi Y, Awano T, Landmesser LT, Monini U (2010)
   Reduced SMN protein in motor neuronal progenitors functions cell autonomously to cause spinal muscular atrophy in model mice expressing the human centromeric (SMN2) gene. J. Neurosci. 30:12005-12019.
   
3. Kastanenka, K.V. and Landmesser, L.T. (2010)
   In-vivo activation of channelrhodopsin-2 reveals that normal patterns of spontaneous activity are required for motoneuron guidance and maintenance of guidance molecules. J. Neurosci. 30:10575-10585..
   
4. Wang, S., Polo-Parada L. and Landmesser, L.T. (2009)
   Characterization of rhythmic Ca2+ transients in early embryonic chick motoneurons: Ca2+ sources and effects of altered activation of transmitter receptors. J. Neurosci. 29:15232-15244..
   
5. Hanson MG, Milner LD, Landmesser LT. (2008)
   Spontaneous rhythmic activity in early chick spinal cord influences distinct motor axon pathfinding decision. Brain Research Reviews. 57:77-85.
   
6. Hata, K., Polo-Parada, L., and Landmesser, L.T. (2007)
   Selective targeting of different Neural cell adhesion molecule isoforms during motoneuron-myotube synapse formation in culture and the switch from an immature to mature form of synaptic vesicle cycling J. Neurosci. 27:14481-14493.
   
7. Hanson MG, Landmesser LT. (2006)
   Increasing the frequency of spontaneous rhythmic activity disrupts pool-specific axon fasciculation and pathfinding of embryonic spinal motoneurons. J. Neurosci. 2006 Dec 6; 26(49):12769-80.
   
8. Li X, Gutierrez DV, Hanson MG, Han J, Mark MD, Chiel H, Hegemann P, Landmesser LT, Herlitze S. (2005)
   Fast noninvasive activation and inhibition of neural and network activity by vertebrate rhodopsin and green algae channelrhodopsin. Proc Natl Acad Sci U S A. 2005 Dec; 6(102(49)):17816-17821. Epub 2005 Nov 23.
   
9. Polo-Parada L, Plattner F, Bose C, Landmesser LT. (2005)
   NCAM 180 acting via a conserved C-terminal domain and MLCK is essential for effective transmission with repetitive stimulation. Neuron. 2005 Jun 16; 46(6):917-931.
   
10. Chan SA, Polo-Parada L, Landmesser LT, Smith C. (2005)
    Adrenal chromaffin cells exhibit impaired granule trafficking in NCAM knockout mice. J Neurophysiol. 2005 Aug; 94(2):1037-1047. 2005/03/30 [aheadofprint].
    
11. Hanson MG, Landmesser LT. (2004)
    Normal patterns of spontaneous activity are required for correct motor axon guidance and the expression of specific guidance molecules. Neuron. 2004 Sep 2; 43(5):687-701.
    
12. Polo-Parada L, Bose CM, Plattner F, Landmesser LT. (2004)
    Distinct roles of different neural cell adhesion molecule (NCAM) isoforms in synaptic maturation revealed by analysis of NCAM 180 kDa isoform-deficient mice. J Neurosci. 2004 Feb 25; 24(8):1852-1864.
    
13. Hanson MG, Landmesser LT. (2003)
    Characterization of the circuits that generate spontaneous episodes of activity in the early embryonic mouse spinal cord. J Neurosci. 2003 Jan 15; 23(2):587-600.
    
14. Polo-Parada L, Bose CM, Landmesser LT. (2001)
    Alterations in transmission, vesicle dynamics, and transmitter release machinery at NCAM-deficient neuromuscular junctions. Neuron. 2001 Dec 6; 32(5):815-828.
    
15. Landmesser LT. (2001)
    The acquisition of motoneuron subtype identity and motor circuit formation. Int J Dev Neurosci. 2001 Apr; 19(2):175-182.
    
16. Rafuse VF, Polo-Parada L, Landmesser LT. (2000)
    Structural and functional alterations of neuromuscular junctions in NCAM-deficient mice. J Neurosci. 2000 Sep 1; 20(17):6529-6539.