Melissa M. Rolls Assistant Professor of Biochemistry and Molecular Biology 118 Life Sciences Building University Park, PA 16802 Telephone: (814) 867-1395 Lab: (814) 867-1396 E-mail:mur22@psu.edu B.S. in Biology, Yale University Ph.D. in Biological and Biomedical Sciences, Harvard University Postdoctoral, University of Oregon Rolls Lab Web Site

Neurons, cell polarity, and Drosophila

Most cells are exquisitely organized. Neurons take compartmentalization to extremes in order to send signals directionally across long distances. Clearly it is important to target synaptic vesicles, neurotransmitter receptors and ion channels to specific regions of axons and dendrites so that signals can be sent, received and integrated. Other aspects of neuronal polarity are more mysterious. For example, microtubule organization in axons and dendrites has been known to be different for a long time, but how this polarity is established, and how it regulates other aspects of neuronal protein targeting, is not clear.

Although neuronal organization has been studied primarily in mammalian neurons, Drosophila neurons share many aspects of neuronal polarity, and offer many advantages for studying this problem. For example, many Drosophila neurons have easily observed and separate axons and dendrites (Figure1).

Major approaches used in the lab

1. live imaging: If you have the right tools, just looking inside a living cell in its normal environment can be incredibly informative. We regularly look at single microtubules growing inside neurons in whole animals, as well as how membranes are transported in axons and dendrites.

2. genetics: The tools for genetic manipulation of Drosophila are unsurpassed in any other metazoan model organism. Particularly relevant for the questions we are studying are techniques that allow the identification of neuronal functions for broadly used genes. Generation of marked mutant clones in neurons allows analysis of the function of almost any gene in neurons in vivo, even genes that are required for early steps in embryogenesis.

3. RNAi: Neuron-specific RNAi can be performed by driving expression of hairpin RNAs in subsets of cells. In this way you can generate neurons with reduced gene function in an otherwise normal animal.

Microtubule polarity in axons and dendrites

We have recently generated a map of microtubule orientation by examining microtubule growth in all major classes of fly neurons. We found that microtubules have opposite orientation in axons and dendrites: all axonal microtubules have plus ends distal to the cell body, and dendritic microtubules have minus ends distal to the cell body. The arrangement  of dendritic microtubules is particularly interesting as microtubules in many model systems used in cell biology are organized with minus ends clustered in the center of the cell around the centrosome. Noncentrosomal microtubule arrays are found in many important cell types including epithelia and neurons, but how they are established and maintained is not well understood.

Some current projects

1. establishment and maintenance of dendritic microtubule arrays

2. role of microtubule minus ends and nucleation sites in setting up neuronal microtubule organization

3. neuronal responses to injury

An example of a fun experiment

Unipolar neurons (these are neurons in which a single process arises from the cell body, and then branches to generate axons and dendrites) are common in insects. The microtubule map we made predicts that there are no microtubules that run from the cell body to the dendrites. This is very surprising as there must be lots of transport back and forth between the cell body and dendrites. So we were very curious how cargoes moved in the region where the dendrites branch off from the single process that comes from the cell body. We used Rab4-RFP as an example cargo and watch it moving in motor neurons, and it could travel smoothly and rapidle between the cell body and axons, but very rarely moved smoothly between the cell body and dendrites (Figure 2). We think that cargoes take indirect routes between the cell body and dendrites, either stopping to regroup at the junction where dendrites arise, or going via the axon.

Representative Publications:

• Stone, M. C., Roegiers, F., Rolls, M. M. (2008) Microtubules have opposite orientation in axons and dendrites of Drosophila neurons. Molecular Biology of the Cell 19: 4122-4129.

• Satoh, D., Sato, D., Tsuyama, T., Saito, M., Ohkura, H., Rolls, M., Ishikawa, F., Uemura, T. (2008) Spatial control of branching within dendritic arbors by dynein-dependent transport of Rab5 endosomes. Nature Cell Biology 10: 1164-1171.

• Rolls, M. M., Satoh, D. Clyne, P. J., Henner, A. L., Uemura, T, Doe, C. Q. (2007) Polarity and intracellular compartmentalization of Drosophila neurons. Neural Development 2:7.

  Rolls, M. M., Doe, C. Q. (2004) Baz, Par-6 and aPKC are not required for axon or dendrite specification in Drosophila. Nature Neuroscience. 7: 1293-1295.

• Rolls, M. M., and Albertson, R. (equal contributors), Shih, H. P., Lee, C.-Y., Doe, C. Q. (2003) Drosophila aPKC regulates cell polarity and cell proliferation in neuroblasts and epithelia. Journal of Cell Biology 163: 1089-1098.

• Rolls, M. M. and Doe, C. Q. (2003) Cell polarity: from embryo to axon. Nature 421: 905-906.

  Rolls, M. M., Hall, D. H., Victor, M., Rapoport, T. A. Targeting of rough endoplasmic reticulum membrane proteins and ribosomes in invertebrate neurons. (2002) Molecular Biology of the Cell 13: 1778-1791.

• Voeltz, G. K. and Rolls, M. M., Rapoport, T. A. (2002) Structural organization of the endoplasmic reticulum.  EMBO Reports 3:944-950.

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