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Michael E. Selzer, M.D., Ph.D.

Professor, Dept of Neurology
School of Medicine
450 Stemmler Hall/6077
(215) 662-3396/898-0180
FAX: (215) 573-2107
email:   selzerm@uphs.upenn.edu

Click here for selected publications since Dr. Selzer's arrival at Penn

RESEARCH INTERESTS

RESEARCH TECHNIQUES

RESEARCH SUMMARY

Unlike axons in the central nervous system of mammals, the axons of the lamprey spinal cord regenerate across a spinal transection, and even grow preferentially through the glial/ependymal scar. This regeneration is specific both with regard to the direction of axon growth and synaptic reconnection. We are attempting to determine the mechanisms involved in the elongation and guidance of regenerating axons. Electron microscopic analysis of intracellularly labeled growth cones showed that they are densely packed with neurofilaments (NFs). This is different from growth cones of most embryonic neurons studied in dissociated cell culture, which grow much faster than the regenerating spinal axons in lamprey and contain no NFs. Thus one of our goals is to determine whether NFs play a role in the axonal elongation during regeneration in the central nervous system. We have shown that the lamprey NF is unique in being a homopolymer of a single 180 kD subunit (NF-180), which has now been cloned and sequenced. Monoclonal antibodies and sense and antisense riboprobes are being used to study the expression of lamprey NF in injured neurons by immunohistochemistry, in situ hybridization and in vivo transfection targeted at manipulating transcription, translation or assembly of NF. We have recently been successful in expressing a reporter gene, beta galactosidase, in brainstem neurons by the use of gene gun bombardment with gold particles coated with plasmid vectors containing the reporter gene DNA. This results in expression of the protein in many neurons lasting at least four weeks, which is as long as we have followed the process. We are now attempting to use this technique to manipulate the expression of NF.

EM analysis also shows that the growth cones of regenerating spinal axons are in disproportionate contact with glial processes. At the margin of an injury, glial cells send thickened, longitudinally oriented fibers into the lesion, forming a glial 'scar' and these fibers precede regenerating axons into the wound, suggesting that glial cells may play an important role in guiding the regeneration. Lamprey glia differ from mammalian astrocytes in containing keratin intermediate filaments instead of glial fibrillary acidic protein and in being interconnected by desmosomes. This phenotype is found in other systems showing axonal regeneration, e.g., the optic nerves of fish and amphibia. We are using molecular cloning to study the CNS expression of keratin and of desmosomal cadherins, the molecules that cement together the halves of desmosomes on opposing cell membranes. A partial sequence for one putative lamprey desmosomal cadherin has been obtained. We are also employing antibodies to desmosomal proteins and calcium chelation to disrupt desmosomes in order to test the hypothesis that these intercellular adhesive junctions suppress migration and proliferation of glial cells near a spinal cord injury, stabilizing the glial architecture and thus providing appropriate channels for the guidance of regenerating axons. Finally, we have recently found evidence for the existence of guidance molecules in the lamprey CNS, including collapsin 1 and 4, netrin 1and the netrin receptors UNC-5 and DCC.. We are now attempting to determine the cellular localization of these molecules, how their expression is modulated following spinal cord transection, and how manipulation of their expression will alter regeneration.

A goal of the laboratory is to be able to image the regeneration of axons in the living animal. Toward that end, we are collaborating with the Division of Neuroradiology to image the spinal cord by MRI and to perform physiological measurements using diffusion weighted imaging and MR spectroscopy. To date we are able to achieve a resolution of 9mm in the isolated spinal cord and thereby image individual large axons.

KEY WORDS:

The Selzer Lab