Human Midsized Neurofilament Expression in Transgenic Mouse-Derived Grafts Facilitates Study of Graft-Host Interactions in Hypogonadal Mice

Abstract

Our previous studies established that targeted axonal outgrowth into the host median eminence (ME) from grafted gonadotropin-releasing hormone (GnRH) neurons is essential for stimulation of reproductive function in hypogonadal (HPG) mice homozygous for a deletion in the GnRH gene. In the current experiments transgenic mice expressing the human midsized neurofilament NF(M) were used as sources of grafts to clarify the extent of transplant-derived innervation of the host that accompanies this dramatic recovery process. Preoptic area (POA) tissue from 1- or 2-day-old transgenic pups was implanted in the third ventricle of adult male HPG mice. Polymerase chain reaction (PCR) analysis verified that each donor bore the transgene. Immunohistochemistry using a monoclonal antibody specific for human NF(M) revealed intensely immunoreactive parvicellular soma and proximal dendrites in the grafts. Interestingly, human NF(M)-positive neurons also migrated out of the graft into the host hypothalamus. In the animal with the most dramatic increase in testicular and seminal vesicle weight, human NF(M)-positive cells were observed within the host ME. In some cases, human NF(M)-positive axonal bundles were present within the wall of the third ventricle of the host as well as in the host ME. These were GnRH negative. More extensive anatomical studies will delineate the degree and patterning of axonal outgrowth. In these early studies, however, it is apparent that neuronal fiber outgrowth into the host brain is not exclusive to GnRH fibers from the preoptic area grafts. This is important information in regard to our studies of the specificity of the signals derived from the ME that may be involved in GnRH fiber targeting.