Use of a new mouse schwannoma tumour model to monitor changes in peripheral nerve morphology in Merlin null Schwann cells
Authors: Srotyr Marie , Laraba Liyam , Harper Glenn M , Lespade Charlotte , Woodhouse Evyn , Lloyd Alison C , Parkinson David B 
Published: October 2021

Abstract

Aims
Our lab is interested in signals that trigger schwannoma tumour formation and we have previously shown that peripheral nerve injury triggers tumour formation in nerves with Schwann cell-specific loss of the Merlin (NF2) tumour suppressor. The Ras/Raf/MAPK/ERK pathway activity in myelinating Schwann cells is involved in nerve regeneration, causing demyelination and recruitment of inflammatory cells in areas of nerve damage, as well as dedifferentiation of myelinating Schwann cells into a repair-competent state. We have used a mouse model expressing a tamoxifen-inducible Raf-Kinase estrogen receptor fusion protein (Raf-TR) in myelinating Schwann cells of the PNS in either a control wild-type Merlin or Merlin-null background. This allows us to determine the effects of an injury-like signal in Schwann cells and its role in generating schwannoma tumour development. We present here a detailed analysis of the proliferation of Schwann cells within the nerve and morphological changes in PNS structure following Raf-TR activation.

Method
The P0-promotor driving the Raf-TR transgene is active in myelinating Schwann cells but inactive in the non-myelinating population, allowing specific targeting of the myelinating Schwann cell population. In addition to the Raf-TR gene, the mice exhibit a separate P0-promotor controlled Cre floxed NF2 gene which undergoes Cre-mediated recombinase at embryonic day 13.5 causing NF2 knockout in all developing Schwann cells. Mice aged between 4-6 weeks received intraperitoneal injections of either 2mg Tamoxifen or oil vehicle for 5 consecutive days and were then studied at either 10 or 21 days post-first injection. The peripheral nervous system of the mice was studied with fluorescent immuno-histochemistry staining, semithin sections and transmission electron microscopy (TEM) on sciatic nerves and dorsal root ganglia (DRG).

Results
Activation of the Ras/Raf/MAPK/ERK pathway in NF2 null Schwann cells led to higher rates of proliferation within sciatic nerves at 10d post-tamoxifen injections. At both 10d and 21d Raf-TR+ NF2-null mice sciatic nerve fascicles were visibly larger with significantly more cell bodies present than controls, however at 21d the rate of proliferation had reduced. In the DRG, proliferation was higher in Raf-TR+ NF2-null mice compared to controls, with proliferation remaining high at 21 days. Quantitative imaging of peripheral nerve semi-thins analysed to date showed no significant difference in the number of myelin rings present in the fascicles between different genotypes. Additionally, dual immuno-histochemistry staining with Myelin Basic Protein and EdU, markers for myelin and proliferation respectively, appeared to show proliferation in the non-myelinating Schwann cell population. Results from staining with other cell markers will also be presented, as well as a detailed analysis of nerve structure using TEM.

Conclusion
While developmental myelination of Merlin-null Schwann cells appears largely normal, the reaction of Merlin-null Schwann cells in the nerve to an injury signal (activation of the Raf-TR) is remarkably different from those of control nerves. The high levels of proliferation in Merlin-null Schwann cells may be indicative of a higher tumorigenesis potential. While the proliferation of Merlin-null cells does reduce over time in the sciatic nerve, further experiments are now testing whether there may be ongoing tumour growth at other locations in the nervous system that are associated with NF2 tumours in human patients.