Axotomy-induced cytoskeleton changes in unmyelinated mammalian central nervous system axons

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Oligodendrocyte-derived myelin retards the ability of CNS axons to regenerate following transection. The intrinsic response of CNS axons to an axotomy insult may be vastly different in the absence of myelin. However, the paucity of adequate experimental models has limited detailed investigation of cellular behaviour following axon transection in an unmyelinated CNS environment. In this study we perform laser-induced axotomy of the porcine retinal ganglion cell axon, a physiologically unmyelinated, mature CNS axon that is structurally similar to humans to infer knowledge about axonal behaviour in the absence of myelin. Axotomy-induced changes to the neuronal cytoskeleton and supporting astrocytes during the early stages after transection are delineated by examining the sequence of neurofilament subunit, microtubule (TUB), microtubule associated protein (MAP), glial fibrillary acidic protein (GFAP) and terminal deoxynucleotidyl transferase biotin-dUTP nick end labelling (TUNEL) modification. Axonal transection induced an increase in the expression of neurofilament light at regions within, and immediately adjacent to, sites of axotomy. Other neurofilament subunits were not altered at sites of transection. Unlike myelinated axons where an increase in GFAP staining within hypertrophic glial scars have been shown to inhibit axonal repair we demonstrate a decrease in GFAP staining within regions of increased or preserved neurofilament expression. The behaviour of TUB and MAP proteins following transection of unmyelinated CNS axons are similar to what has previously been described in myelinated CNS axons. This study provides fundamental insights into astrocyte and axonal behaviour acutely after axotomy and demonstrates a series of degenerative events in unmyelinated CNS axons, which in comparison to prior reports are different to myelinated CNS axons. The findings of this report have relevance to understanding pathogenic mechanisms underlying neuro-degeneration in the CNS.