HTLV-1 Secreted Proteins Induce Axonal Degeneration Through Kinase and Phosphatase Activity Dysregulation

HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP) pathology has been associated with Tax protein secreted from HTLV-1 infected CD4⁺-T-lymphocytes, which interacts with soluble SEMA-4D (Semaphorin 4D) inducing growth cone collapse in neurons. We investigated HTLV-1-induced phenotypic and signaling changes during PC12 neuronal differentiation that may mediate growth cone collapse effects. We measured kinases and phosphatases associated with microtubule-associated proteins and molecular motor functions. Phosphorylation status of proteins that participate in the cytoskeleton and axonal transport such as Tau, microtubule-associated protein 1B (MAP1B), motor proteins (kinesin-1 and dynein) and collapsin response mediator protein (CRMP-2), all involved in neurite extension and branching, were measured. The phosphorylation/dephosphorylation of these proteins is catalyzed by Cyclin-dependent kinase-5 (CDK5), Glycogen synthase kinase-3β (GSK3β), and Protein phosphatase-2 (PP2A). Our results show that viral secreted proteins produced a reduction of neurite extension and branching in PC12 cells during neuronal differentiation. We observed that GSK3β activity increased, while CDK5 and PP2A activities decreased. In addition, we found reduced levels of Tau phosphorylated at Thr¹⁸¹ and increased levels of CRMP-2 phosphorylated at Ser⁵²². No changes in motor proteins or MAP1B phosphorylation were found. Neurotoxic effects of HTLV-1 secreted proteins on neuronal differentiation of PC12 cells include lower CDK5 activity, which could explain the reduced levels of Tau-(pThr¹⁸¹); this could induce conformational changes in Tau protein, altering microtubule dynamics. Increased CRMP-2-(pSer⁵²²) phosphorylation precedes further phosphorylation at Thr⁵⁰⁹/⁵¹⁴ residues by GSK3β. All these phosphorylations are associated with growth cone collapse. The increased CRMP-2-(pSer⁵²²) levels found here suggest that CDK5 activity, even when decreased, is sufficient for this priming phosphorylation. Reduction in PP2A activity could importantly contribute to maintaining the increased phosphorylation in CRMP-2. These results suggest the involvement of extracellular Tax/sSEMA-4D complex in the activation of Plexin1B receptor, activating downstream cascade involving PI3K/AKT/GSK3β/CRMP-2, inducing growth cone collapse.