Extracellular Neuroleukin Enhances Neuroleukin Secretion From Astrocytes and Promotes Axonal Growth in vitro and in vivo

Abstract

Under pathological conditions in the central nervous system (CNS), including spinal cord injury, astrocytes show detrimental effects against neurons. It is also known that astrocytes sometimes exert beneficial effects, such as neuroprotection and secretion of axonal growth factors. If beneficial effects of astrocytes after injury could be induced, dysfunction of the injured CNS may improve. However, a way of promoting beneficial functions in astrocytes has not been elucidated. In the current study, we focused on neuroleukin (NLK), which is known to have axonal growth activities in neurons. Although NLK is secreted from astrocytes, the function of NLK in astrocytes is poorly understood. We aimed to clarify the mechanism of NLK secretion in astrocytes and the functional significance of secreted NLK from astrocytes. Stimulation of cultured astrocytes with recombinant NLK significantly elevated the secretion of NLK from astrocytes. Furthermore, astrocyte conditioned medium treated with NLK increased axonal density in cultured cortical neurons. Recombinant NLK itself directly increased axonal density in cultured neurons. These results indicated that NLK secreted from astrocytes acted as an axonal growth factor and that secretion was stimulated by extracellular NLK. To elucidate a direct binding molecule of NLK on astrocytes, drug affinity responsive target stability (DARTS) analysis was performed. A 78 kDa glucose regulated protein (GRP78) was identified as a receptor for NLK, which was related to the secretion of NLK from astrocytes. When NLK was injected into the lesion site of spinal cord injured mice, axonal density in the injured region was significantly increased and hindlimb motor function improved. These results suggested that NLK-GRP78 signalling was important for the beneficial effects of astrocytes. This study strengthens the potential of astrocytes for use as therapeutic targets in CNS traumatic injury.

Cerebrospinal fluid monocyte chemoattractant protein 1 correlates with progression of Parkinson’s disease

“…Neuroleukin (NLK) is a neurotrophic factor with axonal growth activity and a product of lectin-stimulated T cells 17,18 . Previous studies showed that NLK is upregulated in the brain in patients with Huntington's disease 19 and promotes axonal growth in spinal cord injury 20 , indicating a role of NLK in neurodegenerative processes. Therefore, in the present study, we aimed to assess the potential of the inflammatory proteins MCP-1 and NLK in CSF as biomarkers for diagnosis and progression of PD and MSA.…”

GRP78-Mediated Signaling Contributes to Axonal Growth Resulting in Motor Function Recovery in Spinal Cord-Injured Mice

“…Our previous study also clarified that treatment with recombinant NLK of cultured cortical neurons promoted axonal growth in the absence or presence of CSPGs. A single injection of recombinant NLK to the lesion site increased the axonal density and improved motor function in SCI mice (Tanie et al, 2018). These data reveal that the axonal growth activity of NLK may be beneficial for functional recovery in SCI.…”

“…The concentrations of NLK and antibodies were maintained at approximately 65 ng/ml in the CSF during the administration period. Our previous study demonstrated that 10-100 ng/ml NLK promoted axonal growth in cultured neurons (Tanie et al, 2018). In addition, pretreatment with 10-100 ng/ml neutralizing antibody against GRP78 diminished the axonal growth activity of NLK ( Figure 1).…”

“…Our previous study showed that treatment to cultured neurons with NLK promoted axonal growth in the presence of CSPGs. The single injection of NLK to the lesion site promoted axonal growth and improved motor function in SCI mice (Tanie et al, 2018). In addition, as shown in Figures 3 and 4, cell surface GRP78 was necessary for Akt activation and NLK-induced axonal growth.…”

“…Our previous study indicated that the cell surface GRP78 is a receptor for NLK in astrocytes (Tanie et al, 2018). Therefore, we hypothesized that this molecule may also function as a receptor of NLK in neurons.…”