Induced Neural Activity Promotes an Oligodendroglia Regenerative Response in the Injured Spinal Cord and Improves Motor Function after Spinal Cord Injury.

Myelination in the central nervous system (CNS) is a dynamic process that includes birth of oligodendrocyte progenitor cells (OPCs), their differentiation into oligodendrocytes, and ensheathment of axons. Regulation of myelination by neuronal activity has emerged as a new mechanism of CNS plasticity. Activity-dependent myelination has been shown to regulate sensory, motor, and cognitive functions. In this work, we aimed to employ this mechanism of CNS plasticity by utilizing induced neuronal activity to promote remyelination and functional recovery in a subchronic model of spinal cord injury (SCI). We used a mild contusive SCI at T10, which demyelinates surviving axons of the dorsal corticospinal tract (dCST), to investigate the effects of induced neuronal activity on oligodendrogenesis, remyelination, and motor function after SCI. Neuronal activity was induced through epidural electrodes that were implanted over the primary motor (M1) cortex. Induced neuronal activity increased the number of proliferating OPCs. Additionally, induced neuronal activity in the subchronic stages of SCI increased the number of oligodendrocytes, and enhanced myelin basic protein (MBP) expression and myelin sheath formation in dCST. The oligodendroglia regenerative response could have been mediated by axon-OPC synapses, the number of which increased after induced neuronal activity. Further, M1-induced neuronal activation promoted recovery of hindlimb motor function after SCI. Our work is a proof of principle demonstration that epidural electrical stimulation as a mode of inducing neuronal activity throughout white matter tracts of the CNS could be used to promote remyelination and functional recovery after CNS injuries and demyelination disorders.

Neuronal activity promotes myelination via a cAMP pathway.

Neuronal activity promotes myelination in vivo and in vitro. However, the molecular events that mediate activity-dependent myelination are not completely understood. Seven, daily 1 h sessions of patterned electrical stimulation (ESTIM) promoted myelin segment formation in mixed cultures of dorsal root ganglion (DRG) neurons and oligodendrocytes (OLs); the increase in myelination was frequency-dependent. Myelin segment formation was also enhanced following exposure of DRGs to ESTIM prior to OL addition, suggesting that ESTIM promotes myelination in a manner involving neuron-specific signaling. Cyclic adenosine monophosphate (cAMP) levels in DRGs were increased three-fold following ESTIM, and artificially increasing cAMP mimicked the ability of ESTIM to promote myelination. Alternatively, inhibiting the cAMP pathway suppressed ESTIM-induced myelination. We used compartmentalized, microfluidic platforms to isolate DRG soma from OLs and assessed cell-type specific effects of ESTIM on myelination. A selective increase or decrease in DRG cAMP levels resulted in enhanced or suppressed myelination, respectively. This work describes a novel role for the cAMP pathway in neurons that results in enhanced myelination.

Axon myelination and electrical stimulation in a microfluidic, compartmentalized cell culture platform.

Axon demyelination contributes to the loss of sensory and motor function following injury or disease in the central nervous system. Numerous reports have demonstrated that myelination can be achieved in neuron/oligodendrocyte co-cultures. However, the ability to selectively treat neuron or oligodendrocyte (OL) cell bodies in co-cultures improves the value of these systems when designing mechanism-based therapeutics. We have developed a microfluidic-based compartmentalized culture system to achieve segregation of neuron and OL cell bodies while simultaneously allowing the formation of myelin sheaths. Our microfluidic platform allows for a high replicate number, minimal leakage, and high flexibility. Using a custom built lid, fit with platinum electrodes for electrical stimulation (10-Hz pulses at a constant 3 V with ~190 kΩ impedance), we employed the microfluidic platform to achieve activity-dependent myelin segment formation. Electrical stimulation of dorsal root ganglia resulted in a fivefold increase in the number of myelinated segments/mm² when compared to unstimulated controls (19.6 ± 3.0 vs. 3.6 ± 2.3 MBP+ segments/mm²). This work describes the modification of a microfluidic, multi-chamber system so that electrical stimulation can be used to achieve increased levels of myelination while maintaining control of the cell culture microenvironment.

Electrical stimulation promotes the survival of oligodendrocytes in mixed cortical cultures.

Oligodendrocyte (OLG) death plays a major role in white matter dysfunction and demyelination following injury to the CNS. Axonal contact, communication, and neuronal activity appear to promote OLG survival and function in cell culture and during development. The application of electrical stimulation to mixed neural cultures has been shown to promote OLG differentiation and the formation of myelin in vitro. Here we show that OLG viability can be significantly enhanced in mixed cortical cultures by applying biphasic pulses of electrical stimulation (ESTIM). Enhanced survival via ESTIM requires the presence of neurons and is suppressed by inhibition of voltage-gated sodium channels. Additionally, contact between the axon and OLG is necessary for ESTIM to promote OLG survival. This report suggests that patterned neuronal activity could repress delayed progression of white matter injury and promote CNS repair in neurological conditions that involve white matter damage.