Choosing the right cell for spinal cord repair

There is a rich history of neural progenitor transplantation for repair of the injured central nervous system (Anderson, Howland, & Reier, 1995; Bonner & Steward, 2015; Jakeman & Reier, 2015; Lane, Lepore, & Fischer, 2017; Lu, 2017; Nori, Nakamura, & Okano, 2017; Tetzlaff et al., 2011). Sugar and Gerard (1940) were among the first to transplant developing spinal cord tissue into the injured adult spinal cord to facilitate repair, albeit with limited efficacy. In the 1980’s Reier and colleagues at the University of Florida extended this work to show that transplantation of embryonic spinal cord tissues promoted repair and functional recovery following experimental spinal cord injury (SCI) in mice, rats, and cats (Reier et al., 2002). This work was eventually then translated for a small clinical study in patients undergoing surgery for post-traumatic syringomyelia—the first study of its kind in people with SCI (Reier, 2004; Thompson et al., 2001; Wirth et al., 2001). This prompted subsequent studies that transplanted stem cell derived neural precursor cells into the injured spinal cord, and led to the first FDA approved stem cell trial in the United States.

Initial studies by Reier and colleagues also began to characterize the cytoarchitecture of transplanted tissue to determine how transplanted tissues were able to promote repair and restore tissue continuity (Eng, Reier, & Houle, 1987; Giovanini, Reier, Eskin, Wirth, & Anderson, 1997; Horner, Reier, & Stokes, 1996; Jakeman et al., 1989; Stokes & Reier, 1991; White et al., 2010). The transplanted tissue was shown to produce a heterogeneous mix of mature neurons and glia, become vascularized (Giovanini et al., 1997; Horner et al., 1996), and retained its capacity to produce spinal cord morphology (Jakeman et al., 1989; White et al., 2010) as it matured within the injury site. Within donor neurons alone there is a vast range of spinal interneuronal precursors that have wide ranging excitatory, modulatory, or inhibitory functions. Small, tightly packed populations of donor neurons that are morphologically comparable to the substantia gelatinosa of the mature dorsal horn (Jakeman et al., 1989; White et al., 2010), have been found to be derived from the dorsal (alar) region of the developing spinal cord (White et al., 2010). Donor cells from this region have since also been shown to be innervated by the appropriate host cell axons over time (Dulin et al., 2018). Donor tissues derived from the developing ventral (basal) spinal cord resulted in quite unique donor morphology, with higher levels of myelin (White et al., 2010). However, with this heterogeneity comes the caveat that not all donor components are beneficial for functional improvement, and some recovery may in fact be limited if the wrong donor source is used (White et al., 2010). With this in mind, there has been a growing interest in interneuronal phenotypes that exist within donor tissues, and which may be optimal for transplantation.

Building on the early studies transplanting developing spinal cord tissues, Fischer and colleagues at Drexel University extended this work to culture embryonically derived spinal tissues to select for neuronal and glial restricted progenitors (excluding extracellular matrix and vascular endothelial cells, among other cellular components). Like the tissue transplants, cultured neural progenitors restored tissue continuity and enhanced repair. But an important consideration is that preparing tissue in this more clinically relevant way may also alter their developing neuronal phenotype (Zholudeva, Iyer, et al., 2018a).

Spinal interneurons (SpINs) and their roles in the development of neural circuits, as well as their neuroplastic and therapeutic potential following SCI and disease, are of growing interest within the neurosciences (Zholudeva, Qiang, et al., 2018b). Ventrally derived developing spinal cord is typically considered to comprise neurons with motor functions, while dorsal tissue is more closely associated with sensory functions (Lu, Niu, & Alaynick, 2015). For this reason, and due to the potentially more limited efficacy seen with transplantation of dorsally derived donor tissues, there is growing interest in ventrally-derived phenotypes for spinal cord repair (Brock, Graham, Staufenberg, Im, & Tuszynski, 2018; Spruance et al., 2018; Zholudeva, Iyer, et al., 2018a).

The cover image shows developing rat spinal cord tissue (embryonic day 13.5) that has been transplanted into the injured adult rat spinal cord. The image was taken from a histological section collected through the donor tissue, 3 days after transplantation. This transplanted tissue is rich with developing spinal interneurons (SpINs), which are key cellular elements for neuroplasticity after injury. One subtype of SpINs—the V2a SpINs—is immunohistochemically labeled with a transcriptional factor Chx10 (red), while nestin (green) stains immature neural tissue. This image was inspired by the explosion in research of V2a SpINs in neural development, plasticity and repair, and our hypothesis that these cells could be one of the beneficial elements for repair. The V2a SpINs have gained attention as an excitatory, pre-motor interneuron that might contribute to improved motor function in pre-clinical models of injury (traumatic SCI) or disease (amyotrophic lateral sclerosis; ALS), and can be generated from stem cell populations (Butts et al., 2017; Iyer, Huettner, Butts, Brown, & Sakiyama-Elbert, 2016). They have been shown to contribute to anatomical (Zholudeva, Karliner, Dougherty, & Lane, 2017) and functional (Romer et al., 2016) plasticity in respiratory neural circuits and play a role in locomotor central pattern generator circuits (Dougherty & Kiehn, 2010; Hayashi et al., 2018). More recently, we have shown that enriching donor cells with stem cell derived V2a interneurons can enhance the extent of motor recovery within an anatomically and functionally defined spinal network—the phrenic motor system (Zholudeva, Iyer, et al., 2018a).

Future work will now be able to build upon this work to more extensively characterize the phenotype of SpINs, and even other donor cell types. Such research will significantly advance on previous transplantation studies, and help to determine optimal donor cells for spinal cord repair.

CAPTION

The cover image is of developing rat spinal cord-derived tissue (embryonic day 13.5) 3 days after it has been transplanted into an injured adult rat spinal cord. The injury in this case was a cervical level spinal contusion. This transplanted tissue is rich with developing spinal interneurons (SpINs), which have been shown to be key cellular elements for neuroplasticity after injury. One particular subtype—the V2a SpINs—is immunohistochemically labeled with a transcriptional factor Chx10 (red), while nestin (green) stains immature neural tissue. These cells have been shown to play an important role in neuroplasticity of both respiratory and locomotor circuits after injury or disease and may represent an important therapeutic target.