Delayed death of identified reticulospinal neurons after spinal cord injury in lampreys

Abstract

There is controversy about whether axotomized neurons undergo death or only severe atrophy after spinal cord injury (SCI) in mammals. Lampreys recover from complete spinal transection, but only about half of the severed spinal-projecting axons regenerate through the site of injury. The fates of the unregenerated neurons remain unknown, and until now death of axotomized spinal-projecting neurons has not been described in the lamprey brain. We now report that in animals allowed to survive for 12 or more weeks after spinal cord transection, several identified reticulospinal (RS) neurons were missing in Nissl-stained or neurofilament-immunostained brain whole mounts. At earlier times, these neurons were swollen and pale in Nissl-stained preparations. Retrograde fluorescent labeling from the site of transection combined with TUNEL histochemistry suggested that neuronal death, including that of the identified RS neurons, began in animals 4 weeks posttransection, reaching a peak at 12-16 weeks. This was not seen in untransected animals. The TUNEL positivity suggests that some cells were dying by apoptosis. Of special interest, among the identified neurons, this delayed cell death was restricted to neurons that at earlier posttransection times have a low probability of regeneration. These data show that SCI induces delayed cell death in lamprey spinal-projecting neurons and suggest that the reason why some neurons are "bad regenerators" is that they are already undergoing apoptotic cell death. Thus protection from apoptosis may be necessary in order to enhance axonal regeneration after SCI.

Reducing synuclein accumulation improves neuronal survival after spinal cord injury

“…1B). However, after spinal transection, we and others have reported that a distinct and reproducible subset of the giant RS neurons degenerates, as demonstrated by swelling, loss of Nissl substance, and chromatolysis (Shifman et al, 2008; Busch and Morgan, 2012). The neurons that typically die after injury are M2, M3, I1, Bl, B3, and Mth neurons (Fig.…”

“…To determine the extent to which post-injury synuclein accumulation causes neuronal death, we took advantage of the identified lamprey RS neurons due to their large size, as well as known capacities for degeneration and regeneration after SCI (Shifman et al, 2008; Busch and Morgan, 2012). There are 30 identified giant RS neurons in the lamprey midbrain and hindbrain, which are bilaterally positioned in stereotypical locations (Fig.…”

“…1C, red arrows). These neurons also exhibit other signs of post-injury degeneration, including FluoroJadeC® staining (Busch and Morgan, 2012), TUNEL staining (Shifman et al, 2008), and increased levels of activated caspases (Barreiro-Iglesias and Shifman, 2012; Hu et al, 2013), suggesting a mechanism of apoptosis. In contrast, another subset of giant RS neurons typically survives (e.g.…”

“…In keeping with terminology established in the literature, we refer to these classes of neurons as “poor survivors” and “good survivors,” respectively (Fig. 1A) (Shifman et al, 2008; Busch and Morgan, 2012). The “good survivors” can be retrogradely labeled from a position caudal to the injury site, indicating that their axons readily regenerate (Jacobs et al, 1997; Shifman et al, 2008).…”

Transgenic Zebrafish Lines as Valuable Tools to Understand Successful Spinal Cord Regeneration

“…Recent findings using lampreys have also shown that those brain neurons that are "bad regenerators" usually die after a complete spinal cord transaction [3,5]. Other studies in lampreys have suggested that changes in the expression of axonal guidance molecules or neurotransmitter receptors could be responsible for the success/failure of the regeneration process after injury [11][12][13]. These and other studies have shown that lampreys are great models to study successful spinal cord regeneration.…”