J Neurotrauma. 2015 Jun 1;32(11):753-64. doi: 10.1089/neu.2014.3390.

Endogenous neural stem/progenitor cells stabilize the cortical microenvironment after traumatic brain injury.

Dixon KJ1, Theus MH2, Nelersa CM1, Mier J1, Travieso LG1, Yu TS3, Kernie SG3, Liebl DJ1.

  • 11The Miami Project to Cure Paralysis and Department of Neurological Surgery, University of Miami, Miami, Florida.
  • 22The Department of Biomedical Sciences and Pathobiology, Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg, Virginia.
  • 33Department of Pathology and Cell Biology, Columbia University, New York, New York.

 

Abstract

Although a myriad of pathological responses contribute to traumatic brain injury (TBI), cerebral dysfunction has been closely linked to cell death mechanisms. A number of therapeutic strategies have been studied in an attempt to minimize or ameliorate tissue damage; however, few studies have evaluated the inherent protective capacity of the brain. Endogenous neural stem/progenitor cells (NSPCs) reside in distinct brain regions and have been shown to respond to tissue damage by migrating to regions of injury. Until now, it remained unknown whether these cells have the capacity to promote endogenous repair. We ablated NSPCs in the subventricular zone to examine their contribution to the injury microenvironment after controlled cortical impact (CCI) injury. Studies were performed in transgenic mice expressing the herpes simplex virus thymidine kinase gene under the control of the nestin(δ) promoter exposed to CCI injury. Two weeks after CCI injury, mice deficient in NSPCs had reduced neuronal survival in the perilesional cortex and fewer Iba-1-positive and glial fibrillary acidic protein-positive glial cells but increased glial hypertrophy at the injury site. These findings suggest that the presence of NSPCs play a supportive role in the cortex to promote neuronal survival and glial cell expansion after TBI injury, which corresponds with improvements in motor function. We conclude that enhancing this endogenous response may have acute protective roles after TBI.

PMID: 25290253

 

Supplement:

The human brain is highly susceptible to trauma that can originate from a single or repetitive impact injuries (for example, car accidents and athletes, respectively) or other pathological conditions such as stroke injury. A potential consequence of trauma is long-term functional disability as a result of tissue damage, cell death and the overall disruption of brain circuitry. For the past century, researchers such as Santiago Ramon y Cajal have shown that neurons and glial cells undergo cellular change as a result of brain damage, and the brain responds by attempting to regenerate (1,2). Unfortunately, these responses are minimal and are compounded by the inability of the brain to respond to massive cell death. This is particularly problematic when neurons are lost, since the vast majority of these principle cells have lost their ability to self-renew in the adult brain. For this reason, the consequence of CNS injury can be devastating.

 

In 1962, Joseph Altman first reported that new neurons could be generated in the adult mammalian brain from a couple specialized niches (3), namely the subventricular zone (SVZ) and dentate gyrus of the hippocampus (4). In the SVZ, neural stem cells give rise to neuronal progenitors cells, called neuroblasts, that migrate to the olfactory bulb (OB) where they differentiate into interneurons (5,6). Figure 1a shows that migratory pathway of neuroblasts (red cells) from the SVZ through a tight rostral migratory stream (RMS) to the OB in the adult mouse brain. Under pathological conditions, such as traumatic brain injury (TBI), enhanced neurogenesis (7) can lead to increased numbers of neuroblasts migrating outside the RMS (Fig. 1a’; arrowheads) and traveling to regions of damaged tissue.  However, the influence of neuroblasts on damaged tissues is poorly understood. We hypothesized that SVZ-derived neuroblasts contributed to the acute injury sequelae by protecting residential cells in the local environment from TBI-induced damage.

 

 

Fig. 1

Figure 1. Neuroblast migration outside the rostral migratory stream (RMS) after controlled cortical impact (CCI) injury. (a) Sagittal brain cross-section of doublecortin-labeled neuroblasts (red) migrating from the subventricular zone (SVZ) through the RMS to the olfactory bulb (OB). (a’) shows neuroblasts exiting the RMS (arrowheads) and migrating to regions of cortical damage. Ctx, cortex; Hipp, hippocampus. Bar is 400 mm.

 

 

In our study, we examined the influence of adult neurogenesis on TBI damage to cortical tissues by examining injury progression in the presence and absence of neuroblasts. We injured our animals using a controlled cortical impact (CCI) device that permits researchers to control the degree of cortical damage to ensure reproducibility in the pathophysiological outcomes. To abolish adult neurogenesis, we took advantage of transgenic mice expressing the herpes simplex virus thymidine kinase (TK) gene under the control of the nestin-delta (d) promoter (i.e. nestind-TK). Basically, the delta subunit of the nestin promoter leads to expression of the TK gene specifically in neural stem cells in both neurogenic niches. Following treatment with the drug cytovene, cells expressing the viral TK gene terminate DNA synthesis and undergo cell death.

 

Cytovene treatment prior to CCI injury resulted in the ablation of proliferating neural stem cells in the SVZ and their resulting progeny (i.e. neuroblasts). As a consequence significantly fewer neuroblasts were present in injured tissues, which enabled us to examine the importance of neuroblasts to the dynamics of brain injury.  Specifically, we quantified the number of surviving neurons and glial cells in regions surrounding the injury cavity, called the perilesional cortex, two weeks after injury onset.  Although we did not observe major differences in the size of the cavity between treated and untreated brains, we did observe reduced numbers of neurons and glial cells in neuroblast-deficit mice. This suggested that many populations of cells in damaged tissues are positively influenced by the presence of SVZ-derived neuroblasts. This is likely due to the ability of neuroblasts to express a host of growth and survival factors that, in turn, support residential cells in damaged tissues. Thus, in the absence of neuroblasts fewer growth/survival factors are expressed and many injured cells do not survive.

 

A second observation was that glial cells alter their shape and possibly their function (termed “reactive”) in the absence of SVZ-derived neuroblasts. Glial cells, such as astrocytes and microglia, have diverse functions in the injured brain that can be both beneficial and detrimental to brain recovery. One example of a beneficial influence is their ability to secrete survival factors; however, reactive glial cells can also release pro-inflammatory proteins that enhance tissue damage. Therefore, it’s reasonable to suggest that the presence of neuroblasts in brain injured tissues leads to more but less reactive glial cells that participate in promoting recovery.

 

As it turns out, the cellular alterations associated with the absence of SVZ-derived neuroblasts can augment the functional deficits associated with brain injury. To examine brain function, we tested the motor responses of our mice by examining how long the mice could walk on a rotating cylindrical rod that is accelerating from 10 rpm to 60 rpm over a period of 10 minutes. We found that neuroblast-ablated mice had reduced motor ability to stay on the rotating rod as compared to pre-injury times, wild type mice (TK negative mice), or non-cytovene-treated groups (i.e. neuroblasts present) at 14 days post-injury (Fig. 2). These findings demonstrate that the cellular changes that occur in damaged tissues in the absence of neuroblasts has global effects on brain recovery.

 

Importance of the study: Our data show that the brain has an internal capacity to protect itself from traumatic insults through the enhancement of the stem cell pool and recruitment of young neuroblasts to injured tissues that participate to support residential cells.  Furthermore, understanding the mechanisms to enhance adult neurogenesis could represent an important therapeutic strategy to improve TBI recovery.

 

 

Fig 2Figure 2. Depletion of neuroblasts leads to reduce motor recovery in cytovene-treated nestind-TK mice at 14 days post-injury.

 

References:

(1)   Ramón y Cajal, S. (1895) Textura del sistema nervioso del hombre y de los vertebrados. Madrid; Imprenta y Librería Moya.

(2)   Ramón y Cajal, S. (1969) Degeneration and Regeneration of the Nervous System. New York; Harper Press.

(3)   Altman, J. (1962) Are new neurons formed in the brains of adult mammals? Science 135:1127-1128.

(4)   Alvarez-Buylla, A., Garcia-Verdugo, J.M. (2002) Neurogenesis in adult subventricular zone. J. Neurosci. 22:629-634.

(5)   Luskin, M.B. (1993) Restricted proliferation and migration of postnatally generated neurons derived from the forebrain subventricular zone. Neuron 11:173-189.

(6)   Betarbet, R., Zigova, T., Bakay, R.A., Luskin, M.B. (1996) Dopaminergic and GABAergic interneurons of the olfactory bulb are derived from the neonatal subventricular zone. Int. J. Dev. Neurosci. 14:921–930.

(7)   Chen, X.H., Iwata, A., Nonaka, M., Browne, K.D., and Smith, D.H. (2003) Neurogenesis and glial proliferation persist for at least one year in the subventricular zone following brain trauma in rats. J. Neurotrauma 20:623-631.

 

Acknowledgement: This work was supported by NIH/NINDS NS049545 & NS30291 (DJL), DOD W81XWH-05-1-0061 (DJL), NS007459 & NS064699 (MHT) and the Miami Project to Cure Paralysis.

 

Contact:

Daniel J Liebl, PhD

University of Miami Miller School of Medicine

The Miami Project to Cure Paralysis

1095 NW 14th Terrace

Miami, Fl 33136 USA

dliebl@miami.edu

 

 

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