Translated Abstract
Background:
Traumatic brain injury (TBI), as a high prevalence and potentially devastating disease, is the leading cause of death for persons under age 45 and a critical public health and socio-economic problem throughout the world. Due to the unique biomechanical nature of TBI, varying degrees of diffuse axonal injury (DAI) is ubiquitously accompanied with TBI, and be thought as the main reason which results to long-term coma or even death of TBI patients. Yet with enormous burden of DAI to the society, so far there is no clinically proven therapeutic drag available to limit the neuropathological sequelae. So, more comprehensive understanding of neurobiological mechanisms of DAI is quite urgent.
Even though many progresses about the pathogenesis of DAI have being achieved in recent decades, the more detailed mechanisms of DAI caused axonal degeneration and disconnection remain elusive. During the brain trauma, the shearing force formed during cerebral accelerated or decelerated movement is the mechanical inducement of DAI. And after that, the elevated axoplasmic Ca2+ ([Ca2+]axo) has widely been reported as a key mediator which lead to secondary axonal degeneration and disconnection through several mechanisms including activation of calpain and mitochondrial failure. At present, the mechanism about how Ca2+ accumulates in axoplasm after DAI is not fully understood.
Researches have already proved that axonal stretch can trigger a rapid sharp [Ca2+]axo increase (“initial [Ca2+]axo increase”), which makes calpain activated. Further, calpain mediated proteolysis of voltage gated sodium channels (NaChs) can activate voltage-gated calcium channels (VGCCs) and reverse Na+-Ca2+ exchanger, whereby lead to sustained [Ca2+]axo increase, calpain activation and axonal cytoskeleton breakdown. To date, the mechanism by which mechanical stretch triggers the “initial [Ca2+]axo increase” is not clear. One possibility is mediated by the stretch activated channels (SACs), which can be directly activated by strain force under cell membrane. Besides, it has been reported that Ca2+ release from endoplasmic reticulum (ER) involves in the [Ca2+]axo increase caused by axonal stretch injury. However, it is still unclear whether ER calcium release directly affects axonal degeneration in axonal stretch injury. And if ER Ca2+ depletion occurring, it will activate store-operated calcium channels (SOCCs), as a subsequent pathway mediating extracellular Ca2+ influx. Whether SOCCs also participates in stretch-induced [Ca2+]axo increase and degeneration is unknown.
In this study, a new established in vitro DAI model and mouse cortical neuron cultures were used to comprehensively discussed several potential mechanisms which may participate in [Ca2+]axo increase and axon acute degeneration(occur in 30min post injury) after DAI. Also, we explored the protein expressions of SACs and SOCCs on the axons of mouse cortical neurons. As for both SACs and SOCCs, their channel candidates are still controversial. Relatively widely accepted molecular components for SACs are TRPC1, TRPC6, TRPV1 and TRPV2. And STIM1, Orai1, and TRPC1 are largely being considered as SOCCS components.
Objectives:
1. To establish a brand new DAI in vitro model, and evaluate this new model comprehensively.
2. To determine the dynamic pattern of [Ca2+]axo incrase after DAI
3. To determine whether endoplasmic reticulum Ca2+ release occurs after DAI.
4. To determine whether components of store-operated channels and stretch-activated channels express in axons structurally and functionally.
5. To investigate roles of store-operated channels and stretch-activated channels in the [Ca2+]axo increase and axonal secondary injury processes.
6. To investigate the interactional roles of stretch-activated channels, voltage-gated sodium channels and voltage-gated calcium channels in the [Ca2+]axo increase after DAI.
Methods:
1. Establishment and evaluation of DAI in vitro model: Modified primary neuron cultured metheod was used to conduct compartmental culture of axons and somata. DAI processes were simulated by axonal mechanical stretch. MAP2, Tau and β-tubulin immunofluorescences were used for examation of the purity of cultured neurons. Immunofluorescences of Tau and β-tubulin were used for identification of the axon-only region. Theoretical and real stretch extents were obtained by finite element analysis and length measurement. Axonal cytoskeleton proteins, such as Tau and Tau and β-tubulin were investigated by fluorescent immunocytochemistry. Theoretical and real stretch extents were obtained by finite element analysis and length measurement. Axonal pathological changes caused by stretch injury were observed by confocal microscope. Propidium iodide (PI) staining was used to determine the neuronal cell death after injury.
2. Fluo-4 AM and Cell tracker red were used for Ca2+ imaging, by which dynamic [Ca2+] axo changing pattern was investigated. Ca2+ free extracellular fluid and caffeine were used for study of the endoplasmic reticulum Ca2+ release.
3. Immunofluorescence, Western blot methods were used to study expressions of STIM1, Orai1, TRPC1, TRPC6, TRPV1 and TRPV2 in axons.
4. CPA (a sacro/endoplasmic Ca2+-ATP inhibitor can make ER Ca2+ release) and inhibitor of SOCs (SKF-96365, Pyr-6 and Gd3+) were used to prove SOCs exist in axons functionally.
5. Roles of SOCs and SACs in [Ca2+]axo increase and ASI after DAI were tested by either SKF-96365, Pyr-6 or GsMTx-4 and Gd3+
6. Roles of NaChs and VGCCs in [Ca2+]axo increase after DAI were tested TTX and nifedipine.
7. High potassium extracellular fluid was used to study activity of VGCCs on axons of sham group, injury group and GsMTx-4 treatment group.
Results:
1. Neuons of purity more than 90% can be obtained by this modified primary neuron culture method. Neurites in the axon-only regions exhibit positive staining of Tau, whereas negative staining of MAP2. The maximum stain occurs in the central part of chamber for both Twenty percent stretch and fifty percent stretch. And the real stain of axons under 20% stretch was 14%, whereas the value increase to 42% under 50% stretch. The extent of axonal beading caused by 50% stretch (n=42) was significantly higher than 20% stretch (n=28, P<0.01). Fifty percent stretch caused neuronal cell death determine by PI staining. PI positive staining cells at 1d post 50% stretch (n=9) was significantly higher than sham group (n=6) and 20% stretch group (n=7), whereas PI positive staining cells at 1d post 20% stretch was not significantly higher than the sham group (P>0.05). Besides, fifty percent stretch resulted in the interruption of cytoskeletion proteins, such as Tau (n=6) and β-tubulin (n=6). Inside the beads areas, there were significant higher Tau fluorescent intensities than outside beads areas (P<0.01). However, Inside the beads areas, there were significant lowerβ-tubulin fluorescent intensities than outside beads areas (P<0.01).
2. [Ca2+] axo increasd after 50% stretch. For the non-bead areas of axons (n=42), [Ca2+] axo peaked at 30s after injury (P<0.01, vs sham group, n=19), than [Ca2+] axo decreased gradually. At 30min after injury, [Ca2+] axo of non-bead areas was still higher than sham group (P<0.05). As to bead areas, [Ca2+] axo also increased strongly at 30s post injury (vs sham group, P<0.01). In the following 10-30 minutes, [Ca2+] axo inside bead-areas increased continuously (vs sham group, P<0.01; vs non-bead areas, P<0.01)
3. Fifty percent stretch did not caused [Ca2+]axo increase in the Ca2+-free extracellular fluid condition (n=6) (P>0.05, vs sham, n=19). 30min post injury, number of beads per 100 microns (n=6) was not significantly higher than sham group (n=19) (P>0.05). Caffeine triggered [Ca2+] axo was significant lower in 50% stretch group (n=6), compared with sham group (n=6) (P<0.01).
4. STIM1, Orai1 and TRPC1 all expressed in axons. CPA trigged Ca2+ influx was elicited in axons. And this CPA trigged Ca2+ influx was significantly suppressed by SOCs inhibitors (SKF-96365, n=6, Pyr-6, n=6, Gd3+, n=6) (all P<0.01, vs blank control group, n=4).
5. SKF-96365 (n=9) and Pyr-6 (n=7) significantly decreasd the [Ca2+]axo increase inside bead area at 5, 10 and 30min post injury.(vs injury group, n=42, all P<0.01). Neither SKF-96365 nor Pyr-6 decreased the [Ca2+] axo increase outside bead area at any time point less than 30min post injury (vs injury group, all P>0.05 ). At 30min post injury, overlapped coefficient and Pearson’s correlation r of STIM1-Orai1 and STIM1-TRPC1 were increased significantly (injury group, n=6, vs sham group, n=6, P<0.05)
6. For mouse, TRPC6 and TRPV2, but not TRPV1 expressed in axons. SACs inhibitors, GsMTx-4 (n=8) and Gd3+(n=7), significantly restrained [Ca2+]axo increase at each time points from 30s-30min post injury(vs injury group, n=42, all P<0.01).
7. Both TTX (n=6) and nifedipine (n=7) significantly restrained [Ca2+]axo increase at each time points from 30s-30min post injury(vs injury group, n=42, all P<0.01).
8. At 30s post injury, High K+ triggered [Ca2+] axo increase of injury group (n=7) was significantly lower than sham group (n=13). High K+ triggered [Ca2+] axo increase of injury + GsMTx-4 group (n=5) was significantly higher than injury group (P<0.05). There was no significant difference between injury + GsMTx-4 group and sham group (P>0.05).
Conclusions:
1. A brand new DAI in vitro model was established by mechanical stretch method. Fifty percent stretch is capable to be the ideal option for creating this model, due to it of more potent secondary axonal degeneration and disconnection than 20% stretch. At the same time, model created by fifty percent stretch conforms to clinical pathological characters of DAI.
2. DAI results in uneven [Ca2+] axo increase, which manifest as higher as well as more sustained in bead areas, compared with non-bead areas.
3. DAI results axonal ER Ca2+ release.
4. SOCs exist in axons of mouse cortical neurons structurally and functionally. SOCs play a role in the [Ca2+] axo increase inside beads. However, SOCs inhibitors have no protective effect for axonal secondary injury.
5. SACs components components, such as TRPC, TRPC6 and TRPV2, also exist in axons of mouse cortical neurons structurally. SOCs participate in the [Ca2+] axo increase and axonal secondary injury after DAI.
6. Activations of NaChs and VGCCs are also necessary for [Ca2+] axo increase after DAI. SACs may be the initial procedure leading to [Ca2+] axo increase.
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