Characterization of the hydrogel. To form hydrogels, which are liquid at 4 ℃ and transform into gels at 37 ℃, poloxamer 407 and poloxamer 188 were mixed at different mass ratios. As shown in Fig.1Aa, when the mixture was composed of 25% w/v high poloxamer 407 concentration and 5% w/v low poloxamer (Pol hydrogel), the prepared hydrogel could transform into a gel state at 37 ℃, close to human body temperature. In order to clearly observe the process of Pol at 37 ℃, Evans blue dye was embedded into the Pol hydrogel to form Pol/Eb with blue color. The sol Pol/Eb (4 ℃) was injected into 37 ℃ water, and then the Pol/Eb hydrogel was quickly formed (Fig.1Ab). Furthermore, we investigated the sol-gel transition and injectable behavior of the Pol hydrogel in vivo using the TBI model of Feeney’s weight-drop injury (WDI). The sol pol could transform into a gel pol after 2 min in the wound cavity of a TBI mouse (Fig.1B). The porous structure of the Pol hydrogel was observed by scanning electron microscopy (SEM) (Fig.1C). T1AM at 50 mg/mL contained within the Pol hydrogel did not affect the structure of the hydrogel (Fig.1Ca, b). To test the biodegradation property of this Pol/T hydrogel in vivo, 1, 1′-dioctadecyl–3, 3, 3′, 3′-tetramethylindotricarbocyanine iodide (DiR) was embedded into the Pol hydrogel (Pol/DiR). As shown in Fig.1Da, b, the fluorescence signal was decayed, and the lowest fluorescence intensity was tested 12 hours after injection, which suggested that the Pol hydrogel could biodegrade in the BTE over the 12 hours period.
To further monitor the degradation behavior of the Pol hydrogel in an environment similar to the human TBI environment, we collected the cerebrospinal fluid (CSF) from clinical TBI patients, which have been confirmed to be germfree, and soaked the hydrogel in CSF to verify the biodegradation behavior of Pol in vitro (Fig.1Ea). Pol/Eb was mostly biodegraded by 12 hours (Fig.1Eb), and 89.07 ± 4.38% of T1AM was released from the Pol hydrogel during incubation in CSF within 12 hours (Fig.1F). Taken together, these results demonstrate that the Pol hydrogel could biodegrade and accomplish controlled-release T1AM in a post-traumatic environment.
The safety of materials is the primary problem in clinical application. Pol hydrogels are polymers with good biological safety and are approved by the FDA. In this study, the cytotoxicity of the Pol hydrogel was determined by MTT assay. As shown in fig.S1, the survival rate of cells was higher than 90% when they were incubated for 3, 6, and 12 hours in Pol hydrogel immersion solution, which indicates that the Pol hydrogel had high favorable safety.
The local hypothermia induced by Pol/T hydrogel without leading to the systemic negative effects. T1AM can stimulate TAAR1-expressing cells to produce cAMP, which induces profound hypothermia within minutes (28). Neurons and astrocytes in the brain both express TAAR1 (30). Therefore, we speculated that local administration of free T1AM in the brain would achieve local hypothermia (Fig.2A). To investigate the effect of hypothermia induced by the Pol/T hydrogel, we assessed the temperature variation in the brain and body. Rectal temperature was representative of the body temperature. Intraperitoneal injection of T1AM has been reported to induce a rapid dose-dependent drop in body and brain temperature (28). The dose of free T1AM and that of T1AM (from 50 mg/kg to 100 mg/kg) was evaluated for safety by survival curve of the mice through intraperitoneal injection. As shown in fig.S2, no mice died with the T1AM concentration of 50 mg/kg (consistent with reporting by Thomas S Scanlan), and some mice died with the rise of T1AM concentration beyond 50 mg/kg. When the dose of T1AM reached 100 mg/kg, the mice became inactive, assumed a slightly hunched-back posture and developed ptosis (drooping eyelids), and some mice even died within 1 hour of the intraperitoneal injection (see Movie S1 in supporting information). Nevertheless, when we used Pol as a T1AM carrier to deliver 100 mg/kg T1AM directly to the brain, the behavior of the treated mice was similar to that of normal mice, and the side effects of whole-body cooling were avoided (see Movie S2 in supporting information). According to our experiments and data reported by Thomas S Scanlan (28), 50 mg/kg T1AM was used for all subsequent experiments.
As shown in Fig.2B, the TBI mice injected intraperitoneally with free T1AM had a body and brain temperature 4–7 °C lower than normal, and the low temperature was only maintained for approximately 4 hours in both the brain and body. The brain temperature decreased 2 ℃ from the normal brain temperature for less than 2 hours, while the body temperature was unchanged only in the Pol hydrogel injection group, which suggests that the transition of a 4 ℃ sol to a gel led to a cooler brain temperature. Compared with the other treatments, the brain temperature of the TBI mice treated with the Pol/T hydrogel could be maintained at 30.25±2.25 °C for 12 hours and the body temperature did not noticeably change (36.80±1.75 °C), which suggests that T1AM embedded in the Pol hydrogel was released slowly over time to maintain a cooler brain temperature.
The negative systemic effect of whole-body hypothermia limits its clinical application, but local hypothermia can effectively avoid this negative effect. In this study, heart rate, systolic blood pressure, respiratory rate and oxygen saturation were monitored for 12 hours after the trauma. As shown in Fig.2C, each vital sign showed a similar trend in temperature variation. Free T1AM has already been confirmed to have a negative effect on the heart and causes a decline in cardiac output (28) and thus we observed a significant decline in heart rate and systolic blood pressure in the free T1AM-treated group. As the temperature returned to normal, the heart rate and systolic blood pressure suddenly increased and even exhibited a rebound effect that increased these two vital signs beyond the normal level. Similarly, the respiratory rate and oxygen saturation were significantly reduced with a sharp drop in body temperature as a result of free T1AM. However, when we used the Pol/T hydrogel to induce local hypothermia in the brain, these vital signs did not fluctuate appreciably (Fig.2Ca, b, c, d). These phenomena showed that the Pol/T hydrogel even induced a very low temperature of 28.9±0.9 °C in the brain, but this did not result in any negative behavior in the mice. The above results demonstrated that the Pol/T hydrogel could induce effective and timely local hypothermia after TBI without the side effects.
Effect of the local hypothermia induced by Pol/T hydrogel on trauma-induced neuronal injury. Many studies have reported that therapeutic hypothermia exhibits a long-term neuroprotective effect after TBI (31, 32). Our aim was to test whether the Pol/T hydrogel could lead to local hypothermia and protect neurons from damage induced by TBI. H&E and Nissl staining were performed to investigate damaged neurons in the ipsilateral cortex surrounding the injury site 7 days after TBI (Fig.3A a, b). Nissl staining and H&E staining revealed the lesion volume after TBI (Fig.3B). The contusion volume was 14.00±2.67 mm3 in the TBI control group. In TBI mice that received free T1AM treatment, the contusion volume was reduced to 9.29±2.14 mm3. The contusion volume of TBI mice was reduced to 12.82±1.32 mm3 in the Pol hydrogel treatment group. More importantly, the TBI mice treatment with the Pol/T hydrogel had the smallest contusion volume (6.57±1.14 mm3) of all the treatments. From Fig3.Ab, evident damage was shown in the TBI control group, with loss of Nissl intensity and a decrease in neuron cell number, which resulted from shrinkage necrosis. The neuronal cell number was less in the Pol hydrogel group because it only cooled the brain to 33.75 ± 0.65 ℃ for 2 hours, which resulted in less of a neuroprotective effect after TBI. Compared with the Pol hydrogel group, the severity of neuronal degeneration was alleviated in the free T1AM group, which suggests that T1AM-induced hypothermia protected the neurons after TBI injury. Moreover, the strongest neuroprotective effect was observed in the Pol/T hydrogel group, which demonstrated that local hypothermia lasted for more than 12 hours and effectively improved the protective effect on neurons.
To further validate the neuroprotective effect of the Pol/T hydrogel, apoptosis and necrosis of neurons following TBI were tested. As shown in Fig.3C, D, more TUNEL-positive cells were found in the TBI control group, which demonstrated substantial overall cell death post trauma. We observed that the Pol/T hydrogel group reduced the overall number of TUNEL-positive cells more effectively than free T1AM because the Pol/T hydrogel induced local hypothermia for 12 hours.
It has been shown that Bcl–2 and Bax are a pair of antagonistic factors, where the former has an anti-apoptotic effect, while the latter can promote apoptosis. Bcl–2/Bax regulates mitochondrial function and the release of apoptosis-related proteins (33). We used immunohistochemistry (IHC) to determine the expression of Bcl–2 and Bax on the edge of the injured area. IHC staining showed that the expression of Bcl–2 in mice given the Pol/T hydrogel treatment was obviously greater than that in mice that were given other treatments, but the expression of Bax was obviously weaker than that in mice given other treatments (Fig.3Ea, b). The ratio of anti-apoptosis-related Bcl–2/Bax in the brain tissue of the mice was increased under Pol/T hydrogel treatment (Fig.3F). The larger the ratio, the better the cell survival, and the smaller the ratio, the more likely apoptosis will be induced. This result indicates that the Pol/T hydrogel could inhibit the expression of Bax and upregulate the expression of Bcl–2 after TBI, thereby reducing apoptosis and acting as a neuroprotective agent. Altogether, these results suggest that the Pol/T hydrogel induced local hypothermia and effectively protected neurons from damage caused by TBI.
Effect of the local hypothermia induced by Pol/T hydrogel on BBB permeability and brain edema after TBI. After TBI, brain tissue exhibits a massive disruption in the BBB, which is followed by brain edema and exacerbates the devastating consequences of the final outcome of TBI (34,35). Hypothermia reduces the extent of BBB disruption and reduces the volume of brain edema (10). Therefore, to further explore whether the Pol/T hydrogel that induces local hypothermia has a positive effect on the destruction of the BBB, the BBB permeability following TBI was estimated by the extravasation of Evans blue dye (36). Compared with the TBI-control group, BBB permeability was significantly reduced in the ipsilateral hemisphere in the Pol hydrogel, free T1AM and Pol/T hydrogel groups (Fig.4A, B). Moreover, Pol/T hydrogel treatment obviously inhibited Evans blue leakage and restricted the leakage area to the primary lesion site, which suggests that the Pol/T hydrogel induced local hypothermia and effectively maintained BBB integrity.
Matrix metalloproteinases (MMPs) are also associated with increased BBB permeability, which is indicative the hemorrhagic potential and the extent of brain edema (31). MMP–9 levels were investigated in cerebral cortex (Fig.4C, D). In the sham group, the number of MMP–9-positive cells was very small, but the TBI mice showed a gradually increasing expression level. Compared with the TBI control group, MMP–9 expression was decreased in the Pol hydrogel, free T1AM and Pol/T hydrogel groups. The lowest expression of MMP–9 was observed in the Pol/T hydrogel group, which demonstrated that the Pol/T hydrogel effectively induced local hypothermia and prevented brain impairment after TBI injury.
To further explore the ability of the Pol/T hydrogel to effectively reduce the spread of secondary damage, brain edema was detected by T2-weighted and diffusion-weighted imaging (DWI) of MRI at 12 hours after percussion injury. As shown in Fig.4E, F, brain edema centered around the percussion site was evident in the TBI control group. Compared with the TBI control group, the high-intensity area was restricted to a relatively small region in the Pol hydrogel, free T1AM and Pol/T hydrogel groups. The hyperintensity volume was 13.39 ± 2.91 mm3 in the TBI control group. In TBI mice that received free T1AM treatment, the hyperintensity volume was reduced to 8.9 ± 1.55 mm3. The contusion volume in the TBI mice was reduced to 11.96 ± 1.60 mm3 in the Pol hydrogel treatment group. This phenomenon suggests that the Pol hydrogel and T1AM showed different degrees of neuroprotection. Most importantly, the volume of edema surrounding the damage was the lowest (5.94 ± 1.70 mm3) in the Pol/T hydrogel group. We also assessed brain edema in different groups after 12 hours by measuring brain water content and these results were consistent with the result of T2-weighted MR (fig.S3).
The dispersion of water molecules on the surface of the cerebral cortex was further evaluated by measuring the apparent diffusion coefficient (ADC) values of DWI to determine the edema of brain tissue, as ADC values can reflect the variation trend of edema with high sensitivity (37). The reduction of ADC values reflected the cytotoxic edema gradually occurred and aggravated as the release of neurotoxic substances.(38) The ADC values of mice treated with the Pol hydrogel (0.53 ± 0.10 × 10–3 mm2/sec) and free T1AM (0.57 ± 0.11 × 10–3 mm2/sec) were all higher than those in the TBI control group (0.43 ± 0.15 × 10–3 mm2/sec). Consistent with the trend of hyperintensity volume in T2-weighted images, the ADC values were higher (0.72 ± 0.10 × 10–3 mm2/sec) in mice treated with the Pol/T hydrogel (Fig.4G). Collectively, the Pol/T hydrogel effectively reduced brain edema and protected BBB integrity.
Anti-inflammatory effect of the local hypothermia induced by Pol/T hydrogel. The inflammatory response plays important roles in secondary damage after TBI injury (39). The astrocytes and microglia are quickly activated and the cell bodies are enlarged and branched after brain tissue is damaged. Ionized calcium binding adapter molecule (Iba–1) is the indicator of activated microglia and glial fibrillary acidic protein (GFAP) is the indicator of the activity of the astrocytes (46). Their expressions were detected by IHC. As shown in Fig.5Aa, b, c, d, we observed that the expression of Iba–1 and GFAP was significantly increased on the edge of the injured area in mice of the TBI control group, which was consistent with the results described above. The expression of Iba–1 and GFAP was decreased in the Pol hydrogel, free T1AM and Pol/T hydrogel groups. Importantly, either Iba–1 or GFAP expression surrounding the TBI injury site was the lowest in the Pol/T hydrogel group compared with the other treatment groups. The results show that local hypothermia induced by the Pol/T hydrogel inhibited the chronic secondary astrogliosis inflammatory response at the TBI-injured site.
TBI induced an acute increase in proinflammatory cytokine (TNFα, IL–1β and IL–6) and chemokine (CXCL1) production throughout the brain (Fig.5Ba, b, c, d) (39). According to enzyme-linked immunosorbent assay (ELISA) analysis of protein levels in the brain tissue 12 hours post trauma, durable and stable local hypothermia induced by Pol/T hydrogel could more significantly decrease inflammatory factor expression than transitory whole-body hypothermia via free T1AM. Taken together, the method of hypothermia induced by the Pol/T hydrogel effectively inhibited the inflammatory response after TBI.
Promotion in functional recovery after TBI. The behavioral tests were performed after the hypothermia treatment with the Pol/T hydrogel on 21 days after TBI. The Morris water maze test was used to assess the long-term learning and memory abilities of TBI mice (Fig.6A-G). As shown in Fig.6B, C, during first 5 days of training, the mice treated with the Pol/T hydrogel exhibited comparable improvement in learning capacity, as confirmed by decreasing the searching time for the platform and distance to the platform. On the last day, the platform was removed and the memory capacity of each mouse was evaluated. The mice treated with the Pol/T hydrogel used the shortest distance to the original position of platform (Fig.6D). Correspondingly, the time spent in the target quadrant, the duration time in the platform area and the platform crossing frequency were increased in the Pol/T hydrogel-treated TBI mice (Fig.6E-G). The modified neurological severity score (mNSS) was adopted to evaluate motor and sensory function as well as reflex and balance after TBI. A high score indicated more serious damage in the TBI mice (40). Compared with the TBI and Pol groups, the mNSS scores were lower than in the free T1AM- and Pol/T hydrogel-treated TBI mice (Fig.6H). The Pol/T hydrogel-treated mice had the lowest mNSS score compared with the other groups except the sham group, which demonstrates that the Pol/T hydrogel-treated TBI mice showed enhanced neurological recovery. The hanging-wire-grip test was used to assess motor strength after Pol/T hydrogel treatment. As shown in Fig.6I, Pol/T hydrogel-treated TBI mice exhibited maximum endurance compared with mice in the other treatment groups, which indicates that the motor strength of TBI mice was enhanced after Pol/T hydrogel treatment. These results suggest that injection of the Pol/T hydrogel into TBI mice could efficiently enhance the behavioral ability of these animals