[ARTICLE] Therapeutic Potential of Neurotrophins for Repair After Brain Injury: A Helping Hand From Biomaterials – Full Text

Stroke remains the leading cause of long-term disability with limited options available to aid in recovery. Significant effort has been made to try and minimize neuronal damage following stroke with use of neuroprotective agents, however, these treatments have yet to show clinical efficacy. Regenerative interventions have since become of huge interest as they provide the potential to restore damaged neural tissue without being limited by a narrow therapeutic window. Neurotrophins, such as brain-derived neurotrophic factor (BDNF), and their high affinity receptors are actively produced throughout the brain and are involved in regulating neuronal activity and normal day-to-day function. Furthermore, neurotrophins are known to play a significant role in both protection and recovery of function following neurodegenerative diseases such as stroke and traumatic brain injury (TBI). Unfortunately, exogenous administration of these neurotrophins is limited by a lack of blood-brain-barrier (BBB) permeability, poor half-life, and rapid degradation. Therefore, we have focused this review on approaches that provide a direct and sustained neurotrophic support using pharmacological therapies and mimetics, physical activity, and potential drug delivery systems, including discussion around advantages and limitations for use of each of these systems. Finally, we discuss future directions of biomaterial drug-delivery systems, including the incorporation of heparan sulfate (HS) in conjunction with neurotrophin-based interventions.


Stroke consistently remains a leading cause of death and disability worldwide (Mozaffarian et al., 2015). While stroke preferentially affects older people, stroke can affect anyone of any age, race, or gender (Feigin et al., 2014). In addition, ethnic disparities are known to exist, for instance, New Zealand Māori, and Pacific Island populations experience a stroke at a much younger age than Europeans (Feigin et al., 2006). With the global population continuing to age, the number of people suffering from a stroke and subsequently living with a lasting disability is also expected to rise. As a result, there is an urgent and unmet need to find treatment options to aid in improving recovery of lost functions.

The central nervous system CNS has a limited capacity to regenerate, which is one of the main causes for why stroke patients recover poorly. In addition, there is a lack of spontaneous recovery seen after stroke, leading to a large personal, and societal burden (Mozaffarian et al., 2015). This burden has led research into investigating neural mechanisms of regeneration and repair, in the hope to restore and improve lost functions following stroke. Depending on the neuronal area affected by the stroke (i.e., motor, speech, or language centers), rehabilitation, mental practice, and music therapies have all been shown to increase the capacity to recover after a stroke (Johansson, 2011). However, in patients who have had a large stroke, rehabilitation is less effective in facilitating an improvement in functional recovery (Johansson, 2011).

Significant work has shown that exogenous administration of GFs can help facilitate the repair of injured CNS tissue via their ability to regulate neuronal growth and survival (Connor and Dragunow, 1998Sofroniew et al., 2001Berretta et al., 2014). In particular, BDNF has been highlighted as being a key regulator of rehabilitation-induced recovery after stroke (Ploughman et al., 20072009). Moreover, activity-driven increases in BDNF have also been shown to promote motor recovery after stroke (Fritsch et al., 2010Clarkson et al., 2011).

The regenerative capabilities of neurotrophin-mediated interventions have already been observed in preclinical models of neurodegenerative diseases. Specifically, systemic administration of BDNF, NGF, and NT-3 have been reported to enhance neurite outgrowth, neurogenesis, and functional recovery in rodent models of stroke (Grill et al., 1997Jakeman et al., 1998Ramer et al., 2000Winkler et al., 2000Schäbitz et al., 20042007). However, the translation of such treatments into a clinical setting has been challenged by poor BBB permeability, off-target effects on the PNS, and short half-life (Chan et al., 2017). Approximately 98 percent of all compounds targeting the CNS have failed to cross the BBB (Pardridge, 2005), creating a need for alternative approaches. As a novel solution to this issue, various biomaterials have been engineered to provide an effective, and sustained drug-delivery system to the injured brain. These systems allow stem cells and/or small drug molecules including neurotrophins to bypass the BBB (Aizawa et al., 2008) and be delivered directly to the site of injury.

Heparan sulfate, as the GAG cleaved from its protein backbone, is thought to provide a novel approach for the delivery of neurotrophins to the injured CNS. HS plays a critical role in coordinating GFs and mediating their biological potential (Xu and Esko, 2014). This highly sulfated biopolymer is ideally suited as a component of a biomaterial therapeutic for brain injury. It is chemically stable, physiologically tolerated, has a demonstrated long-term benefit to wound repair by tunable protein-interactions and has been demonstrated to tolerate the level of γ-irradiation treatment required to sterilize sufficiently medical-device-like materials (Smith et al., 2018). In addition, HS has demonstrated a remarkable propensity to stabilize short-lived GF’s in what appears to be a selective fashion (Wang et al., 2014Wijesinghe et al., 2017).

The effects of BDNF and other GFs have previously been investigated in various stroke models and have shown strong regenerative potential (Berretta et al., 2014). In general, BDNF is believed to have a beneficial effect on stroke recovery via several mechanisms: protection against acute ischemic injury (Schäbitz et al., 2007), increased angiogenesis (Kermani and Hempstead, 2007), neurogenesis (Schäbitz et al., 2007), and neural repair (Mamounas et al., 2000) as well as enhanced synaptic plasticity (Waterhouse and Xu, 2009Clarkson et al., 2011). Whilst the use of BDNF and other GFs to promote neuroprotection and minimize the spread of damage during the acute phase of injury has been extensively studied (Pardridge, 2005Wu, 2005Cai et al., 2014), this work will not be reviewed here. This review instead summarizes past experimental evidence that highlights how neurotrophins have shown potential as a delayed treatment option for aiding in repair and regeneration of neural tissue for stroke and other neurodegenerative diseases. In addition, we discuss novel biomaterial delivery systems that have been utilized to enhance the delivery of neurotrophins to the CNS, as utilizing such delivery systems have resulted in a further improvement in functional recovery when treatment is delayed by days to weeks post injury.

Stroke Pathophysiology and Mechanisms of Endogenous Repair

Stroke occurs when blood flow to the brain is either obstructed by an occlusion (ischemic) or following rupturing (hemorrhagic) of a cerebral blood vessel. Lack of oxygen and glucose, and a build-up of toxic by-products, cause the areas of the brain deprived of blood flow to undergo a chain of pathological events (ischemic cascade), ultimately leading to cell death, and loss of function associated with that region of the brain (Dirnagl et al., 1999). As time passes, a lack of reperfusion to the penumbra (regions surrounding the core infarct) results in further expansion until the stroke is fully formed. Whilst neuroprotective agents have shown preclinical success at restoring reperfusion and minimizing cellular damage, they are only effective when administered within the first few hours following stroke onset and have failed to translate into clinical use, with the exception of thrombolytic compounds (Moretti et al., 2015). As a result, much needed stroke research unraveling the mechanisms associated with neuroregeneration and repair in the days to weeks following a stroke have highlighted a number of targetable treatment options, some of which have already resulted in the establishment of clinical trials.

Whilst the CNS shows limited capacity for regeneration and repair, under pathological conditions such as stroke, axonal sprouting, endogenous neurogenesis, and spontaneous functional recovery appear to be enhanced (Carmichael et al., 2001Benowitz and Carmichael, 2010). In addition, Stroemer et al. (1995) reported that both GAP-43 and synaptophysin, two proteins involved with neurite growth and synaptogenesis; are upregulated throughout the neocortex 2 weeks following a focal infarct in rats. Further, these researchers show a positive correlation between improved locomotor function and stroke-induced elevations in GAP-43 and synaptophysin. It has since been established that enhancing neuronal connections throughout the ischemic brain via pharmacological interventions correlates with an improvement in functional motor recovery after stroke (Lee et al., 2004Overman et al., 2012Clarkson et al., 2013Cook et al., 2017).

Neurogenesis occurs continuously in two areas of the healthy adult CNS: the SVZ of the lateral ventricle, and the subgranular zone (SGZ) of the hippocampal dentate gyrus. In these areas, self-renewing multipotent cells known as NSPCs differentiate into both neuronal, and glial cells (Taupin, 2006). Under physiological conditions these NSPCs migrate to the dentate gyrus or olfactory bulb to replenish the continuously dying granule cells or olfactory neurons, respectively. Following acute injury to the CNS, these resident neural progenitors may also be induced in an attempt to replace damaged neurons (Zhang et al., 2005Greenberg, 2007). Supporting this notion, many animal models of stroke have reported the upregulation of SVZ and SGZ neurogenesis, peaking around 7–10 days post-stroke (Iwai et al., 20022003Parent et al., 2002Carmichael et al., 2005). Furthermore, stroke-induced local changes to the microvasculature has been found to attract neuroblasts generated within the SVZ and facilitate their migration to the ischemic boundary zone (peri-infarct), an area lying lateral to the stroke (Carmichael et al., 2005Yamashita et al., 2006Thored et al., 2007).

Interestingly, preclinical research indicates NSPCs offer the majority of their regenerative potential through bystander effects, specifically by providing a rich source of GFs to the injured tissue rather than contributing to structural reconstruction (Chen et al., 2000). Supporting this, preclinical work has shown that MSCs facilitate an improvement in functional recovery, even though they fail to differentiate into mature neurons or glia when grafted into the ischemic boundary zone of rats exposed to a MCAo (Chen et al., 2000). Moreover, at least a portion of the therapeutic potential of MSCs has been ascribed to their ability to enhance endogenous neurogenesis and protect newborn cells from deleterious environments (Yoo et al., 2008), both of which are mechanisms mediated by neurotrophin signaling pathways. Although explicit evidence directly linking enhanced neurogenesis and functional recovery is sparse, a direct causal relationship seems probable with almost all neuroregenerative agents that improve neurological function following stroke also potentiating neurogenesis (Zhang et al., 2001Wang et al., 2004Schäbitz et al., 2007Cook et al., 2017). Of note, HS has also been demonstrated to support the proliferation (without differentiation) of stem cells ex vivo (Dombrowski et al., 2009Wijesinghe et al., 2017).

In addition to changes in neurogenesis, angiogenesis and axonal sprouting, work over the past few years has highlighted a chronic imbalance in brain excitability following stroke (Clarkson, 2012). Specifically, stroke has been reported to weaken transcallosal inhibition from the ipsilesional hemisphere onto the intact, contralesional hemisphere (Murase et al., 2004Duque et al., 2005). This further exacerbates an increase in tonic GABAergic inhibition onto the stroked hemisphere, limiting its potential for cortical plasticity, and spontaneous recovery after stroke (Clarkson et al., 2010). Indeed, restoring neuronal excitability in the ipsilesional stroke hemisphere via increasing AMPA activity, and/or dampening tonic GABAergic inhibition has shown great promise to enhance recovery of lost function (Clarkson et al., 20102015Orfila et al., 2017). Whilst the above evidence collectively supports an intrinsic capability of the ischemic brain to regenerate and repair itself, it must be reiterated that these mechanisms alone are most often insufficient to cause a complete reversal of the functional impairment (Yamashita and Abe, 2012). Furthermore, age-related decreases in neurogenesis (Cooper-Kuhn et al., 2004Kernie and Parent, 2010) and angiogenesis (Sonntag et al., 1997) have been reported, making it even harder for endogenous repair mechanisms to overcome damage and promote recovery. Accordingly, regenerative interventions have begun to target the potentiation of these endogenous mechanisms to try and maximize functional recovery in animal models of stroke and other neurodegenerative conditions such as SCI and Alzheimer’s disease (Wang et al., 2004Piantino et al., 2006Schäbitz et al., 2007Weishaupt et al., 2014Choi et al., 2018). This has been demonstrated in the case of the endogenous stimulation of key GF’s by addition of HS on a biomaterial support to the wound site (Murali et al., 2013).

Neurotrophins for Repair and Regeneration

Neurotrophins are the predominant mediators of neuronal survival and regeneration throughout the CNS, making them of particular interest in neuroregenerative research. The neurotrophin family consists of several main GFs, including NGF, BDNF, and NT-3. All neurotrophins are initially synthesized as precursor proteins, known as pro-neurotrophins. These pro-neurotrophins can then be cleaved intracellularly by furin or proconvertases, or extracellularly by metalloproteases and plasmin, to form stable mature neurotrophins. Whilst mature neurotrophins selectively bind to their respective Trk to exert neurotrophic effects, pro-neurotrophins have been found to conduct somewhat opposing, pro-apoptotic effects through the p75NTR/sortilin receptor (see Figure 1: further information on neurotrophin signaling pathways can be found below; (Nykjaer et al., 2004Teng et al., 2005). This finding has left the scientific world moving away from the passive function of these pro-domains, and has brought a new level of complexity to the role and function of neurotrophin signaling in the CNS (Zanin et al., 2017).

Figure 1. Neurotrophin signaling through the p75NTR and Trk receptors. This diagram depicts the major intracellular signaling pathways associated through each neurotrophin receptor. Each Trk receptor isoform binds mature neurotrophins and acts through three predominant pathways. Activation of PLC-γ1 results in PKC-mediated promotion of synaptic plasticity. Activation of Ras initiates MAPK-mediated promotion of neuronal regeneration and growth. Activation of PI3-K results in activation of Akt and promotion of NF-κB-mediated cell survival. Each of these pathways are also known to regulate genetic transcription, further promoting pro-survival, and regenerative gene expression. The p75NTR receptor also regulates three main pathways. When a mature neurotrophin binds to an isolated p75NTR NF-κB-mediated cell survival is promoted. If the p75NTR is co-expressed with the sortilin receptor, pro-neurotrophins can bind, and cause activation of JNK-c-Jun mediated cell death and degeneration. A receptor complex consisting of Nogo, p75NTR, and Lingo1 can bind both pro- and mature-neurotrophins to alter neurite outgrowth in a RhoA-dependent manner. NGF, nerve growth factor; BDNF, brain-derived neurotrophic factor; NT-3, neurotrophin-3; Trk, tropomyosin receptor kinase; mNT, mature neurotrophin; ProNT, proneurotrophin; p75NTR, pan neurotrophin receptor 75; PLCy1, phospholipase C gamma one; PKC, protein kinase C; MAPK, mitogen-activated protein kinase; TFs, transcription factors; PI3-K, phosphoinositide 3-kinase; NF-kB, nuclear factor kappa-light-chain-enhancer of activated B cells; JNK, c-Jun N-terminal kinase



Continue —> Frontiers | Therapeutic Potential of Neurotrophins for Repair After Brain Injury: A Helping Hand From Biomaterials | Neuroscience

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