Recent advances in nanotherapeutic strategies for spinal cord injury repair

Abstract

Spinal cord injury (SCI) is a devastating and complicated condition with no cure available. The initial mechanical trauma is followed by a secondary injury characterized by inflammatory cell infiltration and inhibitory glial scar formation. Due to the limitations posed by the blood–spinal cord barrier, systemic delivery of therapeutics is challenging. Recent development of various nanoscale strategies provides exciting and promising new means of treating SCI by crossing the blood–spinal cord barrier and delivering therapeutics. As such, we discuss different nanomaterial fabrication methods and provide an overview of recent studies where nanomaterials were developed to modulate inflammatory signals, target inhibitory factors in the lesion, and promote axonal regeneration after SCI. We also review emerging areas of research such as optogenetics, immunotherapy and CRISPR-mediated genome editing where nanomaterials can provide synergistic effects in developing novel SCI therapy regimens, as well as current efforts and barriers to clinical translation of nanomaterials.

Introduction

This review article provides a summary of nanomaterial fabrication and therapeutic effects achieved by the nanoscale systems in pre-clinical studies of spinal cord injury (SCI). Specifically, we discuss strategies to 1) attenuate inflammatory response, 2) target inhibitory components, and 3) enhance axonal regeneration. In addition, efforts and barriers to clinical translation of nanomaterials for SCI repair are discussed. The majority of the nanomaterial studies covered in this review is published in the past 10 years. For the purpose of this review, we employ the United Kingdom House of Lords Science and Technology Committee definition of nanoscale, i.e., up to 1000 nm, or <1 μm (“sub-micron”), in size [1] in discussing various nanotechnology strategies developed to enhance SCI repair. Note that this review does not cover nanomaterials developed for delivery of mammalian cells, as these cells are micron-sized [2] and therefore do not fit within our definition of nanoscale therapies. Readers are directed to other excellent reviews specifically for recent advances in cell transplantation therapies developed for SCI repair [[3], [4], [5], [6], [7]].

Spinal cord injury (SCI) is a debilitating condition as patients often suffer from neurological impairments and a reduction in quality of life such as partial or complete paralysis, respiratory distress, and bladder dysfunction [8]. Currently, there are over 280,000 patients with SCI and approximately 17,000 new cases every year in the United States alone [9]. The economic impact of SCI on patients is huge as the estimated lifetime cost of treatment per patient is well over one million dollars and may become over four million dollars depending on the age of the patient and the severity of the injury [10].

Despite advances in medicine, most treatments for SCI are palliative with minimal functional recovery. After diagnosis and initial stabilization, the patient is given a large systemic dose of 30 mg/kg bolus injection followed by a 5.4 mg/kg·h infusion over 24 h of methylprednisolone (MP), a synthetic corticosteroid [11,12]. Studies have demonstrated that early administration of MP within the first 8 h following SCI helps in reducing acute inflammatory responses [13,14]. Improvement in neurologic recovery in patients with MP administration was also shown in the report from the National Acute Spinal Cord Injury Study II. However, current method of MP administration is largely inefficient as the high dose is associated with severe side effects such as pneumonia, infections, corticosteroid myopathy and gastric bleeding [15,16].

The pathological progression of SCI can be categorized into primary and secondary injuries [[17], [18], [19]]. The primary injury is the result of the initial trauma caused by a physical force, which may take the form of compression, contusion, laceration, or stretch [20]. This initial trauma causes damage to the small vessels carrying blood to the tissue, thus creating a hypoxic environment and oxidative stress [21]. The mechanical insult also causes axonal membrane disruption and subsequent release of inhibitory breakdown products of the myelin sheath: Nogo-A, myelin-associated glycoprotein (MAG), and oligodendrocyte myelin glycoprotein (OMgp) [22]. Finally, inflammatory cells such as neutrophils and pro-inflammatory M1 macrophages migrate to the injury site because of increased permeability of the blood-spinal cord barrier [[23], [24], [25], [26]]. These cells release pro-inflammatory cytokines such as interleukin (IL)-1 alpha, IL-1 beta, IL-6, CD95 ligand and tumor necrosis factor (TNF)-alpha and lead to death of many resident neuronal and glial cell populations in the spinal cord [17,24,25,27].

This cascade of biochemical events causes the spread of the injury into adjacent tissue, resulting in the secondary injury [28]. Secondary injury begins a few hours after the initial injury and could last for a few weeks. Distortion in the local microenvironment following SCI causes glial cells, such as astrocytes and microglia, to undergo dramatic genetic and morphological changes, transforming into reactive phenotypes [29]. These cells deposit excessive amounts of neuro-inhibitory chondroitin sulfate proteoglycans (CSPGs) in the extracellular matrix (ECM) [30,31]. This connective tissue around the lesion boundary is known as a glial scar, and it acts to physically block axon regeneration near the lesion site [32]. Studies have shown that bacterial enzyme chondroitinase ABC (ChABC) effectively removes glial scar and promotes axonal regeneration as well as functional recovery after SCI in animal models [[33], [34], [35], [36]]; however, intrathecal ChABC injection without a proper delivery vehicle poses significant limitations in clinical translation because of thermal instability of ChABC at body temperature [37]. Additionally, reactive astrocytes also have been shown to contribute to prolonged disruption of the blood-spinal cord barrier at the lesion periphery, whereas the blood-spinal cord barrier is restored at the epicenter 2–3 weeks after injury [38]. Overall, the presence of CSPG-rich glial scar and the heterogeneity of blood-spinal cord barrier stability pose challenges in successful delivery of therapeutics to the lesion and improvements in patient outcome.

As alluded to earlier, systemic administration of therapeutics, such as MP and ChABC, in both pre-clinical and clinical studies of SCI raises several complications such as high drug dose required to achieve therapeutic effects, high cost of treatment, waste of drugs, and systemic cytotoxicity and side effects [21,39,40]. To overcome these limitations and deliver therapeutics more efficiently, researchers have developed various nanocarriers that allow localized, slow, and sustained delivery of therapeutics at the site of injury [41,42]. These therapeutic nanocarriers can be encapsulated in injectable scaffolds to be directly delivered to the lesion site, or be specifically designed to extravasate the blood-spinal cord barrier and localize into the injured spinal cord tissues [43,44].

Numerous studies suggest that drugs delivered via nanocarriers could achieve similar therapeutic effects at a lower or the same dose in pre-clinical models, both at the cellular and behavioral levels [45]. In some cases, studies have shown improved patient outcomes in nanocarrier-assisted drug delivery approaches compared to the conventional systemic drug administration. In addition to drug delivery, providing sub-cellular nanotopography is also crucial in nerve repair strategies [46]. Yet, there currently exist no clinical trials designed to assess the effects of nanomaterials specifically for SCI repair [47]. Interestingly, no clinical trials exist for microcarriers in SCI applications either. The only microscale materials listed on ClinicalTrials.gov are micro-electrode array implants to control a prosthesis, mainly for peripheral neural injury treatment [48].