Development of Calpain-specific Inactivators by Screening of Positional Scanning Epoxide Libraries

Non-Canonical Amino Acids in Analyses of Protease Structure and Function

All known organisms encode 20 canonical amino acids by base triplets in the genetic code. The cellular translational machinery produces proteins consisting mainly of these amino acids. Several hundred natural amino acids serve important functions in metabolism, as scaffold molecules, and in signal transduction. New side chains are generated mainly by post-translational modifications, while others have altered backbones, such as the β- or γ-amino acids, or they undergo stereochemical inversion, e.g., in the case of D-amino acids. In addition, the number of non-canonical amino acids has further increased by chemical syntheses. Since many of these non-canonical amino acids confer resistance to proteolytic degradation, they are potential protease inhibitors and tools for specificity profiling studies in substrate optimization and enzyme inhibition. Other applications include in vitro and in vivo studies of enzyme kinetics, molecular interactions and bioimaging, to name a few. Amino acids with bio-orthogonal labels are particularly attractive, enabling various cross-link and click reactions for structure-functional studies. Here, we cover the latest developments in protease research with non-canonical amino acids, which opens up a great potential, e.g., for novel prodrugs activated by proteases or for other pharmaceutical compounds, some of which have already reached the clinical trial stage.

Synthetic Approaches of Epoxysuccinate Chemical Probes

Synthetic chemical probes are powerful tools for investigating biological processes. They are particularly useful for proteomic studies such as activity-based protein profiling (ABPP). These chemical methods initially used mimics of natural substrates. As the techniques gained prominence, more and more elaborate chemical probes with increased specificity towards given enzyme/protein families and amenability to various reaction conditions were used. Among the chemical probes, peptidyl-epoxysuccinates represent one of the first types of compounds used to investigate the activity of the cysteine protease papain-like family of enzymes. Structurally derived from the natural substrate, a wide body of inhibitors and activity- or affinity-based probes bearing the electrophilic oxirane unit for covalent labeling of active enzymes now exists. Herein, we review the literature regarding the synthetic approaches to epoxysuccinate-based chemical probes together with their reported applications, from biological chemistry and inhibition studies to supramolecular chemistry and the formation of protein arrays.

De novo design of tyrosine and serine kinase-driven protein switches

Kinases play central roles in signaling cascades, relaying information from the outside to the inside of mammalian cells. De novo designed protein switches capable of interfacing with tyrosine kinase signaling pathways would open new avenues for controlling cellular behavior, but, so far, no such systems have been described. Here we describe the de novo design of two classes of protein switch that link phosphorylation by tyrosine and serine kinases to protein-protein association. In the first class, protein-protein association is required for phosphorylation by the kinase, while in the second class, kinase activity drives protein-protein association. We design systems that couple protein binding to kinase activity on the immunoreceptor tyrosine-based activation motif central to T-cell signaling, and kinase activity to reconstitution of green fluorescent protein fluorescence from fragments and the inhibition of the protease calpain. The designed switches are reversible and function in vitro and in cells with up to 40-fold activation of switching by phosphorylation.

Calpains for dummies: What you need to know about the calpain family

This review was written in memory of our late friend, Dr. Hiroyuki Sorimachi, who, following the steps of his mentor Koichi Suzuki, a pioneer in calpain research, has made tremendous contributions to the field. During his career, Hiro also wrote several reviews on calpain, the last of which, published in 2016, was comprehensive. In this manuscript, we decided to put together a review with the basic information a novice may need to know about calpains. We also tried to avoid similarities with previous reviews and reported the most significant new findings, at the same time highlighting Hiro's contributions to the field. The review will cover a short history of calpain discovery, the presentation of the family, the life of calpain from transcription to activity, human diseases caused by calpain mutations and therapeutic perspectives.

Insertion sequence 1 from calpain-3 is functional in calpain-2 as an internal propeptide

Calpains are intracellular, calcium-activated cysteine proteases. Calpain-3 is abundant in skeletal muscle, where its mutation-induced loss of function causes limb-girdle muscular dystrophy type 2A. Unlike the small subunit-containing calpain-1 and -2, the calpain-3 isoform homodimerizes through pairing of its C-terminal penta-EF-hand domain. It also has two unique insertion sequences (ISs) not found in the other calpains: IS1 within calpain-3's protease core and IS2 just prior to the penta-EF-hand domain. Production of either native or recombinant full-length calpain-3 to characterize the function of these ISs is challenging. Therefore, here we used recombinant rat calpain-2 as a stable surrogate and inserted IS1 into its equivalent position in the protease core. As it does in calpain-3, IS1 occupied the catalytic cleft and restricted the enzyme's access to substrate and inhibitors. Following activation by Ca²⁺, IS1 was rapidly cleaved by intramolecular autolysis, permitting the enzyme to freely accept substrate and inhibitors. The surrogate remained functional until extensive intermolecular autoproteolysis inactivated the enzyme, as is typical of calpain-2. Although the small-molecule inhibitors E-64 and leupeptin limited intermolecular autolysis of the surrogate, they did not block the initial intramolecular cleavage of IS1, establishing its role as a propeptide. Surprisingly, the large-molecule calpain inhibitor, calpastatin, completely blocked enzyme activity, even with IS1 intact. We suggest that calpastatin is large enough to oust IS1 from the catalytic cleft and take its place. We propose an explanation for why calpastatin can inhibit calpain-2 bearing the IS1 insertion but cannot inhibit WT calpain-3.

Calpain research for drug discovery: challenges and potential

Calpains are a family of proteases that were scientifically recognized earlier than proteasomes and caspases, but remain enigmatic. However, they are known to participate in a multitude of physiological and pathological processes, performing 'limited proteolysis' whereby they do not destroy but rather modulate the functions of their substrates. Calpains are therefore referred to as 'modulator proteases'. Multidisciplinary research on calpains has begun to elucidate their involvement in pathophysiological mechanisms. Therapeutic strategies targeting malfunctions of calpains have been developed, driven primarily by improvements in the specificity and bioavailability of calpain inhibitors. Here, we review the calpain superfamily and calpain-related disorders, and discuss emerging calpain-targeted therapeutic strategies.

Neuroprotective effect of synthetic chalcone derivatives as competitive dual inhibitors against μ-calpain and cathepsin B through the downregulation of tau phosphorylation and insoluble Aβ peptide formation

A series of chalcone derivatives were synthesized and evaluated for their μ-calpain and cathepsin B inhibitory activities. Among the tested chalcone derivatives, two compounds, 7 and 11, showed potent inhibitory activities against μ-calpain and cathepsin B and were selected for further evaluation. Compounds 7 and 11 showed enzyme inhibitory activities at the cellular level and displayed neuroprotective effects against oxidative stress-induced apoptosis in SH-SY5Y cells, a human neuroblastoma cell line. Moreover, compounds 7 and 11 reduced p25 formation, tau phosphorylation and insoluble Aβ peptide formation. Enzyme kinetic experiments and docking studies revealed that compounds 7 and 11 competitively inhibited both μ-calpain and cathepsin B enzymes.

Cysteine proteases as therapeutic targets: does selectivity matter? A systematic review of calpain and cathepsin inhibitors

Cysteine proteases continue to provide validated targets for treatment of human diseases. In neurodegenerative disorders, multiple cysteine proteases provide targets for enzyme inhibitors, notably caspases, calpains, and cathepsins. The reactive, active-site cysteine provides specificity for many inhibitor designs over other families of proteases, such as aspartate and serine; however, a) inhibitor strategies often use covalent enzyme modification, and b) obtaining selectivity within families of cysteine proteases and their isozymes is problematic. This review provides a general update on strategies for cysteine protease inhibitor design and a focus on cathepsin B and calpain 1 as drug targets for neurodegenerative disorders; the latter focus providing an interesting query for the contemporary assumptions that irreversible, covalent protein modification and low selectivity are anathema to therapeutic safety and efficacy.

Mechanism of action of thalassospiramides, a new class of calpain inhibitors

Thalassospiramides comprise a large family of lipopeptide natural products produced by Thalassospira and Tistrella marine bacteria. Here we provide further evidence of their nanomolar inhibitory activity against the human calpain 1 protease. Analysis of structure-activity relationship data supported our hypothesis that the rigid 12-membered ring containing an α,β-unsaturated carbonyl moiety is the pharmacologically active functional group, in contrast to classic electrophilic “warheads” in known calpain inhibitors. Using a combination of chemical modifications, mass spectrometric techniques, site-directed mutagenesis, and molecular modeling, we show the covalent binding of thalassospiramide's α,β-unsaturated carbonyl moiety to the thiol group of calpain's catalytic Cys115 residue by a Michael 1,4-addition reaction. As nanomolar calpain inhibitors with promising selectivity and low toxicity from natural sources are rare, we consider thalassospiramides as promising drug leads.

Neuroprotective strategies against calpain-mediated neurodegeneration

Calpains are calcium-dependent proteolytic enzymes that have deleterious effects on neurons upon their pathological over-activation. According to the results of numerous studies to date, there is no doubt that abnormal calpain activation triggers activation and progression of apoptotic processes in neurodegeneration, leading to neuronal death. Thus, it is very crucial to unravel all the aspects of calpain-mediated neurodegeneration in order to protect neurons through eliminating or at least minimizing its lethal effects. Protecting neurons against calpain-activated apoptosis basically requires developing effective, reliable, and most importantly, therapeutically applicable approaches to succeed. From this aspect, the most significant studies focusing on preventing calpain-mediated neurodegeneration include blocking the N-methyl-d-aspartate (NMDA)-type glutamate receptor activities, which are closely related to calpain activation; directly inhibiting calpain itself via intrinsic or synthetic calpain inhibitors, or inhibiting its downstream processes; and utilizing the neuroprotectant steroid hormone estrogen and its receptors. In this review, the most remarkable neuroprotective strategies for calpain-mediated neurodegeneration are categorized and summarized with respect to their advantages and disadvantages over one another, in terms of their efficiency and applicability as a therapeutic regimen in the treatment of neurodegenerative diseases.

An updated patent review of calpain inhibitors (2012 - 2014)

INTRODUCTION

Calpain is a family of cysteine proteases found in eukaryotes and a few bacteria. There is considerable interest in the search for calpain inhibitors because the enzyme has been implicated in several diseases including ocular disorders, neurodegenerative disorders, metabolic disorders and cancer.

AREAS COVERED

An overview of calpain inhibitors disclosed between 2012 and 2014 is presented. Among these are epoxysuccinates, dipeptide imaging agents, macrocyclic inhibitors, α-helical peptidomimetic inhibitors, carboxamides, 5-azolones and α-mercaptoacrylates. Additionally, preclinical studies of calpain inhibitors in pathologies such blood disorders, ocular disorders, neurological disorders and muscle disorders are discussed.

EXPERT OPINION

Major advances made in calpain inhibitor research between 2012 and 2014 include: i) the discovery of cytosolic-stable carboxamide calpain inhibitors; ii) synthesis of epoxysuccinates with excellent bioavailability; iii) disclosure of the X-ray crystal structures of novel α-mercaptoacrylates bound to the pentaEF hand region from human calpain; and iv) disclosure of calpain inhibitors as anti-sickling agents. Several calpain inhibitors were reported but limited effort was directed towards the discovery of calpain isoform selective agents, which continues to dampen the therapeutic potential of calpain inhibitors.

Understanding the interaction determinants of CAPN1 inhibition by CAST4 from bovines using molecular modeling techniques

HCV-induced CAPN activation and its effects on virus-infected cells in a host-immune system have been studied recently. It has been shown that the HCV-nonstructural 5A protein acts as both an inducer and a substrate for host CAPN protease; it participates in suppressing the TNF-α-induced apoptosis response and downstream IFN-induced antiviral processes. However, little is known regarding the disturbance of antiviral responses generated by bovine CAPN activation by BVDV, which is a surrogate model of HCV and is one of the most destructive diseases leading to great economic losses in cattle herds worldwide. This is also thought to be associated with the effects of either small CAPN inhibitors or the natural inhibitor CAST. They mainly bind to the binding site of CAPN substrate proteins and competitively inhibit the binding of the enzyme substrates to possibly defend against the two viruses (HCV and BVDV) for anti-viral immunity. To devise a new stratagem to discover lead candidates for an anti-BVDV drug, we first attempted to understand the bovine CAPN-CAST interaction sites and the interaction constraints of local binding architectures, were well reflected in the geometry between the pharmacophore features and its shape constraints identified using our modeled bovine CAPN1/CAST4 complex structures. We propose a computer-aided molecular design of an anti-BVDV drug as a mimetic CAST inhibitor to develop a rule-based screening function for adjusting the puzzle of relationship between bovine CAPN1 and the BVDV nonstructural proteins from all of the data obtained in the study.

Calpain-1 inhibitors for selective treatment of rheumatoid arthritis: what is the future?

Effective small-molecule treatment of inflammatory diseases remains an unmet need in medicine. Current treatments are either limited in effectiveness or invasive. The latest biologics prevent influx of inflammatory cells to damaged tissue. Calpain-1 is a calcium-activated cysteine protease that plays an important role in neutrophil motility. It is, therefore, a potential target for intervention in inflammatory disease. Many inhibitors of calpains have been developed but most are unselective and so unsuitable for drug use. However, recent series of α-mercaptoacrylate inhibitors target regulatory domains of calpain-1 and are much more specific. These compounds are effective in impairing the cell spreading mechanism of neutrophils in vitro and raise the possibility of treating rheumatoid arthritis with a pill; however, challenges still remain. Improved bioavailability is needed and solution of their precise mode of action should prompt the development of specific calpain-1 screens for novel classes of inhibitors.

Design, synthesis, and optimization of novel epoxide incorporating peptidomimetics as selective calpain inhibitors

Hyperactivation of the calcium-dependent cysteine protease calpain 1 (Cal1) is implicated as a primary or secondary pathological event in a wide range of illnesses and in neurodegenerative states, including Alzheimer's disease (AD). E-64 is an epoxide-containing natural product identified as a potent nonselective, calpain inhibitor, with demonstrated efficacy in animal models of AD. By use of E-64 as a lead, three successive generations of calpain inhibitors were developed using computationally assisted design to increase selectivity for Cal1. First generation analogues were potent inhibitors, effecting covalent modification of recombinant Cal1 catalytic domain (Cal1cat), demonstrated using LC-MS/MS. Refinement yielded second generation inhibitors with improved selectivity. Further library expansion and ligand refinement gave three Cal1 inhibitors, one of which was designed as an activity-based protein profiling probe. These were determined to be irreversible and selective inhibitors by kinetics studies comparing full length Cal1 with the general cysteine protease papain.

Structure-function relationships in calpains

Calpains are a family of complex multi-domain intracellular enzymes that share a calcium-dependent cysteine protease core. These are not degradative enzymes, but instead carry out limited cleavage of target proteins in response to calcium signalling. Selective cutting of cytoskeletal proteins to facilitate cell migration is one such function. The two most abundant and extensively studied members of this family in mammals, calpains 1 and 2, are heterodimers of an isoform-specific 80 kDa large subunit and a common 28 kDa small subunit. Structures of calpain-2, both Ca2+-free and bound to calpastatin in the activated Ca2+-bound state, have provided a wealth of information about the enzyme's structure-function relationships and activation. The main association between the subunits is the pairing of their C-terminal penta-EF-hand domains through extensive intimate hydrophobic contacts. A lesser contact is made between the N-terminal anchor helix of the large subunit and the penta-EF-hand domain of the small subunit. Up to ten Ca2+ ions are co-operatively bound during activation. The anchor helix is released and individual domains change their positions relative to each other to properly align the active site. Because calpains 1 and 2 require ~30 and ~350 μM Ca2+ ions for half-maximal activation respectively, it has long been argued that autoproteolysis, subunit dissociation, post-translational modifications or auxiliary proteins are needed to activate the enzymes in the cell, where Ca2+ levels are in the nanomolar range. In the absence of robust support for these mechanisms, it is possible that under normal conditions calpains are transiently activated by high Ca2+ concentrations in the microenvironment of a Ca2+ influx, and then return to an inactive state ready for reactivation.

Development of α-helical calpain probes by mimicking a natural protein-protein interaction

We have designed a highly specific inhibitor of calpain by mimicking a natural protein-protein interaction between calpain and its endogenous inhibitor calpastatin. To enable this goal we established a new method of stabilizing an α-helix in a small peptide by screening 24 commercially available cross-linkers for successful cysteine alkylation in a model peptide sequence. The effects of cross-linking on the α-helicity of selected peptides were examined by CD and NMR spectroscopy, and revealed structurally rigid cross-linkers to be the best at stabilizing α-helices. We applied this strategy to the design of inhibitors of calpain that are based on calpastatin, an intrinsically unstable polypeptide that becomes structured upon binding to the enzyme. A two-turn α-helix that binds proximal to the active site cleft was stabilized, resulting in a potent and selective inhibitor for calpain. We further expanded the utility of this inhibitor by developing irreversible calpain family activity-based probes (ABPs), which retained the specificity of the stabilized helical inhibitor. We believe the inhibitor and ABPs will be useful for future investigation of calpains, while the cross-linking technique will enable exploration of other protein-protein interactions.

Design, synthesis, and evaluation of 2-(arylsulfonyl)oxiranes as cell-permeable covalent inhibitors of protein tyrosine phosphatases

A structure-based design approach has been applied to develop 2-(arylsulfonyl)oxiranes as potential covalent inhibitors of protein tyrosine phosphatases. A detailed kinetic analysis of inactivation by these covalent inhibitors reveals that this class of compounds inhibits a panel of protein tyrosine phosphatases in a time- and dose-dependent manner, consistent with the covalent modification of the enzyme active site. An inactivation experiment in the presence of sodium arsenate, a known competitive inhibitor of protein tyrosine phosphatase, indicated that these inhibitors were active site bound. This finding is consistent with the mass spectrometric analysis of the covalently modified protein tyrosine phosphatase enzyme. Additional experiments indicated that these compounds remained inert toward other classes of arylphosphate-hydrolyzing enzymes, and alkaline and acid phosphatases. Cell-based experiments with human A549 lung cancer cell lines indicated that 2-(phenylsulfonyl)oxirane (1) caused an increase in intracellular pTyr levels in a dose-dependent manner thereby suggesting its cell-permeable nature. Taken together, the newly identified 2-(arylsulfonyl)oxiranyl moiety could serve as a novel chemotype for the development of activity-based probes and therapeutic agents against protein tyrosine phosphatase superfamily of enzymes.

Functional imaging of proteases: recent advances in the design and application of substrate-based and activity-based probes

Proteases are enzymes that cleave peptide bonds in protein substrates. This process can be important for regulated turnover of a target protein but it can also produce protein fragments that then perform other functions. Because the last few decades of protease research have confirmed that proteolysis is an essential regulatory process in both normal physiology and in multiple disease-associated conditions, there has been an increasing interest in developing methods to image protease activity. Proteases are also considered to be one of the few 'druggable' classes of proteins and therefore a large number of small molecule based inhibitors of proteases have been reported. These compounds serve as a starting point for the design of probes that can be used to target active proteases for imaging applications. Currently, several classes of fluorescent probes have been developed to visualize protease activity in live cells and even whole organisms. The two primary classes of protease probes make use of either peptide/protein substrates or covalent inhibitors that produce a fluorescent signal when bound to an active protease target. This review outlines some of the most recent advances in the design of imaging probes for proteases. In particular, it highlights the strengths and weaknesses of both substrate-based and activity-based probes and their applications for imaging cysteine proteases that are important biomarkers for multiple human diseases.

Disruption of the axon initial segment cytoskeleton is a new mechanism for neuronal injury

Many factors contribute to nervous system dysfunction and failure to regenerate after injury or disease. Here, we describe a previously unrecognized mechanism for nervous system injury. We show that neuronal injury causes rapid, irreversible, and preferential proteolysis of the axon initial segment (AIS) cytoskeleton independently of cell death or axon degeneration, leading to loss of both ion channel clusters and neuronal polarity. Furthermore, we show this is caused by proteolysis of the AIS cytoskeletal proteins ankyrinG and betaIV spectrin by the calcium-dependent cysteine protease calpain. Importantly, calpain inhibition is sufficient to preserve the molecular organization of the AIS both in vitro and in vivo. We conclude that loss of AIS ion channel clusters and neuronal polarity are important contributors to neuronal dysfunction after injury, and that strategies to facilitate recovery must preserve or repair the AIS cytoskeleton.

On-bead screening of a combinatorial fumaric acid derived peptide library yields antiplasmodial cysteine protease inhibitors with unusual peptide sequences

A new class of cysteine protease inhibitors based on fumaric acid derived oligopeptides was successfully identified from a high-throughput screening of a solid-phase bound combinatorial library. As target enzymes falcipain and rhodesain were used, which play important roles in the life cycles of the parasites which cause malaria (Plasmodium falciparum) and African sleeping sickness (Trypanosoma brucei rhodesiense). The best inhibitors with unusual amino acid sequences not reported before for this type of enzyme were also fully analyzed in detail in solution. K(i) values in the lower micromolar and even nanomolar region were found. Some inhibitors are even active against plasmodia and show good selectivity relative to other enzymes. Also the mechanism of action was studied and could be shown to be irreversible inhibition.

Evaluation of a novel calpain inhibitor as a treatment for cataract

PURPOSE

The aim of this study is to evaluate the therapeutic potential of a newly synthesized calpain inhibitor, CAT0059, using a naturally occurring in vivo sheep cataract model.

METHODS

The selectivity of CAT0059 was investigated by an in vitro protease assay. The efficacy of CAT0059 in preventing proteolysis of lens cytoskeletal proteins by calpain 2 was investigated using a lens-based cell-free method. The cytotoxicity and stability of CAT0059 in physiological conditions were examined using cultured sheep lenses. Protein binding of CAT0059 by ocular proteins was assessed and quantified by a modified high-performance liquid chromatography assay. CAT0059 was formulated in an eye drop solution and as an eye ointment. These were applied in vivo daily to one eye of the cataract lambs, over a 67- and 97-day trial period, respectively. The progression of cataracts in the treated and untreated eyes was assessed by an independent veterinary ophthalmologist using a slit-lamp microscope.

RESULTS

In vitro assays revealed that CAT0059 was selective for cysteine proteases and also protected lens cytoskeletal proteins from degradation. CAT0059 was stable in physiological conditions and non-toxic to the lens. Only 15% of CAT0059 is bound to proteins in the aqueous humour but >90% bound to lens homogenate. The 67-day CAT0059 eye drop treatment was not effective in slowing the rate of cataract development. However, application of CAT0059 in an eye ointment initially slowed cataract development compared with the untreated eye. This effect was temporary.

CONCLUSIONS

In vitro assays confirmed CAT0059 to be a potent calpain inhibitor. The two in vivo trials addressed the ability of CAT0059 to reach the lens and established its limitations as a therapeutic molecule for cataract treatment.

Concerted multi-pronged attack by calpastatin to occlude the catalytic cleft of heterodimeric calpains

The Ca(2+)-dependent cysteine proteases, calpains, regulate cell migration, cell death, insulin secretion, synaptic function and muscle homeostasis. Their endogenous inhibitor, calpastatin, consists of four inhibitory repeats, each of which neutralizes an activated calpain with exquisite specificity and potency. Despite the physiological importance of this interaction, the structural basis of calpain inhibition by calpastatin is unknown. Here we report the 3.0 A structure of Ca(2+)-bound m-calpain in complex with the first calpastatin repeat, both from rat, revealing the mechanism of exclusive specificity. The structure highlights the complexity of calpain activation by Ca(2+), illustrating key residues in a peripheral domain that serve to stabilize the protease core on Ca(2+) binding. Fully activated calpain binds ten Ca(2+) atoms, resulting in several conformational changes allowing recognition by calpastatin. Calpain inhibition is mediated by the intimate contact with three critical regions of calpastatin. Two regions target the penta-EF-hand domains of calpain and the third occupies the substrate-binding cleft, projecting a loop around the active site thiol to evade proteolysis.

Intraspinal MDL28170 microinjection improves functional and pathological outcome following spinal cord injury

Although calpain (calcium-activated cysteine protease) inhibition represents a rational therapeutic target for spinal cord injury (SCI), few studies have reported improved functional outcomes with post-injury administration of calpain inhibitors. This reflects the weak potency and limited aqueous solubility of current calpain inhibitors. Previously, we demonstrated that intraspinal microinjection of the calpain inhibitor MDL28170 resulted in greater inhibition of calpain activity as compared to systemic administration of the same compound. In the present study, we evaluated the ability of intraspinal MDL28170 microinjection to spare spinal tissue and locomotor dysfunction following SCI. Contusion SCI was produced in female Long-Evans rats using the Infinite Horizon impactor at the 200-kdyn force setting. Open-field locomotion was evaluated until 6 weeks post-injury. Histological assessment of tissue sparing was performed at 6 weeks after SCI. The results demonstrate that MDL28170, administered with a single post-injury intraspinal microinjection (50 nmoles), significantly improves both locomotor function and pathological outcome measures following SCI.