Vicki M. Tysseling

BMPR1a and BMPR1b Signaling Exert Opposing Effects on Gliosis after Spinal Cord Injury

Astrogliosis following spinal cord injury (SCI) involves an early hypertrophic response that is beneficial and a subsequent formation of a dense scar. We investigated the role of bone morphogenetic protein (BMP) signaling in gliosis after SCI and find that BMPR1a and BMPR1b signaling exerts opposing effects on hypertrophy. Conditional ablation of BMPR1a from glial fibrillary acidic protein (GFAP)-expressing cells leads to defective astrocytic hypertrophy, increased infiltration by inflammatory cells, and reduced axon density. BMPR1b-null mice conversely develop “hyperactive” reactive astrocytes and consequently have smaller lesion volumes. The effects of ablation of either receptor are reversed in the double knock-out animals. These findings indicate that BMPR1a and BMPR1b exert directly opposing effects on the initial reactive astrocytic hypertrophy. Also, BMPR1b knock-out mice have an attenuated glial scar in the chronic stages following injury, suggesting that it has a greater role in glial scar progression. To elucidate the differing roles of the two receptors in astrocytes, we examined the effects of ablation of either receptor in serum-derived astrocytes in vitro. We find that the two receptors exert opposing effects on the posttranscriptional regulation of astrocytic microRNA-21. Further, overexpression of microRNA-21 in wild-type serum-derived astrocytes causes a dramatic reduction in cell size accompanied by reduction in GFAP levels. Hence, regulation of microRNA-21 by BMP signaling provides a novel mechanism for regulation of astrocytic size. Targeting specific BMPR subunits for therapeutic purposes may thus provide an approach for manipulating gliosis and enhancing functional outcomes after SCI.

Self‐assembling peptide amphiphile promotes plasticity of serotonergic fibers following spinal cord injury

Injection into the injured spinal cord of peptide amphiphile (PA) molecules that self‐assemble and display the laminin epitope IKVAV at high density improved functional recovery after spinal cord injury (SCI) in two different species, rat and mouse, and in two different injury models, contusion and compression. The improvement required the IKVAV epitope and was not observed with the injection of an amphiphile displaying a nonbioactive sequence. To explore the mechanisms underlying these improvements, the number of serotonergic fibers in the lesioned spinal cord was compared in animals receiving the IKVAV‐PA, a nonbioactive PA (PA control), or sham injection. Serotonergic fibers were distributed equally in all three groups rostral to the injury but showed a significantly higher density caudal to the injury site in the IKVAV PA‐injected group. Furthermore, this difference was not present in the subacute phase following injury but appeared in the chronically injured cord. The IKVAV PA‐injected groups also trended higher both in the total number neurons adjacent to the lesion and in the number of long propriospinal tract connections from the thoracic to the lumbar cord. IKVAV PA injection did not alter myelin thickness, total axon number caudal to the lesion, axon size distribution, or total axon area. Serotonin can promote stepping even in complete transection models, so the improved function produced by the IKVAV PA treatment may reflect the increased serotonergic innervation caudal to the lesion in addition to the previously demonstrated regeneration of motor and sensory axons through the lesion.

SDF1 in the dorsal corticospinal tract promotes CXCR4+ cell migration after spinal cord injury

BackgroundStromal cell-derived factor-1 (SDF1) and its major signaling receptor, CXCR4, were initially described in the immune system; however, they are also expressed in the nervous system, including the spinal cord. After spinal cord injury, the blood brain barrier is compromised, opening the way for chemokine signaling between these two systems. These experiments clarified prior contradictory findings on normal expression of SDF1 and CXCR4 as well as examined the resulting spinal cord responses resulting from this signaling.MethodsThese experiments examined the expression and function of SDF1 and CXCR4 in the normal and injured adult mouse spinal cord primarily using CXCR4-EGFP and SDF1-EGFP transgenic reporter mice.ResultsIn the uninjured spinal cord, SDF1 was expressed in the dorsal corticospinal tract (dCST) as well as the meninges, whereas CXCR4 was found only in ependymal cells surrounding the central canal. After spinal cord injury (SCI), the pattern of SDF1 expression did not change rostral to the lesion but it disappeared from the degenerating dCST caudally. By contrast, CXCR4 expression changed dramatically after SCI. In addition to the CXCR4+ cells in the ependymal layer, numerous CXCR4+ cells appeared in the peripheral white matter and in the dorsal white matter localized between the dorsal corticospinal tract and the gray matter rostral to the lesion site. The non-ependymal CXCR4+ cells were found to be NG2+ and CD11b+ macrophages that presumably infiltrated through the broken blood-brain barrier. One population of macrophages appeared to be migrating towards the dCST that contains SDF1 rostral to the injury but not towards the caudal dCST in which SDF1 is no longer present. A second population of the CXCR4+ macrophages was present near the SDF1-expressing meningeal cells.ConclusionsThese observations suggest that attraction of CXCR4+ macrophages is part of a programmed response to injury and that modulation of the SDF1 signaling system may be important for regulating the inflammatory response after SCI.

Design and evaluation of a chronic EMG multichannel detection system for long-term recordings of hindlimb muscles in behaving mice

Mouse models are commonly used for identifying the behavioral consequences of genetic modifications, progression or recovery from disease or trauma models, and understanding spinal circuitry. Electromyographic recordings (EMGs) are recognized as providing information not possible from standard behavioral analyses involving gross behavioral or kinematic assessments. We describe here a method for recording from relatively large numbers of muscles in behaving mice. We demonstrate the use of this approach for recording from hindlimb muscles bilaterally in intact animals, following spinal cord injury, and during the progression of ALS. This design can be used in a variety of applications in order to characterize the coordination strategies of mice in health and disease.

Potential associations between chronic whiplash and incomplete spinal cord injury

Study Design:This research utilized a cross-sectional design with control group inclusion.Objectives:Preliminary evidence suggests that a portion of the patient population with chronic whiplash may have sustained spinal cord damage. Our hypothesis is that in some cases of chronic whiplash-associated disorders (WAD), observed muscle weakness in the legs will be associated with local signs of a partial spinal cord injury of the cervical spine.Setting:University based laboratory in Chicago, IL, USA.Methods:Five participants with chronic WAD were compared with five gender/age/height/weight/body mass index (BMI) control participants. For a secondary investigation, the chronic WAD group was compared with five unmatched participants with motor incomplete spinal cord injury (iSCI). Spinal cord motor tract integrity was assessed using magnetization transfer imaging. Muscle fat infiltration (MFI) was quantified using fat/water separation magnetic resonance imaging. Central volitional muscle activation of the plantarflexors was assessed using a burst superimposition technique.Results:We found reduced spinal cord motor tract integrity, increased MFI of the neck and lower extremity muscles and significantly impaired voluntary plantarflexor muscle activation in five participants with chronic WAD. The lower extremity structural changes and volitional weakness in chronic WAD were comparable to participants with iSCI.Conclusion:The results support the position that a subset of the chronic whiplash population may have sustained partial damage to the spinal cord.Sponsorship:NIH R01HD079076-01A1, NIH T32 HD057845 and the Foundation for Physical Therapy Promotion of Doctoral Studies program.

Characterization of motor units in behaving adult mice shows a wide primary range.

The mouse is essential for genetic studies of motor function in both normal and pathological states. Thus it is important to consider whether the structure of motor output from the mouse is in fact analogous to that recorded in other animals. There is a striking difference in the basic electrical properties of mouse motoneurons compared with those in rats, cats, and humans. The firing evoked by injected currents produces a unique frequency-current (F-I) function that emphasizes recruitment of motor units at their maximum force. These F-I functions, however, were measured in anesthetized preparations that lacked two key components of normal synaptic input: high levels of synaptic noise and neuromodulatory inputs. Recent studies suggest that the alterations in the F-I function due to these two components are essential for recreating firing behavior of motor units in human subjects. In this study we provide the first data on firing patterns of motor units in the awake mouse, focusing on steady output in quiet stance. The resulting firing patterns did not match the predictions from the mouse F-I behaviors but instead revealed rate modulation across a remarkably wide range (10-60 Hz). The low end of the firing range may be due to changes in the F-I relation induced by synaptic noise and neuromodulatory inputs. The high end of the range may indicate that, unlike other species, quiet standing in the mouse involves recruitment of relatively fast-twitch motor units.

Ambulatory function in motor incomplete spinal cord injury: a magnetic resonance imaging study of spinal cord edema and lower extremity muscle morphometry

Study design:This research utilized a cross-sectional design.Objectives:Spinal cord edema length has been measured with T2-weighted sagittal MRI to predict motor recovery following spinal cord injury. The purpose of our study was to establish the correlational value of axial spinal cord edema using T2-weighted MRI. We hypothesized a direct relationship between the size of damage on axial MRI and walking ability, motor function and distal muscle changes seen in motor incomplete spinal cord injury (iSCI).Setting:University-based laboratory in Chicago, IL, USA.Methods:Fourteen participants with iSCI took part in the study. Spinal cord axial damage ratios were assessed using axial T2-weighted MRI. Walking ability was investigated using the 6-min walk test and daily stride counts. Maximum plantarflexion torque was quantified using isometric dynomometry. Muscle fat infiltration (MFI) and relative muscle cross-sectional area (rmCSA) were quantified using fat/water separation magnetic resonance imaging.Results:Damage ratios were negatively correlated with distance walked in 6 min, average daily strides and maximum plantarflexion torque, and a negative linear trend was found between damage ratios and lower leg rmCSA. While damage ratios were not significantly correlated with MFI, we found significantly higher MFI in the wheelchair user participant group compared to community walkers.Conclusions:Damage ratios may be useful in prognosis of motor recovery in spinal cord injury. The results warrant a large multi-site research study to investigate the value of high-resolution axial T2-weighted imaging to predict walking recovery following motor incomplete spinal cord injury.

Firing characteristics of deep dorsal horn neurons after acute spinal transection during administration of agonists for 5-HT1B/1D and NMDA receptors

Spinal cord injury (SCI) results in a loss of serotonin (5-HT) to the spinal cord and a loss of inhibition to deep dorsal horn (DDH) neurons, which produces an exaggerated excitatory drive to motoneurons. The mechanism of this excitatory drive could involve the DDH neurons triggering long excitatory postsynaptic potentials in motoneurons, which may ultimately drive muscle spasms. Modifying the activity of DDH neurons with drugs such as NMDA or the 5-HT1B/1D receptor agonist zolmitriptan could have a large effect on motoneuron activity and, therefore, on muscle spasms. In this study, we characterize the firing properties of DDH neurons after acute spinal transection in adult mice during administration of zolmitriptan and NMDA, using the in vitro sacral cord preparation and extracellular electrophysiology. DDH neurons can be categorized into three major types with distinct evoked and spontaneous firing characteristics: burst (bursting), simple (single spiking), and tonic (spontaneously tonic firing) neurons. The burst neurons likely contribute to muscle spasm mechanisms because of their bursting behavior. Only the burst neurons show significant changes in their firing characteristics during zolmitriptan and NMDA administration. Zolmitriptan suppresses the burst neurons by reducing their evoked spikes, burst duration, and spontaneous firing rate. Conversely, NMDA facilitates them by enhancing their burst duration and spontaneous firing rate. These results suggest that zolmitriptan may exert its antispastic effect on the burst neurons via activation of 5-HT1B/1D receptors, whereas activation of NMDA receptors may facilitate the burst neurons in contributing to muscle spasm mechanisms following SCI.

Soma size and Cav1.3 channel expression in vulnerable and resistant motoneuron populations of the SOD1G93A mouse model of ALS

Although the loss of motoneurons is an undisputed feature of amyotrophic lateral sclerosis (ALS) in man and in its animal models (SOD1 mutant mice), how the disease affects the size and excitability of motoneurons prior to their degeneration is not well understood. This study was designed to test the hypothesis that motoneurons in mutant SOD1G93A mice exhibit an enlargement of soma size (i.e., cross‐sectional area) and an increase in Cav1.3 channel expression at postnatal day 30, well before the manifestation of physiological symptoms that typically occur at p90 (Chiu et al. ). We made measurements of spinal and hypoglossal motoneurons vulnerable to degeneration, as well as motoneurons in the oculomotor nucleus that are resistant to degeneration. Overall, we found that the somata of motoneurons in male SOD1G93A mutants were larger than those in wild‐type transgenic males. When females were included in the two groups, significance was lost. Expression levels of the Cav1.3 channels were not differentiated by genotype, sex, or any interaction of the two. These results raise the intriguing possibility of an interaction between male sex steroid hormones and the SOD1 mutation in the etiopathogenesis of ALS.

Chronic EMGs in treadmill running SOD1 mice reveal early changes in muscle activation

The present study demonstrates that electromyograms (EMGs) obtained during locomotor activity in mice were effective for identification of early physiological markers of amyotrophic lateral sclerosis (ALS). These measures could be used to evaluate therapeutic intervention strategies in animal models of ALS. Several parameters of locomotor activity were shifted early in the disease time course in SOD1G93A mice, especially when the treadmill was inclined, including intermuscular phase, burst skew and amplitude of the locomotor bursts. The results of the present study indicate that early compensatory changes may be taking place within the neural network controlling locomotor activity, including spinal interneurons. Locomotor EMGs could have potential use as a clinical diagnostic tool.