Biology subjects

Meehan, C. F.

Publications and source records attributed to Meehan, C. F..

9 recordsLinked to original sources

bioRxiv2026

Discovery of the Honeycomb Synapse in Spinal Motor Circuits

The structural and molecular diversity of synapses in the nervous system contributes to the specialisation of neural circuits underlying diverse behaviours. We have discovered a morphologically distinct postsynaptic specialisation in the mammalian spinal cord. We have named this the Honeycomb Synapse based on its elaborate postsynaptic nanostructure, comprising rings formed of ~6 scaffolding protein domains that create multiple perforations throughout the large postsynaptic domain. Analysis of the organisation of other synaptic proteins reveals that Honeycomb Synapses harbour mixed signalling properties structurally facilitated by a postsynaptic scaffold matrix supporting chemical transmission, with gap junction proteins occupying some of the perforations. We reveal anatomical diversity in the presence of Honeycomb Synapses on populations of -motoneurons across the lumbar spinal cord in mice from approximately 2 weeks of age through to adulthood. The Honeycomb Synapse was found to be a subtype of synapse within the Ia afferent monosynaptic stretch reflex circuit. Finally, we have identified that Honeycomb Synapses are highly vulnerable to degeneration in two different genetically engineered mouse models of Amyotrophic Lateral Sclerosis (ALS), contributing to monosynaptic stretch reflex circuit dysfunction. These findings suggest that synaptic diversity within circuits may confer selective vulnerability to distinct synaptic subclasses.

neuroscience↗

bioRxiv2026

Activity-dependent structural plasticity of myelinated axons tunes their excitability

Myelinated axons are classically viewed as stable cables of fixed geometry. We show here that a brief subthreshold stimulation of single nodes in mouse dorsal column axons drives a rapid, lasting structural change: the stimulated axons shrink locally along the internode segments adjacent to the node, to about 70% of their diameter within a few seconds, while neighbouring glia swell to fill the space, sparing the node, paranode and more distant internodes. The thinned axon becomes markedly more excitable for hours, by an amount cable theory predicts from its shorter internodal length constant rather than from active currents that tend to decay over long periods. Using a grease-gap signal that reports the axial-conductance change, we find the axon thinning requires under-myelin Kv1 channels, Na/K-ATPase pumps, bicarbonate, and altered pH, consistent with a regenerative periaxonal potassium build-up that osmotically compresses the axon and swells glia, as previously proposed for physiological high-frequency firing. We present evidence that this mechanism may contribute to the long-lasting clinical benefits of spinal cord stimulation. Axonal GABAA receptor activation reproduces and occludes the plasticity induced by stimulation, indicating that they share a common underlying mechanism. Activity-dependent axon thinning thus enables potent physiologically relevant long term control of conduction, with potential clinical applications.

neuroscience↗

bioRxiv2025

Intrinsic mechanisms contributing to the biophysical signature of mouse gamma motoneurons

Precise motor control relies on continuous sensory feedback from muscles, a process in which gamma motoneurons play a key role. These specialized spinal neurons innervate intrafusal muscle fibres, modulating their sensitivity to stretch and maintaining proprioceptive signalling during movement. Gamma motoneurons are characterized by a distinct biophysical profile, including low recruitment thresholds and high firing rates that enable rapid activation of intrafusal fibres at contraction onset. Despite their importance, the intrinsic mechanisms that underlie these properties remain poorly understood. In this study, we analysed published and unpublished data to identify a population of low-threshold, high-gain motoneurons with features consistent with gamma motoneurons, emerging during the third postnatal week in mice. Their low recruitment threshold was linked to lower membrane capacitance, higher input resistance, a more hyperpolarized activation of persistent inward currents (PICs), and a narrower axon initial segment. In contrast, higher firing rates were associated not with PIC amplitude but with shorter action potential durations and smaller medium afterhyperpolarizations. Notably, 92% of putative gamma motoneurons exhibited a sodium pump-mediated ultra-slow afterhyperpolarization (usAHP), which was absent in slow alpha motoneurons. This difference could not be attributed to h-current activity or expression of the alpha 3 subunit of the sodium-potassium ATPase. These findings reveal key intrinsic properties that support the unique excitability of gamma motoneurons, offering new insight into their contribution to motor control. This work provides a foundation for future studies into their development, regulation, and involvement in neuromuscular disorders. Key Points[bullet] A distinct cluster of motoneurons with low recruitment current and high firing gain, characteristic of gamma motoneurons, emerges in the third week of postnatal development. [bullet]Gamma motoneurons have a low recruitment current due to lower capacitance, higher input resistance, and a more hyperpolarized activation voltage for persistent inward currents. [bullet]Their high firing rates are not driven by differences in persistent inward current amplitude but are instead attributed to shorter duration action potentials and smaller amplitude medium afterhyperpolarizations. [bullet]A narrower axon initial segment in gamma motoneurons may contribute to their increased excitability compared to alpha motoneurons. [bullet]Gamma motoneurons present with a higher prevalence of ultra slow afterhyperpolarization than slow alpha motoneurons that cannot be accounted for by differences in h-current or expression of alpha 3 subunits of the sodium potassium ATPase pump.

neuroscience↗

bioRxiv2025

Synaptopathy in the TDP43ΔNLS Mouse Model of Sporadic Amyotrophic Lateral Sclerosis

Sporadic cases of Amyotrophic Lateral Sclerosis (sALS) represent the most common form of motor neuron disease. sALS is characterised by pathological cytoplasmic inclusions of TDP-43, so-called reactive astrocyte pathology, and motor neuron degeneration. Early-stage alterations in certain subpopulations of synapses between neurons are thought to be a key driver of the early pathological mechanisms of ALS. However, we do not have a clear understanding of which types of synapses are impacted in ALS. Identifying vulnerable synapses affected in sALS models may provide insights into the key sites of disease pathogenesis. In this study we have performed quantitative high-resolution microscopy to survey different synapse subtypes, including excitatory (glutamatergic), inhibitory (glycinergic) and modulatory (cholinergic C-Bouton) synapses, in the spinal cord of a mouse model of sALS showing inducible TDP-43 pathology (TDP43{Delta}NLS) restricted to neurons. We have identified changes in cholinergic synapses and a subpopulation of excitatory synapses. Mice display robust neuronal TDP-43 pathology and evidence of TDP-43 changes at cholinergic C-boutons. We also observe no evidence of astrocytic pathology nor changes in the fraction of synapses that are contacted by astrocytes, demonstrating that synapse pathology is driven by cell-autonomous (neuronal) mechanisms. Overall, our findings highlight the selective vulnerability of distinct synapse populations in ALS.

neuroscience↗

bioRxiv2024

TDP-43 pathology is sufficient to drive axon initial segment plasticity and hyperexcitability of spinal motoneurones in vivo in the TDP43-NLS model of Amyotrophic Lateral Sclerosis

A hyperexcitability of the motor system is consistently observed in Amyotrophic Lateral Sclerosis (ALS) and has been implicated in the disease pathogenesis. What drives this hyperexcitability in the vast majority of patients is unknown. This is important to know as existing treatments simply reduce all neuronal excitability and fail to distinguish between pathological changes and important homeostatic changes. Understanding what drives the initial pathological changes could therefore provide better treatments. One challenge is that patients represent a heterogeneous population and the vast majority of cases are sporadic. One pathological feature that almost all ([~]97%) cases (familial and sporadic) have in common is cytoplasmic aggregates of the protein TDP-43 which is normally located in the nucleus. In our experiments we investigated whether this pathology was sufficient to increase neuronal excitability and the mechanisms by which this occurs. We used the TDP-43({Delta}NLS) mouse model which successfully recapitulates this pathology in a controllable way. We used in vivo intracellular recordings in this model to demonstrate that TDP-43 pathology is sufficient to drive a severe hyper-excitability of spinal motoneurones. Reductions in soma size and a lengthening and constriction of axon initial segments were observed, which would contribute to enhanced excitability. Resuppression of the transgene resulted in a return to normal excitability parameters by 6-8 weeks. We therefore conclude that TDP-43 pathology itself is sufficient to drive a severe but reversible hyperexcitability of spinal motoneurones.

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bioRxiv2022

Homeostatic changes maintain the gain control of spinal motoneurones across the lifetime of C57BL/6J mice

Age-related changes in the excitability of spinal motoneurone have been observed in mouse models of neurodegenerative diseases affecting these neurones. How the excitability of spinal motoneurones change with healthy ageing in mice and how this compares with that seen in neurodegenerative diseases is unknown. Therefore, we performed in vivo intracellular recording from identified spinal motoneurones in C57BL/6 mice at three different ages (100, 300-400 and 600-750 days old). Behavioral tests confirmed a linear reduction in motor function across these ages (using the rotorod test). Significant differences were observed with respect to the features of individual somatic action potential with ageing including a decreased rate of rise and fall in aged mice. Surprisingly, the rate of rise of the action potential at the initial segment was altered in middle aged mice. Immunohistochemical labelling of the axon initial segment of the motoneurones confirmed structural changes occurring at middle age (decreased length and diameter) but returning to the earlier parameters in aged mice. To explore the effects on repetitive firing, this was tested across the age groups which showed surprising little difference as the mice aged, with a similar rheobase and I-f gain across all age groups (with the exception of a lower voltage threshold for action potential initiation in middle-aged mice). However, amplitudes of the after-hyperpolarization and the input resistance were both found to be significantly altered with age. We conclude that there are changes occurring in the intrinsic properties of spinal motoneurones that control their excitability over the lifetime of mice, although these do not develop in a linear fashion from young to old. We propose that these changes are homeostatic in nature and are able to compensate for one another to maintain a constant gain control across the lifetime.

neuroscience↗

bioRxiv2022

Cytoplasmic TDP-43 accumulation drives changes in C-bouton number and size in a mouse model of sporadic Amyotrophic Lateral Sclerosis

An altered neuronal excitability of spinal motoneurones has consistently been implicated in Amyotrophic Lateral Sclerosis (ALS) leading to several investigations of synaptic input to these motoneurones. One such input that has repeatedly been shown to be affected is a population of large cholinergic synapses terminating mainly on the soma of the motoneurones referred to as C-boutons. Most research on these synapses during disease progression has used transgenic Superoxide Dismutase 1 (SOD1) mouse models of the disease which have not only produced conflicting findings, but also fail to recapitulate the key pathological feature seen in ALS; cytoplasmic accumulations of TAR DNA-binding protein 43 (TDP-43). Additionally, they fail to distinguish between slow and fast motoneurones, the latter of which have more C-boutons but are lost earlier in the disease. To circumvent these issues, we quantified the frequency and volume of C-boutons on traced soleus and gastrocnemius motoneurones, representing predominantly slow and fast motor pools respectively. Experiments were performed using the TDP-43{Delta}NLS mouse model that carries a transgenic construct of TDP-43 devoid of its nuclear localization signal, preventing its nuclear import. This results in the emergence of pathological TDP-43 inclusions in the cytoplasm, modelling the main pathology seen in this disorder, accompanied by a severe and lethal ALS phenotype. Our results confirmed changes in both the number and volume of C-boutons with a decrease in number on the more vulnerable, predominantly fast gastrocnemius motoneurones and an increase in number on the less vulnerable, predominantly slow soleus motoneurones. Importantly, these changes were only found in male mice. However, both sexes and motor pools showed a decrease in C-bouton volume. Our experiments confirm that cytoplasmic TDP-43 accumulation is sufficient to drive C-bouton changes.

neuroscience↗

bioRxiv2020

Increased axon initial segment length results in increased Na+ currents in spinal motoneurones at symptom onset in the G127X SOD1 mouse model of Amyotrophic Lateral Sclerosis.

Amyotrophic lateral sclerosis is a neurodegenerative disease preferentially affecting motoneurones. Transgenic mouse models have been used to investigate the role of abnormal motoneurone excitability in this disease. Whilst an increased excitability has repeatedly been demonstrated in vitro in neonatal and embryonic preparations from SOD1 mouse models, the results from the only studies to record in vivo from spinal motoneurones in adult SOD1 models have produced conflicting findings. Deficits in repetitive firing have been reported in G93A SOD1 mice but not in presymptomatic G127X SOD1 mice despite shorter motoneurone axon initial segments (AISs) in these mice. These discrepancies may be due to the earlier disease onset and prolonged disease progression in G93A SOD1 mice with recordings potentially performed at a later sub-clinical stage of the disease in this mouse. To test this, and to explore how the evolution of excitability changes with symptom onset we performed in vivo intracellular recording and AIS labelling in G127X SOD1 mice immediately after symptom onset. No reductions in repetitive firing were observed showing that this is not a common feature across all ALS models. Immunohistochemistry for the Na+ channel Nav1.6 showed that motoneurone AISs increase in length in G127X SOD1 mice at symptom onset. Consistent with this, the rate of rise of AIS components of antidromic action potentials were significantly faster confirming that this increase in length represents an increase in AIS Na+ channels occurring at symptom onset in this model. HighightsO_LIIn vivo electrophysiological recordings were made in symptomatic G127X SOD1 mice. C_LIO_LIThere were no deficits in repetitive firing in motoneurones in G127X mice. C_LIO_LIIncreased persistent inward currents were still present in the symptomatic mice. C_LIO_LIResults suggest increases in Na+ currents at axon initial segments (AISs). C_LIO_LIImmunohistochemistry showed that motoneurone AISs were longer and thinner. C_LI

neuroscience↗

bioRxiv2020

Spinal motoneurones are intrinsically more responsive in the adult G93A SOD1 mouse model of Amyotrophic Lateral Sclerosis

In vitro studies from transgenic Amyotrophic Lateral Sclerosis models have suggested an increased excitability of spinal motoneurones. However, in vivo intracellular recordings from adult ALS mice models have produced conflicting findings. Previous publications using barbiturate anaesthetised G93A SOD1 mice suggested that some motoneurones are hypo-excitable, defined by deficits in repetitive firing. Our own previous recordings in G127X SOD1 mice using different anaesthesia, however, showed no repetitive firing deficits, and increased persistent inward currents at symptom onset. These discrepancies may be due to differences between models, symptomatic stage, anaesthesia or technical differences. To investigate this, we repeated our original experiments, but in adult male G93A mice at both presymptomatic and symptomatic stages, under barbiturate anaesthesia. In vivo intracellular recordings from antidromically identified spinal motoneurones revealed no significant differences in the ability to fire repetitively in the G93A SOD1 mice. Motoneurones in G93A SOD1 mice fired significantly more spontaneous action potentials. Rheobase was significantly lower and the input resistance and input-output gain were significantly higher in both presymptomatic and symptomatic G93A SOD1 mice. This was despite a significant increase in the duration of the post-spike after-hyperpolarisation (AHP) in both presymptomatic and symptomatic G93A SOD1 mice. Finally, evidence of increased activation of persistent inward currents was seen in both presymptomatic and symptomatic G93A SOD1 mice. Our results do not confirm previous reports of hypo-excitability of spinal motoneurones in the G93A SOD1 mouse and demonstrate that the motoneurones do in fact show an increased response to inputs. Key Point SummaryO_LIAlthough in vitro recordings using neonatal preparations from mouse models of Amyotrophic Lateral Sclerosis (ALS) suggest increased motoneurone excitability, in vivo recordings in adult ALS mouse models have been conflicting. C_LIO_LIIn adult G93A SOD1 models, spinal motoneurones have previously been shown to have deficits in repetitive firing, in contrast to the G127X SOD1 mouse model. C_LIO_LIOur in vivo intracellular recordings in barbiturate-anaesthetised adult male G93A SOD1 mice reveal no deficits in repetitive firing either prior to or after symptom onset. C_LIO_LIWe show that deficits in repetitive firing ability can be a consequence of experimental protocol and should not be used alone to classify otherwise normal motoneurones as hypo-excitable. C_LIO_LIMotoneurones in the G93A SOD1 mice showed an increased response to inputs, with lower rheobase, higher input-output gains and increased activation of persistent inward currents. C_LI

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