Glial bundles of nerve roots in Werdnig-Hoffmann disease

Glial Activation and Central Synapse Loss, but Not Motoneuron Degeneration, Are Prevented by the Sigma-1 Receptor Agonist PRE-084 in the Smn2B/- Mouse Model of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is characterized by the loss of α-motoneurons (MNs) with concomitant muscle denervation. MN excitability and vulnerability to disease are particularly regulated by cholinergic synaptic afferents (C-boutons), in which Sigma-1 receptor (Sig1R) is concentrated. Alterations in Sig1R have been associated with MN degeneration. Here, we investigated whether a chronic treatment with the Sig1R agonist PRE-084 was able to exert beneficial effects on SMA. We used a model of intermediate SMA, the Smn2B/- mouse, in which we performed a detailed characterization of the histopathological changes that occur throughout the disease. We report that Smn2B/- mice exhibited qualitative differences in major alterations found in mouse models of severe SMA: Smn2B/- animals showed more prominent MN degeneration, early motor axon alterations, marked changes in sensory neurons, and later MN deafferentation that correlated with conspicuous reactive gliosis and altered neuroinflammatory M1/M2 microglial balance. PRE-084 attenuated reactive gliosis, mitigated M1/M2 imbalance, and prevented MN deafferentation in Smn2B/- mice. These effects were also observed in a severe SMA model, the SMNΔ7 mouse. However, the prevention of gliosis and MN deafferentation promoted by PRE-084 were not accompanied by any improvements in clinical outcome or other major pathological changes found in SMA mice.

Mechanisms involved in spinal cord central synapse loss in a mouse model of spinal muscular atrophy

Motoneuron (MN) cell death is the histopathologic hallmark of spinal muscular atrophy (SMA), although MN loss seems to be a late event. Conversely, disruption of afferent synapses on MNs has been shown to occur early in SMA. Using a mouse model of severe SMA (SMNΔ7), we examined the mechanisms involved in impairment of central synapses. We found that MNs underwent progressive degeneration in the course of SMA, with MN loss still occurring at late stages. Loss of afferent inputs to SMA MNs was detected at embryonic stages, long before MN death. Reactive microgliosis and astrogliosis were present in the spinal cord of diseased animals after the onset of MN loss. Ultrastructural observations indicate that dendrites and microglia phagocytose adjacent degenerating presynaptic terminals. Neuronal nitric oxide synthase was upregulated in SMNΔ7 MNs, and there was an increase in phosphorylated myosin light chain expression in synaptic afferents on MNs; these observations implicate nitric oxide in MN deafferentation and suggest that the RhoA/ROCK pathway is activated. Together, our observations suggest that the earliest change occurring in SMNΔ7 mice is the loss of excitatory glutamatergic synaptic inputs to MNs; reduced excitability may enhance their vulnerability to degeneration and death.

Glial bundles in spinal nerve roots: a form of isomorphic gliosis at the junction of the central and peripheral nervous system

A morphologic study of the spinal nerve roots was undertaken in three cases of Werdnig-Hoffmann disease to investigate the phenomenon of glial bundle formation. The glial elements extended along the ventral roots as discrete cylindrical bundles comprising a large number of parallel astrocytic processes and sparsely scattered cell bodies all enclosed by a basal lamina. The bundles tapered off at a variable distance from the root exit zones. The early stage of glial bundle formation was characterized by the protrusion of astrocytes into the neurilemmal tubes containing degenerated myelinated axons. It was concluded that axonal degeneration, evoking a glial reaction, was the initial event in this process. Subsequently, the reactive astrocytes from the vicinity of the root exit zones enter the neurilemmal tubes previously occupied by myelinated axons and migrated into the domain of the peripheral nervous system in an orderly fashion. Thus glial bundle formation might be considered a special form of isomorphic gliosis occurring in Werdnig-Hoffmann disease and also in several other conditions all sharing a common feature, namely, degeneration of axons within the spinal nerve roots.

Neurodegenerative diseases of infancy and childhood

The neurodegenerative diseases of infancy and childhood include disorders in which there is progressive loss of neurological function due to structural abnormalities of the central nervous system. Well over six hundred disorders, many of which are rarely seen, can be included in this category. Yet, the conditions represent collectively over one-fourth of all admissions to pediatric neurology services. Five-year samples of admission characteristics of 1218 patients from two medical centers over twenty-two years permit an estimate of the frequency of the neurodegenerative diseases. The six most-encountered diagnoses, in declining order, were: subacute sclerosing panencephalitis; neuronal ceroid lipofuscinosis; tuberous sclerosis with degeneration; West disease, or idiopathic degenerative encephalopathy associated with infantile spasms; Werdnig-Hoffmann disease, and hereditary spastic paraplegia. A classification is offered grouping the neurodegenerative disorders into five major categories: polioencephalopathies, leukoencephalopathies, corencephalopathies, spinocerebellopathies, and diffuse encephalopathies. Disorders in each subgroup may be either genetic or nongenetic. Neurodegenerative diseases have multiple causes, including metabolic, viral, immunopathic, environmental, and epileptogenic. The cause of many remains unknown.

Glial outgrowth along spinal nerve roots in amyotrophic lateral sclerosis

Formation of glial bundles in the proximal portion of the ventral nerve roots is described in a 51-year-old patient with the sporadic form of amyotrophic lateral sclerosis (ALS). Although the bundles were relatively fewer, they were identical in morphology and distribution to those consistently found in Werdnig-Hoffmann disease (WHD). The occurrence of glial bundles in ALS, albeit rare, indicates that this phenomenon is not a unique feature of WHD. Similar changes have been observed in several other unrelated conditions, always in association with degeneration of neurons or axons. Thus, outgrowth of astrocytes in the form of glial bundles should be considered a special type of astrocytic reaction at the interface of the central and peripheral nervous systems.

Congenital hypomyelination neuropathy: glial bundles in cranial and spinal nerve roots

Autopsy examination of a 3 1/4-year-old child with a severe congenital hypomyelination neuropathy showed the anterior spinal nerve roots and motor cranial nerves to be almost devoid of myelin in their subarachnoid course. The posterior spinal nerve roots and peripheral nerves were less severely affected. Onion bulb formation was minimal and was present only in the sural nerve. There was extensive glial overgrowth in cranial nerves and spinal nerve roots adjacent to the brainstem and spinal cord. The extent and severity of glial overgrowth were similar to that described in Werdnig-Hoffmann disease and morphologically appeared as glial bundles. These glial bundles are most likely secondary to chronic myelin and axonal damage.