Lesnick E. Westrum

Bergmann glia expression of polyglutamine-expanded ataxin-7 produces neurodegeneration by impairing glutamate transport

Non-neuronal cells may be pivotal in neurodegenerative disease, but the mechanistic basis of this effect remains ill-defined. In the polyglutamine disease spinocerebellar ataxia type 7 (SCA7), Purkinje cells undergo non-cell-autonomous degeneration in transgenic mice. We considered the possibility that glial dysfunction leads to Purkinje cell degeneration, and generated mice that express ataxin-7 in Bergmann glia of the cerebellum with the Gfa2 promoter. Bergmann glia–specific expression of mutant ataxin-7 was sufficient to produce ataxia and neurodegeneration. Expression of the Bergmann glia–specific glutamate transporter GLAST was reduced in Gfa2-SCA7 mice and was associated with impaired glutamate transport in cultured Bergmann glia, cerebellar slices and cerebellar synaptosomes. Ultrastructural analysis of Purkinje cells revealed findings of dark cell degeneration consistent with excitotoxic injury. Our studies indicate that impairment of glutamate transport secondary to glial dysfunction contributes to SCA7 neurodegeneration, and suggest a similar role for glial dysfunction in other polyglutamine diseases and SCAs.

Fine structural aspects of the synaptic organization of spinal trigeminal nucleus (pars interpolaris) of the cat

Abstract Pars interpolaris of the spinal trigeminal nuclear complex has been studied electron microscopically in normal adult cats and after total ipsilateral retrogasserian rhizotomy. Small cell body profiles are numerous and have few axo-somatic synapses. Larger cell body profiles are few in number but have several axo-somatic synapses. Axo-dendritic synaptic contacts are numerous and axo-axonic serial synapses are seen at all levels. Presynaptic profiles containing predominantly round vesicles are usually characterized by an asymmetrical postynaptic specialization and those with many flattened vesicles usually have a symmetrical specialization. A small proportion of profiles with flattened vesicles are associated with an asymmetrical specialization. Axo-axonic synapses have flattened vesicle profiles presynaptic to round vesicle ones. Degeneration of axons and presynaptic processes are seen at 30 h survival and become rare after 7 days. Most degenerated processes are electron-dense and associated with an asymmetrical specialization. In serial synapses the round vesicle profile degenerates. After 3 days many degenerated endings are replaced by glial and other processes which are applied to the denervated postsynaptic specialization. After 7 days survival some unaltered flattened vesicle profiles appear to migrate into the denervated postsynaptic sites. Later survivals show such endings applied to both symmetrical and asymmetrical specializations on the same dendritic profile. Other origins for unaltered presynaptic processes are considered. A correlation between denervated postsynaptic sites and neuronal hyperactivity is discussed, as is the apparent synaptic reorganization or re-innervation of these denervated sites.

Transganglionic degeneration in the spinal trigeminal nucleus following removal of tooth pulps in adult cats

It is generally thought that transganglionic degeneration of central axons following lesions of their peripheral processes does not occur in adult nervous systemS, 6. For this reason there has been a lack of anatomical descriptions of the central distribution of specific groups of peripheral sensory axons. We, however, have found reproducible axonal and terminal degeneration in specific regions of the spinal trigeminal nucleus in adult cats following tooth pulpectomies 12. Healthy adult cats were used and the absence of dental pathology was confirmed prior to operation and in the controls. Under pentobarbital anesthesia and using appropriate aseptic dental procedures, the pulps were completely removed from all of the teeth on one side of 12 animals. Following postoperative survival times of 7, 9, 11, 14, 21, 28 and 100 days the animals were perfused with 4~o formaldehyde in a phosphate buffer 11. Transverse frozen sections through the brain stem, including the major parts of the spinal trigeminal nucleus, were stained by either the NautaGygax v or the Fink-Heimer (I) 4 technique. Two normal, unoperated animals were sacrificed and processed in the same fashion to serve as controls. Fig. 1 illustrates the pattern of degeneration observed at 1 l days. Unequivocal axonal and terminal degeneration (Fig. 2) is seen predominantly in the ventral half to one-third of the subnucleus interpolaris of the spinal trigeminal complex, near and immediately rostral to the obex, with very much less in the adjacent subnuclei (caudalis and oralis). The distribution is bilateral, but heaviest on the side of the lesion. The occurrence of contralateral degeneration is surprising in view of what is known of the organization of this system 2, but it is a consistent finding here. A crossing pathway of degenerating fibers cannot be identified with certainty. The amount of degeneration is less at shorter or longer survivals, but the pattern is similar in each. Control animals, with healthy teeth, show no degeneration.

Microtubules, dendritic spines and spine apparatuses

SummaryUsing techniques for enhanced microtubular preservation, including albumin pretreatment (Gray, 1975), occipital cortex of rats was studied electron microscopically at various ages of development. A close structural relationship was seen between microtubules, sacs of SER and the postsynaptic “thickening” in primordial spines and with the dense “plate” material of spine apparatuses. Stereoscopic preparations in addition show a more complicated substructure than previously described for the “plate”. Microtubules may contribute to the formation of the “plate” of the spine apparatus which in turn is associated with the postsynaptic “thickening” of the mature spine. Possible functional correlates are discussed.

Platelet-derived growth factor B-chain-like immunoreactivity in the developing and adult rat brain

Platelet-derived growth factors (PDGFs) are growth-regulatory molecules that stimulate chemotaxis, proliferation and increased metabolism, primarily of connective tissue cells. In our previous paper, we have demonstrated the ubiquitous localization of PDGF B-chain-containing proteins in neurons and expression of transcripts for PDGF A-chain, B-chain and the two forms of the PDGF receptor in the brains of non-human primates. In the present study, the cellular localization of PDGF B-chain in developing and adult rat brains was analyzed using immunocytochemistry with a PDGF B-chain-specific monoclonal antibody. Intense PDGF B-chain immunoreactivity (PDGFB-I) was distributed around the continuously regenerating primary olfactory neurons at all stages of development from embryo to adult. The major part of PDGFB-I associated with neurons appeared some time after birth and increased with age. PDGFB-I appeared in several nerve fiber systems during earlier stages of development and gradually decreased with age. In conjunction with other data showing the existence of functional PDGF receptor beta-subunits in the neurons, these data suggest a possible role for PDGF B-chain as a neurotrophic or neuroregulatory factor in both developing and mature brains.

Electron microscopy of synaptic structures in olfactory cortex of early postnatal rats

SummaryLayer 1 of the rat olfactory cortex has been studied with the electron microscope at birth and at several consecutive postnatal days up to 14 days of age. Special attention was directed towards synaptic structures and axons of the lateral olfactory tract (LOT). Numerous mature synapses are seen at birth and estimates were made of their subsequent increase in number. In addition, immature synapses are seen and mature postsynaptic sites occur with atypical, partial, multiple or no contact. The findings suggest: (1) considerable prenatal synaptogenesis in contrast to other cortical systems; (2) the maturation of the postsynaptic site may precede that of the presynaptic contact and vesicle accumulation; (3) there may be competition by more than one process for one postsynaptic specialization; (4) the non-innervated sites may result from deafferentation caused by prenatal cell death, although no degeneration was seen, and the atypical contacts may be a stage in the reinnervation of these sites; (5) the LOT develops in parallel with the synaptic neuropil and (6) by 14 days of age the area closely resembles adult tissue.

The fine structural synaptic organization of the cat lateral cuneate nucleus. A study of sequential alterations in degeneration

Summary The fine structural, synaptic anatomy of the lateral cuneate nucleus of Clarke-Monakow (LCN) has been investigated for the first time in both normal adult cats and animals subjected to dorsal root deafferentation by C1-T7 unilateral, extradural rhizotomies. The normal nucleus contains two distinguishable cell types, the larger of which probably represents the origin of the cuneocerebellar tract. Few axosomatic synapses are present on either cell type and profiles with flattened synaptic vesicles predominate on each. The populations of presynaptic profiles in general are divisible into two types: a large (LR) variety (2–6 μm) with round synaptic vesicles and an asymmetrical contact; a second (SF) type of smaller processes (0.6–2.5 μm) with flattened synaptic vesicles and a symmetrical type of contact. Both types contact dendrites of all sizes. Axoaxonic synapses are present and have the SF presynaptic to the LR bouton. Operated material with survival times of 13 h to 16 days after C1-T7 dorsal root deafferentation reveals a sequential transition from electron-lucent to electron-dense presynaptic and axonal profiles which seems to involve the entire population of LR boutons. It is suggested that these profiles represent the terminals of the primary dorsal root afferents which correspond to both group I and group II muscle afferents. The possible functional significance of these findings is discussed. The observed morphologic transition of presynaptic profiles is correlated to the general process of axonal degeneration in the nervous system. Potential origins for the unaltered (SF) presynaptic profiles are considered. Some aspects of the frequently observed deafferented specialization sites are discussed.

Early forms of terminal degeneration in the spinal trigeminal nucleus following rhizotomy

SummaryPars interpolaris of the spinal trigeminal nucleus in adult cats has been studied with the electron microscope at several early and consecutive postoperative survival times following retrogasserian rhizotomy. The results show that several different forms of degenerative changes occur in the axon terminals and some of these forms may be interpreted as stages in certain sequences of terminal degeneration.Synaptic vesicle depletion and mitochondrial alterations are probably the earliest changes in some terminals, but the occurrence of completely different forms also at very early survival times suggest that other processes may be involved. Electron-dense forms of terminal degeneration of later survivals are suggested as arising from any of at least four possible types or classes of earlier alterations based upon presence or absence of vesicles and neurofilaments and on the mitochondrial changes. Electron-lucent forms probably progress to dense types with time but may account for only a proportion of the later dense forms.All types of neuroglia, or reactive glia, and a possible ‘third glia’ are involved in early and rapid phagocytosis and denudation of post-synaptic sites at times earlier than previously reported.The study also confirms a previous study of a time-related degeneration pattern in the same area and offers an explanation for this pattern.

Each canine tooth projects to all brain stem trigeminal nuclei in cat

Abstract The aim of this study was to determine in cats the precise sites of representation in the brain stem for specific teeth and their periodontal structures. Horseradish peroxidase was implanted in the pulpal chambers of either maxillary or mandibular canines and its transganglionic transport to the brain stem was studied light microscopically in tetramethyl benzidine preparations. The findings showed some differences between projections of maxillary and of mandibular canines, but a much more extensive ipsilateral central representation than previously reported for the individual teeth. The results are discussed in relation to previous anatomic and physiologic studies.

Nerve growth factor receptor expression in the young and adult rat olfactory system

Nerve growth factor (NGF) and its receptor (NGFR) are proteins that have a role in the normal development and survival of neurons in the peripheral and central nervous systems. During development, NGF is necessary for outgrowth of axons and establishment of synapses, and NGFR is the transmembrane protein that binds NGF and brings it into the cell. NGF and NGFR expression in the rat olfactory system have been studied previously, and age differences in NGFR are explored further in this study, using immunocytochemistry and immunoelectron microscopy to determine the changes in two different ages: postnatal day 5 and the adult. Dramatic differences were found in the distribution of NGFR immunoreactivity in the olfactory system of each of the two ages studied. Electron microscopy revealed that glial cells were responsible for this immunoreactivity.