Expression of Semaphorins, Neuropilins, VEGF, and Tenascins in Rat and Human Primary Sensory Neurons after a Dorsal Root Injury.

Dorsal root injury is a situation not expected to be followed by a strong regenerative growth, or growth of the injured axon into the central nervous system of the spinal cord, if the central axon of the dorsal root is injured but of strong regeneration if subjected to injury to the peripherally projecting axons. The clinical consequence of axonal injury is loss of sensation and may also lead to neuropathic pain. In this study, we have used in situ hybridization to examine the distribution of mRNAs for the neural guidance molecules semaphorin 3A (SEMA3A), semaphorin 3F (SEMA3F), and semaphorin 4F (SEMA4F), their receptors neuropilin 1 (NP1) and neuropilin 2 (NP2) but also for the neuropilin ligand vascular endothelial growth factor (VEGF) and Tenascin J1, an extracellular matrix molecule involved in axonal guidance, in rat dorsal root ganglia (DRG) after a unilateral dorsal rhizotomy (DRT) or sciatic nerve transcetion (SNT). The studied survival times were 1-365 days. The different forms of mRNAs were unevenly distributed between the different size classes of sensory nerve cells. The results show that mRNA for SEMA3A was diminished after trauma to the sensory nerve roots in rats. The SEMA3A receptor NP1, and SEMA3F receptor NP2, was significantly upregulated in the DRG neurons after DRT and SNT. SEMA4F was upregulated after a SNT. The expression of mRNA for VEGF in DRG neurons after DRT showed a significant upregulation that was high even a year after the injuries. These data suggest a role for the semaphorins, neuropilins, VEGF, and J1 in the reactions after dorsal root lesions.

Complement receptor 2 is up regulated in the spinal cord following nerve root injury and modulates the spinal cord response.

BACKGROUND: Activation of the complement system has been implicated in both acute and chronic states of neurodegeneration. However, a detailed understanding of this complex network of interacting components is still lacking. METHODS: Large-scale global expression profiling in a rat F2(DAxPVG) intercross identified a strong cis-regulatory influence on the local expression of complement receptor 2 (Cr2) in the spinal cord after ventral root avulsion (VRA). Expression of Cr2 in the spinal cord was studied in a separate cohort of DA and PVG rats at different time-points after VRA, and also following sciatic nerve transection (SNT) in the same strains. Consequently, Cr2 (-/-) mice and Wt controls were used to further explore the role of Cr2 in the spinal cord following SNT. The in vivo experiments were complemented by astrocyte and microglia cell cultures. RESULTS: Expression of Cr2 in naïve spinal cord was low but strongly up regulated at 5-7 days after both VRA and SNT. Levels of Cr2 expression, as well as astrocyte activation, was higher in PVG rats than DA rats following both VRA and SNT. Subsequent in vitro studies proposed astrocytes as the main source of Cr2 expression. A functional role for Cr2 is suggested by the finding that transgenic mice lacking Cr2 displayed increased loss of synaptic nerve terminals following nerve injury. We also detected increased levels of soluble CR2 (sCR2) in the cerebrospinal fluid of rats following VRA. CONCLUSIONS: These results demonstrate that local expression of Cr2 in the central nervous system is part of the axotomy reaction and is suggested to modulate subsequent complement mediated effects.

Unbiased expression mapping identifies a link between the complement and cholinergic systems in the rat central nervous system.

The complement system is activated in a wide spectrum of CNS diseases and is suggested to play a role in degenerative phenomena such as elimination of synaptic terminals. Still, little is known of mechanisms regulating complement activation in the CNS. Loss of synaptic terminals in the spinal cord after an experimental nerve injury is increased in the inbred DA strain compared with the PVG strain and is associated with expression of the upstream complement components C1q and C3, in the absence of membrane attack complex activation and neutrophil infiltration. To further dissect pathways regulating complement expression, we performed genome-wide expression profiling and linkage analysis in a large F2(DA × PVG) intercross, which identified quantitative trait loci regulating expression of C1qa, C1qb, C3, and C9. Unlike C1qa, C1qb, and C9, which all displayed distinct coregulation with different cis-regulated C-type lectins, C3 was regulated in a coexpression network immediately downstream of butyrylcholinesterase. Butyrylcholinesterase hydrolyses acetylcholine, which exerts immunoregulatory effects partly through TNF-α pathways. Accordingly, increased C3, but not C1q, expression was demonstrated in rat and mouse glia following TNF-α stimulation, which was abrogated in a dose-dependent manner by acetylcholine. These findings demonstrate new pathways regulating CNS complement expression using unbiased mapping in an experimental in vivo system. A direct link between cholinergic activity and complement activation is supported by in vitro experiments. The identification of distinct pathways subjected to regulation by naturally occurring genetic variability is of relevance for the understanding of disease mechanisms in neurologic conditions characterized by neuronal injury and complement activation.

Axonal regeneration after sciatic nerve lesion is delayed but complete in GFAP- and vimentin-deficient mice.

Peripheral axotomy of motoneurons triggers Wallerian degeneration of injured axons distal to the lesion, followed by axon regeneration. Centrally, axotomy induces loss of synapses (synaptic stripping) from the surface of lesioned motoneurons in the spinal cord. At the lesion site, reactive Schwann cells provide trophic support and guidance for outgrowing axons. The mechanisms of synaptic stripping remain elusive, but reactive astrocytes and microglia appear to be important in this process. We studied axonal regeneration and synaptic stripping of motoneurons after a sciatic nerve lesion in mice lacking the intermediate filament (nanofilament) proteins glial fibrillary acidic protein (GFAP) and vimentin, which are upregulated in reactive astrocytes and Schwann cells. Seven days after sciatic nerve transection, ultrastructural analysis of synaptic density on the somata of injured motoneurons revealed more remaining boutons covering injured somata in GFAP(-/-)Vim(-/-) mice. After sciatic nerve crush in GFAP(-/-)Vim(-/-) mice, the fraction of reinnervated motor endplates on muscle fibers of the gastrocnemius muscle was reduced 13 days after the injury, and axonal regeneration and functional recovery were delayed but complete. Thus, the absence of GFAP and vimentin in glial cells does not seem to affect the outcome after peripheral motoneuron injury but may have an important effect on the response dynamics.

Neuronal myosin-X is upregulated after peripheral nerve injury and mediates laminin-induced growth of neurites.

The successful outcome of peripheral neuronal regeneration is attributed both to the growth permissive milieu and the intrinsic ability of the neuron to initiate appropriate cellular responses such as changes in gene expression and cytoskeletal rearrangements. Even though numerous studies have shown the importance of interactions between the neuron and the extracellular matrix (ECM) in axonal outgrowth, the molecular mechanisms underlying the contact between ECM receptors and the cellular cytoskeleton remain largely unknown. Unconventional myosins constitute an important group of cytoskeletal-associated motor proteins. One member of this family is the recently described myosin-X. This protein interacts with several members of the axon growth-associated ECM receptor family of integrins and could therefore be important in neuronal outgrowth. In this study, using radioactive in situ hybridization, we found that expression of myosin-X mRNA is upregulated in adult rat sensory neurons and spinal motoneurons after peripheral nerve injury, but not after central injury. Thus, myosin-X was upregulated after injuries that can be followed by axonal regeneration. We also found that the protein is localized to neuronal growth cones and that silencing of myosin-X using RNA interference impairs the integrin-mediated growth of neurites on laminin, but has no effect on non-integrin mediated growth on N-cadherin.

The extent of synaptic stripping of motoneurons after axotomy is not correlated to activation of surrounding glia or downregulation of postsynaptic adhesion molecules.

Synapse elimination in the adult central nervous system can be modelled by axotomy of spinal motoneurons which triggers removal of synapses from the cell surface of lesioned motoneurons by processes that remain elusive. Proposed candidate mechanisms are removal of synapses by reactive microglia and astrocytes, based on the remarkable activation of these cell types in the vicinity of motoneurons following axon lesion, and/or decreased expression of synaptic adhesion molecules in lesioned motoneurons. In the present study, we investigated glia activation and adhesion molecule expression in motoneurons in two mouse strains with deviant patterns of synapse elimination following axotomy. Mice deficient in complement protein C3 display a markedly reduced loss of synapses from axotomized motoneurons, whereas mice with impaired function of major histocompatibility complex (MHC) class Ia display an augmented degree of stripping after axotomy. Activation of microglia and astrocytes was assessed by semiquantative immunohistochemistry for Iba 1 (microglia) and GFAP (astrocytes), while expression of synaptic adhesion molecules was determined by in situ hybridization. In spite of the fact that the two mouse strains display very different degrees of synapse elimination, no differences in terms of glial activation or in the downregulation of the studied adhesion molecules (SynCAM1, neuroligin-2,-3 and netrin G-2 ligand) could be detected. We conclude that neither glia activation nor downregulation of synaptic adhesion molecules are correlated to the different extent of the synaptic stripping in the two studied strains. Instead the magnitude of the stripping event is most likely a consequence of a precise molecular signaling, which at least in part is mediated by immune molecules.

Mitofusin 2 is necessary for striatal axonal projections of midbrain dopamine neurons.

Mitochondrial dysfunction is implicated in aging and degenerative disorders such as Parkinson's disease (PD). Continuous fission and fusion of mitochondria shapes their morphology and is essential to maintain oxidative phosphorylation. Loss-of-function mutations in PTEN-induced kinase1 (PINK1) or Parkin cause a recessive form of PD and have been linked to altered regulation of mitochondrial dynamics. More specifically, the E3 ubiquitin ligase Parkin has been shown to directly regulate the levels of mitofusin 1 (Mfn1) and Mfn2, two homologous outer membrane large GTPases that govern mitochondrial fusion, but it is not known whether this is of relevance for disease pathophysiology. Here, we address the importance of Mfn1 and Mfn2 in midbrain dopamine (DA) neurons in vivo by characterizing mice with DA neuron-specific knockout of Mfn1 or Mfn2. We find that Mfn1 is dispensable for DA neuron survival and motor function. In contrast, Mfn2 DA neuron-specific knockouts develop a fatal phenotype with reduced weight, locomotor disturbances and death by 7 weeks of age. Mfn2 knockout DA neurons have spherical and enlarged mitochondria with abnormal cristae and impaired respiratory chain function. Parkin does not translocate to these defective mitochondria. Surprisingly, Mfn2 DA neuron-specific knockout mice have normal numbers of midbrain DA neurons, whereas there is a severe loss of DA nerve terminals in the striatum, accompanied by depletion of striatal DA levels. These results show that Mfn2, but not Mfn1, is required for axonal projections of DA neurons in vivo.

Reduced removal of synaptic terminals from axotomized spinal motoneurons in the absence of complement C3.

Complement proteins C1q and C3 play a critical role in synaptic elimination during development. Axotomy of spinal motoneurons triggers removal of synaptic terminals from the cell surface of motoneurons by largely unknown mechanisms. We therefore hypothesized that the complement system is involved also in synaptic stripping of injured motoneurons. In the sciatic motor pool of wild type (WT) mice, the immunoreactivity (IR) for both C1q and C3 was increased after sciatic nerve transection (SNT). Mice deficient in C3 (C3(-/-)) showed a reduced loss of synaptic terminals from injured motoneurons at one week after SNT, as assessed by immunoreactivity for synaptic markers and electron microscopy. In particular, the removal of putative inhibitory terminals, immunopositive for vesicular inhibitory amino acid transporter (VIAAT) and ultrastructurally identified as type F synapses, was reduced in C3(-/-) mice. In contrast, lesion-induced removal of nerve terminals in C1q(-/-) mice appeared similar to WT mice. Growth associated protein (GAP)-43 mRNA expression in lesioned motoneurons increased much more in C3(-/-) compared to WT mice after SNT. After sciatic nerve crush (SNC), the C3(-/-) mice showed a faster functional recovery, assessed as grip strength, compared to WT mice. No differences were detected regarding nerve inflammation at the site of injury or pattern of muscle reinnervation. These data indicate that a non-classical pathway of complement activation is involved in axotomy-induced adult synapse removal, and that its inhibition promotes functional recovery.

Karolinska institutet 200-year anniversary. Symposium on traumatic injuries in the nervous system: injuries to the spinal cord and peripheral nervous system - injuries and repair, pain problems, lesions to brachial plexus.

The Karolinska Institutet 200-year anniversary symposium on injuries to the spinal cord and peripheral nervous system gathered expertise in the spinal cord, spinal nerve, and peripheral nerve injury field spanning from molecular prerequisites for nerve regeneration to clinical methods in nerve repair and rehabilitation. The topics presented at the meeting covered findings on adult neural stem cells that when transplanted to the hypoglossal nucleus in the rat could integrate with its host and promote neuron survival. Studies on vascularization after intraspinal replantation of ventral nerve roots and microarray studies in ventral root replantation as a tool for mapping of biological patterns typical for neuronal regeneration were discussed. Different immune molecules in neurons and glia and their very specific roles in synapse plasticity after injury were presented. Novel strategies in repair of injured peripheral nerves with ethyl-cyanoacrylate adhesive showed functional recovery comparable to that of conventional epineural sutures. Various aspects on surgical techniques which are available to improve function of the limb, once the nerve regeneration after brachial plexus lesions and repair has reached its limit were presented. Moreover, neurogenic pain after amputation and its treatment with mirror therapy were shown to be followed by dramatic decrease in phantom limb pain. Finally clinical experiences on surgical techniques to repair avulsed spinal nerve root and the motoric as well as sensoric regain of function were presented.

Evidence of hypothalamic degeneration in the anorectic anx/anx mouse.

Mice homozygous for the anorexia (anx) mutation are characterized by poor food intake and death by three to five weeks after birth. By P21 these mice display lower density of hypothalamic neuropeptides, including Agouti gene-related protein (AGRP). The AGRP/neuropeptide Y (NPY) system of the anx/anx mice develops normally until postnatal day (P) 12, then the normal increase in fiber density ceases, in some areas even distinctly decreases. This overlaps with activation of microglia, indicating an inflammatory and/or degenerative process. Here we studied, by in situ hybridization and immunohistochemistry (IHC), the expression of major histocompatibility complex (MHC) class I-related molecules and markers for cellular reactivity in hypothalamus of anx/anx mice. MHC class I transcript and -related proteins were found in arcuate nucleus (Arc), presumably both in neurons and glia, the latter also in areas innervated by AGRP (NPY) neurons. In the anx/anx hypothalamus, using TUNEL labeling, significantly higher number of apoptotic cells were found compared with +/+ mice, and active caspase 6 immunoreactivity was detected in degenerating NPY-fibers as well as signs of "microglia-associated cell death". In addition, Y1 receptor-labeled processes and soma of pro-opiomelanocortin (POMC) neurons, were markedly decreased at P21. These results support the hypothesis of degeneration of hypothalamic arcuate neuron populations in the anx/anx mice, whereby the AGRP system may be first affected, the changes in the POMC system being secondary in this process.

Netrin G-2 ligand mRNA is downregulated in spinal motoneurons after sciatic nerve lesion.

Netrin G-2 ligand (NGL-2) and synaptic adhesion like molecules induce synapses in vitro. We investigated the expression of these molecules in a model of CNS synaptic detachment and restoration in vivo. After axotomy of spinal motoneurons, synapses are lost from the somata of lesioned motoneurons. We could not detect any synaptic adhesion like molecule mRNA in the spinal cord, but signal for NGL-2 mRNA was seen in motoneurons. The signal for NGL-2 decreased after sciatic nerve transection and sciatic nerve crush. After regeneration, the levels of NGL-2 mRNA were partially restored after sciatic nerve transection, but completely restored after sciatic nerve crush. We conclude that axotomized motoneurons decrease their NGL-2 expression and that the restoration of NGL-2 expression mirrors the restoration of synaptic inputs.

Classic major histocompatibility complex class I molecules: new actors at the neuromuscular junction.

The presence and function of immune molecules in the central nervous system (CNS) have been under debate for a long time. There is mounting evidence that molecules fundamental for immune function are indeed expressed by both neurons and glia and that such molecules may have important nonimmunological function for the organization and stability of synaptic connections. Here, we present data showing that the classic form of major histocompatibility complex (MHC) class I molecules is expressed in spinal motoneurons, in particular in their axons and presynaptically at their synapses with skeletal muscles, the neuromuscular junctions (NMJs). The expression is strongly increased after axon lesion in the peripheral nerve. In the absence of classic MHC I, the organization of NMJs is disturbed with NMJs in higher numbers than normal, thereby equipping single muscle fibers with multiple NMJs. It is suggested that these effects are mediated by the classic MHC class I in the motor axons, possibly through effects mediated by the peripherally myelinating Schwann cells, which express receptors for classic MHC class I. The presence of immune molecules normally used by other cells for antigen presentation in peripheral motor axons may have implications for the onset of specific motoneuron disease.

SynCAM1 expression correlates with restoration of central synapses on spinal motoneurons after two different models of peripheral nerve injury.

SynCAM1 and neuroligins (NLGs) are adhesion molecules that govern synapse formation in vitro. In vivo, the molecules are expressed during synaptogenesis, and altered NLG function is linked to synapse dysfunction in autism. Less is known about SynCAM1 and NLGs in adult synapse remodeling. CNS synapse elimination occurs after peripheral nerve injury, which causes a transient decrease in synapse number on spinal motoneurons. Here we have studied the expression of SynCAM1 and NLGs in relation to changes in synaptic covering on spinal motoneurons. We performed sciatic nerve transection (SNT) or crush (SNC), axotomy models that result in poor or good conditions for axon regeneration, respectively. The two lesions resulted in similar synapse elimination and in poor (SNT) and good (SNC) return of synapses after 70 days. Functional recovery was good after SNC but absent after SNT. SynCAM1 mRNA decreased after 14 days in both models and was restored 70 days after SNC, but not after SNT. NLG2 and -3 mRNAs decreased to a smaller degree after SNC than after SNT. Synaptophysin immunoreactivity correlated with SynCAM1 mRNA 70 days after SNT and NLG2 mRNA 70 days after SNC. Surprisingly, an inverse correlation was seen between NLG3 mRNA and Vglut2, a marker for excitatory synapses, 70 days after SNT. We conclude that 1) SynCAM1 mRNA levels seem to reflect the loss and restoration of synapses on motoneurons, 2) down-regulation of NLGs is not a prerequisite for synapse elimination, and 3) expression of SynCAM1 and NLGs is regulated by different mechanisms during regeneration.

Classical major histocompatibility complex class I molecules in motoneurons: new actors at the neuromuscular junction.

Major histocompatibility complex (MHC) class I molecules have fundamental functions in the immune system. Recent studies have suggested that these molecules may also have non-immune functions in the nervous system, in particular related to synaptic function and plasticity. Because adult motoneurons express mRNAs for MHC class I molecules, we have examined their subcellular expression pattern in vivo and their role for the synaptic connectivity of these neurons. We observed immunoreactivity for classical MHC class I (Ia) protein in motoneuron somata, but the predominant expression was found in axons and presynaptically at neuromuscular junctions (NMJs). Peripheral nerve lesion induced a strong increase of motoneuron MHC class Ia (H2-K(b)/D(b)) mRNA, indicating a role for MHC class Ia molecules during regeneration. Accordingly, there was an accumulation of MHC class Ia proteins at the cut ends and in growth cones of motor axons after lesion. In K(b-/-)D(b-/-) mice (lacking MHC class Ia molecules), the time course for recovery of grip ability in reinnervated muscles was significantly delayed. Muscles from K(b-/-)D(b-/-) mice displayed an increased density and a disturbed distribution of NMJs and fewer terminal Schwann cells/NMJ compared with wild-type mice. A population of Schwann cells in sciatic nerves expressed the paired Ig receptor B, which binds to MHC class I molecules. These results provide the first evidence that neuronal MHC class Ia molecules are present in motor axons, that they are important for organization of NMJs and motor recovery after nerve lesion, and that their actions may be mediated via Schwann cells.

Altered expression of nectin-like adhesion molecules in the peripheral nerve after sciatic nerve transection.

Following axotomy several processes involving cell-cell interaction occur, such as loss of synapses, axon guidance, and remyelination. Two recently discovered families of cell-cell adhesion molecules, nectins and nectin-like molecules (necls) are involved in such processes in vitro and during development, but their roles in nerve injury have been largely unknown until recently. We have previously shown that axotomized motoneurons increase their expression of nectin-1 and nectin-3 and maintain a high expression of necl-1. We here investigate the expression of potential binding partners for motoneuron nectins and necls in the injured peripheral nerve. In situ hybridization (ISH) revealed a decreased signal for necl-1 mRNA in the injured nerve, whereas no signal for necl-2 was detected before or after injury. The signals for necl-4 and necl-5 mRNA both increased in the injured nerve and necl immunoreactivity displayed a close relation to axon and Schwann cell markers. Finally, signal for mRNA encoding necl-5 increased in axotomized spinal motoneurons. We conclude that peripheral axotomy results in altered expression of several necls in motoneurons and Schwann cells, suggesting involvement of the molecules in regeneration.

Integrin-laminin interactions controlling neurite outgrowth from adult DRG neurons in vitro.

A prerequisite for axon regeneration is the interaction between the growth cone and the extracellular matrix (ECM). Laminins are prominent constituents of ECM throughout the body, known to support axon growth in vitro and in vivo. The regenerative capacity of adult neurons is greatly diminished compared to embryonic or early postnatal neurons. Since most lesions in the nervous system occur in the adult, we have examined neurite outgrowth from adult mouse DRG neurons on four laminin isoforms (laminin-1/LM-111, laminin-2/LM-211, laminin-8/LM-411 and laminin-10/LM-511) in vitro. The growth on laminin-1 and -10 was trophic factor-independent and superior to the one on laminin-2 and -8, where growth was very poor in the absence of neurotrophins. Among other ECM proteins, laminins were by far the most active molecules. Using function-blocking antibodies to laminin-binding integrins, we identified non-overlapping functions of integrins alpha3beta1, alpha7beta1 and alpha6beta1 on different laminin isoforms, in that alpha3beta1 and alpha7beta1 integrins appeared to be specific receptors for both laminin-1 and-2, whereas integrin alpha6beta1 was a receptor for laminin-8 and-10. Lastly, by use of immunohistochemistry, expression of subunits of laminin-1, -2, -8 and -10 in sensory organs in the human epidermis could be demonstrated, supporting an important role for these laminins in relation to primary sensory axons.

MHC I expression and synaptic plasticity in different mice strains after axotomy.

The success of axonal regeneration has been attributed to a co-operation between the severed neurons and the surrounding environment, including non-neuronal cells and the extracellular matrix. Important differences regarding the regeneration potential after injury have been described among inbred mice strains. To date, there is only limited knowledge of how such variation can be linked with the genetic background. It has recently been demonstrated that MHC class I molecules have an influence on the spinal cord synaptic plasticity elicited by a peripheral lesion, and the regenerative capacity following such a lesion. Therefore, in the present work we compared the MHC I expression after axotomy in three isogenic mice strains, namely C57BL/6J, Balb/cJ, and A/J, and investigated the fine ultrastructure of the synaptic elimination process that follows such lesion. The results show that C57BL/6J mice, that have a comparatively poor regenerative potential, display a lower upregulation of MHC I in the spinal cord, coupled with a slower synaptic stripping. On the other hand, A/J mice, which have been shown to have a stronger axonal regrowth potential, showed a clear upregulation of MHC I and a sharp acute loss of afferents, at 1 week after lesion. Our results suggest that a more prominent expression of MHC I in the first week after lesion may positively influence the regenerative outcome associated with a more effective axonal regrowth.

MHC class I expression and synaptic plasticity after nerve lesion.

An axon lesion to a bulbar or spinal motoneuron is followed by a typical retrograde response at the cell body level, including the removal or 'stripping' of synapses from the perikaryon and dendrites of affected cells. Both activated microglia and astrocytes have been attributed roles in this process. The signalling pathways for this 'synaptic stripping' have so far been unknown, but recently a classical set of immune recognition molecules, the MHC class I molecules, have been shown to have a strong influence on the strength and pattern of the synapse elimination response. Thus, when MHC class I signalling is severely impaired in mice lacking the MHC class I subunit beta2-microglobulin (beta2m) and transporter associated with antigen processing 1 (TAP 1) genes, both of which are necessary for surface expression of MHC class I, there is a stronger elimination of synapses from injured neurons, with the surplus elimination directed towards clusters of putatively inhibitory synapses. Moreover, the regenerative capacity of motoneurons in such mice is lower than in wild-type animals. The expression of MHC class I, as well as MHC class I-related receptors in both neurons and glia, lend support to a hypothesis that classical immune recognition signalling between neurons and glia underlie part of the 'stripping' response.

The microglial networks of the brain and their role in neuronal network plasticity after lesion.

Microglia are the resident inflammatory cells of the central nervous system (CNS) extending a network of processes in the CNS parenchyma. Following axon lesion to neurons, most extensively studied in motoneurons, there is a typical retrograde response at the cell body level, including the removal or 'stripping' of synapses from the perikaryon and dendrites of affected cells. Microglia have been attributed a main and active role in this process, although also an involvement of activated astrocytes has been suggested. The signaling pathways for this 'synaptic stripping' have so far been unknown, but recently some classical immune recognition molecules, the MHC class I molecules, have been shown to have a strong influence on the strength and pattern of the synapse elimination response. Since there is an expression of MHC class I in both neurons and glia, in particular microglia, as well as MHC class I related receptors in axons and microglia, there are good reasons to believe that classical immune recognition signaling between neurons and glia underlies part of the 'stripping' response. A role for microglia in an interplay with synapses based on this type of signaling is further exemplified by the fact that, in the absence of some MHC class I related receptors normally found on microglia during development, profound effects on synaptic function and biochemistry have been demonstrated. Such effects may be linked to the development of various disorders of the CNS, such as degenerative disease.