Retinal degenerative diseases, such as glaucoma and retinitis pigmentosa (RP), are the leading causes of blindness in adults. In Japan, glaucoma is a leading cause, and RP is third major cause of acquired blindness. Specific types of neurons are injured in the patients of glaucoma and RP. Retinal ganglion cells (RGC) are specifically degenerated in glaucoma. Excitotoxicity caused by excess glutamate in the retinal extracellular space is thought to be one of the mechanisms of RGC death induced by glaucoma and retinal central artery occlusion. Retinal ischemia-reperfusion, intravitreal NMDA injection, intravitreal NO donor injection and knock out of glutamate aspartate transporter, which are used as the experimental models of glaucoma, are known to induce RGC death. RGCs are vulnerable for excess glutamate and oxidative stress related to NO, and this vulnerability may be involved in pathogenesis of glaucomatous optic neuropathy. RP, which is characterized by progressive photoreceptor-selective degeneration, is caused by mutation of the genes related to the function of photoreceptor and retinal pigment epithelium. It has not been thoroughly clarified how the mutations induce specific photoreceptor death. Tunicamycin is widely known to induce ER stress, and intravitreal tunicamycin cause photoreceptor-specific degeneration. Therefore, ER stress may cause photoreceptor-selective degeneration in RP.
Different and selective vulnerability among motor neuron subtypes are a fundamental, but unexplained, feature of amyotrophic lateral sclerosis (ALS): fast-fatigable (FF) motor neurons are the most vulnerable, and fast fatigue-resistant/slow (FR/S) motor neurons are relatively resistant. We identified that osteopontin (OPN) can serve as a marker of FR/S motor neurons, whereas matrix metalloproteinase-9 (MMP9) is expressed by FF motor neurons in mice. In SOD1G93A ALS model mice, as the disease progressed, OPN was secreted and accumulated as granular deposits in the extracellular matrix. We also detected OPN/MMP9 co-expressed motor neurons around the disease onset. These double positive motor neurons showed the expression of αvβ3 integrin (OPN receptor) and up-regulation of ER stress markers. We discovered that the double positive motor neurons are remodeled FR/S motor neurons, which compensated for FF motor neuron degeneration (the first wave of degeneration). Genetic ablation of OPN delayed the onset of disease, but later accelerated disease progression. This reflects two modes of OPN involvement in the pathogenesis of ALS: cell-autonomous and non-cell-autonomous effects on motor neuron vulnerability. Our study suggests that OPN expressed in FR/S motor neurons is involved in the second wave of motor neuron degeneration in ALS, and an OPN-αvβ3 integrin-MMP9 axis could be a potentially useful therapeutic target for ALS.
Narcolepsy is a kind of sleep disorder featured by selective loss of orexin neurons in the lateral hypothalamus. Several lines of evidence, including association with specific HLA haplotypes, gene polymorphism in T cell receptor and detection of autoantibodies in a subpopulation of patients, suggest that autoimmune responses play an important role in the pathogenesis of this disorder. Potential relationship with influenza virus infection has also been a matter of interest. However, these events may not be able to explain all cases of narcolepsy. Based on the structural features of orexin, in addition to the findings on the characteristics of orexin neurons obtained from studies in organotypic hypothalamic slice cultures, we proposed novel mechanisms potentially involved in selective degeneration of orexin neurons. Increase in local production of nitric oxide induced by several life style-related conditions such as shortage of sleep and intake of high fat diet leads to inactivation of protein disulfide isomerase. Consequently, abnormal aggregates of orexin and/or its precursor that possess two intra-molecular disulfide bonds accumulate within orexin neurons. In addition to the increase in endoplasmic reticulum stress, accumulation of orexin as abnormal aggregates leads to increased excitability of orexin neurons by shutdown of feedback inhibition resulting from deficits in orexin release. These mechanisms may provide a clue to understand the pathogenic mechanisms of various neurological and psychiatric disorders accompanied by a decrease of orexin.
G-protein-coupled receptor 3 (GPR3) is a member of the class A rhodopsin-type GPCR family and is highly expressed in various neurons. A unique feature of GPR3 is its ability to constitutively activate the Gαs protein without the addition of ligands, which results in the elevation of the basal level of intracellular cAMP. During the development of the cerebellum, GPR3 expression is upregulated in cerebellar granular neurons (CGNs) and maintained thereafter. In our previous studies, we showed that the intrinsic expression of GPR3 in CGNs is highly associated with neurite outgrowth, neurite differentiation, and neuronal survival. Recently, we have focused on the possible signaling pathways associated with GPR3-mediated neurite outgrowth in CGNs. Interestingly, GPR3-mediated neurite outgrowth is mediated by not only PKA-dependent signaling pathways but also PI3K-mediated signaling pathways. Moreover, the Gβγ-mediated signaling pathway is involved in GPR3-mediated neurite outgrowth. These results suggested that neural expression of GPR3 stimulates multiple downstream signaling pathways, contributing to the maintenance of homeostasis in neurons. Further precise analyses of constitutively active GPCRs may help in unveiling novel neuronal functions.
Adaptor molecules (adaptor proteins) have indispensable roles in cellular signaling, essential for cellular proliferation, development and metabolism. Shc (Src homology and collagen homology)-family molecule is a group of adaptor molecules, and indicated to be involved in intracellular phosphotyrosine signaling. Shc family has 4 subtypes, ShcA-ShcD, and there are long and short isoforms in ShcA and ShcC whereas ShcB and ShcD have short isoform only. There are three domains conserved in all Shc-family isoforms: phosphotyrosine-binding (PTB) domain, collagen-homology 1 (CH1) domain and Src-homology 2 (SH2) domain, from the N-terminal to C-terminal. PTB and SH2 domains recognize and bind to phosphotyrosine in other molecules, and CH1 domain is recognized and bind to SH2 domain in Grb2, an adaptor molecule, when the tyrosine residues in the domain are phosphorylated. Expression of ShcA is observed in all tissues except for brain in adult animals, although ShcA mRNA is detected in brain during embryonic days. On the other hand, in adult brain, expressions of ShcB, ShcC, and ShcD are observed. Analysis of single knockout mice (ShcA (neuron specific), ShcB, ShcC) and double knockout mice for ShcB and C indicated essential roles of Shc-family molecules in proliferation and survival of cells in various brain regions as well as synaptic plasticity and higher brain functions such as learning and memory. Studies on multiple-knockout mice of Shc-family molecules may further clarify possible involvements of Shc family in physiological and pathophysiological functions in brain.
Brain can be roughly divided into two parts, cerebrum and cerebellum. Cerebrum controls higher brain functions including memory, emotion and cognition, while cerebellum is important for motor coordination. The only output neuron in cerebellum, Purkinje cell, regulates long term depression (LTD). LTD and morphology of Purkinje cells are important for motor function. So far, disorder of protein kinase C (PKC) α and γ, which are expressed in Purkinje cells, impaired LTD, morphology of Purkinje cells and motor coordination. Diacylglycerol kinase (DGK) γ phosphorylates diacylglycerol (DG) and is abundantly expressed in Purkinje cells. In other words, DGKγ can attenuate PKC activity by reducing amount of DG and may contribute to motor coordination. However, its physiological role has not been elucidated. Therefore, we developed DGKγ knockout (KO) mice and investigated their LTD, morphology of Purkinje cells, and cerebellar motor coordination. We found that cerebellar motor coordination and LTD were impaired in the DGKγ KO mice and the morphology of Purkinje cells from DGKγ KO mice was significantly retracted. Interestingly, abnormal activation of PKCγ was involved in impairment of the morphology of Purkinje cells from DGKγ KO mice. These results indicated that DGKγ was involved in cerebellar LTD and morphology of Purkinje cells, and DG signaling is important for cerebellar motor coordination.
Rodent laboratory animals, such as mice and rats have been greatly contributing to biomedical research. Although its usefulness would not change in the future, nonhuman primates (NHPs) also offer excellent models for preclinical research to assess safety and efficacy of developing novel therapeutic approaches because of their similarities of genetics, metabolism and physiological characteristics to humans. Recent years, the gene modification technology in nonhuman primates has been developed. In fact, pre-clinical studies using nonhuman primates are increasing in the world, especially in the neuroscience research field. Among the NHPs, the common marmoset (Callithrix jacchus) is one of a suitable NHP laboratory animal for producing genetically modified models because they are fecund animal. This article outlines the common marmoset and that of the disease models.