| Amblyopia is a common disease which can not be correceted to normal level andcaused by blurring visual pattern stimulation during the sensitive period of visualdevelopment. With high incidence, the lower visual function causes many difficulties in thejob, study and life of the amblyopia patients. The disease incidence in adults is2.9%, as thechief reason of the monocular impaired vison among people aging from20-70. Thedevelopment and treatment of this disease have closly related to the visual cortex plasticity.Within the critical period of plasticity, the visual function of ambyopia patients can berestored by some suitable methods. However, after the termination of critical period(usually after8-12years old), although abnormal visual enviroment can not causeamblyopia, but the impaired visual function can not recover. Thus it is very important tomake research on the mechanism of visual plasticity and know more about the pathogenesisof amblyopia, improving the cure rate of adult amblyopia patients and explore new drugsfor treating this disease.Around the end of the critical period, the CSPGs in the ECM are gradually wrappingthe somata and dentrites of neurons, forming PNNs, especially around parvalbumin neurons.Degradation of CSPGs by the injection of chABCase can succesfully restore the movementof ocular dominance, by decreasing the inhitory function of GABA neurons. CSPGs havethe common core proteins and are secreted by glia cells and neurons, which could inhibitthe growth of axon and regeneration. Our previous work shows that the synapictransmission mediated by GABA receptors increased along with the development of vision.Degradation of CSPGs by ChaseABC in the rat visual cortex, and combined with the patchclamp technology, we found that the index of the PSCs did not change in layer Ⅳ,indicating that the degradation of CSPGs could not influence the characteristics of synaptic transmission in layer Ⅳ, but could affect the inhibitory synaptic transmissioin specifically.These results implied that the inhibition function of CSPGs in the termination of the criticalperiod might be the structural basis of the maturation of GABA neuronal circuits. Althoughit is noticed that CSPGs are the crucial factors in the termination of the visual cortex, theunderlying molecular mechanisms of CSPGs playing its inhibitory role are not clear.Protein tyrosine phosphatase σ (PTPσ) is a member of LAR family, and comprisedof Ig and FNⅢ repeats, widely expressed in the glands and nervous system (highlyexpressed in hippocampus, cerebrum, bulbus olfactorius, retina and subependymal layer)participating in the process of neural development and regeneration, especially in theinhibition of the growth of the axon and the formation of the glia scar. PTPσ binds withhigh affinity to neural CSPGs, binding involves the chondroitin sulfate chains and a specificsite on the first immunoglobulin-like domain of PTPσ. Similarly to ChaseABC treatment,functional ablation of Ptprs, the gene encoding RPTPσ, promotes neurite outgrowth in thepresence of CSPGs in vitro and enhances axonal growth into CSPG-rich scar tissuefollowing SCI in vivo.Recent research shows that PTPσ plays the role to negatively modulate the axonalgrowth as the functional receptor of the inhibitory function of CSPGs. However, it is stillunknown that whether PTPσis involved in the termination of critical period of visualcortex plasticity, as well as the correlation of CSPGs and PV neurons. Actin is closelyrelated to the molecular mechanism of the visual plasticity. The cytoskeleton of the neuronare microfilament, microtubule and neurofilament, only microfilament in the action network could befound in the developing growth cone and the spines in mature central nervous system. Growthcone is aactive movement microstructures, and participate in the genesis and growth of the nervous process,special pathway of axon, and formation of the synapses. Importantly, the growth and elongation of theneuritis can only be found in the growth cone. As a family member of the synaptically localized celladhesion molecules (SAMs), N-cadherin exists at the presynptic and postsynaptic terminals,and combined with β-catenin through the cellular structural domain. β-catenin combinedwith α-catenin, thus making the connection between N-cadherin and actin. Throughcontrolling the aggregation of synaptic proteins and the morphous of dentritic spines,N-cadherin and catenins participate in the process of the maturation of synapses. Siu alsofound that N-cadherin and PTPσ are highly expressed in dorsal root ganglion cells, and modulate the elongation of the neurites.Besides, no report has been published on the changes of the downstream molecularsN-cadherin andβ-catenin during development and the reactivation of plasticity in adults.Therefore, we applied the following methods and techniques to investigate therelationship between PTP σ and the termination of the critical period of visualdevelopment, as well as the possible underlying molecular mechanisms.Methods:1. With the use of Q-PCR and immunofluorescence histochemistry, we observed thePTPσ mRNA levels and the expression of PTPσ in the visual cortex during devlopment..2. With the use of immunofluorescence histochemistry to double labell PTPσ and PNNs,PTPσ and PV interneuron, triple immunostaining PTPσ, PNNs and PV neurons, wedetected the expression of PV, PNNs, PTPσ and their relationship during the developmentand the critical period.3. By forming the binocular formation deprivation animal model to reactivating thevisual cortex plasticity in adult rats and being tested by P-VEPs to confirm the reactivationof the visual plasticity in the adult visual cortex, with the use of Q-PCR andimmunohistochemistry, we detected the change of PTPσ, PNNs and PV expression in alllayers of the visual cortex and also tested the change of N-cadherin and β-catenin mRNAlevels during development and after BFD in adult rats.Results:1. The expression and distribution of PTPσduring the development and the criticalperiod(1) At PW1, the expression of PTPσ was at the highest level and then decreasedobviously after PW3in all layers, and stayed at a relatively low level during the criticalperiod, indicating that the expression of PTPσ is related to visual experience.(2) The expression of PTPσ in adulthood increased to the same level of that at thebeginning of the critical period, giving proof that PTPσmay participate in the terminationof critical period.(3) Cellular PTPσ was expressed in all layers of the VC through development. PTPσimmunolabeling appeared to be mainly localized within the cytoplasm and apical orprimary dendrites. PTPσ positive cells were found in all layers of visual cortex, which is mostly abundant in layer Ⅴ-Ⅵ. Interestingly, most PTPσ positive cells were neurons inthe visual cortex.2. Research on PTPσmodulating PV neurons to terminate the visual cortical plasticity(1) Most of the neurons wrapped by PNNs expressed PTPσ, indicating that CSPGsmay play its inhibitory role through its receptor PTPσ by inhibiting the axonal growth andmodulating the development and maturaion of PV neurons, which could be an importantmolecular mechanism of the termination of visual plasticity.(2) Most of the PV neurons expressed PTPσ during the development, which suggestedthat PTPσ may be involved in the modulation of PV function until the adulthood. PV/PTPσdouble labelled cells were more intense in layer Ⅳ compared to other layers, whish was inconsisitence with the location feature of PNNs and PNN/PTPσ double immunostainingcells.(3) The neurons expressed PTPσ and PV were also wrapped by PNNs, givingmorphological proof that CSPGs inhibit the axonal growth and the contact of synapses ofPV neurons by combining with PTPσ. The ratio of PV/PNN/PTPσ triple immunostainingneurons in PV neurons increased from the lowest ratio at PW3to the highest percentage atPW9, indicating that PTPσ gradually participate in the CSPG modulation of the PV neuronfunction, which was the important mechanism of the termination of visual plasticity.3. Research on the participation of PTPσ and its downstream molecules in therestoration of plasticity in adult visual cortex(1) Compared with the control rats (PW9, PTPσ mRNA level and the density of PTPσpositive cells in all layers were decreased in BFD group, suggesting that binocular formdeprivation could decrease the expression of PTPσ in the visual cortex of the adult rats.(2) Binocular form deprivation did not change the number of PV neurons in all thelayers of the visual cortex.(3) Binocular form deprivation could change the number of PNN postive cells in layerⅡ-Ⅲ and layer Ⅳ which are closly related to the visual cortical plasticity.(4) The expression of N-cadherin mRNA was highest at PW1, then decreased until thelower peak at PW7, followed by the lower level in adulthood, suggesting that theexpression of N-cadherin mRNA was infuluenced by the development and visualexperience with related to plasticity. However, BFD didn’t change the N-cadherin mRNA level in the visual cortex.(5) The expression of β-catenin mRNA was highest at PW1, then decreased until thelower peak at PW7, followed by the lower level in adulthood, suggesting that theexpression of β-catenin mRNA was infuluenced by the development and visual experiencewith related to plasticity. BFD increased the level of β-catenin mRNA to promote thegrowth of neurites, which may serve as the mechanism of the restoration of adult visualcortex plasticity.Conclusion:1. Our results found the expression of PTPσ during the development in the visualcortex for the first time. The expression of PTPσ was declined after PW1, and maintained atthe lowest level during the critical period. Interestingly, after the critical period (PW9), theexpression of PTPσ recovered to the same level at the start of the critical period (PW3).These results indicates that PTPσ is involved in the termination of the visual cortexplasticity.2. Most of the PV neurons expressed PTPσ, which countinued to the end of the criticalperiod, even in the adulthood, indicating that PTPσ could modulate the function of PVneurons during the development of the visual cortex. The neurons expressed PTPσ and PVwere also wrapped by PNNs, and its ratio in PV neurons increased from the lowest at PW3(the beginning of the critical period) to the highest ratio at PW9(the adulthood), indicatingthat PTPσ participates in the modulation of the termination of visual cortex plasticity byparticipating in the CSPG modulation of the function of PV neurons.3. We found that BFD14d could modulate the expression and distribution of CSPGsand PTPσ, suggesting that binocular form deprivation could decrease the expression ofCSPGs in the extracellular matrix and PTPσ, weaken the effect of the CSPG inhibition,could be one of the mechanism of the reactivation of the visual plasticity in the visualcortex of the adult rats.4. The expression of β-catenin mRNA was found highest at PW1, then decreased withage. At the end of the critical period (PW7), it increased to the second peak, but with asignificant decrease in the adulthood (PW9), indicating that the expression of β-cateninmRNA could be modulated by factors of age and visual experience which are related to thetermination of visual cortex plasticity. However, binocular form deprivation could significantly increase the level of β-catenin mRNA, which could faciliate the growth of theneurite and might be an important mechanism of the restoration of visual cortex plasticityin adult rats. |
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