MCU-enriched dendritic mitochondria regulate plasticity in distinct hippocampal circuits.

Mitochondria are dynamic organelles that are morphologically and functionally diverse across cell types and subcellular compartments in order to meet unique energy demands. Mitochondrial dysfunction has been implicated in a wide variety of neurological disorders, including psychiatric disorders like schizophrenia and bipolar disorder. Despite it being well known that mitochondria are essential for synaptic transmission and synaptic plasticity, the mechanisms regulating mitochondria in support of normal synapse function are incompletely understood. The mitochondrial calcium uniporter (MCU) regulates calcium entry into the mitochondria, which in turn regulates the bioenergetics and distribution of mitochondria to active synapses. Evidence suggests that calcium influx via MCU couples neuronal activity to mitochondrial metabolism and ATP production, which would allow neurons to rapidly adapt to changing energy demands. Intriguingly, MCU is uniquely enriched in hippocampal CA2 distal dendrites relative to neighboring hippocampal CA1 or CA3 distal dendrites, however, the functional significance of this enrichment is not clear. Synapses from the entorhinal cortex layer II (ECII) onto CA2 distal dendrites readily express long term potentiation (LTP), unlike the LTP- resistant synapses from CA3 onto CA2 proximal dendrites, but the mechanisms underlying these different plasticity profiles are unknown. We hypothesized that enrichment of MCU near ECII-CA2 synapses promotes LTP in an otherwise plasticity-restricted cell type. Using a CA2-specific MCU knockout (cKO) mouse, we found that MCU is required for LTP at distal dendrite synapses but does not affect the lack of LTP at proximal dendrite synapses. Loss of LTP at ECII-CA2 synapses correlated with a trend for decreased spine density in CA2 distal dendrites of cKO mice compared to control (CTL) mice, which was predominantly seen in immature spines. Moreover, mitochondria were significantly smaller and more numerous across all dendritic layers of CA2 in cKO mice compared to CTL mice, suggesting an overall increase in mitochondrial fragmentation. Fragmented mitochondria might have functional changes, such as altered ATP production, that might explain a deficit in synaptic plasticity. Collectively, our data reveal that MCU regulates layer-specific forms of plasticity in CA2 dendrites, potentially by maintaining proper mitochondria morphology and distribution within dendrites. Differences in MCU expression across different cell types and circuits might be a general mechanism to tune the sensitivity of mitochondria to cytoplasmic calcium levels to power synaptic plasticity. MAIN TAKE HOME POINTS: The mitochondrial calcium uniporter (MCU) regulates plasticity selectively at synapses in CA2 distal dendrites.The MCU-cKO induced LTP deficit correlates with a trending reduction in spine density in CA2 distal dendrites.Loss of MCU in CA2 results in ultrastructural changes in dendritic mitochondria that suggest an increase in mitochondrial fragmentation. These ultrastructural changes could result in functional consequences, such as decreased ATP production, that could underlie the plasticity deficit.Dendritic mitochondrial fragmentation in MCU cKO occurred throughout the dendritic laminae, suggesting that MCU is dispensable for establishing layer-specific mitochondrial structural diversity.

GABAB receptor-dependent bidirectional regulation of critical period ocular dominance plasticity in cats.

Gama amino butyric acid (GABA) inhibition plays an important role in the onset and offset of the critical period for ocular dominance (OD) plasticity in the primary visual cortex. Previous studies have focused on the involvement of GABAA receptors, while the potential contribution of GABAB receptors to OD plasticity has been neglected. In this study, the GABAB receptor antagonist SCH50911 or agonist baclofen was infused into the primary visual cortex of cats concurrently with a period of monocular deprivation (MD). Using single-unit recordings we found that the OD shift induced by four days of MD during the critical period was impaired by infusion of the antagonist SCH50911, but enhanced by infusion of the agonist baclofen. In contrast, seven days of MD in adult cats did not induce any significant OD shift, even when combined with the infusion of SCH50911 or baclofen. Together, these findings indicate that an endogenous GABAB receptor-mediated inhibition contributes to juvenile, but not adult, OD plasticity.

Adult visual experience promotes recovery of primary visual cortex from long-term monocular deprivation.

Prolonged visual deprivation from early childhood to maturity is believed to cause permanent visual impairment. However, there have been case reports of substantial improvement of binocular vision in human adults following lifelong visual impairment or deprivation. These observations, together with recent findings of adult ocular dominance plasticity in rodents, led us to re-examine whether adult primary visual cortex (V1) is capable of any recovery following long-term monocular deprivation starting in development. Using mice as a model, we find that monocular deprivation from early development to mature ages (well past the critical period) severely impaired binocular vision by reducing the amplitude of responses elicited by stimulation of the deprived eye. Surprisingly, we find little effect on nondeprived eye responses. Restoration of binocular vision in mature adults yields modest but significant improvement of visual responses in V1. Remarkably, we find that when binocular vision is followed by occlusion of the nondeprived eye, visual responses in V1 recover almost fully, as measured by visual evoked potential amplitude, spatial frequency threshold, and single-unit activity. We conclude that adult V1 can recover from long-term deprivation when provided with an optimal regimen of visual experience.

Monocular deprivation in adult mice alters visual acuity and single-unit activity.

It has been discovered recently that monocular deprivation in young adult mice induces ocular dominance plasticity (ODP). This contradicts the traditional belief that ODP is restricted to a juvenile critical period. However, questions remain. ODP of young adults has been observed only using methods that are indirectly related to vision, and the plasticity of young adults appears diminished in comparison with juveniles. Therefore, we asked whether the newly discovered adult ODP broadly reflects plasticity of visual cortical function and whether it persists into full maturity. Single-unit activity is the standard physiological marker of visual cortical function. Using a more optimized protocol for recording single-units, we find evidence of adult ODP of single-units and show that it is most pronounced in deep cortical layers. Furthermore, using visual evoked potentials (VEP), we find that ODP is equally robust in young adults and mature adults and is observable after just one day of monocular deprivation. Finally, we find that monocular deprivation in adults changes spatial frequency thresholds of the VEP, decreasing the acuity of the deprived pathway and improving the acuity of the non-deprived pathway. Thus, in mice, the primary visual cortex is capable of remarkable adaptation throughout life.

Experience-driven plasticity of visual cortex limited by myelin and Nogo receptor.

Monocular deprivation normally alters ocular dominance in the visual cortex only during a postnatal critical period (20 to 32 days postnatal in mice). We find that mutations in the Nogo-66 receptor (NgR) affect cessation of ocular dominance plasticity. In NgR-/- mice, plasticity during the critical period is normal, but it continues abnormally such that ocular dominance at 45 or 120 days postnatal is subject to the same plasticity as at juvenile ages. Thus, physiological NgR signaling from myelin-derived Nogo, MAG, and OMgp consolidates the neural circuitry established during experience-dependent plasticity. After pathological trauma, similar NgR signaling limits functional recovery and axonal regeneration.

Reversible blockade of experience-dependent plasticity by calcineurin in mouse visual cortex.

Numerous protein kinases have been implicated in visual cortex plasticity, but the role of serine/threonine protein phosphatases has not yet been established. Calcineurin, the only known Ca2+/calmodulin-activated protein phosphatase in the brain, has been identified as a molecular constraint on synaptic plasticity in the hippocampus and on memory. Using transgenic mice overexpressing calcineurin inducibly in forebrain neurons, we now provide evidence that calcineurin is also involved in ocular dominance plasticity. A transient increase in calcineurin activity is found to prevent the shift of responsiveness in the visual cortex following monocular deprivation, and this effect is reversible. These results imply that the balance between protein kinases and phosphatases is critical for visual cortex plasticity.

Requirement for the RIIbeta isoform of PKA, but not calcium-stimulated adenylyl cyclase, in visual cortical plasticity.

The cAMP-dependent protein kinase (PKA) signaling pathway plays a key role in visual cortical plasticity. Inhibitors that block activation of all PKA regulatory subunits (RIalpha,RIbeta, RIIalpha, RIIbeta) abolish long-term potentiation (LTP) and long-term depression (LTD) in vitro and ocular dominance plasticity (ODP) in vivo. The details of this signaling cascade, however, including the source of PKA signals and which PKA subunits are involved, are unknown. To investigate these issues we have examined LTP, LTD, and ODP in knock-out mice lacking either the two cortically expressed Ca2+-stimulated adenylyl cyclases (AC1 and AC8) or the predominant neocortical subunit of PKA (RIIbeta). Here we show that plasticity remains intact in AC1/AC8-/- mice, whereas ODP and LTD, but not LTP, are absent in RIIbeta-/- mice. We conclude that (1) plasticity in the visual cortex does not require the activity of known Ca2+-stimulated adenylyl cyclases, (2) the PKA dependence of ODP and LTD, but not LTP, is mediated by RIIbeta-PKA, and (3) multiple isoforms of PKA contribute to LTD.

Reduced ocular dominance plasticity and long-term potentiation in the developing visual cortex of protein kinase A RII alpha mutant mice.

The cAMP-dependent protein kinase (PKA) signalling pathway has been shown to play an important role in long-term potentiation (LTP) and depression (LTD), and ocular dominance plasticity in the visual cortex. In order to investigate further the involvement of individual PKA subunits in visual cortical plasticity, LTP and LTD in vitro and ocular dominance plasticity in vivo in the developing visual cortex were examined in mice lacking the RII alpha subunit of PKA. Here we show that LTP in layers II/III was decreased in RII alpha knockout mice, but LTD was almost unaffected, and the ocular dominance shift induced by monocular deprivation was also partially blocked. These data provide evidence that RII alpha is involved in LTP and ocular dominance plasticity, and further suggest that different afferent inputs could selectively activate particular subunits of PKA and thereby direct specific aspects of visual cortical plasticity.

Blockade of cyclic AMP-dependent protein kinase does not prevent the reverse ocular dominance shift in kitten visual cortex.

Monocular deprivation (MD) during the critical period for the development of visual cortex causes a loss of binocular response of neurons and a shift to the open eye, a normal ocular dominance (OD) shift. However, when MD is combined with chronic inactivation of the visual cortex by muscimol, the OD distribution of the neurons shifts to the deprived eye (reverse OD shift). We have previously shown that the normal OD shift is abolished by chronic infusion of the protein kinase A (PKA) inhibitor, 8-chloroadenosine-3', 5'-cyclic monophosphorothioate, Rp-isomer (Rp-8-Cl-cAMPS), into kitten visual cortex. In this study, we investigated the effect of this inhibitor on the reverse OD shift. Combination of MD and muscimol infusion into the visual cortex of 6-wk-old kittens caused a reverse OD shift that was comparable to that seen in previous studies. However, a reverse OD shift was also seen with concurrent infusion of the PKA inhibitor with muscimol. The strongest OD shift was observed in layer IV regardless of the presence or absence of the PKA inhibitor. This suggests that the dissociation of pre- and postsynaptic activities, which occurs mainly at thalamocortical synapses, induces the reverse OD shift and that inhibition of PKA does not prevent it. Presumably, an inhibition of PKA has no effect in silent cortex. We conclude that 1) an activation of PKA is not required for the induction of the reverse OD shift, and 2) the intracellular signaling mechanism underlying MD-induced OD plasticity differs between normal and reverse OD shifts.

Orientation selectivity is reduced by monocular deprivation in combination with PKA inhibitors.

We have previously shown that the protein kinase A (PKA) inhibitor, 8-chloroadenosine-3',5'-monophosphorothioate (Rp-8-Cl-cAMPS), abolishes ocular dominance plasticity in the cat visual cortex. Here we investigate the effect of this inhibitor on orientation selectivity. The inhibitor reduces orientation selectivity in monocularly deprived animals but not in normal animals. In other words, PKA inhibitors by themselves do not affect orientation selectivity, nor does monocular deprivation by itself, but monocular deprivation in combination with a PKA inhibitor does affect orientation selectivity. This result is found for the receptive fields in both deprived and nondeprived eyes. Although there is a tendency for the orientation selectivity in the nondeprived eye to be higher than the orientation selectivity in the deprived eye, the orientation selectivity in both eyes is considerably less than normal. The result is striking in animals at 4 wk of age. The effect of the monocular deprivation on orientation selectivity is reduced at 6 wk of age and absent at 9 wk of age, while the effect on ocular dominance shifts is less changed in agreement with previous results showing that the critical period for orientation/direction selectivity ends earlier than the critical period for ocular dominance. We conclude that closure of one eye in combination with inhibition of PKA reduces orientation selectivity during the period that orientation selectivity is still mutable and that the reduction in orientation selectivity is transferred to the nondeprived eye.