LTD-like molecular pathways in developmental synaptic pruning

Abstract

In long-term depression (LTD) at synapses in the adult brain, synaptic strength is reduced in an experience-dependent manner. LTD thus provides a cellular mechanism for information storage in some forms of learning. A similar activity-dependent reduction in synaptic strength also occurs in the developing brain and there provides an essential step in synaptic pruning and the postnatal development of neural circuits. Here we review evidence suggesting that LTD and synaptic pruning share components of their underlying molecular machinery and may thus represent two developmental stages of the same type of synaptic modulation that serve different, but related, functions in neural circuit plasticity. We also assess the relationship between LTD and synaptic pruning in the context of recent findings of LTD dysregulation in several mouse models of autism spectrum disorder (ASD) and discuss whether LTD deficits can indicate impaired pruning processes that are required for proper brain development.

Figure 1

Experience-dependent pruning shapes the cortical circuit architecture. (a) Synaptic density as a function of age in the human primary visual cortex. (b) The stabilization or elimination of cortical spines depends on the level of input activity and is controlled by Hebbian plasticity rules. Large and small lightning bolts symbolize synaptic input strength. LTD at weakly active synapses may result in synapse and spine pruning. Both LTD and pruning are activity-dependent processes that require activity at a threshold level. Panel a adapted from ref. , Elsevier.

Figure 2

Impaired CF synapse elimination and deficient PF LTD in mGluR1 knockout mice. (a) Schematic of the patch-clamp configuration used to record from an mGluR1 knockout Purkinje cell (PC) that is illustrated with multiple CF innervation. (b) Gross anatomy of the cerebellum (top, Nissl staining) and morphology of PC dendrites (bottom, calbindin immunostaining) are normal in mGluR1 knockout mice. (c) Persistent multiple CF innervation in adult mGluR1 knockout mice (P22–P75). CF-mediated EPSCs (left) and frequency distribution histogram showing the number of discrete CF EPSC steps at increasing stimulus strength (right), representing the number of CF inputs. (d) LTD at PF PC synapses is deficient in adult mGluR1 knockout mice. In wild-type mice, EPSPs elicited by PF stimulation undergoes LTD after conjunctive PF and CF stimulation (CJS) at 1 Hz for 5 min (top). In contrast, PF EPSPs are not depressed by CJS in mGluR1 knockout mice (bottom). All values are shown as mean ± s.e.m. (e) mGluR1 signaling pathway. CaMKII activation contributes to LTD through an indirect blockade of PP2A. CaMKII may similarly contribute to CF synaptic pruning, but this has not yet been verified. mGluR1, type 1 metabotropic glutamate receptor; Gαq, G-protein αq; PLCβ4, phospholipase Cβ4; PKC, protein kinase C; αCaMKII, α isoform of calcium/calmodulin-dependent kinase II; PP2A, protein phosphatase 2A; Arc, activity-regulated cytoskeleton-associated protein (also known as Arg3.1). Panels b,c adapted from ref. , Elsevier; d from ref. , AAAS.

Figure 3

Developmental CF synapse elimination in the rodent cerebellum. (a) Schemes representing CF innervation of Purkinje cells (PCs) at three stages of postnatal development. At around P3, the PC soma is innervated by multiple CFs with similar synaptic strengths. At around P9 to P17, a single winner CF extends its innervation territory from the soma to the growing PC dendrite, whereas the loser CFs maintain synapses on the soma. After P18, most of the somatic CF synapses are eliminated and a single winner CF innervates the PC, forming synapses on spines located on the primary dendrite. (b,d) Triple fluorescence labeling at P9 (b) and P12 (d) for BDA (biotinylated dextran amine, a tracer labeling a CF subset), VGluT2 (type 2 vesicular glutamate transporter, a CF terminal marker) and CB (calbindin, a PC marker). (b) At P9, the soma is innervated by BDA and VGluT2 double-positive CFs (yellow puncta) and BDA-negative and VGluT2-positive CF terminals (arrows, green puncta), indicating innervation by two different CFs. (d) At P12 the PC dendrites are innervated by BDA and VGluT2 double-positive CFs (yellow puncta) and the somata are contacted by BDA-negative and VGluT2-positive CFs (arrows, green puncta), indicating single strong CF inputs on PC dendrites and additional weak CF inputs on the somata. (c,e) Three-dimensionally reconstructed image of CF innervation from serial electron microscopic analysis of a PC at P9 (c) and P12 (e). Scale bars in b and d, 10 µm. Panels b–e adapted from ref. , Elsevier.

Figure 4

LTD at climbing fiber synapses depends on mGluR1–PKC signaling. (a) Recording configuration. Inset traces show typical CF responses recorded in voltage-clamp mode (CF EPSC; upper left) and current-clamp mode (complex spike; lower left). (b) CF LTD results from CF stimulation at 5 Hz for 30 s (n = 15; filled dots), but the EPSC amplitudes remain stable in the absence of tetanization (n = 5; open dots). (c) Model scheme of molecular events involved in CF LTD and CF pruning. (d) Top: CF LTD is absent in the presence of the group 1 mGluR antagonist AIDA (1 mM), which was bath-applied at the time indicated by the horizontal bar (n = 6). Bottom: CF LTD is also prevented in the presence of the bath-applied PKC inhibitor chelerythrine (10 µM; n = 5). Arrows indicate the time point of tetanization. All values are shown as mean ± s.e.m. Panels b,d adapted from ref. , Elsevier.

Figure 5

LTD dysregulation and impaired CF pruning in a mouse model of the human 15q11–13 duplication. (a) Schematic showing the corresponding regions of human chromosome 15 (left) and mouse chromosome 7 (right). Conserved linkage groups are shown by connecting lines between the human and mouse chromosomes. Genes that in wild-type mice are paternally expressed, maternally expressed and non-imprinting are labeled with blue, red and green, respectively. Dotted lines indicate the borders of the duplication regions. (b) Calbindin staining of a sagittal cerebellar section (top; scale bar: 1 mm) and Golgi staining of a Purkinje cell (PC) obtained from patDp/+ mice (bottom; scale bar: 50 µm). (c) Patch-clamp recording configuration. (d) CF pruning is impaired in patDp/+ mice. Left: typical traces showing discrete CF EPSC steps (holding potential: −10 mV) in slices from a P11 wild-type mouse (top) and a P11 patDp/+ mouse. Right: percentage of P10–12 wild-type (n = 55, from six mice) and patDp/+ (n = 56; from six mice) PCs showing one, two and three CF EPSC steps, which are taken as a measure of the number of CF inputs. (e) PF LTD dysregulation in patDp/+ mice. Top: typical PF EPSC traces before and after application of the tetanization protocol. PF LTD is induced in wild-type mice, but is absent from patDp/+ mice. Bottom: time graph showing PF LTD in wild-type mice (n = 10), and a potentiation in patDp/+ mice (n = 7). Arrow: time of tetanization. Error bars are mean ± s.e.m. Panels a,b,d,e adapted from ref. , Nature Publishing Group.