No abstract

We report a type of synaptic modulation that involves retrograde signaling from postsynaptic metabotropic glutamate receptors (mGluRs) to presynaptic cannabinoid receptors. Activation of mGluR subtype 1 (mGluR1) expressed in cerebellar Purkinje cells (PCs) reduced neurotransmitter release from excitatory climbing fibers. This required activation of G proteins but not Ca2+ elevation in postsynaptic PCs. This effect was occluded by a cannabinoid agonist and totally abolished by cannabinoid antagonists. Depolarization-induced Ca2+ transients in PCs also caused cannabinoid receptor-mediated presynaptic inhibition. Thus, endocannabinoid production in PCs can be initiated by two distinct stimuli. Activation of mGluR1 by repetitive stimulation of parallel fibers, the other excitatory input to PCs, caused transient cannabinoid receptor-mediated depression of climbing fiber input. Our data highlight a signaling mechanism whereby activation of postsynaptic mGluR retrogradely influences presynaptic functions via endocannabinoid system.

White adipocytes are unique in that they contain large unilocular lipid droplets that occupy most of the cytoplasm. To identify genes involved in the maintenance of mature adipocytes, we expressed dominant-negative PPARγ in 3T3-L1 cells and performed a microarray screen. The fat-specific protein of 27 kDa (FSP27) was strongly downregulated in this context. FSP27 expression correlated with induction of differentiation in cultured preadipocytes, and the protein localized to lipid droplets in murine white adipocytes in vivo. Ablation of FSP27 in mice resulted in the formation of multilocular lipid droplets in these cells. Furthermore, FSP27-deficient mice were protected from diet-induced obesity and insulin resistance and displayed an increased metabolic rate due to increased mitochondrial biogenesis in white adipose tissue (WAT). Depletion of FSP27 by siRNA in murine cultured white adipocytes resulted in the formation of numerous small lipid droplets, increased lipolysis, and decreased triacylglycerol storage, while expression of FSP27 in COS cells promoted the formation of large lipid droplets. Our results suggest that FSP27 contributes to efficient energy storage in WAT by promoting the formation of unilocular lipid droplets, thereby restricting lipolysis. In addition, we found that the nature of lipid accumulation in WAT appears to be associated with maintenance of energy balance and insulin sensitivity.

PKC gamma is highly expressed in Purkinje cells (PCs) but not in other types of neurons in the cerebellum. The expression of PKC gamma changes markedly during cerebellar development, being very low at birth and reaching a peak around the third postnatal week. This temporal pattern of PKC gamma expression coincides with the developmental transition from multiple to single climbing fiber innervation onto each PC. In adult mutant mice deficient in PKC gamma, we found that 41% of PCs are still innervated by multiple climbing fibers, while other aspects of the cerebellum including the morphology and excitatory synaptic transmission of PCs appear normal. Thus, elimination of multiple climbing fiber innervation appears to be specifically impaired in the mutant cerebellum. We suggest that the developmental role of PKC gamma may be to act as a downstream element in the signal cascade necessary for the elimination of surplus climbing fiber synapses.