Modeling non-syndromic autism and the impact of TRPC6 disruption in human neurons.

An increasing number of genetic variants have been implicated in autism spectrum disorders (ASDs), and the functional study of such variants will be critical for the elucidation of autism pathophysiology. Here, we report a de novo balanced translocation disruption of TRPC6, a cation channel, in a non-syndromic autistic individual. Using multiple models, such as dental pulp cells, induced pluripotent stem cell (iPSC)-derived neuronal cells and mouse models, we demonstrate that TRPC6 reduction or haploinsufficiency leads to altered neuronal development, morphology and function. The observed neuronal phenotypes could then be rescued by TRPC6 complementation and by treatment with insulin-like growth factor-1 or hyperforin, a TRPC6-specific agonist, suggesting that ASD individuals with alterations in this pathway may benefit from these drugs. We also demonstrate that methyl CpG binding protein-2 (MeCP2) levels affect TRPC6 expression. Mutations in MeCP2 cause Rett syndrome, revealing common pathways among ASDs. Genetic sequencing of TRPC6 in 1041 ASD individuals and 2872 controls revealed significantly more nonsynonymous mutations in the ASD population, and identified loss-of-function mutations with incomplete penetrance in two patients. Taken together, these findings suggest that TRPC6 is a novel predisposing gene for ASD that may act in a multiple-hit model. This is the first study to use iPSC-derived human neurons to model non-syndromic ASD and illustrate the potential of modeling genetically complex sporadic diseases using such cells.

Adult hippocampal neurogenesis: regulation by HIV and drugs of abuse.

New dentate granule cells are continuously generated from neural progenitor cells and integrated into the existing hippocampal circuitry in the adult mammalian brain through an orchestrated process termed adult neurogenesis. While the exact function remains elusive, adult neurogenesis has been suggested to play important roles in specific cognitive functions. Adult hippocampal neurogenesis is regulated by a variety of physiological and pathological stimulations. Here we review emerging evidence showing that HIV infection and several drugs of abuse result in molecular changes that may affect different aspects of adult hippocampal neurogenesis. These new findings raise the possibility that cognitive dysfunction in the setting of HIV infection or drug abuse may, in part, be related to alterations in hippocampal neurogenesis. A better understanding of how HIV and drugs of abuse affect both molecular and cellular aspects of adult neurogenesis may lead to development of more effective therapeutic interventions for these interlinked epidemics.

Acute morphogenic and chemotropic effects of neurotrophins on cultured embryonic Xenopus spinal neurons.

Neurotrophins constitute a family of trophic factors with profound effects on the survival and differentiation of the nervous system. Addition of brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3), but not nerve growth factor (NGF), increased the survival of embryonic Xenopus spinal neurons in culture, although all three neurotrophins enhanced neurite outgrowth. Here we report that neurotrophins also exert acute actions on the morphology and motility of 1-day-old cultured Xenopus spinal neurons. Bath application of BDNF induced extensive formation of lamellipodia simultaneously at multiple sites along the neurite shaft as well as at the growth cone. The BDNF-induced lamellipodia appeared within minutes, rapidly protruded to their greatest extent in about 10 min, and gradually disappeared thereafter, leaving behind newly formed thin lateral processes. When applied as microscopic concentration gradients, both BDNF and NT-3, but not NGF, induced the growth cone to grow toward the neurotrophin source. Our results suggest that neurotrophic factors, when delivered to responsive neurons, may serve as morphogenic and chemotropic agents during neuronal development.

cAMP-induced switching in turning direction of nerve growth cones.

Development of the nervous system depends on the correct pathfinding and target recognition by the growing tip of an axon, the growth cone. Diffusible or substrate-bound molecules present in the environment may serve as either attractants or repellents to influence the direction of growth-cone extension. Here we report that differences in cyclic-AMP-dependent activity in a neuron may result in opposite turning of the growth cone in response to the same guidance cue. A gradient of brain-derived neurotrophic factor normally triggers an attractive turning response of the growth cone of Xenopus spinal neurons in culture, but the same gradient induces repulsive turning of these growth cones in the presence of a competitive analogue of cAMP or of a specific inhibitor of protein kinase A. This cAMP-dependent switch of the turning response was also found for turning induced by acetylcholine, but not for the turning induced by neurotrophin-3 (NT-3). Thus, in the presence of other factors that modulate neuronal cAMP-dependent activity, the same guidance cue may trigger opposite turning behaviours of the growth cone during its pathfinding in the nervous system.

Turning of retinal growth cones in a netrin-1 gradient mediated by the netrin receptor DCC.

Netrin-1 promotes outgrowth of axons in vitro through the receptor Deleted in Colorectal Cancer (DCC) and elicits turning of axons within embryonic explants when presented as a point source. It is not known whether netrin-1 alone can elicit turning nor whether DCC mediates the turning response. We show that Xenopus retinal ganglion cell growth cones orient rapidly toward a pipette ejecting netrin-1, an effect blocked by antibodies to DCC. In vitro, netrin-1 induces a complex growth cone morphology reminiscent of that at the optic nerve head, a site of netrin-1 expression in vivo. These results demonstrate that netrin-1 can function alone to induce turning, implicate DCC in this response, and support the idea that netrin-1 contributes to steering axons out of the retina.

cAMP-dependent growth cone guidance by netrin-1.

Netrin-1 is known to function as a chemoattractant for several classes of developing axons and as a chemorepellent for other classes of axons, apparently dependent on the receptor type expressed by responsive cells. In culture, growth cones of embryonic Xenopus spinal neurons exhibited chemoattractive turning toward the source of netrin-1 but showed chemorepulsive responses in the presence of a competitive analog of cAMP or an inhibitor of protein kinase A. Both attractive and repulsive responses were abolished by depleting extracellular calcium and by adding a blocking antibody against the netrin-1 receptor Deleted in Colorectal Cancer. Thus, nerve growth cones may respond to the same guidance cue with opposite turning behavior, dependent on other coincident signals that set the level of cytosolic cAMP.