Doctor of Philosophy, S.U.N.Y. State University at Buffalo (2010)
Neuroligins are postsynaptic cell-adhesion molecules that bind to presynaptic neurexins. Although the general synaptic role of neuroligins is undisputed, their specific functions at a synapse remain unclear, even controversial. Moreover, many neuroligin gene mutations were associated with autism, but the pathophysiological relevance of these mutations is often unknown, and their mechanisms of action uninvestigated. Here, we examine the synaptic effects of an autism-associated neuroligin-4 substitution (called R704C), which mutates a cytoplasmic arginine residue that is conserved in all neuroligins. We show that the R704C mutation, when introduced into neuroligin-3, enhances the interaction between neuroligin-3 and AMPA receptors, increases AMPA-receptor internalization and decreases postsynaptic AMPA-receptor levels. When introduced into neuroligin-4, conversely, the R704C mutation unexpectedly elevated AMPA-receptor-mediated synaptic responses. These results suggest a general functional link between neuroligins and AMPA receptors, indicate that both neuroligin-3 and -4 act at excitatory synapses but perform surprisingly distinct functions, and demonstrate that the R704C mutation significantly impairs the normal function of neuroligin-4, thereby validating its pathogenicity.Molecular Psychiatry advance online publication, 17 March 2015; doi:10.1038/mp.2015.20.
View details for DOI 10.1038/mp.2015.20
View details for PubMedID 25778475
Direct conversion of nonneural cells to functional neurons holds great promise for neurological disease modeling and regenerative medicine. We previously reported rapid reprogramming of mouse embryonic fibroblasts (MEFs) into mature induced neuronal (iN) cells by forced expression of three transcription factors: ASCL1, MYT1L, and BRN2. Here, we show that ASCL1 alone is sufficient to generate functional iN cells from mouse and human fibroblasts and embryonic stem cells, indicating that ASCL1 is the key driver of iN cell reprogramming in different cell contexts and that the role of MYT1L and BRN2 is primarily to enhance the neuronal maturation process. ASCL1-induced single-factor neurons (1F-iN) expressed mature neuronal markers, exhibited typical passive and active intrinsic membrane properties, and formed functional pre- and postsynaptic structures. Surprisingly, ASCL1-induced iN cells were predominantly excitatory, demonstrating that ASCL1 is permissive but alone not deterministic for the inhibitory neuronal lineage.
View details for DOI 10.1016/j.stemcr.2014.05.020
View details for PubMedID 25254342
Direct lineage reprogramming is a promising approach for human disease modeling and regenerative medicine, with poorly understood mechanisms. Here, we reveal a hierarchical mechanism in the direct conversion of fibroblasts into induced neuronal (iN) cells mediated by the transcription factors Ascl1, Brn2, and Myt1l. Ascl1 acts as an "on-target" pioneer factor by immediately occupying most cognate genomic sites in fibroblasts. In contrast, Brn2 and Myt1l do not access fibroblast chromatin productively on their own; instead, Ascl1 recruits Brn2 to Ascl1 sites genome wide. A unique trivalent chromatin signature in the host cells predicts the permissiveness for Ascl1 pioneering activity among different cell types. Finally, we identified Zfp238 as a key Ascl1 target gene that can partially substitute for Ascl1 during iN cell reprogramming. Thus, a precise match between pioneer factors and the chromatin context at key target genes is determinative for transdifferentiation to neurons and likely other cell types.
View details for DOI 10.1016/j.cell.2013.09.028
View details for Web of Science ID 000326571800016
Recent studies suggest that induced neuronal (iN) cells that are directly transdifferentiated from nonneuronal cells provide a powerful opportunity to examine neuropsychiatric diseases. However, the validity of using this approach to examine disease-specific changes has not been demonstrated. Here, we analyze the phenotypes of iN cells that were derived from murine embryonic fibroblasts cultured from littermate wild-type and mutant mice carrying the autism-associated R704C substitution in neuroligin-3. We show that neuroligin-3 R704C-mutant iN cells exhibit a large and selective decrease in AMPA-type glutamate receptor-mediated synaptic transmission without changes in NMDA-type glutamate receptor- or in GABAA receptor-mediated synaptic transmission. Thus, the synaptic phenotype observed in R704C-mutant iN cells replicates the previously observed phenotype of R704C-mutant neurons. Our data show that the effect of the R704C mutation is applicable even to neurons transdifferentiated from fibroblasts and constitute a proof-of-concept demonstration that iN cells can be used for cellular disease modeling.
View details for DOI 10.1073/pnas.1316240110
View details for PubMedID 24046374
We and others have reported the successful conversion of human fibroblasts into functional induced neuronal (iN) cells; however the reprogramming efficiencies were very low. Robust reprogramming methods must be developed before iN cells can be used for translational applications such as disease modeling or transplantation-based therapies. Here, we describe a novel approach in which we significantly enhance iN cell conversion efficiency of human fibroblast cells by reprogramming under hypoxic conditions (5% O2). Fibroblasts were derived under high (21%) or low (5%) oxygen conditions and reprogrammed into iN cells using a combination of the four transcription factors BRN2, ASCL1, MYT1L and NEUROD1. An increase in Map2 immunostaining was only observed when fibroblasts experienced an acute drop in O2 tension upon infection. Interestingly, cells derived and reprogrammed under hypoxic conditions did not produce more iN cells. Approximately 100% of patched cells fired action potentials in low O2 conditions compared to 50% under high O2 growth conditions, confirming the beneficial aspect of reprogramming under low O2. Further characterization showed no significant difference in the intrinsic properties of iN cells reprogrammed in either condition. Surprisingly, the acute drop in oxygen tension did not affect cell proliferation or cell survival and was not synergistic with the blockade of GSK3β and Smad-mediated pathways. Our results showed that lowering the O2 tension at the initiation of reprogramming is a simple and efficient stratergy to enhance the production of iN cells which will facilitate their use for basic discovery and regenerative medicine.
View details for DOI 10.1016/j.jneumeth.2013.03.020
View details for PubMedID 23562599
Available methods for differentiating human embryonic stem cells (ESCs) and induced pluripotent cells (iPSCs) into neurons are often cumbersome, slow, and variable. Alternatively, human fibroblasts can be directly converted into induced neuronal (iN) cells. However, with present techniques conversion is inefficient, synapse formation is limited, and only small amounts of neurons can be generated. Here, we show that human ESCs and iPSCs can be converted into functional iN cells with nearly 100% yield and purity in less than 2 weeks by forced expression of a single transcription factor. The resulting ES-iN or iPS-iN cells exhibit quantitatively reproducible properties independent of the cell line of origin, form mature pre- and postsynaptic specializations, and integrate into existing synaptic networks when transplanted into mouse brain. As illustrated by selected examples, our approach enables large-scale studies of human neurons for questions such as analyses of human diseases, examination of human-specific genes, and drug screening.
View details for DOI 10.1016/j.neuron.2013.05.029
View details for Web of Science ID 000320743400006
We recently showed that defined sets of transcription factors are sufficient to convert mouse and human fibroblasts directly into cells resembling functional neurons, referred to as "induced neuronal" (iN) cells. For some applications however, it would be desirable to convert fibroblasts into proliferative neural precursor cells (NPCs) instead of neurons. We hypothesized that NPC-like cells may be induced using the same principal approach used for generating iN cells. Toward this goal, we infected mouse embryonic fibroblasts derived from Sox2-EGFP mice with a set of 11 transcription factors highly expressed in NPCs. Twenty-four days after transgene induction, Sox2-EGFP(+) colonies emerged that expressed NPC-specific genes and differentiated into neuronal and astrocytic cells. Using stepwise elimination, we found that Sox2 and FoxG1 are capable of generating clonal self-renewing, bipotent induced NPCs that gave rise to astrocytes and functional neurons. When we added the Pou and Homeobox domain-containing transcription factor Brn2 to Sox2 and FoxG1, we were able to induce tripotent NPCs that could be differentiated not only into neurons and astrocytes but also into oligodendrocytes. The transcription factors FoxG1 and Brn2 alone also were capable of inducing NPC-like cells; however, these cells generated less mature neurons, although they did produce astrocytes and even oligodendrocytes capable of integration into dysmyelinated Shiverer brain. Our data demonstrate that direct lineage reprogramming using target cell-type-specific transcription factors can be used to induce NPC-like cells that potentially could be used for autologous cell transplantation-based therapies in the brain or spinal cord.
View details for DOI 10.1073/pnas.1121003109
View details for Web of Science ID 000300489200073
View details for PubMedID 22308465
Activation of group I metabotropic glutamate receptors (mGluRs) has been suggested to modulate development of auditory neurons. However, the acute effects of mGluR activation on physiological response properties are unclear. To address this, we studied the effects of mGluRs in bushy cells (BCs) of the mammalian anteroventral cochlear nucleus (AVCN). Activation of mGluRs with dihydroxyphenylglycine (DHPG) caused depolarization of BCs in mice as old as P42, but did not affect neurotransmitter release by presynaptic auditory nerve (AN) fibers. Application of mGluR antagonists indicated that mGluRs are tonically active, and are highly sensitive to small elevations in ambient glutamate by the glutamate reuptake blocker threo-?-benzyloxyaspartic acid (TBOA). mGluR-mediated depolarization enhanced the firing probability in response to AN stimulation, and reduced the latency and jitter. Furthermore, excitation through postsynaptic mGluRs can significantly counterbalance the inhibitory effects of presynaptic GABA(B) receptors. Thus, interaction between these two modulatory pathways may provide additional flexibility for fine-tuning the BC relay.
View details for DOI 10.1523/JNEUROSCI.1193-11.2011
View details for Web of Science ID 000290716600023
View details for PubMedID 21593328
One important model for understanding neuronal computation is how auditory information is transformed at the synapses made by auditory nerve (AN) fibers on the bushy cells (BCs) in the anteroventral cochlear nucleus (AVCN). This transformation is influenced by synaptic plasticity, the mechanisms of which have been studied primarily using postsynaptic electrophysiology. However, it is also important to make direct measurements of the presynaptic terminal to consider presynaptic mechanisms. Here we introduce a technique for doing that using calcium imaging of presynaptic AN terminals, by injecting dextran-conjugated fluorophores into the cochlea. To measure the calcium transients, we used calcium-sensitive fluorophores, and measured the changes in fluorescence upon stimulation. As an example of the application of this technique, we showed that activation of GABA(B) receptors reduces presynaptic calcium influx. This technique could be further extended to study the effects of activity- and other neuromodulator-dependent plasticities on AN terminals.
View details for DOI 10.1016/j.jneumeth.2010.11.008
View details for Web of Science ID 000287073400004
View details for PubMedID 21108967
Modulation of synaptic strength by ?-aminobutyric acid receptors (GABARs) is a common feature in sensory pathways that contain relay cell types. However, the functional impact of these receptors on information processing is not clear. We considered this issue at bushy cells (BCs) in the cochlear nucleus, which relay auditory nerve (AN) activity to higher centers. BCs express GABA(A)Rs, and synaptic inputs to BCs express GABA(B)Rs. We tested the effects of GABAR activation on the relaying of AN activity using patch-clamp recordings in mature mouse brain slices at 34°C. GABA affected BC firing in response to trains of AN activity at concentrations as low as 10 ?M. GABA(A)Rs reduced firing primarily late in high-frequency trains, whereas GABA(B)Rs reduced firing early and in low-frequency trains. BC firing was significantly restored when two converging AN inputs were activated simultaneously, with maximal effect over a window of <0.5 ms. Thus GABA could adjust the function of BCs, to suppress the relaying of individual inputs and require coincident activity of multiple inputs.
View details for DOI 10.1152/jn.00474.2010
View details for Web of Science ID 000282649900022
View details for PubMedID 20702743
Postsynaptic receptor desensitization has been observed to contribute to depression in immature synapses. However, it is not clear whether desensitization persists and causes depression in mature synapses. We investigate this issue at the endbulb of Held, the synapse made by auditory nerve (AN) fibers onto bushy cells (BCs) of the anteroventral cochlear nucleus, where depression could influence the processing of sound information. Experiments using cyclothiazide (CTZ) have implicated desensitization in endbulbs from postnatal day 16 (P16) to P21 mice, but application of ?-D-glutamylglycine (DGG) did not reveal desensitization in endbulbs >P22. To reconcile these findings, we have studied the effects of both CTZ and DGG on endbulbs from P5 to P40 CBA/CaJ mice. In paired-pulse protocols, both CTZ and DGG reduced depression in all ages at intervals <10 ms, consistent with their effects preventing desensitization. However, DGG increased depression at intervals >20 ms, consistent with DGG's use to prevent saturation. DGG application revealed receptor saturation even under conditions of very low release probability. Preventing desensitization by CTZ occluded the effects of DGG on desensitization and revealed the effects of saturation at short intervals. We developed an approach to separate DGG's effect on saturation from its effect on desensitization, which showed that desensitization has an impact during bursts of auditory nerve activity. Dynamic-clamp experiments indicated that desensitization can reduce BC spike probability and increase latency and jitter. Thus desensitization may affect sound processing in the mature auditory system.
View details for DOI 10.1152/jn.00751.2009
View details for Web of Science ID 000276555800021
View details for PubMedID 20107122
Complexins (CPXs I-IV) presumably act as regulators of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex, but their function in the intact mammalian nervous system is not well established. Here, we explored the role of CPXs in the mouse auditory system. Hearing was impaired in CPX I knock-out mice but normal in knock-out mice for CPXs II, III, IV, and III/IV as measured by auditory brainstem responses. Complexins were not detectable in cochlear hair cells but CPX I was expressed in spiral ganglion neurons (SGNs) that give rise to the auditory nerve. Ca(2+)-dependent exocytosis of inner hair cells and sound encoding by SGNs were unaffected in CPX I knock-out mice. In the absence of CPX I, the resting release probability in the endbulb of Held synapses of the auditory nerve fibers with bushy cells in the cochlear nucleus was reduced. As predicted by computational modeling, bushy cells had decreased spike rates at sound onset as well as longer and more variable first spike latencies explaining the abnormal auditory brainstem responses. In addition, we found synaptic transmission to outlast the stimulus at many endbulb of Held synapses in vitro and in vivo, suggesting impaired synchronization of release to stimulus offset. Although sound encoding in the cochlea proceeds in the absence of complexins, CPX I is required for faithful processing of sound onset and offset in the cochlear nucleus.
View details for DOI 10.1523/JNEUROSCI.0632-09.2009
View details for Web of Science ID 000267339000006
View details for PubMedID 19553439