A comparative study of neocortical development between humans and great apes

The neocortex is the most recently evolved part of the mammalian brain which is involved in a repertoire of higher order brain functions, including those that separate humans from other animals. Humans have evolved an expanded neocortex over the course of evolution through a massive increase in neuron number (compared to our close relatives-­‐‑ the chimpanzees) in spite of sharing similar gestation time frames. So what do humans do differently compared to chimpanzees within the same time frame during their development? This dissertation addresses this question by comparing the developmental progression of neurogenesis between humans and chimpanzees using cerebral organoids as the model system. The usage of cerebral organoids, has enabled us to compare the development of both the human neocortex, and the chimpanzee neocortex from the very initiation of the neural phase of embryogenesis until very long periods of time. The results obtained so far suggest that the genetic programs underlying the development of the chimpanzee neocortex and the human neocortex are not very different, but rather the difference lies in the timing of the developmental progression. These results show that the chimpanzee neocortex spends lesser time in its proliferation phase, and allots lesser time to the generation of its neurons than the human neocortex. In more scientific terms, the neurogenic phase of the neocortex is shorter in chimpanzees than it is in humans. This conclusion is supported by (1) an earlier onset of gliogenesis in chimpanzees compared to humans which is indicative of a declining neurogenic phase, (2) an earlier increase in the chimpanzee neurogenic progenitors during development, compared to humans, (3) a higher number of stem cell– like progenitors in human cortices compared to chimpanzees, (4) a decline in neurogenic areas within the chimpanzee cerebral organoids over time compared to human cerebral organoids

A comparative study of neocortical development between humans and great apes

The neocortex is the most recently evolved part of the mammalian brain which is involved in a repertoire of higher order brain functions, including those that separate humans from other animals. Humans have evolved an expanded neocortex over the course of evolution through a massive increase in neuron number (compared to our close relatives-­‐‑ the chimpanzees) in spite of sharing similar gestation time frames. So what do humans do differently compared to chimpanzees within the same time frame during their development? This dissertation addresses this question by comparing the developmental progression of neurogenesis between humans and chimpanzees using cerebral organoids as the model system. The usage of cerebral organoids, has enabled us to compare the development of both the human neocortex, and the chimpanzee neocortex from the very initiation of the neural phase of embryogenesis until very long periods of time. The results obtained so far suggest that the genetic programs underlying the development of the chimpanzee neocortex and the human neocortex are not very different, but rather the difference lies in the timing of the developmental progression. These results show that the chimpanzee neocortex spends lesser time in its proliferation phase, and allots lesser time to the generation of its neurons than the human neocortex. In more scientific terms, the neurogenic phase of the neocortex is shorter in chimpanzees than it is in humans. This conclusion is supported by (1) an earlier onset of gliogenesis in chimpanzees compared to humans which is indicative of a declining neurogenic phase, (2) an earlier increase in the chimpanzee neurogenic progenitors during development, compared to humans, (3) a higher number of stem cell– like progenitors in human cortices compared to chimpanzees, (4) a decline in neurogenic areas within the chimpanzee cerebral organoids over time compared to human cerebral organoids

A comparative study of neocortical development between humans and great apes

The neocortex is the most recently evolved part of the mammalian brain which is involved in a repertoire of higher order brain functions, including those that separate humans from other animals. Humans have evolved an expanded neocortex over the course of evolution through a massive increase in neuron number (compared to our close relatives-­‐‑ the chimpanzees) in spite of sharing similar gestation time frames. So what do humans do differently compared to chimpanzees within the same time frame during their development? This dissertation addresses this question by comparing the developmental progression of neurogenesis between humans and chimpanzees using cerebral organoids as the model system. The usage of cerebral organoids, has enabled us to compare the development of both the human neocortex, and the chimpanzee neocortex from the very initiation of the neural phase of embryogenesis until very long periods of time. The results obtained so far suggest that the genetic programs underlying the development of the chimpanzee neocortex and the human neocortex are not very different, but rather the difference lies in the timing of the developmental progression. These results show that the chimpanzee neocortex spends lesser time in its proliferation phase, and allots lesser time to the generation of its neurons than the human neocortex. In more scientific terms, the neurogenic phase of the neocortex is shorter in chimpanzees than it is in humans. This conclusion is supported by (1) an earlier onset of gliogenesis in chimpanzees compared to humans which is indicative of a declining neurogenic phase, (2) an earlier increase in the chimpanzee neurogenic progenitors during development, compared to humans, (3) a higher number of stem cell– like progenitors in human cortices compared to chimpanzees, (4) a decline in neurogenic areas within the chimpanzee cerebral organoids over time compared to human cerebral organoids

Mutants in phospholipid signaling attenuate the behavioral response of adult Drosophila to trehalose

In Drosophila melanogaster, gustatory receptor genes (Grs) encode putative G-protein-coupled receptors (GPCRs) that are expressed in gustatory receptor neurons (GRNs). One of the Gr genes, Gr5a, encodes a receptor for trehalose that is expressed in a subset of GRNs. Although a role for the G protein, Gsα, has been shown in Gr5a-expressing taste neurons, there is the residual responses to trehalose in Gsα mutants which could suggest additional transduction mechanisms. Expression and genetic analysis of the heterotrimeric G-protein subunit, Gq, shown here suggest involvement of this Gα subunit in trehalose perception in Drosophila. A green fluorescent protein reporter of Gq expression is detected in gustatory neurons in the labellum, tarsal segments, and wing margins. Animals heterozygous for dgq mutations and RNA interference-mediated knockdown of dgq showed reduced responses to trehalose in the proboscis extension reflex assay and feeding behavior assay. These defects were rescued by targeted expression of the wild-type dgqα transgene in the GRNs. These data together with observations from other mutants in phospholipid signaling provide insights into the mechanisms of taste transduction in Drosophila

Mutants in Drosophila TRPC channels reduce olfactory sensitivity to carbon dioxide.

BACKGROUND: Members of the canonical Transient Receptor Potential (TRPC) class of cationic channels function downstream of Gαq and PLCβ in Drosophila photoreceptors for transducing visual stimuli. Gαq has recently been implicated in olfactory sensing of carbon dioxide (CO(2)) and other odorants. Here we investigated the role of PLCβ and TRPC channels for sensing CO(2) in Drosophila. METHODOLOGY/PRINCIPAL FINDINGS: Through behavioral assays it was demonstrated that Drosophila mutants for plc21c, trp and trpl have a reduced sensitivity for CO(2). Immuno-histochemical staining for TRP, TRPL and TRPγ indicates that all three channels are expressed in Drosophila antennae including the sensory neurons that express CO(2) receptors. Electrophysiological recordings obtained from the antennae of protein null alleles of TRP (trp(343)) and TRPL (trpl(302)), showed that the sensory response to multiple concentrations of CO(2) was reduced. However, trpl(302); trp(343) double mutants still have a residual response to CO(2). Down-regulation of TRPC channels specifically in CO(2) sensing olfactory neurons reduced the response to CO(2) and this reduction was obtained even upon down-regulation of the TRPCs in adult olfactory sensory neurons. Thus the reduced response to CO(2) obtained from the antennae of TRPC RNAi strains is not due to a developmental defect. CONCLUSION: These observations show that reduction in TRPC channel function significantly reduces the sensitivity of the olfactory response to CO(2) concentrations of 5% or less in adult Drosophila. It is possible that the CO(2) receptors Gr63a and Gr21a activate the TRPC channels through Gαq and PLC21C

Expression of TRPC proteins in CO₂ sensing neurons located in the third antennal segment of adult <i>Drosophila</i>.

<p>TRP, TRPL and TRPγ are expressed in CO₂ responsive neurons in the adult <i>Drosophila</i> antenna. A) Frozen antennal sections (14 µm thick) from <i>Gr21aGAL4/UASH2bRFP</i> animals stained with anti-TRP, anti-TRPL and anti-TRPγ antibodies showing expression of TRP, TRPL and TRPγ respectively along the membranes of the Gr21a receptor neurons, marked by anti- RFP staining in red. The first panel shows the localization of Gr21a neurons in the antenna after staining with anti- RFP. B) Frozen antennal sections (14 µm thick) from the null mutants of <i>trpl</i> and <i>trp</i> stained with anti-TRPL and anti-TRP antibodies respectively. No expression of TRPL and TRP proteins could be observed in the respective mutant strains. mAb22C10 (anti-futch, microtubule protein) staining in red served as a neuronal marker.</p

Null mutants of <i>trp</i> and <i>trpl</i> show reduced behavioral avoidance towards CO₂.

<p>A) The mean avoidance index towards 5% CO₂ in a Y-maze behavioral assay is shown for the indicated genotypes. The ability of <i>trpl</i> null homozygotes (<i>trpl³⁰²/trpl³⁰²</i>), to discriminate between 5% CO₂ and air is significantly reduced (<i>p<</i>0.0001) as compared to the heterozygous control. The phenotype of the null mutant is rescued by expressing a wild type <i>trpl</i> transgene in Gr21a receptor neurons (<i>trpl³⁰²/trpl³⁰²; Gr21a GAL4/UAS trpl⁺</i>) (<i>p<</i>0.0001). B) Null mutant of <i>trp</i> (<i>trp³⁴³/trp³⁴³</i>) has reduced avoidance to 5% CO₂ in the Y-maze assay. The avoidance response of the double null mutant (<i>trpl³⁰²/trpl³⁰²; trp³⁴³/trp³⁴³</i>) is also reduced but not significantly different from the single null homozygotes (<i>p></i>0.05). Error bars indicate SEM in A and B.</p

Reduced sensitivity to CO₂ is not a developmental defect.

<p>A) RNAi lines grown at the restrictive temperature of 18°C (active GAL80) show normal electrophysiological responses to CO_2, since the CO₂ receptor neuron specific GAL4 remains inactive (absence of RNAi expression; <i>p></i>0.05). RNAi lines grown at the permissive temperature of 29°C (inactive GAL80) show reduced electrophysiological responses to CO₂ due to active GAL4 and RNAi expression (n = 10; <i>p<</i>0.0001). The RNAi heterozygotes in the absence of <i>Gr63aGAL4</i> show normal responses to CO₂ at 29°C. Error bars indicate SEM. B) Whole antennal mounts showing CO₂ sensory neurons marked using <i>UAS RedStinger</i> driven by <i>Gr21aGAL4</i> in wild type, <i>plc21C^P319/plc21C^P319</i> and <i>trpl³⁰²/trpl³⁰²</i> mutant lines. C) Quantification of CO₂ sensory neurons in the adult antennae of the same lines (n = 14; <i>p</i> value not statistically significant).</p

Electrophysiological recordings from the antennae of <i>plc21C</i> mutants.

<p>A) Representative traces of field recordings obtained from the basiconica rich region of the 3^rd antennal segment of 3 to 4 days old flies. Individual genotypes are indicated. Both the <i>plc21C</i> mutants show reduced electrophysiological responses to the three concentrations of CO₂ tested as compared to the wild type flies (n = 10, <i>p<</i>0.0001). <i>Gr63a</i> null mutants (<i>Gr63a−/−</i>) and an RNAi knockdown of Gαq in CO₂ sensitive neurons (<i>Gr21aGAL4>UASGαq^1F1</i>) were included as test controls. B) Quantification of the field recordings for the genotypes tested (n = 10; <i>p<</i>0.0001). Error bars indicate SEM.</p