Genetic analysis of anterior posterior expression gradients in the developing mammalian forebrain

Cortical gene expression architecture links healthy neurodevelopment to the imaging, transcriptomics and genetics of autism and schizophrenia

Human brain organization involves the coordinated expression of thousands of genes. For example, the first principal component (C1) of cortical transcription identifies a hierarchy from sensorimotor to association regions. In this study, optimized processing of the Allen Human Brain Atlas revealed two new components of cortical gene expression architecture, C2 and C3, which are distinctively enriched for neuronal, metabolic and immune processes, specific cell types and cytoarchitectonics, and genetic variants associated with intelligence. Using additional datasets (PsychENCODE, Allen Cell Atlas and BrainSpan), we found that C1-C3 represent generalizable transcriptional programs that are coordinated within cells and differentially phased during fetal and postnatal development. Autism spectrum disorder and schizophrenia were specifically associated with C1/C2 and C3, respectively, across neuroimaging, differential expression and genome-wide association studies. Evidence converged especially in support of C3 as a normative transcriptional program for adolescent brain development, which can lead to atypical supragranular cortical connectivity in people at high genetic risk for schizophrenia.

Synaptic genes and neurodevelopmental disorders: From molecular mechanisms to developmental strategies of behavioral testing

Synaptopathies are a class of neurodevelopmental disorders caused by modification in genes coding for synaptic proteins. These proteins oversee the process of neurotransmission, mainly controlling the fusion and recycling of synaptic vesicles at the presynaptic terminal, the expression and localization of receptors at the postsynapse and the coupling between the pre- and the postsynaptic compartments. Murine models, with homozygous or heterozygous deletion for several synaptic genes or knock-in for specific pathogenic mutations, have been developed. They have proved to be extremely informative for understanding synaptic physiology, as well as for clarifying the patho-mechanisms leading to developmental delay, epilepsy and motor, cognitive and social impairments that are the most common clinical manifestations of neurodevelopmental disorders. However, the onset of these disorders emerges during infancy and adolescence while the behavioral phenotyping is often conducted in adult mice, missing important information about the impact of synaptic development and maturation on the manifestation of the behavioral phenotype. Here, we review the main achievements obtained by behavioral testing in murine models of synaptopathies and propose a battery of behavioral tests to improve classification, diagnosis and efficacy of potential therapeutic treatments. Our aim is to underlie the importance of studying behavioral development and better focusing on disease onset and phenotypes.

Roles of dual specificity tyrosine-phosphorylation-regulated kinase 2 in nervous system development and disease

Dual specificity tyrosine-phosphorylation-regulated kinases (DYRKs) are a group of conserved eukaryotic kinases phosphorylating tyrosine, serine, and threonine residues. The human DYRK family comprises 5 members (DYRK1A, DYRK1B, DYRK2, DYRK3, and DYRK4). The different DYRKs have been implicated in neurological diseases, cancer, and virus infection. Specifically, DYRK2 has been mainly implicated in cancer progression. However, its role in healthy and pathological nervous system function has been overlooked. In this context, we review current available data on DYRK2 in the nervous system, where the available studies indicate that it has key roles in neuronal development and function. DYRK2 regulates neuronal morphogenesis (e.g., axon growth and branching) by phosphorylating cytoskeletal elements (e.g., doublecortin). Comparative data reveals that it is involved in the development of olfactory and visual systems, the spinal cord and possibly the cortex. DYRK2 also participates in processes such as olfaction, vision and, learning. However, DYRK2 could be involved in other brain functions since available expression data shows that it is expressed across the whole brain. High DYRK2 protein levels have been detected in basal ganglia and cerebellum. In adult nervous system, DYRK2 mRNA expression is highest in the cortex, hippocampus, and retina. Regarding nervous system disease, DYRK2 has been implicated in neuroblastoma, glioma, epilepsy, neuroinflammation, Alzheimer's disease, Parkinson's disease, spinal cord injury and virus infection. DYRK2 upregulation usually has a negative impact in cancer-related conditions and a positive impact in non-malignant conditions. Its role in axon growth makes DYRK2 as a promising target for spinal cord or brain injury and regeneration.

Early Dendritic Morphogenesis of Adult-Born Dentate Granule Cells Is Regulated by FHL2

Dentate granule cells (DGCs), the progeny of neural stem cells (NSCs) in the sub-granular zone of the dentate gyrus (DG), must develop and functionally integrate with the mature cohort of neurons in order to maintain critical hippocampal functions throughout adulthood. Dysregulation in the continuum of DGC development can result in aberrant morphology and disrupted functional maturation, impairing neuroplasticity of the network. Yet, the molecular underpinnings of the signaling involved in adult-born DGC maturation including dendritic growth, which correlates with functional integration, remains incompletely understood. Given the high metabolic activity in the dentate gyrus (DG) required to achieve continuous neurogenesis, we investigated the potential regulatory role of a cellular metabolism-linked gene recently implicated in NSC cycling and neuroblast migration, called Four and a half LIM domain 2 (FHL2). The FHL2 protein modulates numerous pathways related to proliferation, migration, survival and cytoskeletal rearrangement in peripheral tissues, interacting with the machinery of the sphingosine-1-phosphate pathway, also known to be highly active especially in the hippocampus. Yet, the potential relevance of FHL2 to adult-born DGC development remains unknown. To elucidate the role of FHL2 in DGC development in the adult brain, we first confirmed the endogenous expression of FHL2 in NSCs and new granule cells within the DG, then engineered viral vectors for genetic manipulation experiments, investigating morphological changes in early stages of DGC development. Overexpression of FHL2 during early DGC development resulted in marked sprouting and branching of dendrites, while silencing of FHL2 increased dendritic length. Together, these findings suggest a novel role of FHL2 in adult-born DGC morphological maturation, which may open up a new line of investigation regarding the relevance of this gene in physiology and pathologies of the hippocampus such as mesial temporal lobe epilepsy (MTLE).

Lost in Translation: Traversing the Complex Path from Genomics to Therapeutics in Autism Spectrum Disorder

Recent progress in the genomics of non-syndromic autism spectrum disorder (nsASD) highlights rare, large-effect, germline, heterozygous de novo coding mutations. This distinguishes nsASD from later-onset psychiatric disorders where gene discovery efforts have predominantly yielded common alleles of small effect. These differences point to distinctive opportunities for clarifying the neurobiology of nsASD and developing novel treatments. We argue that the path ahead also presents key challenges, including distinguishing human pathophysiology from the potentially pleiotropic neurobiology mediated by established risk genes. We present our view of some of the conceptual limitations of traditional studies of model organisms, suggest a strategy focused on investigating the convergence of multiple nsASD genes, and propose that the detailed characterization of the molecular and cellular landscapes of developing human brain is essential to illuminate disease mechanisms. Finally, we address how recent advances are leading to novel strategies for therapeutics that target various points along the path from genes to behavior.

Interplay Between Amphetamine and Activity Level in Gene Networks of the Mouse Striatum

The psychostimulant amphetamine can be prescribed to ameliorate the symptoms of narcolepsy, attention-deficit hyperactivity disorder and to facilitate weight loss. This stimulant can also have negative effects including toxicity and addiction risk. The impact of amphetamine on gene networks is partially understood and this study addresses this gap in consideration of the physical activity. The striata of mice exposed to either amphetamine or saline treatment were compared in a mouse line selected for home cage physical overactivity, a phenotype that can be mitigated with amphetamine, and in a contemporary control line using RNA-seq. Genes presenting opposite expression patterns between treatments across lines included a pseudogene of coiled-coil-helix-coiled-coil-helix domain containing 2 gene (Chchd2), ribonuclease P RNA component H1 (Rpph1), short stature homeobox 2 (Shox2), transient receptor potential melastatin 6 (Trpm6), and tumor necrosis factor receptor superfamily, member 9 (Tnfrsf9). Genes presenting consistent treatment patterns across lines, albeit at different levels of significance included cholecystokinin (Cck), vasoactive intestinal polypeptide (Vip), arginine vasopressin (Avp), oxytocin/neurophysin (Oxt), thyrotropin releasing hormone (Trh), neurotensin (Nts), angiotensinogen (Agt), galanin (Gal), prolactin receptor (Prlr), and calcitonin receptor (Calcr). Potassium inwardly rectifying channel, subfamily J, member 6 (Kcnj6), and retinoic acid-related (RAR)-related orphan receptor alpha (Rora) were similarly differentially expressed between treatments across lines. Functional categories enriched among the genes presenting line-dependent amphetamine effect included genes coding for neuropeptides and associated with memory and neuroplasticity and synaptic signaling, energy, and redox processes. A line-dependent association between amphetamine exposure and the synaptic signaling genes neurogranin (Nrgn) and synaptic membrane exocytosis 1(Rims1) was highlighted in the gene networks. Our findings advance the understanding of molecular players and networks affected by amphetamine in support of the development of activity-targeted therapies that may capitalize on the benefits of this psychostimulant.

Genetic influences on cortical myelination in the human brain

Cortical myelination, which is essential for interneuronal communication and neurodevelopment, has been reported to be under genetic control. However, the degree to which genes contribute to the variability of myelination, the pattern of genetic control, and how genes influence the organization of myelination are largely unknown. To answer these questions, the present study calculated heritability estimates for myelination of the cortical regions using the high quality structural magnetic resonance imaging (MRI) scans from the Human Connectome Project pedigree cohort (n = 873, 383/490 M/F, 22-36 years of age). Then, we used transcriptional profiles to evaluate the contribution of myelination-related genes (data from the Allen Human Brain Atlas) to explain interregional variations in cortical myelination. Our results showed that all the cortical areas were modestly to moderately influenced by genetic factors (h² = 29%-66%, all Ps < 0.05 after Bonferroni correction). The genetic control of cortical myelination showed bilateral symmetry and an anterior-to-posterior gradation. A bivariate model indicated that the regions are strongly genetically correlated with their homologs in the opposite cerebral hemisphere. A cross-modal analysis did not find a correlation between cortical myelination and the expression levels of myelination-related genes. This could have been due to the small number of samples with expression data in each cortical region. Overall, our findings suggest that cortical myelination is shaped by genetic factors and may be useful to bridge the underlying genetic variants and the cognitive functioning and related neuropsychiatric disorders.

Distinct multivariate brain morphological patterns and their added predictive value with cognitive and polygenic risk scores in mental disorders

The brain underpinnings of schizophrenia and bipolar disorders are multidimensional, reflecting complex pathological processes and causal pathways, requiring multivariate techniques to disentangle. Furthermore, little is known about the complementary clinical value of brain structural phenotypes when combined with data on cognitive performance and genetic risk. Using data-driven fusion of cortical thickness, surface area, and gray matter density maps (GMD), we found six biologically meaningful patterns showing strong group effects, including four statistically independent multimodal patterns reflecting co-occurring alterations in thickness and GMD in patients, over and above two other independent patterns of widespread thickness and area reduction. Case-control classification using cognitive scores alone revealed high accuracy, and adding imaging features or polygenic risk scores increased performance, suggesting their complementary predictive value with cognitive scores being the most sensitive features. Multivariate pattern analyses reveal distinct patterns of brain morphology in mental disorders, provide insights on the relative importance between brain structure, cognitive and polygenetic risk score in classification of patients, and demonstrate the importance of multivariate approaches in studying the pathophysiological substrate of these complex disorders.

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Linked ICA showed six independent multivariate morphology patterns sensitive to SZ.

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Machine learning used to compare brain structure, cognitive and genetic scores.

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Cognition showed highest prediction of SZ, boosted by brain structure or genetics.

FHL1 mutations that cause clinically distinct human myopathies form protein aggregates and impair myoblast differentiation

FHL1 mutations cause several clinically heterogeneous myopathies including Reducing Body Myopathy (RBM), Scapuloperoneal Myopathy (SPM) and X-Linked Myopathy with Postural Muscle Atrophy (XMPMA). The molecular mechanisms underlying the pathogenesis of FHL1 myopathies are unknown. Protein aggregates designated “Reducing Bodies” (RBs) containing mutant FHL1 are detected in RBM muscle but not several other FHL1 myopathies. Here RBM, SPM and XMPMA FHL1 mutants were expressed in C2C12 cells and showed equivalent protein expression to wild-type FHL1 and formed aggregates positive for the RB stain Menadione-NBT, analogous to RBM muscle aggregates. However HCM and EDMD FHL1 mutants generally exhibited reduced expression. Wild-type FHL1 promotes myoblast differentiation however RBM, SPM and XMPMA mutations impaired differentiation, consistent with loss-of-normal function. Furthermore, SPM and XMPMA mutants retarded myotube formation relative to vector control consistent with a dominant-negative/toxic function. Mutant FHL1 myotube formation was partially rescued by expression of the FHL1-binding partner constitutively-active NFATc1. This is the first study to show FHL1 mutations identified in several clinically distinct myopathies lead to similar protein aggregation and impaired myotube formation suggesting a common pathogenic mechanism despite heterogenous clinical features.

Dyrk kinases regulate phosphorylation of doublecortin, cytoskeletal organization, and neuronal morphology

In a neuronal overexpression screen focused on kinases and phosphatases, one "hit" was the dual specificity tyrosine phosphorylation-regulated kinase (Dyrk4), which increased the number of dendritic branches in hippocampal neurons. Overexpression of various Dyrk family members in primary neurons significantly changed neuronal morphology. Dyrk1A decreased axon growth, Dyrk3 and Dyrk4 increased dendritic branching, and Dyrk2 decreased both axon and dendrite growth and branching. Kinase-deficient mutants revealed that most of these effects depend on kinase activity. Because doublecortin (DCX), a microtubule-binding protein, regulates cytoskeletal dynamics and neuronal morphogenesis, we investigated the possibility that DCX is a target of Dyrks. We found that overexpression of Dyrk2 and Dyrk3, but not Dyrk1A or Dyrk4, can change DCX phosphorylation status. Mutation of a consensus phosphorylation site for Dyrk kinases at Serine 306 (Ser306) in DCX indicated that this is one target site for Dyrk2 and Dyrk3. Overexpression of Dyrk2 restored altered DCX distribution in the growth cones of dendrites and axons, and partially reversed the morphological effects of DCX overexpression; some of these effects were abrogated by mutation of Ser306 to alanine. These studies implicate Dyrks in the regulation of cytoskeletal organization and process outgrowth in neurons, and suggest that DCX is one relevant Dyrk target.

Transcriptional programs in transient embryonic zones of the cerebral cortex defined by high-resolution mRNA sequencing

Characterizing the genetic programs that specify development and evolution of the cerebral cortex is a central challenge in neuroscience. Stem cells in the transient embryonic ventricular and subventricular zones generate neurons that migrate across the intermediate zone to the overlying cortical plate, where they differentiate and form the neocortex. It is clear that not one but a multitude of molecular pathways are necessary to progress through each cellular milestone, yet the underlying transcriptional programs remain unknown. Here, we apply differential transcriptome analysis on microscopically isolated cell populations, to define five transcriptional programs that represent each transient embryonic zone and the progression between these zones. The five transcriptional programs contain largely uncharacterized genes in addition to transcripts necessary for stem cell maintenance, neurogenesis, migration, and differentiation. Additionally, we found intergenic transcriptionally active regions that possibly encode unique zone-specific transcripts. Finally, we present a high-resolution transcriptome map of transient zones in the embryonic mouse forebrain.

Spinogenesis and Pruning in the Anterior Ventral Inferotemporal Cortex of the Macaque Monkey: An Intracellular Injection Study of Layer III Pyramidal Cells

Pyramidal cells grow and mature at different rates among different cortical areas in the macaque monkey. In particular, differences across the areas have been reported in both the timing and magnitude of growth, branching, spinogenesis, and pruning in the basal dendritic trees of cells in layer III. Presently available data suggest that these different growth profiles reflect the type of functions performed by these cells in the adult brain. However, to date, studies have focused on only a relatively few cortical areas. In the present investigation we quantified the growth of the dendritic trees of layer III pyramidal cells in the anterior ventral portion of cytoarchitectonic area TE (TEav) to better comprehend developmental trends in the cerebral cortex. We quantified the growth and branching of the dendrities, and spinogenesis and pruning of spines, from post-natal day 2 (PND2) to four and a half years of age. We found that the dendritic trees increase in size from PND2 to 7 months of age and thereafter became smaller. The dendritic trees became increasingly more branched from PND2 into adulthood. There was a two-fold increase in the number of spines in the basal dendritic trees of pyramidal cells from PND2 to 3.5 months of age and then a 10% net decrease in spine number into adulthood. Thus, the growth profile of layer III pyramidal cells in the anterior ventral portion of the inferotemporal cortex differs to that in other cortical areas associated with visual processing.

Gene expression in the rat brain: high similarity but unique differences between frontomedial-, temporal- and occipital cortex

BACKGROUND

The six-layered neocortex of the mammalian brain may appear largely homologous, but is in reality a modular structure of anatomically and functionally distinct areas. However, global gene expression seems to be almost identical across the cerebral cortex and only a few genes have so far been reported to show regional enrichment in specific cortical areas.

RESULTS

In the present study on adult rat brain, we have corroborated the strikingly similar gene expression among cortical areas. However, differential expression analysis has allowed for the identification of 30, 24 and 11 genes enriched in frontomedial -, temporal- or occipital cortex, respectively. A large proportion of these 65 genes appear to be involved in signal transduction, including the ion channel Fxyd6, the neuropeptide Grp and the nuclear receptor Rorb. We also find that the majority of these genes display increased expression levels around birth and show distinct preferences for certain cortical layers and cell types in rodents.

CONCLUSIONS

Since specific patterns of expression often are linked to equally specialised biological functions, we propose that these cortex sub-region enriched genes are important for proper functioning of the cortical regions in question.

A lifespan analysis of intraneocortical connections and gene expression in the mouse I

A hallmark of mammalian evolution is the structural and functional complexity of the cerebral cortex. Within the cerebral cortex, the neocortex, or isocortex, is a 6-layered complexly organized structure that is comprised of multiple interconnected sensory and motor areas. These areas and their precise patterns of connections arise during development, through a process termed arealization. Intrinsic, activity-independent and extrinsic, activity-dependent mechanisms are involved in the development of neocortical areas and their connections. The intrinsic molecular mechanisms involved in the establishment of this sophisticated network are not fully understood. In this report (I) and the companion report (II), we present the first lifespan analysis of ipsilateral intraneocortical connections (INCs) among multiple sensory and motor regions, from the embryonic period to adulthood in the mouse. Additionally, we characterize the neocortical expression patterns of several developmentally regulated genes that are of central importance to studies investigating the molecular control of arealization from embryonic day 13.5 to postnatal day (P) 3 (I) and P6 to 50 (II). In this analysis, we utilize novel methods to correlate the boundaries of gene expression with INCs and developing areal boundaries, in order to better understand the nature of gene-areal relationships during development.

A lifespan analysis of intraneocortical connections and gene expression in the mouse II

The mammalian neocortex contains an intricate processing network of multiple sensory and motor areas that allows the animal to engage in complex behaviors. These anatomically and functionally unique areas and their distinct connections arise during early development, through a process termed arealization. Both intrinsic, activity-independent and extrinsic, activity-dependent mechanisms drive arealization, much of which occurs during the areal patterning period (APP) from late embryogenesis to early postnatal life. How areal boundaries and their connections develop and change from infancy to adulthood is not known. Additionally, the adult patterns of sensory and motor ipsilateral intraneocortical connections (INCs) have not been thoroughly characterized in the mouse. In this report and its companion (I), we present the first lifespan analysis of ipsilateral INCs among multiple sensory and motor regions in mouse. We describe the neocortical expression patterns of several developmentally regulated genes that are of central importance to studies investigating the molecular regulation of arealization, from postnatal day (P) 6 to P50. In this study, we correlate the boundaries of gene expression patterns with developing areal boundaries across a lifespan, in order to better understand the nature of gene-areal relationships from early postnatal life to adulthood.

Cortical thickness is influenced by regionally specific genetic factors

BACKGROUND

Although global brain structure is highly heritable, there is still variability in the magnitude of genetic influences on the size of specific regions. Yet, little is known about the patterning of those genetic influences, i.e., whether the same genes influence structure throughout the brain or whether there are regionally specific sets of genes.

METHODS

We mapped the heritability of cortical thickness throughout the brain using three-dimensional structural magnetic resonance imaging in 404 middle-aged male twins. To assess the amount of genetic overlap between regions, we then mapped genetic correlations between three selected seed points and all other points comprising the continuous cortical surface.

RESULTS

There was considerable regional variability in the magnitude of genetic influences on cortical thickness. The primary visual (V1) seed point had strong genetic correlations with posterior sensory and motor areas. The anterior temporal seed point had strong genetic correlations with anterior frontal regions but not with V1. The middle frontal seed point had strong genetic correlations with inferior parietal regions.

CONCLUSIONS

These results provide strong evidence of regionally specific patterns rather than a single, global genetic factor. The patterns are largely consistent with a division between primary and association cortex, as well as broadly defined patterns of brain gene expression, neuroanatomical connectivity, and brain maturation trajectories, but no single explanation appears to be sufficient. The patterns do not conform to traditionally defined brain structure boundaries. This approach can serve as a step toward identifying novel phenotypes for genetic association studies of psychiatric disorders and normal and pathological cognitive aging.

Evolution of the neocortex: a perspective from developmental biology

The enlargement and species-specific elaboration of the cerebral neocortex during evolution holds the secret to the mental abilities of humans; however, the genetic origin and cellular mechanisms that generated the distinct evolutionary advancements are not well understood. This article describes how novelties that make us human may have been introduced during evolution, based on findings in the embryonic cerebral cortex in different mammalian species. The data on the differences in gene expression, new molecular pathways and novel cellular interactions that have led to these evolutionary advances may also provide insight into the pathogenesis and therapies for human-specific neuropsychiatric disorders.

Integrating microRNA and mRNA expression profiles of neuronal progenitors to identify regulatory networks underlying the onset of cortical neurogenesis

BACKGROUND

Cortical development is a complex process that includes sequential generation of neuronal progenitors, which proliferate and migrate to form the stratified layers of the developing cortex. To identify the individual microRNAs (miRNAs) and mRNAs that may regulate the genetic network guiding the earliest phase of cortical development, the expression profiles of rat neuronal progenitors obtained at embryonic day 11 (E11), E12 and E13 were analyzed.

RESULTS

Neuronal progenitors were purified from telencephalic dissociates by a positive-selection strategy featuring surface labeling with tetanus-toxin and cholera-toxin followed by fluorescence-activated cell sorting. Microarray analyses revealed the fractions of miRNAs and mRNAs that were up-regulated or down-regulated in these neuronal progenitors at the beginning of cortical development. Nearly half of the dynamically expressed miRNAs were negatively correlated with the expression of their predicted target mRNAs.

CONCLUSION

These data support a regulatory role for miRNAs during the transition from neuronal progenitors into the earliest differentiating cortical neurons. In addition, by supplying a robust data set in which miRNA and mRNA profiles originate from the same purified cell type, this empirical study may facilitate the development of new algorithms to integrate various "-omics" data sets.

Functional and evolutionary insights into human brain development through global transcriptome analysis

Our understanding of the evolution, formation, and pathological disruption of human brain circuits is impeded by a lack of comprehensive data on the developing brain transcriptome. A whole-genome, exon-level expression analysis of 13 regions from left and right sides of the mid-fetal human brain revealed that 76% of genes are expressed, and 44% of these are differentially regulated. These data reveal a large number of specific gene expression and alternative splicing patterns, as well as coexpression networks, associated with distinct regions and neurodevelopmental processes. Of particular relevance to cognitive specializations, we have characterized the transcriptional landscapes of prefrontal cortex and perisylvian speech and language areas, which exhibit a population-level global expression symmetry. We show that differentially expressed genes are more frequently associated with human-specific evolution of putative cis-regulatory elements. These data provide a wealth of biological insights into the complex transcriptional and molecular underpinnings of human brain development and evolution.

New horizons for the subplate zone and its pioneering neurons

Transitional neuronal layers are a hallmark of the prenatal and neonatal brain yet their contribution to the development of higher functions is not clear. Evidence accumulated over the last 3 decades shows that early connectivity and functional activity start in a transitional layer called the subplate zone (SPZ). The SPZ is host to a heterogeneous population of neurons and its evolutionary complexity peaked in the human brain. In this issue of Cerebral Cortex, three reports (Hoerder-Suabedissen et al., 2008; McKellar and Shatz, 2008; Moore et al., 2008) present new data and evidence in three species (mouse, rat, human) as to the function of the SPZ, to the heterogeneity of its cellular composition, and to the genetic basis of its development.

Gene expression analysis of neural cells and tissues using DNA microarrays

DNA microarrays pose specific challenges to those studying the central and peripheral nervous systems. Probably the most important involve difficulty in obtaining appropriate tissue for study, as well as the problems posed by cellular heterogeneity. This unit describes advances in the available technologies and provides protocols for cDNA microarray hybridization, including the use of PCR amplicons. Protocols are also provided for the two major methods for limiting cellular heterogeneity by study of RNA from single cell populations in high-throughput microarray studies, laser capture microdissection (LCM), and automated fluorescent cell sorting (FACS-array).