Postmortem brain: an underutilized substrate for studying severe mental illness

Abstract

We propose that postmortem tissue is an underutilized substrate that may be used to translate genetic and/or preclinical studies, particularly for neuropsychiatric illnesses with complex etiologies. Postmortem brain tissues from subjects with schizophrenia have been extensively studied, and thus serve as a useful vehicle for illustrating the challenges associated with this biological substrate. Schizophrenia is likely caused by a combination of genetic risk and environmental factors that combine to create a disease phenotype that is typically not apparent until late adolescence. The complexity of this illness creates challenges for hypothesis testing aimed at understanding the pathophysiology of the illness, as postmortem brain tissues collected from individuals with schizophrenia reflect neuroplastic changes from a lifetime of severe mental illness, as well as treatment with antipsychotic medications. While there are significant challenges with studying postmortem brain, such as the postmortem interval, it confers a translational element that is difficult to recapitulate in animal models. On the other hand, data derived from animal models typically provide specific mechanistic and behavioral measures that cannot be generated using human subjects. Convergence of these two approaches has led to important insights for understanding molecular deficits and their causes in this illness. In this review, we discuss the problem of schizophrenia, review the common challenges related to postmortem studies, discuss the application of biochemical approaches to this substrate, and present examples of postmortem schizophrenia studies that illustrate the role of the postmortem approach for generating important new leads for understanding the pathophysiology of severe mental illness.

Figure 1

A role for postmortem studies for investigating severe mental illness. Schizophrenia is caused by a combination of genetic risk and environmental factors that combine to create a disease phenotype that is typically not apparent until late adolescence. The complexity of this illness and its etiology creates challenges for hypothesis testing aimed at understanding the pathophysiology of the illness as well as developing new and more effective treatments. Postmortem brain tissues collected from individuals with schizophrenia reflect neuroplastic changes from a lifetime of severe mental illness (and the life stressors associated with severe medical illness) as well as treatment with antipsychotic medications. While there are significant concerns with studying postmortem brain, such as the postmortem interval, it confers a translational element that is difficult to recapitulate in animal models. Data derived from animal models typically provides specific mechanistic and behavioral measures that cannot be generated in the human condition. Convergence of these two approaches is providing important insights for understanding the molecular deficits and their causes in this often devastating illness.

Figure 2

Laser-capture microdissection coupled to protein assays. Nanoscale microcapillary electrophoresis (using the Nanopro1000 device, Protein Simple Corp., Santa Clara, CA, USA) analysis of (a) HeLa cell lysates, (b) 600 laser-capture microdissected (LCM) pyramidal neurons from the frontal cortex, or (c) cortical brain homogenate. The HeLa lysate (positive control) expresses all isoforms of ERK detectible by the NP1000 pan-ERK assay. The coefficient of variability for ERK2 in the pyramidal neuron LCM sample (n=6 replicates) was 8.1%.

Figure 3

Assembly and modification of neurotransmitter receptors from the nucleus to the dendrite. Neurotransmitter receptors such as the NMDA and AMPA glutamate receptors are synthesized and assembled in the ER before moving through the Golgi. Assembled receptors such as the AMPA and NMDA subtype glutamate receptors may then be phosphorylated and interact with motor proteins which bind the receptors to the microtubules for transport along the dendrite. The receptors then enter the spine where they interact with other anchoring proteins and are inserted at the synapse. AMPA receptor insertion into the synapse consists of direct insertion from the Golgi or indirect insertion and lateral translocation from the constitutive pool. AMPA receptors may also be localized in the regulated pool. NMDA receptors may be directly inserted and are anchored at the postsynaptic density by myriad proteins or may be internalized following phosphorylation. AMPA, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid; CaMKII, Ca²⁺/calmodulin-dependent protein kinases II; CK2, Casein Kinase 2; DLC, Dynein Light Chain; GKAP, Guanylate-kinase-associated Protein; GRIP/ABP, glutamate receptor AMPAR binding protein; KIF, Kinesin Family Member; mLin, Mammalian Lin; MyoV, Myosin V; NF-L, Neurofilament Light Chain; NMDA, N-Methyl-D-aspartate; nNOS, Neuronal Nitric Oxide Synthase; NSF, N-ethylmalemide sensitive factor; PKA, Protein Kinase A; PKC, Protein Kinase C; PP1, Protein Phosphatase 1; PRKCA 1 PICK1, Protein interacting with Protein Kinase C Alpha; PSD95; Post-Synaptic Density 95; SAP97, Synapse-Associated Protein 97; SAP102, Synapse-Associated Protein 102; Stg, Stargazin; 4.1N, Protein 4.1N.

Figure 4

Insertion of neurotransmitter receptors at the synapse. AMPA and NMDA receptors as examples of the complexity of receptor trafficking in the brain. AMPA receptor insertion into the synapse consists of direct insertion from the Golgi or indirect insertion and lateral translocation from the constitutive pool. AMPA receptors may also be localized in the regulated pool. NMDA receptors may be directly inserted and are anchored at the postsynaptic density by myriad proteins or may be internalized following phosphorylation. AMPA, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid; CaMKII, Ca²⁺/calmodulin-dependent protein kinases II; CK2, Casein Kinase 2; GKAP, Guanylate-kinase-associated Protein; GRIP/ABP, glutamate receptor AMPAR binding protein; NF-L, Neurofilament Light Chain; NMDA, N-Methyl-D-aspartate; nNOS, Neuronal Nitric Oxide Synthase; NSF, N-ethylmalemide sensitive factor; PKA, Protein Kinase A; PKC, Protein Kinase C; PP1, Protein Phosphatase 1; PRKCA 1 PICK1, Protein interacting with Protein Kinase C Alpha; PSD95; Post-Synaptic Density 95; SAP97, Synapse-Associated Protein 97; SAP102, Synapse-Associated Protein 102; Stg, Stargazin; 4.1N, Protein 4.1N.