Lower levels of cortical [3H]pirenzepine binding to postmortem tissue defines a sub-group of older people with schizophrenia with less severe cognitive deficits

Higher levels of AKT-interacting protein in the frontal pole from people with schizophrenia are limited to a sub-group who have a marked deficit in cortical muscarinic M1 receptors

We are studying the molecular pathology of a sub-group within schizophrenia (∼ 25 %: termed Muscarinic Receptor Deficit subgroup of Schizophrenia (MRDS)) who can be separated because they have very low levels of cortical muscarinic M1 receptors (CHRM1). Based on our transcriptomic data from Brodmann's area ((BA) 9, 10 and 33 (controls, schizophrenia and mood disorders) and the cortex of the CHRM1-/- mouse (a molecular model of aberrant CHRM1 signaling), we predicted levels of AKT interacting protein (AKTIP), but not tubulin alpha 1b (TUBA1B) or AKT serine/threonine kinase 1 (AKT1) and pyruvate dehydrogenase kinase 1 (PDK1) (two AKTIP-functionally associated proteins), would be changed in MRDS. Hence, we used Western blotting to measure AKTIP (BA 10: controls, schizophrenia and mood disorders; BA 9: controls and schizophrenia) plus TUBA1B, AKT1 and PDK1 (BA 10: controls and schizophrenia) proteins. The only significant change with diagnosis was higher levels of AKTIP protein in BA 10 (Cohen's d = 0.73; p = 0.02) in schizophrenia compared to controls due to higher levels of AKTIP only in people with MRDS (Cohen's d = 0.80; p = 0.03). As AKTIP is involved in AKT1 signaling, our data suggests that signaling pathway is particularly disturbed in BA 10 in MRDS.

Lower levels of kainate receptors, but not AMPA or NMDA receptors, in Brodmann's area (BA) 9, but not BA 10, from a subgroup of people with schizophrenia who have a marked deficit in cortical muscarinic M1 receptors

In a previous study on ionotropic glutamate receptors, we have shown that [3H]kainate, but not [3H]AMPA or [3H]NMDA, receptor binding was lower in Brodmann's area (BA) 9 from people with schizophrenia. Subsequently, we defined a subgroup within the syndrome of schizophrenia who are termed the Muscarinic Receptor Deficit subgroup of Schizophrenia (MRDS) as they have markedly lower levels of [3H]pirenzepine binding to the muscarinic M1 receptor. The previous glutamate receptor study did not contain enough people with MRDS and other forms of schizophrenia (non-MRDS) to study any subgroup-specific differences. Hence, in this study we first measured [3H]pirenzepine binding to the muscarinic M1 receptor to confirm the MRDS subgroup, then measured [3H]kainate, [3H]AMPA and [3H]NMDA receptor binding using autoradiography in BA 9 from people with MRDS, non-MRDS and controls. We also measured binding in BA 10 as our gene expression study indicated that BA 10 is disproportionally affected by the molecular pathology of schizophrenia. As expected, due to case-selection criteria, [3H]pirenzepine binding to the M1 receptor was lower in BA 9 and BA 10 from people with MRDS, although more profound in BA 10. [3H]kainate receptor binding was lower only in BA 9 from people with MRDS, while [3H]AMPA and [3H]NMDA receptor binding was not altered in either region. Muscarinic M1 receptors and kainate receptors are both located on glutamatergic pyramidal neurons so a perturbation in both receptors could indicate altered excitatory neurotransmission in BA 9 from people with MRDS.

Changes in levels of the zinc transporter SLC39A12 in Brodmann's area 44: Effects of sex, suicide, CNS pH and schizophrenia

Disturbed CNS zinc homeostasis is suggested as part of the pathophysiology of schizophrenia. Our data, from multiple studies, suggests levels of cortical RNA for the solute carrier family 39 member 12 (SLC39A12), a putative zinc transporter, is higher in people with schizophrenia and is more perturbed in a sub-group of people with the disorder that can be separated because they have very low levels of muscarinic M1 receptors (MRDS). In this study qPCR was used to measure levels of two RNA splice variants of SLC39A12 (a and b) in Brodmann's area (BA) 44 from new cohorts of controls and people with schizophrenia. For the first time, in our study cohort as a whole, we report levels of both splice variants of SLC39A12 are lower in females compared to males and there are correlations between levels of SLC39A12 a and b and CNS pH. Levels of both splice variants were also lower in people with schizophrenia who were suicide completers compared to those who were not. Accounting for these factors, we showed levels of SLC39A12 a and b were higher in BA 44 in schizophrenia compared to controls. In further analyses, with and without our previous data on SLC39A12 a and b, we confirmed changes in levels of SLC39A12 RNAs were more profound in MRDS. In conclusion, our study argues there are higher levels of SLC39A12 a and b in BA 44 in schizophrenia which could be contributing to the breakdown in CNS zinc homeostasis suggested as part of the pathophysiology of schizophrenia, particularly in those with MRDS.

Lower levels of soluble β-amyloid precursor protein, but not β-amyloid, in the frontal cortex in schizophrenia

We identified a sub-group (25%) of people with schizophrenia (muscarinic receptor deficit schizophrenia (MRDS)) that are characterised because of markedly lower levels of cortical muscarinic M1 receptors (CHRM1) compared to most people with the disorder (non-MRDS). Notably, bioinformatic analyses of our cortical gene expression data shows a disturbance in the homeostasis of a biochemical pathway that regulates levels of CHRM1. A step in this pathway is the processing of β-amyloid precursor protein (APP) and therefore we postulated there would be altered levels of APP in the frontal cortex from people with MRDS. Here we measure levels of CHRM1 using [3H]pirenzepine binding, soluble APP (sAPP) using Western blotting and amyloid beta peptides (Aβ1-40 and Aβ1-42) using ELISA in the frontal cortex (Brodmann's area 6: BA 6; MRDS = 14, non-MRDS = 14, controls = 14). We confirmed the MRDS cohort in this study had the expected low levels of [3H]pirenzepine binding. In addition, we showed that people with schizophrenia, independent of their sub-group status, had lower levels of sAPP compared to controls but did not have altered levels of Aβ1-40 or Aβ1-42. In conclusion, whilst changes in sAPP are not restricted to MRDS our data could indicate a role of APP, which is important in axonal and synaptic pruning, in the molecular pathology of the syndrome of schizophrenia.

Muscarinic M1 and M4 receptor agonists for schizophrenia: promising candidates for the therapeutic arsenal

INTRODUCTION

Successful phase 3 trials of KarXT in people with schizophrenia herald a new era of treating the disorder with drugs that do not target the dopamine D2 receptor. The active component of KarXT is xanomeline, a muscarinic (CHRM) M1 and M4 agonist, making muscarinic receptors a viable target for treating schizophrenia.

AREAS COVERED

This review covers the process of taking drugs that activate the muscarinic M1 and M4 receptors from conceptualization to the clinic and details the mechanisms by which activating the CHRM1 and 4 can affect the broad spectrum of symptoms experienced by people with schizophrenia.

EXPERT OPINION

Schizophrenia is a syndrome which means drugs that activate muscarinic M1 and M4 receptors, as was the case for antipsychotic drugs acting on the dopamine D2 receptor, will not give optimal outcomes in everyone within the syndrome. Thus, it would be ideal to identify people who are responsive to drugs activating the CHRM1 and 4. Given knowledge of the actions of these receptors, it is possible treatment non-response could be restricted to sub-groups within the syndrome who have deficits in cortical CHRM1 or those with one of the cognitive endophenotypes that may be identifiable by changes in the blood transcriptome.