Damian Sendler: We use a variety of experimental models to study the origin, development, and therapy of complex human brain illnesses. Human dermal fibroblast (HDF) cultures obtained from patients have recently re-emerged as a potential in vitro functional system for studying different cellular, molecular, metabolic and (patho)physiological states and features of mental illnesses. To supplement postmortem and animal investigations, HDF studies may provide valuable information regarding the alterations in brain tissue homeostasis. Patients with schizophrenia, depression, bipolar disorder and other mental illnesses have contributed greatly to our knowledge of these dreadful diseases via the analysis of HDFs. For example, these studies show conclusively that the HDF model may be used to study signal transduction, redox homeostasis and circadian rhythms. This underused patient biomaterial, in combination with contemporary molecular biology methods, may also have diagnostic and prognostic value, including the prediction of response to therapy drugs, according to the results.
Damian Jacob Sendler: Simplicity, isolation of important pathophysiological components and experimentation are all advantages of models. The model must be able to examine cause and effect links in order to investigate a system. This means that we must choose a model that best fits our needs, and then we must sacrifice some of the less important variables in order to keep things simple. In order to get conclusive evidence and acquire new information, it is essential to choose a model that is based on the tested hypothesis.
Dr. Sendler: As non-invasive methods can only be used to study the brain pathophysiology of psychiatric disorders in humans, animal models are the preferred choice to study behavioral disturbances and associated structural and functional brain alterations that result from genetic manipulations of the human brain (Brown et al., 2015; Schmidt et al., 2014; Schmidt and Mirnics, 2012). Because mental diseases are unique human circumstances, it is impossible to analyze these studies without knowing what is happening in a patient’s sick tissue (Nestler and Hyman, 2010).
Damian Sendler
To keep mature neurons alive after brain biopsies from people with mental health issues is incredibly difficult to do (Hayashi-Takagi et al., 2014). In order to investigate disease-related alterations in the transcriptome, proteome, and metabolome, postmortem investigations are the method of choice. However, the most crucial functional experiments, such as assessing geneenvironment (GE) interactions, stress resilience, or medication effects, cannot be performed using these tissue samples. Individual lifestyle variations, comorbidity and drug misuse, medication usage, postmortem interval, hormonal state and many other variables hamper the interpretation of results in this sample of opportunity (Mirnics et al., 2006; Schmitt et al., 2008). In addition, postmortem tissue’s limited diagnostic and therapeutic value is likely its biggest inherent restriction.
Neurochemical, cellular, neurochemical, molecular, systemic, psychological and social factors all contribute to the pathology and pathophysiology of psychiatric illnesses (Schmitt et al., 2014). It’s important to note, however, that all human-based in vitro experimental systems have significant limitations, but also significant advantages: they allow mechanistic experimentation on patient biomaterial, which is especially important because none of the in-vivo rodent models of psychiatric disorders can fully recapitulate human genetic diversity and overall disease phenotype (Sarnyai et al., 2011; Kaiser and Feng, 2015).
Biological specimens obtained from patients are in high demand, especially those that can be multiplied without affecting their genetic make-up. Psychiatric disorders like schizophrenia (SZ) can be studied in cell cultures because (1) they show high heritability, suggesting a significant genetic contribution to the disease, which would be preserved in an in vitro model; and (2) psychiatric disorders appear to be a disease of the whole body (and not just the brain), so many of the disease-associated immune, endocrine, and microbial processes can be studied in cell cultures.
For decades, researchers have been using patient-derived functional models to conduct research. As of now, the most commonly used patient-derived models include (6) human dermal fibroblasts (HDFs), (7) immortalized peripheral lymphocytes, (8) induced pluripotent stem cells (iPSCs), and (9) human peripheral blood leukocytes (PBLs). These models are all derived from patients, and they have been used to study a variety of diseases (Hayashi-Takagi et al., 2014).
Damian Jacob Sendler
Researchers have traditionally used peripheral leukocytes and mononuclear cells obtained from peripheral blood in psychiatric studies because they believe that the immune and neurological systems communicate in both directions (Khandaker and Dantzer, 2015; Naydenov et al., 2007). The proteome, signaling profile, and hormone and drug responsiveness of peripheral leukocytes and brain cells have been hypothesized to be comparable (Gladkevich et al., 2004; Iga et al., 2008). Peripheral leukocyte count and function were shown to be altered in mental illnesses in many investigations, making it an effective ex vivo model (reviewed in Gladkevich et al., 2004 and Iga et al., 2008). Both external and internal stimuli may be used to monitor the disease condition with the use of peripheral leukocytes/mononuclear cells (PLM/MNC) paradigm. There are several limitations to the PBC models in terms of individual variability and susceptibility to non-neuropsychiatric variables such as lifestyle, hormone state, circadian rhythm and exposure to infections (Druzd et al., 2014; Gladkevich et al., 2004). Psychopharmacological therapy also affects the amount and function of leukocytes in circulation (Munzer et al., 2013; Tourjman et al., 2013). All of these elements contribute to a large amount of “biological noise,” which may make it difficult to find modest effects or distinguish unambiguous causation (Krieger et al., 1998). However, peripheral blood cells may still be used to conduct ex vivo functional experiments for one generation.
Damian Jacob Markiewicz Sendler: There are currently two methods for obtaining brain cells: iPSC differentiation (Hu et al., 2010; direct transdifferentiation; and lineage conversion) (Pang et al., 2011). IPS cells may be reprogrammed into neural progenitor cells (NPCs) and adult neurons by the addition of growth factors or exogenous growth factors (reviewed by Kim et al., 2014). They can be readily cultivated and preserved since the acquired cells are believed to follow the same neuronal growth pathways as those found in the brain (Song et al., 2013; Hu et al., 2010). Cell lineage conversion has been developed as an alternative iPSC production technique because of the technical difficulties connected with iPSC research (i.e. generation time and resource needs, (epi)genetic alterations and population heterogeneity and oncogenesis) (Pang et al., 2011). Two microRNAs or a single transcription factor may be all that is needed to drive neural cell destiny in human postmitotic somatic cells and create functioning neurons, according to new research (Chanda et al., 2014; Yoo et al, 2011).
Neurons in the brain have receptor expression and an electrophysiologic profile similar to that of iPSC-derived and transdifferentiated iNCs (Belinsky et al., 2014; Chanda et al., 2014). GABAergic cortical interneurons (Liu et al., 2013), glutamatergic pyramidal neurons (Liu and colleagues, 2013), dopaminergic midbrain neurons, and spinal motor neurons may all be created (Zeng et al., 2010). Because of the possibility of de novo copy number variations (CNVs) occurring during the somatic cell – iPSC – iNC transition (Corrales et al., 2012) and because neurons mature over the course of six to twelve weeks, it is necessary to validate the iNC models using a variety of genetic, electrophysiologic, and pharmacologic approaches. Extensive culture periods typically result in diverse, non-synchronic cell populations made up of progenitors, glial cells, mature neurons, and immature neurons throughout this time period (Dage et al., 2014; Fathi et al, 2011). (Belinsky et al., 2014). iPSCs “reset” in their epigenetics, telomere processing, and mitochondrial architecture, but transdifferentiated neurons preserve the donor’s age-related signature (Rohani et al., 2014).
Damien Sendler: Neurotransmitter systems have permitted the creation of psychiatric medications that alleviate symptoms, but they have not shed light on the complete pathophysiology of these illnesses. They have not been fully understood. When this transition occurred, researchers began to focus less on neurotransmitter levels and more on intracellular dysregulations. It is clear from the results of these investigations that signal transduction abnormalities may change neuronal function and predispose to mental and emotional illnesses (Akin et al., 2005; Dwivedi et al., 2002; Zhan et al., 2011).
Receptor and effector expression, the presence of co-regulatory factors, epigenetic alterations and final effectors all play a role in the cell type-specific response to stimuli (most commonly a transcription factor). Many cell types have similar second messenger systems and intracellular signaling cascades in common. Nevertheless. As Rieske and colleagues found, HDF cells exhibit transcription factors, signaling molecules and receptors that are extremely comparable to those found in cells of neuro-ectodermal origin.
There is a growing body of research indicating that the patient-derived HDF model is an excellent starting point for the development of innovative molecular biology tools and a good cell system for understanding the molecular underpinnings of mental illness. HDFs are valuable because of the simplicity with which patient cell lines can be obtained and established, because they have molecular similarities with CNS cells, and because they can be multiplied without the need of extra genetic alterations. As a platform for examining G*E interactions, signal transduction, and metabolic dysfunctions, HDFs aid in our knowledge of brain illnesses by enabling us to compare in vitro data to pathophysiology of the CNS. HDF may also be used to discover indicators of brain pathology and hence stress the organic elements of neuropsychiatric diseases. HDFs may have a significant impact on pharmacogenetics and personalized medicine in the near future because of these properties.
