Ernest Arenas (1962-2024): a legacy of advancing dopaminergic neurogenesis research Free

The scientific community mourns the loss of Ernest Arenas, a visionary neuroscientist and pioneer in the field of stem cell neurobiology who, throughout his career, displayed a deep dedication to unravelling the mysteries of the human brain. Ernest passed away on 15 September 2024 at the age of 62, leaving behind a profound legacy of groundbreaking discoveries. Renowned for his pivotal contributions to understanding the development and function of dopaminergic neurons, particularly in the context of Parkinson's disease, Ernest's work has not only advanced our knowledge of neurodegenerative disorders but has also paved the way for potential therapeutic interventions. Beyond his scientific acumen, Ernest was esteemed for his deep commitment to mentorship and collaboration, for his warm and approachable personality, and for nurturing the next generation of neuroscientists and fostering a spirit of innovation and excellence within his research group. His pioneering work has left an indelible mark on the field, inspiring numerous researchers to continue pushing the boundaries of stem cell neurobiology.

Ernest was born on the 13 June 1962 in Lleida (Catalonia, Spain). He spent his childhood in Manresa – the capital of Bages county, located in the central region of Catalonia – an industrial area with textile, metallurgical and glass industries. Ernest was diagnosed with a kidney condition at a very young age, but he demonstrated remarkable resilience and strengthened his character with a pragmatic and decisive optimistic outlook on life. This condition did not hinder his active scientific pursuits or his rich personal life with great dedication to others, especially to his family, his wife Carmen and his daughters Clara and Julia.

Ernest commenced his academic journey by studying medicine at the University of Lleida, culminating in his graduation in 1986. Ernest continued to pursue his academic and research interests, undertaking doctoral studies focused on the intricate mechanisms of neurotransmission in the basal ganglia, an area critically implicated in neurological disorders, especially movement-related conditions such as Parkinson's disease. This led to the successful completion of his PhD in 1991 under the mentorship of Professor Jordi Alberch, where he received a Cum Laude distinction and a thesis award from the University of Barcelona. His early studies investigated the regulation of neostriatal cholinergic function by tachykinins, revealing that neurokinin B was the most potent in eliciting endogenous acetylcholine release in the neostriatal system (Arenas et al., 1991).

Ernest honed his research expertise during his postdoctoral studies as an European Molecular Biology Organization Fellow from 1991 to 1993 in the group of Dr Håkan Persson at the Karolinska Institutet (Stockholm, Sweden), after which he became an Assistant Professor at the same institute in 1994, later achieving the status of Associate Professor in 1997. His exceptional scientific contributions were further acknowledged with a tenure academic appointment in 1998. In 2002, he was appointed as a Professor of Stem Cell Neurobiology.

In his early years at the Karolinska Institutet, Ernest studied the role of several members of the neurotrophin family of proteins. Ernest described how neurotrophin 3 (NT3) was found to prevent the degeneration of noradrenergic neurons in the locus coeruleus, a brain region affected in Alzheimer's disease. This discovery suggested that NT3 could potentially be used as a therapeutic agent with the potential for preventing the death of noradrenergic neurons in the locus coeruleus (Arenas and Persson, 1994).

The Karolinska Institutet has been at the forefront of pluripotent stem cell research, contributing significantly to our understanding of these cells and their potential applications, and it was an optimal environment for Ernest's efforts to study stem cell biology and how to control cell fate. One of Ernest's notable achievements was the elucidation of key signalling pathways involved in the differentiation of pluripotent stem cells, with special focus on dopaminergic neurogenesis. His studies on the interplay between WNT signalling and neural stem cell fate determination have revolutionized our understanding of the molecular mechanisms governing neurogenesis (Castelo-Branco et al., 2003, 2006). These findings have paved the way for the development of novel strategies to harness the regenerative capacity of neural stem cells for repairing damaged neural circuits. Throughout their research, Ernest and his team consistently emphasized the crucial role of WNT signalling in midbrain dopaminergic neuron development and its potential implications for developing improved protocols to differentiate pluripotent stem cells into midbrain dopaminergic neurons, particularly in enhancing the quantity and quality of stem cell-derived dopaminergic neurons for drug discovery and cell replacement therapy in Parkinson's disease (Parish et al., 2008; Nishimura et al., 2023).

Over the years, Ernest and his team delved into the intricate mechanisms governing early human brain development and the differentiation of specific cell types. Ernest realized early on the huge potential of single-cell omics and adapted single-cell RNA sequencing to analyse ventral midbrain development in both human and mouse brains. His work revealed new molecularly defined human cell types and delineated gene expression dynamics (La Manno et al., 2016). This foundational study not only illuminated the molecular mechanisms governing human midbrain development but also laid the groundwork for the advancement of cell replacement therapies. Ernest knew well that a cell is not a physically isolated entity, and the environment and the communication with other cells is key. To this end, he studied how specific extracellular matrix proteins, such as RSPO2 and SPON1, impact the development of midbrain dopaminergic neurons, how signalling pathways such as CXCL12/CXCR4 regulate the migration and orientation of neural processes, and how the use of certain proteins and inhibitors, such as laminin-511 and GSK3 inhibition, significantly improved the development and differentiation of dopaminergic neurons (Gyllborg et al., 2018; Yang et al., 2013; Zhang et al., 2017; Nishimura et al., 2023). These findings revealed previously unknown roles of these molecules and suggested potential applications to enhance midbrain dopaminergic neurogenesis and differentiation. These findings, supported by single-cell RNA sequencing analysis, not only shed light on the developmental dynamics of the human midbrain, but also offer promise for future applications of precisely defined cell types in treating Parkinson's disease. Interestingly, Ernest was among the early adopters of using GSK3 inhibitors to modulate WNT signalling in vitro, thereby enhancing the generation of dopaminergic neurons (Castelo-Branco et al., 2004; Tang et al., 2010), a practice now commonly employed in many stem cell differentiation protocols.

Another area of focus for Ernest's research involved uncovering previously unreported roles of transcription factors and ligands in governing the specification and survival of midbrain dopaminergic neurons. His studies showed the crucial role of different transcription factors, such as PBX1, in midbrain dopaminergic neuron specification and survival. Ernest and colleagues uncovered the role of the PBX1 transcription factor in regulating a transcriptional network crucial for midbrain dopaminergic neuron specification and survival (Villaescusa et al., 2016). They also identified SREBP1 as part of a cluster of pro-neural transcription factors controlling midbrain dopaminergic neurogenesis (Toledo et al., 2020). Furthermore, Ernest and his team also unravelled the complex regulatory networks governed by the Liver X Receptors (LXR) and ligands in neurogenesis. Controlling the growth of precursor cells and the production of new neurons is crucial for stem cell-based treatments. Ernest investigated how two nuclear receptors (LXRα and LXRβ) and their activators, oxysterols, impact cell division, brain cell development and the creation of dopamine-producing neurons in the ventral midbrain (Sacchetti et al., 2009). Deleting these receptors led to fewer dopaminergic neurons at birth, while activating them increased dopaminergic neuron numbers in both mouse and human embryonic stem cell differentiation protocols. Ernest and his team also identified cholic acid as a new LXR ligand and highlighted 24(S),25-epoxycholesterol as the most potent and abundant LXR ligand in the developing mouse midbrain (Theofilopoulos et al., 2013). Both LXR ligands were found to promote neural development in a cell type-specific manner. These findings introduce a new class of highly selective regulators of neurogenesis and neuronal survival.

Notably, Ernest's efforts extended beyond understanding developmental factors to directly impacting Parkinson's disease treatment. In 2017, Ernest introduced a novel approach for treating Parkinson's disease by directly converting astrocytes into induced dopaminergic neurons (iDANs) using a specific set of transcription factors and a microRNA (NeAL218) (Rivetti di Val Cervo et al., 2017). Through this method, human astrocytes achieved a considerable reprogramming efficiency, generating iDANs with appropriate midbrain markers and excitability. In a mouse model, NeAL218 successfully reprogrammed adult striatal astrocytes into iDANs, showing promise in addressing motor impairments. This approach offers potential for clinical therapies by delivering genes rather than cells.

In addition to his groundbreaking research, Ernest was an influential figure in numerous international scientific conferences, where he shared his insights and fostered collaborations with leading researchers in the field. His keynote presentations on the therapeutic potential of stem cells have inspired and guided the next generation of scientists, fuelling enthusiasm for exploring innovative approaches to neurological repair.

Ernest was an essential part of several European Consortia for stem cell therapy in neurodegenerative diseases, including Neurostemcell, Neurostemcellrepair and, most recently, NSC-Reconstruct. Through collaborative efforts within these consortia over the years, Ernest and his colleagues have significantly contributed to the advancement of research exploring innovative approaches to neurological repair. The collaborative efforts within these consortia have catalysed transformative advancements in regenerative medicine and neural repair, positioning Europe at the forefront of stem cell-based therapies for neurological disorders. Their collective work has definitely contributed to the recent initiation of the first European clinical trial in Parkinson's disease, marking a significant milestone in the translation of stem cell therapy for neurodegenerative conditions.

In recognition of his outstanding academic and research contributions, Ernest was awarded a Researcher position by the Swedish Royal Academy of Sciences in 2001, as well as the prestigious INGVAR Award from the Swedish Foundation for Strategic Research. His leadership and administrative capabilities were evident as he assumed the role of Chairman of the Department of Medical Biochemistry and Biophysics at the Karolinska Institutet from 2010 to 2015. Ernest's remarkable influence and expertise were further recognized by his election as a Member of the Nobel Assembly in Medicine or Physiology, starting in 2010, a testament to his standing in the scientific community. Additionally, in 2016, he took on the role of Co-director of the Strategic Research Area in Stem Cells and Regenerative Medicine at the Karolinska Institutet, solidifying his commitment to advancing transformative research in the field. In 2020, Ernest Arenas was awarded with the prestigious European Research Council Advanced Grant to customize dopamine-producing brain cells specifically for individuals with Parkinson's disease. Through his illustrious career, Ernest has demonstrated a tireless dedication to pioneering research, education, and leadership in stem cell biology and regenerative medicine.

In summary, Ernest Arenas and his team have made significant strides in unravelling the complexities of human brain development, particularly in the context of Parkinson's disease. His passion for discovery and tireless commitment to scientific inquiry have inspired countless colleagues and young researchers, nurturing a new generation of scientists devoted to unravelling the complexities of the nervous system. Ernest was highly esteemed in his research group for his deep dedication to mentorship and his approachable personality. His door was always open, at any time, for all members of his group. He was a gentle authority on midbrain development, and his deep knowledge and scientific commitment will continue inspiring us.