Homozygous SMN1 loss causes spinal muscular atrophy (SMA), the most common lethal genetic childhood motor neuron disease. SMN1 encodes SMN, a ubiquitous housekeeping protein, which makes the primarily motor neuron-specific phenotype rather unexpected. SMA-affected individuals harbor low SMN expression from one to six SMN2 copies, which is insufficient to functionally compensate for SMN1 loss. However, rarely individuals with homozygous absence of SMN1 and only three to four SMN2 copies are fully asymptomatic, suggesting protection through genetic modifier(s). Previously, we identified plastin 3 (PLS3) overexpression as an SMA protective modifier in humans and showed that SMN deficit impairs endocytosis, which is rescued by elevated PLS3 levels. Here, we identify reduction of the neuronal calcium sensor Neurocalcin delta (NCALD) as a protective SMA modifier in five asymptomatic SMN1-deleted individuals carrying only four SMN2 copies. We demonstrate that NCALD is a Ca-dependent negative regulator of endocytosis, as NCALD knockdown improves endocytosis in SMA models and ameliorates pharmacologically induced endocytosis defects in zebrafish. Importantly, NCALD knockdown effectively ameliorates SMA-associated pathological defects across species, including worm, zebrafish, and mouse. In conclusion, our study identifies a previously unknown protective SMA modifier in humans, demonstrates modifier impact in three different SMA animal models, and suggests a potential combinatorial therapeutic strategy to efficiently treat SMA. Since both protective modifiers restore endocytosis, our results confirm that endocytosis is a major cellular mechanism perturbed in SMA and emphasize the power of protective modifiers for understanding disease mechanism and developing therapies.

Since its introduction as a genetic vertebrate model system approximately 30 years ago, the focus of zebrafish research has increasingly shifted to questions that are also relevant for human development and disease. Here, we review the potential of the zebrafish as a model for human endocrine systems. A recent review compared the functions of the different endocrine systems and glands in zebrafish with those in other vertebrates, including humans, coming to the conclusion that major aspects are conserved. Here, we present an updated overview of this rapidly growing field of zebrafish research, focusing on the hypothalamo-pituitary axis, which links the central nervous system with the endocrine systems, and on major processes that are under (neuro)endocrine control and are the subject of intensive current research in other endocrine model organisms, such as feeding circuits and energy homeostasis, sleep, stress, reproduction, osmoregulation, and calcium homeostasis. Finally, we summarize the strengths and weaknesses of zebrafish as a model for studying endocrine systems.

Vertebrate dopaminergic neurons develop in distinct neural territories to constitute one of the major neuromodulatory systems. We have identified a zebrafish mutation in the bHLH-PAS family member arnt2, based on a strong reduction in cell number of specific dopaminergic neuron groups in the hypothalamus and posterior tuberculum. Knockdown of sim1 causes a dopaminergic phenotype similar to arnt2 mutants, suggesting that Sim1 acts as a binding partner of Arnt2, similar to their role in hypothalamic neuroendocrine cell specification. sim1, arnt2 and otp are co-expressed in dopaminergic neurons, and combined overexpression of Sim1 and Otp leads to formation of supernumerary dopaminergic neurons in the ventral diencephalon. Arnt2, Sim1 and Otp thus are core components of a conserved transcriptional network, which specifies neuroendocrine as well as A11-related dopaminergic neurons in the fish hypothalamus and posterior tuberculum. Our data suggest a common evolutionary origin of specific hypothalamic neuroendocrine and dopaminergic systems.

Dopaminergic and noradrenergic neurons constitute some of the major far projecting systems in the vertebrate brain and spinal cord that modulate the activity of circuits controlling a broad range of behaviors. Degeneration or dysfunction of dopaminergic neurons has also been linked to a number of neurological and psychiatric disorders, including Parkinson's disease.Zebrafish (Danio rerio) have emerged over the past two decades into a major genetic vertebrate model system,and thus contributed to a better understanding of developmental mechanisms controlling dopaminergic neuron specification and axonogenesis. In this review, we want to focus on conserved and dynamic aspects of the different catecholaminergic systems, which may help to evaluate the zebrafish as a model for dopaminergic and noradrenergic cellular specification and circuit function as well as biomedical aspects of catecholamine systems.