Amyotrophic lateral sclerosis (ALS) is a progressive, fatal neurodegenerative disease that leads to the death of motor and cortical neurons. The clinical manifestations of ALS are heterogenous, and efficacious treatments to significantly slow the progression of the disease are lacking. Cortical hyper-excitability is observed pre-symptomatically across disease-causative genetic variants, as well as in the early stages of sporadic ALS, and typically precedes motor neuron involvement and overt neurodegeneration. The causes of cortical hyper-excitability are not yet fully understood but is mainly agreed to be an early event. The identification of the nucleotide repeat expansion (GGGGCC)_nin theC9ORF72gene has provided evidence that ALS and another neurodegenerative disease, frontotemporal dementia (FTD), are part of a disease spectrum with common genetic origins. ALS and FTD are diseases in which synaptic dysfunction is reported throughout disease onset and stages of progression. It has become apparent that ALS/FTD-causative genes, such asC9ORF72, may have roles in maintaining the normal physiology of the synapse, as mutations in these genes often manifest in synaptic dysfunction. Here we review the dysfunctions of the central nervous system synapses associated with the nucleotide repeat expansion inC9ORF72observed in patients, organismal, and cellular models of ALS and FTD.

Before neuronal degeneration, the cause of motor and cognitive deficits in patients with amyotrophic lateral sclerosis (ALS) and/or frontotemporal lobe dementia (FTLD) is dysfunction of communication between neurons and motor neurons and muscle. The underlying process of synaptic transmission involves membrane depolarization-dependent synaptic vesicle fusion and the release of neurotransmitters into the synapse. This process occurs through localized calcium influx into the presynaptic terminals where synaptic vesicles reside. Here, the protocol describes fluorescence-based live-imaging methodologies that reliably report depolarization-mediated synaptic vesicle exocytosis and presynaptic terminal calcium influx dynamics in cultured neurons. Using a styryl dye that is incorporated into synaptic vesicle membranes, the synaptic vesicle release is elucidated. On the other hand, to study calcium entry, Gcamp6m is used, a genetically encoded fluorescent reporter. We employ high potassium chloride-mediated depolarization to mimic neuronal activity. To quantify synaptic vesicle exocytosis unambiguously, we measure the loss of normalized styryl dye fluorescence as a function of time. Under similar stimulation conditions, in the case of calcium influx, Gcamp6m fluorescence increases. Normalization and quantification of this fluorescence change are performed in a similar manner to the styryl dye protocol. These methods can be multiplexed with transfection-based overexpression of fluorescently tagged mutant proteins. These protocols have been extensively used to study synaptic dysfunction in models of FUS-ALS and C9ORF72-ALS, utilizing primary rodent cortical and motor neurons. These protocols easily allow for rapid screening of compounds that may improve neuronal communication. As such, these methods are valuable not only for the study of ALS but for all areas of neurodegenerative and developmental neuroscience research.

Background and Objectives: Neuropathy is a debilitating disorder characterized by peripheral nerve dysfunction and damage to sensory, motor, and autonomic neurons and their axons. While homozygous mutations in DNAJB2/HSJ1 have been linked to early-onset neuropathy, a heterozygous DNAJB2 c.823+6C>T was discovered in an adult patient with severe sensory–motor polyneuropathy. This mutation is predicted to affect both isoforms of the protein. DNAJB2 (HSP40), a key member of the heat shock protein family, plays a critical role in cellular protection and stress, including response to heat shock. DNAJB2 traffics unfolded proteins to another heat shock protein, HSP70, and activates its ATPase activity to result in a correctly folded protein(s). In this study, we aimed to investigate the effects of the heterozygous DNAJB2 c.823+6C>T mutation on the stress response of DNAJB2 in fibroblasts obtained from the neuropathy patient. Methods: The fibroblasts were subjected to one hour of heat shock at 42 °C, and the time course of expression levels of DNAJB2 was established. Additionally, we evaluated the therapeutic efficacy of Cystamine, which has been shown to modulate DNAJB2 levels in cellular and animal models of Huntington’s disease. Results: Our results revealed reduced baseline levels of DNAJB2 between the mutant and control fibroblasts. Importantly the mutant cells exhibited a diminished response to heat shock. Thus, the mutation affects the upregulation of DNAJB2 under stress, possibly contributing to the pathogenesis of sensory–motor polyneuropathy. A 48-h pretreatment with 150 μM of Cystamine increased the levels of DNAJB2 in both the control and patient’s fibroblasts. Conclusions: To the best of our knowledge, this is the first study to explore this mutant form of DNAJB2 in neuropathy. The study demonstrated that the heterozygous DNAJB2 c.823+6C>T mutation leads to impaired DNAJB2 response to heat shock in the fibroblasts. Cystamine showed promise in restoring DNAJB2 expression, highlighting the need for further research into targeted therapeutic strategies for DNAJB2-related disorders.